Author's Accepted Manuscript
Wear resistance investigation of titanium nitridebased coatings Eleonora Santecchia, Erfan Zalnezhad, A.M.S. Hamouda, Farayi Musharavati, Marcello Cabibbo, Stefano Spigarelli
PII: DOI: Reference:
S0272-8842(15)00911-6 http://dx.doi.org/10.1016/j.ceramint.2015.04.152 CERI10559
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Received date: Revised date: Accepted date:
28 January 2015 14 April 2015 27 April 2015
Cite this article as: Eleonora Santecchia, Erfan Zalnezhad, A.M.S. Hamouda, Farayi Musharavati, Marcello Cabibbo, Stefano Spigarelli, Wear resistance investigation of titanium nitride-based coatings, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2015.04.152 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Wear resistance investigation of titanium nitride-based coatings Eleonora Santecchiaa*, Erfan Zalnezhadb*, A.M.S. Hamoudaa, Farayi Musharavatia, Marcello Cabibboc, Stefano Spigarellic a
Mechanical and Industrial Engineering Department, College of Engineering, Qatar University, 2713, Doha, Qatar. b
Department of Mechanical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 133-791, Korea.
Dipartimento di Ingegneria Industriale e Scienze Matematiche (DIISM), Università Politecnica delle Marche, I-60131, Ancona, Italy.
Abstract The wear of components while they are in service is a predominant factor controlling the life of machine components. Metal parts are often damaged because of wear-driven failures causing the loss of dimensions and functionality. In order to reduce wear, researchers follow two paths: (i) use new, wear resistant materials, or (ii) improve the wear resistance of materials by adding alloying elements or performing surface treatments. Thin film hard nitride coatings are seen as a viable way to enhance the wear resistance of metallic materials, thus extending the lifespan of products. This paper reviews the wear resistance of titanium nitride-based coatings obtained using physical vapor deposition (PVD), chemical vapor deposition (CVD), and thermal spraying techniques. The results of thin film coatings deposition on the wear performance and on the coefficient of friction are investigated. The advantages and disadvantages of coating methods are discussed. Finally, recent developments and new
possibilities for coating manufacturers to produce films with enhanced wear performance are briefly discussed.
Keywords: Films; Wear resistance; Friction; Wear parts.
* Corresponding authors: Eleonora Santecchia, [email protected]
, Phone: +974 44036696, Fax: +974 44034300; Erfan Zalnezhad, [email protected]
, Phone: + 82-2-2220-0431, Fax: + 82-2-2292-3406.
1. Introduction The scientific research on advanced materials is unstoppable. Many research groups from all over the world have pushed the performances of materials beyond their theoretical limits, in order to achieve the best results possible . When manipulating raw materials becomes too challenging, surface coating is the best way to improve the performance and the durability of materials, especially those exposed to aggressive environments or extreme working conditions. For several years, thin hard coatings were considered to be crucial in the production of mechanical parts and tools, owing to their hardness and wear resistance properties [2-5]. Nitride coatings are the most frequently used because they combine features such as high bond strength to the substrate [6-7] and excellent resistance to wear, erosion, and corrosion [6-7]. In particular, transition metal nitrides are widely used because of their excellent intrinsic properties such as: (i) good conductivity, (ii) hardness, (iii) high melting point, (iv) chemical stability, and (v) wear resistance. For these reasons, transition metal nitrides have been used as diffusion barriers and in hard, wear resistant, and anti-corrosion coatings [8-23]. Titanium nitride (TiN) is a hard and versatile ceramic material, known to crystallize in the B1 NaCl structure. It exists as a solid solution with a nitrogen concentration in the range of 37.5%-50% [24-25]. TiN has good wear and corrosion resistant properties [26-27], and it is widely applied on cutting tools in order to increase their lifespan . Titanium nitride has biocompatible properties [29-31] as well as a combination of high ductility and hardness, leading to its use on medical implants such as orthopedic and dental prosthesis [32-36]. The application of titanium nitride based coatings on machining tools is essential because there are limitations related to the use of lubricating oils to minimize the
environmental impact of industrial machining operations [37-39]. Moreover, ecycling cleansing agents and disposal of waste remain critical safety and pollution issues . Dry machining represents a viable alternative to wet machining  but it requires surface engineering techniques that provide low friction and wear resistant surfaces, in order to get the same advantages provided by cutting fluids, acting as transfer mediums for chip removal, coolants, and lubricants . From a technological point of view, high speed machining  is another industrial process that is attracting more and more attention. Consequently, combining these two cutting methods was inevitable and has resulted in the evolution of dry high-speed cutting systems [46-47]. The major issue related to dry cutting and high speed machining is local heat generation, implying the use of tools with high heat resistance, and having heat insulating coatings on tool surfaces . During dry cutting procedures, high temperatures cause hardness changes, metallurgical transformations, and even chemical composition variations caused by deforming materials and the steps taken to overcome sliding friction between tools, work pieces, and chips. This behavior has a dramatic influence on tool life and on the finished product, in terms of surface integrity and the accuracy of the dimensions and shape [49-51]. The goal of this paper was to provide a broad overview of the wear resistance of titanium nitride based coatings obtained using different preparation techniques including physical vapor deposition (PVD), chemical vapor deposition (CVD), and spraying techniques. Physical vapor deposition (PVD) techniques are used extensively to deposit a wide range of materials on a number of substrates. The general classification of PVD techniques is made according to the methods used to evaporate the target (material to be
deposited) . Titanium nitride-based coatings are mostly deposited by magnetron sputtering, cathodic-arc and pulsed laser deposition techniques . The wide diffusion of PVD techniques for industrial applications is due to the resulting coatings, hard and durables that can be deposited on organic or inorganic substrates. The high temperatures involved in the deposition process and the high vacuum required, are the major drawbacks of these methods . Chemical vapor deposition (CVD) techniques are based on the use of chemical precursors to form a thin film on a substrate by reacting in an isolated chamber. CVD techniques can be divided in groups according to the method used to activate the chemical precursors . Plasma enhanced and plasma assisted chemical vapor depositions (PECVD, PACVD), as well as thermal chemical vapor deposition, are the most used techniques to obtain TiN-based thin film coatings. CVD techniques allow the synthesis of very pure and dense thin films enabling high deposition rates on objects with complex geometries and shapes. However, the complexity of some chemical processes, as well as the toxicity of exhausted gases represent the main drawbacks associated with these techniques. PVD and CVD both suffer from the greatest film thickness achievable, which is usually around 10 µm . This issue can be overcome using spraying techniques. Thermal spraying and reactive spraying are used to obtain titanium nitride based films .
2. Deposition techniques 2.1 Physical Vapor Deposition (PVD) techniques Physical vapor deposition (PVD) is a group of techniques widely used to obtain thin films. Generally, during a PVD process the material to be deposited is evaporated from
a solid or liquid source (target) and is carried in the form of plasma to the substrate, where it condenses. According to the method applied to obtain the evaporation of the target material, the most common PVD techniques are classified as follows: (i) sputtering deposition [54-55], (ii) cathodic-arc deposition [56-57], (iii) electron beam physical vapor deposition (EB-PVD) [58-59], (iv) pulsed laser deposition (PLD) , and (v) ion beam assisted deposition (IBAD) [62-64]. In addition, plasma assisted or enhanced physical vapor deposition has grown in importance owing to a number of advantages it offers compared to classical PVD methods, such the chance to deposit alloy compounds, and the ability to vary coating characteristics continuously throughout the film, providing functionally graded coatings . Fig. 1 illustrates a schematic for a PVD system. In sputtering deposition, Direct Current, DC (diode and triode), and Alternate Current, AC (radiofrequency) are the two most common operating conditions. Both operating conditions can perform deposition in two different configurations, namely magnetron balanced and magnetron unbalanced sputtering [66-67]. DC reactive magnetron sputtering from an elemental target is a popular technique that can produce films with controllable stoichiometry  and composition  at high deposition rates and on an industrial scale [14, 36]. Single layers as well as multilayers TiN-based coatings have been deposited successfully with sputtering techniques on a number of substrates [25, 36, 68-81]. When two or more elements besidestitanium must be sputtered, the cosputtering technique was proved by several authors [82-85]. Magnetron sputtering efficiency can also be improved
with plasma, resulting in a plasma-enhanced
magnetron sputtering (PEMS) deposition. During the PEMS process in the classical magnetron sputtering configuration, an electron source (i.e. a hot filament) heated by an
AC power supply, is introduced in order to generate electrons. When a DC power supply (discharge) is applied between the filament and the chamber wall, the electrons accelerate to the wall. The presence of gas (Ar) in the vacuum chamber leads to electron-neutral collisions, resulting in ionization and plasma generation [86-90]. Cathodic-arc is one of the oldest known vacuum coating techniques. Improvements to coating properties
given by this technique are linked to the production of high
quantities of ions that provide thin films with enhanced adhesion, density, and composition stoichiometry for compound coatings . The deposition can be performed using continuous (DC) deposition or pulsed deposition. Commercially, cathodic arc deposition in DC mode is the most commonly used method and it is particularly suitable for large areas and for the high-throughput deposition of relatively thick films because it is capable of very high deposition rates. On the other hand, pulsed deposition enhances the properties of the deposited films and their controllability. Moreover, pulsed deposition requires much less cooling because the average power can be kept lower than 1 kW, and the out coming deposition rates are relatively small . In order to reduce the incorporation of “macroparticles” generating high compressive stresses in the films, appropriate filters should be applied [57, 91]. A strong correlation between substrate bias and Ti nitride based thin films has been reported in literature  along with the properties and performance of these films when they are deposited on tool steel [93-96]. In the electron beam physical vapor deposition (EB-PVD) process, beams of focused high-energy electrons are generated from electron guns and are used to melt and evaporate ingots besidespreheating the substrate in the vacuum chamber. Electron beam physical vapor deposition produces graded coatings with a high level of reliability and
durability, generating a graded transition zone instead of a flat interface between layers, and achieving a good adhesion to the substrate. Moreover, the cost of this technique is lower than that of the traditional multistage technologies. EB-PVD has been successfully used to deposit titanium nitride based coatings [97-98] and is particularly suitable for thermal barrier coatings applications [99-100]. Pulsed laser deposition (PLD) is a very powerful branch of PVD. The outstanding features of the PLD technique are the stoichiometry transfer between target and deposited film and the high deposition rate (about 0.1 nm per pulse) [60-61,101]. The first property is due to the target surface being heated very quickly by the intense laser beam , ensuring that all the target components evaporate at the same time irrespective of their partial binding energies . In other words, when the density of the laser energy is sufficiently high, each laser pulse vaporizes (or ablates) a small amount of the target material. Then, the ablated material is ejected from the target in a high, forward-directed plasma plume, resulting in a flux that leads to film growth . The pulsed nature of this technique allows for the production of complex composite materials because the laser conditions for each target can be varied . Additionally, the inert gas atmosphere where PLD takes place allows the film properties to be adjusted by varying the kinetic energy of the deposited particles [60, 61,101]. Hard titanium nitride based thin films have been successfully deposited by PLD [103-108] and the effects of the gas pressure on the properties of the thin films have been recently studied . Ion beam assisted deposition (IBAD) is a technique that usually combines sputtering or electron beam evaporation with the ion implantation concurrent ion beam bombardment, producing a final coating with a highly intermixed interface  and
with a less built-in strain compared to other PVD techniques [28,111-113]. The effect of IBAD on the coating depends on many parameters such as: (i) ion energy, (ii) ion to atom arrival ratio, (iii) angle of ion incidence, (iv) pressure, and (v) temperature . The main advantage of IBAD is that it allows adherent coatings to be applied at low temperatures. In practice, the additional bombardment with ions is combined with a conventional deposition process, enhancing the mobility of the atoms on the surface of the samples and the properties of the coatings [115-116]. Single-layer and multilayer TiN-based coatings and nanocrystalline coatings [123-124] have been deposited using IBAD [117-122].
2.2 Chemical Vapor Deposition (CVD) techniques Chemical vapor deposition processes are widely used in industry because they deposit a wide range of elements and compounds in a number of structures, from amorphous to epitaxial. In a CVD process, the chemical volatile precursors reacts or decomposes (or both) on the surface of the substrate, in order to generate the desired thin film . The process of activating the precursors defines the type and the characteristics of the chemical vapor deposition techniques. The most common CVD technologies are classified as follows: (i) thermal (conventional) [52,126], (ii) low pressure chemical vapor deposition (LCVD) [52,127], and (iii) plasma activated/assisted chemical vapor deposition (PACVD) or plasma enhanced chemical vapor deposition (PECVD) [52,128130].
Conventional chemical vapor deposition (thermal CVD) can occur in a hot wall reactor or in a cold wall reactor. In a hot wall reactor, the reactor is placed inside a resistance heater or a tube furnace so that the substrate and the reactor’s wall are at the same temperature. In a cold wall reactor, only the substrate is subjected to heating . A schematic draw of the CVD process and the design of the hot and cold wall reactors can be found in Fig. 2(a) and Fig. 2(b). Several studies have examined TiN and TiN-based thin films deposited by conventional CVD, at different temperature levels and on several substrates such cemented carbides, tools steel, and silicon wafers [131-135]. Depending on the deposition parameters and on the materials involved, typical values of low temperatures CVD can go from less than 300 °C to ~700 °C [52,134-135], while moderate-high temperatures are in the range of 800 - 1600 °C [134-136]. It has been shown that low deposition temperatures create textured TiN coatings . The poor mechanical properties of Ti-Al-N coatings deposited at high temperatures with thermal CVD have been also reported . LPCVD is similar to conventional CVD, but the lower pressure is its only distinguishing parameter. The main advantage of lower pressures is the ratio of the mass transport velocity and the velocity of reaction on the surface, while the major drawback is that LPCVD can only occur at high temperature [125,138]. Low pressure CVD has been used since early 90s’, for the deposition of titanium nitride thin films [139-142]. More recently, Endler et al. examined a new LPCVD process for preparing TiAlCN layers involving moderate temperatures (800 °C - 900 °C) and pressures below 10 kPa . The issue of the high temperatures required by thermal CVD and low pressure CVD can be overcome with the introduction of plasma-based techniques in CVD systems. In
these situations, gas phase chemistry can be used because the energy comes from a plasma discharge rather than a thermal source, as in high temperature processes. During a plasma-based chemical vapor deposition, the vapor phase constituents react to form a thin solid film, thanks to an electric discharge, and involve temperatures typically lower than 300 °C. Plasma has a twofold role: it enhances the formation of compound films by activating the reactions between the evaporating metal species and the reactive gas, and it modifies growth kinetics, structure, morphology and therefore the physical properties of the final coating [52,144-145]. Plasma-based CVD is widely known as plasma enhanced CVD (PECVD), or plasma activated/assisted CVD (PACVD) . The name “plasma induced CVD (PICVD)” has also been used . PECVD allows homogeneous coatings to be applied on objects with complicated shapes, while ensuring high adhesion and good morphology of the layers. Recently, a high power pulsed PECVD (HiPP-PECVD) process has been developed and it is a promising tool for low temperature deposition of films with tailored properties such as those found in the hard coatings industry [146,148]. When the substrate is placed far from the plasma generation zone, or rather not directly subjected to the plasma discharge generated between anode and cathode, PECVD is performed in a remote configuration. This is a more gentle technique owing to the lower degree of particle bombardment  and it is frequently used in the CVD process known as atomic layer deposition (ALD) . Titanium-based nitrides deposited by PECVD for mechanical applications are attracting the attention of the scientific community [151-154]. Tools coated with TiN-based hard coatings deposited by plasma-based techniques have been used in several industrial applications [155-159]. Low-friction titanium nitride based
single-layers and multilayers have been obtained using the PACVD/PECVD technique [160-162]. This technique has been successfully used for
the deposition of TiN
coatings for biomedical applications as reported in literature [163-164].
2.3 Thermal Spraying Thermal spraying is a class of deposition techniques that is gaining more and more importance, owing to its versatility and to the wide range of materials that can be used as substrates (metals, ceramics, and cermet) [33,165]. During a thermal spraying process, the material is taken to a plastic or molten state using heat generated by combustion gases or the electric arc. Then, the heated material is accelerated using a compressed gas through a gun, and the confined stream of particles is conveyed to the surface of the substrate, where they cool down and build up the coating (lamellar structure) [166-168]. Despite the short dwell time and the steep temperature gradients in a DC plasma jet, carbides, borides, and nitrides can be synthesized from reactive gases or solids using atmospheric or low-pressure thermal plasma [169-170]. The most commonly used method in the field of Ti-based nitrides is reactive plasma spraying (RPS). However, a few authors have also studied the performance of TiN obtained by cold spraying  and high velocity oxygen-fueled spraying [172-173]. Reactive plasma spraying (RPS) is a promising technology that allows for the development of dense composite coatings with a metallic or an intermetallic matrix and having finely dispersed ceramic phases [53,174-176]. During RPS, a high frequency spark starts an electric arc that acts as the energy source for plasma spraying. In a DC plasma gun, the arc burns between a cylindrical tungsten cathode and a radial concentric copper anode, the latter being also the nozzle where arc gases (usually Ar, He, N2, or
mixtures of these) are introduced. Passing through the arc, the gas molecules react with the electrons in the cathodic source, dissociating or ionizing the gas, thus leading to the plasma state creation [52,177]. A schematic of a typical RPS system can be seen in Fig. 3. The major benefits of the RPS technique on PVD or CVD methods are the following: (i) higher deposition thickness (PVD and CVD usually cannot exceed 10 µm thickness [53,178]), (ii) higher speed of the process , (iii) high toughness, and (iv) high quality of the obtained coatings [33,179-180]. Concerning the film thickness, achievable values are usually higher than 50 microns . Borgioli et al.  obtained films with an average thickness of 120 µm, while Ma et al.  and Yanchun et al.  reached values of ~300 µm and ~400 µm, respectively. The results of TiN and TiNbased coatings deposition by reactive plasma spraying can be easily found in literature [180-182]. The effects of powder preparation has also been studied . The use of a supersonic plasma jet has also been reported .
3. Wear resistance tests The formation of the inherently fragile titanium oxide (TiO2) rutile phase on the surface of TiN-based coatings is one of the reasons for their degradation (delamination) . Oxygen penetration through the coating generates pores and weakens the mechanical properties . Adding aluminum to TiN coatings improves wear resistance because it forms a dense and highly adherent protective oxide layer of Al2O3 that prevents oxygen diffusion in the coated material [185-187].
The incorporation of alloying elements such as B, Si, Al, and Cr affects ffects the properties of thin films including: (i) grain size, (ii) coating morphology, (iii) texture, (iv) chemical and phase composition, and (v) volume fractions of crystalline and amorphous constitutions [188-193]. In other words, alloying has a strong impact on the mechanical, chemical, and tribological behavior of the coatings. The use of multilayered, multicomponent and nanostructured coatings is growing in importance and it is a key method for increasing the protective properties of different industrial products, as well as their hardness, wear, and corrosion resistance under the influence of high temperatures and severe working conditions [194-200]. However, it has been shown that increasing the amount of metal in metal/metal nitride multilayers beyond a certain threshold, leads to a decrease in hardness and wear resistance . There are many types of accelerated test equipments that allow for the control of key friction factors including: (i) applied load, (ii) sample geometry, (iii) sliding velocity, (iv) temperature, and (v) humidity [202-205]. The wear performance of a material when sliding against a counter body is evaluated with measurements performed in a tribometer. Many configurations are available to measure the wear resistance of materials but when coatings are tested, the operating modes depend on the type of mutual movement of the two parts. The two most common operating mode are pin-ondisc (rotating) and pin-on-flat (reciprocating). In pin-on-disc, a pin is held stationary and is forced to impinge on a disc made of the testing material rotating at a fixed speed. In pin-on-flat, a flat plate moves relative to a stationary pin in a reciprocating motion. In some cases, the flat plate is stationary and the pin reciprocates [204-205]. In both configurations, the tip can be a ball, a hemispherically tipped pin, or a flat-ended cylinder and it can be made of different materials, from steel to alumina (Al2O3) or
Cr100, depending on the sample. The ASTM standard designation for wear testing with a pin-on-disc apparatus is G 99 . A schematic overview of all the possible configurations used to study sliding contact between surfaces is shown in Fig. 4 .
3.1 Wear resistance of TiN-based coatings deposited by PVD As wear-resistant or protective layer, the first generation of PVD coatings was made of transition metal nitrides with a NaCl-type structure, namely TiN . The good results provided by TiN coated tools motivated improvements in their performance in terms of hardness and wear resistance, leading to the development of the second and third generation of PVD coatings, (Ti(C,N) and (Ti,Al)N), respectively [209-211]. Ternary nitride films such as (Ti,Al)N are very attractive thanks to the mechanism of solid solution strengthening . The tribological behavior and the wear mechanisms occurring in titanium nitride have been long studied [213-214] and the effects of various parameters such as substrates , coating thickness , and deposition methods  have been addressed. Some examples can be found in Fig. 5, where the effect of nanoindentation on TiN films deposited on different substrates is seen. In order to improve the properties of TiN, various alloying elements have been added, giving rise to common coatings such as TiCN, TiAlN, AlTiN , TiSiN , and TiCNO [220-221]. Tanno et al.  conducted a study on the effect of grain orientation, controlled by changing the substrate bias voltage, on the friction coefficient of TiN coatings. In this study, coatings deposited using arc ion plating (AIP) PVD on SKD11 tool steel were
tested in a ball-on-disc tribometer using a SUJ2 ball as counter body, a normal load of 5 N, a sliding speed of 0.2 cm/s, and a sliding distance of 10 m. The results indicated that TiN coatings with (111) preferred orientation deposited at -200V had the lowest friction coefficient values owing to the formation of a Ti oxide layer with good lubricity on the surface of the counter body that prevented adhesion phenomena. Fig. 6 highlights the significant effect of the substrate bias on the coefficient of friction (COF), in particular at short sliding distances (below 8 m). A steep decrease in the COF is clearly visible shifting the bias from -100 V to -150 V and -200 V. TiAlN coatings are deposited mainly by reactive evaporation of Ti–Al intermetallic target materials using nitrogen and either sputter evaporation or arc evaporation because of the dense structures with smooth top surface found in the single-layer arc-TiAlN [223-224]. The tribological behavior of TiN, TiAlN, and TiCN deposited by arc-evaporation PVD, both on high speed steels (HSS) M2 and cemented carbides (WC/Co) substrates [4,225] has been the subject of a number of studies. In a study conducted by Rodriguez et al. , TiCN provided the best results during friction tests. Aihua et al.  reported that the addition of Al led to a higher friction coefficient (TiAlN > TiN). However, the wear mechanisms of TiN failed because of oxidation and abrasive wear. The wear mechanism TiAlN was a mixture of abrasive wear, oxidation, and micro-grooves. Fig. 7 shows the typical microstructure of the TiN (Fig. 7(a)), TiAlN (Fig. 7(b)), and AlTiN (Fig. 7(c)) coatings before sliding. The white “droplets” visible on the TiN and TiAlN surfaces seen in Fig. 7(a-b) and less in Fig. 7(c), are typical defects of the cathodic arcevaporation technique and are due to and incorrect reaction between N and the evaporated metal macro particles.
Deposited TiN and TiAlN coatings by means of arc ion plating (AIP) on AISI D2 substrates were tribologically characterized under two different contact loads (1 N and 5 N), and three different sliding speeds (0.1, 0.3, 0.5 m/s) against steel and alumina balls using a conventional ball-on-disc sliding wear apparatus . Tribological tests performed against the steel ball showed adhesive wear for both coatings. Tribological tests conducted using an alumina ball showed an abrasive-like wear behavior. The different wear behaviors can be easily observed in Fig. 8 (a-d). When sliding against the steel counter body, wear debris are smeared on the wear track (Fig. 8(a) and (b)), especially in the case of the TiN coating. Against the alumina ball (Fig. 8(c) and (d)), both coatings showed an accumulation of debris at the boundaries of the wear track. Consequently, the TiAlN coatings were more favorable than TiN coatings for high speed machining owing to the lower friction coefficient at the highest sliding speed . Ball-on-disc as well as reciprocating tests have been performed by Zhu et al.  on TiAlN deposited by arc PVD on cemented carbide substrates. The results of this test demonstrated only fine distinctions between the two different sliding modes. The tribological performance of TiN/TiAlN multilayer coatings deposited on M2 high speed steel by a pulsed bias arc ion plating system  were tested in a pin-on-disc apparatus at room temperature against a hardened GCr15 steel ball with a revolution rate of 50 rpm and a normal load of 100 g. The relationship between the friction coefficient and the sliding distance was continuously recorded. The results showed that TiN/TiAlN multilayer coatings fabricated at 50% duty ratio of pulsed bias had the lowest friction coefficient, making them quite favorable for industrial applications.
Ti-based thin films (TiN, TiCN (Nrich), TiCN (C-rich), TiAlN, AlTiN, TiSiN and TiCNO) deposited by magnetron sputtering on M2 steel were subjected to ball-on-disc wear tests (ASTM G99) under a normal load of 2.5 N with a speed of 20 cm/s, against alumina balls . Analyzing the wear debris determined that the wear experienced by all the Ti-based coatings was due to the formation of TiO2 while AlTiN was subjected to ploughing wear. Fig. 9 shows the optical micrographs of the wear tracks of the TiSiN (Fig. 9(a)) and AlTiN (Fig. 9(b)) coatings where the traces left by small abrasive particles detached from the coating via brittle failure play the role of a third body  giving rise to the ploughing wear mechanism. Ti(C,N) and Ti(C,N,O) coatings deposited using the same technique with different oxygen and nitrogen flow rates were characterized in a pin-on-disk tribometer (ASTM G99) against an alumina ball under a normal load of 5 N at 30 cm/s . The results demonstrated that the lowest wear rate was reached with an oxygen flow rate of 4 sccm during the deposition of the TiCNO coating. An increase in oxygen flow rate caused a decrease in hardness, adhesion, and wear resistance, besides an increase of friction coefficient. Andersen et al.  suggested three different tests in order to characterize the tribological behavior of TiN-TiAlN multilayers and single-layers deposited on AISI D2 substrates by unbalanced magnetron sputtering in a pin-on-disc apparatus. These tests are: (i) friction test against a fine ground flat pin of hardened 100Cr6, sliding speed of 0.05 m/s, normal load 0.5 N and sliding distance 200 m, (ii) step-load test, using a hardened bearing ball (100Cr6) as counter body, using a sliding speed of 0.3 m/s, a normal load increased stepwise from 5 N to 10, 20, 40, 80, 120, 160 and 200 N for each 50 m of sliding (total sliding distance of 400 m), and (iii) wear test, using a ceramic
Al2O3 bearing ball as counterpart, a sliding speed of 0.3 m/s and a normal load of 5 N. This study showed that the friction coefficients of the multilayered coatings were larger compared to those of the single-layered TiN and TiAlN. Improvements in the wear properties were observed for the multilayers, owing to the significantly reduced abrasive wear. PVD-obtained AlCrN, TiN, TiAlN single-layer coatings as well as TiAlN/AlCrN, AlN/TiN nanomultilayer coatings were subjected to wear tests in a ball-on-disc tribometer at a speed of 10 and 100 m/min, under a normal load of 5N for a sliding distance of 200 m, both in ambient air and vacuum (1× 10-5 Pa) conditions . Out of all the single layer coatings, TiN showed the highest lubrication (Fig. 10(a) and (b)) and the multilayer coating that exhibiting the lowest coefficient of friction was AlCrN/TiAlN (Fig. 10(c) and 10(d)). The most severe wear was experienced by TiAlN because of a long running-in process, a high steady state COF, and a relatively low hardness, compared to the nano-coatings. TiN, TiAlN/AlCrN and AlN/TiN exhibited similar wear resistance values but had higher wear resistance than TiAlN . TiN/TaN  as well as TiN/CrN, TiN/MoN, TiN/NbN  multilayers deposited by electron-beam physical vapor deposition (EBPVD) on HSS and cemented carbide substrates have been the subject of several studies. Micro abrasive analyses conducted using a ‘crater/dimple grinder test’ [232-233] showed that the tribological properties of the TiN/TaN coatings were much better than what was suggested by the properties of the two homogenous materials (rule of mixture). Moreover, TiN/TaN was the best choice for applications where the predominant wear mechanism is abrasive wear [197, 231].
Using the same dimple grinder test apparatus (Fig. 11) typically used in TEM specimen preparation, Bemporad et al.  demonstrated that the thickness of the Ti/TiN multilayers deposited by cathodic arc-PVD on AISI M2 tool steel led
to a wear
resistance depending only on the total amount of Ti. They also found that coatings produced with optimized ductile layers distribution (i.e. increasing towards coating/surface interface Ti quantity) possessed the greatest wear resistance, leading to longer substrate protection for severe wear applications. The wear performances of hard nanostructured Ti-based ternary (Ti–B–N), quaternary (Ti–Cr–B–N, Ti–Si–B–N, Ti-Al-B-N), and Ti–Al–Si–B–N sputtered coatings, have been documented by several authors [189-191]. Ti–B–N, Ti–Cr–B–N, Ti–Si–B–N and Ti–Al–Si–B–N were deposited on single crystal silicon (100) and hard alloy (TT8K6 brand mark) substrates several times. They were then tested in a ball-on-disc tribometer under a normal load of 5 N, against a WC+6% Co ball, for 10000 cycles at a sliding speed of 0.1 m/s. All the coatings showed friction coefficients ranging from 0.39 to 0.6, depending on the Ar and N partial pressures used during the deposition process. The combination of low friction, high hardness, and adhesion strength makes the Ti–B–N, Ti–Cr–B–N, Ti–Si–B–N, and Ti–Al–Si–B–N coatings appropriate for various tribological applications . Details regarding deposition and ion implantation parameters, as well as tribological characteristics are reported in Table 1. Titanium-boron nitride films synthesized using ion beam assisted sputter deposition (IBAD) from a TiB2 target were tested under normal loads of 1 and 5 N, at a speed of 0.1 m/s, using mirror-polished steel ball as counter body . For coatings synthesized at a bombarding energy of about 200 eV, a remarkable anti-wear improvement (two orders of magnitude lower in the wear rate) was achieved. This result was connected to
the unique multiphase architecture characterized by nanosized hard phases (TiB2 and TiN) bounded within the lubricating matrix (h-BN) . Ion beam assisted deposition (IBAD) obtained Ti-Cr-N coating on Si (100) wafers and 440C stainless steel  were tested in a ball-on-disc tribometer against 440C SS ball, under a normal load of 50 N, at a speed of 180 rpm for 1.5 million cycles (about 6 days for each test), under marginal lubricated conditions using 600P neutral oil. These test conditions simulated well drilling conditions and demonstrated the excellent wear properties of the coating. The effect of the coating on the wear performance of the sample is reported in Fig. 12. The effects of W , Nb, C , Si  and V  ion implantation on the tribological behavior of TiN coatings have been reported by the authors of several recent studies. Tribological tests were conducted in a ball-on-disc tribometer against GCr15 ball (AMST A-295 52100 bearing steel), except for the W-ion implanted TiN films. The W-ion implanted TiN films were tested using a Si3N4 ball as the counterpart. Rotating wear tests showed that TiN coatings with high doses of W implantations lead to low friction coefficients, owing to the formation of lubricating tungsten oxides and to the longer lubrication time of titanium oxides during the wear process . The wear performances of Nb and C ion-implanted TiN thin films were tested in reciprocating configuration, under a normal load of 30 N and at a frequency of 25 Hz, for 5 minutes. The results showed that implantation lead to the formation of NbN and TiC phases, remarkably improving the wear behavior of the coatings and positively effecting the Nb + C dual ion implantation where a carbonaceous layer acted as solid lubricant . The wear behavior of Si-ions implanted TiN coatings tested in a ball-on-disc apparatus (15 N, 300 (r/min), 70.7 m), resulted to be improved at the dose of 5×1016 ions/cm2.
When the implanted dose reached 1×1017 ions/cm2, the wear behavior worsened because the surface softened due to a Si implantation overdose . Vanadium ionsimplanted TiN coatings where subjected to the same wear test as Si-implanted TiN and they showed adhesive wear and low friction coefficients thanks to the formation of VN and vanadium oxide, leading to increased wear resistance . A feasible way to improve surface wear resistance is to combine plasma nitriding and PVD coating, or rather to employ a process also known as duplex surface treatment . Concerning this particular treatment, TiN and TiAlN hard coatings have been deposited by PVD on AISI 4140 steel substrates subjected to different plasma surface treatments [239-240]. The wear resistance properties of this material were evaluated using a pin-on-disc tribometer. Duplex-treated flat-ended pin samples were loaded against an uncoated disc made of ball bearing steel (100Cr6, hardened to 700 HV0.5). Unlubricated tribological tests were conducted at a sliding speed of 1 m/s for a sliding distance of 100 m, under normal loads of 30 N and 60 N. The results showed that by a controlling the nitriding conditions, a uniform, homogeneous, and highly adherent compound layer is obtained that can reduce the hardness and the stress gradient between substrate and coating, resulting in superior sliding wear properties. Quesada et al.  deposited TiAlN on ASTM A36 steel after various pre-treatment techniques, including plasma nitriding. The wear resistance was tested in a nonconventional apparatus, where a ball made of hardened steel AISI D6 rotated on the sample with a velocity of 550 rpm, at a constant charge of 1 Kg, applied during variable time intervals from 5 to 15 seconds. The greatest wear resistance was observed for the pre-nitrided substrates.
Novak et al.  deposited TiAlN using magnetron sputtering on plasma nitrided PM tool steel. The wear resistance of the obtained coatings was evaluated with a modified pin-on-disc method that moved the samples against a grinding paper P1200, for a sliding distance of 2500 m and under a normal load of 5.8 N. The tests showed improved wear resistance and adhesion of TiAlN layers on nitrided substrates similar to those of TiAlN deposited on non-nitrided substrates. Tillman et al.  deposited Ti/TiAlN multilayer coatings with various Ti interlayer thicknesses on different plasma nitrided and non-nitrided steels. The tribological properties were tested in a ball-on-disc apparatus using a sphere of WC/Co as counterpart, a linear velocity of 0.4 m/s for a distance of 100 m, and a normal load of 5 N. SEM micrographs of the multilayer coatings (Fig. 13). The tests showed that the greater hardness of Ti/TiAlN (10/500)5 compared to that of Ti/TiAlN (200/500)5 led to a higher wear resistance for Ti/TiAlN (10/500)5. Luo et al.  studied the mechanical properties and wear performances of newly developed TiAlN/VN and TiAlCrYN coatings deposited on a low alloy cold-mould steel P20, following a pulse plasma nitriding pre-treatment. Wear tests were conducted in air at a relative humidity of 30–35% using a pin-on-disc tribometer, with an alumina ball as a counterpart, sliding at a 0.1 m/s speed, under normal loads of 5 and 10 N. The results showed that nano-structured multilayer TiAlN/VN coatings exhibited excellent wear resistance with friction coefficients between 0.43 and 0.55 depending on the applied load. The obtained wear coefficients were in the region of 10-17 m3/Nm and were significantly lower than the wear coefficients for the monolithically grown TiAlCrYN .
TiN thin films were synthesized using reactive pulsed laser deposition (RPLD) and were subjected to low-load (25–100 mN range) friction and wear measurements using a nanotribometer . The tests showed that nanocrystalline samples had a lower friction coefficient compared to that of the microcrystalline samples. The wear performance of TiN/Ti/a-C:H multilayer coatings produced by hybrid PLD (Pulsed Laser Deposition + magnetron sputtering) was tested in a ball-on-disc tribometer by Major  who applied 1 N load normal for 20000 cycles. The results found that more than one type of wear mechanisms occurred simultaneously during the wear process including cracking, layer-by-layer removal, and tribofilm formation. The TEM micrographs reported in Fig. 14 show that the crack was moving from the bottom to the top part of the coating or in the opposite direction, depending on the distance from the indenter. The stress concentration motion caused the 'layers motion' . In order to assess the behavior of PVD-obtained hard coatings at high temperatures, tribological tests were performed on a variety of coatings at temperatures up to 900ºC [6, 13,246-247]. Fateh et al.  reported the tribological behavior of TiN coatings deposited on highspeed steel, at temperatures from ∼25ºC (room temperature, RT) up to 900ºC, using a ball-on-disc tribometer, performing dry sliding tests in ambient air (relative humidity of 30–40%), under a normal load of 5 N, at a sliding speed of 0.1 m/s, for a sliding distance of 100 m, involving alumina ball as counterpart. These tests showed that wear increased when the temperature increased with the evident formation of rutile TiO2 on the coating surface at 700ºC. In order to overcome this drawback, TiAlN based nanoscale multilayer coatings were designed by Hovsepian et al. . Fig. 15 shows SEM cross-sectional micrographs of the wear tracks after the tribo-test at 700 °C, at low
(Fig. 15(a)) and high (Fig. 15(b)) magnification. Fig. 15 clearly shows that melting occurred on the near-surface region only. This result also implied that the bulk of the TiAlN/VN coating remained intact at the tested temperatures . High-temperature wear tests were conducted on TiAlCrN/TiAlYN and TiAlCrN coatings deposited by a combined cathodic arc/unbalanced sputtering technique called the arc bond sputtering (ABS) technique . Reciprocating sliding ball-on-disc tribological tests were performed against a tungsten carbide ball, under a normal load of 15 N, at reciprocating stroke of 2 mm, and 15 Hz of frequency for 60 minutes. Samples were heated to 400, 600, 800, and 900ºC. The results showed that in the case of TiAlCrN, the coefficient of friction was high and the rise the temperature from 400 ºC to 900 ºC resulted in a significant increase of the wear crater depth (from 0.076 µm to 0.97 µm). Until 600 ºC, the TiAlCrN/TiAlYN coating underwent an initial increase in the wear depth but then showed a significant decrease of the wear depth at higher temperatures. The lowest value of 0.82 µm was reached at 900ºC. The friction coefficient of TiAlCrN/TiAlYN showed a similar behavior according to the temperature level . TiCN coatings obtained by unbalanced magnetron sputtering and deposited on austenitic steel substrates were subjected to tribological tests at high temperature in a pin-on-disc apparatus using balls of 100Cr6 bearing steel and Si3N4 as counter bodies . Some of the material was used at temperatures less than 200 ºC, while other material was used at temperatures up to 500 ºC. All the wear tests were performed under a normal load of 15 N, at linear speeds ranging from 0.04 m/s to 0.3 m/s, for up to 30000 cycles. The results showed that a rise in the temperature increased the friction coefficient and the wear rate of the TiCN coating, when sliding against the 100Cr6
balls. By contrast, friction and wear behavior was not dependent only on temperature for the Si3N4 balls. Polcar et al.  carried out tribological tests at high temperatures on TiN and TiCN coatings, in order to establish which one was the most suitable for applications involving high temperatures. TiCN was deposited by unbalanced magnetron sputtering while TiN was arc-deposited. Sliding tests were performed against spheres of 100Cr6 bearing steel or ceramic Si3N4, at temperatures up to 500 ºC, under a load of 5 N, at a linear speed of 4 cm/s, and for 5000 cycles. Fig. 16 shows the friction coefficients variations for TiCN (Fig. 16(a)) and TiN (Fig. 16(b)) coatings, according to rising temperatures. When testing TiCN against 100Cr6 ball, the results showed that wear occurred at room temperature. By contrast, wear was only is measurable for TiN when the temperature rose above 100ºC. The friction coefficient of TiN was higher than that of TiCN while its wear rate was lower. This indicates that the latter coating can perform as a wear resistant coating at high temperatures . TiAlN/VN coatings were deposited by reactive unbalanced magnetron sputtering on BM2 tool steel coupons then subjected to high-temperature tribological tests in a ballon-disc tribometer against α-Al2O3 ceramic ball at the following temperature levels: RT (~25 ºC), 70, 100, 120, 150, 200, 300, 400, 500, 600, and 700ºC . All the measurements were performed at 0.1 m/s sliding speed. In order to measure the steadystate friction coefficient, part of the tests was performed under 1 N normal load for approximately 15,000 sliding cycles. The wear coefficient measurements were performed under a normal load of 5 N and ran much longer (e.g. up to 150,000 cycles) to get a measurable wear track depth. Fig. 17 shows the overall relationship between the steady-state friction coefficient and the temperature level. Steep variations from 100 to
200 ºC (Region II), from 400 to 500 ºC, and from 600 to 700 ºC, were detected. Regions I and III show relatively temperature independent friction coefficients (µ = 0.60–0.61 and µ = 0.86–0.89, respectively). The highest coefficient of friction was detected in Region IV (µ= 1.1 at 500 ºC), while the lowest (µ= 0.24 at 700 ºC) was found in Region V . Three PVD nitride coatings, TiN, Ti55Al45N, and Ti35Al65N, were deposited on cemented carbide substrates by cathodic arc-evaporation, by Jianxin et al. . Sliding wear tests were conducted in a high-temperature ball-on-disc apparatus at temperatures less than 600 °C, in ambient air conditions, using SiC ceramic balls as counterpart, at sliding speed of 100 m/min, for 2000 cycles, and under a normal load of 10 N. As reported in Fig. 18, the wear rates of the TiN coating showed a gradual increase that rose with the temperature, while Ti55Al45N and Ti35Al65N exhibited a lower wear rate, especially at temperatures higher than 400 °C. Ti35Al65N was the coating with the best wear resistance. The results also showed that wear resistance behavior depended on tribological oxidation behavior. The improved wear resistance of the Ti55Al45N and Ti35Al65N coatings could be attributed to the tribo-chemical properties of the formed Al2O3 oxide layer, which were superior to the properties of the TiO2 layer formed on the TiN coatings . Recently, Wang et al.  discussed the tribological performances of TiCN-coated WC cemented carbide substrates in air and water. Measurements were taken in a ballon-disc tribometer against SiC and SUJ2 / SUS440C steel balls, under a normal load varying from 3 N to 12 N, and at a sliding velocity in the range of 0.1 m/s to 0.4 m/s, covering a total sliding distance of 1000 m. TiCN coatings sliding against SiC in water resulted in a friction coefficient and a specific wear rate that increased with a normal
load and decreased linearly with the sliding velocity. Moreover, the tribological properties of the TiCN/SiC tribo-pair were better in water than in air . The tribological properties of super hard coatings prepared by PVD techniques have been the subject of recent studies [64,252]. Tu et al.  deposited TiN/a-C composite films with different graphite to Ti ratios (G/Ti) using a conventional DC magnetron sputtering system, equipped with a single graphite/Ti combined target, on steel substrates. The tribological properties of the coatings were tested in a ball-on-disc tribometer under un-lubricated conditions, at air humidity of RH = 60%, against quench-and-tempered GCr15 steel ball (HRC62), under a normal load of 1.0 N, at a sliding velocity of 0.11 m/s, and for 1×104 cycles. The results showed that the presence of amorphous carbon in the composite films significantly reduced the friction coefficient and wear rate . Table 2 reports the deposition and tribological characteristics of some TiN-based coatings that can be found in literature. The features reported in brackets in the “Counter body” coloumn are employed to highlight the differences in the tribotests performed by the different authors [4,6,13,189,190,221,225-228,234-237,239-240,243244,247,251]. From the data reported in Table 2, the best wear performance in terms of the coefficient of friction was 0.34, and it was obtained by TiN deposited by cathodic arc-evaporation on HSS M2, when dry sliding against 100Cr6 ball at 70% humidity . A result quite close to this one was reached by depositing TiN using arc ion plating (AIP) on AISI D2 substrate, when dry sliding against alumina, under a normal load of 5 N, an at 0.1 m/s sliding speed . The addition of carbon in PVD deposition resulted in a better, or at least comparable, wear behavior with respect to PVD-deposited TiN, as reported in Table 2 [4,221,226,251]. Very good results have been achieved with
substrate’s surface pretreatments [239-240]. Magnetron sputtering and cathodic arc evaporation are the most commonly used PVD techniques, but ion plating is essential to improve the wear properties of single layer coatings [226,234-237]. Magnetron sputtering, (balanced, unbalanced and DC) was most often used when multilayers wear resistant coatings were deposited [229,243-244,252].
3.2 Wear resistance of Ti-based coatings deposited by CVD TiN coatings for wear resistance applications synthesized by chemical vapor deposition, have been studied and used for many years [157,253]. TiN coatings deposited by thermal CVD at temperatures between 850 and 1050 °C by Wagner et al.  on cemented carbide cutting inserts. Dry-sliding wear tests were performed on polished and un-polished samples, using a ball-on-disc tribometer in ambient air, against alumina balls, with 5 mm wear track radius, under a normal load of 5 N, at 10 cm/s, and for a total sliding distance of 1000 meters. Unpolished (textured) coatings deposited at lower temperatures showed better wear resistance as the finegrained structure was responsible for the brittle and ductile wear mechanisms. By contrasts, coatings deposited at high temperatures suffered from micro-chipping (brittlefracture). In both cases, polishing the surfaces did not improve wear properties. Ternary Ti-B-N coatings with different concentrations of B and N were successfully deposited by plasma enhanced CVD (PECVD) on AISI 304 stainless steel, using different flow rates for the gaseous precursors (TiCl4, BCl3, H2, N2) , and on SKD 61(H13) tool steel . In the first instance, wear resistance tests were performed in a ball-on-disc apparatus against a steel ball at a sliding speed of 0.157 m/s and under a
normal load of 5 N. The results indicated that lowest friction coefficient was obtained by the Ti−41%B−N coating (Fig. 19(e)). This corresponded to a nanocomposite microstructure made up of an amorphous BN phase and fcc-TiN nanocrystallites (Fig. 19(a)-(d)). The addition of boron in Ti-N systems has been studied in order to obtain multiphase structures that can optimize the coatings. Boron is well known for its grain refinement effect [254-255]. The formation of high amounts of TiB2 improved hardness and oxidation resistance, although TiB2 suffered from poor chemical stability when in contact with steel . Wagner et al.  deposited Ti–B–N coatings using thermal CVD with the greatest concentration reaching 35.1 at % B. The effects of the boron on the wear rate of the samples were evaluated in a high-temperature ball-on-disc tribometer at 25, 500 and 600 °C, against alumina balls in ambient air (relative humidity of 35±5%), under a normal load of 5 N, at a sliding speed of 0.1 m/s, using a heating time of 2 hours. The combination of wear-track-radius, sliding distance, and the related number of cycles, were differed for each temperature. The results of wear rate and friction coefficient are reported in Fig. 20. Quaternary Ti-Si-C-N coatings were deposited by Ma et al.  by means of plasma enhanced chemical vapor deposition (PECVD) on high speed steel (HSS) substrates. Five samples with different amounts of C and Si were deposited. The wear resistance properties of the samples were evaluated in a conventional pin (ball)-on-disk tribometer under a load of 5 N, at 20 cm/s sliding speed, for a total distance of 500 m, and involved hardened (HRC 60–62) GCr15 steel balls as counterparts. Tests were carried out at 25 °C and 550 °C, without lubricant. The results revealed that at room temperature, the friction coefficient decreased as the carbon content increased and the silicon content
decreased. At the higher temperature, the friction coefficients of coatings containing a large amount of carbon and little silicon, increased from 0.17–0.35 to about 0.6, owing to the oxidation of the amorphous carbon. The best nanocomposite coating, Ti-12 at.% Si-30 at.% C-20 at.% N, had a friction coefficient of about 0.3 and a low wear rate of 4.5×10-5 mm3/Nm at 550 °C, making it suitable for high-speed and dry machining applications . Kessler et al.  tested the wear resistance of TiN-coated steels subjected to induction surface hardening, performed in order to restore mechanical properties after the coating was deposited. Four different AISI hardened steels were used as substrates: (i) 4140, (ii) 52100, (iii) A2, and (iv) D2. Wear tests on as-deposited and after-induction samples were performed in a pin-on-disc tribometer using Al2O3 balls as counterparts, under a normal load of 20 N, at a revolution speed of 240 1/min, for 7200 total revolutions (30 minutes). The results showed that the as-deposited compounds, CVD TiN/4140, CVD TiN/52100, and CVD TiN/A2, increased the wear resistance in a way that was almost independent from the substrate, heating atmosphere, or quenching medium. By contrast, after induction surface hardening in air only, the wear resistance of the CVD TiN/D2 compound decreased slightly. The lower initial wear resistance appears to originate from the presence of a thin oxide layer . Nie et al.  used an inclined impact–sliding wear tester [261-262] in order to study the failure behavior of TiN/Al2O3/TiCN multilayer deposited on cemented carbide substrates by CVD. The results showed that the multilayer coating arrangement had excellent wear resistance properties because TiN reduced material transfer build-up from the counter body and TiCN has good wear resistance properties.
Kamaraj et al.  focused on issues that occurred while machining hypereutectic aluminum-silicon (Al-Si) alloys, which are used as materials in internal combustion engines. These alloys are usually machined by WC-Co tools. In order to improve the lifespan of the tools, WC-Co substrates were coated with multilayer TiCN/Al2O3/TiN and diamond, using hot filament chemical vapor deposition (HFCVD). Fig. 21 clearly shows the surface of the TiN coating. In the inset the overall multilayer structure is highlighted. The wear behavior of TiN and diamond-coated samples, as well as the bare WC-Co substrates, was studied in a pin-on-disc tribometer under a normal load of 8 kg, at a rotational speed of 400 rpm, keeping a track diameter of 30 mm, and focusing on three sliding distances: 1500, 2000 and 2500 m. The discs were made of AlSi alloy and the pins were the three samples examined in this study (bare WC-Co, TiN/WC-Co, and diamond/WC-Co). The wear tests showed that the diamond coated WC–Co substrates had a wear loss that is one order less than that of bare WC–Co substrates. Moreover, TiN-coated WC-Co samples showed a weight gain instead of a weight loss, owing to the phenomena of adhesion and pileup of the Al–Si counter-face material . Raoufi et al.  studied the effect of a plasma nitriding active screen (ASPN) pretreatment [265-266] on the wear behavior of TiN, obtained by plasma assisted chemical vapor deposition (PACVD). AISI H13 discs were used as substrates. The wear performance of the samples was evaluated in a pin-on-disc tribometer using pins of bearing steel and WC–Co. The tests were performed at 27 °C, under 10 N normal applied force, at a rotating speed of 0.1 m/s, and for a total sliding distance of 1000 m. The results on the weight loss by the samples and pins are reported in Fig. 22. Weight loss is a key measure of wear resistance and the results in Fig. 22 suggest that wear
resistance increased up to 10 times when the ASPN pretreatment was applied. However, during the tribological tests against the steel pin, the ASPN pretreatment did not have any significant influence on the friction coefficient but when using the WC–Co pin, the friction coefficient increased up to 0.8 in the non-ASPN case, owing to the removal of some of the TiN coating . Perry et al. [267,268] studied the wear resistance of metal ion post-implanted TiN deposited by CVD on cemented carbide inserts. Wear tests were performed against ruby balls in a ball-on-disc tribometer, at 22 °C, under a normal load of 1 N, at a speed of 0.125 m/s, on a wear radius of 4 mm, and for a total number of 9550 cycles (total sliding distance of 239 m). The tests revealed a reduced friction coefficient after metal ion implantation. Concerning superhard coatings, Vepřek et al.  made a very interesting comparison between the tribological behavior of nc-TiN/a-Si3N4 coatings prepared by plasmainduced chemical vapor deposition (PCVD) and reactive magnetron sputtering (RMS). Wear tests were performed in a pin-on-disc apparatus against a GCr15 steel ball (HRC 6062), at 25 °C and at a high temperature of 550 °C, under a load of 5 N, at a sliding speed of 20 cm/s, for a total sliding distance of 1 km. At 25 °C, the friction coefficient did not show any clear dependence on silicon content. By contrast, at 550 °C, an increase in the silicon content led to a decrease of COF in the case of RMS deposition, while an increase in the friction coefficient values was observed in samples deposited by PCVD, owing to the presence of chlorine and TiSi2 phases . Table 3 summarizes the tribological performances of some TiN-based coatings obtained by different chemical vapor deposition methods [136,154,257-258,263-264]. The different substrates employed and the counter bodies involved in the tribotests are also
highlighted. From the results reported in Table 3, the best performance in terms of friction coefficient was obtained by multi-component coatings, deposited either by plasma enhanced chemical vapor deposition (PECVD) [154,258] or thermal CVD .
3.3 Wear resistance of Ti-based coatings deposited using spraying techniques The wear resistance of TiN coatings obtained by reactive plasma spraying (RPS) has been studied by several authors [176,270]. Feng et al.  deposited a TiN coating using RPS on AISI M2 high-speed steel substrates, after the deposition of a Ni–10 wt.% Al self-melting alloy bond layer (~100µm) that was used to increase the adhesive strength between the coating and the substrate. Wear tests were performed in a block-on-ring apparatus. The wear ring (38 mm external diameter) was made of AISI E52100 steel, heat-treated to an average hardness of about 60 HRC and moving at a constant sliding speed of 0.4 m/s. The vertical loads applied at the top of specimens ranged from 490 and 1470 N. The RPS TiN coating showed a high level of wear resistance and a low friction coefficient under a high load in non-lubricated conditions. The coefficient of friction of the M2 steel was about 18.5 times of that of the RPS TiN coating under a 1470 N load . Yanchun et al.  prepared nanocrystalline TiN coatings by spraying Ti powder (30∼40 µm in size) using a plasma spray gun with a self-made reactive chamber, filled with N2. Wear resistance was evaluated in the apparatus reported by Feng et al.  at the same sliding speed and against the same material, but under applied loads ranging
from 100 to 1470 N. The wear performance was compared to that of bare M2 highspeed steel and Al2O3 coatings, and TiN resulted to be the best oneA TiN-matrix coating was deposited using reactive high-velocity oxy-fuel (HVOF) spraying on a medium carbon steel (0.42% − 0.50% C steel) butterfly type substrate . Wear tests were performed using the apparatus reported in Ref. . A GCr15 ring (about 60HRC) was used as the counterpart wear ring. The rotational speed was set at 200 r/min and three loads (196, 294 and 735 N) were chosen. The wear coefficients assumed values in the range of 0.49−0.5. Under light and heavy loads, the wear mechanism was three-body abrasive wear, while failure occurred for particle spallation under light loads and for cracking between inter-lamellae under heavy loads . Borgioli et al.  studied the sliding wear resistance of thick Ti-TiN coatings deposited by reactive plasma spraying (RPS) of Ti powders in a N-containing plasma gas. The coatings were characterized by a titanium matrix with microdispersed TiN reinforcing phases (Fig. 23). The wear performance was evaluated using a block-on-ring configuration in dry sliding conditions, under applied loads in the range of 45-100 N, at sliding velocities in the range of 0.4–2.0 m/s, and for a sliding distance of 6000 m. Hardened and stress-relieved AISI O2 steel discs were used as counterparts. The results showed that under a low load (45 N) the wear volume was low, but increased slightly when the sliding velocity increased. When a heavy load (100 N) was used with low sliding velocities, adhesion to the counterpart and detachment of weakly bonded particles was observed, resulting in high wear volume. At high sliding velocities, increasing the temperature in the contact zone promoted the oxidation of both the coating and the counterpart, inhibiting the adhesion phenomenon, thus resulting in a lower wear volume and friction coefficient .
The same authors proposed air oxidation as a post-coating treatment, in order to enhance the wear and corrosion resistance of Ti-TiN coatings deposited using RPS on prismatic AISI 1040 carbon steel substrates . The post-coating oxidation treatment was performed inside an air-circulating furnace at 973 K for 2 hours and was followed by a calm air-cooling. The temperature was below the phase transition temperature of both the titanium (α-β) and the steel substrate (α-γ). Wear tests were performed in the same block-on-disc configuration and at the same conditions reported above, except for the load, which was set to 100 N. The post-coating oxidation treatment led to a decrease in the wear volume up to ~2.7 times with respect to the as-sprayed samples, especially at low sliding velocities (0.4, 0.8 m/s) . SEM micrographs of the worn surfaces of the as-sprayed (Fig. 24(a)) and sprayed and oxidized (Fig. 24(b)) coatings after wear tests at 0.4 m/s, show that no plough tracks are visible in the latter sample indicating an improvement in wear resistance . Tului et al.  recently developed a new graded coating system, created using two consecutive steps: (i) deposition of a thick (hundreds of µm) composite Ti/TiN coating on titanium based substrate by reactive plasma spraying (RPS)  and (ii) PVD deposition of a thin, hard TiN coating on the already deposited thick coating. The deposition of the Ti/TiN interlayer reduced the abrupt change in properties at the substrate/coating interface, thus limiting the deformation of the system. A Ti–6Al–4V alloy was used as substrate. Wear resistance was evaluated in a pin-on-disc apparatus using a ball of Si3N4 as counterpart, at a constant sliding distance of 3 m/s, for a total distance of 720 m, corresponding to 1800 seconds. Tests were performed on bare PVDTiN, RPS-Ti/TiN deposited samples, as well as on duplex PVD-RPS-deposited samples,
under the conditions reported in Table 4, where the results of pin-on-disc tests are also provided. The tribological tests showed that the structural integrity and load carrying capacity of the graded system significantly improved, opening up the number of potential applications and industrial sectors where Ti-based materials could be employed . The coatings were deposited on sprockets and their performance was tested in the bench test reported in Fig. 25(a). Tests were conducted at a constant and a variable mechanical rate with the application of an increasing resistant torque (by a brake). The sprocket was in contact with a transmission chain made of carburized steel . The good results provided by the real sprockets (Fig. 25(b)) led to their installation on motorbikes. Proudhon et al.  studied the influence of porosity on the wear behavior of a Ti– 6Al–4V/TiN coating deposited by high-pressure reactive plasma spraying (HPRPS) in a nitrogen environment, on Ti–6Al–4V substrates. The wear performance was evaluated in a ball-on-disc tribometer in a reciprocating setup (linear motion) against Ti–6Al–4V ball, under a normal load of 50 or 100 N, with ± 2 mm displacement amplitude. The test durations ranged from 6 to 12 hours (216 and 432 cycles). A low frequency was used in order to avoid the effect of the local contact heating. Six tests were performed. The results showed a predominant abrasive wear mechanism and a linear evolution of the wear volume. Scanning electron microscopy (SEM) observation showed that the pores acted as weak points in the coating, leading to large particle detachment during wear testing (Fig. 26). The deposition details and the tribological properties of some sprayed coatings reported in literature, are shown in Table 5 [172,174,181,270-272,274]. The reported results clearly show that coatings deposited with spraying techniques led to low values of
friction coefficient. The lowest COF value was obtained by TiN deposited by reactive plasma spraying on steel . Very good COF results were obtained by Ti-TiN coatings deposited by RPS .
4. Summary Thin hard coatings are a key feature for the future of machining industry. Many different deposition techniques can be used to achieve the desired characteristics of: (i) microstructure, (ii) composition, (iii) thickness, (iv) hardness, (v) coefficient of friction, and (vi) adhesion. Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are well established techniques used to synthesize thin films. Their major drawbacks are the high temperatures involved during deposition and the utmost film thickness (around 10 µm) . A viable way to overcome these issues is represented by spraying techniques, in particular reactive plasma spraying (RPS). The wear resistance performance of coatings was assessed using a number of tests. The most used apparatus was the tribometer and the most common classical configurations for wear tests were pin (ball)-on-disc (rotating) and pin-on-flat (reciprocating) [204,205]. The overall performance of hard coatings as well as their wear resistance and behavior are strongly influenced by many key factors including: (i) substrate, (ii) deposition technique, (iii) temperature and humidity, (iv) counter body properties, (v) applied load, and (vi) testing speed. When multilayer coatings are needed, physical vapor deposition (PVD) techniques or reactive plasma spraying (RPS) should be chosen. On the other hand, when the
geometry of the substrate material is complicated, chemical vapor deposition (CVD) techniques are preferred. Spraying techniques are most appropriate for thick coatings. Moreover, these techniques are appropriate when dealing with complicated geometries or large surfaces. There is no perfect choice of TiN-based thin hard coatings and each working condition should be specifically addressed. Of the friction coefficients values observed in this study, only spraying techniques never reached a value of 1.00. These powerful and new deposition methods require further testing. However, the results achieved so far indicate a bright future for spraying techniques.
Acknowledgement This research was made possible by a NPRP award NPRP 5–423–2–167 from the Qatar National Research Fund (a member of The Qatar Foundation). The authors also acknowledge support of the Hanyang University's research fund with No. 201500000000438.
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List of figures and tables captions Fig. 1. A schematic representation of a PVD setup. Fig. 2. Schematic of the CVD process in the (a) cold wall reactor, and (b) hot wall reactor. In both cases, the chemical precursor reacted to the substrate surface and began building the coating. Fig. 3. Schematic of a plasma spraying system. Fig. 4. Schematics of the classic interface geometries used for sliding friction and wear tests: (a) pin-on-disk, (b) pin-on-flat, (c) pin-on-cylinder, (d) thrust washers, (e) pininto-bushing, (f) rectangular flats on rotating cylinder, (g) crossed cylinders, and (h) four-ball . Fig. 5. A 50 mN nanoindentation in the TiN on (a) 304L substrate, and (b) a copper substrate. In Fig. (a) the indentation is dominated by classic plasticity and there is no evidence of cracking. In Fig. (b) there is a visible cracking at the bottom of the indentation; it can be also seen an extensive ‘pin-cushioning’ of the indentation
geometry compared to (a). This is a consequence of the high E/Y ratio of the substrate material . Fig. 6. Coefficient of friction as a function of the sliding distance, for different substrate bias . Fig. 7. Scanning Electron Microscopy micrographs of the three coatings surfaces: (a) TiN, (b), TiAlN, and (c) AlTiN [adapted from 225]. Fig. 8. Morphology analysis after the wear tests for TiN ((a), (c)), and TiAlN ((b), (d)). The upper row show surfaces after sliding against the steel ball, while the lower row shows the results after sliding against alumina ball [adapted from 226]. Fig. 9. Optical micrographs of the wear tracks of: (a) TiSiN, and (b) AlTiN . Fig. 10. Variation of coefficient of friction in dry sliding at 100 m/min for the singlelayer coatings in (a) vacuum and (b) air, and for the nano-multilayer coatings in (c) vacuum and (d) air . Fig. 11. Schematic of the ‘crater/dimple grinder’ abrasive wear test . Fig. 12. Wear data for the coated and uncoated substrates . Fig. 13. SEM micrographs of the morphology of Ti/TiAlN (10/500)5 and Ti/TiAlN (200/500)5 . Fig. 14. Bright field TEM microstructure image of TiN/Ti/a-C:H multilayer coating in cross-section after a mechanical ball-on-disc test. This image reveals the layers motion mechanism in the cracking process; (a) view of the total crack propagating through the coating; (b) fragment of the coating showing the layers reunion after cracking; and (c) fragment of the coating showing a role of plastically deformed Ti metallic layers .
Fig. 15. SEM micrographs of the (a) cross-section overview of the wear track on a TiAlN/VN superlattice coating after a tribo-test at 700 °C, and (b) detail of the oxidized coating surface from the highlighted area in (a) of the wear track . Fig. 16. Graphs reporting the friction coefficient variations with temperature (ceramic ball counterpart) for: (a) TiCN, and (b) TiN . Fig. 17. Graph of the temperature dependent average friction coefficient of the TiAlN/VN coating in the steady-state sliding period . Fig. 18. Wear rates values for TiN, Ti55Al45N, and Ti35Al65N coatings at different temperature levels . Fig. 19. Cross-sectional HRTEM micrographs, selected area diffraction patterns (SADP), dark-field TEM images of Ti−41% B−N coating (a, b) and Ti−63% B−N coating (c, d), and (e) coefficients of friction of TiN and Ti−B−N coatings as a function of the B content [adapted from 154]. Fig. 20. (a) Influence of B content on the wear rate of Ti-N-B coatings deposited within the partial pressure of BCL3 (p(BCL3))-variation and the deposition temperature (Tdep)-variation at 25, 500 and 600 ºC; b) influence of B content on the friction coefficient at 25, 500, and 600 °C . Fig. 21. SEM image of the TiN coating deposited on the WC–Co substrate. The inset reports the cross-sectional SEM micrograph of the TiN multilayer coating . Fig. 22. Results of the weight lost from samples and abrasive pins . Fig. 23. SEM images of the as-sprayed coating: (a) surface, (b) cross-section . Fig. 24. SEM images of the worn surfaces of samples tested at 0.4 m/s sliding velocity, 100 N coupling load and 6000 m sliding distance: (a) as-sprayed and (b) sprayed and oxidised .
Fig. 25. (a) Test bench for sprockets, and (b) a coated sprocket after testing (adapted from ). Fig. 26. Micrograph of the wear track (100 N load, 216 cycles) . Table 1. Deposition parameters, mechanical properties, and tribological characteristics of the coatings reported in Ref. . Table 2. Data for the coatings deposited by PVD techniques. Table 3. Coefficients of friction reported in some of the above-cited papers about coatings deposited by CVD techniques. Table 4. Pin-on-disc test condition and related results for all the sample typologies [adapted from 272]. Table 5. Coefficients of friction reported in some of the above-cited papers about coatings deposited by spraying techniques.
1 2 3 4 5 6 7 8
Table 1 Deposition parameters, mechanical properties and tribological characteristics of the coatings reported in Ref. . Deposition Methods Substrates Targe Magnetron sputtering Ion Si Hard alloy µ Lc , t implantation (TT8K6) N T, Ubias ξ t, P, t, I, U, H, E, We H, E, We °C ,V mi Pa mi m K GP GP , % GP GP , % n n A V a a a a TiBN 25 -250 0 80 0.2 32 260 67 34 380 60 0.6 30 0 25 -250 0.1 80 0.2 24 220 61 31 380 59 0.49 50 0 4 20 -200 0.1 60 0.0 18 200 54 20 280 49 0.4- 41 0 4 7 0.5 20 -200 0.1 60 0.0 10 10 20 20 220 53 23 300 46 0.4- 46 0 4 7 0.5 20 -200 0.1 60 0.0 60 10 20 16 200 42 19 270 45 0.4- 46 0 4 7 0.5 TiCrB 20 -125 0.1 80 0.2 29 260 61 30 370 53 0.52 22 0 4 20 -250 0.1 80 0.2 34 270 63 33 370 58 0.45 0 4 TiSiB 20 -200 0.1 60 0.0 16 210 45 19 270 43 0.47 87 0 4 7 0.55 20 -200 0.1 60 0.0 10 10 20 19 170 53 25 340 47 0.39 >9 0 4 7 -0.5 0 20 -200 0.1 45 0.0 45 10 14 190 41 17 280 40 0.44 67 0 4 7 0.55 TiAlSi 20 -200 0.1 60 0.0 21 220 51 23 300 48 0.52 54 B 0 4 7 20 -200 0.1 60 0.0 10 10 20 17 180 51 20 260 46 0.39 37 0 4 7 0.47 20 -200 0.1 60 0.0 60 10 20 16 190 48 19 280 44 0.4- 41 0 4 7 0.5 T is the deposition temperature, Ubias is the applied bias, t is the time for the deposition and/or the ion implantation, ξ is the nitrogen partial pressure, P is the total pressure, I is the ion implantation current, H, E and We are the hardness, elastic modulus and elastic recovery properties of the substrates, and µ and Lc are the friction coefficient and the critical load of the deposited coatings.
Table 2 Data reported on the coatings deposited by PVD techniques. Coating
Arc Ion Plating (AIP)
Coating Thickne ss [μm] 3
up to 0.70 0.34 0.46
100Cr6 (70%) 100Cr6 (50%) 100Cr6 (30%) 100Cr6 (20%) WC (70%)
[222 ] 
100Cr6 (70%) 100Cr6 (50%) 100Cr6 (30%) 100Cr6 (20%) WC (70%)
100Cr6 (70%) 100Cr6 (50%) 100Cr6 (30%) 100Cr6 (20%) WC (70%)
    
    [225 ] [225 ] [225 ] [226 ] [226 ]
>1 >1 >1
2.25 2.25 2.25
>1 >1 >1
Arc Ion Plating (AIP)
Steel (1 N, 0.1 m/s)
Steel (1 N, 0.5 m/s)
Arc Ion Plating (AIP)
WC+Co(10%) 2 ± 0.1
Steel (5 N, 0.1 m/s)
Steel (5 N, 0.5 m/s)
Al2O3 (1 N, 0.1 m/s)
Al2O3 (1 N, 0.5 m/s)
Al2O3 (5 N, 0.1 m/s)
Al2O3(5 N, 0.5 m/s)
Steel (1 N, 0.1 m/s)
Steel (1 N, 0.5 m/s)
Steel (5 N, 0.1 m/s)
Steel (5 N, 0.5 m/s)
Al2O3 (1 N, 0.1 m/s)
Al2O3 (1 N, 0.5 m/s)
Al2O3 (5 N, 0.1 m/s)
Al2O3(5 N, 0.5 m/s)
Si3N4 ∼0.70 (reciprocatin g) Si3N4 ∼0.85 (rotating)
Pulsed bias arc ion plating
up to 0.9
Closed field unbalanced reactive DC magnetron sputtering
TiCN (N rich)
TICN (C rich)
[226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [226 ] [227 ] [227 ] [228 ] [221 ] [221 ] [221 ] [221 ] [221 ] [221 ] [221 ]
Magnetron Sputtering / Ion implantation assisted magnetron sputtering (IIAMS)
Si (100) wafer / Hard alloy
[189 ] [189 ] [189 ] [189 ] [190 ]
Ion beam assisted sputter deposition
Multi Arc Ion Plating
Si (100) wafer / HSS SKH-51 316L SS
Magnetic filter arc ion plating (MFAIP)
Magnetic filter arc ion plating (MFAIP)
up to 0.68
up to 0.60
up to 0.25
Magnetic filter arc ion plating (MFAIP)
up to ∼0.65
up to ∼0.71
up to ∼0.66
Plasma nitriding + γ'
Plasma nitriding + γ' + TiN
Plasma nitriding + γ' + TiAlN
∼0.39 (average) ∼0.36 (average)
[234 ] [234 ] [234 ] [235 ] [235 ] [235 ] [235 ] [235 ] [236 ] [236 ] [236 ] [237 ] [237 ] [237 ] [239 ] [239 ] [239 ] [239 ] [239 ]
Plasma nitriding +TiN
Plasma nitriding +TiAlN
Hardening + TiN
Hardening + TiAlN
Plasma nitriding + γ'
Plasma nitriding + γ' + TiN
Plasma nitriding + TiN
Hardening- C + TiN
Hardening- D + TiN
Plasma nitriding + γ' + TiAlN
Plasma nitriding + TiAlN
Hardening- C + TiAlN
Hardening- D + TiAlN
Plasma nitriding + γ' + ta-C
Plasma nitriding + ta-C
Hardening- C + ta-C
Hardening- D + ta-C
2.7 ± 0.2
[239 ] [239 ] [239 ] [239 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [240 ] [243 ] [243 ] [243 ] [243 ] [243 ]
56NiCrMoV7 -N Ti/TiAlN (200/500)5
Unbalanced magnetron sputtering
∼0.60 3.6 ± 0.2
Unbalanced magnetron sputtering
WC + 15 wt.%TiC + 6 wt.% Co
0.10 - 0.30
0.13 - 0.3
0.07 - 0.6
Reactive unbalanced magnetron sputtering
Reactive unbalanced magnetron sputtering
3 (averag e)
3 (averag e)
  
Al2O3 (300 °C) Al2O3 (400 °C) Al2O3 (500 °C) Al2O3 (600 °C) Al2O3 (700 °C) Al2O3 (RT)
Al2O3 (300 °C) Al2O3 (400 °C) Al2O3 (500 °C) Al2O3 (600 °C) Al2O3 (700 °C)
[243 ] [243 ] [243 ] [243 ] [243 ] [243 ] [243 ] [244 ] [244 ] [244 ] [244 ] 
Cathodic arc V-ion etching + Reactive unbalanced magnetron sputtering
Enhanced cathodic arc WC magnetron sputtering
Al2O3 (0-80 °C)
Al2O3 (80160 °C)
0.60 - 0.85
Al2O3 (160440 °C)
Al2O3 (440540 °C)
up to 1.10
Al2O3 (540700 °C)
0.24 - 1.00
0.47 (mean 0.1 m/s)
0.24 (mean 0.1 m/s)
[247 ] [247 ] [247 ] [247 ] [251 ] [251 ]
Table 3 Coefficients of friction reported in some of the above-cited papers about coatings deposited by CVD techniques. Coating Deposition Substrate Coating Counter COF Ref. technique Thickness body [μm] TiN (as-deposited) Thermal CVD WC 5 ± 0.5 Al2O3 0.90 - 1.10  TiN (polished) TiN
0.80 - 1.00 0.56 (average)
Ti−63%B−N Ti-B-N (35.1 at.%B Thermal CVD - 25 °C Tdep) Ti-B-N (2.9 at.%B 25 °C Tdep) Ti-Si-C-N (13 at.% PECVD Si) Ti-Si-C-N (4 at.% Si)
0.44 (average) 0.38 (min)
  
GCr15 (25 °C) GCr15 (25 °C) GCr15 (25 °C) GCr15 (25 °C) GCr15 (550 °C) GCr15 (550 °C) GCr15 (550 °C) GCr15 (550 °C) Al-Si alloy
∼0.75 (steady state) 
Si (100)/AISI 304 SS
WC+10 wt.% Co
5 ± 1.5
Ti-Si-C-N (3 at.% Si) Ti-Si-C-N (12 at.% Si) Ti-Si-C-N (13 at.% PECVD Si) Ti-Si-C-N (4 at.% Si)
Ti-Si-C-N (3 at.% Si) Ti-Si-C-N (12 at.% Si) WC-Co (no coating) HFCVD
WC + 8 wt% Co
2 / 5 / 15
∼0.35 (steady state)  ∼0.15 (steady state)  ∼0.35 (steady state)  ∼0.52 (steady state)  ∼0.65 (steady state)  ∼0.60 (steady state)  ∼0.32(steady state)
    
0.80 - 1.00 0.60 - 0.80
Table 4 Pin-on-disc test condition and related results for all the sample typologies [adapted from 260].
Substrate Coating typology
5 2 5 10 10 20 30 10 20 30
0.48 0.70 0.48 0.51 0.72 0.72 0.61 0.70 0.65 0.66
Weight loss (mg) 4.9 <0.1 <1 <1 <1 <0.1 <1 <1
Coating conditions Undamaged Completely damaged Completely damaged Slightly damaged Slightly damaged Slightly damaged Undamaged Slightly damaged TiN coating completely worn RPS layer slightly damaged
Table 5 Coefficients of friction reported in some of the above-cited papers about coatings deposited by spraying techniques. Coating
Reactive Plasma Spraying (RPS)
Reactive Plasma Spraying (RPS)
Substrate Coating s Thickne ss [μm] C45 Steel ~400
High-velocity oxy-fuel (HVOF) Spraying
AISI E52100 steel 0.37∼0.41
[176 ] [176 ] [270 ] [270 ] [270 ] [270 ] [172 ]
Medium carbon steel (0.42%−0.50 %C steel)
AISI E52100 steel (490 N load)
AISI E52100 steel (1470 N load)
AISI E52100 steel (490 N load)
AISI E52100 steel (1470 N load)
GCr15 (196 N, no coating)
GCr15 (196 N)
GCr15 (294 N) GCr15 (735 N) Ti-TiN
Reactive Plasma Spraying (RPS)
Reactive Plasma Spraying (RPS)
Reactive Plasma Spraying (RPS)
Reactive Plasma Spraying (RPS) + Air Oxidation
120 (averag e)
120 (averag e)
AISI O2 steel (45 N, 0.4 m/s)
[172 ] 0.49 [172 ] 0.50 [172 ] ∼0.44 [181 ]
AISI O2 steel (45 N, 0.8 m/s)
AISI O2 steel (45 N, 1.6 m/s)
AISI O2 steel (45 N, 2.0 m/s)
AISI O2 steel (100 N, 0.4 m/s)
AISI O2 steel (100 N, 0.8 m/s)
AISI O2 steel (100 N, 1.6 m/s)
AISI O2 steel (100 N, 2.0 m/s)
AISI O2 (0.4 m/s)
AISI O2 (> 0.4 m/s)
AISI O2 (0.4 m/s)
[181 ] [181 ] [181 ] [181 ] [181 ] [181 ] [181 ] [271 ]
[271 ] ∼0.40 [271 ]
Ti/TiN + TiN
Reactive Plasma Spraying (RPS)
Reactive Plasma Spraying (RPS) + PVD-arc
High-Pressure Reactive Plasma Spraying (HPRPS)
151 ± 11
AISI O2 (> 0.4 m/s)
Si3N4 (10 N)
Si3N4 (20 N)
Si3N4 (30 N)
Si3N4 (10 N)
Si3N4 (20 N)
Si3N4 (30 N)
Ti–6Al–4V (50 N, 12 h)
Ti–6Al–4V (50 N, 6 h)
Ti–6Al–4V (50 N, 12 h)
Ti–6Al–4V (50 N, 6 h)
Ti–6Al–4V (100 N, 12 h)
Ti–6Al–4V (100 N, 6 h)
[271 ] [272 ] [272 ] [272 ] [272 ] [272 ] [272 ] [274 ] [274 ] [274 ] [274 ] [274 ] [274 ]