Tribo-mechanical and electrochemical properties of plasma nitriding titanium

Tribo-mechanical and electrochemical properties of plasma nitriding titanium

SCT-20300; No of Pages 10 Surface & Coatings Technology xxx (2015) xxx–xxx Contents lists available at ScienceDirect Surface & Coatings Technology j...

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SCT-20300; No of Pages 10 Surface & Coatings Technology xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Tribo-mechanical and electrochemical properties of plasma nitriding titanium F.M. El-Hossary a, N.Z. Negm a, A.M. Abd El-Rahman a,⁎, M. Raaif a, A.A. Seleem b, A.A. Abd El-Moula a a b

Physics Department, Faculty of Science, Sohag University, Sohag, Egypt Zoology Department, Faculty of Science, Sohag University, Sohag, Egypt

a r t i c l e

i n f o

Article history: Received 20 November 2014 Revised 16 May 2015 Accepted in revised form 1 June 2015 Available online xxxx Keywords: Titanium nitride RF plasma Tribo-mechanical properties Surface wettability Electrochemical performance

a b s t r a c t Titanium nitrides have good tribo-mechanical and biomedical properties. They are employed to harden and protect cutting and sliding surfaces for industrial purpose and as a non-toxic outer-surface for bio-medical applications. In this study, pure titanium was nitrided using RF plasma technique. The microstructural, mechanical, tribological, electrochemical and biomedical properties of nitrided titanium were investigated. The X-ray diffraction demonstrates the formation of ε-Ti2N and the cubic δ-TiN phases after plasma nitriding. The microhardness of the nitride samples increases as the plasma-processing power increases up to 1300 HV0.1. That represents approximately 7-fold increment in the microhardness in comparison with the untreated titanium. High nitriding rate of 0.17 μm2/s was recorded for the sample that was treated at 650 W. The wear and corrosion resistance are improved after plasma nitriding. Moreover, the friction coefficient is reduced from nearly 0.75 for the untreated titanium to 0.25 for the nitride one. An enhancement in the biocompatibility of the nitrided titanium has been achieved. The number of grown mesenchymal stem cells was higher for nitrided substrates compared to that of the untreated titanium. The improved tribo-mechanical and electrochemical performance of the nitrided titanium can be attributed to the formation of super-hard titanium nitrided phases. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Titanium and titanium alloys have excellent properties including lightweight, high strength-to-weight ratio and outstanding corrosion performance [1–3]. Further, they are suitable to work under high stress conditions and indicated no toxicity effects with biological environments [4]. For these attractive properties, titanium and its alloys are widely used for aerospace, chemicals and petrochemicals, automotive, orthopedic implants, dental and endodontic instruments and other industrial and biological applications. However, they are still suffering from tribological drawbacks; mainly the low wear resistance and high friction coefficient that limit some of their practical applications [5]. Different surface treatment techniques have been successfully developed to overcome these drawbacks and to provide these surfaces with more desired properties and functionalities for exceptional applications. In this regard, plasma-based nitriding and ion nitriding are well-known technologies used for many years among various plasma surface engineering techniques. Plasma nitriding [6–9], plasma carburizing [10] and plasma carbonitriding [11] are typical methods used for surface treatment purposes. The significant advantages of plasma nitriding over conventional nitriding methods include a reduce in operating coast (gas and energy consumption), and a complete elimination of

⁎ Corresponding author.

environmental pollution. Further, controlling the treatment temperature during the process leads to control the formation of nitrided layer with a specific phase composition and less shape distortion with free porous zone [12,13]. As reported in a previous literature, the surface treatment of titanium is better to be performed at low temperature range, up to ~950 °C in order to reduce the fatigue strength of the treated titanium [14]. Plasma nitriding of titanium based on thermal diffusion mechanism produces a compound layer formed from δ-TiN on top and ε-Ti2N beneath; giving a hardness of about 1500–3000 HV [15]. A diffusion layer of solid solution phase α-Ti(N) can create underneath as a consequence of incorporation of nitrogen into titanium matrix which results in hardening of dislocation-pinning effects [16]. It has been found that, the nitrided titanium surfaces lead to significant changes in surface topography beside the physiochemical features and tribo-mechanical properties [14,17,18]. A reduction in the wear rate to a value of 4.8 × 10−7 mm3/Nm has been reported for nitrided titanium [19,20]. From another side, such nitrided layers have interesting features in biomedical applications especially towards cell adhesion, proliferation, and differentiation and ultimately the interfacial tissue formation [21–25]. The current study focuses on improving the tribo-mechanical properties of commercial titanium by RF plasma nitriding. Further, it was extended to study the surface energy characterization and corrosion behavior of the nitrided layers as a function of plasma processing power. Furthermore, cell adhesion and cell spreading were correlated

http://dx.doi.org/10.1016/j.surfcoat.2015.06.003 0257-8972/© 2015 Elsevier B.V. All rights reserved.

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to the physiochemical and surface topography features in order to preliminary evaluate the biocompatibility performance. 2. Experimental 2.1. Sample treatment Titanium with purity of 99.96% was cut into small coupons with dimensions of 10 mm × 10 mm × 1 mm. The samples were ground and polished mirror like, washed in ethanol for 15 min using an ultrasonic cleaner and then installed in the RF plasma system. Detailed information about the RF nitriding process can be found elsewhere [26,27]. Fig. 1 displays a schematic of the nitriding system. This system comprises a quartz tube (reactor) with 500 mm in length and 41.5 mm in diameter evacuated by a two stage rotary pump to a base pressure of 1.0 × 10−2 mbar. The titanium samples were centered in the RF coil on a supported titanium bar of 2.9 cm length and 1 cm diameter which is fixed on a water cooled copper sample holder. Nitrogen (N2) gas was fed into the reactor tube to establish a working gas pressure of 7.5 × 10−2 mbar; measured by means of a capacitance manometer. The induction copper coil was energized by a 13.65 MHz RF power generator (model HFS 2500 D) via a tunable matching network. The samples were nitrided at a varied plasma-processing power from 400 up to 650 W and for a processing time of 15 min. It is important to state that this treatment process was performed without using any external source of heating. The sample temperature was measured by means of a Chromel–Alumel thermocouple just placed close to the surface of the sample. At the end of the nitriding process, the samples were allowed to cool down to room temperature in the evacuated reactor tube. The processing temperature of the treated substrates increases rapidly to steady state temperature within the first 2 min of plasma processing with an average heating rate of approximately 8 °C/s. A fast temperature stabilization and good plasma stability has been reported previously in plasma surface treatment using inductively-coupled RF power source [28]. Fig. 2 displays a linear trend of increasing the treatment temperature with increasing plasma-processing power. As one can observe from this figure, the treatment temperature increased from 850 °C at a plasma-processing power of 400 W to reach a maximum value of 1050 °C at a plasma power of 650 W. The treatment temperature plays a significant role in affecting the structure, nitriding rate and the tribo-mechanical properties of titanium. 2.2. Sample characterization Different characterization techniques have been used in order to study and correlate the properties of the untreated and treated samples.

Fig. 2. Sample temperature as a function of RF plasma-processing power.

X-ray diffraction (XRD) using Philips-PW1710 diffractometer with Co Kα radiation of λ = 1.78896 Å was used to characterize the crystallographic configuration of the samples. The XRD scan was run between 35° and 95°, with step interval of 0.02° and scan rate of 2°/min. The treated samples were exposed to the standard metallo-graphic procedure including sectioning, mounting, grinding, polishing and etching. The etching process was performed using 100 ml H2O + 50 ml ethanol + 2 g ammonium hydrogen fluoride for time of few seconds to and up to 1 min. The surface and cross-section morphologies of the treated samples were investigated using Olympus BX51 optical microscope. Vickers microhardness measurements of the untreated and nitrided titanium were carried out using a Leitz Durimet microhardness tester with a contact load of 100 gf. The microhardness measurements were performed according to ASTM E384-11 standard test method at temperature of 25 °C ± 3 °C [29]. The microhardness tester has been accredited according to ISO/IEC 17025:2005 requirements. The wear and friction coefficient measurements were conducted using a ballon-disk tribometer at a mean sliding speed of 2 mm/s with a normal load of 1 N. A 6 mm diameter Al2O3 ball was used as a counterpart against the untreated and treated Ti without lubrication. The environmental conditions were T = 25 ± 3 °C and 35% relative humidity. The wear measurements were performed according to ASTM G 133-10 standard test method (linearly reciprocating ball-on flat sliding wear). The oscillating ball-on-disk type tribometer is accredited according to ISO/IEC 17025:2005 requirements. The surface roughness of the investigated samples was performed using a Form Talysurf 50, which has been accredited according to ISO/IEC 17025:2005 requirements. The water contact angle measurement, at room temperature, was performed using Phoenix 300 (Contact Angle Analyzer manufactured by S.E.O. Co.

Fig. 1. A schematic of the nitriding system.

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Ltd). The Phoenix 300 utilized a precision camera and advanced PC technology to capture the static droplet image and calculate the contact angle measurement by Sessile Drop method. The electrochemical experiments were performed in Ringer's solution using the potentiodynamic technique at temperature of 25 °C ± 3 °C and humidity of 38% ± 5%. The effective area of samples exposed to corrosive solution was fixed at 0.36 cm2. The corrosion test was performed using threeelectrodes; silver–silver chloride saturated electrode as a reference electrode, platinum as a counter electrode and the investigated sample as a working electrode. The potential–current corrosion curve is recorded and plotted with potential scan rate of 1 mV/s using Gill AC instrument and ACM program version 5. In order to examine the biocompatibility performance, the adhesion of mesenchymal stem cells (has been taken from Wharton's jelly of the human umbilical cord) on the untreated and treated titanium has been examined. Before starting the biocompatibility test, the samples were cleaned by alcohol 70% and sterilized in autoclave (KGemmyFA-260MA) for 40 min. Then, they seeded into 24 well plat at appropriate concentration of 100,000 cells/well. The mesenchymal stem cells were cultured in flasks using RPMI and DEEM Medium supplemented with 10% fetal bovine serum and incubated at 37 °C and 5% CO2 under humidified conditions. After 48 h, the samples were washed and the cells fixed by 4% para-formaldehyde. Acridine Orange fluorescent staining was applied to demonstrate the cells adhesion on the examined surface [30]. The surface was investigated by a fluorescent microscope [31].

3. Results and discussion 3.1. Microstructure analysis The XRD of the untreated and nitrided titanium at different plasmaprocessing powers is shown in Fig. 3. One can get from the figure that, titanium is characterized as a hexagonal close packed structure (α-Ti phase) with lattice parameters of a = 0.295 nm and c = 0.4686 nm. As a consequence of this structure, titanium shows prismatic and pyramidal slip systems, which cause comparatively low shear strength and high friction coefficient [32]. As observed after nitriding titanium, the chemical compounds of ε-Ti2N and δ-TiN have been identified beside some residual from α-Ti phase. The ε-Ti2N phase was existed close to 31.5 at.% N with the same (111), (200), (002), (202) and (321) diffraction planes [33,34]. It was inspected that the pre-treated peak intensity for all Ti2N diffractions increases with increasing the treatment temperature and it has a preferred orientation of (111) direction. The second observed phase is δ-TiN and it has lattice parameter of a = 0.424 nm. The TiN phase has multiple orientations (111), (200), (220), (311)

Fig. 3. XRD of the un-treated and nitrided titanium at different plasma-processing powers.

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and (222) and it has a preferred orientation of (200). Since the stoichiometry of δ-TiN is not determined, it is generally termed as δ-TiNx, where x lies between 0.5 and 1.0. At high nitrogen concentrations, the conversion from α-Ti(N) into δ-TiN would be occurred and can be explained by the sliding of (001) plane of the hexagonal structure to become (111) plane of the fcc structure [34]. The TiN peak positions are shifted with respect to the peaks of the stoichiometric δ-TiN powder pattern by a value of 0.8%. This shift could be elucidated to the deviations from the 1:1 stoichiometry of TiN or to the presence of internal stress in the lattice [35]. δ and ε-phases are expected to be the reason for the good mechanical and tribological properties of the nitrided Ti [36,37]. Moreover, δ-TiN layer is characterized by resistant to wear [38] and good biocompatibility performance [39]. One can monitor from the X-ray diffractogram that, the TiN phase is the predominating phase for all nitrided samples. Therefore, the texture coefficient (Tc) for the TiN phase should be calculated according to the following formula [40]. Tcð200Þ ¼ Ið200Þ=

X

Ið111Þ þ Ið200Þ þ Ið220Þ þ Ið311Þ þ Ið222Þ

ð1Þ

The value of Tc conveniently determines the degree of preferred orientation or texture of diffraction planes. Fig. 4 illustrates the calculated texture coefficient of TiN (200) as a function of RF plasma-processing power. One can observe from the figure that, the texture coefficient of TiN (200) increases as the plasma-processing power increases up to a maximum value of 0.56 for the sample that was nitrided at a plasmaprocessing power of 550 W. Afterward, it decreases as the plasmaprocessing power increases up to a value of 0.414 at a plasmaprocessing power of 650 W. The results of the texture coefficient (Tc) lead to confirm that all treated samples have a preferred orientation of TiN along (200) direction. These results agree well with Pohrelyuk et al. [41]. The higher strength of plane (200) than plane (111) diffraction reflects an increase in the surface hardness of the nitrided samples [42]. Plasma surface interactions introduce a suggested conception of how the texture is affected by RF plasma nitriding as follows: The sample is immersed in an RF generated plasma. Then, the energetic plasma species (excited nitrogen atoms or ions) are adsorbed and diffused into the metal substrate surface. As it is expected, with the increase in the plasma processing power, the total energy flux and the density of plasma-generated species can be increased. This leads to increase the total energy transfer and the momentum flux towards the Tisubstrate, which, in turn affect the surface properties of the treated substrate including the surface topography and texture of chemical compounds. Further, the plasma ion energy and density increase with the increase of plasma processing power, which, in turn increase the sputtering rate and offering texture effects. However, the decrease in texture coefficient with further increase of plasma power higher than

Fig. 4. Texture coefficient of TiN (200) as a function of RF plasma-processing power.

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550 W remains an open question. On the other hand, it is important to announce that the activation energy of nitrogen diffusion into the titanium matrix is different for relatively low power range compared to that calculated for high power range [43]. It might be also responsible for changing the texture coefficient, especially for sample treated at relatively high processing power. 3.2. Cross-section morphology and nitriding rate Fig. 5 shows typical cross-section morphology of plasma nitrided titanium at different plasma-processing powers for 15 min processing time. The optical micrographs of the cross-sections reveal uniform nitrided layers with sharp interface with the bulk titanium. One can detect from the figure that, these compound layers are observed without different contrasts or distinct sub-layers. Further, the nitrided layer thickness increases up to approximately 13 μm as the plasma processing-power increases up to 650 W. By measuring the thickness of the compound layer (d) in μm, the nitriding rate can be calculated as d2 /t, where t is the time of nitriding process in s. Fig. 6 represents the nitriding rate as a function of plasma-processing power. One can inspect from the figure that, the rate of nitriding increases with increasing the plasma-processing power. 0.17 μm2/s is the highest nitriding rate that can be obtained at plasma-processing power of 650 W. The formation of nitrided layer in titanium and titanium alloys is a complicated process and involves several reactions taking place concurrently at the boundary between the gas and the metal. As well known, the nature of these physical and chemical reactions was found to be dependent on the kind of plasma technique that used for nitriding. In the

Fig. 6. Nitriding rate as a function of plasma-processing power.

glow discharge plasma generated with an inductively coupled radio frequency ionization source, the plasma is composed of uncountable energetically active species including, electrons, free radicals, charged monoatomic species and photons [44]. Adsorption of excited neutral atoms, ion bombardment and a slight sputtering due the self-bias are the main reactions carried out on the surface of the Ti-substrate. The adsorption of neutral atoms leads to chemical reactions with the Ti atoms followed by thermal diffusion into Ti-surface and the TiN is formed. This is typically the proposed model of Tibbetts [45]. However, the mean free path of plasma nitrogen species is too large to cause any collision with the sputtered titanium atoms, preventing re-deposition of any chemical compounds of titanium nitride. Further inelastic reactions are expected to form where fast ions interact with lattice electrons to yield optical radiations with secondary electron emission. On the other hand, the kinetics of the diffusion process of nitriding have been studied by several research groups [46–49]. The high nitriding rate, in this work, can be explained according to Raaif et al. conception [50] which combines the diffusion mechanism reported by Malinov et al. [51] with the microcrack mechanism reported by El-Hossary [52]. Malinov et al. [51] reported that, for titanium material located in plasma active nitrogen environment, the nitrogen species will chemically and physically react with the solid. The adsorbed nitrogen at the surface diffuses into the titanium forming α-Ti(N) phase. This process can continue as long as the α-titanium matrix can react with nitrogen at the gas/ solid interface. If the concentration of nitrogen at the gas/metal interface increased, a new phase of Ti2N occurred. With further increase of nitrogen concentration at the gas/metal interface, the Ti2N transforms to TiN. The TiN and Ti2N forms a compound layer, while α(N) is a diffusion zone. The phase transitions of the sample surface during nitriding can be written as: α‐Ti⇒αðNÞ−Ti⇒Ti2 N⇒TiN:

Fig. 5. Cross-section morphology of plasma nitrided titanium at different plasma-processing powers for plasma-processing time of 15 min.

ð2Þ

However, the above diffusion mechanism is insufficient to interpret the high value of nitriding rate that has been achieved in this study. Therefore, Raaif et al. conception [49] has adapted the diffusion mechanism reported by Malinov et al. [51] together with the formed microcrack mechanism reported by El-Hossary [52]. At certain temperature range, depending on the treated material, and under high concentration of nitrogen species and high energy of plasma the chance for surface microcracks and high penetration of nitrogen species is high. The nitrogen diffuses into the material surface mainly through the grain boundaries and the wall of the formed microcracks. The diffusion of nitrogen in titanium alloy is by interstitial or vacancy mechanism depending on the sample temperature and activation energy which equal 1.4 eV for interstitial mechanism and 0.42 eV for vacancy mechanism as previously measured [43].

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Fig. 9. The hardness of nitrided titanium as a function of peak area of δ-TiN.

Fig. 7. Microhardness values of the untreated and nitrided titanium samples as a function of plasma-processing power.

3.3. Tribo-mechanical properties 3.3.1. Microhardness Fig. 7 shows the microhardness values of the untreated and nitrided titanium samples as a function of plasma-processing power. It has been found that, the microhardness value is significantly increased with the increase in the plasma-processing power up to 500 W. Subsequently, the microhardness is gradually decreased with further increase in the plasma-processing power. The microhardness recorded a maximum value of nearly 1300 HV0.1 at a plasma-processing power of 500 W, which represents of nearly 7-fold increase in the microhardness in comparison with the untreated titanium. The high value of microhardness is mostly attributed to the formation of ε-Ti2N and δ-TiN super-hard phases. The behavior of microhardness is correlated with the amount and the texture of the cubic δ-TiN in the treated samples. Fig. 8 characterizes the peak area of the cubic δ-TiN in the nitrided samples as a function of plasma-processing power. One can monitor from the figure that the peak area of the cubic TiN increases with increasing the plasmaprocessing power up to 500 W, afterward it decreases as the plasmaprocessing power increases. This in reality represents the same trend of microhardness with plasma-processing power. Moreover, the plotting of peak area of the cubic δ-TiN with the microhardness reflects the increase of the microhardness as the peak area of the cubic TiN increases as one can see in Fig. 9. Further, from the X-ray diffractograms, one can scrutinize that, there is a tendency to appear a preferred orientation for the cubic δ-TiN along (200) diffraction. Table 1 represents the relation between the Tc (200) and the microhardness. One can notice from the table that, there is a coloration between microhardness and texture coefficient. The strength along the plane (200) is higher than

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along plane (111) which reflects the increase of the surface hardness of the nitrided samples as the plasma-processing power increases [42]. 3.3.2. Wear and friction coefficient measurements The wear behavior of untreated and nitride titanium samples was evaluated using oscillating ball-on-disk type tribometer. The wear tests were executed with 6 mm ball of alumina moves at a mean sliding speed of 2 mm/s with a normal load of 1 N. Fig. 10 characterizes the optical micrograph of the wear track of the untreated and nitride titanium substrates at different plasma-processing powers. One can monitor from this figure that, the track width of the nitrided samples is narrower than that of the untreated titanium, signifying the enhancement in the wear resistance for the plasma nitrided titanium. This improvement is recognized to the surface strengthening resulting from the formation of hard phases of ε-Ti2N and δ-TiN precipitated in the near-surface region of Ti matrix. The high hardness of these treated layers with the high ductility of the base material imparts significant strength to the surface and consequently improves the wear resistance. During the wear measurements, the recording of the friction coefficient was continuously measured using a force sensor. Fig. 11 stands for the friction coefficient of the untreated and plasma nitrided titanium at different plasma-processing powers. From this figure, the friction coefficient decreases from nearly 0.75 for the untreated titanium to nearly 0.25 for all nitrided titanium. That represents a factor of 3 reductions in the friction coefficient. The change in the microstructure and chemical composition of the surface treated layer is the reason for the decrease in the friction coefficient. Furthermore, the tribo-oxide film of TiO2 (formed during sliding wear) is considered as another contributing factor in decreasing the friction coefficient. The TiO2 layer has low shear strength, leading to decline in the friction coefficient [53]. Further, one can observe from Fig. 12, which represents the variation of friction coefficient with the sliding distance for the untreated and nitrided titanium that, no brittle failure in the nitrided layer was observed after 2000 tracks and the friction coefficient increases slightly as the sliding distance increases. This behavior could be ascribed to the variation of nitrogen content along the nitride layer, which was found to be decreased when moving

Table 1 The relation between the Tc (200) and the microhardness.

Fig. 8. The peak area of the cubic δ-TiN as a function of plasma-processing power.

Plasma-processing power (W)

Treatment temperature (°C)

Vickers microhardness (HV)

Texture coefficient (Tc)

400 450 500 550 600 650

850 910 950 975 1015 1050

788 994 1300 1203 1183 1154

0.51 0.52 0.55 0.56 0.53 0.41

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Fig. 10. Optical micrograph of the wear track of the untreated and nitrided titanium at different plasma-processing powers.

3.3.3. Wettability and roughness measurements The wettability and surface energy analysis of the biomaterials are important aspects in assessing the biocompatibility features and in turn the rate of osseointegration [58,59]. The wettability can be evaluated using contact angle technique, which is based on Sessile Drop Method. Low contact angle means high wettability and high surface energy; high contact angle means low wettability and low surface energy [60]. Fig. 13 corresponds to the relation between the water contact angle and surface energy of the untreated and nitrided titanium samples as a function of plasma-processing power. One can observe from the figure that, the water contact angle of all surfaces is less than 90°, indicating

the hydrophilic surface for all samples. The contact angle decreases from 70° for the untreated titanium to reach a value of 34.5° for the sample that was treated at a plasma-processing power of 500 W. Afterward, it increases to reach a value of 47.5° at a plasma-processing power of 650 W. The contact angle for the untreated titanium is compared with other related work. Alves et al. [20] recorded a value of 50° for the untreated titanium. The difference between the two values is attributed to the change in the surface roughness. In the present study, the surface roughness of the untreated titanium is approximately 0.015 μm, while Alves et al. [20] recorded a value of 0.2 μm. One can find from the figure that, the water contact angle for all nitrided samples is less than that of the untreated titanium, indicating the higher wettability for the treated surfaces. The higher wettability and more hydrophilic surfaces are favorable for the adhesion, spread and proliferation of cells [61]. On the other hand, the surface energy increases from 30.2 mN/m for untreated Ti-substrate up to nearly 75 mN/m as the plasma-processing

Fig. 11. The friction coefficient of the untreated and nitrided titanium samples at different plasma-processing powers.

Fig. 12. The relation between friction coefficient and sliding distance for the untreated and nitrided titanium samples.

towards the bulk material [54]. Furthermore, the increase in the friction coefficient is ascribed to the role of the wear debris created during the peel-off of the protective nitride layer, which acts as an abrasive interface between the counter bodies [55–57].

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Table 2 The correlation between Rp/Rz ratio and the surface wettability of the untreated and nitrided titanium samples at different plasma-processing powers.

Fig. 13. The relation between the water contact angle and surface energy of the untreated and nitrided titanium samples as a function of plasma-processing power.

power increases up to 500 W. After that, it decreases down to 62 mN/m when the plasma-processing power increases up to 650 W. A closer value for the untreated Ti-substrate (33.3 mN/m) is previously recorded by Wang et al. [62]. It has been found that, the surface energies of Tinitride layers are higher compared to that achieved for Ti-surface oxidized at 400 °C which resulted in a value of 55.3 mN/m [62]. Further, they found to be higher in comparison to that of the TiN coatings (40 mN/m) with smother surface (Ra = 3.5 nm) [63]. The large difference between the values of surface energy of TiN is mostly ascribed to the higher roughness of the nitrided titanium surface (Ra N 300 nm). Furthermore, the surface energy variations are observed to be in a good trend with the surface microhardness. There are numerous reports demonstrating that the surface energy increases with increasing the surface microhardness [50,64–66]. The formation of α-Ti(N), ε-Ti2N, and δ-TiN hard phases increases the surface strengthening and consequently increases the surface energy. It is well known that, the untreated Ti-substrate ace wettability can be affected by the surface topography, including roughness and the varied chemical composition related to the structure of the treated surface. The Ra parameter describes the surface roughness and is defined as the mean height of peaks and valleys on the surface. Numerous reports demonstrated a direct relationship between Ra and wettability of the surface [67–69]. Fig. 14 reflects the relation between the Ra and plasma-processing power for the untreated and nitrided titanium samples. One can observe from the figure that, the surface roughness of the nitrided titanium is significantly higher than that of the untreated titanium, which has a mirror-like surface finish. Further, the roughness of the treated surface is gradually increased with the increase in plasma-

Fig. 14. The relation between Ra of the untreated and nitrided titanium samples as a function of plasma-processing power.

Samples

Rv (μm)

Rz (μm)

Rp (μm)

Rp/Rz

Ti-untreated 400 W 450 W 500 W 550 W 600 W 650 W

0.052 1.45 1.48 1.79 1.66 1.10 1.11

0.19 2.855 2.87 3.42 3.30 2.10 2.12

0.14 1.40 1.39 1.63 1.64 0.99 1.01

0.7 0.49 0.48 0.47 0.49 0.47 0.47

processing power. The increase in surface roughness of the nitrided samples in comparison to that of the untreated one could be ascribed to the increase in the treatment temperature, which in turn increases the vibrational motion of the titanium atoms, making them more vulnerable for sputtering by ions in the presence of self-bias potential. On the other hand, the increase of plasma power lead to an increase in energy and plasma ion density, which in turn increase the surface irregularities and topography during plasma surface interactions [20]. Numerous reports verified that, both the early fixation and long-term mechanical stability of the prosthesis can be improved by a high roughness profile compared to smooth surfaces [70,71]. Nevertheless, the change in the Ra parameter does not reflect all surface roughness variations. Numerous reports defining other roughness parameters called Rp, Rz and Rv are connected with the surface wettability. Rp is obtained from mean peak height in relation to the central line in five consecutive readings. Rz is calculated by the sum of maximum peak heights (Rp) and maximum valley depths (Rv) in the sampling length [72]. The Rp/Rz is an important ratio in describing the surface shape. A ratio greater than 0.5 means sharp peaks whereas value lower than 0.5 indicates a surface with round peaks [72]. Round peaks favor to spread out the liquids over the surface. According to this hypothesis, the influence of Rp/Rz ratio on the surface wettability is evaluated in the present study. Table 2 illustrates the correlation between Rp/Rz ratio and the surface wettability values for the untreated and nitrided samples. From this table one can monitor that, the treated samples have Rp/Rz values lower than 0.5 and consequently exhibit round peaks with high wettability and surface energy. Therefore, plasma nitriding can be beneficially used in changing the surface topography which in turn affects the surface wettability. 3.3.4. Corrosion performance Fig. 15 represents the potentiodynamic polarization curves for the untreated and nitrided titanium in Ringer's solution at room temperature. Moreover, Table 3 listed the average values of the corrosion

Fig. 15. The potentiodynamic polarization curves for the untreated and nitrided titanium in Ringer's solution at room temperature.

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Table 3 Corrosion data for untreated and treated samples investigated in Ringer's solution. Plasma-processing power (W)

Icorr (×10−5 mA/cm2)

Ecorr (mv)

Corrosion rate (×106 mm/year)

Untreated-Ti 400 450 500 600 650

13 1 6 4 7 5.9

48 −484 36 −219 −611 −227

2.26 0.174 1.04 0.69 1.22 1.03

potential (Ecorr) and the corrosion current density (Icorr) which are calculated from the polarization curves for all tested samples. In general, the results show that the untreated titanium has the highest corrosion potential and the highest corrosion current density in comparison with that of the nitrided samples. This indicates higher corrosion resistance for the treated samples with respect to that of untreated titanium. On the other hand, the corrosion current density for all nitrided samples has found to be one order of magnitude lower than that for the untreated titanium substrate. Moreover, the current density of anodic dissolution for all nitrided samples is higher than that of the untreated one. This behavior is ascribed to the increase in the surface roughness, which sequentially increases with increasing the plasma-processing power [41]. Fig. 16 represents the optical micrograph of the treated titanium samples at different plasma-processing powers before and after immersing in corrosive media. As observed from this figure, no pitting corrosion sites have been detected on the treated titanium after immersing in NaCl solution. The corrosion results scrutinize that, the corrosion performance has been improved for the nitrided titanium samples in comparison with that of the untreated one. As reported in other previous studies, the nitride phases impeded in the treated layers with high strength of the chemical bonds create anticorrosive protection [41,73].

3.4. Biocompatibility tests Ti and Ti-based alloys are well established biomaterials where the biocompatibility and corrosion resistance of these alloys are superior [74,75]. They are widely used for hard-tissue implants which are particularly important for load bearing application such as total joint replacement, bone cement accessories, orthopedic implants and instrumentation [75,76]. In the present work, the growing of mesenchymal stem cells on the untreated and nitrided titanium is considered as a biocompatibility test. Fig. 17 shows the growing of mesenchymal stem cells on the untreated and nitrided titanium samples at different plasmaprocessing powers. It is generally accepted that, cells adhere on surface mainly via adsorbed proteins, e.g., fibronectin and vitronectin [77]. These proteins are formed by the cells as part of their own extracellular matrix [78]. It is worth mentioning to observe that, the number of grown cells is higher on the nitrided substrates compared to that of the untreated titanium. The number of grown cells increases with increasing the RF plasma-processing power. This is attributed to the surface topography and physicochemical features of the nitrided Ti, which affects the cell adhesion and cell spreading [17,79]. The increase of plasma power raises the plasma ion density and treatment temperature which leads accordingly to form a stable nitride phases (TiN and Ti2N) with high intensity. Further, the surface roughness of treated Tisurface samples increases as the plasma temperature increases in comparison with untreated one. The rougher surface exhibits higher cell adhesion and lower cell spreading. The high difference in the surface roughness between the untreated Ti-surface and the treated ones at plasma powers 500 and 600 W, makes the surface topography plays an important role in cell adhesion. However, the small difference between the surface roughness values of these treated surfaces has to be considered the effect of the physicochemical nature of the treated surface at different plasma powers. These results correlate well with other previous work [17,18,80]. 4. Conclusion The obtained results concluded that plasma-processing power could control the structure of the nitrided compounds, which consequently has a massive effect on the physical and electrochemical properties of the nitrided titanium. The mechanical, wear and corrosion resistance are improved after plasma nitriding of titanium. For biocompatibility test, the number of grown mesenchymal stem cells is higher for nitride substrates compared to that of the untreated titanium. The improved surface properties of the nitrided titanium can be attributed to the high reactivity of titanium with nitrogen and the formation of αTi(N), ε-Ti2N and the cubic δ-TiN hard phases in the titanium matrix. Based on the results obtained, the best case, in terms of tribomechanical properties and biocompatibility performance was the Ti-substrate treated by RF plasma nitriding at relatively moderate processing power of 500 W. However, the higher case depth obtained for Tisamples nitrided at relatively higher processing power ≥550 W, ensure good tribomechanical performance in dry environments. In conclusion, the good properties of titanium nitride provide a good performance for different biomedical and industrial applications. Acknowledgments The work has been carried out as part of the research project of “Plasma Technology for Biomedical Applications”. This project was supported financially by the Science and Technology Development Fund (STDF), Egypt, Grant 3894. References

Fig. 16. Optical micrograph of the treated titanium samples at different plasma-processing powers before and after immersing in Ringer's solution.

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Fig. 17. Mesenchymal stem cells growing on the untreated and nitrided titanium samples at different plasma-processing powers.

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