Effects of tailored nitriding layers on comprehensive properties of duplex plasma-treated AlTiN coatings

Effects of tailored nitriding layers on comprehensive properties of duplex plasma-treated AlTiN coatings

Author’s Accepted Manuscript Effects of Tailored Nitriding Layers on Comprehensive Properties of Duplex Plasmatreated AlTiN coatings Yang Deng, Chao-l...

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Author’s Accepted Manuscript Effects of Tailored Nitriding Layers on Comprehensive Properties of Duplex Plasmatreated AlTiN coatings Yang Deng, Chao-lin Tan, Yi Wang, Ling Chen, Pan-pan Cai, Tong-chun Kuang, Shu-mei Lei, Kesong Zhou www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)30586-2 http://dx.doi.org/10.1016/j.ceramint.2017.03.209 CERI14981

To appear in: Ceramics International Received date: 1 March 2017 Revised date: 27 March 2017 Accepted date: 30 March 2017 Cite this article as: Yang Deng, Chao-lin Tan, Yi Wang, Ling Chen, Pan-pan Cai, Tong-chun Kuang, Shu-mei Lei and Ke-song Zhou, Effects of Tailored Nitriding Layers on Comprehensive Properties of Duplex Plasma-treated AlTiN c o a t i n g s , Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.209 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.

Effects of Tailored Nitriding Layers on Comprehensive Properties of Duplex Plasma-treated AlTiN coatings Yang Deng1 (#), Chao-lin Tan1, 2 (#), Yi Wang3, Ling Chen1, Pan-pan Cai1, Tong-chun Kuang1*, Shu-mei Lei1, Ke-song Zhou1 # These authors contributed equally to this work 1 2

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China;

Guangdong Institute of New Materials, National Engineering Laboratory for Modern Materials Surface Engineering Technology, Key Lab of Guangdong for Modern Surface Engineering Technology, Guangzhou 510651, China.

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Faculty of Mechanical and Electrical Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou, 510225, China;

Abstract: Duplex-treated AlTiN coatings were deposited by advanced plasma assisted arc (APA-Arc) technology on pre-plasma nitrided AISI-H13 steel substrates using different N2/H2 flow ratios. The microstructures and properties of the AlTiN coatings were comprehensively characterized and analyzed. The results show that the N2/H2 flow ratios can tailor the thickness of compound layer during plasma nitriding process and the bright nitriding layer without compound layer is achieved. The properties of duplex-treated AlTiN coatings are well improved comparing with monolayer AlTiN coating. The adhesion of the AlTiN coating is well enhanced by duplex treatment process, and adhesion grade increases from HF3-4 for monolayer AlTiN coating to HF1 for composite coatings. Moreover, the composite coatings with various thickness compound layers show different load-bearing capacities, and the interfacial adhesion force of the composite coating without compound layer reaches 61 N. The hardness of AlTiN coating is also enhanced by duplex treatment with the highest hardness of 2935 HV0.05 for composite AlTiN coating. Meanwhile, tribological properties of AlTiN coatings are also improved by duplex treatments.

Keywords: Plasma nitriding; Duplex treatment; Arc; Compound layer; AlTiN; Adhesion strength.

# Co-first authors: Yang Deng and Chao-lin Tan contributed equally to this work * Corresponding authors: E-mail: [email protected] (T.C. Kuang) . 1

1. Introduction AlTiN hard coating has been widely used in cutting tools, dies, molds and mechanical key components due to its excellent oxidation resistance, superior chemical inertness, good tribological properties and well mechanical properties [1-4]. However, the extensional application of AlTiN coatings deposited on steel substrates in some harsh conditions are limited due to the “eggshell failure” caused by hardness inconformity and poor adhesion between the coating and the substrate [5, 6]. In order to solve the problem of simplex surface treatment, Korhnen et al [7] proposed a new technology of duplex treatment, which combines plasma nitriding (PN) and physical vapor deposition (PVD). However, during the study of PN-PVD duplex treatment processes, the influence of nitriding treatment on the adhesion of the coating is still controversial. Although, the nitriding zone can serve as an interfacial transition zone and overcome the“eggshell failure” phenomenon in return. But the existence of the γ′-Fe4N or ε-Fe2,3N compound layer in the nitriding layer may weaken the adhesion strength [8-10]. Therefore, the effects of preparation parameters and different nitriding layers on the performances of PN-PVD composite coatings are the research focuses of the duplex treatment. Plasma nitriding, as an important surface treatment technology, has many advantages, such as lower temperature, higher diffusion rate, environmentally friendly, flexible process controllability, etc. And it can significantly improve the surface hardness and wear resistance of metal materials[10]. Besides, the arc plasma assisted nitriding is a newly emerged plasma nitriding technique, which equipped with arc discharge, instead of traditional glow discharge. The nitriding pressure of plasma nitriding is about 0.4~4 Pa, while the plasma density reaches up to 1010~1011cm-3. The working pressure of arc plasma assisted nitriding decreases by 2~3 orders of magnitude compared with the conventional plasma nitriding, which can effectively avoid the surface micro-arc discharge and inhomogenous temperature caused by hollow cathode effect [11, 12]. Meanwhile, the advanced plasma-assisted arc (APA-Arc) technique has many prominent advantages, such as a higher plasma density, a higher deposition rate, a better adhesion, a denser microstructure with a smoother coating surface and a lower consumption of targets compared with the tradiontional arc ion plating [10, 13].

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In this paper, the different nitriding layers were tailored by plasma nitriding with various N2/H2 flow ratios on polished AISI-H13 steel. The AlTiN coatings were deposited on different plasma nitrided and merely polished non-nitrided H13 steel by APA-Arc technology, respectively. The effects of N2/H2 flow ratios on the microstructures of nitriding layers were detailedly analyzed. The influence of different nitriding layers on microstructures and properties of duplex-treated AlTiN coatings were comprehensively investigated.

2. Experimental 2.1 Plasma nitriding and coatings deposition The H13 steel substrates were mirror polished after vacuum heat treatment (1050℃ vacuum quenching, double 600ºC×2h tempering). The hardness of the original substrates is 48~50HRC and the roughness value Ra is approximately 0.05μm. The specimens were cleaned in an alcoholic solution with ultra-sonic agitation for 15min at room temperature. After that, the specimens dried by hot wind were loaded onto a three-fold rotation turntable (diameter 300mm×400mm height) within the chamber of an industrial capacity Sulzer Metaplas Domino mini PVD system, as shown schematically in Fig. 1a. The PVD system is equipped with arc enhanced glow discharge (AEGD), Plasma Nitriding, APA-Arc and HIPIMS technologies. The specimens were pre-cleaned by highly ionized Ar+ with AEGD ion etching technology, as shown schematically in Fig. 1b, followed by the preparation of different nitriding layer by using three different N2/H2 flow ratios. The plasma nitriding was enhanced by the generated energetic nitrogen ions with glow discharge plasma for increasing the diffusion capacity. Finally, AlTiN coating were deposited on the polished H13 and the different nitrided H13 by the APA-Arc technique. The details of the plasma nitriding and coating deposition parameters for the AlTiN coating are listed in Table 1. After specimens fabrication, the monolayer coating was marked as AlTiN, and the duplex-treated coatings with N2/H2 flow ratios of 50/25, 38/38 and 25/50 sccm during plasma nitriding process were marked as PN1/AlTiN, PN2/AlTiN and PN3/AlTiN, respectively. 2.2 Characterization The surface and the cross-sectional microstructures were investigated by a field emission scanning electron microscope (FE-SEM, ZEISS Merlin, Germany) fitted with an energy dispersive spectrometer (EDS, Oxford X-MaxN20, England). The surface roughness was measured by an 3

atomic force microscope (AFM, Bruker Multimode 8, Germany) with an area of 10µm×10µm. The chemical compositions of the coatings were determined by an electron probe micro-analyser (EPMA, Shimadzu EPMA-1600, Japan). The phases, orientation and crystallite size of the coating were determined by X-ray diffraction (XRD, Philips X’Pert, Netherlands) with the Cu target, at 40 kV and 40 mA in a 2θ range of 30-90° using a step size of 0.02°. The microhardness of the specimens were measured by a micro Vickers tester (Shimadzu HMV-2T, Japan) with a load of 50 g for 10 s and estimated by an average value from 5 measured points. Hardness Rockwell C (HRC) indentation measurements were conducted to evaluate the coatings adhesion according to the Verein Deutscher Ingenieure (VDI) standard 3198. The scratch tests were conducted by a MFT-4000 instrument and equipped with a radius of 200 μm diamond indenter, the loading of indenter increased linearly from 0 to 100N and the loading rate was 50N/min. Tribological tests were carried out using a ball-on-disc tribometer (Bruker UMT-3, Germany) with a 4 mm diameter silicon nitride (Si3N4) ball as counterpart at room temperature in an ambient atmospheric condition. The testing parameters were set as: constant normal loading force 10 N, rotating speed 318 rpm, rotational diameter 6 mm and sliding for 30min. The worn surfaces were characterized by SEM and the wear tracks of the worn surfaces were investigated by a 3-D model optical profilometer (BMT SMS Expert, Germany).

3. Results and discussion 3.1 Surface morphology Fig. 2 shows the SEM surface micrographs of the polished substrate, the nitriding layers with various N2:H2 flow ratios and the duplex-treated AlTiN coatings with different nitriding layers. The ion etching effects of original substrate surface are obvious during plasma nitriding process (Figs. 2 b~d), and the surface roughness increased significantly from Ra 0.05μm to 0.41~0.50μm (Fig. 3). The nitrided specimen with N2:H2 flow ratios of 38/38 presented highest Ra value (0.50μm), which revealed a novel ion etching capability of this kind of nitriding parameters. The surface conditions of substrate play an important role in affecting the adhesion between the substrate and the coating [14]. On the one hand, the substrate with a certain level of surface roughness, will change the shrinkage force direction of the coatings to be in parallel with the substrate surface and decrease the interfacial stress, which reduces the cracking tendency of the coating in return; on the other hand, 4

the surface micro-roughening could increase the effective contact area between the coating and the substrate, it is of great benefits to promote the chemical and physical reactions occurring at the interface. The micropores and large particles are the most concerned issues in the arc ion plating coating [15]. As shown in Figs. 2 e~f, the relatively small quantities of micropores and particles on the surface of the AlTiN coatings are smaller than 2μm, which revealed high-qualities of the coatings and novel parameters of AlTiN coating deposition process. Compared with the monolayer AlTiN coating, the pits morphologies of the composite coatings are more obvious due to ion bombardments during the nitriding process, which is consistent with the roughness results shown in Fig.3. The roughness of monolayer AlTiN coating is 0.13μm, while the roughness values duplex-treated AlTiN coatings with Ra ranging from 0.25 to 0.33μm are obviously larger than that of the monolayer AlTiN coating. Meanwhile, the surface roughness of the duplex-treated coatings varies for different nitriding layers. The PN3/AlTiN coating with Ra of 0.25μm, reaches lowest roughness value, while the PN2/AlTiN coating reaches highest roughness value with Ra 0.33μm among the composite coatings. The roughness value sequence of coatings is in accordance with the roughness value alignment of the corresponding nitriding layer, which reflects the conformal coating effect of PVD deposition. 3.2 Interface Structure and Element Analysis Fig.4 shows the optical and SEM micrographs for the cross-section of the nitriding layers with various N2:H2 flow rations and the duplex-treated AlTiN coatings with different nitriding layers. The AlTiN coating, the compound layer and the nitriding layer can be respectively recognized clearly, the AlTiN coatings are about 6.6±0.2 μm in thickness and the nitriding layers reach 30~40 μm in thickness. The nitrided specimen with N2/H2 flow ratio of 25:50 has the highest thickness of nitriding layer and achieved about 42μm after 2-hour nitriding process, so the nitriding rate reached 21μm/h, much higher than that of the traditional plasma nitriding (5~10 μm/h). Different N2/H2 flow ratios during nitriding process, caused various thickness compound layers. The nitriding layer with N2/H2 flow ratio of 50:25 has a compound layer of 0.8μm in thickness, the nitriding layer with N2/H2 flow ratio of 38:38 has 1.5μm compound layer and reaches the highest thickness, while the compound layer thickness of N2/H2=25:50 flow ratio nitriding layer is only 0~0.3μm. Compared

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with the nitrided samples, the thickness of compound layer decreased after AlTiN coating deposition, and there is almost no compound layer in the PN3/AlTiN coated sample. The reason is the AEGD ion etching process before AlTiN depositon etched and removed some of the compound layers. SEM images for fractured cross-section of AlTiN coated specimen is revealed in Fig. 5a, it could be clearly observed at the interface that AlTiN coating developed crystal bondings with the substrate. The morphologies of a great majority columnar crystals and nanocrystals are easily observed in Fig. 5a, the nucleation and growth processes can be described as: ions will preferential nucleation begins at the defects on the substrate; with ion groups depositing, crystals grow up to islands in the three-dimensional direction; with continuous ion depositing, the system energy forces islands dissociating, forming, and diffusing to the valleys between columns, it is the most important stage that determines the formation of columnar crystals. The stage is mainly determined by growth temperature, surface energy, incident energy and deposition rate [16]. The Volmer-Weber growth mode could also account for the decreased surface roughness after coating deposition, the ion groups preferential begins nucleation at defects caused by nitriding process, then decreased the roughness value in return. Fig.5b shows element compositions for the cross-sections of duplex-treated AlTiN coating. The element compositions of AlTiN coating is Al35.03Ti30.04N34.93 (at. %), the Al/Ti atomic ratio of coating is about 1.17, which is similar with the Al55Ti45 at.% target with Al/Ti ratio of 1.22. EPMA analysis of nitrogen line distributions for the cross-sections of duplex-treated coatings with different nitriding layers were shown in Fig. 5c, the nitrogen element is gradient distributed alone the cross sections with nitrogen content progressively decreased and the thicknesses of nitriding layer are ranging about from 35 to 45μm. As mentioned above, the nitriding layer with N2/H2 flow ratio of 38:38 with compound layer of 1.5μm in thickness reached the highest value among them, so the peak of nitrogen is wider than the others. Therefore, the nitrogen distributions are related to the thickness of the compound layer, the higher nitrogen content of the compound layer was, the thicker the compound layer might present. 3.3 Phase structure analysis The AlxTi1-xN phase is based on the NaCl-TiN structure with titanium atoms being substituted by

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aluminum atoms. Due to the radius of the Al atom (0.143nm) is smaller than that of the Ti atom (0.146nm), the crystal structure is distorted and the peak position is shifted to the high angle position [17]. Fig.6 shows the XRD patterns of the duplex-treated coatings with different nitriding layers. The compound layers consist of Fe3N phase, and there is almost no diffraction peak of Fe3N observed in the PN3/AlTiN coating, which is in accordance with the results that no compound layer observed from cross-sectional SEM micrographs in Fig.4. The diffraction peaks of AlTiN coatings present (111), (200), (220) and (311) textures, for the cubic structure of the hard film, the preferred orientation is dominated by the energy of the deposited particles, including the surface energy and strain energy. According to the theoretical calculation, the degree of the surface energy (Shkl) of TiN crystal is S200
0 𝐼ℎ𝑘𝑙 ⁄𝐼ℎ𝑘𝑙 0 (1⁄𝑛) ∑[𝐼ℎ𝑘𝑙 ⁄𝐼ℎ𝑘𝑙 ]

0 where 𝐼ℎ𝑘𝑙 and 𝐼ℎ𝑘𝑙 are the integrated intensities of the reflections measured for an experimental

specimen and a standard powder, respectively, while n is the total number of the reflection planes. When the TC of one plane is larger than unity, then a preferred orientation with the (hkl) planes parallel existences. The TC of the (200) plane for the AlTiN, PN1/AlTiN, PN2/AlTiN and PN3/AlTiN coatings is 1.70, 1.61, 1.70 and 1.55, respectively, larger than unity, indicating that the coatings exhibit the (200) preferred orientation. Therefore, during the coating growth, the surface energy is dominant. As we know, the internal compressive stresses in the films mainly originated from the ‘‘atomic peening’’ mechanism, therefore, the internal stress correlates to the strain energy in the deposited films [18, 20]. In this work, the dominant position of surface energy rather than strain energy would alleviate internal stress. 3.4 Hardness analysis Fig. 7 shows micro-hardness of the monolayer AlTiN and the duplex-treated AlTiN coatings, as well as hardness profiles cross the sections of nitriding layers. The hardness of the duplex-treated

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AlTiN coatings is higher than that of the monolayer AlTiN coating due to the enhanced substrate by nitriding process (Fig. 7a). Meanwhile, duplex-treated coatings present different hardness due to different nitriding layers, and the sequential hardness is PN2/AlTiN> PN1/AlTiN> PN3/AlTiN. As revealed in Fig. 7b, the microhardness profile taken from the cross-sections depicted the hardness features. The curve is divided into two distinct parts, marked as nitriding zone and substrate. The nitriding zone reached more than 1100 HV0.05, which served as transition zone of hardness. Moreover, the nitriding zone between the harder coating and the softer substrate could relieve the eggshell-like failure mechanisms of the substrate-coating system. 3.5 Adhesion strength analysis SEM micrographs of Rockwell-C indentations of the monolayer AlTiN and duplex-treaed AlTiN coatings are shown in Fig. 8. The radial and annular cracks as well as spallations appear at the edge of indentation crate of the monolayer AlTiN coating, as shown in Fig.8a. The poor adhesion of the monolayer AlTiN is recognized as HF3~4. On contrast, Figs.8b~d show that the number of radial and annular cracks of the AlTiN composite coatings are significantly reduced, and there is scarcely any obvious coating peelings, so the AlTiN composite coatings with HF1 exhibited preferable adhesion strength. Relying on the obtained results, it can be interpreted as the hardness of substrate was greatly increased by the nitriding layer and provided sufficient support for the followed hard coatings even under heavily loading. Besides, one can note that the bonding strength of PN3/AlTiN coating is the best among these three composite coatings with qualitative judgements from the number of cracks, while PN2/AlTiN coating shows the worst adhesion due to the highest thickness of compound layer. Because, the hard and brittle compound layer, where cracks tend to initiate and expend, will weaken the adhesion and loading capacity of the coating, this was also confirmed by Zhang [8]. The results of scratch tests are shown in Fig. 9. Three critical loads can be determined in scratch test: Lc1 is the minimum load where cracks occurs; Lc2 is the minimum load where the coating starts to peel off, which is generally regarded as the critical point of coating failure; Lc3 is the minimum load where the coating is fully stripped off [21]. So Lc2 is considered as the adhesion value of coating-substrate system among them [22]. The adhesion forces (Lc2) of the AlTiN, PN1/AlTiN, PN2/AlTiN and PN3/AlTiN are 35N, 39N, 43N and 61N, respectively. The results of scratch tests

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are in conformity with those of indentation tests. Besides, it can be note from the corresponding peeling morphologies that the width of scratch track for the monolayer AlTiN is obviously larger than that for the composite coatings, and the delaminations around the scratch track are evident in the AlTiN, PN1/AlTiN and PN2/AlTiN coatings, while there is almost no delamination in PN3/AlTiN. In general, coatings on the harder nitrided substrates will exhibit better adhesion, due to the increased deformation resistance and load bearing capacity of the substrates during nitriding process. However, the hard but brittle compound layer, on the top of nitriding layer, will easily prone to initiate cracks and propagation at the interface under highly compressive stress condition. This is why the adhesion force of PN1/AlTiN and PN2/AlTiN are lower than that of PN3/AlTiN. 3.6 Tribological properties analysis Fig.10 shows the friction coefficient and the wear track profile of the monolayer and duplex-treated AlTiN coatings. As observed in Fig.10a, the friction coefficient of composite coatings are stable and about 0.7~0.75, while the monolayer AlTiN is rather unstable and ranges from 0.7 to 0.79. Besides, among the composite coatings, the friction coefficient of the PN3/AlTiN is slightly lower than the others. The wear track profiles are revealed in Fig.10b, the wear track area of composite coatings are much smaller than that of monolayer AlTiN, and the wear track depth of composite coatings (~2.5μm) are much smaller than that of monolayer AlTiN (~4.3μm). Therefore, the wear resistance of AlTiN coating are well improved by duplex treatments. SEM morphologies of worn tracks are illustrated in Fig. 11, the wear track are relatively shallow and almost no wear debris adhered on their middle zone. The wear track of the monolayer coating is wider than that of the composite coatings under the same loading condition. A small amount of tearing as well as minor shallow scratches caused by abrasive particles, indicating that the AlTiN coatings mainly suffers from a wear mechanism of adhesive wear at room temperature. Meanwhile, debris cluster at the edge of the wear track was formed by accumulation of ploughed out debris from the wear track during sliding. The EDS analysis results for the wear tracks are listed in Table 2, it show that the main components of the wear track is Al, Ti elements of the coating, and the high content of O element at the edge of wear track indicate that the wear mechanism of the duplex-treated coating is a mixture of adhesion wear and oxidation wear as reported earlier [10].

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In addition, the elemental transfer phenomenon of the Si3N4 ball material to the coating surface has also been confirmed from the EDS analysis.

4. Conclusions The duplex-treated AlTiN coatings were successfully deposited on H13 steel by two-hour plasma nitriding and one-hour APA-Arc deposition for the AlTiN top layer. The primary conclusions can be summarized as follows: (1) The plasma nitriding rate reached about 20 μm/h and deposition rate of AlTiN coating reached about 6.6 μm/h with the plasma assistance during nitriding and coating deposition process. (2) The N2/H2 flow ratios can tailor the thickness of compound layer during plasma nitriding process and the bright nitriding layer without compound layer was achieved with N2/H2 flow ratios of 25:50sccm. The compound layer can be decreased and even removed by AEGD ion etching technology before coating deposition process. (3) The hardness of the duplex-treated AlTiN coatings is higher than that of the monolayer coating. The adhesion grade of the monolayer coating is HF3-4, while the adhesion grade of the duplex-treated coatings reached to HF1. The duplex-treated PN3/AlTiN coating without compound layer exhibited the highest adhesion force of 61N in scratch tests. The tribological properties of AlTiN coatings were also improved by duplex treatment and the composite coatings without compound layer (PN3/AlTiN) have the lowest friction coefficient.

Acknowledgements The authors are pleased to acknowledge the support of senior engineer Shiheng Yin, the Analysis & Test Center at South China University of Technology and senior engineer Mingzhen Mo, Guangdong Entry-Exit Inspection and Quarantine Bureau for their kind help in experiment tests. We also gratefully acknowledge the financial support of the National Nature Science Foundation of China (51471071), the Science & Technology Program of Guangdong Province (2013B010403008) and the scientific research project of Guangdong Entry-Exit Inspection and Quarantine Bureau (2014GDK54). This work was also supported by Technological Projects of Zhongshan-GDAS

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(2016GIFC0004) and Sciences Platform Environment and Capacity Building Projects of GDAS (2016GDASPT-0206).

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Figures Fig.1. Sectional view of the Sulzer deposition system (a) and the schematic diagram of AEGD process (b). Fig.2. SEM surface micrographs of the polished substrate, the nitriding layers with various N2:H2 flow ratios and the duplex-treated AlTiN coatings: (a) polished substrate; (b) PN1 N2:H2=50:25; (c) PN2 N2:H2=38:38; (d) PN3 N2:H2=25:50; (e) AlTiN; (f) PN1/AlTiN; (g) PN2/AlTiN and (h) PN3/AlTiN. Fig.3. Surface roughness values of the polished substrate, the nitriding layers with various N2:H2 flow ratios and the corresponding duplex-treated coatings: (a) polished substrate and AlTiN coated; (b) N2:H2=50:25 nitrided and corresponding PN1/AlTiN coated; (c) N2:H2=38:38 nitrided and corresponding PN2/AlTiN coated and (d) N2:H2=25:50 nitrided and corresponding PN3/AlTiN coated specimens. Fig.4. Optical and SEM micrographs of the cross-section of the nitriding layers with various N2:H2 flow rations and the duplex-treated AlTiN coatings with different nitriding layers. Fig.5. Fractured SEM images (a) and element composition (b) of the cross-section of AlTiN coating, and EPMA analysis of nitrogen line distributions of the cross-sections of duplex-treated coatings with different nitriding layers (c). Fig.6. XRD patterns of the monolayer and duplex-treated AlTiN coatings. Fig.7. Micro-hardness of the monolayer and the duplex-treated AlTiN coatings (a) and hardness profiles cross the sections of nitriding layers with various N2:H2 flow ratios (b). Fig.8. SEM micrographs of Rockwell-C indentations of the monolayer and duplex-treaed AlTiN coatings: (a) AlTiN, (b) PN1/AlTiN, (c) PN2/AlTiN and (d) PN3/AlTiN. Fig.9. Scratch tests of the monolayer and duplex-treated AlTiN coatings with different nitriding layers: (a) AlTiN; (b) PN1/AlTiN; (c) PN2/AlTiN and (d) PN3/AlTiN. 12

Fig.10. The friction coefficients (a) and the cross-sectional wear track morphologies (b) of the monolayer and duplex-treaed AlTiN coatings. Fig.11. SEM morphologies of worn tracks of the monolayer and duplex-treated AlTiN coatings.

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Fig.1

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Fig.2

Polished substrate

PN1: N2/H2=50/25

PN2: N2/H2=38/38

AlTiN

PN1/AlTiN

PN2/AlTiN

PN3: N2/H2=25/50

PN3/AlTiN

15

Fig.3

16

Fig.4

17

Fig.5

18

Fig.6

19

Fig.7

20

Fig.8

21

Fig.9

(a) AlTiN

Lc2

(b) PN1/AlTiN

Lc2

(d) PN3/AlTiN

(c) PN2/AlTiN

Lc2

Lc2

22

Fig.10

23

Fig.11 Middle

Edge

PN3/AlTiN

PN2/AlTiN

PN1/AlTiN

AlTiN

Wear track

Tables Table 1 Parameters for plasma nitriding and AlTiN coatings deposition. Steps 1 2 3 4 5

Process Vacuum Heating Vacuum Heating Vacuum

PN parameters Pressure= 4.0×10-5mbar Temperature= 600 ºC, duration=30min Pressure=4.0×10-5mbar Temperature=550 ºC, duration=30min Pressure=4.0×10-5mbar

PVD parameters Same as the left. Same as the left. Same as the left. Same as the left. Same as the left. 24

6

AEGD

7

Plasma nitriding or coating deposition

8

Cooling down

Temperature=500ºC, cathode=Ti (99.99%), Ti target evaporation current=85A, process gas=Ar, 1×10-2mbar, substrate d.c./pulse bias =−300/20V, duration=60min Pressure=1×10-2mbar, process gas= Ar+N2+H2, N2/H2 =PN1= 50/25 sccm, PN2=38/38sccm, PN3 =25/50 sccm, duration=120min, others as above. Cooling water temperature =18 ºC, pressure=1.0×10-7Pa

Same as the left.

Temperature=500ºC, cathodes= Ti55Al45 (at.%), TiAl target evaporation current =150A, process gas=N2, pressure= 8.5×10-2mbar, substrate d.c. bias= -40~-80 V, duration=60min. Same as the left.

Table 2 Element analysis results for the wear tracks of the monolayer and duplex-treated AlTiN coatings. Elemental composition(wt%) AlTiN PN1/AlTiN PN2/AlTiN PN3/AlTiN

Middle Edge Middle Edge Middle Edge Middle Edge

Al

Ti

N

O

Si

27.47 22.61 21.59 21.02 20.67 22.26 20.86 22.64

43.05 35.70 33.21 32.47 33.14 35.61 33.29 36.37

24.14 20.48 16.49 15.18 15.40 19.62 15.93 20.97

3.64 16.54 23.38 25.90 25.76 17.93 23.96 14.18

0.04 0.39 1.05 1.05 1.16 0.53 1.02 0.39

25