A new method to improve the tribological performance of metal nitride coating: A case study for CrN coating

Journal Pre-proof A new method to improve the tribological performance of metal nitride coating: A case study for CrN coating Jingwen Zhang, Zechao Li, Yongxin Wang, Shengguo Zhou, Yixuan Wang, Zhixiang Zeng, Jinlong Li PII:

S0042-207X(19)32412-1

DOI:

https://doi.org/10.1016/j.vacuum.2019.109158

Reference:

VAC 109158

To appear in:

Vacuum

Received Date: 1 October 2019 Revised Date:

20 December 2019

Accepted Date: 23 December 2019

Please cite this article as: Zhang J, Li Z, Wang Y, Zhou S, Wang Y, Zeng Z, Li J, A new method to improve the tribological performance of metal nitride coating: A case study for CrN coating, Vacuum, https://doi.org/10.1016/j.vacuum.2019.109158. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

A new method to improve the tribological performance of metal nitride coating: A case study for CrN coating Jingwen Zhanga,b, Zechao Lia, Yongxin Wanga,1, Shengguo Zhoub, Yixuan Wanga,c, Zhixiang Zenga, Jinlong Lia. a

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key

Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China b

School of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, PR China;

c

Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, PR China

Abstract:

Taking CrN coating as an example, a new refinement method was performed to improve the tribological performance of metal nitride coatings in the low temperature

1

Corresponding author: Tel: +86 0574 86697306, E-mall: [email protected]; 1

thermal cycling in humidity condition. After the low temperature thermal cycling post-treatment of -20 to 60 ℃ in high humidity environment of 80 %, the structural and performance changes of the monolayer CrN coating fabricated by the multi-arc ion plating deposition technique were evaluated by using high-resolution transmission electron microscopy, scanning electron microscope equipped with EDS analyzer, X-ray diffraction, X-ray photoelectron spectroscopy and ball-on-disk tribometer. TEM observations and inverse Fourier-filtered images revealed dislocation-mediated mechanism in the treated state CrN coating, which was a key mechanistic process to enhance mechanical properties and wear resistance of CrN coatings after low temperature thermal cycling treatment. Moreover, the more physisorption and chemisorption of oxygen in treated CrN coating can be released and then pass through the defects and the channels between columnar crystal structure, finally arriving at the surface of coating to continuously participate in the tribo-contact interface reaction and produce more oxides as lubricants to effectively reduce friction coefficient and wear rate during dry-sliding. Keywords: Low temperature thermal cycling; CrN coating; Dislocation-mediated mechanism; Physisorption and chemisorption; Tribological performance

1. Introduction The metal nitride (Me(X)N, X means doping element, such as, TiN, CrN, ZrN, TiSiN, CrAlN, TiCrAlN, etc.) coatings, which was normally fabricated by physical vapor deposition (PVD) technology, had great potential to be used in the variety fields 2

of protection due to their high hardness, high thermal stability, low friction, good wear resistance as well as superior corrosion resistance [1-4]. To achieve higher performance for Me(X)N coatings, various microstructure features were designed, including multi-elements composite, gradient transitional microstructure, multilayered coating structure, nanocrystalline mixed with amorphous phases and so on [5-8]. Though special microstructures were formed during the deposition process to enhance the Me(X)N matrix, there were still growth defects such as microcavities, microparticles on surface and pores, pinholes within the coating. The growth defects and locally coherent interfaces could cause cracks propagation and expose the substrate to the environment, which accelerated Me(X)N coatings failure [9-11]. The acceleration was rather bad or even fatal when the Me(X)N coatings were used as protective surfaces in harsh conditions, for example, high temperature, corrosive fluid, pressure water [3, 12, 13]. Hence, in order to inhibit these negative effects of microstructure defects to the tribological performance of Me(X)N coatings, many researchers tried many efforts to modify the as-fabricated coatings after fabrication or interfacial designs including substrate pretreatment, interlayer architecture, elements doping, coating annealing and so on. For example, Härkönen et al. successfully used the ALD layers to seal the defects in hard CrN surface, which further enhanced wear resistance and anti-corrosion properties [14]. Park et al. reported that the magnetron sputtering process could be interrupted several times by an intermediate plasma etching process to increase the corrosion resistance of CrN coating [15]. Reinhard et al. proved that the ion etching pre-treatment prior to CrN/NbN coating deposition 3

could significantly improve coating adhesion [16]. However, these typical treatments still have many disadvantages for the follow-up applications of metal nitrides coatings. Though the high temperature in the heat treatment process could relax or strengthen the coating matrix due to the microstructure evolution, there would be a huge risk to change the substrate. Even the temperature during the sealing process could be adjusted into a low range, extra cost would be unavoidable. As for the plasma implanting, it was hard to say good or bad because of the enhanced internal stress for some applications. In the present work, the typical CrN coating was taken as an example and performed a new coating refinement method which was low temperature thermal cycling in humidity condition (LTC) to improve the tribological performances of Me(X)N coating. The low temperature level of -20 to 60 ℃ has little effect on many kinds of substrates. The microstructure evolution would occur without significant cost since it was a rather simple process. More importantly, the CrN coating exhibited much lower friction and wear after the treatment. Undoubtedly, this microstructural modification opened a window to improve the Me(X)N coatings materials by means of a simple, harmless and friendly process. 2. Experimental details 2.1. Coating deposition The CrN coating was fabricated by multi-arc ion plating deposition method (Hauzer Flexicoat 850) with the chromium targets (purity ＞ 99.5 wt%, Φ 63 mm). Before the deposition process, the 316L stainless steel substrates were ultrasonically 4

cleaned in an ultrasonic bath of acetone and ethanol for 20 min, respectively, and then mounted on the substrate holder at 10 cm in front of the targets to ensure the uniform growth of the coatings. When the chamber was pumped down to a base pressure of 4×10−3 Pa, the substrates were etched by an arc-enhanced glow discharge in argon plasma for 2 min with substrate bias voltages of -900, -1100 and -1200 V separately to remove thin oxide layers and contaminants on the substrate surface. The Cr targets with −20 V substrate bias and currents of 60 A were firstly used for the fabrication of thin Cr interlayer for 10 min in high purity Ar gas with a flow rate of 350 sccm. Then N2 was introduced into the chamber at 600 sccm gas flow. The target current of 60 A and the substrate bias of −20 V were applied for 2 h to fabricate CrN layer. The deposition temperature was 400 ℃ and the substrate holder rotation speed was 3 rpm. 2.2. The low temperature alternating treatment After deposition, the CrN coating was treated by the low temperature alternating (hot and humidity test chamber HS-050D). Under normal pressure, the temperature cycle firstly started from room temperature to high temperature (60 ℃) with a lifting temperature rate of 2 ℃/min and then reduced to low temperature (-20 ℃) with the cooling rate of 1 ℃/min. The temperature deviation was lower than ±2 ℃. When the alternating cycle time was 6, the samples were kept at constant temperature for 12 h before the next cycle. The total number of alternating cycle was 84 (two weeks) and the humidity of chamber maintained at 80% all the time. 2.3. Microstructure characterizations 5

The field emission scanning electron microscope (FEI QUANTA 250 FEG) equipped with EDS analyzer (OXFORD X-Max) was used to investigate the surface morphology and elemental concentration changes of CrN coating during thermal cycling. In order to investigate the microstructure changes of CrN coating before and after low temperature alternating, the investigation of cross-sectional HRTEM, as well as the inverse fast Fourier transform were carried out by the transmission electron microscope (FEI Tecnai F20). The X-ray diffraction (BRUKER D8 DISCOVER) with a Cu Kα radiation (wavelength = 0.154056 nm) was performed to determine the crystal structure changes of CrN coatings before and after low temperature alternating. And the internal stress in the as-deposited and treated coatings was calculated by the sin2ψ method. The CrN (400) peak at the angle of 2θ ~95.929° was used for 12 inclination angles ψ (0° to 60°). The elasticity modulus and a poisson ratio (υ = 0.25) of CrN coatings were used for stress calculation [17, 18]. 2.4. Mechanical properties and tribological behavior The mechanical properties changes of CrN coatings before and after low temperature alternating treatment were measured using a Vickers indenter (MVS-1000D1 China) and a MTS Nano Indenter G200 system with the Oliver-Pharr method. By the Vickers indentation method, the toughness of hard coatings could be evaluated, in which the surface of polished CrN coatings was penetrated by a Vickers pyramidal indenter at loads of 200, 300, 500 g, respectively. The Oliver-Pharr method was a generally accepted method to calculate the hardness and elastic modulus of coating. The maximum indentation depth was 1000 nm and 6 indentations in different 6

areas of each sample were performed to acquire the mean value and the standard deviation. Tribological properties of CrN coatings were performed by a reciprocating ball-on-disk tribometer (CETR UMT−3MT, USA) and the SiC balls with a diameter of 3 mm were selected as counterparts. The load of 10 N and a sliding stroke of 5 mm was applied in the experiments with the high stroke frequency of 5 Hz, and the friction coefficient was continuously recorded during testing lasted for 60 min. An Alpha-Step IQ profilometer was used to detect the wear track depth profiles at several locations and the wear loss can be obtained after the sliding tests. Then the formula K = V/FS was used to calculate the wear rate. The laser confocal scanning microscope (Zeiss-material type, Germany) and the scanning electron microscope (Carl Zeiss EVO 18, Japan) equipped with EDS analyzer (OXFORD X-Max) were employed to acquire morphology of wear tracks and elemental analysis at the friction interface. Moreover, the multifunctional X-ray photoelectron spectroscopy (XPS Kratos AXIS UTLTRADLD, UK) with a monochromatic Al kα X-Ray source (hν = 1486.6 eV) can be used for chemical state analysis of wear tracks surface. The power was 120 W and the analysis spot size was set to 700 × 300 µm. The pass energy of XPS analyzer was wet at 20 eV, the scan step was 0.05 eV. In which, the electrostatic lens mode and analyzer entrance were selected in the Hybrid and Slot modes. Before the measurement, Ar+ sputtering etching was conducted on the wear tracks surface in order to clean up impurities and contaminants. Ar+ ion energy was 2 kV, incidence angle was 60° used for sputter cleaning together with the size of cleaned area was 4 mm and sputtering time was 5 min. 7

3. Results and discussion 3.1. Micro-structure, morphologies and element content of the coatings before and after low temperature alternating

Fig. 1. Surface morphology FESEM observations of CrN coatings before and after low temperature alternating, with (c and d) the corresponding cross-sectional micrographs and the O element area mapping results (inset). As shown in Fig. 1a, there are many half-spherical micro-cavities and cauliflower-like micro-droplets randomly distributed on the surface of CrN coating in as-deposited state, which are the characteristics of multi arc ion plating deposition technique. All features are also seen on the treated CrN coating (after low temperature alternating treatment) (Fig. 1b) and no deformations were found on the surface appearance. According to the EDS result from pink dotted box in Fig. 1(a and b), oxygen (O) content on the CrN coating surface more than doubled after low temperature

alternating

treatment,

which 8

implies

more

physisorption

and

chemisorption of oxygen on CrN coating surface [19]. Moreover, the chemical reactions also easily and rapidly occur between the CrN coating and water molecules in the cycling chamber with high humidity and temperature to form thin oxide surface layer, such as Eqs. (1) and (2) [20]. 2CrN + 3H2O = Cr2O3 + 2NH3

(1)

∆

(2)

= −250.10 Where ∆

∙

is the reaction Gibbs free energy of formation at 298 K.

Fig. 1(c and d) show the corresponding O element cross-section distribution in CrN coating before and after low temperature cycling. The EDS results estimate that O content in CrN coating before and after low temperature alternating treatment are 6.41 and 12.63 at. %, respectively. The O contents is increased by 97% in CrN coating with low temperature alternating treatment, which again indicates the present of absorbed molecules. In particular, comparing with as-deposited state CrN coating, a plenty of O elements were located in the defects of CrN coating such as pores and pinholes after low temperature alternating cycling. As have been explained qualitatively or semi-quantitatively by many researchers, the sorption kinetics of oxygen atoms perpendicular to the coatings surface mainly attribute to the large gradient of the chemical potential in the region of near the surface [21-23]. The porosity of CrN coatings and high humidity also accelerate the penetration of O element. Based on the above characterizations and analyses, the O element is located in the treated CrN coating in forms of Cr-oxides atoms or ions, which is a key factor as a lubricant to effectively reduce of friction coefficient during dry-sliding. 9

Fig. 2. Typical bright field TEM images (a and b) of coating before and after low temperature alternating treatment, the corresponding HRTEM observation (c and d) and inverse fourier-filtered images from inside the pink loop frames in (c) and (d). The bright-field TEM observations (Fig. 2a) show that the columnar crystals with vertically alignment characteristics remain intact in the as-deposited CrN coating. Compared with as-deposited CrN coating, many fine cracks (indicated by arrows) are clearly appeared at the bottom and boundary of columnar crystals after low temperature alternating (Fig. 2b), which indicates that the high residual stress are generated in the treated CrN coatings. The HRTEM images (Fig. 2c and d) clarify the presence of some lattice distortions and piles-up of dislocations at the grain 10

boundaries of treated state CrN coating. Meanwhile, the inverse Fourier-filtered image (inset) from the yellow box more clearly displays that the dislocation (yellow T) is trapped inside the grain boundary (GB) of treated state CrN. Due to the high-angle GBs with lower thermal and mechanical stability [24, 25], the TEM images confirm that CrN coating is mainly mediated by the nucleation and motion of dislocations under higher stress raised during cooling and heating stages of temperature alternative cycling. During the low temperature alternating process, because of the anisotropy of CrN structure and the large coefficient of thermal expansion (CTE) mismatch between the substrate and the coating [26, 27], the internal stress is gradually increased, which makes further efforts to cause accumulation of the defects and cracks initiation and propagation.

Fig. 3. X-ray diffraction patterns from as-deposited and treated CrN coatings. The XRD patterns of the CrN coating before and after low temperature alternating treatment are shown in Fig. 3. The XRD patterns for all CrN coatings clearly indicate a face centered cubic (fcc) structure (JCPDS11-0065 [28]) and a strong preferred orientation with the CrN (200) (PDF 65-9001) at 2θ value of 42.7°. 11

After the low temperature thermal cycling in humidity condition, all peaks are obviously shifted toward higher diffraction angle values and broadened significantly with lower intensity compared with the as-deposited coatings. This is mainly due to the increasing compressive residual stress and lattice distortion in the CrN coatings [29-31], which are consistent with TEM observations. Furthermore, according to the sin2ψ method, the internal compressive stress of as-deposited state CrN coating after low temperature alternating treatment are -0.44 and -1.25 GPa, respectively. This also verifies the stress intensification in the CrN coatings after low temperature alternating treatment. The accumulation of dislocations is a key mechanistic process that can potentially enhance the strength of coatings [32, 33]. And the strain hardening effect is finally achieved by the accumulation of necessary dislocations, which also indicates a great potential to improve wear resistance of the CrN coating [34]. 3.2. Mechanical property

Fig. 4. Vickers indentation SEM morphologies of as-deposited (a) and treated (b) CrN coatings. 12

The toughness, one important mechanical parameter in the coating materials design, generally characterizes the fracture resistance of hard coatings [35, 36]. And the Vickers indentation technique is one of the most common methods for evaluating the fracture toughness of hard coatings [37-39]. The SEM observations of the indentation are shown in Fig. 4. With the increase of loads, the indentations are affected by the substrates. As shown in Fig. 4a, few edge (nested) cracks and radial cracks were observed at the edge and center of the as-deposited CrN coating indentations. For the treated CrN coating (Fig. 4b), a large amount of delamination gradually penetrate into the indentation and the some circumferential cracks are observed, which indicate higher hardness, brittle nature and lower toughness than the as-deposited coatings [37, 40-43].

Fig. 5. Variation of hardness with displacement into surface from as-deposited and treated CrN coatings. According to the Oliver-Pharr method, the average hardness and elastic modulus 13

are calculated and the variation of the hardness with displacement into surface is presented in Fig. 5. As shown in Fig. 5, the hardness of treated CrN coating is obviously higher than that of as-deposited coating and its hardness reaches up to 28.6 GPa. With increasing displacement into the surface, the hardness of coating in different states decrease due to the effect of soft 316L substrate. Through the low temperature alternating treatment, the average elastic modulus and hardness of the CrN coatings show obviously increase from 329.2 to 412.8 GPa and 18.6 to 24.2 GPa, respectively. The H/E and H3/E2 as important mechanical indexes are convenient to evaluate the durability, elastic strain resistance and plastic deformation resistance of coatings [44, 45]. The H/E and H3/E2 values of the treated CrN coatings are 0.059 and 0.084 GPa, respectively, which are also apparently higher than as-deposited CrN coating’s H/E (0.056) and H3/E2 (0.059 GPa) ratios. As is known, the higher H/E ratio well means the better ability of coatings to elastic deformation, and the high H3/E2 implies high resistance to cracks initiation and propagation, which both indicate a potential excellent wear resistance [46-49]. The nano-indentation test reveals that there is a significant improvement in mechanical properties for monolayer CrN coating by the low temperature alternating treatment, which is believed to originate from a dislocation-mediated mechanism and the increase of residual stress. 3.3. Tribological behavior

14

Fig. 6. Friction coefficient curves of the CrN coatings. The inset showing 3D profiles of the two wear tracks on the CrN coatings before and after low temperature alternating treatment. Fig. 6 shows the friction coefficient curves of CrN coating. At the beginning of the friction test, particles protruding from the surface can cause solid-solid contact, resulting in a high coefficient of friction. In addition, since local pressure is high at the beginning of the sliding, a welded joint is formed at the peak of the particles, so the adhesive wear may dominate the wear mechanism and lead to a high COF. The second part is the rapid wear stage and the friction coefficient is unstable. The decrease after the running-in period may be attributed to the rapid increase in the corresponding ball and coating, which results in a rather smooth interface between the friction pairs. As the sliding process progresses, loose particles are removed and then act as wear debris. The particles are not immediately discharged from the contact area, but are surrounded by the lubricating medium and may roll or slide on the surface. 15

This process may result in a slight increase in the coefficient of friction. As the sliding time increases, the contact of the friction pair becomes smooth, and the wear debris may act as a rolling ball in the wear track, thereby reducing the friction coefficient. Furthermore, the friction pair is continuously removed by sliding to obtain an extremely smooth contact surface. On the basis of the curve characteristics, the friction coefficient of as-deposited CrN coating rapidly decreases to a maximum value of 0.27 at the beginning of the test, and then gradually increase to a stable value of about 0.37, finally a big continuous fluctuation of COF around 0.37 occur until the end of test. The increase of friction coefficient is mainly due to the abrasive plowing action of the hard CrN coating debris [50, 51]. For the treated state CrN coating, a similar trend is observed. A faster running-in stage indicates more steady friction coefficient. In the duration test, the friction coefficient in treated state CrN coating is consistently lower than the as-deposited CrN coatings. In the initial state (before low temperature alternating treatment), the CrN coatings exhibit high average COF values of 0.43. After the low temperature alternating cycles, the CrN coatings persent lower average COF of 0.28, which decrease by 31.8 %. The insets of Fig. 6 show 3D profiles observations of the wear tracks on CrN coating. Comparing with as-deposited CrN coating, it can be clearly observed that the width of wear track is also significantlly narrowed and shallow after low temperature alternating. According to the formula, the wear rates of CrN coatings in different states are calculated after friction test. The as-deposited CrN coating exhibits a high wear rate of 11.8 × 10−7 mm3/(Nm). After 84 low temperature thermal cycles, the wear rate of CrN coatings 16

significantly decreases to 5.7 × 10−7 mm3/(Nm). The wear rate decreases by 51.6 %, which can be attributed to the strain hardening and hardness increase caused by the accumulation of dislocation [52]. As shown in Fig. 7a, a broad width was observed and a significant amount of debris were accumulated along the wear track, and the furrow phenomena parallel to the sliding direction in the wear tracks has obviously disappeared, which indicate that as-deposited CrN coating is mainly controlled by the abrasive wear mechanism during the dry sliding. For the treated coating (Fig. 7b), the wear track shows some irregular peeling pits and wear debris without furrows, which reveals adhesion wear mechanism. Furthermore, by comparing the EDS result from the wear tracks surface area of CrN coating (blue dotted box) in different states, the more oxygen element (O) in the wear track of treated CrN coating (23.57 %) is greatly higher than that of as-deposited coating (12.42 %). It also indicates that prominent oxidative wear existed at the wear interface of treated CrN coating.

Fig. 7. SEM observation and EDS spectra of the wear tracks on the CrN coatings 17

before (a) and after low temperature alternating treatment (b). Table 1 Elements content acquired from wide scanned spectra of the XPS testing Name Position FWHM Area C 1s 284.8 1.69 1768.8 The wear tracks on Cr 2p 574.7 3.08 9085.1 as-deposited CrN O 1s 532.1 3.89 1989.8 N 1s 396.3 1.06 1739.7 Si 2p 103.1 2.4 142.8 C 1s 284.8 2.22 1245.0 The wear tracks on Cr 2p 574.6 3.23 8531.1 treated CrN O 1s 531.5 3.50 4727.3 N 1s 396.3 1.03 1468.0 Si 2p 103.3 2.77 847.83

At % 38.02 22.34 15.24 21.79 2.60 17.81 22.72 30.75 15.61 13.11

Fig. 8. Cr 2p (a), Si 2p (b), O 1s (c), N 1s (d) and C 1s (e) XPS spectra of the wear track on the treated CrN coating after ion bombardment. The existence of oxides at the wear interface can be similarly detected according to the XPS spectra analysis. Before the XPS measurement, Ar+ sputtering etching was conducted on the wear tracks surface in order to clean up the impurities and 18

contaminants. According to elements content acquired from the XPS testing (Table 1), it can be clearly seen that oxygen (O), silicon (Si) and chromium (Cr) element contents on the wear tracks of treated CrN coating are much higher than that of the as-deposited coating, which can powerful demonstrate the active participation of oxygen at the tribo-contact interface during dry sliding and then more oxides are produced. As shown in Fig. 8, high resolution XPS spectra of Cr 2p, Si 2p, O 1s, N 1s (d) and C 1s (e) core levels are analyzed by using CasaXPS program in order to obtain a more detailed chemical state analysis of friction products on the wear track of treated CrN coating. All spectra has been curve-fitted by using a non-linear least squares fitting method with the Gaussian-Lorentzian function [53, 54]. For the Cr 2p peak (Fig. 8a), the peak separation of the components (Cr 2p3/2 and Cr 2p1/2) and its intensity ratios are restricted to 9.2 ~ 9.4 eV and 1:2, respectively. The Cr 2p3/2 is fitted to three peaks at 577.5, 575.6 and 574.5 eV, which are ascribed Cr2O3, CrOxNy and CrN, respectively [55, 56]. In Fig 8b, the Si 2p is convoluted into three peaks: a major peak at 102.9 eV and two weak peaks at 101.2 and 100.7 eV, corresponding to the chemical compound of SiO2, silicate and SiC [53, 54, 57, 58]. In Fig 8c, the O 1s core level spectra is fitted to four peaks at 531.8, 531.6, 530.6 and 530.3 eV, which are ascribed SiO2, C=O, CrOxNy and Cr2O3, respectively [59]. In Fig 8d, the N 1s core level spectra is fitted to two peaks at 398.1 and 397.6 eV, corresponding to CrN and oxynitride [59]. In Fig. 8e, the C 1s spectra is deconvolved into four peaks at 289.9, 287.3, 285.5 and 282.9 eV, which are ascribed C=O, C-C and SiC, respectively [53, 59]. Based on the above the XPS spectra analysis, it further verifies that more oxygen 19

participate in the tribochemical reaction during dry sliding process and more chemical compound of SiO2 and Cr2O3 are formed at the tribo-contact interface of treated CrN coating. It can be imaged that such low friction coefficients and excellent wear resistance displayed by the CrN coating after low temperature alternating is largely due to an unusual state of contact interface between coating and SiC ball during the sliding. The high humidity and temperature in the cycling chamber can accelerate the Cr2O3 formed on the surface of CrN coating, which effectively reduces the friction coefficient at the start of the test. In the meantime, more physisorption and chemisorption of oxygen in CrN coating can be released and then pass through the defects and the channels between columnar crystal structure to arrive at the surface of coating during the sliding test, which can participate in the interface reaction on the later stage of sliding testing (Fig. 9). Hence, a large amount of oxidations such as chromium oxides and silicon oxides (Eqs. 3-6) can be formed at the tribo-contact interface which is a key factor as excellent lubricants to effectively reduce friction coefficient and wear rate during dry-sliding [60, 61]. 4CrN + 3O2 = 2Cr2O3 + 2N2

(3)

∆

∙

(4)

SiC + 2O2 = SiO2 + CO2

(5)

∆

(6)

= −1767.32

= −399.21

∙

20

Fig. 9. Oxygen is released to form chromium oxides and participate in the interface reaction during the sliding friction test. 4. Conclusions In summary, the effect of the low temperature thermal cycling in humidity condition on the oxygen element distribution and the structure of monolayer CrN coating prepared by multi-arc ion plating deposition method has been examined in this studied. High-resolution transmission electron microscopy and inverse Fourier-filtered image investigation directly show that the dislocation-mediated mechanism plays a dominant role to achieve strain hardening effect which resultes in excellent wear resistance for CrN coatings after low temperature alternating treatment. After low temperature thermal cycling treatment, more oxygen physisorption and chemisorption in CrN coating can continuously participate in the tribo-contact interface reaction between CrN coatings and SiC ball to form the oxides, which has a great role in lubrication to evidently decrease the coefficient of friction and improve wear resistance during dry-sliding. This simple but unique coating retreatment method is much desirable to achieve 21

low friction and improve wear resistance of the CrN coating. And the evolution of structures and properties for CrN and other metal nitride coatings at different low temperature alternating processing times and different temperature intervals need to be further investigated and explained.

Acknowledgments The authors gratefully acknowledge the financial support from the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA13040602), the National Basic Research Program of China (973 Program) (Grant no. 2014CB643302), the National Natural Science Foundation of China (Grant no. 51475449 and no. 51505467), the Key research and development program of Jiangsu Province (Grant no. BE2016115). References [1] Y. Kitamika, H. Hasegawa, Mechanical, tribological, and oxidation properties of Si-containing CrAlN films, Vacuum 164 (2019) 29-33. [2] Z. Li, Y. Wang, X. Cheng, Z. Zeng, J. Li, X. Lu, L. Wang, Q. Xue, Continuously Growing Ultrathick CrN Coating to Achieve High Load-Bearing Capacity and Good Tribological Property, ACS Appl Mater Interfaces 10(3) (2018) 2965-2975. [3] D. Wang, M. Hu, D. Jiang, Y. Fu, Q. Wang, J. Yang, J. Sun, L. Weng, The improved corrosion resistance of sputtered CrN thin films with Cr-ion bombardment layer by layer, Vacuum 143 (2017) 329-335. [4] H. Ju, L. Yu, D. Yu, I. Asempah, J. Xu, Microstructure, mechanical and tribological properties of TiN-Ag films deposited by reactive magnetron sputtering, 22

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Research Highlights

► CrN coatings were treated by low temperature thermal cycling in humidity condition. ► Tribological performance of CrN was improved remarkably after LTC treatment. ► Dislocation-mediated mechanism could be observed by TEM images in treated-CrN coating. ► Physisorption and chemisorption of oxygen in CrN participate in tribo-contact interface.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: