Crystal structure and tribological properties of ZrAlMoN composite films deposited by magnetron sputtering

Accepted Manuscript Crystal structure and tribological properties of Zr-Al-Mo-N composite films deposited by magnetron sputtering

Hongbo Ju, Dian Yu, Junhua Xu, Lihua Yu, Bin Zuo, Yaoxiang Geng, Ting Huang, Ling Shao, Letian Ren, Chengzhong Du, Hongfei Zhang, Hongzhao Mao PII:

S0254-0584(19)30271-8

DOI:

10.1016/j.matchemphys.2019.03.071

Reference:

MAC 21510

To appear in:

Materials Chemistry and Physics

Received Date:

04 December 2018

Accepted Date:

26 March 2019

Please cite this article as: Hongbo Ju, Dian Yu, Junhua Xu, Lihua Yu, Bin Zuo, Yaoxiang Geng, Ting Huang, Ling Shao, Letian Ren, Chengzhong Du, Hongfei Zhang, Hongzhao Mao, Crystal structure and tribological properties of Zr-Al-Mo-N composite films deposited by magnetron sputtering, Materials Chemistry and Physics (2019), doi: 10.1016/j.matchemphys.2019.03.071

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 proof before it is published in its final 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.

ACCEPTED MANUSCRIPT

Crystal structure and tribological properties of Zr-Al-Mo-N composite films deposited by magnetron sputtering Hongbo Ju1, 2*, Dian Yu1, Junhua Xu1, Lihua Yu1*, Bin Zuo1, 3, Yaoxiang Geng1, Ting Huang1, Ling Shao4, Letian Ren2, Chengzhong Du2, Hongfei Zhang2, Hongzhao Mao2 1. School of Materials Science and Engineering, Jiangsu University of Science and Technology, Mengxi Road 2, Zhenjiang, Jiangsu Province, 212003, China 2. Yangzijiang Shipbuilding (Holdings) Ltd., 1# lianyi Road, Jiangyin-Jingjiang Industry Zone, Jingjiang, Jiangsu Province, China

3. College of physics, Jilin Normal University, Changchun 130000, Jilin Province, China 4. Research Institute of Zhejiang University-Taizhou, West Section of Shifu Avenue 618, Taizhou, Zhejiang Province, 318000 China

*corresponding author: H. Ju, e-mail: [email protected] L. Yu, e-mail: [email protected] Abstract: Quaternary zirconium aluminum molybdenum nitride (Zr-Al-Mo-N) composite films with different molybdenum content were deposited using reactive magnetron sputtering. The results showed that the films at <6.7 at.% molybdenum exhibited a single face center cubic (fcc) ZrN phase, while further incorporation of molybdenum led the film to exhibit a dual phase of fcc-ZrN and fcc-Mo2N. Zr-Al-Mo-N films’ room temperature friction coefficient decreased gradually due to the lubricant molybdenum and the formation of Mo2N phase in the film at > 6.7 at.% molybdenum. The room temperature wear rate first maintained its value of ~3.5×10-7 mm-3·N-1·mm-1 at 0-8.6 at.% molybdenum and then dropped to ~5.0×10-8 mm-3·N-1·mm-1 at 12.6 at.% molybdenum, while further increasing in molybdenum content results in the increase of wear rate. The roughness of the film and the appearance of fcc-Mo2N phase were considered as the main factors of the wear rate. For the tribological properties of the film at 13.8 at.% molybdenum, the increase in temperature could induce the main wear mechanism changing to oxidation wear. The 1

ACCEPTED MANUSCRIPT change in the wear mechanism resulted in the decreasing in the wear rate as well as also led to the friction coefficient increased at first from RT to 500 °C and then started decreasing at further elevated temperature. Keyword: reactive magnetron sputtering, Zr-Al-Mo-N films, tribological properties

1. Introduction Composite design is an effective method to enhance the phase stability and compressive properties of transition metal nitride (TMN) films [1-6,32-33]. The commercial titanium aluminum nitride composite film has been successfully used in large-scale application. On the basis of above, Aluminum as an adding element has been widely incorporated into TMN based films (such as Zr-Al-N [7], Cr-Al-N [8], Nb-Al-N [9], Mo-Al-N [10] and V-Al-N [11]) to improve the thermal stability, hardness and wear resistance at room temperature as well as at elevated temperature. The Zr-Al-N composite film is one of the representative TM-Al-N systems and has been widely studied. For example, P. H. Mayrhofer et al. [7] synthesized a series of Zr-Al-N composite films and investigated the influence of reactive and non-reactive sputtering on structure, mechanical properties and thermal stability of the films. L. Rogstron et al. [12] deposited the Zr1-xAlxN films (x=0-0.83) on WC-10 wt.% Co and investigated the cutting property using a longitudinal turning operation. The results showed that the film’s cutting property is correlated to its composition and the film with higher aluminum content exhibits the best cutting property due to its high thermal stability. S. H. Sheng et al. [13] investigated the phase stabilities and thermal decomposition in the Zr1-xAlxN films using ab initio density functional theory and the results showed that the film with x below 0.43 is stable in a face center cubic structure. The Magnéli phase considered as a solid self-lubrication in many applications of the oxide materials. Several research activities have been carried out to investigate the 2

ACCEPTED MANUSCRIPT formation and function mechanism of Magnéli phases in hard metal nitride films [14, 15]. Molybdenum is usually considered as additional elements to introduce into some TMN based films due to the formation of Magnéli phase of molybdenum oxide during the wear test and the relevant report relates to some system such as Ti-Mo-N [16], Ti-Mo-Si-N [17], Ti-Al-Mo-N [18], Cr-Mo-N [19] and W-Mo-N [20]. On the basis of the above research results, the addition of aluminum below ~50 at.% into the zirconium nitride film could improve the film’s thermal stability, hardness and wear resistance. The TMN based films containing molybdenum could exhibit self-lubrication characteristics. The synthesis and crystal structure of Zr-Al-N system and TM-Mo-N system films have been widely studied and the relationship between the microstructure and properties has been already obtained. However, relatively seldom effort has been made to investigate the microstructure, mechanical and tribological properties of Zr-Al-Mo-N films. In this paper, Zr-Al-N film with the highest hardness and lowest room temperature wear rate was chosen as a matrix and molybdenum was incorporated into it to investigate the influence of Mo content on the crystal structure, mechanical and tribological properties of the films. Experiment details 2.1 Sample preparation A series of Zr-Al-Mo-N composite films with different Mo contents were deposited with a reactive magnetron sputtering system on both polished stainless steel (AISI 304; composition: C<0.08, Si<1.00, Mn<2.00, S<0.030, P<0.045, Cr 17.00-20.00, Ni: 8.00-10.50; austenitic; hardness: 192 Hv) and wafer Si substrates. The 3D model of the magnetron sputtering system is shown in Fig. 1. There are three targets including zirconium (99.9 at.%), aluminum (99.9 at.%) and molybdenum (99.9 at.%) were sputtered with three radio frequency powers. The diameter of the target was 75 mm. The distance between the targets and the substrate was 75 mm. The substrates were cleaned by acetone and alcohol respectively. The substrate holder in the chamber rotates about an axis perpendicular to the horizontal plane. The sample table kept substrates rotating at a speed of 15 rpm. The base pressure was lower than 6×10-4 Pa. 3

ACCEPTED MANUSCRIPT A layer of zirconium interlayer with a thickness of ~150 nm was deposited on the substrate. The working pressure was kept at 0.4 Pa and the ratio of argon to nitrogen (flow rate) was kept at 10:5. The film with different Mo contents was deposited by fixing the power of the zirconium and aluminum target at 150 and 40 W, adjusting the power of molybdenum targets from 0 to 60 W. No bias voltage was applied and the substrate was not heated during the deposition. The deposition time of all the above films was 2 hours. The film deposited on the Si wafer was used to investigate the elemental composition, microstructure and mechanical properties, and the film on the stainless steel was used to investigate the tribological properties. 2.2 Characterization method The electron probe microanalysis (EPMA, CAMECA SX-50, France) was used to characterize the elemental composition of the deposited films. The X-ray diffraction (XRD, Shimazu-6000, Shimadzu, Japan) was used to explore the crystal structure of Zr-Al-Mo-N films which having a Cu Kɑ radiation, operating at 40 kV and 35 mA and the scanning speed was 4°/min. The average grain size of Zr-Al-Mo-N was calculated by the Debye-Scherer equation [21] and the lattice constant was calculated by the Nelson Riley function [22]. The microstructure of the films was investigated by transmission electron microscopy (TEM, JEOL-2100F, JEOL, Japan) at an accelerating voltage of 200 kV. The roughness of the films was studied by Atomic force microscope (AFM, MFP-3D Infinity, OXFORD, UK). The hardness and elastic modulus of the films were determined by a nano-indenter (CPX+NHT2+MST, CSM, CH) equipped with a diamond Berkovich indenter tip. In order to minimize the substrate’s influence on the hardness of the films, the maximum load of 4 mN was selected. A minimum of nine indentations was made for each sample and the mean value taken. The automatic indentation mode was programmed to place dents in the 3×3 array. The indenter was calibrated relative to the reference sample of fused silica. The tribological properties of the films were investigated by the high temperature wear tester (UMT-2, CETR, USA). The wear counterpart was alumina ball with 9 mm in diameter. The wear test was carried at a speed of 50 rpm at room temperature, 300, 500 and 700 ℃ for 30 minutes under a constant load of 3 N. The length of the 4

ACCEPTED MANUSCRIPT distance during the wear test was ~377 m based on above experiment data. Wear tracks were circular tracks with a diameter of 8 cm. After the wear test, the profilometer (DektakXT, BRUKER, Germany) was applied to measure the wear volume of the films using the probe scanning the wear track. The wear rate of the film was calculated by Archard's classical wear equation [23]. The surface morphology of the wear track at elevated temperature and its element compositions were measured using the scanning electron microscope (SEM, Merlin Compact*, Carl Zeiss, Germany) and the energy dispersive spectroscopy (EDS, EDAX DX-4 energy dispersive analyzer, Oxford, UK) respectively. The residual stress (  ) of the films was calculated by Stoney's equation [24]:  

t s2 1   6t f E

1 1     R R s  

where E–elastic modulus of the substrate (E = 170 GPa). υ–Poisson's ratio of the substrate (υ= 0.3) ts–the thickness of the substrate tf–the thickness of the film R–the substrate curvature radii of the Si wafer Rs–the curvature radii of the film The curvature radii were also measured using Bruker DEKTAK-XT profilometer. 2. Results and discussion 3.1 Crystal structure Table 1 illustrates the elemental compositions of Zr-Al-Mo-N films as a function of molybdenum target power. As shown in table 1, by increasing the molybdenum target power drops the zirconium and aluminum content gradually, while the molybdenum content and Mo/(Zr+Al+Mo) ratio increase with the increase in molybdenum target power. Besides this, the oxygen content is independent on the molybdenum target power and its value is in the range of 1.7 at.%-2.8 at.%. Fig. 2 shows XRD patterns of Zr-Al-N, Mo2N and Zr-Al-Mo-N films with different 5

ACCEPTED MANUSCRIPT molybdenum content. As shown in Fig. 2, the XRD pattern of Zr-Al-N film exhibits five diffraction peaks at ~34°, ~40°, ~58°, ~70° and ~72° corresponding to face centered cubic (fcc) ZrN (JCPDF card 35-0753) (111), (200), (220), substrate silicon and fcc-ZrN (222) respectively. The Zr-Al-N film is an fcc-(Zr,Al)N solid solution. The XRD pattern of binary molybdenum nitride film showed six diffraction peaks, those peaks refer to fcc-Mo2N (JCPDF card 25-1366) (111), (200), (220), substrate silicon, fcc-Mo2N (222) and (311) respectively [30, 31]. The XRD patterns of all Zr-Al-Mo-N films regardless of molybdenum content exhibit a diffraction peak at ~32° corresponding to the silicon substrate. The incorporation of molybdenum of 6.7 at.% into the Zr-Al-N matrix induces the fcc-(Zr,Al)N peaks shifting to high angle, and the film still exhibits a single fcc-(Zr,Al)N phase. However, a further increase in molybdenum content leads to the appearance of other three peaks corresponding to fcc-Mo2N (111), (222), (311) and all diffraction peaks shift to the high angle. The shift of the Zr-Al-Mo-N films’ diffraction peaks might be attributed by the solid solution of the molybdenum atoms into the Zr-Al-N matrix and the solid solution of zirconium and aluminum atoms into the Mo2N matrix. Beside this, the reduction of the Zr/Al ratio in the films might leads to the diffraction peak shift. Lattice distortion induced by the solution of molybdenum also influences the residual internal stress of the films. Therefore, the diffraction peak shifting to a higher angle, which is attributed by the solution molybdenum into the Zr-Al-N lattice, leads to the compressive internal stress. The lattice constant of Zr-Al-Mo-N films with different molybdenum content was calculated according to the XRD data and the result is shown in Table 1. The lattice constant of Zr-Al-N and Mo2N is ~0.471 and ~0.422 nm respectively. The lattice constant of binary zirconium nitride film deposited under the same condition is ~0.479 nm. Ternary Zr-Al-N film’s lattice constant is smaller than that of binary zirconium nitride film due to the formation of substitutional solid solution of (Zr, Al)N that the zirconium in zirconium nitride lattice is substituted by the smaller aluminum. The lattice constant of Zr-Al-Mo-N phase decreases from ~0.460 nm at 6

ACCEPTED MANUSCRIPT 6.7 at.% molybdenum to ~0.434 nm at 18.6 at.% molybdenum, and the tendency of Mo2N lattice constant is similar to that of Zr-Al-Mo-N and its value decreases gradually from ~0.436 nm at 8.6 at.% molybdenum to ~0.425 nm at 18.6 at.% molybdenum. The decreasing in lattice constant of Zr-Al-Mo-N phase might be attributed by the formation of the substitutional solid solution of (Zr-Al-Mo)N where the smaller molybdenum atoms replacing the bigger zirconium in Zr-Al-N lattice. Increasing the molybdenum content above 6.7 at.% induces the appearance of fcc-Mo2N phase. At this time, another substitutional solid solution of (Mo-Zr-Al)2N is formed in the films and leads to the decrease of lattice constant of Mo2N with the increase of molybdenum content in the films due to the difference of atomic radius between the zirconium and molybdenum. Transmission electron microscope (TEM) was used to further investigate the microstructure of the film, and the cross-sectional TEM image, its corresponding selected area electron diffraction (SAED) pattern and HRTEM image of Zr-Al-Mo-N film at 6.7 at.% molybdenum are shown in Fig. 3. Fig. 3(a) confirms that the film has a dense columnar structure, and the SAED pattern exhibits four diffraction rings and it corresponds to fcc-ZrN (111), (200), (220) and (311) from inner to outer. No other diffraction rings referring to zirconium is detected in the SAED pattern. This result is in good agreement with that obtained from XRD. The HRTEM image of the film at 6.7 at.% molybdenum is shown in Fig. 3(b), and one group of clear lattice fringe with a spacing of ~0.1520 nm is detected. This lattice fringe refers to fcc-ZrN (220) plane based on the JCPDF card 35-0753. The inserted Fast Fourier Transformation (FFT) pattern reveals a single cubic structure. Based on the results above, as for the Zr-Al-Mo-N film at <6.7 at.% molybdenum, zirconium in the Zr-Al-N lattice is substituted by the smaller molybdenum and this attributes to the drop in lattice constant. The film is the substitutional solid solution of (Zr,Al,Mo)N and exhibits a single of fcc-ZrN phase. A further increase in molybdenum content induces another phase of fcc-Mo2N because the solution of 7

ACCEPTED MANUSCRIPT molybdenum into Zr-Al-N lattice could not consume all molybdenum deposited to the substrate. The film with the molybdenum content above 6.7 at.% molybdenum is consisted of a substitutional solid solution of (Zr,Al,Mo)N and (Mo,Zr,Al)2N and exhibits a mixture phase of fcc-ZrN and fcc-Mo2N. 3.2 Mechanical properties Fig. 4 illustrates the residual stress of Zr-Al-Mo-N films with different molybdenum content. The residual stress of Zr-Al-N film is in the compressive state and its value is ~-1.9 GPa. The Mo2N film’s residual stress is in the tensile state with a value of ~+0.2 GPa. The amalgamation of molybdenum below 6.7 at.% into the Zr-Al-N matrix has a little effect on the residual stress of the film and its value remains stable at ~-1.7 GPa, while by further increasing in molybdenum content into the films, induces the appearance of fcc-Mo2N and as a results residual stress changed from compressive state to tensile state. Besides this, the residual stress of the film is also influenced by the kinetic energy of bombarding and condensing particles, incident on the surface of the film and forming the film Ep [25]. Ep=Ebi≈(Usis)/aD Where Us–the substrate bias is–the substrate ion current density aD–the film deposition rate In this research work, while preparing the samples the value of bias voltage was kept at 0 V. Therefore, the value of Ep is relatively low and is not the main influencing factor of the residual stress. Fig. 5 shows the hardness (H) and elastic modulus (E) of Zr-Al-Mo-N films with different molybdenum content. The value of H and E for Zr-Al-N film are ~29 GPa and ~360 GPa respectively, and for Mo2N film their values are ~29 GPa and ~400 GPa respectively. The incorporation of molybdenum into the Zr-Al-N matrix has a little effect on the H and its value is in the range of 27-30 GPa, while E increases 8

ACCEPTED MANUSCRIPT gradually from ~366 GPa at 6.7 at.% molybdenum to ~390 GPa 18.6 at.% molybdenum. The Zr-Al-Mo-N films’ H is mainly influenced by the residual stress and the solution strengthening. Solution strengthening plays an important role in improving the mechanical properties, while the residual stress changing from compressive to tensile state drops the value of H. They two co-work results in the stable value of H. The increase in E is mainly attributed to the appearance of Mo2N phase in the films. J. Hao et al. addition molybdenum into the Ta-Al-N matrix using the co-sputtering method to synthesize a series of Ta-Al-Mo-N films and also found the addition molybdenum could improve the hardness of the films [26]. H/E* ratio which is widely considered as the plasticity index usually relates to the elastic strain to failure. The H/E* value of the films is illustrated in table 1. The results show that the H/E* value of the Zr-Al-N and Mo2N film is 0.087 and 0.061 respectively. The H/E* value of the Zr-Al-Mo-N film decreases gradually with the increase of the molybdenum content in the film. H3/E*2 ratio is another important mechanical parameter, since it relates to the resistance to the plastic deformation. Table 1 also illustrates the H3/E*2 value of the films. The result shows that the value of H3/E*2 initially maintained its value of 0.19 GPa for Zr-Al-N film and Zr-Al-Mo-N film at 6.7 and 8.6 at.% molybdenum and then decreases gradually with a further increase in molybdenum in the films. The elastic recovery (We) is the ratio of the recovered displacement after unloading to the total indentation displacement and can be calculated from load—displacement curves [22]. The We of the Zr-Al-Mo-N film is almost unaffected by the molybdenum content and its value for all films is ~44% that is regardless of molybdenum content. J. Musil et al. [27] fabricated a series of films using magnetron sputtering and this kind of film was composed of elements that crystallize in different crystal structure and was a class of heterostructural film. The heterostructural film exhibits more excellent properties such as high hardness and high We than that of homostructural films. Although the Zr-Al-Mo-N film at>6.7 at.% molybdenum presents a dual phase, 9

ACCEPTED MANUSCRIPT the film doesn’t exhibit excellent mechanical properties. This might be attributed by its two-phase isostructure. 3.3 Tribological properties Fig. 6 shows the friction coefficient (μ) and wear rate (WR) at room temperature of Zr-Al-Mo-N films with different molybdenum content. The value of μ and WR for Zr-Al-N films are ~0.88 and ~3.5×10-7 mm3/(N.mm) respectively, while for Mo2N films their values are ~0.30 and ~6.0×10-7 mm3/(N.mm) respectively. The incorporation of molybdenum into the Zr-Al-N matrix drops the value of μ gradually. However, WR of the Zr-Al-Mo-N films remains stable of ~3.5×10-7 mm3/(N.mm) as the molybdenum content increases from 0 to 8.6 at.% and then drops to ~5.0×10-8 mm3/(N.mm) at 12.6 at.% molybdenum. By further increasing in molybdenum content, the value of WR starts rising again. Fig. 7 illustrates the RT wear track optical microscope image of Zr-Al-Mo-N films. As shown in Fig. 7(a), a lot of ploughings are detected in the wear track of the film at 0 at.% molybdenum and the width of the wear track is ~200 μm. The wear mechanism of the film might be abrasive wear. Increasing molybdenum content to 12.6 at.%, as shown in Fig. 7(b), results in the width decreasing to ~150 μm. Besides this, little ploughings are detected on the center of the wear track and the obvious cracks appear on the right side of the wear track. Therefore, the wear resistance is improved significantly by the incorporation of molybdenum into the Zr-Al-N matrix. However, by further increasing in molybdenum content to 18.6 at.%, as shown in Fig. 7(c), the obvious ploughings appear on the surface of the wear track again like the film at 0 at.% molybdenum. The roughness of the films is illustrated in Table 1, and its value initially decreases and then remains stable with the increase of molybdenum content in the films. The drop of roughness might be attributed by the formation of fcc-Mo2N phase in the films due to the low value of roughness of binary Mo2N film. Based on the results above, for the RT tribological properties, the monotone decrease in μ is attributed by the increase of the molybdenum content in the films. Besides this, 10

ACCEPTED MANUSCRIPT binary Mo2N film’s μ is only ~0.3, therefore, the appearance of fcc-Mo2N phase in the film at >6.7 at.% molybdenum might be the main factor resulting in the decrease in μ of the film at >6.7 at.% molybdenum. The counterpart first contacts the asperity of the wear track during the wear test, and some asperity fracture under a load of counterpart and form wear debris [28]. This hard wear debris induces the appearance of ploughing on the surface of the wear track. Therefore, the roughness of the film causing the change of wear mechanism could be considered as the main factor of WR, since the hardness is stable against Mo variation in films. In addition, the appearance of fcc-Mo2N with high WR in the films also attributes to the increase in WR of the Zr-Al-Mo-N film at>13.8 at.% molybdenum. In addition, the residual stress of the films also influences the value of WR, since the film with a compressive residual stress usually exhibits a better toughness. Therefore, the increase in WR of the Zr-Al-Mo-N film at>13.8 at.% molybdenum is also attributed by the residual stress changing to tensile state. Besides this, the drop in the H/E* and H3/E*2 ratios of the film at>13.8 at.% molybdenum might attribute to the increase in WR. The Zr-Al-Mo-N film at 12.6 at.% molybdenum is chosen to investigate its high temperature tribological properties since it exhibits the relatively high hardness and relatively low values of μ and WR at RT. Fig. 8 shows the values of μ and WR Zr-Al-Mo-N film at 12.6 at.% molybdenum as a function of testing temperatures. As shown in Fig. 8, the film’s μ increases slightly as the temperature increases from RT to 500 °C and then drops sharply to ~0.5 when the temperature further increases to 700 °C. WR of the film increases monotonously with the increase in temperature. Fig. 9 shows SEM images of the wear track after the wear test at different temperatures. As the temperature increases to 300 °C, as shown in Fig. 9(a), much more cracks appear in the wear track and no obvious deep scratch is detected. As shown in Fig. 9(b), both the crack and deep scratch disappear after the wear test at 500 °C, and wear debris could be detected on some certain area of the wear track. Further increase the temperature to 700 °C (Fig. 9c) induces a lot of debris on the 11

ACCEPTED MANUSCRIPT surface of the wear track. The oxygen content (O/(Zr+Al+Mo+O), at.%) of the wear track at different temperatures was measured using EDS and its value is 14.3, 12.9, 23.1 and 39.4 at.% with the corresponding testing temperature of RT, 300, 500 and 700 °C. This points that all wear tracks are oxidized, and an increase in temperature causes the oxidation to be more severe. Tribo-films play a significant role in the high temperature tribological properties, and Fig. 10 shows the XRD pattern of the wear track after performing the wear test at different temperatures. The XRD pattern of the substrate after annealing at the same temperature is also shown in Fig. 10 as a reference. As shown in Fig. 10, the diffraction peak at ~33°, ~38° and ~34° corresponding to MoO3 appears except for the peaks of as-deposited film and substrate at RT, 300 °C and 500 °C. Increasing the temperature to 700 °C induces another diffraction peak at ~31° corresponding to ZrO2 except for the diffraction peaks of as-deposited film, substrate, and MoO3. Besides this, the appearance of the iron oxide phase confirmed that the substrate is oxidized. The softening of the substrate of the hard brittle Zr–Al–Mo-N films might be due to an undesirable effect on the tribological properties However, no obvious crack is detected on the surface of the wear track at elevated temperatures because of the relatively low load and the appearance of soft oxide based phase in the wear test. Therefore, substrate oxidation could be considered as a minor factor which have influence on the tribological properties of the film. MoO3 is considered as one of solid self-lubrication material and could be formed easily during the wear test at RT as well as elevated temperatures. MoO3 consists of double layers of distorted edge-sharing MoO6 octahedra parallel to (010) planes [16]. Successive layers are held together by weak Van der Waals forces due to which MoO3 has low shear strength. During the wear test, MoO3 can be worn away easily by the counterpar [16]. However, more than one tribo-phase appears on the wear track at 700 °C, and we could be determined lubricating ability of MoO3 and ZrO2 with the help of 12

ACCEPTED MANUSCRIPT principles of crystal chemistry [29]. Oxides with a high value of ionic potential always have low μ while the ionic potential value of MoO3 and ZrO2 is 8.9 and 5.6 respectively. Hence, it is clear that, the tribo-phase of MoO3 play the main role of dripping μ. For the high temperature tribological properties of the film at 12.6 at.% molybdenum, increasing the temperature from RT to 300 °C leads to the interaction between counterpart and wear track more intense and the appearance of much more cracks on the wear track, although the lubricious tribo-film MoO3 always exists in the wear track. This might be attributed by the evaporation of the moisture since the oxygen content in the wear track remains stable at this stage. The main wear mechanism is abrasive wear accompanied by oxidation wear. Hence both μ and WR increase with the temperature increasing to 300 °C. Further increase in the temperature induces the large-scale oxidation on the wear track and the oxygen content on the surface wear track increases gradually. The main wear mechanism changes to oxidation wear. The sufficient content of tribo-films could play an effective lubrication role during the wear test. However, the layered structure of MoO3 is easily worn away by the counterpart. Besides this, the oxidation on the wear track drops the hardness of the film. Therefore, at this stage, μ decreases gradually at the cost of increasing WR. 3. Conclusion Molybdenum was incorporated into (Zr0.795Al0.205)N matrix to improve the mechanical and tribological properties using a magnetron sputtering system and the influence of molybdenum content on the crystal structure, mechanical properties and tribological properties at RT and elevated temperature were studied. The main conclusion was as follows: (1) (Zr0.795Al0.205)N film exhibited a single of face centered cubic (fcc) ZrN structure. The incorporation of molybdenum below 6.7 at.% into the matrix had little effect of the crystal structure of the film and the film still exhibited a single fcc-ZrN 13

ACCEPTED MANUSCRIPT phase. A further increasing in molybdenum content induced the appearance of another phase of fcc-Mo2N and the film consisted of dual phase. (2) Zr-Al-Mo-N films’ hardness was influenced by the molybdenum content slightly and its value regardless of molybdenum content was in the range of 27 GPa-30 GPa. Both of solution strengthening and residual stress co-work resulted in the stable value of hardness. (3) Zr-Al-Mo-N films’ RT friction coefficient (μ) decreased gradually with the increase of lubricant molybdenum in the films. RT wear rate (WR) first remained its value of ~3.5×10-7 mm3/(N.mm) as the molybdenum content increases from 0 to 8.6 at.%, and then dropped to ~5.0×10-8 mm3/(N.mm) at 12.6 at.% molybdenum, further increasing in molybdenum content results in the increase in WR to ~5.1×10-7 mm3/(N.mm) at 18.6 at.% molybdenum. The roughness of the film and the appearance of fcc-Mo2N phase were considered as the main factors of the WR. (4) For the elevated temperature μ and WR of the Zr-Al-Mo-N film at 13.8 at.% molybdenum, the main wear mechanism is abrasive wear accompanied with oxidation wear from RT to 500 °C, and the interaction between counterpart and wear track becomes more and more intense. Both μ and WR increase with the temperature increasing to 500 °C. Further increase in the temperature induces the main wear mechanism changes to oxidation wear and the sufficient content of tribo-films could play an effective lubrication role during the wear test. Therefore, at this stage, μ decreases gradually at the cost of increasing WR. Based on above results, Zr-Al-Mo-N film at 13.8 at.% molybdenum exhibited excellent mechanical and tribological properties and could be applied in the cutting tools coatings. Acknowledgement

14

ACCEPTED MANUSCRIPT Supported by the National Natural Science Foundation of China (51801081, 51574131), National Key R&D Program of China (No.2016YFB1100103) and China Postdoctoral Science Foundation (2018M632251). References [1] J. Musil, Š. Kos, S. Zenkin, β-(Me1,Me2) and MeNx films deposited by magnetron sputtering: Novel heterostructural alloy and compound films, Surf. Coat. Technol. 337 (2017) 75. [2] H. Ju, N. Ding, J. Xu, L. Yu, Y. Geng, G. Yi, T. Wei, Improvement of tribological properties of niobium nitride films via copper addition, Vacuum, 158 (2018) 1. [3] H. Ju, S. He, L. Yu, I. Asempah, J. Xu, The improvement of oxidation resistance, mechanical and tribological properties of W2N films by doping silicon, Surf. Coat. Technol. 317 (2017) 158. [4] H. Ju, X. He, L. Yu, J. Xu, The microstructure and tribological properties at elevated temperatures of tungsten silicon nitride films, Surf. Coat. Technol. 326 (2017) 255. [5] H. Ju, D. Yu, L. Yu, N. Ding, J. Xu, X. Zhang, Y. Zheng, L. Yang, X. He, The influence of Ag contents on the microstructure, mechanical and tribological properties of ZrN-Ag films, Vacuum. 148 (2018) 54. [6] H. Ju, L. Yu, S. He, I. Asempah, J. Xu, Y. Hou, The enhancement of fracture toughness and tribological properties of the titanium nitride films by doping yttrium, Surf. Coat. Technol. 321 (2017) 57. [7] P. H. Mayrhofer, D. Sonnleitner, M. Bartosik, Structural and mechanical evolution of reactively and non-reactively sputtered Zr-Al-N thin films during annealing, Surf. Coat. Technol. 244 (2014) 52. [8] L. Chen, Z. Liu, Y. Xu, Y. Du, Influence of Zr on structure, mechanical and thermal properties of Cr–Al–N coatings, Surf. Coat. Technol. 275 (2015) 289. [9] H. Ju, P. Jia, J. Xu, L. Yu, I. Asempah, Y. Geng, Crystal structure and high temperature tribological behavior of niobium aluminum nitride films, Materialia, (2018) https://doi.org/10.1016/j.mtla.2018.08.025. [10] J. Xu, H. Ju, L. Yu, Microstructure, oxidation resistance, mechanical and tribological properties of Mo–Al–N films by reactive magnetron sputtering, Vacuum. 103 (2014) 21. [11] H. Ju, P. Jia, J. Xu, L. Yu, Y. Geng, Y. Chen, M. Liu, T. Wei, The effects of adding aluminum on crystal structure, mechanical, oxidation resistance, friction and wear properties of nanocomposite vanadium nitride hard films by reactive magnetron sputtering, Mater. Chem. Phys. 215 (2018) 368. [12] L. Rogström, M. Johansson-Jõesaar, L. Landälv，M. Ahlgren, M. Odén, Wear behavior of ZrAlN coated cutting tools during turning, Surf. Coat. Technol. 282 (2015) 180. [13] S. H. Sheng, F. Zhang, S. Veprek, Phase stabilities and thermal decomposition in 15

ACCEPTED MANUSCRIPT the Zr1−xAlxN system studied by ab initio calculation and thermodynamic modeling, Acta Mater. 56 (2008) 968. [14] W. Tillmann, D. Kokalj, D. Stangier, Investigation on the oxidation behavior of AlCrVxN thin films by means of synchrotron radiation and influence on the high temperature friction, Appl. Surf. Sci. 427 (2018) 511. [15] G. Gassner, H. Mayrhofer, K. Kutschej, Magnéli phase formation of PVD Mo-N and W-N coatings, Surf. Coat. Technol. 201 (2006), 3335. [16] Q. Yang, R. Zhao, P. Patnaik, X. Zeng, Wear resistant TiMoN coatings deposited by magnetron sputtering, Wear. 261 (2006) 119. [17] J. Xu, H. Ju, L. Yu, Influence of silicon content on the microstructure, mechanical and tribological properties of magnetron sputtered Ti–Mo–Si–N films, Vacuum. 110 (2014) 47. [18] S. Sergevnin, V. Blinkov, O. Volkhonskii, S. Belov, V. Kuznetsov, V. Gorshenkov, A. Skryleva, Wear behaviour of wear-resistant adaptive nano-multilayered ti-al-mo-n coatings, Appl. Surf. Sci. 388 (2016) 13. [19] D. Qi, H. Lei, T. Wang, Z. Pei, J. Gong, C. Sun, Mechanical, Microstructural and Tribological Properties of Reactive Magnetron Sputtered Cr–Mo–N Films, J. Mater. Sci. Technol. 31 (2015) 55. [20] J. Yang, Z. Yuan, G. Zhang, X. Wang, Q. Fang, Manufacture, microstructure and mechanical properties of Mo–W–N nanostructured hard films, Mater. Res. Bull. 44 (2009) 1948. [21] H. Ju, D. Yu, J. Xu, L. Yu, Y. Geng, T. Gao, G. YI, S. Bian, Microstructure, mechanical, and tribological properties of niobium vanadium carbon nitride films, Vac. Sci. Technol. A 36 (2018) 031511. [22] H. Ju, N. Ding, J. Xu, L. Yu, I. Asempah, J. Xu, G. Yi, Crystal structure and the improvement of the mechanical and tribological properties of tungsten nitride films by addition of titanium, Surf. Coat. Technol. 345 (2018) 132. [23] T. Hanninen, S. Schmidt, I. Ivanov, J. Jensen, L. Hultman, Silicon Carbonitride Thin Films Deposited by Reactive High Power Impulse Magnetron Sputtering, Surf. Coat. Technol. 335 (2018) 227. [24] G. Janssen, M. Abdalla, F. Keulen, B. Pujada, B. Venrooy, Cerebrating the 100th anniversary of the Stoney equation for film stress: developments from polycrystalline steel strips to single crystal silicon wafers, Thin Solid Films, 517 (2009) 1858. [25]J. Musil, Hard nanocomposite coatings: thermal stability, oxidation resistance and toughness, Surf. Coat. Technol. 207 (2012) 50. [26]J. Hao, Y. Zhang, P. Ren, K. Zhang, J. Chen, S. Du, M. Wang, M. Wen, Spinodal decomposition in the Ta-Mo-Al-N films activated by Mo incorporation: toward enhanced hardness and toughness, Ceram. Int. 44 (2018) 21358. [27]J.Musil,Š.Kos, S. Zenkin, Z. Čiperová, D. Javdošňák, R. Čerstvý. β- (Me1, Me2) and MeNx films deposited by magnetron sputtering: Novel heterostructural alloy and compound films, Surf. Coat. Technol. 337 (2018) 75. [28] H. Ju, L. Yu, D. Yu, I. Asempah, J. Xu, Microstructure, mechanical and trobological properties of TiN-Ag films deposited by reactive magnetron sputtering, Vacuum. 141 (2017) 82. 16

ACCEPTED MANUSCRIPT [29]A. Erdemir, A crystal-chemical approach to lubrication by solid oxides, Tribol. Lett. 8 (2–3) (2000) 97. [30]I. Jauberteau, A. Bessaudou, R. Mayet, J. Cornette, J. Jauberteau, P. Carles, Molybdenum nitride films: crystal structures, synthesis, mechanical, electrical and some other properties, Coatings. 5 (2015) 656. [31]A. A. Bagdasaryan, A. V. Pshyk, L. E. Coy, P. Konarski, M. Misnik, V. I. Ivashchenko, M. Kempinski, N. R. Mediukh, A. D. Pogrebnjak, V. M. Beresnev, S. Jurga, A new type of (TiZrNbTaHf)N/MoN nanocomposite coating: Microstructure and properties depending on energy of incident ions, Comp. Part B, 146 (2018) 132. [32]A. M. Abd El-Rahman, W. Ronghua, Effect of ion bombardment on structural, mechanical, erosion and corrosion properties of Ti–Si–C–N nanocomposite coatings, Surf. Coat. Technol. 258 (2014) 320-328. [33]A. M. Abd El-Rahman, Synthesis and annealing effects on the properties of nanostructured Ti-Al-V-N coatings deposited by plasma enhanced magnetron sputtering, Mater. Chem. Phys. 149-150 (2015) 179-187.

17

ACCEPTED MANUSCRIPT

Figure captions Table 1 Elemental compositions, film thickness and roughness of Zr-Al-Mo-N films as a function of molybdenum target power Fig. 1 3D model of the magnetron sputtering system Fig. 2 XRD patterns of Zr-Al-N, Mo2N and Zr-Al-Mo-N films with different molybdenum content Fig. 3 Cross-sectional TEM, its corresponding SAED pattern and HRTEM image of Zr-Al-Mo-N film at 6.7 at.% molybdenum Fig. 4 Residual stress of Zr-Al-Mo-N films with different molybdenum content Fig. 5 Hardness and elastic modulus of Zr-Al-Mo-N films with different molybdenum content Fig. 6 Friction coefficient and wear rate of Zr-Al-Mo-N films with different molybdenum content at room temperature Fig. 7 Wear track optical microscope image of Zr-Al-Mo-N films with different molybdenum content at RT: (a) 0 at.% molybdenum, (b) 12.6 at.% molybdenum and (c) 18.6 at.% molybdenum Fig. 8 Friction coefficient and wear rate of Zr-Al-Mo-N film at 12.6 at.% molybdenum as a function of temperatures Fig. 9 SEM images of the wear track at 12.6 at.% molybdenum at different temperatures: (a) 300 °C, (b) 500 °C and (c) 700 °C Fig. 10 XRD patterns of the wear track of the film at 12.6 at.% molybdenum after wear test at RT, 300 °C, 500 °C and 700 °C

18

Table 1 Elemental compositions, film thickness and roughness of Zr-Al-Mo-N films as a function of molybdenum target power Molybdenum power (W)

Elemental compositions (at.%)

Lattice constant (nm)

roughness (nm)

H/E*

H3/E*2(GPa)

/

6.9±0.4

0.087

0.19

0.460±0.024

/

6.9±0.4

0.082

0.20

1800±90

0.444±0.022

0.436±0.022

6.1±0.3

0.082

0.20

34.0±1.7

1900±95

0.440±0.022

0.431±0.022

5.2±0.3

0.078

0.16

2.5±0.1

40.5±2.0

2050±102

0.437±0.022

0.429±0.022

4.8±0.3

0.074

0.15

56.7±3.2

2.4±0.1

54.8±2.7

2100±105

0.434±0.022

0.425±0.022

3.2±0.2

0.073

0.15

48.8±3.3

1.7±0.1

100

1700±85

/

0.422±0.022

2.1±0.1

0.061

0.08

Zr

Al

Mo

N

O

Mo/(Zr+Al+Mo) ratio

0

34.2±1.7

8.8±0.4

0

54.3±2.7

2.7±0.1

0

15

28.4±1.4

8.1±0.4

6.7±0.4

54.2±2.7

2.6±0.1

25

23.7±1.1

7.6±0.4

8.6±0.4

57.8±2.8

35

21.1±0.8

7.3±0.4

12.6±0.6

45

20.9±0.6

7.4±0.4

60

15.2±0.4

150

0

Film thickness (nm)

ZrN phase

Mo2N phase

1700±85

0.471±0.024

15.5±0.8

1750±88

2.3±0.1

21.5±1.1

56.2±3.0

2.8±0.2

13.8±0.7

55.4±3.0

7.1±0.4

18.6±0.9

0

49.5±1.6

Fig. 1 3D model of the magnetron sputtering system

Fig. 2 XRD patterns of Zr-Al-N, Mo2N and Zr-Al-Mo-N films with different molybdenum content

Fig. 3 Cross-sectional TEM, its corresponding SAED pattern and HRTEM image of Zr-Al-Mo-N film at 6.7 at.% molybdenum

Fig. 4 Residual stress of Zr-Al-Mo-N films with different molybdenum content

Fig. 5 Hardness and elastic modulus of Zr-Al-Mo-N films with different molybdenum content

Fig. 6 Friction coefficient and wear rate of Zr-Al-Mo-N films with different molybdenum content at room temperature

Fig. 7 Wear track optical microscope image of Zr-Al-Mo-N films with different molybdenum content at RT: (a) 0 at.% molybdenum, (b) 12.6 at.% molybdenum and (c) 18.6 at.% molybdenum

Fig. 8 Friction coefficient and wear rate of Zr-Al-Mo-N film at 12.6 at.% molybdenum as a function of temperatures

Fig. 9 SEM images of the wear track of the film at 12.6 at.% molybdenum at different temperatures: (a) 300 °C, (b) 500 °C and (c) 700 °C

Fig. 10 XRD patterns of the wear track of the film at 12.6 at.% molybdenum after wear test at RT, 300 °C, 500 °C and 700 °C

ACCEPTED MANUSCRIPT Highlights 1. The incorporation of molybdenum into Zr-Al-N matrix decreased the room temperature friction coefficient. 2. Zr-Al-Mo-N films at 34.0-40.5 at.% molybdenum exhibited excellent the room temperature wear resistance. 3. Zr-Al-Mo-N film at 13.8 at.% molybdenum exhibited a friction coefficient of ~0.5 at 700 °C against a alumina counterpart.