Tribological properties of Cr- and Ti-doped MoS2 composite coatings under different humidity atmosphere

Solid-lubricant MoS₂ coatings have been successfully applied in high vacuum and aerospace environments. However, these coatings are very sensitive to water vapor and not suitable for applications in moist environments. In this work, Cr- and T-doped MoS₂ composite coatings were developed. The results demonstrated that these composite coatings are promising for applications in high humidity environments.MoS₂-Cr and MoS₂-Ti composite coatings with different Cr or Ti content were deposited on high speed steel substrate by unbalanced magnetron sputtering. The composition, microstructure, and mechanical properties of the as-deposited MoS₂-metal composite coatings were analyzed by energy dispersive analysis of X-ray (EDX), X-ray diffraction (XRD), and nanoindentation experiments. The tribological properties of the coatings were evaluated against an alumina ball under different relative humidity atmosphere using a ball-on-disc tribometer. The MoS₂-Cr and MoS₂-Ti coatings showed a maximum hardness of 7.5 GPa and 8.4 GPa at a dopant content of 16.6 at.% Cr or 20.2 at.% Ti, respectively. The tribological test results showed that, with a small amount of Cr and/or Ti doping, the tribological properties of MoS₂ coatings under humid atmosphere could be significantly improved. The optimum doping level was found to be around 10 at.% for both MoS₂-Cr coatings and MoS₂-Ti coatings to show the best tribological properties, with both the lowest friction coefficient and wear rate. The excellent tribological properties of the MoS₂-Cr and MoS₂-Ti coatings with an appropriate metal doping level in moist atmosphere are found due to their ability to form stable transfer layer on the surface of the counterbody, which supplies lubrication for the contact surface.     

Synthesis and structural properties of Mo-Se-C sputtered coatings

Transition metal dichalcogenides have attracted considerable attention due to their self-lubricant properties. Their drawbacks, such us low load-bearing capacity or environmental sensitivity, have been partially overcome by alloying or doping with metals, carbon, or nitrides. Nevertheless, there is still a considerable potential for further improvement, since the majority of studies has been aimed at MoS₂ and WS₂ based coatings and the properties of diselenides remain almost unknown.Mo-Se-C coatings were prepared by non-reactive r.f. magnetron sputtering from carbon target with embedded MoSe₂ pellets. The carbon content and Se/Mo ratio determined by electron probe microanalysis increased from 29 to 68 at.% and from 1.7 to 2.0, respectively, as a function of the decreasing number of pellets. The coating structure analyzed by X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy showed that Mo-Se-C was a mixture of amorphous carbon and Mo-Se phases, since no traces of molybdenum carbides were observed. Linear increase of the hardness from 0.7 (29 at.% C) to 4.1 GPa (68 at.% C) showed a significant improvement compared to values typical for pure MoSe₂ coating.     

爆炸喷涂WC-12Co/MoS_2复合涂层的摩擦磨损性能

为进一步提高爆炸喷涂WC-12Co涂层的耐磨性,在WC-12Co合金粉末中添加不同比例的MoS2粉末,利用爆炸喷涂技术在Q235钢表面制备了系列WC-12Co/MoS2复合涂层.采用金相显微镜、扫描电子显微镜、X射线衍射仪、显微硬度计及摩擦磨损试验机对WC-12Co/MoS2复合涂层的微观组织形貌、结构、显微硬度、摩擦磨损性能进行了研究.结果表明,MoS2均匀的分布于复合涂层中,当MoS2含量为2%时,复合涂层的硬度、致密度变化不大,但摩擦系数和磨损率大幅度下降,分别为WC-12Co涂层的50%和36%.随着MoS2含量的增加,复合涂层的摩擦系数和磨损率均呈上升趋势.     

脉冲磁控溅射MoS2涂层的组织结构及摩擦学性能

采用单极性脉冲磁控溅射技术在A286基体表面制备MoS₂低摩擦系数涂层(LFC)。利用XRD、SEM等手段表征涂层的成分与微观组织;采用原位纳米力学测试系统、球-盘式摩擦磨损试验机分析涂层的力学和摩擦学性能,并探讨了脉冲偏压对涂层结构、力学和摩擦学性能的影响。结果表明,脉冲偏压由300V增加到600V,MoS₂涂层择优取向发生了(002)向(100)转变,当脉冲偏压增至800V时又恢复(002)择优取向,;随着脉冲偏压的增加,涂层的硬度及弹性模量出现先减少后增大趋势,摩擦系数在0.065~0.076范围内波动,呈现出先增加后减小趋势;偏压为800V的涂层摩擦学性能最佳,其磨损率仅为基体的13.5%。     

Computer controlled detonation spraying of WC/Co coatings containing MoS2 solid lubricant

Composite WC/Co + MoS₂ coatings were deposited onto steel substrates by Computer Controlled Detonation Spraying using three spraying modes: very cold, cold and normal. Maximal content of MoS₂ in a sprayed powder was 10 wt.%. Characterization of coatings was made with chemical and phase analyses, microhardness measurement, morphology and microstructure investigation. X-ray diffraction study shows that residual MoS₂ exists only in coatings obtained at very cold and cold spraying modes. At normal spraying mode complete decomposition of the solid lubricant occurs during spraying. From the engineering point of view, the coating applied at the cold mode using a powder containing 10 wt.% MoS₂ is the most promising. Such a coating has microhardness of 650 HV_0.2 and a porosity of 10%.     

Friction and wear mechanisms in MoS2/Sb2O3/Au nanocomposite coatings

Fundamental phenomena governing the tribological mechanisms in sputter deposited amorphous MoS₂/Sb₂O₃/Au nanocomposite coatings are reported. In dry environments the nanocomposite has the same low friction coefficient as pure MoS₂ (～0.007). However, unlike pure MoS₂ coatings, which wear through in air (50% relative humidity), the composite coatings showed minimal wear, with wear factors of ～1.2–1.4 × 10^?7 mm³ Nm^?1 in both dry nitrogen and air. The coatings exhibited non-Amontonian friction behavior, with the friction coefficient decreasing with increasing Hertzian contact stress. Cross-sectional transmission electron microscopy of wear surfaces revealed that frictional contact resulted in an amorphous to crystalline transformation in MoS₂ with 2H-basal (0 0 0 2) planes aligned parallel to the direction of sliding. In air the wear surface and subsurface regions exhibited islands of Au. The mating transfer films were also comprised of (0 0 0 2)-oriented basal planes of MoS₂, resulting in predominantly self-mated “basal on basal” interfacial sliding and, thus, low friction and wear.     

脉冲激光沉积WSx/a-C复合薄膜的结构与摩擦学性能

采用脉冲激光烧蚀石墨/WS₂组合靶,在硅基片上沉积不同碳质量分数的WSx/a-C复合膜。用能谱仪、扫描电子显微镜和X射线衍射仪对薄膜的成分、形貌和微观组织进行了表征。采用纳米压痕仪、涂层附着力划痕仪和球-盘式摩擦磨损试验机对薄膜的硬度、结合力和大气中(相对湿度50~55%)的摩擦学性能进行了测试。结果表明,薄膜的S/W比稳定在2.0左右且形成了(002)择优取向的WS₂相。随着薄膜中碳质量分数的增加,薄膜的硬度在36.1%C时出现最高值,结合力随之增大且在52.4%C时达到最高值,摩擦因数先降低后增加,在41.2%C时有最小值0.144。薄膜磨损率在(0.91~1.61)×10_-15 m₃N_-1m_-1范围内变化,36.1%C的WSx/a-C复合膜具有最佳耐磨性能。     

Tribological properties of MoSi2–MoS2 coatings coupling with SAE 52100 steel under reciprocating sliding

Tribological properties of MoSi₂–MoS₂ coatings coupling with SAE52100 steel were tested under reciprocating sliding. Effects of normal load, sliding speed and MoS₂ content on the coatings tribological properties were studied. Worn surfaces of the coatings were analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The friction coefficient of the coatings was approximately 0.45 and a little lower than that of the monolithic MoSi_2. The friction did not vary with the sliding time, sliding speed and load. Coating with 12 wt.% MoS₂ had the lowest friction. Wear rate of the coatings increased with the sliding speed and normal load and was higher than that of the monolithic MoSi₂. Wear rate of the coatings did not vary with MoS₂ content. Worn surface of the coatings and the coupling steel ball was covered by a SiO₂ and MoO₃ wear debris layer. Wear mechanism of the coatings was microfracture.     

Micro-scale sliding contacts on Au and Au-MoS2 coatings

The microtribological properties of Au and Au-MoS₂ coatings were examined using a nanoindentation instrument. MoS₂ was chosen for this study as an additive to Au due to its positive influence on the mechanical and tribological properties. Reciprocating microscratch tests were performed using a diamond indenter with a tip radius of 50 μm and a range of normal loads between 0.2 mN and 5.0 mN. The friction and wear results, with respect to the two coatings, were correlated to different velocity accommodation modes and levels of adhesion. It was found that the addition of 20 mol% MoS₂ to Au reduced the adhesion and limiting friction and also improved the wear resistance significantly. This coating shows potential for applications in microcomponents and microswitches due to its wear resistance, relatively low friction and good electrical conductivity.     

Molybdenum disulfide coating deposited by hybrid treatment and its friction-reduction performance

To obtain the MoS₂ film, a hybrid method, ion plating plus low temperature ion sulfuration, is utilized. A layer of metal molybdenum with 2–3 μm thick was first deposited by the multi-arc ion plating, and then the layer of metal molybdenum was treated by the low temperature ion sulfuration to obtain the composite MoS₂ film. The friction and wear experiment results showed that the MoS₂ film possessed an excellent performance of friction-reduction. SEM equipped with EDAX was adopted to analyze the morphologies and compositions of surface and worn scar of the MoS₂ film. XPS was used to detect the valence states of the film surface, and a nano-tester was employed to measure the nano-hardness and nano-modulus of the films.     

Al_2O_3/MoS_2复合涂层的制备及摩擦磨损性能

以大气等离子喷涂工艺制备的Al_2O_3陶瓷涂层为模板,利用陶瓷涂层中存在的孔隙和微裂纹,采用水热反应在其内部原位合成具有润滑特性的MoS_2,制备出Al_2O_3/MoS_2的复合涂层。结果表明,通过水热反应在陶瓷涂层原有的微观缺陷中成功合成了MoS_2,合成的MoS_2固体粉末呈类球形状,并且这球状的粉末是由纳米片层状的MoS_2搭建组成的。摩擦试验结果表明,与纯Al_2O_3涂层相比,复合涂层中由于MoS_2润滑膜的形成,其摩擦因数和磨损率都显著降低,且载荷越大,复合涂层的摩擦性能越好。     

TiOxNy coatings grown by atmospheric pressure metal organic chemical vapor deposition

Titanium oxynitride coatings were deposited on various substrates by an original atmospheric pressure metal organic chemical vapor deposition (MOCVD) process using titanium tetra-iso-propoxide as titanium and oxygen precursors and hydrazine as a nitrogen source. The films composition was monitored by controlling the N₂H₄ mole fraction in the initial reactive gas phase. The variation of the N content in the films results in significant changes in morphological, structural and mechanical properties. When a large excess of the nitrogen source is used the resulting film contains ca 17  at % of nitrogen and forms dense and amorphous TiO_xN_y films. Growth rates of these amorphous TiO_1.5N_0.5 coatings as high as 14 μm/h were obtained under atmospheric pressure. The influence of the deposition conditions on the morphology, the structure, the composition and the growth rate of the films is presented. For the particular conditions leading to the growth of amorphous TiO_1.5N_0.5 coatings, first studies on the mechanical properties of samples grown on stainless steel have revealed a high hardness, a low friction coefficient, and a good wear resistance in unlubricated sliding experiments against alumina which make them very attractive as protective metallurgical coatings.     

Nanoscale triboactivity: The response of Mo–Se–C coatings to sliding

Mo–Se–C films were deposited by sputtering from a carbon target with pellets of MoSe₂. In addition to the standard evaluation of their chemical composition, structure, morphology, hardness and cohesion/adhesion, the core objective of this paper was to analyze the tribological behavior of these films, particularly in the high-load regime. The carbon content varied from 29 to 68 at.% which led to a progressive increase of the Se/Mo ratio and the hardness. The friction coefficient of Mo–Se–C coatings clearly decreased with load from ∼0.15 to ∼0.05. The excellent friction properties were attributed to the formation of a thin molybdenum diselenide film on the top of the wear track of the coating and on the counterpart surface, while the role of the carbon in the sliding process is only secondary by increasing the coating hardness and thus its wear resistance.     

Mechanical and tribological properties of sputter deposited nanostructured Cr-WS2 solid lubricant coatings

Solid lubricant coatings of WS₂ and Cr-WS₂ (15-50 at.% Cr) prepared using an unbalanced magnetron sputtering system were evaluated for their mechanical and tribological properties. Nanoindentation results indicated that addition of Cr helped in improving the mechanical properties and the elastic recovery ability of Cr-WS₂ coatings. The adhesive strengths of Cr-WS₂ coatings were evaluated using a nanoscratch tester and from the nanoscratch profiles, critical load values and optical images, it was evident that the adhesion of Cr-WS₂ coatings increased with an increase in the Cr content. Further analysis of the nanoscratch data indicated that WS₂ coatings exhibited large amount of plastic deformation compared to Cr-WS₂ coatings which showed a combination of elastic-plastic deformation. However, micro-tribometer measurements at a load of 2 N showed that the tribological properties of Cr-WS₂ coatings deteriorated with an increase in the Cr content. For example, Cr-WS₂ coatings prepared at Cr content ≥ 33 at.% failed after a sliding distance of 1 m. On the other hand, WS₂ and Cr-WS₂ coatings prepared at low Cr contents (15-23 at.% Cr) exhibited a stable friction coefficient (50-60% relative humidity) in the range of 0.10-0.13 for a sliding distance of 14 m. Micro-Raman spectroscopy data of the worn films taken after a sliding distance of 14 m indicated the presence of WS₂ transfer films for WS₂ and Cr-WS₂ coatings prepared at low Cr contents. For Cr-WS₂ coatings with Cr content ≥ 33 at.%, the worn films consisted predominantly of WO₃. After an extended sliding distance of 50 m, Cr-WS₂ coatings (15-23 at.% Cr) outperformed WS₂ coating which failed after 20 m. Further, the coatings prepared at low Cr contents did not show any failure even after a sliding distance of 200 m. At a higher load of 7 N, Cr-WS₂ coating with 15 at.% Cr exhibited the best performance with a friction coefficient of 0.07 up to a sliding distance of 72 m. These results indicate that the amount of Cr in the WS₂ matrix needs to be controlled judiciously to obtain improved mechanical and tribological properties in Cr-WS₂ solid lubricant coatings.     

等离子喷涂 NiCr-Cr3C2基复合自润滑涂层的组织及摩擦学性能

将NiCr-Cr3C2复合粉和Ni包MoS2粉按不同比例混合,制成三种喷涂粉末,采用等离子喷涂技术在304不锈钢表面制备复合自润滑涂层,并对涂层的物相组成、显微组织及摩擦磨损性能进行了研究。结果表明:三种涂层的物相组成相同,主相均为Cr7C3,Ni和MoS2;涂层与基体的结合为机械结合,孔隙率较低,表面有少量微裂纹;喷涂粉末中的Ni包MoS2粉偏少或偏多都会导致涂层的摩擦磨损性能变坏,Ni包MoS2粉质量分数为30%时,涂层的摩擦系数及磨损率最低,分别约为0.36和3.3×10-4mg/s。     

Characterization and frictional behavior of nanostructured Ni–W–MoS2 composite coatings

Ni–W–MoS₂ composite coatings were obtained by pulse plating from a Ni–W electrolyte containing suspended MoS₂ particles. The coating composition, morphology, crystalline structure, microhardness and frictional behavior were studied as a function of MoS₂ concentration. The results obtained in this study indicate that co-deposited lubricant particles strongly influenced the composite Ni–W coating properties. It was found that increasing co-deposited MoS₂ diminished tungsten content in the coating and consequently increased the average grain size. Ni–W nanostructured coatings with high MoS₂ content have a porous sponge-like structure, high surface roughness and irregular frictional behavior. However, the friction coefficient of Ni–W coatings is reduced to half its value with low MoS₂ content.     

Composition and structure-property relationship of low friction, wear resistant TiN-MoSx composite coating deposited by pulsed closed-field unbalanced magnetron sputtering

Effect of MoS_x content has been studied in TiN-MoS_x composite coating deposited by closed-field unbalanced magnetron sputtering (CFUBMS) using separate MoS₂ and Ti target in N₂ gas environment. Pulsed dc power was applied for both the targets as well as for substrate biasing. Crystallographic orientation and structure of the coating was analysed by grazing incidence X-ray diffraction (GIXRD) technique. The surface morphology and coating fractograph were studied with field emission scanning electron microscopy (FESEM) whereas the composition of the coating was determined by energy dispersive spectroscopy (EDS) by X-ray. Scratch adhesion test, Vickers microhardness test and pin-on-disc test with cemented carbide (WC-6%Co) ball were carried out to investigate mechanical and tribological properties of the coating. Increase in MoS_x content (from 6.22 wt.% to 30.43 wt.%) was found to be associated with decrease in grain size (from 63 nm to 24 nm). Maximum hardness of 32 GPa was obtained for TiN- MoS_x composite coating. Film substrate adhesion was also observed to depend on MoS_x content of the composite coating. Significant improvement in tribological properties was observed. With optimal MoS_x content, it was possible to achieve low friction (µ = 0.02-0.04) and wear resistant (wear coefficient = 5.5 × 10^− 16 m³/Nm) composite solid lubricant coating.     

Mechanical and tribological properties of coatings sputtered from SiC target in the presence of CH4 gas

Amorphous hydrogen-free silicon carbide (a-SiC) coatings demonstrate good adhesion to different steel substrates, low intrinsic stress and high hardness however show quite high coefficient of friction in comparison with carbon-based coatings. Some addition of carbon to SiC can promote the decrease of friction coefficient.In the present work the amorphous hydrogenated silicon-carbide (a-SiC:H) films with different C/Si ratio were prepared at room temperature using DC magnetron sputtering in two ways: (i) sputtering of silicon target; (ii) sputtering of SiC target, both in the gas mixture of Ar and CH₄. In the latter case the films contained less hydrogen at the same C/Si ratio. The mechanical and tribological properties of these films were studied to find their optimum combination.The hardness, elastic modulus (nanoindentation), intrinsic stress (Stoney's formula) and coefficient of friction (pin on disc tribometer) were examined in dependence on the technological parameters, film structure and composition (Raman spectra, electron probe microanalysis). An increase of carbon in the films from 50 to 70 at.% resulted in decrease of hardness and friction coefficient. In the first case (i) the hardness decreased from 13 to10 GPa and in the second case (ii) from 23 to 16 GPa. Thus sputtering of SiC target in the gas mixture of Ar and CH₄ allows obtaining at room temperature the films with C/Si > 1 in which relatively high hardness (16-18 GPa) and low friction coefficient (~ 0.15) are combined.     

Microstructure and tribological properties of diamond-like carbon and TiAlSiCN nanocomposite coatings

Diamond-like carbon (DLC) and TiAlSiCN nanocomposite coatings were synthesized by multi-plasma immersion ion implantation and deposition. The DLC content in the composite coating was controlled by the flow ratio of N₂ to C₂H₂ during the deposition process. The microstructure and tribological properties of the as-deposited coatings were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), nanoindentation and ball-on-disk friction tests. The TEM results show that all the DLC-TiAlSiCN coatings had a two phase composite structure of the TiCN nanocrystals embedded in an amorphous matrix consisting of a-Si₃N₄, a-SiC, a-CN and DLC. TEM observations also reveal that the spacing between the adjacent nanocrystals increases with DLC content. In addition, the DLC-TiAlSiCN nanocomposite coating with a small crystalline spacing of about 0.6 nm shows a higher hardness up to 50 GPa and a larger friction coefficient. An increase in the DLC content of the coating benefits its friction coefficient while its hardness decreases. The friction coefficient reduces to 0.14 when the DLC content is about 31%.     

Evaluation of adhesion and fatigue of MoS2-Nb solid-lubricant films deposited by pulsed-dc magnetron sputtering

The thin film deposition for tribological applications becomes more and more widespread. The tribological performance of the overlay coatings correlates with coating-substrate adhesion. Hence, it is important to measure adhesive strength. The scratch adhesion test for thin films is extensively used. In this work, MoS₂-Nb coatings deposited by closed-field unbalanced magnetron sputtering (CFUBMS) have been scratch tested in two modes. A multi-mode operation was used as sliding-fatigue, like multi-pass scratching in the same track at different fractions of critical load (unidirectional sliding) and a standard mode using progressive load operation. Failure mechanisms are discussed according to examination of response of very dense microstructure and the adhesion value. The critical load to the first failure (L_C1) was 15 N but the final adhesion value from the film and substrate interface was 120 N (L_C2) as function of the coating thinning. The coefficient of friction (COF) from the multi-scratch for MoS₂-Nb started at a very high value of around 0.067, 0.073, and 0.093 under 5, 8, and 15 N loads respectively and then drops to 0.006, 0.035, and 0.065 at the end of the 1000 cycles. The most significant finding in the test is that when the multi-scratch passes reached to 1000 cycles, micro scale fatigue failures disappeared.