Study On Properties And Structures Of The Nanoscale MoS₂-based Coatings

Regarding to the problems of relatively low thermal stability, insufficient wearing resistance in the wide application of low friction coefficient of the solid lubricating MoS₂-Ti coatings, the aim is to design a kind of novel coating which maintain the excellent coating properties of MoS₂-Ti coating and, at the same time, to overcome its shortages.Coatings were prepared using a Teer closed-field unbalanced magnetron sputter ion-plating technique. While depositing MoS₂-Ti coatings on domestic W6Mo5Cr4V2 high-speed steel substrate, co-deposition of high hardness and high stability of ceramic phase TiB₂ was also performed on this substrate to obtain MoST-TiB₂ coatings. Ti was added as intermediate layer to increase the combing strength of coatings and substrate.The mechanical and thermodynamic properties, such as surface morphology, composition distribution, microstructure and hardness, wear resistance, thermal stability, were characterized and studied by comprehensive utilization of the optical microscope, microhardness tester, friction and wear spectrometer, electron probe (EPMA) and X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA) and differential scanning calorimetry analysis (DSC) technology.MoST-TiB₂ coatings consist mainly of nanocrystalline and amorphous composition, the grain size is about 100nm. The crystal growth direction of MoST-TiB₂ and MoS₂-Ti coatings are basically the same, the growth along with MoS₂ Crystal plane(002) is preferred orientation.MoS₂ plays the role of coating substrate. The nucleation and growth of TiB₂ along the MoS₂ crystal plane(002), embedded in MoS₂ layers, formed coherency/half coherency interface, and the combining interface is MoS₂ crystal plane (002) and TiB₂ crystal plane (002) Crystal plane.The embedded TiB₂ blocked the growth trend of MoS₂ columnar crystal along the crystal plane (100), and promoted the growth of coating along MoS₂ crystal plane (002). The coating shows the alternate layered structure of MoS₂-TiB₂-MoS₂. TiB₂ played the role of skeleton, reduced the gaps between layers of MoS₂, improved the density of coating, and strengthened the coating adhesion between the layers.MoST-TiB₂ coating hardness is 706HV. In MoST-TiB₂ coating, TiB₂ embedded into MoS₂ layers with a MoS₂-TiB₂-MoS₂ layered structure, and its contribution to hardness is less than the solid solution strengthening effect of Ti in MoS₂ layers in MoS₂-Ti coating, and this leaded to lower hardness than that in MoST-TiB₂ coating, which is 980 HV.MoST-TiB₂ coatings kept the low friction properties of MoS₂ coatings, and the average friction coefficient is 0.145. TiB₂ has wear resistant skeleton effect. The coating has excellent lubricating property and abrasion resistance, so the service life of the coating is improved.The initial decomposition temperature of MoST-TiB₂ coating is about 550℃, which is higher than that of MoS₂-Ti coating, which is 503.8℃. Compared with the solid solution of Ti in layers of MoS₂ in MoS₂-Ti coatings, the formation of coherency/half coherency interface in MoST-TiB₂ coatings induced the stronger bonding among crystals inside, which restrained the oxidation decomposition of MoS₂ and slowed the speed of oxidation.MoST-TiB₂ coatings prepared by closed-field unbalanced magnetron sputter ion-plating technology possess the advantages of hard coatings TiB₂ and soft coatings MoS₂-Ti, which has excellent friction coefficient, abrasion resistance and thermal stability. MoST-TiB₂ coating, a kind of excellent solid lubricant, has wide applications in the future.