Effects Of Ti And Al Doping On Microstructure And Properties Of Molybdenum Disulfide-based Lubricant Coating

The MoS₂-base lubricating coating with excellent lubricating properties has been widely studied after the successes in the space field.The applications in daily lives and industries have been limited due to the lubricating failure caused by oxygen and water in air.The various dopants,such as metal,metal oxide and metal sulfide,have been used to prepare MoS₂-base lubricating coatings with low friction and high abrasion resistance.The comprehensive service performance of the co-doped MoS₂-based lubricating coating is usually better but the interactions among the components are more complex,too.And related research work needs to be further improved.Based on the grain refining,chemical reaction and synergistic effects in the multicomponent MoS₂lubricating coating,Ti,Al doped Cu-MoS₂,CuS-MoS₂,and ZnO-MoS₂ coatings were designed,respectively.With the optimizing of the main technical parameters of magnetron sputtering,gradient process control and vacuum annealing heat treatment process,MoS₂-base solid lubrication coating with low friction coefficient and high durability under the condition of room temperature and humidity were developed.Effects of Ti and Al doping on different MoS₂-based lubricant coatings with different processing techniques and underlying mechanism of tribological performance in the multi-component MoS₂-based lubricant coatings were systematically studied.The synergistic lubrication of Cu-MoS₂,ZnO-MoS₂ coatings,and effects of thermal decomposition of CuS were described.Influence of pre-deposited ZnO thin film using atomic layer deposition techniques on the microstructure and properties of the subsequent coatings were discussed.Worn-model of the coatings were constructed based on factors of ZnO thin film-substrate binding force,brittleness of ZnO thin film,and chemical adsorption between the metal-ZnO-MoS₂ coatings and ZnO thin film layers.Firstly,due to low hardness and lubricating properties of Cu,the synergistic lubrication effect in Cu-MoS₂ coating further reduced the friction coefficient of the pure MoS₂ coating,but the nano-hardness of the Cu-MoS₂ coating was not significantly improved.Ti dopants increased the compactness of Ti-Cu-MoS₂ coating obviously,but bonding force and lubrication performance of the coating before and after annealing were general.After substrate heating and coating annealing,the Al₂S₃ and?₂-Cu₉Al₄ phases formed in Al-Cu-MoS₂ coating,which lead to increase of nano-hardness to 1.97 GPa.The average friction coefficient could be as low as 0.076 after annealing at400°C.XPS results also show that metal dopants have a good antioxidant property on the surface of the Al-Cu-MoS₂ coating,and hydrolysis reaction of MoS₂ was also weakened.Therefore,the mechanical properties and tribological properties of the Cu and Al co-doped MoS₂ coatings were improved by the dispersed microcrystalline phase due to the annealing effect.The metal-CuS-MoS₂ coating was prepared with consideration of the CuS decomposing into Cu₂S and S at 220°C.Since CuS had no lubricating property,the lubricating property of CuS and MoS₂ co-sputtering coating decreased,and the lubricating properties of CuS-MoS₂ coating doped with metals improve unobviously.However,during the annealing process,the decomposition of CuS in the metal-CuS-MoS₂ coating has a great influence on the microstructure of the coating.By analyzing the relationship between S:Mo and lubrication performance of the MoS₂-base lubricating coating,it is found that the S:Mo value is correlated with the lubrication performance of the single-metal-doping coating,and the coating has good lubrication performance when the S:Mo ratio is between 1.2 and 1.6.However,it is found that there is no definite logic relationship between S:Mo ratio and friction coefficient of the multi-component co-doped MoS₂-base lubricating coating,especially in the CuS-MoS₂ coatings.Ti or Al-doped ZnO-MoS₂ coatings were prepared based on the synergistic lubrication of ZnO and MoS₂,that is,ZnS generated when S atom enters the vacancy of ZnO.It is found that Ti-doped ZnO-MoS₂ coating can inhibit the growth of ZnO grains,which is beneficial to the compactness of the coating,and the friction coefficient of the coating is as low as 0.08,slightly lower than that of the coating before doping.However,the lowest friction coefficient of Al-doped coating is only 0.13.Metals doping has less effect on improving the lubricating properties of metal-ZnO-MoS₂ coatings.The gradient process carried out on metal-doped ZnO-MoS₂ coatings further optimize the microstructure of the coatings and improve the bonding force between film and substrate,which reduce the friction coefficient and improve the nano-hardness of the coatings.Ti-doped gradient coating is more compact,and Al-doped gradient coating has better oxidation resistance and higher toughness.The underlying mechanisms of Ti and Al dopants on the ZnO-MoS₂ coating are obviously different.In order to further analyze the effects of nanocrystalline on the lubrication performance of ZnO-MoS₂ coatings,the crystalline ZnO thin film was pre-deposited on the substrate using atomic layer deposition,and then the metal-ZnO-MoS₂ coatings were deposited by magnetron sputtering.ZnO thin films obtained by atomic layer deposition have ZnO?100?orientation growth,while the different thickness of ZnO films has different crystallization degree too.Due to induction the effect of ZnO orientation growth,Al-ZnO-MoS₂ coating becomes denser on the basis of previous toughness improvement.The friction coefficient of the Al-ZnO-MoS₂ gradient coating reduces to0.09.Nano-hardness of the Al-ZnO-MoS₂ gradient coating increase to 3.95 GPa after annealing at300°C.The XPS results also show that the oxidation resistance and moisture resistance of the ZnO-MoS₂ series coating after annealing were improved,which was beneficial to improve the service life of MoS₂-base lubricating coating under normal conditions.The MoS₂-based solid lubricant coatings had low friction coefficient and long life under the condition of room temperature and humidity were prepared.The different underlying mechanisms of the metal-doped synergetic lubricating Cu-MoS₂ and ZnO-MoS₂ coatings,and CuS-MoS₂coating decomposed after annealing were analyzed.The findings showed that different metal-doped coatings lead to the forming of mesophase,refining the grain,improvement of hardness,toughness,oxidation resistance and moisture resistance,reduction of the friction coefficient,etc.The results further improve the theory system of metal-doped MoS₂-based lubricant coating,which is helpful to guide the deposition process of soft lubricant coating.The reseach has the value of popularization and application in the industrial field.