Structural Evolution and Friction Loss Effect Analysis of TiB2/WS2 Composite Films under Annealing Treatment
The exploration of structural evolution and failure patterns in WS2-based composite films,especially regarding their friction and wear reduction capabilities under harsh aerospace conditions,still requires further investigation.To widen their application across various temperature ranges,TiB2/WS2 composite films,aimed at adapting to a broad temperature domain,were deposited onto silicon wafers and 718 high-temperature alloy substrates using a magnetron sputtering method.These films were then annealed at 200 ℃,450 ℃,and 600 ℃ in atmospheric conditions.Detailed analyses of the films,both before and after the annealing processes,were performed using a range of analytical techniques,including scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),Raman spectroscopy,nanoindentation,scratch testing,and ball-on-disk high-temperature friction testing.The study focused on the effects of annealing temperatures,ranging from 0 to 600 ℃,on the films'elemental composition,microstructure,hardness,and frictional wear properties.It was determined that the films,deposited by non-equilibrium magnetron sputtering,were characterized by a cabbage-like morphology on the surface,a columnar growth structure in the cross-section,and an amorphous state overall.An increase in annealing temperature was associated with an accelerated decomposition rate of sulfur elements,a significant reduction in the S/W ratio,a noticeable rise in film oxidation,and a wear rate that decreased initially but then increased with the annealing temperature.At an annealing temperature of 200 ℃,the films exhibited the lowest number of surface defects and the most compact structure,which slightly enhanced mechanical properties and maintained stable tribological performance,demonstrating effective friction and wear reduction(with a coefficient of friction＜0.075 and wear rate of 9.21 × 10-6 mm3/m·N).Excellent tribological properties were exhibited by the film during dynamic and continuous temperature increases(RT of approximately 600 ℃),and thereby,excellent thermal stability and continuous lubricity were realized.Mechanisms of destabilization in composite films before and after undergoing annealing treatments at various temperatures were observed:A low and stable coefficient of friction was maintained by the unannealed film during the friction process due to the formation of a shear-slip friendly WS2(002)crystal orientation structure.Increased densities and significantly improved hardness,along with optimal tribological properties,were observed after the annealing treatment at 200 ℃.A significant fluctuation in the coefficient of friction was displayed by the composite film after the annealing treatment at 450 ℃,as WS2 starts to oxidize at 450 ℃,leading to the formation of a small amount of oxide phases WO3 and TiO2 on the composite film surface.These oxide phases,being higher in the coefficient of friction than the lubricant phase WS2,partake in the friction process,causing the coefficient of friction to fluctuate and gradually increase,and the wear rate to rise.Failure of the composite films annealed at 600 ℃ occurs within a very short period,attributed on one hand to the destruction of the microstructure and mechanical properties by high-temperature annealing,and on the other hand to the instantaneous failure of the S element in the composite films due to a high temperature decomposition rate of 92.9％and the difficulty in forming the lubricant phase during the friction process in the absence of the S element.The evolution of frictional wear loss at different temperatures was clarified by investigating the changes in the microstructure of the composite films before and after the annealing treatment.
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