Research On Tribological Properties Of TZ3Y20A Composites Containing Strontium Molybdate And Strontium Vanadate

Self-lubricating composites play a crucial role in many modern industrial sectors including aerospace and airspace devices,automobiles,gas turbine seals,air-foil bearings,and many others.The oxide ceramics have been considered as desired matrix for self-lubricating materials due to their excellent mechanical properties and oxidation resistance.However,the high-temperature tribological properties of oxide ceramics are poor.In the past decades,oxysalts(sulfates,chromates,molybdates and vanadates)have attracted more and more attention due to their excellent lubricity,which is attributed to their low shear strength at elevated temperature.In this thesis,Sr MoO4 and Sr3V2O8 powders were prepared by chemical precipitation method and solid-state reaction method,respectively.Their phase composition,microstructure and thermal stability were examined.Zr O2(3 mol.%Y2O3)-20 wt.% Al2O3(TZ3Y20A)was used as ceramic matrix.The as-obtained Sr MoO4 and Sr3V2O8 powders were used as the solid lubricant.Ag,Mo S2 and Sr SO4 were also added into the composites as the second lubricant additive,respectively.The composites were all fabricated by spark plasma sintering(SPS)method.Then the phase composition,microstructure,relative densities,hardness and friction behavior of the composites were investigated.Finally,the wear mechanism and self-lubricating mechanism of the composites were identified.The as-synthesized Sr MoO4 powders with uniform spherical shape had a narrow size distribution of 13.10 ?m in diameter.The morphology of the as-obtained Sr MoO4 particles consisted of hierarchical agglomerates.The pure Sr3V2O8 powders were fabricated by solid-state reaction method under 1150 o C.Both of the Sr MoO4 and Sr3V2O8 powders had good thermal stability from room temperature to 1200 o C.The TZ3Y20 A ceramic fabricated by SPS method consisted of t-Zr O2 and?-Al2O3,which had a hardness of 14.78 GPa and a relative density of 99.8%.Compared with the TZ3Y20 A ceramic,both of the hardness and relative densities of the composites decreased after adding the solid lubricants.The tribological properties of the TZ3Y20 A ceramic and the composites with different solid lubricant were tested by ball-on-disk friction tester.The results showed that the wear mechanism of TZ3Y20 A ceramic was brittle fracture from room temperature to 600 o C.At 800 o C,the main wear mechanism of TZ3Y20 A ceramic was recognized as brittle fracture and delamination.Whenadding 20 wt.% Sr MoO4 into TZ3Y20 A ceramic,the friction coefficients of the composite were in the range of 0.22-0.55 from room temperature to 800 oC,which were lower than those of TZ3Y20 A ceramic.However,the wear rates of the composites reached the higher value of 10-4 mm3/(N·m)at 400 and 600 o C as contrast with TZ3Y20 A ceramic.10 wt.% Ag,Mo S2 and Sr SO4 were also added into TZ3Y20A-20 wt.% Sr MoO4 composites as the second lubricant additives for improving the wear resistance of the composite,respectively.The composites exhibited excellent tribological properties at high temperature,which was ascribed to the synergistic lubricating effect of the multiple lubricants.The TZ3Y20 A composite doped with 10 wt.% Sr3V2O8 and 10 wt.% Ag showed excellent tribological properties with the friction coefficients of 0.31-0.61 and the wear rates of 10-8-10-5 mm3/(N·m)from room temperature to 600 o C.The synergistic lubricating effect of Ag,Ag VO3 and Ag3VO4 produced by tribo-reaction dominated the obvious improvement of the tribological properties of the TZ3Y20A-Sr3V2O8-Ag composite.