High frequency reciprocating sliding wear behavior and mechanisms of quaternary metal oxide coatings

Abstract

Wear resistant coatings based on quaternary metal oxides were studied with comparisons made to more traditional ternary metal oxides. While much is known about ternary metal oxide wear behavior and mechanisms from room to higher temperatures, little is known about quaternary oxides; for instance, the role of the fourth element in determining the coating crystalline state and defect structure and how they control tribological properties. To this end, the system (ZnTiZr)xOy was deposited by atomic layer deposition (ALD) and compared to previously studied wear resistant nanocrystalline ALD ZnTiO3 coatings. X-ray diffraction determined that both as-deposited and ex situ 550°C annealed ternary and quaternary coatings exhibit ZnTiO3 phase or solid solution Zn(Ti,Zr)O3 phase only, respectively. However, the coatings have completely different growth structures where the ternary ZnTiO3 coatings exhibit textured (104) nanocolumnar grains and the quaternary Zn(Ti,Zr)O3 coatings are predominantly amorphous with some nanocrystalline grains. High frequency reciprocating sliding on these coatings revealed that the growth structure did not influence the wear rate (~1×10−6mm3/Nm). However, the crystal structure-dependent deformation mechanisms were different as revealed by FIB–SEM and HRTEM analyses inside worn surfaces. It was determined that both coatings show surface deformation due to ductile layering/smearing with no evidence of brittle fracture. Wear reduction of the ZnTiO3 coating was due to nanoscale sliding-induced plastic deformation when (104) stacking faults were sheared along the sliding direction resulting in an intrafilm shear velocity accommodation mode. Conversely, wear reduction in the quaternary Zn(Ti,Zr)O3 coating was a result of a tribofilm/mechanically mixed layer, composed of amorphous transferred silica from the sliding Si3N4 counterface and refined nanocrystalline Zn(Ti,Zr)O3 grains from the coating. Both wear surfaces contained solid cylindrical rolls or roll-ups often found in ceramic–ceramic reciprocating sliding. New insights revealed that the rolls originate in the tribofilm, with similar composition and structure, and evolve to the surface with continued sliding.