Processing-structure-tribological property interrelationships of zinc titanate coatings grown by atomic layer deposition

The growth defect structure and properties of the zinc titanate (ZnxTiyOz) coating system have been tailored via the selective variation of atomic layer deposition (ALD) parameters to obtain different crystallographic zinc titanate phases, namely hexagonal ilmenite ZnTiO3 and cubic spinel Zn2TiO4. Amongst the phases with textured or random grain orientations, the (104) textured ZnTiO3 coatings with a defective (low stacking fault energy) structure exhibited excellent tribological behavior for an oxide coating. Steady-state sliding friction coefficients of ~ 0.12 and ~ 0.2 were measured in humid air and dry nitrogen, respectively, even after ex situ annealing at 550 °C. High resolution TEM with selected area electron diffraction revealed that the (104) planes have extensive stacking faults bordered by partial dislocations, which serve as a pathway for dislocations to glide parallel to the sliding direction and hence achieve low interfacial shear/friction, i.e. “deck-of-cards” shear. These ternary coatings with low crystallographic shear strength exhibit ductile behavior during sliding contact and thereby generate smoother wear surfaces compared to brittle microcrystalline oxide coatings, and thus are a potential candidate for high temperature solid lubrication.