Sliding wear and rolling contact fatigue behavior of M50 steel coated with atomic layer deposited lubricious oxide nanolaminates

Atomic layer deposition (ALD) was employed to deposit highly conformal and uniform nanolaminate coatings of ZnO/Al2O3/ZrO2/Al2O3 on M50 bearing steel substrates for sliding wear testing, as well as on CrN coated M50 bearing steel cylindrical rods and cups for three-ball on rod rolling contact fatigue (RCF) testing. The nanocrystalline ZnO top layer was structurally-engineered to achieve low surface energy, textured (0002)-oriented grains. To achieve this preferred orientation, amorphous Al2O3 was deposited as a seed layer, while crystalline ZrO2 acted as a high toughness/load bearing layer. The ALD nanolaminates reduced both the sliding wear rate (2 × 10-7 mm3/N·m) and enhanced the RCF resistance of M50 steel out to 6 million rolling contact fatigue cycles with no signs of spall. Based on cross-sectional transmission electron microscopy studies inside the wear surfaces, it was determined that sliding and rolling-induced plastic deformation was observed in ZnO layer where basal plane stacking faults were sheared parallel to the sliding and rolling directions. This intrafilm shear velocity accommodation mode mitigates the friction, reduces wear, and inhibits brittle fracture. Therefore, ALD ZnO/Al2O3/ZrO2/Al2O3 nanolaminates are candidate coatings that provide both sliding and RCF resistance in moving mechanical assemblies that require thin (∼10-300 nm), uniform and conformal solid lubricants.