Intrinsic Hardness Evolution and Indentation Size Effect Modeling in Cu/Co Doped ZnO, TiO ₂ , and SnO ₂

ABSTRACT Metal oxides such as ZnO, TiO 2 , and SnO 2 are widely used in structural and functional applications; however, their micromechanical performance is often limited by pronounced indentation size effects and surface‐related deformation behavior. In this study, the influence of single Cu and Co doping as well as Cu/Co co‐doping on the intrinsic hardness and indentation size effect of these metal oxides is systematically investigated. Dopant incorporation leads to significant hardness enhancement in ZnO and TiO 2 , whereas SnO 2 exhibits the most stable mechanical response, accompanied by a strongly suppressed indentation size effect. The observed mechanical behavior is correlated with dopant‐induced microstructural modifications, including crystallite size evolution, lattice distortion, and enhanced intergranular connectivity. Indentation modeling using the Hays–Kendall, elastic‐plastic deformation, and modified proportional specimen resistance approaches reveals a clear material‐dependent reliability of hardness models, with SnO 2 exhibiting near‐ideal load‐independent behavior. Overall, these findings enable a realistic comparison of ZnO, TiO 2 , and SnO 2 in terms of microstructural modification, hardness enhancement, and indentation size effect, providing deeper insight into the factors governing the true intrinsic hardness of metal oxide ceramics.