Scratch and wear resistance of hydrophobic CeO_2-x coatings synthesized by reactive magnetron sputtering

CeO_2-x coatings were deposited under variable oxygen flow ratios (%f_O2) onto Si substrates by reactive magnetron sputtering. Nanoindentation testing revealed an increase in the hardness, elastic modulus, H/E and H³/E² ratio with increasing oxygen flow ratio, which in turn increased the adhesion and tribological performance of the coatings. Scratch testing yielded the highest critical load (L_C2 = 28.8 N) and CPR_S = 103 for the coating deposited with the highest oxygen flow ratio (57 %f_O2). Cracking events during scratch testing were initiated by tensile forces behind the scratch stylus, which led to the formation of semi-circular ring cracks. As the normal load increased, transverse cracks emerged extending outwards from the scratch track towards the edge causing the exposure of substrate. Beyond L_C2, severe spallation of the CeO_2-x coatings led to coating failure. Furthermore, the specific wear rates of the CeO_2-x coatings were determined to be within the ~10⁻¹⁵ m³/Nm range influenced by three-body abrasive wear. In-depth analyses from scratch and wear data indicates that these coatings possess good adhesion and durability.