In situ observation of fracture sequence of physical vapor deposited TiN film on (1120) sapphire

The critical parameters for the structural stability of physical vapor deposited TiN film on (112̄0) sapphire were investigated by analyzing the adhesion strength and failure mechanism through in situ observations of the fracture sequence during scratch tests and static normal indentation. TiN was deposited by arc ion plating on (112̄0) sapphire, and the thickness of the TiN film was controlled to 700 nm. Delamination of TiN film was monitored in situ from below the contact through a transparent sapphire substrate, using zoom optics mounted into a video imaging sensor. In situ observation enables us to detect the failure origin of TiN coating on sapphire. The failure origin of TiN film on (112̄0) sapphire was identified as both rhombohedral and basal twinning of the sapphire substrate. Rhombohedral twinning was initiated first, and basal twinning ensued. Twinning-induced plastic deformation of the sapphire substrate triggered the initiation of interfacial delamination of the TiN coating. The plastic deformation of the substrate ultimately induced failure of the protective coating.