What is Scratch Hardness

Magnificently developed Al2O3, 3YSZ, 8YSZ, and CNT-based thermal barrier coatings (TBCs) were subjected to study multi-length scale tribological behaviour (fretting wear and micro-scratch) of the composite coatings. Subsequently, the role of constituents of the composite on the tribological behaviour of the coatings has been recognized. Fretting wear rate and the dominative wear mechanism are identified. The fretting wear behaviour is evaluated with a distinct representation (frictional force mapping) to understand the transition of regimes. Further, micro-scratching is used to assess the composite coatings against the sharp edges. The critical load of failure and scratch characteristics (scratch hardness, toughness, and scratch resistance) are appraised to find the suitability of the composite in TBC. Notably, the role of CNT in a multi-length scale is reported quantitatively. Thus, the chapter provides a comprehensive overview of the Al2O3-based composites that deal with the understanding of the multi-length scale tribological properties at room temperature.