**4.4. Other aspects contribute to the contact load-bearing**

Our previous study [31] using micropillar compression testing has shown the deformationinduced precipitation of nanocrystals in the outer A/NC layer. The interaction of microcracks and shear bands with these nanocrystals allowed high ductility in this layer. This plasticity was evident in the present study by the indentation-induced deformation in the A/NC layer (**Figure 4a** and **e**). Previous studies demonstrated that the inelastic deformation occurring in the graded ceramics can contribute toward the cone-crack suppression [11, 12]. The susceptibleness of the A/NC layer to deform plastically reduces the propensity for cracking under indentation condition.

Residual stress may also be contributing to crack suppression in the deformed MHSed Ti structure. Residual surface compressive stress can be induced by various techniques to improve the contact damage resistance and strength of many brittle materials [32]. Further, multilayering and mechanical grading of a surface is known produce complex residual stresses [33, 34]. The residual stress–depth profile analysis [20] revealed that the MHSed Ti has residual compressive stresses through the layers and a tensile stress at the NG layer/UFG core interface. The residual compressive stress in A/NC and NG layers act to arrest crack development and increase damage resistance, however the residual tensile stress at NG/UFG junction theoretically increases the propensity for crack initiation under contact loading. Given the absence of cracking at the NG/UFG interface, it appears the reduction of maximum stress and the attendant of redistribution of the stresses arising from multilayering and mechanical grading more than compensate for the residual tensile stress.
