Correlation of friction and depth in ramped nano-scratch test with a 5 µm probe on a 1.1 µm nanocomposite TiN/Si3N4 coating on Si. 

Coating failure and chipping of the silicon substrate results in periodic cracking in the scratch track where abrupt increases in depth with friction reduction (due to loss of contact area) are followed by a gradual increase in friction before the next failure event.

Variation in friction with wear cycles for three hard carbon coatings deposited on hardened tool steel in 500 mN reciprocating tests vs. 25 µm diamond probe.

Although the friction is initially very similar with continued cycles significant differences emerge. These are due to the relative importance of fracture and deformation on the wear process.