Generalized approach of scratch adhesion testing and failure classification for hard coatings using the concept of relative area of delamination and properly scaled indenters

Failure characteristics and adhesion evaluation of hydrogen-free amorphous carbon coatings (a-C and ta-C) were examined by scratch testing using different indenter tip radii. Based on previous work of some of the authors, this article provides an extended description of main failure modes occurring for brittle coatings, with respect to the ratio of indenter radius R to coating thickness h. For best control over expected coating failure, it is shown that the failure mode of the coating can be reliably predicted and adjusted by the choice of the indenter radius. Failure modes of interest are predominant spallations (failure mode II) and predominant bending cracks (failure mode II). Same failure modes were typically found within certain ranges of R/h: Failure mode II occurred for 20 < R/h < 40 and failure mode III for 100 < R/h < 200. For an enhanced adhesion evaluation, the concept of adhesion assessment by comparing the relative size of delaminated area was expanded and generalized for variable indenter sizes and failure mechanisms. Thus, this approach enables direct comparison of scratch results in case of similar R/h, especially allowing adhesion comparison for a wide range of coating thicknesses.