Can scratch testing be used as a model for the abrasive wear of hard coatings?

Hardness testing has often been used to give a guide to the abrasive wear resistance of materials since an approximately linear correlation between wear resistance and hardness has been observed for broadly similar materials. However, the abrasive wear behaviour of a range of thin hard coatings has been found to vary quite dramatically and no reliable correlation with hardness exists. Scratch testing provides a closer approximation to the processes taking place during abrasive wear and as such might be expected provide better assessment data for the abrasion resistance of thin hard coatings. This paper discusses the relationship between abrasive wear rate (as measured by the Taber test) and hardness and scratch testing parameters for TiN coatings on 304 stainless steal deposited by three different deposition technologies. The correlation between abrasive wear resistance and hardness is poor but the correlation with interfacial shear strength determined in the scratch test is only marginally better. In general, as the interfacial shear strength increases the abrasive wear rate of the system decreases. However, the balance between the deformation mechanisms which control this shear strength (fracture, plasticity and coating detachment) changes as the coating thickness (and substrate hardness) increases and choosing an appropriate scratch test load to mimic this is not possible. The scratch test in its current form can thus be used to generate the same damage mechanisms as are observed in abrasive wear but cannot simply be used to make a quantitative prediction of wear rate. (C) 1999 Elsevier Science S.A. All rights reserved.