Evaluation of toughness of hard coatings

Repeated impact has been introduced to evaluate different properties of hard coatings, such as impact wear resistance, bonding strength, fatigue, etc. The load and impact number relationships are similar to that of fatigue curves, and close to some real applications of coated parts. Cr-Cu-N coatings were performed on a hardened steel substrate and the repeated impact tests were conducted on the coating substrate system. At a low impact load, the intrinsic strength of the coating layer and the bonding strength between the coating and substrate are in competition. Coating layer cracking or delamination might occur and the strength plays an important role like that of high cycle fatigue. When the load is increased and towards the low cycle fatigue regime, toughness of the coating layer turns out to be predominant. The cohesive failure in high load regime preferentially depends on the toughness of the coating layer, and the failure cycles can be employed as a toughness criterion.