In-plane tensile strength and residual stress in thick Al2O3 coatings on aluminum alloy

Three types of strength are important for coating materials; bond (adhesive) strength, cohesive strength and in-plane strength. The distinction between the first two can be made by failure location; if failure occurs at the coating/substrate interface, the corresponding strength value is adhesive; and if it occurs within the ceramic coating, the strength value is cohesive. Between the two, adhesive strength is the most important since coatings often fail by debonding at the interface. In fact, there is a standard test procedure to determine both of these strengths, ASTM C633-79. In-plane strength characterizes the point of coating failure under in-plane applied loads. Since it is not the primary failure parameter, little attention has been given for its measurement. Therefore, work in the area of in-plane properties is limited. However, the knowledge of in-plane strength could be important for weaker coatings, but there is no simple way of measuring it. In this study as attempt was made to determine in-plane tensile strength of thick Al{sub 2}O{sub 3} coatings on Al-alloy through straightforward strength of materials approach. The residual stresses, which invariably occur in coatings, affect the in-plant strength. The magnitude of apparent average residual stress in Al{sub 2}O{sub 3} was determined indirectlymore » by comparing the in-plane strength of coating/substrate system with the intrinsic strength of coating (no substrate).« less