A method to assess the fracture toughness of intermetallic coatings by ultramicroindentation techniques: applicability to coated medical stainless steel

Abstract

The design of coatings in the field of engineering applications aims at a progressive shift to the development of “hard but tough” coatings. The difficulty in assessing their mechanical behaviour by conventional methods is behind the growing relevance of “in situ” experiments using instrumented microindentation techniques. Determination of fracture toughness with existing models is only possible if cracks are formed during indentation. In the case of metallic coatings, however, the low loads of indentation required to avoid the involvement of the substrate usually prevent the coating from cracking. In this investigation we propose a novel method to determine the fracture toughness of metallic coatings by microindentation with a cube-corner tip using small cyclic loads, assuming that the indented coatings resembles the pattern for the fracture mode type I considered in the classical fracture toughness tests. The method is investigated for the growth of intermetallic coatings on medical stainless steel by hot dipping in an Al–12.6 wt.% Si alloy. In addition to hardness and Young’s modulus, residual stresses within the coating are determined as a function of the immersion time. We show that hardness and compressive residual stresses decrease with increasing immersion time. Toughening of the coating (up to about 25.79 MPa) in the shortest immersion time is achieved from the highest level of compressive residual stresses, which make greater tensile efforts necessary to generate a crack. These experiments allow the correlation of microstructure–mechanical properties and residual stresses, which is an important step before considering any load-bearing application.