Determination of fracture toughness of a platinum-modified nickel aluminide coating by micromechanical testing

Abstract

The fracture toughness of an industry benchmark platinum-modified nickel aluminide coating (RT22LT) is determined by microcantilever bending tests. Cantilevers are prepared in three microstructurally-different zones across the coating cross-section by focused ion beam machining and then bent in-situ by a nanoindenter in a scanning electron microscope. Elastic-plastic fracture mechanics, combined with finite element analysis, are implemented to compute the fracture toughness. It is found that the fracture toughness of the coating varies through its thickness: the middle zone has the highest toughness (7.62 MPam), followed by the surface zone (6.59 MPam), and the interdiffusion zone has the lowest toughness (4.89 MPam). The main reason for such difference is the amount of plastic deformation prior to fracture and its corresponding contribution to the energy release rate vary from zone to zone. Observations of fracture surface show that fracture in the β-phase matrix is predominantly transgranular. However, the fracture morphology depends on the precipitate content of each zone and a higher precipitate content gives rise to more interfacial decohesion between the β-matrix and the precipitates. Analysis of microstructural-compositional features and their correlation with the plastic component of the energy release rate suggests that the precipitate content plays a major role in variation of the fracture toughness between the three zones. The fracture behaviours of the coating are compared with those reported in the literature and their difference is rationalised by the Weibull analysis.