THE EFFECT OF RESIDUAL-STRESSES ON THE DEBONDING OF COATINGS .2. AN EXPERIMENTAL-STUDY OF A THERMALLY SPRAYED SYSTEM

Specimens have been produced by plasma spraying of boron carbide coatings about 1 mm in thickness on to titanium alloy substrates about 3 mm thick. The residual stress distributions in these specimens have been calculated using a numerical process model and also estimated from observed changes in curvature on debonding. Good agreement was observed between the two methods, with both suggesting the average substrate stress to be about + 20 MPa and the average coating stress to be about - 60 MPa. In both constituents, there was a significant positive gradient of stress level through the thickness. These specimens were loaded in four point bending until cracks propagated along the interface between substrate and coating. From the load/displacement plots obtained during this testing, and taking account of the effect of relaxation of the residual stresses during debonding, the critical strain energy release rates of the interfaces, G(ic), were estimated to be approximately 0.2-0.5 kJ m-2. Substantial errors would have resulted from neglect of the presence of the residual stresses. Also of significance is the effect of the residual stresses on the mode mixity of interfacial loading, as characterised by the phase angle, psi, since G(ic) has been often observed to vary with psi. The value of psi for the four point bend test in the absence of residual stress is about 47-degrees, whereas for the specimens tested here it was estimated to cover the complete range from 90-degrees (pure shear) to 0-degrees (pure opening) as the applied load was increased. The quoted values of G(ic) were obtained in a regime where psi is similar to 30-degrees.