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 coati ngs 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 ob served changes in curvature on debonding. Good agreement was observed between the two methods, with both suggesting the average substrate st ress to be about + 20 MPa and the average coating stress to be about - 60 MPa. In both constituents, there was a significant positive gradie nt of stress level through the thickness. These specimens were loaded in four point bending until cracks propagated along the interface betw een substrate and coating. From the load/displacement plots obtained d uring this testing, and taking account of the effect of relaxation of the residual stresses during debonding, the critical strain energy rel ease rates of the interfaces, G(ic), were estimated to be approximatel y 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 load ing, as characterised by the phase angle, psi, since G(ic) has been of ten observed to vary with psi. The value of psi for the four point ben d test in the absence of residual stress is about 47-degrees, whereas for the specimens tested here it was estimated to cover the complete r ange from 90-degrees (pure shear) to 0-degrees (pure opening) as the a pplied load was increased. The quoted values of G(ic) were obtained in a regime where psi is similar to 30-degrees.