Determining the Critical Strain Energy Release Rate of Plasma‐Sprayed Coatings Using a Double‐Cantilever‐Beam Technique

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Abstract
A double‐cantilever‐beam (DCB) method for determining critical strain energy release rate (GIc) values from plasma‐sprayed coatings is described in detail. This approach, involving acoustic emission (AE) methodology, yielded up to 25 results per specimen and was successful in providing cohesive GIc values for plasma‐sprayed coatings of Al2O3‐2.5 wt% TiO2, Ni‐20 wt% Al, as well as two ostensibly identical, 99.5% commercially pure Al2O3 coatings. Adhesive GIc data from Al2O3‐40 wt% TiO2 coatings was also obtained. Results showed a GIc dependence on crack length, and a number of possibilities, based on fractography, the AE response of the coatings during testing, and crack velocity measurements, are advanced to explain this occurrence. Differences in GIc values between coatings were found to correlate with differences in powder/coating properties. The DCB method was also used to investigate batch differences and the effect on toughness of sealing an alumina coating. Problems associated with this method of testing are addressed.

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