TWI Industrial Member Report Summary 542/1996
F J Blunt
Adhesion testing is a basic requirement in procedure development, but how can we evaluate coatings which defy testing using the ASTM C633-79 method?
While there is wide acceptance of the ASTM C633-79 adhesion test, it has several limitations. The most serious of these is that it requires an adhesive to bond the coating to a loading fixture. Several types of high performance coating are now available whose adhesion is better than the cohesion of the best adhesives on the market. Such coatings include tungsten carbide-cobalt mixtures sprayed using high velocity oxyfuel (HVOF) or detonation gun processes. Thus the ASTM test cannot be used to assist in procedure development for these coatings. The ASTM test loads the coating in simple tension using a fixture which has been glued on, and a valid result is obtained if failure occurs either in the coating itself, or at the interface between the coating and substrate.
Failure of the bond between a coating and a substrate can also be induced in a variety of other ways. The bond could be loaded in tension without an adhesive, or could be loaded in shear. The fracture of the interface could be studied by driving a wedge between substrate and coating, by bending the sample or by scoring a track across a cross section. Lastly, ultrasonic methods might be used for a non- destructive examination of the boundary between coating and substrate.
All of these methods have been applied to sprayed coatings, and this report looks at the most promising methods for characterisation of adhesion of coatings, and what limitations each appears to have. In particular, their potential for ranking coatings of adhesion in excess of 70MPa was assessed.
- To review the principal methods of adhesion testing to be found in the literature.
- To discuss their potential for ranking high adhesion coatings.