MEASUREMENT OF ADHESION FOR THERMALLY SPRAYED MATERIALS

Thermally sprayed coatings have a distinctive microstructure which can be described as 'a three-dimensional layered structure of discs which are interlaced to form a material of composite nature'. The coatings are normally greater than 25 mu m in thickness and can thus be describ ed as bulk coatings. The minimum microstructural detail would be a sin gle splat (often described as a lamella), which is about 5 mu m in thi ckness and up to 80 mu m in diameter. This paper focuses on methods us ed to define and measure; the adhesion of coatings or deposits formed by thermal spray technology. The properties distinguished include thos e of strength and toughness. Measurements such as the tensile adhesion (according to ASTM C633) and double cantilever beam (DCB) tests will be addressed to illustrate the relevance (if any) of such methods to p resent industrial practice. Acoustic emission studies have also assess ed a function termed as the 'crack density function', i.e. a product o f the number of cracks and crack size. Other measuring methods applied to this technology include microhardness and scratch testing. The for mer technique has demonstrated that the material properties of coating s are anisotropic, and the latter method is being considered within th e biomedical industry to assess the adhesion of hydroxyapatite to orth opedic prostheses. These techniques, among others, may be used for bot h fundamental understanding of coating performance (i.e. life predicti on and cracking mechanisms)and as tests for quality control.