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 µm in thickness and can thus be described as bulk coatings. The minimum microstructural detail would be a single splat (often described as a lamella), which is about 5 µm in thickness and up to 80 µm in diameter. This paper focuses on methods used to define and measure the adhesion of coatings or deposits formed by thermal spray technology. The properties distinguished include those 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 present industrial practice. Acoustic emission studies have also assessed a function termed as the ‘crack density function’, i.e. a product of the number of cracks and crack size. Other measuring methods applied to this technology include microhardness and scratch testing. The former technique has demonstrated that the material properties of coatings are anisotropic, and the latter method is being considered within the biomedical industry to assess the adhesion of hydroxyapatite to orthopedic prostheses. These techniques, among others, may be used for both fundamental understanding of coating performance (i.e. life prediction and cracking mechanisms) and as tests for quality control.