Measurement and analysis of adhesion strength for thermally sprayed coatings

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 (Ώm in thickness and can thus be described as bulk coatings. The minimum microstructural detail would be a single splat (ofter described as a lamella) which is about S (Ώm (approximately 0.0002 in.) in thickness and up to 80 Ώm (approximately 0.003 in.) 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 strength and toughness. Measurements such as the tensile adhesion test (according to ASTM C633), the double cantilever beam test, and the scratch test are detailed to illustrate their relevance to present industrial practice. Acoustic emission studies have also been used to assess the "crack density function," a product of the number of cracks and crack size. Indentation techniques have been used to determine the fracture toughness of coatings and to demonstrate that the material properties of coatings are anisotropic. These techniques, among others, may be used to gain a fundamental understanding of coating performance or for quality control. A further focus of this paper concerns the highly variable nature of the material properties of coatings. Such variation leads to poor reproducibility during service and can cause unpredictable performance. Therefore, a section is presented on the statistical analysis of thermal spray coatings, with particular reference to the Weibull distribution.