The four-point bend test as a tool for coating characterization

Abstract

One property of hard coatings that limits their use is the tendency to crack under load. Currently, no standardized test exists to measure the toughness of a coating. At the same time, coating technology has advanced to the point where traditional methods of characterization, specifically hardness, metallography and tensile bond tests, are no longer adequate means of discriminating coatings. It is proposed that bend testing be added as a means of evaluating coating integrity and toughness. This paper describes a four-point bend test used to measure the strain to fracture of detonation gun coatings. The coating is applied to a beam of rectangular cross-section and the composite beam is tested with the coating in tension. Cracking is detected with the use of acoustic emission.In seperate experiments, the residual stress of coatings was measured using the blind hole test. Residual stress and strain to fracture data for several coatings are presented and, as expected, there is a linear relationship between the two properties. By creating coatings with low tensile residual stress, tougher coatings have been created. Other coating properties such as hardness and wear behavior are also presented and it is shown that they do not adequately differentiate the coatings.