Abstract
Erosion damage of aero-engine compressor blades and vanes is a serious concern to engine operators and manufacturers because it reduces engine performance and increases fuel consumption and operating cost. The Canadian Forces (CF) are considering the application of titanium nitride (TiN) coatings to the compressor blades and vanes of some of their engines to improve the erosion resistance of these components. In support of the CF`s interest in protective coatings, the Structures, Materials and Propulsion Laboratory (SMPL) has developed test facilities and techniques to evaluate the properties of candidate coatings and to qualify promising ones by bench and engine tests. One such facility is the Nanomechanical Probe (NMP) which can be used in the indentation and scratch modes to evaluate various micro-mechanical properties of coatings, including their adhesion to candidate substrates. It is clear that in order for TiN coatings to provide the necessary protection, they must adhere strongly to the substrate material. A popular method for evaluating the adhesion strength between a coating and its substrate is the scratch test which was first proposed by Heavens and later introduced by Benjamin and Weaver. In this test, the critical load, P{sub cr}, needed to remove the coating from the substratemore » is determined and used to describe the adhesion between coatings and substrates onto which they are deposited. However, P{sub cr} was found to depend on factors other than just the interfacial adhesive strength. Some of these factors include elastic and plastic properties of the coating and substrate, friction, residual stresses, coating thickness and loading rate and scratch speed. Hence, these factors should all be taken into consideration when evaluating coating adhesion in terms of the critical load determined by the scratch test. This can be done for example by calculating the interfacial strength, work of adhesion or interfacial fracture toughness.« less
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