Effects of intrinsic properties of TiN coatings on acoustic emission behavior at scratch test
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Acoustic emission (AE) behavior at scratch test has been studied for TiN coatings that deposited in an ion plating system. The AE intensity of the coatings measured by the scratch test depended not only on the adhesion of a coating-substrate interface but also on the intrinsic properties of a deposited film. In this work, a careful separation of coating parameters was investigated to clarify the relationship between the AE intensity and the intrinsic properties of coated films. For identical coating conditions, the AE intensity increased with coating thickness since the elastic energy released during the scratching is proportional to film thickness. It has been found that the coatings prepared at higher substrate temperature and/or at higher substrate bias voltage had a larger crystallite size and lower internal stress. With increasing bias voltage and substrate temperature, the AE intensity of TiN coatings decreased. An explanation for this behavior has been proposed in terms of the internal stress and the toughness of the deposited films.
I. INTRODUCTION Coatings of hard materials, which are subjected to severe working conditions, allow reduced friction and increased wear and corrosion resistance. The expected performance of the coated products can be achieved only if the adhesion and the intrinsic cohesion of the coating film itself are sufficient. Titanium nitride (TiN) coatings, produced either by chemical vapor deposition (CVD) or physical vapor deposition (PVD) are widely used in wear-resistant applications. In this paper we shall consider only PVD coatings produced by the process known as cathodic arcion plating.1 This coating technique is gaining in interest as it is possible to get good adhesion and very high dense films at the lower substrate temperature. Among the various techniques proposed for the evaluation of adhesion between the coated film and the substrate, the scratch test appears to be the only available practical method to study the adhesion of hard tribological coatings. In this test, a loaded diamond stylus is drawn over the coated surface under a progressively increasing normal force. The critical load is defined as the load at which the coated film is removed from the substrate. The critical load is determined by optical or scanning electron microscopy, and, in the case of hard coatings, acoustic emission measurement. The acoustic emission (AE) is detected by a piezoelectric detector fixed at a position above a diamond stylus. The AE method is suitable for routine testing of real coating products. Although many authors2'3 have studied the relationship between coating adhesion (critical load) and the deposition parameters using a scratch tester with acoustic detector, they have not investigated the 2240 http://journals.cambridge.org
J. Mater, Res., Vol. 7, No. 8, Aug 1992 Downloaded: 14 Mar 2015
quantitative relationship between the acoustic emission behavior and the intrinsic characteristics of coated films such as morphology, texture, and the grain size. In order to analyze the AE beha
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