Tribo-oxidation of a TiN coating sliding against corundum
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This paper is aimed at understanding the tribo-oxidation of a physical vapor-deposited TiN coating when sliding against a corundum ball. This is achieved through a compositional and structural analysis of the wear debris. Wear debris particles generated at three different sliding speeds were analyzed with micro-Raman spectroscopy, transmission electron microscopy, and electron probe microanalysis. The analysis showed that the wear debris when formed at the low and medium sliding speed consist of TiO2 with a nanocrystalline structure containing both anatase and rutile structural elements. Only rutile structural elements could be observed in the debris formed at the high sliding speed. These results on the characterization of the wear debris are interpreted with calculations of the flash temperature in the tribo-contact and with recent ball-on-disk results on the wear rate of TiN as a function of the sliding speed to propose a mechanistic view of the tribo-oxidation and wear process. The relation with previous and recent experimental results on the static oxidation of TiN is also given.
I. INTRODUCTION TiN coatings increase tool life in a large number of metal working and cutting applications.1-2 Still, the wear mechanisms of TiN under various operating conditions need to be understood further. In studying wear processes, tribologists have realized that the wear debris bear the signature of the wear process.3 Surface analytical techniques have revealed that some material transfer always takes place between two sliding first bodies, even when sliding at extremely low loads or speeds.4 The transferred material can be a mixture of debris from either of the two first bodies or a reaction product from the first bodies with the environment. Examples of chemically controlled wear are steel sliding on steel in air where different types of iron oxides are formed depending on the humidity of the environment.5 Another example is the case of TiN sliding against steel with the development of a transfer layer consisting of iron oxides or titanium oxides, depending on the operating conditions of the test. 67 Interesting cases of the effect of a transfer layer formation on the wear behavior are the improved wear resistance of silicon nitride under increased humidity,8 and the self-lubricating action of MoS 2 coatings in the presence of traces of oxygen in a vacuum environment.9 Singer et al.10 have studied the debris resulting from the sliding of TiN against a sapphire ball in the sliding speed range between 10"4 and 0.08 m/s. They concluded from a transmission electron microscopy study of the a)
Present address: Koninklijke/Shell Exploratie en Produktie Laboratorium, Rijswijk, The Netherlands.
992 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 4, Apr 1994 Downloaded: 11 Mar 2015
wear debris that tribo-oxidation of the TiN occurs and that the reaction product is exclusively rutile, the stable TiO2 polymorph. Recently, it was shown that the TiN coating wear volume when sliding against corundum depends on the sliding s
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