Surface and Coatings Technology 177–178 (2004) 341–347 0257-8972/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2003.09.024 Calorimetric evidence for frictional self-adaptation of TiAlN y VN superlattice coatings P.H. Mayrhofer *, P.Eh. Hovsepian , C. Mitterer , W.-D. Munz a, b a b ¨ Department of Physical Metallurgy and Materials Testing, University of Leoben, Materials Center Leoben, Franz Josef Strasse 18, a Leoben A-8700, Austria Materials Research Institute, Sheffield Hallam University, Howard Street, Sheffield S1 1WB, UK b Abstract TiAlNyVN superlattices are potential candidates for dry machining due to their high hardness and excellent tribological properties. It has been reported that VN easily oxidizes at relatively low temperatures and forms V O having lubricious properties. 25 The aim of this work is to investigate the effects of vanadium oxides on tribological properties of TiAlNyVN superlattices at high temperatures. During differential scanning calorimetry (DSC) in an argonyoxygen atmosphere it was found that the coatings oxidize at approximately 450 8C. Melting and boiling point of the formed oxides could be determined by DSC to approximately 635 8C and 1400 8C, respectively. It was observed by DSC and X-ray diffraction that the melting phase, a V O containing oxide, 25 transforms into a VO containing oxide causing the loss of the liquid phase. Dry sliding tests showed that up to 500 8C the 2 friction coefficient increases from 0.55 to 0.95. It drops to approximately 0.18 at 700 8C and remains there as long as a liquid surface oxide is present. If most of V O is converted into lower-oxidized vanadium the friction coefficient increases to a steady 25 state value of approximately 0.55 at 700 8C. The results obtained show that addition of VN to hard coatings has high potential to achieve a low friction effect due to the formation and melting of a V O containing oxide, in addition to its conversion into 25 easily shearable lower-oxidized vanadium phases.  2003 Elsevier B.V. All rights reserved. Keywords: TiAlNyVN; Superlattice; Oxidation; Differential scanning calorimetry; Tribology 1. Introduction Tribological properties of hard coatings for tooling applications are the topic of many papers w1–8x. Espe- cially, TiAlN is well known for its excellent wear and oxidation resistance enabling improved machining pro- cesses like high speed and dry cutting w1,9–18x. Due to their supersaturated metastable phase, coatings within the Ti–Al–N system show age hardening effects, which effectively increase their hardness at higher temperatures w19x. In high speed and dry cutting applications low-friction and lubricating mechanisms of the coating itself are required in addition to excellent mechanical properties. Intrinsic solid lubricants often begin to fail in their tribological effectiveness with increasing temperature, in humid atmosphere or due to oxidation. Analogous phe- *Corresponding author. Tel.: q43-3842-402449; fax: q43-3842- 402737. E-mail address: paul.mayrhofer@unileoben.ac.at (P.H. Mayrhofer). nomena happen to liquid lubricants that are a product of chemical reactions between coating and moisture of ambient atmosphere w4,20–22x. Thus, oxide materials with ‘easy’ crystallographic shear planes have attracted increasing interest. Such so called ‘Magneli-phase ´ oxides’ promise higher phase, oxidation and thermal stability. Vanadium is known to form with oxygen Magneli-phases w23x. Consequently, a lot of effort has ´ been made to develop TiAlNyVN superlattice coatings combining many beneficial properties. TiAlNyVN has proven to be an excellent candidate in protecting machine wear parts and cutting tools due to its high wear resistance and low friction w1,13–18x. The latter is expected to result from the formation of V O showing 25 lubricious properties w1,13x. The purpose of this study was to investigate the oxidation behavior of TiAlNyVN with a superlattice period l of approximately 3.5 nm. Main emphasis was laid on oxide-formation, especially of V O due to its 25 low melting point and their effects on the tribological properties at high temperatures.