Oxidation Behavior and Mechanisms of TiAlN/VN Coatings
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TITANIUM nitride (TiN) with the B1 NaCl structure has been widely used as a hard wear-protective coating since the 1980s. However, a major limitation of TiN coatings for high-speed machining applications is that they oxidize rapidly to rutile, TiO2, at temperatures above 500 C. Initial oxidation is parabolic, with O diffusion through TiO2 to the oxide/nitride interface as the rate-limiting step.[1] The large difference in molar volumes of TiO2 and TiN results in spallation of the oxide scale, resulting in a transition from parabolic to pseudo-linear kinetics.[2,3] TiAlN coatings provide much improved high-temperature oxidation resistance, up to 750 C to 900 C, and have consequently been used extensively for high-temperature cutting operations with minimum use of lubricant or dry machining.[4] Titanium aluminum nitride (Ti1-xAlxN) has the B1 NaCl structure with 0 £ x £ 0.52, but as x is increased, phase decomposition occurs into B1 NaCl and wurtzite AlN structures.[5,6] Ti0.5Al0.5N exhibits stable parabolic oxidation kinetics, associated with an oxide comprised of a passive double Z. ZHOU, Postdoctoral Research Assistant, W.M. RAINFORTH, Professor, C. RODENBURG, Royal Society Research Fellow, and N.C. HYATT, Lecturer, Dr., are with the Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, United Kingdom. D.B. LEWIS, Lecturer, and P.E. HOVSEPIAN, Professor, are with the Materials Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, United Kingdom. Contact e-mail: [email protected] Manuscript submitted July 27, 2006. Article published online September 14, 2007. 2464—VOLUME 38A, OCTOBER 2007
layer oxide, which is a result of outward diffusion of Al to form Al-rich oxide at the topmost surface and inward diffusion of O to form Ti-rich oxide at the interface to TiAlN.[7] However, during sliding wear tests, the friction coefficient of TiAlN is usually high, typically l = 0.8 for TiAlN measured from the pin-on-disc test against alumina ball.[8] Both the friction and wear performance of TiAlN can be improved through the quaternary addition of V, as monolithic Ti-Al-V-N coatings or as TiAlN/VN multilayers. The VN, which also has the B1 NaCl structure, exhibits continuous solid solubility with a number of metal nitrides and carbides, such as TiN, TiC, TiAlN, VC, and NbN.[9] Knotek et al.[10] found that the addition of V to TiN and Ti-Al-N enhanced wear performance, with the best wear resistance obtained in a Ti-V-N coating with 29 at. pct V. TiAlN/VN multilayers, which incorporate VN into TiAlN as a nanoscale multilayer (also referred to as a ‘‘superlattice’’) coating, have exhibited excellent sliding wear resistance (1.26 · 10-17 m3ÆN-1 m-1) with a lower friction coefficient at room temperature (l = 0.4, pin-on-disc test, Al2O3 ball counterpart) in comparison to other wearresistant coatings,[8] e.g., TiAlN/CrN(sliding wear coefficient 2.4 · 10-16 m3ÆN-1 m-1, l = 0.7 to 0.9). The oxidation of VN has been investigated by several authors.[11–13] Oxidation is complex, with columnar V2O5
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