High-Temperature Sliding Wear Testing of Cathodic Arc Physical Vapor Deposition AlTiN- and AlTiON-Coated Hot Work Tool S
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INTRODUCTION
HIGH-TEMPERATURE wear and oxidation, along with thermal fatigue, make up a very hostile environment, which disapproves conventional hot work tool steels for tooling applications in semisolid processing of steels.[1–7] Tooling materials that are superior to those available today are thus needed. While several replacements with superior performance have been identified in recent years,[6,8–20] cost considerations favor coating hot work tool steels over employing high-temperature alloys for thixoforming dies. Such coatings must offer thermal and chemical stability as well as adequate resistance to thermal fatigue, oxidation, and wear at thixoforming temperatures. Physical vapor deposition (PVD) is a popular surface engineering route employed to extend die life in metal processing, since it is capable of depositing thin hard coatings without impairing the mechanical strength of tool steels.[21–24] PVD coatings offer an attractive combination of high hardness, good wear resistance, and chemical stability and have thus been widely employed on forming and cutting tools in recent years.[25–32] However, there have been very few attempts to explore the potential of PVD coatings in thixoformYUCEL BIROL, Senior Scientist, is with the Materials Institute, Marmara Research Center, TUBITAK, Kocaeli, Turkey. Contact _ e-mail: [email protected] DUYGU ISLER, Researcher, and MUSTAFA URGEN, Professor, are with the Department of Metallurgy and Materials Engineering, Istanbul Technical University, Istanbul, Turkey. Manuscript submitted March 15, 2010. Article published online December 15, 2010 3316—VOLUME 42A, NOVEMBER 2011
ing tools for high-temperature alloys.[2,33–35] AlTiN type PVD coatings are particularly identified for high-temperature use owing to an excellent oxidation resistance.[36–44] The present work was undertaken to investigate the high-temperature sliding wear behavior of AlTiN- and AlTiON-coated hot work tool steel at 1023 K (750 °C), measured to be the maximum die cavity surface temperature encountered in steel thixoforming experiments.[3]
II.
EXPERIMENTAL
The cathodic arc physical vapor deposition process was employed to deposit AlTiN and AlTiON coatings on X32CrMoV33 hot work tool steel, which was heat treated to a hardness of 45 HRC (Table I). 99.99 at. pct purity Cr and 67/33 at. pct Al-Ti cathodes were used. The chamber was evacuated to approximately 10–3 Pa before the deposition step. Pure chromium was deposited on the substrate at a bias voltage of –150 V for 1 minute to improve the adhesion of hard films. High-purity (99.999 pct) nitrogen and oxygen were used for AlTiN and AlTiON coatings, respectively. The coating deposition parameters used in the present work are listed in Table II. The coating thickness was measured by a ball cratering test unit. Elemental depth profiles of the coatings were obtained with a glow discharge–optical emission spectrometer. A scanning electron microscope, equipped with an energy dispersive spectroscopy (EDS) unit, was employed to analyze the chemistry of the coat
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