Wear resistance of arc ion-plated chromium nitride coatings
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Wear resistance of arc ion-plated chromium nitride films has been studied. It has been found that texture and phases composing the films depend much on bias voltage and nitrogen gas pressure at the deposition. A phase diagram was constructed as a function of these two parameters, which indicated that three categories exist: CrN single, CrN and Cr 2 N dual, and CrN and Cr dual phased regions, respectively. Results of Falex No. 2 test showed that the wear resistance of CrN single phased films is superior to others, especially when (220) preferred orientation is developed. Since hardness and surface morphology do not differ much between these films, a high toughness of CrN single phased film is considered to make a difference by suppressing abrasion wear.
I. INTRODUCTION Coatings of hard materials, which are subjected to severe working conditions, allow reduction of friction and increase wear and corrosion resistance. Chromium nitride films on steels are well known to be hard and to increase wear resistance.1'2 Moreover, Kawana and Ichimura3 have reported that CrN films have high hot corrosion resistance compared with TiN films. Therefore, chromium nitride coatings are expected to get good wear resistance under the severe tribological conditions such as high applied pressure and high temperature. Chromium nitride has two kinds of crystal structures: Cr2N (hexagonal) and CrN (fee). These have been reported in the studies of chromium nitride films. Kashiwagi et al.A and Wang and Oki5 have reported that the component ratio of Cr, CrN, and Cr 2 N and the morphology of deposited films are controlled by changing the pressure of nitrogen gas and the substrate bias voltage. However, there are hardly any reports on the relationships between those films and the wear resistance. Wear resistance is strongly influenced by the intrinsic properties of film such as morphology, hardness, the coefficient of friction, and cohesion. Furthermore, wear behavior depends on tribological conditions such as applied loads, sliding speeds, and working atmospheres. The aim of this work is to clarify the relationships between the wear resistance properties and ion-plated chromium nitride phases, and to discuss the wear mechanism of those films. II. EXPERIMENTAL DETAILS Chromium nitride films were deposited onto test pieces (pin, block, ring, and plate) made from austenite J. Mater. Res., Vol. 8, No. 5, May 1993
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stainless steel using a cathodic arc ion-plating system.6 Prior to coating, test pieces were mechanically polished to less than 0.1 /u,m of maximum surface roughness. Then the test pieces were cleaned ultrasonically in an ethanol bath and subjected to Freon drying. The test pieces were mounted in a vacuum chamber and pumped down to a pressure of 10~5 Torr. The distance from the Cr cathode to a test piece was 180 mm. The test pieces were treated by the Cr ion bombardment to heat and to remove the surface oxide layer. Nitrogen was introduced to maintain a chamber pressure from 3 to 70
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