Case studyWear behavior of Cr1−xAlxN PVD-coatings in dry running conditions
Introduction
Over the last two decades the market for wear reducing and friction lowering coatings has grown tremendously. The development of wear reducing coatings has started with tool coating. Hereby, hard coatings like titanium nitride (TiN) [1] or titanium aluminum nitride (TiAlN) [2], [3] made tool coating popular because of their excellent performance. Today, there exist a broad variety of tool coatings for nearly every demand. Nowadays component coatings are a great topic for the coating industry. Chromium based coatings are a promising choice for components. These coatings combine good wear properties and a high corrosion resistance. Especially, the ternary system chromium aluminum nitride is of high interest for tool and component applications [4], [5], [6]. Chromium aluminum nitride shows high oxidation resistance next to improved wear resistance [7]. Experimental investigations in lubricated contacts showed good tribological results [8], [9].
Within this study the tribological behavior of Cr1−xAlxN coatings was investigated for dry running applications. Ceramics like Al2O3 and Si3N4 were chosen as counterpart materials for ball-on-disc (BoD) tests. The main focus was set on the wear phenomena of the different tribological systems. Friction and wear were regarded with respect to the coating's properties and tribological interactions with the counterpart material.
Section snippets
Experimental details
The Cr1−xAlxN coatings were deposited on hardened (HRC 63) and polished round blanks of 100Cr6 (1.3505, SAE 52100, see Table 1) with a diameter of 20 mm. The samples had a surface roughness of Ra 0.06 μm and Rz,ISO 0.8 μm. Before coating the samples were steam blasted and cleaned in a multi-stage ultrasonic supported alkaline bath. The coatings were applied by pulsed reactive magnetron sputtering. A commercial facility CC800/9, CemeCon AG, was used for the deposition processes. The process
Coating characteristics
The deposited Cr1−xAlxN coatings were investigated as mentioned above. The results of the tests are summed up in Table 2. The deposited coatings showed different coating thicknesses. This fact has to be considered for the tribological investigations because for comparability reasons. The results of Rockwell test showed that there was no loss of hardness at the substrate material. That means all process temperatures did not exceed 160 °C. All coatings reached the adhesion class 1 according to VDI
Conclusion
The tribological investigations were carried out at a ball-on-disc tribometer. Ceramics, Al2O3 and Si3N4, were chosen as counterpart materials for dry running conditions. The results of wear track investigation and inspection of wear debris showed that AlN is worn by a mainly adhesive wear mechanism, while the wear mechanism of CrN is based on a combination of tribooxidation and adhesive wear what results in abrasive particles. Hard oxide particles lead to a microcutting of the coating. Cr0.77Al
Acknowledgements
The authors would like to express their gratitude for the financial support of the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) within the project SFB 442 “Environmental friendly tribosystems”. Special thanks to Mr. U. Mohr (IWM, RWTH Aachen) for his technical support.
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