Low friction W-C:H coatings for wear resistance in roller bearings
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Low friction W-C:H coatings for wear resistance in roller bearings Thorsten Kacsich1, Peter Werner Gold2 and Jörg Loos2 Metaplas Ionon GmbH, Bergisch Gladbach, Germany 2 Institute for Machine Elements and Machine Design, RWTH Aachen, Germany
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ABSTRACT Life time limiting effects in roller bearings and gear wheel applications are wear, sizing, and fatigue damages like micro pitting. These mechanisms are related to a number of parameters: lubrication conditions, surface roughness, and the hardness of the bulk material. Nowadays, MeDLC coatings like MAXIT W-C:H offer surface protection under dry, mixed, and hydrodynamic conditions. This allows the increase of load capacities, respectively the decrease of gearing size. Moreover, a reduction of toxic lubrication additives can be achieved. The performance of the MAXIT W-C:H coatings in roller bearings was investigated on the FE8 test rig under mixed friction conditions. The W-C:H was deposited via the PVD technique of magnetron sputtering onto case hardened steel. The coating thickness was varied from 1.5 to 5 µm with respect to optimised wear resistance. The wear was drastically reduced by powers of ten as compared to uncoated roller bearings. INTRODUCTION Amorphous carbon based low friction coatings are introduced into industrial manufacturing since years. Metal free a-C:H (respectively DLC, diamond like carbon), and metal containing Me-C:H (respectively Me-DLC) derivations have proven their wear reducing capabilities, for example on engine components like pistons and tappets, or gear wheels of wind power stations. Also for roller bearing applications, these coatings have been applied successfully: The aim of a German research project called ‘SFB 442’ [1] is the replacement of toxic additives in bearing lubricants by the use of such coatings. In FE8 bench tests, tungsten based Me-C:H thin films performed best [2]. In this context, the influence of the layer thickness on the wear protection of cylindrical roller bearings was investigated. EXPERIMENTAL MAXIT W-C:H was deposited at 180oC onto case hardened washers and roll-bodies of FAG Kugelfischer 81212 bearings. The depositions were performed in a METAPLAS IONON MZR 353 facility, using the PVD technique of magnetron sputtering. The coating thickness was 1.5, 3 and 5 µm, respectively. The coating properties are summarised in table 1. The coated washers were tested together with uncoated roll-bodies on a FE8 test bench, where two cylindrical roller thrust bearings can be examined at the same time. The test parameters, indicating mixed friction conditions (high load –low velocity, no protective additives) are given in table 2. The weight loss due to wear was measured in dependence of test duration and coating thickness.
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Table 1. Properties of MAXIT W-C:H
Table 2. FE8 test conditions
Hardness (HV) E-Module Critical Load LC2 Adhesion class (VDI 3198)
Axial load Power density Rotational speed Slippage Friction condition Lubrication
1200 170 GPa 40 N HF 1
Temperature Material Roughness Test duration
80 kN
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