Tribology of wire arc spray coatings under the influence of regenerative fuels

In order to further optimize the efficiency of today's internal combustion engines, specific coatings are used on functional surfaces to reduce internal engine friction and wear. In the current research projects oxymethylene ether (OME) is discussed because it is CO2 neutral and it has a strong soot-reducing effect as fuel or fuel additive. In some operation regimes of the internal combustion engine a dilution of engine oil by fuel must be assumed. In this paper, the frictional contact between piston ring and cylinder raceway is modelled using a pin-disk tribometer and the friction and wear behavior between a diamond-like carbon coating (DLC) and a thermal spray coating is characterized. The wear of the spray layer could be continuously detected by radionuclide technology (RNT). With the aid of photoelectron spectroscopic measurements (XPS), the steel thermal spray coating was chemically analyzed before and after the tribometer tests and the oxidative influence of OME was investigated. In addition, confocal microscopy was used to assess the topographies of the specimens. The measurements showed that the addition of OME to the lubricant reduced the viscosity and load-bearing capacity of the lubricating film, which led to an increase in the coefficient of friction. While almost no wear on the pin could be detected at 10% OME, the first visible material removal occurs at an OME content of 20 % and the layer delaminated at 30% OME. The evaluation of the RNT wear tests showed that both the tests with engine oil and with engine oil plus 20 % OME achieved very low wear rates. No corrosion of the thermal spray coating could be detected by XPS. Only the proportion of engine oil additives in the friction track increased with increasing OME concentration.