Enhancing the tribological performance under biodiesel lubrication using CVD diamond coated parts

One important issue in internal combustion engines is the performance of injection pumping systems that is related to the energy efficiency of the engines. Considering that the trend is to increase the combustion pressure, new materials of lower weight, harder, wear and corrosion resistant, and compatible with the fuel environment, are needed. In this work, ball-on-flat reciprocating tests were performed with bare and CVD diamond coated Si3N4 materials lubricated by a commercial soybean-derived biodiesel. Experiments with AISI 52100 steel were also accomplished. Two types of coatings were tested: (i) a single layer (SL) of microcrystalline diamond (MCD) and (ii) a multilayered (ML) system made of three bilayers of MCD/nanocrystalline diamond (NCD). Results show that diamond coatings give lower friction coefficient values (COF) of 0.07, as the minimum, than that of the steel tribopair, which presents a COF above 0.10, which can limit engineering applications. The SL tribopair presents a higher initial COF peak (0.35) than ML (0.15) explained by the difference in surface roughness. Regarding the wear behaviour, all the tested materials have shown similar mild wear coefficients around 10−8 mm3 N−1 m−1, the same observed in dry tests with CVD diamond coatings, which constitutes a further advantage when the system is running in lubricant starved conditions.