On the performance of thin hard coatings for gearing applications

Thin hard coatings are possible candidates,for the enhancement of scuffing and micro-pitting preformance of gears. They exhibit very high levels of surface hardness (typically 1000 HV) combined with a low traction coefficient (typically 0.2) against dry steel. The paper presents the results of an experimental program carried out in order to assess the scuffing performance of thin hard coatings for various combinations of substrates and surface finish under realistic engineering conditions of sliding speed, oil temperature and contact pressure. Two types of coating were assessed: diamond like carbon (DLC) coatings and boron carbide coatings. Although the overall performance improvement is promising, caution may be required when using thin hard coatings with case carburised substrates where adhesion problems may occur. Their use with nitrided steel as a substrate seems to be particularly advantageous, possibly because a higher process temperature may be used. The coatings tested also gave better performance at relatively low sliding speeds (less than or equal to 16 m/s), At higher levels of sliding the predominant effect is one of lubrication failure, the material parameter becoming secondary.