Effects of lubricant additives on micro-pitting behaviours of gear metals

For achievement of carbon neutrality by reducing carbon dioxide emissions, the increase of the range of electric vehicles is important. One promising method is reducing churning loss in transaxles by viscosity reduction of lubricants. However, it generally leads to thin oil films, which often results in micro-pitting, especially at gear contacts. Since micro-pits are known to be initiated on metal surface, lubricant additives which form tribo-films on the surfaces affect their formation. One of such additives is a calcium detergent. Thus, it is significantly important to clarify its influence on the micro-pitting behaviours of metals. In this study, slip-rolling tests were carried out using twin disc tribometers to evaluate the degree of micro-pitting on steel substrates. Two types of over-based calcium detergents, calcium sulphonate or salicylate were used with phosphates to investigate the effects of their chemical structures. The calcium salicylate showed lower friction and larger roughness reduction compared to calcium sulfonate during the test and a smaller micro-pit area after the tests. Different surface analyses revealed that both the calcium detergents formed tribo-films of calcium phosphate. To elucidate the working mechanisms, three additional experiments were further performed regarding the wear at high contact pressure and the formation rate and the shear strength of the tribo-film. Based on these results, two mechanisms regarding fewer micro-pits formed by calcium salicylate were proposed. The first mechanism is the decrease of contact stress achieved by roughness reduction. This resulted from the truncation of micro asperities (wear at high contact pressure) due to the slower formation of tribo-films. The second mechanism is related to a smaller friction force. The smaller shear strength of the tribo-films formed by calcium salicylate reduces friction force, which also lowers the contact stress. The knowledge gained in this study helps to design lubricant formulations which can reduce viscosity and still prevent micro-pitting.