Influence of triboinduced films on failures and frictional behaviour of gears with particular focus on running-in processes - experimental and analytical investigations

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The formation of triboinduced films (tribofilms) by interaction of lubricant additives with gear flank surfaces can have significant influence on the failure mechanisms and frictional behaviour of gears. While in part I of the research programme a practical and optimized running-in method for increasing the load carrying capacity of spur gears has been developed, the complex mechanisms of tribofilm formation during running-in and operation have not been considered in detail. However, since all results have shown a clear dependency on the investigated lubricant additives, in part II of the research programme the connection between formation and evolution of tribofilms with load carrying capacity and frictional behaviour of gears is investigated systematically. This requires detailed knowledge about local stresses and permissible stresses of the tribological system: On the one hand, the physical and chemical composition of the tribofilm and of the near surface region is investigated by suitable surface analysis techniques and on the other hand the local stresses are determined by means of detailed numerical simulations of the lubricated contact at the research partner IMK Magdeburg. Hence, the local formation of tribofilms on technical surfaces can be connected to the local stresses in the lubricated contact. Based on the conducted investigations, models and concepts are developed in order to clarify, which tribofilms cause high or low friction and high or low load carrying capacities in gear applications. With respect to the investigations on the load carrying capacity of gears, the focus is on the failure modes pitting and scuffing. It is also investigated, if tribofilms can have positive effects on all investigation areas (scuffing, pitting, frictional behaviour) at the same time. As tribofilms are effective solely by the use of lubricant without elaborate design modifications and additional efforts, the results of part II of the research programme make an important contribution to resource conservation. The findings of the research programme provide an approach for selecting appropriate lubricant additives and for optimising existing lubricant additive formulations for gear drive applications for the additive and lubricant manufacturers as well as for end users.