Gears are an integral part of modern life, necessary for both production and transport. The
compact and efficient transmission offered by gears made their usage predominant compared
to other drives. Recent development have increased both the efficiency and durability of gears,
especially in the automotive industry. Still, enhanced performance is required due to global
demands on sustainability and energy consumption. Actually, one billion cars are rolling on the
streets around the globe, without counting trucks and busses. This means even small increase
in efficiency could significantly reduce the energy usage.
Efficiency and durability determines the gear performance, which highly relies on the surface
integrity characteristics topography, residual stresses and microstructure. These are generated
by hard finishing in the last manufacturing step using one of the methods grinding, honing or
superfinishing. Running-in (initial cycles of operation) further alter the surface characteristics
and presets the gear for service. Nevertheless, every finishing process produces unique
characteristics, which responds differently to the operating conditions, and in turn influences
the surface integrity of gears accordingly. In this context, a detailed investigation has been
performed to understand the surface characteristics generated by the different hard finishing
methods and their evolution during usage. The role of microstructure in the failure mechanism
micropitting was also studied. The overall aim is to contribute to improved gear performance.
To summarize, surface roughness was found to be the most influential factor among all the
characteristics. During running-in the peaks on the rougher surface for ground and honed gears
were locally deformed. This induced localized microstructutal changes that led to crack
initiation and ultimately micropitting. Clearly, higher running-in load enhanced this pattern.
Instead, for smoother superfinished gears no cracks or structural changes occurred even after
200.000 cycles. The microstructural changes (PDR and localized deformation of martensite
lath) associated with micropitting started already within 200 cycles and cracks within 2000
cycles. Thus, from a materials perspective the running-in of gears can be as short as 2000 cycles.
Compressive residual stresses, beneficial for fatigue life, varied between finishing methods,
with the highest stresses for honed gears. Mostly for the rougher gears, stresses evolved during
testing but always remaining compressive. The initial increase in compressive residual stresses
was linked to retained austenite transformation and its later decrease to crack formation. The
formation of lubricating tribofilm from oil additives was promoted by roughness and runningin
load. All in all this study showed how the surface roughness influences gear performance
and can initiate gear failure after 200 cycles even for load conditions when failure occurs after
22 million cycles.