Modeling hydrogen diffusion in a tribological scenario: A failure analysis of a thrust bearing
A coupled diffusion-mechanical finite element simulation model was developed to study the diffusion of hydrogen in a cylindrical roller thrust bearing (CRTB). The simulations enabled obtaining qualitative information pertaining to stress-assisted diffusion in tribological loading and under the influence of residual stresses. The mechanical behavior of bearing steel was obtained from experiments and supported by literature. In parallel, rolling-contact fatigue (RCF) tests on CRTBs lubricated with a fully additivated transmission oil were conducted for 25 h and 50 h to investigate their premature failure modes. Post-RCF rheological analysis indicated decrease of the lubricant viscosity due to degradation. Carrier gas hotextraction analysis indicated a significant increase in hydrogen conten t in the bearings tested for 50 h. Whereas, serial cross-sectional analysis revealed the formation of subsurface White Etching Crack (WEC) networks associated with White Etching Areas (WEA); the cracks breached the surface on multiple positions causing flaking and eventual failure. On the other hand, no signs of damage were observed in the bearings tested for 25 h. The simulations revealed insignificant hydrogen accumulation due to stress-assisted diffusion in comparison to concentration gradient driven diffusion; hydrogen trapping was pronounced in zones undergoing plastic deformation due to the formation of deformation induced trapping sites. However, residual stresses had an evident influence on the subsurface accumulation of hydrogen. A comparison between the zones of elevated hydrogen concentration due to residual stresses and RCF induced subsurface damage yielded a good correlation.