James Nygaard

Secondary hardened steels are used in the manufacture of bearings for aircraft gas-turbine engines; here, contact stresses approach 2GPa and can lead to fatigue failure.

The rolling elements are a high-carbon M50 tool steel which in the heat-treated condition has a hardness of 850 HV (64 HRC). The raceways are low-carbon, carburised M50NiL steel with added Ni for improved fracture toughness to cope with tensile stresses arising from fitting and centrifugal effects.

After tens of millions of fatigue cycles, rolling contact fatigue failures can initiate at imperfections on the meeting surfaces and also beneath the point of contact where the shear stresses are maximum. Here in the subsurface cracks nucleate at stress raising defects; traditionally at interfaces between non-metallic inclusions and the matrix. To compensate, aeroengine bearing steels are produced under vacuum for high purity and low oxygen content (< 5 ppm). Modern bearing failures are more concerned with surface tribology where failures due to lubrication contamination have become dominant.

This PhD research is carried out in collaboration with Rolls-Royce and SKF Group, and funded by the EPSRC and Rolls-Royce.