Damage models of different hardness layers subjected to point contact

In many tribologically and mechanically complex machine parts such as gears, camshafts, bearings, the contact stress plays a crucial role on their lifetime. This paper contributes to the characterization of the strengthening and damage mechanisms pertaining to Rolling-Contact-Fatigue (RCF). A comparison of damage mechanisms and load capacity under contact stress of case-hardened, plasma-nitrided and tempered martensitic materials of different hardening depths with different materials is shown in the present work. The methodology involves Ballon-Rod (BoR) tests, along with image analysis and two-dimensional finite element calculations to assess local effects and damage. For point contact, nitrided, case hardened and tempered martensitic materials showed different damage mechanisms that have an influence on the lifetime of machine parts. The crack initiation for nitrided materials generally occurred on the contact surface, while they occurred below the contact surface for case hardened and tempered martensitic materials. For high contact stress, the case-hardened material 20MnCr5 is clearly preferable than the nitrided versions. At low contact stresses, case hardened materials generally have a somewhat shorter life than the tempered martensitic roller bearing materials.