Characterization and residual stress analysis of wear resistant Mo thermal spray-coated steel gear wheels

The determination of residual stress (RS) in case-hardened steel gear truck synchronisers coated with thermal sprayed molybdenum was carried out using neutron and synchrotron X-ray diffraction. Two samples with different coating thicknesses (about 120 μm and 1.4 mm) and different steel substrates (16MnCr5 and SAE4140, respectively) were investigated. Microanalysis revealed substantial porosity in both samples and some debonding was observed between the thin coating and the substrate. The bulk hardness of the SAE 4140 proved to be much higher than the 16MnCr5 and the surface case-hardening increased it by a further 20%. The full three-dimensional stress depth-profile was determined by neutron diffraction (ND) in both the coatings and the substrates, while synchrotron radiation allowed a depth strain scan in the thinner sample. Corrections were applied to the ND data to overcome the surface effect and data reduction was performed to coherently interpret the results, including the determination of the centre-of-mass of the diffracting volume. The results show how thermal stresses play an eminent role in this system, especially in the case of the thicker coating. Quenching stresses mitigate their effect in the thinner coating. Stresses as high as 800 MPa are reached in the sample with the thicker coating, while they are limited to 250 MPa in the sample with the thinner coating. This can be explained by the radically different yield strength of the two steels and by the different coating characteristics.