Fracture and Residual Characterization of Tungsten Carbide Cobalt Coatings on High Strength Steel

Tungsten carbide cobalt coatings applied via high velocity oxygen fuel thermal spray deposition are essentially anisotropic composite structures with aggregates of tungsten carbide particles bonded with both amorphous and crystalline cobalt phases. X-ray diffraction was used to characterize the residual stresses within the coatings to understand the crack initiation and propagation behavior of samples subjected to axial fatigue loads. Diffraction was also used to establish a baseline stress state of the uncoated high strength steel fatigue specimens. Stress states were evaluated for bare metal, and coated hourglass fatigue specimens that were subjected to low, medium and high cyclic fatigue conditions. Scanning electron microscopy was used to determine coating crack initiation and propagation paths within the coating as well as substrate fatigue site origins After finish grinding observed coating cracks were determined to have started at surface defects and were observed to propagate in the radial direction towards the substrate along splat and interfacial boundaries within the softer cobalt coating matrix. These boundaries provide paths for cracks around WC particles, which contain high compressive residual stresses. High magnification inspection also confirmed that substrate fatigue cracks initiate at subsurface inclusions when subjected to low stress conditions and substrate-coating interface defects when subjected to high stress conditions. Subsurface defects are inclusions and impurities from the steel manufacturing process whereas the interfacial defects are imbedded aluminum oxide particles from the grit blasting process used to prepare the substrate surface for coating application. Cracks in the coating from applied axial stresses do not propagate beyond the coating-substrate interface and did not provide preferential sites for substrate fatigue crack initiation.