Abstract:In the present work, the premature rupture of the chromium carbide coating on a freewheel used in a helicopter main reducer was analyzed by stereomicroscope visual examination, scanning electron microscopy examination, structure analysis, hardness test and X-ray diffraction analysis(XRD). The results showed that the fracture of the coating was a mechanical contact fatigue mechanism, the main cause of the flaking of the coating were the microcracks caused by micro porosities under the condition of cyclic loading. With the development of the microcracks, the lightly flaking area initiated on the surface, which causes stress concentration in the adjacent area and the through coating crack in the border of flaking area. The flaking of the coating reduced the loading area which increased the stress and new flaking areas initiated, at last, with the extension of different flaking areas, the throughout flaking formed.