直升机主减速器用离合器自由轮表面铬碳相渗层 失效分析

直升机主减速器用离合器自由轮在使用过程中出现了表面渗层剥落失效问题，本研究采用体式显微镜观察、扫描电镜观察、金相分析、硬度分析、XRD分析等方法，对铬碳相渗层的剥落原因进行了分析。分析结果表明，渗层剥落的机理为接触疲劳剥落，渗层内部存在的显微孔洞在循环载荷的作用下产生的微裂纹扩展，是造成渗层剥落产生的主要原因，随着微裂纹的扩展，表面出现轻微剥落区域，剥落区域的出现造成了邻近区域的的应力集中，促进了剥落区域边缘贯穿裂纹的形成，加剧了剥落。同时，渗层的局部剥落，造成的承载区域减小，应力上升，促进了新剥落区域的产生，最终多个剥落区域扩展，形成贯穿性渗层剥落。

Failure Analysis of Chrome Carbide Coating on a Freewheel used in Helicopter Main Reducer

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.