Study on Contact Fatigue Crack Propagation Behavior of Cr7C3 Coatings

Abstract

Abstract. In this paper, the contact fatigue crack propagation behavior of Cr7C3 coatings was studied by means of extended finite element method and linear elastic fracture mechanics. The contact fatigue model of simplified geometric model was established by commercial software ABAQUS. The influence of the thickness of coatings, the elastic modulus of the coatings, the elastic modulus ratio of the coatings and the substrate on the contact fatigue life of the coatings was studied. The optimal coatings design parameters were obtained. IntroductionA Positive continuous engagement (PCE) overrunning clutch is the essential component of helicopter transmission system. It is a critical element to ensure the correct combination and disengaged of the main reducer from the engine. A PCE overrunning clutch is composed of wedges, cages and a spring. Its function is to transfer the torque of the output shaft of the engine to the input shaft of the main reducer. Using chemical vapor deposition (CVD) method to plate Cr7C3 coatings on the surface of wedges aims to reduce the wear of wedges. During the flight of helicopter, we found that the actual life of the coatings is much less than the design life of the coatings.According to the stress analysis, it can be seen that the wedges are subjected to cyclic normal pressure and alternating tangential sliding friction force. The surface of the coatings was observed by scanning electron microscope (SEM). After the analysis, it can be concluded that the mechanism of the fatigue spallation of Cr7C3 coatings is as follows. At first, the distribution of shear stress in the coatings will occur under the action of alternating contact load. The location of the maximum shear stress is the weak position of the coatings, and the micro cracks or micro defects at this location are the sources of fatigue crack initiation. Then, the cracks will extend in the direction of 45 degrees under the maximum shear stress. In the process of crack propagation, the crack is fused with other micro cracks, and finally extends to the coatings' surface. Finally, the coatings spalled under the action of the surface force.In order to improve the fatigue life of the coatings, the influence of coatings parameters on the crack propagation behavior of the coatings is studied. Extended finite element method (XFEM) can simulate crack propagation effectively [1,2]. Sukumar uses the XFEM and the fast stepping method to simulate the 3D fatigue crack propagation, and uses the fast stepping method and the Paris formula to simulate the crack tip movement [3]. Baydoun studies of the reliability of different crack propagation criterion under the XFEM and 3D fracture mechanics method, concludes that the maximum energy release rate criterion and the maximum circumferential tensile stress criterion can be used to accurately describe the crack propagation process [4,5]. Baietto combines the experimental results and the XFEM to simulate 2D and 3D contact fatigue crack growth [6].In this paper, the XFEM in ABAQUS and Paris formula are...

The method of mathematical description of the phase composition diagrams

“…It is divided via usual triangulation by 5 elementary polytopes (triangles) of the co-existing phases: Fe -Cr 3 Si -Сr (1), Fe -Cr 3 Si -FeSi (2), FeSi -Cr 3 Si -Cr 5 Si 3 (3), FeSi -Cr 5 Si 3 -CrSi 2 (4), FeSi -CrSi 2 -Si (5). Table 1 includes the coordinates of triangle 2 phases in % (mass.)…”

“…In the same way, to obtain wear-resistant cast iron, it is necessary to provide forming of Cr 7 C 3 carbide in it, not only presence of definite chromium amount [3][4][5]. Operating the chemical composition is a rather suitable tool for calculation of charges and taking into account loading of materials in industrial units.…”