The Thermal Fatigue Life Prediction of Ellipsoidal Ceramic Ni Base Alloy Composite Coating

摘要

Ni base alloy ceramic composite coating fabricated through vacuum fusion sintering process can acquire high strength and good thermal fatigue resistance. Composite coating was mainly composed of Ni base alloy and ellipsoidal ceramic particles with random orientation. If the thermal expansion coefficients and elastic modulus of Ni base alloy and ceramic particles are different, there will be thermal stresses between particles and matrix in thermosyphon. The thermal stress will arouse the initiation and growth of thermal fatigue crack. The three-phases model is used to study the thermal stress in composite coating. First, the three-phases model was used to determine effective eigen strain of two-phases cell formed by the ellipsoidal matrix shell being around a ellipsoidal ceramic particle, then disturbance strain in two-phases cell aroused thermal inconsistency is obtained. Then thinking average stress in effective matrix vanish, the thermal stress field in Ni base alloy and particles can be gotten by considering random orientation of two-phases cells. It can be shown that the more difference of thermal expansion coefficients between Ni base alloy and ceramic particles is, the bigger the thermal stress is. The thermal stress relates to volume fractions and elastic constants of Ni base alloy and ceramic particles. Finally, based on low-cycle fatigue crack growth rate formation, the thermal fatigue life was computed. The bigger thermal stress is, the smaller thermal fatigue life is. Thermal life is an exponential function of crack initiation length and critical length.