Thermal Fatigue Behavior Of Electroless Co-P Alloy Coating Surface

In this experiment, the Co-P alloy was plated at the surface of steel bonded carbide–GJW50. The thermal fatigue test of Co-P alloy coating was done by the method of self-restricted thermal fatigue test. The sample was heated by resistance furnace and cooled by water. During the thermal fatigue test, the cyclic temperature was set 20℃←????→400℃, 20℃←????→600℃and 20℃←????→680℃. In the experiment, the surface morphology and the phase transformation of electroless Co-P alloy coating surface and the process of thermal fatigue cracks during the thermal cycling were characterized by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS) and micro-hardness testing. In this thesis, I focus on analysis the surface of Co-P alloy coating and the mechanism of initiation and propagation of thermal fatigue cracks. Research results showed that there is a clear incubation period during the thermal fatigue crack initiation at the surface of the coating, under the conditions of 20℃←????→400℃thermal cycles and after 200 times, I find out that the district office near the V-notch and the coating surface have plastic deformation, with the thermal cycling going on, the thermal fatigue crack initiate at the bottom of V-notch and propagate follow the direction of thermal cycling. In addition, the thermal fatigue cracks initiate and propagate accompanied with oxidative corrosion. At the same time, oxidation corrosion shorter the incubation period of thermal crack and promote the speed of thermal crack initiation. Thermal crack propagation patterns are the radial, parallel-like and reticular.In the initial thermal cycling, the thermal crack of Co-P alloy coating surface has a longer gestation period. The coating delaying the thermal crack initiate and propagate and playing a protective effect on the base material. The result show that the thermal fatigue properties of Co-P alloy coating is better than that of the original materials.During the thermal cycling, the electroless Co-P alloy coating surface has the phenomenon of cyclic softening, which tends an overall downward trend in micro-hardness. The distribution of micro-hardness values has a certain gradient in the vicinity of V-notch root. Nearer the gap, the micro-hardness become lower and the degree of cyclic softening are greater. The lowest point of micro-hardness is at the front of thermal crack. The results indicated that the corrosion resistance of the electroless Co-P alloy coating was gradually weakened with the thermal cycling going on.