Features of eddy-current testing of the fatigue degradation of laser clad cobalt-nickel-chromium coating under contact loading

Abstract

The fatigue spalling due to contact stresses is one of the main types of failure of the surface protective coatings. Therefore, an important task is to study the contact endurance and to develop nondestructive techniques for testing of the fatigue degradation of such coatings. The possibility of using the eddy-current technique to test the fatigue degradation under contact loading of the cobalt-nickel-chromium PG-10K-01 coating obtained through a gas-powder laser cladding has been investigated. It has been shown that the eddy-current testing of the fatigue degradation under contact loading of the PG-10K-01 coating is possible, but has certain limitations due to non-monotonic changes in the eddy-current device readings α depending on the number of loading cycles. While on the one hand, it is possible to test a sharp increase in the size of contact damages, which, under the used loading conditions, occurs after 5 ×10 4 and 5 ×10 5 cycles and results from the formation of a large number of peripheral ring cracks in the failure zone and the corresponding decrease in the value of α due to the increasing resistivity of the coating. On the other hand, it is possible to test the achievement of a certain number of loading cycles in the range of 8 ×10 5-1×10 6 , when the eddy-current device readings exceed the initial values. This increase in the value of α is caused by the compaction of the coating as a result of healing of discontinuities and a decrease in its resistivity. The testing can be performed by measuring the eddy-current device readings at high excitation frequencies of the-eddy current probe f = 72-120 kHz. In this case, the impact of the ferromagnetic steel base on the eddy-current device readings is minimal and mainly surface layers are analyzed, where the processes of crack formation, mechanical hardening, and compaction that affect the physical characteristics of the coating are developed more intensively.