FATIGUE RESISTANCE OF A MEDIUM-CARBON STEEL WITH A WEAR-RESISTANT THERMAL SPRAY COATING

The fatigue behaviour of a Ni-Cr-base powder flame-spray coating on a 0.4% C steel is investigated. Fatigue tests were carried out using mil d hour-glass profile specimens. Cracks were detected and measured usin g plastic replicas and an image analysis system. Coated specimens show ed a slightly lower fatigue endurance than plain specimens under torsi on loading, while the opposite was observed for push-pull loading. Mic rocracks in coated specimens invariably form at pores. Contrary to the usual case of stage I shear growth for a plain 0.4% C steel in tensio n or torsion loading, the coated specimens show initial crack growth f rom pores along directions perpendicular to the maximum tensile stress . The crucial behaviour of short cracks, and their growth rates, relat ive to the thickness of the coating, are discussed in some detail.