Effect of size of coating defect on fatigue properties of steel thermally sprayed with Ni-based self-fluxing alloy

To clarify the effects of fusing on fatigue properties of thermally sprayed steel, four types of sprayed specimen with different heating length in the fusing process and with different coating thickness were prepared. After polishing the specimen surface, fatigue tests were carried out with a rotational bending machine at room temperature. Results showed that the size and amount of the coating defects were determined by fusing length; the longer the heating time, the larger the defect size and amount. Defect size and amount were also affected by the thickness of coatings. A thinner coating included smaller and less defects as compared to thicker coating. Since fatigue cracks originate from coating defects and propagate through them, the larger the size and amount of defects, the lower the fatigue strength. Therefore, the fatigue strength of the longer heating-length specimen was lower than that of the specimen treated with shorter length fusing. The fatigue strength of the specimen with thin coating was greater than that of the thick coating specimen. This was due to the difference of size and amount of defects in the coating. Consequently, the fatigue strength of a coated specimen is determined by the size and amount of defects in the coating. This means that it is very important to limit the size and amount of coating defects to achieve high quality sprayed coating.