Friction Properties of Chromium Base Hardfacing Materials in High Temperature Liquid Sodium

The frictional properties of three kinds of coating materials, Cr3C2+Cr2O3, Cr3C2 and Cr2O3, on Type 304 stainless steel were tested for sliding parts of fast breeder reactor components. The temperature dependence of the kinetic friction coefficient and the continuity of lubricants were tested for each coating in sodium with 5 mass ppm of oxygen at temperatures of 473-823 K.The kinetic friction coefficients of all these materials were about 0.4 when the sodium temperature was below 683 K and were reduced to 0.2 at higher sodium temperatures. This was because NaCrO2 was formed on the coating surfaces and worked as a good lubricant. Its lubricating property was best in the Cr3C2+Cr2O3 coating, followed by the Cr3C2 and Cr2O3 coatings in order. In the Cr3C2+Cr2O3 coating, a NaCrO2 was formed from Cr2O3 in the interior of the coated layer, and was supplied on the sliding surface continuously. In Cr3C2 coating, very thin NaCrO2 layer was formed, but in Cr2O3 a very thick and brittle NaCrO2 layer was formed and was easily broken away from the surface during sliding.