Friction characteristics of nitrided layers on AISI 430 ferritic stainless steel obtained by various nitriding processes

Abstract

The influence of plasma, gas and salt-bath nitriding techniques on the friction coefficient of AISI 430 ferritic stainless steel was studied in this paper. Samples were plasma nitrided in 80 % N-2 + 20 % H-2 atmosphere at 450 degrees C and 520 degrees C for 8 h at a pressure of 2 mbar, gas nitrided in NH3 and CO2 atmosphere at 570 degrees C for 13 h and salt-bath nitrided in a cyanide-cyanate salt-bath at 570 degrees C for 1.5 h. Characterisation of nitrided layers on the ferritic stainless steel was carried out by means of microstructure, microhardness, surface roughness and friction coefficient measurements. Friction characteristics of the nitrided layers on the 430 steel were investigated using a ball-on-disc friction-wear tester with a WC-Co ball as the counter-body under dry sliding conditions. Analysis of wear tracks was carried out by scanning electron microscopy. Maximum hardness and maximum case depth were achieved on the plasma nitrided sample at 520 degrees C for 8 h. The plasma and salt-bath nitriding techniques significantly decreased the average surface roughness of the 430 ferritic stainless steel. The friction test results showed that the salt-bath nitrided layer had better friction-reducing ability than the other nitrided layers under dry sliding conditions. Furthermore, the friction characteristic of the plasma nitrided layer at 520 degrees C was better than that of the plasma nitrided layer at 450 degrees C.