Microstructure and unlubricated sliding wear behavior of active screen plasma nitrided FeAl40 alloy against WC-6Co under ambient conditions

This study examines the microstructure of the nitride layer formed on FeAl40 alloy during active screen plasma nitriding (ASPN) at 575 °C for 4 h in a nitrogen-hydrogen mixture (30 vol% nitrogen) and its effect on the tribological behavior under severe wear conditions. The resulting tribological properties are evaluated using unlubricated linear-reciprocating sliding ball-on-disc wear tests against WC-6Co ball-counterbodies (ϕ6 mm) applying various normal loads F_N (10 N, 30 N, 50 N) and sliding distances s (100 m, 1000 m) under ambient conditions. During ASPN a complex-structured nitride layer forms, consisting of a compound bilayer and a diffusion layer. Notably, the microstructure of the compound bilayer consists of an outer sublayer with mainly iron nitrides and an inner sublayer with additional aluminum nitride, which differs significantly from the “white” compound layer formed by gas nitriding or conventional plasma nitriding, respectively, on FeAl40 alloy. The diffusion layer contains a very high nitrogen content up to 38 at.% nitrogen forming nitride phases of Fe and Al within the layer, resulting in an increase in surface hardness up to 1550 HV0.05. The presence of the nitride layer causes a change in the wear mechanism of the FeAl40 alloy, where the two-body abrasive wear is dominant. As a result, the nitrided samples exhibit excellent wear resistance with a reduction of the specific wear coefficient (k) with increasing F_N, in contrast to the base material.