Stress-overstress-surface fatigue: new approach in the understanding of the tribology of plasma coated sintered steels

This paper deals with the tribological evaluation of plasma nitrocarburizing of commercially sintered low alloy steels. The samples were tested in dry sliding conditions against a cast iron pin. A model for particle detachment in the coatings is proposed, which takes into account the volume expansion in the coating process as a source of internal stresses in the compound layer. Two different routes were used for plasma nitrocarburizing—a ferritic (560°C) and an austenitic (600°C) treatment—and the tribological behaviour of the coatings was found to reflect the stress condition in the compound layer as a function of its thermal history. The experimental wear data fit well with the model, and the ferritic treatment was found to induce more stress in the coating, as predicted. Comparative results of tribological tests (wear and friction data) for as-received, coated and polished samples reinforce the ‘stress-overstress-fatigue’ sequence.