Ion nitrided AISI H13 tool steel -: Part 2 -: High temperature performance under sliding wear conditions

In the present work, study on a systematic variation of the processing parameters such as gas mixture, time of nitriding and current density has been carried out in order to determine how these will influence the sliding wear behaviour of plasma nitrided layers on AISI H13. Optical profilometry and scanning electron microscopy were used to characterise the worn sample and to determine the wear mechanism. The results revealed a strong influence of the current density on the formation of different diffusion and nitrided layers, which explained the wear behaviour of the studied samples, especially for those tested at room temperature. It was found that the wear volume of the nitrided samples without the existence of a compound layer was similar to 200 times smaller than the wear volume of the untreated samples and nearly seven times smaller than the wear volume of nitrided samples when the compound layer was present. When the tests were carried out at 300 degrees C, the wear behaviour was worse for the nitrided samples than for the untreated ones, owing to the formation of higher amount of debris as a consequence of the breakdown of the compound layer and the subsequent removal of the enriched Cr-N from the ferritic matrix, generating a severe abrasion wear mechanism. All the samples tested at 600 degrees C presented a similar tribological behaviour irrespective of the processing parameters values chosen for nitriding, as a consequence of the formation of a uniform and 'lubricious' iron oxide layer which, however, was insufficient to protect the steel against wear at this temperature to the levels which were obtained for the samples tested at room temperature.