Plasma-Assisted Nitriding of Aluminum-Alloy Parts

Plasma-assisted nitriding of aluminum alloys promises to become a suitable alternative to variants of thermochemical treatment used for raising the wear resistance of these materials especially from the standpoint of the environmental effect. Process performed in vacuum with the use of nitrogen and an argon-hydrogen mixture as the process gas provides for the formation of a layer of compounds (aluminum nitrides) with enhanced hardness on various aluminum alloys. However, until the present the process has been used only at the laboratory scale. No data can be found in the literature, for example, on the effect of the batch size on the uniformity of the thickness of the nitrided layer or on the wear resistance of parts from aluminum alloys. The present work is a study of plasma-assisted nitriding of commercially produced parts (pistons) fabricated from an aluminum alloy. The parameters of the process are chosen after the determining optimum conditions for preliminary sputtering treatment. Special attention is devoted to provision of temperature uniformity within a batch and the possibility of improving the surface quality by increasing the thickness of the compound layer. The topography of the surface and the thickness, chemical composition, and hardness of the compound layer are studied by the methods of scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), metallography, and measurement of ultramicrohardness. The performance characteristics of untreated, nitrided, and anodized pistons are evaluated with the help of vibration wear tests with a rider fabricated from a standard material. The results of the tests show that the wear resistance of some nitrided pistons is comparable to that of anodized parts.