No abstract

The high passivation capacity of austenitic stainless steels results in excellent corrosion resistance in a wide variety of corrosion media. However, the tribological properties of these steels are insufficient due to their low hardness and high susceptibility to adhesive wear. To improve the wear behavior of austenitic steels while maintaining corrosion resistance, thermochemical processes such as nitriding or nitrocarburizing can be used. Such processes can be carried out as single procedures or in combination with coating techniques. Herein, the results of a new combination treatment consisting of the electron beam cladding (EBC) of a Co‐based alloy with subsequent gas nitrocarburizing (GNC) are presented. The effects of the beam deflection technique on the microstructure and phase formation of the cladded wear protection layer, the resulting hardness as well as the dilution ratio and the bonding of this layer to the base material are demonstrated. The subsequent gas nitrocaburizing leads to the formation of an ≈10 μm‐thick surface layer enriched with nitrogen and carbon. The significant improvement in wear resistance compared with the untreated base material is demonstrated by abrasive and adhesive‐abrasive wear tests. Corrosion resistance is investigated by recording current density—potential curves and long‐term tests in corrosive media.

In principle, the plasma nitriding of Al based substrates is a well-known process, though it remains extremely challenging from both the technological point of view and the aspect of stress loading conditions. In order to improve the latter, a duplex treatment consisting of plasma nitriding and subsequent surface remelting using electron beam technology was employed. The focus of this paper (part I) was on the characterisation of the initial microstructure after plasma nitriding. This should create the basis for a better understanding of the processes taking place or changes in the subsequent duplex treatment. This was done with the help of high-resolution imaging and analysis tools in the scanning and transmission electron microscope as well as XPS analyses. Special attention was paid to the nitriding mechanism at the interface as a function of the local microstructural constituents of the hypereutectic Al alloy substrate (Al solid solution, primary silicon, and intermetallic phases). While the main part of the nitride layer formed consisted of AlN and small fractions of pure Al in the diffusion paths, other nitrides and oxides could also be detected in the area of the interface.

Plasma nitriding of aluminum alloys is a suitable method for improving wear resistance because of the hard ceramic AlN layer formed. However, the surface's load‐bearing behavior is greatly limited by the low hardness of the Al base material. New investigations regarding improved load support of the thin AlN layer examine the treatment sequence of nitriding and subsequent EB remelting. Because of its broad range of beneficial alloying elements (Si, Fe, Cu, Mg), a hypereutectic Al–Si alloy (DISPAL® S232) − made by spray forming − was used as the base material. The electron beam remelting process is carried out on samples with a nitride layer thickness of approx. 3 μm. As a result of the newly formed phases, grain refinement, and oversaturation of the aluminum solid solution, the surface hardness beneath the nitride layer can be increased by up to three times compared to that of the initial base material. The estimated enhancement in load support is evaluated by unlubricated wear tests using a pin‐on‐disc configuration and scratch tests under constant loading conditions. Furthermore, the wear mechanisms are investigated by means of detailed SEM examination of the remelted surface layer.

Nitriding of Al alloys leads to the formation of a thin, hard nitride layer (AlN) on the surface. A subsequent EBR can both eliminate the nitriding-related cavities under the nitride layer and increase the hardness of the substrate without melting or destroying the nitride layer. This paper deals with investigations regarding the influence of the energy/heat input on the microstructure within both the AlN layer and the remelted Al substrate. Of particular interest was the interface between the AlN and the Al substrate, which changed to a transition zone with a depth of approximately 80 µm. A range of high-resolution imaging and analytical tools for both scanning and transmission electron microscopy were used for these investigations. Based on the findings from the microstructural investigations, a schematic model was developed of the processes occurring within the nitride layer and at the interface as a result of remelting.