Surface carburization of aluminum alloys by excimer laser

The excimer laser cementation process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. It would be interesting to use aluminum alloys in the automotive industry widely because of their low density, corrosion resistance and good workability. The motor weight can be reduced by replacing usual materials such as iron-steel by light alloys treated to increase their wear resistance. Ceramic materials generally exhibit great strength, resistance to wear and oxidation. The use of laser beams allows surface treatment to be located at the parts strongly exposed to wear and friction. The surface undergoes a transformation leading to an increase in hardness without changing the dimensions of the piece, thus avoiding post machining after treatment. The laser process is especially suitable for environment protection as there is no pollution by chemical solvent or emanation. An excimer laser beam is focused onto the alloy surface in a cell containing I bar methane or propylene gas. A vapor plasma expands from the surface and a shockwave dissociates and ionizes the ambient gas. It is assumed that nitrogen or/and carbon from the plasma in contact with the surface penetrates into the depth. Thus, it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The carbon concentration profiles are determined from Rutherford backscattering spectroscopy (RBS) and scanning electron microscopy (SEM). Crystalline quality is evidenced by grazing incidence X-ray diffraction (GIXD) technique. Transmission electron microscopy (TEM) gives the in-depth microstructure. The polycrystalline cemented layer obtained is several micrometers thick and composed of a pure composition (columnar microstructure) top layer (200-500-nm thick) standing on a diffusion layer (grains). Fretting test measurements exhibit an improvement of the surface mechanical behavior for some experimental conditions. (C) 2001 Elsevier Science B.V. All rights reserved.