Plasma Process to Harden the Surface of Aluminum and Alloys

Aluminum and its alloys are valued in structural applications for their excellent strength to weight ratio and several other desirable properties. However, their tribological properties in sliding friction are poor due to the softness of the bulk combined with the fragility of the native oxide. A method to harden the surface of small aluminum components has been developed. The process consists in immersing the object to be treated in a pulsed low pressure plasma using oxygen as working gas, and applying to it a high negative voltage (typically 30 kV). This drives the oxygen ions of the plasma into the piece to a depth of tens of nanometers (∼10^-6 inch). This results in the formation of a layer of extremely fine-grained oxide precipitates in an aluminum matrix. The thickness of the layer is of the order of 0.1 μm (4 × 10^-6 inch) and the precipitate size much less. The optimum results are obtained with a layer composition of about 50% oxide and 50% metal. The hardness of the treated layer, as measured by nanoindentation, is increased several fold up to values of 3 - 5 GPa (400 - 700 kpsi), while retaining good elasticity and ductilily. Nanoscratch test results show reductions in the scratch depths and the friction coefficients by nearly the same factors.