Low hardness and wear resistance of aluminum alloys limit their use in practical application to automotive parts. Formation of hard aluminum nitride (AlN) layer on the surface can prolong the life time of aluminum automotive parts. Plasma nitriding was selected in the present study to form AlN layer on aluminum alloys. This processing is an environmental friendly method because of its low gas and energy consumption. Normal plasma nitriding requires long processing time to successfully form AlN layer. For advancing this surface treatment, refining microstructure and micro-alloying processes are proposed to activate the formation of AlN by plasma nitriding. Bulk Mechanical Alloying is used not only to make grain-size refinement but also to carry out micro alloying with addition of 1 mass%Ti. The formation rate of AlN layer is improved from 4:2 Â 10 À5 mm/s to 20:8 Â 10 À5 mm/s by microstructure refining. In particular, since the co-formed TiN with AlN works as a template in the initial state nitriding, the micro-alloyed aluminum can be nitrided even without pre-sputtering.

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Cast aluminum with 99.99% purity was successfully plasma nitrided using nitrogen and hydrogen mixed gas. Pre-sputtering was carried out prior to plasma nitriding in order to eliminate surface oxide film. Sputtering and nitriding durations were varied from 3.6 to 18 ks and 72 ks to 252 ks, respectively. The samples were nitrided at 823 and 873 K to observe the effect of nitriding temperature. The nitrided samples were analyzed by GIXD, XPS, and TEM. Through GIXD and XPS results, formation of AlN was distinctly detected as a nitrided surface. Crosssectional microstructure of nitrided samples showed that AlN was formed with the thickness up to 3-5 mm. AlN formation is controlled by the diffusion process. The thickness of AlN layer was determined by the nitriding time and temperature. Partial degradation of AlN in the vicinity of the free surface occurred due to its reaction with moisture in air. Partial detachment of AlN layer occurred due to the residual thermal stress, which was caused by the difference in thermal expansion coefficient between AlN and substrate of Al.

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Plasma nitriding for aluminium and aluminium alloys is a promising processing to improve the wear resistance for automotive parts. Normal plasma nitriding is characterised by three processes: presputtering, aluminium nitride nucleation and nitrided layer growth processes. N2+ presputtering is used to effectively eliminate the preexisting oxide films of Al2O3, covering the surface of aluminium matrix. Relatively long incubation time is required for nucleation process to form AlN islands or nodules on its surface. In addition, formation rate becomes very slow owing to low nitrogen diffusion coefficient in the nitrided layer. Physical and chemical modification methods to this normal nitriding processing are proposed to accelerate the formation rate of nitrided layer. Refinement of grain size in the aluminium matrix increases the formation rate by enlarging grain boundary area as a diffusion path. Crystallographic coherency between TiN and AlN reflects on enhancement of nucleation process by coformation of TiN with AlN. Standing on the nitriding design by physical and chemical modification of inner nitriding mechanism, an alternative plasma nitriding is proposed as the third processing for copper bearing aluminium alloys. In this processing, reduction of duration for nucleation and acceleration of growth rate are attained with the aid of the precipitate, Al2Cu. Crystallographic coherency between AlN and Al2Cu is effective to enhance the formation of AlN nodules and islands. Solid state reaction between Al2Cu and penetrating nitrogen is also significant to form the fine interfacial boundaries as a nitrogen diffusion path and to accelerate the formation rate of nitrided layer.