SURFACE MODIFICATION BY PLASMA ELECTROLYTIC OXIDATION (PEO) OF THE AA 2618/20 % Al2O3p COMPOSITE: TRIBOLOGICAL BEHAVIOUR IN DRY SLIDING CONDITIONS

Plasma Electrolytic Oxidation (PEO) is a surface modification treatment recently developed for light alloys (Al, Ti or Mg alloys), which allows the formation Of an oxidised layer with low porosity and excellent adhesion. The PEO treatment is a conversion treatment based on anodic oxidation at low temperature (T<60 degrees C) in a dilute aqueous electrolyte with the application of a low-frequency (typically 50 Hz) AC electric field. In these conditions, micro-arc discharge events take place on the growing oxide, thus locally increasing the temperature and contributing to the densification the oxidised layer which, due to rapid solidification after discharge, may consist of both amorphous and nanocrystalline phases. The structure of PEO-treated surfaces typically consist of different layers: mainly an external layer (the so-called "technological layer", about 20-30% of the total thickness) with high roughness and low hardness, and a hard and dense inner layer ("functional layer"). Basically, the PEO process is able to produce a thick (up to about 100 mu m), compact and well-adhered conversion layer with uniform thickness, which can act as a thermal barrier as well as increase the wear and corrosion resistance of engineering components. Among the advantages of PEO, also the low environmental impact of the dilute electrolyte baths is worth mentioning. Therefore, PEO might be usefully applied not only to light alloys but also to metal matrix composites based on light alloys, so as to expand the field of application for these materials. In particular, aluminium matrix composites are widely used in the automotive field for components such as pistons or brake disks, where tribological properties have a very important influence on the in-service behaviour. For these reasons, in the present work the effect of the PEO-treatment on the tribological behaviour of the aluminium-matrix composite AA2618/20%vol.Al2O3p (composition of the matrix in Table 1; microstructure of the untreated composite in Fig. 1) has been evaluated by dry sliding tests (slider on cylinder contact geometry). The PEO-treated slider was tested against a rotating cylinder consisting of induction hardened UNI C55 (AISI 1055) steel (hardened layer of about 400 mu m, 650 HV1, Ra=0.15 mu m). The applied loads ranged from 10 to 50 N, with sliding speeds of 0.6 and 1.8 m/s and a total sliding distance Of 10 km. During the tests, the friction force and total wear (i.e. cumulative wear of both fixed slider and rotating cylinder) were continuously measured by means of a load cell and a linear variable displacement transducer (LVDT), respectively, and the data were recorded as a function of sliding distance. After the tests, separated values of the wear scar depth on both slider and countermaterial, were evaluated by means of a stylus profilometer. Worn surfaces and wear debris were observed and analysed by SEM, in order to identify the dominant wear mechanisms. In the present work, the PEO treatment was carried out on AA2618/20%vol. Al2O3p in an industrial plant (AC power supply with 50 Hz frequency amplitude modulation, constant current density of 20 A dm(-2), in Silicate electrolyte at (45 degrees C). The average thickness of the oxide layer was 30 5 mu m and it was identified by XRD as a mixture of alpha-Al2O3 and gamma-Al2O3, with both amorphous phases and compounds due to the interaction of the oxidised layer with the electrolyte bath. The PEO treatment induced a noticeable increase of both surface roughness, from 1.5 to 2.1 mu m, as a consequence of the typical surface morphology of the PEO coating (Fig. 2) and hardness (from 141 HB to 1100 HV0.1) of the composite substrate. Intrinsic hardness of the dense "functional" PEO layer was also evaluated after mechanical abrasion of the external layer (-10 mu m) from load and indentation depth data, while the local stiffness was determined from the unloading response, using the conventional Oliver and Pharr technique: a hardness of 8.1 +/- 1.5 GPa and an elastic modulus of 170 +/- 15 GPa were measured. The oxide layer also benefited in terms of adhesion from the presence of reinforcing particles across the oxide/substrate interface (Fig. 3). The alumina particles were present at the interface due to the incorporation of the particles themselves in the layer, during the conversion of the composite into oxide. The results of the dry sliding tests showed that the untreated composite underwent a mild tribo-oxidative wear only at the lowest applied load and sliding speed (10 N - 0.6 m/s) (Fig. 4, 5, 6a), whereas, with increasing applied load and sliding speed, a transition to severe delamination wear was observed: Fig. 4, 6b, 7). This wear transition is probably due to a decrease of the plastic flow resistance of the matrix when a critical temperature is attained, as well as to fragmentation and avulsion of the reinforcing particles. The PEO treatment, thanks to the hardness increase, the high adhesion to the substrate and the thermal barrier effect, significantly improved the tribological behaviour of the composite by moving the wear transition towards higher values of both applied load and sliding speed (Fig. 8-10).