Wear behaviour of aluminium matrix composites manufactured by diecasting

The possibility to substitute the traditional cast iron for manufacturing cylinder liners in automotive applications with different materials, which can afford a significant decrease of the weight of nowadays engines, is particularly interesting. To this end both Al matrix composites and hypereutectic Al-Si alloys are possible candidates, in particular having in mind the need for wear resistance. While the wear behaviour of Al matrix composites has been extensively studied, the feasibility of the real workpieces has not been so far sufficiently investigated, in order to achieve a satisfactory stability of properties in large scale production. The present work was focussed on the evaluation of the wear properties of Al-10 vol.% SiCp (composition given in Table 1) with pins obtained from a test-scale production diecast components (Figs 1-2), aimed at assessing their potential tribological resistance. The tribological tests were carried out by means of dry sliding wear tests with the pin-on-cylinder configuration, against a grey cast iron cylinder (microstructure shown in Figs 4-5). Subsequently pins obtained from spray formed Al-25 wt.% Si hypereutectic billets (composition given in table 2, microstructure shown in Fig. 3) were tested against the same cylinder material in order to make a comparison between the two couples Al-SiCp/grey cast iron and Al-25wt.% Si grey cast iron. Preliminary microsiructural and microhardness evaluations were performed on the materials, followed by accurate pin manufacturing, as well as accurate polishing of the grey cast iron counterpart were the sliding tracks were to be performed. In all cases the surfaces were given a final polishing just prior to the tribological test. Both the original and final weight of the pins were recorded (Table 3) in order to evaluate weight variations, after 10 km dry sliding against the cast iron cylinder, carried out at different speeds, that is 0.55 m/s, 1.58 m/s and 4.23 m/s (Fig. 6). Moreover the bulk temperatures developed in the pins, the friction coefficient, the temperature and humidity of the air in the laboratory were continuously monitored and recorded. Finally multiple profile measurements, performed transversally on each wear track on the surface of the cylinder, were acquired in order to evaluate the volume of the worn-out material and compute the corresponding wear rates (Fig. 7). On the basis of the weight variations of the pins a corresponding evaluation was carried out. The identification of the wear mechanisms involved in the single runs was carried out on the basis of the examination of the debris stuck onto the surface of the corresponding pins, and on the information given by the related plots showing temperature / friction coefficient vs. sliding distance. For the Al-10 vol. % SiCp vs. grey cast iron tests at sliding speeds of 0.55 m/s and 1.58 m/s a tribo-oxidative mechanism acting on the grey cast iron counterpart is evident, while at higher sliding speed the wear mechanism is mostly abrasive (Figs 8-10). The effect of the hypereutectic Al-25 wt.% Si pins on the grey cast iron counterpart appear to be less severe. In this case the third body partially adhering to the pin sets up an instable layer which affects both the friction coefficient and the temperature causing remarkable fluctuations.