The effect of discontinuous fibre orientation on the wear properties of composites

Fibre-reinforced aluminium matrix composites possess good mechanical and tribological properties and are applied successfully in the aerospace and automobile industry. Their wear properties and mechanisms have been investigated for several decades. One important aspect of these is the effect of ®bre orientation on the wear properties of composites. For continuous-®bre-reinforced metal matrix composites, it is found in general that the composites with ®bres normal to the wear surface exhibit superior wear resistance to those with ®bres parallel to the wear surface [1±3]. However, different conclusions have been obtained by different studies on the effects of discontinuous ®bre orientation on the wear resistance of composites. Arikan and Murphy [4] concluded that the a-Al2O3y(AlyZn) composites with normal-orientation ®bres exhibited superior wear resistance to those with parallelorientation ®bres. Asanuma and Hirohashi [5] obtain the same conclusion by the investigation on the wear behaviour of SiC whiskers (SiCw)yAl composites under lubricated sliding conditions. However, Pan and Fine [6] found that the wear rate of SiCwyAl alloy 2124 composites is lower when the ®bres are parallel rather than normal to the wear surface. Wang and Rack [7] concluded that the initial wear rates of SiCwyAl alloy 7091 composites depend strongly upon whisker orientation; the highest wear rates were observed on normal-orientation whisker composites. The steady-state wear rates of the composites were, however, generally independent of reinforcement orientation, with the exception of wear at 3:6 m sy1 where the parallel-orientation composites was superior. In the present letter, the effect of discontinuous ®bre orientation on the wear properties of composites was studied by a number of tests and a statistical method. Mullite (3Al2O3 . 2SiO2)-®bre reinforced Al±Si composites were fabricated by squeeze casting. The ®bre, with a mean diameter of 3.0±8.0 im and a mean length of 40±100 im, was obtained from the Luoyang Institute of Refractories Research Ministry (People's Republic of China). The Al±Si alloys used were Al±12 wt % Si±Cu±Mg±Ni. The composites was treated by T6 treatment after casting. The ®bres possess a planar-random distribution in the composites, as shown in Fig. 1. Wear tests under lubricated sliding conditions were carried out on a block-on-ring type of wear apparatus at room temperature. The sliding ring (width, 10 mm; outer diameter, 50 mm) was steel 45 with a surface Rockwell C hardness of 56 HRC. The stationary block was a metal matrix composite (MMC) specimen, 10 mm wide, 10 mm high and 17 mm long. The rectangular faces of the MMC specimens were placed in contact with the sliders. The lubricant used was a grade 20 hydraulic oil, which was continuously sprayed onto the outside surface of the sliding ring during the experiment by a spray gun. The nomal load, sliding velocity and distance were 60 N, 1.5 m sy1 and 3768 m, respectively. Composites with ®bres parallel (face A in Fig. 1) and normal (face B in Fig. 1) were tested 44 times. The detailed data have been given in [8]. Using the Shapiro±Wilk method [9], which is a non-parametric method for data-normality tests, it was found that wear volumes of each kind of ®bre orientation composites agree with the normal