Wear resistance of aluminium matrix composites reinforced with Al2O3 fibers and graphite
Krzysztof Naplocha, Andrzej Janus Politechnika Wrocławska, Instytut Technologii Maszyn i Automatyzacji, ul. Łukasiewicza 3/5, 50-371 Wrocław
Quarterly No. 1, 2006 pages 3-7
DOI:
keywords: composite, fibers, graphite, wear
abstract Tribological characteristics of aluminium matrix composite reinforced with alumina fibres and graphite have been reported. Investigation deals with preparation of preforms with about 6.5÷15.0 vol.% Al2O3 fibres (Saffil) and 1.5÷12.0 vol.% graphite which were next infiltrated using squeeze casting method. Porous preforms possess suitable permeability, good strength and reveal semi-oriented arrangement of fibres and graphite flakes. The graphite is not wetted by the binder used to join alumina fibres and harden preform, it was possible (Fig. 1) to achieve wetting of alumina fibres only. The composite microstructures exhibit regular arrangement of fibres and rather poor bonding between matrix and graphite ( Fig. 2). Dry wear test were carried out using wear tester (Fig. 3) at room temperature. The effects of applied load and graphite volume fraction on dry sliding friction and wear properties of the hybrid composites were investigated at constant sliding velocity of 0.92 m/s and various loads which in relation to 8 mm diameter of samples corresponds to pressure of P1 = 0.81; P2 = 1.23; P3 = 1.53 MPa. Comparison of wear loss for monolithic Al-Si7 alloy and its composites reveals that alumina fibres considerably improved this property but addition of 5 vol.% of graphite also protected from seizure (Fig. 4). Figure 5 shows the effect of sliding load on volume losses of composite reinforced with 1.9; 2.7; 4.6 vol.% C fibres and 8 vol.% alumina fibres. Under smaller loads composites reveal constant wear loss similar to composites reinforced with carbon flakes (Fig. 6). It is worthy of notice that the composites reinforced with graphite fibres were less sensitive to applied load then both the matrix and the composites with graphite flakes. Wear surface of composite and microstructure near this surface were examined to explain wear behaviour. Crush of graphite fibres and segments of alumina fibres embeded in matrix on wear surface was frequently observed (Fig. 7a). In the case of composite with graphite flakes weak layers of matrix fractured and delaminated above pocket of graphite (Fig. 7b). It produced cavities of graphite which was next mixed with debris and as the wear test continued this holes joined together along direction of counterpart movement (Fig. 8a). Graphite flakes located under the grooves of the wear surface weaken composite and accelerate wear process (Fig. 8b).