Tribological Response of SPS-Prepared Mg–Ti–Al2O3 Nanofibre–GNP Composites with a Graphite Surface Layer

Magnesium-based hybrid composites reinforced with Ti, graphene nanoplatelets (GNPs), and short Al2O3 nanofibres were investigated with respect to their microstructure, hardness and dry sliding tribological behaviour. Four materials were studied: M0 (100Mg), M1 (70Mg–30Ti), M2 (69Mg–30Ti–1GNPs) and M3 (63Mg–30Ti–6Al2O3nf–1GNPs). Prior to tribological testing, a temporary graphite layer was mechanically applied to the polished surfaces. Reciprocating sliding tests were performed against a 6 mm ZrO2 ball at a load of 6 N, a frequency of 1 Hz and a stroke length of 5 mm for 1000 cycles. The sequential incorporation of Ti, GNPs, and Al2O3 nanofibres increased the hardness from approximately 58 HV0.3 for M0 to 111 HV0.3 for M3. The unreinforced Mg exhibited unstable friction and severe wear, whereas the reinforced composites maintained a low and stable coefficient of friction of approximately 0.14–0.16. The specific wear rate decreased from 1.17 × 10−3 mm3 N−1 m−1 for M0 to 9.0 × 10−6 mm3 N−1 m−1 for M3. SEM/EDS and confocal microscopy confirmed a transition from severe abrasive–adhesive wear and delamination to mild wear. Under the investigated conditions, the combined presence of Ti, GNPs, and Al2O3 nanofibres in M3 provided the highest hardness, the most stable low-friction response, and the lowest wear among the investigated materials.