Effect of metalloid silicon addition on densification, microstructure and thermal–physical properties of Al/diamond composites consolidated by spark plasma sintering

• Mechanism of Si addition facilitates formation of molten metal is newly explained.
• Si addition improves densification and that causes precipitation of eutectic-Si.
• Eutectic-Si weakens negative effect of thermal stress on interface at cooling stage.
• Improved interface and eutectic-Si are together responsible for optimized properties.
• Advanced Al/diamond composites can be fabricated by SPS with Si addition.

Aluminum matrix composites reinforced with diamond particles were consolidated by spark plasma sintering. Metalloid silicon was added (Al–Si/diamond composites) to investigate the effect. Silicon addition promotes the formation of molten metal during the sintering to facilitate the densification and enhance the interfacial bonding. Meanwhile, the alloying metal matrix precipitates the eutectic-Si on the diamond surfaces acting as the transitional part to protect the improved interface during the cooling stage. The improved interface and precipitating eutectic-Si phase are mutually responsible for the optimized properties of the composites. In this study, for the Al–Si/diamond composite with 55 vol.% diamonds of 75 μm diameter, the thermal conductivity increased from 200 to 412 Wm−1 K−1, and the coefficient of thermal expansion (CTE) decreased from 8.9 to 7.3 × 10−6 K−1, compared to the Al/diamond composites. Accordingly, the residual plastic strain was 0.10 × 10−3 during the first cycle and rapidly became negligible during the second. Additionally, the measured CTE of the Al–Si/diamond composites was more conform to the Schapery’s model.