When a metal matrix composite is fabricated using a liquid phase, the wettability of the reinforcement by the metal matrix is very important. However, the wetting of ceramics by molten metals is generally poor, and this fact is a major obstacle in this process. This study focused on the combination of BN and aluminum, which has been reported to have a good wetting. The contact angle of this system is 0°, which is very unusual for ceramic/metal systems. However, the mechanism for such an excellent wetting is still unresolved. Based on these situations, the purposes of this study are to obtain an accurate contact angle of aluminum and BN and to elucidate the mechanisms of its good wettability.
99.999% Al and 99.5% h-BN were used for the molten metal and the ceramic substrate, respectively. 98.0% AlB12and 99.0% AlN substrates were also used to investigate the wetting mechanism. The wettability was evaluated by measuring the contact angle using the modified sessile drop method. This method enables us to measure the changes over time from the moment of the contact between the substrate and the droplet and to obtain a clear image even under strong self-emitting conditions at high temperatures using He-Ne laser light sources and high resolution digital cameras in two directions perpendicular to each other. The microstructure of the sample after the wetting measurement was analyzed using SEM, EDS and XRD.
The contact angle was measured in a vacuum of 1 × 10-4Pa at 1100 °C. The contact angle θ on the BN substrate decreases with time, and becomes 0 ° after 800 seconds. The reaction layer at the interface was observed in detail, because the contact angle change with time is considered to be caused by a chemical reaction at the interface. As a result, the formation of the AlB12reaction layer was identified on the AlN reaction layer, and the top surface of the substrate was revealed to be AlB12.