Wetting of boron nitride by molten Al-Si alloys

An improved sessile drop method which prevents aluminum for oxidizing, was used order to obtain accurate values for the contact angles between boron nitride and aluminum or aluminum-silicon alloys. The experimental temperature and atmosphere were 1373 K and an purified He+3%H2 mixed gas respectively. The contact angle of each system changed with time through the four phases (I, II, III, IV) similar to other typical ceramic/metal systems. As the quantity of Si increases, the original contact angle (Phase II) decreases. When more than about 11 mass%Si is added, the original contact angle is smaller than 90°. The equilibrium contact angle (Phase IV) increases as the quantity of Si increases. However, it has become clear that the increase in the observed equilibrium contact angle was apparently caused by the crystallization of Si at the solid/liquid/vapor interface and that the true equilibrium contact angle is equal to or around 0 degrees. Therefore, it is concluded from the viewpoint of wetting that the BN/Al-Si system is a suitable combination for fabricating a composite material. Furthermore, the results indicate, when a sessile drop method is used to measure contact angles, the evaporation of the liquid and/or the change in the composition of the liquid can apparently change the contact angles under the conditions where the true contact angles are constant.