The wetting characteristics of liquid iron on dense MgAlON-based composite ceramics were investigated using X-ray sessile drop technique. The contact angles were measured on substrates of different composites as functions of temperature and varying partial pressures of oxygen. The results with pure argon gas showed that contact angles kept almost constant in the temperature range 1823 to 1873 K. The contact angle was found to show a slight increase with increasing boron nitride (BN) content in MgAlON-BN composites. These are attributed to the higher contact angle between BN substrate and liquid iron drop compared with that obtained for MgAlON substrate. When the CO-CO 2 -Ar gas mixtures were introduced into the system, the contact angle showed an initial quick decrease followed by a slow decrease and then a period of nearly constant contact angle at a given temperature corresponding to the steady-state condition. Even in this case, BN seemed to cause an increase in the equilibrium contact angle. The equilibrium contact angle was found to decrease with increasing temperature. XRD results indicated that the substrate was oxidized and the oxidation products combined with FeO formed by the oxidation of the iron drop to form FeAl 2 O 4 and Mg 1Ϫ x Fe x O. These were likely to form a ternary FeO-Al 2 O 3 -MgO slag or a quaternary slag by combining with B 2 O 3 . An interesting observation is that the iron drop moved away from the original site, probably due to the Marangoni effect.

Thermal diffusivity and heat capacity of MgAlON and MgAlON-BN composites were measured in the temperature range of 25°C to 1300°C using a laser flash technique and a differential scanning calorimeter (DSC) technique, respectively. Based on these measurements, effective thermal conductivity of the composites was calculated using the values measured earlier in the same substance. The experimental effective thermal conductivity results of the composites containing different BN contents were found to show the similar trend, which decreased rapidly with increasing temperature below 900°C followed by a slow decrease with further increasing temperature. This can be explained by the fact that thermal conduction in both components, MgAlON and BN, was dominated by phonons. The phonon mean free path decreased with increasing temperature, limited by the characteristic length between two neighboring atoms. The BN addition has significant influence on the effective thermal conductivity. The effective thermal conductivity of the composites containing BN exhibited a small degree of anisotropy with respect to preferred orientation of the BN phase. The degree of anisotropy of the composites increased with increasing BN content, which is particularly pronounced at the higher BN additions. An equation suitable for the present composites has been derived based on Luo's model. The model was slightly modified in the present article. The predicted values calculated by the model were in good agreement with experimental results.