Fully dense MoAlB bulk samples are not easy to obtain under hot pressing conditions, possibly due to the existence of small pores caused by the evaporation of MoO3 or B2O3 formed with the absorbed O at sintering temperatures. For example, a relative density of 94 % was achieved for the MoAlB samples by hot pressing of MoB and Al powders at 1200°C with 39 MPa for 5.8 h in vacuum [3]. In the present study, a relative density of about 93% for MoAlB prepared by hot pressing of a mixture of Mo, Al and B at 1200°C with 25 MPa for 1 h in Ar. No full density obtained in the SiC/MoAlB composites should be not only ascribed to the above reason, but also to the absence of sintering reactions between SiC and MoAlB phases. This has been confirmed by the SEM and TEM examinations (Figs. 2 and 3). The densification of composites without sintering reactions has always been a major challenge. For example, relative densities of 94.4% and 95% were obtained, respectively, in monolithic CrB2 and 5 wt. % MoSi2/CrB2 composite after hot pressing at 1400 °C under 35 MPa for 2 h [16]. To further increase the density of SiC/MoAlB composites, hot isostatic pressing should be considered. It is reasonable to believe if the density of SiC/MoAlB composite was improved, the oxidation resistance would be further enhanced.
Upon high temperature oxidation, it is reasonable to assume the following oxidation reactions are:
2MoAlB (s) + 6O2 (g) = Al2O3 (s) + B2O3 (g) + 2MoO3 (g) (1)
SiC (s) + 2O2 (g) = SiO2 (s) + CO2 (g) (2)
For reaction (1), the sequence of phase formation at temperatures below 1400°C is Al2O3 > B2O3 > MoO3 [17]. Before the formation of a dense and continuous Al2O3 scale, B2O3 and MoO3 evaporate, and thus cause the weight loss upon oxidation at temperatures above 1200°C [18]. Once the dense and continuous Al2O3 scale formed, it acted as a barrier to prevent the inward diffusion of O and the outward diffusion of B and Mo, decreasing the evaporation rate of MoO3 and B2O3. Therefore, the weight gain was predominant in the MoAlB material during oxidation at high temperatures.
For the oxidation of SiC/MoAlB composites, both reactions (1) and (2) occur. The reaction (2) induces a fluid glassy phase of SiO2 at above 1200°C [19]. This fluid phase fills some pores and accelerates the formation of dense and continuous oxide scale, decreasing the oxidation rate and leading to the thinner oxide scale on the SiC/MoAlB as compared to the pure MoAlB material.
A cross-sectional back-scattered SEM micrograph presents the microstructure of scale. It should be noted that the formed SiO2 was just over a SiC particle (Fig. 8a), confirmed by the EDS mapping (Fig. 8b). This feature suggests that the SiC particles on the surface is gradually oxidized to SiO2. The small and round pore can be found in the SiO2 particle, which should be resulted from the evaporation of gas phases of CO2 according to Eq. (2) at 1300°C. In addition, it can be concluded that the larger SiC particles induce both the larger SiO2 particles and pores formed at high oxidation temperatures on the basis of the microstructures shown in Figs. 7 and 8. It is reasonable to believe that fine SiC particles in the MoAlB will further improve the oxidation resistance of composites due to the fact that the fine reinforcement leads to the rapid formation of SiO2 which seal the panels in the Al2O3 scale to retard the evaporation of B2O3 and MoO3 in the initial oxidation stage.
Based on the above results, a proposed oxidation resistance mechanism for SiC/MoAlB was present in Fig. 9. Before a continuous Al2O3 and SiO2 scale formation, the evaporation of MoO3, B2O3, and CO2 at above 1200°C induces the weight loss at early oxidation stage. However, the fluid glass of SiO2 formed at above 1200°C can fill phase boundaries and small pores to seal the channels for the inward diffusion of oxygen. Hence, the oxidation resistance of SiC/MoAlB is better than that of MoAlB even at early oxidation stage. Once the continuous and dense scale forms, the inward diffusion rate of oxygen is retarded, and a low oxygen partial pressure under the scale generates. As a result, the oxidation of Mo and B followed by evaporation of MoO3 and B2O3 is suppressed leading to weight gain. In addition, the fluid glass of SiO2 can also heal microcracks and small pores in the scale, and further increase the bonding strength between the oxide scale and the matrix. This explains the reason why SiC/MoAlB has thinner dense oxide scales than MoAlB after oxidation at 1200°C and 1300°C.