An highly dense HfB2–SiC composite, containing 5.8v/o Si3N4 as sintering aid, was successfully fabricated by hot-pressing. The resistance to oxidation was tested by applying several heat treatments in laboratory air, monitoring not only the weight change but also the modifications to the microstructure induced by the oxidation. The thermogravimetric data for an isothermal test at 1450 ◦C agreed with a paralinear law over 10 h of exposure. Afterwards, linear kinetics seemed to prevail. The main oxidation products were HfO2 and a borosilicate glass. The release of volatile oxidation products had an appreciable impact for the thermal treatments below 1350 ◦C. Above 1400 ◦C, the presence of the SiC particles markedly improved the oxidation resistance of the composite, because the oxidation of SiC provides significant amount of silica, forming a thick protective borosilicate glass. This glassy coating plugs pores, covers the sample surfaces and efficaciously seals grain boundaries depleted of the original secondary phases.
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