Unmasking the anomalous rapid oxidation of refractory TiB2 at low temperatures

As a refractory transition-metal diboride, monolithic TiB2 oxidizes at temperatures below 400 degrees C. Here, we performed detailed microstructural investigations to study the low-temperature oxidation mechanism of monolithic TiB2. An anomalous rapid oxidation behavior is observed at similar to 500 degrees C, where the oxidation rate is much higher than that at a higher temperature of 650 degrees C. The anomalous rapid oxidation behavior originates from an unreported bi-layer oxide scale consisting of outer homogeneous B2O3/TiO2 mixed layer with a suprananometre-sized dual-phase amorphous-crystal microstructure and inner unstable Ti-B-O amorphous layer. By contrast, the oxide scale changes from homogeneous mixed microstructure to a laminated configuration at 650 degrees C, restoring the superior oxidation resistance. Our experimental results indicate that it is the configuration of the oxide scale that determines primarily the oxidation behavior of TiB2, which has been seldom envisaged in previous studies.