Oxidation of multi-principal component diborides at 2300 °C under ultrafast heating

Multi-principal component diboride (MEB) ceramics are promising candidates for ultrahigh-temperature thermal protection systems. However, their intrinsic oxidation behavior above 2000 °C remains poorly understood because conventional testing methods have limitations. Here, an ultrafast heating oxidation test (UHOT) with a heating rate of 100 °C/s was applied to investigate the oxidation behavior of (Zr_1/4Hf_1/4Ta_1/4Ti_1/4)B₂, (Zr_1/4Hf_1/4Ta_1/4Nb_1/4)B₂, (Zr_1/3Hf_1/3Ta_1/3)B₂, and monolithic ZrB₂ ceramics at 2300 °C under cyclic oxidation conditions. After five 1-min cyclic exposures at this temperature, totaling 5 min, MEB ceramics form multi-layered oxide scales, while ZrB₂ ceramic develops only a porous and cracked single-layer scale. Among them, the Ti-containing composition exhibits the thinnest scale, the densest outer layer, the least mass loss, and intact scale integrity. Beyond the thermodynamic contribution of configurational entropy, this superior performance mainly arises from the dual role of the functional element Ti. First, it forms a liquid Ti-containing oxide phase that densifies the outer (Zr,Hf)O₂-based layer derived from the structural elements Zr and Hf, enabling the outer layer to act as an effective oxygen diffusion barrier. Second, it promotes the formation of a porous needle-like Ta₂O₅ network in the inner layer during cooling, which can relieve thermal stress and prevent interfacial separation during cyclic heating and cooling. This work establishes a rapid-heating evaluation protocol and shows that oxidation resistance in these materials is governed by elemental functionality rather than entropy alone, offering a design principle for both equimolar and nonequimolar multiprincipal component nonoxide ceramics for reusable thermal protection applications.