Grain growth behavior and properties of high-entropy pseudobrookite (Mg,Co,Ni,Zn)Ti₂O₅ ceramics

It is well known that the grain size of high-entropy ceramics is quite small owing to the sluggish diffusion effect. However, abnormal grain growth often occurs in high-entropy pseudobrookite ceramics, ultimately resulting in the formation of many abnormally grown grains with a grain size as large as 50 μm. To study this phenomenon, the grain growth behavior of high-entropy pseudobrookite ceramics was systematically investigated in this paper. The results demonstrate that the starting material powders first react with each other to form a high-entropy intermediate phase and calcined TiO₂ powders (TiO₂-1100 °C), and then as the sintering temperature increases, the formed high-entropy intermediate phase further reacts with TiO₂-1100 °C to form high-entropy pseudobrookite ceramics. Thus, in this system, in addition to the sluggish diffusion effect, the grain sizes of the high-entropy intermediate phase and TiO₂-1100 °C also affect the morphology of high-entropy pseudobrookite. Compared to nanosized TiO₂, micron-sized TiO₂ has a lower sintering activity. Therefore, the high-entropy intermediate phases (Mg,Co,Ni,Zn)TiO₃ and TiO₂-1100 °C prepared with micron-sized starting materials exhibit lower grain sizes, finally resulting in the formation of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with small grain sizes. Moreover, nano-indentation and thermal conductivity tests were carried out on high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with different morphologies. The results show that the hardness of high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ increases from 6.05 to 9.95 GPa as the grain size increases, whereas the thermal conductivity decreases from 2.091±0.006 to 1.583±0.006 W·m⁻¹·K⁻¹. All these results indicate that high-entropy (Mg,Co,Ni,Zn)Ti₂O₅ with a small grain size is a potential material for thermal protection.