Zirconium diboride (ZrB2) as a vital member of ultra-high temperature ceramics (UHTCs) classes is a subject of interest due to its high melting point, high hardness, superior strength, good electric and thermal conductivities, resistance to corrosion, as well as superb antioxidant ablation performance[1–4]. All these features make ZrB2 and ZrB2-based ceramics attractive for ultra-high temperature applications, such as sharp nosecones, leading edges and propulsion components, thermal protection system for hypersonic aerospace vehicles and advanced reusable atmospheric reentry vehicles [5–8].
As with other nonoxide ceramics, ZrB2 would be easy to be oxidized when exposed to air under an elevated temperature, the addition of second phase SiC particles would enhance the oxidation resistance by decreasing oxidation rate of ZrB2 [2, 9, 10]. Generally, ZrB2-SiC ceramics are usually fabricated by hot pressing or spark plasma sintering , which limit the fabrication of samples with relatively simple geometrical and moderate size, and requires costly and time-consuming electrical machining to obtain complex-shaped components. Recently, a great number of colloidal processing have been used to prepare near-net shaped components, such as tape casting , slip casting , centrifugal casting  and gel casting . Among the colloidal processing, the most suitable process is gel casting that enables the materials to achieve higher microstructural homogeneity and high strength in green and sintered parts with complex shapes. Gel casting is a generic method based on a combination of traditional ceramics forming and polymer chemistry, including the homogeneous dispersion of ceramic powders into dissolved colloidal solutions and then casting into a sealed mould with in situ polymerization to form a macromolecular gel network to hold the ceramic particles together [16, 17]. However, the wet gelcasted green bodies were prone to shrink or crack owing to the non-uniform contraction in the process of removing the moisture . He et al.  pointed out that a high solid loading in the suspensions could reduce shrinkage during drying and sintering processes. However, it must be noted that when the slurry viscosity increased as a consequence of the increase in the solid loading, the problem with air bubbles trapped in the suspensions become more critical . Vacuum deairing would not work as efficiently as in the case of low viscosity slurries and it was essential to combine the mechanical stirring process with vacuum deairing to minimize the air bubbles .
Centrifugal casting process is one of the colloidal processings that has been used for many years of the fabrication of the ceramics . A well-dispersed suspension was poured into a mould and then particles packing and casting was motivated by the centrifugal forces. The centrifugal field on the slurries could act as a purifying factor to eliminate air bubbled trapped in the suspensions and disrupt the larger heavy inclusions. Centrifugal casting with a high centrifugal acceleration was an effective way to eliminate significant mass segregation of particles for processing highly concentrated and multicomponent slurries. However, the number of the heterogeneities among particles was increased using centrifugal casting process, resulting in slightly detracting from the packing density and final properties of the ceramics . In addition, the centrifugal accelerations used in centrifugal casting process to fabricate advanced ceramic materials were usually in the order of 2000–5000 g, which was reasonable to use higher spinning speeds instead of longer radius of centrifugation to achieve the specified centrifugal accelerations due to the equation of α = ωr2. Besides, the time required for the centrifugal processing of submicrometer-sized concentrated slurries was as long as 1–2 hours , which was too long to keep the slurry under such severe centrifugation condition due to high energy consumption and high degree of machinery depreciation.
In this paper, we present a novel process called “centrifugal gel casting” (CGC), which combined the advantages of centrifugal casting and gel casting processes, enhancing particle packing and eliminating bubbles trapped in the green body via high centrifugal forces and in situ polymerization of the three-dimensional gel networks to cover particles. In the combined process, neither high centrifugal accelerations are required nor a long time for cast formation. The dispersion behaviour and rheology properties of ZrB2-SiC suspensions were studied in detail, and the centrifugal parameters on the microstructure and relative density distribution of green bodies were analysed. In addition, the microstructure and mechanical properties of pressureless sintered parts were also characterized.