Investigation And Performance On Centrifugal-gelcasted ZrB₂-SiC Ceramics

ZrB₂-Si C ultra-high temperature ceramics are capable to exploit in extreme heat chemical environments of hypersonic vehicle owing to its high melting point, high hardness, superb anti-oxidation ablative and excellent room and elevated temperature mechanical properties, which make them one of the most promising candidates for ultra-high temperature structural applications, such as sharp nosecones and leading edges. The dry methods are traditionally used to consolidate ZrB₂-Si C ceramics and then densify by hot pressing. However, the size and shape of ceramics are limited by the specification of dies, and needing for costly post-machining process to realize fabrication of complex-shaped ceramic structures. Wet forming could realize near-net shaping to fabricate complex-shaped ceramics, and reduce cost for post-machining processing as well as cycle for preparation. However, much amount of colloidal content could lead to a low relative density of green ceramic bodies, and the green bodies were prone to occur crack initiation phenomenon. Fortunately, the centrifugal-gelcasting process could throw away the remaining holes and excessive colloid of ceramic slurry to inprove the particle packing, thus the ZrB₂-Si C green bodies with high density and low organic content were fabricated successfully, and promote pressureless sintering densification. Therefore, it is of great significance to carry out centrifugal-gelcasting process of ZrB₂-Si C ceramics.Firstly, in this paper, the effects of p H value, dispersant and volume solid loading on the stability and rheological properties of ZrB₂-Si C ceramic slurry were investigated systematically. With p H value adjusting to 6 and 0.6wt.% PAA dispersant added, ZrB₂-Si C ceramic slurry could achieve the optimum dispersion state due to maximum absolute value of Zeta potential, and the median diameter about Dv50 of ZrB₂ and Si C powders approached to minimum. Moreover, the sedimentation heights of ZrB₂-Si C suspension reached to the lowest. Different solid loading suspensions all showed a shear thinning effect. When solid loading of ZrB₂-Si C suspension reached up to 48 vol.%, the viscosity approached to the minimum value of 1.50 Pa?s at 60 S-1 shear rate, that was favorable for centrifugal-gelcasting process.The influence of colloidal contents and centrifugal parameters on the microstructure of centrifugal-gelcasting ZrB₂-Si C green bodies were investigated systematically. The ZrB₂-Si C green bodies with high green density（60.5%）, homogeneous particle packing and minimum density gradient of various parts were achieved successfully when the AM content, ratio of MBAM to AM, cnetrifigal speed and centrifugal time were 3.5 wt.%, 1:10, 9000 r/min and 10 min, respectively, which could ensure the requirements of strength for post-machining.The drying and debinding process of ZrB₂-Si C green bodies were analysised in detail. The drying process was conducted in two stages: the initial drying shrinkage of green bodies conducted at a fast drying rate, which was attribute to rapid evaporation of surface moisture; and then tended to slow uniform shrink due to capillary action of the green bodies. Furthermore, it was benefit for densification process after bindering at 800oC completely.The effect of monomer contents and initiator additions on pressureless densification of ZrB₂-Si C ceramics were investigated. The results showed that the particle distribution of sintered parts was homogenous and no phase separation phenomenon occurred when pressureless sintered at 1900℃/2h, which was attribute to less monomer content（3.5wt.%）. While pressureless sintering at 2000℃/1h, ZrB₂-Si C ceramics with optimized initiator addition（4wt.%） had homogeneous microstructure and superb mechanical properties, which its density, flexural strength and fracture toughness were 94.8%, 394±14 MPa and 4.03±0.10 MPa?m1/2 respectively.