Low-temperature Densification Behavior And Properties Of ZrB₂-based Ultra-high Temperature Ceramic Composites

ZrB₂ is one of potential candidates for ultra-high temperature thermal protection and structural materials due to its high melting point,high strength,high electrical and thermal conductivity,excellent oxidation and ablation resistance,which makes ZrB₂ can be widely used on nose and leading edges of hypersonic aircrafts.However,the inherent brittleness and inferior thermal shock resistance of such material limit its application in extream environments.The most effective method to solve this problem is to introduce carbon fibers into ZrB₂-based ultra-high temperature ceramics?UHTCs?.While the structural damage and performance degradation of carbon fibers caused by high temperature sintering become the technical bottleneck of such material in fabrication,which could be avoided by reducing the sintering temperature of ZrB₂-based UHTCs and/or introducing a coating on the carbon fibers.The main research contents and methods of the present study are as follows: ZrB₂-based UHTCs were sintered at low temperatures by the selection of nanosized ZrB₂ powders with appropriate particle size,optimization of sintering parameters,oxygen removal from raw powders and the selection of sintering additives.Then the ZrB₂-Si C-Csf composites were successfully fabricated by low-temperature hot pressing,in which the volume fraction of carbon fibers and the sintering parameters were optimized based on the investigation of densification behaviors and mechanical properties of such materials.The compositions and microstructures of the composites were investigated by means of X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and energy dispersive spectroscopy?EDS?.Meanwhile,the thermal shock resistance and ablation/oxidation resistance of the ZrB₂-Si C-Csf composites were evaluated.The influence of the ZrB₂ particle size on the densification and grain growth as well as the mechanical properties of ZrB₂-Si C ceramic was researched and the results indicated that the relative densities of ZrB₂-Si C were above 95% when sintered at 1450 ?C using nanosized ZrB₂ powders with an average particle size of 200 nm.Specially,ZrB₂-Si C ceramic reached full densification at low temperature?1800 ?C?and the flexural strength and fracture toughness were 1044 MPa and 6.21 MPa?m1/2,respectively,significantly higher than those of conventional ZrB₂-Si C sintered by microsized ZrB₂ powders.The sintering parameters of ZrB₂-Si C were further optimized according to its densification behavior.ZrB₂-Si C ceramic with improved relative densiy,refined microstructure and increased mechanical properties was fabricated at low temperature?1500 ?C?by a novle two-step hot pressing method.The incorporation of appropriate amount of oxygen-removal additive?carbon?to ZrB₂ powders was benefit to improve the densification and restrain the grain growth.Based on the investigation of the influence of the carbon amount on the densification and mechanical properties of ZrB₂-Si C ceramic,the relative density of ZrB₂-Si C reached >98% when hot pressed at 1600 ?C with addition of 1.5 wt.% carbon,and only little amount of residual carbon could be found in the sintered material,which demonstrated that the introduction of 1.5wt.% carbon enhanced the densification and inhibited the grain growth of ZrB₂-Si C.The relative density of ZrB₂-Si C was further improved using polycarbosilane?PCS?instead of Si C particles,and it reached 95.4¹00% while hot pressed at 1450¹800 ?C.Moreover,the average grain size of ZrB₂ was refined to 0.30¹.07 ?m,while the flexural strength and fracture toughness decreased slightly.The results indicated that the optimization of the powder size,oxygen-removal additive and the introduction of PCS were the main mechanisms for the low-temperature densification of ZrB₂-based UHTCs.The influence of sintering temperature and sintering time on the microstructure evolution of carbon fibers in carbon fiber toughened ZrB₂-based composites was investigated.The degradation of carbon fibers was effectively inhibited by reducing sinterting temperature,while the sintering time had minor effect on the microstructure evolution of carbon fibers.The results of the thermodynamic calculation decleared that the dominant factors causing the degradation of carbon fibers were the impurities existed on the surfaces of the powders such as Zr O2,B2O3 and Si O2,and thus we concluded that the sintering temperature of ZrB₂-Si C-Csf composites should be below 1500 ?C.The influence of carbon fiber contents on the relative densities,microstructures and mechanical properties of ZrB₂-Si C-Csf composites by hot pressing at 1450 ?C was investigated.The results demonstrated that the degradation of carbon fibers was effectively inhibited owing to the low sintering temperature.The damage strain of the composite remarkably increased while the relative density decreased with the increasing of the carbon fiber content,and the composites containing ?30 vol.% Csf exhibited non-brittle fracture features.The optimized composition of ZrB₂-20 vol.% Si C-30 vol.% Csf composite had excellent performance with high relative density of 97.1% and fracture toughness of 6.12±0.12 MPa?m1/2 as well as a high work of fracture?321 J/m2?,dramatically higher than those of conventional ZrB₂-based UHTCs.Furthermore,the fracture toughness of such material further improved to 6.16±0.15 MPa?m1/2 when the carbon fibers were coated by a carbon layer.The thermal shock resistance and the oxidation/ablation resistance of ZrB₂-Si C-Csf composite were also investigated.The critical thermal shock temperature difference of ZrB₂-Si C-Csf composite containing 30 vol.% Csf was as high as 740 ?C,and it was further improved to 773 ?C as the carbon fiber coating was introduced,significantly higher than the reported values for ZrB₂-based UHTCs,indicating an excellent thermal shock resistance of such material.Dense oxide layers generated on the surfaces of ZrB₂-Si C-Csf composites containing ?40 vol.% Csf after oxidation at 1500 ?C for 1 h in static air,implying a good oxidation resistance.The results of oxyacetylene ablation experiments for ZrB₂-Si C-Csf composite showed that the composites were intact after ablation at ¹600 ?C or ¹900 ?C for long time?1000 s?,and the linear ablation rates of such materials after ablation at different temperatures were-2.0×10-4^-1.1×10-4 mm/s,showing a near-zero ablation feature.