Reactive hot pressing of ZrB2-based composites with changes in ZrO2/SiC ratio and sintering conditions. Part I: Densification behavior

ZrB₂–SiC–ZrO₂ composites were hot pressed in order to investigate the effects of adding nano-sized ZrO₂ particles as well as the hot pressing parameters on the densification behavior of ZrB₂–SiC composites. An L9 orthogonal array of the Taguchi method was employed to study the significance of each parameter such as the sintering temperature, time, the applied external pressure, and ZrO₂/SiC volume ratio on the densification process. The statistical analyses revealed that among the mentioned parameters, the hot pressing temperature had a great influence over the densification. By being hot pressed at 1850 °C for 90 min under 16 MPa, fully dense ZrB₂-based composites were obtained. The relative density of the composites decreased at first and then enhanced as a function of ZrO₂/SiC ratio. Microstructural investigation of the fracture surfaces of the composites, which was carried out using the SEM analysis, showed the formation of new phases on the surfaces of SiC grains. The EDS and XRD analyses identified the ZrC as the newly formed interfacial phase due to the reaction between nano-ZrO₂ and SiC. The ZrC acted as an adhesive interphase between the ZrB₂/SiC grains, which could assist the sintering process.     

Effect of SiC addition on the formation of ZrB2 through reaction of ZrO2, boric acid (H3BO3) and carbon black

ZrB₂-SiC powders with different amounts of SiC (10–30 wt%) were in-situ synthesized at 1600 °C for 90 min in Ar atmosphere. Effects of SiC addition on the formation of ZrB₂ via carbothermal reduction of ZrO₂, H₃BO₃ and carbon black were investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The grain size of ZrB₂ in final powders decreased with adding SiC. Columnar ZrB₂ and granular SiC were combined interactively when the SiC content was 25 wt%. Layer-like hexagonal SiC was obtained in the product containing 30 wt% SiC, whereas the ZrB₂ grain growth was strongly inhibited. Furthermore, the growth mechanisms of ZrB₂ and SiC were studied.     

Densification,microstructure and mechanical properties of hot pressed ZrB2–SiC ceramic doped with nano-sized carbon black

The effect of nano-sized carbon black on densification behavior, microstructure, and mechanical properties of zirconium diboride (ZrB₂) – silicon carbide (SiC) ceramic was studied. A ZrB₂-based ceramic matrix composite, reinforced with 20 vol% SiC and doped with 10 vol% nano-sized carbon black, was hot pressed at 1850 °C for 1 h under 20 MPa. For comparison, a monolithic ZrB₂ ceramic and a ZrB₂–20 vol% SiC composite were also fabricated by the same processing conditions. By adding 20 vol% SiC, the sintered density slightly improved to ~93%, compared to the relative density of ~90% of the monolithic one. However, adding 10 vol% nano-sized carbon black to ZrB₂–20 vol% SiC composite meaningfully increased the sinterability, as a relatively fully dense sample was obtained (RD=~100%). The average grain size of sintered ZrB₂ was significantly affected and controlled by adding carbon black together with SiC acting as effective grain growth inhibitors. The Vickers hardness, flexural strength and fracture toughness of SiC reinforced and carbon black doped composites were found to be remarkably higher than those of monolithic ZrB₂ ceramic. Moreover, unreacted carbon black additives in the composite sample resulted in the activation of some toughening mechanisms such as crack deflections.     

Fabrication of SiCw reinforced ZrB2-based ceramics

ZrB₂-based ceramics with SiC_w were produced by hot pressing at 1750 °C for 1 h from mixed powders after adding liquid polycarbosilane. The obtained ZrB₂-SiC_w composites had toughness up to 7.57 MPa m^1/2, which was much higher than those for monolithic ZrB₂, SiC particles reinforced ZrB₂ composites, and other ZrB₂–SiC_w composites directly sintered at high temperatures. The added liquid polycarbosilane could reduce the sintering temperatures and restrict the reaction of matrix with whisker, which led to fewer damages to the whisker and high fracture toughness.     

Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part I: Densification behavior

The effects of processing variables on densification behavior of hot pressed ZrB₂-based composites, reinforced with SiC particles and short carbon fibers (C_sf), were studied. A design of experiment approach, Taguchi methodology, was used to investigate the characteristics of ZrB₂–SiC–C_sf composites concentrated upon the hot pressing parameters (sintering temperature, dwell time and applied pressure) as well as the composition (vol% SiC/vol% C_sf). The analysis of variance recognized the sintering temperature and SiC/C_sf ratio as the most effective variables on the relative density of hot pressed composites. The microstructural investigations showed that C_sf can act as a sintering aid and eliminate the oxide impurities (e.g. B₂O₃, ZrO₂ and SiO₂) from the surfaces of raw materials. A fully dense composite was achieved by adding 10 vol% C_sf and 20 vol% SiC to the ZrB₂ matrix via hot pressing at 1850 °C for 30 min under a pressure of 16 MPa. Moreover, the in-situ formation of interfacial ZrC, which also improves the sinterability of ZrB₂-based composites, was studied by energy-dispersive X-ray spectroscopy analysis and verified thermodynamically.     

Reactive hot-pressing of ZrB2-ZrC-SiC ceramics via direct addition of SiC

A series of ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol% were prepared by reactive hot-pressing using Zr, B₄C and SiC as raw materials. Self-propagating high-temperature synthesis (SHS) occurred, and ZrC grains connected each other to form a layered structure when the SiC content is below 20 vol%. The evolution of microstructure has been discussed via reaction processes. The composite with 10 vol% SiC presents the most excellent mechanical properties (four-point bending strength: 828.6±49.9 MPa, Vickers hardness: 19.9±0.2 GPa) and finest grain size (ZrB₂: 1.52 µm, ZrC: 1.07 µm, SiC: 0.79 µm) among ZrB₂-ZrC-SiC composites with various SiC content from 0 to 20 vol%.     

Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part II: Grain growth

The synergetic effects SiC particles and short carbon fibers (C_sf) as well as hot pressing parameters (sintering temperature, dwell time and applied pressure) on the grain growth of ZrB₂-based composites were investigated. Taguchi methodology was employed for the design of experiments to study the microstructure and grain growth of ZrB₂–SiC–C_sf ceramic composites. Three hot pressing parameters and SiC/C_sf ratio were selected as the scrutinized variables. The sintering temperature and SiC/C_sf ratio were identified by ANOVA as the most effective variables on the gain growth of ZrB₂-based samples. Removal of oxide impurities from the surface of starting particles by the reactant C_sf, not only hindered the extraordinary grain growth of ZrB₂ matrix, but also improved the sinterability of the ceramics. A fully dense ceramic with an average grain size of 8.3 µm was obtained by hot pressing at 1850 °C for 30 min under 16 MPa through adding 20 vol% SiC and 10 vol% C_sf to the ZrB₂ matrix. SEM observations and EDS analysis verified the in-situ formation of ZrC which can restrain the growth of ZrB₂ particles, similar to the role of SiC, by the pinning of grain boundaries as another stationary secondary phase.     

High temperature erosion behavior of spark plasma sintered ZrB2-SiC composites

Damage of structural components of hypersonic vehicles by atmospheric particles demands thorough understanding on their wear behavior. In the present work, dense ZrB₂-SiC (10, 20, and 30 vol%) composites are prepared by spark plasma sintering at 55 MPa in two stages: 1400 °C for 6 min followed by 1600 °C for 2 min. With increase in SiC content, microstructures of sintered composites reveal strongly bonded ZrB₂ grains with SiC particles. A combination of maximum hardness of 23 GPa, elastic modulus of 398 GPa and fracture toughness of 5.4 MPa m^1/2 are obtained for the composite containing 30 vol% SiC particles. It is found that cracks are bridged or deflected by SiC particles in the composites. When the composites are subjected to SiC particle erosion at 800 °C, a 14% decrease in erosion rate is obtained with increase in SiC content from 10 to 30 vol%. The formation of large extent of boro-silicate rich viscous surface on eroded surfaces is attributed to reduced fracture or removal of ZrB₂ grains of the composites with increased SiC content.     

Microstructure,hardness and fracture toughness of spark plasma sintered ZrB2–SiC–Cf composites

The combined effects of SiC particles and chopped carbon fibers (C_f) as well as sintering conditions on the microstructure and mechanical properties of spark plasma sintered ZrB₂-based composites were investigated by Taguchi methodology. Analysis of variance was used to optimize the spark plasma sintering variables (temperature, time and pressure) and the composition (SiC/C_f ratio) in order to enhance the hardness of ZrB₂–SiC–C_f composites. The sintering temperature was found as the most effective variable, with a significance of 83%, on the hardness. The hardest ZrB₂-based ceramic was achievable by adding 20 vol% SiC and 10 vol% C_f after spark plasma sintering at 1850 °C for 6 min under 30 MPa. Fracture toughness improvement were related to the simultaneous presence of SiC and C_f phases as well as the in-situ formation of nano-sized interfacial ZrC particles. Crack deflection, crack branching and crack bridging were detected as the toughening mechanisms. A Vickers hardness of 14.8 GPa and an indentation fracture toughness of 6.8 MPa m^1/2 were measured for the sample fabricated at optimal processing conditions.     

Silicon carbide–titanium diboride ceramic composites

The effect of TiB₂ content on mechanical properties of silicon carbide–titanium diboride ceramic composites was studied. The hardness of the ceramics decreased from 27.8 GPa for nominally pure SiC to 24.4 GPa for nominally pure TiB₂. In contrast, fracture toughness of the ceramics increased from 2.1 MPa m^1/2 for SiC to ～6 MPa m^1/2 for SiC with TiB₂ contents of 40 vol.% or higher. Flexure strengths were measured for three composites containing 15, 20, and 40 vol.% TiB₂ and analyzed using a two parameter Weibull analysis. The Weibull modulus increased from 12 for 15 vol.% TiB₂ to 17 for 20 and 40 vol.% TiB₂. Microstructural analysis revealed microcracking in the ceramics containing 20 and 40 vol.% TiB₂. The ceramic containing 40 vol.% TiB₂ had the best combination of properties with a fracture toughness of 6.2 MPa m^1/2, hardness of 25.3 GPa, Weibull modulus of 17, and a strength of 423 MPa.     

Spark plasma sintering and characterization of ZrC-TiB2 composites

ZrC-based composites were consolidated from ZrC and TiB₂ powders by the Spark Plasma Sintering (SPS) technique at 1685 °C and 1700 °C for 300 s under 40-50-60 MPa. Densification, crystalline phases, microstructure, mechanical properties and oxidation behavior of the composites were investigated. The sintered bodies were composed of a (Zr,Ti)C solid solution and a ZrB phase. The densification behaviors of the composites were improved by increasing the TiB₂ content and applied pressure. The highest value of hardness, 21.64 GPa, was attained with the addition of 30 vol% TiB₂. Oxidation tests were performed at 900 °C for 2 h and the formation of ZrO₂, TiO₂ and B₂O₃ phases were identified by using XRD.     

Enhanced oxidation resistance of ZrB2/SiC composite through in situ reaction of gadolinium oxide in patterned surface cavities

Micro-cavities on the surface of dense ZrB₂/20 vol.% SiC composites, machined by ultra-fast laser ablation, were filled with Gd₂O₃ nanopowder and oxidized in static air at 1600 °C. Optimized rectangular pattern of cavities, 10 μm diameter and deep, 20 μm apart conferred improved oxidation resistance compared to the untreated ZrB₂/20 vol.% SiC due to the formation of glasses of higher viscosity with lower oxygen diffusivities. Reduction of the oxidized depth was revealed by a significant decrease of 10 μm (60%) in the extent of the protective layer. The filled-cavity strategy leads to better protection against oxygen diffusivity into the composite without altering the bulk properties.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

ZrB2-based composites toughened by as-received and heat-treated short carbon fibers

In order to improve the fracture toughness of ZrB₂ ceramics, as-received and heat treated short carbon fiber reinforced ZrB₂-based composites were fabricated by hot pressing. The toughening effects of the fibers were studied by investigating the relative density, phase composition, microstructure and mechanical properties of the composites. It was found that the densification behavior, microstructure and mechanical properties of the composites were influenced by the fibers’ surface condition. The heat treated fiber was more appropriate to toughen the ZrB₂-based composites, due to the high graphitization degree, low surface activity and weak interfacial bonding. As a result, the fracture toughness of the composites with heat-treated fiber is 7.62 ± 0.12 MPa m^1/2, which increased by 10% as compared to the composites with as-received fiber (6.89 ± 0.16 MPa m^1/2).     

Preparation and characterization of ZrB2 and TaC containing Cf/SiC composites via Polymer-Infiltration-Pyrolysis process

To improve the thermo-chemical resistance of PIP–C_f/SiC composites, the SiC matrix is modified by adding ZrB₂ and Ta powder to the pre-ceramic slurry to form C_f/SiC–ZrB₂–TaC composites. Within this study the modified composites are investigated regarding their microstructure, chemical composition and physical properties (density = 2,39–2,72 g/cm³; porosity = 20,3–24,8 vol.-%; fiber volume content = 52–57 vol.-%). Mechanical properties are investigated in order to ensure that there is no negative influence by ZrB₂ and TaC matrix modification. The matrix modification is followed by an improvement in bending strength (up to 27% increase), Young’s modulus (up to 28% increase) and for interlaminar shear strength (up to 22% increase). Finally the thermo-chemical behavior of the C_f/SiC–ZrB₂–TaC composites is evaluated in a combustion chamber-like environment using the Airbus Group long-term material test facility (Environmental Relevant Burner Rig-Kerosene, ERBURIG^K). The results show that the thermo-chemical resistance of C_f/SiC–ZrB₂–TaC composites is improved and the oxygen permeability through the composite is decreased (from 5 to 1 layer).     

Densification of ZrB2-based composites and their mechanical and physical properties: A review

This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB₂ ceramics and ZrB₂-based composites. Hot-pressing is the most commonly used densification method for the ZrB₂-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB₂-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB₂ ceramics and ZrB₂-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB₂-based composites is in the range of 2–6 MPa m^1/2. Fine-grained ZrB₂ ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi₂. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB₂-based ceramics. In addition, the ZrB₂-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB₂-based composites attractive candidates for high-temperature thermomechanical structural applications.     

Electrical and thermophysical properties of ZrB2 and HfB2 based composites

Electrical resistivities, thermal conductivities and thermal expansion coefficients of hot-pressed ZrB₂–SiC, ZrB₂–SiC–Si₃N₄, ZrB₂–ZrC–SiC–Si₃N₄ and HfB₂–SiC composites have been evaluated. Effects of Si₃N₄ and ZrC additions on electrical and thermophysical properties of ZrB₂–SiC composite have been investigated. Further, properties of ZrB₂–SiC and HfB₂–SiC composites have been compared. Electrical resistivities (at 25 °C), thermal conductivities (between 25 and 1300 °C) and thermal expansion coefficients (over 25–1000 °C) have been determined by four-probe method, laser flash method and thermo-mechanical analyzer, respectively. Experimental results have shown reasonable agreement with theoretical predictions. Electrical resistivities of ZrB₂-based composites are lower than that of HfB₂–SiC composite. Thermal conductivity of ZrB₂ increases with addition of SiC, while it decreases on ZrC addition, which is explained considering relative contributions of electrons and phonons to thermal transport. As expected, thermal expansion coefficient of each composite is reduced by SiC additions in 25–200 °C range, while it exceeds theoretical values at higher temperatures.     

Spark plasma sintering of TiN–TiB2 composites

TiN–TiB₂ composites were fabricated by spark plasma sintering at 1773–2573 K. Effects of TiN and TiB₂ content on relative density, microstructure, and mechanical properties were investigated. Above 2373 K, TiN–TiB₂ composites exhibited relative densities over 95%. A high density of 99.7% was obtained at 2573 K with 20–30 vol% TiB₂. Shrinkage of the TiN–70 vol% TiB₂ composite was the highest at 1573–2473 K. For the TiN–70 vol% TiB₂ composite prepared at 1973–2373 K, TiN grains were small, while at 2573 K, TiB₂ became a continuous matrix, in which irregular-shaped TiN dispersed. hBN was formed in the TiN–TiB₂ composite containing 50–60 vol% TiB₂ above 2373 K. The maximum Vickers hardness and fracture toughness obtained for the TiN–80 vol% TiB₂ composite sintered at 2473 K was 26.3 GPa and 4.5 MPa m^1/2, respectively.     

Reaction joining of SiC ceramics using TiB2-based composites

SiC ceramics were reaction joined in the temperature range of 1450–1800 °C using TiB₂-based composites starting from four types of joining materials, namely Ti–BN, Ti–B₄C, Ti–BN–Al and Ti–B₄C–Si. XRD analysis and microstructure examination were carried out on SiC joints. It is found that the former two joining materials do not yield good bond for SiC ceramics at temperatures up to 1600 °C. However, Ti–BN–Al system results in the connection of SiC substrates at 1450 °C by the formation of TiB₂–AlN composite. Furthermore, nearly dense SiC joints with crack-free interface have been produced from Ti–BN–Al and Ti–B₄C–Si systems at 1800 °C, i.e. joints TBNA80 and TBCS80, whose average bending strengths are measured to be 65 MPa and 142 MPa, respectively. The joining mechanisms involved are also discussed.