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.     

Strength of hot pressed ZrB2–SiC composite after exposure to high temperatures (1000–1700 °C)

Residual strength (room temperature strength after exposure in air at high temperatures) of hot pressed ZrB₂–SiC composites was evaluated as function of SiC contents (10–30 vol%) as well as exposure temperatures for 5 h (1000–1700 °C). Multilayer oxide scale structures were found after exposures. The composition and thickness of these multilayered oxide scale structure was dependent on exposure temperature and SiC contents in composites. After exposure to 1000 °C for 5 h, the residual strength of ZrB₂–SiC composites improved by nearly 60% compared to the as-hot pressed composites with 20 and 30 vol% SiC. On the other hand, the residual strength of these composites remained unchanged after 1500 °C for 5 h. A drastic degradation in residual strength was observed in composites with 20 and 30 vol% SiC after exposure to 1700 °C for 5 h in ZrB₂–SiC. An attempt was made to correlate the microstructural changes and oxide scales with residual strength with respect to variation in SiC content and temperature of expsoure.     

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.     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

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.     

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).     

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.     

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.     

Fabrication and properties of ZrB2–SiC–BN machinable ceramics

ZrB₂–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB₂–SiC–BN (40 vol%ZrB₂–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m^1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB₂–SiC (80 vol%ZrB₂–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB₂–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB₂–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB₂–20 vol%SiC and 43.1 vol%ZrB₂–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm², respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB₂–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB₂–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB₂–SiC–BN.     

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.     

Influence of sol-gel derived ZrB2 additions on microstructure and mechanical properties of SiBCN composites

A simple method for introducing ZrB₂ using sol-gel processing into a SiBCN matrix is presented in this paper. Zirconium n-propoxide (ZNP), boric acid and furfuryl alcohol (C₅H₆O₂) (FA) were added as the precursors of zirconia, boron oxide and carbon forming ZrB₂ dispersed in a SiBCN matrix. SiBCN/ZrB₂ composites with different contents of ZrB₂ (5, 10, 15, and 20 wt%) were formed at 2000 °C for 5 min by spark plasma sintering (SPS). The microstructures were carefully studied. TEM analysis showed that the as formed ZrB₂ grains were typically 100–500 nm in size and had uniform distribution. HRTEM revealed clean grain boundaries between ZrB₂ and SiC, however, a separation of C near the SiC boundary was observed. The flexural strength, fracture toughness, Young's modulus and Vicker's hardness of composites all improved with the ZrB₂ contents and SiBCN matrix containing 20 wt% of ZrB₂ could reach 351±18 MPa, 4.5±0.2 MPa m^1/2, 172±8 GPa and 7.2±0.2 GPa, respectively. The improvement in fracture toughness can be attributed to the tortuous crack paths due to the presence of reinforcing particles.     

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%.     

Significance of hot pressing parameters and reinforcement size on sinterability and mechanical properties of ZrB2–25 vol% SiC UHTCs

The influences of hot pressing parameters and SiC particle size on the bulk density, the average ZrB₂ grain size and Vickers hardness of ZrB₂–25 vol% SiC ultrahigh temperature ceramic composites were investigated. In this paper, the Taguchi methodology (An L9 orthogonal array) was used to specify the contributions of four parameters: the hot pressing temperature, holding time, applied pressure and SiC particle size. The experimental procedure included nine tests for four parameters with three levels which were employed to optimize the process parameters. The statistical analyses recognized the hot pressing pressure and temperature as the most consequential parameters affecting the density and hardness of ZrB₂–SiC composites. The SiC particle size and holding time were specified as the most effective parameters on the average ZrB₂ grain size. The bulk density, average ZrB₂ grain size, Vickers hardness and fracture toughness of the sample, hot pressed at optimal conditions (1850 °C, 90 min, 16 MPa and 200 nm), reached about 5.36 g/cm³, 10.03 µm, ~17.1 GPa and 5.9 MPa m^1/2, respectively. The confirmation test, carried out under optimum conditions, showed that the experimental results were relatively equal to the predicted values from the Taguchi prediction model. Finally, the mechanisms of enhanced fracture toughness of the hot pressed ZrB₂–SiC ceramic composites were discussed.     

Influence of silicon carbide addition on the microstructural development of hot pressed zirconium and titanium diborides

The influences of adding SiC on the microstructure and densification behavior of ZrB₂ and TiB₂ ceramics, hot pressed at 1850 °C for 60 min under 20 MPa, were investigated. The sintered samples were characterized by SEM, EDS and XRD methods. A fully dense TiB₂-based ceramic was obtained by adding 30 vol% SiC. The grain size of ZrB₂ or TiB₂ matrices in the final microstructures decreased with increasing SiC content. The XRD analyses, microstructural characterization as well as thermodynamical calculations proved the in-situ formation of TiC in the SiC reinforced TiB₂-based composites. The interfaces between ZrB₂ and SiC grains in the SiC reinforced ZrB₂-based composites were free of any impurities or tertiary interfacial phases such as ZrC. This result was consistent with the X-ray diffraction pattern and thermodynamics.     

Improved fracture toughness of laminated ZrB2-SiC-MoSi2 ceramics using SiC whisker

In this study, SiC whisker (SiC_w) was introduced to ZrB₂ matrix layer of laminated ZrB₂/BN ceramics to improve fracture toughness. Laminated ZrB₂-SiC_w/BN ceramics were prepared by tape casting and spark plasma sintering. For comparison, monolithic ZrB₂-SiC_w and laminated ZrB₂-SiC_p/BN ceramics were also prepared using the same method. The introduction of SiC whiskers increased fracture toughness of laminated ZrB₂-SiC_w/BN ceramics to 13.31?±?0.33?MPa?m^1/2 for all samples. This was related to the multi-scale toughening mechanism, including delamination and crack deflection issued from the laminate structure at the macroscopic level, as well as whiskers bridging and pullout at the microscopic view. The R-curve behaviors of all samples revealed improved resistance to crack propagation of laminated ZrB₂-SiC_w/BN when compared to ZrB₂-SiC_p/BN and ZrB₂-SiC_w issued from multi-scale toughening design.     

Oxidation of ZrB2–SiC ultra-high temperature composites over a wide range of SiC content

The oxidation performance of ZrB₂–SiC ultra-high temperature ceramics with SiC content ranging from 20 to 80 vol% has been evaluated at 1773 K for 50 h and at 2073 K for 20 min. Oxidation reaction pathways were interpreted using volatility diagrams of the ZrB₂–SiC system. At 1773 K for 50 h, all ZrB₂–SiC composites from 20 to 80 vol% SiC formed a protective SiO₂ surface coating. Samples with ≤50 vol% SiC developed a distinguishable SiC-depleted layer at 1773 K and 2073 K. High temperature torch testing for 20 min at approximately 2073 K revealed that samples with ≥65 vol% SiC exhibit a depression under the torch flame. Samples rich in ZrB₂ were dominated by a ZrO₂ layer after a similar exposure. The overall weight density of ultra-high temperature ceramics can be reduced with improved oxidation performance at 1773 K by adding at least 65 vol% SiC.     

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.     

In situ studies of oxidation of ZrB2 and ZrB2–SiC composites at high temperatures

High temperature oxidation of ZrB₂ and the effect of SiC on controlling the oxidation of ZrB₂ in ZrB₂–SiC composites were studied in situ, in air, using X-ray diffraction. Oxidation was studied by quantitatively analyzing the crystalline phase changes in the samples, both non-isothermally, as a function of temperature, up to ～1650 °C, as well as isothermally, as a function of time, at ～1300 °C. During the non-isothermal studies, the formation and transformation of intermediate crystalline phases of ZrO₂ were also observed. The change in SiC content, during isothermal oxidation studies of ZrB₂–SiC composites, was similar in the examined temperature range, regardless of sample microstructure and composition. Higher SiC content, however, markedly retarded the oxidation rate of the ZrB₂ phase in the composites. A novel approach to quantify the extent of oxidation by estimating the thickness of the oxidation layer formed during oxidation of ZrB₂ and ZrB₂–SiC composites, based on fractional conversion of ZrB₂ to ZrO₂ in situ, is presented.     

ZrB2–SiC laminated ceramic composites

The oxidation behavior for ZrB₂–20 vol% SiC (ZS20) and ZrB₂–30 vol% SiC (ZS30) ceramics at 1500 °C was evaluated by weight gain measurements and cross-sectional microstructure analysis. Based on the oxidation results, laminated ZrB₂–30 vol% SiC (ZS30)/ZrB₂–25 vol% SiC (ZS25)/ZrB₂–30 vol% SiC (ZS30) symmetric structure with ZS30 as the outer layer were prepared. The influence of thermal residual stress and the layer thickness ratio of outer and inner layer on the mechanical properties of ZS30/ZS25/ZS30 composites were studied. It was found that higher surface compressive stress resulted in higher flexural strength. The fracture toughness of ZS30/ZS25/ZS30 laminates was found to reach to 10.73 MPa m^1/2 at the layer thickness ratio of 0.5, which was almost 2 times that of ZS30 monolithic ceramics.