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.     

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.     

Fabrication and properties of reactively hot pressed ZrB2–SiC ceramics

ZrB₂–SiC ceramics with relative densities >99% were fabricated by ‘in situ’ reactive hot pressing from ZrH₂, B₄C and Si. The reaction was studied using two processes, (1) powder reactions at temperatures from 1150 to 1400 °C and (2) reactive hot pressing between 1600 and 1900 °C. The products from the reaction of a 2ZrH₂:1B₄C:1Si molar mixture were ZrB₂, SiC, ZrO₂ and ZrC. Modification of the composition to 2ZrH₂:1.07B₄C:1.16Si resulted in the elimination of the undesired ZrO₂ and ZrC phases. The final composition was approximately ZrB₂–27 vol% SiC with no undesired phases detected by X-ray diffraction, and only low concentrations of B₄C detected by scanning electron microscopy. Elimination of the undesired phases was accomplished by removing surface oxides through chemical reactions at elevated temperatures. Reactively hot pressed samples consisting of ZrB₂ with 27 vol% SiC had a Young's modulus of 508 GPa, a flexure strength of 720 MPa, a fracture toughness of 3.5 MPa m^1/2 and a Vickers’ hardness of 22.8 GPa.     

Oxidation behavior of hot pressed ZrB2–SiC and HfB2–SiC composites

Non-isothermal, isothermal and cyclic oxidation behavior of hot pressed ZrB₂–20 (vol.%) SiC (ZS) and HfB₂–20 SiC (HS) composites have been compared. Studies involving heating in thermogravimetric analyzer have shown sharp mass increases at 740 and 1180 °C for ZS, and mass gain till 1100 °C followed by loss for HS. Isothermal oxidation tests for 1, 24 and 100 h durations at 1200 or 1300 °C have shown formation of partially and completely stable oxide scales after ～24 h exposure for ZS and HS, respectively. X-ray diffraction, scanning electron microscopy and energy or wavelength dispersive spectroscopy has confirmed presence of ZrO₂ or HfO₂ in oxide scales of ZS or HS, respectively, besides B₂O₃–SiO₂. Degradation appears more severe in isothermally oxidized ZS due to phase transformations in ZrO₂; and is worse in HS on cyclic oxidation at 1300 °C with air cooling, because of higher thermal residual stresses in its oxide scale.     

High temperature strength of hot pressed ZrB2–20 vol% SiC ceramics based on ZrB2 starting powders prepared by different carbo/boro-thermal reduction routes

ZrB₂–20 vol% SiC (ZS) ceramics based on ZrB₂ starting powders obtained by different boro/carbo-thermal reductions involving ZrO₂ + B₄C, ZrO₂ + B₄C + C, and ZrO₂ + B, were fully densified by hot pressing at 1900–2000 °C. The flexural strength of these ZS ceramics was measured from room temperature up to 1600 °C. At 1600 °C, the flexural strength of the ceramics is 460 ± 31, 471 ± 32 and 345 ± 11 MPa, respectively. The evolution of the strength as function of temperature is explained in terms of the differences in oxygen content, nature of fracture, grain sizes, grain boundary phases and microstructural defects.     

High-temperature bending strength,internal friction and stiffness of ZrB2–20 vol% SiC ceramics

Dense ZrB₂–20 vol% SiC ceramics (ZS) were fabricated by hot pressing using self-synthesized high purity ZrB₂ and commercial SiC powders as raw materials. The high temperature flexural strength of ZS and its degradation mechanisms up to 1600 °C in high purity argon were investigated. According to the fracture mode, crack origin and internal friction curve of ZS ceramics, its strength degradation above 1000 °C is considered to result from a combination of phenomena such as grain boundary softening, grain sliding and the formation of cavitations and cracks around the SiC grains on the tensile side of the specimens. The ZS material at 1600 °C remains 84% of its strength at room temperature, which is obviously higher than the values reported in literature. The benefit is mainly derived from the high purity of the ZrB₂ powders.     

The ablation behavior of ZrB2/Cu composite irradiated by high-intensity continuous laser

Ultra high temperature ceramics (UHTCs) based composite ZrB₂/20 vol.% Cu was prepared by spark plasma sintering (SPS) at 1650 C° for 3 min. The ablation behavior of composite irradiated for 2–20 s by 20 MW/m² high-intensity continuous laser was investigated. The phase and microstructure evolution of ZrB₂/20 vol.% Cu during ablation was demonstrated by XRD and SEM, respectively. The results reveal that no macroscopic damage but only one ablated layer with 40 μm in thickness is observed even after being ablated for 20 s. It implies that ZrB₂/20 vol.% Cu composite exhibits good ablation resistance against high-intensity continuous laser. The continuous Cu in composite evaporates preferentially, which impacts on the following ablation behavior. The generated ZrO₂ at the spot center shows different forms such as closely packed nano-ZrO₂, micron ZrO₂ or melting ZrO₂ for different ablation time. The melting ZrO₂ is helpful to promote the ablation resistance of ZrB₂/20 vol.% Cu.     

SPS densification and microstructure of ZrB2 composites derived from sol–gel ZrC coating

A technique for densifying ultra high temperature ceramic composites while minimising grain growth is reported. As-purchased ZrB₂ powder was treated with a zirconia-carbon sol–gel coating. Carbothermal reduction at 1450 °C produced 100–200 nm crystalline ZrC particles attached on the surface of ZrB₂ powders. The densification behaviour of the sol–gel coated powder was compared with both the as-purchased ZrB₂ and a compositionally similar ZrB₂–ZrC mixture. All three samples were densified by spark plasma sintering (SPS). The ZrB₂ reference sample was slow to densify until 1800 °C and was not fully dense even at 2000 °C, while the sol–gel modified ZrB₂ powder completed densification by 1800 °C. The process was studied by ram displacement data, gas evolution, SEM, and XRD. The sol–gel coated nanoparticles on the ZrB₂ powder played a number of important roles in sintering, facilitating superior densification by carbothermal reduction, nanoparticle coalescence and solid-state diffusion, and controlling grain growth and pore removal by Zener pinning. The sol–gel surface modification is a promising technique to develop ultra-high temperature ceramic composites with high density and minimum grain growth.     

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%.     

Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra-high temperature ceramics

Thermal properties of La₂O₃-doped ZrB₂- and HfB₂-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB₂- and HfB₂-based UHTCs and monolithic ZrB₂ and HfB₂. Thermal conductivities of La₂O₃-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range.     

Synthesis of ZrB2–SiC–ZrC nanocomposite by spark plasma in ZrSiO4/B2O3/C/Mg system

A novel reduction process by spark plasma is proposed for the synthesis of homogeneous intermixed ZrB₂–SiC–ZrC nanostructure powder using ZrSiO₄, B₂O_3, C and Mg as starting materials. In the present research, the reaction mechanisms were investigated by the displacement–temperature–time (DTT), the displacement rate–temperature and the displacement rate–time diagrams which were obtained during spark plasma cycles. The synthesis process of the composite was completed after 15 min at temperature of 400 °C. The total shrinkage of 5.5 mm was observed due to the rearrangement and reaction phenomenon. The main shrinkage occurs within a period of less than 1 min at 400 °C similar to the self propagating high temperature synthesis. Finally using Rietveld method, the mean crystallites sizes of about 26, 25 and 22 nm were calculated for ZrB₂, SiC and ZrC phases, respectively.     

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.     

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.     

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.     

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.     

Characterization investigations of ZrB2/ZrC ceramic powders synthesized by mechanical alloying of elemental Zr,B and C blends

ZrB₂/ZrC ceramic powders were fabricated by mechanical alloying (MA) of zirconium (Zr), amorphous boron (B) and graphite (C) powder blends prepared in the mole ratios of Zr/B/C: 1/1/1, 1/2/1, 1/1/2, 1/2/2 and 2/2/1. MA runs were carried out in a vibratory ball mill using hardened steel vial/balls. The effects of Zr/B/C mole ratios and milling duration on the formation and microstructure of ZrB₂/ZrC ceramic powders were examined. Gibbs free energy change-temperature relations of the reactions and moles of the products were interpreted by thermochemical software. Zr/B/C: 1/1/1, 1/2/1, 1/1/2 and 1/2/2 powder blends MA’d for 2 and 3 h contain unreacted Zr and C, ZrB₂, ZrC and B₄C particles. Synthesis of ZrB₂/ZrC ceramic powders was completely accomplished after MA of Zr/B/C: 2/2/1 powder blend for 2 h. ZrC and ZrB₂ particles were obtained ranging in size between 50 and 250 nm in the presence of FeB contamination (<1 wt.%).     

Reactive hot pressing of ZrB2–ZrCx ultra-high temperature ceramic composites with the addition of SiC particulate

Starting with non-stoichiometric Zr–B₄C powder mixture ZrB₂–ZrC matrix composites with SiC particulate addition have been made. It was found that variable amounts (5–25 vol%) of SiC could be incorporated and reactively hot pressed (RHPed) to relative densities of 97–99% at 1400–1500 °C. This technique has the potential to fabricate ZrB₂-based matrices at low temperatures with a variety of reinforcements whose composition and volume fraction are not limited by stoichiometric considerations. The hardness of the composites is in the range of 17–22 GPa.     

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.     

Flexural strength and fracture behavior of ZrB2–SiC ultra-high temperature ceramic composites at 1800 °C

ZrB₂–15 vol.%SiC and ZrB₂–30 vol.%SiC composites with smaller starting particle sizes in which the particle sizes of ZrB₂ and SiC are 2 μm and 0.5 μm, respectively, demonstrated marked plasticity and significant reduction in the flexural strength at 1800 °C. The flexural strengths of these two composites are 112 ± 12 MPa and 48 ± 10 MPa, respectively, and their corresponding strength retentions are 13% and 7%, respectively. Large ZrB₂ grains were commonly observed in the samples containing 15 vol.%SiC, which are always the sites for the crack initiation. Cavities were found in the samples containing 30 vol.%SiC and the grain boundaries are the main sites for the crack and cavity nucleation. To improve ultra-high temperature strength, larger starting particle sizes (ZrB₂ and SiC are 5 μm and 2 μm, respectively) were used for the preparation of ZrB₂–15 vol.%SiC. This sample fractured in an elastic manner up to 1800 °C and showed a very high strength with a value of 217 ± 16 MPa.     

Oxidation of ZrB2–SiC–ZrSi2 ceramics in oxygen

The oxidation of ZrB₂–SiC and ZrB₂–SiC–ZrSi₂ ceramics of different composition has been studied experimentally at 1500 °C in pure oxygen for up to 50 h. ZrB₂–SiC–ZrSi₂ ceramics proved to be the most oxidation-resistant at ZrSi₂ contents of less then 4 wt%. These ceramics were more oxidation-resistant than ZrB₂–SiC ceramics. An analytical model of growth kinetics for a multilayered scale based on an oxidation–diffusion balance was developed and tested.