Combination of SHS and SPS Techniques for fabrication of fully dense ZrB2-ZrC-SiC composites

A novel method for the fabrication of fully dense ZrB₂-ZrC-SiC Ultra High Temperature Ceramic (UHTC) materials is proposed. It consists of first synthesizing ZrB₂-40 vol.% ZrC-12 vol.% SiC powders by Self-propagating High-temperature Synthesis (SHS) and subsequently consolidating them by Spark Plasma Sintering (SPS). Specifically, when starting from Zr, B₄C, Si, and graphite, the SHS technique leads to the complete conversion of reactants to the desired product. In addition, the use of the SPS apparatus allows for the full consolidation of the SHS powders. This result is achieved under the optimal conditions of 10 min total time and with a maximum temperature of 1800 °C. The proposed method is particularly rapid and convenient as compared to other techniques available for the preparation of analogous materials and for the consolidation of commercial ZrB₂, ZrC, and SiC, using the same SPS conditions.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

Oxidation of ZrB2- and HfB2-based ultra-high temperature ceramics: Effect of Ta additions

Several compositions of ZrB₂- and HfB₂-based Ultra-High Temperature Ceramics (UHTC) were oxidized in stagnant air at 1627°C in ten minute cycles for times up to 100 min. These compositions include: ZrB₂-20 vol% SiC, HfB₂-20 vol% SiC, ZrB₂-20 vol% SiC-20 vol% TaSi₂, ZrB₂-33 vol% SiC, HfB₂-20 vol% SiC-20 vol% TaSi₂, and ZrB₂-20 vol% SiC-20 vol% TaC. The weight change due to oxidation was recorded. The ZrB₂-20 vol% SiC-20 vol% TaSi₂ composition was also oxidized in stagnant air at 1927°C and in an arc jet atmosphere. Samples were analyzed after oxidation by X-ray diffraction, field emission scanning electron microscopy, and energy dispersive spectroscopy to determine the reaction products and to observe the microstructure. The ZrB₂-20 vol% SiC-20 vol% TaSi₂ showed the lowest oxidation rate at 1627°C, but performed poorly under the more extreme tests due to liquid phase formation. Effects of Ta-additions on the oxidation of the diboride-based UHTC are discussed.     

Effect of hot pressing time and temperature on the microstructure and mechanical properties of ZrB<Subscript>2</Subscript>–SiC

Structure–property relations were examined for ZrB₂ containing 30 volume percent SiC particulates. Two grades of ZrB₂ with initial particle sizes of 2 and 6 μm were used. Billets of ZrB₂–SiC were produced by hot pressing at 1850, 1950 or 2050 °C for 45 min. In addition, the material prepared from ZrB₂ with an initial particle size of 2 μm was hot pressed at 2050 °C for 90 and 180 min. Microstructures and mechanical properties were characterized to determine the effects of the initial particle size, hot pressing time, and hot pressing temperature on the final grain size and morphology. The average grain size of the ZrB₂ phase ranged from 2.2 to 4.7 μm. Similarly, the average grain size of the SiC phase ranged from 1.2 to 2.7 μm. Hardness and modulus of elasticity were not affected by the processing conditions with average values of 22 and 505 GPa, respectively. However, flexural strength decreased as grain size increased from a maximum of ∼1050 MPa for the finest grain sizes to ∼700 MPa for the largest grain sizes. Analysis suggested that the strength of ZrB₂–SiC was limited by the size of the SiC inclusions in the ZrB₂ matrix.     

Hot pressed ZrB2–SiC–C ultra high temperature ceramics with polycarbosilane as a precursor

ZrB₂–SiC–C ultra high temperature ceramics (UHTCs) have been produced by hot pressing pyrolyzed mixtures of ZrB₂ and polycarbosilane (PCS). Samples with SiC contents of 0%, 5% and 16% in volume derived from PCS were prepared. The phase composition, microstructure and mechanical properties were characterized for composites hot pressed at 2073 K for 60 min under the pressure of 20 MPa in an argon atmosphere. Analysis showed that the addition of PCS improved the relative density from 78% (without PCS addition) to ∼ 100% (with 16% SiC derived from PCS addition). Hardness and fracture toughness of the composite were also improved.     

Preparation and mechanical properties of ZrB2-based ceramics using MoSi2 as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Oxidation resistance of hafnium diboride—silicon carbide from 1400 to 2000?°C

Oxidation resistance tests were carried out on HfB₂-20 vol.% SiC prepared by spark plasma sintering. The dense samples were exposed from 1400 to 2000 °C in an ambient atmosphere for 1 h. For comparison, the same material was tested using an arc jet to simulate an atmospheric reentry environment. The oxidation properties of the samples were determined by measuring the weight gain per unit surface area and the thicknesses of the oxide scale. The oxide scale consists of a SiO₂ outer layer, porous HfO₂ layers, and an HfB₂ layer depleted in SiC. A transition in HfO₂ morphology from equixed to columnar and a decrease in SiO₂ viscosity between 1800 and 1900 °C accompanied a rapid increase in weight gain and scale thickness.     

Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Preparation and characterization of ZrB<Subscript>2</Subscript>-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Synthesis and characterization of dense ultra-high temperature thermal protection materials produced by field activation through spark plasma sintering (SPS): I. Hafnium Diboride

The consolidation of HfB₂ by sintering and reactive sintering using the field-activated process of the spark plasma sintering (SPS) method was investigated. Sintering of the diboride at 1900°C under a pressure of 95 MPa did not result in dense materials. In contrast, reactive sintering at 1700°C produced about 98% dense HfB₂ with a 10 min hold at temperature. In contrast to previous observations, the reaction between the elements and the consolidation of the resulting diboride did not coincide, the latter occurring at a much higher temperature. The reaction mechanism between B and Hf during reactive sintering was investigated. Measured rates of growth of the HfB₂ were found to be in agreement with calculated values from diffusion couple experiments, suggesting that the current did not play a significant role in the reactivity.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Strength and fracture toughness of in situ-toughened silicon carbide

Fine β-SiC powders either pure or with the addition of 1 wt % of α-SiC particles acting as a seeding medium, were hot-pressed at 1800 °C for 1 h using Y₂O₃ and Al₂O₃ as sintering aids and were subsequently annealed at 1900 °C for 2, 4 and 8 h. During the subsequent heat treatment, the β → α phase transformation of SiC produced a microstructure of “in situ composites” as a result of the growth of elongated large α-SiC grains. The introduction of α-SiC seeds into the β-SiC accelerated the grain growth of elongated large grains during annealing which led to a coarser microstructure. The sample strength values decreased as the grain size and fracture toughness continued to increase beyond the level where clusters of grains act as fracture origins. The average strength of the in situ-toughened SiC materials was in the range of 468–667 MPa at room temperature and 476–592 MPa at 900 °C. Typical fracture toughness values of 8 h annealed materials were 6.0 MPa m^1/2 for materials containing α-SiC seeds and 5.8 MPa m^1/2 for pure β-SiC samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

Spark Plasma Sintering of ZrB2-SiC-ZrC ultra-high temperature ceramics at 1800 °C

The compositional effects in ZrB₂-SiC-ZrC ultra high temperature composites with four different compositions were investigated via Spark Plasma Sintering (SPS) at a maximum temperature of 1800 °C. Density, Rockwell hardness, and thermal conductivity were measured, along with structural X-ray diffraction (XRD) and microstructural characterization. The relative amounts of SiC and ZrC had an influence on the composites’ density, mechanical and thermal properties.     

Spark plasma sintering of ultra refractory compounds

Spark plasma sintering experiments were conducted on Zr- and Hf-based borides and carbides with the addition of 1, 3, and 9 vol% MoSi₂ as sintering aid. For comparison, as-received ZrC, HfC, ZrB₂, HfB₂ powders were also sintered. The microstructural features were investigated by means of scanning electron microscop–energy dispersive spectroscopy technique. Silicon carbide was detected in all the doped compositions along with significant amounts of oxide species (Hf/ZrO₂, and SiO₂). The effect of the MoSi₂ content on densification, microstructure, and mechanical properties is analyzed.     

Synthesis and mechanical properties of Ti3AlC2 by spark plasma sintering

In this paper, spark plasma sintering (SPS), after hot isostatically pressing (HIP) method was reported as a new approach to prepare bulk polycrystalline samples of Ti₃AlC₂. The ternary carbide was fabricated by spark plasma sintering (SPS) at a pressure of 22 MPa and temperature of 1250°C. The raw materials, elemental powders of Ti, Al and activated carbon, were pretreated in the following different ways prior to SPS: one way was to obtain porous Ti₃AlC₂ by self-propagating high-temperature synthesis (SHS) from mixture of Ti, Al and C, and then densify the product by SPS; the second way was to synthesize Al₄C₃ from Al and C firstly, and then mix powders of Ti and C with synthesized Al₄C₃ to fabricate bulk Ti₃AlC₂ by SPS. Obtained polycrystalline Ti₃AlC₂ ceramics had excellent mechanical properties: density was 4.24 ± 0.02 g/cm³, flexural strength was 552 ± 30 MPa and fracture toughness (K _IC) was 9.1 ± 0.3 MPa · m^1/2. It could be concluded that SPS method was a useful method to synthesize bulk Ti₃AlC₂ with excellent properties in a very short time and easily sintering process. The optimal conditions to synthesize Ti₃AlC₂ were also discussed.     

Fabrication of silicon carbide composites with carbon nanofiber addition and their fracture toughness

The authors have examined the fabrication conditions of SiC composites containing carbon nanofiber, i.e., vapor-grown carbon nanofiber (VGCF), to enhance the fracture toughness. Commercially available ultrafine SiC powder (specific surface area: 47.5 m² g⁻¹) was mixed with VGCF and sintering aid in the Al₄C₃–B₄C system. Approximately 1.5 g of the mixture was uniaxially pressed at 50 MPa to obtain a compact with a diameter of 20 mm and a thickness of approximately 1.5 mm. The resulting compact was hot-pressed at 1800 °C for 1 h in Ar atmosphere under a pressure of 62 MPa. The relative density of hot-pressed SiC composite decreased from 98.0 to 96.3%, whereas the fracture toughness was enhanced from 3.8 to 5.2 MPa m^1/2, as the amount of VGCF increased from 0 to 6 mass%. Furthermore, an acid treatment of VGCF was conducted to enhance its dispersibility within the SiC matrix, owing to the formation of COO⁻ groups on the VGCF surface. As a result of this treatment, the relative density and fracture toughness of hot-pressed SiC composite with 6 mass% acid-treated VGCF addition increased to 99.0% and 5.7 MPa m^1/2, respectively.     

Microstructure and mechanical properties of an HfB2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB₂–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB₂ + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min⁻¹ heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO₂. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB₂. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.