Low temperature consolidation for fine-grained zirconium carbide from nanoparticles with ZrH2 as sintering additive

Zirconium carbide (ZrC_0.84O_0.13) nanopowders were consolidated using plasma activated sintering with 0–8 wt% ZrH₂ as the sintering additive to improve the sinterability. Compared with pure ZrC sintering, ZrH₂ additive led to the higher sintering kinetics and lower sintering temperature. This improvement was attributed to the increased carbon-vacancy concentration in the non-stoichiometric ZrC in the presence of ZrH₂ additive during the sintering process. Fully dense and fine-grained ZrC ceramics (1.3 ± 0.2 μm) were achieved at 1650 °C with 6 wt.% ZrH₂. The final product exhibited the Vicker’s hardness of 21.2 ± 1.0 GPa and fracture toughness of 2.2 ± 0.3 MPa m^1/2.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.     

ZrB2–SiC composites prepared by reactive pulsed electric current sintering

Fully densified ZrB₂–20 vol% SiC composites were produced by reactive pulsed electric current sintering (PECS) of a powder mixture containing ZrH₂, B, SiC and B₄C within a total thermal cycle time of only 50 min. During the combined synthesis and sintering process, the ZrH₂ powder decomposed gradually from ZrH₂ into ZrH_m and finally metal Zr that reacted with elemental B to form the ZrB₂ matrix. Reducing the ZrH₂ particle size by attritor milling significantly enhanced densification and allowed initiation of self-propagating high temperature synthesis (SHS) during PECS. The PECS grades exhibited a slightly textured structure, with ≤17% of the ZrB₂ grains oriented with their (0 0 1) planes perpendicular to the direction of pressure and DC current. Because of the ZrB₂ grain orientation, anisotropic mechanical properties were observed. Ceramics prepared from attritor milled powders and PECS with a pressure applied after 5 min upon reaching 1900 °C achieved excellent flexural strengths of 901–937 MPa. The hardness and fracture toughness were respectively 19.7–19.8 GPa and 4.0–4.7 MPa m^1/2 in the direction parallel and 20.2–21.3 GPa and 3.8–3.9 MPa m^1/2 in the direction perpendicular to the applied pressure.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Ablation behavior of C/C-ZrC and C/SiC-ZrC composites fabricated by a joint process of slurry impregnation and chemical vapor infiltration

C/C-ZrC and C/SiC-ZrC composites were fabricated by a joint process of slurry impregnation and chemical vapor infiltration, in which ZrC matrix was obtained by slurry impregnation process, while C or SiC matrix was introduced by chemical vapor infiltration process. The as fabricated C/C-ZrC and C/SiC-ZrC composites have densities of 1.67 g cm^?3 and 1.91 g cm^?3 respectively. Tensile strength is 89.4±8.4 MPa and 182.2±14.0 MPa respectively for the as prepared C/C-ZrC and C/SiC-ZrC. Ablation behavior of the C/C-ZrC and C/SiC-ZrC composites under air plasma was studied and compared in detail. Due to different oxidation resistance and heat transfer capacity of the matrix, these two ZrC based composites showed various ablation behavior. The linear erosion rate is 48 µm s^?1 and 39 µm s^?1 respectively for C/C-ZrC and C/SiC-ZrC composites.     

Anisotropy of functional properties of SiC composites with GNPs,GO and in-situ formed graphene

This paper reports on anisotropy of functional properties of different silicon carbide-graphene composites due to preferential orientation of graphene layers during sintering. Dense silicon carbide/graphene nanoplatelets (SiC/GNPs) and silicon carbide/graphene oxide (SiC/GO) composites were sintered in the presence of yttria (Y₂O₃) and alumina (Al₂O₃) sintering additives at 1800 °C in vacuum by the rapid hot pressing (RHP) technique. It is found that electrical conductivity of SiC/GNPs and SiC/GO composites increases significantly in the perpendicular direction to the RHP pressing axis, reached up to 1775 S/m in the case of SiC/GO (for 3.15 wt.% of rGO). Also, thermal diffusivity was found to increase slightly by the addition of GNPs in the SiC/GNPs composites in the perpendicular direction to the RHP pressing axis. But, in the parallel direction, the addition of GNPs showed a negative effect. The formation of graphene domains was observed in reference sample SiC-Y₂O₃-Al₂O₃ sintered by RHP, without any addition of graphene. Their presence was confirmed indirectly by increasing electrical conductivity about three orders of magnitude in comparison to the reference sample sintered by conventional hot press (HP). Raman, SEM and TEM analysis were used for direct evidence of presence of graphene domains in RHP reference sample.     

The processing and properties of (Zr,Hf)B2–SiC nanostructured composites

(Zr, Hf)B₂–SiC nanostructured composites were fabricated by high energy ball milling and reactive spark plasma sintering (RSPS) of HfB₂, ZrSi₂, B₄C and C. Highly dense composites with homogeneously intermixed ultra-fine (Zr, Hf)B₂ and SiC grains (100–300 nm) were obtained after RSPS at 1600 °C for 10 min. The densification was promoted by high energy ball milling and ZrSi₂ additive. The additives were almost completely transformed into ZrB₂ and SiC during densification. The improvement of flexural strength and fracture toughness (641 MPa and 5.36 MPa m^1/2, respectively) was achieved. The relationships between the ultra-fine microstructure and mechanical properties were discussed.     

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.     

In-situ fabrication of Al2O3–SiC nanocomposites using B2O3 as sintering aid

Al₂O₃–SiC nanocomposites with 5 and 10 vol% SiC have been in-situ fabricated by sol-gel method followed by carbothermal reduction of alumina–silica gel using B₂O₃ as sintering aid. Green bodies were formed by cold isostatic pressing of calcined gel, which was prepared by an aqueous sol-containing aluminum chloride, TEOS, sucrose and boric acid. Pressureless sintering was carried out in Ar–12%H₂ atmosphere at 1700 °C. Addition of B₂O₃ (1 or 3 wt%) was an effective densification aid in the Al₂O₃–5 vol% SiC composites, while the densification of Al₂O₃–10 vol% SiC composites was not affected by adding B₂O₃. The composite material containing 5 vol% SiC doped with 3 wt% B₂O₃ reached 98.7% of full density. Nano-sized β-SiC particles were formed in-situ by means of a reaction between mullite and carbon at 1600 °C. Scanning electron microscopy revealed that the spherical in-situ synthesized SiC nanoparticles were well distributed through the composite and located predominantly to the interior of alumina matrix grains.     

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.     

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%.     

Microstructure and properties of B4C-SiC composites prepared by polycarbosilane-coating/B4C powder route

B₄C-SiC composites with fine grains were fabricated with hot-pressing pyrolyzed mixtures of polycarbosilane-coated B₄C powder without or with the addition of Si at 1950 °C for 1 h under the pressure of 30 MPa. SiC derived from PCS promoted the densification of B₄C effectively and enhanced the fracture toughness of the composites. The sinterability and mechanical properties of the composites could be further improved by the addition of Si due to the formation of liquid Si and the elimination of free carbon during sintering. The relative density, Vickers hardness and fracture toughness of the composites prepared with PCS and 8 wt% Si reached 99.1%, 33.5 GPa, and 5.57 MPa m^1/2, respectively. A number of layered structures and dislocations were observed in the B₄C-SiC composites. The complicated microstructure and crack bridging by homogeneously dispersed SiC grains as well as crack deflection by SiC nanoparticles may be responsible for the improvement in toughness.     

Effects of high-energy ball milling and reactive spark plasma sintering on the densification of HfC-SiC composites

The combined effects of high-energy ball milling (HEBM) and reactive spark plasma sintering (R-SPS) of HfSi₂ and C powder mixture on the densification and microstructure of nanostructured HfC-SiC composites were investigated. HEBM significantly promoted the densification and improved the microstructure of the HfC-SiC composites. In contrast, the reactions between HfSi₂ and C did not directly promote the densification of the HfC-SiC composites. While the reaction was mostly completed at 1300 °C, the onset temperature of significant densification was 1610 °C. Fine and homogeneously distributed HfC and SiC particles formed by HEBM and R-SPS were the key factors for promoting the densification of the HfC-SiC composites. The fine particles had high surface energy, which provided enough driving force for densification. In addition, the homogeneously distributed SiC particles effectively suppressed the growth of HfC matrix grains during densification.     

Fabrication and microstructure of 3D Cf /ZrC-SiC composites: through RMI method with ZrO2 powders as pore-making agent

3D C_f/ZrC–SiC composites were prepared by a combination process of slurry infiltration and reactive melt infiltration. ZrO₂ powders and ZrSi₂ alloy, both of which reacted with amorphous carbon, were used as pore-making agent and infiltrator, respectively. After carbothermal reduction at 1650 °C, X-ray diffraction analysis revealed that ZrO₂ powders were completely converted into ZrC by reacting with amorphous carbon, and an in-situ formed submicron porous configuration was observed at the areas containing ZrO₂. Results showed that the matrix in composites mainly consisted of SiC, ZrC and a small quantity of residual metal. SEM and TEM images revealed the formation of ZrC or SiC intergranular particles in the matrix and the characteristic around the residual resin carbon. The composites had a bending strength of 94.89±16.7 MPa, fracture toughness of 11.0±0.98 MPa m^1/2, bulk density of 3.36±0.01 g/cm³, and open porosity of 4.64±0.40%. The formation mechanisms of ZrC–SiC dual matrix and intrabundles׳ structure were discussed in the article.     

Reaction Synthesis and Mechanical Properties of TiB2–AlN–SiC Composites

TiB₂–AlN–SiC (TAS) ternary composites were prepared by reactive hot pressing at 2000°C for 60 min in an Ar atmosphere using TiH₂, Si, Al, B₄C, BN and C as raw powders. The phase composition was determined to be TiB₂, AlN and β-SiC by XRD. The distribution of elements Al and Si were not homogeneous, which shows that to obtain a homogeneous solid solution of AlN and SiC in the composites by the proposed reaction temperatures higher than 2000°C or time duration longer than 60 min are needed. The higher fracture toughness (6·35±0·74 MPa·m^1/2 and 6·49±0·73 MPa·m^1/2) was obtained in samples with equal molar contents of AlN and SiC (TAS-2 and TAS-5) in the TAS composites. The highest fracture strength (470±16 MPa) was obtained in TAS-3 sample, in which the volume ratio of TiB₂/(AlN+SiC) was the nearest to 1 and there was finer co-continuous microstructure. ©     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

SiC whisker reinforced multi-carbides composites prepared from B4C and pyrolyzed rice husks via reactive infiltration

SiC whisker reinfored carbide-based composites were fabricated by a reactive infiltration method by using Si as the infiltrate. Rice husks (RHs) were pyrolyzed to SiC whiskers, particles and amorphous carbon, and were then mixed with different contents of B₄C as well as Mo powders. The mixtures were molded to porous preforms for the infiltration. The SiC whiskers and particles in the preform remained in the composite. Molten Si reacted with the amorphous carbon, B₄C as well as Mo in the preform during the infiltration, forming newly SiC, B₁₂(C,Si,B)₃ as well as MoSi₂. The upper values of elastic modulus, hardness and fracture toughness of the composites are 297.8 GPa, 16.8 ± 0.8 GPa, and 3.8 ± 0.2 MPa m^1/2, respectively. The influence of the phase composition of the composites on the mechanical properties and the fracture mechanism are discussed.     

High-strength ZrB2-based ceramics prepared by reactive pulsed electric current sintering of ZrB2–ZrH2 powders

Fully densified ZrB₂-based ceramic composites were produced by reactive pulsed electric current sintering (PECS) of ZrB₂–ZrH₂ powders within a total thermal cycle time of only 35 min. The composition of the final composite was directly influenced by the initial ZrH₂ content in the starting powder batch. With increasing ZrH₂ content, ZrB₂–ZrO₂, ZrB₂–ZrB–ZrO₂ and ZrB₂–ZrB–Zr₃O composites were obtained. The ZrB₂–ZrB–ZrO₂ composite derived from a 9.8 wt% ZrH₂ starting powder exhibited an excellent flexural strength of 1382 MPa combined with a Vickers hardness of 17.1 GPa and a fracture toughness of 5.0 MPa m^1/2. The high strength was attributed to a fine grain size and the removal of B₂O₃ through reaction with Zr. Higher ZrH₂ content starting powders were densified through solution-reprecipitation resulting in the formation of coarser angular ZrB₂–ZrB composites with a Zr₃O grain boundary phase with a fracture toughness of 5.0 MPa m^1/2 and an acceptable strength in the 852–939 MPa range.     

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).     

Si/SiC coated Cf/SiC composites via tape casting and reaction bonding: The effect of carbon content

In this paper, a novel surface modification method for C_f/SiC composites is proposed. Si/SiC coating on C_f/SiC composites is prepared by tape casting and reaction bonding method. The effects of carbon content on the rheological property of the slurries along with the microstructure of the sintered coatings are investigated. The best result has been obtained by infiltrating liquid silicon into a porous green tape with a carbon density of 0.84 g/cm³. In addition, the effect of sintering parameters on the phase composition of the coatings is studied. Dense Si/SiC coating with high density as well as strong bonding onto the substrate is obtained. This Si/SiC coating exhibits an excellent mechanical property with HV hardness of 16.29±0.53 GPa and fracture toughness of 3.01±0.32 MPa m^1/2. Fine surface with roughness (RMS) as low as 2.164 nm is achieved after precision grinding and polishing. This study inspires a novel and effective surface modification method for C_f/SiC composites.