Microstructural development during spark plasma sintering of ZrB2–SiC–Ti composite

The present study investigates the effect of Ti addition on the microstructure development and phase evolution during spark plasma sintering of ZrB₂–SiC ceramic composite. A ZrB₂–20?vol% SiC sample with 15?wt% Ti was prepared by high-energy milling and spark plasma sintering at 2000?°C for 7?min under 50?MPa. The X-ray diffraction test, microstructural studies and thermodynamic assessments indicated the in-situ formation of several compounds due to the chemical reactions of Ti with ZrB₂ and SiC. The Ti additive was completely consumed during the sintering process and converted to the ceramic compounds of TiC, TiB and TiSi₂. In addition, another refractory phase of ZrC was also formed as a result of sidelong reaction of ZrB₂ and SiC with the Ti additive.     

TEM characterization of spark plasma sintered ZrB2–SiC–graphene nanocomposite

The interfacial behavior of spark plasma sintered ZrB₂–SiC nanocomposite doped with graphene nano-platelets was investigated by transmission electron microscopy (TEM). A powder mixture including ZrB₂ matrix, 20?vol% SiC and 10?vol% graphene was used as the starting material. X-ray diffraction analysis did not exhibit any in situ phase formation in the prepared nanocomposite. TEM observations verified the diffusion-controlled sintering. This study clarifies that graphene nano-platelets additive in the prepared nanocomposite did not engage in reactive sintering process, unlike many previous research studies addressing reactive sintering role for carbon additives.     

Spark plasma sintering of ZrC–SiC ceramics with LiYO2 additive

Spark plasma sintering (SPS) of ZrC–SiC composite powders in the presence of LiYO₂ sintering additive was studied. The starting powders were obtained by a carbothermal reduction (CTR) of natural mineral zircon (ZrSiO₄), which provided an intimate mixing of in-situ created ZrC and SiC powders. This composite powder and LiYO₂ as additive were densified by spark plasma sintering. Microstructural features of the composite were investigated by XRD, SEM/EDS and AFM analysis. The sintered composite material possesses promising mechanical properties and excellent cavitation resistance which was observed with a cavitation erosion test. The values of Vickers microhardness and fracture toughness of the composite material are 20.7 GPa and 5.07 MPam^1/2, respectively.     

Spark plasma sintering of quadruplet ZrB2–SiC–ZrC–Cf composites

Spark plasma sintering (SPS) route was employed for preparation of quadruplet ZrB₂–SiC–ZrC–C_f ultrahigh temperature ceramic matrix composites (UHTCMC). Zirconium diboride and silicon carbide powders with a constant ZrB₂:SiC volume ratio of 4:1 were selected as the baseline. Mixtures of ZrB₂–SiC were co-reinforced with zirconium carbide (ZrC: 0–10 vol%) and carbon fiber (C_f: 0–5 vol%), taking into account a constant ratio of 2:1 for ZrC:C_f components. The sintered composite samples, processed at 1800 °C for 5 min and 30 MPa punch press under vacuumed atmosphere, were characterized by densitometry, field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry as well as mechanical tests such as hardness and flexural strength measurements. The results verified that the composite co-reinforced with 5 vol% ZrC and 2.5 vol% C_f had the optimal characteristics, i.e., it reached a relative density of 99.6%, a hardness of 18 GPa and a flexural strength of 565 MPa.     

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.     

Spark plasma sintering of TiC–SiCw ceramics

Silicon carbide whiskers (SiC_w) in TiC had impressive impacts on the properties and made it possible for special applications which generally would not be conceivable with TiC alone. In the present work, SiC_w reinforced TiC based composites were prepared by spark plasma sintering (SPS) technique, at the temperature of 1900 °C under the pressure of 40 MPa for sintering time of 7 min. To test out the effects of different amount of SiC whisker (0, 10, 20 and 30 vol%) on the characteristics of TiC, the sintered samples were investigated about sinterability and physical-mechanical properties. Microstructure observations and density measurements confirmed that the composites were dense with uniformly distributed reinforcement, and the specimen doped with higher than 10 vol% SiC_w could attain higher relative density (>100%) than pure TiC and TiC–10 vol% SiC_w. Also, the highest values for hardness (29.04 GPa) and thermal conductivity (39.2 W/mK) were achieved in specimen containing 30 vol% SiC_w, whereas the optimum bending strength (644 MPa) was recorded in material containing 20 vol% SiC_w. It seems that one of the reasons which contributes to this trend of properties variation is the generation of near-stoichiometric TiC_x phase and new Ti₃SiC₂ compound.     

ZrB2–SiC多层陶瓷的抗氧化性

用传统陶瓷的流延工艺制备ZrB2–SiC多层陶瓷。用Archimedes法测定ZrB2–SiC多层陶瓷的相对密度。用扫描电子显微镜观察其显微结构,并进行循环抗氧化性能评价。结果表明：ZrB2–SiC多层陶瓷在1 950℃烧结的致密度达到99.7%,材料的抗氧化过程主要可分为两个阶段：第一阶段低熔点相的挥发,出现质量损失;第二阶段氧化层的形成,降低进一步氧化速率。抗氧化性能较ZrB2–SiC复相陶瓷有很大提高。     

Microstructure characterization of ZrB2–SiC composite fabricated by spark plasma sintering with TaSi2 additive

Dense ZrB₂–SiC composite was synthesized by spark plasma sintering with 10 vol.% TaSi₂ additive. When sintered at 1600 °C, core–shell structure was found existing in the sample. The core was ZrB₂ and the shell was (Zr,Ta)B₂ solid solution. This result was ascribed to the decomposition of TaSi₂ and the solid solution of Ta atoms into ZrB₂ grains. The solid solution process probably decreased the boride grain boundary active energy, contributing to the formation of coherent structure of grain boundaries. Additionally, the existence of dislocations in the boride grains indicated that the applied pressure also imposed an important effect on the densification of composite. When sintered at 1800 °C, owing to the atom diffusion, Ta atoms homogeneously distributed in the boride grains, leading to the disappearance of core–shell structure. The boundaries between (Zr,Ta)B₂ grains, as well as between boride grains and SiC particles, were still clear without amorphous phase existing.     

Spark plasma sintering of TaC–HfC UHTC via disilicides sintering aids

Ta_0.8Hf_0.2C ceramic has the highest melting point among the known materials (4000 °C). Spark plasma sintering is a new route for consolidation of materials, specially ultra high temperature ceramics (UHTCs), which are difficult to be sintered at temperatures lower than 2000 °C.The purpose of this study is to consolidate Ta_0.8Hf_0.2C by spark plasma sintering at low temperature using MoSi₂ and TaSi₂ as sintering aid. In this regard, effect of different amounts of sintering aids and carbides ratio on densification behavior and mechanical properties of Ta_1?xHf_xC were investigated.Fully consolidation of Ta_0.8Hf_0.2C was achieved in presence of 12 vol.% sintering aid after sintering at 1650 °C for 5 min under 30 MPa. The first stage of sintering was due to plastic deformation of sintering aids particles and consequent rearrangement. The second stage was occurred via Ta_1?xHf_xC solid solution and liquid phase formation.     

Mechanical properties of ZrB2–SiC ceramics prepared by polymeric precursor route

ZrB₂–SiC composite ceramics with varying compositions (6.4, 22.3, and 61.5 vol% ZrB₂–SiC) were synthesized and spark plasma sintered (SPS) for 30 min under argon atmosphere. Ceramics showed relatively uniformly distributed phases with small spherical crystallized grains. Vickers hardness and fracture toughness of ceramics were measured, and scratch and tribological behaviors of sintered ceramic specimens were also investigated. According to experimental results, materials having different inter- and trans-granular fractures showed different wear loss, friction efficient, and tribofilm morphology. Ceramics chemically reacted with moisture while being tribotested, leading to the formation of a tribofilm on the bottom of wear track. Characteristics of silica/hydride silica revealed the formation of tribofilms with different morphologies, thereby implying that several key factors are involved in determining the efficiency of this process.     

Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics

New high temperature negative temperature coefficient (NTC) thermistor ceramics based on a xMgAl₂O₄–(1 − x)YCr_0.5Mn_0.5O₃ (x = 0.1, 0.4, 0.6) composite system have been successfully fabricated through spark plasma sintering (SPS) with a low sintering temperature and a short sintering period. The X-ray diffraction analysis indicates that the SPS-sintered composite ceramics consist of a cubic spinel MgAl₂O₄ phase and an orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase isomorphic to YCrO₃. The SPS-sintered composite ceramics have high relative density ranging from 94.1 to 97.4% of the theoretical density. X-ray photoelectron spectroscopy analysis corroborates the presence of Cr³⁺, Cr⁴⁺, Mn³⁺, and Mn⁴⁺ ions on lattice sites, which may result in the hopping conduction. The obtained ρ₂₅, B_25–150, and B_700–1000 of the SPS-sintered composite NTC thermistors are in the range of 1.53 × 10⁶–9.92 × 10⁹ Ω cm, 3380–5172 K, and 7239–9543 K, respectively. These values can be tuned by adjusting the MgAl₂O₄ concentration.     

Ablation of C/SiC,C/SiC–ZrO2 and C/SiC–ZrB2 composites in dry air and air mixed with water vapor

C/SiC composites with different additives (ZrO₂ and ZrB₂) were fabricated by CVI and CVD and their oxidation and ablation properties at 1700–1800 °C were investigated. Two different ablation test conditions, dry air and air mixed with water vapor, are compared. The ablation test results are reviewed, the weight loss rates are presented and the corresponding micro-structures are investigated in detail. The results show that in dry air, the weight loss rate of C/SiC composites is greater than those with ZrO₂ and ZrB₂ additives. However, in air mixed with water vapor (5 wt%) to simulate the hygrothermal condition, the weight loss rates of these three composites all become relatively smaller. A model is proposed to predict the weight loss of C/SiC composites and it agrees well with the experimental data.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

C/SiC–ZrB2–ZrC composites fabricated by reactive melt infiltration with ZrSi2 alloy

C/SiC–ZrB₂–ZrC composites were prepared by reactive melt infiltration (RMI) combined with vacuum pressure impregnation (VPI) method. B₄C–C was first introduced into C/SiC composites with a porosity of about 30% by impregnating the mixture of B₄C and phenol formaldehyde resin, followed by pyrolysis at 900 °C. The molten ZrSi₂ alloy was then infiltrated into the porous C/SiC–B₄C–C to obtain C/SiC–ZrB₂–ZrC composites. The flexural strength was tested. The ablation behavior was investigated under an oxyacetylene torch flame. It has been found that the C/SiC–ZrB₂–ZrC showed a high flexural strength and an excellent ablation resistance. The reactions between ZrSi₂ alloy and B₄C–C were studied, and a model based on these reactions was built up to describe the formation mechanism of the matrix.     

ZrB2–SiC gradient oxidation protective coating for carbon/carbon composites

To improve the oxidation protective ability of carbon/carbon composites, ZrB₂–SiC gradient coating was prepared on the surface of C/C composites by an in-situ reaction method. The ZrB₂–SiC gradient coating consisted of an inner ZrB₂–SiC layer and an outer ZrB₂–SiC–Si coating. The phase composition and microstructures of the multiphase coating were characterized by XRD, EDS and SEM. Results showed that the inner coating is mainly composed of ZrB₂ and SiC, while the outer multiphase coating is composed of ZrB₂, SiC and Si. The multilayer coating is about 200 μm in thickness, which has no penetration crack or big hole. The oxidation behavior of the coated C/C composites at 1773 K in air was investigated. Results show that the gradient ZrB₂–SiC oxidation protective coating could protect C/C from oxidation for 207 h with only (4.56±1.2)×10⁻³ g/cm² weight loss, owing to the compound silicate glass layer with the existence of thermally stable phase ZrSiO₄.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Compressive creep behavior of spark plasma sintered La2O3–YSZ composite

The present paper describes compressive creep behavior of cubic 8 mol% yttria stabilized zirconia+10 mol% La₂O₃ (fabricated by Spark Plasma Sintering) in the temperature range of 1300–1330 °C at a stress level of 45–78 MPa in vacuum. The pre- and post-creep microstructures, relative magnitudes of the stress exponent (n=1.7–2.1) and the activation energy (540–580 kJ/mol) suggest that grain boundary sliding aided by inter-diffusion of La and Zr leading to the formation of pyrochlore La_1.6Y_0.4Zr₂O₇ phase at the grain boundaries during creep is the active creep mechanism in this composite.     

Spark plasma sintering and high-temperature strength of B6O–TaB2 ceramics

Tantalum diboride – boron suboxide ceramic composites were densified by spark plasma sintering at 1900 °C. Strength and fracture toughness of these bulk composites at room temperature were 490 MPa and 4 MPa m^1/2, respectively. Flexural strength of B₆O–TaB₂ ceramics increased up to 800 °C and remained unchanged up to 1600 °C. At 1800 °C a rapid decrease in strength down to 300 MPa was observed and was accompanied by change in fracture mechanisms suggestive of decomposition of boron suboxide grains. Fracture toughness of B₆O–TaB₂ composites showed a minimum at 800 °C, suggestive a relaxation of thermal stresses generated from the mismatch in coefficients of thermal expansion.Flexural strength at elevated temperatures for bulk TaB₂ reference sample was also investigated.Results suggest that formation of composite provides additional strengthening/toughening as in all cases flexural strength and fracture toughness of the B₆O–TaB₂ ceramic composite was higher than that reported for B₆O monoliths.     

Pressureless sintering of ZrB2–SiC composite laminates using boron and carbon as sintering aids

Abstract

A processing method common to composite ceramics with very different ZrB₂/SiC ratios was developed in order to exploit ZrB₂–SiC laminates comprising alternate layers with different compositions for thermal protection systems of re-entry vehicles. Ceramic laminates were made using SiC, ZrB₂ and composites with a SiC/ZrB₂ ratio ranging from 100 vol.-%SiC to 100 vol.-%ZrB_2. The preparation was performed by tape casting of a slurry, layer stacking, debinding and pressureless sintering. Boron and carbon proved to be suitable sintering aids for SiC laminates as well as for composite laminates containing SiC. In the case of composites with a ZrB₂ matrix, the second phase of SiC acted as a sintering aid. This process obtains very similar densification for specimens with very different compositions (ranging from 100%SiC to 80ZrB₂–20SiC). The stiffness of ZrB₂–SiC laminates increased with the ZrB₂ content increase, while the bending strength was not affected by the ZrB₂/SiC ratio.     

Effect of ball milling on reactive spark plasma sintering of B4C–TiB2 composites

The influence of mechanical activation by ball milling (BM) of Ti, B and graphite powders mixture on the synthesis of dense B₄C-41% vol. TiB₂ composite by Spark Plasma Sintering (SPS) is investigated. BM treatment produces grains size refinement (50–150 nm) in the processing powders and the formation of TiB and TiB₂, when milling times are longer than 6 h.