Effect of Annealing Conditions on Microstructural Development and Phase Transformation in Silicon Carbide

The effect of annealing with and without applied pressure on the microstructural development and phase transformation was investigated in fine-grained β-SiC ceramics containing α-SiC seeds. Materials annealed without pressure had a microstructure consisting of elongated grains, while materials annealed with pressure showed a duplex microstructure consisting of small matrix grains and some of elongated grains. However, annealing with pressure (25 MPa) was found to greatly retard phase transformation from β→α polytypes and inhibit grain growth. This change in lattice parameter suggests that the retardation of phase transformation and grain growth might be attributed to a reduced mass transport rate, which is the result of Al being introduced into the SiC by the annealing pressure.     

Microstructural Development of Silicon Carbide Containing Large Seed Grains

Fine (}0.1μm) β-SiC powders, with 3.3 wt% large (}0.44μm) α-SiC or β-SiC particles (seeds) added, were hot-pressed at 1750°C and then annealed at 1850°C to enhance grain growth. Microstructural development during annealing was investigated using image analysis. The introduction of larger seeds into β-SiC accelerated the grain growth of elongated large grains during annealing, in which no appreciable β→α phase transformation occurred. The growth of matrix grains in materials with β-SiC seeds was slower than that in materials with α-SiC seeds. The material with β-SiC seeds, which was annealed at 1850°C for 4 h, had a bimodal microstructure of small matrix grains and large elongated grains. In contrast, the material with α-SiC seeds, also annealed at 1850°C for 4 h, had a uniform microstructure consisting of elongated grains. The fracture toughnesses of the annealed materials with α-SiC and β-SiC seeds were 5.5 and 5.4 MPa·^1/2, respectively. Such results suggested that further optimization of microstructure should be possible with β-SiC seeds, because of the remnant driving force for grain growth caused by the bimodal microstructure.     

Novel process for recrystallized silicon carbide through β-α phase transformation

A novel process was investigated to produce recrystallized silicon carbide through β-α phase transformation. The specimen was prepared from carbon and β-SiC powder mixture, first by infiltration with liquid silicon at 1500 °C to form β-SiC preform with a high density, and then by heating it further up to 2200 °C. When the β-SiC particles were transformed into α-SiC at 2200 °C, the rapid grain growth occurred of the α-SiC by consuming β-SiC particles, resulting in an interconnected network structure with huge and elongated α-SiC grains. The specimen recrystallized at 2200 °C had a measured density of 2.7 g/cm³ and strength of 134 MPa. The infiltration behavior, microstructure evolution and mechanical properties of the recrystallized silicon carbide were examined and discussed.     

Microstructural Analysis of Liquid-Phase-Sintered β-Silicon Carbide

The microstructures of fine-grained β-SiC materials with α-SiC seeds annealed either with or without uniaxial pressure at 1900°C for 4 h in an argon atmosphere were investigated using analytical electron microscopy and high-resolution electron microscopy (HREM). An applied annealing pressure can greatly retard phase transformation and grain growth. The material annealed with pressure consisted of fine grains with β-SiC as a major phase. In contrast, the microstructure in the material annealed without pressure consisted of elongated grains with half α-SiC. Energy-dispersive X-ray analysis showed no differences in the amount of segregation of aluminum and oxygen atoms at grain boundaries, but did show a significant difference in the segregation of yttrium atoms at grain boundaries along SiC grains for the two materials. The increased segregation of yttrium ions at grain boundaries caused by the applied pressure might be the reason for the retarded phase transformation and grain growth. HREM showed a thin secondary phase of 1 nm at the grain boundary interface for both materials. The development of a composite grain consisting of a mixture of β/α polytypes during annealing was a feature common to both materials. The possible mechanisms for grain growth and phase transformation are discussed.     

Effect of annealing on grain growth and Y segregation behavior in tetragonal ZrO2 thin film

In order to fabricate tetragonal yttria stabilized zirconia samples with large grain size, 3 mol% Y₂O₃ doped zirconia thin films were grown on (0001) α-Al₂O₃ substrate by pulsed laser deposition (PLD) followed by subsequent high temperature annealing. The thin film samples were annealed at 1200°C, 1250°C, 1300°C, and 1350°C in order to obtain larger grain size without Y segregation. The microstructure and chemical composition of these annealed films were analyzed using atomic force microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The as-grown thin film was found to be composed of [111]-oriented grains of ∼100 nm connected with small-angle tilt boundaries. Based on analysis of annealed thin films, it was revealed that grain growth of tetragonal zirconia occurred anisotropically. Cross section scanning transmission electron microscopy observations revealed that such grain growth behavior is affected by the step-terrace structures of the sapphire substrate. Energy-dispersive X-ray spectroscopy showed that Y was found to distribute almost uniformly below 1300°C but to segregate at the grain boundaries at 1350°C. As a conclusion, the 1300°C-annealed sample shows the largest grain size with homogeneous Y distributions.     

Mechanical Characterization of Annealed ZrB2–SiC Composites

Composites consisting of 70 vol% ZrB₂ and 30 vol% α‐SiC particles were hot pressed to near full density and subsequently annealed at temperatures ranging from 1000°C to 2000°C. Strength, elastic modulus, and hardness were measured for as‐processed and annealed composites. Raman spectroscopy was employed to measure the thermal residual stresses within the silicon carbide (SiC) phase of the composites. Elastic modulus and hardness were unaffected by annealing conditions. Strength was not affected by annealing at 1400°C or above; however, strength increased for samples annealed below 1400°C. Annealing under uniaxial pressure was found to be more effective than annealing without applied pressure. The average strength of materials annealed at 1400°C or above was ~700 MPa, whereas that of materials annealed at 1000°C, under a 100 MPa applied pressure, averaged ~910 MPa. Raman stress measurements revealed that the distribution of stresses in the composites was altered for samples annealed below 1400°C resulting in increased strength.     

Formation of porous SiC ceramics via recrystallization

In this study, porous SiC ceramics with interconnected huge plate-like grains were fabricated from oxidized β-SiC powder with 1 wt% B₄C. When the β–α SiC phase transformation occurred at 2100 °C, rapid grain growth of α-SiC consumed the unstable β-SiC matrix resulting in an interconnected network structure with huge plate-like grains. The oxidation of β-SiC powder and the addition of B₄C are necessary conditions for rapid grain growth. The observed results are discussed based on thermodynamic considerations. The measured porosity of the specimens sintered at 2100 °C for 30 min was 47% and the mean pore size was 6–7 μm. The strength of the sintered specimen was 45 ± 5 MPa.     

Effect of LaB6 addition on compressive creep behavior with simultaneous oxide scale growth of spark plasma sintered ZrB2-SiC composites

A study has been carried out to examine the effect of LaB₆ addition on the compressive creep behavior of ZrB₂-SiC composites at 1300–1400°C under stresses between 47 and 78 MPa in laboratory air. The ZrB₂-20 vol% SiC composites containing LaB₆ (10% in ZSBCL-10 and 14% in ZSBCL-14) besides 5.6% B₄C and 4.8% C as additives were prepared by spark plasma sintering at 1600°C. Due to cleaner interfaces and superior oxidation resistance, the ZSBCL-14 composite has exhibited a lower steady-state creep rate at 1300°C than the ZSBCL-10. The obtained stress exponent (n ∼ 2 ± 0.1) along with cracking at ZrB₂ grain boundaries and ZrB₂-SiC interfaces are considered evidence of grain boundary sliding during creep of the ZSBCL-10 composite. However, the values of n ∼ 1 and apparent activation energy ∼700 kJ/mol obtained for the ZSBCL-14 composite at 1300–1400°C suggest that ZrB₂ grain boundary diffusion is the rate-limiting mechanism of creep. The thickness of the damaged outer layer containing cracks scales with temperature and applied stress, indicating their role in facilitating the ingress of oxygen causing oxide scale growth. Decreasing oxidation-induced defect density with depth to a limit of ∼280 μm, indicates the predominance of creep-based deformation and damage at the inner core of samples.     

High temperature properties of the monolithic CVD β-SiC materials joined with a pre-sintered MAX phase Ti3SiC2 interlayer via solid-state diffusion bonding

Monolithic high purity CVD β-SiC materials were successfully joined with a pre-sintered Ti₃SiC₂ foil via solid-state diffusion bonding. The initial bending strength of the joints (∼ 220 MPa) did not deteriorate at 1000 °C in vacuum, and the joints retained ∼ 68 % of their initial strength at 1200 °C. Damage accumulation in the interlayer and some plastic deformation of the large Ti₃SiC₂ grains were found after testing. The activation energy of the creep deformation in the temperature range of 1000 – 1200 °C in vacuum was ∼ 521 kJmol⁻¹. During the creep, the linkage of a significant number of microcracks to form a major crack was observed in the interlayer. The Ti₃SiC₂ interlayer did not decompose up to 1300 °C in vacuum. A mild and well-localized decomposition of Ti₃SiC₂ to TiC_x was found on the top surface of the interlayer after the bending test at 1400 °C in vacuum, while the inner part remained intact.     

The effect of SPS parameters on the production of yttria transparent ceramic

In this work, the spark plasma sintering (SPS) of commercial yttria nanopowder is investigated. The SPS parameters such as sintering temperature, applied pressure, and dwell time are varied. Densification without grain growth occurring at occurred up to a sintering temperature of 1400°C and grain growth without further densification taking place at the higher temperature. The optimum sample was obtained at a temperature of 1400°C with a pressure of 70 MPa and dwelling time of 15 minutes. The highest relative density of 99.8% and the average grain size of 1.26 μm were obtained at 1400°C. The yttria ceramic annealed at 1200°C had the in-line transmission of 5%-70% and 70%-82% in the visible and infrared wavelength region, respectively. The measurements of hardness and fracture toughness characteristics of the transparent yttria ceramic showed 9.2 GPa and 2.24 MPa.m^1/2, respectively.     

Effect of β-to-α Phase Transformation on the Microstructural Development and Mechanical Properties of Fine-Grained Silicon Carbide Ceramics

Ultrafine β-SiC powders mixed with 7 wt% Al₂O_3, 2 wt% Y₂O₃, and 1.785 wt% CaCO₃ were hot-pressed and subsequently annealed in either the absence or the presence of applied pressure. Because the β-SiC to α-SiC phase transformation is dependent on annealing conditions, the novel processing technique of annealing under pressure can control this phase transformation, and, hence, the microstructures and mechanical properties of fine-grained liquid-phase-sintered SiC ceramics. In comparison to annealing without pressure, the application of pressure during annealing greatly suppressed the phase transformation from β-SiC to α-SiC. Materials annealed with pressure exhibited a fine microstructure with equiaxed grains when the phase transformation from β-SiC to α-SiC was <30 vol%, whereas materials annealed without pressure developed microstructures with elongated grains when phase transformation was >30 vol%. These results suggested that the precise control of phase transformation in SiC ceramics and their mechanical properties could be achieved through annealing with or without pressure.     

Temperature and processing effects on lithium ion conductivity of solution-deposited lithium zirconium phosphate (LiZr2P3O12) thin films

Lithium zirconium phosphate (LiZr₂P₃O₁₂) thin films have been prepared on platinized silicon substrates via a chemical solution deposition approach with processing temperatures between 700°C and 775°C. Films that were subject to a single high-temperature anneal were found to crystallize at temperatures above 725°C. Crystallization was observed in films annealed after each deposited layer at 700°C and above. In both cases, grain size was found to increase with annealing temperature. Ion conductivity was found to increase with annealing temperature in singly annealed films. In per-layer annealed films ion conductivity was found to initially increase then decrease with increasing annealing temperature. A maximum ion conductivity of 1.6 × 10⁻⁶ S/cm was observed for the singly annealed 775°C condition, while a maximum ion conductivity of 5.8 × 10⁻⁷ S/cm was observed for the 725°C per-layer annealed condition. These results are consistent with an increasing influence of cross-plane, internal interface resistance and vapor phase carrier loss in the per-layer annealed samples. This work demonstrates that post-deposition processing methods can strongly affect the ion conducting properties of LiZr₂P₃O₁₂ thin films.     

In S/Yu-Toughened Silicon Carbide-Titanium Carbide Composites

A process based on liquid-phase sintering and subsequent annealing for grain growth is presented to obtain in situ -toughened SiC-30 wt% TiC composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiC powders with the liquid forming additives of A1₂O₃ and Y₂O₃ by hot pressing. During the subsequent heat treatment, the β→α phase transformation of SiC led to the in situ growth of elongated α-SiC grains. The fracture toughness of the SiC-30 wt% TiC composites after 6-h annealing was 6.9 MPa-m^1/2, approximately 60% higher than that of as-hot-pressed composites (4.4 MPa-m^1/2). Bridging and crack deflection by the elongated α-SiC grains appear to account for the increased toughness of this new class of composites.     

Spark plasma sintered ZrC-Mo cermets: Influence of temperature and compaction pressure

The microstructure analysis and mechanical characterisation were performed on a ZrC-20 wt%Mo cermet that was spark plasma sintered at various temperatures ranging between 1600 and 2100 °C under either 50 or 100 MPa of compaction pressure. The composite reached ~98% relative density for all experiments with an average grain size between 1 and 3.5 µm after densification. The nature of SPS technology caused a faster densification rate when higher compaction pressures were applied. The difference in compaction pressures produced different behaviors in densification and grain structure: 1900 °C, 100 MPa produced excessive grain growth in ZrC; 1600 °C, 50 MPa revealed a very clear ZrC grain structure and Mo diffusion between carbide grains; and 2100 °C, 50 MPa exhibited the highest overall mechanical properties due to small clusters of Mo phases across the microstructure. In fact, this particular sintering regime gave the most optimal mechanical values: 2231 HV10 and 5.4 MPa*m^1/2, and 396 GPa Young's modulus. The compaction pressure of SPS played a pivotal role in the composites’ properties. A moderate 50 MPa pressure caused all three mechanical properties to increase with increasing sintering temperature. Conversely, a higher 100 MPa pressure caused fracture toughness and Young modulus to decrease with increasing sintering temperature.     

Effect of AlN-content on the microstructure and fracture toughness of hot-pressed and heat-treated LPS–SiC ceramics

The influence of additive content on the microstructural development of hot-pressed and heat-treated LPS–SiC has been investigated using AlN–Y₂O₃ mixtures at a molar ratio of 80:20, varying the total amount from 5, 10, 15 to 20 wt.%. Specimen were hot-pressed at 1900 °C for 1 h in nitrogen atmosphere under an applied pressure of 25 MPa and subsequently heat-treated at 2000 °C for 1, 2, 4 and 8 h.It has been found that the transformation rate of β- into α-SiC is retarded by higher AlN-contents and the formation of the 6H α-SiC polytype is favored. Furthermore, grain growth during annealing is also effectively inhibited. While hardness remained almost unchanged, fracture toughness varied with additive content and/or duration of the heat-treatment. Fracture toughness increased during the first 1 or 2 h of annealing,depending on the AlN-content, and diminishing for more prolonged treatments. The maximum fracture toughness has been determined for samples containing 10 wt.% of additives, hot-pressed and annealed during 1 h.     

Effect of pressure and temperature on densification,microstructure and mechanical properties of spark plasma sintered silicon carbide processed with β-silicon carbide nanopowder and sintering additives

The effects of applied pressure and temperature during spark plasma sintering (SPS) of additive-containing nanocrystalline silicon carbide on its densification, microstructure, and mechanical properties have been investigated. Both relative density and grain size are found to increase with temperature. Furthermore, with increase in pressure at constant temperature, the relative density improves significantly, whereas the grain size decreases. Reasonably high relative density (~96%) is achieved on carrying out SPS at 1300 °C under applied pressure of 75 MPa for 5 min, with a maximum of ~97.7% at 1500 °C under 50 MPa for 5 min. TEM studies have shown the presence of an amorphous phase at grain boundaries and triple points, which confirms the formation of liquid phase during sintering and its significant contribution to densification of SiC at relatively lower temperatures (≤1400 °C). The relative density decreases on raising the SPS temperature beyond 1500 °C, probably due to pores caused by vaporization of the liquid phase. Whereas β-SiC is observed in the microstructures for SPS carried out at temperatures ≤1500 °C, α-SiC evolves and its volume fraction increases with further increase in SPS temperatures. Both hardness and Young׳s modulus increase with increase in relative density, whereas indentation fracture toughness appears to be higher in case of two-phase microstructure containing α and β-SiC.     

Microstructure evolutions of SiC whiskers at high temperature and its effects on silicon carbide ceramics

SiC whisker with a single-crystal structure is promising in enhancing the strength and toughness of advanced structural ceramics, owing to its excellent properties. However, studies on its microstructure evolution at high temperature (>2000 °C) are scarce. Herein, SiC whiskers were calcined at 2100 °C, and XRD, SEM, and TEM were employed to analyze microstructure evolutions. Compared with raw whiskers, XRD results indicated serious annihilation of stacking faults after calcination. The annihilation led to the fracture of whiskers and the formation of β-SiC grains, and then partial grains underwent the phase transformation to form hexagonal prism and triangular prism α-SiC grains with diameters of about 10 μm, according to SEM and TEM results. Furthermore, SiC ceramics containing different whisker contents were innovatively fabricated by pressureless solid-state sintering. The flexural strength and fracture toughness of SiC ceramic containing 10 vol% whiskers were 540 MPa and 5.1 MPa m^0.5, resulting in 38% and 11% higher values than those without whiskers, respectively.     

Complex impedance spectroscopy study of a liquid-phase-sintered α-SiC ceramic

The electrical response of a liquid-phase-sintered (LPS) α-SiC with 10 wt.% Y₃Al₅O₁₂ (YAG) additives was studied from near-ambient temperature up to 800 °C by complex impedance spectroscopy. The electrical conductivity of this LPS SiC ceramic was found to increase with increasing temperature, which was attributed to the semiconductor nature of the SiC grains. It was concluded that the contribution of the SiC grains to the electrical conductivity of the LPS SiC ceramic at moderate temperatures (<450 °C) is a somewhat greater than that of the YAG phase. In contrast, at higher temperatures the SiC grains control the electrical conductivity of the LPS SiC ceramic. It was also found that there are two activation energies for the electrical conduction process of the α-SiC grains. These are 0.19 eV at temperatures lower than ∼400 °C and 2.96 eV at temperatures higher than ∼500 °C. The existence of two temperature-dependence conduction regimes reflects the core–shell substructure that develops within the SiC grains during the liquid-phase sintering, where the core is pure SiC (intrinsic semiconductor) and the shell is mainly Al-doped SiC (extrinsic semiconductor).     

Additive-free superhard B4C with ultrafine-grained dense microstructures

A unique combination of high-energy ball-milling, annealing, and spark-plasma sintering has been used to process superhard B₄C ceramics with ultrafine-grained, dense microstructures from commercially available powders, without sintering additives. It was found that the ultrafine powder prepared by high-energy ball-milling is hardly at all sinterable, but that B₂O₃ removal by gentle annealing in Ar provides the desired sinterability. A parametric study was also conducted to elucidate the role of the temperature (1600–1800 °C), time (1–9 min), and heating ramp (100 or 200 °C/min) in the densification and grain growth, and thus to identify optimal spark-plasma sintering conditions (i.e., 1700 °C for 3 min with 100 °C/min) to densify completely (>98.5%) the B₄C ceramics with retention of ultrafine grains (∼370 nm). Super-high hardness of ∼38 GPa without relevant loss of toughness (∼3 MPa m^1/2) was thus achieved, attributable to the smaller grain size and to the transgranular fracture mode of the B₄C ceramics.     

The influence of different SiC amounts on the microstructure,densification, and mechanical properties of hot-pressed Al2O3-SiC composites

The hot pressing process of monolithic Al₂O₃ and Al₂O₃-SiC composites with 0-25 wt% of submicrometer silicon carbide was done in this paper. The presence of SiC particles prohibited the grain growth of the Al₂O₃ matrix during sintering at the temperatures of 1450°C and 1550°C for 1 h and under the pressure of 30 MPa in vacuum. The effect of SiC reinforcement on the mechanical properties of composite specimens like fracture toughness, flexural strength, and hardness was discussed. The results showed that the maximum values of fracture toughness (5.9 ± 0.5 MPa.m^1/2) and hardness (20.8 ± 0.4 GPa) were obtained for the Al₂O₃-5 wt% SiC composite specimens. The significant improvement in fracture toughness of composite specimens in comparison with the monolithic alumina (3.1 ± 0.4 MPa.m^1/2) could be attributed to crack deflection as one of the toughening mechanisms with regard to the presence of SiC particles. In addition, the flexural strength was improved by increasing SiC value up to 25 wt% and reached 395 ± 1.4 MPa. The scanning electron microscopy (SEM) observations verified that the increasing of flexural strength was related to the fine-grained microstructure.