Phase Transformation Of SiC Research Articles

Fabrication and Property of SiC Ceramic with Large Thickness/Diameter Ratios

Silicon carbide ceramics with different thicknesses/diameter ratios were prepared by using ultra-fine silicon carbide powder with the sintering additives of 1.0 wt% boron and 1.5 wt% carbon. The influence of thickness/diameter ratio on the microstructure and density of SiC ceramics was investigated in detail. The experimental results show that the addition of boron and carbon sintering aids can promote the densification process of SiC ceramic, leading to the low sintering temperature and improve mechanical properties. At 1950 °C, SiC ceramic with a density of 99% exhibits Young’s modulus, hardness, and flexural strength of 476 MPa, 28.3 GPa, and 334 MPa, respectively. It is found that long holding time has a positive effect on the uniformity of the microstructure and density distribution of SiC ceramics with large thickness/diameter ratios. Additionally, the sintering additive of boron can solid-solve into SiC, and then facilitate the phase transformation of SiC to form 6H-SiC and 4H-SiC composite ceramics.

Harmonized toughening and strengthening in pressurelessly reactive-sintered Ta0.8Hf0.2C-SiC composite

Ta0.8Hf0.2C-27 vol%SiC (99.0% in relative density) composite was toughened and strengthened via pressurelessly in-situ reactive sintering process. HfC and β-SiC particles were formed after reaction of HfSi2 and carbon black at 1650 °C. Ta0.8Hf0.2C was obtained from solid solutioning of HfC and commercial TaC. The β-α phase transformation of SiC proceeded below 2200 °C. High aspect ratio, platelet-like α-SiC grains formed and interconnected as interlocking structures. Toughness and flexural strength values of 5.4 ± 1.2 MPa m1/2 and 443 ± 22 MPa were measured respectively. The toughening mechanisms by highly directional growth of discontinuous α-SiC grains were crack branching, bridging and deflection behaviors.

Raman characterization of damaged layers of 4H-SiC induced by scratching

Recent development of device fabrication of SiC is awaiting detailed study of the machining of the surfaces. We scratched 4H-SiC surfaces with a sliding microindenter made of a SiC chip, and characterized machining affected layers by micro-Raman spectroscopy. The results of the Raman measurement of the scratching grooves revealed that there were residual stress, defects, and stacking faults. Furthermore, with heavy scratching load, we found clusters of amorphous SiC, Si, amorphous carbon, and graphite in the scratching grooves. Analysis of the Raman spectra showed that SiC amorphization occurs first and surface graphitization (carbonization) is subsequently generated through the phase transformation of SiC. We expect that the Raman characterization of machined surfaces provides information on the machining mechanism for compound semiconductors.

Effect of grain growth on electrical properties of silicon carbide ceramics sintered with gadolinia and yttria

The effect of grain growth on electrical properties of SiC ceramics sintered with a new additive system (Gd2O3–Y2O3) was investigated. Hot-pressing of β-SiC with 3vol% Gd2O3–Y2O3 in a nitrogen atmosphere resulted in highly conductive SiC ceramics (∼10−2–10−3Ωcm), whereas hot-pressing of the same specimen in an argon atmosphere resulted in semi-insulating SiC ceramics (∼108Ωcm). This difference was due to the nitrogen (N) doping in SiC ceramics in the former case. The N-doping in SiC ceramics was achieved by grain growth of SiC grains via a solution-reprecipitation mechanism. The electrical resistivity of N-doped SiC ceramics decreased with an increase in grain size of SiC ceramics if there is no β→α phase transformation of SiC. The lowest electrical resistivity obtained was 8.9×10−3Ωcm after 6h sintering at 2000°C under an applied pressure of 40MPa in nitrogen.

Structural Phase Transformation in SiC and PtC under Pressure

The study of pressure induced structural phase transition of silicon carbide and platinum carbide which crystallize in zinc blende structure (B3), has been carried out using the well described three body interaction potential model (TBIPM). Our present TBIP model consists of long range Coulombic, three body interaction and the short range overlap repulsive forces operative up to next nearest neighbor ions. These materials exhibit a first order phase transition from their ZnS (B3) to NaCl (B1) structure. The phase transition pressure for SiC and PtC are 94.5 GPa and 50GPa respectively.

TEM investigation of hot pressed ‐10 vol.%SiC–ZrB2 composite

The polytypism of SiC, phase transformation of ZrB2 and the interfaces between SiC and ZrB2 were investigated using high resolution TEM in a hot pressed 10 vol.%SiC‐ZrB2 composite. In most cases, no grain boundary interphases between hexagonal ZrB2 and 6H‐SiC phases were observed with SiC being both inter‐ and intragranular. Occasionally, 6H‐SiC transformed into 3C and 15R and hexagonal ZrB2 transformed into cubic ZrB. High resolution TEM showed no grain boundary interphases in most regions. Energy dispersive X‐ray spectroscopy and electron energy‐loss spectroscopy analyses showed the presence of oxygen throughout the sample. The phase transformation of SiC and ZrB2, and the interphase formation between SiC and ZrB2 grains are discussed.

고온단조에 의한 액상소결 탄화규소의 미세구조 및 기계적 특성

Two kind of -SiC powders of different particle sizes ( and ), containing 7 wt% , 2 wt% and 1 wt% MgO as sintering additives, were prepared by hot pressing at for 1 h under applied pressures, and then were hot-forged at for 6 h under 40 MPa in argon. All the hot-pressed specimens consisted of equiaxed grains and were developed grain growth after hot-forging. The smaller starting powder was developed the finer microstructure. The microstructures on the surfaces parallel and perpendicular to the pressing direction of the hot-forged SiC were similar to each other, and no texture development was observed because of the lack of massive to phase transformation of SiC. The fracture toughness (), hardness (~ 25.2 GPa) and flexural strength (480 MPa) of hot-forged SiC using larger starting powder were higher than those of the other.

Processing of porous silicon carbide with toughened strut microstructure

Porous SiC ceramics were fabricated by the carbothermal reduction of polysiloxane-derived SiOC containing hollow microspheres followed by sintering. The effect of the Al2O3–Y2O3–CaO (AYC) content on the microstructure and flexural strength of the porous SiC ceramics were investigated. The growth of large platelet α-SiC grains increased with increasing additive content due to enhanced grain growth and the β → α phase transformation of SiC. In situ toughened microstructures consisting of large platelet grains and fine equiaxed grains were obtained when 10 or 20 wt% AYC was added. The porosity generally decreased with increasing the additive content, whereas the flexural strength increased. The typical flexural strength of the porous SiC ceramics sintered with 10 wt% AYC was 61 MPa at a porosity of 44%.

Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxide Additives

SiC was sintered with AlN and Y2O3 as sintering additives by spark plasma sintering (SPS). Using nano-sized β-SiC powder as the starting material, the sintered density reached about 95% of theoretical at 1800-2000oC for 10min. The β to α phase transformation of SiC was not found by XRD. The secondary phase such as Y2O3 decreased as the firing temperature was elevated, and β-SiC monophase was identified at 2000oC. It seems that the residual intergranular glassy phase is present between the SiC grains. Significant SiC grain growth was observed at 1800-1900oC by SEM. The grain size decreased with increasing amount of AlN additive. The maximum value of flexural strength of the sample reached approximately 800MPa. These results are discussed on the basis of the liquid-phase sintering mechanism in AlN-Y2O3 and Al2O3-Y2O3 systems.

Atomic-level simulation of epitaxial recrystallization and phase transformation in SiC

A nano-sized amorphous layer embedded in an atomic simulation cell was used to study the amorphous-to-crystalline (a-c) transition and subsequent phase transformation by molecular-dynamics computer simulations in 3C–SiC. The recovery of bond defects at the interfaces is an important process driving the initial epitaxial recrystallization of the amorphous layer, which is hindered by the nucleation of a polycrystalline 2H–SiC phase. The kink sites and triple junctions formed at the interfaces between 2H– and 3C–SiC provide low-energy paths for 2H–SiC atoms to transform to 3C–SiC atoms. The spectrum of activation energies associated with these processes ranges from below 0.8 eV to about 1.9 eV.

Al-B-C 첨가 탄화규소의 스파크 플라즈마 소결에 의한 미세구조 발달

Densification of SiC powder with additives of total amount of 2,4,8 wt% Al-B-C was carried out by Spark Plasma Sintering (SPS). The unique features of the process are the possibilities of a very fast heating rate and a short holding time to obtain fully dense materials. The heating rate and applied pressure were kept at 100℃/min and 40 MPa, while the sintering temperature and holding time varied from 1700-1800℃ for 10-40 min, respectively. The SPS-sintered specimens with different amount of Al-B-C at 1800℃ reached near-theoretical density. The 3C→6H, 15R→4H phase transformation of SiC was enhanced by increasing the additive amount. The microstructure of SiC sintered up to 1750℃ consisted of fine equiaxed grains. In contrast, the growth of large of large elongated grains in small matrix grains was shown in sintered bodies at 1800℃, and the plate-like grains interlocking microstructure had been developed by increasing the holding time at 1800℃. The grain growth rate decreases with increasing amount of Al-B-C in SiC starting powder, however, the both of volume fraction and aspect ratio of large grains in sintered body increased.

Microstructure stability of fine-grained silicon carbide ceramics during annealing

Fine-grained silicon carbide ceramics with an average grain size of ∼140 nm or smaller were prepared by low-temperature hot-pressing of very fine β-SiC powders using Al2O3-Y2O3-CaO (AYC) or Y-Mg-Si-Al-O-N glass (ON) as sintering additives. The microstructure stability of the resulting fine-grained SiC ceramics was investigated by annealing at 1850°C and by evaluating quantitatively the grain growth behavior using image analysis. The β → α phase transformation of SiC in AYC-SiC was responsible for the accelerated abnormal grain growth of platelet-shaped grains. In contrast, the β → α phase transformation in ON-SiC was suppressed, which resulted in a very stable microstructure.

Microstructure and fracture toughness of in-situ-toughened SiC composites

Silicon carbide composites with toughened microstructure were prepared by two-step process based on liquid-phase sintering for densification and subsequent annealing for grain growth. The resulting composites consist of uniformly distributed elongated α-SiC grains, matrix-like TiC grains, and an amorphous grain boundary phase. The in-situ toughening is a result of the β-to-α phase transformation of SiC or the controlled grain growth of SiC during annealing. Crack deflection by the elongated α-SiC grains appears to account for the increased toughness of this new class of composites.

Effect of initial α-phase content of SiC on microstructure and mechanical properties of SiC–TiC composites

By using α- and β-SiC starting powders, the effects of initial α-phase content of SiC on microstructure and mechanical properties of the hot-pressed and subsequently annealed SiC–30 wt.% TiC composites were investigated. The microstructures developed were analyzed by image analysis. Their microstructures consist of uniformly distributed elongated α-SiC grains, equiaxed TiC grains and an amorphous grain boundary phase. During annealing, the β→α phase transformation of SiC leads to the in-situ growth of elongated α-SiC grains. The average diameter of SiC increases with increasing α-SiC content in the starting powder and the aspect ratio shows a maximum at 1% α-SiC and decreases with increasing α-SiC content in the starting powder. Such results suggest that microstructure of SiC–TiC composites can be controlled by changing α-SiC content in the starting powder. The strength increased with increasing α-SiC content when α-SiC content is higher than 10% while the fracture toughness decreased with increasing α-SiC content, i.e. the same trend with the variation of aspect ratio of SiC in the composites.

β⇒ α Phase transformation in SiC: link between polytypism, dihedral angle and nucleation mechanism of α twins

This paper deals with the β⇒ α phase transformation in SiC. The mechanism is studied in materials in which the transformation results in α twins, called feathers. The dihedral angle (2 α) between the basal planes of the twin grains is related to the mean polytype of the feathers. The 15R polytype is predominant (about 67%) in the studied materials. An analysis of the 15R diffraction pattern shows that four different stackings could be expected in 15R feathers. The stacking in both branches of 15R feathers is studied by electron diffraction and HRTEM. Only one out of the four stackings expected is observed when 2 α is large and two when 2 α is small. Based on this result and on the symmetry elements of feathers whatever their mean polytype is, a discussion of the nucleation mechanism is developed. ©

Silicon carbide ceramics prepared by pulse electric current sintering of β–SiC and α–SiC powders with oxide and nonoxide additives

Using a pulse electric current sintering (PECS) method, β–SiC and α–SiC powders doped with a few weight percent of Al2O3–Y2O3oxide or Al4C3–B4C–C nonoxide additives were rapidly densified to high densities (95.2–99.7%) within less than 30 min of total processing time. When Al2O3–Y2O3additive was used, both ceramics resulting from β–SiC and α–SiC had fine, equiaxed microstructures. In contrast, when Al4C3–B4C–C additive was used, the ceramic resulting from α–SiC had a coarse, equiaxed microstructure, whereas the ceramic resulting from β–SiC was composed of large elongated grains whose formation was accompanied by the β →?α phase transformation of SiC. Compared with the Al2O3–Y2O3-doped SiC ceramics, the Al4C3–B4C–C-doped SiC ceramics had higher densities, lower fracture toughness, and higher hardness. The fracture mode of the oxide-doped SiC was mainly intergranular, whereas the nonoxide-doped SiC exhibited almost complete intragranular fracture that was attributed to the higher interfacial bonding strength.

Effects of additive amount on microstructure and fracture toughness of SiCTiB 2 composites

Powder mixtures of β-SiC-30 wt% TiB 2 containing 5 wt% (ST1), 10 wt% (ST2), and 20 wt% (ST3) Al 2O 3Y 2O 3 as sintering additives were liquid-phase sintered at 1850 °C for 1 h by hot-pressing and subsequently annealed at 1950 °C for 2, 6, and 12 h. These materials had a microstructure of ‘in situ-toughened composites’ as a result of the β→α phase transformation of SiC during annealing. The introduction of larger amount of additives accelerated the grain growth of elongated α-SiC grains with higher aspect ratio, resulting in the improved fracture toughness. The fracture toughnesses of 12-h annealed materials with 5, 10, and 20 wt% additives were 5.7, 5.9, and 7.1 MPam sol1 2 , respectively.

In situ enhancement of toughness of SiC-TiB2 composites

A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC–30 wt%, 50 wt%, and 70 wt% TiB2 composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, relatively equiaxed TiB2 grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiB2 powders with the liquid forming additives of Al2O3 and Y2O3 by hot-pressing at 1850°C and subsequent annealing at 1950°C. The annealing led to the in situ growth of elongated α-SiC grains, due to the β→α phase transformation of SiC, and the coarsening of TiB2 grains. The fracture toughness of the SiC–50 wt% TiB2 composites after 6 h annealing was 7.3 MPa m1/2, approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m1/2). Bridging and crack deflection by the elongated α-SiC grains and coarse TiB2 grains appear to account for the increased toughness of the composites.

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 m1/2 for materials containing α-SiC seeds and 5.8 MPa m1/2 for pure β-SiC samples.

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‐m1/2, approximately 60% higher than that of as‐hot‐pressed composites (4.4 MPa‐m1/2). Bridging and crack deflection by the elongated α‐SiC grains appear to account for the increased toughness of this new class of composites.