Elevated-Temperature Fracture Resistance of a Sintered α-Silicon Carbide

The fracture toughness of a dense, sintered commercial α-silicon carbide was determined for temperatures from 20° to 1400°C using both straight- and chevron-notched test specimens and also controlled-surface-microflaw specimens, all in three-point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R -curve behavior, both of which are independent of temperature from 20° to 1400°C.     

Sea-Salt Corrosion and Strength of a Sintered α-Silicon Carbide

A commercially available, sintered silicon carbide was exposed to a temperature of 982°C for up to 50 h in a burner rig pressurized to 500 kPa. Synthetic sea salt added to the flame (5 ppm) resulted in the deposition of sodium sulfate and formation of a sodium magnesium silicate corrosion product. A 16% reduction in room-temperature strength occurred after 5 h of exposure; this reduction was due to the formation of surface pits. Exposure for longer times resulted in continued strength reduction, up to 56% at 25 h. Samples exposed for 50 h were so degraded that mechanical tests could not be conducted. The strength after 25 h of exposure to a salt concentration of 2 ppm was similar to the as-received strength, whereas exposures to 10 ppm of salt resulted in strengths similar to that observed with 5 ppm of salt.     

Nonequiaxial Grain Growth and Polytype Transformation of Sintered α-Silicon Carbide and β-Silicon Carbide

α(6 H )- and β(3 C )-SiC powders were sintered with the addition of AlB₂ and carbon. α-SiC powder could be densified to ∼98% of the theoretical density over a wide range of temperatures from 1900° to 2150°C and with the additives of 0.67–2.7 mass% of AlB₂ and 2.0 mass% of carbon. Sintering of the β-SiC powder required a temperature of >2000°C for densification with these additives. Grains in the α-SiC specimens grew gradually from spherical-shaped to plate-shaped grains at 2000°C; the 6 H polytype transformed mainly to 4 H . On the other hand, grains in the β-SiC largely grew at >2000°C; the 3 C polytype transformed to 4 H , 6 H , and 15 R . The stacking faults introduced in grains were denser in β-SiC than in α-SiC. The rapid grain growth in the β-SiC specimen was attributed to polytype transformation from the unstable 3 C polytype at the sintering temperature.     

Effects of Sodium Silicate Exposure at High Temperature on Sintered α-Silicon Carbide and Siliconized Silicon Carbide

Sintered α-silicon carbide and siliconized silicon carbide were exposed to combustion off-gas containing sodium silicate vapors and particulates in a combustion test facility for 24 to 373 h at 900° to 1050°C. Degradation was evaluated by measuring dimensional changes, by measuring loss in strength due to changes in flaw population, and by evaluating surface corrosion morphology. It is suggested that passive oxidation and dissolution of the silica oxidation scale play an important role in the corrosion process. These mechanisms were enhanced by the continuous removal and replenishment of corrosive material by the high-velocity gas. These degradation phenomena caused surface pitting and an approximately 50% reduction in strength for both materials after long-term exposure (>100 h). Morphological evaluation suggested that the grain boundaries in the α-silicon carbide were oxidized more rapidly than the grains, while for the case of the siliconized silicon carbide the silicon phase was oxidized rapidly along with preferential oxidation of the silicon carbide grains parallel to the {0001} plains.     

Plasma-Induced Morphological Changes in α-Silicon Carbide

Significant alterations in the surface morphology of α-silicon carbide powders have been observed after exposure to a low-temperature Ar plasma. Surface alterations were observed in 3 min at 1900°C and 30 min at 1600°C. Powders heated with plasma exposure were characterized by contact development and particle rounding whereas without the plasma substantial faceting was observed at higher temperatures.     

Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: III, Sintered α-Silicon Carbide

The operative and controlling mechanisms of steady-state creep in sintered α-SiC have been determined both from kinetic data within the ranges of temperature and constant compressive stress of 1670 to 2073 K and 138 to 414 MPa, respectively, and from the results of extensive TEM and other analytical analyses. Dislocations in glide bands, B₄C precipitates, and the interaction of these two entities were the dominant microstructural features of the crept material. The stress exponent increased from 1.44 to 1.71 with temperature; it was not a function of stress at a given temperature. The curves of In ɛ vs 1/ T showed a change in slope at 1920 ± 20 K. The respective activation energies below and above this temperature interval were 338 to 434 and 802 to 914 kJ/mol. A synthesis of all this information leads to the conclusion that the controlling creep mechanism at low temperatures is grain-boundary sliding accommodated by grain-boundary self-diffusion; at high temperatures, the controlling mechanism becomes grain-boundary sliding accommodated by lattice diffusion. The parallel mechanism of dislocation glide contributes increasingly to the total strain as the number/volume of precipitates declines as a result of progressive coalescence with increasing temperature.     

Densification and Properties of α-Silicon Carbide

Densification of a-Sic powders with no premixed sintering aids (type 1) and with premixed B and C (type 2) was investigated by sintering them at 2150° to 2200°C for 30 min. Flexure strengths, Weibull moduli, and fracture flaws were characterized for type 2 α-SiC only. The results were compared with those for a state-of-the-art sintered a-Sic material.     

Thermoelastic Micromechanical Stresses Associated with a Large α-Silicon Carbide Grain in a Polycrystalline β-Silicon Carbide Matrix

The thermoelastic micromechanical stresses associated with a single large hexagonal α-SiC grain within a fine-grain-size cubic (3C) β-SiC matrix were calculated. The naturally occurring residual stresses which are created during cooling from the processing temperatures and the effects of superimposed applied external stresses are both considered. A significant effect of the shape or geometry of the α-SiC grain is revealed, with the largest residual stresses associated with the naturally occurring tabular or platelet structure. The stresses are compared with the published strength results for these materials, which suggests that the residual stresses assume a significant role in the strength reduction that is observed.     

Active Corrosion of Sintered α-Silicon Carbide in Oxygen–Chlorine Gases at Elevated Temperatures

The active corrosion of sintered α-silicon carbide from heat exchanger tubes in the temperature range 900° to 1100°C in gas mixtures containing 2% Cl₂ by volume with additions of O₂ or H₂ has been investigated by thermogravimetric analysis and subsequent examination of the corrosion products. The presence of a small amount of oxygen accelerated rapid active corrosion in chlorine-containing gas mixtures, but the corrosion was suppressed by an active-to-passive transition when the concentration of oxygen in the gas mixture was too high. Low rates of attack were observed in the environments containing H₂ even when the chlorine potential was high. The concentration of oxygen necessary to produce the active-to-passive transition was found to vary from one material to another and may be related to the amount of excess carbon in the sintered silicon carbide.     

Effects of Active Oxidation on the Flexural Strength of α-Silicon Carbide

The effects of oxygen partial pressure ( P _o2) on the oxidation behavior and room-temperature flexural strength of sintered α-SiC were investigated. Groups of flexure bars were exposed at 1400°C to flowing Ar containing various levels of oxygen ( P _o2 ranging from 7.5 × 10⁻⁷ to 1.5 × 10⁻⁴ MPa). The changes in weight, flexural strength, and surface morphology of the samples were strongly influenced by the P _o2 level. When the P _o2 was higher than 3 × 10⁻⁵ MPa, SiO₂ was formed on the surface (i.e., passive oxidation occurred) and the strengths of the samples were not significantly affected. However, when the P _o2 was lower than 2 × 10⁻⁵ MPa, material loss occurred (active oxidation), decreasing the weight and strength of the samples. Both the reduction in strength and the weight loss resulting from active oxidation were proportional to the P _o2. An approximately 50% reduction in strength was observed in the SiC after oxidation for 20 h at a P _o2 of 1.5 × 10⁻⁵ MPa, a level that is slightly lower than the P _o2 at which the transition from active to passive oxidation occurs. Large pits formed during exposure were responsible for the reduction in strength.     

Origin of Density Gradients in Sintered β-Silicon Carbide Parts

Sintered densities of β-silicon carbide compacts were observed to decrease as the thickness of the compact increased. The chemistry and density gradients of these compacts were analyzed, as well as the weight loss and the CO evolution during sintering. The sintering cycle was altered to include a vacuum hold between the temperatures of 1400° and 1700°C, which resulted in increased density. Depending on the temperature of the vacuum soak, densities of 98% could be achieved on tiles with a thickness greater than 2.5 cm. The density gradients within the compact were significantly reduced, but not totally eliminated. It is suggested that these gradients are due to the effect of microstructural coarsening which occurs simultaneously with CO evolution.     

Creep of 6H α-Silicon Carbide Single Crystals

The compressive creep behavior of single-crystal 6H α-SiC was measured for orientations parallel to and at 45° to [0001]. Deformation of the 45° orientation was dominated by basal slip. Steady-state creep rates above 10^-7/s were measured at temperatures as low as 800°C. An activation energy of 277 kJ/mol and a stress exponent of 3.32 were determined. Creep testing with applied stresses parallel to [0001] was performed at 1650°C to 1850°C, yielding a stress exponent and activation energy of 4.93 and 180 kJ/mol, respectively. The occurrence of basal slip in the [0001] specimens suggested that significant off-axis stresses were present during testing.     

Occurrence and Distribution of Boron-Conitaining Phases in Sintered ş-Silicon Carbide

A variety of optical and analytical instruments have been employed to observe and characterize the mlcrostructure and composition of the B-containing phases which occur in sintered α-SiC as a result of either their use as a nensification aid or that which evolves as a result of annealing well below the sintering temperatures. The former have been identified as B₄C containing a small amount of Si. The latirr occur as ∼20-nm precipitates which have also been tentatively identified as B₄C and are believed to contain trace quantities of Si. No B phase was observed on the SiC grain boundaries; furthermore, the precipitate formation was not enhanced by the application of stress.     

Reactions between α-Silicon Carbide Ceramic and Nickel or Iron

The interactions of very pure and practically 100% dense α-SiC with nickel and iron at 850°C were investigated. Large reaction zones, consisting of carbon and silicides of Ni and Fe, were examined by microprobe analysis. By means of marker experiments, the diffusion mechanisms were investigated: only Ni and Fe migrate through the reaction layer. Isothermal sections of the ternary phase diagrams Ni-Si-C and Fe-Si-C at 850°C are presented.     

Correlation Between Microstructure and Creep Behavior in Liquid-Phase-Sintered α-Silicon Carbide

The influence of increasing the sintering time from 1 to 7 h on the microstructure evolution and the mechanical properties at high temperature was studied in α-silicon carbide (α-SiC) sintered in argon atmosphere with Y₂O₃–Al₂O₃ (10% weight) as liquid phase (LPS-α-SiC). The density decreased from 98.8% to 94.9% of the theoretical value, the grain size increased from 0.64 to 1.61 μm, and some of the grains became elongated. The compression tests were performed in argon atmosphere, between 1450°C and 1625°C and stresses between 25 and 450 MPa, with the strain rate being between 4.2 × 10⁻⁸ and 1.5 × 10⁻⁶ s⁻¹. The stress exponent n and the activation energy Q were determined, finding values of n between 2.4±0.1 and 4.5±0.2 and Q =680±35 kJ/mol for samples sintered for 1 h, and n between 1.2±0.1 and 2.4±0.1 and Q =710±90 kJ/mol for samples sintered for 7 h. The correlation between these results and the microstructure indicates that grain-boundary sliding and the glide and climb of dislocations, both accommodated by bulk diffusion, may be two independent deformation mechanisms operating. At the temperatures of the tests, the existence of solid-state reactions between SiC and the sintering additives is responsible of the microstructural changes observed. These effects are not a consequence of the process of deformation, but rather they are because of the thermal treatment of the material during the creep.     

Influence of Pyridine–Borane on the Sintering Behavior and Properties of α-Silicon Carbide

In the present study, α-SiC powder is coated with pyridineborane (BH₃·C₅H₅N), a liquid molecular compound, which forms a boron carbonitride (BC_3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d ₅₀= 1.3 μm and a narrow grain size distribution ( d ₁₀= 0.6 μm, d ₉₀= 2.2 μm) are detected in the liquid-phase-processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.     

Effect of Atmosphere on Weight Loss in Sintered Silicon Carbide during Heat Treatment

Heat treatment was performed on β-SiC with different sintering additives in the temperature range 1873–2073 K, in both argon and nitrogen-gas atmospheres. In the case of the specimens heat-treated at 2073 K in argon, the weight loss was more than the total weight of the sintering additives, except for B,C-doped β-SiC. On the other hand, weight loss was suppressed by about one-third to one-half in nitrogen gas. Weight loss depended mainly on the reaction at the interface between the SiC grains and the grain-boundary phase.     

Corrosion Kinetics and Strength Degradation of Sintered δ-Silicon Carbide in Potassium Sulfate Melts

Pressureless sintered α-SiC ceramics containing carbon and boron as sintering aids and hot-pressed SiC containing aluminum nitride as a sintering aid were corroded in K₂SO₄ melts at 1080° to 1150°C. SiC ceramics were oxidized and dissolved into K₂SO₄ melts. Since the corrosion of SiC ceramics in K₂SO₄ melts proceeded autocatalytically, a reaction product such as K₂S_1.44 was suspected to promote the corrosion reaction. The corrosion resistance of SiC containing AlN in K₂SO₄ melt was superior to that of SiC containing boron and carbon. Apparent activation energies for the corrosion of SiC ceramics were 309 to 331 kJ mol^-1. The fracture strength of the specimens corroded by K₂SO₄ melt degraded to 40% to 70% of the original values up to a 20% weight loss and then was almost constant up to 45% weight loss.     

Dislocations in as-Grown CVD α-Silicon Nitride

CVD αSi₃N₄ has been studied by transmission electron microscopy, and dislocations are found to occur in as-grown material. Two-beam contrast analysis has been used to determine that these dislocations have Burgers vectors of (0001), 1/3(2110), and 1/3(2113). Because of their smaller Burgers vector, (0001) dislocations are dominant. Their dissciation into half partials has been observed.