Fracture Toughness and Flexural Strength of Chemically Vapor‐Deposited Silicon Carbide As Determined Using Chevron‐Notched and Surface Crack in Flexure Specimens

Abstract

Two different techniques used to measure the fracture toughness of silicon carbide (SiC) produced by chemical vapor deposition (CVD) are compared, and the influence of the material growth direction on toughness and flexural strength are evaluated. Fracture toughness values for monolithic CVD SiC are found to be independent of the CVD growth direction and test temperature from ambient to 1100°C with values ranging from 2.8 to 5.5 MPa·m1/2. This isotropic nature is notably different from hot‐pressed α‐SiC and is likely a result of the cubic β‐SiC structure produced by the CVD process and the transgranular fracture mode. The range of fracture toughness results obtained using a chevron‐notched specimen (2.8–4.0 MPa·m1/2) was less than values obtained using a surface crack in flexure method (3.7–5.5 MPa·m1/2), and these differences are statistically significant. These differences can be attributed to difficulties in resolving the true precrack size for the surface crack in the flexure specimen. Flexural strength values for monolithic CVD SiC are found to be isotropic with respect to the CVD growth direction from room temperature to 1090°C, as observed for the fracture toughness values. Flexural strength values measured at 1090°C are significantly higher (69% to 10% higher) than those measured at room temperature, while the modulus measured at 1090°C is less (−25% to −10% lower) than that measured at room temperature. Since the fracture toughness values are independent of temperature between room temperature and 1090°C, the increased flexural strength at elevated temperature is difficult to explain on the basis of classical fracture mechanics and must be influenced by other factors. Surface machining flaws on flexure specimens are shown to produce a 35% to 50% decrease in the flexure strength of monolithic CVD SiC that was quantified using the fracture toughness and flaw‐size data.

Processing and Testing of RE2Si2O7 Fiber–Matrix Interphases for SiC–SiC Composites

“…Taking elastic modulus and fracture toughness of SiC at 400 GPa and 3 MPa·m 1/2 , respectively, and considering:the fracture energy of SCS‐0 fiber ᴦ f = 22.5 J/m 2 .…”

Flexural Strength and Shear Strength of Silicon Carbide to Silicon Carbide Joints Fabricated by a Molybdenum Diffusion Bonding Technique

“…Flexural tests of monolithic CVD SiC and joined SiC were performed using a 4‐point bend fixture with an inner span of 19.05 mm and an outer span of 38.1 mm at composite testing and analysis (CT&A), State College, PA in accordance with ASTM C1161 22,23 methods, which have been described 26,27 . The tests were performed under displacement control at a constant crosshead displacement rate of 0.05 cm/min for the flexure tests and 0.072 cm/min for the shear tests using a precision load cell calibrated to a 2 kN force range for the flexure tests and 10 kN for the shear tests.…”

“…A linear voltage differential transformer (LVDT) was used to determine displacement for the flexure tests with a SiC rod in direct contact with the center of the outer span of the flexure specimen. This is not a true three‐probe extensometer, and the measured strain and Young's modulus determined from the flexure tests are influenced by the compliance of the load‐train, but are relative measures that can be used for comparisons among the data sets reported herein 26,27 . Displacement for the shear tests was determined from the crosshead speed based on time.…”

Production of Silicon Carbide Pieces by Immersion of Silicon Preforms in Carbonaceous Powders

“… SiC powder : α‐SiC is produced via solid‐state reaction between silicon dioxide and carbon (C) at very high temperatures (above 2200°C) in an electric arc furnace (Acheson method), 8 with various accessories that allow acceleration of the reaction process. Direct‐sintered SiC 9–11 (DSSiC) : Also commonly called pressureless sintered silicon (Si): This procedure is carried out at 2050°C–2175°C and comprises the addition of sintering aids: B+C or Al+C. Converted SiC : Si+C=SiC. A gas phase containing Si is infused in a C‐based solid. CVD process 12–16 : By thermally decomposing commercially available Si and C source gases SiH 4 +CH 4 =SiC+4H 2 . The precursor method 17 : Involves the reaction between phenol resins and silicate‐based liquid components to produce SiC. Silicon nitride bonded SiC 18–20 (NBSiC) : A mixture of SiC grain and finely divided elemental silicon is processed in a nitrogen atmosphere at temperatures below the melting point of silicon. Reaction bonded SiC 21,22 (RBSiC) : Produced by adding molten silicon to a mixture of SiC and C, and heating it in vacuum or atmosphere‐controlled furnaces at temperatures between 1500° and 1750°C. Composite‐bonded SiC : A high‐purity fine grain SiC is blended with elemental Si and a binder system. It is processed in nitrogen atmospheres at temperatures exceeding 1350°C. Oxy/nitride‐bonded SiC 23 : Similar to NBSiC, except for a slightly higher porosity, lower density and performance in wear applications. Clay‐bonded SiC 24 : A mixture of SiC grain and clay, which forms upon cooling a glass bond.…”

“…CVD process [12][13][14][15][16] : By thermally decomposing commercially available Si and C source gases SiH 4 1CH 4 5 SiC14H 2 .…”