Effects of Postdeposition Treatments on the Mechanical Properties of a Chemical‐Vapor‐Deposited Silicon Carbide

Abstract

The influence of different postdeposition treatments such as water quench and thermal heating in air, nitrogen, and vacuum on mechanical properties of chemical-vapor-deposited (CVD) silicon carbide was investigated. The results showed that these postdeposition treatments increased the flexural strength by as much as 60% but did not significantly change other properties such as hardness and fracture toughness. The strength increase was achieved by treatments performed in both the oxidizing and nonoxidizing environments. Compressive residual stresses in CVD SIC increased because of these treatments, but this increase was not large enough to explain fully the observed increase in the flexural strength. It is proposed that these thermal treatments led to strength increase via healing of surface machining flaws. Thermal treatments in nonoxodizing environments reduced or blunted the flaws through the rearrangement of atoms and restoration of damaged crystal structure in SIC, while in oxidizing environments, passive oxidation may have served as an additional flaw healing mechanism.

Handbook of SiC properties for fuel performance modeling

“…The fracture toughness of SiC has been widely investigated by various test techniques: micro-and nano-indentation [32,71,76,78,87,[90][91][92]94,110,112,113,117,118], surface crack in flexure (SCF) [29,71,72,77,82,105,109,112,[119][120][121][122][123], double cantilever beam (DCB) [117], double torsion (DT) [75,106,117,124], single edge notched beam (SENB) [54,58,88,117,120,121,[125][126][127][128][129][130][131][132], Chevron notched beam (CNB) [121], and fractography [120]. The representative values determined by each test are sum...…”

“…The hardness of SiC was investigated by Vicker's [29,32,71,72,76,78,80,82,84,91,94,106,[109][110][111][112][113], Knoop [32,75,76,81] and nano-indentation techniques, [90][91][92][93] and summarized in Table 5. There appears to be no significant difference between Vicker's and Knoop hardness, while nano-hardness gives slightly higher values.…”

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

“…[1][2][3][4] Mechanical property data, such as fracture toughness and flexural strength, are needed to perform a brittle fracture analysis and evaluate the use of CVD SiC. Although fracture toughness values have been reported for many types of SiC over a range of temperatures and orientations, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] a limited amount of data are available for CVD SiC which are generally limited to room temperature. All the fracture toughness data reported for CVD SiC have been determined using an indentation or surface crack in flexure method.…”

“…All the fracture toughness data reported for CVD SiC have been determined using an indentation or surface crack in flexure method. 1,5,12 The objective of this work is to determine the fracture toughness of monolithic CVD SiC from room temperature to 1090°C using both chevron-notched and surface-cracked specimens to determine the change in fracture toughness with temperature and evaluate the influence of the test technique in determining the fracture toughness of this material. Flexural test results from monolithic CVD SiC are also used with the fracture toughness data to quantify the role of machining defects at the surface.…”

High Thermal Conductivity Materials

“…The flexural strength of CVD-SiC increased by 37% to 53% when it was heated in the temperature range of 600 • C to 1000 • C and quenched in water or annealed in flowing N 2 . In the case of thermal shock by water quenching, the increase started at 600 • C and became relatively constant above 800 • C. This increase was attributed to an increase in compressive residual stresses and partially due to healing of machining flaws [50].…”