Mechanical‐ and Physical‐Property Changes of Neutron‐Irradiated Chemical‐Vapor‐Deposited Silicon Carbide

Abstract

Indentation and density measurements have revealed important changes in the mechanical and physical properties of silicon carbide (SiC) due to neutron irradiation. Specifically, the changes in the elastic modulus, hardness, fracture toughness, and density with irradiation have provided an understanding of the expected performance of SiC and SiC composites in nuclear applications. After the accumulated damage has saturated, these mechanical properties were affected primarily by the irradiation temperature. Chemical-vapor-deposited (CVD) SiC was irradiated above the saturation fluence and yielded volumetric swelling of 2.6% and 1.3% for irradiation temperatures of 100°-150°C and 500°-550°C, respectively. At the same respective temperatures, the elastic modulus decreased from an unirradiated value of 503 GPa to ∼420 and 450 GPa. Conversely, the hardness increased from 36 GPa for the unirradiated CVD SiC to 38 and 40 GPa for the samples irradiated at 100°-150°C and 500°-550°C, respectively. Interestingly, these two independent properties approached almost-constant levels after exposure to a fluence of 0.5 × 10 25 n/m 2 , E > 0.1 MeV. Indentation fracture toughness measurements, which were within the range of values in the literature for conventional fracture toughness procedures for SiC, increased from ∼2.8 MPaؒm 1/2 for the unirradiated samples to 3.7 and 4.2 MPaؒm 1/2 for the samples that were irradiated at 100°-150°C and 500°-550°C, respectively.

Evaluation of neutron irradiated silicon carbide and silicon carbide composites

“…It was reported that HiNicalon™ fibers underwent shrinkage of approximately 1.8% following neutron irradiation at 500-550°C [18], while identically irradiated CVD SiC underwent swelling of approximately 1.1% [18] in agreement with the 500°C irradiation of this work. The magnitude of swelling of the composites reinforced with Hi-Nicalon™ fibers seems high considering the shrinkage of the fiber, if fiber/matrix interfacial bonding was rigid.…”

“…The reduction of elastic modulus has not been previously reported by flexural evaluation, although some decrease of elastic modulus has been reported by application of indentation techniques [18,22,23] to neutron irradiated materials. As discussed by Katoh elsewhere [24] the slight reduction in elastic modulus is consistent with and a result of the lattice expansion.…”

Evaluation of Fiber/Matrix Interfacial Strength of Neutron Irradiated SiC/SiC Composites Using Hysteresis Loop Analysis of Tensile Test

“…4.1 Neutron irradiation effects on tensile elastic modulus The slight loss of tensile elastic modulus after neutron irradiation at 1273 K was similar to the decrease irradiated at 1073 K. 8) According to previous reports, moduli of HiNicalonÔ Type-S single fiber by tensile test 14) and CVD-SiC by nano-indentation techniques, 15,16) decreased $10% at 1 dpa, and kept this level up to at least 10 dpa in the temperature range from 423 to 823 K. At 1023 to 1273 K, …”

Mechanical Testing of Silicon Carbide on MISSE-7

“…This, in turn, will be a function of manufacturing, so even the same type of SiC may have different strengths according to processing conditions. Previous research has shown that the mechanical [5], physical [6], and electronic [7] properties of SiC may be altered by radiation in the form of high-energy neutrons more typical of a nuclear reactor than LEO. The effect of atomic oxygen on the optical properties of polished SiC in a simulated LEO environment showed that oxidation of SiC, especially at pre-existing microcracks, degraded the reflectance of the material [8].…”