Structural, thermal and mechanical properties of reaction bonded silicon carbide/silicon and diamond/silicon carbide composites
Date
2020
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Reactive sintering or Si infiltration is a proven technology for fabricating reaction bonded SiC/Si (RBSC) and diamond/SiC (RBSD) composites. Due to their outstanding properties of monolithic SiC and diamond, RBSC and RBSD promise great potentials in a variety of applications such as high energy laser (HEL) mirrors, armors, semiconductor processing platforms, and heat exchangers. This dissertation focuses on the relationship between microstructural, thermal and mechanical properties of RBSC and RBSD, as emphasized in the following three topics: determination of phase components and structural evolution of RBSC and RBSD, thermal transport characteristics in RBSC and RBSD, and investigation of interfacial strength of diamond/SiC. ☐ RBSC and RBSD composites have attracted significant research efforts in the past decade due to their outstanding and tailorable properties endowed by the corresponding monolithic counterparts. Just like any material systems, the phase components and distributions in RBSC and RBSD determine the composite properties and behaviors. Moreover, microstructural specifications, such as grain size and interfacial condition, are the common variables that have significant impact on the thermal and mechanical properties of the composites. In this work, the phase components and microstructures of RBSCs with a SiC fraction of 80 vol% or 90 vol% and RBSDs with a diamond fraction of 12 vol%, 17 vol%, 27 vol%, or 39 vol% were extensively examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD analysis indicates that RBSC consists of α-SiC, β-SiC, and Si and RBSD contains diamond, α-SiC, β-SiC, Si, and graphite, in which β-SiC and graphite are reaction formed phases. SEM study suggests that the reaction formed β-SiC can coherently grow on the preexisting SiC to form a core-rim structure. In addition, TEM characterization indicates that β-SiC includes large number of structural defects, such as dislocations and stacking faults. The formation mechanisms of core-rim SiC and defects are explained by a carbon dissolution and saturation mechanism, in which SiC precipitates from a carbon supersaturated Si solution and grows coherently on the preexisting SiC. The precipitation process is extremely fast and the rapid growth of β-SiC is responsible for the formation of defects inside. TEM reveals the existence of an amorphous Si transition layer in between SiC and Si grains and the amorphous Si is found to significantly affect the thermal conductivity of RBSC. Characterization of diamond/SiC interfacial region indicates that a thin graphite layer lies between diamond and SiC grains. The formation of graphite interlayer is related to the crystallization of amorphous carbon originally coated on the diamond grain as well as the diamond graphitization during fabrication. Moreover, the structurally disordered graphite interlayer limits the grain growth of reaction formed SiC, resulting in nanocrystalline SiC grains near diamond/SiC interface. At sporadic locations, Al4C3 phase exists near the interfacial area of diamond/SiC, which is believed to form during infiltration and solidification due to the existence of impurities in raw materials, a factor that has implications on the interfacial properties. ☐ Heat dissipation remains a significant challenge in applications for extreme environment and extreme precision, such as HEL mirrors, semiconductor wafer chucks and armors, since heat accumulation leads to device malfunction or failure. To establish RBSC and RBSD as premier candidates of material choices in these fields, it is essential to understand the thermal conductivity (κ) of the composites as a function of temperature and evaluate their ability of heat transfer in a wide range of temperatures. In this study, both experimental and computational methods are employed to investigate the thermal transport behaviors in RBSC and RBSD. The thermal conductivity κ of RBSCs and RBSDs from room temperature to 1000 °C was measured by laser flash technique. As expected, κ decreases as the temperature increases and the highest κ at room temperature is found to be 211 W/m∙K and 329 W/m∙K for RBSC with a SiC volume fraction of 90 vol% and RBSD with a diamond volume fraction of 39 vol%, respectively. By performing variance-reduced Mont Carlo simulation, together with the interfacial thermal resistance data calculated based on diffused mismatch model (DMM), the κ values and temperature profiles of the composites were calculated respectively for the composites over the same temperature range. The calculated κ values have good agreement with the experimental measurement and the calculated temperature profile across multiple grains indicates that interfaces and Si grains are primary thermal barriers in RBSD. The κ of RBSD shows a strong correlation with diamond fractions. When diamond fraction increases from 12 vol% to 39 vol%, the κ value of RBSD increases from 249 W/m∙K to 329 W/m∙K. A level-off of κ value at high diamond fraction above 27 vol% in RBSD reflects increased interfacial thermal resistance associated with the quantity of interphases. The composite structural evolution is evaluated for post-annealing samples and is also investigated by TEM in-situ heating experiments. Systematic assessments are further made for the effect of phase transformation on composite κ. The results indicate that amorphous Si crystallization and SiC grain growth contribute to the improvement of κ in RBSC, while diamond graphitization at high temperature significantly degrades the κ value of RBSD. Thermogravimetric analysis (TGA) is further performed to assess the thermal stability of RBSD under air condition for up to 1000 °C. Appreciable oxidation occurs at about 700 °C and the oxidation deteriorates significantly at 900 °C, especially for RBSD with higher diamond volume fraction. ☐ In addition to the composite systems, model systems of SiC thin films were prepared for investigating the thermal conductivity of nanocrystalline SiC and the interfacial thermal conductance of SiC/Si. The thermal conductivity of SiC was found to be around 1.3 W/m∙K and the highest thermal conductance of SiC/Si interface was measured to be 69.9 MW/m2∙K. ☐ Data in published literature suggest that RBSD composite can have excellent mechanical properties. However, the property does not generally follow the simple rule of mixtures presumably due the to interfacial conditions left from high temperature reaction of various species related to raw materials including graphite interlayer, nanocrystalline SiC, and Al4C3. There are limited research and data in evaluating the effects of the interfacial structures on the strength of RBSD. To better understand the correlation between microstructure and property, it is essential to explore the role of interface and the mechanistic failure characteristics during mechanical loading. Sub-micron specimens of Si from single crystal Si wafer and interfacial specimens of diamond/SiC from bulk RBSD were therefore prepared in adequate shape and size by utilizing Nano Patterning and Visualization Engine (NPVE) with Ga focused ion beam (FIB) in an Auriga® 60 FIB/SEM. A novel SEM in-situ tensile test was performed to study the tensile properties of single crystal Si and the interfacial tensile strength of diamond/SiC. The Si tensile specimen is used as reference for the purpose of validation of the tensile test. The measured tensile strength and strain of single crystal Si are larger than the reported data, reflecting a gauge volume effect. A brittle fracture was observed to happen at the interfacial area and the average interfacial tensile strength of diamond/SiC interface was measured to be 0.73 ± 0.22 GPa, significantly lower than that of monolithic diamond and SiC as well as that of SiC/Si interface, indicating the diamond/SiC interface is a weak link in RBSD. The low interfacial strength can be related to the interphases including graphite and Al4C3. X-ray computed tomography (CT) reveals that the crack not only propagates along the interface in composite but also penetrates across diamond and SiC grains, exhibiting both transgranular and intergranular failure modes. ☐ In summary, XRD indicates that RBSC consists of α-SiC, β-SiC, and Si and RBSD contains diamond, α-SiC, β-SiC, Si, and graphite. SEM study reveals a core-rim structure of SiC and a carbon dissolution and saturation mechanism is proposed to explain the core-rim formation. A graphite interlayer, amorphous Si, and Al4C3 were detected by TEM and these reaction formed interphases were further confirmed to have significant impacts on the thermal and mechanical properties of the composites. The κ values of RBSCs and RBSDs as a function of temperature were measured by laser-flash technique and κ decreases as temperature increases in both cases. The measured κ value of RBSD shows good agreement with the value calculated by solving BTE. In addition, as the diamond fraction increases in RBSD, so is the composite κ value. However, the increment becomes less appreciable when the diamond fraction is over 27 vol%. This limitation relates to a dictating thermal resistance induced by an increasing fraction of graphite and nanocrystalline SiC. Calculated temperature profile across multiple grains and interfaces reflects significant temperature drops at interfaces and in Si grains, which indicates that interfaces and Si grains are the primary thermal barriers in RBSD. The highest interfacial conductance in SiC/Si model system is found to be 69.9 MW/m2∙K and the conductance is believed to relate to the crystallinity of SiC, which determines the phonon band match of constituting phases across the interface. High temperature annealing induces phase transformation in RBSC and RBSD, which further affects the κ value of composites. Finally, the interfacial strength of diamond/SiC interface was measured by SEM in-situ tensile test and the study indicates the diamond/SiC interface is a weak link in RBSD.
Description
Keywords
In-situ microscopy, Interfacial strength, Microstructural characterization, RBSC, RBSD, Thermal conductivity