Silicon Carbide Diamond

Silicon carbide diamond is an exceptional material with exceptional wear resistance that is particularly appealing for applications involving wear resistance. Production can take place without pressure by infiltrating porous carbon bonded diamond preforms with liquid silicon.

The infiltration process results in the formation of a dense graphitic layer at the diamond/silicon interface that offers exceptional strength.

Thermal Conductivity

Thermal conductivity is one of the key properties that makes silicon carbide diamond an appealing material choice for high-performance electronic devices, including semiconductor lasers, high power transistors, optical amplifiers and power LEDs. A high thermal conductivity in diamond/SiC composite allows efficient heat transfer through it while minimizing energy losses at its interfaces with diamond particles. Its superior thermal conductivity results primarily from efficient formation of carbon in SiC phases as well as sound interfacial bonding between diamond and SiC phases.

To achieve maximum thermal conductivity of diamond/SiC composites, three structural parameters must be taken into account: 1) volume content of diamond and free silicon; 2) grain size of diamond; and 3) interface structure between diamond and SiC phase. To produce high-quality composites using diamond/SiC technology, its synthesis conditions should be tailored in such a way as to protect its diamond particles – for instance by sintering at temperatures higher than melting point of silicon powder as bedding material and using mixture of a-Si3N4 and silicon powder as bedding material for an amorphous b-SiC.

So that diamond and graphite can form an impenetrable matrix suitable for infiltrating with liquid silicon, an FAST/SPS furnace can be used at elevated temperatures with short sintering times (less than 30 seconds) in order to generate a thin graphite layer at the interface between diamond and b-SiC that significantly enhances thermal conductivity.

Composition was determined using polished cross sections and scanning electron microscopy (SEM). SEM images reveal that b-SiC is completely embedded within diamond, without a gap between them. Furthermore, an ESB fracture experiment shows that crack propagation mostly takes place through diamond after initiating at a graphite interlayer between b-SiC and diamond; hence achieving an overall residual silicon content of 5 vol% is possible.

Fracture Toughness

Silicon Carbide (SiC) is one of the hardest, strongest advanced ceramic materials with excellent erosion and abrasion resistance, exceptional thermal conductivity, low coefficient of thermal expansion and exceptional thermal conductivity – making it perfect for high temperature applications. However, SiC can become susceptible to oxidation at very high sintering temperatures. This degradation in hardness and fracture toughness limits its usefulness in demanding applications but this problem can be remedied by applying refractory layers such as diamond to its surface.

Crystalline diamond has exceptional mechanical properties that can greatly increase fracture toughness of silicon carbide, beyond just its resistance to abrasion. Utilizing both x-ray diffraction and Raman spectroscopy, researchers were able to measure both its morphology and distribution of diamond particles within a fully dense SiC-diamond composite made at GPa pressure; its hardness measured 45GPa while Knoop hardness came in at 42GPa with an incredible Young’s modulus of 560GPa!

Ball-milling the original SiC powder with a mixture of silicon and graphite followed by high-pressure sintering at GPa created an interlayer of diamonds which enhanced hardness by 35GPa while also significantly increasing bend strength to 3 GPa while nearly doubling fracture toughness.

Vickers indenters were used to measure fracture toughness of samples, and it was observed that cracks propagated predominantly through diamond phase cantilevers. Furthermore, most cantilevers displayed tilted interfaces due to either different elastic constants between interfaced phases or local damage at diamond/SiC interface. This may be explained by differences in elastic constants or local damage at diamond/SiC interface.

Experimental data was analysed in order to examine whether or not TiO2-diamond composites’ fracture toughness had any correlation with energy dissipation during an indentation cycle, and to assess whether its high hardness and fracture toughness can be attributed to enhanced bonding between interfaced phases; also noted was their sharp contrast to smooth fracture surfaces seen with non-diamond interlayer specimens.

Electrical Conductivity

Silicon carbide diamond has an extremely high electrical conductivity; however, this material is currently unsuitable for most applications due to sintering process required to fully densify it leading to 20% shrinkage of body structure, making machining tight tolerances impossible and needing expensive tools and equipment for production.

Sintering also produces a graphitic interlayer between diamond and SiC matrix, which serves to protect it from reacting with liquid silicon and producing SiC. Furthermore, its presence limits diamond’s ability to absorb heat from surrounding matrix material and leads to lower thermal conductivity.

Materials scientists face a key challenge today in creating compact, cost-effective heat sinks for high-power semiconductor devices like power transistors and photodiodes that require extensive cooling surface area, such as power transistors or photodiodes. Aluminum and copper have high thermal conductivities but differ significantly in their linear thermal expansion coefficients from those of their intended semiconductor devices they are meant to cool – currently their linear thermal expansion coefficients differ by over 20%!

An alternative approach involves using a composite of natural diamond and nanocrystalline SiC. A boron-modified version has been created that can be sintered under higher pressures and temperatures without producing graphite interlayer, providing excellent thermal and mechanical properties as well as low electrical resistivity.

SiC-diamond composite showed excellent fracture toughness of 12 MPa-m1/2 when put through a bending test, as evidenced by its ESB image shown in Figure 1. Crack propagation started from near the graphite-diamond interface – an indicator of short crack paths which reduce stress on the composite.

The X-ray diffraction pattern and Raman spectrum of an unsintered composite show that ball milled powder contains both crystalline and amorphous silicon. With increasing temperature sintering temperature, the frequency of an amorphous silicon peak decreases with increasing sintering temperature indicating transformation into crystallinity silicon then SiC during sintering process.

Mechanical Properties

Silicon carbide diamonds’ mechanical properties depend on both the strength of bond between diamond and silicon carbide and quality of diamond/graphite interface. The latter plays an integral part in producing materials with superior fracture toughness while simultaneously decreasing thermal conductivity compared to monocrystalline diamonds due to conversion of carbon from its crystalline state to its amorphous state at interface and presence of graphite boundary layers and micropores.

To increase fracture toughness of these materials, a new preparation method has been devised that eliminates the need for infiltration under low vacuum conditions. Ball milling produces a powder composed of diamond, graphite and amorphous silicon which is then mixed with silicon carbide powder and compressed under high pressure to form a preform preform with excellent fracture toughness suitable for various applications.

In order to further optimize the mechanical properties of these materials, we conducted extensive studies on the effects of diamond grain size and polymodal distribution on their mechanical properties. After conducting these investigations, we discovered that an optimal material is composed of diamond with a maximum grain size of 10 mm which ensures all amorphous silicon is completely transformed without overloading material with free silicon particles; furthermore, its effect does not negatively influence diamond/graphite interface interactions.

Field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy were employed to analyze the results obtained in this study. Figure 5 depicts an example prepared according to EXAMPLE E, featuring an FE-SEM image showing diamond particles dispersed spherically throughout its matrix; there were no large gaps between particles, suggesting carbonization had tightened preforms further. X-ray diffraction revealed largely amorphous silicon with two minor peaks at 2Th = 28deg and 52deg that indicate small amounts of crystallized silicon.

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