Silicon Carbide Whiskers

Silicon carbide whiskers, single-crystal micron-sized particles with outstanding physical and chemical properties, have garnered significant research interest due to their wide-ranging applications across many fields including high temperature structural materials and tool ceramics.

Mullite ceramic is a highly chemically stable refractory material with superior mechanical and physical properties such as high hardness and temperature creep resistance, yet its fracture toughness remains relatively low. SiC whiskers significantly enhance this property.

Physical properties

Silicon carbide whiskers are micron-sized single crystal fibers resembling diamond in their structure, which have the unique qualities of high strength, stiffness, chemical resistance and temperature stability that make them an excellent reinforcement material for ceramics, metals and polymer composites.

Mineral fillers can be used to strengthen ceramics, while also increasing their ductility and hardness, oxidation resistance, thermal expansion rate and fracture toughness. Furthermore, they may be added to polymer matrix composites to reduce filler requirements while making them more resistant to cracking, denting and fatigue damage.

Whiskers can be manufactured through various techniques, including carbothermal reduction of silica, chemical reaction between silicon halides and CCl4, or CVD with metallic catalysts. Unfortunately, their quality and dimension is often restricted by agglomeration and anisotropy (Wright, 2006). A more advanced process involves vapor-liquid-solid growth which enables whiskers to be placed deep within pores in porous substrates and provides desirable properties.

Whiskers were studied and compared with airborne industrial by-product fibers to ascertain any risks to workers handling them, with no significant respiratory hazards being found within SiC whiskers. The results of the research concluded that SiC whiskers do not pose any major hazards when handled.

Chemical properties

Silicon carbide was first synthesized artificially by Edward Acheson in 1891 as an abrasive material known as carborundum (Encyclopedia Britannica, 2014). Since then it has been mass produced using quartz sand and carbon heated at high temperatures to form large scale batches of silicon carbide which finds use across several industrial fields due to its exceptional strength, weight advantage and stability at high temperatures as well as being highly resistant to radiation exposure; ceramic seals and sandblast nozzles use silicone carbide, while durability replacement of asbestos substances among others.

Silicon Carbide (SiC) is a semimetallic compound comprised of silicon and carbon with a melting point of 2260 degC and low vapor pressure that renders it insoluble in water, yet soluble in alcohol and ether; however, due to air exposure at room temperature it quickly degrades rapidly resulting in rapid degradation over time. SiC can be found as an abrasive in sandpaper as well as coating tools such as sandblasting tools as well as grinding wheels or other machining equipment.

SiC whiskers can be dangerously toxic to cells, disrupting cell membranes and leading to cell death. Studies conducted on mouse fibroblast cells demonstrated their cytotoxicity by inducing DNA mutations, increasing total cell DNA content, and leading to malignant transformation – these effects being diminished when coated with non-toxic surfactant. When tested against human ciliated tracheal epithelia cells they proved more toxic than industrial by-product fibres such as asbestos and nickel sulphide while being less harmful than talc and titanium dioxide.

Mechanical properties

Silicon carbide whiskers are an ideal reinforcement material, ideal for toughening ceramics, metals and polymer composites. Their properties include high strength, hardness, chemical inertness and temperature resistance as well as improved oxidation resistance of materials such as silicon oxycarbide ceramic materials which have been widely studied as tools, engine components and wear resistant ceramics – depending on chemistry, structure and size of crystals synthesised via carbothermal reduction, CVD or combustion of waste products such as rice husks.

However, their mechanical properties fall far short of those seen with monolithic crystalline silicon (c-Si). To improve tensile and fatigue strengths of these materials, researchers have modified them with organic materials or inorganic fibers known as SiC whiskers to improve compressive and tensile strengths as well as to decrease stress concentration around them during fracture. This modification has significantly increased compressive strengths while simultaneously decreasing stress concentration during fracture.

Addition of SiC whiskers to a glass ceramic matrix significantly improves its strength and toughness while maintaining ease of glass formation and improving tribological performance. Furthermore, it can increase ductility as well as the Hugoniot elastic limit by several orders of magnitude; furthermore its fracture morphology shows evidence of this by showing many dimples and tear edges, indicative of its ductile behaviour.

Thermal properties

Silicon Carbide (SiC) is an extremely hard chemical compound comprised of silicon and carbon that naturally occurs as moissanite gemstone, but mass-produced versions have been produced since 1893 in powder and crystal form for use as an abrasive. Silicon Carbide also finds use in applications requiring high endurance such as ceramic brake discs for cars and bulletproof vests as well as pump shaft seals; its melting point allows it to withstand extreme temperatures without cracking or deforming over time.

SiC whiskers’ thermal properties depend heavily on their microstructure, surface chemistry and size. With low vapor pressure and stability up to 1650 degC, these whiskers can be used as reinforcements in various materials like metals and polymers; improving strength, toughness, chemical inertness and oxidation resistance while extending service life.

Analysis of SiC whisker microstructure can be performed using scanning electron microscopy and X-ray diffraction. They consist of polycrystalline structures consisting of 3C beta-SiC and 6H-SiC polytypes; further examination has revealed that the crystalline regions possess higher planar defect densities than amorphous regions due to microtwins formed on 1 1 1 planes that exert significant effects on both tensional and bending fracture surfaces of PI-SiCw nanocomposites.

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