SiC whisker-reinforced ceramic composites were an innovation that came into prominence for potential structural applications because of the significant improvements in the mechanical properties these materials offered as compared to the monolithic materials. The incorporation of SiC whiskers into alumina ceramics resulted in increases in strength, fracture toughness, thermal conductivity, thermal shock resistance and high temperature creep resistance. These discoveries initiated several years of intense study into this class of composites.

Background

SiC whiskers used for reinforcement are discontinuous, rod- or needle-shaped fibers in the size range of 0.1 to 1 µm in diameter and 5 to 100 µm in length. Because they are nearly single crystals, the whiskers typically have very high tensile strengths (up to 7 GPa) and elastic modulus (up to 550 GPa).

The first commercially available SiC whiskers were introduced in the early 1960s. The use of SiC whiskers to reinforce materials was originally applied to metal matrices, such as aluminum. However, the first application of whisker reinforcement to ceramics did not occur until the 1980's. Several methods and numerous starting materials can be used to grow SiC whiskers. Much of the early work prior to the mid-1970s employed the vapor-liquid-solid (VLS) mechanism to produce small quantities of whiskers. Later production methods used carbothermic reduction reactions of low-cost silica and carbon precursors, such as rice hulls, to produce large quantities of whiskers at reasonable cost. This allowed SiC whiskers to become economically viable as reinforcing agents in components for large-scale, high-volume applications .

Using the rice-hull technology, Advanced Composite Materials Corporation (ACMC), which had been previously owned by Arco Metals, developed production capability to manufacture large quantities of SiC whiskers in the USA. The major market for the whiskers was to be for reinforcing metal matrix composites as had been done in the past. However, there was also interest in alternate markets, so to investigate the potential for whisker-reinforcement in other materials, ACMC interacted with personnel at Oak Ridge National Laboratory (ORNL). This led to the first whisker-reinforced alumina composites being fabricated at ORNL by hot-pressing in 1982. The initial results showed such promise that large-scale development programs were started to exploit the materials and several years of research followed.

Properties of SiC Whisker-Reinforced Ceramics

The mechanical property improvements observed with the incorporation of SiC whiskers into ceramic matrices were unprecedented. For example, the fracture toughness of alumina was increased from ~3.0 MPaÃm to 8.5 MPaÃm with the addition of 20 v/o whiskers. This was accompanied by fracture strengths of 700-800 MPa versus <= 400 MPa in unreinforced alumina. Just as importantly, these property improvements were retained to elevated temperatures, unlike some other toughened ceramic systems. Remarkably improved thermal shock and creep resistance were also observed. Subsequent studies have examined the SiC whisker-reinforcement of numerous ceramic matrix systems, including mullite, zirconia, glass, spinel, cordierite, silicon nitride, boron carbide, and combinations of these materials.

Research into the toughening behavior responsible in the composite materials shows that crack-whisker interaction resulting in crack bridging, whisker pullout and crack deflection are the major toughening mechanisms. One of the keys to the behavior of SiC whisker reinforced composites, is that for this mechanism to operate, debonding along the crack-whisker interface must occur during crack propagation and allow the whiskers to bridge the crack in its wake. Scientifically, the understanding gained in studies of the mechanical behavior of these composites has been used to develop other ceramic systems (e.g., the self-reinforced ceramics) having increased toughness and strength.

Applications of SiC Whisker-Reinforced Ceramics

During the same time period that the early research efforts were being done after the initial laboratory results, product development efforts were being carried out at ACMC. This work led to the use of SiC whisker-alumina composites for cutting tool applications. The first commercial cutting tools based on this technology were introduced in the USA by Greenleaf Corporation in April 1985. ACMC and Greenleaf received several awards for the pioneering development of this cutting tool including an R&D Magazine IR-100 Award and the Corporate Technical Achievement Award from the American Ceramic Society. At the present time, Sandvik AB of Sweden also markets a SiC whisker-reinforced cutting tool in other parts of the world.

In the area of cutting tools, it was found that the addition of SiC whiskers to alumina not only improved the strength and fracture toughness, but did so without compromising the hot hardness of the matrix. This combination revolutionized machining of high-nickel alloys that are used in the jet engine industry. The SiC whisker-reinforced alumina enabled a ten-fold increase in metal removal rates at increased speeds. For example, in one reported case history, changing from a conventional tool to a SiC whisker-alumina one reduced a 5-hour machining operation of inconel to 20 minutes. In addition to cutting tools, SiC whisker reinforced composites are employed in a wide variety of applications in can forming punches, extrusion dies and other wear environments.

The remarkably fast development of SiC whisker-reinforced alumina from initial laboratory samples to a competitive commercial product took under just three years. The short development time was possible because the materials found a niche market in the cutting of nickel-based alloys where they had exceptional performance.