Commercial SiC whiskers

Fig. 8. Typical morphologies of commercial SiC whiskers produced by Tateho (top photograph) and American Matrix (bottom photograph). Magnified 5000 X. ...

The characteristics of commercial SiC whiskers as claimed by producers are given in Table 2. 

Commercially available as Grade WG-300 from Greenleaf Corporation, Saegertown, PA. This material contains SiC whiskers which are 0.2-0.7 pun in diameter and up to 25 pun in length. The average grain size of the alumina matrix is about 1.3 pun. 

Despite the fact that many different ceramic whiskers have been produced in research laboratories [93] only SiC and Si3N4 have gained real commercial significance so far. Among whisker-reinforced ceramics the best results were obtained with Al203-SiC whiskers and Si3N4-SiC whiskers. 

SiC whiskers are used commercially in a number of different applications. Alumina reinforced with 25-30 wt% SiC whiskers is the material of choice for inserts used in high-speed cutting of nickel-based superalloys (for aerospace applications). However, whiskers do have a number of disadvantages over particles. It is difficult to produce homogeneous dispersions as the whiskers tend to form entwined agglomerates and, even if well dispersed, some orientation of the whiskers occurs leading to anisotropic properties. 

SiC whiskers are the strongest materials known that are produced in commercial volumes. There are two methods that are used ... 

S.3.2.2.3 Vapor-Solid Reaction At present the principal commercial method for SiC whisker production is the carbothermic reduction of low cost silica sources at temperatures of 1500-1700°C. The reaction for the formation of VS-cubic (3SiC whiskers occurs in two steps ... 

Among the oxide composites, SiC whisker-reinforced alumina deserves some discussion. It was first introduced in the 1980s, and there was high level of enthusiasm for whisker composites. In addition to SiC whiskers, work had been done with whiskers of TiC, TiCN, and TiB2 showing improved performance in laboratory (Pettersson and Johnsson 2003) (Fig. la), but these did not get much commercial success due to lack of significant differentiation in performance and lack of whisker availability. Commercially available composites have used SiC as whisker for the last two decades. Improvements have been made in xmderstanding the whisker-matrix relationship. Whisker is the cost driver and is a health hazard. 

SiC whiskers were initially developed in the early 1960s. However, their use was originally to be applied to the reinforcement of metal matrices, such as aluminum. These metal matrix composites were only a small commercial success, mainly because of the high cost of the whiskers. The first application of whisker reinforcement to ceramics, and in particular alumina, did not occur until the 1980 s. For the purposes of this article, SiC whiskers will be defined to be acicular or needle-like shaped, discontinuous, nearly single crystals. 

With the development of some composite materials and the commercial success of some of these products, the demand for whiskers has increased. Numerous companies experimented with different whisker growth processes over the years and several patents have been issued in the area. At the present time, commercial sale of whiskers is limited to a few companies in the world with SiC whiskers the most predominant. In addition, whiskers ofTiC.TiN, Al203,mullite, Si3N4, and B4C have also been produced and studied. However, SiC remains the whisker of most interest and commercial success. 

The whiskers are normally received from commercial sources in an agglomerated form and appear to be spray-dried balls. Deagglomeration and dispersion of the SiC whiskers has been found to be extremely important in the fabrication of composites with good mechanical... 

The initial development of SiC whisker reinforced alumina was done in the early 1980 s and the first commercial product was introduced in 1985. The remarkably fast development of SiC whisker-reinforced alumina from initial laboratory samples to a competitive... 

Vapor Phase Formation and Condensation In order to grow whiskers by vapor phase formation and condensation, the bulk SiC is first vaporized by heating to very high temperatures (>2200 °C), usually under reduced pressure. Upon cooling, a-SiC whiskers form on the nucleation sites. The addition of lanthanum, yttrium, neodymium, or zirconium leads to an increase in the growth rates. However, whiskers are no longer produced commercially using this sublimation process. 

SiC whiskers having a diameter of 4-6 p,m and a length of 5 mm have been reported to possess an averaged tensile strength of 8.40 GPa and an average elastic modulus of 581 GPa [40]. Commercially available SiC whiskers are usually smaller, and typically have a diameter of approximately 1 p.m and a length of several tens of microns or more (see Figure 15.1.5). 

Nicalon and Tyranno ceramic fibers, two well-known preceramic derived commercial products, are marketed for structural applications. Nicalon is a SiC based ceramic fiber processed using chemistry and techniques first developed by Yajima and coworkers [6-14]. Tyranno fibers are SiC/TiC based fibers produced via novel modifications to the original Yajima work [15-17]. Elastic moduli and tensile strengths for both fibers are of the order of 250-300 GPa and 2-3 GPa respectively. Textron s CVD SiC fibers (not preceramic) offer tensile strengths of up to 4 GPa [18]. The elastic modulus of sintered, hot pressed SiC is in the range of 400-450 GPa [19]. These compare with tensile strengths of =< 8 GPa and an elastic modulus of= 580 GPa for single crystal, SiC whiskers [18]. 

SiC whiskers can be fabricated by the reaction of silicon and carbon to form a gaseous species that can be transported and reacted in the vapor phase. This type of formation is referred to as a vapor-sohd reaction and occurs by the following multistep process 1) SiOj + C-SiO + CO and 2) SiO -I-2C-SiC -I- CO. Although the raw materials used in these carbothermal processes vary widely, nearly aU of the commercial and high-volume SiC whisker products are formed by this method. These reactions occur at temperatures greater than 1400°C and in an inert or nonoxidizing atmosphere. In addition, a catalyst is added to assure the formation of whiskers rather than particulate during the reaction. 

Miaterial The materials used in this study were obtained by glass encapsulated HIPing (as developed by ASEA Cerama, Robertsfors, Sweden) of slip cast or pressed green bodies. Japanese SiC-whiskers (Tokawhisker. Tokai Carbon, Japan) and commercial AI2O3 powder (A16SG, Alcoa, USA) were used. The processing (24) (25) and some mechanical... 

Non-whisker SiC fibers are also of imporfance in MMCs, and they are currently available in two commercial forms Tyranno and Nicalon . As with the whisker form, the primary advantages of SiC fibers is their oxidation resistance and high-temperature mechanical property retention relative to other fibers. The high-temperature strength of three commercially available Nicalon SiC fibers is shown in Figure 5.111. 

More than 100 different materials have to date been produced as whiskers and characterized. However, only SiC- (50 to 100 t, 1996) and potassium titanate whiskers (ca. 5000 t) have achieved commercial importance. Important properties of whiskers are summarized in Table 5.2-18. 

Vapor-phase techniques are not as widely used as growth from the melt or solution because the growth rate is generally slow or only small crystals can be grown. However, two vapor-phase techniques have commercial application the growth of whiskers or small islands (e.g., SiC and GaN) by the VLS process and the growth of SiC and nanotubes by sublimation processes. 

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