Whisker mechanical properties

The term s plastic, polymer, resin, elastomer, and reinforced plastic (RP) are some-what synonymous. However, polymer and resin usually denote the basic material. Whereas plastic pertains to polymers or resins containing additives, fillers, and/or reinforcements. Recognize that practically all materials worldwide contain some type of additive or ingredient. An elastomer is a rubberlike material (natural or synthetic). Reinforced plastics (also called composites although to be more accurate called plastic composites) are plastics with reinforcing additives, such as fibers and whiskers, added principally to increase the product s mechanical properties. 

Table 5.9 Selected Mechanical Properties of Some Common Whiskers...

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. 

Chu, C.Y. and Singh, J.P. Mechanical properties and micro structure of Si3N4-whisker-reinforced Si3N4 matrix composites , Ceram. Eng. Sci. Proc., 11 [7—8] (1990) 709-720. 

Desmarres, J.M., Goursat, P, Besson, J.L. Lespade, P. and Capdepuy, B. SiC whiskers reinforced Si3N4 matrix composites Oxidation behavior and mechanical properties , J. Eur. Ceram. Soc., 7 (1991) 101-108. 

Figure 3.25 shows a SEM micrograph of a well-consolidated 30 wt% SiC whisker-reinforced composite [10]. Table 3.18 gives the mechanical properties... 

The impact of thermal shock on the properties of a ceramic or a CMC is assessed by means of both destructive and non-destructive testing methods. Flexural or tensile (mainly for CMCs) tests of suitably-sized thermally shocked specimens are usually employed to measure retained mechanical properties as a function of the temperature difference. The temperature differential for which a significant drop in property values is observed is the A A- For monolithic ceramics and particle- or whisker-reinforced CMCs the property under investigation is usually strength, whereas in fibre-reinforced CMCs a drop in Young s modulus is usually a better indication of the onset of damage. 

Zhao, Z., Johnson, M., Shen, Z. (2002), Microstructure and mechanical properties of titanium carbonitride whisker reinforced P-sialon composites , Materials Research Bulletin, 37, 1175-1187. 

Goto, Y., Tsuge, A., Mechanical properties of unidirectionally oriented SiC-whisker-... 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

G. Pezzotti, et al., Processing and Mechanical Properties of Dense Si3N4-SiC-Whisker Composites without Sintering Aids, J. Am. Ceram. Soc., 72[8], 1461-1464 (1989). 

K. Xia and T. G. Langdon, The Mechanical Properties at High Temperatures of SiC Whisker-Reinforced Alumina, in High Temperature/High Performance Composites, Materials Research Society Symposium Proceedings, Vol. 120, eds. F. D. Lemkey, S. G. Fishman, A. G. Evans, and J. R. Strife, Materials Research Society, Pittsburgh, PA, 1988, pp. 265-271. 

E. R. Fuller, Jr., R. F. Krause, Jr., J. Kelly, R. N. Kacker, E. S. Lagergren, P. S. Wang, J. Barta, P. F. Jahn, T. Y. Tien, and L. Wang, Microstructure, Mechanical Properties, and Machining Performance of Silicon Carbide Whisker-Reinforced Alumina, J. Research NIST, in press. 

Whiskers are single crystalline fibers with a diameter of several tenths of a im and lengths of 10 to 200 pm. They are characterized by maximum achievable mechanical properties. Thus, for example, tensile strengths of over 10 GPa have been measured with SiC-whiskers. 

The different fabrication routes result in A1203 powders and SiC whiskers with different surface characteristics. The interfacial chemical compositions vary depending on the combination of whiskers and A1203. This causes the formation of a liquid phase and the chemical reactions at the Al203-SiC interface to occur at different processing temperatures. Therefore, conditions selected to achieve full density also have a critical influence on interfaces and on material mechanical properties. Some combinations of A1203 and SiC work better than others, but all require individual optimization of processing conditions. 

The effect of whisker-matrix interfaces on mechanical properties becomes clear when the same Al203-SiC materials are hot-pressed at different temperatures. While maintaining similar density, similar sizes of alumina grains, and identical whisker volumes, these composites have different fracture toughnesses as shown in Fig. 19. The differences in Klc values between A1203 with ACMC SC-9 SiC and A1203 with Tateho SCW... 

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