Silicon nitrides strength ceramics

Figure 16.5 Data from Wachtman, J.B. (1996) Mechanical Properties of Ceramics, Wiley, New York, p. 358. Figure 16.9 Data from Zeng, J., Tanaka, 1., Miyamoto, Y., Yamada, O., andNiihara, K. (1992) High-temperature strength and cavitation threshold of silicon nitride-silica ceramics, J. Am. Ceram. Soc. 75, 195.

Not unlike the case of superplastic ceramics, ductility and strength relations are influenced by strain rate. The conditions of the experiment must be above the DBT to observe plastic flow, which is different for various ceramics. An illustration of the effect of strain rate and temperature on the strain (ductility) at some stress level can be seen in monolithic Si-C-N. Silicon-nitride-based ceramics are quite promising candidates for mechanical applications at elevated temperatures. Specimens were prepared by hot isostatic pressure (henceforth HIP) of pyrolyzed powder compact at 1500 °C and 950 MPa, without any sintering additives. These compression tests were conducted at temperatures from 1400 to 1700 °C in a nitrogen atmosphere with a servo-hydraulic-type testing machine at constant crosshead speed in an induction heating furnace. In Fig. 2.5, stress-strain curves... 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

Considerable recent activity in the area of ceramic processing is aimed toward the formulation of materials with high strengths, comparable to the room temperature strength of metal alloys, at high temperatures (of the order of 2000 K). The impetus comes from the significant gains made in the last 20 years with materials formed from submicron powders of silicon nitride and silicon carbide and the promise of similar improvements in the near future. 

The carbides and nitrides are well known for their hardness and strength, and this section will briefly compare a number of these properties with those of the pure metals. Concentration will be placed here on the first row compounds, since these constitute a complete series, and Mo and W, since these are the most commonly studied metals. As will be shown, the physical and mechanical properties of carbides and nitrides resemble those of ceramics not those of metals. Comparisons will be made with boron carbide (B4C), silicon carbide (SiC), aluminium nitride (AIN), silicon nitride (Si3N4), aluminium oxide (A1203), and diamond, as representative ceramic materials. 

Tajima Y, Urashima K (1994) Improvement of Strength and Toughness of Silicon Nitride Ceramics. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics, NATO ASI Series, Series E Vol 276. Kluwer Academic Publishers, Dordrecht, p 101... 

Nishimura N, Masuo E, Takita K (1991) Effect of Microstructural Oxidation on the Strength of Silicon Nitride after High Temperature Exposure. In Carlsson T, Johansson T, Kahlman T (eds) Proc 4th Int Symp Ceram Mater Components for Engines. Elsevier, London, p 1139... 

Final Report 2004, EC/BBW Contract No. ICA-CT-2000-10020, FP5 INCO-Copemicus project LAMINATES (Silicon Nitride Based Laminar and Functionally Graded Ceramic Composites for Engineering Applications), project partners University of Warwick (UK), FCT Technologie (Germany), Institute for Problems of Materials Science (Ukraine), Materials Research Center Ltd (Ukraine), Institute for Problems of Strength (Ukraine), Institute of Chemical Physics (Armenia), Drexel University (USA), EMPA (Switzerland). 

Kawai, C., and Yamakawa, A., Machinability of high-strength porous silicon nitride ceramic, J. Ceram. Soc. Jap., 106(11) 1135-1137 (1998). 

Rendtel, A., Htibner, H., Hermann, M., Schubert, C., Silicon nitride/silicon carbide nanocomposite materials II, Hot strength, creep and oxidation resistance, J. Am. Ceram. Soc., 81(5), 1998, 1109-1120. 

Lange, F.F. (1973), Relation between strength, fracture energy, and microstructure of hot-pressed silicon nitride , J. Am. Ceram. Soc., 56(10), 518-22. 

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. Morscher, P. Pirouz, and A. H. Heuer, Temperature Dependence of Interfacial Shear Strength in SiC-Fiber-Reinforced Reaction-Bonded Silicon Nitride, 7. Am. Ceram. Soc., 73[3], 713-720 (1990). 

---