Hot-pressed silicon carbide

50 MPa. The conventional method requires large amounts of energy and mold material, and can normally be used only for parts vdth simple, uncomplicated shapes. 

Due to these limitations, precision parts can only be produced by machining with diamond tools [269], and these, of course, are expensive. 

The post-densification of SSiC by HIP is less complicated because no gastight encapsulation is necessary. Hence, not only is it possible to achieve more than 99% of the theoretical density, but the variation of density and strength can also be reduced. Pressureless sintering and HIP can now be performed within the same cycle, in a process termed sinter-HIP, and this offers clear economic benefits [273-275]. 

Although the importance of hot-pressed SiC has decreased considerably since the introduction of pressureless sintering, it is currently still the most suitable method for obtaining the best mechanical properties for pure, monoUthic SiC. 

6 Chemical Vapor- and Physicsil Vapor-DeposHed Silicon Carbide 

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Only highly dense ceramics with high thermal shock resistance can be utilized for such strongly mechanically and thermally stressed components. These demands are met by hot-pressed silicon carbide produced at pressures of 350 bar and sintering temperatures of 1900 to 2000°C. The necessary, but very expensive, mechanical finishing is disadvantageous for industrial manufacture of hot-pressed silicon carbide. 

W. Hinze, W. C. Tripp and H. C. Graham, The High-Temperature Oxidation of Hot-Pressed Silicon Carbide, pp. 409-19 in Mass Transport Phenomena inCeramic. Edited by A. R. Cooper and A. H. Heuer. Plenum, New York (1975). 

K. Yoshida, M. Imai, T. Yano, Microstructure and mechanical properties of hot-pressed silicon carbide fiber-reinforced silicon carbide composite. Key Eng. Mater. 164-165 (1999) 217-220. 

Singhal S C, Oxidation kinetics of hot-pressed silicon carbide , J Mater Sci, 1976 11 1246-1253. 

Silicon carbide, hot pressed Alumina, dense sintered Boron nitride, hot pressed Silicon nitride, hot pressed Boron carbide, hot pressed... 

The tensile fracture strengths of three different structural ceramics are listed below hot-pressed silicon nitride (HPSN), reaction-bonded silicon nitride (RBSN), and chemical vapor-deposited silicon carbide (CVDSC), measured at room temperature. 

Values for ceramics range from below 1 for glasses and most single crystals 1 to 3 for glass-ceramics, clay-based ceramics and MgO 2.5. to 4 for early engineering ceramics (alumina, boron carbide, silicon carbides, RBSN) 4 to 6 for hot-pressed silicon nitride and SRBSN and for transformation-toughened alumina... 

Hot-pressed silicone nitride (HPSN) versus tungsten carbide =320 GPa =320 GPa... 

It is not always possible to obtain a low-porosity body by pressureless sintering , i.e. by sintering at atmospheric pressure. For example, difficulties are experienced with silicon nitride and silicon carbide. More commonly it may prove difficult to combine the complete elimination of porosity with the maintenance of small crystal size. These problems can usually by overcome by hot-pressing, i.e. sintering under pressure between punches in a die, as shown in Fig. 8.9. The pressure now provides the major part of the driving force eliminating porosity and the temperature can be kept at a level at which crystal growth is minimized. 

Care has to be taken in selecting materials for the die and punches. Metals are of little use above 1000 °C because they become ductile, and the die bulges under pressure so that the compact can only be extracted by destroying the die. However, zinc sulphide (an infrared-transparent material) has been hot pressed at 700 °C in stainless steel moulds. Special alloys, mostly based on molybdenum, can be used up to 1000 °C at pressures of about 80 MPa (5 ton in-2). Alumina, silicon carbide and silicon nitride can be used up to about 1400 °C at similar pressures and are widely applied in the production of transparent electro-optical ceramics based on lead lanthanum zirconate as discussed in Section 8.2.1. 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

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