Hot-pressing

The powders used for hot pressing need not be of any specific size distribution, and they need not be uniformly mixed. The densification can be 

Only simple parts, in the shape of flat plates, blocks, and cylinders, are made by hot pressing. We go for this method only when the parts require small grain size, low porosity, or low impurity level, owing to the high cost and low productivity. 

When diree spherical particles are sintered together, die volume between them decreases as the necks increase until a spherical cavity is left. The source of material to promote further neck growth is now removed by die coalescence of 

Assuming that the average diffusion length of particles to the cavity from the shell surface is equal to tire thickness of the shell, and conversely tlrat there is a counter-cutTent of vacancies from tire cavity to the shell surface, the rate of densification dp/dt is given by 

This latter equation defines the relative roles in hot pressing densification of volume and grain boundary processes. 

It was shown earlier that the NabaiTO-Hemirg model of creep in solids involved the migration of vacancies out of the stressed solid accompanied by counter-migration of atoms to reduce dre length of the solid in the direction of the applied stress. This property could clearly contribute to densification under an external pressure, given sufficient time of application of the stress 

Typical microstructures developed by solid state sintering, liquid phase sintering and firing a porcelain are shown in Fig. 5.20. 

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. 

Although hot-pressing is usually regarded as an expensive process and only simple shapes with a wide tolerance on dimensions can be made, it provides the only route for several valuable materials. Continuous hot-pressing methods have been developed for some magnetic ferrites and piezoelectric niobates. They olfer higher production rates but tool wear is very severe. 

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The first industrial hardboard was developed by W. Mason in the mid-1920s he found that a mat of wet fiber pressed in a hot press would produce a self-bonded flat panel with good strength, durabiUty, and stabiUty. The product was patented in 1928, trademarked as Masonite, and commercial production began. Over time several other processes for producing hardboards have been developed from modifications of the original process. Brief descriptions of these processes foUow and a flow chart of the process is shown in Figure 5. 

M. H. Leipold, "Hot Pressing," ia F. F. Y. Wang, ed.. Treatise on Materials Science and Technology, Vol. 9 (Ceramic Fabrication Processes), Academic Press, New York, 1976. 

Fig. 2. Strength as a function of temperature for representative SiC stmctural ceramics A, sintered (Y2O2 added) , hot-pressed (2% AI2O2) sintered...

Reaction-bond ed Sintered Sintered beta alpha Hot-pressed (AI2O3)" Sintered... 

Fig. 3. Stress mpture behavior in air at 1200°C for SiC stmctural ceramics —hot-pressed -, reaction-bonded , sintered alpha —sintered beta. To...

Fig. 4. Thermal diffusivity of silicon-based stmctural ceramics (a) reaction-bonded SiC (b) hot-pressed and sintered SiC (c) hot-pressed (1% MgO,...

Using hot-pressing, shaping and densiftcation occur in a single process step. The temperatures are in the range of 1650—1800°C and appHed pressures are from 30—40 MPa (4000—6000 psi) (45), resulting in parts of high quaHty. This method is limited to simple shapes and low production volumes, however, and the process may also impart anisotropic characteristics to the material (46). 

Property Reaction-bonded Sintered Hot-pressed isostaticaHy Hot-pressed ... 

Magnesium fluoride optical crystals are made by hot-pressing (14) high quaUty MgF2 powder. The optical quaUty powder is made by the NH4HF2... 

Hot pressing with a smooth plate has an advantage in smoothing the grain, and the heat can be used to cure the resin of the finish. The hot pressing is anticipated in the design of the finish system and in the choice of the resins by the finish manufacturer. 

Hot pressing to produce substantial texture and magnetic anisotropy via plastic deformation is accompHshed by a process referred to as... 

Hot Pressing. Hot pressing may be used either to consoHdate a powder that has poor compactabiHty at room temperature, or to combine compaction and sintering in one operation. The technique is essentially the same as described for unidirectional die compacting. The powder is heated by either heating the entire die assembly in a furnace or by induction heating. In most instances, a protective atmosphere must be suppHed. 

Hot pressing produces compacts that have superior properties, mainly because of higher density and finer grain size. Closer dimensional tolerances than can be obtained with pressing at room temperature are also possible. Hot pressing is used only where the higher cost can be justified. It has been usehil in producing reactive materials. One use is the combination of P/M and composites to produce hot-pressed parts that are fiber reinforced. 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Silicon Nitride. SiUcon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, siUcon nitride parts. a-SiUcon nitride, used in the manufacture of Si N intended for hot pressing, can be obtained by nitriding Si powder in an atmosphere of H2, N2, and NH. Reaction conditions, eg, temperature, time, and atmosphere, have to be controlled closely. Special additions, such as Fe202 to the precursor material, act as catalysts for the formation of predorninately a-Si N. SiUcon nitride is ball-milled to a very fine powder and is purified by acid leaching. SiUcon nitride can be hot pressed to full density by adding 1—5% MgO. 

Annual production of powdered BN is ca 180—200 metric tons per year and its cost is 50—250/kg, depending on purity and density. The price of cubic boron nitride is similar to that of synthetic diamond bort. Hot-pressed, dense BN parts are 3—10 times more expensive than reaction-sintered parts. 

Annual production of sihcon nitride is ca 100—200 t. Utility-grade sihcon nitride costs 4—5/kg in 100- to 500-kg quantities. The reaction-sintered parts are sold for 120 to 300/kg, depending on complexity of shape. Hot-pressed, fully dense Si N parts are priced 5—10 times higher than reaction-sintered parts. 

Sihcon nitride is one of the few nonmetaUic nitrides that is able to form alloys with other refractory compounds. Numerous soHd solutions of P-Si N and AI2O2 have gained technical interest. Many companies have begun to mass produce reaction-sintered and hot-pressed Si N parts. 

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