Densification

The driving force for densification is the reduction in surface energy as the free surfaces of particles disappear and how this is accomplished defines the terms firing , solid state sintering and liquid phase sintering. 

To obtain a dense, tough polycrystalline aggregate it is necessary to eliminate the voids between the particles of the ferrite and form grain boundaries. Although there is a continuous evolution of the microstructure 

Stage 1 Contact area between particles increases 

Stage 2 Porosity changes from open to closed porosity 

Stage 3 Pore volume decreases grains grow 

The presence of insoluble impurities may produce local differences in densification rate, which result in mechanical stresses. Sintering damage such as cracks and arrays of pores are associated with the elimination of these stresses (Hsueh, Evans McMeeking, 1986). 

---

Sintering invoives the densification and microstmcture deveiopment that transfonns the iooseiy bound particies in a powder compact into a dense, cohesive body [, 70, 71, 72 and 73]. The end-... 

Figure C2.11.6. The classic two-particle sintering model illustrating material transport and neck growtli at tire particle contacts resulting in coarsening (left) and densification (right) during sintering. Surface diffusion (a), evaporation-condensation (b), and volume diffusion (c) contribute to coarsening, while volume diffusion (d), grain boundary diffusion (e), solution-precipitation (f), and dislocation motion (g) contribute to densification.

Because densification occurs via tire shrinkage of tliennodynamically unstable pores, densification and microstmcture development can be assessed on tire basis of tire dihedral angle, 0, fonned as a result of tire surface energy balance between tire two solid-vapour and one solid-solid interface at tire pore-grain boundary intersection [, 78, 79 and 80],... 

Liquid-phase sintering is significantly more complex tlian solid-state sintering in tliat tliere are more phases, interfaces, and material transport mechanisms to consider. In general, densification will occur as long as it is... 

Densification during liquid-phase sintering occurs in tliree stages. Initially, liquid fonns at particle intersections and redistributes tliroughout the particulate mass under the influence of the capillary action. Shear stresses due to the... 

Trapped gas in closed pores often limits densification when sintering witlr a liquid or viscous (glass) phase because rapid material transport tlirough tlie liquid often results in pore closure early in tlie sintering process. 

Ewsuk K G 1986 Finai stage densification of aiumina during hot isostatic pressing PhD Thesis The Pennsyivania State University... 

Ewsuk K G and Messing G L 1986 A theoreticai and experimentai anaiysis of finai-stage densification of aiumina during hot isostatic pressing Hot IsostatIc Pressing Theories and Applications ed R J Schaefer and M Linzer (Materiais Park, OH ASM internationai) pp 23-33... 

Ewsuk K G 1992 Effects of trapped gases on ceramic-fiiied-giass composite densification Solid State Phenomena voi 25-26, ed A C D Chakiader and J A Lund (Brookfieid, VT Trans-Tech) pp 63-72 (Proc. Sintering 91)... 

The mats are moved along the line to the press loader. When the loader is filled and the press opens to remove the load of freshly pressed boards, the loader pushes the new boards into the unloader and deposits the load of mats on the press platens. The press closes as quickly as possible to the desired panel thickness. More pressure, as much as 4.8—6.9 MPa (700—1000 psi) is required to press high density dry-process hardboard, because the dry fiber exhibits much more resistance to compression and densification than wet fiber. Press temperatures are also higher, in the range of 220—246°C. No screens are used in the dry-process, but the moisture in the mats requires a breathe cycle during pressing to avoid blowing the boards apart at the end of the cycle. Because no screens are used, the products are called smooth-two-sides (S-2-S), in contrast to the wet-process boards, which have a screen pattern embossed into the back side and are known as smooth-one-side (S-l-S). 

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

Y. Murata and R. Smoak in S. Somiya and S. Saito, eds., Proc. Int. Sjmp. of Factors in Densification and Sintering of Oxide and Nonoxide Ceramics, Gakujutsu Biinken Fukyu-Kai, Tokyo, 1979, p. 382. 

Ceramics. The properties of ferroelectrics, basically deterrnined by composition, are also affected by the microstmcture of the densifted body which depends on the fabrication method and condition. The ferroelectric ceramic process is comprised of the following steps (10,24,25) (/) selection of raw oxide materials, (2) preparation of a powder composition, (J) shaping, (4) densification, and (5) finishing. 

The quantity of resin appHed to the reinforcing ply to achieve a state of full densification varies inversely with the laminating pressure. Therefore, high pressure laminates pressed at about 7 MPa (1000 psi) need only about 25—30% phenoHc resin in kraft paper, whereas low pressure (1 MPa = 145 psi) laminates need 50—60% resin in the reinforcing ply if all voids are to be filled in the final product. 

---