Use of Liquid-Phase Sintering

A second-phase that forms a liquid at the firing temperature can provide a fast diffusion path for densification but grain growth by the Ostwald ripening process may also be enhanced. In this case, high density is normally accompanied by appreciable grain growth. This commonly used fabrication approach is the subject 

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A new type of liquid-phase sintered SiC using yttria [1314-36-9J, Y20, as the oxide additive and submicrometer SiC powder for enhanced densification, produces a material which can be densified without the application of pressure (13). This material, sintered from cold isostatically pressed billets, appears to be comparable to silicon nitride in strength and fracture toughness. 

The manufacturing process (especially the cold and hot compaction stages) of pcBN is very similar to that of pcD, summarized in Section 1.3.3. However, the types of binders used and liquid phase sintering mechanisms can be quite different. In pcBN production, reactive sintering plays a major role in terms of driving the densification process, whereas in pcD, dissolution and precipitation and/or adhesion/coalescence are the main driving mechanisms. A typical example for pcBN would be the following reaction [162]. 

Due to the tailored properties of liquid phase sintered silicon carbide (LPSSiC) it is used as dewatering elements in the paper machinery and as rings for highly stressed gas seals. It is a price competitive alternative to silicon nitride materials and outperforms alumina and tungsten carbide materials. In addition, LPSSiC is proposed as neutral matrix in ceramic matrix composites containing plutonium to burn the world s stockpiles of military plutonium in thermal or fast reactors [278]. 

Kim, Y.-P., Jung, S.-W., Kim, B.-K. and Kang, S.-J. L., Enhanced densification of liquid phase sintered WC-Co by use of coarse WC powder experimental support for the pore filling theory, unpubUshed work (2003), to be published. 

Fig. 16. Schematic of a joint formed using transient liquid phase sintering conductive adhesives, (a) Initial state after chip attachment and before temperature reach the melting point of the low-melting-alloy filler, (b) The low-melting-alloy fillers start to melt and dissolve the high-melting-alloy filler, (c) After the high-melting-alloy fillers are completely dissolved, the liquid metal phase solidifies and forms a three-dimensional network in the joint.

Pressure sintering can also be useful to further density sintered ceramics. The density of liquid-phase sintered PZT was improved from —97% to -98% of theoretical density by HIPing for 7.5—60 min at 1300 °C and 7—21 MPa. Consistent with the microstructural evidence of liquid formation and redistribution (Figure 5.7), and the preferential annihilation of the fine, submicrometer porosity within the micro-structure, the majority of densification occurs rapidly, within the first few minutes of HIPing. 

Cemented carbides and nitrides are manufactured by a powder metallurgy process using liquid phase sintering (Figure 2.1). The cobalt or nickel binder metal, in the form of fine powder, is blended with the carbide or nitride powders in ball mills or attritor mills lined with carbide sleeves. 

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