AB xC AC yBEq lb

The driving force for the reaction is the negative heat of mixing of the forming compound. This results in a release of energy (as heat), which sustains and propagates the reaction through the body of the reactants. A complete description of the thermodynamics, wave propagation, and phenomenological aspects of combustion synthesis can be found in the article "Combustion Synthesis of Advanced Materials" in this Volume. Commonly, the product of a combustion synthesis reaction is porous and therefore easily milled into powder.

Evolution of Porosity During Combustion Synthesis. Porosity is induced during combustion synthesis from several factors (Ref 4, 5, 6, 7, 8, 9, 10, and 11):

• Kirkendall and Frenkel effects. The diffusivity of the elements is unbalanced (i.e., element A diffuses more rapidly into element B than B diffuses into A; as a consequence, after the reaction propagates through the reactants a pore forms at prior particle A powder sites).

• Gas evolution. The heat generated by the initiation of the combustion synthesis reaction may be great enough to volatilize residual impurities attached to the surfaces of the starting powders. The evolution of gaseous species may be violent enough to prevent densification of the product or can be entrapped within the product resulting in porosity.

• Molar volume changes. The product phase has a different molar volume than the reactants. Typically, the product phase is denser than the reactants. For aluminides, the density increase is typically less than 12%, while for ceramics, the increase is quite substantial, ranging from 15 to 30% (see Table 1, Ref 8). The resulting volumetric change is intrinsic, as it is a direct result of the inherent atomic structure and bonding of the resultant product versus those of the starting constituents. Unless the change in the dimensions of the product (e.g., shrinkage due to sintering) is equal to the intrinsic volume change resulting from the reaction, porosity will be generated in the product.

• Initial porosity. The starting green compact of pressed powders contains 20 to 50 vol% porosity. This must to be eliminated during the sintering operation for product densification to occur.

The aim of reactive sintering is to form dense intermetallic and ceramic compounds, alloys, or composites directly from elemental powders. As discussed below, during combustion synthesis of certain compounds, the formation of porosity can be eliminated by careful control over the reaction and/or through the application of external pressure during reactive sintering by reactive hot pressing (RHP) or reactive hot isostatic pressing (RHIP).

Table 1 Intrinsic volume and density changes during formation of ceramic and intermetallic products from

elemental powders

Product

Volume change ( AV), %

MoSi2

-40.6

1.39

SiC

-2S.4

1.39

TiSi2

-27.5

TiC

-24.4

1.32

WC

-23.S

1.31

TiB2

-23.3

1.29

NiAl

-12.6

TiAl

-5.3

Ni3Al

-5.2

ReHf

-2.2

3TiC + 2Al2O3

-22 to -28

1.28 to 1.39

3TiB2 + 5Al2O3

-27 to -28

1.37 to 1.39

9Fe + 4Al2O3

-22

1.28

3Fe + Al2O3

-19

1.23

Source: After Ref 8

Source: After Ref 8

(<l) Pj/Pi is theoretical density of product(s)/theoretical density of reactants (i.e., density of nonporous compact of reactants).

Reactive Sintering. Coble (Ref 12) defines seven classes of reactions that can occur during sintering:

A1

A2

(1)

Crystallographic phase change

A

+ B Solid-solution formation

(2)

Solid-solution formation

ß

(3)

A

+ B -n + Liquid phase sintering

liquid

(4)

A

+ B Compound formation

AB

(5)

A1

+ liquid —A2 + P ► ftSiJN4, with liquid

liquid

(6)

Asolid + Bliquid '"ABsolld

(7)

Disappearing liquid phase, reactive infiltration

Reactions 2 and 3 are simple eutectic systems at temperatures below the eutectic temperature. The final product is either single phase (n ) or two phase (£1 + '■*). The fourth reaction is analogous to the third reaction, except at temperatures above the eutectic temperature. The sixth reaction is for phase transformation via solution in a liquid. An example of this is B ■ >O Si3N4 transformation in magnesium silicate liquid. This article is concerned with sintering when reactions of type 5 occur and both starting constituents are in solid form. Reactive liquid infiltration of powder preforms is not within the scope of this article.

In many instances, the combustion synthesis reaction is accompanied by the formation of a transient liquid phase (Ref 13, 14). As with any liquid phase sintering operation, densification occurs due to capillary effects associated with the formation of the liquid. Figure 1 illustrates the reactive sintering process for forming the compound AB from a mixture of A and B powders (Ref 13). The initial compact is composed of mixed elemental powders, which diffusionally interact during heating. When the lowest liquidus temperature is reached (e.g., a eutectic or the melting point of one of the constituents), partial melting of the compact occurs. The liquid flows into pores between the solid particles resulting in densification. Also, the liquid is a rapid diffusion path between the elements, which facilitates compound formation. The final product is single phase and dense AB.

Fig. 1 A binary phase diagram and the typical sequence of events expected in reactive sintering of compound AB from a mixture of A and B powders. Source: Ref 13

Formation of the transient liquid phase during processing does not guarantee a dense product. Shrinkage porosity may develop during the solidification of the liquid phase (Ref 8). Control over the formation of the liquid phase, hence the reaction, through judicial selection of the characteristics of the starting powders and the process parameters is paramount for producing dense articles with useful microstructures by reactive sintering. Further difficulties can arise during reactive sintering when the adiabatic flame temperature of the reaction exceeds the melting point of the forming compound. Nevertheless, monolithic aluminides, ceramics, and composites have been fabricated by reactive sintering and its variants, as discussed in detail below.

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