Growth into Particulate Preforms

Of the two most commonly used preform materials, one, alumina, is chemically inert during DMO while the other, silicon carbide, is reactive. We first examine the growth of a DMO composite into an alumina particulate, followed by that into SiC preforms, and finally the DMO infiltration of a suitably coated fibrous preform. Coatings of calcium sulfate with calcium silicate may be used to ensure that oxidative growth ceases at the external boundary of the preform, thereby ensuring near-net-shape fabrication. The mechanism by which these barriers poison the oxidation without impeding the flow of oxygen is not clear at this time. 

it was argued that the oxygen supply through the pores of the alumina preform was the rate-limiting step, and that growth became progressively easier as the alloy percolation channels approached the free surface. The oxidation rate was also found to decrease with increase in the silicon content of the melt. 

When excessive reactive wetting of the preform leads to metal migration over particle surfaces ahead of the main reaction front, residual porosity becomes a ubiquitous feature of DM0 infiltration. The surface of a partially-infiltrated region then becomes sealed before the underlying pores are filled with the advancing composite. In the case of oxidation in air, a similar phenomenon can lead to the entrapment of nitrogen-containing pores. 

The crystallinity of alumina formed by DMO infiltration into SiC particulate preforms follows the expected behavior of heterogeneous nucleation on SiC particles and large monocrystalline regions with [0001] parallel to the growth direction within the larger voids [83,86]. Manor et al. [82] found that the infiltration rate increased as the SiC particle size decreased. However, no systematic trend was found in another work [83], most probably due to widely dilferent silicon concentrations in the percolation channels resulting from reduction of the silica layer during infiltration. 

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MPam /, respectively. The decrease observed after growth into the particulate preform is inconsistent with the results of Pickard et al. but it is unlikely that the alloys used for the two composites were the same. 

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