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IMPROVED HIGH-TEMPERATURE CORROSION RESISTANT SILICON-NITRIDE CARBIDE COMPOSITES

Objective

1.To determine the surface reactivity of silicon nitride matrices and corresponding silicon nitride-silicon carbide composites in environments representative of fossil fuel conversion systems.
2.To determine how the presence of impurities such as sulphur vapour, sulphur dioxide and chlorine within the corrosion environment affects the surface reactivity of silicon nitride and silicon nitride-silicon carbide composites.
3.To elucidate the mechanisms of high temperature chemical degradation in corrosive environments, which contain either oxygen or sulphur or chlorine.
4.To optimise the composition of composites in terms of additive and silicon carbide content for use in aggressive situations.
The objectives of the project are:
to determine the surface reactivity of silicon nitride matrices and corresponding silicon nitride silicon carbide composites in environments representative of fossil fuel conversion systems;
to determine how the presence of impurities such as sulphur vapour, sulphur dioxide and chlorine within the corrosion environment affects the surface reactivity of silicon nitride and silicon nitride silicon carbide composites;
to elucidate the mechanisms of high temperature chemical degradation in corrosive environments, which contain either oxygen or sulphur or chlorine;
to optimise the composition of composites in terms of additive and silicon carbide content for use in aggressive situations.

Achievements to date include the acquisition of oxidation data relating to the datum matrix material (silicon nitride densified with 7 w/o yttria, 4 w/o) and the datum composite material (ie datum matrix material containing 10 w/o silicon carbide particles) and chloridation data relating to the data matrix and composite materials.

The effects of silicon nitride matrix composition and silicon carbide additions on the corrosion of silicon nitride based materials have been studied. Six beta-sialon matrices with z-values of 0.2 0.5 1.0 1.5 2.0 and 3.0 and five composites comprising beta-sialon matrices with z-values of 0.2 0.5 1.0 2.0 and 3.0 plus 10% by weight silicon carbide particles (below 5 um) were prepared by pressureless sintering. In addition an aluminium free hot isostatic press (HIP) processed beta-silicon nitride and reaction bonded silicon nitride were tested. Corrosion results obtained at 1150 and 1350 C in environments comprising:
argon 20%, oxygen 2%, chlorine;
hydrogen 10%, hydrogen sulphide 2%, water vapour;
nitrogen 15%, carbon dioxide 4%, oxygen 0.2%, sulphur dioxide
showed that for each of the three environments the HIP processed beta-silicon nitride and the z=3.0 matrix and composite materials exhibited the best corrosion resistance. The composite materials showed no significant improvements in corrosion resistance and room temperature indentation toughness which would offset increased fabrication difficulties and accordingly matrix materials are to be preferred for corrosion resistance. The composite materials showed no significant improvements in corrosion resistance and room temperature indentation toughness which would offset increased fabrication difficulties and accordingly matrix materials are to be preferred for corrosion resistant materials. The mechanism of corrosion at 1350 C in each of the 3 environments involves the formation of large volumes of low viscosity liquid phases. These large volumes of liquid enable the solution of silicon nitride/sialon grains to occur and it is this process which is responsible for corrosion.

Coordinator

University of Limerick
Address
Plassey Technological Park
Limerick
Ireland

Participants (1)

University of Northumbria at Newcastle
United Kingdom
Address
Ellison Place
NE1 8ST Newcastle Upon Tyne