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98198 OXIDATION AND ALKALI-METAL SULPHATE CORROSION PROCESSES IN SILICON NITRIDE - BASED MATERIALS

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Product Number: 51300-98198-SG
ISBN: 98198 1998 CP
Author: M. J. Pomeroy, P. Byrne, R. Ramesh
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sialon materials (Si 6-z AlzOzN(8-z)) densified with a YSiAlON glass and having z-values of 0.2, 0.5, 1.0, 1.5, 2.0 and 3.0 have been subject to oxidation and hot corrosion experiments. From the results obtained, the process by which oxidation occurs at a temperature of 1350°C involves direct solution of p-sialon grains by an ahtminosilicate liquid. This liquid forms by the preferential diffusion of yttrium and/or altinium from the bulk of the ceramic into the scale - ceramic interface. In the case of higher z-value materials (=>1.0), liquid formation is also facilitated by the reaction of yttrium aluminium garnet, formed by the devitrihcation of the YSiAlON glass during oxidation, and silica formed as an oxidation product. Hot corrosion experiments carried out using a crucible testing technique at 1150°C showed that only the z=O.2 and 3.0 materials do not undergo catastrophic corrosion by molten sodium, potassium sulphate. The z=0.2 material is partially protected by a high viscosity surface. film, whilst the z=3.0 material is protected by the formation of a nepheline (RAlSi04 [R= alkali -metal]) scale layer. Corrosion morphologies for these two materials show that the reaction interface is at the scale - ceramic interface and that a liquid phase is in direct contact with the ceramic. Accordingly, the mechanism by which hot corrosion occurs, like oxidation at the higher temperature, is by solution of b-silaon grains. Whilst the z=3.0 material is protected by the formation of nepheline, the five to six fold difference between the coefficients of thermal expansion for this phase and the z=3.0 material results in significant mechanical damage to the scale during thermal cycling. Accordingly, the nepheline phase may have only limited protective ability.
sialon materials (Si 6-z AlzOzN(8-z)) densified with a YSiAlON glass and having z-values of 0.2, 0.5, 1.0, 1.5, 2.0 and 3.0 have been subject to oxidation and hot corrosion experiments. From the results obtained, the process by which oxidation occurs at a temperature of 1350°C involves direct solution of p-sialon grains by an ahtminosilicate liquid. This liquid forms by the preferential diffusion of yttrium and/or altinium from the bulk of the ceramic into the scale - ceramic interface. In the case of higher z-value materials (=>1.0), liquid formation is also facilitated by the reaction of yttrium aluminium garnet, formed by the devitrihcation of the YSiAlON glass during oxidation, and silica formed as an oxidation product. Hot corrosion experiments carried out using a crucible testing technique at 1150°C showed that only the z=O.2 and 3.0 materials do not undergo catastrophic corrosion by molten sodium, potassium sulphate. The z=0.2 material is partially protected by a high viscosity surface. film, whilst the z=3.0 material is protected by the formation of a nepheline (RAlSi04 [R= alkali -metal]) scale layer. Corrosion morphologies for these two materials show that the reaction interface is at the scale - ceramic interface and that a liquid phase is in direct contact with the ceramic. Accordingly, the mechanism by which hot corrosion occurs, like oxidation at the higher temperature, is by solution of b-silaon grains. Whilst the z=3.0 material is protected by the formation of nepheline, the five to six fold difference between the coefficients of thermal expansion for this phase and the z=3.0 material results in significant mechanical damage to the scale during thermal cycling. Accordingly, the nepheline phase may have only limited protective ability.
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