The aim of this project is to improve oxidation and corrosion resistanc e of non-oxide ceramics of the silicon nitride and related types, through systematic, basic research. Sintered silicon carbides of good oxidation resistance will be used as reference materials, and it would be expected that the final quality of the silicon nitrides will approach that of the silicon carbides. To provide guidelines for the direction of this basic study, and later to assist in the effective quantitative evaluation of material improvements, one potential application, metal forming dies, has been selected identifying a specific set of oxidation and corrosion conditions. This industrial application of non-oxide ceramics, and the silicon nitride based materials in particular, would appear to be ideally suited to their known high-temperature mechanical properties. The immediate economies to be achieved will result in large measure from the reductions in 'down-time' resulting from less frequent routine die replacement and premature die failure. The economic value of the increased basic understanding of the oxidation and corrosion behaviour of the silicon nitrides, and of the resulting improved oxidation and corrosion resistance, cannot be easily quantified. It is certain, however, to have a very significant impact on the future acceptance of these materials as substituents for metals in a wide range of high temperature applications.
Current theory suggests that the oxidation rate of materials that are highly resistance to oxidation is related to the type and amount of additives used during sintering.
The first route was to add yttrium oxide to silicon nitride via hot isostatic pressing (HIP). The weight change measured after oxidation in air at temperatures between 1400 1500 C showed a logarithmic kinetics. Passivation may be related to the high crystallinity of the oxide film formed on the surface of the ceramic.
The second route involved the use of an aluminium oxide-titanium oxide system as a densification aid to silicon nitride. This was chosen on the theoretical basis that both materials should be in thermodynamic equilibrium during the intergranular phase which should overcome the anticipated problem of intergranular phase cation release. The results of testing were encouraging. After oxidations carried out at 1300, 1400 and 1460 C, this material showed negligible weight gain.
The new, densified silicon nitride was used in the preparation of metallic die ceramic insert assemblies. These were then used in laboratory and factory environments to produce extrusion forged components for the rear axle of a truck. The configuration and strength of the metal die holder was inadequate to contain the elastic deformation of the ceramic liner caused by the hoop stresses created. During extrusion, there was severe adhesion of metal and metal oxide to the ceramic liner surface. Both problems may be controlled in both types of materials tested by the careful selection and control during the intergranular phase composition of the silicon nitride. A more complete solution may found in the development of special lubricant or fluxing systems. These would minimize the contact between the metal oxide and the non-oxide ceramic surface of the die.
SINTERED SILICON CARBIDES OF GOOD OXIDATION RESISTANCE WILL BE USED AS REFERENCE MATERIALS, AND IT WOULD BE EXPECTED THAT THE FINAL QUALITY OF THE SILICON NITRIDES WILL APPROACH THAT OF THE SILICON CARBIDES.
TO PROVIDE GUIDELINES FOR THE DIRECTION OF THIS BASIC STUDY, AND LATER TO ASSIST IN THE EFFECTIVE QUANTITATIVE EVALUATION OF MATERIALS'IMPROVEMENTS, ONE POTENTIAL APPLICATION, METAL FORMING DIES, HAS BEEN SELECTED, IDENTIFYING A SPECIFIC SET OF OXIDATION AND CORROSION CONDITIONS. THIS INDUSTRIAL APPLICATION OF NON-OXIDE CERAMICS, AND THE SILICON NITRIDE BASED MATERIALS IN PARTICULAR, WOULD APPEAR TO BE IDEALLY SUITED TO THEIR KNOWN HIGH-TEMPERATURE MECHANICAL PROPERTIES.
THE IMMEDIATE ECONOMIES TO BE ACHIEVED WILL RESULT IN LARGE MEASURE FROM THE REDUCTIONS IN "DOWN-TIME" RESULTING FROM LESS FREQUENT ROUTINE DIE REPLACEMENT AND PREMATURE DIE FAILURE.
THE ECONOMIC VALUE OF THE UNDERLYING INCREASED BASIC UNDERSTANDING OF THE OXIDATION AND CORROSION BEHAVIOUR OF THE SILICON NITRIDES, AND OF THE RESULTING IMPROVED OXIDATION AND CORROSION RESISTANCE CANNOT BE SO EASILY QUANTIFIED. IT IS CERTAIN, HOWEVER, TO HAVE A VERY SIGNIFICANT IMPACT ON THE FUTURE ACCEPTANCE OF THESE MATERIALS AS SUBSTITUENTS FOR METALS IN A WIDE RANGE OF HIGH TEMPERATURE APPLICATIONS.
Funding SchemeCSC - Cost-sharing contracts
LS2 9JT Leeds