Joining of silicon nitride ceramics to itself and to other materials using chemically compatible interlayers

The joining of Si{dollar}rmsb3Nsb4{dollar} ceramics to itself and to other materials is an important technologically process. Current methods of joining limit the temperature of use to less than 500{dollar}spcirc{dollar}C due to the softening of the joint. This dissertation presents methods of joining Si{dollar}rmsb3Nsb4{dollar} ceramics for use at room and elevated temperatures.; The first part discusses the joining of Si{dollar}rmsb3Nsb4{dollar} ceramics to itself using sintering aids as the interlayer. The rationale behind this approach is that sintering aids are effective in bonding individual grains of Si{dollar}rmsb3Nsb4{dollar}, hence they should also be effective in joining bulk pieces of Si{dollar}rmsb3Nsb4{dollar}. The interlayer is a mixture of rare-earth oxide (RE{dollar}rmsb2Osb3{dollar}) and silica (SiO{dollar}sb2{dollar}) so as to form a crystalline rare-earth disilicate (RE{dollar}rmsb2Sisb2Osb7{dollar}) intergranular phase upon processing. The joining process is further optimized such that the microstructure and chemistry of the joint are similar to that of the bulk, effectively "eliminating" the joint. The room temperature strength of these joints is as high as 950 MPa, which is more than 90% the strength of the bulk Si{dollar}rmsb3Nsb4{dollar}.; The second part of the dissertation is on the joining of Si{dollar}rmsb3Nsb4{dollar} to Al{dollar}rmsb2Osb3{dollar}. Due to the differences in the coefficient of thermal expansion (CTE), the materials shrink and expand to different volumes upon thermal cycling. This volume difference introduces stresses at the joint resulting in low strength or failure. The residual stresses can be controlled and hence minimized by using functionally graded materials (FGM) as the interlayer. The FGM used has a gradient in composition with Si{dollar}rmsb3Nsb4{dollar} at one end and Al{dollar}rmsb2Osb3{dollar} at the other. This composition gradient causes a gradient in the CTE and shrinkage, which can accommodate the volume changes during thermal cycling. To fabricate the FGM, a new processing method has been developed based on differential sedimentation. The powders used in the sedimentation process is chosen using a model developed to predict the composition profile as a function of the initial powder distribution. The FGMs fabricated by this process is then used to join Si{dollar}rmsb3Nsb4{dollar} to Al{dollar}rmsb2Osb3{dollar}.