The constitutive response of brazing alloys and the residual stresses in ceramic-metal joints

Nowadays the joining of dissimilar materials is often the only solution to fulfill the complex requirements of high technology applications. One of the fields in which the research activity is more intense and promising is that of the brazing of ceramics with metals. The performance of brazed ceramic-metal joints is limited by residual stresses which develop in the bonded assembly as it cools down after brazing. The magnitude and influence of these stresses can be particularly high because of the large difference between the thermomechanical properties of the two joining partners. Given the base materials and the joint geometry the relief of residual stresses mainly depends on the thermomechanical properties of the filler metal. To study how variations of the properties of the braze alloy influence the residual stresses, an active brazing filler metal, Incusil™ABA®, was reinforced with SiC particles and was used to join the ceramic composite Si3N4/TiN to steel. The characterization of the properties of the filler was carried out with a combined experimental and numerical approach: an experimental procedure for the production and mechanical testing of bulk specimens of reinforced and unreinforced braze filler metal was established and in parallel a three dimensional elastoplastic homogenization model was developed. The model was used both in an inverse homogenization approach, to identify the elastoplastic behaviour of the matrix alloys in the composite fillers at room temperature, and in the framework of straightforward homogenization, to obtain their temperature dependent properties. Once the production procedures for the fabrication of joints were established, the experimental investigation on the joints followed two main directions: on one hand the residual strains were measured by X-ray diffraction while on the other hand the joint performance was evaluated by 4-point bend tests. Furthermore, optical microscopy and SEM/EDX investigations were carried out to compare the composite filler microstructures in the joints and in the tensile specimens. In parallel, finite element models were developed both for the prediction of residual stresses and for the evaluation of the evolution of the stress state in the joints during the bend tests. Finally a parametric study was carried out to study different joint layouts.