Bonding of Ceramics: an Analysis of Recent Joint Specimens

In the community of the ceramic materials scientists, there is a great interest in the joints of ceramic parts (or ceramic-coated parts), due to their high strength, thermal stability and leakproofness. An example of application of such a technology is in the construction of nuclear components. For high temperature applications, glass-ceramics are used as adhesive. This generates the issue of test methods suitable to assess joint strength. A first case of interest is the hourglass test, originated from the modification of an ASTM standard, with the aim of obtaining failure under a pure shear state in the bondline subjected to torsion. Unfortunately, due to the brittle nature of the materials involved in the joint, if the strength of the adhesive is not enough lower than that of the substrates the failure does not remain confined in the bond and also the substrates break. Thus, the results are of arguable application for design purposes. A second significant case is the ISO test, in which two tiny bars are bonded to form a cross-shaped specimen that can be used to load the bond either in peel or in shear mode. But, in both conditions the stress distribution is non-uniform and in the latter (similar to a lap joint) also peel stress is applied. Again, application of the results is difficult. The proposed work, by means of detailed analyses of the loading conditions in the specimens and accounting for the brittle nature of the materials, gives an insight on these tests and the interpretation of their results.