Crystallization Toughening


IN ANOTHER approach, using glass adhesives, joints can be prepared with thicknesses of 20-100 um, substantially exceeding those of the grain boundary (GB) phases in polycrystalline ceramics. Such joints can be subsequently crystallized to increase their fracture toughness and stress corrosion resistance. Formation of interlocking grains (needles) across the joint is expected to result in toughening. The composition of the adhesive chosen must be based on the possible effect of interfacial reactions and should be optimized for a material system of interest.
    If feasible, bulk glasses should be prepared which have the composition of the GB phase and this should be followed by determination of Tg, a, E and possibly of the viscosity change of the glass as function of T. Joining could then be carried out at a certain viscosity level thus ensuring optimum reaction rate between the glass and ceramic until the excess of glass phase has either migrated away from the joint, or crystallized. Such approach offers a potential for manufacturing assemblies free from macrostructural discontinuity at the joint. Once the microstructure of the adhesive layer is properly controlled by crystallization treatment, the toughness of adhesive joints can equal or exceed that of adherend ceramic bodies. This concept has been proved for alumina ceramics where substantial toughening was induced by crystallization of the GB phase [32]. The success of this approach came from a thorough understanding of the crystallization processes in the MgO-Al₂0₃- SiO₂ system [33]. Examples of investigations related to crystallization of grain boundaries in ceramics can be found in Ref. 34.
    However, one major difficulty encountered in ceramic/glass joints was the persistent porosity in the joints. It is possible that porosity may be caused by the evolution of gases absorbed in the ceramic. Swelling of dense ceramics by gas evolution have been described [35]. Glass phase can also contain dissolved SO" and NO_x. These sporadic anions are potential gas-formers at high T. Vacuum joining should ameliorate some of these problems, provided that the systems are thermodynamically stable under reduced pressure.
    Before these method are industrially used, refinement of the joining and toughening procedures is required. This can be pursued based or experience related to crystallization processes in glasses and melts, crystal-liquid interactions, the role of molecular and capillary forces, solution-precipition reactions, etc. [36]. During crystallization, impurities in the melt are commonly rejected to the melt/crystal interface and affect the grain morphology. Therefore grain growth at the interface can be controlled by altering the degree of constitutional supercooling by adding selected dopands to the joining material.

Recommendations

Advances in ceramic joining by glass adhesives require detailed understanding of glass/ceramic interactions at elevated temperatures. Studies of interfacial reactions (dissolution-precipitation, effect of capillary forces, nucleation and growth, etc.) are recommended. These should involve confined spaces, e.g. in bicrystals, which simulate morphologically the grain boundaries existing in polycrystalline ceramics.


To be continued