Metal-ceramic bonding in dicalcium silicate composites with nickel

The advantages of ceramic coatings for high temperature applications are limited by the difficulty in integrating ceramic pieces with existing metal technology. One major constraint is the general difference in thermal expansion behavior between common metals and ceramics which results in poor thermal shock and fatigue resistance in metal-ceramic bonds. In this study bonding between a conventional metal (Ni) and a novel high thermal expansion composite ceramic (C{dollar}sb2{dollar}S-CZ) has been examined.; High thermal expansion dicalcium-silicate (Ca{dollar}sb2{dollar}SiO{dollar}sb4{dollar} or C{dollar}sb2{dollar}S) based ceramics may be produced by stabilizing {dollar}beta{dollar}-C{dollar}sb2{dollar}S in a non-reactive matrix such as calcium zirconate (CaZrO{dollar}sb3{dollar} or CZ). 15v% C{dollar}sb2{dollar}S composites produced by three reaction-sintering processes were examined for use in the metal-bonding experiments. The direct reaction sintering of calcia and zircon to form C{dollar}sb2{dollar}S-CZ composites was found to be limited by solid state reaction kinetics which favored the formation of amorphous calcia-silica complexes over complete C{dollar}sb2{dollar}S formation. Multiple firings with intervening rehomogenization were found to induce more complete C{dollar}sb2{dollar}S formation and strengths on the order of 260 MPa were obtained. Alumina additions were also found to expedite C{dollar}sb2{dollar}S formation through a liquid phase sintering mechanism. Reaction was essentially complete in such composites after a single firing and good strength ({dollar}sim{dollar}240 MPa) and density ({dollar}sim{dollar}4 gm/cm{dollar}sp3{dollar}) were obtained. Composites containing 15v% and 56v% C{dollar}sb2{dollar}S were produced using single stage firing with alumina additions for use in bonding experiments.; Dicalcium silicate-calcium zirconate (C{dollar}sb2{dollar}S-CZ) composite ceramics with 56v% C{dollar}sb2{dollar}S content were successfully bonded to Ni under oxidizing conditions. Microstructural characterization has shown that the bond is essentially mechanical in nature and formed by growth of NiO into surface defects inherent to the composite ceramic. A 10{dollar}mu{dollar}m thick layer of NiO grown upon Ni in contact with the polished C{dollar}sb2{dollar}S-CZ was necessary for successful bonding. Bonding attempts with less nickel oxidation were unsuccessful due to insufficient interlocking of NiO and composite ceramic. Similarly, bonding was not achieved between Ni and composites of lower C{dollar}sb2{dollar}S content (e.g. 15v%) as C{dollar}sb2{dollar}S is responsible for the composites' surface roughness.; This bonding method resulted in an interfacial strength greater than that of the bulk composite ceramic due to significant mechanical interlocking between NiO and C{dollar}sb2{dollar}S-CZ. Extensive NiO penetration into defects on the C{dollar}sb2{dollar}S-CZ surface resembled a three-component NiO-C{dollar}sb2{dollar}S-CZ composite microstructure extending along the length of the interface. The bonding technique should be applicable to any oxidizable nickel alloy and ceramic composite of sufficient C{dollar}sb2{dollar}S content (e.g. C{dollar}sb2{dollar}S-MgO composites).