MICROSTRUCTURAL CHARACTERIZATION OF COFIRED TUNGSTEN-METALLIZED HIGH-ALUMINA ELECTRONIC SUBSTRATES

Microstructural characterization of a high-Al2O3 substrate containing cofired thick-film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al2O3 grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO2 and reduced rutile (TiO2-x). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al2O3 grains by the glass phase which penetrated the porous W layers during sintering; there was no interfacial reaction or diffusion zone. The mechanical properties of the W metallization did not limit interfacial strength. Heat treatments of the substrate at 1400 K in air and under vacuum resulted in the devitrification of the intergranular glass. The most abundant devitrification product was anorthite (CaAl2Si2O8), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, alpha-cristobalite (SiO2), and alpha-cordierite (Mg2Al4Si5O18). In addition, small rutile particles precipitated within the Al2O3 grains.