Microstructural Characterization of Cofired Tungsten‐Metallized High‐Alumina Electronic Substrates

Microstructural characterization of a high‐Al 2 O 3 substrate containing cofired thick‐film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al 2 O 3 grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO 2 and reduced rutile (TiO 2‐ x ). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al 2 O 3 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 (CaAl 2 Si 2 O 8 ), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, α‐cristobalite (SiO 2 ), and α‐cordierite (Mg 2 Al 4 Si 5 O 18 ). In addition, small rutile particles precipitated within the Al 2 O 3 grains.