Conference paper

High-strength ceramic substrates for thick-film sensor applications

Force and pressure sensors based on the piezoresistive properties of thick- film resistors have recently found wide application due to their low cost, reliability, and relatively good precision. Achieving high elastic mechanical strains is important to maximise the sensor response, and better substrates than the standard 96% alumina such as yttrium-stabilised zirconia (YSZ) and zirconia-toughened alumina (ZTA) have therefore been investigated in this work. The substrate materials were characterised with respect to compatibility with thick-film resistors, and to the effect on the mechanical strength of processing steps (scoring and breaking, laser cutting) and of the presence of overlying thick films. Two factors were found to considerably reduce the short-term mechanical strength of the substrate materials: processing (laser scored or cut edges), and the presence of heavily alloyed (3:1) Ag:Pd metallisation. This was respectively attributed to the presence of microcracks at the edges, and to the glass frit in the Ag:Pd. On the other hand, the Au conductor, resistor and overglaze layers used in the miniature test force sensor had little effect on the strength. Compatibility with thick-film piezoresistive sensors seems to be better for ZTA than for YSZ: ZTA has less influence on the properties of the resistors, has better thermal expansion matching, and higher thermal conductivity (less thermal drift).

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