Piezoresistive properties of low-firing temperature thick-films on steel sensors

Thick-film materials are very advantageous for piezoresistive pressure and force sensors because of ease of processing, reliability and low cost. The standard substrate material used with thick-film technology for sensing is alumina, but its elastic modulus is high and its strength rather low. Steels offer better mechanical properties and permit assembly without an elastomer seal, which is required for pressure sensing in severe conditions. In order to use the steel as substrate, the standard firing temperature of thick-films has been decreased. In previous studies, we have developed and characterized 2 low-firing thick-film systems (dielectrics, resistors and conductors) compatible with austenitic and ferritic materials. We have formulated these systems to achieve to chemical and thermal expansion compatibility. Other parameters like adherence, soldering properties and process, have been optimized too in order to be adapted on high-performance sensors. In this work, we will present the characterization of 2 steel sensors based on low fired thick-film technology: a high-performance pressure sensor based on high-strength steel substrate chemically similar to the ferritic steel, and a force sensor used in surgical operation of total knee arthroplasty (TKA) based on a medical alloy comparable to the austenitic steel. Key words: thick film system, high strength steel, pressure sensors, low-temperature firing.