Novel fabrication processes for thin film vapour deposited strain gauges on mild steel

Pressure measurement using a strain gauge bonded with epoxy adhesive to a metallic mechanical support has been, and still is, extensively employed, however, for some applications the use of an epoxy is inadequate, especially when temperatures exceed 120C. There is therefore particular interest in the use of thin film techniques to vacuum deposit strain gauges directly on metallic substrates. Such devices are highly cost effective when produced in large quantities due to the manufacturing techniques involved. This makes them ideally suited for use in large-volume products such as electronic weighing scales and pressure transducers. In this thesis, new techniques for fabricating thin film vapour deposited strain gauge transducers on metal substrates for application as novel pressure sensors in the fastener industry are developed. Clearly, for a vapour deposited strain gauge to function correctly, it is essential that it be deposited on a defect free, high quality electrically insulating film. This was a significant challenge in the present study since all available physical vapour deposition (PVD) equipment was direct current (DC) and insulators of around 4 um thick were needed to electrically isolate the strain gauges from metal. As a result, several methods of depositing insulators using DC were developed. The first involved the use of DC magnetron sputtering from an aluminium target to reactively deposit up to 4 um thick AlN. DC magnetron discharges suffer arc instability as the AlN forms on the target and this limits the maximum thickness that can be deposited. Consequently, the arc instability was suppressed manually by increasing argon gas flow at the onset of arcing. Although the deposited AlN showed a high insulating resistance, it was found that the breakdown voltage could significantly increase by (a) utilising a metallic interlayer between the thin film insulator and the metallic substrate and (b) annealing in air at 300C. A second deposition method involved the use of DC magnetron sputtering to deposit modulated thin film insulators in which an aluminium target was used to reactively deposit alternating layers of aluminium nitride and aluminium oxide. These films showed significant increases in average breakdown voltage when the number of layers within the composite film was increased. The third method involved the deposition of AlN thin film insulators using partially filtered cathodic arc evaporation with shielding. Initially, AlN was deposited under partially filtered conditions to obtain a relatively thick (~ 4 um) coating then, while still depositing under partially filtered conditions, a smooth top coating was deposited by using a shielding technique. The deposition of metal macroparticles is an inherent problem with cathodic arc deposition and shielding is one form of macroparticle filtering. Such particles are highly undesirable in this study as they are electrically conductive. A fourth coating technique for depositing insulators on steel was based on thermal spray technology. Insulating films of Al2O3 were plasma sprayed and then polished to thereby fabricate viable electrical insulators for vapour deposited strain gauges. With respect to depositing strain gauges two methods were employed. The first involved the sputter deposition of chromium through a shadow mask to form a strain gauge with gauge factor sensitivity of around 2. The second used cathodic arc evaporation to fabricate a multi-layered strain gauge composed of alternating CrN and TiAlN layers that yielded a gauge factor of around 3.5. The technique achieves better compatibility between gauge and insulator by allowing a wider selection of materials to form the gauge composition. Finally, a novel pressure sensor in the form of a load cell was developed that consisted of a chromium strain gauge on a steel washer electrically insulated with AlN thin film. The load cell showed good performance when tested under compressive load.