Hard anodisation of aluminium and its application to sensorics

Hard anodising is normally carried out with electrolytes at subzero temperatures and with a very high current density. This may sometimes lead to burning and powdered deposits. In this work, a method of hard anodising at a higher temperature (10°C) using a pulsed power supply is investigated. The quality of coating obtained with this process is better than that obtained with the conventional method. Furthermore, it minimises the refrigeration requirements for cooling the electrolyte and eliminates the problem of burning and powdery coatings. The application of hard anodic coating with insulation resistance 1–30 GΩ up to 100 V dc to a pressure sensor as an insulating base layer is explored. The pressure sensor diaphragm is hard anodised, and this is followed by the deposition of a thin platinum–tungsten (92Pt–8W) film as strain sensor. The fabrication, characterisation and calibration of the pressure sensor and its applications are discussed.