Strain sensitivity and temperature behavior of invar alloy films

The electrical resistance–strain characteristics of Invar films has been studied for their possible application as strain gauges. The Invar films were prepared by sputtering technique and the necessary details of film preparation are given. Temperature coefficient of resistance (TCR) of the films has been measured by systematic annealing of films in vacuum. It has been found that TCR of film is of the order of 10−3/°C. The gauge factor which is an index of the strain sensitivity, was measured for as-deposited and annealed films. Films exhibited linear electrical resistance versus strain characteristics. The gauge factor for relatively thinner films was found to be as high as 5.5 and for films with thickness greater than 500 Å, it was lower. For films annealed by heating up to temperature less than 300°C, the gauge factor value did not change. However, films annealed at temperature above 300°C, showed reduction in gauge factor.