Electrical and structural stability of highly strain-sensitive metal-carbon thin films

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Metal-carbon thin films (Me:a-C:H or Me:a-C) are able to react up to 15 times more sensitive on strain (gauge factor up to 30) than materials which are normally used in conventional strain gauges (gauge factor of 2). The strain sensitivity of the electrical resistance depicts a linear behavior as a function of the applied strain, furthermore the temperature coefficient of resistance can be adjusted to the material of the test body. Hence Me:a-C:H thin films provide the potential to be used as the sensitive element of strain gauges. They are particularly of interest for applications in which small strains need to be detected. The highest strain sensitivities (gauge factor of 30) are obtained when nickel is used as the metal (Ni:a-C:H). However, with this thin film type we noted an increasing intrinsic creep when temperature is increased: i.e. after deformation a systematic change in the electrical resistance occurs with time at constant strain. In addition, the electrical resistance at higher temperatures is drifting without load. Those instabilities limit the application of Ni:a-C:H in sensors. Instabilities are reduced when nickel is partially replaced by chromium (NiCr:a-C:H). Unfortunately the strain sensitivity of NiCr:a-C:H is only 5 times higher than that of conventional strain gauges. Transmission electron microscopy (TEM) investigations of Ni:a-C:H have shown that the films are constituted of nanoscaled metallic particles, which are embedded in structures of graphitic carbon onion shells, and of regions of amorphous carbon between these structures. First differences in the morphology of Ni:a-C:H and NiCr:a-C:H were noted: no graphitic carbon onion shells were observed in NiCr:a-C:H thin films. We aim to identify the film structures and processes, responsible for the high strain sensitivity and instabilities of Ni:a-C:H, and to decrease the creep rate at higher temperatures by preserving the high strain sensitivity. As a working hypothesis we assume that gliding processes in the graphitic carbon network are responsible for the observed changes of resistance. We will first investigate, how chromium is modifying the film growth and the mechanical-electrical properties. In situ annealing TEM and x-ray-diffraction experiments will be used to identify thermal induced diffusion and rearrangement processes. Modelling the metal carbon thin film and Monte Carlo simulations are planned to investigate the electrical conductivity and the implications of deformation. Different approaches are envisaged to modify the thin film composition in order to harden the graphitic carbon network and to prevent gliding processes.