IMPROVED LANGMUIR PROBE SURFACE-COATINGS FOR THE CASSINI SATELLITE

A Langmuir probe will be used on the space research satellite Cassini in order to monitor electron density fluctuations in space plasmas surrounding Saturn. In order to obtain well-defined Langmuir probe characteristics a surface with uniform work function PHI must be used. A graphite coating has been used on earlier earth-bound satellite probes, but for the Cassini satellite to Saturn a hard, wear-resistant coating must be used. In this contribution we report on investigations of titanium nitride (TiN) coatings for this purpose. TiN coatings have been prepared by high temperature nitriding in pure nitrogen gas and by reactive magnetron sputtering on plane titanium and titanium alloy substrates. The resulting coatings have been analysed with laterally resolved, relative PHI measurements, and the tribological properties of the coatings have been investigated by solid particle erosion and scratch testing. In the tests, the graphite coating DAG 213 used on earlier satellite probes was used as a reference material. The results show that the TiN coatings are superior to the graphite coating with regard to both photoelectric properties and solid particle impact resistance. The sputtered coatings exhibit the lowest lateral work function variation, but the nitrided coatings have superior erosion resistance. The results demonstrate that TiN coatings are suitable for space plasma probes.