RADIO-FREQUENCY SPUTTER-DEPOSITION AND PROPERTIES OF CALCIUM-FLUORIDE THIN-FILMS

Calcium fluoride thin films have been deposited on various substrates by radio frequency sputtering of a hot-pressed CaF2 target in a pure argon plasma using load-locked sputtering equipment. The sputtering power was less than 100 W to avoid crack formation and fracture of the target. As a result, the maximum deposition rate of films was only 2.1 nm/min. The films analyzed by Rutherford backscattering spectroscopy (RBS) were found to be fluorine deficient; depending on the deposition conditions, the content of oxygen atoms incorporated in the films can be as low as the RBS detection limit of 2 at. %. The crystallographic structure of the deposited material was identified to be a CaF2 fluorite phase by x-ray diffraction technique; this phase was preferentially oriented in the (111) direction for films of 0.4-0.5 mum in thickness. This preferred orientation vanished with increasing film thickness. Films of 2.5 mum in thickness were slightly (110) oriented. The sputter-deposited CaF(x) films (with x less-than-or-equal-to 1.85) exhibited compressive residual stresses. The apparent Knoop hardness of the films deposited on silicon substrates (harder than CaF2) and measured under a load of 10 and 20 g was about 3500 MPa, i.e., twice the hardness of the bulk CaF2 compound. The surface and cross-section morphology of CaF(x) films was examined by scanning electron microscopy. These films were resistant to oxidation in air at 500-degrees-C. Tribological characteristics of CaF(x) films sputter-deposited on cobalt-based alloy disks were determined by pin-on-disk tests. The friction coefficient and wear resistance of these films sliding against Cr-C coated pin specimens in air at 500-degrees-C were found to be encouraging. These films are potential candidates to serve as solid lubricant films for precision components (ball bearings) operating at high temperatures in oxidizing ambient.