Evaluation of diamond-like carbon films for lubrication in space

Abstract

Solid coatings for lubrication of mechanisms operating in space are widely used because of the rather severe operating conditions. These include the presence of an ultrahigh vacuum, a wide operating temperature range, an absent of gravity, a lack of oxygen, and ionising radiation. Currently, sputtered molybdenum disulfide (MoS2) is an established solid lubricant for space applications. However, diamond-like carbon coatings have been increasingly studied as potential candidates for preparation of wear resistant materials with low friction in vacuum conditions.
This thesis describes in-vacuum investigations made on a variety of formulations of diamond-like carbon (DLC) coatings for their suitability as a lubricant for application in mechanisms operating in space conditions. Specifically, high-hydrogenated siliconcontaining DLC coatings (a-CHSi or Si-DLC) were assessed. These coatings were produced by plasma-assisted chemical-vapour deposition (PACVD) by the supplier VITO, a Belgian research institute. Testing of the coatings was performed in a pin-on-disc machine at the European Space Tribology Laboratory (ESTL). The test conditions were a high vacuum (air pressure below 5 x 106 mbar) under a load of 5 Newton and at rotational speeds of 10rpm (0.01ms') and 200rpm (0.2 ms'). Tribological results arising from the tests showed that some of these coatings offer promising characteristics (low and stable friction coefficients, low wear rates and long life) for application in space mechanisms.
Scanning electron microscopy (SEM) technology was applied for post-test inspection of the specimens, and the effect that the deposition factors (precursor, bias voltage and hydrogen content) had on the resulting tribological characteristics was discussed.