This letter presents experimental results on unlubricated friction, which suggests that stick–slip is described by self-organized criticality (SOC). The data, obtained with a pin-on-disc tribometer examines the variation of the friction force as a function of time—or sliding distance. This is the first time that standard tribological equipment has been used to examine the possibility of SOC. The materials were matching pins and discs of aluminium loaded with 250, 500 and 1000 g masses, and matching M50 steel couples loaded with a 1000 g mass. An analysis of the data shows that the probability distribution of slip sizes follows a power law. In addition, the frequency power spectrum follows a 1/fα pattern with α in the range 1.1–1.3. We perform a careful analysis of all the properties, beyond the two just mentioned, which are required to imply the presence of SOC. Our data strongly support the existence of SOC for stick–slip in dry sliding friction.

Four research areas at the NASA Glenn Research Center involving the tribology of space mechanisms are highlighted.These areas include: soluble boundary lubrication additives for perfluoropolyether liquid lubricants, a Pennzane dewetting phenomenon, the effect of ODC-free bearing cleaning processes on bearing lifetimes and the development of a new class of liquid lubricants based on silahydrocarbons.

Generation of Stable Phosphorus Heterofullerenes in a Radiofrequency Furnace For the first time, phosphorus heterofullerenes, mainly C59P and C69P have been produced, and identified by mass spectroscopy. Their chemical properties have been characterized with respect to their behaviour at sublimation as well as to solubility and to sensivity to air. The formation of these heterofullerenes was achieved by evaporating phosphorus and carbon simultaneously, however, at different positions, that is at different temperatures, in a radiofrequency furnace.

Historically, space mechanism lubricant choices were based on space heritage rather than on the latest technology or best available materials. With the limited mission lives and minimal duty cycles of the early space programme, this strategy was highly successful. As missions extended, other spacecraft components, such as electronics, batteries, and computers, failed before lubricated mechanisms; however, during the 1980s and 1990s, these ancillary components vastly improved and tribological systems became a main factor limiting spacecraft reliability and performance. Although tribological components represent only a small fraction of the spacecraft's cost, they are often single-point failures that cripple or debilitate expensive spacecraft. This paper will discuss the following: common space lubricants, mechanism components, testing, and factors affecting lubricant selections.

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