Platinum Metals Rev., 1968, 12, (2), 61
Frictional Characteristics of the Refractory Platinum Metals
A recent report by D. H. Buckley of the Lewis Research Centre (NASA Tech. Note TN D–4152, 1967, (Sept.), 1–15) conveys at first glance the impression that osmium and ruthenium, the hexagonal refractory metals, have, when rubbed against themselves, lower coefficients of friction than their cubic counterparts rhodium and iridium, and are therefore to be preferred for sliding electrical contacts. The author states dogmatically that ‘from the results of this investigation, it would appear that ruthenium, with its hexagonal crystal structure, would certainly be highly superior to the face centred cubic metal rhodium in sliding electrical contact applications’. Detailed study of the paper, however, shows that this conclusion is based entirely on experiments made under ultra high vacuum conditions. The superiority of ruthenium over rhodium is only apparent at pressures below 10−8 torr and results reported show that at higher pressures rhodium has a much lower coefficient of friction than ruthenium. No information on the frictional characteristics of iridium or osmium at pressures above 10−8 torr is given in this report.
Within the context of space applications covered by NASA, it is perhaps logical to assume that atmospheric pressures below 10−8 torr constitute a perfectly normal ambient environment. Such clean conditions should certainly help when attempts are made to correlate surface properties such as friction with the crystallographic characteristics of pure metals. It is disappointing, therefore, to find in this report no indication of the purity of the materials studied or the way in which the test specimens were fabricated and prepared for examination. In subsequent publications it is hoped that fuller details will be supplied of the process, whereby the disc and rider specimens were ‘finished to a roughness of 4 to 8 micro inches and then fully annealed’.