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The frictional forces generated during rubbing contact of metals greatly influence the ignitability of the bulk material when exposed to high-pressure oxygen environments. Previous works have ranked the ignition susceptibility of metals by frictional heating tests and by promoted combustion tests. Variations in the rankings by these two methods have been explained by differences in the friction coefficient and the resulting differences in frictional heat generation. A material with a higher coefficient of friction will produce more heat and will therefore achieve its ignition temperature at a lower PV product, where P is the normal load divided by the rubbing area and V is the relative linear velocity between the samples. A ranking of frictional ignition data taking into account the friction coefficient was shown to agree quite well with promoted combustion rankings. To further investigate the effects of the friction coefficient on ignition, a series of pin-on-disk friction tests were performed on five selected metals. These tests measured the friction and wear of each metal during the pre-ignition phase, while exposed to high-pressure oxygen. Wear characteristics appeared dramatically different for the pin-on-disk tests compared to the hollow cylinder configuration of the frictional heating test for some materials. Among the metals tested, three types of wear were observed: a severe adhesive wear resulting in high frictional forces; a mild wear characterized by a glazed oxide coating and low friction coefficients; and an adhesive-delamination wear combined with the release of many burning wear particles. The wear configuration was found to be extremely important when assessing the frictional heating properties that lead to ignition in oxygen-enriched atmospheres.
friction, wear, friction coefficient, PV product, frictional heating, ignition, oxygen
Senior Engineer, Lockheed-ESC, White Sands Test Facility, Las Cruces, NM
Aerospace Engineer, NASA, White Sands Test Facility, Las Cruces, NM