Development of a new cryogenic tribotester and its application to the study of cryogenic wear of AISI 316 stainless steel

Abstract

A pin-on-disk tribotester has been designed and constructed that enables cryogenic friction and wear testing to be conducted either in liquid nitrogen (wet tests) or in dry sliding conditions. The tester does not require a sealed, cooled cryostat for the cryogenic tests. For wet tests, both pin and disk specimens are immersed in liquid nitrogen (LN2), whereas for the dry cryogenic tests, the disk specimen is only partially submerged in LN2 while the pin specimen remains dry in gaseous nitrogen.To prove the capabilities of the tribotester, tests were run using AISI 316 stainless steel pins in sliding contact with disks made of yttria-stabilized zirconia. Friction and wear tests were run under three conditions: dry sliding in room temperature air, cryogenic dry sliding in which bulk pin temperatures remained less than 115 K, and wet cryogenic tests in LN2 at 77 K. All tests were run at two different sliding speeds, 0.1 and 1.0 m/s, for a sliding distance of 1 km. It was found that wear rates of the stainless steel material were slightly lower at cryogenic temperatures than at room temperature, with wear being greater at low sliding speed than at high speed for all test conditions. Friction coefficients in the cryogenic tests were generally slightly lower than those at room temperature. X-ray diffraction analysis of the worn AISI 316 stainless steel pin surfaces showed that phase transformation from austenite to martensite had occurred during all cryogenic wear tests, whether in liquid nitrogen or during dry sliding, as well as during tests in air at room temperature. More martensite was produced during low speed sliding tests, both wet and dry, when the wear rates were higher, than at high sliding speeds. X-ray photoelectron spectroscopy analysis of worn pin surfaces was conducted to determine the role of oxides in the wear process at both room and cryogenic temperatures.