EET Validation – Dry  Preliminary EET testing was conducted at STP conditions to establish operating procedures and confirm  traceability to low-speed testing. Once the traceability and operating procedures were established, the EET  was operated at high speeds to expand the wear coefficient database. A summary of pertinent wear  coefficient data for both low- and high-speed EET operation in air is included in Table 2. High-speed wear  coefficients measured in air at ambient conditions were equivalent to those measured on the commercial  tribometers. This provided confidence in the EET design and operation.  Surrogate Fluid Wear Coefficient  The wear coefficient database was expanded to include flooded liquid conditions at high speeds. Initially,  water was used in the high-speed flooded liquid tests. These initial tests uncovered a nontrivial issue in the  test operation. Hydrodynamic lift was being generated between the carbon pin to chrome runner interface.  This lift was the result of several factors, including low axial runout in the rotating disk, low surface  roughness on the chrome, low surface roughness on the worn carbon bearing material, high absolute  viscosity of the water at room temperature, and relatively high speeds. During the testing initial phase, wear  was observed during the initial (i.e., high contact pressure) phase of pin wear. It was postulated that even  at relatively high contact pressures the pin-disc interface could develop sufficient hydrodynamic lift to  prevent further wear. To overcome this limitation for high-speed flooded operation, several modified test  techniques were developed without success to disrupt the hydrodynamic film. The only reliable technique  for eliminating hydrodynamic lift in these tests was reduction of the absolute fluid viscosity through elevated  temperature.   Once an acceptable test technique was developed, surrogate fluid testing was conducted. Water, 4 percent  ammonia in water, and 28 percent ammonia in water solutions were tested at temperatures below their  boiling points. The evolution of wear coefficient with reduced contact pressure was tracked during this test  campaign. A summary of the test data is included in Table 2.  Anhydrous Ammonia Measured Wear Coefficients  The final phase of testing on the EET encompassed the exact conditions found in the failed ISS pump from  2010, high contact speeds in flooded anhydrous ammonia. Data is presented in Table 3 along with data for  a 28% ammonia in water mixture and bone-dry, gaseous nitrogen at -45°C.   The wear coefficient database enables calculations to be performed to evaluate the failure progression  theory for the ISS active thermal control pump. By combining the available on-orbit operations data from  the failed pump with the new wear coefficient database, the closure calculation was within 5% of the  observed wear found during the failure investigation. This compares to calculations based on estimates of  wear coefficients that ranged from -98% to over 5 times the observed wear volume.   Cold, Dry Material Performance  One final set of tests was conducted after the anhydrous ammonia data were obtained. The objective of  this test was to evaluate the effect of low temperature on the wear rates because the EET was operated at  approximately –45°C during the anhydrous ammonia tests to keep the internal pressure low and minimize  the risk of a thermal runaway and boil-off situation. The final cold test was conducted in a dry gaseous  nitrogen environment at –45°C. The measured wear coefficients were three orders of magnitude higher  than those measured in the flooded anhydrous ammonia tests. Three potential causes for these high wear  coefficients were postulated. The resin binding the graphitic carbon could go through a secondary glass  transition temperature, causing embrittlement. The differential thermal expansion between resin and carbon  may cause stress cracking and induce a weaker composite material. The surface layer of the carbon in  rubbing contact may be highly sensitive to humidity and trace hydrogen.  NASA/CP—2018-219887 506