The first theory was investigated with differential calorimetry. Specimens were sent to a polymers lab to  search for an embrittlement temperature between room temperature and –100°C. None were found.  The second theory of differential thermal expansion causing microcrack formation at the resin-carbon bond  was investigated using two methods. First, high-power microscopy was used to search for cracks, but none  were observed. The second technique was to continue testing the specimens in ambient conditions. At  ambient conditions, the measured wear coefficients returned to normal levels, which indicated that a  reversible phenomenon was occurring, thus eliminating the microcracking theory.  The third cause of the high wear rate coefficients was investigated by testing in dry nitrogen gas at ambient  temperatures. Under these conditions, the high wear rates were repeated, which gave credibility to the  sensitivity of this material to humidity and trace hydrogen to achieve low wear rates.  Discussion  A new pin on disc apparatus was designed and fabricated to provide high-speed wear coefficient data for  hydrodynamic bearings in canned motors. These motors and bearing arrangements are commonly used in  actively pump thermal control circuits for manned and robotic spacecraft. The dramatic increase in desired  life of these pumps has led to the need to understand the high-speed wear phenomenon of the bearings.  Test data in surrogate fluids has reveal important trends in the wear coefficients. Surrogate fluids are  significant because they may ease the burden of testing and provide a level of similarity between different  coolants used in various spacecraft. The significant trends include:  •Twenty-eight percent ammonia hydrate solution is a suitable surrogate fluid for 100 percent anhydrous ammonia for the material wear couple present in the PCVP bearings. •Four percent ammonia hydrate solution behaves similarly to pure water for the material wear couple present in the PCVP bearings. •Wear rates measured in water are two orders of magnitude higher than wear rates measured in anhydrous ammonia. •Wear rates measured in dry nitrogen gas were two orders of magnitude higher than those measured in air at ambient temperature, pressure, and humidity. •At rubbing speeds greater than 4 m/s, the wear rate decreased with increasing speed for flooded ammonia conditions. •Based on testing, the high-speed wear coefficient of the ISS pump bearings to be in the range of 1.4 to 8.3E-09 (mm3/(N m)). Additionally, certain best practices were determined to be useful for future, long duration canned motor  bearing designs.  •Document the bearing groove orientations with respect to key motor housing features if bearing pads are used. •Preclude the secondary flow paths from generating rotor-bearing asymmetric loading. •Maintain a symmetric journal bearing design to the greatest extent possible. The EET and the test technique developed to measure high-speed wear coefficients is available to future  spaceflight programs that may utilize actively pumped thermal control circuits. Furthermore, the database  generated from the initial EET test program was able to validate the failure mode of the 2010 ISS thermal  control pump failure to within 5% of the observed material loss, which was measured during the failure  NASA/CP—2018-219887 507