Liquid phase sintering (LPS) sample cartridges were flown on IML-2 mission by the Principal Investigator (PI), Prof. Randall M. German , from The Pennsylvania State University. The design from this previous mission is being used for sintering experiments on the MSL-1 mission, classifying the present hardware as series hardware. As such, the creep rupture verification requirement could be satisfied based on data provided from the previous mission. However, the composition of the samples for the MSL-1 mission are such that more liquid phase forms at temperature and higher vapor pressures are anticipated. So in consultation with the PI, the project management team decided to conduct a creep rupture test of a cartridge with the new composition samples. These samples were supplied by the PI, along with their alumina crucibles.

The creep rupture test on the Liquid Phase Sintering cartridge was conducted in the Creep Lab in Building 105 at the Lewis Research Center from September 13 through September 21, 1996. The system consists of a creep furnace, controller, diffusion pump, vacuum gauge, and strip chart recorder. The furnace, BREW #3, was custom built to LeRC specifications, contains tungsten mesh heating elements, and molybdenum thermal reflection shields. Its maximum temperature is 4500 °F (2482 °C) in a vacuum. Its operating vacuum is 4xl0-6 torr. The furnace can accommodate a sample 2 inches in diameter by 8 inches high (5.05 cm x 20.3 cm, respectively). The furnace is controlled by a series IP SCR power controller, single phase, with output voltage proportional to the input signal. The voltage is regulated to ± 1 % with a ± 10% line voltage change. The four inch diffusion pump operates at a speed of 750 liters per second for air and can pump down to lxlO-9 torr. The vacuum is measured with an ionization gauge. The temperature is recorded on a 6 point dot printer strip chart recorder from the signal of a W-5Rh vs W-25%Re thermocouple with a reported accuracy of ± 1 %. The thermocouple was located midway along the sample cartridge assembly axis, protruding perpendicular to the cartridge but not in contact with it.

The intent was to duplicate the creep rupture test conducted on the LPS cartridge of IML-2 mission by lshikawajima-Harima Heavy Industries (IHI), the Large Isothermal Furnace (LIF) developer. The procedure has the cartridge sample assembly ramped up to the sintering temperature of 1500 °C, held there for the maximum sintering time (120 minutes for the IML-2 mission), and then ramped up to 1755 °C in twenty minutes before shutting down the furnace power. This temperature profile simulates the worst possible flight scenario, where the temperature runs away just as the Shuttle goes into a 20 minute loss of signal (LOS) period and peaks at the ultimate temperature before power cuts out, see figure 1. Note that while the ultimate and limiter temperatures, 1755 °C and 1717 °C. respectively, refer to the heater elements, the temperature actually recorded is between the heater elements and the sample cartridge assembly.

The sample assembly of cartridge G2-6 (DWG No. 60010DMA0526) was tested, see figure 2. This assembly contained the seven LPS samples (each in an alumina crucible) enclosed in a boron nitride (BN) ampoule. The BN ampoule was enclosed in three concentric tantalum tubes, thus providing three layers of containment, see figure 3. Each tantalum tube was electron beam (EB) welded in a vacuum. The sample cartridge assembly was suspended from the creep rupture grip by a tungsten wire as shown in figure 4. The maximum sintering time for these flight experiments is 10 hours. The sintering temperature is 1500 °C. The first attempt fell short of the peak temperature, 1566 °C instead of 1755 °C, see Table I (a). The problem was in the controller power limiter, which was set for 40% of maximum. A setting of 65% was used in the second attempt. The new setting was determined by a dry run in which the cartridge sample assembly was replaced by a tungsten dummy load, 0.75 inches in diameter and 8.0 inches long. The temperature was taken to 1500 °C and held there for an hour and then ramped up to the peak temperature, while adjusting the power limiter. The same sample cartridge assembly used in the first attempt was used in the again in the second creep rupture run. This time the target temperatures were met, within the accuracy of the thermocouple. However, the sample cartridge assembly had accumulated much more time at temperature than originally planned. Nevertheless, visual examination of the tantalum tubes showed no signs of rupture nor bulging. Measurements of the diameter of the inner most tantalum tube were within original specified dimensions, see Table 1 (b).