Ultrahigh Temperature Materials and Components

Work is currently under development on ultrahigh temperature materials for space and aerospace applications. This research is focused on the use of ultrahigh temperature materials at temperature ranges beyond that encountered in today's state-of-the-art systems.

 

Potential Applications

These advanced materials will be used to increase the performance of aerospace and space engines or space based power generating systems. Such improvements would be in the form of faster and more efficient jet airplanes for civilian and military applications. Advances in materials would also make possible and increase interest in new missions for space exploration. The development of an aerospace plane for service missions to the International Space Station will require improved performance capabilities of aerospace engines and thrusters over the current state-of-the-art. Higher temperatures allow for increased performance from rockets that would be used for possible manned missions to the moon and mars. Rockets that might take a crew to the moon and mars would operate more efficiently at higher temperatures reducing the overall mission time. This would help reduce the cost of a manned mission and reduce the risk to the astronauts. Finally, if man is to colonize space, we will require sufficient energy systems to operate equipment and systems necessary for maintaining habitability. Because of restrictions on design for space applications, these power systems would necessarily operate at higher temperatures than comparable terrestrial power systems.

 

Review of Past Work

INSPI is currently investigating chromium and chromium-alloys as a candidate for use as an ultrahigh temperature material. Chromium is a high temperature material with a melting point of 2136 K. Other favorable characteristics include high oxidation and creep resistance at high temperatures. However, its brittleness at room temperature prevents it from being used in any significant manner. A joint research effort with the Russian Scientific Institute, LUTCH, has produced single crystals of chromium with improved ductility. Further, alloys with certain metals such as vanadium, rhenium, titanium, and niobium have also been shown to improve the ductility of chromium. Because of difficulties controlling process variables in their production facilities, in particular the uniformity of the temperature in the reaction chamber, samples could not be obtained that were entirely mono-crystalline. Also, the distribution of the alloying elements was not uniform over the samples. This initial investigation provided useful experience in dealing with these difficulties and pointed to the need for a more sophisticated apparatus with better control over process variables.

Current Investigation Based on this initial lead, INSPI has constructed its own production facilities for the production of chromium and chromium alloys by chemical vapor deposition (CVD). The demands on such a system are great because of the requirements for high temperature and a pure/controlled environment for the reaction to take place. A schematic of the CVD apparatus constructed is shown in Fig. 1. This reactor and various auxiliary components have been constructed and fine tuning of the systems is currently underway. Once this is complete, samples can be produced for testing.  CVD apparatus showing reactor and furnace

Figure 1: Schematic diagram showing the various components and systems involved in the CVD process.

The process is illustrated in Fig. 2 and works as follows. A rough vacuum (about 10-4 torr) is achieved in the reactor. The iodine gas is allowed to flow into the reactor and over the chromium pieces surrounded by a resistance furnace that heats the chromium and iodine up to 1100 K. At this temperature the chromium and iodine react to form CrI2 (gas). The molybdenum ribbon attached to two nickel electrodes is heated electrically to 1500 K. The CrI2 (gas) dissociates at the ribbons surface at that temperature and deposits chromium on the ribbon's surface and I gas is returned for further transport of chromium to the ribbon surface. In addition to CVD, work has been done on arc melting of samples. This method will be used to produce the first polycrystalline alloys for testing. Several samples of intermetallic, Cr2Nb, have been produced. The samples are undergoing testing and x-ray analysis has already shown that indeed the intermetallic phase was produced.

Figure 2: Functional diagram showing the different zones in the CVD process and their relevant process information.

Testing

Once samples have been produced, they can be removed for environmental and mechanical testing. Sample coupons about one inch square will be tested over a range of high temperatures under various types of atmospheres to measure their oxidation resistance. Samples will further be evaluated for their tensile strength and fracture toughness.

New Directions In Chromium Alloys and Single Crystals

The fracture resistance of the Cr₂Nb intermetallic and the Cr₂Nb-Nb composite has been reported to be inadequate. Ternary systems involving the addition of a third element such as Ti to improve the ductility of the Cr-Nb system is yet another area of investigation. This work would follow up on previous joint research efforts with LUTCH and fully characterize the effects of alloying elements on fracture toughness. For CVD of alloys, research must be done on transport agents to find an acceptable transport agent(s) and temperature regime capable of depositing both elements simultaneously on the same substrate. Once the method of producing chromium by CVD has been established, the procedure of single crystal chromium and chromium alloys can be investigated.

Chromium cylinders produced by chemical vapor deposition.