An alloy of 88Cu/8Cr/4Nb (numbers indicate atomic percentages) has been proposed for use in fabricating rocket-engine combustion chambers by vacuum plasma spraying (VPS). Relative to other techniques that could be used to fabricate rocket-engine combustion chambers, VPS is inexpensive. In VPS, a powder feedstock of the desired alloy becomes melted and deposited at high speed onto a substrate. Heretofore, those rocket-engine combustion chambers that have been fabricated by VPS have typically been made of the copper-based alloy NARloy-Z.
The 88Cu/8Cr/4Nb alloy was selected as a candidate to replace NARloy-Z because previous research had revealed that this alloy exhibits excellent high-temperature strength, resistance to creep, and low cycle fatigue behavior, all combined with exceptional thermal stability. Specimens of 88Cu/8Cr/4Nb fabricated by powder metallurgy (PM) had been found to exhibit better mechanical properties at a temperature of 1,200 °C than does NARloy-Z at 1,000 °C. The problem then became one of determining whether the superiority of PM 88Cu/8Cr/4Nb would carry over to VPS 88Cu/8Cr/4Nb.
In the first stage of an effort to investigate the properties of VPS 88Cu/8Cr/4Nb, VPS parameters to obtain an optimal deposit of this alloy on a cylindrical substrate were developed, then VPS was carried out to form a shell with a wall thickness of 0.5 in. (1.3 cm), and length of 6 in. (15 cm). Samples were machined from the 88Cu/8Cr/4Nb shell and analyzed for density and hardness. Some of the samples were further treated by vacuum annealing and/or hot isostatic pressing (HIP) prior to analysis.
The table presents the results of these analyses. For comparison, the table also presents the results of previous research on extruded PM 88Cu/8Cr/4Nb. The comparison reveals that the density and hardness of VPS 88Cu/8Cr/4Nb treated with vacuum annealing and HIP were greater than those of extruded PM 88Cu/8Cr/4Nb. These superior properties of the VPS 88Cu/8Cr/4Nb are expected to translate into superior tensile and low-cycle fatigue properties; experiments to verify this expectation had not been completed at the time of reporting the information for this article.
This work was done by Richard R. Holmes and Frank Zimmerman of Marshall Space Flight Center and George P. Beason, Jr., and Timothy N. McKechnie of Plasma Processes, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the category.