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Stability Characterization of Refractory Materials Under High Velocity Atmospheric Flight Conditions. Part 3, Volume 3: Experimental Results of High Velocity Hot Gas/Cold Wall Tests.

Report Number: AFML TR 69-84 Part 3 Volume 3
Author(s): Kaufman, Larry; Nesor, Harvey
Corporate Author: ManLabs Inc.
Laboratory: Air Force Materials Laboratory
Date of Publication: 1970-02
Pages: 365
Contract: AF 33(615)-3859
Project: 7312
Task: 731201
AD Number: AD0867307

Abstract:
The oxidation of refractory borides, graphites ahd JT composites, hypereutectic carbide-graphite composites, refractory metals, coated refractory metals, metal oxide composites, and iridium coated graphites in air over a wide range of conditions was investigated over the spectrum of conditions encountered during reentry or high velocity atmospheric flight, as well as those employed in conventional furnace tests. Elucidation of the relationship between hot gas/cold wall (HG/CW) and cold gas/hot wall (CG/HW) surface effects in terms of heat and mass transfer rates at high temperatures was a principal goal.

Arc plasma exposures have been performed at Mach Numbers between 0.1 and 3.2 stagnation pressures between 0.01 and 1.0 atm., stagnation enthalpies up to 16,000 BTU/lb, cold wall heat flux up to 1200 BTU/ft^2sec, exposure times up to 23,400 seconds and surface temperatures between 2100 and 6500F. Data include material recession, metallographic and X-ray analysis, radiated heat flux and normal total emittance. In addition, color motion picture coverage was provided. Materials forming solid oxides show lower recessions in the HG/CW tests at a stated surface temperature than in CG/HW tests. The reverse is true for ablating materials. Temperature gradients of 800 to 1500F through 30-50 mil oxides are observed. The practical implications of this finding are substantial (if the gradients exist under free flight conditions). Since the temperature level experienced by the substrate is substantially below that predicted, strength and load carrying capacity of the substrate would be much higher than for the case where gradients are ignored. Long-time cyclic exposures of diboride composites in the Model 500 and ROVERS facilities for trajectories typified by FDL-7MC provide a striking illustration of the reuse capability of boride composites for lifting reentry applications.

A HfB2+SiC composite was exposed for thirteen cycles at 0.07 atm (1 psi) stagnation pressure, a stagnation enthalpy of 10,200 BTU/lb and a cold wall heat flux of 495 BTU/ft^2sec. Each cycle was about 1800 seconds long with a total exposure time of 22,500 seconds at a surface temperature ne 5300 R. Total material recession was 15 mils. A ZrB2+SiC composite was exposed for four cycles at 1.0 atm (15 psi) stagnation pressure, a stagnation enthalpy of 5000 BTU/lb and a cold wall heat flux of 380 BTU/ft^2sec. Each cycle was 1800 seconds long, total exposure time was 7200 seconds. The surface temperatures were near 5000 R. Total material recession was 26 mils. Under similar conditions graphite and tungsten would exhibit recessions of 7 to 14 inches.

These results illustrate the reuse capability of boride composites for lifting reentry application, since they exceed the range of conditions for FDL-7MC. This capability is unrivaled by any other materials system.

Surface temperature calculated from stream conditions and radiation equilibrium agree with observed temperatures on melting. When solid coatings are present, surface temperatures are below computed values. Silicon carbide bearing materials achieve lower temperatures than predicted from stream conditions and exhibit superior behavior.

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