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Stability Characterization of Refractory Materials Under High Velocity Atmospheric Flight Conditions Part II. Volume I: Facilities and Techniques Employed for Characterization of Candidate Materials

Report Number: AFML TR 69-84 Part 2 Volume 1
Author(s): Larry Kaufman and Harvey Nesor
Corporate Author: ManLabs Inc.
Laboratory: Air Force Materials Laboratory
Date of Publication: 1969-12
Pages: 100
Contract: AF 33(615)-3859
Project: 7312
Task: 731201
AD Number: AD0865317

Abstract:
The oxidation of refractory borides, graphites, and 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 studied 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 of this investigation.

This report describes the candidate materials which were obtained from commercial sources and represent state of the art materials. Available processing information is included. Characterization of materials was per4formed by qualitative spectrographic, wet chemical and vacuum (or inert gas) fusion, metallographic, X-ray, electron microprobe and pycnometric analysis. Standard analysis of refractory boride, carbide and silicide composites were employed. However, considerable difficulties were encountered in the chemical analysis of JT graphite composites due to formation of ZrSiO4 or HfSiO4 on combustion. In order to avoid this complication a novel method was developed.

Nondestructive testing of candidate materials included radiography, gamma radiometry, die penetrant inspection and maeasurement of ultrasonic velocity. Film radiography was used to detect the prescence of voids, inclusions and local gross changes in composition. Radiometric density gauging used to measure local densities within each specimen and alcohol penetrant tests were employed to disclose tight surface cracks which are not visible at moderate magnifications.

The measurement of ultrasonic velocity was utilized for establishing correlations between quantitative NDT measurements and materials properties. Process variations leading to modulus changes, (such as preffered orientation in elastically anisotropic materials or small amounst of "stiffening" impurities) change sound velocity. These techniques are capable of a precision of about 1%. Moreover, ultrasonic energy is reflected at solid material/aire interfaces. Such interfaces exist as cracks, bursts, voids, etc., present in solids.

The results of nondestructive testing of samples prior to arc plasma testing is reported. Test results are provided for a series of hemispherical shells of diboride composites. Graphite composites, silicon carbide and hafnium-tantalum alloy were also tested prior to exposure. In several instances, flaws which caused failures on eposure were detected by means of dye penetrant and radiographic techniques. The latter methods proved to be most effectiv of the NDT techniques employed in this study.

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