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The Role Of Emittance In Refractory Metal Coating Performance: Part I - Review And Analysis

Report Number: AFML TR 66-55 Part 1
Author(s): Bartsch, K. O.; Huebner, A.
Corporate Author: North American Aviation
Laboratory: Air Force Materials Laboratory
Date of Publication: 1966-01
Pages: 170
Contract: AF 33(615)-3039
Project: 7312
Task: 731201
AD Number: AD0801274

The role of emittance in refractory metal coating performance was reviewed and analyzed for the purpose of orienting the planning of a comprehensive program to provide accurate and proper emittance data for thermal calculations needed in the design of advanced aerospace vehicles and engines. The following was found: Coated refractory metals are used and are planned for future use on reentry and hypersonic cruise vehicles which are cooled almost solely by the radiation of heat to space. Space engine nozzles and extensions fabricated of coated refractory metals may be cooled to advantage by radiation. Emittance values have a profound effect on the rate of heat rejection, and thereby, on the metal-coating system temperature which, in turn, controls coating life and metal strength. The emittance of a coated refractory metal is a function of its environment which includes temperature, time, partial pressures of the atmospheric constituents, and the free-stream velocity of the atmosphere under extreme temperature conditions. Total hemispherical emittance values are needed for thermal calculations involving evterior vehicle and engine surfaces that radiate to space, but for calculations between surfaces, the use of spectral emittance values is indicated if emittance varies appreciably with wavelength. Most emittance data in the literature are total normal data which do not necessarily equal total hemispherical data unless the coatings radiate diffusely. The extent to which refractory metal coatings display diffuse radiation is not known.

As an aid in planning the comprehensive program, a number of preliminary emittance experiments were performed. Heating as-received silicide-coated columbium and molybdenum alloys and one tin-aluminum-coated tantalum alloy to 2000 F in air cause a marked increse in emittance which, for the silicide coatings, was associated with oxide formation. The total normal emittance calculated by integrating the spectral curve (as a function of a 2000 F graybody) inferred from room temperature reflectance data. This indicates that the shape of the spectral curve at 2000 F is not a strong function of temperature.

A comprehensive emittance program is presented for possible future action. It covers the effect of cyclic environmental exposure on emittance, including exposure in a hypersonic tunnel; the relationship of coating process control parameters to emittance; the significance of various types of emittance properties in thermal calculations; and recommendations concerning the determination of special emittance properties such as angula emittance.

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