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Macroetching of carbon fiber-phenolic composites
Metallography
83
Macroetching of Carbon Fiber-Phenolic Composites
C. A. GAULIN AND J. H. RICHARDSON
Aerospace Corporation, Los Angeles, California
The carbon fiber-phenolic matrix materials are one of the more important composites used in space and missile systems both as a nozzle insert and as re-entry heat shields. It is often necessary to examine the structure of these materials at low magnifications. For example, it is important to study the fabrication steps such as the "lay-up" of the carbon cloth. These low-magnification studies are also important in the testing of these composites, such as the correlation of fabric orientation with physical and mechanical properties and ablation characteristics. Since these materials are characterized by their inertness to most common solvents, a study was conducted to develop a suitable etchant for use in photomacrography. A number of organic based solvents have been studied recently by Magariello 1 for the solution of the cured phenolic resin in connection with a resin content analysis. One of these solvents, a solution of nitric acid in dimethyl sulfoxide (DMSO) was found to produce an excellent macroetch on the carbon fiberphenolic materials. The macro specimen of the carbon fiber-phenolic composite is ground to a 600-grit surface. This specimen is then placed in a beaker and covered with a solution consisting of 75 ml of dimethyl sulfoxide and 25 ml of concentrated nitric acid. The temperature of the solvent is maintained at 75 ° to 80°C for an etching period of 4 hours. After etching, the specimen is removed from the solvent with tongs, rinsed in running water, and dried with an acetone wash. Figure 1 shows a cross section of a typical composite. Figure l(a) is an untreated composite. Figure l(b) is the same composite etched with the above solution. The carbon fiber cloth is in sharp contrast to the phenolic matrix in the etched specimen. Figure 2 shows the effect of the etchant on a surface parallel to the carbon fiber cloth. In this case the weave of the cloth is quite apparent on the etched surface.
It is important to note that liquid dimethyl sulfoxide is readily absorbed through Metallography, 4 (1971) 83-85 [Short Communication] Copyright © 1971 by American Elsevier Publishing Company, Inc.
(a)
(b) FIo. 1. Cross sections of a carbon fiber cloth-phenolic composites (a) in the unetched condition and (b) after etching. Magnification 16 x (reduced 50%).
Fiber-Phenolic Composites
85
FIG. 2. Surfaces of a carbon fiber cloth-phenolic composite parallel to the cloth. The upper specimen is in the unetched condition, the lower specimen is etched. Magnification 16 < (reduced 50%).
the skin, and the vapor is absorbed through both the skin and the lungs. In admixture with nitric acid the D M S O will also act as a carrier of the acid, thereby aggravating the effects of contact with the liquid acid or inhalation of the fumes. It is strongly recommended that all etching operations be carried out in an efficient fume hood and that surgical rubber gloves be used in manipulation. An effective macroetchant has been described for the carbon fiber-phenolic composites which clearly delineates the weave and lay-up structure of the fibers in the phenolic matrix.
Reference 1. E. Magariello, SAMPEff., 3 (June/July, 1967), 28.
Accepted September 1, 1970
83
Macroetching of Carbon Fiber-Phenolic Composites
C. A. GAULIN AND J. H. RICHARDSON
Aerospace Corporation, Los Angeles, California
The carbon fiber-phenolic matrix materials are one of the more important composites used in space and missile systems both as a nozzle insert and as re-entry heat shields. It is often necessary to examine the structure of these materials at low magnifications. For example, it is important to study the fabrication steps such as the "lay-up" of the carbon cloth. These low-magnification studies are also important in the testing of these composites, such as the correlation of fabric orientation with physical and mechanical properties and ablation characteristics. Since these materials are characterized by their inertness to most common solvents, a study was conducted to develop a suitable etchant for use in photomacrography. A number of organic based solvents have been studied recently by Magariello 1 for the solution of the cured phenolic resin in connection with a resin content analysis. One of these solvents, a solution of nitric acid in dimethyl sulfoxide (DMSO) was found to produce an excellent macroetch on the carbon fiberphenolic materials. The macro specimen of the carbon fiber-phenolic composite is ground to a 600-grit surface. This specimen is then placed in a beaker and covered with a solution consisting of 75 ml of dimethyl sulfoxide and 25 ml of concentrated nitric acid. The temperature of the solvent is maintained at 75 ° to 80°C for an etching period of 4 hours. After etching, the specimen is removed from the solvent with tongs, rinsed in running water, and dried with an acetone wash. Figure 1 shows a cross section of a typical composite. Figure l(a) is an untreated composite. Figure l(b) is the same composite etched with the above solution. The carbon fiber cloth is in sharp contrast to the phenolic matrix in the etched specimen. Figure 2 shows the effect of the etchant on a surface parallel to the carbon fiber cloth. In this case the weave of the cloth is quite apparent on the etched surface.
It is important to note that liquid dimethyl sulfoxide is readily absorbed through Metallography, 4 (1971) 83-85 [Short Communication] Copyright © 1971 by American Elsevier Publishing Company, Inc.
(a)
(b) FIo. 1. Cross sections of a carbon fiber cloth-phenolic composites (a) in the unetched condition and (b) after etching. Magnification 16 x (reduced 50%).
Fiber-Phenolic Composites
85
FIG. 2. Surfaces of a carbon fiber cloth-phenolic composite parallel to the cloth. The upper specimen is in the unetched condition, the lower specimen is etched. Magnification 16 < (reduced 50%).
the skin, and the vapor is absorbed through both the skin and the lungs. In admixture with nitric acid the D M S O will also act as a carrier of the acid, thereby aggravating the effects of contact with the liquid acid or inhalation of the fumes. It is strongly recommended that all etching operations be carried out in an efficient fume hood and that surgical rubber gloves be used in manipulation. An effective macroetchant has been described for the carbon fiber-phenolic composites which clearly delineates the weave and lay-up structure of the fibers in the phenolic matrix.
Reference 1. E. Magariello, SAMPEff., 3 (June/July, 1967), 28.
Accepted September 1, 1970