Preparing nickel-boron and other composites for optical-metallographic examinations

Preparing nickel-boron and other composites for optical-metallographic examinations

Metallography 293 Preparing Nickel-Boron and other Composites for Optical-Metallographic THERESA United States Army, Examinations V. BRASSARD...

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Metallography

293

Preparing

Nickel-Boron

and other Composites

for Optical-Metallographic THERESA United

States

Army,

Examinations

V. BRASSARD

Watervliet

Arsenal,

Watervliet,

The recent interest in the use of boron multifilaments

New

York

for composite applica-

tions has necessitated devising suitable techniques for their preparation for microstructural examination and study. This discussion will present a relatively simple method using very little time for polishing hard boron filaments in a soft nickel matrix. First, the entire specimen is mounted in liquid epoxy resin,8 preparatory

to

cutting. Caution is exercised during cutting, as the boron filament is brittle and cracks readily. With copious amounts metallographic sample.

of water and a slow cutting

cutoff wheel proved to be adequate

in producing

rate, our a suitable

To maintain a reasonably flat interface between the filament and the matrix, diamond disks and abrasives are used exclusively during polishing. The surface to be examined is ground sequentially using medium pressure on medium-grade diamond disks?, and 280 mesh 45 micron (fine), the rotating disks being lubricated with ample flow of water. We have observed during preliminary grinding that there is less “chipping” of the filaments, faster cutting action, and better retention of the interface with the use of diamond disks rather than with conventional

silicon carbide papers.

Next the specimen

is polished on rotating paper laps (we use high-quality

tablet paper) impregnated successively with 15-p, 8-p, 3-p, and 1-p diamond paste abrasive,c a small amount of diamond paste thinner being used as a lubricant.

Care must be taken to use medium pressure with fairly fast rotation

of the sample on a medium-speed (2800-rpm) polishing wheel to ensure a mirrorlike finish with negligible relief. As soon as “smearing” is observed on the specimen surface, the paper is replaced with a new paper freshly charged with abrasive. a Hysol resin and hardener, Dexter Corporation, H Minnesota Mining & Manufacturing. p Elgin diamond abrasives.

Olean, New York.

Metallography, Copyright

0

2 (1969) 293-296

1968 by American Elsevier Publishing Company,

Inc.

Theresa

294

V. Brassurd

c.

FIG.

1.

As-polished

nonporous

specimen.

Bright-field

illumination.

Magnilication

175 x.

FIG.

2.

Etched

“porous”

specimen.

Bright-field

illumination.

Magnification

175

X .

Nickel-Boron

FIG.

and other Composites

3.

As-polished

FIG. 4.

Etched

FIG.

5.

Lightly

specimen.

specimen.

295

Bright-field

Bright-field

etched specimen.

Polarized

illumination.

illumination.

Magnification

Magnification

light illumination.

1000

X.

1000 x

Magnification

1000

x

Theresa V. Brassard

296

As a final polish, the specimen is held for a very short time, using circular motion with medium pressure on microclothd (rotating at 220 rpm) charged with a slurry of 0.1-p aluminad and 1-p diamond paste. Although this last step employing the use of a coarser diamond paste abrasive with a finer alumina powder may violate basic metallographic and therefore

can be justified.

The

principles,

the method yields results

sample is then washed thoroughly

with

alcohol and dried. The time required for polishing varies with the specimen but is of the order of 5 to 15 minutes. During the polishing procedure the soft nickel has a tendency to “flow” and conceal the voids formed during the electro-forming

fabrication

of the com-

posite.’ Therefore, etching is necessary to expose these voids and uncover the true microstructure. A number of etchants were tried. Swabbing with 60 parts of acetic and 40 parts of nitric acid solution proved to be the most effective for our laboratory. An example of an unetched sample where flowed metal has obscured voids which were subsequently revealed by etching is shown in Fig. 1 and 2, respectively. If present, the above etchant will severely attack the diffusion zone surrounding the boron filament. Consequently, specimen

be observed

illumination.

first in the as-polished

it is recommended condition

using

that the

bright-field

Figures 3 and 4 clearly illustrate the severity of the etch attack on a

portion of the reacted layer immediately adjacent to the filament. At this time microprobe analysis is being made in an attempt to identify the constituents of these reaction zones. At high magnification

using polarized light illumination,

grain structure

can

sometimes be seen in the reacted layer encircling the filament. This can be seen in Fig. 5. In some instances under polarized light a structure is also visible within the fiber, signifying that a reaction is occurring within the filament itself. The

polishing

procedure

described

has also been utilized on other metal-

matrix composite systems such as tungsten-nickel. In addition, the absence of relief and the resulting resolution of the diffusion zone between the filament and matrix material make this technique most suitable in preparing samples for the electron microprobe I wish to express appreciation on this short communication.

to Dr. Thomas

analyzer.

E. Davidson for discussion and advice

1. W. A. Wallace and V. P. Greco, Electra-Forming High-Strength Continuous FiberReinforced Composites. Presented at American Electroplaters Society 1969 Convention at Detroit, Michigan. Also see J. A. Snide, L. G. Bates, F. A. Ashdown, and J. R. Myers, J. Compos. Mater., 2 (1968) 509.

Accepted

July 29, 1969

* Buehler microcloth and polishing abrasives.