METALS BY TRANSMISSION 



advances downward. When perforation has was carried out immediately in methyl alco- 



extended about half-way down, the specimen hoi kept below — 20°C to prevent attack by 



is removed and washed, and foils are cut from the anodic layer before it was completely re- 



the edge of the hole. Tomlinson states that mo^'ed. 



the foils are more uniform in thickness over Another techniciue employing the viscous 

 larger regions if the current is switched on polishing layer has been developed by Bran- 

 and off rapidly near the end of the polishing, don and Nutting (26) for use with iron. A 

 No explanation of the mechanism of this sheet of material 50-100 n thick was electro- 

 phenomenon has been given. polished down to ^^20 ju in the usual way. 



Better foils are obtained if the specimen is If perforation occurred during this polish- 

 removed as soon as perforation begins at the ing, its progress was stopped by coating with 

 top, and the perforated edge coated with lacquer. When the thickness of the specimen 

 lacquer. The specimen is then in\-erted, re- had been sufficiently reduced, one surface 

 placed, and polished until perforation occurs was painted with non-conducting lacquer to 

 at the other end of the specimen. This can give a "figure-of-eight" outline. The speci- 

 be repeated until there is a narrow band at men was further electropolished and perfora- 

 the center of the specimen from which the tion occurred at the top of the specimen 

 foils are cut. and under the "arms" of the "eight," leav- 



The advantages of both the "window" ing the portion between the "arms" suitable 



and the Bollmann methods have been incor- for thin foils. 



porated in a low-temperature polishing tech- To a\'oid the preferential rounding off of 

 nique for copper alloys, developed by Swann the edges of holes, Mirand and Saulnier (27) 

 and Nutting (25). The "window" technique have backed the specimen with a piece of 

 is used first, the specimen being inverted the same material; thus, when a hole ap- 

 after each successive perforation, until a pears, attack continues on the backing metal, 

 band about half the width of the original The polishing was continued until fragments 

 specimen is left. The Bollmann techniciue is dropped from the specimen into the electro- 

 then used to thin the center of this band lyte, where they were collected for examina- 

 until perforation occurs. The final polishing tion. This technique works well for titanium 

 is done with the "window" technique. alloys, but the difficulty in extending it to 



In producing thin foils of copper alloys, other metals lies in finding an appropriate 

 there are two main difficulties. First, the so- polishing solution. Most electropolishing so- 

 lution used polishes very rapidly at room lutions will etch the metal, unless a current 

 temperature and this gives little control over is applied, and therefore etch the fragments 

 the final stages of thinning; secondly, the as soon as they fall from the specimen. Elec- 

 anodic layer is so reactive that even imme- trolyte may also seep between the specimen 

 diate washing after removal of the specimen and the backing piece, thus etching the back 

 from the solution is not enough to prevent of the specimen. However, when suitable 

 etching and oxidizing of the specimen. To conditions have been established the tech- 

 slow down the rate of attack and give the nique of Mirand and Saulnier provides a 

 required degree of control over polishing, the rapid and easy method of preparing thin 

 temperature of the bath was kept below foils. 



— 20°C. Just before the specimen was re- The production of exact geometrical 



moved, sufficient liquid nitrogen was poured shapes and the preservation of certain pro- 



onto the top of the solution to form a layer, files durhig electropolishing have been investi- 



through which the specimen was withdrawn, gated by Michel (28). This work gives an 



without switching off the current. Washing indication of the methods to be adopted if 



185 



