KI.K( IKON MKHOSCOPY 



understanding of the conditions necessary to 

 produce a uniform thin foil. 



Tiie main cUfhculty in i)r()ducing a thin 

 foil is that of attaining random removal of 

 metal from the surface of the specimen. In 

 order to achieve this during electropolishing, 

 the current density at all points on the speci- 

 men must be the same. Since it is difficult 

 to measure the thickness of the metal in 

 the region 200-2000 A, the criterion that the 

 specimen is thin enough is usually the ap- 

 pearance of one or more holes. Thin regions 

 occur near the edges of the holes, but as soon 

 as a hole is formed the current-density dis- 

 tribution is disturbed, with the result that 

 the edges of the holes become rounded and 

 the thin regions lost. 



Wlien a sheet-metal specimen is placed 

 vertically as the anode in an elect ropolish- 

 ing solution, with a flat vertical cathode, it is 

 found that the current density is maximum 

 at the edges of the specimen and at the 

 "waterUne" of the solution. In addition, 

 elect ropolishing solutions can be divided into 

 two classes: 



(i) Solutions forming a viscous layer which 

 flows down under gravity. 



(ii) Solutions which evolve bubbles of gas 

 at the anode. 



Solutions of the first type will give en- 

 hanced attack at the top of the specimen, 

 since the viscous layer is flowing away from 

 that region and is increasing in thickness as 

 it flows down the specimen. Solutions of the 

 second type will form a blanket of gas bub- 

 bles which is thin at the bottom of the speci- 

 men and thick at the top; as a result metal 

 is removed more quickly from the bottom of 

 the specimen. A horizontal anode eliminates 

 the effect of gravity on the viscous layer and 

 on the bubbles, but now the conditions are 

 different on the two sides of the specimen. 

 Hence, in this case it wiU be difficult to at- 

 tain the correct polishing conditions on both 

 sides of the specimen, especially if the cur- 

 rent-density range for polishing is small for 

 the electrolyte. 



To eliminate the effect of preferential pol- 

 ishing at the edges of the specimen, these 

 may be coated with a non-conducting lac- 

 fiucr. The result of this is preferential polish- 

 ing at the edge of the lacquer coating, leav- 

 ing the center of the specimen thicker than 

 the outside. 



To overcome this difficulty, BoUmann (21, 

 22) used pointed cathodes mounted close to 

 the center of a metal disc (^^2 cm. in dia. 

 and 25-200 /x thick). The specimen was elec- 

 tropolished until a hole appeared in the 

 center, when the electrodes were moved 

 ^^1-2 cm. away and polishing was continued. 

 Perforation then occurred at the edge of the 

 lacquer coating and continued toward the 

 central hole. The specimen was then removed 

 and washed, and foils were cut from the 

 regions near the junction of the two holes. 



To avoid removing the specimen from the 

 solution to adjust the position of the cath- 

 odes, a modification of the electrode assem- 

 bly has been designed in which there are 

 two sets of cathodes, a set of pointed cath- 

 odes close to the specimen and a set of flat 

 cathodes far away from the specimen. Either 

 set may be connected to the negative of the 

 power supply, so that the same sequence of 

 events can be followed merely by switching 

 from one set to the other. 



The Bollmann technique is hmited to 

 polishing solutions with a low throwing 

 power and may produce wedge-shaped foils, 

 but otherwise the method is simple and ver- 

 satile. 



The preferential polishing of a vertical 

 specimen due to the formation of a heavy 

 viscous layer has been utilized in the "win- 

 dow" technique developed by Nicholson, 

 Thomas, and Nutting (23) for use with alu- 

 minium alloys, and later by Tomlinson (24) 

 for making thin foils of Mg, Ni, Al, Cu, Fe, 

 and Co. This consists of lacquering the edges 

 of a thin sheet (25-200 ^l thick) and mount- 

 ing it vertically with a vertical cathode in an 

 electropolishing solution. Perforation occurs 

 at the top edge of the lacquer coating and 



184 



