REPLICAS, REMOVAL FROM SURFACES 



6. Hassard, G. and Scott, N. W., J. Opt. Soc. 



Am., 179, 39 (1949). 



7. Bradley, D. E., Brit. J. Appl. Phys., 5, 65 



(1954). 



8. KoNiG, H. AND Helwig, G., W. Phys., 129, 



491 (1951). 



9. Bradley, D. E., Brit. J. Appl. Phys., 5, 96 



(1954). 



10. Bradley, D. E., J. Inst. Metals, 83, 35 (1954). 



11. Heidenreich, R. D. and Peck, V. G., /. 



Appl. Phys., 14, 23 (1943). 



12. Pfeiffer, I., Naturwiss., 42, 508 (1955). 



13. Smith, B. A. and Nutting, J., Brit. J. Appl. 



Phys.,7,2U (1956). 



14. Hyam, E. D. and Nutting, J., Brit. J. Appl. 



Phijs., 3, 173 (1952). 



15. Nankivell, J. A., Brit. J. Appl. Phys., 4, 141 



(1953). 



16. Booker, G. R., Brit. J. Appl. Phys., 5, 349 



(1954). 



17. Bradley, D. E., Brit. J. Appl. Phys., 6, 430 



(1955). 



18. Halldal, p., Markali, J., and Naess, T., 



Mikroskopie, 9, 197 (1954). 



19. Bradley, D. E., Mikroskopie, 12, 257 (1957). 



20. Williams, R. C. and Wyckoff, R. W. G., J. 



Appl. Phys., 17,23 (1946). 



21. Labaw, L. W. and Wyckoff, R. W. G., Na- 



ture, 176, 455 (1955). 



22. Bradley, D. E., Brit. J. Appl. Phys., 10, 198 



(1959). 



23. HiBi, T. AND Yada, K., "Proc. 3rd Interna- 



tional Conf. Electron Microscopy," London, 

 1954. 



D. E. Bradley 



REPLICAS, REMOVAL FROM SURFACES 



The carbon replica technique of examining 

 surfaces with the electron microscope is 

 widely used on ceramic and metalhc solids. 

 The technique consists of first shadowing the 

 surface to be examined with a noble metal 

 such as platinum or palladium at an angle of 

 10° to 30° to the surface. The shadow is then 

 backed by evaporating a carbon film at 90° 

 to the surface to complete the replica. To be 

 usable this replica must then be removed 

 from the surface by etching away the sample 

 from beneath the carbon and shadowing 

 material. It is this step which is the most 

 difficult since the fragile carbon film with its 



attached shadow must be floated off the sam- 

 ple by some etchant and the floating repUca 

 picked up on a grid. In the case of ceramic 

 materials, strong etchants of low pH, such 

 as aqua regia or HF-HNO3 mixtures, are 

 frequently required to effect a separation 

 within a reasonable period of time. The elec- 

 tron microscope grids, usually of copper, 

 stainless steel or titanium, are attacked by 

 the low pH solutions when they are placed 

 in contact with the replica; this often re- 

 sults in severe tearing of the carbon film. 

 Diluting of the solution in order to raise the 

 pH frequently causes sufficient turbulence 

 to damage the replica. The technique de- 

 scribed herein has been found successful in 

 circumventing these obstacles. It consists 

 essentially of (1) placing the specimen in a 

 wedge-shaped specimen holder which senses 

 to maintain the specimen at the proper angle 

 with respect to the surface of the etchant, 

 thus permitting progressive solution of the 

 substrate and causing the replica to float on 

 the surface of the liquid as it is released; and 

 (2) subsequently diluting the etchant so- 

 lution without appreciably disturbing the 

 floating repfica. 



This technique was developed during an 

 investigation of fused ceramic coatings on 

 tungsten wires in which it was necessary to 

 examine the interface between the ceramic 

 coating and the tungsten. Because of the 

 chemical stability of the ceramic material 

 and the tungsten, it was necessary to use 

 etchants containing 4% nitric acid and 

 8-12 % hj^drofluoric acid by volume to sep- 

 arate the replica from the sample. As was 

 previously noted, these solutions attacked 

 the grids and caused severe tearing of the 

 replicas. This necessitated diluting the 

 etchant after the separation was accom- 

 plished. 



Because of the corrosive nature of the 

 releasing etchant, the samples were first 

 mounted in a block of a thermoplastic 

 material and then polished by ordinary 

 ceramographic techniques to produce a flat 



239 



