Carbon Extraction Replica Preparation

The carbon extraction replica technique was very useful for the identification or counting of carbide phases in the metallic system. The main advantages of replicas over foils were that they eliminated any effects due to the steel matrix and thus enabled the chemical composition of the carbides to be measured more accurately, and problems associated with working with a magnetic specimen in the TEM were avoided.

Figure 1: a) Initial sample, b) sample after chenical etching, c) carbon deposit and d) final replica.

Single stage carbon extraction replicas were prepared using the method described by Smith and Nutting (1957) from surfaces prepared as for optical microscopy. For initial etching of the sample (before carbon coating), a 2% nital solution (or Villela) was used to give a light etch (carbides just being resolved by optical microscopy). A piece of acetate was dipped in acetone and then put on the specimen after etching as stripping cleaning. The mounting bakelite was well masked before carbon coating. The specimen was placed in a carbon evaporator with a high vacuum. It is important to ensure that the surface of the mounted specimen is clean and free from grease to give a good adhesion of the deposited carbon film (colour bluebrown). Carbon is deposited from a height of 100 mm directly above the specimen surface to obtain as even a coverage as possible.

The etchant used for releasing the carbon film from the surface of the specimen was a 6% nital solution (or Villela). The specimen was scored with a sharp blade into squares approximately 3 mm across to allow reasonable access for the etchant to bare metal surface, and to ensure that the carbon replica pieces would be of suitable size. As soon as the carbon layer had peeled off the metal surface, the specimen was washed in methanol, and the fragments of carbon layer were floated using the surface tension provided by a 20% methanol and 80% distilled water mixture. Each replica was then collected onto a 3 mm diameter, 200 square mesh copper grid and was ready for analysis in the TEM.

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