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IMPORTANCE OF REPLICA SAMPLES

11.0. IMPORTANT OF REPLICA AND AS REQUESTED BY CLIENT

11.1. BASIC PRINCIPLE OF REPLICA:

Non-destructive metallography of surfaces makes it possible to analyses a material microstructure and a surface condition as well as various surface damages occurring due to overloading or improper tribological circumstances. Machine or tool parts inspected are not damaged during the surface preparation. It is also required that the replica applied is strong and elastic enough not to get damaged when removed. As the replica does not damage the machine part and does not chemically affect its condition, the machine part can further operate if the inspection performed confirms its quality. The way of preparing the machine part, i.e., the area to be inspected depends on the requirements or objectives of the examination to be performed and on the accessibility of the area to be inspected. The inspection of the surface condition or microstructure can be performed in two ways:

·         If the part to be examined and its environment provide enough room for the surface preparation and observation, then the surface inspection can be performed directly with an optical microscope. For this purpose adapted optical microscopes are available.

·         In the opposite case, a replica is produced, which is then observed with a light or electronic microscope in a laboratory.

The optical microscope makes it possible to observe the replica in bright or dark field. The area at the machine part or the tool part to be examined shall be prepared with a suitable preparatory technique which is the same as the usual surface preparation for the optical microscopy. The area concerned shall be first mechanically treated by grinding and/or polishing or electrolytic polishing, and then it shall be etched so that the microstructure of the surface becomes uncovered. The surface preparation is similar to that used in the general metallographic analysis with the optical microscope. In case the machine part has already been fine-ground or polished because of functional requirements, it is needed to etch it for the microstructure analysis. It is different, however, with the analysis of the surface condition due to tribological conditions, in which case the surface shall only be cleaned and a replica shall be made for subsequent observation. Electro polishing and etching can be carried out with  a portable unit produced by Struers. This is followed by the production of a replica and finally by the observation of the microstructure with the optical or electronic microscope.

For an analysis of topographical features of the surface of a machine or tool part the surface shall be thoroughly cleaned, without any preliminary mechanical treatment. The preparation of both simple and exacting surfaces may be made easier with the application of admissible chemical media for the elimination of colour, grease, and other impurities. One finds commercially available various types of replicas made of different materials. The materials available are practical for application and permit the production of a replica of the surface concerned in a few minutes. One finds commercially available also metal-coated plastic foils prepared in advance for the production of replicas. The metal coating provides efficient light reflection when the replica surface is inspected with the optical microscope. This also provides a higher quality of the surface image, and additional mechanical hardening of the replica results in easier and safer handling. Consequently, the modern replicas are made of two-part silicon rubber. The latter is squeezed from an adapted gun with a piston to the area to be examined. The surface preparation technique requires moderate to deep etching of the surface to be examined in order to obtain a well-profiled replica for a good and efficient topographical analysis of the surface.

In addition to the employment of a mass for replicas, the employment of backing paper is also recommended. It allows the production of an even thinner replica. Such a replica is then stuck, with a double-sided adhesive tape, to a flat glass slide, which enables subsequent high-quality observation of the surface with the optical microscope. The backing paper makes it possible to produce thin replicas of particularly exacting surfaces, which can be analyzed with the optical microscope. The replicas produced with the Struers backing paper can also be shadowed and subsequently analyzed with a scanning electron microscope.

Figure shows the components of the RepliSet unit for the production of replicas. The RepliSet system consists of a dosing pistol with a piston for squeezing-out, a container with two-part silicon rubber, and a mixing nozzle for the application of the replica mass to the surface to be analyzed at the machine part. The mixing nozzle shall be replaced after each application because rests of the mass remain in the nozzle, become hard, and block the nozzle. For the production of a quality replica it is important to know the temperature of the surface of which a replica is to be made. Silicon rubber can be applied to a surface of machine parts both standing still and operating in a temperature range from -10 to 180 °C. The procedure of making a replica is following

1. The surface of the machine part concerned shall be cleaned and degreased. Degreasing is usually accomplished with ethyl alcohol, similarly as in the common optical microscopy for the observation of the surface topography.

2. When the condition of a surface or a crack reaching to the surface is to be observed, cleaning of the surface with acetone is recommended. Irrespective which cleaning medium is used, its remnants shall be carefully and thoroughly removed from the surface so as not to react with silicone rubber used in making the replica.

3. With exacting microstructures, surfaces or cracks, the cleaning medium shall be additionally rinsed and, as circumstances require, the surface dried with hot air.

4. Then follows the application of the replica mass capable of hardening, i.e. having favorable strength and elastic properties, and thus being capable of efficient removal from the surface.

5. Good preparation of the surface, a quality mass and careful making of the replica results in high-quality production and observation of the replica with the optical or electron microscope.

11.2. NON-DESTRUCTIVE MICROSTRUCTURE ANALYSIS THROUGH REPLICA TESTING

Metallographic examination can also be performed on site by means of the replica testing method. Results are then analyzed in the microscope. Also called in-situ metallography, replica testing is a procedure that consists in reproducing the microstructure of a metal’s surface. It enables experts to determine creep damage and to estimate the remaining service lifetime of a piece of equipment. Also on site hardness analysis on metal´s surface e.g. on welding can be done.

Although replica testing first requires sample preparation and the etching of the examined surface, this testing method is nearly non-destructive since the sampling is only superficial. The microstructure is imprinted by means of a foil, which is further analyzed through a microscope. Upon completion of the sample examination, results are provided in the form of a written report and picture documentation. Hardness testing can also be conducted on site.

On the strength of years of experience in materials testing and inspection services for global customers, NDE FLAW TECHNOLOGIES PVT. LTD. offers a comprehensive portfolio of services including a full range of non-destructive testing methods. For any client from any industry, we can test, approve and certify your materials and products for quality and compliance with local and international requirements.

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