The scratch test: Different critical load determination techniques

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Abstract

Different techniques for determining the critical load of coated products (microscopy, acoustic emission, and normal, tangential and lateral forces) are presented; the suitability of each is discussed.

It appears that the most straightforward technique is microscopic observation. Among the methods based on the use of a physical measurement, acoustic emission detection is the most effective. The dynamics ratio between the signals below and above the critical load for the acoustic emission (much greater than 100) is well above that obtained with the normal, tangential and lateral forces (from less than 5 to 10). Finally, the fields of application of the scratch test as an adhesion test are discussed.

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    Subsequently, this critical normal force (Fc) is taken as a measure of adhesion strength. The onset of coating failure is revealed by a sudden increase in the friction force and acoustic emission, which can be monitored by optical microscopy, acoustic emission, and friction force measurements [153,155]. The procedure of scratch test for testing the adhesion and cohesion strengths is described in the European Standard prEN 1071-3 [161].

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    In the latter case, transitions in the tangential force signal may be more appropriate. Early work on automated scratch testing methods [38] described the utility of recording three forces during a scratch test, namely normal force (Fn), tangential force (Ft) and lateral force (Fl). The tangential force is the frictional or scratching force which acts in the direction of scratching and is usually measured by a load sensor mounted underneath the sample, acting in the direction of motion.

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Paper presented at the 15th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 11 – 15, 1988.

Present address: NASA Lewis Research Center, 21000 Brookpark Road, Cleveland, OH 44135, U.S.A.

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