Scratch Testing

Scratching has been a well known tool for obtaining a material hardness since 1812 when the German mineralogist Friedrich Mohs put forth his scale of the mineral hardness.This scale contains ten minerals (1. talc, 2. gypsum, 3. calcite, 4. fluorite, 5. apatite, 6.orthoclase feldspar, 7. quartz, 8. topaz, 9. corundum, 10. diamond), ordered from the softest to the hardest mineral. This scale is based on the simple idea that harder material can visibly scratch another material, but not contrarily. Yet the scale itself might be sufficient to acquire basic idea of minerals hardness it does not meet the requirements of industrial practice.

Hardness is defined as an ability of material to resist penetration or abrasion by other materials. In agreement with this definition can be performed various tests that differ from the technique and value of the hardness. Evaluating of the hardness can be divided into the three main types: rebound hardness, indentation and scratch. Rebound hardness is evaluated by measuring of the bounce of hammer dropped from the fixed height onto the material. Indentation hardness is evaluated according to the dimensions of indent left by the indenter. On the field of engineering is the most common Vickers’s, Brinnel’s and Rockwell’s test. Scratch hardness is often assessed in the case of surface films or as a comparative method.

Hardness can be evaluated on the basis of three different scales: macro, micro and nano scale. Specimen tested on the macro-scale usually undergoes test load higher than 10 N and in this scale are also included aforementioned Vickers’s, Brinnel’s and Rockwell’s tests. Development of the micro and nano-scale tests has been driven by the need of material science to test samples on the smaller scale e.g. hard thin coats, separately test elements of composed material or when only a limited amount of material is available. Macro-scale test would not be possible to perform under such circumstances. Interestingly, it has been observed that due to flaws occurred in a material, hardness measured on the micro and nano-scale is higher than hardness measured on the macro-scale.

This paper is focused on the discussion over the various types of scratch tests for different purposes and is divided into the two sections. In the first section will be discussed scratch test method itself and scratch tests for different purposes according to the tested material. Second part will be devoted to the practical evaluation of the scratch test.

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