Magnetic tape consists of a film of PET with a thin coating of iron oxide magnetic particles suspended in a polymeric matrix. The durability of the magnetic film is a function of its mechanical properties. For layer wear resistance, a hard surface layer is important. Layer adhesion depends on the level of interfacial shear stress. To minimise this stress the layer and substrate modulii must be matched. It is important, therefore, to be able to measure the mechanical properties of the magnetic layer.
These mechanical properties have been investigated using a micro-indentation technique. A diamond indenter was loaded onto the surface of the tape to depths of around 100 nm. The load and depth of penetration were monitored continuously.
The hardness of the surface layer was determined from the loading part of the curve. The unloading is an elastic process; the elastic modulus may be obtained from the form of this curve. The magnetic layer is very thin (less than 2 microns); indentations at this depth are subject to experimental error. Deep indentations are dominated by substrate properties, whilst shallow indentations are dominated by layer properties. We carried out a series of indentations at reducing depth and used an extrapolation technique to estimate layer properties.
The hardness of the magnetic layer was measured as 350-400MPa, whilst the reduced modulus of the layer was 7-8GPa and of the substrate was 3-4GPa. The latter figure was consistent with estimates from other experimental techniques.
Both the substrate and layer materials were found to exhibit rate dependent deformation. We analysed this by carrying out a series of tests at varying loading rates. This data can then be used to determine the creep behaviour of the materials.
We are grateful to Mitsubishi Chemical Corporation, the Royal Society of Great Britain, and the Japan Society for the Promotion of Science for sponsorship of this work. This work was carried out in the Department of Mechanical Science and Engineering at Kyushu University. RDJ would like to express his gratitude to Prof Murakami for hosting the visits.
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