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Development of Indentation Methods and Numerical Models for Measuring (Thin) Coating Material Properties

Yielding and creep characteristics can be obtained from indentation data [1, 2], via iterative use of finite element models.  Current work is aimed at extending these methodologies to (relatively thin) coatings of, for which the effect of the presence of the substrate cannot be ignored. Work has been done recently on copper films (with thickness of the order of a few hundred µm) attached to steel substrates, indented with a tungsten carbide ball to a range of depths (up to 50% of the thickness of the film).  Maximum loads as a function of indentation depth were recorded and compared with the indentation response obtained from tests on the copper in its bulk form.  These data have been successfully used to infer the yield stress and work hardening rate of the copper. The methodology has also been successfully applied to a hard on soft coating system consisting of a stainless steel coating on a copper substrate.  

jlr49 FEM and experiments
Comparison between FE simulations and experimental data for a known material system with a 1.5mm radius ball bearing.

The methodology has now been enhanced by the addition of "goodness of fit" parameter, which minimises the discrepancy between the predicted and experimental curves. By making iterative changes to the input yielding characteristics, a full picture of the sensitivity to input parameters can be built up over the entirety of  parameter space. Work is ongoing to understand the uniqueness and accuracy of extracted plasticity parameters using this method. 

This methodology will now be extended to thinner copper and aluminium coatings (between 6 µm and 12 µm), using a smaller indenter.  This presents additional challenges relating to the effects of oxide films, surface contamination, surface roughness etc. 

This project falls under the"Fine Scale Mechanical Interrogation" research theme, where published papers in this area can be found.

References

[1]      J Dean, JM Wheeler & TW Clyne, Use of Quasi-static Nanoindentation Data to obtain Stress–strain Characteristics for Metallic Materials, Acta Mater., 58 (2010) 3613-3623.

[2]      J Dean, A Bradbury, G Aldrich-Smith & TW Clyne, A Procedure for Extracting Primary and Secondary Creep Parameters from Nanoindentation Data, Mechanics of Materials, 65 (2013) 124-134

 

 

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