Stiffness of ultrathin coatings

A key engineering parameter of thin coatings is their stiffness. Stiffness characterisation of ultrathin coatings with a nanometer scale thickness is experimentally challenging. In a new work, three feasible methods have been used to estimate the Young’s modulus of metal coatings on polymer films. The methods are: (1) nanoindentation, (2) strain-induced elastic buckling and (3) peak-force measurements integrated in atomic force microscopy.

Necessity of a sufficiently sharp indenter tip

The samples were prepared by atomic layer deposition of TiO₂ (6 and 20 nm thick) and mixed oxides of TiO₂ and Al₂O₃ (4 and 20 nm thick). The differences in estimated Young’s modulus are interpreted in terms of the underlying assumptions and test conditions. Their specific advantages and drawbacks are also compared and discussed. In particular, the nanoindentation necessitates a sufficiently sharp indenter tip to make localised measurements dominated by the coating.

Higher Young’s modulus for TiO₂ coating

The strain-induced elastic buckling method is simple in practice, but showed a large scatter due to variation in local coating thickness and irregular deformation patterns. The stiffness characterisation using atomic force microscopy gave the most consistent results, due to a sharp tip with a radius comparable to the thinnest coating thickness. All methods gave a higher Young’s modulus for the TiO₂ coating than for the mixed oxide coating, with a variation within one order of magnitude between the methods.

The study is published in: Journal of Coatings Technology and Research  July 2018, Volume 15, Issue 4, pp 743–752.