Ultrasonic Measurements of the Elastic Properties of Polymer Coatings

Prof. Rob Dwyer-Joyce
Dr Neil Hankinson

Engineering is about improving efficiency and reducing cost. One way to do this is to make components smaller and to make sure they have strength only in the places where it is really needed. These economic drivers have resulted in a big increase in the number of thin film and surface treatment products in many industries, automotive, aerospace, domestic aplliances etc. Condition monitoring is an essential tool for ensuring optimisation and safe operation of these products. Ultrasound is a useful, non-destructive source of such information.

This new area of investigation is currently focused upon developing an ultrasonic instrument to measure the elastic properties of a thin coating on a dissimilar substrate. Currently these are determined semi-destructively using nano-hardness indentors. This does not yield Young's Modulus and Poisson's ratio independently. An ultrasonic technique has been successfully employed to measure these parameters for a polymer coating of thickness 0.15mm or greater.

Any interface between two media will cause a proportion of ultrasound to be reflected. The percentage that is reflected is governed by the ratio of their acoustic impedances; the product of density and speed of sound. With a layer of material there exists two interfaces, the front face and the back face. Both these interfaces will reflect ultrasound and are observable as "echoes".

The echo separation is dependent upon the layer thickness and the acoustic properties of the layer medium. For thin layers the echoes will overlap and hence interfere. One phenomenon of ultrasound through layers is that when the wavelength is equal to the layer thickness it is entirely transmitted and none is reflected. Our investigations are performed using broadband transducers, i.e. emitting a range of frequencies simultaneously. The transmitted wavelengths are observed as resonant frequency drops, as shown in the figure, whose value is a function of the layer thickness and the acoustic properties of the layer.

The resonant frequencies can be measured with both longitudinal and transverse waves. The ratio of these two resonant frequencies is a function of the coating's normal to lateral response and hence Poisson's ratio. If the thickness of the coating is known then these in turn can be related to the Young's Modulus. Advancement of the work involves developing an inherent method to also measure the thickness, hence increasing the number of variables measureable through the technique.

 

For thinner coatings the resonant frequency becomes too high to measure without the use of expensive transducers and wave generators. This developing area of research involves adapting a current fluid film ultrasonic measurement technique to be used on solid coatings. Measurements of stiffness through the use of hardened contactors of known surface shape, such as that shown, will enable the layer's parameters to be isolated and hence compared for longitudinal and transverse waves. This in turn will enable the elastic properties to be measured for very thin hard solid layers. We have also been experimenting using a liquid metal called Galinstan as the contactor. This makes complete contact but has a high enough impedence to get good acoustic coupling.

Publications:

  1. Dwyer-Joyce, R.S. and Hankinson, N., (2005), "Ultrasonic Measurement of the Elastic Properties of Soft Surface Coatings", Tribology International, Vol. 39, No.4 pp 326-331.