Thin coatings, modulus

It should be noted that for thin coatings the dependence imder consideration is approximately hnear. Extrapolation to thinner coatings results in a modulus of elasticity of (60—70) x 10 MPa for the coating on the aluminum substrate and of (26—29) x 10 MPa for the coating... 

Workers at the Institute for Dynamic Materials Testing in Ulm [58] have developed a new method for determining viscoelastic characteristics of thin coatings. Elasticity and damping components of the dynamic shear modulus as a function of temperature or time can be determined from the resonance frequency and damping of the natural vibration of a coated aluminium carrier plate. 

This paper discusses the type of fracture events which occur for hard coatings on soft and hard substrates and outlines the effect that fracture events have on other mechanical properties measured by indentation (such as hardness and Young s Modulus). It also introduces the use of acoustic emission generation to monitor fracture events in very thin coatings where it is difficult to identify them by conventional microscopy (both scanning electron and atomic force). 

Fig. 40. Indentation hardness, at A, 19 )J.ni B, 47 jJm and C, 108 ]lni thickness, and Young s modulus, E, of a free film (as functions of temperature) of an acryhc coating (249). These results show the dependence of thickness that occurs with thin films. To convert MPa to psi, multiply by 145.

It has been also shown that when a thin polymer film is directly coated onto a substrate with a low modulus ( < 10 MPa), if the contact radius to layer thickness ratio is large (afh> 20), the surface layer will make a negligible contribution to the stiffness of the system and the layered solid system acts as a homogeneous half-space of substrate material while the surface and interfacial properties are governed by those of the layer [32,33]. The extension of the JKR theory to such layered bodies has two important implications. Firstly, hard and opaque materials can be coated on soft and clear substrates which deform more readily by small surface forces. Secondly, viscoelastic materials can be coated on soft elastic substrates, thereby reducing their time-dependent effects. 

Jao, S.H. and McGarry, F.J. (1992a) On the failure work and modulus of composites reinforced by thin elastomer-coated fibers. J. Composite Mater. 26, 2632-2654. 

In this section we will describe how a proper accounting for film dynamics, based on a model of the thin-film/acoustic-wave interactions, can be used to quantitatively evaluate the shear modulus values as a function of temperature. As described in Section 3.1, an equivalent-circuit model can be used to relate the measured TSM electrical characteristics to the elastic properties, density, and thickness of a polymer film coating the device. Consequently, measurements made with polymer-coated TSM devices can be used to extract the shear elastic properties of the film. 

Efforts to improve the compressive properties of rigid-rod polymer fibers have involved introduction of cross-linking in the transverse direction (Bhattacharya, 1989 Spillman et al, 1993) and coating the fiber surface with a thin layer of a high modulus material (McGarry and Moalli, 1991,1992). 

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