Nanoindentation of particulate coatings
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Nanoindentation of particulate coatings M. J. Adams, A. Akram, B. J. Briscoe,a) C J. Lawrence, and D. Parsonage Particle Technology Group, Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, London SW7 2BY, United Kingdom (Received 28 August 1998; accepted 24 February 1999)
A knowledge of the formation and rupture mechanisms for agglomerates is essential when seeking to model equipment designed to produce and process such materials. In the work described here, nanoindentation of “two-dimensional” agglomerate films, basically particulate coatings, was carried out to establish a means of identifying the generic breakage mechanisms for agglomerates. Selected applied load and penetration depth data in the range (0.02 mN and 700 nm, respectively) are provided as a function of the loading time during continuous loading for a model system composed rather of monodispersed colloidal silica particles (20–24 nm diameter) bound with a poly(methyl methacrylate) at 5 vol%. It is argued that these data enable the sequence of binder bridge failures to be observed, thus giving an indication of the breakage mechanism of the agglomerate and also the strength of the individual junctions. These data are also incorporated into a mechanical model that describes the rupture and deformation behavior of these planar agglomerate systems.
I. INTRODUCTION
Indentation is a widely practiced method for determining certain bulk material properties, such as the hardness and elastic modulus, of a broad range of dense materials including metals, ceramics and organic polymers. The method has also been used to study powder compacts and foams as well as a range of “thick” and “thin” coatings. Historically, this method has involved relatively large depth indents and adopted either the imaging or contact compliance techniques. More recently it has been possible to study reliably the near surface characteristics of materials and the properties of thin films by the use of nanoindentation methods.1–3 The scale of the deformation is much smaller; depths and forces are in the range of a few nm and mN, respectively. The “nanomethods” exclusively adopt the contact compliance method. Direct imaging of the indent, while possible, is generally not a viable experimental procedure. The present paper seeks to exploit the unique features of the established nanoindentation method as a basis for characterizing the origins of the mechanical integrity of rather porous particle agglomerated systems. A model system comprising colloidal silica, particle diameter approximately 20–24 nm, bound with a glassy polymer, poly(methyl methacrylate) (PMMA), is used for this purpose. While it is possible to perform such nanoindentations directly upon agglomerated particles, here a preliminary study will be reported for 2-dimensional agglomerates in the form of thin particulate coatings. a)
Address all correspondence to this author. e-mail: [email protected]
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