Subsurface cracking during indentation on hybrid coatings on polycarbonate
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Jose´ C.M. Brokken-Zijp Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Dutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, The Netherlands
Gijsbertus de With Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands (Received 26 July 2002; accepted 21 November 2002)
Subsurface cracking after nano- and Vickers indentation on transparent silica–(meth)acrylate hybrid coatings deposited on a polycarbonate substrate was observed for the first time. It appeared that after initiation at the interface, these cracks propagated toward the surface and in radial directions as the indentation load increased. It was found that, for chemically identical coatings, the thicker the coating, the higher load necessary to initiate these cracks. Subsurface cracks formed during nanoindentation occur at lower loads than the normally observed surface cracks and are accompanied by a very small change in the slope of the loading curve.
I. INTRODUCTION
Nowadays load and depth-sensing indentation or nanoindentation1 receives an increasing interest from material scientists. It has been shown that nanoindentation is an attractive technique to evaluate mechanical properties of treated surfaces, thin films, and thick coatings. Among its advantages, the technique has a nano-/microscale of measurement and a simple sample preparation and provides detailed load–displacement data for the whole indentation process. For a coating–substrate system during indentation various types of mechanical failure may occur, i.e., in the coating, in the substrate, and/or in the interface. All failures typically result in particular changes in the loading or/and unloading curve. As was reported earlier failures such as radial and annular cracking at the surface are accompanied by a small discontinuity,2–5 which changes the slope of the loading curve,6–8 as well as by more significant abrupt changes in the loading curve.8 Furthermore, radial9 and annular5 cracking formed upon unloading was also reported. It was observed that both pop-in into a specimen10–14 (also referred to as “excursion”15,16) and
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 2, Feb 2003
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pop-out12,13 result in a discontinuity in the loading and unloading curve, respectively. Prolonged indenter contact at a fixed load, i.e., creep, also leaves its fingerprint on the load–displacement curves.17–21 Normal delamination, which has been earlier shown to be present experimentally 22,23 as well as theoretically,24,25 occurs when a circular part of the coating, directly under the contact area, is lifted up from the substrate or from the part of the coating. This results typically in a change in the unloading curve at a negligible residual indentation depth. For bulk material, a similar type of fracture was referred t
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