Atomic force microscopy study of nanoindentation deformation and indentation size effect in MgO crystals
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Atomic force microscopy study of nanoindentation deformation and indentation size effect in MgO crystals K. Sangwala) Institute of Physics, Technical University of Lublin, ul. Nadbystrzycka 38, 20-618 Lublin, Poland
P. Gorostiza, J. Servat, and F. Sanz Department of Physical Chemistry, University of Barcelona, Marti i Franques 1, 08028 Barcelona, Spain (Received 20 April 1999; accepted 28 July 1999)
The dependences of various nanoindentation parameters, such as depth of penetration d, indentation diameter a, deformation zone radius R, and height h of hills piled up around indents, on applied load were investigated for the initial (unrecovered) stage of indentation of the (100) cleavage faces of MgO crystals by square pyramidal Si tips for loads up to 10 N using atomic force microscopy. The experimental data are analyzed using theories of elastic and plastic deformation. The results revealed that (i) a, R, and h linearly increase with d; (ii) the development of indentation size and deformation zone and the formation of hills are two different processes; (iii) the load dependence of nanohardness shows the normal indentation size effect (i.e., the hardness increases with a decrease in load); and (iv) there is an absence of plastic deformation involving the formation of slip lines around the indentations. It is found that Johnson’s cavity model of elastic–plastic boundary satisfactorily explains the experimental data. The formation of hills around indentations is also consistent with a new model (i.e., indentation crater model) based on the concept of piling up of material of indentation cavity as hills.
I. INTRODUCTION
Indentation hardness testing is frequently used to assess the mechanical properties of crystalline and noncrystalline substances. However, despite numerous studies devoted to the understanding of the physical nature of hardness, there are several phenomena associated with indentation hardness and deformation occurring beneath and around indents that are poorly understood. Among the most widely discussed and controversial topics in the area of indentation hardness are (i) the dependence of microhardness on applied load, a phenomenon known as indentation size effect (ISE),1–12 (ii) the nature of deformation beneath and around indentations,1,11,13–19 and (iii) the mechanism responsible for the appearance of hills piled up around indents.17,19 The earlier investigations dealt with microindentation, but recently papers have also been devoted to the study of nanoindentation deformations,3,10,17,19,20 comparison of deformation in small volumes with continuum plastic and Hertzian elastic theories,17,19 and phase transitions15,16,20,21 and twinning induced by indentation deformation.11 Moreover,
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 10, Oct 1999
http://journals.cambridge.org
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with an increase in load or indentation size both a decrease9,10,20,22,23 and an increa
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