Strength of Coherently Strained Nanolayers Under High Temperature Nanoindentation
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Strength of Coherently Strained Nanolayers Under High Temperature Nanoindentation K. M. Y. P'ng, X. D. Hou, D. J. Dunstan, and A. J. Bushby Centre of Materials Research, Queen Mary, University of London, Mile End Road, London, E1 4NS, United Kingdom
ABSTRACT Semiconductor strained layer superlattices are an ideal model material to study the effects of coherency strain in plasticity, due to the fine control of nanolayer thickness and internal strain afforded by MBE deposition. Previously, nanoindentation of bulk InGaAs at 300K gave a yield pressure of 6GPa (Jayawera et al Proc. Roy Soc, A459, 2049, 2003) while bending at 500 centigrade gave a yield value of 30MPa (P’ng et al Phil. Mag. 85, 4429, 2005). In contrast, coherently strained InGaAs superlattices gave nanoindentation values of 3GPa at room temperature and bending at 500oC gave a yield value also around 3GPa. It appears that the coherency strain can impart an athermal strengthening to the superlattice. It is clearly necessary to do mechanical testing over the range 300-800K that will be able to link the room temperature nanoindentation with the results from the high temperature bending experiment and to determine the relationship between strength, coherency strain and temperature. Preliminary experiments on these samples at elevated temperatures using a hot stage and the UMIS nanoindentation system is difficult but feasible with the help of AFM to verify the contact area.
INTRODUCTION Research in the mechanical properties of nanoscale materials is expedited using the nanoindentation technique [1, 2]. Nanoindentation has the accuracy and sensitivity of measuring forces accurate to 0.01mN and depth measurements up to ± 0.1nm using either a pointed or spherical tip that is microns in dimension [1-3]. Therefore, the volume of the material studied is very small; making it suitable to examine nanostructured materials. However, nanoindentation at high temperature presents a problem that is difficult to solve due to thermal fluctuation and thermal expansion that causes instability in the depth measurement. Therefore, mechanical data from high temperature nanoindentation are often useless which confines its functionality to room temperature measurements. It is crucial to overcome this limitation because nanoindentation can at times be the only practical method of obtaining mechanical property data of nanostructured materials. In this paper, we propose a novel method of obtaining mechanical property data by hot indentation and present preliminary experimental results that provide an alternative solution to the problem. At high temperatures, the nanoindenter can still make indents with accurate force measurements. The nanoindenter applies a force on the material and measures the depth at each force increment. Upon reaching its specified maximum load, it retracts, leaving a permanent indent if it has exceeded its yield pressure. Since the depth measurement is unreliable, the only useful data is the maximum force reading. To obtain indentation stre
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