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Research Papers

Extraction of Anisotropic Mechanical Properties From Nanoindentation of SiC-6H Single Crystals

[+] Author and Article Information
Amit Datye, Lin Li, Wei Zhang

Department of Materials Science
and Engineering,
University of Tennessee,
Knoxville, TN 37996

Yujie Wei

LNM,
Institute of Mechanics,
Chinese Academy of Sciences,
Beijing 100190, China

Yanfei Gao

Department of Materials Science
and Engineering,
University of Tennessee,
Knoxville, TN 37996;
Materials Science and Technology Division,
Oak Ridge National Laboratory,
Oak Ridge, TN 37831
e-mail: ygao7@utk.edu

George M. Pharr

Department of Materials Science
and Engineering,
University of Tennessee,
Knoxville, TN 37996;
Materials Science and Technology Division,
Oak Ridge National Laboratory,
Oak Ridge, TN 37831
e-mail: pharr@utk.edu

1Corresponding authors.

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received May 2, 2016; final manuscript received May 29, 2016; published online June 22, 2016. Editor: Yonggang Huang.

J. Appl. Mech 83(9), 091003 (Jun 22, 2016) (7 pages) Paper No: JAM-16-1222; doi: 10.1115/1.4033790 History: Received May 02, 2016; Revised May 29, 2016

Because brittle solids fail catastrophically during normal tension and compression testing, nanoindentation is often a useful alternative technique for measuring their mechanical properties and assessing their deformation characteristics. One practical question to be addressed in such studies is the relationship between the anisotropy in the uniaxial mechanical behavior to that in the indentation response. To this end, a systematic study of the mechanical behavior the 6H polytype of a hexagonal silicon carbide single crystal (SiC-6H) was performed using standard nanoindentation methods. The indentation elastic modulus and hardness measured using a Berkovich indenter at a peak load of 500 mN varied over a wide range of crystal orientation by only a few percent. The variation in modulus is shown to be consistent with an anisotropic elastic contact analysis based on the known single crystal elastic constants of the material. The variation in hardness is examined using a single crystal plasticity model that considers the anisotropy of slip in hexagonal crystals. When compared to experimental measurements, the analysis confirms that plasticity in SiC-6H is dominated by basal slip. An anisotropic elastic contact analysis provides insights into the relationship between the pop-in load, which characterizes the transition from elasticity to plasticity during nanoindentation testing, and the theoretical strength of the material. The observations and analyses lay the foundations for further examination of the deformation and failure mechanisms in anisotropic materials by nanoindentation techniques.

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Figures

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Fig. 1

(a) Schematic diagram showing the zenith angle defined in the lattice prism relative to the indentation direction. In the experiments, the zenith angle was varied in plane 1. Plane 2 is also defined to indicate maximum possible deviation of the indentation direction from the plane 1. ((b) and (c)) Depth dependence of the indentation modulus and hardness obtained using the Oliver–Pharr method for NC SiC-6H single crystals with four zenith angles.

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Fig. 2

(a) Effective indentation modulus and (b) hardness for both Cree and NC SiC-6H single crystals plotted as a function of zenith angle. The measured effective indentation moduli agree well with those predicted from anisotropic elastic contact analysis.

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Fig. 3

A comparison of the experimentally measured hardnesses to those predicted from the crystal plasticity model assuming a CRSS, τCRSS  = 4.8 GPa, when only one type of slip is used: (a) basal, (b) prismatic, (c) pyramidal 〈a〉, and (d) pyramidal 〈a + c〉. The comparison suggests that the basal slip system dominates the deformation and thus the hardness anisotropy.

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Fig. 4

Values of the maximum resolved shear stress on the basal slip system as a function of spherical indenter radius for Cree SiC-6H single crystals in two crystallographic orientations. The pop-in stresses are near the theoretical strength. No pop-ins were observed in tests using indenter with radii of 7.5 μm and 50 μm because the pop-in forces exceed the machine load capacity (500 mN).

Grahic Jump Location
Fig. 5

(a) Indentation Schmid factor and (b) the predicted pop-in load as a function of the zenith angle in the two planes defined in Fig. 1(a) when only basal slip is allowed. Indentations in the c and a directions give almost the same indentation Schmid factor and pop-in load.

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