薄膜与小尺度材料及力学性能研究组

Effects of focused ion beam milling on the compressive behavior of directionally solidified micropillars and the nanoindentation response of an electropolished surface

S. Shim^{a, b}, H. Bei^a, M.K. Miller^a, G.M. Pharr^{a, b} and E.P. George^{a, b}

1.Effects of pre-strain on the compressive stress–strain response of Mo-alloy single-crystal micropillars
Pages 4762-4770
H. Bei, S. Shim, G.M. Pharr and E.P. George

2.Sample shape and temperature strongly influence the yield strength of metallic nanopillars
Pages 4816-4828
Ajing Cao and E. Ma

3.Void formation in nanocrystalline Cu film during uniaxial relaxation test
Pages 4921-4931
Junya Inoue, Yosuke Fujii and Toshihiko Koseki 

Effects of pre-strain on the compressive stress–strain response of Mo-alloy single-crystal micropillars

H. Bei^a, S. Shim^{a, b}, G.M. Pharr^{a, b} and E.P. George^{a, b}

^aMaterials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

^bDepartment of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA

Received 20 February 2008; revised 14 May 2008; accepted 14 May 2008. Available online 21 June 2008.

Abstract

A NiAl–Mo eutectic was directionally solidified to produce composites with well-aligned single-crystal Mo-alloy fibers embedded in a NiAl matrix. They were pre-strained by compressing along the fiber axis and then the matrix was etched away to expose free-standing micropillars having different sizes (360–1400 nm) and different amounts of pre-strain (0–11%). Compression testing of the pillars revealed a variety of behaviors. At one extreme were the as-grown pillars (0% pre-strain) which behaved like dislocation-free materials, with yield stresses approaching the theoretical strength, independent of pillar size. At the other extreme were pillars pre-strained 11% which behaved like the bulk, with reproducible stress–strain curves, relatively low yield strengths, stable work-hardening and no size dependence. At intermediate pre-strains (4–8%), the stress–strain curves were stochastic and exhibited considerable scatter in strength. This scatter decreased with increasing pre-strain and pillar size, suggesting a transition from discrete to collective dislocation behavior.

Keywords: Plastic deformation; Yield phenomena; Crystal plasticity; Size dependence; Mechanical properties

Influence of indenter angle on cracking in Si and Ge during nanoindentation

Jae-il Jang^a and G.M. Pharr^{b, c}

^aDivision of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea

^bDepartment of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200, USA

^cMaterials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Received 11 January 2008; revised 1 May 2008; accepted 1 May 2008. Available online 2 June 2008.

Abstract

The influence of indenter angle on the nanoindentation cracking behavior of single crystal Si and Ge was systematically explored through nanoindentation experiments with a series of triangular pyramidal indenters with different centerline-to-face angles in the range 35.3–85.0°. The relationships between indentation load, crack length and indentation size and their dependence on indenter angle were carefully examined and compared with previous indentation cracking studies. The results are discussed in terms of ways to estimate fracture toughness and indentation cracking threshold loads more precisely through nanoindentation.

Keywords: Nanoindentation; Toughness; Cracking; Semiconductors

Extraction of flow properties of single-crystal silicon carbide by nanoindentation and finite-element simulation

Sanghoon Shim^{a, b}, Jae-il Jang^c and G.M. Pharr^{a, b}

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

^bOak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN 37831, USA

^cHanyang University, Division of Materials Science and Engineering, Seoul 133-791, Republic of Korea

Received 22 February 2008; revised 4 April 2008; accepted 6 April 2008. Available online 28 May 2008.

Abstract

A method is presented for estimating the plastic flow behavior of single-crystal silicon carbide by nanoindentation experiments using a series of triangular pyramidal indenters with five different centerline-to-face angles in combination with two-dimensional axisymmetric finite-element (FE) simulations. The method is based on Tabor’s concepts of characteristic strain and constraint factor, which allow indentation hardness values obtained with indenters of different angles to be related to the flow properties of the indented material. The procedure utilizes FE simulations applied in an iterative manner in order to establish the yield strength and work-hardening exponent from the experimentally measured dependence of the hardness on indenter angle. The methodology is applied to a hard, brittle ceramic material, 6H–SiC, whose flow behavior cannot be determined by conventional tension or compression testing. It is shown that the friction between the indenter and the material plays a significant role, especially for very sharp indenters.

Keywords: Nanoindentation; Hardness; Plastic deformation; Finite-element modeling (FEM); Single-crystal silicon carbide