Shear-activated indentation crack in GaAs single crystal
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X. Wu Department of Mechanical and Engineering, Wayne State University, Michigan 48202 (Received 29 May 2001; accepted 1 August 2001)
Cracks induced by 0.049N load indentation on the surface of gallium arsenide single crystal were investigated before and after annealing. The results revealed that the crack initiation essentially relates to the shear deformation during indentation. There were dislocation generation, lattice distortion, and the transformation from crystalline to disordered structure, leading to the occurrence of an amorphous band in front of the crack tip. After being annealed at 500 °C for 60 min, the amorphous band disappeared, and instead, recrystallized grains appeared along the crack-propagation direction. It is reasonable to propose that crack propagation is the result of the decohesion of the amorphous band rather than the direct debonding along a certain atomic plane.
I. INTRODUCTION
The fracture of brittle materials has been the subject of numerous theoretical and experimental studies in the past decades. Many attempts have been made to observe a crack tip, including optical microscopy,1,2 x-ray topography,3,4 transmission electron microscopy (TEM),5,6 and high-resolution electron microscopy (HREM).7–10 Of these techniques, the resolution of optical microscopy and x-ray topography is too poor. Although the resolution of TEM is much better than that of the former cases, it is also not high enough to obtain the information at a crack tip on an atomic scale. By contrast, HREM is the most useful method for the observation of a crack tip. Up to now, most results showed that the crack tip is atomically sharp and the crack propagation results from the sequential rupture of cohesive bonds.5–9 This atomically sharp model, in fact, is in agreement with the earlier idea of “lattice-trapped” proposed by Thomson et al.11 and the cohesive force theory suggested by Barenblatt.12 Indentation techniques are important for studying the mechanical behavior of brittle materials. In its most widely used form, the indentation test allows hardness to be determined and provides a means of introducing localized cracks. Some in situ observations have been made to study the fractures induced by indentation in brittle materials,13,14 but could not reveal the
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 10, Oct 2001
deformation characteristics ahead of the crack tip and the mode of crack propagation at atomic scale due to the limitation of the resolution. GaAs is a semiconductor material with a sphalerite structure and has characteristically very low fracture toughness, which is 0.46 MPa m1/2 as reported by Yasutake et al.15 Therefore, GaAs is a suitable material used to study the brittle fracture, in particular to understand the structure at the crack tip. The purpose of this article is to study the characteristics of the initiation and propagation of a crack induced by low-load indentation in GaAs single crystal.
II. EXPERIMENTAL PROCEDURES
Two 3-mm-diameter
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