Indentation-induced deformation and microcracking of highly textured superconducting (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x cerami
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Jie Luo and R. Stevens School of Materials, University of Leeds, Leeds LS2 9JT, United Kingdom (Received 6 June 1994; accepted 12 November 1994)
A high Tc ceramic superconductor has been prepared having a high degree of preferred orientation, this having been generated by means of a large magnetic field during the early stages of processing. The texture of the material has been characterized using XRD textural analysis and the microstructure by high resolution electron microscopy. Pyramidal indentation (Vickers indent), deformation, and cracking have been investigated using high resolution SEM. Semispherical deformation zones were formed under indents, a result of slipping of adjacent ab/ab (basal) plane grain boundaries and some microcrack development, together with lattice deformation on the basal plane. Densification of the green compact was also observed in the deformation zones, where lateral cracking has been seen to take place along the ab/ab plane grain boundaries. Radial cracking has been seen to take place along the low angle grain boundaries of the lattice ab/ab plane when indentations were made on the surface perpendicular to the preferred texture surface of the materials and also at the highly deformed corners of indent diagonals. The stresses generated resulted in the fracture of the flake-shaped grains and initiated small radial cracks. The Vickers microhardness of the textured ceramics was found to decrease with increasing load.
I. INTRODUCTION A key parameter for the successful application of high temperature superconductors is the critical current density attainable in the materials. ( B u P b ^ S ^ C a ^ u s O ^ (denoted as BSCCO-2223), which is one of a family of high temperature superconductors, has considerable potential for practical applications in the near future, since in the form of silver clad tapes a high critical current density has been achieved for the composite tape.1"8 As the thickness of the tapes is usually of the order of tens of microns, they are mechanically so fragile that degradation of the properties due to unavoidable stressing during normal handling is a constant hazard. Stressing of the BSCCO-2223 tapes by Lorentz forces can also occur in the superconducting state when a large electric current is passing through the superconductor in a magnetic field. To achieve a high critical current density in the BSCCO-2223 tapes, it has been shown that texturing of the superconductor grains is essential.1"8 Texturing of the BSCCO-2223, either in the form of powder or the sintered body, has usually been achieved by the application of a mechanical force such as occurs in cold pressing. There has been little discussion of the mechanical parameters involved in reorganization of the crystals as they compact, specifically of how the application of mechanical force transforms a BSCCO2223 material with randomly oriented grains into a
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J. Mater. Res., Vol. 10, No. 3, Mar 1995
http://journals.cambridge.org
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highly textured state. Consequently, there is a need
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