Mechanical Properties of Nanostructured hard coating of ZrO 2
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Mechanical Properties of Nanostructured hard coating of ZrO2. R. F. Sabirianov1, F. Namavar2, X.C. Zeng3, J. Bai3, W.N. Mei1 1
Department of Physics, University of Nebraska - Omaha, Omaha, NE 68182 USA Department of Orthopaedic Surgery and Rehabilitation,University of Nebraska Medical Center, Omaha, NE 68198 USA 3 Department of Chemistry and Center for Materials Research Analysis,University of Nebraska Lincoln, Lincoln, NE 68588 USA 2
Abstract Nano-crystalline films of pure cubic ZrO2 have been produced by ion beam assisted deposition (IBAD) processes which combine physical vapor deposition with the concurrent ion beam bombardment in a high vacuum environment and exhibit superior properties and strong adhesion to the substrate. Oxygen and argon gases are used as source materials to generate energetic ions to produce these coatings with differential nanoscale (7 to 70 nm grain size) characteristics that affect the wettability, roughness, mechanical and optical properties of the coating. The nanostructurally stabilized chemically pure cubic phase has been shown to possess hardness as high as 16 GPa and a bulk modulus of 235 GPa. We examine the mechanical properties and the phase stability in zirconia nanoparticles using first principle electronic structure method. The elastic constants of the bulk systems were calculated for monoclinic, tetragonal and cubic phases. We find that calculated bulk modulus of cubic phase (237GPa) agrees well with the measured values, while that of monoclinic (189GPa) or tetragonal (155GPa) are considerably lower. We observe considerable relaxation of lattice in the monoclinic phase near the surface. This effect combined with surface tension and possibly vacancies in nanostructures are sources of stability of cubic zirconia at nanoscale. Introduction Tough, wear resistant, refractory zirconia ceramics are used to manufacture parts operating in aggressive environments, like extrusion dyes, valves and port liners for combustion engines, low corrosion, thermal shock resistant refractory liners or valve parts in foundries. High temperature ionic conductivity makes zirconia ceramics suitable as solid electrolytes in fuel cells and in oxygen sensors. Good chemical and dimensional stability, mechanical strength and toughness, coupled with a Young’s modulus in the same order of magnitude of stainless steel alloys is the origin of the interest in using zirconia as a ceramic biomaterial.[1] Zirconia ZrO2 has three distinct polymorphous phases at ambient pressure. It exhibits cubic phase (fluorite Fm3m) above 2640K and melts at about 2950K. The cubic zirconia transforms into tetragonal structure (P42/mnc) below 2640K. The tetragonal zirconia changes into monoclinic structure (P21/C) at 1440K. Because of its high strength and stability at high temperature ZrO2 has numerous industrial applications such as oxygen sensor, high temperature fuel cell, and gate material for semiconductor device. Among them, cubic and tetragonal polymorphs of zirconia are not stable at room temperature in the
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