Fractography of Advanced Ceramics II

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Authors: Peter Zimovčák, Tibor Köves, Ján Dusza, Ladislav Pešek, Francis Chalvet, Goffredo de Portu
Abstract: Micro- and macro-hardness of the individual layers of an Al2O3 / Al2O3 + ZrO2 layered composite have been investigated. The indentation load size effect has been studied in a range of indentation load of 25 mN – 100 N using depth sensing indentation technique and conventional Vickers hardness testers. Fracture toughness values of the layers have been measured using indentation fracture method. The effect of residual stresses on the KIC value has been analyzed.
Authors: L. Ceniga
Abstract: The paper deals with crack formation of thermal stresses in an isotropic multi-particle-matrix system of homogeneously distributed spherical particles in an infinite matrix divided to cubic cells containing a central particle. Originating during a cooling process as a consequence of the difference in thermal expansion coefficients between a matrix and a particle, the thermal stresses are thus investigated within the cubic cell and extreme at the critical particle volume fraction. Resulting from the derived crack formation condition related to an ideal-brittle particle, the critical particle radius considering the critical particle volume fraction is presented along with an application to the thermalstress- strengthened SiC-Si3N4 ceramics.
Authors: Alessio Beverina, Antonio Javier Sanchez-Herencia, N. Hernández, Rodrigo Moreno
Abstract: Nickel-reinforced alumina composites have been manufactured by aqueous slip casting and pressureless sintered under flowing atmosphere of argon with 0,36 and 1% of oxygen in order to force interfacial reactions leading to the formation of a nickel-aluminum spinel. Colloidal stability of concentrated suspensions of alumina with 5, 10 and 15 vol% of nickel has been studied in terms of zeta potential, rheometry and packing density. The processed composites show a high dispersion of the nickel into the alumina matrix and green densities of 60-70 %th. The effect of sintering temperature and atmosphere on the mechanical behaviour of the composites has been investigated through Vickers indentation and fractographic SEM observations.
Authors: Monica Popa, José-Maria Calderón Moreno, Pavol Hvizdoš, Raúl Bermejo, Guy Anné
Abstract: In order to understand the mechanical behavior of layered composites with compositional gradient, it is necessary to determine their state of residual stresses. Compositionally graded materials can offer the advantage of eliminating abrupt changes in composition between layers having different thermal expansion coefficients. The existence of a compositional gradient may reduce discontinuities in thermal residual stresses, something beneficial from the point of view of the mechanical properties. We present here a study of the state of the residual stresses in a layered material made of thicker (several mm) homogeneous layers of alumina and alumina-20%zirconia separated by a thinner (less than 300 µm) intermediate graded alumina-zirconia layer, obtained by controlled deposition of powders from a solution using an electrophoretic deposition method. The thermal residual stresses generated during cooling after sintering were measured in the homogeneous layers at each side, and at steps of about 30 µm in the graded layer along the direction of the compositional gradient, by using fluorescence ruby luminescence piezo-spectroscopy. Results show that the hydrostatic stresses on alumina grains vary continuously, indicating the absence of discontinuities in thermal residual stresses along the compositionally graded layer and at the interfaces of the homogeneous layers.
Authors: Pavol Hvizdoš, José-Maria Calderón Moreno, Jan Očenášek, Luca Ceseracciu, Guy Anné
Abstract: The mechanical properties of a quasi-symmetrical planar functionally graded material (FGM) of Al2O3 / Al2O3+20%ZrO2 / Al2O3+5%ZrO2 prepared by electrophoretic deposition and pressureless sintering has been studied. Hardness and fracture toughness were measured using indentation methods on cross sections of samples. From the difference between lengths of cracks parallel and perpendicular to layers the residual stresses (arisen due to the thermal expansion coefficient mismatch) were calculated, their profile along the cross section has been estimated and compared with values obtained using a finite element model.
Authors: G. Guidoni, Y. Torres Hernández, Marc Anglada
Abstract: Four point bending tests have been carried out on a thermal barrier coating (TBC) system, at room temperature. The TBC system consisted of a plasma sprayed Y-TZP top coat with 8 % in weight of Yttria, a bond coat of NiCrAlY and a Ni-based superalloy Inconel 625 as substrate. The TBC coating was deposited on both sides of the prismatic specimens. Efforts have been done in detecting the damage of the coating by means of Maltzbender et al [1] model.
Authors: Johann Brandstetter, Petr Glogar, Dieter Loidl, Karl Kromp
Abstract: Unidirectionally reinforced ceramic matrix composites (CMC) with matrix derived by pyrolysis of polymethylsiloxane or polymethylphenylsiloxane resins and reinforced with Nicalon or Nextel fibres were investigated. In order to assess and mutually compare their mechanical properties and stability when exposed to hot air up to 1200°C their elasticity moduli were measured prior to and after this exposure. The oxidation - induced damage of the material was monitored also in terms of microstructural features. Moreover, an attempt was undertaken to assess the bonding level by monitoring the push - through of macroscopic bundles of fibres using a flat indentor.
Authors: Miroslav Černý, D. Bednářová, Petr Glogar, Ján Dusza, Emőke Rudnayová
Abstract: Mechanical and fracture properties of unidirectional composites reinforced with R-glass fibres and utilizing various commercially available polysiloxane resins as matrix precursors were investigated. As the matrix becomes more brittle after the pyrolysis the impact toughness and flexural strength of the composites fall. On the other hand, the shear modulus rises after the pyrolysis as the matrix becomes stiffer in shear. The appearance of fracture surfaces generated during the flexural strength at room temperature (RT) and elevated temperatures is discussed.
Authors: Sabina Beranič Klopčič, Saša Novak, Tomaž Kosmač, H.G. Richter, S. Hecht-Mijic
Abstract: Functionally graded alumina/zirconia-toughened alumina (ZTA) ceramics for biomedical applications were designed with a hard, wear-resistant alumina surface and a tough, zirconia-rich core. The main goal was to achieve an improved mechanical performance by introducing a residual compressive stress in the outer surfaces. Symmetrical, 7-layered samples were prepared by the sequential slip-casting of specimens from suspensions prepared at the inherent pH values, close to the pH values at which the maximum (negative) zetapotential was measured. No sintering or tunneling cracks were observed in the samples, but large defects, such as big agglomerates, air bubbles and large circular pores at layer interfaces, which resulted from preparing the samples using the slip-casting technique, were readily observed. The strength of the symmetrical composites is influenced by the presence of defects, which most probably originate from the shaping process. Therefore, the defects have to be regarded as the reason why the strength of the 7-layered composites is low.
Authors: J. Briančin, Ľubomír Medvecký, Helena Bruncková, M. Kmecová
Abstract: [Pb(Zr,Ti)(OAc)x(OR)y] acetate-alkoxide precursor was prepared by chelating from organic Pb, Zr and Ti alkoxides by their dissolution in various amounts of acetic acid (AcOH). The optimum mole ratio of (Pb+Zr+Ti) to AcOH for preparation gel is 1:7 and a pure perovskite phase is formed at 500 °C. At a low AcOH concentration, two-phase (pyrochlore and perovskite) regions are formed during gellation. During sintering, the decomposition of a pyrochlore phase and the formation of a perovskite phase takes place Dependences of dielectric permitivities and losses on temperature in final PZT ceramics were very similar and they were not influenced by the type of calcinate.

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