Microstructure and mechanical properties of yttria-stabilized tetragonal zirconia polycrystals containing dispersed TiC particles

The microstructure and mechanical properties of hot-pressed yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramics containing up to 30 vol % TiC particles were studied. Adding TiC particles to Y-TZP improved the bending strength and fracture toughness. With 20 vol% TiC particles the maximum bending strength and fracture toughness reached 1073±30.4 MPa and 14.56±0.25 MPa m^1/2, respectively. The residual tensile stress induced by the thermal expansion difference between ZrO₂ and TiC must have inhibited the tetragonal-monoclinic transformation. The stress-induced phase transformation was therefore not the dominant toughening mechanism. High-densities of dislocations within TiC particles and microcracking were detected by TEM. The improved toughness of the materials is considered to be the result of crack deflection, crack bowing of TiC particles and microcracking toughening of ZrO₂.     

EPR structural study on hydrothermally aged yttria-doped tetragonal zirconia polycrystals

EPR measurements on yttria-doped tetragonal zirconia polycrystals (Y-TZP) were performed in order to study the modifications of the structure of the material following hydrothermal treatments. By X-ray irradiation of the samples, the structural defects near the oxygen vacancies were magnetically activated by electron trapping. Y₂O₃ aggregates were identified by EPR signals of defects of F⁺ type, coordinated with yttrium ions, and defects such as Y²⁺ (4d¹) in four-fold coordination, randomly tetragonally distorted. The analysis of these defect sites, used as local probes, supports the existence of structural modification of the material and the formation of -Y(OH)₃ clusters.     

Structural changes and mechanical properties of CeO2-doped tetragonal zirconia polycrystals

IICT Communication No. 2528     

Multiphase composites of tetragonal zirconia agglomerate dispersed into alumina and alumina-zirconia matrices

Multiphase composites of yttria- and ceria-doped tetragonal zirconia agglomerates (10–50 m) dispersed into an alumina or alumina-zirconia matrix were sintered at 1500–1600 °C in air, followed by post-Hot Isostatic Pressing (HIP) at 1450°C and 150 MPa in an Ar gas atmosphere. The relative density of the recovered composites was above 98% of the theoretical density. By chemically etching on the surface of zirconia agglomerates, the sinterability of composites was apparently improved; and no microcracks nor pores were observed at the interface of agglomerate and matrix. According to scanning electron microscopy (SEM) observation, tetragonal and tetragonal-monoclinic zirconia agglomerates were highly dispersed into the alumina or alumina-zirconia matrix. The multiphase composites containing 10 vol% spherical agglomerates demonstrate the relatively low value of bending strength, < 400 MPa, and a high value of fracture toughness, > 11 MPa m^1/2. The crack propagation introduced by Vickers indentation was efficiently suppressed and deflected by the agglomerates.     

Preparation and characterization of precursor powders for yttria-doped tetragonal zirconia polycrystals (Y-TZP) and Y-TZP-Al2O3 composites

Precursor powders for the preparation of tetragonal 2.5 mol% Y₂O₃-ZrO₂ containing 0 to 30 wt% Al₂O₃ were made by coprecipitation. The behaviour of this powder during calcination from room temperature to 1200° C was studied using differential thermal analysis. X-ray diffraction and transmission electron microscopy methods, and measurements of surface area. The uncalcined powder was essentially amorphous. On heating alumina-free powder, zirconia crystallized at 485° C: for increasing alumina content, zirconia crystallized from an amorphous aluminous matrix at increasing temperatures (850° C for 20 wt% Al₂O₃), while the crystallite size decreased and the surface area of the powder increased. The zirconia first crystallized as cubic, but transformed to the tetragonal form near 1100° C. The alumina crystallized as corundum at 1200° C. No monoclinic zirconia could be detected when calcined aluminous material was cooled to room temperature. The sintering behaviour of the calcined powder is discussed.     

Microstructure and mechanical properties of ZrO2-Gd2O3 tetragonal polycrystals

The phases, transformability, microstructure and mechanical properties of ZrO₂-Gd₂O₃ polycrystals containing 1.75–8 mol% Gd₂O₃ were studied. The samples were prepared by a coprecipitation route followed by sintering at 1400°C for 2 hours. The grain size was in the range of 0.1–0.2 m except for some large grains at high Gd₂O₃ contents. Only a tetragonal phase was observed between 2–4 mol% Gd₂O₃ and a cubic phase for compositions containing 9.6 mol% Gd₂O₃. A peak K _IC of 12 MPa m^1/2 and a strength of 800 MPa were obtained in the 2 mol% Gd₂O₃ alloy for which the t m transformation on the fracture surface was also found to be maximum. Transformation toughening is able to account for most of the toughness of the samples.     

Frequency dispersion of the internal friction in tetragonal and cubic zirconia polycrystals stabilized with yttria

A forced vibration method for measuring the frequency dispersion of internal friction under cyclic-tensile-compressive stress on the single axis was developed so that various mechanical tests can be conducted using a specimen having the same shape, machining, and sintering conditions. With this new method, frequency dispersions of the internal friction for tetragonal and cubic zirconia polycrystals stabilized with yttria were measured. The activation energy for the relaxation time was determined. As a result of comparison with the activation energy for the relaxation time measured by a conventional method, we conclude that the results of the internal friction measured in this study are valid. The activation energy for the ionic conductivity was determined, and compared with that for the relaxation time. The activation energies for the relaxation time in 2.6YSZ, 3YSZ, and the lower temperature peak of 10YSZ are very similar to those for the conductivity. As a result, we conclude that the internal friction peaks of 2.6YSZ and 3YSZ, and the lower temperature peak of 10YSZ are derived from rearrangement of point defects associated with oxygen ion vacancies.     

Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams

Silicon nitride bonded silicon carbide foams have been produced by nitridation of the foamed compacts containing silicon carbide and silicon powders. When no nitridation additive was used the ceramic foams nitrided at all temperatures studied contained a significant amount of whisker phase α-Si₃N₄ formed both inside and outside the cell walls leading to a loose microstructure and a low mechanical strength. When the Al₂O₃ and Y₂O₃ were used as nitridation additives, the ceramic foams nitrided at temperatures of 1360 and 1395 °C containing certain amount of Si₂N₂O and whisker α-Si₃N₄ phases that are bonded by a glassy phase and behave as reinforcements for the ceramic foams exhibited a much higher mechanical strength. At nitridation temperature of 1430 °C, the ceramic foam showed the locally formed β-Si₃N₄ as the main nitrided phase that caused no increase in bonding area between the nitrided phase and the silicon carbide particles. Thus, a relatively lower mechanical strength was observed for the ceramic foam.     

Nanocrystalline alumina dispersed in nanocrystalline nickel: enhanced mechanical properties

Nickel–alumina nano-composites have been electrochemically deposited by pulse plating from a suspension of nano-Al₂O₃ in a Watts-type electrolyte. The influence of duty cycle and amount of suspended Al₂O₃ on the content of Al₂O₃ in the deposit was studied. With an optimized set of plating parameters, the influences of additives on wear resistance, hardness and the deformation behaviour (quasistatic and dynamic compression tests) of these nickel–alumina nano-composites in comparison to pure nano-nickel were investigated. The addition of Al₂O₃ tripled the yield stress and improved the hardness up to twice the value of pure nickel. Due to its high hardness and stiffness, the nickel–alumina composites deposited with the additive naphthalene-1,3,6-trisulfonic acid trisodium salt in the electrolyte are appropriate to wear resistant coating. Nickel–alumina nano-composites deposited without any additives are hard and at the same time ductile and so considered as ideal structural materials.