Phase Constitution and Dynamic Properties of Spark Plasma‐Sintered Alumina–Titanium Composites

Al₂O₃/Ti composites of various metal to ceramic ratios were fabricated by the spark plasma sintering (SPS) technique. The effects of titanium concentration in the initial mixture on phase composition, and on the static and dynamic (planar impact testing) mechanical properties of the SPS‐processed composites were investigated. It was observed that the significant alumina dissolution in titanium takes place during SPS treatment. The composites fabricated from starting alumina/titanium powder mixtures with a mass fraction of titanium less than 0.8 consisted of two phases, alumina and a solid solution of oxygen and aluminum in titanium. For starting mixtures with higher titanium content, the presence of a Ti₃Al intermetallic phase with a relatively low fraction of dissolved oxygen was detected. Changes in phase composition could explain the effect of titanium content in the starting mixture on physical and mechanical properties of the composites. Mechanisms governing the dynamic response of the composites under loading of different intensities are discussed.     

The Effect of Grain Size on the Mechanical and Optical Properties of Spark Plasma Sintering‐Processed Magnesium Aluminate Spinel MgAl2O4

The study deals with the effect of the SPS parameters and LiF doping on the mechanical and optical properties polycrystalline magnesium aluminate spinel (PMAS) with emphasis on the grain size of the final product. Sintering at 1300°C of undoped powder yielded fully dense submicrometer (0.4–0.6 μm) samples with elevated mechanical properties (1600HV and 300MPa bending strength). Doped samples had a larger, 40 μm grain size, lower, 1450HV, hardness and 150MPa bending strength. The transmittance of the doped samples (80% at 500 nm wavelength) was higher than that of the undoped ones. Thus, the required functionality of the ceramic dictates the choice of parameters for the fabrication of dense transparent PMAS.     

Randomized or probabilistic Hough transform: unified performance evaluation

Rapid computation of the Hough transform is necessary in very many computer vision applications. One of the major approaches for fast Hough transform computation is based on the use of a small random sample of the data set rather than the full set. Two different algorithms within this family are the randomized Hough transform (RHT) and the probabilistic Hough transform (PHT). There have been contradictory views on the relative merits and drawbacks of the RHT and the PHT. In this paper, a unified theoretical framework for analyzing the RHT and the PHT is established. The performance of the two algorithms is characterized both theoretically and experimentally. Clear guidelines for selecting the algorithm that is most suitable for a given application are provided. We show that, when considering the basic algorithms, the RHT is better suited for the analysis of high quality low noise edge images, while for the analysis of noisy low quality images the PHT should be selected.     

Influence of coatings on microstructure and mechanical properties of preceramic paper-derived porous alumina substrates

Preceramic papers loaded with inorganic fillers may be used as preforms in a novel manufacturing technique to fabricate lightweight ceramic structures. In order to reduce the porosity caused by burning out cellulosic fibers and organics, porous preceramic paper-derived alumina substrates were post-treated via two different coating routes using silica suspension or methylphenylvinylhydrogen polysiloxane. Sintering of the alumina-filled preceramic papers in air at 1600 °C for 2 h resulted in a non-uniform distributed open porosity ranging from 23 to 26%. After coating and infiltration, all samples were additionally heat treated up to at 1500 °C for 2 h. Thermal analysis (DTA/TG) was applied to determine the pyrolysis temperature of polysiloxane. Microstructure and phase analysis were performed respectively by SEM and XRD. After sintering, water absorption, apparent density and open porosity of test pieces were determined, and mechanical properties of the substrates were evaluated before and after coating. For the samples coated with silica suspension, the mechanical strength remained in the same range of those for uncoated samples, while for the polysiloxane coated samples the mechanical strength steadily increases after repeated impregnation steps, reaching ～350 MPa.     

Corrugated glass–ceramics from LZSA cast tapes

The aim of this work was to fabricate corrugated lightweight structures from green tapes of Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) glass–ceramics. The green tapes were cast, thermopressed, corrugated, and sintered at 800 °C for 1 h, yielding low porous and homogenous microstructures confirmed by electron microscopy analysis. Dilatometric analysis demonstrated three linear regions, each one exhibiting a specific thermal expansion coefficient (TEC) in the 25–800 °C temperature range. It was also concluded that 600 °C was the critical temperature above which the LZSA glass–ceramic structure could not be assembled. The fabricated bodies were also characterized macroscopically and geometrically to verify the integrity and the morphology of the structures.     

Preceramic Paper Derived Alumina/Zirconia Ceramics

Preceramic paper offers a novel approach for manufacturing of lightweight ceramic structures applying versatile paper shaping technologies. Substitution of bioorganic pulp fibers in alumina loaded preceramic paper by inorganic short zirconia fibers was investigated. A retention of more than 90 wt% was achieved by a combination of cationic and anionic retention aids. Powder packing density in the paper sheet decreased with increasing amount of non‐deformable zirconia fibers used to substitute highly deformable pulp fibers. Applying post‐pressing, however, resulted in a pronounced improvement of packing density while retaining excellent flexibility and strength of the preceramic paper preform. Thus, preceramic paper containing a zirconia fiber volume fraction of 19% sintered at 1 600 °C attained a rupture strength measured by ball‐on‐three‐balls loading of 120 ± 17 MPa (porosity 44%) which after pressure consolidation of the paper increased to 180 ± 49 MPa (porosity 28%).