Grain Size Dependence of Hardness in Dense Submicrometer Alumina
TL;DR: In this article, pressureless sintered alumina compacts with a submicrometer microstructure exhibit a hardness that approaches or even exceeds the level of advanced hot-pressed composites of Al{sub 2 O{sub 3} + 35 vol% TiC, whereas the strength of both ceramics is approximately the same.
Abstract: Pressureless sintered alumina compacts with a submicrometer microstructure exhibit a hardness that approaches or even exceeds the level of advanced hot-pressed composites of Al{sub 2}O{sub 3} + 35 vol% TiC, whereas the strength of both ceramics is approximately the same. The combination of reduced dislocation mobility (due to the small grain size), high density, and density homogeneity are the prerequisites for the surprisingly high hardness. Quasi-conventional powder processing is used to produce these outstanding alumina bodies.
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TL;DR: In this article, a systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al 2 O 3 powder.
Abstract: A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al 2 O 3 powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al 2 O 3 could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21-22 GPa and fracture toughness values of 3.5 ± 0.5 MPa.m 1/2 were found for the compacts containing submicrometer-sized Al 2 O 3 grains.
729 citations
TL;DR: The structure, properties, applications, and limitations of the ceramics that have been used in orthopedic bearings are reviewed, and the new ceramic composite materials and surface treatments that will be available for joint replacement surgery in the near future are described.
Abstract: The most commonly used bearing couple in prosthetic hip or knee joint replacements consists of a cobalt–chrome (CoCr) metal alloy articulating against ultrahigh-molecular-weight polyethylene. Ceramics have been used as an alternative to metal-on-polyethylene in joint replacement surgery of arthritic hips and knees since the 1970s. In prosthetic hip and knee bearings, ceramic surfaces offer a major benefit of drastically reduced wear rates and excellent long-term biocompatibility, which can increase the longevity of prosthetic hip and knee joints. This benefit is important clinically because hip and knee replacement has become a very common surgical procedure, particularly in the United States, and because these procedures are being increasingly performed in younger patients who place greater demands on the prosthetic bearings. However, ceramics are brittle and the risk of catastrophic bearing failure in vivo, while rare, is a major concern. Improvements in material quality, manufacturing methods, and implant design have resulted in a drastic reduction of the incidence of such failures, so that modern ceramic bearings are safe and reliable if used with components of proven design and durability. Future material improvements are actively being investigated to reduce the risk of ceramic-bearing failures even further. The purpose of this article is to review the structure, properties, applications, and limitations of the ceramics that have been used in orthopedic bearings, and to describe the new ceramic composite materials and surface treatments that will be available for joint replacement surgery in the near future.
301 citations
TL;DR: A review of recent developments in the design and manufacture of precision, fixed-abrasive tools is presented in this paper, with examples showing how the components have been enhanced to achieve their current high levels of performance.
204 citations
TL;DR: In this paper, the authors developed a method to associate minimum grain sizes at highest densities with the lowest population of macro-defects by using powders with particle sizes in the range of 100-200 nm.
Abstract: Sintered corundum components with submicrometer grain sizes exhibit properties which enable numerous new applications. Wet powder processing is developed to associate minimum grain sizes at highest densities with the lowest population of macrodefects. A closest ratio of powder particle size and sintered grain size is important for obtaining most fine-grained microstructures. This target was approached best by using powders with particle sizes in the range of 100-200 nm rather than with smaller nanoparticles.
198 citations
TL;DR: In this article, fuel porosity, volume pore density and pore size distributions were determined as a function of the radial position for specimens with average burn-ups between 40 and 67 GWd/tM.
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TL;DR: In this paper, transmission electron microscopy was used to show that high densities of dislocations are produced within the near surface regions by mechanical polishing with a fine diamond compound (0.25 μm).
Abstract: Transmission electron microscopy provided direct evidence that plastic deformation occurs during the room-temperature indentation and abrasion of Al2O3. Examination of single-crystal and polycrystalline specimens showed that high densities of dislocations are produced within the near-surface regions by mechanical polishing with a fine diamond compound (0.25 μm) and that plastic deformation by both slip and mechanical twinning occurs during the placement of Vickers microhardness indentations. The occurrence of plastic deformation in this normally brittle material is considered to be a consequence of the nature and magnitude of the local stresses developed under pointed indenters and irregularly shaped abrasive particles. Preliminary results on the effect of annealing on the retained substructure are also presented. Annealing at 900°C and higher resulted in the reduction of residual stresses through the motion of dislocations and their rearrangement into lower-energy configurations.
249 citations
TL;DR: In this article, a solute-clustering model is proposed for grain boundary quench-hardening based on preferential solutevacancy interactions and non-ideal thermodynamic behavior of the binary system.
226 citations
TL;DR: Al2O3-TiC, Al2O 3-HfB2 powders were synthesized by the aluminothermic reduction of oxides in the presence of carbon or boron, and reacted powders are milled to reduce the size of agglomerates and subsequently densified without applied pressure to near theoretical density.
Abstract: Al2O3-TiC, Al2O3-TiC05N05, Al2O3-WC, Al2O3-SiC, and Al2O3-HfB2 powders were synthesized by the aluminothermic reduction of oxides in the presence of carbon or boron The reacted powders were milled to reduce the size of agglomerates and subsequently densified without applied pressure to near-theoretical density Microstructures and mechanical properties of composites made from exothermically reacted powders were compared with similar ceramics made from commercially available powders In situ sintering was possible in the Al2O3-TiC system using a closed graphite crucible to contain reaction gases The synthesis of β-SiC at temperatures above 1400°C via the direct reaction of the elements (SHS) was compared with SiC made by the magnesiothermic reduction of SiO2 in the presence of C after removing the MgO by leaching
49 citations
47 citations