Paul Blank

Bio: Paul Blank is an academic researcher from Fraunhofer Society. The author has contributed to research in topics: Ceramic & Grain size. The author has an hindex of 8, co-authored 23 publications receiving 874 citations.

Transparent Sintered Corundum with High Hardness and Strength

Abstract: Commercial corundum powder and a liquid-shaping approach are used for manufacturing complex hollow components and large flat windows of sintered and hot isostatically pressed Al2O3 ceramics having grain sizes of 0.4–0.6 μm at relative densities of >99.9%. High macrohardness (HV10 = 20–21 GPa) and four-point bending strength (600–700 MPa; 750–900 MPa in three-point bending) are associated with a real in-line transmission of 55%–65% through polished plates. The submicrometer microstructure and the optical properties can be retained for use at >1100°C using dopants that shift the sintering temperature to high values without additional grain growth.

Grain Size Dependence of Hardness in Dense Submicrometer Alumina

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.

Processing of High‐Density Submicrometer Al2O3 for New Applications

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.

The Influence of shaping method on the grain size dependence of strength in dense submicrometre alumina

Abstract: The grain size dependence of the strength of pressureless sintered aluminas is investigated with specimens fabricated by uniaxial pressing, cold isostatic pressing, pressure filtration, gel casting, and combinations thereof. The strength depends on the flaw population and the observed grain size effect is different with different shaping approaches: with reduced grain sizes the diversity of measured strength averages broadens including increasingly high values for some of the shaping procedures only. Submicrometre grain sizes are, however, not an indispensable prerequisite for a high strength: grain sizes of 1–2 μm are sufficient to achieve 800–900 MPa by pressureless sintering, but present microstructures with grain sizes

Advances in the Grinding Efficiency of Sintered Alumina Abrasives

Abstract: The study relates the grinding power of different grades of sintered alumina abrasives to their microstructures and to basic mechanical properties in comparison with conventionally fused electrocorundum and with an electrofused alumina/ zirconia eutectic. Contrary to the traditional approach of the Battelle test, the fracture toughness KIc of individual grains is measured by a quantitative indentation analysis. Compared with fused corundum, sintered alumina grits exhibit an increased toughness and grinding efficiency, but the further increase of KIc in the eutectic does not improve the grinding performance. The key parameter for grinding is the inherent hardness of the abrasive. The elimination of flaws by a new approach results in a strong increase in the grinding power of sintered alumina abrasives.

Transparent Alumina: A Light-Scattering Model

Abstract: A model based on Rayleigh-Gans-Debye light scattering theory has been developed to describe the light transmission properties of fine-grained, fully dense polycrystalline ceramics consisting of birefringent crystals. This model extends light transmission models based on geometrical optics, which are only valid for coarse-grained microstructures, to smaller crystal sizes. We verify our model by measuringthe light transmission properties of fully dense (>99.99%) polycrystalline alpha-alumina (PCA) with mean crystal sizes ranging from 60 mm down to 0.3 mm. The remarkable transparency exhibited by PCA samples with small crystal sizes (< 2 mm) is very well explained by thismodel.

Spark Plasma Sintering of Alumina

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.

Transparent Sintered Corundum with High Hardness and Strength

Abstract: Commercial corundum powder and a liquid-shaping approach are used for manufacturing complex hollow components and large flat windows of sintered and hot isostatically pressed Al2O3 ceramics having grain sizes of 0.4–0.6 μm at relative densities of >99.9%. High macrohardness (HV10 = 20–21 GPa) and four-point bending strength (600–700 MPa; 750–900 MPa in three-point bending) are associated with a real in-line transmission of 55%–65% through polished plates. The submicrometer microstructure and the optical properties can be retained for use at >1100°C using dopants that shift the sintering temperature to high values without additional grain growth.

Ceramics for Prosthetic Hip and Knee Joint Replacement

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.