Nils Claussen

Strength and fracture toughness of aluminum/alumina composites with interpenetrating networks

The mechanical properties of metal reinforced ceramics, especially Al/Al2O3 composites with interpenetrating networks, are described. Key parameters to tailor the characteristics of these materials are the ligament diameter and volume fraction of ductile reinforcement. Fracture strength and fracture toughness data are given as a function of both variables and are compared with the corresponding values for the porous preforms. A simple model accounts for the influence of metal volume and metal ligament diameter on the plateau toughness of the composites. The increase in fracture strength from the porous preform to the composite is found to be much larger than the gain which can be predicted from the increase in fracture toughness alone. A discussion of fracture strength in these composites therefore must include at least two issues, crack propagation through the matrix as well as crack initiation at metal filled pores.

Strengthening strategies for ZrO2-toughened ceramics at high temperatures☆

Abstract Transformation-toughened (i.e. ZrO2-toughened) ceramics represent a new class of high performance ceramics with spectacular strength properties at low and intermediate temperatures. However, at temperatures above about 700°C, most of these tough oxide-base ceramics can no longer be used as load-bearing engineering parts because of characteristic deficiencies. The aim of the present paper is to provide and discuss microstructural design strategies which may enable ZrO2-toughened ceramics to be applied to higher temperatures. From the various strategies suggested, three appear to show good prospects, namely (i) the prevention of glassy intergranular films, (ii) the addition of hard high modulus particles and (iii) whisker or fibre reinforcement. Experimental approaches are presented for some ZrO2-toughened ceramics, e.g. tetragonal ZrO2 polycrystals and ZrO2-toughened cordierite, spinel and mullite.

Low-shrinkage reaction-bonded alumina

Abstract A novel method of manufacturing strong Al 2 O 3 bodies with low ( 2 O 3 powder mixtures are heat treated at temperatures between 1200 and 1550°C such that the expansion due to the Al→Al 2 O 3 reaction and the shrinkage on sintering of Al 2 O 3 is nearly balanced. Depending on processing parameters such as Al/Al 2 O 3 ratio, compaction pressure and heating cycle, a variety of different microstructures can be developed. In spite of some residual porosity, strengths of ∼300 MPa are attained.

Reaction bonding of aluminum oxide (RBAO) composites: Processing, reaction mechanisms and properties

Abstract As an alternative to conventional Al 2 O 3 manufacturing, a novel technique, reaction bonding of aluminum oxide (RBAO), has been successfully developed. Although the technique is in its infancy, it has already demonstrated interesting characteristics, such as high green strength without organic additives, low-to-zero shrinkage tailorability, superior mechanical properties, superplastic transformability and broad microstructural versatility. One of the important technical potentials is to use RBAO as a matrix for large-scale second-phase particles, e.g. platelets and fibers, without causing the harmful residual stresses normally encountered with shrinking matrix materials.

REACTION SINTERING OF ALUMINA-ALUMINIDE ALLOYS (3A)

A novel pressureless reaction sintering process is presented for the fabrication of Al{sub 2}O{sub 3}{endash}aluminide alloys (3A). Compacts of intensively milled metal oxide{endash}aluminum mixtures are heat-treated in vacuum or inert atmosphere such that the exothermic reactions take place in a controlled manner essentially at temperatures below the melting point of Al. Dense, homogeneous microstructures were obtained with a variety of Al{sub 2}O{sub 3}-matrix systems with interpenetrating networks of aluminides of Ti, Fe, Nb, Mo, Zr, Ni, etc. By adding modifiers in the form of oxides or metals, volume and phase composition as well as properties can be tailored in a wide range. {copyright} {ital 1996 Materials Research Society.}

Reaction synthesized Al2O3-based intermetallic composites

Abstract Intermetallics-reinforced Al 2 O 3 composites have been fabricated by reaction sintering compacts of intensively milled powder mixtures containing either metal oxides and Al or elemental metals, Al and Al 2 O 3 . During carefully controlled heat-treatment in non-oxidizing atmosphere, the metal oxides, for instance TiO 2 , Fe 2 O 3 and Nb 2 O 5 , are reduced by Al to form the respective aluminides (Ti x Al y , Nb x Al y ,). Alternatively, aluminides can also be formed through reaction between Al and elemental metals (e.g. Fe). By adding Al 2 O 3 to the starting mixture, the intermetallic/ceramic ratio of these alumina-aluminide alloys (3A) can be adjusted within a wide range. At intermetallic volume fractions ⪢~20 vol. %, both phases are continuous, exhibiting a microstructure similar to that of products of directed metal oxidation or reactive metal penetration processes. In this paper, systems based on Ti x Al y , FeAl and NbAl 3 are emphasized. The principles of the reaction synthesis process and the influence of the processing parameters on microstructural development and mechanical properties are described.

Ni3AlAl2O3 composites with interpenetrating networks

Homogenous microstructures of Ni3Al/A2O3 composites can be produced by gas pressure infiltration of the intermetallic phase into the porous ceramic. Since the nickel aluminide is single crystalline in the reinforcing ligaments, it exhibits ductile deformation and provides appreciable toughening for the composite. The mechanical property data of this material at room temperature and at 1073 K are comparable.

Corrosion screening tests of high-performance ceramics in supercritical water containing oxygen and hydrochloric acid

Abstract The corrosion of various ceramic materials in simulated supercritical water oxidation (SCWO) environment was measured. Supercritical water with 0.44 mol kg −1 oxygen and 0.05 mol kg −1 hydrochloric acid was used to simulate typical SCWO conditions after the decomposition of the organic material. The experimental temperature was 465 °C and the pressure 25 MPa. The experiments were performed within a reactor with an inner surface made of alumina. In this very corrosive fluid only a few Al 2 O 3 - and ZrO 2 -based materials did not corrode severely. Homogeneous surface attack and grain boundary diffusion were observed. HIP-BN, B 4 C, TiB 2 , Y 2 O 3 and Y-TZP disintegrated. SiC and Si 3 N 4 -based materials showed a large weight loss, up to above 90%.

Reliability of alumina ceramics : effect of grain size

The grain size dependence of fracture strength and Weibull modulus of alumina using a high purity, commercial starting powder was investigated. In the regime of an average grain size between 1.7 and 11 μm, fracture strength increases with decreasing grain size. This behaviour could be explained quantitatively by modelling the failure-causing defect as either a spherical pore or a hemispherical surface pit with a circumferential crack proportional to the average grain size. No dependence of Weibull modulus on average grain size and hence on R-curve behaviour could be observed. Theoretical considerations show that the closure stresses in alumina responsible for the R-curve behaviour are too low to affect the Weibull modulus.

Yttria- and ceria-stabilized tetragonal zirconia polycrystals (Y-TZP, Ce-TZP) reinforced with Al2O3 platelets

Abstract Composites of yttria- and ceria-stabilized tetragonal zirconia polycrystals (Y-TZP, Ce-TZP) with up to 20 vol% alumina platelets were fabricated by hot isostatic pressing. The microstructure and mechanical properties of these materials were investigated. The fracture toughness of Y-TZP increased by addition of platelets, e.g. KIc of 2Y-TZP from 8·2 M Pa m to 9·5 M Pa m with 5 vol% Al2O3-platelets. The fracture strength of this composite, however, decreased from 1430 MPa to 735 MPa. Such decrease in strength was generally observed in Y-TZP platelet composites investigated in this work. Platelet-reinforced Ce-TZP exhibited a slight increase in strength and a decrease in fracture toughness in comparison to the non-reinforced matrix.