Jacques Crampon

Superplastic deformation mechanisms during creep of alumina-zirconia composites

Abstract The high temperature creep of three alumina-zirconia composites (alumina volume fraction: 20%, 50% and 80%) and of the end-constituents (Y-PSZ and alumina) has been studied as a function of stress (4–200 MPa), temperature (1275–1400°C) and grain size (0.4–2 μm). The deformation of the end-constituents results from two serial mechanisms (interface reaction and diffusion, preponderant at low or high stress, respectively). The shape of the creep rate-stress curves of composites was similar to that of the end-constituents, suggesting similar deformation mechanisms. The influence of the alumina volume content on the creep rate of the composites has been analysed by various models. The model that accounts best for the composite behaviour is a “three-phase” model. The influence of grain size shows that the creep rate accommodation is connected with alumina-zirconia interfaces.

Elaboration of low shrinkage mullite by active filler controlled pyrolysis of siloxanes

Abstract The active filler controlled pyrolysis of polymers has been used to synthesize mullite from Al 2 O 3 -filled, Al-filled and Al/Al 2 O 3 -filled siloxanes. Due to the presence of transition alumina and to a finer scale of mixing of the mullite precursors provided from active Al-filler, the beginning of mullitization temperature was lowered in the two last systems (1200–1250°C) in comparison to the first one (1400°C). The mullite equiaxed-grains obtained from the polymer/Al 2 O 3 (PAO) system contained a large amount of dislocations whereas in polymer/Al (PA) and polymer/Al/Al 2 O 3 (PAAO) derived samples they were primarily dislocation free. The presence of small amounts of residual glassy phase was observed as glassy pockets at triple-junctions in the whole of the materials. The improved microstructure obtained with the Al/Al 2 O 3 -filled mixture indicates that this system seems to be a good choice for a low shrinkage and low defect mullite, in comparison to mullite obtained from Al-filled and Al 2 O 3 -filled siloxanes.

Investigation of grain boundary sliding during superplastic deformation of a fine-grained alumina by atomic force microscopy

The contribution of grain boundary sliding to total strain has been investigated in a superplastically deformed fine-grained alumina by atomic force microscopy in contact mode. The analysis of the surface relief and of its change with strain allowed us to determine the vertical component of grain boundary sliding as a function of strain up to 16%. Grain boundary sliding has been thus estimated to contribute to about 70% of total strain. This value is compared with results published on alumina ceramics or superplastic metallic alloys. The reliability and the limitation of the method are also discussed.

High temperature creep behaviour of β′-Si3N4α-YSiAlON ceramics

Abstract The improvement in high temperature mechanical properties of silicon nitride ceramics has been explored by fabrication of a β′-Si 3 N 4 α-YSiAlON ceramic densified, without additives and without HIP treatment, by sintering of a mixture of α-Si3N4 and α- YSiAlON powders under a nitrogen pressure of 5 MPa. The high temperature creep behaviour was investigated in the ranges 100–400 MPa and 1280–1400 ° C by compressive tests. The creep performances were very impressive. The activation energy (Q = 770 kJ/mol) and the stress exponent (n = 1) are consistent with a deformation mechanism resulting from grain boundary sliding accommodated by volume diffusion. Microstructure was not affected by the deformation conditions, in particular no cavity formation was observed after creep at 1400 °C and 300 MPa. This behaviour is related to the probable absence of a thick boundary film which may result from crystallization during cooling of small glass pockets and from internal stresses caused by the volume change of crystallizing pockets.

Characterization of grain boundary sliding in a fine-grained alumina–zirconia ceramic composite by atomic force microscopy

Abstract Grain boundary sliding has been investigated on the three kinds of interface during the superplastic deformation of a fine-grained alumina–zirconia ceramic composite by atomic force microscopy. The changes in surface topography on both sides of the same boundaries have been characterized throughout specimen deformation. In the experimented deformation conditions (1325°C, 60 MPa) the three kinds of interface exhibit similar sliding mobilities, within 20%, and in average grain boundary sliding contributes to more than 80% of the total strain. The stress redistribution, associated to the difference in phase ductility, has been estimated. The results are in agreement with the values that can be calculated by an isostrain model or a three phase model.

Formation at low temperature with low shrinkage of polymer/Al/Al2O3 derived mullite

Abstract The active filler controlled pyrolysis of polymer has been used to synthesize mullite from Al/Al 2 O 3 -filled siloxane. This monolith has been compared to mullite synthesized in the same conditions from Al-filled and Al 2 O 3 -filled siloxanes. Due to the presence of transition alumina and to a finer scale of mixing of the mullite precursors provided from the active Al-filler, the initial mullitization temperature was lowered in the two first systems (1200–1250°C) in comparison to the last one (1400°C). However, the improved microstructure obtained with the Al/Al 2 O 3 -filled mixture indicates that this system has advantages of low temperature and shrinkage, and low defect mullite, in comparison to mullite obtained from Al-filled and Al 2 O 3 -filled siloxanes.

Grain boundary sliding-induced deformation in a 30 wt% zirconia-spinel composite : Influence of stress

Abstract The contribution of grain boundary sliding to total strain has been investigated in a 21 vol% zirconia–spinel composite crept under stresses of 12 and 90 MPa. To this goal, the surface topography and its changes with strain were investigated on a face parallel to the compression axis by atomic force microscopy in contact mode. Due to the low zirconia content, only sliding on spinel–spinel (S–S) and spinel–zirconia (S–Z) boundaries really contributes to strain and was consequently analysed. Insensitive to stress value, boundary sliding can account for 70–80% of the total strain. However, if the two investigated interfaces behave similarly at 90 MPa, at 12 MPa sliding on S–Z boundaries is larger than on S–S ones. That difference is to relate to a stress–strain rate sensitivity dependent on stress, 1.8 and 4.2 at 90 and 12 MPa, respectively, an increase in the stress exponent able to be induced by the existence of a threshold stress that would concern spinel–spinel boundaries.

Upgrading superplastic deformation performance of fine-grained alumina by graphite particles

The process of ceramic part forming is often accompanied by grain growth, which in turn limits the deformation capabilities. Alumina ceramics are not usually good candidates for superplasticity as the grain growth reduces quickly their deformation performance. In order to impede the grain boundary mobility and to limit the grain growth, zirconia has often been associated with the alumina since the early 80s. In this work, a dense MgO-doped α-alumina containing a small amount of carbon has been produced. The sintering and the deformation behaviour of this material are compared to those of a MgO-doped alumina. The presence of small carbon particles reduces greatly the grain growth during the sintering and during the compression experiments. The superplasticity of the carbon-containing alumina is greatly improved.

Microstructural superplastic deformation in MgO · Al2 O3 spinel

Abstract Polycrystals of hot-pressed and then hot-isostatically-pressed MgO · Al2O3 spinel, having large grains of about 10–20 μm embedded in a fine-grained matrix of 610 nm grain size, have been deformed in uniaxial compression. The results have been analysed by taking into account grain growth. The variation in stress exponent n suggests a transition from interface-reaction (n = 2) at low stresses to diffusional mechanism (n = 1) at high stresses, but cavitational creep was responsible for the non-linear behaviour at stresses above 80 MPa. The grain size exponent found at 60 MPa (p = 2.7), the absence of dislocations within fine grains and the similarity of grain shapes before and after large flow (55%), resemble the characteristics of “microgram superplastic flow”. The values of the grain size exponent and of the activation energy suggest that grain-boundary diffusion (p = 3) makes a significant contribution to the mixed deformation mechanism.

High temperature deformation of a 5 wt.% zirconia-spinel composite: influence of a threshold stress

Abstract Creep experiments performed on a 5 wt.% zirconia- MgAl 2 O 4 spinel material, in the stress and temperature ranges 8–200 MPa and 1350–1410°C, have shown the importance of grain boundaries in deformation of this material. Deformation can be analysed as the result of two sequential contributions. At low stress, an increase in the apparent stress exponent and the occurrence of a threshold stress, whose value roughly varies inversely proportional to spinel grain size, were observed. At high stress, grain boundary diffusion is the most likely mechanism that controls the grain boundary sliding. These observations are consistent with previous experiments showing that sliding of spinel/spinel boundaries is more difficult than sliding of spinel/zirconia boundaries in the low stress range. The plastic flow is analysed by means of grain boundary dislocations whose density increases with stress. At low stress, when the density of boundary dislocations is low, creep rates are interface-controlled while at high stress, when the boundary dislocation density is large, rates are limited by the long-range diffusion process.