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Martin P. Harmer - One of the best experts on this subject based on the ideXlab platform.

  • effect of annealing environment on the crack healing and mechanical behavior of silicon carbide reinforced alumina Nanocomposites
    Journal of the American Ceramic Society, 2005
    Co-Authors: Irene A Chou, Helen M. Chan, Martin P. Harmer
    Abstract:

    The crack healing and strength behavior of an alumina-silicon carbide (Al2O3-SiC) nanocomposite (Al2O3+ 5 vol% 0.2 μm SiC particles) has been studied, as a function of the crack size and the annealing environment. Results show that annealing treatments can significantly increase the indentation strength. The annealing atmosphere has a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon results in a strength increase of 50%, whereas annealing in air yields a three-fold improvement in the indentation strength. Scanning electron microscopic observation has shown that healing of indentation cracks occurs in both environments, with the greater degree of healing occurring during annealing in air. Implications of the findings to the strengthening mechanism in Al2O3 (SiC) Nanocomposites will be discussed.

  • evaluation of subgrain formation in al2o3 sic Nanocomposites
    Journal of Materials Science, 1997
    Co-Authors: Jianxin Fang, Martin P. Harmer, Helen M. Chan
    Abstract:

    Both theoretical analysis and transmission electron microscopy (TEM) complementary studies have been conducted to evaluate the possible role of subgrain formation as a strengthening mechanism in a nanocomposite consisting of Al2O3 and 5 vol % 0.15 μm SiC particles. The theoretical calculation predicted that the residual stresses due to thermal expansion mismatch between Al2O3 and SiC are insufficient to induce the extensive plastic deformation required for subgrain formation upon annealing. This prediction was consistent with TEM observations that the bulk of the material was completely free from subgrains, and that only a low density of dislocations was present in isolated areas. The results suggest, therefore, that microstructure refinement through subgrain formation cannot account for the superior mechanical behaviour of the nanocomposite reported in previous studies.

  • machining induced surface residual stress behavior in al2o3 sic Nanocomposites
    Journal of the American Ceramic Society, 1996
    Co-Authors: Irene A Chou, Helen M. Chan, Martin P. Harmer
    Abstract:

    The machining and subsequent annealing behavior of an Al{sub 2}O{sub 3}-SiC nanocomposite (Al{sub 2}O{sub 3} + 5 vol% 0.2 {micro}m SiC particles) was compared to that of single-phase Al{sub 2}O{sub 3}. The machining-induced residual line force was determined by measuring the extent of elastic bending in thin disk specimens, and the surface roughness was evaluated by profilometry. The results showed that, when the two materials were subjected to the same grinding conditions, they developed compressive residual stresses and surface roughness values of similar magnitude. The maximum thickness of the residual stress layers was estimated to be {approximately}10 {micro}m for the Al{sub 2}O{sub 3} and {approximately}12 {micro}m for the nanocomposite. A direct linear correlation was observed between the residual force and the surface roughness for different machining treatments. Annealing of the machined samples produced complete relaxation of residual stresses in the single-phase Al{sub 2}O{sub 3}, whereas only partial stress relaxation occurred for the nanocomposite.

  • crack healing and stress relaxation in al2o3 sic Nanocomposites
    Journal of the American Ceramic Society, 1995
    Co-Authors: Mark A Thompson, Martin P. Harmer, Helen M. Chan, Robert E Cook
    Abstract:

    The crack-healing behavior of Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3}-SiC nanocomposite was studied using Vickers indentations to generate precracks. After annealing in argon for 2 h at 1,300 C, radial cracks in the nanocomposite healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip. In contrast, radial cracks in alumina grew when exposed to the same annealing treatment. The different responses are attributed to the fracture mode and toughening mechanism in each material: In the nanocomposite, the cracks close as the residual stresses surrounding the indentations relax. Radial cracks open and grow in Al{sub 2}O{sub 3} because microstructural toughening is diminished during heating to the annealing temperature. An implication is that strength-limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this ``nanocomposite`` system.

  • processing and microstructure development in al2o3 sic Nanocomposites
    Journal of The European Ceramic Society, 1992
    Co-Authors: Laura C Stearns, Junhong Zhao, Martin P. Harmer
    Abstract:

    Abstract Composites consisting of Al 2 O 3 + 5 vol.% 0·15 μm SiC particles were prepared by pressureless sintering. The optimum conditions for achieving dense and uniform microstructures by conventional ceramic processing are given in detail. The SiC particles were found to strongly inhibit grain growth of the Al 2 O 3 matrix. Densification was also significantly retarded by these ultra-fine particles, and possible explanations for this behavior are discussed.

Helen M. Chan - One of the best experts on this subject based on the ideXlab platform.

  • effect of annealing environment on the crack healing and mechanical behavior of silicon carbide reinforced alumina Nanocomposites
    Journal of the American Ceramic Society, 2005
    Co-Authors: Irene A Chou, Helen M. Chan, Martin P. Harmer
    Abstract:

    The crack healing and strength behavior of an alumina-silicon carbide (Al2O3-SiC) nanocomposite (Al2O3+ 5 vol% 0.2 μm SiC particles) has been studied, as a function of the crack size and the annealing environment. Results show that annealing treatments can significantly increase the indentation strength. The annealing atmosphere has a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon results in a strength increase of 50%, whereas annealing in air yields a three-fold improvement in the indentation strength. Scanning electron microscopic observation has shown that healing of indentation cracks occurs in both environments, with the greater degree of healing occurring during annealing in air. Implications of the findings to the strengthening mechanism in Al2O3 (SiC) Nanocomposites will be discussed.

  • evaluation of subgrain formation in al2o3 sic Nanocomposites
    Journal of Materials Science, 1997
    Co-Authors: Jianxin Fang, Martin P. Harmer, Helen M. Chan
    Abstract:

    Both theoretical analysis and transmission electron microscopy (TEM) complementary studies have been conducted to evaluate the possible role of subgrain formation as a strengthening mechanism in a nanocomposite consisting of Al2O3 and 5 vol % 0.15 μm SiC particles. The theoretical calculation predicted that the residual stresses due to thermal expansion mismatch between Al2O3 and SiC are insufficient to induce the extensive plastic deformation required for subgrain formation upon annealing. This prediction was consistent with TEM observations that the bulk of the material was completely free from subgrains, and that only a low density of dislocations was present in isolated areas. The results suggest, therefore, that microstructure refinement through subgrain formation cannot account for the superior mechanical behaviour of the nanocomposite reported in previous studies.

  • machining induced surface residual stress behavior in al2o3 sic Nanocomposites
    Journal of the American Ceramic Society, 1996
    Co-Authors: Irene A Chou, Helen M. Chan, Martin P. Harmer
    Abstract:

    The machining and subsequent annealing behavior of an Al{sub 2}O{sub 3}-SiC nanocomposite (Al{sub 2}O{sub 3} + 5 vol% 0.2 {micro}m SiC particles) was compared to that of single-phase Al{sub 2}O{sub 3}. The machining-induced residual line force was determined by measuring the extent of elastic bending in thin disk specimens, and the surface roughness was evaluated by profilometry. The results showed that, when the two materials were subjected to the same grinding conditions, they developed compressive residual stresses and surface roughness values of similar magnitude. The maximum thickness of the residual stress layers was estimated to be {approximately}10 {micro}m for the Al{sub 2}O{sub 3} and {approximately}12 {micro}m for the nanocomposite. A direct linear correlation was observed between the residual force and the surface roughness for different machining treatments. Annealing of the machined samples produced complete relaxation of residual stresses in the single-phase Al{sub 2}O{sub 3}, whereas only partial stress relaxation occurred for the nanocomposite.

  • crack healing and stress relaxation in al2o3 sic Nanocomposites
    Journal of the American Ceramic Society, 1995
    Co-Authors: Mark A Thompson, Martin P. Harmer, Helen M. Chan, Robert E Cook
    Abstract:

    The crack-healing behavior of Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3}-SiC nanocomposite was studied using Vickers indentations to generate precracks. After annealing in argon for 2 h at 1,300 C, radial cracks in the nanocomposite healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip. In contrast, radial cracks in alumina grew when exposed to the same annealing treatment. The different responses are attributed to the fracture mode and toughening mechanism in each material: In the nanocomposite, the cracks close as the residual stresses surrounding the indentations relax. Radial cracks open and grow in Al{sub 2}O{sub 3} because microstructural toughening is diminished during heating to the annealing temperature. An implication is that strength-limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this ``nanocomposite`` system.

Irene A Chou - One of the best experts on this subject based on the ideXlab platform.

  • effect of annealing environment on the crack healing and mechanical behavior of silicon carbide reinforced alumina Nanocomposites
    Journal of the American Ceramic Society, 2005
    Co-Authors: Irene A Chou, Helen M. Chan, Martin P. Harmer
    Abstract:

    The crack healing and strength behavior of an alumina-silicon carbide (Al2O3-SiC) nanocomposite (Al2O3+ 5 vol% 0.2 μm SiC particles) has been studied, as a function of the crack size and the annealing environment. Results show that annealing treatments can significantly increase the indentation strength. The annealing atmosphere has a profound influence on the extent of crack healing and the degree of strength recovery. Annealing in argon results in a strength increase of 50%, whereas annealing in air yields a three-fold improvement in the indentation strength. Scanning electron microscopic observation has shown that healing of indentation cracks occurs in both environments, with the greater degree of healing occurring during annealing in air. Implications of the findings to the strengthening mechanism in Al2O3 (SiC) Nanocomposites will be discussed.

  • machining induced surface residual stress behavior in al2o3 sic Nanocomposites
    Journal of the American Ceramic Society, 1996
    Co-Authors: Irene A Chou, Helen M. Chan, Martin P. Harmer
    Abstract:

    The machining and subsequent annealing behavior of an Al{sub 2}O{sub 3}-SiC nanocomposite (Al{sub 2}O{sub 3} + 5 vol% 0.2 {micro}m SiC particles) was compared to that of single-phase Al{sub 2}O{sub 3}. The machining-induced residual line force was determined by measuring the extent of elastic bending in thin disk specimens, and the surface roughness was evaluated by profilometry. The results showed that, when the two materials were subjected to the same grinding conditions, they developed compressive residual stresses and surface roughness values of similar magnitude. The maximum thickness of the residual stress layers was estimated to be {approximately}10 {micro}m for the Al{sub 2}O{sub 3} and {approximately}12 {micro}m for the nanocomposite. A direct linear correlation was observed between the residual force and the surface roughness for different machining treatments. Annealing of the machined samples produced complete relaxation of residual stresses in the single-phase Al{sub 2}O{sub 3}, whereas only partial stress relaxation occurred for the nanocomposite.

Robert E Cook - One of the best experts on this subject based on the ideXlab platform.

  • crack healing and stress relaxation in al2o3 sic Nanocomposites
    Journal of the American Ceramic Society, 1995
    Co-Authors: Mark A Thompson, Martin P. Harmer, Helen M. Chan, Robert E Cook
    Abstract:

    The crack-healing behavior of Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3}-SiC nanocomposite was studied using Vickers indentations to generate precracks. After annealing in argon for 2 h at 1,300 C, radial cracks in the nanocomposite healed: The cracks closed and there was a small degree of rebonding in the vicinity of the crack tip. In contrast, radial cracks in alumina grew when exposed to the same annealing treatment. The different responses are attributed to the fracture mode and toughening mechanism in each material: In the nanocomposite, the cracks close as the residual stresses surrounding the indentations relax. Radial cracks open and grow in Al{sub 2}O{sub 3} because microstructural toughening is diminished during heating to the annealing temperature. An implication is that strength-limiting machining flaws in these materials behave similarly, thereby accounting for the strengthening effect of annealing in this ``nanocomposite`` system.

Sg Roberts - One of the best experts on this subject based on the ideXlab platform.

  • Effects of Y2O3 additives and powder purity on the densification and grain boundary composition of Al2O3/SiC Nanocomposites
    'Elsevier BV', 2009
    Co-Authors: Ip Shapiro, Ri Todd, Jm Titchmarsh, Sg Roberts
    Abstract:

    Sub-micron sized SiC additions can be used to increase the wear resistance and change the fracture mode of Al2O3. However, these additions also restrict sintering. Al2O3 and Al2O3-5%SiC 'Nanocomposites' were prepared from alumina powders of high purity and of commercial-purity, with or without the addition of Y2O3. The effects of these compositional variables on sintering rate, final density and grain boundary composition were investigated. A direct comparison with Al2O3-SiO2 composites was also made, as it has been proposed that SiC partially oxidises during processing of Al2O3-SiC Nanocomposites. The addition of 5 vol.% SiC to Al2O3 hindered densification, as did addition of 0.15 wt.% Y2O3 or 0.1 wt.% SiO2. In contrast, the addition of 0.15 wt.% Y2O3 to Al2O3-5% SiC Nanocomposites improved densification. The composition of Al2O3-Al2O3 grain boundaries in these materials was studied using STEM and EDX microanalysis. The addition of SiC and SiO2 caused segregation of Si, and Y2O3 addition caused segregation of Y. The segregation of each element was equivalent to

  • Effects of Y2O3 additives and powder purity on the densification and grain boundary composition of Al2O3/SiC Nanocomposites
    2009
    Co-Authors: Ip Shapiro, Ri Todd, Jm Titchmarsh, Sg Roberts
    Abstract:

    Sub-micron sized SiC additions can be used to increase the wear resistance and change the fracture mode of Al2O3. However, these additions also restrict sintering. Al2O3 and Al2O3-5%SiC 'Nanocomposites' were prepared from alumina powders of high purity and of commercial-purity, with or without the addition of Y2O3. The effects of these compositional variables on sintering rate, final density and grain boundary composition were investigated. A direct comparison with Al2O3-SiO2 composites was also made, as it has been proposed that SiC partially oxidises during processing of Al2O3-SiC Nanocomposites. The addition of 5 vol.% SiC to Al2O3 hindered densification, as did addition of 0.15 wt.% Y2O3 or 0.1 wt.% SiO2. In contrast, the addition of 0.15 wt.% Y2O3 to Al2O3-5% SiC Nanocomposites improved densification. The composition of Al2O3-Al2O3 grain boundaries in these materials was studied using STEM and EDX microanalysis. The addition of SiC and SiO2 caused segregation of Si, and Y2O3 addition caused segregation of Y. The segregation of each element was equivalent to <10% of a monolayer at the grain boundary. However, if SiC and Y2O3 were simultaneously added the segregation increased to 40% of a monolayer. The enhanced segregation was attributed to increased oxidation of SiC in the presence of Y2O3 allowing formation of a SiO2-Al2O3-Y2O3 eutectic phase or a segregated layer which may explain the improvement in sintering rate when Y2O3 was added to Nanocomposites. © 2008 Elsevier Ltd. All rights reserved