John Hallloran | University of Michigan (original) (raw)
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Papers by John Hallloran
Key Engineering Materials, 2010
Recent ideas on the oxidation of ZrB2-SiC are presented, emphasizing the behavior of the boria-si... more Recent ideas on the oxidation of ZrB2-SiC are presented, emphasizing the behavior of the boria-silica-zirconia liquid (BSZ liquid) which forms during oxidation and flows to transport silica and zirconia to the surface of the scale. Oxide film microstructure is strongly influenced by the boron oxide, which acts as a volatile flux.
Journal of the American Ceramic Society, 2004
Journal of the European Ceramic Society, 2010
The oxidation of ZrB 2-SiC composites at 1450-1650 • C was directly observed with in situ optical... more The oxidation of ZrB 2-SiC composites at 1450-1650 • C was directly observed with in situ optical microscopy. Video frames showed the flow of silicate liquids, the formation of zirconia deposits, and the growth and collapse of gaseous bubbles on the oxide surface. Contrast in the incandescence of in situ images is analyzed as spatial variations in hue and intensity and related to differences in emissivity of the oxide scale surface features by comparing these hot images with room temperature images. Above 1450 • C, gaseous bubbles were observed to grow and collapse causing perturbations in the liquid oxide on the surface. The bubbles are associated with the evolution of CO from SiC oxidation and the onset is related to the critical temperature where the partial pressure of CO under the oxide scale exceeds atmospheric pressure.
Journal of Materials Science, 2000
Preferred orientation was measured in Si 3 N 4 /BN fibrous monolithic ceramics using x-ray diffra... more Preferred orientation was measured in Si 3 N 4 /BN fibrous monolithic ceramics using x-ray diffraction. The materials were manufactured by co-extrusion of polymer binder/ceramic blends which were subsequently pyrolized and then hot-pressed to produced a fully dense ceramic composite. A very strong modified wire texture was present in the BN with the basal planes aligned parallel to the axis of extrusion due to shear-induced reorientation of the platelet-shaped BN particles during co-extrusion. Texture was also observed in the Si 3 N 4 and was attributed to a combination of co-extrusion and hot-pressing. After hot pressing, the basal planes of the rod-shaped β-Si 3 N 4 were observed to be preferentially aligned perpendicular to the extrusion direction. Measurements prior to hot-pressing revealed that a small amount (≈5%) of β-Si 3 N 4 was present in the α-Si 3 N 4 starting powder. Although texturing of the predominant α-Si 3 N 4 did not occur during co-extrusion, significant texturing of the β-Si 3 N 4 was observed. During subsequent hot-pressing, the pre-existing textured β-Si 3 N 4 particles appeared to act as seeds for transformation and preferred growth of rod-shaped β grains parallel to the axis of extrusion.
Journal of the American Ceramic Society, 2005
Journal of the American Ceramic Society, 2005
Journal of the American Ceramic Society, 2004
Monazite (LaPO 4 ) has been proposed as an interphase to promote debonding between the reinforcem... more Monazite (LaPO 4 ) has been proposed as an interphase to promote debonding between the reinforcement and the matrix during the fracture of oxide-based composites. The correlation between fracture behavior and micromechanical properties in model alumina/monazite (Al 2 O 3 /LaPO 4 ) multilayer laminates has been investigated in this study. The delamination fracture energy (⌫ i ) was dependent on crack length, which is consistent with previous results; the initial value of ⌫ i was ϳ10 J/m 2 . The interfacial frictional sliding resistance increased as the normal stress on the interface increased. Using a Coulombic friction model, the coefficient of static friction between the Al 2 O 3 and LaPO 4 layers was determined to be 0.63. The influence of ⌫ i and flaw size in the Al 2 O 3 layers on fracture path has been predicted, using an existing model, and confirmed experimentally. The results indicate that, in addition to satisfying energybased fracture criteria, several other factors affect whether LaPO 4 is a suitable interphase for oxide composites. M. Thouless-contributing editor Manuscript No. 189421.
Key Engineering Materials, 2010
Recent ideas on the oxidation of ZrB2-SiC are presented, emphasizing the behavior of the boria-si... more Recent ideas on the oxidation of ZrB2-SiC are presented, emphasizing the behavior of the boria-silica-zirconia liquid (BSZ liquid) which forms during oxidation and flows to transport silica and zirconia to the surface of the scale. Oxide film microstructure is strongly influenced by the boron oxide, which acts as a volatile flux.
Journal of the American Ceramic Society, 2004
Journal of the European Ceramic Society, 2010
The oxidation of ZrB 2-SiC composites at 1450-1650 • C was directly observed with in situ optical... more The oxidation of ZrB 2-SiC composites at 1450-1650 • C was directly observed with in situ optical microscopy. Video frames showed the flow of silicate liquids, the formation of zirconia deposits, and the growth and collapse of gaseous bubbles on the oxide surface. Contrast in the incandescence of in situ images is analyzed as spatial variations in hue and intensity and related to differences in emissivity of the oxide scale surface features by comparing these hot images with room temperature images. Above 1450 • C, gaseous bubbles were observed to grow and collapse causing perturbations in the liquid oxide on the surface. The bubbles are associated with the evolution of CO from SiC oxidation and the onset is related to the critical temperature where the partial pressure of CO under the oxide scale exceeds atmospheric pressure.
Journal of Materials Science, 2000
Preferred orientation was measured in Si 3 N 4 /BN fibrous monolithic ceramics using x-ray diffra... more Preferred orientation was measured in Si 3 N 4 /BN fibrous monolithic ceramics using x-ray diffraction. The materials were manufactured by co-extrusion of polymer binder/ceramic blends which were subsequently pyrolized and then hot-pressed to produced a fully dense ceramic composite. A very strong modified wire texture was present in the BN with the basal planes aligned parallel to the axis of extrusion due to shear-induced reorientation of the platelet-shaped BN particles during co-extrusion. Texture was also observed in the Si 3 N 4 and was attributed to a combination of co-extrusion and hot-pressing. After hot pressing, the basal planes of the rod-shaped β-Si 3 N 4 were observed to be preferentially aligned perpendicular to the extrusion direction. Measurements prior to hot-pressing revealed that a small amount (≈5%) of β-Si 3 N 4 was present in the α-Si 3 N 4 starting powder. Although texturing of the predominant α-Si 3 N 4 did not occur during co-extrusion, significant texturing of the β-Si 3 N 4 was observed. During subsequent hot-pressing, the pre-existing textured β-Si 3 N 4 particles appeared to act as seeds for transformation and preferred growth of rod-shaped β grains parallel to the axis of extrusion.
Journal of the American Ceramic Society, 2005
Journal of the American Ceramic Society, 2005
Journal of the American Ceramic Society, 2004
Monazite (LaPO 4 ) has been proposed as an interphase to promote debonding between the reinforcem... more Monazite (LaPO 4 ) has been proposed as an interphase to promote debonding between the reinforcement and the matrix during the fracture of oxide-based composites. The correlation between fracture behavior and micromechanical properties in model alumina/monazite (Al 2 O 3 /LaPO 4 ) multilayer laminates has been investigated in this study. The delamination fracture energy (⌫ i ) was dependent on crack length, which is consistent with previous results; the initial value of ⌫ i was ϳ10 J/m 2 . The interfacial frictional sliding resistance increased as the normal stress on the interface increased. Using a Coulombic friction model, the coefficient of static friction between the Al 2 O 3 and LaPO 4 layers was determined to be 0.63. The influence of ⌫ i and flaw size in the Al 2 O 3 layers on fracture path has been predicted, using an existing model, and confirmed experimentally. The results indicate that, in addition to satisfying energybased fracture criteria, several other factors affect whether LaPO 4 is a suitable interphase for oxide composites. M. Thouless-contributing editor Manuscript No. 189421.