Novel joining of dissimilar ceramics in the Si{sub 3}N{sub 4}-Al{sub 2}O{sub 3} system using polytypoid functional gradients

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A unique approach to crack-free joining of heterogeneous ceramics is demonstrated by the use of sialon polytypoids as Functionally Graded Materials (FGM) as defined by the phase diagram in the system, Si3N4-Al2O3. Polytypoids in the Al2O3-Si3N4 system offer a path to compatibility for such heterogeneous ceramics. The first part of the dissertation describes successful hot press sintering of multilayered FGM's with 20 layers of thickness 500 mm each. Transmission Electron Microscopy was used to identify the polytypoids at the interfaces of different areas of the joint. It has been found that the 15R polytypoid was formed in the Al2O3-contained layers ... continued below

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Lee, Caroline Sunyong August 22, 2001.

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A unique approach to crack-free joining of heterogeneous ceramics is demonstrated by the use of sialon polytypoids as Functionally Graded Materials (FGM) as defined by the phase diagram in the system, Si3N4-Al2O3. Polytypoids in the Al2O3-Si3N4 system offer a path to compatibility for such heterogeneous ceramics. The first part of the dissertation describes successful hot press sintering of multilayered FGM's with 20 layers of thickness 500 mm each. Transmission Electron Microscopy was used to identify the polytypoids at the interfaces of different areas of the joint. It has been found that the 15R polytypoid was formed in the Al2O3-contained layers and the 12H polytypoid was formed in the Si3N4-contained layers. The second part of the dissertation discusses the mechanical properties of these polytypoidally joined Si3N4-Al2O3. The thermal stresses of this FGM junction were analyzed using a finite element analysis program (FEAP) taking into account both coefficient of thermal expansion (CTE) and modulus variations. From this analysis, the result showed a dramatic decrease in radial, axial and hoop stresses as the FGM changes from three layers to 20 graded layers. Scaling was considered, showing that the graded transition layer should constitute about 75 percent or more of the total sample thickness to reach a minimal residual stress. Oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were minimally or not deflected at the sialon layers, implying strong interfaces. Finally, flexural testing was conducted at room temperature and at high temperature. The average strength at room temperature was found to be 581 MPa and the average strength at high temperature (1200 degrees C) was found to be 262 MPa. Scanning electron microscope observation of fracture surfaces at a different loading rates indicated that the strength loss at higher temperatures was consistent with a softening of glassy materials present at grain junctions.

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OSTI as DE00791793

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  • Other Information: TH: Thesis (Ph.D.); Submitted to University of California, Berkeley, CA (US)

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  • Report No.: LBNL--48986
  • Grant Number: AC03-76SF00098
  • Office of Scientific & Technical Information Report Number: 791793
  • Archival Resource Key: ark:/67531/metadc738341

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  • August 22, 2001

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  • Oct. 19, 2015, 7:39 p.m.

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  • April 4, 2016, 1:46 p.m.

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Lee, Caroline Sunyong. Novel joining of dissimilar ceramics in the Si{sub 3}N{sub 4}-Al{sub 2}O{sub 3} system using polytypoid functional gradients, thesis or dissertation, August 22, 2001; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc738341/: accessed October 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.