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Mechanical Properties and Microstructures of hot-pressed MgAlON-BN Composites
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2007 (English)In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 27, no 1, 319-326 p.Article in journal (Refereed) Published
Abstract [en]

The relationship between the mechanical properties and microstructure of hot-pressed MgAlON-BN composite materials was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and X-ray diffraction (XRD) techniques. The phase compositions of hot-pressed samples prepared from starting mixtures of Al2O3, AlN, MgO and h-BN consisted of MgAlON phases as a matrix and BN phases as the second phase. The density, bending strength at room temperature, fracture toughness and Vickers hardness were measured. The results indicated that the density, strength and Vickers hardness decrease with increasing h-BN content due to the non-reactive nature and layered structure of h-BN. The fracture toughness, however increased with increasing h-BN addition, reaching a maximum of 3.64 MPa m(0.5); it decreased with further increase of BN content. The increase of fracture toughness was attributed to the presence of microcracks and the decrease was considered to be the discontinuous microstructure of the MgAlON phases. Temperature dependence of bending strength remained constant at low temperature, followed by an increase at 800 degrees C and then, dropped quickly. The increase in the bending strength of the composite was attributed to the decrease of residual stress and to the interwoven microstructure of the composites which prevented grain boundary slip and reduced the attenuation rate of high temperature strength. The machinability of the composites was examined. The results indicate that the composite materials with BN content more than 15 vol.% exhibit excellent machinability and could be drilled using conventional hard metal alloy drills

Place, publisher, year, edition, pages
2007. Vol. 27, no 1, 319-326 p.
Keyword [en]
mechanical properties, microstructure, hot pressing, composites, refractories, BN, MgAlON
National Category
Information Science
Identifiers
URN: urn:nbn:se:kth:diva-5478DOI: 10.1016/j.jeurceramsoc.2006.04.184ISI: 000242565400044Scopus ID: 2-s2.0-33751066193OAI: oai:DiVA.org:kth-5478DiVA: diva2:9858
Note
QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2017-11-21Bibliographically approved
In thesis
1. Synthesis and Characterization of MgA1ON-BN refractories
Open this publication in new window or tab >>Synthesis and Characterization of MgA1ON-BN refractories
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In order to meet the need of metallurgical industry in the world, a new MgAlON-BN composite which can be used for example in special refractory nozzles, tubes and break rings for the continuous casting of steel was studied in the present thesis. The aim was to understand the mechanism of synthesis and their physicochemical properties during the application. Thus, the thermodynamic properties, synthesis process, mechanical properties, thermal shock behaviour, thermal diffusivity/conductivity as well as corrosion resistance to molten iron containing oxygen and molten slag of MgAlON and MgAlON-BN composites have been investigated.

The Gibbs energy of formation of MgAlON was estimated using the method proposed by Kaufman. The phase stability diagram of Mg-Al-O-N-B was investigated, and consequently the synthesis parameters were determined. MgAlON and MgAlON-BN composites were fabricated by hot-pressing method. The composites obtained this way were characterized by XRD, SEM, TEM and HREM analyses. A Matrix-flushing method was employed in the quantitative XRD analysis for the multi-component samples to understand the mechanism of synthesis.

The relationship between mechanical properties and microstructure of the composites was investigated. The experimental results indicated that BN addition has significant influence on the mechanical properties of the composites. These can be explained by the fact that BN has low Young’s modulus, density and non-reactive nature as well as considerable anisotropy of many properties such as thermal expansion, thermal diffusivity/conductivity. Thus, the addition of BN in MgAlON is likely to lead to the presence of microcracks caused by the mismatch of thermal expansion coefficient. The microcracks result in the enhancement of the strength at elevated temperature and thermal shock durability of the composites.

Effective thermal conductivities were evaluated from the present experimental results of thermal diffusivities, heat capacity and density. A model suitable for present composites has been derived based on Luo’s model. The predicted lines calculated by the model were in good agreement with experimental results.

The reactions between the composites and molten iron as well as the slag were investigated by ‘‘finger’’ experiments and sessile drop experiments. Both experimental results indicated that the BN addition has positive influence on the corrosion resistance. These are attributed to the excellent corrosion resistance of BN to molten iron and slag, such as the higher contact angle between BN substrate and liquid iron and molten slag compared with that obtained for pure MgAlON.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. xii, 43 p.
Keyword
MgAlON-BN composites, Mechanical properties, Thermal shock durability, Thermal diffusivity/conductivity, Corrosion resistance.
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-3883 (URN)91-7178-271-0 (ISBN)
Public defence
2006-03-31, Salongen, KTHB, Osquars backe 31, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100929Available from: 2006-03-15 Created: 2006-03-15 Last updated: 2010-09-29Bibliographically approved

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