An irreversible disappearance of the mechanical loss peak of an yttria-doped silicon nitride
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K. Yamada and N. Kamiya Toyota Central R&D Laboratories., Inc., Nagakute, Aichi 480-1131, Japan
(Received 19 February 2004; accepted 18 May 2004)
The internal friction of a silicon nitride with a sintering aid of yttrium oxide was measured from room temperature to 1400 °C. A mechanical loss peak was typically observed at 1000 °C on heating the as-sintered specimen; however, it disappeared on cooling. Also, the peak was not observed upon heating a specimen that had been heated above 1400 °C. When we carried out thermal cycling, raising terminal temperatures from 1050 to 1400 °C, the integrated intensity of the peak gradually decreased due to the progressive crystallization of a grain-boundary amorphous phase.
The internal friction of silicon nitride ceramics sintered with a liquid-densification aid at elevated temperatures has been eagerly investigated to estimate the viscosity of a grain-boundary amorphous phase.1–16 To estimate the viscosity of a grain-boundary amorphous phase gives evidence on the high-temperature creep resistance and/or strength by the detection of a grainboundary slide at the beginning of plasticity. In previous studies, a mechanical loss peak assigned to the viscous deformation of the intergranular amorphous phase was typically observed at about 1000 °C, on an exponential increase of internal friction, with an increase in temperature. The peak was frequently observed when heating the as-sintered specimen; but occasionally upon cooling, the amplitude strongly diminished.3,4,7 The crystallization of the amorphous phase is shown to be the reason for the diminution of the mechanical loss peak. However, it has not been clear enough that the transitional change in the peak is due to progressive crystallization. Thus, it is necessary to more fully investigate the crystallization behavior of the grain-boundary amorphous phase in silicon nitride ceramics, because the crystallization drastically improves the mechanical properties of this material at elevated temperatures.16 In this study, the vanishing
a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/publications/jmr/policy.html DOI: 10.1557/JMR.2004.0329 2532
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J. Mater. Res., Vol. 19, No. 9, Sep 2004 Downloaded: 29 Mar 2015
process of the mechanical loss peak of an yttria-doped silicon nitride was examined in detail through internal friction measurement with thermal cycling. The crystallization of the grain-boundary amorphous phase was monitored with x-ray diffraction (XRD) between each thermal cycling. We will discuss the correlation between the disappearance of the mechanical loss peak and the crystallization of the amorphous phase. Silicon nitride ceramics were processed by hotpressing silicon nitride powders (SN-E10, UBE Chemicals Co., Ltd., Ube, Japan) mixed with 6 wt% of yttria
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