Silicon nitride nanoceramics densified by dynamic grain sliding
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Zhijian Shena) Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-106 92 Stockholm, Sweden
Ingrid Schulz Institut fu¨r Werkstoffwissenschaft, Technical University of Dresden, Dresden D-01307, Germany
Jianfeng Hu Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-106 92 Stockholm, Sweden
Bostjan Jancar Jozef Stefan Institute, Advanced Materials Department K-9, Jamova 39, SI-1000 Ljubljana, Slovenia (Received 3 September 2010; accepted 22 September 2010)
The densification behaviors of two silicon nitride nanopowder mixtures based respectively on a-Si3N4 and b-Si3N4 as the major phase constituent were studied by spark plasma sintering. Sintering conditions were established where a low viscous liquid not in equilibrium with the main crystalline constituent(s) stimulated the grain sliding yet did not activate the reprecipitation mechanism that unavoidably yields grain growth. By this way of dynamic grain sliding full densification of silicon nitride nanoceramics was achieved with no noticeable involvement of a- to b-Si3N4 phase transformation and grain growth. This processing principle opens the way toward flexible and precise tailoring of the microstructures and properties of Si3N4 ceramics. The obtained silicon nitride nanoceramics showed improved wear resistance, particularly under higher Hertzian stresses.
I. INTRODUCTION
It has been known since the late 1950s that the covalently bonded crystalline silicon nitride powders are hard to sinter due to their high volatility yet low atomic lattice and grain-boundary diffusion rates at the temperature necessary for densification. Still, this category of ceramics is possible to be densified by the addition of liquid-forming additive(s), preferably under a mechanical pressure.1 During such a sintering process, known as liquid phase assisted hot pressing, the additive(s), oxides, or nitrides, react partially with silicon nitride to form a liquid phase, which subsequently facilitates two densification processes, namely (i) the grain sliding lubricated by the liquid phase and (ii) the mass transport via grain boundaries enabled by the solution-diffusionreprecipitation of the silicon nitride phase(s). Early hot-pressing studies discovered the important role of grain sliding, sometimes defined as grain boundary sliding, in facilitating of a rapid particle rearrangement thus also in enhancing the densification of silicon nitride ceramics.2,3 It was demonstrated that the degree of grain a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0313
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J. Mater. Res., Vol. 25, No. 12, Dec 2010
sliding is determined by the amount of liquid phase formed in the grain boundaries. The grain sliding may proceed without yielding grain growth, but it operates only up to relative densities of about 0.65 to 0.70. This means that it is impossible to achieve full densification only by the grain sliding mechanism. On the other hand, full densification becom
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