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Development of the Microstructure of the Silicon Nitride Based Ceramics

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Development of the Microstructure of the Silicon Nitride Based Ceramics Materials Research, Vol. 2, No. 3, 165-172, 1999. © 1999 Development of the Microstructure of the Silicon Nitride Based Ceramics J.C. Bressiani, V. Izhevskyi*, Ana H. A. Bressiani Instituto de Pesquisas Energéticas e Nucleares, IPEN, C.P. 11049 Pinheiros 05422-970 São Paulo - SP, Brazil Received: August 15, 1998; Revised: March 30, 1999 Basic regularities of silicon nitride based materials microstructure formation and development in interrelation with processing conditions, type of sintering additives, and starting powders properties are discussed. Models of abnormal or exaggerated grain growth are critically reassessed. Results of several model experiments conducted in order to determine the most important factors directing the microstructure formation processes in RE-fluxed Si3N4 ceramics are reviewed. Existing data on the mechanisms governing the microstructure development of Si3N4-based ceramics are analyzed and several principles of microstructure tailoring are formulated. Keywords: silicon nitride, microstructure, phase transformation, liquid phase sintering 1. Introduction the Weibull modulus of the fracture strength of silicon nitride. A small transverse grain size was shown to give Mechanical properties of the Si3N4-based ceramics higher Weibull modulus than the coarse microstructures. such as strength and fracture toughness as well as Weibull The observed increase in fracture resistance was mainly modulus are strongly dependent on the microstructure. attributed to crack deflection, and crack bridging mecha- Formation of the resulting microstructure is dictated by the nism3-6. In some cases crack branching was also observed. densification mechanism, i.e., reactive liquid phase sinter- An important role in fracture resistance improvement is ing. The α-Si3N4 in the starting powder compact is trans- played also by the secondary phase. If on cooling or after formed to β-Si3N4 via solution-reprecipitation in an post-sintering heat treatment tensile stress is induced in the oxynitride liquid-phase sintering medium. The final β- material due to thermal expansion coefficient mismatch Si3N4 grain size and shape distribution in the material are between Si3N4 and the secondary phase or due to volume controlled by the Si3N4 starting powder characteristics, the changes upon phase transformation, the fracture toughness metal oxide sintering additives, and the processing condi- may be diminished. Under such conditions the crack is not tions. Microstructural observations have shown that the deflected by elongated grains of β-phase and a transgranu- β-Si3N4 grains grow as elongated hexagonal prisms in lar crack development is characteristic. Considering the 1 suitable liquid-phase environment . importance of the microstructure tailoring for the further It is generally agreed that for superior mechanical prop- improvement of the Si3N4-based ceramics properties the erties a fine microstructure with a high proportion of elon- present paper has the goal of systematizing and analyzing gated β-grains, the so called high aspect ratio grains, or the existing data and concepts in this field. β even more preferably fibrous -phase grains is necessary. 2. Factors Governing Microstructure The aspect ratio characteristic means the ratio of length to Formation in Si3N4-based Ceramics width of the elongated hexagonal prismatic β-Si3N4 grains. Detailed studies of the grain morphology on fracture One of the main factors of elongated grains formation toughness and strength of silicon nitride ceramics have is the viscosity of the liquid phase formed during sintering been reviewed recently by Becher et al.2. Briefly, the which, in turn, depends on the type of the metal oxide(s) fracture toughness increases monotonically with the longi- used as a sintering aid. Grain growth in Si3N4-based mate- tudinal directions of the elongated grains. Both grain size rials is diffusion-controlled and can be considered in terms and the grain aspect ratio must be controlled to optimize of Ostwald ripening7. Therefore, it is to be expected that *On leave from the Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev, Ukraine 166 Bressiani et al. Materials Research the liquid phase with higher viscosity will yield β- grains secondary phases, revealing a high thermal stability with higher aspect ratios. The higher viscosity hinders both field12,13, are already formed on the early stage of densifi- the heterogeneous nucleation and the material transport by cation. During subsequent sintering, these phases remain means of diffusion during solution-reprecipitation, thus stable up to their melting point, while the eutectic liquid promoting preferential growth of the pre-existing intrinsic had already formed at a lower temperature and facilitates β-grains. This, in fact, was proved by observations in Y2O3- initial densification. The eutectic temperature in the Si3N4- 10,14 and Yb2O3-doped Si3N4 ceramics. In Yb2O3-doped mate- Y2O3-SiO2 ternary system lies at 1550 °C . However, rials higher average aspect ratios were observed in com- the eutectic temperature is lower, owing to the Al2O3 parison with the Y2O3-doped ones. It is a well known fact content of the starting powder blend. With increasing tem- that ytterbia as a sintering aid produces the liquid phase of perature, dissolution-reprecipitation occurs simultaneously the highest viscosity among all the other rare-earth (RE) with the α to β silicon nitride phase transformation, which oxides. Moreover, for the Y2O3-doped ceramics a pro- is complete at about 1650 °C for these materials. At nounced diminishing of the β- grains aspect ratios was 1630 °C the secondary H-phase (Fig. 3) is still crystalline. observed with the increase of alumina addition. Alumina is Even at 1730 °C this phase still lies outside the liquid phase known to diminish the viscosity of the liquid phase formed field. It is thought that the crystalline secondary phase only during sintering. It must be emphasized that in order to melts at higher sintering temperatures, forming excess liq- produce the favorable microstructure the presence of im- uid. The latter is inhomogeneously distributed over entire purities, which decrease the viscosity of the molten liquid sample, owing to inhomogeneous distribution of crystalline phase, i.e. iron, alkali and alkaline-earth metals, etc., must secondary phase at lower sintering temperatures, which is be avoided. thought to initiate both increased densification and the The effect of such impurities on microstructure forma- onset of abnormal grain growth. A strong increase in den- tion can be seen on Fig. 1, where microstructures of sam- sification rate at temperatures about 1800 °C was experi- ples of silicon nitride doped with 5 wt.% of Al2O3 and mentally confirmed by dilatometry measurements during β 5 wt. % of Y2O3 with different amounts of model impuri- sintering. Moreover, the formation of large, elongated - 8 ties (Si and FeSix) are presented . It is clearly seen that with Si3N4 grains, randomly oriented in the sintered body, was the increase of either one of the model impurities micro- also reported10. It is thought that some of the excess liquid structure refinement and morphological homogenization will progress through the reaction bonded silicon nitride occurs, the amount of large β-Si3N4 grains with high aspect (RBSN) pore channels by capillary forces and simultane- ratios substantially diminishes. The effect is much more ously permit densification, but it is presumed in this model pronounced if FeSix is added. that excess liquid remains at the site containing the original As it was mentioned before, there are other factors that metastable Y-Si oxynitride phase. This liquid would act as control the microstructure development and may cause the a flux for the rapid growth of highly elongated β-Si3N4 so-called exaggerated or abnormal grain growth, that leads grains by a dissolution - reprecipitation process10. to the formation of extremely large β-Si3N4 grains, which This model is based on two requirements. Firstly, upon may act as crack initiation sites9. Several factors are be- liquid formation the wetting liquid should not be drawn lieved to contribute to such an occurrence: inhomogeneous completely out of the central reservoir into the surrounding 10 distribution of crystalline secondary phases , β-Si3N4 porous RBSN matrix. The situation could arise if the vol- grain morphology, the β-Si3N4 nuclei density, and the ume of liquid exceeds the porosity in the RBSN and also, if densification occurs simultaneously with capillary extru- β-Si3N4 grain size distribution within the starting powder β mixtures11. sion. Secondly, exaggerated -Si3N4 grain growth and elongation is required to occur at the liquid reservoir, 3. Models of Exaggerated Grain Growth in whereas normal coarsening should prevail in the surround- Si3N4-based Ceramics and Model ing matrix. To prove this requirements two model experi- Experiments ments were carried out by Dressler et al.11. One of the models of exaggerated grain growth which In the first one a large piece of pre-synthesized crystal- is mostly attributed to reaction-bonded silicon nitride but line secondary phase (about 5 mm in diameter) was embed- ded in a silicon nitride powder compact and subsequently is also fully applicable to all Si3N4-based ceramics is based on the assumption of intermediate (transient) crystalline consolidated, as schematically depicted on Fig. 4. secondary phase formation on the early stages of nitridation It was expected
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