Significant improvements in the fracture resistance of self-reinforced silicon nitride ceramics have been obtained by tailoring the chemistry of the intergranular amorphous phase. First, the overall microstructure of the material was controlled by incorporation of a fixed amount of elongated s-Si3N4 seeds into the starting powder to regulate the size and fraction of the large reinforcing grains. With controlled microstructures, the interfacial debond strength between the reinforcement and the intergranular glass was optimized by varying the yttria-to-alumina ratio in the sintering additives. It was found that the steady-state fracture toughness value of these silicon nitrides increased with the Y:Al ratio of the oxide additives. The increased toughness was accompanied by a steeply rising R-curve and extensive interfacial debonding between the elongated s-Si3N4 grains and the intergranular glassy phase. Microstructural analyses indicate that the different fracture behavior is related to the Al (and O) content in the s´-SiAlON growth layer formed on the elongated s-Si3N4 grains during densification. The results imply that the interfacial bond strength is a function of the extent of Al and Si bonding with N and O in the adjoining phases with an abrupt structural/chemical interface achieved by reducing the Al concentration in both the intergranular phase and the s´-SiAlON growth layer. Analytical modeling revealed that the residual thermal expansion mismatch stress is not a dominant influence on the interfacial fracture behavior when a distinct s´-SiAlON growth layer forms. It is concluded that the fracture resistance of self-reinforced silicon nitrides can be improved by optimizing the sintering additives employed.

Microstructural design of silicon nitride with improved fracture toughness: II. Effects of yttria and alumina additives

Silicon nitride ceramics is a generic term for a variety of alloys of Si3N4 with additional compounds necessary for a complete densification of the Si3N4 starting powder. They are heterogeneous, multicomponent materials characterised by the inherent properties of the crystalline modifications α and β of Si3N4 and the significant influence of the densification additives. With a view to ability of the α and β modification to form solid solutions α-Si3N4 (αss) and β-Si3N4 (βss) solid solutions can be distinguished. Each group contains engineered materials with interesting properties for special applications. Phase relations and micro-structures determine the properties decisively. Composition of the phases, the distribution of the grains, their aspect ratio and the grain boundary phase are pronounced microstructural features. The formation of the microstructure strongly depends on the one hand on the quality of the Si3N4 starting powders, which closely is related to the chemistry of the production process, and on the other on the liquid phase sintering as the most important step in the densification route. The interrelation between pure Si3N4, the densification of the powder including the role of sintering additives, microstructural engineering, physicochemical properties of the sintered Si3N4 ceramics (SSN, GPSN, HPSN, HIP-SSN, HIP-SN) are described in more detail and compared to reaction bonded Si3N4 ceramics (RBSN), which are produced by nitridation of silicon powders.

Silicon Nitride Ceramics

Silicon nitride materials that contained different mixtures of sintering aids were investigated with respect to microstructure development and resulting fracture toughness. Postsintering annealing at 1850°C for various times was adopted in order to coarsen the respective microstructures. Although constant processing conditions were used, a marked variation in fracture toughness of the Si3N4 materials was evaluated. With a larger grain diameter of the Si3N4 grains, an increase in fracture resistance was generally observed. However, a correlation between fracture toughness and apparent aspect ratio could not be established. The observed changes in microstructure were in fact caused by the difference in secondary-phase chemistry. Si3N4 grain growth was dominated by diffusion-controlled Ostwald ripening and was hence affected by the viscosity of the liquid at processing temperature. In addition, crystallization at triple pockets also depends on the sintering additives employed and was found to influence fracture toughness by altering the crack-propagation mode as a consequence of local residual microstresses at grain boundaries. The stress character (compressive vs tensile) is governed by the type of crystalline secondary phase formed. Moreover, a variation in interface chemistry changes the glass network structure on the atomic level, which can promote transgranular fracture, i.e., can result in a low fracture resistance even in the presence of favorable large Si3N4 matrix grains. Therefore, secondary-phase chemistry plays a dominant role with respect to the mechanical behavior of liquid-phase-sintered Si3N4. Fracture toughness is, in particular, influenced by (i) altering the residual glass network structure, (ii) affecting the secondary-phase crystallization at triple pockets, and (iii) changing the Si3N4 grain size/morphology by affecting the diffusion rate in the liquid. The first two effects of secondary-phase chemistry are superimposed on the merely structural parameters such as grain diameter and apparent aspect ratio.

Microstructure and Fracture Toughness of Si3N4 Ceramics: Combined Roles of Grain Morphology and Secondary Phase Chemistry

Thermal expansion coefficients ( α ), glass-transition ( T g ) and Littleton ( T L ) temperatures, hardness and density of the RE–Si–Mg–O–N glasses (RE=Sc, Y, La, Nd, Sm, Gd, Yb, and Lu) were investigated to separate the effects of rare-earth elements and nitrogen on the properties of bulk glasses. These properties depend approximately linearly on nitrogen content, density of the glass and ionic radius of the corresponding lanthanide. Lanthanides modify hardness of the studied oxynitride glasses with 20–24 eq.% N by 11.4–14.4%, α by ∼13%, T g by 31–35 °C, and T L by 33–43 °C. Deviations from linearity found in Nd-, Sm- and Yb-doped glasses were possibly due to valence change. Non-lanthanide dopants, Sc and Y, often do not fit the dependence but result in comparable properties. The effects of nitrogen and rare-earth dopants are independent and additive.

Thermal expansion and glass transition temperature of the rare-earth doped oxynitride glasses

Debonding of interfaces between beta-silicon nitride whiskers and SiAlY oxynitride glasses

The effect of thermal expansion mismatch stress between silicon nitride and different grain boundary phases on the fracture toughness of silicon nitride was investigated. Different sintering aids in the Y-Mg-Si-Al-O-N system produced silicon nitride specimens with very similar microstructures but different grain boundary phase compositions and different values of fracture toughness. The fracture toughness of the silicon nitride increased as the thermal expansion coefficient of the grain boundary phase increased. The presence of tensile residual stress at the grain boundary caused by thermal expansion mismatch between the silicon nitride and the grain boundary phase enhanced crack deflection and grain bridging.

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Effect of the Grain Boundary Thermal Expansion Coefficient on the Fracture Toughness in Silicon Nitride

The thermal expansion coefficients and glass transition temperatures of Y-Mg-Si-Al-O-N glasses were measured using dilatometry. The thermal expansion coefficients of these glasses could be calculated on the basis of composition, using experimentally determined thermal expansion prefactors
.

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Thermal Expansion and Glass Transition Temperatures of Y-Mg-Si-Al-O-N Glasses

Baikowski International), A1,0, (Sumitomo, AKP-50), SiO, (99.9%), Si,N, (Ube, SN-E10, N > 38.0 wt%, 0 = 1.4 wt%), and AIN (Starck, N 33.5%). Powders were weighed and mixed in an agate mortar under isoprenyl for 1.5 h. The mixtures were dried and then isostatically pressed into a pellet (5 mm X 10 mm in diameter) under pressure of 300 MPa. Solid-state reaction was carried out by firing the pellets embedded in BN powder for 2 h. Some pellets were also fired for 2 h in a BN powder bed under 1.5 MPa nitrogen pressure (GPS) at 1640°C. Some of the compositions were hot pressed in a BN-lined graphite die for 1 h in flowing nitrogen under a pressure of 35 MPa at temperatures 1400” or 1550°C. After firing, some melted samples were annealed at 1300°C for 20 h to crystallize the crystal phase. The samples with weight loss <4 wt% after firing and <2 wt% after hot pressing were used for analysis. The X-ray diffraction technique was used for phase identification. The N-phase composition was also verified using electron probe microanalysis (EPMA). 111. Results and Discussion

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Irene M. Peterson

Papers

Effect of the Grain Boundary Thermal Expansion Coefficient on the Fracture Toughness in Silicon Nitride

Thermal Expansion and Glass Transition Temperatures of Y-Mg-Si-Al-O-N Glasses

Formation of N-phase and Phase Relationships in MgO–Si2N2O–Al2O3 System