Effect of amount and composition of additives on the fracture toughness of silicon nitride (original) (raw)
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Determination of fracture toughness and elastic module in materials based silicon nitride
Ingeniería Investigación y Tecnología
The knowledge of the mechanical properties of any material subjected to loads is necessary for its use in structural applications. Silicon nitride (Si3N4) ceramics are well-known materials used in engineering applications due to their outstanding combination of high strength and fracture toughness. The most studied mechanical properties of Si3N4 are hardness, fracture toughness and mechanical resistance. Recent advances in the production processes that incorporate high purity rare earth elements as sintering additives have improved these mechanical properties. Using Vickers indentation method, the elastic module and fracture toughness of Si3N4 based materials modified with La2O3, Y2O3 and Al2O3 were determined as a function of the cracking system type that prevails under the effect of load. The results indicate that adding rare earth to the matrix increased the fracture toughness the Si3N4 base ceramic Samples containing La2O3+Y2O3 showed higher values of fracture toughness than the...
Journal of the American Ceramic Society, 2004
The Mode I fracture toughness (K IC ) of a small-grained Si 3 N 4 was determined as a function of hot-pressing orientation, temperature, testing atmosphere, and crack length using the single-edge precracked beam method. The diameter of the Si 3 N 4 grains was <0.4 µm, with aspect ratios of 2-8. K IC at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPaؒm 1/2 for the T-S and T-L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in K IC was observed through 1200°C, to ∼4.1 MPaؒm 1/2 , before increasing rapidly to 7.5-8 MPaؒm 1/2 at 1300°C. The decrease in K IC through 1200°C was a result of grain-boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in K IC . Crack healing was observed in specimens annealed in air. No R-curve behavior was observed for crack lengths as short as 300 µm at either 25°or 1000°C.
Journal of the American Ceramic Society, 2005
The use of self-reinforcement by larger elongated grains in silicon nitride ceramics requires judicious control of the microstructure to achieve high steady-state toughness and high fracture strength. With a distinct bimodal distribution of grain diameters, such as that achieved by the addition of 2% rodlike seeds, the fracture resistance rapidly rises with crack extension to steady-state values of up to 10 MPaؒm 1/2 and is accompanied by fracture strengths in excess of 1 GPa. When the generation of elongated reinforcing grains is not regulated, a broad grain diameter distribution is typically generated. While some toughening is achieved, both the plateau (steady-state) toughness and the R-curve response suffer, and the fracture strength undergoes a substantial reduction. Unreinforced equiaxed silicon nitride exhibits the least R-curve response with a steady-state toughness of only 3.5 MPaؒm 1/2 coupled with a reduced fracture strength.
Ceramics International
Composite materials based on Si 3 N 4 were synthesized by introducing previously prepared b-Si 3 N 4 seeds into a mixture of a-Si 3 N 4 and 10 wt% of sintering additive. The densification process during pressureless sintering was studied in the presence of CeO 2 additive, which is less frequently used as sintering aid to Si 3 N 4 . The effect of seed concentration and sintering time on the densification and fracture toughness was followed. The results showed that the addition of seeds retarded densification. On the other hand, fracture toughness passes through maximum at the content of 3 wt% seeds. #
Effect of β-Si3N4 seeds on densification and fracture toughness of silicon nitride
Ceramics International, 2006
Composite materials based on Si 3 N 4 were synthesized by introducing previously prepared b-Si 3 N 4 seeds into a mixture of a-Si 3 N 4 and 10 wt% of sintering additive. The densification process during pressureless sintering was studied in the presence of CeO 2 additive, which is less frequently used as sintering aid to Si 3 N 4 . The effect of seed concentration and sintering time on the densification and fracture toughness was followed. The results showed that the addition of seeds retarded densification. On the other hand, fracture toughness passes through maximum at the content of 3 wt% seeds. #
Correlation between fracture toughness and microstructure of seeded silicon nitride ceramics
Journal of Materials Science, 2007
Microstructure development and fracture toughness of Si3N4 composites were studied in the presence of seeds and Al2O3 + Y2O3 as sintering aids. The elongated β-Si3N4 seeds were introduced into two different α-Si3N4 matrix powders; one was the ultra fine powder matrix and the other was the coarse powder matrix. The amount of seeds varied from 0 to 6 wt%. The grain growth inhibition and the mechanism of toughening were discussed and correlated with microstructure. The maximum fracture toughness of 9.0 MPa m1/2 was obtained for ultra fine powder with 5 wt% seeds hot pressed at 1,700 °C for 6 h.
Fracture toughness of Si3N4 processed by gas pressure sintering and hot pressing
This present work evaluates the influence of microstructure on the fracture toughness of two types of silicon nitride. The two microstructural types of silicon nitride were processed using the gas pressure sintering (GPS) and hot pressing (HP) pathways. The fracture toughness was measured using the Single Edge V-Notch Beam (SEVNB) and Chevron Notch Beam (CNB) methods. The results from both methods for the two forms were in close agreement (with a maximum variation of 5.8%); the KIc of the material processed by HP was 35% higher than that of GPS and the grain length had a direct influence on the fracture toughness.
2002
To better understand the high temperature properties of the ESIS SiliconNitride Reference Material, the secondary phases it contains are investigated using X-raydiffraction, Differential Scanning Calorimetry and optical microscopy image analysis. TheImpulse Excitation Technique was used to determine the elastic and damping properties,both at room and elevated temperature.Tests revealed the presence of a substantial amount of amorphous intergranular phase,which passes a glass transition around 950°C. This observation is used to interpret thehigh temperature fracture strength of the silicon nitride, as determined by other partners inthe Reference Material Testing Program. It is also shown that the amorphous intergranularphase has limited or no tendency to crystallise, which will facilitate interpretation of timeandloading-rate-dependent and long term behaviour at elevated temperature.Differences between surface and core of the sintered plates are observed. The content ofa crystalline ...
MRS Proceedings, 2002
ABSTRACTThe microstructural evolution and structural characteristics and transitions in the thin grain-boundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si3N4-2wt% Y2O3 ceramic for ∼200 hr at 1400°C in air, the fracture toughness can be increased by ∼100%, coincident with a change in fracture mechanism from transgranular to intergranular. Structural phase transformations occur in the thin grain boundaries during oxidation that are revealed by XRD, EDX, Raman and EELS analyses. They affect the local bonding of atoms. It is concluded that only specific crystal “building blocks”, i.e., tetrahedra, are transformed along the grain boundary and the resulting difference in the atomic structure of the oxide interface is seen directly to alter the macroscopic fracture behavior.