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 MPa{center_dot}m{sup 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 MPa{center_dot}m{sup 1/2} coupled with a reduced fracture strength.

Microstructural Design of Silicon Nitride with Improved Fracture Toughness: I, Effects of Grain Shape and Size

The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β-Si3N4 singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation Dn–D0n=kt, with growth exponents of 3 and 5 for the c-axis direction and the a-axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m1/2, retaining high strength levels of about 1 GPa.

Microstructure Control of Silicon Nitride by Seeding with Rodlike β‐Silicon Nitride Particles

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

Different microstructures in Si{sub 3}N{sub 4} ceramics containing Y{sub 2}O{sub 3} and Al{sub 2}O{sub 3} as sintering additives were prepared by two-step sintering. Pull-out and elastic bridging were most frequently observed as the toughening mechanisms in samples with fine-grained microstructures having needlelike {beta}-Si{sub 3}N{sub 4} grains with diameters of 1 {micro}m. The values of fracture toughness were varied from 6.1 to 8.2 MPa {center_dot} m{sup 1/2} with respect to the microstructural characteristics, characterized by the volume fraction of needlelike grains and their diameter.

Relationship between Microstructure, Toughening Mechanisms, and Fracture Toughness of Reinforced Silicon Nitride Ceramics

https://www.tandfonline.com/doi/pdf/10.1088/1468-6996/9/3/033001

Textured silicon nitride: processing and anisotropic properties

Improved KIC values of monolithic Si3N4 ceramics will be expected by growing-in a matrix of microcrystals of β-phase Si3N4-an appreciable amount of extensively large, rod-shape crystals of the same phase. It is anticipated that the large grains should play as crack impediments like whiskers mixed in composite ceramics. The test materials prepared by changing firing conditions had such well-grown crystals of different sizes, ranging from 2 to 10 microns in diameter. The fracture toughness (measured by the SEPB method) increases up to 11.3MPa·m1/2 with the increase in the grain size. Prominent crack bridging along the fracture surface behind the propagating crack tip is found to be the possible toughening mechanism.

Grain Size Dependence of the Fracture Toughness of Silicon Nitride Ceramics

A base material (1) of an exhaust valve is laminated, in orderfrom an uppermost sintered ceramics (2) with a bonding metal material comprising an active metal (3) and an insert material (4) for moderating thermal stress which is composed as a main of low thermal expansion material. The materials are different in thermal serving function between the base material and the sintered ceramics, so that generation of thermal stress is moderated during cooling after bonding.

Components for engines and production therefor

Method and structure of joining sintered ceramic to metal

Long and short fatigue crack growth behavior in silicon nitride.

PURPOSE:To enhance the extent of joint strength in a ceramic material, by joining a sintered ceramic seat member to a sheet part forming position of a metallic atomizer body by means of solid phase diffused junction after interposing two types of intermediate members composed of a specified material between the seat member and the said body CONSTITUTION:A metallic atomizer body 1 of a fuel injection valve is provided with a needle valve insertion hole 2, a fuel oil supply hole 3, a seat surface 4, and a fuel injection nozzle 5 When a sintered ceramic seat member 6 is joined to a seat part forming position of the atomozier body 1 like this, both first and second intermediate members 7 and 8 are interposed between the seat member 6 and the atomizer body 1 from the seat member side, and the seat member 6 is joined to the seat part forming position by means of solid phase diffused junction And, the first intermediate member 7 is made up of an Al-Al alloy and/or a Cu-Cu alloy or a compound material subjected to a compound layer structure, while the second intermediate member 8 is made up of Ni With this constitution, joint strength in the seat member 6 is enhanced and durability in the injection valve is improved

Hiromi Okamoto

Papers

Grain Size Dependence of the Fracture Toughness of Silicon Nitride Ceramics

Components for engines and production therefor

Method and structure of joining sintered ceramic to metal

Long and short fatigue crack growth behavior in silicon nitride.

Manufacture of fuel injection valve