Preparation and toughening characterization of high fracture toughness Si3N4 ceramic with rodlike structure

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Compressive creep of Si3N4/MgO alloys

The compressive creep behaviour of four compositions within the Si₃N₄-Mg₂SiO₄-Si₂N₂O compatibility triangle were studied in air at 1400° C. Strain rate ( ) versus stress () was analysed to determine the stress exponent, n ( ). Cavitation during creep was determined by precise (sink-float) density measurements. Compositions close to the Si₃N₄-Si₂N₂O tie line exhibited no cavitation and had n1, whereas compositions close to the Si₃N₄-Mg₂SiO₂ tie line exhibited extensive cavitation and had n2. Test results are interpreted in terms of the volume fraction of the viscous phase present.     

Compressive creep of Si3N4/MgO alloys

The activation energy of two Si₃N₄/MgO materials, fabricated in the Si₃N₄-Si₂N₂O-Mg₂SiO₄ compatibility triangle of the Si-Mg-O-N system, was determined between 1300 and 1400° C under a compressive stress of 275 MPa. The activation energy of the material previously shown to exhibit pure diffusional creep was determined to be 660 kJ mol^–1, whereas the material shown to exhibit extensive cavitational creep had an activation energy of 1080 kJ mol^–1. These results are compared with results obtained by others for densification and slow crack growth.     

Controlled surface-flaw-initiated fracture in reaction-bonded Si3N4

Reproducible controlled surface flaws were introduced in reaction-bonded Si₃N₄ by Knoop microhardness indentation. Fracture was initiated at a single suitably oriented flaw on the tensile surface of a four-point bend specimen, and the stress required to propagate the controlled flaw catastrophically was used to calculate the critical stress intensity factor K _IC from standard fracture-mechanics formulae for semi-elliptical surface flaws in bending. K _IC was unchanged up to 1200° C, but values above this temperature together with those at room temperature after vacuum annealing were reduced approximately 25%. This reduction is thought to be due to reaction with oxygen concentrations present in the vacuum atmosphere rather than to a change in material properties. Although flaw healing was observed for annealing in air, it was accompanied by a decrease in fracture stress. It is suggested that this is due to volume mismatch between Si₃N₄ and the oxidation product, cristobalite, which introduces tensile stresses in the matrix material.     

Compression creep of Si3N4/MgAl2O4 alloys

The compression creep behaviour in air of pressureless sintered MgAl₂O₄-doped Si₃N₄ was studied at temperatures between 1473 and 1593 K, and under stresses between 50 and 300 M Pa. The variation of strain rate with stress and temperature was analysed to determine the stress exponent and the activation energy. Microstructural change was investigated by transmission electron microscopy. Up to 1533 K and 200 M Pa (low temperature, low stress) viscous creep (n 1) appeared as the predominant mechanism of deformation. At 1593 K and 200 to 300 M Pa (high temperature, high stress), extensive cavitation in intergranular vitreous phase produces the fracture of the samples. In the low-temperature region, strain whorls which are characteristic of grain-boundary sliding were observed at grain-boundaries in crept specimens.     

Preparation and dielectric properties of Si3N4/SiCw composite ceramic

Gelcasting was employed to fabricate Si₃N₄/SiC whisker (SiCw) composite ceramics, and the effects of heat-treatment temperature on the length-to-diameter ratio of the whiskers and SiCw content on microwave dielectric properties were studied. Compared with pure SiCw of spherical structure obtained at temperature of 1,750 °C(Ar), pure SiCw treated at 1,600 °C(Ar) showed rod-like structure, higher dielectric properties and more evenly distribution in Si₃N₄/SiCw composite ceramics. Both the real (ε′) and imaginary (ε″) permittivity of Si₃N₄/SiC whisker (SiCw) composite ceramics decreased with increasing frequency and increased as the whisker content raised owing to the interface and SiCw playing a role of dipole in the frequency range of 8.2–12.4 GHz. In addition, comparing the ceramics with lower content of SiCw, the reflectivity of the composite ceramics moved to a lower frequency; the maximum absorption peak reached ?22.4 dB at the whisker content of 15 wt%.     

The fractal geometry of Si3N4 wear and fracture surfaces

Good mechanical properties and chemical stability at high temperatures make silicon nitride a good candidate as an advanced engine material. Much research has been done to characterize the mechanical strength and resistance of crack propagation in this material. In this paper, we use fractal analysis to study the geometry of Si3N4 fracture and wear surfaces. We found that the geometries of the failure surfaces as characterized by the fractal dimensional increment, D*, under different failure stress states are similar for the same brittle material, but different for different brittle materials. The similar D* in an identical brittle material implies that the failure process in the material is the same regardless of loading mode, i.e., mode I or mixed-mode stress. The fractal technique is shown to be useful for correlating the fractal dimension to the material properties and fracture-surface topography. This revised version was published online in November 2006 with corrections to the Cover Date.     

h-BN/Si3N4陶瓷复合材料的断裂行为及断裂韧性

以亚微米级α-Si3N4和h-BN粉末为原料,Y2O3-Al2O3为助烧剂,采用热压烧结制备了h-BN/Si3N4陶瓷复合材料.研究3h-BN含量对h-BN/Si3N4陶瓷复合材料断裂韧性及其断裂行为的影响.结果表明:随着h-BN含量增加,柱状β-Si3N4晶粒的直径和长径比均下降;未加h-BN时,β-Si3N4陶瓷以沿晶断裂为主,添加体积含量为6%和8%的h-BN后,复合材料出现明显的沿晶和穿晶断裂,而添加10%h-BN的陶瓷复合材料则以沿晶断裂为主.随着h-BN含量增加,h-BN/Si3N4陶瓷复合材料的断裂韧性下降,但由于h-BN颗粒对裂纹扩展的影响,因而其下降程度不大.     

熔盐热析出反应金属化Si3N4与Si3N4的连接

在采用熔盐热析出反应在Si₃N₄陶瓷表面沉积钛金属膜的基础上,对CuAg合金在金属化表面的润湿性进行了研究,结果表明,CuAg合金能对采用该方法金属化的Si₃N₄陶瓷实现良好润湿．在此基础上,成功实现了钛金属化Si₃N₄陶瓷与Si₃N₄陶瓷的连接并对连接工艺进行了系统研究．连接界面的TEM研究发现,界面上广泛存在Ti-Cu－Si-N相并对这种相对连接强度的影响进行了讨论．     

High-resolution interface analysis of SiC-whisker-reinforced Si3N4 and Al2O3 ceramic matrix composites

Whisker/matrix interfaces between -SiC whiskers and -Si₃N₄ or -Al₂O₃ matrices in composites were examined by high-resolution electron microscopy (HREM), and electron energy loss (ELS) and energy dispersive X-ray (EDS) spectroscopies. Most whisker/matrix interfaces were crystalline, with whiskers directly bonded to matrix crystals. Some whisker/matrix interface regions contained amorphous thin films and these occurred more often in the Si₃N₄ composite, which contained sintering additives, than in the Al₂O₃ matrix composite, which did not. No evidence for light element segregation at crystalline whisker/matrix interfaces was detected by ELS or EDS at 5 nm spatial resolution. Impurities were concentrated in glassy regions in matrix grain boundaries, triple junctions, or at infrequent whisker/matrix interfaces containing amorphous films.     

Indentation mechanics and fracture behavior of metal/ceramic nanolaminate composites

Composite laminates on the nanoscale have unique properties, such as high strength, high wear resistance, and biocompatibility. In this paper we report on the nanoindentation behavior of a model metal–ceramic nanolaminate consisting of alternating layers of aluminum and silicon carbide (Al/SiC) processed by PVD on a Si substrate. Composites with different layer thicknesses were fabricated and the effect of layer thickness on Young’s modulus and hardness was quantified. The effect of indentation depth on modulus and hardness was studied. The damage that took place during nanoindentation was examined by cross-sectioning the samples by focused ion beam (FIB) technique and imaging the surface using scanning electron microscopy (SEM). Finite element modeling (FEM) of nanoindentation of nanolaminates was conducted. The damage patterns observed in experiments were qualitatively supported by the numerical simulations.