Shirley B. Waters

Debonding behavior between β-Si3N4 whiskers and oxynitride glasses with or without an epitaxial β-SiAlON interfacial layer

Abstract In order to gain insight on the influence of intergranular glass on the fracture toughness of silicon nitride, the debonding behavior of the interface between the prismatic faces of β-Si3N4 whiskers and oxynitride glasses was investigated in model systems based on various Si–(Al)–Y(Ln)–O–N (Ln: rare-earth) oxynitride glasses. It was found that while the interfacial debonding strength increased when an epitaxial β′-SiAlON layer grew on the β-Si3N4 whiskers, the critical angle for debonding was lowered with increasing Al and O concentrations in the SiAlON layer. Only in the absence of a SiAlON epitaxial layer, were debonding conditions altered by residual stresses imposed on the interface due to thermal–mechanical mismatch. A possible explanation for the effect of SiAlON formation and its composition on the debonding behavior is suggested by first-principles atomic cluster calculations. It is concluded that by tailoring the densification additives and hence the chemistry of the intergranular glass, it is possible to improve the fracture resistance of silicon nitride.

Properties of siliconaluminumyttrium oxynitride glasses

Abstract The thermal and mechanical properties of two SiAlYON systems, including linear thermal expansion coefficient, glass transition temperature, elastic modulus, hardness and fracture toughness, were characterized. Results are compared with those previously reported in the literature in an attempt to expand our knowledge of silicon oxynitride glasses. It is found that increasing the nitrogen content generally results in decreasing the linear thermal expansion coefficient while increasing the glass transition temperature, the elastic modulus and the hardness. These observations are consistent with previous studies on similar oxynitride glasses. Present work also shows that the Y:Al cation ratio has a stronger effect on the thermal properties of the glasses than does the nitrogen content, a factor not systematically examined before. The outcome of this work adds to the database on oxynitride glasses which is essential in research on silicon nitride ceramics.

Ceramic composites with a ductile Ni3Al binder phase

Abstract Composites of B-doped ductile Ni3Al alloys with both non-oxide (WC, TiC) and oxide (Al2O3) ceramic powders were produced by hot-pressing. The Ni3Al alloys wet the non-oxide ceramic powders well and form a semi-continuous intergranular phase. However, the Ni3Al alloys do not wet the oxide powders well and tend to form discrete “islands” of the metallic phase. Mechanical property testing showed the flexural strength is retained to temperatures of at least 800 °C. The fracture toughness and hardness were found to be equal to or higher than comparable Co-based hardmetal systems. Initial corrosion tests showed excellent resistance to acid solutions.

Fracture strength characterization of tubular ceramic materials using a simple c-ring geometry

The potential application of the c-ring compression specimen for strength determinations involving tubular ceramic samples was investigated. Mathematical descriptions of the stress distributions generated within the c-ring were derived using several approaches. The resulting equations were used in conjunction with a Weibull analysis to predict a relationship between failure probability and fracture strength. Finally, the validity of the c-ring test was examined for a SiC ceramic by comparing the c-ring strength data with that obtained from four-point bend samples.

Thermal expansion anisotropy in hot-pressed SiC-whisker-reinforced alumina composites

The measured linear thermal expansion coefficients α of hot-pressed SiC-whisker-reinforced aluminas decrease as the SiC whisker content is increased from 1 to 60 vol.%. However, owing to the preferred orientation of the whiskers, the α value for each SiC whisker content is greater in the direction parallel compared with the direction perpendicular, to the hot-pressing axis. The observed behavior is generally consistent with that predicted by a model based on the effects of internal thermal mechanical mismatch strains in composites with oriented reinforcements.

Control of interface fracture in silicon nitride ceramics: influence of different rare earth elements

The toughness of self-reinforced silicon nitride ceramics is improved by enhancing crack deflection and crack bridging mechanisms. Both mechanisms rely on the interfacial debonding process between the elongated {Beta}-Si{sub 3}N{sub 4} grains and the intergranular amorphous phases. The various sintering additives used for densification may influence the interfacial debonding process by modifying the thermal and mechanical properties of the intergranular glasses, which will result in different residual thermal expansion mismatch stresses; and the atomic bonding structure across the {Beta}-Si{sub 3}N{sub 4} glass interface. Earlier studies indicated that self-reinforced silicon nitrides sintered with different rare earth additives and/or different Y{sub 2}O{sub 3}:AI{sub 2}0{sub 3} ratios could exhibit different fracture behavior that varied from intergranular to transgranular fracture. No studies have been conducted to investigate the influence of sintering additives on the interfacial fracture in silicon nitride ceramics. Because of the complexity of the material system and the extremely small scale, it is difficult to conduct quantitative analyses on the chemistry and stress states of the intergranular glass phases and to relate the results to the bulk properties. The influence of different sintering additives on the interfacial fracture behavior is assessed using model systems in which {Beta}-Si{sub 3}N{sub 4}whiskers are embedded in SIAIRE (RE: rare-earth) oxynitride glasses. By systematically varying the glass composition, the role of various rare-earth additives on interfacial fracture has been examined. Specifically, four different additives were investigated: Al{sub 2}0{sub 3}, Y{sub 2}0{sub 3}, La{sub 2}O{sub 3}, and Yb{sub 2}O{sub 3}. In addition, applying the results from the model systems, the R- curve behavior of self-reinforced silicon nitride ceramics sintered with different Y{sub 2}0{sub 3}:AI{sub 2}0{sub 3} ratios was characterized.

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained ∼90% α-SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10 × 10−10 s−1) at temperatures ≤1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (∼0.8 to 3 × 10−9 s−1) at stresses below a threshold value of ∼190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n = 17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature ≤1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.

Tailoring the Intergranular Phases in Silicon Nitride for Improved Toughness

Intergranular glass phases can have a significant influence on fracture resistance (R-curve behavior) of Si nitride ceramics and appears to be related to debonding of the {beta}-Si{sub 3}N{sub 4}/oxynitride-glass interfaces. Applying the results from {beta}- Si{sub 3}N{sub 4}-whisker/oxynitride-glass model systems, self- reinforced Si nitrides with different sintering additive ratios were investigated. Si nitrides sintered with a lower Al{sub 2}O{sub 3}: Y{sub 2}O{sub 3} additive ratio exhibited higher stead-state fracture toughness together with a steeply rising R-curve. Analytical electron microscopy suggested that the different fracture behavior is related to the Al content in the SiAlON growth band on the elongated grains, which could result in differences in interfacial bonding structures between the grains and the intergranular glass.