Paul F. Becher

Observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions.

Silicon nitride (Si3N4) ceramics are used in numerous applications because of their superior mechanical properties. Their intrinsically brittle nature is a critical issue, but can be overcome by introducing whisker-like microstructural features. However, the formation of such anisotropic grains is very sensitive to the type of cations used as the sintering additives. Understanding the origin of dopant effects, central to the design of high-performance Si3N4 ceramics, has been sought for many years. Here we show direct images of dopant atoms (La) within the nanometre-scale intergranular amorphous films typically found at grain boundaries, using aberration corrected Z-contrast scanning transmission electron microscopy. It is clearly shown that the La atoms preferentially segregate to the amorphous/crystal interfaces. First-principles calculations confirm the strong preference of La for the crystalline surfaces, which is essential for forming elongated grains and a toughened microstructure. Whereas principles of micrometre-scale structural design are currently used to improve the mechanical properties of ceramics, this work represents a step towards the atomic-level structural engineering required for the next generation of ceramics.

Surface/Interface-Related Conductivity in Nanometer Thick YSZ Films

Results of the electrical conductivity study of highly textured, ultrathin (15 nm) cubic yttria-stabilized zirconia (YSZ) thin films are presented for the first time. A nanoscale effect that results in exceptionally high ionic conductivity at moderate temperatures is detected in films less than 60 nm thick. The conductivity increases continuously below this level and reaches 0.6 S/cm at 800°C for a 15 nm thick film, which represents the highest reported value for the YSZ system. The observed behavior is attributed to an increasingly significant contribution of the surface/interface conductivity with decreasing film thickness. These observations can have important implications for the development of nanostructured electrochemical devices with enhanced performance.

Relation of transformation temperature to the fracture toughness of transformation-toughened ceramics

AbstractThe stress induced tetragonal to monoclinic ZrO2 martensitic transformation contribution to fracture toughness is described in terms of the required external strain energy and the thermo-dynamic stability of the constrained tetragonal phase. The strain energy, derived from an externally applied stress acting on the main crack, required to achieve transformation toughening is shown to be a function of the term (T - Ms) whereT is the test temperature andMs is the martensite start temperature for the case ofT > Ms. Thus for a givenT (T > Ms), the transformation toughening component increases asMs approachesT and for a fixedMs, the toughness decreases asT increases. Experimental data for partially stabilized zirconia ceramics confirm these results and show that increasing tetragonal precipitate size is the primary feature which affects an increase inMs. In the case ofT ⩽Ms, autotransformation occurs, resulting in decreasing toughness with decrease inT due to a continuous loss in the tetragonal phase content. A temperature region is thus obtained over which transformation toughening exhibits a maximum in its contribution. The temperatures over which this occurs then is shown to be dependent on theMs temperature of the material.

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.

Using Microstructure to Attack the Brittle Nature of Silicon Nitride Ceramics

The evolution of silicon nitride ceramics over the last two decades has brought about the advancement of materials which were first fabricated by the application of mechanical pressure and temperature (i.e., hot pressing) resulting in high flexure strengths (e.g., 700–800 MPa) but rather poor resistance to creep at temperatures of ~1200°C. At the same time, these ceramics remained quite brittle with fracture-toughness values of 4–5 MPa m½, such that strengths were very sensitive to flaw or crack sizes. As a result, measured strengths exhibited considerable scatter, as reflected by a low Weibull modulus. In the ensuing years, approaches were sought to develop more economical methods of fabricating silicon nitride components by densifying to near-net shape. Methods were also sought for increasing the elevated-temperature reliability by minimizing the additives employed to promote densification and by utilizing additives that produced more stable and refractory grain boundary phases. The application of gas-pressure sintering methods, utilizing gaseous environments of 10–100 atmospheres, led to the ability to produce dense near-net shaped components with very high fracture strengths (e.g., ≥1000 MPa). At the same time, advances in processing and additive chemistry, sometimes combined with additional fabrication methods (e.g., hot isostatic pressing), have resulted in ceramics with excellent creep resistances at temperatures in excess of 1300°C. Some of these silicon nitride ceramics exceed the elevated-temperature capability of superalloys by 200°C. The initial desire for light-weight ceramic components that could sustain tensile loads for high-temperature applications is, indeed, beginning to bear fruit. One of the most impressive examples of the development of a complexly shaped lightweight component is the silicon nitride turbocharger rotor used in a number of Japanese automobiles, which is currently manufactured at a cost approaching that of the opposing superalloy rotor and provides exceptionally high mechanical reliability and production yields. Currently, there are also earnest efforts to incorporate silicon nitride valves for engines, as well as in a variety of other components (e.g., combustion swirl chambers, valve-lifter pads, etc.). The acceptance and use of this class of brittle materials, which were once considered prohibitively expensive for fabrication into complex shapes and not suited for such applications, is a remarkable testimony of the progress that has been made.

The importance of amorphous intergranular films in self-reinforced Si3N4 ceramics

Abstract High-fracture-strength and high-toughness β-Si 3 N 4 ceramics can be obtained by tailoring the size and number of the elongated bridging grains. However, these bridging mechanisms rely on debonding of the reinforcing grains from the matrix to increase toughness. Interfacial debonding is shown to be influenced by sintering aids incorporated in the amorphous intergranular films. In one case, the interface strength between the intergranular glass and the reinforcing grains increases with the aluminum and oxygen content of an interfacial epitaxial β-SiAlON layer. In another, the incorporation of fluorine in the intergranular film allows the crack to circumvent the grains. Atomic cluster calculations reveal that these two debonding processes are related to (1) strong Si–O and Al–O bonding across the glass/crystalline interface with an epitaxial SiAlON layer and (2) a weakening of the amorphous network of the intergranular film when difluorine substitutes for bridging oxygen.

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.

Microstructural Contributions to the Fracture Resistance of Silicon Nitride Ceramics

To achieve toughening by the crack bridging process the introduction of large elongated grains by fracture resistance is necessary but not sufficient. While increasing the diameter of the elongated grains can increase the toughening effect, this requires that fracture occur along grain interfaces rather than through the grains. This interface debonding process appears to be modified by the chemistry of the oxynitride glass at the grain boundaries. Experiments show that increasing the yttria to alumina ratio or decreasing the ntirogen content of Si-AI-O-N glasses promotes interfacial debonding. The crack bridging contributions to the R-curve behavior is also a function of the content and size of the bridging reinforcement as noted in whisker-reinforced ceramics. Thus, control of micrstructure and interfacial phases is critical to the development of toughened silicon nitiride ceramics.

Residual thermal stresses in ceramic composites. Part I: with ellipsoidal inclusions

Abstract Residual thermal stresses in ceramic matrix composites containing ellipsoidal inclusions are analyzed using a modified Eshelby model. Closed-form analytical solutions are obtained; however, their formulations are formidable. When the inclusion is disc-shaped, spherical, or fiber-shaped, simple analytical solutions can be obtained using different models, and they are in excellent agreement with those obtained from the modified Eshelby model. The analytical solutions are compared with the experimental and finite element results. Also, effects of the aspect ratio and the volume fraction of inclusions on residual thermal stresses are examined.