Compositions in the α'-SiAlON-√-Si3N4 solid solution region in the system Si,Al,Y/N,O which contained a third phase as a sintering aid were prepared. Mixtures of starting materials were reaction sintered to full densities without applied pressure. Phases were identified and lattice parameters of β' and √ph ases were measured and compared with standards. The results were used to construct β'-SiAION-√-Si3N4 solidsolution tie lines in the two phase region. As expected, the specimens with a higher β'-SiAION content showed higher hardness and lower toughness values.

High density silicon oxynitride ceramics were fabricated by reaction-sintering a mixtureof silicon nitride and silica without the use of sintering aids. Precursor silicon nitride compacts were prepared by conventional means after which they were subjected to a lowtemperature oxidation heattreatment (∼1000ºC) producing a composite silicon nitride/silica compact. Oxidized compacts were then reaction-sintered in a nitrogen atmosphere at temperatures between 1400 and 1800ºC using a range of protective ‘powder-bed’ compositions. A ‘powder-bed’ comprising a mixture of boron nitride, silicon nitride and silica was found to be most effective in preventing decomposition and subsequent weight loss. Nearly complete reactive transformation to silicon oxynitride was observed under optimised sintering conditions.

The densification behavior and formation characteristics of R-α'-Sialons (R = Nd, Sm, Gd, Dy, Er and Yb) hot-pressed in the temperature range of 1500ºC-1750ºC have been studied. The temperatures required for full densification decrease and the content of the α-Sialon phase increases with an increasing atomic number of R.

μ-silicon nitride forms a range of solid solution Mx(Si,Al)12(0,N)16 where x = 2 and M is Li+, Ca2+, y3+ or certain lanthanide cations. This paper reports the formation of Ln-α'-sialons by the reaction of Si3N4 with AIN and Ln2O3 where Ln=Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y. Transformation reactions of Nd, Sm, Gd, Ho, Er and Y cations occurring during liquid phase pressureless sintering are reported. The effects of the volume and viscosity of the liquid phase and the amountof secondary N-melilite on sintering are discussed.

This article is a review of the selected activities since 1979 in using non-oxide additives as sintering aids for silicon nitride. Emphasis was placed on the work done in the authors' laboratory, and it is shown that ZrN and AIN in combined use can result in producing silicon nitride with good properties.

Raw β-Si3N4 powder was gas-pressure sintered with Y2 O3-Nd2O3additives at > 1700ºC. Graingrowth behavior was investigated in relation to sintering conditions. Selective growth of large grains was accomplished by sintering the powder at high temperatures with small amounts of additives. As a result, in-situ composites were obtained from β-powder.

The desired material properties have been attained by controlling the microstructural design using large grains. Materials with high reliability, having a Weibull modulus of about 50, were fabricated by maintaining a uniform size and distribution of elongated grains. Tough materials, having fracture toughness of, were developed by increasing the diameter of elongated grains. This method was applied to the sintering of refractory grade powder with the aim of lowering sintered material cost. Fairly good mechanical properties have been obtained even with impure powders.

The advantage of self-reinforced silicon nitride is the in-situ control of the microstructure. This control is provided in large degree by the chemistry of glassy phase which can be adjusted to tailor the morphology of silicon nitride grains as well as the matrix - reinforcement interface. The presence of high aspect ratio silicon nitride grains is necessary but not sufficient condition to produce materials with optimum properties. For maximum flexure strength and fracture toughness an optimized glass matrix is required.

Sinter/HIP consolidation with systematic variation of all relevant process parameters was performed for low additive (Y2O3-A12O3) silicon nitride material. With SEM-investigations as well as bending and creep tests the relevance of process parameters on microstructure development and mechanical properties was studied.

The microstructures of unreinforced and reinforced Si3N4 ceramics have been characterized by analytical electron microscopy in combination with quantitative microscopy. Special attention was paid to the effect which different additives, viz. metal oxides and SiC- and β-Si3N4-whiskers, have upon matrix morphology and intergranular microstructure. It was demonstrated that SiC-whiskers may suppress Si3N4 grain growth while an addition of β-Si3N4-whiskers results in a coarser Si3N4 microstructure. The different microstructures have been related to the mechanical properties of the ceramics.

A homogeneous mixture of Y2O3, CeO2 and MgO with a final weight ratio of 3:1: 2 was prepared by the alkoxide method. The powder mixture was then added into Si3N4 powder in amounts ranging from 4 to 12 wt%, andconsolidated by hot-pressing. Microstructure and mechanical properties of the sintered bodies were determined and compared to those of materials prepared by the conventional route of mixing the oxide powders as sintering aids individually in essentially same composition. The β-fraction (modification ratio) in same composition was higher in thesintered bodies made through the alkoxide method than those made through the conventional one. The room temperature flexural strength was maximized with 6wt% addition of the alkoxide derived oxide, whereas, 12wt% addition of the total oxide was required to maximize the strength by conventional processing.

A novel characterization method is applied to study the evolution of microstructures during densification of silicon nitride ceramics. This characterization method involves an immersion liquid for making green and sintered bodies transparent, and a subsequent direct optical microscopic examination. Granules were prepared with the spray drying processand formed into green bodies by CIP. After sintering at various temperatures, the specimens were examined for microstructural evolution. Large pores were located at the center and boundary regions of granules left in the green bodies; they were not removed by densification and resulted in large pores in the sintered body, possibly forming fracture origin in ceramics.

Sintered Si3N4 (with 2 w/o A12O3 and 6 w/o Y2O3) was studied by acoustic emission method during indentation testing and during heating to and cooling from 920ºC in air. The acoustic emission activity was discontinous and its magnitude and duration increased with load applied during indentation testing. Intense acoustic emission signals were detected on cooling Si3N4 ceramics to temperatures below 590ºC, particularly below 450ºC. X-ray diffraction studies showed the presence of SiO2 (Quartz) after heating the sample to 920ºC during acoustic emission study, compared to single phase β Si3N4 before the heat treatment.

Vickers and Knoop indentation methods were used to determine the deformation behavior and fracture toughness of single crystal α-Si3N4 at room temperature. The tests were performed on (0001), (1100) and (1120) faces; both hardness and toughness values were found to be independent of orientation within statistical variations. Thus, the mechanical properties of silicon nitride are essentially isotropic at room temperature.

Single crystals of α-Si3N4 were also compressed at 1760 and 1820°C at a strain rate of The sample tested at 1760°C showed yielding at a stress of 200MPa but it fractured before indicating a significant amount of plastic deformation. On the other hand, the sample compressed at 1820°C deformed to a strain of 2.7% at a stress of 4OMPa before fracturing. A significant density of dislocations was observed by transmission electron microscopy. From conventional g.b analysis, the Burgers vector of the dislocations was determined to be 1/3<1120>. From the line direction of the dislocations, the primary slip system of α-Si3N4 is determined to be {1101}<1120>.

Tensile fatigue behavior of a hot-isostatically-pressed (HIPed) silicon nitride was investigated over ranges of constant stresses, constant stress rates, and cyclic loading at 1150-1370°C. At 1150°C, static and dynamic fatigue failures were governed by a slow crack growth mechanism. Creep rupture was the dominant failure mechanism in static fatigue at 1260 and 1370°C. A transition of failure mechanism from slow crack growth to creep rupture appeared at stress rates ≤10−2 MPa/s for dynamic fatigue at 1260 and 1370°C. At 1 150-1370°C, cyclic loading appeared to be less damaging than static loading as cyclic fatigue specimens displayed greater failure times than static fatigue specimens under the same maximum stresses.

Cyclic and static fatigue properties of pressure-less sintered silicon nitride were evaluated at 1000° C in air and in nitrogen using four-point bending mode. The data of cyclic fatigue tests or static fatigue tests and the morphology of the fractured surfaces in nitrogen were compared with those in air. The cyclic fatigue behavior was remarkably influenced by the atmosphere, while the static fatigue was less influenced. Crack healing effect due to the oxidation around the crack are thought to be the most probable mechanism to affect the cyclic fatigue rate in air.

We have characterized the evolution of cavities during tensile creep of a Y2O3-hot isostatically pressed Si3N4, using precision density measurements, small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). The cavities are bimodally distributed in size. Lenticular, 200 nm-size cavities are common, and lie primarily on two-grain boundaries. Irregularly shaped 500-1000 nm-size cavities are rare and lie at multi-grain junctions, but comprise approximately half of the total volume fraction of cavities. Although the material shows a continuous decrease in strain rate with strain, the cavity volume fraction evolves linearly with strain. Cavities account for approximately 85% of the total strain at any point during creep.

The room temperature fracture behavior for in situ reinforced (ISR) silicon nitride is correlated to its microstructure and R-curve behavior. The relation of strength to fracture origin suggests that stable growth of the intrinsic flaw precedes catastrophic fracture. Grainbridging that generates a rising bridging stress behind the crack-tip has been proposed as the cause for stable crack growth, which in turn reduces the strength dependency on initial flaw size. As a result of strong bridging by the acicular β-Si3N4 grains, ISR Si3N4 is characterized for high Weibull modulus. At elevated temperatures, the material's tensile creep rupture behavior follows the Monkman-Grant type plot. A tensile creep rate of -10−9s−1 at 1260°C/250 MPa, 1300°C/180 MPa, and 1350°C/90 MPa has been recorded. This relatively strong creep resistance is related to the sliding-resistance of the acicular grains and the properties of the amorphous film between the grains in ISR Si3N4.

Fracture process zone in SiCw/Si3N4 ceramic composite was studied by a hybrid experimental-numerical analysis employing moire interferometry and finite element analysis. A chevron-notched, wedge-loaded double cantilever beam specimen was used to obtain a stable crack growth. The relation between crack closure stress and crack opening displacement which govern fracture process zone was obtained.

The mechanical properties of SiC whisker, SiCw, reinforced β-Sialon composite were investigated. β-Sialon with 20 vol% SiCw showed excellent high temperature strength, 610 MPa at 1350°C, and fracture toughness, 5.3 MPa √m (Chevron Notched Beam, CNB, method). This fracture toughness value was an improvement of 75% over that of β-Sialon (monolithic material). This composite also showed excellent oxidation-resistance; the weight gain due to oxidation after 200 hr at 1350°C in dry air was 0.3 mg/cm2, and after oxidation the high temperature strength, 1350°C, was 683 MPa, an improvement over the pre-oxidation value. Also, in high temperature creep testing (200 hr at 1350°C and 300 MPa) deformation of the SiCw reinforced β-Sialon composite, SiCw/β-Sialon was 66% smaller than that of β-Sialon.

In the present paper, the effect of La2O3-Y2O3-Al2O3 (LYA) and La2O3-Y2O3-MgO (LYM) additives on the mechanical properties of hot pressed SiC whisker-reinforced Si3N4 matrix composites and the relationship between whisker content and additive content were investigated in detail. The results show obvious strengthening in the case of LYA additives containing ≤2.4 wt% Al2O3. When the Al2O3 content of the additive was > 1.2 wt%, the fracture toughness decreased accompanied by decreased interfacial debonding and whisker pull-out. With LYM additives containing 2.4 wt% MgO, both the room temperature strength and toughness increased. A combination of Y2O3 + MgO (YM) with LYA additives produced substantial improvements in strength and toughness, up to 1050 MPa and ∼11 MPa√m, respectively. The Weibull modulus of a SiC whisker-reinforced composite was found to be greater than that of either the unreinforced Si3N4 or Si3N4 containing SiC particles. The experiments also demonstrated that in order to achieve full density the additive content had to be increased with increased whisker content. Preliminary but encouraging results on strength retention at elevated temperatures were also reported.