Creep of reaction-bonded silicon nitride

The high temperature mechanical properties, mainly the creep behaviour of reaction-bonded silicon nitride (RBSN), a new engineering ceramic for the gas turbine, have been a point of considerable interest. During the recent development a remarkable increase of the creep resistance of RBSN has been reached and the latest data show creep rates of below 10^–6 h^–1 at 1300° C and 70 to 100 MM m^–2. Activation energies between 540 and 700 kJ mol^–1 and stress exponents of 1in vacuo. Methods to determine the amount of internal oxidation,namely X-ray diffraction analysis, electron microprobe analysis and Rutherford backscattering of -particles were used. The deleterious effects of the internal oxidation are explained in terms of the microstructure, mainly porosity and pore size distribution, and ways to avoid this effect are discussed.     

Quantitative microstructural analysis of reaction-bonded silicon nitride

Quantitative analysis of the essential microstructural parameters of injection-moulded reaction-bonded silicon nitride was performed. The effect of silicon grain size on the porosity, the pore size distribution, the degree of reaction and the proportions of the alpha and beta modifications and the grain size of these phases was studied under constant processing conditions. With increasing silicon grain size the degree of reaction and the total porosity and density remain unaffected. The percentage of beta phase, the grain size of alpha and beta modifications as well as the mean pore diameter increase with increasing grain size of the starting powders. The effect of the structural parameters on the room-temperature strength is discussed.     

Oxidation protection of porous reaction-bonded silicon nitride

Oxidation kinetics of both as-fabricated and coated reaction-bonded silicon nitride (RBSN) were studied at 900 and 1000°C with thermogravimetry. Uncoated RBSN exhibited internal oxidation and parabolic kinetics. An amorphous Si-C-0 coating provided the greatest degree of protection to oxygen, with a small linear weight loss observed. Linear weight gains were measured on samples with an amorphous Si-N-C coating. Chemically vapour deposited (CVD) Si₃N₄ coated RBSN exhibited parabolic kinetics, and the coating cracked severely. A continuous-SiC-fibre-reinforced RBSN composite was also coated with the Si-C-O material, but no substantial oxidation protection was observed.     

Fracture strength of low-pressure injection-moulded reaction-bonded silicon nitride

Reaction-bonded silicon nitride (RBSN) samples were fabricated via a low-pressure injection-moulding technique. Sample batches of 58, 68, and 70 vol % silicon solids loading were moulded using a multicomponent, nonaqueous binder. These samples were analysed in terms of their nitrided bulk density, flexural strength, and microstructure. Bulk densities of 2.9 g cm^–3 (91% theoretical density) and three-point moduli of rupture in excess of 304 MPa (44×10³ p.s.i.) were achieved. These results indicate a potential use of low-pressure injection moulding as a forming technique for the fabrication of RBSN components.     

Vapour transport of magnesia into reaction-bonded silicon nitride

Magnesia, as a sintering additive, can be introduced into reaction-bonded silicon nitride (RBSN) via vapours-phase transport. The principal process variables were studied; a controlling factor was found to be the amount of silica on the internal surfaces of the ceramic. Through a controlled oxidation of the RBSN, the amount of magnesia introduced to the compact was increased to 2 wt% allowing a post-sintered density of 93% theoretical to be achieved. Further increases in internal oxidation, and consequent magnesia uptake, were limited by the oxidation of Si₃N₄ whiskers on the surface of the RBSN and in its pore structure.     

The permittivity and dielectric loss of reaction-bonded silicon nitride

Using recently developed coaxial line methods values of permittivity and dielectric loss have been determined over the frequency range 0.5 to 7 GHz for a series of reaction-bonded silicon nitride specimens in which the degree of nitridation has been varied. For fully nitrided material (having a weight gain of 62% and a volume porosity of 19%) the measured permittivity was 4.60 and was almost independent of frequency; fitting both the permittivity and loss data to the Universal Law of dielectric response confirmed that the limiting condition of lattice loss applied withn=0.98±0.02. Reduction of the degree of nitridation caused progressive increases in permittivity and loss, both of which closely approached the quoted values for pure silicon at weight gains below about 40%.     

Cyclic fatigue of reaction-bonded silicon nitride at elevated temperatures

Cyclic and static loading tests were performed on reaction-bonded silicon nitride from 1000–1400 °C in air. This porous, fine-grained material contained no glassy grain-boundary phase and exhibited no slow crack growth at room temperature. Under cyclic loading, the crack-growth behaviour at 1000 °C was similar to room-temperature results; however, at 1200 and 1400 °C crack-growth rates increased significantly. Under static loading, significant crack growth was detected at 1000 °C and increased with temperature. Most of the crack growth under cyclic loading was attributed to slow crack-growth mechanisms, but evidence of cyclic crack-growth mechanisms were also observed. Oxidation played a major role in crack-growth velocity at high temperature. This revised version was published online in November 2006 with corrections to the Cover Date.     

Some factors influencing the formation of reaction-bonded silicon nitride

Several salient factors influencing the formation of reaction-bonded silicon nitride (RBSN) compacts have been studied. These include the effects of mullite and alumina furnace tubes typically employed during high-purity nitridation studies, pre-sintering of green silicon compacts, free powder versus compact nitridation, and the influence of metal/metal oxide additions. The latter studies have provided experimental evidence for enhancement due to dissociated nitrogen, and suggest that (1) -Si₃N₄ formation does not necessarily require a liquid phase, (2) atomic nitrogen stimulates -phase formation, and (3) the liquid phase provides an efficient source for volatile silicon, promoting -Si₃N₄. These conclusions are consistent with accepted mechanisms for the formation of the two phases.     

Fracture of flash oxidized,yttria-doped sintered reaction-bonded silicon nitride

The oxidation behaviour of a slip cast, yttria-doped, sintered reaction-bonded silicon nitride after flash oxidation was investigated. It was found that both the static oxidation resistance and flexural stress rupture life (creep deformation) were improved at 1000° C in air compared to those of the same material without flash oxidation. Stress rupture data at high temperatures (1000 to 1200° C) are presented to indicate applied stress levels for oxidation-dependent and independent failures.     

Structure,formation mechanisms and kinetics of reaction-bonded silicon nitride

This paper reports on models which have been developed to explain the formation mechanism and kinetics of the major microconstituents of reaction-bonded silicon nitride. These models are based on information obtained from a detailed microstructural study during various stages of reaction and a knowledge of the reaction conditions which encourage formation of each particular microconstituent. It has become clear that there are at least two independent mechanisms and that they are governed by independent rate laws. The kinetics of nitridation of a silicon compact is, therefore, the superposition of at least two independent rate laws. Experimental evidence obtained thus far is in good agreement with this hypothesis.     

High-temperature deformation and fracture processes in sintered reaction-bonded silicon nitride

Studies of the high-temperature deformation behaviour of sintered reaction-bonded silicon nitride (SRBSN) materials were conducted at 1200 °C in air under selected stress levels, which were applied at a single stress or as a sequence of stepwise increasing stresses. The objective was to evaluate the effects of the fabrication methods (conventional versus microwave heating process), microstructure, and precursor silicon powder purity on the deformation and fracture processes during creep loading of SRBSN materials containing a mixture of 3 wt% Al₂O₃ and 9 wt% Y₂O₃ sintering additives. Results indicated that all of the SRBSN materials exhibited a threshold stress above which the dominant process underwent transition from creep to extensive creep-assisted crack growth (CACG) from existing pores. In addition, the microwave SRBSN materials exhibited a better resistance (higher threshold stress) to CACG process, compared with those fabricated by conventional heating with the same metallurgical grade of silicon powder. The higher threshold stress observed in microwave SRBSN is mainly associated with the increased number density of elongated grains and the related higher fracture toughness. However, the minimum creep rates and stress exponents obtained in the creep regime were independent of the heating method. The microwave SRBSN material fabricated with lower purity silicon also exhibited a higher threshold stress for multiple crack formation and growth as compared with that processed with higher purity silicon. Conversely, the creep rate of microwave SRBSN materials was decreased by decreasing the impurity level (i.e. iron) in silicon powder.     

Relationships between processing,microstructure and properties of dense and reaction-bonded silicon nitride

Some aspects of processing, microstructure and properties of the various types of silicon nitride are discussed. Special emphasis is placed on the relationships between powder properties, process conditions, densification and microstructure, as well as the interdependence between microstructure and properties. After summarizing the areas of crystal structure and thermodynamic properties, and processing of the different types of Si₃N₄, the state-of-the-art of dense and reaction-bonded silicon nitride is given. For both types the formation mechanisms and microstructure, relationships between powder properties, additives (in the case of dense Si₃N₄), process conditions, and densification and microstructure, as well as data and microstructural effects of various mechanical, thermal and thermo-mechanical properties, are outlined. Advanced processing techniques, such as sintering, gas-pressure sintering, post-sintering, and the different routes of hot-isostatic pressing (starting with powder compacts, reaction-bonded Si₃N₄ or pre-sintered Si₃N₄ and the resulting properties, are discussed.     

Effects of interface coating and nitride enhancing additive on properties of Hi-Nicalon SiC Fiber reinforced reaction-bonded silicon nitride composites

Strong and tough Hi-Nicalon SiC fiber reinforced reaction-bonded silicon nitride matrix composites (SiC/RBSN) have been fabricated by the fiber lay-up approach. Commercially available uncoated and PBN, PBN/Si-rich PBN, and BN/SiC coated SiC Hi-Nicalon fiber tows were used as reinforcement. The composites contained 24 vol% of aligned 14 m diameter SiC fibers in a porous RBSN matrix. Both one- and two-dimensional composites were characterized. The effects of interface coating composition, and the nitridation enhancing additive, NiO, on the room temperature physical, tensile, and interfacial shear strength properties of SiC/RBSN matrix composites were evaluated. Results indicate that for all three coated fibers, the thickness of the coatings decreased from the outer periphery to the interior of the tows, and that from 10 to 30 percent of the fibers were not covered with the interface coating. In the uncoated regions, chemical reaction between the NiO additive and the SiC fiber occurs causing degradation of tensile properties of the composites. Among the three interface coating combinations investigated, the BN/SiC coated Hi-Nicalon SiC fiber reinforced RBSN matrix composite showed the least amount of uncoated regions and reasonably uniform interface coating thickness. The matrix cracking stress in SiC/RBSN composites was predicted using a fracture mechanics based crack bridging model.