Sintered Reaction-bonded Silicon Nitride Research Articles

The Effect of Processing Variables and Composition on the Nitridation Behavior of Silicon Powder Compact

The effect of compositional and processing variables on a nitriding reaction of silicon powder compact and subsequent post sintering of RBSN (Reaction-Bonded Silicon Nitride) was investigated. The addition of a nitriding agent enhanced nitridation rate substantially at low temperatures, while the formation of a liquid phase between the nitriding agent and the sintering additives at a high temperature caused a negative catalyst effect resulting in a decreased nitridation rate. A liquid phase formed by solely an additive, however, was found to have no effect on nitridation for the additive amount used in this research. The original site of a decomposing pore former was loosely filled by a reaction product (Si₃N₄), which provided a specimen with nitriding gas passage. For SRBSN (Sintered RBSN) specimens of high porosity, only a marginal dimensional change was measured after post sintering. Its engineering implication for near-net shaping ability is discussed.

Effects of processing method and additive composition on microstructure and thermal conductivity of Si 3N 4 ceramics

A comparative investigation of sintered reaction-bonded silicon nitride (SRBSN) and sintered silicon nitride (SSN) was carried out by using two additive compositions of Y 2O 3–MgO and Yb 2O 3–MgO. The effects of additive composition on the processing, microstructure and thermal conductivity of both SRBSN and SSN materials were evaluated. It is shown that the replacement of Yb 2O 3 by Y 2O 3 has no significant effect on the densification, microstructure and thermal conductivity of SSN materials, but it has a large effect on the processing, microstructure and thermal conductivity of SRBSN materials. The replacement of Yb 2O 3 by Y 2O 3 results in highly bimodal microstructural development during post-sintering and this may be attributed to the formation of large rodlike β-nuclei during nitridation. Although the replacement of Yb 2O 3 by Y 2O 3 leads to the production of fully dense SRBSN materials, but it leads to a decrease in thermal conductivity, which is consistent with microstructural difference. More importantly, the present work implies that SRBSN materials with comparable thermal conductivity to SSN materials could be achieved.

Microstructure and Properties of Sintered Reaction Bonded Silicon Nitride with Aligned Whisker Seeds

Sintered reaction bonded silicon nitride with aligned whisker seeds was prepared by tape casting silicon slurry with 5 wt% b-Si3N4 whisker seeds followed by nitridation and sintering. Three different sintering additives were used for the samples; 7 wt% Y2O3, 6 wt% Y2O3 + 1 wt% Al2O3 and 5 wt% Y2O3 + 2 wt% Al2O3. The sample with 5 wt% Y2O3 + 2 wt% Al2O3 showed the fastest a to b phase transformation after nitridation and the highest fracture toughness and flexural strength after gas pressure sintering among the samples. It also had finer microstructure than the other samples after sintering at 2248 K and at 2273 K. The finer microstructure was related to the faster phase transformation after nitridation, which resulted in the higher flexural strength.

ON THE NATURE OF MACHINING CRACKS IN GROUND CERAMICS: PART II, COMPARISON TO OTHER SILICON NITRIDES AND DAMAGE MAPS

ABSTRACT The size and severity of grinding machining cracks in a sintered reaction bonded silicon nitride are compared to data for other silicon nitrides. Crack sizes follow a similar trend with grinding wheel grit size despite differences in microstructures, strengths, and fracture toughnesses. Silicon nitrides with enhanced fracture toughness actually develop deeper machining cracks than in less tough silicon nitrides. Machining damage maps for silicon nitride are presented.

ON THE NATURE OF MACHINING CRACKS IN GROUND CERAMICS: PART I: SRBSN STRENGTHS AND FRACTOGRAPHIC ANALYSIS

ABSTRACT Machining cracks in ground sintered reaction-bonded silicon nitride (SRBSN) rods and bars were analyzed by fractographic techniques. Grinding flaw sizes were as small as 12 µm and as large as 80 µm and correlated strongly with grinding direction and wheel grit size. Some grinding treatments had no deleterious effect on strength since the machining cracks were very small and fracture occurred from the material's inherent flaws. The telltale signs of machining damage may be detected with conventional low power optical microscopy using simple fractographic techniques. The telltale signs are summarized in a new series of schematic drawings which will aid pattern recognition for engineers and fractographers.

Crack‐Tip Toughness Measurements on a Sintered Reaction‐Bonded Si3N4

An evaluation of measurements of crack opening displacements (CODs) on a commercial sintered reaction‐bonded silicon nitride (SRBSN) was performed. To determine the intrinsic fracture toughness of this material, a new evaluation method is presented, which takes into account not only the near tip CODs, but the CODs of the complete crack profile. The method is applied on through‐thickness cracks in bend bars and contrasted to CODs of Vickers radial cracks. A crack‐tip toughness of ∼1.7 MPa·m1/2 is obtained.

Effect of Sintering Additive Composition on the Processing and Thermal Conductivity of Sintered Reaction‐Bonded Si3N4

Two compositions of the Y2O3–MgO (YM) and Yb2O3–MgO (YbM) systems were chosen to study the effect of the sintering additive composition on the processing and thermal conductivity of sintered reaction‐bonded silicon nitride (SRBSN). The nitridation, postdensification, microstructural evolution, and thermal conductivity of SRBSN were found to depend strongly on the sintering additive composition. The RBSN materials with YbM exhibited a poor sinterability, whereas those with YM exhibited an excellent sinterability. However, the SRBSN materials with YbM showed a higher thermal conductivity than those with YM. This was associated primarily with the isolated distribution and lower amount of secondary phase and the higher percentage of large grains in the former materials.

Aqueous Processing of Sintered Reaction‐Bonded Silicon Nitride: I, Dispersion Properties of Silicon Powder

An aqueous‐based system (Si‐Al2O3‐Y2O3‐Fe2O3) for processing sintered reaction‐bonded silicon nitride (SRBSN) was investigated with an emphasis on chemical control of suspension component interactions. Chemical stability and dispersion properties of a commercial silicon powder were characterized using electroacoustic, adsorption isotherm, and rheological measurements. The interactions of silicon with nitriding agent, sintering aids, dispersants, and binder were considered. The effects of pH, electrolyte, aging, particle size, and solids loading were examined. The suspension properties of the silicon powder were influenced by the native oxide film and powder treatment history. The silicon—oxide composite particles exhibit dispersion behavior similar to silica, characterized by a negative surface potential above pH 2. A method to improve the dispersion and homogeneity of suspension components based on the use of quaternary amine dispersants is proposed.

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.

Reactive atomization of silicon to formin situ oxide sintering aids

The present investigation demonstrated the feasibility of using reactive atomization to produce Si powder within situ oxide sintering aids. With further process optimization, this powder may be an alternative starting material to the conventional, mechanically blended, Si-plus-oxide powder used to produce commercial sintered reaction bonded silicon nitride (SRBSN). In the reactive atomization approach, yttrium and aluminum additives were introduced into silicon metal during induction melting. Reactive atomization was accomplished using a N2-5 pct O2 mixture as the atomization gas. During atomization, oxygen in the atomization gas reacted with Y and Al in the Si melt to produce Y2O3 and Al2O3, which act asin situ sintering aids. The reactive atomized powder demonstrated a Gaussian distribution with a mean diameter of 36 μm. The powder fines (<38 μm) were used to produce cold isostatically pressed compacts that were subsequently reaction bonded and sintered. The results demonstrate that β-Si3N4 formed during reaction bonding and sintering. The density of the SRBSN was 77 pct of theoretical. Transmission electron microscopy (TEM) studies indicated the presence of a glassy phase on the grain boundaries, which is typical in SRBSN and indicative of the presence of thein situ sintering aids. A kinetic model was used to study the influence of processing parameters, such as droplet temperature and oxygen partial pressure, on the kinetics of oxide formation during reactive atomization. The results suggest that the volume fraction of oxides increases with increasing droplet temperature and oxygen partial pressure in the atomization gas mixture.

Scale-Up of the Nitridation and Sintering of Silicon Preforms Using Microwave Heating

AbstractScale-up studies were performed in which microwave heating was used to fabricate reactionbonded silicon nitride and sintered reaction-bonded silicon nitride (SRBSN). Tests were performed in both a 2.45 GHz, 500 liter and a 2.45 GHz, 4000 liter multimode cavities. A variety of sizes, shapes, and compositions of silicon preforms were processed in the studies, including bucket tappets and clevis pins for diesel engines. Up to 230 samples were processed in a single microwave furnace run. Data were collected which included weight gains for nitridation experiments, and final densities for nitridation and sintering experiments. For comparison, nitridation and sintering studies were performed using a conventional resistance-heated furnace.

Comparison of Properties of Sintered and Sintered Reaction-Bonded Silicon Nitride Fabricated by Microwave and Conventional Heating

ABSTRACTA comparison of microwave and conventional processing of silicon nitride-based ceramics was performed to identify any differences between the two, such as improved fabrication parameters or increased mechanical properties. Two areas of thermal processing were examined: (1) sintered silicon nitride (SSN) and (2) sintered reaction-bonded silicon nitride (SRBSN). The SSN powder compacts showed improved densification and enhanced grain growth. SRBSN materials were fabricated in the microwave with a one-step process using cost-effective raw materials. The SRBSN materials had properties appropriate for structural applications. Observed increases in fracture toughness for the microwave processed SRBSN materials were attributable to enhanced elongated grain growth.

Sintered Reaction-Bonded Silicon Nitride by Microwave Heating

ABSTRACTSintered silicon nitride has many desired properties, however, for most applications these materials are too expensive to compete with metal parts. Sintered reaction-bonded silicon nitride (SRBSN) is more economical, with raw material costs &lt;27% those of comparable high-purity materials, making it competitive with metal parts. Conventional processing of SRBSN requires long nitridation times and a two-step firing process. Microwave (MW) heating reduces the reaction times and is performed in a one-step process, thereby simplifying the operation. The flexural strength of the MW-SRBSN is equivalent to the strength of some materials made from higher-cost powders. Thus, these materials may be appropriate for a number of applications.