Sintering process of Y2O3-added Si3N4

The sintering process of Y₂O₃-added Si₃N₄ has been investigated by dilatometry and microstructural observations. Densification was promoted above 1440 ° C by the formation of eutectic melts in the Y₂O₃-SiO₂-Si₃N₄ triangle. However, the dilatometric curves indicated no shrinkage corresponding to the rearrangement process, despite liquid-phase sintering. The kinetic order for The Initial-stage sintering was 0.47 to 0.49. These values indicated that the phase-boundary reaction was rate controlling. The apparent activation energy (323 kJ mol^–1) was smaller than the dissociation energy for the Si-N bond (435 kJ mol^–1). ESR data and lattice strain indicated that the disordered crystalline structure of the Si₃N₄ starting powder promoted the reaction of Si₃N₄ with eutectic melts. After a period of initial-stage sintering, the formation of fibrous -Si₃N₄ grains resulted in interlocked structures to interrupt the densification.     

Dynamic compaction of silicon nitride powder

Dynamic compaction experiments were carried out on fine Si₃N₄ powder, that contained no additives, using maximum pressures of from 20 to 77 GPa. With pressures of from 20 to 64 G Pa the relative densities of the resulting Si₃N₄ compacts were the same: 96% of the theoretical density, but their microhardness values differed significantly. The optimum shock pressure for the Si₃N₄ powder with an initial density of 60% was near 44 G Pa. At this pressure, sintered Si₃N₄ compacts with a density of 96% of the theoretical density and a microhardness of 21.2 G Pa were obtained. However, at 64 G Pa, -Si₃ N₄ was transformed to -Si₃N₄ as a result of the high temperatures experienced during the compaction process. Because of this transformation, the microhardness of the compacted Si₃N₄ was reduced significantly.     

Development of microstructure during the fabrication of Si3N4 by nitridation and pressureless sintering of Si:Si3N4 compacts

The technique for the fabrication of Si₃N₄ which was investigated involves the nitridation of Si:Si₃N₄ powder compacts containing additions of sintering aids (e.g. Y₂O₃ and Al₂O₃) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si₃N₄ forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. The- to-Si₃N₄ phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fine-Si₃N₄ grains. The grains are surrounded by a thin intergranular amorphous film.     

Behaviour of B-N-Si under high pressure and high temperature

The phase relations of the B-N-Si system have been studied using a quenching method up to 10GPa and 2000 °C using a high-pressure apparatus of the octahedral anvil type. Pressure-temperature conditions for obtaining z-BN (diamond analogue of boron nitride) were delineated for turbostratic BN (t-BN), t-BN/amorphous Si₃N₄ and t-BN/-Si₃N₄. These conditions shift toward higher regimes of temperature as amorphous Si₃N₄ or -Si₃N₄ is incorporated into t-BN. Spontaneous sintering occurringin situ at high pressure yields z-BN-based composite compacts.     

Processing and microstructural development of reaction-bonded sintering of a Si3N4/intermetallic composite

This paper focuses on the reaction-bonded sintering of a Si₃N₄/intermetallic composite. The preparation process and the microstructural characterization of this composite with different initial contents of Ni₃Al have been investigated. The effects of Ni₃Al addition on the nitridation of silicon and the phase transformation of Si₃N₄ formed, as well as the densification of the composite, also have been discussed. The addition of Ni₃Al particles accelerates the nitriding rate of silicon and results in the formation of -Si₃N₄ phase at low temperature due to the reaction occurring between silicon and Ni₃Al. Consequently, a completely nitrided and denser composite, compared with reaction-bonded sintered Si₃N₄, was obtained at low temperature. For comparison, pure silicon simultaneously processed was also investigated.     

Effect of grain growth and measurement on fracture toughness of silicon nitride ceramics

Two kinds of -Si₃N₄ powders with different properties (high purity and low purity) were pressureless sintered using MgAl₂O₄-ZrO₂ as sintering additives. The grain growth was controlled by sintering time and temperature. The fracture toughness was determined using indentation microfracture (IM) and single-edge-precracked-beam (SEPB) methods. A discrepancy of fracture toughness was found between the values obtained by these two methods, and the SEPB method provided higher fracture toughness than the IM method. Materials with more large elongated Si₃N₄ grains gave higher fracture toughness, R-curve behavior and larger discrepancy. This is contributed to the effects of grain bridging, pull-out and deflection by the large elongated Si₃N₄ grains. Comparing the specimens from two kinds of -Si₃N₄ powders with different purity level, both gave high fracture toughness of over 9 MPa·m^1/2 by the SEPB method, while the E10 samples have a little higher flexural strength.     

Hot-press sintering studies of amorphous silicon nitride powders

The hot-press sintering behaviour of amorphous Si₃N₄ powders prepared from the ammonia pyrolysis of polycarbosilane or hydridopolysilazane polymers was studied. In the presence of yttria and alumina, the amorphous powders sintered to > 98% of theoretical density at 2023 K. Both the microstructure and average four-point MOR bend strengths, of test bars machined from the sintered compacts were comparable to those obtained from UBE SN-E-10 Si₃N₄ powder processed under the same conditions. However, in contrast to commercial crystalline powders, between approximately 1690 and 1700 K the amorphous Si₃N₄ powders underwent a rapid shrinkage corresponding to 50–60% of the total densification. In this narrow temperature regime, a radical change in morphology and phase composition of the amorphous powder occurred. Prior to 1690 K, the Si₃N₄ powders were totally amorphous as determined by X-ray diffraction analysis and consisted of angular shards with an average particle size of 2–3 m. Samples quickly cooled after heating to 1700 K, consisted of a 53/47 mixture of equiaxed and Si₃N₄ crystallites with an average particle size of 0.1–0.3 m. Thus, the rapid densification at R~ 1700 K is identified with the amorphous to ( + ) transition. Beyond 1700 K, these samples gradually densified to the maximum density as mentioned. The fully densified samples consisted of 100% -Si₃N₄ phase.     

Si3N4 ceramics formed by HIP using different oxide additions — relation between microstructure and properties

Si₃N₄-based ceramic materials formed by glass encapsulation and hot isostatic pressing (HIP) using different additions of Al₂O₃, Y₂O₃ and ZrO₂ have been characterized by analytical electron microscopy and X-ray diffractometry. The microstructures have been related to formation process and to room temperature hardness and fracture toughness of the ceramics. A high volume fraction of retained -Si₃N₄ after processing at 1550 °C gave the Si₃N₄ ceramics high hardness. The equi-axed grain morphology of the Si₃N₄ matrices in these materials, which contained only small amounts of residual glass, resulted in comparatively low fracture toughness values. Processing at 1750 °C reduced the amount of retained -Si₃N₄ substantially. When Y₂O₃ was added, the microstructure contained a comparatively large volume fraction of residual glass, and the Si₃N₄ was present mainly as high aspect ratio -Si₃N₄ grains. This type of microstructure gave an Si₃N₄ ceramic material with high fracture toughness combined with a lower hardness. Additions of ZrO₂ and/or Al₂O₃ resulted also at 1750 °C in an extremely small volume fraction of residual glass, and a major part of the Si₃N₄ was present as equi-axed grains. These ceramics exhibited medium hardness and toughness values, however, larger additions of ZrO₂ appeared to slightly increase toughness.     

Preparation and characterisation of MgSiN2 powders

The powder preparation of MgSiN₂ was studied using several starting mixtures (Mg₃N₂/Si₃N₄, Mg/Si₃N₄ and Mg/Si) in the temperature range 800–1500°C in N₂ or N₂/H₂ atmospheres. The phase formation was followed with TGA/DTA and powder X-ray diffraction (XRD). At 1250°C Mg/Si mixtures did not yield single phase MgSiN₂ whereas for Mg/Si₃N₄ and Mg₃N₂/Si₃N₄ mixtures nearly single-phase powders were obtained. The Mg/Si₃N₄ mixtures yielded MgSiN₂ at the lowest processing temperature but the Mg₃N₂/Si₃N₄ mixtures yielded the most pure MgSiN₂ powder with respect to secondary phases. The main secondary phase detectable with XRD was MgO when starting from Mg₃N₂/Si₃N₄ or MgO and metallic Si when starting from Mg/Si₃N₄ mixtures. When the processing starting from Mg₃N₂/Si₃N₄ mixtures was optimised MgSiN₂ powders containing only 0.1 wt% O could be prepared. Using XRD the solubility of oxygen in the MgSiN₂ lattice was estimated to be at maximum 0.6 wt%. The MgSiN₂ powder was oxidation resistant in air till 830°C. The morphology and particle size were studied with the scanning electron microscope (SEM) and the sedimentation method. Two different kinds of morphology were observed determined by the morphology of the Si₃N₄ starting material.     

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.     

Estimate of polytype fractions and dislocation density in SiC before and after sintering in Si3N4 matrix

A quantitative characterization of polytype fractions and dislocation morphology and density is presented for two -SiC powders. The tools were X-ray diffraction (XRD) and etch-pit analysis carried out before and after hot-isostatic-press (HIP) sintering in an Si₃N₄ matrix at 2050 °C under 180 MPa. Results are compared with data from transmission electron microscopy and electron diffraction previously obtained on the same powders. To avoid overlapping of the major XRD peaks with that of the Si₃N₄ matrix and to make possible the observation of the Si plane during etch-pit analysis in the powders after sintering, a chemical etching procedure to separate nitride and carbide phases without damage was developed. The morphology and density of pits and dislocations were analysed to get quantitative information about the crystal structures of the SiC crystallites and their modifications after the HIP cycle. The polytype fractions were found to be unchanged after sintering. It was also determined that polytypes 6H, 4H and 15R generally share part of the surface in a single crystallite rather than existing as single crystallites themselves, the 15R polytype generally being a hosted structure by a 6H or 4H matrix. A high density of dislocations (10¹³–10¹⁴cm^–2) was found in both the SiC powders after HIP sintering compared with the raw materials.     

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant

Silica (SiO₂) bonded porous silicon nitride (Si₃N₄) ceramics were fabricated from α-Si₃N₄ powder in air at 1200–1500 °C by the oxidation bonding process. Si₃N₄ particles are bonded by the oxidation-derive SiO₂ and the pores derived from the stack of Si₃N₄ particles and the release of N₂ and SiO gas during sintering. The influence of the sintering temperature and holding time on the Si₃N₄ oxidation degree, porosity, flexural strength and dielectric properties of porous Si₃N₄ ceramics was investigated. A high flexural strength of 136.9 MPa was obtained by avoiding the crystallization of silica and forming the well-developed necks between Si₃N₄ particles. Due to the high porosity and Si₃N₄ oxidation degree, the dielectric constant (at 1 GHz) reaches as low as 3.1.     

Effects of ZrO2 and Y2O3 dissolved in zyttrite on the densification and theα/β phase transformation of Si3N4 in Si3N4-ZrO2 composite

In Si₃N₄-ZrO₂ composite, the effects of zirconia and Y₂O₃ dissolved in zyttrite on the densification and the/ phase transformation of Si₃N₄ were studied using hot-pressing of Si₃N₄ with the addition of pure, 3, 6, and 8 mol% Y₂O₃-doped zirconia. Reaction couples between Si₃N₄ and ZrO₂ of zyttrite were made to observe the reaction phenomena. The addition of pure zirconia was not effective to obtain full density of the Si₃N₄-ZrO₂ composite. However, Y₂O₃ diffused from the added zyttrite promoted densification; the density of Si₃N₄ with 5 vol% pure ZrO₂ composite was 71% theoretical, and nearly full density (>97%) could be obtained in Si₃N₄ with 5 vol% 6, 8 mol% Y₂O₃-doped ZrO₂ composite. On the basis of observations of the Si₃N₄-pure ZrO₂ reaction couple, the reaction between Si₃N₄ and ZrO₂ resulted in the formation of Si₂N₂O phase, and the/ phase transformation of Si₃N₄ occurred via this Si₂N₂O phase. From the XRD analysis of the reaction layer between Si₃N₄ and zyttrite, it is suggested that the reaction products, Y₂Si₂O₇ and Y₂Si₃N₄O₃ phases, play an important role in the densification of Si₃N₄-zyttrite composite.     

Self-monitoring of fracture and strain in titanium carbide reinforced silicon nitride ceramics

Si₃N₄ Ceramic Composites with TiC powder have been fabricated by gas-pressure sintering and their electrical conductivity has been investigated. The ceramic composites with different electrical resistivity consist of Si₃N₄ powder as an insulating matrix, and TiC as electrically conductive additive. Under tensile loading or compressive unloading, the R/R of TiC/Si₃N₄ composites reversibly increased. Under compressive loading, the R/R decreased gradually with the increasing of loading up to fracture. The results suggest the possibility of self-monitoring fractures and strains in the composites under tensile and compressive loading.     

Phase transformation and microstructural changes of Si3N4 during sintering

Changes of density, the - phase transformation, and composition of grains and grain boundaries during sintering of Si₃N₄ with various sintering conditions using additives of Y₂O₃ and Al₂O₃ were investigated. The phase determination of individual Si₃N₄ grains was performed by convergent beam electron diffraction. The relations between densification and transformation were divided into two groups, depending on the additive compositions. Aluminium dissolution into Si₃N₄ grains occurred mostly during - transformation process. The concentrations of aluminium and oxygen in the grain boundaries decreased as the - transformation progressed.     

AEM investigation of ceramic/lncoloy 909 diffusional reactions after joining by HIP

Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si₃N₄ and either 60 vol% TiN or 50 vol% TiB₂, while three were monolithic materials, namely Si₃N₄ with 2.5 wt% Y₂O₃ as a sintering additive, Si₃N₄ without additives, and Si₂ N₂O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 m in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.     

Pressureless sintering of SiAlON ceramics

Pressureless sintering of β-SiAlON ceramics has been carried out successfully without the use of additives. Grinding the raw material powders (AlN, Si₃N₄ and Al₂O₃) for long periods was shown to be an important factor in obtaining ceramics of high quality. The problem of volatilisation of sub-oxides such as “SiO” has been solved by carrying out the sintering in a special closed crucible. The sintering time is approximately 2 hours at temperatures between 1740 and 1780°C. In this way a material having a density corresponding to 99 % of the theoretical density was obtained. Bend strengths of 420 MN/m² have been measured on the best SiAlON obtained. The ease of sintering depends on the composition of the particular SiAlON being apparently more difficult as the mixture becomes richer in Si₃N₄.     

Effect of grain size distribution on the strength of porous Si3N4 ceramics composed of elongated β-Si3N4 grains

The grain size distributions (diameter and aspect ratio) of porous Si₃N₄ ceramics composed of elongated -Si₃N₄ grains were evaluated statistically, and their effect on the pore size distribution and the flexural strength of the porous Si₃N₄ was investigated. Porous Si₃N₄ ceramics having porosities of 27 to 43% and median pore diameters of 0.56 to 0.96 m were used as specimens. The grain diameter distribution was well correlated to the pore size distribution of the porous Si₃N₄ ceramics. We concluded that the strength of the porous Si₃N₄ ceramics increased with increasing grain length of -Si₃N₄ as well as with decreasing porosity.     

Development and characterisation of SiAlON composites with TiB2, TiN, TiC and TiCN

30 vol% of TiB₂, TiCN, TiN or TiC was added to a sialon matrix with an X-phase sialon (Si₁₂Al₁₈O₃₉N₈) and an Al₂O₃–Si₃N₄ (77/23 wt%) starting powder composition and hot pressed at 1650°C in vacuum. The microstructures of the obtained composites were characterised by means of X-ray diffraction and electron microscopy, and the mechanical properties; E-modulus, hardness, bending strength and fracture toughness were measured and evaluated.Fully dense composites with an X-phase sialon or a polyphase Al₂O₃–-sialon–X-sialon matrix with 30 vol% of TiB₂, TiN and TiCN were obtained. TiC, added as a dispersed phase, however reacts with the nitrogen from the Si₃N₄ during liquid phase sintering, with the formation of TiC_1–x N _x , SiC and a changed sialon matrix composition. In the case of the X-phase sialon starting composition, a mullite matrix is obtained after sintering. The microstructural observations with respect to the sialon-TiC composites are found to be in agreement with the thermodynamic calculations.     

Liquid-Phase Sintering and Properties of Si3N4–TiN Composites

Densification during liquid-phase sintering of Si₃N₄–TiN was studied in the presence of Y₂O₃. The content of TiN was varied from 0–50 mass%. During the densification Y-silicate was formed. The amount of silicate increased with both decreasing fraction of TiN and increasing isothermal heating time. Density, fracture toughness, and electrical resistivity were measured as a function of TiN content. It was found that the density and fracture toughness increased with increasing TiN content. The electrical resistivity drops drastically, from 10¹⁰ m for sintered Si₃N₄ to 10^–3 m for sintered Si₃N₄–TiN composite containing 30 vol% TiN.