Effects of α-Si3N4 and AlN addition on formation of α-SiAlON by combustion synthesis

Preparation of Yb α-SiAlON was investigated by self-propagating high-temperature synthesis (SHS) from α-Si₃N₄- and α-Si₃N₄/AlN-diluted powder compacts under nitrogen of 2.17 MPa. For the AlN-free samples, the molar ratio of Si₃N₄/Si varies between 0.22 and 0.5. The starting stoichiometry of the AlN-added samples comprises a constant proportion of Si₃N₄/Si equal to 0.22, but a broad range of AlN/Al from 0.33 to 1.0. The self-sustaining combustion wave propagated in the spinning mode on account of highly diluted samples adopted in this study. The overall reaction exothermicity increases with Si₃N₄/Si ratio for the AlN-free samples, while decreases with AlN/Al ratio for the AlN-added powder compacts. As a result, the amount of unreacted Si left in the final product was significantly reduced and the formation of nearly single-phase Yb α-SiAlON was achieved in the sample with Si₃N₄/Si = 0.5. Moreover, the growth of elongated α-SiAlON grains was enhanced in the samples with high contents of Si₃N₄. In contrast, the nitridation of Si was only improved to a certain extent with the addition of AlN and no further improvement was attained by increasing the AlN content. Due to the lack of sufficient liquid phases during combustion and the weak reaction exothermicity, the samples with high contents of AlN were inclined to produce α-SiAlON grains in a fine equiaxed form.     

Densification, microstructure, and fracture behavior of TiC/Si3N4 composites by spark plasma sintering

TiC/Si₃N₄ composites were prepared using the β-Si₃N₄ powder synthesized by self-propagating high-temperature synthesis (SHS) and 35 wt.% TiC by spark plasma sintering. Y₂O₃ and Al₂O₃ were added as sintering additives. The almost full sintered density and the highest fracture toughness (8.48 MPa·m^½) values of Si₃N₄-based ceramics could be achieved at 1550°C. No interfacial interactions were noticeable between TiC and Si₃N₄. The toughening mechanisms in TiC/Si₃N₄ composites were attributed to crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the Si₃N₄ matrix. However, increasing microcracks easily led to excessive connection of microcracks, which would not be beneficial to the strength.     

Effects of α- and β-Si3N4 as precursors on combustion synthesis of (α + β)-SiAlON composites

Preparation of (α + β)-SiAlON composites from the powder compacts of Si, Si₃N₄, SiO₂, Al, and AlN was investigated by self-propagating high-temperature synthesis (SHS) in nitrogen of 2.17 MPa. Test samples adopted not only pure α- and β-Si₃N₄, but also a mixture of (α + β)-Si₃N₄. The combustion temperature and flame-front propagation velocity decreased with increasing ratio of Si/Si₃N₄, but they increased with proportion of α/β-Si₃N₄. For the sample containing pure α-Si₃N₄, the synthesis reaction yielded only α-SiAlON with various morphologies including equiaxed crystals, elongated grains, and fine whiskers. As a mixture of (α + β)-Si₃N₄ was employed, the resulting products were (α + β)-SiAlON composites, within which the content of β-SiAlON increased with increasing β-Si₃N₄. For the sample adopting 100% β-Si₃N₄, comparable amounts of α- and β-SiAlON were produced. Additionally, the morphology of (α + β)-SiAlON composites was dominated by elongated grains with a high aspect ratio.     

The fracture behaviour of A356 alloys with different iron contents under resonant vibration

The fracture behaviour of A356 alloys with different iron contents under resonant vibration has been investigated by examining the morphology of eutectic silicon and Fe-rich intermetallic phases. The test materials were prepared by varying the iron content within the range of 0.14 to 0.97 wt%. The experimental results shows that while the iron content is below 0.57 wt%, the Fe-rich intermetallic phases are mainly π-Al₈Mg₃FeSi₆ and α-Al₁₅Fe₃Si₂ and there is no obvious difference in the fracture resistance under resonant vibration. However, when the specimen has higher iron content (0.97 wt%Fe), the fracture resistance drops due to the formation of β-Al₅FeSi. Hence, the β-Al₅FeSi morphology is more detrimental to the fracture resistance than that of π-Al₈Mg₃FeSi₆ or α-Al₁₅Fe₃Si₂. The results indicate that the vibration fatigue cracks initiate at the surface and the fracture paths go predominantly through the silicon particles, and occasionally through the Fe-rich intermetallic phase particles. The detrimental β-Al₅FeSi becomes influential in the specimen with higher iron content. The results also show that the line intercepted density (LID) and projected crack intercepted density (PCID) values of Fe-rich intermetallic phases increased with increasing area fraction of Fe-rich intermetallic phases (or iron content). This shows that an increase in area fraction of Fe-rich intermetallic phases (or iron content) may promote the crack propagation and growth under resonant vibration.     

Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si₃N₄-TaC and Si₃N₄-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600 °C using a resistance-heated hot-pressing technique. The addition of 20–40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40 mol% TaC or ZrC at 1500 °C, 0–80 mol% TaC at 1600 °C, and 0–60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si₃N₄ was larger than that of β-SiAlON; α-Si₃N₄ transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si₃N₄-to-β-SiAlON phase transformation. Young's modulus for the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics increased with the carbide amount, and the hardness of dense Si₃N₄-ZrC and Si₃N₄-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si₃N₄ content. Dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics, with larger quantities of α-Si₃N₄ sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si₃N₄ proportion. Both the hardness and fracture toughness of the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics were strongly related to the proportion of α-Si₃N₄ in the sintered body.     

Synthesis of fibrous β-Si3N4 structured porous ceramics using carbothermal nitridation of silica

Porous Si₃N₄ ceramics with a fibrous interlocking microstructure were synthesized directly by carbothermal nitridation (CT/N) of SiO₂. Carbon black was used as the carbon source and α-Si₃N₄ was used as seed. The cold-pressed samples of powder mixtures were heated at nitrogen pressures above 0.6 MPa and temperatures exceeding 1600 °C. The addition of the α-Si₃N₄ to the initial powder mixtures had important effects on the microstructure and mechanical properties of the porous Si₃N₄. Fine elongated fibrous β-Si₃N₄ grains were developed in the seeded samples with stoichiometric C–SiO₂ ratio and Y₂O₃ as the sintering additive, when sintered at 1700–1750 °C. A sample of outstanding strength resulted, five times stronger than the seed-free samples. Such a technique offers the possibility of synthesizing highly porous and strong Si₃N₄ materials at considerably lower cost than at present.     

High α-β phase transition and properties of YbF3-added porous Si3N4 ceramics obtained by low temperature pressureless sintering

In this paper, we used YbF₃ as a sintering additive to get a high α-β phase transition in porous Si₃N₄ ceramics. The mechanism of YbF₃ as sintering additives as well as the relationship between microstructure and mechanical properties have been investigated in detail. In addition, we used pressureless sintering to lower the temperature to 1550 °C. YbF₃ makes α-Si₃N₄ completely transform to β-Si₃N₄, whereas only 41.1% β-Si₃N₄ could be obtained with Yb₂O₃. This process yielded ceramics with more flexural strength and increased fracture toughness using less energy. In addition, using YbF₃ substituted for part Yb₂O₃ could promote sintering behaviors of Si₃N₄ ceramics at low temperature to increase α-β phase transition rate and improve the properties of silicon nitride ceramics significantly. In particular, when we used YbF₃-Yb₂O₃ as additives, we obtained a flexural strength of 269.87 MPa and a fracture toughness of 4.59 MPa·m^1/2.     

A comparative ab initio study of the ‘ideal’ strength of single crystal α- and β-Si3N4

In this study, the ideal tensile and shear strengths of single crystal α- and β-Si₃N₄ were calculated using an ab initio density functional technique. The stress–strain curve of the silicon nitride polymorph was calculated from simulations of predefined strain deformation in various directions. In particular, the ideal strength calculated for an applied tensile γ₁₁ strain, in the [1 0 0] plane, was estimated to be ∼51 and 57 GPa, for α- and β-Si₃N₄, respectively. Using a reported empirical method an estimate was also made of the Vickers indentation hardness of the α- and β-Si₃N₄ single crystals, ∼23.0 and 20.4 GPa, respectively. Moreover, a hardness estimate has been reported for Si₃N₄ in the literature: ∼21.0 GPa.     

Preparation and characterization of Si3N4/TiN nanocomposites ceramic tool materials

Si₃N₄/TiN nanocomposites ceramic tool materials were sintered under 30 MPa at 1650 °C for 40 min. The effects of nano-scale TiN on the mechanical properties and microstructure were investigated. The strengthening and toughening mechanisms of Si₃N₄/TiN nanocomposites were studied by observing the fracture surfaces, samples for TEM and cracks. The oxidation resistance of Si₃N₄/TiN nanocomposites was also discussed based on the observation of microstructure. The results showed that the elongated Si₃N₄ grains which were pulled out from the fracture surfaces were reduced with the increase of the addition of nano-scale TiN, and many intragranular TiN grains were observed in Si₃N₄/TiN nanocomposites. The greater crack deflection and microcrack provided the contributions to the strengthening and toughening mechanisms for Si₃N₄/1 vol%TiN nanocomposite. However, Si₃N₄/TiN nanocomposites were oxidized strongly at higher temperature.     

Growth Kinetics and Microstructure Evolution of Intermediate Phases in MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-WSi<Subscript>2</Subscript>/Mo Diffusion Couples

The growth kinetics and silicon diffusion coefficients of intermediate silicide phases in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple prepared by spark plasma sintering were investigated in temperatures ranging from 1200 to 1500 °C. The intermediate silicide phases were characterized by x-ray diffraction. The microstructures and components of the MoSi₂-Si₃N₄-WSi₂/Mo composites were investigated using scanning electron microscope with energy-dispersive spectroscopy. A special microstructure with MoSi₂ core surrounded by a thin layer of (Mo,W)Si₂ ring was found in the MoSi₂-Si₃N₄-WSi₂ composites. The intermediate layers of Mo₅Si₃ and (Mo,W)₅Si₃ in the MoSi₂-Si₃N₄-WSi₂/Mo diffusion couples were formed at different diffusion stages, which grew parabolically. Activation energy of the growth of intermediate layers in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple was calculated to be 316 ± 23 kJ/mol. Besides, the hindering effect of WSi₂ addition on the growth of intermediate layers was illustrated by comparing the silicon diffusion coefficients in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo and MoSi₂-3.5 vol.% Si₃N₄/Mo diffusion couples. MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂ coating on Mo substrate exhibited a better high-temperature oxidation resistance in air than that of MoSi₂-3.5 vol.% Si₃N₄ coating.     

Development of Si3Na/Al composite by pressureless melt infiltration

Pressureless infiltration process to synthesize Si3N4/Al composite was investigated. Al-2%Mg alloy was infiltrated into Si3N4 and Si3N4 containing 10% Al2O3 preforms in the atmosphere of nitrogen. It is possible to infiltrate Al-2%Mg alloy in Si3N4 and Si3N4 containing 10% Al2O3 preforms. The growth of the dense composite of useful thickness was facilitated by the presence of magnesium powder at the interface and by flowing nitrogen. During infiltration Si3N4 reacted with aluminium to form Si and AIN, the growth of composite was found to proceed in two ways, depending on the Al2O3 content in the initial preform. Firstly, preform without Al2O3 content gives rise to AIN, Al3.27Si0.47 and Al type phases after infiltration. Secondly, perform with 10% Al2O3 content gives rise to AIN-Al2O3 solid solution phase （AION）, MgAl2O4, Al and Si type phases. AlON phase was only present in composite, containing 10% Al2O3 in the Si3N4 preforms before infiltration.     

Effect of Pyrolysis Temperature on Properties of Porous Si3N4-BN Composites Fabricated Via PIP Route

The Si₃N₄-BN composites have been prepared via die pressing and the precursor infiltration and pyrolysis route using borazine as the precursor. The Si₃N₄-BN composites are composed of h-BN, α-Si₃N₄, and β-Si₃N₄ produced at a pyrolysis temperature from 1200 to 1750 °C with only 0.17-3.9 wt.% phase transition of Si₃N₄. The effect of pyrolysis temperature on properties of the composites has been investigated. The density and mechanical properties of the composites, at both room temperature and 1000 °C, increase along with the elevating of the pyrolysis temperature. The density of the composites achieves 2.33 g/cm³ at 1750 °C with the porosity of 14.1%. The flexural strength, elastic modulus, and fracture toughness at room temperature of the Si₃N₄-BN composites pyrolyzed at 1750 °C are 219.1 MPa, 75.5 GPa, and 2.62 MPa m^1/2, respectively. A desirable flexural strength of 184.9 MPa with a residual ratio of 84.4% has been obtained when the composites are exposed at 1000 °C in the air. Micrographs of SEM and TEM illustrate the bonding structure of the pyrolysis BN and Si₃N₄ grains.     

Corrosion behaviour of stoichiometric Fe3Si alloy in liquid zinc

The corrosion behaviour of stoichiometric Fe₃Si alloy in a liquid zinc bath for 3 and 62 h at 500°C was examined. The corrosion products at the Fe₃Si/liquid zinc interface were investigated by using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The corrosion process was controlled by the diffusion of iron and zinc atoms. There were scarcely any silicon atoms diffusing at Fe₃Si/liquid zinc interface from the matrix to liquid zinc. The phase transition process of the stoichiometric Fe₃Si alloy in liquid zinc was Fe₃Si, α, α+ FeSi+δ, FeSi+δ, and the main corrosion products were periodic array of FeSi and δ phase.     

Effect of wear debris of Al2O3, ZrO2 and WC balls on Y-α/β-SiAlON ceramics consolidated by spark plasma sintering

Three types of Y-α/β-SiAlON powders were prepared by mixing and milling the raw materials for 20 h using Al₂O₃, ZrO₂, and WC balls, thereafter denoted as SNA, SNZ and SNW, respectively. The three types of specimens were sintered using spark plasma sintering (SPS) at 1510 °C for 5 min under 30 MPa in a vacuum. α-phases (SiAlON and Si₃N₄) and β-SiAlON phase were observed in the SNA and SNZ specimens, but only the β-SiAlON phase existed in the SNW specimen. The wear debris of the WC balls affected the grain boundary properties and eventually promoted the full phase transformation of α-Si₃N₄ into α- and β-SiAlON phases. However, SNA and SNZ were partially transformed into α- and β-SiAlON phases. The Vickers hardness of SNA was the highest (18.7 GPa), due to its having the highest content of α-phase, but its fracture toughness was the lowest (4.09 MPam^1/2) due to its having the lowest content of β-SiAlON phase. The wear debris and secondary phases existed at the grain boundary, mostly at the triple junction, and also affected the color. The color of the sintered specimen was quite different depending on the milling media.     

Thermally induced crystallization of Fe73.5Cu1Nb3Si15.5B7 amorphous alloy

Thermally induced crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ amorphous alloy occurs in two well-separated stages: the first, around 475 °C, corresponds to formation of α-Fe(Si)/Fe₃Si and Fe₂B phases from the amorphous matrix, while the second, around 625 °C, corresponds to formation of Fe₁₆Nb₆Si₇ and Fe₂Si phases out of the already formed α-Fe(Si)/Fe₃Si phase. Mössbauer spectroscopy suggests that the initial crystallization occurs through formation of several intermediate phases leading to the formation of stable α-Fe(Si)/Fe₃Si and Fe₂B phases, as well as formation of smaller amounts of Fe₁₆Nb₆Si₇ phase. X-ray diffraction (XRD) and electron microscopy suggest that the presence of Cu and Nb, as well as relatively high Si content in the as-prepared alloy causes inhibition of crystal growth at annealing temperatures below 625 °C, meaning that coalescence of smaller crystalline grains is the principal mechanism of crystal growth at higher annealing temperatures. The second stage of crystallization, at higher temperatures, is characterized by appearance of Fe₂Si phase and a significant increase in phase content of Fe₁₆Nb₆Si₇ phase. Kinetic and thermodynamic parameters for individual steps of crystallization suggest that the steps which occur in the same temperature region share some similarities in mechanism. This is further supported by investigation of dimensionality of crystal growth of individual phases, using both Matusita–Sakka method of analysis of DSC data and texture analysis using XRD data.     

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

High-fracture-strength and high-toughness β-Si₃N₄ 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 and fracture processes of Si3N4/ Si3N4 joints brazed using Cu–Zn–Ti filler alloy

Abstract

Si₃N₄ ceramic was jointed to itself using a filler alloy of Cu-Zn-Ti at 1123-1323 K for 0.3-2.7 ks. Ti content in the Cu-Zn-Ti filler alloy was varied from 5 to 20 at.-%. The effect of brazing parameters, such as brazing temperature, holding time and Ti content, on the mechanical properties and facture processes of the Si₃N₄/Si₃N₄ joint were investigated. The results indicated that the increased brazing temperature, holding time and Ti content increase the thickness of the interfacial reaction zone in the Si₃N₄/filler alloy, and the size and amount of the reaction phases in the filler alloy. Their increases lead to increasing shear strength of the joint. The fracture behaviour of the Si₃N₄/Si₃N₄ joint greatly depends on the microstructure of the joint. A suitable thick reaction zone with reaction phases yields the high strength of the Si₃N₄/ Si₃N₄ joint.     

Morphology and ablation mechanism of Cf/Si3N4 composites ablated by oxyacetylene torch at 2200°C

Abstract

Si₃N₄ ceramic matrix composites reinforced by carbon fibres (C_f/Si₃N₄) were prepared by low pressure chemical vapour infiltration at 1250°C using SiCl₄ and NH₃ as precursor. The as prepared C_f/Si₃N₄ composites were ablated to determine the mechanism of the ablation resistance and oxidisation resistance by oxyacetylene torch at 2200°C. The morphology and microstructure of the composites were examined by scanning electron microscopy. The phase compositions of the composites were confirmed by energy dispersive X-ray spectroscopy and X-ray diffraction. The results indicated that the matrix of the C_f/Si₃N₄ composites was composed of the amorphous Si₃N₄ and nanometre α-Si₃N₄. A central ablation region and a ring oxidisation region appeared on the surface of the as ablated C_f/Si₃N₄ composites. Sublimation of the Si₃N₄ matrix and oxidation of the carbon fibres are the main ablation behaviours in the central region. Oxidation of the Si₃N₄ matrix and deposition of SiO₂ particles are the main ablation behaviour in the ring region. A large number of SiO₂ liquid droplets produced during ablation were retained and formed spherical solid particles on the surface of the ring region after ablation. For the mismatch of the coefficient of thermal expansion of the carbon fibres and the Si₃N₄ matrix, Si₃N₄ matrix was cracked under the thermal impact of the oxyacetylene flame. With the passive oxidation of the as cracked surface, the continuous SiO₂ liquid was formed in the ring region. Subsequently, some residual Si₃N₄ particles were covered by transparent SiO₂ layer to form an amber-like microstructure.