Kinetics of phase transformations in SiAlON Ceramics: II. Reaction Paths

Kinetics of various α/β-Si₃N₄→α′/β′-SiAlON transformations were investigated for SiAlON ceramics with additions of rare-earth cations. It was determined that α-Si₃N₄ starting powders lead to faster Si₃N₄→α′/β′-SiAlON transformations than β-Si₃N₄ powders. In reactions with a two-phase α′/β′-SiAlON product, the formation of a SiAlON that is structurally similar to the majority starting powders is initially favored, and such SiAlON has a solute composition leaner than the final equilibrium composition. These results suggest that the transformation kinetics is largely governed by the heterogeneous nucleation that is strongly sensitive to the driving force, dictated by the majority phase of the starting powders, and that coherent nucleation is often favored when abundent isostructural nucleation sites are available.     

Effect of starting Si3N4 powder type and sintering conditions on grain morphology of α-SiAlON ceramics

Abstract

Nitrogen rich multication α-SiAlON ceramics doped with Y-Ce have been densified by gas pressure sintering using α-Si₃N₄ or mixed β/α-Si₃N₄ starting powder. The effects of α-SiAlON nucleation and growth mechanisms were investigated by using di fferent sintering cycles. X-ray diffraction studies after sintering revealed that 21R polytype phase was present in addition to α -SiAlON matrix phase. Microstructural characterisation of sintered materials prepared using α-Si₃N₄ powder in the starting composition revealed a typical equiaxed grain morphology, as expected if the α-SiAlON nucleation step was not applied before grain growth. However, needlelike α-SiAlON grains were observed if a nucleation step was carried out before final sintering. In starting powders containing mixed β/α-Si₃N₄, needlelike grain morphology was also observed. The effects of different Si₃N4 starting powders and sintering conditions on the grain morphology and mechanical properties are discussed.     

Effect of ZrB2 and its oxide impurities (ZrO2 and B2O3) on hot-pressed Si3N4 ceramics at low temperature

Based on the previous work on Si₃N₄-ZrB₂, the addition of 2.5 vol.% ZrB₂ promoted α- to β-Si₃N₄ phase transformation, bimodal microstructure, and toughness and strength improvement after hot-pressing at 1500 °C. However, the mechanism needed to be further explored. In the present work, the effect of ZrB₂ and its oxide impurities (ZrO₂ and B₂O₃) on phase composition, microstructure and mechanical properties of Si₃N₄ ceramics with MgO-Yb₂O₃ additives were studied. Results showed that the addition of B₂O₃ had no influence on Si₃N₄ ceramics, whereas the addition of ZrO₂ inhibited the α- to β-Si₃N₄ phase transformation, formed an uniform equiaxed microstructure, and deceased the toughness and strength. The positive effect of oxide impurities can be eliminated. Based on the STEM analysis, the possible reason was that the addition of ZrB₂ led to the formation of Si-Mg-O-N-Yb-Zr-B liquid phase, and then promoted α- to β-Si₃N₄ phase transformation.     

Fabrication of Si3N4-based composite containing needle-like TiN synthesized using NH3 nitridation of TiO2 nanofiber

A Si₃N₄ composite containing needle-like TiN particles (7 vol%) was fabricated. Needle-like TiN particles several micrometers long were synthesized using NH₃ nitridation of TiO₂ nanofiber, which was obtained using hydrothermal treatment. A mixed powder of α-Si₃N₄ and the needle-like TiN particles with additives was hot pressed at 24 MPa and 1850 °C for 1 h in N₂ atmosphere. Mechanical properties of the composite were compared with those of a composite containing rounded TiN particles and a monolithic β-Si₃N₄ ceramic. The Si₃N₄ matrix of the composites containing TiN was mainly a-phase, suggesting that the α–β phase transformation of Si₃N₄ was inhibited by the presence of TiN. Although fracture strength of the composites was lower, fracture toughness was comparable to that of monolithic β-Si₃N₄ ceramics. Hardness of the composites was about 19 GPa and was greater than that of the monolithic β-Si₃N₄ ceramic.     

Spark plasma sintering behavior of combustion-synthesized (Y,Ca)-α-SiAlON

This paper describes the sintering behavior of combustion-synthesized (Y, Ca)-α-SiAlON powders when using spark plasma sintering (SPS) technology. The effects of sintering temperature, heating rate, and holding time on the densification behavior, α→β phase transformation, and Vickers hardness were investigated in detail. Fully dense Y-α-Si_12−(m+n)Al_m+nO_nN_16−n (m=1.2, n=0.6, Y-α-SiAlON) and Ca-α-Si_12−(m+n)Al_m+nO_nN_16−n (m=1.0, n=0.5, Ca-α-SiAlON) were obtained during sintering for 10 min at final temperatures as low as 1400 °C and 1500 °C, respectively. X-ray diffraction results showed that more than 50 mass% of α-phase transformed to the β-phase for Y-α-SiAlON after SPS, whereas no obvious α→β phase transformation was observed for Ca-α-SiAlON, even after sintering at a high temperature of 1600 °C. A maximum Vickers hardness of 18.56 GPa and 19.95 GPa was reached for Y-α-SiAlON and Ca-α-SiAlON, respectively.     

Wear behaviour of α- and α/β-SiAlON ceramics stabilized with Nd2O3 and Y2O3

α- and α/β-SiAlON compositions, doped with Y₂O₃ or Nd₂O₃, were densified by gas pressure sintering (GPS). The wear and mechanical properties of the materials were investigated and compared to β-Si₃N₄ materials. Microstructure evolution and its change with composition as well as the influence of the microstructural changes on the mechanical and tribological properties were reported. Wear tests were performed using a tribometer with ball-on-plate geometry in reciprocating sliding contact under dry condition. It was observed that α/β-SiAlON compositions, doped with Y₂O₃, have better wear properties in comparison to α-SiAlON and β-Si₃N₄ samples. Field emission scanning electron microscopy (FESEM) was used to analyse the worn surfaces after wear tests. The results showed that α/β-SiAlON and β-Si₃N₄ materials have different wear behaviour.     

Corrosion of HIPed β-Si6−zAlzOzN8−z (z =0, 1, 2, 3) ceramics by NaCl vapor

Tests for the corrosion of β-Si_6−zAl_zO_zN_8−z (z =0, 1, 2, 3) (β-Si₃N₄ and β-SiAlON) ceramics were carried out at 1300 °C for 3–24 h in NaCl vapor of various concentrations (1.67 × 10⁻², 3.33 × 10⁻², 5.0 × 10⁻² g l⁻¹), which was carried by flowing Ar gas. The densified Si₃N₄ and SiAlON ceramics were fabricated by HIPing under N₂ of 150 MPa at 1700 °C. The corroded surface was observed by optical and scanning electron microscopy (OM and SEM). The phases produced during corrosion were identified by X-ray diffraction. The thickness of the corroded scale was determined by cross sectional SEM observation. The Si₃N₄ ceramics were hardly corroded by NaCl vapor, while the z=1 and 2 SiAlONs were slightly corroded with the formation of bubbles on the surface; the z=3 ceramics were severely corroded with the formation of Al₂O₃ needle crystals and fine mullite crystals, depending on the NaCl vapor concentration. Quantitative X-ray microanalysis showed that 2 at.% Na is contained in all the scales of the SiAlONs. The severe corrosion of the z=3 SiAlON was explained on the basis of the kinetic results for the thickness of the scale.     

Formation of α/β-Si3N4 nanoparticles by carbothermal reduction and nitridation of geopolymers

Silicon nitride (Si₃N₄) particles with various α/β-Si₃N₄ ratios were fabricated from geopolymer (GP)-carbon compositions (M₂O·Al₂O₃·4.5SiO₂·12H₂O+18C), where M is an alkali ion (Na⁺, K⁺ and Cs⁺). They were made by carbothermal reduction and nitridation at 1400°, 1500°, and 1600°C for 2 hours under flowing nitrogen. Characterization of carbothermally reacted GP-carbon compositions was based on XRD, SEM-EDS, HRTEM, and selected area electron diffraction analyses. Depending on the alkaline composition of GP, the carbon content and the reaction temperature, a compositionally variable α/β-Si₃N₄ or SiAlON was achieved. Crystallization of the GPs gradually increased by heat treatment over 1400°C with corresponding weight loss. It was found that NaGP, KGP, and CsGP crystallized into a major phase of α-Si₃N₄, β-Si₃N₄, and SiAlON, respectively. Prolonged heating at 1600°C led to an increase in the α/β-Si₃N₄ ratio in NaGP due to the formation of aluminum nitride, while it led to a decrease in α/β-Si₃N₄ ratio in KGP. In the case of CsGP, SiAlON replaced the pollucite which mainly formed at lower temperatures. Transmission electron microscopy revealed that the needle-like particles were of ~0.5 µm in size and consisting of α/β-Si₃N₄ mixtures.     

Incorporation of the transition metals (Cr and Fe) into β-SiAlON crystal structure

Outstanding properties for SiAlON ceramics can be obtained by tailoring the microstructure through α-SiAlON and β-SiAlON phase content as well as a type of secondary phases. It is so far well known that while some of the elements could be accommodated in α-SiAlON structure, β-SiAlON does not easily accommodate different elements in its structure. In this work, SiAlON ceramics were produced by using β-Si₃N₄ starting powder containing small amount of iron and chromium and the possible incorporation of iron and chromium into β-SiAlON structure was investigated by using analytical transmission electron microscopy (TEM) techniques. As a result of analytical TEM analysis, it is found that transition metals (Cr and Fe) can enter into the β-SiAlON crystal structure.     

Enhanced thermal conductivity in Si3N4 ceramic by addition of a small amount of carbon

In order to fabricate Si₃N₄ ceramic with enhanced thermal conductivity, 93 mol%α-Si₃N₄-2 mol%Yb₂O₃-5 mol%MgO powder mixture was doped with 5 mol% carbon, and sintered firstly at 1500 °C for 8 h and subsequently at 1900 °C for 12 h under 1 MPa nitrogen pressure. During the first-step sintering, the carbothermal reduction process significantly reduced the oxygen content and increased the N/O ratio of intergranular secondary phase, resulting in the precipitation of Yb₂Si₄O₇N₂ crystalline phase, higher β-Si₃N₄ content and larger rod-like β-Si₃N₄ grains in the semi-finished Si₃N₄ sample. After the second-step sintering, the final dense Si₃N₄ product acquired coarser elongated grains, lower lattice oxygen content, tighter Si₃N₄-Si₃N₄ interfaces and more devitrified intergranular phase due to the further carbothermal reduction of oxynitride secondary phase. Consequently, the addition of carbon enabled Si₃N₄ ceramic to gain a significant increase of ∼25.5% in thermal conductivity from 102 to 128 W∙m⁻¹ K⁻¹.     

Phase formation and microstructural evolution of Ca α-sialon using different Si3N4 starting powders

Silicon nitride has two polymorphous structures, α-Si₃N₄ and β-Si₃N₄. In this study three different Si₃N₄ starting powders (∼100%α, 40%α+60%β, ∼100%β) were used to prepare Ca α-sialon with the composition Ca_1.8Si_6.6Al_5.4O_1.8N_14.2 by pressureless sintering. Comparison was made concerning the densification process, reaction sequence and microstructure of the corresponding materials. The sluggish reactivity of β-Si₃N₄ resulted in poorer densification during sintering. All the three starting powders produced a similar final phase assembly, namely α-sialon together with a small amount of AlN and AlN polytypoid except that traces of unreacted β-Si₃N₄ remained until 1800° in samples prepared with ∼100%β-Si₃N₄ powders. Elongated α-sialon grain morphology has been identified in the samples prepared using all the three different Si₃N₄ starting powders. Coarser elongated α-sialon grains with lower aspect ratio were found in samples using higher β phase starting powders.     

Feasibility of SiAlON–Si3N4 composite ceramic as a potential bone repairing material

In this study, SiAlON–Si₃N₄ composite ceramic are prepared by direct nitridation and investigated to overcome the limitations associated with ceramic Si₃N₄, which includes the difficulty in fabricating ceramic Si₃N₄ into shaped parts for use in the human body. Phase composition and microstructure of the SiAlON–Si₃N₄ composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, and the porosity, bulk density, compressive strength, and ion release were also measured. The biological properties were evaluated by bone cell cultures on the ceramic surfaces. Results show that Si₄Al₂O₂N₆ is formed by the reaction of Al, Si, and Al₂O₃ with nitrogen at high temperature that forms Si₃N₄, thereby fabricating SiAlON–Si₃N₄ composite ceramics. Some α-Si₃N₄ grains underwent a phase transition from α-to β-Si₃N₄ fiber at high temperature. Porosity of the samples increases with increasing Si₃N₄ content, while the bulk density of the samples decreases. The compressive strength increases and then slightly decreases with increasing Si₃N₄ content. Water leaching experiments of the SiAlON–Si₃N₄ composite ceramics reveal that the composites exhibit outstanding chemical stability. Studies using bone cell culture indicate that the cells present a fusiform and extend two or three thin pseudopodia. The phenomena demonstrate that MC3T3-E1 cells have excellent growth activity and have the potential ability to proliferate to osteocytes on the surfaces of the samples, thus suggesting that SiAlON–Si₃N₄ based ceramics are biocompatible and could be implemented as a potential bone-repairing material.     

Influence of starting material composition and carbon content on the preparation of Mg-α SiAlON powders by carbothermal reduction-nitridation

This paper investigated the synthesis of Mg–α SiAlON from Talc (Mg₃(Si₂O₅)₂(OH)₂) and halloysite clay (Al₂Si₂O₅(OH)₄). Talc, halloysite and carbon black as a reducing agent were used as the starting materials, and 3 wt.% α-Si₃N₄ was added as the seed in all cases. The mixtures were heated in flowing N₂ (gas) to synthesize Mg–α SiAlON powders. The chosen molecular ratios of talc to halloysite were 1.0, 1.5, 2.0 and 2.5. The influences of various reaction parameters such as the carbon content, temperature and holding time at the top temperature were studied. The results showed that the synthesized powders were composed of α-SiAlON, β-SiAlON, β-SiC, 15R-AlN polytypoid and AlN phases; the phases and the particle morphology greatly depended on starting material composition and synthesis parameters. A larger amount of talc was needed to compensate for Mg evaporation loss in the starting composition. Higher carbon content seemed to retard the reaction rate, resulting in coarse particle size with an irregular grain shape. The highest content of Mg–α SiAlON, 90 wt.%, was achieved at 1480 °C for 4 h at talc to halloysite ratios of 1.5 and 2.0.     

Evolution of core–rim structures and phase transformations in infra-red transmitting Y-α-SiAlON ceramics

Core–rim structures were observed as common features in Y-α-SiAlON ceramics hot-pressed between 1550?1950 °C. We found most dopants were taken into α’-rims, and a transition layer grown first on α-cores from liquid-phase over-saturated with metal solutes. Elongated β’-grain were formed as minor phase with α’- or AlN-cores thus only after the α’ matrix had consumed up all Y solutes, revealing that the α’ → β’ transformation is controlled by the transient liquid-phase and similar defects and dangling bonds could be detected in both SiAlON phases by cathodoluminescence. Quantitative assessment of Y_m/3Si_12?(m+n)Al_m+nO_nN_16?n demonstrates the multiphase evolution, initiated by over-saturation of Y solutes at low temperatures thus retaining α-phase as cores to lower the infra-red transmittance, dictated by homogenization of Al solutes at higher temperature. The elimination of those phase boundaries leads to better dopant and sintering design for achieving transparent and high-performance SiAlON ceramics.     

The effect of silicon nitride powder characteristics on SiAlON microstructures,densification and phase assemblage

The surface characteristics, particle size distribution and impurities of starting Si₃N₄ powders exert a very significant influence on the microstructure of sintered silicon nitride based ceramics. Even a change of the processing conditions such as milling liquid media (water or isopropyl alcohol) and milling time can have a substantial effect on particle surface groups, and hence on the microstructure of sintered samples. In this study, SEM, XRD, FTIR, BET, elemental analysis and laser diffraction techniques were used for the comprehensive characterization of Si₃N₄ powders which were produced by diimide, direct nitridation and combustion synthesis, in as received state, and after milling in different liquid media (aqueous or alcohol), for various milling durations. The correlation of the surface characteristics and properties of the Si₃N₄ powders with sintering behavior, and microstructural evolution, densification and phase assemblages of the resulting SiAlON ceramics were reported. The milling conditions affected the surface chemistry of Si₃N₄ powders and the subsequent microstructural evolution. The microstructures evolved from the coarser β-Si₃N₄ powders were coarser, but the fine β-Si₃N₄ powders yielded a bimodal microstructure. The critical particle diameter of the β-Si₃N₄ powder for the formation of needle like SiAlON grains was determined to be less than 0.5 µm.     

Influence of β-Si3N4 particle size and heat treatment on microstructural evolution of α:β-SiAlON ceramics

In the present study, we report the effects of starting β-Si₃N₄ particle sizes and post-sintering heat treatment on microstructure evolution and mechanical properties of prepared α-β SiAlON ceramics. Three different β-Si₃N₄ starting powders, with particle sizes of 2, 1 and 0.5 μm were used to prepare α-β SiAlON ceramics by gas-pressure sintering. Elongated β-SiAlON grain morphology was identified in the samples prepared using 0.5 μm particle size β-Si₃N₄ powder. Low-aspect ratio grain morphology was observed in samples prepared from starting powders with coarse particles (2 μm and 1 μm). The sintered samples were further heat treated to develop desired microstructure with elongated grains. The hardness and indentation fracture toughness values of sintered and heat treated samples were found to lie in the range of 12.4-14.2 GPa and 5.1-6.4 MPa m^1/2 respectively. It was revealed that fracture toughness increases with decrease in particle size of starting β-Si₃N₄ powder.     

Influence of powder mixing processes on phase composition,microstructure, and mechanical properties of α/β-SiAlON ceramic tool materials

α/β-SiAlON composite ceramics using powders mixed through different processes were prepared via spark plasma sintering. Effects of different powder mixing methods (ball-milling and ultrasonic vibration with mechanical stirring), ball-milling media (Al₂O₃ and Si₃N₄ balls), and mixing times (12 and 24 h) on the components and morphology of mixed powders, phase composition, microstructure, and mechanical properties of α/β-SiAlON ceramics were studied. Results indicate that additional oxides are mixed to powders when ball-milled with Al₂O₃ balls, leading to an increase in the amount of amorphous glass and a decline in α/β ratio and mechanical properties. These effects were reduced when powders were ball-milled with Si₃N₄ balls, and α-SiAlON content and fracture toughness increased significantly. Moreover, α/β-SiAlON ceramics that underwent ultrasonic vibration with mechanical stirring exhibited the highest hardness and oxidation resistance at high temperature, and their α/β ratio was close to expected value.     

Effect of nano- and micro-sized Si3N4 powder on phase formation,microstructure and properties of β′-SiAlON prepared by spark plasma sintering

The phase formation behavior of β′-SiAlON with the general formula Si_6-zAl_zO_zN_8-z was studied comprehensively for z values from 1 to 3 using spark plasma sintering (SPS) as the consolidation technique at synthesis temperatures from 1400 to 1700 °C. The samples were prepared close to the β′-SiAlON composition line: Si₃N₄ ? 4/3(AlN·Al₂O₃) in the phase diagram using (A) nano-sized amorphous Si₃N₄ and (B) micro-sized β-Si₃N₄ precursors. Field-emission scanning electron microscopy (FESEM) was used for microstructural analysis.Most compositions reached almost full density at all SPS temperatures. Compared with the micro-sized β-Si₃N₄ precursor, the nano-sized amorphous Si₃N₄ precursor accelerated the reaction kinetics, promoting the formation of dense β′-SiAlON + O′-SiAlON composites after SPS at synthesis temperatures of 1400–1500 °C. This resulted in very high values of Vickers hardness (Hv₁₀) = 18.2–19.2 GPa for the z = 1 composition related to the hardness of the O′-SiAlON component phase.In general, for samples synthesized from nano-sized amorphous Si₃N₄, which were almost fully dense, containing >95% β′-SiAlON, the hardness values were 13.4–13.8 GPa with a fracture toughness of 3.5–4.6 MPa m^1/2. For equivalent samples synthesized from micro-sized β-Si₃N₄, hardness was in the range 13.9–14.4 GPa with a fracture toughness of 4.3–4.5 MPa.m^1/2. These values are comparable with fully dense β′-SiAlONs, usually containing intergranular glass phase which has been sintered by HIP and other processes at much higher temperatures for longer times.     

Reaction and formation mechanism of Fe-Si3N4 composite prepared by flash combustion synthesis

In this paper, in order to improve the performance of Fe–Si₃N₄ composite synthesized by flash combustion, the detailed nitridation mechanism and formation process were discussed. In the process of high temperature nitriding, ξ phase rapidly melted to form Fe-Si melt and the cycle of rapid surface nitriding → rupture of nitridation shells → new melt exposing and nitriding occurred continually on the surface of Fe-Si melt. Since ions have different activities on the surface and internal, Fe ions near the surface migrated inwards and Si ions inside moved out; meanwhile, the Fe-Si melt kept shrinking. The nitriding reaction of the Fe-Si melt finished till the overall activity a_Si approached 0, leaving the atom ratio of [Fe] to [Si] at 3:1. During the falling of the formed Si₃N₄ and Fe₃Si melt in the N₂ flow, the surface of Si₃N₄ was oxidized to form a SiO₂ film. The nitridation product fell into the product pool, loosely stacking and adhering together by the SiO₂ film. α-Si₃N₄ dissolved and precipitated to form β-Si₃N₄ crystals, and the β-Si₃N₄ crystals kept growing to form radioactive elongated crystals. As the temperature decreased, the Fe₃Si melt cooled down; the Si-N-O melt, α-Si₃N₄ and the roots of elongated β-Si₃N₄ crystals formed the dense areas.     

Enhanced mechanical properties of Si3N4 ceramics with ZrB2-B binary additives prepared at low temperature

Phase composition, microstructures, and mechanical properties of silicon nitride (Si₃N₄) ceramics were investigated with ZrB₂ and B additives. Results showed that the addition of ZrB₂ and/or B in 2.5 and 5 vol.% promoted the phase transformation of α- to β-Si₃N₄ phase and the formation of bimodal microstructure after hot-pressing at 1500 °C. With the introduction of 2.5 vol.% (ZrB₂-B) binary additives, fracture toughness and strength of Si₃N₄ ceramics increased significantly from 5.2 MPa m^1/2 and 384 MPa to 7.2 MPa m^1/2 and 675 MPa, respectively. However, the hardness of ceramics decreased slightly from 23.5 GPa to 21.3 GPa, which was still higher than typical values reported on Si₃N₄ ceramics (15˜17 GPa).