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 β-Si3N4 starting powder size on elongated grain growth in β-Si3N4 ceramics

The microstructural evolution of pressureless sintered silicon nitride ceramics prepared from different particle sizes of β-Si₃N₄ as starting powders, has been investigated. When the specimen prepared from as-received β-powder of 0.66 μm in average size, was sintered at 1850°C, equiaxed β-Si₃N₄ grains were observed. As the size of the initial β-powder went down to 0.26 μm, however, the growth of elongated grains was enhanced, which resulted in a whisker-like microstructure similar to that made from α-starting powder. When the sintering temperature was increased to 2000°C, the elongated grains were also developed even in the specimen made from 0.66 μm β-powder. The observed results were discussed with relation to the two dimensional nucleation and growth theory for faceted crystals. In addition, fracture toughness of the specimen consisting of elongated grains, which was prepared from finer powders, increased.     

Effect of particle size and shape of diluent on the reaction process of combustion synthesis of α-Si3N4 powder

α-Si₃N₄ powder was prepared by combustion synthesis using different particle sizes and shapes of Si₃N₄ diluent. The effects of different diluents on combustion temperature, phase composition, and microstructure of the product were investigated. The role of diluents in combustion synthesis is discussed. When no ammonium salt was added, because of the higher reaction temperature, the phase transformation of the fine particle diluent with the best barrier effect was also enhanced, and the α content of the product was the lowest. When the ammonium salt is added, the liquid phase Si content decreases at high temperatures, the lower reaction temperature and the Si₃N₄ generated before Si melting make the barrier effect of the diluent fully play. Finally, Si₃N₄ powder with 86% α content was synthesized by combustion with 2 μm Si₃N₄ diluent.     

Effect of the addition of β-Si3N4 nuclei on the thermal conductivity of β-Si3N4 ceramics

Various microstructures of β-Si₃N₄ were fabricated, with or without the addition of β-Si₃N₄ seed particles to high-purity β-Si₃N₄ powder, using Yb₂O₃ and ZrO₂ as sintering additives, by gas-pressure sintering at 1950 °C for 16 h. The thermal conductivity of the specimen without seeds was 140 W·(m·K)⁻¹, and the specimen exhibited a bimodal microstructure with abnormally grown grains. The thermal conductivity of the specimen with 24 vol.% seed addition was 143 W·(m·K)⁻¹, and this specimen had the bimodal microstructure with finer grain size than that without the seeded material, but maintained the same amount of large grains (⩾2 μm in diameter) as in the specimen without the seeds. This finding indicates that the thermal conductivity of β-Si₃N₄ is controlled by the amount of reprecipitated large grains, rather than by the grain size of the β-Si₃N₄.     

Effect of large β-Si3N4 particles on the thermal conductivity of β-Si3N4 ceramics

Mean-field micromechanics model, the rule of mixture is applied to the prediction of the thermal conductivity of sintered β-Si₃N₄, considering that the microstructure of β-Si₃N₄ is composed of a uniform matrix phase (which contains grain boundaries and small grains of Si₃N₄) and the purified large grains (⩾2 μm in diameter) of Si₃N₄. Experimental results and theoretical calculations showed that the thermal conductivity of Si₃N₄ is controlled by the amount of the purified large grains of Si₃N₄. The present study demonstrates that the high thermal conductivity of β-Si₃N₄ can be explained by the precipitation of high purity grains of β-Si₃N₄ from liquid phase.     

Effect of the residual phases in β-Si3N4 seed on the mechanical properties of self-reinforced Si3N4 ceramics

In this work, the self-reinforced silicon nitride ceramics with crystal seed of β-Si₃N₄ particles were investigated. Firstly, the seeds were prepared by heating of α-Si₃N₄ powder with Yb₂O₃ and MgO, respectively. Then the self-reinforced silicon nitride ceramics were obtained by HP-sintering of α-Si₃N₄ powder, Yb₂O₃ and the as-prepared seeds which were not treated with acid and/or alkali solution. The results indicated that the introduction of seed with Yb₂O₃ could obviously increase the toughness and room temperature strength of the ceramics. Furthermore, its high temperature strength (1200 °C) could nearly keep higher value as the one of room temperature measured from unreinforced ceramic. However, the seed with MgO abruptly decrease the high temperature strength of the ceramics. The SEM and TEM characterization showed that the rod-like seed particle could favor the toughness and the presence of the Mg promote the formation of crystalline secondary phase.     

Synthesis and spark plasma sintering of the α-Si3N4 nanopowder

A two-step process (milling and then heat treatment) was used for the preparation of α-Si₃N₄ nanopowder. The influence of the milling time and heat treatment temperature as processing parameters were investigated on the formation of α-Si₃N₄. Silicon nitride ceramic was produced by spark plasma sintering at 1700 °C for 15 min, using MgSiN₂ additive. The optimum sample was produced in a 30 h milling time, heat treatment at 1300 °C, and a 22 °C/min heating rate conditions. X-ray fluorescence analysis showed that the purity of the final product is above 98%. Nanoindentation hardness and Young’s modulus of the SPS-ed sample were measured as 17±2.0 GPa and 290±11.0 GPa, respectively.     

Influence of β-Si3N4 whiskers on sintering and crystallization of fused silica ceramics

Fused silica ceramics are widely applied for radome materials, crucibles, and vanes, but the mechanical properties were deteriorated due to the cristobalite crystallization. The fused silica ceramics added with by β-Si₃N₄ whiskers were prepared by a slip-casting method to retard the cristobalite crystallization. The influences of the sintering environments and the β-Si₃N₄ whiskers on the microstructure and phase structure were investigated. The silanol (Si-(OH)_n) and oxygen vacancies (V_O) in the fused silica in formed in different conditions were studied by Fourier Transform Infra-Red (FT-IR) and X-ray photoelectron spectroscopy (XPS), and the results indicated that the ball-milled produced a large amount of the silanol groups onto the surface of the fused silica particles. The fused silica heated in the vacuum created the maximum oxygen vacancies (24.2%) on the surfaces. Silanol groups reacted with the β-Si₃N₄ whiskers, and the O atoms in the silanol groups were fixed into the bulk materials. And the crystallization kinetics and the activation energy of Si₃N_4w/SiO₂ ceramics at the temperature ranging from 1200 to 1400°C were calculated based on the JMA(Johnson-Mehl-Avrami) model. The activation energy of the fused silica ceramics with the addition of the β-Si₃N₄ is 506.2 kJ/mol, increased by 23.6% than that of the pure fused silica ceramic.     

Fabrication and properties of a dense SiC NWs-toughened α-Si3N4 composite coating on porous Si3N4 ceramics

A dense SiC nanowires-toughened α-Si₃N₄ coating was prepared using a two-step technique for protecting porous Si₃N₄ ceramic against mechanical damage, and effect of SiC nanowires content on microstructures and properties of the coating were investigated. XRD, SEM and TEM analysis results revealed that as-prepared coatings consisted of α-Si₃N₄, O'-Sialon, SiC nanowires and Y–Al–Si–O–N glass phase. Furthermore, Vickers hardness of the coated porous Si₃N₄ ceramics increased gradually with the increasing SiC nanowires content from 0 to 10 wt%, which is attributed to the gradual improvement in intrinsic elastic modulus (E), hardness (H) and H³/E² of the coatings. But, when the SiC nanowires content was 15 wt%, the thickness of the coating became relatively thinner, so that its protective ability was weakened and Vickers hardness started to decrease accordingly. Meanwhile, the assistance of SiC nanowires enhanced fracture toughness of the coatings obviously because SiC nanowires in the coatings can produce various toughening mechanisms during mechanical damage. When the SiC nanowires content was 10 wt%, its fracture toughness reached the maximum value, which was 6.27 ± 0.05 MPa·m^1/2.     

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.     

Mechanical properties of Si3N4 ceramics from an in-situ synthesized a-Si3N4/β-Si3N4 composite powder

Sintered Si₃N₄ ceramics were prepared from an ɑ-Si₃N₄/β-Si₃N₄ whiskers composite powder in-situ synthesized via carbothermal reduction at 1400–1550 °C in a nitrogen atmosphere from SiO₂, C, Ni, and NaCl mixture. Reaction temperatures and holding time for the composite powder, and mechanical properties of sintered Si₃N₄ were investigated. In the synthesized composite powder, the in-situ β-Si₃N₄ whiskers displayed an aspect ratio of 20–40 and a diameter of 60–150 nm, which was mainly dependent on the synthesis temperature and holding time. The flexural strength, fracture toughness and hardness of the sintered Si₃N₄ material reached 794±136 MPa, 8.60±1.33 MPa m^1/2 and 19.00±0.87 GPa, respectively. The in-situ synthesized β-Si₃N₄ whiskers played a role in toughening and strengthening by whiskers pulling out and crack deflection.     

Fabrication,thermal shock resistance,and dielectric property of α-Si3N4-based ceramic coating on porous Si3N4 ceramics

A dense α-Si₃N₄-based ceramic protective coating was successfully prepared on porous Si₃N₄ ceramics by a liquid infiltration and filling method. The coating composed of a primary α-Si₃N₄ phase and secondary O'-Sialon, β-Sialon, and Y–Si–Al–O–N glass phase. After thermal shock at ΔT = 1000°C for five times, cracks were produced, but the tip of crack stopped inside the coating; so the coated porous Si₃N₄ ceramics still had a good waterproof ability and its water absorption was only 7%. During thermal shock, toughening mechanisms involving needle-like O'-Sialon particle bridging, crack deflection, and rough fracture, occurred within the cracks, contributing to thermal shock resistance of the coating. The dielectric constant of the coated porous Si₃N₄ ceramics showed a slow increase trend with increasing temperature, and it reached the maximum value of 3.57 at 1100°C at the frequency of 11 GHz. The dielectric loss increased slowly as the temperature increased from room temperature to 900°C, but it started to increase evidently when the temperature was over 900°C.     

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.     

In situ measurement of combustion wave propagation during combustion synthesis of α-Si3N4 powder

α-Si₃N₄ powder was prepared by combustion synthesis method. The propagation characteristics of combustion wave were investigated by thermocouple temperature measurement and “resistance—combustion wave displacement response device.” The results show that the combustion reaction takes place in a narrow area, and there is a maximum temperature gradient of 180°C/mm in the combustion front. The “resistance—combustion wave displacement response device” was innovatively used to realize the in situ measurement of combustion wave propagation process. The test results showed the pulse combustion mode in which the combustion front took 10 s as a cycle, and the quantitative data of Si–N₂ discrete combustion characteristics were obtained for the first time.     

Influence of α-Si3N4 coarse powder on densification,microstructure, mechanical properties,and thermal behavior of silicon nitride ceramics

Silicon nitride (Si₃N₄) ceramics, with different ratios of fine and coarse α-Si₃N₄ powders, were prepared by spark plasma sintering (SPS) and heat treatment. Further, the influence of coarse α-Si₃N₄ powder on densification, microstructure, mechanical properties, and thermal behavior of Si₃N₄ ceramics was systematically investigated. Compared with fine particles, coarse particles exhibit a slower phase transition rate and remain intact until the end of SPS. The remaining large-sized grains of coarse α-Si₃N₄ induce extensive growth of neighboring β-Si₃N₄ grains and promote the development of large elongated grains. Noteworthy, an appropriate number of large elongated grains distributed among fine-grained matrix forms bimodal microstructural distribution, which is conducive to superior flexural strength. Herein, Si₃N₄ ceramics with flexural strength of 861.34 MPa and thermal conductivity of 65.76 W m⁻¹ K⁻¹ were obtained after the addition of 40 wt% coarse α-Si₃N₄ powder.     

Low temperature heat capacity measurements of β-Si3N4 and γ-Si3N4: Determination of the equilibrium phase boundary between β-Si3N4 and γ-Si3N4

Isobaric heat capacities of β-Si₃N₄ and γ-Si₃N₄ were measured at temperatures between 1.8 and 309.9 K with a thermal relaxation method. The measured heat capacities of γ-Si₃N₄ are smaller than those of β-Si₃N₄ in this temperature range. Using these data, we determined the standard entropies of β-Si₃N₄ and γ-Si₃N₄ to be 62.30 J·mol⁻¹ K⁻¹ and 51.79 J·mol⁻¹ K⁻¹, respectively. The equilibrium phase boundary between β-Si₃N₄ and γ-Si₃N₄ was calculated using these values and thermodynamic parameters reported in previous studies. The obtained equilibrium phase transition pressure at 2000 K is 11.4 GPa. It is lower than the experimental pressures at which γ-Si₃N₄ was synthesized in previous studies. The calculated Clapeyron slope at this temperature is 0.6 MPa K⁻¹, which is consistent with those of theoretical studies.     

Thermal shock resistance of Si3N4 and Si3N4–SiC ceramics with rare-earth oxide sintering additives

The influence of various rare-earth oxide additives and the addition of SiC nanoparticles on the thermal shock resistance of the Si₃N₄ based materials was investigated. The location of SiC particles inside the Si₃N₄ grains contributed to a higher level of residual stresses, which caused a failure at the lower temperature difference compared to the composites with a preferential location of the SiC at the grain boundaries. A critical temperature difference increased with an increasing ionic radius of RE³⁺ for both the composites and the monoliths. The critical temperature difference for the composite (580 °C) and the monolith (680 °C) sintered with La₂O₃ was significantly higher compared to the composite and the monolith doped with Lu₂O₃ (430 °C). A good agreement was found between the results of the critical temperature difference estimated by the indentation quench test and that obtained by the strength retention method.     

Effect of lattice defects on the thermal conductivity of β-Si3N4

Two types of β-Si₃N₄ were sintered at 1900 °C one for 8 h and the other for 36 h by using Yb₂O₃ and ZrO₂ as sintering additives. The latter specimen was further annealed at 1700 °C for 100 h to promote grain growth. The microstructures of the sintered materials were investigated by SEM, TEM, and EDS. The thermal conductivities of the specimens were 110 and 150 Wm⁻¹K⁻¹, respectively. The sintered material which possessed 110 Wm⁻¹K⁻¹ had numerous small precipitates that consisted of Yb, O and N elements and internal dislocations in the β-Si₃N₄ grains. In the sintered material with 150 Wm⁻¹K⁻¹ neither precipitates nor dislocations were observed in the grains. The microscopic evidence indicates that the improvement in the thermal conductivity of the β-Si₃N₄ was attributable to the reduction of internal defects of the β-Si₃N₄ grains with sintering and annealing time as the grains grew.     

Effect of Ta2O5 additions on phase formation during synthesis and sintering of β-alumina based ceramics

Conclusions We demonstrated the effect of the Ta₂O₅ additive on the process of phase formation during the synthesis of the powders and during sintering of -alumina ceramics (as compared to stoichiometric sodium polyaluminate Na₂O·5Al₂O₃). The presence of the additive increases the quantity of -Al₂O₃ phase and stabilizes it during low-temperature isothermal synthesis; during high-temperature sintering, it hinders the formation of the nonconducting -Al₂O₃ phase; this is one of the main conditions required for obtaining -alumina ceramics having a high ionic conductivity.Translated from Ogneupory, No. 2, pp. 13–15, February, 1991.     

Effect of lattice impurities on the thermal conductivity of β-Si3N4

Effect of impurities in the crystal lattice and microstructure on the thermal conductivity of sintered Si₃N₄ was investigated by the use of high-purity β-Si₃N₄ powder. The sintered materials were fabricated by gas pressure sintering at 1900 °C for 8 and 48 h with addition of 8 wt.% Y₂O₃ and 1 wt.% HFO₂. A chemical analysis was performed on the loose Si₃N₄ grains taken from sintered materials after the chemical treatment. Aluminum was not removed from Si₃N₄ grains, which originated from the raw powder of Si₃N₄. The coarse grains had fewer impurities than the fine grains. Oxygen was the major impurity in the grains, and gradually decreased during grain growth. The thermal conductivity increased from 88 Wm⁻¹ K⁻¹ (8 h) to 120 Wm⁻¹ K⁻¹ (48 h) as the impurities in the crystal lattice decreased. Purification by grain growth thus improved the thermal conductivity, but changing grain boundary phases might also influence the thermal conductivity.