Effects of sintering atmosphere on grain morphology of liquid-phase-sintered SiC with Al2O3 additions

The effects of sintering atmospheres of Ar and N₂ on grain morphology were investigated for pressureless liquid-phase-sintered (LPS) SiC with Al₂O₃ additions. When increasing the sintering temperature, the SiC grain size and its aspect ratio increased in both sintering atmospheres. With a 2 mass% Al₂O₃ addition, no distinct difference was observed between the grain morphology of SiC sintered in the Ar atmosphere and that sintered in the N₂ atmosphere. With a 15 mass% Al₂O₃ addition, sintering in a N₂ atmosphere led to retarded grain growth and this resulted in a fine homogeneous microstructure, whereas sintering in an Ar atmosphere enhanced the grain growth compared with that in 2 mass% Al₂O₃. The effects of atmosphere on the grain morphology depend on the amount of Al₂O₃ addition, and this also affects the grain growth process of solution-reprecipitation. The mechanical properties of the SiC are also considered.     

Inhibition of grain growth in liquid-phase sintered SiC ceramics by AlN additive and spark plasma sintering

SiC ceramics were prepared from nanosized β-SiC powder with different compositions of AlN and Y₂O₃ sintering additives by spark plasma sintering (SPS) at 1900 °C for 600 s in N₂. The relative density of the sintered SiC specimens increased with increasing amount of AlN, reaching a relative density higher than 99%, while at the same time grain size decreased significantly. The smallest average grain size of 150 nm was observed for SiC sample sintered with 10 vol% of additives consisting of 90 mol% AlN and 10 mol% Y₂O₃. Fully dense nanostructured SiC ceramics with inhibited grain growth were obtained by the AlN additive and SPS technique. The flexural strength of the SiC body containing 70 mol% AlN and 30 mol% Y₂O₃ additives reached the maximum value of 1000 MPa. The SiC bodies prepared with AlN and Y₂O₃ additives had the fracture toughness of around 2.5 MPam^1/2.     

Effect of post-sintering heat-treatment on thermal and mechanical properties of Si3N4 ceramics sintered with different additives

To improve the thermal conductivity of Si₃N₄ ceramics, elimination of grain-boundary glassy phase by post-sintering heat-treatment was examined. Si₃N₄ ceramics containing SiO₂–MgO–Y₂O₃-additives were sintered at 2123 K for 2 h under a nitrogen gas pressure of 1.0 MPa. After sintering, the SiO₂ and MgO could be eliminated from the ceramics by vaporization during post-sintering heat-treatment at 2223 K for 8 h under a nitrogen gas pressure of 1.0 MPa. Thermal conductivity of 3 mass% SiO₂, 3 mass% MgO and 1 mass% Y₂O₃-added Si₃N₄ ceramics increases from 44 to 89 Wm⁻¹ K⁻¹ by the decrease in glassy phase and lattice oxygen after the heat-treatment. Relatively higher fracture toughness (3.8 MPa m^1/2) and bending strength (675 MPa) with high hardness (19.2 GPa) after the heat-treatment were achieved in this specimen. Effects of heat-treatment on microstructure and chemical composition were also observed, and compared with those of Y₂O₃–SiO₂-added and Y₂O₃–Al₂O₃-added Si₃N₄ ceramics.     

Effect of sintering additives on the oxidation behavior of Si3N4 ceramics at 1300 °C

This paper describes the results of systematic investigation of the oxidation behavior of Si₃N₄ based ceramics. The tests were carried out at 1300 °C for 2000 h in a high-temperature dry air environment. The Si₃N₄ specimens tested include the following: (a) S-1: Si₃N₄ added 8 mass% Y₂O₃, (b) S-2: Si₃N₄/SiC added 8 mass% Y₂O_3, (c) S-3: Si₃N₄ added 5 mass% Y₂O₃ and 3 mass% Al₂O_3, (d) S-4: Si₃N₄/SiC added 5 mass% Y₂O₃ and 3 mass% Al₂O_3. Several interesting conclusions were obtained as follows: (1) the thicknesses of oxidized layer of S-3 and S-4 were much thicker than S-1 and S-2, (2) oxidation kinetics of S-1 and S-2 obeyed a parabolic law on the whole, while those of S-3 and S-4 had a break, (3) the yttrium (Y) concentration under the oxidized layer decreased significantly. The Y-decreased zone was defined as a diffused layer. The thicknesses of the diffused layers of S-3 and S-4 samples were very large. (4) Primarily, the crystalline phases in the oxidized layer were SiO₂ and Y₂Si₂O_7. (5) The effect of SiC composition on the oxidation behavior was small.     

Effect of Si addition on the mechanical and thermal properties of sintered reaction bonded silicon nitride

Advanced silicon nitride (Si₃N₄) ceramics were fabricated using a mixture of Si₃N₄ and silicon (Si) powders via conventional processing and sintering method. These Si₃N₄ ceramics with sintering additives of ZrO₂ + Gd₂O₃ + MgO were sintered at 1800 °C and 0.1 MPa in N₂ atmosphere for 2 h. The effects of added Si content on density, phases, microstructure, flexural strength, and thermal conductivity of the sintered Si₃N₄ samples were investigated in this study. The results showed that with the increase of Si content added, the density of the samples decreased from 3.39 g/cm³ to 2.92 g/cm³ except for the sample without initial Si₃N₄ powder addition, while the thermal diffusivity of the samples decreased slightly. This study suggested that addition of Si powder, which varied from 0 to 100%, in the starting materials might provide a promising route to fabricate cost-effective Si₃N₄ ceramics with a good combination of mechanical and thermal properties.     

Pressureless sinterability of slip cast silicon nitride bodies prepared from coprecipitation-coated powders

The sinterability of compositions from different powder preparation methods (coprecipitation-coating of Si₃N₄ powder or mechanical mixing of Si₃N₄ with Y₂O₃ and Al₂O₃) and compaction routes (dry pressing or slip casting) was compared. Both the coating method and the slip casting process improved silicon nitride sinterability over the mechanical mixing method and dry pressing route. However, the minimisation of powder agglomeration in the green bodies achieved by slip casting is more determinant to the sintering behaviour than the homogeneous distribution of the sintering additives around the Si₃N₄ offered by the coated powder. The coating powder method in combination with the slip casting process is the most favourable processing route, leading to a homogeneous and fully dense microstructure by pressureless sintering at a relatively low temperature of 1750°C. This technique produced materials with hardness of 15·2 GPa, fracture toughness of 7 MPa  m^1/2 and flexural bending strength of 910 MPa.     

Spark plasma sintering and pressureless sintering of SiC using aluminum borocarbide additives

Aluminum borocarbide powders (Al₃BC₃ and Al₈B₄C₇) were synthesized, and the ternary powders were used as a sintering additive of SiC. The densification of SiC was nearly completed at 1670 °C using spark plasma sintering (SPS) and pressureless sintering was possible at 1950 °C. The sintering behavior of SiC using the new additive systems was nearly identical with that using the conventional Al–B–C system, but grain growth was suppressed when adding the borocarbides. In addition, oxidation of the fine additive powders did not intensively occur in air, which has been a problem in the case of the Al–B–C system for industrial application. The hardness, Young's modulus and fracture toughness of a sintered SiC specimen were 21.6 GPa, 439 GPa and 4.6 MPa m^1/2, respectively. The ternary borocarbide powders are efficient sintering additives of SiC.     

Low temperature pressureless sintering of dense silicon nitride using BaO-Al2O3-SiO2 glass as sintering aid

Dense Si₃N₄ ceramic with BaO-Al₂O₃-SiO₂ low temperature glass powders as sintering aids were prepared by pressureless sintering techniques at a relatively low temperature (1550 °C). Four kinds of glass powders of compositions melting at 1120 °C, 1300 °C, 1400 °C and 1500 °C, respectively, have been introduced as sintering aids. XRD results demonstrate that the BaO-Al₂O₃-SiO₂ glass powders reacted with BaAl₂O₄ and converted into hexagonal celsian, which is a high-temperature phase with melting point of 1760 °C, so being beneficial to the high temperature properties of the materials. In addition, a portion of α-Si₃N₄ transformed to rod like β-Si₃N₄ with high aspect ratio as shown by XRD and SEM analysis. The bulk density increased with the rise of the melting temperature of the BaO-Al₂O₃-SiO₂ glass powders, the sample obtained with the BaO-Al₂O₃-SiO₂ glass powder melting at 1500 °C reaching a maximum density of 98.8%, an high flexural strength (373 MPa) and a fracture toughness (4.8 MPa m^1/2).     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Optimization of spark plasma sintering parameters of Si3N4-SiC composite using response surface methodology (RSM)

The Si₃N₄-SiC micro-nano composites were fabricated via the spark plasma sintering method using MgSiN₂ as an additive. Response surface methodology and central composite design were applied to optimize the spark plasma sintering process for the fabrication of Si₃N₄-SiC/MgSiN₂ with improved density. The relation between the three parameters of sintering including temperature, pressure, and holding time was modeled and the optimized parameters were obtained. The best sintering results obtained for the sintering temperature, holding time, and pressure are 1700 °C, 487 s, and 49 MPa, respectively. The addition of MgSiN₂ as an additive and SiC as a secondary phase were also investigated in the present work. The Si₃N₄−5 vol% SiC composite exhibited high hardness (19 GPa) and fracture toughness values (6.5 MPa m^1/2) at room temperature.     

Preparation and characterization of Si3N4-based composite ceramic tool materials by microwave sintering

Si₃N₄-based composite ceramic tool materials with (W,Ti)C as particle reinforced phase were fabricated by microwave sintering. The effects of the fraction of (W,Ti)C and sintering temperature on the mechanical properties, phase transformation and microstructure of Si₃N₄-based ceramics were investigated. The frictional characteristics of the microwave sintered Si₃N₄-based ceramics were also studied. The results showed that the (W,Ti)C would hinder the densification and phase transformation of Si₃N₄ ceramics, while it enhanced the aspect-ratio of β-Si₃N₄ which promoted the mechanical properties. The Si₃N₄-based composite ceramics reinforced by 15 wt% (W,Ti)C sintered at 1600 °C for 10 min by microwave sintering exhibited the optimum mechanical properties. Its relative density, Vickers hardness and fracture toughness were 95.73 ± 0.21%, 15.92 ± 0.09 GPa and 7.01 ± 0.14 MPa m^1/2, respectively. Compared to the monolithic Si₃N₄ ceramics by microwave sintering, the sintering temperature decreased 100 °C,the Vickers hardness and fracture toughness were enhanced by 6.7% and 8.9%, respectively. The friction coefficient and wear rate of the Si₃N₄/(W,Ti)C sliding against the bearing steel increased initially and then decreased with the increase of the mass fraction of (W,Ti)C., and the friction coefficient and wear rate reached the minimum value while the fraction of (W,Ti)C was 15 wt%.     

Toughening of SiC matrix with in-situ created TiB2 particles

SiC–TiB₂ composites with up to 50 vol% TiB₂ were fabricated by in-situ reaction between TiO₂, B₄C and C. The densification of the uniaxially pressed samples was done using pressureless sintering in the presence of sintering aids consisting of Al₂O₃ and Y₂O₃. The influence of the volume fraction of TiB₂ and sintering temperature on density and fracture toughness was examined. It was found that fracture toughness is strongly affected by the volume fraction of TiB₂. The presence of TiB₂ particles suppresses the grain growth of SiC and facilitates different toughening mechanisms to operate which, in turn, increases fracture toughness of the composite. The highest value for fracture toughness of 5.7 MPa m^1/2 was measured in samples with 30 vol% TiB₂ sintered at 1940 °C.     

Study of processing routes for WC-MgO composites with varying MgO contents consolidated by FAST/SPS

Two different preparation routes were applied to process WC-MgO composites with varying MgO contents (4.1 wt.% and 5.9 wt.% MgO). WC-MgO powder mixtures were synthesized by a milling process at 600 rpm for 6 h of partially oxidized WC (WC + WO₃), Mg₃N₂ and C. Alternatively, WC and MgO as initial powders were used. For consolidation of the powder mixtures the field-assisted sintering technology (FAST) was used. X-ray diffraction shows that samples out of different powder mixtures and sintered between 1600 °C and 1750 °C exhibited WC, MgO and the W₂C phase independent of the preparation route of the powder mixtures. A higher density and better mechanical properties (hardness and indentation fracture toughness) of WC-MgO were achieved of pure WC and MgO as initial powders were consolidated by FAST. It was found that a lower MgO content results in higher hardness values and in a slightly decreased indentation fracture toughness.     

Properties of Si3N4–TiN composites fabricated by spark plasma sintering by using a mixture of Si3N4 and Ti powders

Si₃N₄–TiN composites were successfully fabricated via planetary ball milling of 70 mass% Si₃N₄ and 30 mass% Ti powders, followed by spark plasma sintering (SPS) at 1250–1350 °C. The sintering mechanism for SPS was a hybrid of dissolution–reprecipitation and viscous flow. The electrical resistivity decreased with increasing sintering temperature up to a minimum at 1250 °C and then increased with the increasing sintering temperature. The composites prepared by SPS at 1250–1350 °C could be easily machined by electrical discharge machining. Composite prepared by SPS at 1300 °C showed a high hardness (17.78 GPa) and a good machinability.     

Nano/macro-hardness and fracture resistance of Si3N4/SiC composites with up to 13 wt.% of SiC nano-particles

Relations between composition and mechanical properties of the Si₃N₄/SiC micro/nano-composites were studied by combination of nano-indentation and Vickers indentation techniques. The Si₃N₄/SiC composites were prepared from crystalline Si₃N₄ powder doped with SiNC amorphous precursor and yttria as the sintering aid. During sintering the SiNC precursor crystallised to yield both SiC and Si₃N₄. The in situ formed SiC particles were located both inter- and intra-granularly. The presence of SiC nano-particles enhanced the nano- and macro-hardness, and the fracture toughness of the composites. The nano-hardness of Si₃N₄/SiC composites ranged between 20 and 24 GPa, and depends on the volume fraction of SiC. The nano-hardness of individual Si₃N₄ grains exhibited large scatter as the consequence of the presence of intra-SiC inclusions, which directly influence the measured values as the harder phase, or by generating large thermal stresses within Si₃N₄ grains. Consequently the scatter of nano-hardness was much larger than in case of macro-hardness where the measured values are averaged over large area. The nano-indentation of grain boundaries indicates that the boundaries are much softer than the surrounding matrix phase. Apart of indentation size effect (ISE) this is believed to be an additional reason why the measured values of macro-hardness are lower than the nano-hardness. The maximum fracture toughness (5.8 MPa m^1/2) was achieved for the composite with the total amount of 8 wt.% SiC, where a percolating network of intergranular SiC particles was formed, as indicated by the measurement of electrical resistivity.     

Fabrication of continuously porous SiC–Si3N4 composite using SiC powder by extrusion process

Large amounts of waste SiC sludge containing small amounts of Si and organic lubricant were produced during the wire cutting process of single crystal silicon ingots. Waste SiC sludge was purified by washing it with organic solvent and purified SiC powder was used to fabricate the continuously porous SiC–Si₃N₄ composites, using an extrusion process, in which carbon, 6 wt% Y₂O₃ + 2 wt% Al₂O₃ and ethylene vinyl acetate were added as a pore-forming agent, sintering additives and binder, respectively. In the burning-out process, the binder and carbon were fully removed and continuously porous SiC–Si₃N₄ composites were successfully fabricated. The green bodies containing waste SiC, Si powder and sintering additives were nitrided at 1400 °C in a flowing N₂ + 10% H₂ gas mixture. The continuously porous composites contained SiC, α-Si₃N₄, β-Si₃N₄ and few Fe phases. The pore size of the second passed and third passed SiC–Si₃N₄ composites was 260 μm and 35 μm in diameter, respectively. The values of bending strength and hardness in the second passed and third passed samples were 62.97 MPa, 388 Hv and 77.82 MPa, 423 Hv, respectively.     

Rare-earth element doped Si3N4/SiC micro/nano-composites—RT and HT mechanical properties

Dense Si₃N₄/SiC micro/nano-composites with varying grain boundary phase composition were fabricated by hot-pressing under the same conditions. Six different sintering aids (Lu₂O₃, Yb₂O₃, Y₂O₃, Sm₂O₃, Nd₂O₃ and La₂O₃) were used. The formation of SiC nano-inclusions was achieved by in situ carbothermal reduction of SiO₂ by C during the sintering process. Room temperature, fracture toughness, hardness and strength tended to increase when the cation radius of the rare-earth element used in the oxide additive decreased (i.e. from La³⁺ to Lu³⁺). The composite material with Lu₂O₃ sintering additive showed the highest hardness and had reasonably high fracture toughness and strength. The same micro/nano-composite also possessed the highest creep resistance in the temperature range from 1250 °C to 1400 °C and with loads in the range 50–150 MPa.     

Spark plasma sintering of ZrC–SiC ceramics with LiYO2 additive

Spark plasma sintering (SPS) of ZrC–SiC composite powders in the presence of LiYO₂ sintering additive was studied. The starting powders were obtained by a carbothermal reduction (CTR) of natural mineral zircon (ZrSiO₄), which provided an intimate mixing of in-situ created ZrC and SiC powders. This composite powder and LiYO₂ as additive were densified by spark plasma sintering. Microstructural features of the composite were investigated by XRD, SEM/EDS and AFM analysis. The sintered composite material possesses promising mechanical properties and excellent cavitation resistance which was observed with a cavitation erosion test. The values of Vickers microhardness and fracture toughness of the composite material are 20.7 GPa and 5.07 MPam^1/2, respectively.     

Microstructure and mechanical properties of silicon carbide pressureless sintered with oxide additives

It is known that SiC powders can be densified at relatively low temperatures (1850–2000 °C) with some oxide additions. In this work the densification behavior, microstructure and mechanical properties (bending strength, fracture toughness, hardness) of SiC ceramics pressureless sintered with different additions chosen from oxide groups: Al₂O₃ + Y₂O₃, Al₂O₃ + Y₂O₃ + MgO, were investigated. It was found that oxide additives facilitate densification of sinters and significantly improve mechanical properties of SiC ceramics. The best activating oxide additions have been identified.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.