Nanostructure and bimodal structure of Si3N4 ceramics developed by spark plasma sintering method

Abstract

The densification of high energy ball milled Si₃N₄ nanopowders through spark plasma sintering was investigated. Nanoceramics of Si₃N₄, with fine microstructure, comprising of grains with a diameter of ～70 nm, were produced after sintering at 1600°C for 5 min in N₂ atmosphere with a fast heating rate of 300 K min^?1. The size and the aspect ratio of Si₃N₄ grains increased with decreasing heating rate and increasing holding time and temperature. Post-annealing of sintered ceramics at 1850°C for 3 h favoured development of a self-reinforced bimodal microstructure containing large elongated grains.     

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti₃Al(Si)C₂-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti₃Al(Si)C₂ phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti₃Al(Si)C₂-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m^1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al₂O₃ phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al₂O₃ phases.     

Effect of talc and bauxite on sintering,microstructure, and refractory properties of Egyptian dolomitic magnesite

Abstract

Refractories produced from mixes of Egyptian dolomitic magnesite with talc and low cost Chinese bauxite have been assessed as potential substitutes for the high temperature basic refractory bricks currently used in steelmaking and other industries. Fifteen batches of different compositions were prepared by firing for 2 h up to 1580° C. Six samples were selected for further investigation on the basis of their phase composition and densification parameters. The mineralogical composition and microstructure, pore size distribution, and mechanical and refractory properties of these samples were investigated. All six samples consisted mainly of MgO and MA spinel refractory phases, plus some calcium silicates and aluminoferrite phases. The latter phases contribute to densification during firing by promoting liquid phase formation. Most samples showed high refractoriness under load, good spalling resistance, better mechanical properties than current refractories, and compact microstructure. It is concluded that these new refractories are good candidate materials for the production of shaped linings for industrial kilns and furnaces.     

Influence of mechanical alloying on Ti3SiC2 formation via spark plasma sintering technique from Ti/SiC/C powders

Ti₃SiC₂ was elaborated by two different methods: (i) Spark plasma sintering of 5Ti/2SiC/C powders and (ii) mechanical alloying of powders followed by Spark plasma sintering. The results showed that mechanical alloying was not advantageous for pure Ti₃SiC₂ formation but it can significantly improve the density of the obtained bulk material via the particles refinement as well as the microhardness by increasing the TiC content. It was found that the relative density was increased up to 98.58% for the sintered mechanically alloyed sample whereas it was not more than 96.04% for the sintered 5Ti/2SiC/C starting powders. The Vickers microhardness measured for both bulk samples demonstrates a high improvement for the previously mechanically alloyed powder mixture, as it was of about 1282 Hv and only 581.2 Hv for the alloy obtained from 5Ti/2SiC/C starting powders.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

Synthesis of Al2O3 coatings on Ti(C,N)-based cermets by microwave plasma CVD using Al(acac)3

Aluminum acetylacetonate (Al(acac)₃) was used as a precursor to synthesize aluminum oxide (Al₂O₃) coatings on Ti(C, N)-based ceramic by microwave plasma CVD (MPCVD). Al₂O₃ coatings transformed from γ phase to δ phase and α phase and as microwave power (p_M) and total pressure (P_tot) increased. The effects of p_M and P_tot on the microstructure of the Al₂O₃ coating and oxidation of the substrate have been investigated. The relationship between phase structure and adhesive strength of the coatings was also studied. Coatings deposited at p_M = 1.0-1.2 kW and P_tot = 400 Pa exhibited good adhesion strength (Class 1).     

Rapid synthesis of dense Ti3SiC2 by spark plasma sintering

Ti₃SiC₂ was rapidly synthesized and simultaneously consolidated from the starting mixture of Ti/Si/2TiC by spark plasma sintering (SPS). An intensive reaction leading to the formation of Ti₃SiC₂ occurred at the measured temperature of around 1200 °C, which is several hundreds degrees lower than that of conventional reactive hot pressing. The phase composition of the product could be tailored by adjusting the process parameters. An axisymmetric preferred orientation of the Ti₃SiC₂ grains with well-developed (008) planes was formed, resulting in an anisotropic hardness in respect to the textured product.     

Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering

SPS (spark plasma sintering) process was used to sinter nanocrystalline ZrO₂ (3Y). It was found to be different with the usual rapid sintering method, the density of the samples kept increasing with the rising of the sintering temperature. A higher density could be reached at a lower temperature and shorter dwelling time than that by hot-pressing under the similar pressures. In contrast to the samples with a differential densification from edge to center prepared by a rapid hot-pressing, no obvious densification gradient could be found in the samples sintered by SPS. The grain sizes of the Y-TZP obtained by SPS were smaller than those by the pressureless sintering method, while the grain growth speed was much higher under SPS conditions. All these unique sintering behaviors were explained by the special sintering process of SPS.     

High strengthening effects and excellent wear resistance of Ti3Al(Si)C2 solid solutions

The Ti₃Al_1.2−xSi_xC₂ (x = 0, 0.2, 0.4) powders were synthesized from Ti, Al, Si, and TiC powders, and nearly pure Ti₃Al_1.2−xSi_xC₂ bulks were fabricated by the means of two-time hot-pressing method. Significant strengthening effect in bulks was found after the addition of 0.2 Si and 0.4 Si to form Ti₃Al(Si)C₂ solid solutions. The flexural strengths of Ti₃AlSi_0.2C₂ and Ti₃Al_0.8Si_0.4C₂ were 485 and 554 MPa, 14% and 30% larger than the strength of Ti₃AlC₂, respectively. The Vickers hardness of these compounds were separately, 6.95 and 7.57 GPa, representing the enhancements of 37% and 49% over those of Ti₃AlC₂. The tribological behavior was studied by dry-sliding method with a S45C steel at the sliding speed of 30 m/s and the normal load of 20-80 N. The results showed that after incorporating different contents of Si, the friction coefficient was between 0.22 and 0.30, correspondingly lower wear rate was 3.19-2.61 × 10⁻⁶ mm³/Nm. These excellent tribological performances were attributed to the presence of continuous self-generated oxidized films during tribological examination. Finally, the phase compositions and microhardness of the oxidized films were analyzed and characterized.     

Effect of powder size on microstructure and mechanical properties of Ti(C,N) based cermets

Abstract

Effect of the particle size of TiC and TiN on the microstructure and mechanical properties of Ti(C,N) based cermets has been evaluated. Ti(C,N)–WC–Co cermets made from four groups of mixed raw powders of different sizes were manufactured by vacuum sintering. The microstructure and composition were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDX). The result shows that the four samples have the typical microstructures of 'black core/grey rim'. The mechanical properties of the cermet manufactured from submicron TiC and nano TiN are the best among the four samples.     

Microwave Sintering of Ti3Si(Al)C2 Ceramic

In this work, microwave sintering (MWS) method was successfully applied for fabrication of dense layered ternary Ti₃Si(Al)C₂ ceramic. Compared to conventional pressureless approaches, MWS could significantly decrease preparation temperature from 1600°C to 1400°C. The activation energy of the MWS process was estimated as 233 ± 18 kJ/mol, which was much lower than those in previous sintering techniques. The low sintering temperature likely originates from the low activation energy during MWS process. Such low temperature do not only make the as‐received Ti₃Si(Al)C₂ ceramic much smaller grain size and better mechanical properties, but also indicate higher energy converting efficiency during the sintering processes. Wide application of MWS techniques in MAX phases is expected to promote the practical applications of these materials and contribute to the energy saving during sintering process.     

Fabrication of transparent Y2O3 ceramics by two-step spark plasma sintering

Transparent Y₂O₃ ceramics were successfully fabricated by spark plasma sintering applying a two-step pressure and heating profile. Through the shrinkage curve of the single-step SPS profile, it was confirmed that shrinkage occurred at 800°C–1250°C, and it was selected as the two-step pressure profile. After the first-step SPS stage at 1250°C, the second-step SPS stage, which had the highest real in-line transmittance, was completed at 1500°C. The two-step SPS profile improved the shrinkage behavior and was able to achieve sufficient densification without excessive coarsening. As a result, the normalized real in-line transmittance to 1 mm was 80.6% at 1100 nm, which is close to the theoretical transmittance of 81.6%. The two-step pressure and heating profile in the SPS process was a significant advantage in manufacturing ceramics that were transparent and had sufficient densification.     

Al对等离子放电烧结法合成Ti3SiC2的影响研究

以元素为原料，Al为助剂，采用等离子放电烧结(SPS)工艺合成Ti3SiC2块体材料，通过X射线衍射分析和对SPS过程参数的研究表明：适量A1能促进Ti3SiC2的反应合成，提高合成材料的纯度，但Al也会使Ti3SiC2的热稳定性降低。     

Synthesis and characterization of ternary layered i-MAX (Mo2/3Y1/3)2AlC ceramics fabricated by spark plasma sintering

In this paper, the i-MAX phase (Mo_2/3Y_1/3)₂AlC ceramic with high purity of 98.29 wt% (1.13 wt% Y₂O₃ and 0.58 wt% Mo₂C) and high relative density of 98.59% was successfully synthesized by spark plasma sintering (SPS) at 1500°C with the molar ratio of n(Mo):n(Y):n(Al):n(C) = 4:2:3.3:2.7. The positions of C atoms in the crystal of (Mo_2/3Y_1/3)₂AlC were determined. Microstructure and physical and mechanical properties of (Mo_2/3Y_1/3)₂AlC ceramic were systematically investigated. It was found that the obtained (Mo_2/3Y_1/3)₂AlC ceramic had an average grain size of 32.1 ± 3.1 μm in length and 14.2 ± 1.7 μm in width. In terms of physical properties, the measured thermal expansion coefficient (TEC) of (Mo_2/3Y_1/3)₂AlC was 8.99 × 10⁻⁶ K⁻¹, and the thermal capacity and thermal conductivity at room temperature were 0.43 J·g⁻¹·K⁻¹ and 13.75 W·m⁻¹·K⁻¹, respectively. The room temperature electrical conductivity of (Mo_2/3Y_1/3)₂AlC ceramic was measured to be 1.25 × 10⁶ Ω⁻¹·m⁻¹. In terms of mechanical properties, Vickers hardness under 10 N load was measured as 10.54 ± 0.29 GPa, while flexural strength, fracture toughness, and compressive strength were determined as 260.08 ± 14.18 MPa, 4.51 ± 0.70 MPa·m^1/2, and 855 ± 62 MPa, respectively, indicating the promising structural applications.     

Optimal sintering parameters for Al2O3 optoceramics with high transparency by spark plasma sintering

Transparent Polycrystalline Alumina (PCA) optical ceramics were fabricated at a high heating rate and low temperature by spark plasma sintering (SPS). Maximum pressure (100?MPa) at dwell time keeps the grain size small irrespective of the dwell time. A heating and cooling rate of 100°C?min^?1 at the sintering temperature of 1150°C for a dwell time of 1?h at 100?MPa yielded highly densified samples with the good transparency of 63 and 83% in visible and infra-red region, respectively. Optoceramics yielded a mechanical hardness of (3000 Hv)/ 29.42?GPa and a thermal conductivity of 21?Wm^?1?K^?1.     

Phase decomposition of (Ti,Zr)(C,N) solid solutions prepared by spark plasma sintering

(Ti, Zr)(C, N) solid solutions with 10–90 mol% of ZrC were prepared by spark plasma sintering (SPS) using TiCN and ZrC powders as starting materials. The decomposition behavior of the (Ti, Zr)(C, N) solid solutions as a function of heat treatment temperature (1273–2173 K) was investigated. (Ti, Zr)(C, N) solid solutions with 20–80 mol% of ZrC were decomposed into TiCN-rich (Ti, Zr)(C, N) and ZrCN-rich (Zr, Ti)(C, N) phases when heat-treated in the temperature ranged from 1373 to 2173 K for 3.6 ks, respectively. After heat treatment, lamellar microstructure was formed with an orientation relationship of TiCN-rich (Ti, Zr)(C, N) {100} // ZrCN-rich (Zr, Ti)(C, N) {100}. The Vickers hardness and fracture toughness simultaneously increased with increasing heat-treatment temperature and showed the maximum values of 25.0 GPa and 2.5 MPa m^1/2, respectively, for Ti_0.5Zr_0.5C_0.75N_0.25 at the heat-treatment temperature of 1873 K.     

Ti3Si(Al)C2-based ceramics fabricated by reactive melt infiltration with Al70Si30 alloy

Dense Ti₃Si(Al)C₂-based ceramics were synthesized using reactive melt infiltration (RMI) of Al₇₀Si₃₀ alloy into the porous TiC preforms. The effects of the infiltration temperature on the microstructure and mechanical properties of the synthesized composites were investigated. All the composites infiltrated at different temperatures were composed of Ti₃Si(Al)C₂, TiC, SiC, Ti(Al, Si)₃ and Al. With the increase of infiltration temperature from 1050 °C to 1500 °C, the Ti₃Si(Al)C₂ content increased to 52 vol.% and the TiC content decreased to 15 vol.%, and the Vickers hardness, flexural strength and fracture toughness of Ti₃Si(Al)C₂-based composite reached to 9.95 GPa, 328 MPa and 4.8 MPa m^1/2, respectively.     

Microstructure and mechanical properties of (Ti,W)C cermets prepared by ultrafast spark plasma sintering

To explore the impact of the sintering rate on the microstructure and mechanical properties of cermets, the preparation of (Ti,W)C cermets by ultrafast sintering via spark plasma sintering (SPS) is reported. Compared with a slow heating rate, the electric field produced by an ultrafast heating rate enhances the liquid phase mass transfer of the metal binder phase, thus achieving rapid densification of (Ti,W)C cermets and effectively inhibiting abnormal grain growth. However, an excessive heating rate will lead to an “overflow” phenomenon, which reduces the grain growth difficulty and the bonding strength between grains. The results show that when the heating rate is 500 °C/min, the liquid phase mass transfer is moderate, the densification degree is the highest and the mechanical properties are excellent. The flexural strength, Vickers hardness and fracture toughness are 1340.90 ± 23.55 MPa, 18.42 ± 0.46 GPa and 11.96 ± 0.23 MPa?m^1/2, respectively.     

Al2O3-3YTZP-Graphene multilayers produced by tape casting and spark plasma sintering

This work aims to establish a colloidal route to obtain laminates of alumina–zirconia combining layers with and without graphene. Green tapes of alumina, alumina with 5 vol.% of 3Y-TZP and alumina with 5 vol.% of 3Y-TZP and graphene-oxide (2 vol.%) were obtained by aqueous tape casting. It is possible to design materials for different structural applications with a controlled microstructure with a high number of different layers. The tapes were punched into 20-mm discs, joined to form laminates alternating up to 18-layers, and sintered in one-step by spark plasma sintering (SPS) at 1400 °C. It has demonstrated that there is a significant graphite diffusion provoked by the required graphite holders into the SPS-furnace. Dense laminates with layer thicknesses ∼100 μm and good cohesion between layers were obtained. Nanoindentation results showed that hardness and elastic modulus values were higher than 27 GPa and 300 GPa, respectively, and similar for all layers.     

Fabrication,microstructure and mechanical properties of novel bulk binderless (Ti0.8Zr0.2)C carbides prepared by mechanical alloying and spark plasma sintering

Nanocrystalline (Ti_0.8Zr_0.2)C powder consisting in grains of about 200 nm in diameter obtained by mechanical alloying was sintered by a spark plasma sintering (SPS) process without the addition of any binder phase. The microstructure, Vickers micro hardness and density in relation to the sintering time and temperature are carefully described. The most suitable sintering condition under pressure of 50 MPa is 1650 °C for 5 min. In this sintering condition, the hardness can reach 2760 Hv and the relative density can reach 98%.