Gas Pressure Sintering Research Articles

Effect of La2O3-MgO ratio on the microstructure and properties of Si3N4 ceramics fabricated via digital light processing

Due to the low powder content characteristic of digital light processing in the preparation of ceramic green bodies, significant challenges arise in obtaining high-density, high-performance Si3N4 ceramic. Sintering aids exert a critical influence on the densification and properties of Si3N4 ceramics. To further improve the performance of 3D-printed Si3N4 ceramics, Si3N4 ceramics were fabricated using DLP technology combined with gas pressure sintering employing different ratios of La2O3-MgO as sintering aids. The ceramic slurries, microstructures, and ceramic performance were studied systematically. The results showed that increasing the proportion of· La2O3 powder enhanced the rheological and curing properties of Si3N4 ceramic slurries. Varying the La2O3-MgO ratio affected the composition of the liquid phase, leading to differences in the densification and grain growth of Si3N4 ceramics. The average grain size increased with higher ratios of La2O3-MgO, and reached a maximum value of 0.91 μm with a ratio of 9:1. With the increase of the La2O3-MgO ratio, the bulk density and shrinkage rate of the ceramics initially increased and then declined. In addition, the flexural strength and fracture toughness of the ceramics peaked at a ratio of 3:7 for La2O3-MgO, measuring 577 ± 16.28 MPa and 5.84 ± 0.17 MPa m1/2, respectively. These results demonstrate that high-performance Si3N4 ceramics can be effectively prepared using DLP technology by adapting the ratio of sintering additives.

Influence of the sintering methods on the electrical properties of cerium-doped aluminium nitride ceramics

This article investigates the densification of AlN ceramics through both Gas Pressure Sintering (GPS) and Spark Plasma Sintering (SPS) methods, employing cerium aluminates (CeAlO3) as sintering aids and comparing their influence to that of the usual cerium oxide (CeO2). While sintering aids like CeO2 promote densification, CeAlO3 exhibited lower reactivity during both SPS and GPS sintering. Chemical reactions between cerium oxide and aluminium oxide primarily involved the reduced phase as cerium sesquioxide (Ce2O3). On the basis on the Ce2O3–Al2O3 pseudo-binary system, the formation of secondary phases, such as CeAlO3 and CeAl11O18, during sintering was explained and confirmed by XRD. From complementary characterizations, it has been shown that sintering significantly impacted secondary phase composition and distribution. By employing specific densification cycles, SPS yielded smaller grains and thicker secondary phase cordons which led to enhanced electrical conductivity. Conversely, GPS produced coarser microstructures including larger grains and a network of secondary phases and some agglomerations at the triple points. These modifications influenced the overall conductivity. SPSed samples with 3 wt.% CeO2 and short dwelling times demonstrated higher electrical conductivity, exceeding by about 6 orders of magnitude the electrical conductivity of those obtained by GPS.

Densification, microstructure, thermal and mechanical properties of Si3N4 ceramics: Effect of Y2Si4N6C and MgSiN2 content

The rapid development of third-generation semiconductors urgently requires ceramic substrates with excellent performance. In this work, silicon nitride (Si3N4) ceramics were successfully prepared via gas-pressure sintering (GPS) using Y2Si4N6C and MgSiN2 as additives. The influence of Y2Si4N6C–MgSiN2 content on the liquid phase properties and microstructure of Si3N4 ceramics was clarified. The use of Y2Si4N6C–MgSiN2 not only did not introduce additional oxygen but also consumed SiO2 and purified the β-Si3N4 lattice. The increase in N/O formed a nitrogen-rich liquid phase, which led to the microstructure of Si3N4 ceramics having a bimodal distribution. The relationship between different degrees of bimodal microstructure and properties was explored. When the Y2Si4N6C–MgSiN2 content was 9 wt%, Si3N4 ceramics obtained excellent thermal and mechanical properties due to lattice purification, Si3N4/Si3N4 continuity increase, the largest average grain diameter, and a bimodal distribution microstructure with moderate grain size distribution. The thermal conductivity was 116.7 W m−1 K−1, the fracture toughness was 10.7 ± 0.35 MPa m1/2, and the bending strength was 767 ± 29.6 MPa.

Investigation of the Machined Surface Integrity of WC-High-Entropy Alloy Cemented Carbide

A fine-grained WC-15wt%Al0.5CoCrFeNi cemented carbide was prepared through a vacuum and gas pressure sintering. For achieving high surface integrity, diamond wheel grinding serves as the primary molding process for the machining of WC cemented carbide. To reveal the influence of grinding on the surface integrity of fine-grained WC-HEA cemented carbide, studies were conducted on grinding force, surface microstructure, surface roughness, residual stress, microhardness, and bending strength. The morphological analysis of the ground surface indicated a transition in the material removal mechanism of WC-HEA cemented carbide from ductile removal to brittle removal, with brittle removal becoming predominant as the depth of grinding increases. With the increasing depth of grinding, the grinding force increases, and the grinding force increases while the surface roughness decreases. Correspondingly, there is an improvement in both hardness and bending strength. Additionally, grinding induces high residual compressive stress on the surface, with a maximum compressive stress of 1795 MPa. The bending strength of the material is found to be dependent on the residual stress.

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

AbstractIn this paper, high thermal conductivity Si3N4 ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si3N4 slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si3N4 ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm−3 were prepared by the investigation and optimization of the Si3N4 slurry formulation. After gas pressure sintering, Si3N4 ceramics with a density of 3.23 g cm−3, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si3N4 ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si3N4 ceramics was 100.5 W m−1 K−1, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m1/2.

Silicon Nitride Bioceramics Sintered by Microwave Exhibit Excellent Mechanical Properties, Cytocompatibility In Vitro, and Anti-Bacterial Properties.

Silicon nitride is a bioceramic with great potential, and multiple studies have demonstrated its biocompatibility and antibacterial properties. In this study, silicon nitride was prepared by a microwave sintering technique that was different from common production methods. SEM and pore distribution analysis revealed the microstructure of microwave-sintered silicon nitride with obvious pores. Mechanical performance analysis shows that microwave sintering can improve the mechanical properties of silicon nitride. The CCK-8 method was used to demonstrate that microwave-sintered silicon nitride has no cytotoxicity and good cytocompatibility. From SEM and CLSM observations, it was observed that there was good adhesion and cross-linking of cells during microwave-sintered silicon nitride, and the morphology of the cytoskeleton was good. Microwave-sintered silicon nitride has been proven to be non-cytotoxic. In addition, the antibacterial ability of microwave-sintered silicon nitride against Staphylococcus aureus and Escherichia coli was tested, proving that it has a good antibacterial ability similar to the silicon nitride prepared by commonly used processes. Compared with silicon nitride prepared by gas pressure sintering technology, microwave-sintered silicon nitride has excellent performance in mechanical properties, cell compatibility, and antibacterial properties. This indicates its enormous potential as a substitute material for manufacturing bone implants.

Mechanical and thermal properties of Si3N4 ceramics prepared by gelcasting using high-solid-loading slurries

In this study, according to the Dinger model, aqueous Si3N4 slurries with a solid loading of 63 vol% and a viscosity lower than 1.0 Pa s were successfully prepared using two-grade Si3N4 powders with D50 = 7.2 μm and 9.4 μm as starting materials. These resulting slurries exhibited good rheological properties and suspension stability. Notably, the presence of β-Si3N4 seeds existed in the starting materials effectively reduced the nucleation barrier by transforming homogeneous nucleation into heterogeneous nucleation. Consequently, it facilitated the reconstructive phase transformation of α-Si3N4 to β-Si3N4, promoted the anisotropic growth of β-Si3N4 nucleation, and resulted in the final formation of the bimodal microstructure. This unique microstructure significantly enhanced the mechanical properties of Si3N4 ceramics, resulting in a high flexural strength of 1011.4 MPa and a fracture toughness of 14.8 MPa m1/2. These excellent properties were achieved through the combination of gelcasting and gas pressure sintering. Simultaneously, the Si3N4 ceramics exhibited a low thermal conductivity of 47.7 W m−1 K−1, mainly owing to the low density of 95.3 %, the presence of low-thermal-conductivity silicate-based noncrystalline substances, and the possible existence of lattice oxygen in Si3N4 crystals.

Experimental Study on ELID Grinding of Silicon Nitride Ceramics for G5 Class Bearing Balls

This study has focused on analyzing the impact of material characteristics and grinding conditions on the surface roughness in ELID grinding of ceramic materials intended for bearing balls. The main research objective was to examine the feasibility of achieving the required surface roughness for G5 class bearing balls through a high-efficiency and high-precision ELID grinding process. Three types of silicon nitride specimens and two types of grinding wheels with cBN and diamond abrasives were prepared for the experiments. An HP (high-pressure) specimen was fabricated through high-temperature and high-pressure sintering at 1700 °C for 2 h, containing a composition of Y2O3 and MgO in Si3N4, while GPS 1hr and GPS 6hr specimens were prepared using gas-pressure sintering for 1 h and 6 h, respectively. From the experimental results, it has been confirmed through surface morphology and surface roughness analysis that material characteristics and grinding parameters affect the surface roughness of silicon nitride ceramics during the grinding process. The surface ground with a #2000 diamond wheel is at a level that can satisfy the required surface roughness, 0.014 um or less in G5 class bearing balls. Based on the analysis of surface morphology and roughness in grinding processes, the #325 cBN wheel exhibited excellent performance in rough grinding, while the #2000 diamond wheel demonstrated highly effective surface finishing performance, indicating that the combination of these two abrasives can be effectively utilized for high-efficiency and high-precision nanosurface machining of silicon nitride ceramics.

Effect of the ratio of Y2O3 and MgSiN2 sintering additives on the microstructure, thermal and mechanical properties of Si3N4 ceramics

To investigate the mechanism of the influence of the sintering additive ratio on the microstructure evolution of silicon nitride (Si3N4) ceramics, Si3N4 ceramics were prepared by gas pressure sintering using different ratios of Y2O3/MgSiN2 as additives. Varying the ratio of Y2O3/MgSiN2 changed the composition of the liquid phase, thereby affecting the densification process and phase transformation rate of Si3N4 ceramics, and ultimately changing their microstructure. The reduction in the Y2O3/MgSiN2 ratio raised the N/O and Si/O ratios in the system, which increased the liquid phase viscosity. The Si3N4 grain distribution varied from "normal to bimodal". In the case of increasing the proportion of MgSiN2 and prolonging the sintering time, more of the glass phase evaporated, which reduced the content of the low thermal conductivity phase and increased the continuity of Si3N4/Si3N4. Therefore, Si3N4 ceramics with 1 wt% Y2O3 and 5 wt% MgSiN2 exhibited excellent properties after sintering at 1900 °C for 12 h, with thermal conductivity, bending strength, and fracture toughness of 106.7 Wm−1K−1, 776 MPa, and 11.66 MPa m1/2, respectively.

Copper-graphite-TiC composites-synthesis and microstructure investigation

Copper-graphite-TiC composites were prepared by gas pressure sintering, using copper-coated graphite, pure copper, and Ti3AlC2 as the raw materials. The phase composition and microstructure of copper-graphite-TiC composites have been studied. The results indicate that a strong interdiffusion occurred between the copper matrix and Ti3AlC2 during the sintering process. Ti3AlC2 was completely decomposed into TiC, and a TiC interface transition layer was formed to enhance the bonding strength between the copper matrix and graphite.

Review on the thermal characteristics and applications of silicon nitride ceramics

As the heat generation problem is predicted to intensify due to the trend of integration and high density of the power semiconductor power module responsible for the electric drive of the electric vehicle, which has recently been in full swing, high-reliability materials and It is essential to secure large-area heat dissipation substrate manufacturing process technology, and technical obstacles to maintain reliability even in environmental changes such as severe cold/excessive heat are becoming issues.In the case of silicon nitride ceramic material, which is in the spotlight as a heat dissipation substrate material, a balance that meets the user’s needs is required. In order to realize excellent heat dissipation performance, it is necessary to reduce the thickness of the silicon nitride substrate, increase the thickness of the metal junction, and improve the thermal conductivity of the silicon nitride material. Therefore, the task of technological progress beyond the complementary relationship between heat conduction-intensity still remains.In this paper, various technical considerations for increasing the thermal conductivity of silicon nitride ceramics are described, and the direction of technological progress is described along with detailed examples. In order to improve thermal conductivity, it is necessary to minimize the inflow of impurities into the raw material powder, appropriately select sintering additives required for liquid phase sintering, and optimize the microstructure through minimization of the amorphous glass phase and control of grain growth by the gas pressure sintering process.

Thermal conductivity of Ca α‐SiAlON ceramics with varying m and n values

Abstractα‐SiAlON is well known for its excellent mechanical properties, but its thermal properties are less investigated. In this work, we investigate the thermal conductivity of Ca α‐SiAlON ceramics with varying m and n values and prepared samples by spark plasma sintering (SPS) at 1600°C or gas pressure sintering (GPS) at 1800°C and 1900°C. The prepared Ca α‐SiAlON ceramics showed thermal conductivities ranging from 4.6 to 10.7 W/(mK) at 25°C. It was found that the thermal conductivity of the α‐SiAlON samples could be reduced by increasing either m or n or both. Besides, the samples prepared by SPS had lower thermal conductivity compared with those by GPS due to their smaller grain size, and the effect of m and n would be minor after m reached 2. These results provide basic knowledge for tuning α‐SiAlON's thermal conductivity for applications concerning thermal management.

Microstructure evolution of Si3N4 ceramics with high thermal conductivity by using Y2O3 and MgSiN2 as sintering additives

Silicon nitride (Si3N4) ceramics were prepared by gas-pressure sintering using Y2O3–MgSiN2 as a sintering additive. The densification behavior, phase transition, and microstructure evolution were investigated in detail, and the relevance between the microstructure and the performance (including thermal conductivity and mechanical properties) was further discussed. A significant change from a bimodal to a homogeneous microstructure and a decreased grain size occurred with increasing Y2O3–MgSiN2 content. When the small quantity of preformed β-Si3N4 nuclei grew preferentially and rapidly in a short time, an obvious bimodal microstructure was obtained in the sample with 4 mol% and 6 mol% Y2O3–MgSiN2. When more β-Si3N4 nuclei grew at a relatively rapid rate, the sample with 8 mol% Y2O3–MgSiN2 showed a microstructure consisting of numerous abnormally grown β-Si3N4 grains and small grains. When more β-Si3N4 nuclei grew simultaneously and slowly, there was a homogeneous microstructure and smaller grains in the sample containing 10 mol% Y2O3–MgSiN2. Benefitting from the completely dense, significant bimodal microstructure, low grain boundary phase, and excellent Si3N4–Si3N4 contiguity, the sample containing 6 mol% Y2O3–MgSiN2 exhibited great comprehensive performance, with a maximum thermal conductivity and fracture toughness of 84.1 W/(m⋅K) and 8.97 MPa m1/2, as well as a flexural strength of 880.2 MPa.

A novel thermally matched conductive phase of silicon nitride ceramics

Tungsten was first proposed as a conductive phase that better matches the thermal expansion of silicon nitride ceramics. Silicon nitride ceramics containing 20 vol% tungsten particles were fully densified via gas pressure sintering under a following nitrogen atmosphere. After introducing the tungsten particles, the thermal conductivity is increased two-fold and thermal expansion coefficient (CTE) almost remains as low as the monolithic silicon nitride ceramics. Most importantly, the composite ceramics achieve good electrical conductivity and a small and complex shape was machined via wire-electrode cutting to confirm its good wire-electrical discharge machinability. Tungsten particles have been proven to be a new ideal conductive phase for silicon nitride ceramics, and a new wire-electrical discharge machinable silicon nitride-based ceramic was prepared.

Investigating the effect of Al, Mo or Mn addition to CoCrFeNi entropy alloys on the interface binding properties of WC/HEA cemented carbides

At present, it is found that the mechanical properties of WC/HEA cemented carbide are mainly affected by the interfacial bonding properties of hard phase and bonding phase. Therefore, the effect of adding Al, Mo or Mn on the interfacial bonding properties of WC/HEA cemented carbides was studied by first-principles simulation. The results show that adding Mn can enhance the interface bonding performance, while adding Al and Mo is the opposite. Compared with Al and Mo atoms, Mn atoms are more likely to form strong bonds with W atoms at WC/HEA cemented carbide interface. In order to verify the rationality of the calculation, WC/HEA cemented carbide samples were prepared by mechanical alloying and gas pressure sintering. It is found that WC/CoCrFeNiMn has the best mechanical properties. Its hardness, fracture toughness and strength are 1562 HV, 9.3 MPa·m1/2, 850 MPa. The experimental results further confirm that adding Mn can improve the interfacial bonding properties of WC/HEA cemented carbide.

Grain growth modeling for gas pressure sintering of silicon nitride based ceramics

Grain growth modeling for gas pressure sintering of silicon nitride based ceramics

Development of WC-NiCrMo hardmetals

The demand for improved components with a wide range of properties is a reality in today's hardmetal market. Furthermore, supply shortages of raw materials such as W and especially Co resulted in rapid price fluctuations. Combined with stricter environmental legislation and health protection, these facts prompt the demand for alternative or modified binders. Therefore, in this work, WC-NiCrMo composites were developed as a potential alternative to conventional WC-Co hardmetals. To evaluate this new hardmetal composite, prototypes of submicrometric WC with approximately 15 vol% of NiCrMo binder were produced by conventional powder metallurgy route. Thermodynamic assessment, wettability testing and constant heating rate dilatometry were performed to design an adequate sintering route. A detailed characterization of the mechanical properties (Young's modulus, Vickers hardness and fracture toughness), together with corrosion resistance assessment (OCP, polarization curves, EIS) and abrasive wear (ball-cratering) resistance evaluation were undertaken. Good wettability of molten NiCrMo binder on WC surface was observed, and highly dense compacts could be successfully attained by gas pressure sintering. The new WC-NiCrMo composite has lower hardness but higher corrosion resistance and better wear resistance than the conventional WC-Co hardmetals.

Study on sintering process of Ti2SnC modified copper-graphite composites

Ti2SnC modified copper-graphite composites were fabricated by spark plasma sintering (SPS) and gas pressure sintering with 40 wt% Cu, 10 wt% graphite, and 50 wt% Ti2SnC as components. The phase composition, microstructure, and relative density of the composites were analyzed in detail. The results show that Sn atoms diffuse out from Ti2SnC and into the copper matrix in the composite sample and those fabricated by SPS are stronger than that of gas pressure sintering. The bonding of copper and graphite is favorable, but the pores created by the agglomeration of Ti2SnC grains are difficult to eliminate by using SPS to fabricate the composites. In contrast, air pressure sintering is beneficial for eliminating the pores caused by the agglomeration of Ti2SnC grains, but perfect mechanical interlocking between copper and graphite is lacking.

Properties and ballistic tests of strong B4C-TiB2 composites densified by gas pressure sintering

B4C-TiB2 composites with classical 75/25 vol ratio were sintered by pressureless sintering with and without gas pressure application in the final stage of densification, using a novel prototypal furnace. A small fraction of WC was introduced through high energy milling of the starting powders with WC-Co spheres. High energy milling facilitated the densification thanks to incorporation of WC impurities acting as sintering aid, and size reduction of the starting powders. Strength, stiffness and toughness of the ceramic densified at 2050 °C via gas pressure sintering were even better than hot pressed composites at 1900 °C. Depth of Penetration tests on plates with 3–5 mm thickness demonstrated that the gas pressure sintered material had a superior performance compared to the hot pressed one. This work also revealed that hardness was not the property spotting the best ballistic performance.

Tuning the dielectric properties of porous BN/Si3N4 composites by controlling porosity

To solve the shortcomings of traditional layered structure for broadband wave-transmitting applications, a novel porosity gradient structure is proposed. The precise turning of the relative permittivity of each layer and their simultaneous shrinkage during sintering are the keys to design and fabrication. Porous BN/Si3N4 composites with different porosities were prepared by incorporation of pore former, tape casting and gas pressure sintering. Results show that the obtained composites with different porosities exhibit low and similar shrinkage after sintering at 1900 °C, which is mainly due to the bridging effect between rapidly growing β-Si3N4 rod. When the porosity increases from 33.2% to 59.4%, the relative permittivity decreases from 4.00 to 2.38. Moreover, the composites with different porosities have excellent mechanical properties, with bending strength and fracture toughness of 180 ± 10–36 ± 4 MPa and 3.2 ± 0.30–0.85 ± 0.10 MPa·m−1/2, respectively, which is attributed to the bridging and pull-out strengthening effects of the elongated β-Si3N4 grains.