Pressureless Sintering Conditions Research Articles

Enhancing mechanical properties and density of WC-Co with pressureless sintering via shell structure binder jetting additive manufacturing

The binder jetting (BJT) additive manufacturing (AM) process is suitable for producing complex WC-Co cemented carbide parts. To achieve near-full-density in the final parts, the hot isostatic pressing (HIP) sintering process is typically employed to enhance densification. However, HIP sintering requires high equipment specifications and is relatively more costly compared to conventional pressureless sintering. To improve the density of WC-Co parts printed by BJT AM under the pressureless sintering environment, the shell structure printing method was used in this study. The shell structure printing only jets binder to the model's outer profile, creating a shell that encapsulates the central powder, mitigates binder pyrolysis and residual effects on particle densification, and enhances post-sintering density. In this work, the effect of shell thickness, binder type, and printing layer thickness on the density and mechanical properties of the WC-12Co parts under pressureless sintering conditions were investigated. The results show that the density and mechanical properties of the WC-12Co parts significantly improved with the decreasing shell thickness. Specifically, the WC-12Co printed using shell structure achieves a relative density of over 98% and a transverse rupture strength (TRS) of 1954 MPa after pressureless sintering, a result comparable to BJT printed samples treated with HIP sintering. Moreover, the results indicate using a binder with lower residual carbon content and smaller printing layer thickness has a more positive effect on improving density and mechanical strength when employing the shell structure printing method. Microstructure morphology and phase analysis results indicate that the shell structure printing method does not affect the growth of WC grains in the shell and center regions, nor does it influence its phase composition. The shell structure printing method offers a new approach to the manufacturing of near-full-dense WC-Co materials using the BJT process.

Preparation of spherical AlON powder and high transparent ceramics by pressureless sintering

AbstractThe sphericity and fluidity of the powder play a crucial role in the forming and sintering of ceramics. High‐performance AlON spherical powder was prepared for the first time by spray drying of powder fired by solid‐phase reaction, and the preparation of AlON transparent ceramics was realized under pressureless sintering conditions. The effect of AlN dosage on the purity of AlON powder was investigated, and the powder fluidity was improved by spray drying technique. Then the best spray drying process was selected by scanning electron microscopy, AD/TD, green density, MIP and transmittance measurements. The AlON particles obtained under the optimal granulation process have good dispersibility, high green density after pressing, and uniform pore distribution. After dry pressing and sintering at 1900°C for 20 h, the AlON ceramics with linear transmissivity of 84.3% at 2 μm wavelength can be obtained. The excellent pressing and sintering properties of AlON granulated particles make it possible for large‐scale production.

Microstructure of Ag Nano Paste Joint and Its Influence on Reliability

In this paper, the microstructure of Ag nano paste joint was investigated in pressure-less sintering conditions, and the influence of the microstructure on the joint’s reliability was studied. Firstly, silver nanoparticles (Ag NPs) were synthesized using the redox reaction method. To tightly stack the Ag NPs in nano paste, Ag NPs with sizes of 30~50 nm and submicron-sized Ag particles were mixed. It was found that increasing the sintering temperature or sintering time can reduce the porosity of the bonding layer and the interfacial crack simultaneously, resulting in higher shear strength. When sintering at a temperature of 250 °C, a complete bonding interface was formed, with a 0.68 μm interdiffusion layer. At a higher temperature (300 °C), the bonding interface reached 1.5 μm, providing 35.9 ± 1.7 MPa of shear strength. The reliability of the die attachment was analyzed under thermal shocking from −65 °C to 150 °C for 50 cycles. As the crack could quickly grow through the interfacial defects, the separation ratio was 85% and 67% when sintered at 150 °C and 200 °C, respectively. Because of the reliable bonding interface between the die and the substrate, the Ag nano paste joint formed a slight crack on the edge of the die when sintering at 250 °C. When the joint was sintered at 300 °C, the small voids became large voids, which featured lower resistance to crack growth. Thus, instead of further improved reliability, the separation ratio increased to 37%.

Pressureless and low-temperature sinter-joining on bare Si, SiC and GaN by a Ag flake paste

In this work, we applied a Ag flake formed paste for sinter-joining on bare Si, SiC, and GaN surfaces. The sintered joints possess a high shear strength of over 40 MPa under a pressureless sintering condition at 250°C. The high bonding strength of the joints is achieved by a tight adhesion between Ag and bare surfaces, which attributed to an excellent sinter-joining ability of the Ag flake. The flakes, acquired by mechanical milling, can be rapidly sintered into a homogenous porous structure at a pleasureless and low-temperature sintering condition due to its dislocation-rich nano grain structure. During sintering, a drastic morphology reconstruction of Ag flake, from a flattened flake to a drop-like particle, can introduce robust interfacial connection structures, which is important for the robust bonding. This Ag flake paste can be regarded as a promising sinter-joining material for the connection of bare surfaces in high-temperature applications.

Thermoelectric performance of n-type polycrystalline SnSe with surface depletion by pressureless sintering

In this study, polycrystalline SnSe samples were prepared by a series of mechanical alloying (MA), uniaxial compaction, and subsequently pressureless sintering. It was found that the grains in the resultant SnSe samples grow along the (111) plane direction, resulting in forming a layered structure. Depending on the sintering time, the SnSe samples show the conversion of p-type conduction with the Seebeck coefficient value of +503 μV/K to n-type conduction with the Seebeck coefficient value of −1500 μV/K. The samples were characterized using X-ray diffraction and Raman spectroscopy to reveal that the transition of the Seebeck properties is due to the imbalanced composition of SnSe caused by impurities (SnSe2, SnSe1-x) formed during pressureless sintering. Also, the experiments and characterization results demonstrated a possibility to enhance thermoelectric properties if the pressureless sintering conditions would be well-controlled.

Improved sinterability of particles to substrates by surface modifications on substrate metallization

Abstract Sintering under pressure has been in the forefront of the research and development over the past decade as an alternative to high temperature soldering and die-attach bonding for high temperature electronics. However, high bonding pressure is a deterrent to mass industrialization due to the high costs involved in the design of special tooling and complex process control parameters. Further, it can cause device cracking, especially while working with sensitive high power optoelectronics devices (e.g. high power light emitting diodes). Therefore, alternatives to enhance sinterability are highly requested. Substrate metallization is observed to play an important role while sintering. An innovative low cost method to have nanostructured surface modifications on the substrates is realized and presented here. The method is applied to enhance sinterability of Cu particles to substrate. Shear tests on samples with surface modified substrates are promising with results of ca. 25 MPa, which is 24% better than sintering on unmodified bare Cu substrate. Sintering was enabled by in-house developed hybrid Cu paste under pressureless sintering conditions of 300°C, for 60 min, and under N2 atmosphere.

Silver Sintered Joint Property Between Silicon Carbide Device and Ceramic Substrate for Electric Vehicle Power Module

Highly reliable sintering technologies operating in extreme conditions are being extensively researched nowadays to improve the efficiency of power modules based on silicon carbide (SiC) devices for use in electric vehicles. In this study, the strength of silver (Ag) sintered joints used in SiC/direct bonded copper (DBC) was recorded at 21.9–23 MPa, at a pressure of 0.2 MPa and zero pressure, at 210°C using 83 wt.% and 88 wt.% Ag paste. Al2O3 and AlN DBCs with copper and Ag finishes were used. The inter-diffusion of the Cu-Ag and the Ag-Ag sintered interfaces contributed to increasing bonding strength. Based on the relationship between bonding layer thickness and densification in the pressurized and pressureless sintering conditions, if densification was at least 93%, the strength of the Ag joint strength was as high as 23.8 MPa. From the results of a simulation of thermal deformation based on the mismatch in the coefficients of thermal expansion of materials without considering heat transfer, the Al2O3 DBC material (rather than AlN) exhibited a small and dispersed deformation of the SiC/DBC sintered module. We also observed that Al2O3 DBC was favorable in terms of warpage.

Pressureless sintering of chromium diboride using spark plasma sintering facility

Sinterability of monolithic CrB2 was investigated under pressureless sintering condition using spark plasma sintering facility (SPS). Monolithic chromium diboride (CrB2) was sintered in a modified die setup instead of traditional/conventional plunger and die assembly. This kind of assembly creates pressureless sintering conditions in spark plasma sintering unit. The main objective of this modified setup is to couple the combined aspect of conventional pressureless sintering with fast heating. Densification experiments using CrB2 were conducted at temperatures in the range of 1600°C to 2000°C with different dwelling period (1–30min) under no load condition. The effect of multi-step sintering treatment on the densification behaviour of CrB2 was also investigated. The density and the resulted microstructures of the sintered samples are presented and discussed in this paper.

Dielectric and Ferroelectric Behavior in 8/40/60 PLZT Ceramics

This paper describes the relationship between sintering time conditons, tetragonality from phase formation, dielectric and ferroelectric characteristics of tetragonal PLZT (lanthanum modified lead zirconate titanate) ceramics with the pressureless sintering conditions. Ferroelectric compounds of Pb0.92La0.08(Zr0.4,Ti0.6)0.98)O3, PLZT 8/40/60, were prepared using mixed-oxide synthetic route via a rapid vibro-milling method. X-ray diffraction, is used to observe the tetragonality characteristic and the presence of pyrochlore. When sintering time was increased (≥ 8 hours), pyrochlore was developed. It can be observed that with the increased sintering time, it were changed the structure and ferroelectric behavior of the PLZT ceramics.

Reaction-bond sintering of C/SiC functionally graded nanostructured materials (FGNMs)

In this study, bulk C/SiC functionally graded nanostructured materials (FGNMs) having a compositional gradient from carbon (C) to silicon carbide (SiC) were obtained by applying the powder-stacking method under pressureless sintering condition. During the sintering process, the reaction-bonded SiC was formed in the presence of pure silicon (Si) which led to increase in the density and strength of the FGNMs. At the sintering stage, the compacted bodies were heated at 1525°C for 2h in an argon atmosphere. Results showed that the number of graded layers had a marked effect on the microstructure and properties of the sintered C/SiC FGNMs. With increasing number of graded layers, the bend strength of the samples improved significantly. Moreover, SEM micrographs illustrated that the formation of SiC nanorods between adjacent particles improved the bending strength of the samples.

Rapid sintering of crack-free zirconia ceramics by pressure-less spark plasma sintering

Heating of a ceramic green body is a key step in sintering. We have created inside a spark plasma sintering apparatus pressure-less sintering conditions that allow homogeneous and extremely rapid heating. Dense and crack-free zirconia ceramic was sintered at heating rates of up to 500 °C min -1 and dwell times of 2 min. This extremely fast sintering process is accompanied by extremely rapid grain growth, indicating a non classical sintering mechanism. No grain size gradients were observed inside the sintered zirconia ceramics.

Evaluation of alumina effects on the mechanical property and translucency of nano-zirconia all-ceramics

To study the effects of alumina content on sintered density, mechanical property and translucency of zirconia nanocomposite all-ceramics. Specimens of zirconia nanocomposite all-ceramics were divided into five groups based on their alumina content which are 0% (control group), 2.5%, 5.0%, 7.5% and 10.0% respectively. The sintered densities were measured using Archimedes' method. Specimens' bending strengths were measured with three-point bending test (ISO 6872). The visible light transmittances were measured with spectrophotometric arrangements and the fractured surfaces were observed using scanning electron microscope (SEM). The control group of pure zirconia could be sintered to the theoretical density under pressure-less sintering condition. The bending strength was (1100.27 ± 54.82) MPa, the fracture toughness was (4.96 ± 0.35) MPa×m(1/2) and the transmittance could reach 17.03%. The sintered density and transmittance decreased as alumina content increased from 2.5% to 10%. However, the fracture toughness only increased slightly. In all four alumina groups, the additions of alumina had no significant effect on samples' bending strengths (P > 0.05). When the content of alumina was 10%, fracture toughness of specimens reached (6.13 ± 0.44) MPa×m(1/2) while samples' transmittance declined to 6.21%. SEM results showed that alumina particles had no significant effect on the grain size and distribution of tetragonal zirconia polycrystals. Additions of alumina to yttria-tetragonal zirconia polycrystals could influence its mechanical property and translucency. Additions of the other phase to zirconia ceramics should meet the clinical demands of strength and esthetics.

Hydroxyapatite nanopowders: Synthesis, densification and cell–materials interaction

Hydroxyapatite (HA) nanopowders with different aspect ratios were synthesized using reverse micelle template system. Nanopowders were characterized using X-ray diffraction (XRD), BET specific average surface area analysis and transmission electron microscopy (TEM). It was observed that increase in aqueous to organic ratio (A/O) and pH decreased the aspect ratio of the nanopowders. HA nanopowders with the highest aspect ratio (rod-shaped) of 7.2 ± 3.2 and the lowest aspect ratio (spherical) of 1.3 ± 0.3 were synthesized for processing dense compacts. Effect of powder morphology on densification at 1250 °C was studied with different amount of rod-shaped and spherical nanopowders. It was observed that an increase in high aspect ratio powder content in the compacts decreased sintered density under pressureless sintering condition. Also, due to excessive grain growth, no nanoscale morphology could be retained in the sintered microstructure. Mineralization study in simulated body fluid (SBF) showed formation of apatite layer on the entire surface of both compacts made with spherical and rod-shaped particles. Cytotoxicity result with OPC1 human osteoblast cells showed excellent cell attachment and cell spreading on samples after 5 days in culture.

硬質材料 高強度窒化ケイ素セラミックスの開発と製品化

Due to its superior resistance to thermal shock, wear and oxidation, and high toughness, Si3N4 ceramics are expected to be used in industrial applications. However, before these Si3N4 ceramics can become better established on the market, their fracture strength, fracture toughness characteristics and reliabilty must be improved.In this study, the formation mechanisms of the inside pores at the pressureless sintering conditions that yield the thick body of uniform structure were invesigated.The creep properties of sintered body were studied by analyzing the results of internal friction measurements.Hugoniot-compression curve for Si3N4 ceramics with controlled microstructure were measured to study the yeilding properties under shock compression. And the correlations with Hugoniot-elastic limits and microstructure of Si3N4 ceramics were considered.The degradation characteristics of bending strength of specimens after processing under various grindingg condition were evaluated.