Multi-step Sintering Research Articles

Spherical nanocrystalline ScYSZ thermal barrier material by a novel supersonic plasma spheroidization method: Simple synthesis and excellent performance

The performance of plasma sprayed thermal barrier coatings (TBCs) is strongly dependent on the quality of the feedstock powders, which usually are fabricated by conventional spray drying and multi-step sintering methods. In this work, a novel supersonic plasma spheroidization technology was employed to fabricate the spherical nanocrystalline Sc2O3-Y2O3 co-stabilized ZrO2 (ScYSZ) powder. The deformation process from the irregular powder to the spherical one was stimulated by the COMSOL phase field simulation. The results suggested that the original irregular pyrolysis products of ScYSZ precursors were aggregated into spherical powders by plasma spheroidization technology. The COMSOL simulations verified that the powder changed from irregular particles to spherical ones. The spheroidized ScYSZ particles exclusively consisted of non-transformed t’-ZrO2 phase and showed good fluidity, smooth surface and high apparent density, which were expected to have a broader application prospect in the field of TBCs and other ceramic coatings.

Effect of the cycle number of multistep sintering on the properties of FeSe0.4Te0.6 superconductors

AbstractFeSe0.4Te0.6 polycrystal bulks were synthesized using the solid‐state sintering method. We developed a novel multistep sintering process and studied the impact of the sintering cycle number. Different cycles of multistep sintering, ranging from one to four cycles were systematically arranged. It was found that the superconducting properties of the FeSe0.4Te0.6 polycrystal bulk using three cycles of sintering surpassed those of other samples, reaching a maximum of = 13.4 K, = 14.7 K, and a minimum of ∆ = 1.3 K. And the superconducting current density at 4 K reached 1.77 × 104 A/cm2, exhibiting a weak fishtail effect. At 9 K, a composite pinning effect emerged, consisting of normal point pinning and ∆κ surface pinning. The enhancement in superconductivity is attributed to improve uniformity in grain arrangement and more effective element diffusion. After multiple sintering cycles, the stacked grains exhibited increased thickness and the grain arrangement became dense.

Zero sintering-induced shrinkage of porous oxide ceramics

Porous oxide ceramics are widely used in extreme working conditions owing to their excellent resistance to high temperatures and corrosion. However, sintering is an inevitable process applied to ceramics, from the green body to the final product. The highly complex structures exacerbate the shrinkage-induced irregular deformation and crack formation in the sintering process. A pioneering approach is developed in this study to achieve zero shrinkage for porous alumina ceramics during multistep sintering, using a combination of active fillers - ZrAl3 and Al75Si25. The response surface method is used to optimize the material compositions and sintering process, to achieve shrinkages of less than 0.05% for the entire process. The sintering expansion mechanism is investigated by analyzing the pyrolysis and microstructures of samples at different temperatures. The combination of ZrAl3 and Al75Si25 can attain the continuous expansion of the matrix in a wide temperature range of 600–1400 °C. Furthermore, typical alumina components are fabricated and used to verify the effectiveness of the proposed approach. Owing to shrinkage suppression, the profile deviation of the samples is less than 0.1 mm, and the proportion of microcracks is reduced by 97.8%. The suggested approach shows potential applications in near-net, low-defect fabrication of complex fine ceramic components.

Fabrication of a Porous Fe–Al Intermetallic Alloy with Ultrahigh Open Porosity

Herein, a porous Fe–Al intermetallic material with ultrahigh open porosity is prepared via the multistep sintering process using a Fe–Al–Mn powder mixture as the raw material. The microstructure, phase composition, Mn sublimation, and open porosity of the sintered Fe–Al–Mn compacts are investigated. The compacts sintered at temperatures ≤700 °C are composed of the newly formed Fe/Mn‐based solid solutions, intermetallic phases, and the original elemental particles without the sublimation of Mn. The open porosities of the 650‐ and 700 °C‐sintered compacts are ≈39.83 and ≈47.19 vol%, respectively. After sintering at 1250 °C, Mn is rarely left in the porous body, and a single Fe–Al intermetallic phase with an even chemical composition is generated. The open porosity of the 1250 °C‐sintered compacts is ≈66.67 vol%, superior to other reported porous Fe–Al materials, and its strength is ≈22.5 MPa, which is competitive as well. This work may open a new strategy to design and optimize the high‐quality porous Fe–Al alloy in future.

Effects of dispersoid preforming via multistep sintering of oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy

• A newly designed ODS manufacturing process, multi-step sintering process, consisting of a dispersoid preforming heat treatment followed by sintering to achieve the dispersoid refinement in HEA matrix was suggested. • An atypical form of Y 2 O 3 , FCC-Y 2 O 3 , formed adjacent to the dislocation inside the matrix grain during the dispersoid preforming heat treatment of multi-step sintering process with a cube-on-cube orientation relationship with the matrix. • The FCC-Y 2 O 3 formed in the multi-step sintered bulk acts as an effective obstacle to grain boundary and dislocation movement resulting in a microstructure refinement and the increment of mechanical strength. • In the bulk prepared by the conventional ODS manufacturing process, coarsened dispersoids consisting of a complex structured oxide with BCC-Y 2 O 3 as the core were predominantly formed along the grain boundary. • Cr-C-O region with a crystal structure of Cr 23 C 6 formed in the HEA matrix serves as an oxygen-supplying reservoir for dispersoids, promoting coarsening of the dispersoids. Dispersoid formation and microstructural evolution in an oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy (HEA) using a newly designed multistep sintering process are investigated. The proposed multistep sintering consists of a dispersoid preforming heat treatment of as-milled 0.1wt% Y 2 O 3 -CoCrFeMnNi high-entropy alloy powders at 800°C, followed by sintering at 800–1000°C under uniaxial pressure. In the conventional single-step sintered bulk, the coarsened BCC Y 2 O 3 dispersoids mainly form with an incoherent interface with the HEA matrix. In contrast, finer FCC Y 2 O 3 dispersoids, an atypical form of Y 2 O 3 , are formed in the matrix region after multistep sintering. Nucleation of FCC Y 2 O 3 dispersoids is initiated on the favorable facet, the {111} plane of the austenitic matrix, with the formation of a semi-coherent interface with the matrix during the dispersoid preforming heat treatment and it maintains its refined size even after sintering. It is found that dispersoid preforming prior to sintering appears promising to control the finer dispersoid formation and refined grain structure.

Pixelated sintering of α-Al2O3

The sintering of α-alumina by a brand new and innovative technique, called pixelated sintering (PS), is here studied. Densification and grain growth by PS of perfectly controlled granular compacts are analysed and compared to results obtained using Spark Plasma Sintering (SPS) and Pressure-Less SPS (PL-SPS). Materials are exposed to the same temperature profiles whatever the sintering technique used in order to assess the potential of PS in terms of microstructure control. It is shown that PS can be used as an alternative technique to SPS for fast sintering with the advantages of a much simpler and cost-effective set-up, as well as a better control of the localised heat input. PS also appears to be a very modular technology in the way it controls the temperature gradients allowing its implementation for multi-step sintering approaches, as well as for the fabrication of large and complex parts.

Dual-grained Mo2FeB2-based cermets containing WC fabricated via multi-step sintering approach: Microstructure, mechanical properties and interface characteristics

In this study, a novel method was proposed for preparing dual-grained Mo2FeB2-based cermets by partially replacing pre-sintered Mo2FeB2 particles with WC via multi-step sintering. The microstructure, mechanical properties, and interface structures of the cermets were systematically investigated. The results reveal that the final microstructure of the as-prepared cermets contained fine Mo2FeB2 grains and coarse Fe3(W,Mo)3C grains, and the average grain size exhibited a typical bimodal distribution. Interestingly, a thin film was formed at the rim/binder phase boundary between Mo2FeB2 grains and Fe phases, which alleviated the lattice distortion. Furthermore, the formation of coherent interfaces was observed at the grain/binder phase boundary between Fe3(W,Mo)3C grains and Fe phases, and at the rim/rim grain boundary between neighboring Mo2FeB2 grains, resulting in a significant enhancement in interfacial bonding strength. The unique interfacial structures were conducive to reinforce strength and toughness of Mo2FeB2-based cermets. As a result, superior hardness (90.3 ± 0.1 MPa) and fracture toughness (25 ± 0.5 MPa m1/2) were obtained, without losing lots of transverse rupture strength (1853 ± 21 MPa).

Interface-reinforced Mo2FeB2-based cermets prepared by multi-step sintering under rapid cooling

A novel approach was proposed to synthesize Mo2FeB2-based cermets using multi-step sintering combined with rapid cooling. The results revealed that the rapid cooling resulted in the formation of several nanocrystalline regions at grain boundaries, which corresponded to the γ-Fe phase without allotropic transformation, and greatly increased the crack propagation resistance. In addition, two completely different highly coherent interfaces were observed due to the sudden decrease in diffusion driving force, improving the interfacial bonding strength. The current study presented the phase composition, interfacial characteristics and performance of Mo2FeB2-based cermets.

Influence of WC content on microstructure and mechanical properties of Mo2FeB2-based cermets fabricated by multi-step sintering

Herein, Mo2FeB2-based cermets, with nominal composition of Mo2FeB2-2.06Fe-2.9Ni-2.5Cr-xWC-0.04C (x = 0, 2.5, 5, 10 wt %), are fabricated by multi-step sintering to investigate influence of tungsten carbide (WC) content on microstructure and mechanical properties. Results reveal that the microstructure of WC-free cermets resembles that of traditional cermets. However, microstructure of WC-containing cermets (x = 2.5 wt %) is composed of fine black core/grey rim Mo2FeB2 grains and coarse white-colored coreless Fe3(W,Mo)3C grains. Furthermore, at x ≥ 5 wt %, most of white coreless Fe3(W,Mo)3C grains are transformed into a white core/black rim structure. From these observations, it can be concluded that growth of nearly round-shaped Mo2FeB2 grains is through diffusion-control, leading to normal grain growth (NGG), whereas the growth of polyhedral Fe3(W,Mo)3C grains is dictated by the interfacial reactions, resulting in 2D nucleation and partially abnormal grain growth (AGG). In addition, grain size distribution of WC-containing cermets exhibits distinct bimodal characteristics, where average grain size is smaller than that of WC-free cermets. More importantly, presence of WC significantly improves the mechanical properties, resulting in superior fracture toughness of 25.1 ± 0.5 MPa m1/2 and hardness of 90.3 ± 0.1 HRA, at x = 5 wt %, without compromising much of transverse rupture strength.

Multistep sintering: its role on density, phase homogeneity, microstructure and electrical properties of (1-x-y)BaTiO3-xPbZrO3-yNaVO3 System

Barium Titanate synthesized by the oxalate coprecipitation method was doped by other perovskite-based material PbZrO3 and NaVO3 to obtain a system of (1-x-y)BaTiO3-xPbZrO3-yNaVO3 with x = 0.15 and y = 0.025. TGA characterization shows an important weight loss that starts at 400°C and finished at 650°C. XRD analysis of calcined powder at 650°C shows that the samples mostly consist of single-phase BaTiO3 perovskite. Sintering process was carried out in two ways; first, a single step sintering up to 1250°C for 2 hours shows increasing secondary phases, until the BaTiO3 phase becomes the minority. Secondly, multistep sintering starting at 1050°C, holding for 2 hours, down to room temperature, and then repeated with similar processes for 1150°C and 1250°C. Characterizations including XRD, SEM for microstructure, and permittivity analysis of the sample after this thermal treatment shows that the majority phase is BaTiO3 with minor secondary BaVO2.9 phase and its electrical properties are enhanced.

Self-reinforced and high-thermal conductivity silicon nitride by tailoring α-β phase ratio with pressureless multi-step sintering

A self-reinforced Si3N4 ceramic with high thermal conductivity was prepared through multi-step pressureless sintering with Y2O3 and MgO additives. The steps consisted of phase control and densification stages to provide additional crystallization sites using a partial α-β phase transformation, which also established a bimodal microstructure. The optimal temperature for the intermediate stage that exhibited the maximum flexural strength was determined by comparing the phase content and linear shrinkage. Thus, 1400 °C, which resulted in 14.4 wt% β-Si3N4, was found to be optimal. After final sintering, a flexural strength of 932 MPa and a thermal conductivity of 74 W/mK were achieved, which are 11.5% and 8.0% higher, respectively, than those obtained using conventional pressureless sintering. The rate constant and activation energy of the pressureless sintering with the Y2O3–MgO additive system were also calculated to evaluate the potential of the α-β transformation to obtain the proper quantity of β-Si3N4 seeds.

Enhancement of the laser emission efficiency of Yb:Y2O3 ceramics via multi-step sintering method fabrication

Highly transparent 5.0 at.% Yb:Y2O3 ceramics were successfully fabricated by using an optimal content of La2O3 and ZrO2 sintering additives and multi-step sintering method. The sintering process of the ceramics was based on a combination of two-step sintering in air followed by vacuum sintering. In-line optical transmission reached 78.5% at 1100 nm and 80% at 2000 nm. The absorption cross-section at 978 nm and the stimulated emission cross-section at 1032 nm were determined to be 2.23 × 10−20 cm2 and 1.42 × 10−20 cm2, respectively. Laser emission at 1.03 μm with a high slope efficiency of 33% was obtained from a 1.5 mm thick Yb:Y2O3 uncoated ceramic medium under quasi-continuous wave pumping at 971 nm with a fiber-coupled diode laser.

High-Pressure Pool-Boiling Heat Transfer Enhancement Mechanism on Sintered-Particle Wick Surface

We experimentally study high-pressure pool-boiling heat transfer enhancement using a copper sintered-particle wick structure in water. The wicks are fabricated using the multi-step sintering process using 200 μm sintered copper powder. Thermophysical properties of water change at elevated pressure, therefore changing the bubble dynamics. For the monolayer wick, the Critical Heat Flux (CHF) 179.63 W/cm2, 182.42 W/cm2, and 198.16 W/cm2 are found at 0, 103.4, and 206.8 kPa, respectively. The maximum CHF is found at 206.8 kPa with the wick superheat of 9.89 K. A 110% enhancement is found in the CHF for 206.8 kPa, compared to 0 kPa. While we observe the CHF is 1.8 times higher as compared to the plain copper surface at 0 kPa. The maximum heat transfer coefficient, 252.46 W/cm2 K is found at heat flux 100 W/cm2 for 206.8 kPa. The high-pressure boiling result for the monolayer wick shows that the heat transfer coefficient is enhanced by 100% compared to 0 kPa. We suggest the reasons of enhancements in the pool-boiling performance are primarily due to high rate of bubble generation, high bubble release frequency and reduced thermal-hydraulic length modulation, and enhanced thermal conductivity due to the sintered wick layer. Our analysis suggests that the Rayleigh-critical wavelength decreases by 4.67 % with varying pressure, which may cause the bubble pinning between the gaps of sintered particles preventing bubble coalescence. Similarly, the dominant role of pressure over the wicking layer is also analyzed and found that the critical flow length, λu reduces by three orders of magnitude with 200 μm particles. We suggest that the porous wick layer provides a capillary-assist to liquid flow effect, and delays the surface dry out. The surface modification and the pressure amplify the boiling heat transfer performance. All these reasons may contribute to the CHF, and HTC enhancement in the wicking layer at high pressure.

Influence of Cr and W addition on microstructure and mechanical properties of multi-step sintered Mo2FeB2-based cermets

The influence of Cr and W addition on microstructure and mechanical properties of Mo2FeB2-based cermets, prepared by multi-step sintering, is systematically evaluated by XRD, SEM, EDS, and TEM. After liquid-phase sintering, the final microstructure of Cr-doped Mo2FeB2-based cermets is similar to that of traditional Mo2FeB2-based cermets. The cermet, with 2.5% Cr content, exhibits superior comprehensive mechanical properties, i.e., Rockwell hardness, transverse rupture strength (TRS) and fracture toughness of 87.6 HRA, 2179 MPa and 21.7 MPa m1/2, respectively. In contrast, for W-doped Mo2FeB2-based cermets, a large amount of W-enriched (Mo,Fe,W)3B2 solid solutions precipitated around undissolved Mo2FeB2 particles, resulting in the formation of black core/gray rim structure. Particularly, a transition layer, with several atomic layer thickness, is observed between (Mo,Fe,W)3B2 rim phase and Fe-based binder phase, which corresponds to B-deficient (Mo,Fe,W)3B2−x orthorhombic boride phase. Moreover, the lattice mismatch between (Mo,Fe,W)3B2 and (Mo,Fe,W)3B2−x is found to be 1.9%. When doped with 5 wt% W, Mo2FeB2-based cermet exhibit superior comprehensive mechanical properties with Rockwell hardness of 84.6 HRA, TRS of 2088 MPa and fracture toughness of 24.4 MPa m1/2. The increase of fracture toughness (KIC) can be ascribed to the presence of W-rich transition layer.

A dynamic analysis of the aluminum titanate (Al2TiO5) reaction-sintering from alumina and titania, properties and effect of alumina particle size

Aluminum titanate Al2TiO5 materials were successfully processed from different fine commercial powders and characterized. Particularly, two calcined aluminas were compared through a multitechnique approach including differential thermal analysis and dilatometry together with structural, microestructural, and mechanical characterization. This allowed the description of all the thermochemical processes during thermal treatment. Developed phases were established. Relatively dense ceramics were obtained, and complex microstructures were described with interlocked grains and an interconnected microcrack matrix that do not jeopardize the material integrity. Multistep sintering and reaction sintering processes were observed in both samples. The first stage consists of the sintering of the starting powders (alumina and titania). A second sintering stage of the starting powders was observed for both samples as well. Once advanced, the second one is overlapped with Al2TiO5 formation that starts at 1380 °C and finishes at 1440 °C. They affect crack development and, in consequence, the thermal behavior. The lower alumina particle size enhances the sintering and reaction advance processes. In the technological temperature range (room temperature—1000 °C), low or even negative thermal expansion behaviors were observed in the developed materials. This, together with the mechanical behavior, encourages structural applications with high thermomechanical solicitations of Al2TiO5 based materials.

Grain growth suppression of ZTA by multi-step sintering

Effect of two-step sintering of Zirconia Toughened Alumina (ZTA) on microstructure and mechanical properties were studied in the present work. Al2O3 and 3YSZ nanoparticles were mixed by high speed vibrating mill, and powders were compacted by a uniaxial pressing protocol at 150 MPa, and sintered through both conventional sintering schedule (CSS) and two-step sintering (TSS) schedules. The composition containing 95 wt% Al2O3 and 5 wt% 3YSZ sintered at two-step sintering (TSS-2; 1600 °C/10 min followed by 1500 °C/6 h) exhibits higher Vickers hardness (1835 HV), fracture toughness (4.95 ± 0.1 MPa.√m), compressive strength (900 MPa) and Diametral tensile strength (125 MPa), respectively. The ZTA composite shows relative density 99.2% with grain size 4.5 µm during CSS at 1600 °C for 4 h. However, the TSS-2 facilitates to achieve relative density 98.4% consist of 1.4 µm alumina grain size that eventually experiences ∼21% fracture toughness increment through zirconia phase assisted transformation toughening.

Enhancement of the laser emission efficiency of Yb:Y2O3 ceramics via multi-step sintering method fabrication

Enhancement of the laser emission efficiency of Yb:Y₂O₃ ceramics via multi-step sintering method fabrication

Microstructure evolution in high density AZO ceramic sputtering target fabricated via multistep sintering

Aluminium doped zinc oxide (AZO) is a functional ceramic with growing relevance as part of future economical optoelectronic devices taking the dual role of transparent electrode and plasmonic material. Here we present a pressureless multistep sintering method for high density and low resistivity AZO ceramic sputtering targets consisting of 98% w/w submicron ZnO powder with 2% w/w Al2O3 powder (average particle size 80 nm) as nanoscale dopant. Effect of sintering temperature on crystal structure, elemental composition, conductivity and micromorphology was investigated with XRD, EDAX, 4-point-probe and FESEM analysis. An optimum value for resistivity of 7.53×10−4Ω⋅cm and relative density of 99.2% for the target sintered at 1300°C was obtained that rivals AZO ceramic sputtering targets protocols in the literature fabricated involving higher sintering temperatures, co-precipitation and hot isostatic press. In addition, main and secondary phase interaction has a high influence on the properties of polycrystalline materials, but has not been studied in depth in AZO ceramic sputtering targets. EDAX micrographs confirm that the secondary phase has higher aluminium compared to the main phase. Grain size distributions for both phases provide a distinctive insight into the recrystallization and exsolution mechanism. It is expected that the unique approach to ceramic target fabrication and characterisation will pave the way for an appraisal of functional ceramic sputtering targets.

Densification process of silica microspheres by multi-step sintering

Dense silica microspheres were fabricated by spray drying and multi-step sintering. The effect of sinter temperatures on densification, dispersibility and inner morphology of silica spheres was analyzed by SEM, and the densification process was deeply discussed. The results showed that single-step sintering at 1100 °C made silica spheres seriously coherent each other and spherical morphology irregular. However, a multi-step process made the sinter temperature increase to 1190 °C, while silica spheres still kept uniformly spherical and dispersed. The increased temperature resulted in that the silica particles obtained dense inner morphology, and their density reached 2.18 g · cm–3, corresponding to a relative density of 98.6% (in comparison with the density of 2.21 g · cm–3 for noncrystal silica).

Preparation of Mo2FeB2-based cermets with a core/rim structure by multi-step sintering approach

A novel multi-step sintering approach was adopted to eliminate the boride reticular structure from Mo2FeB2-based cermets. First, Mo2FeB2 hard phase was synthesized by reaction boronizing sintering and pulverized into powders. Subsequently, the pre-sintered powders were mixed with other compositions, and then the mixture was pressed and sintered. During solid-state sintering, most of the coarse Mo2FeB2 hard phase particles remained undissolved. The boride complex solid solutions containing small amounts of Cr and W precipitated around the remaining Mo2FeB2 hard phase particles during liquid phase sintering, resulting in a black core/gray rim structure. The SAED analysis revealed that the black cores and the gray rims of Mo2FeB2 grains possessed the same crystal structure with similar lattice parameters. Notably, the gray rims enriched in W and Cr improved the wettability of the system, leading to the elimination of large pores. Furthermore, the as-prepared Mo2FeB2-based cermet demonstrated superior mechanical properties, i.e., transverse rupture strength of 2219 MPa and fracture toughness of 23.1 MPa m1/2, which were 15% and 25.5% higher than the traditional Mo2FeB2-based cermet, respectively.