Two-step Pressureless Sintering Research Articles

Manufacturing of High Purity Cr2AlC MAX Phase Material and Its Characterization

AbstractPresent study discusses about a technique for producing high-purity Cr2AlC MAX phase materials and gaining insight into their thermal behavior for high-temperature applications. The research conducted involved synthesizing a pure layered ternary carbide Cr2AlC MAX phase material by mixing powders of Chromium, Aluminum, and Carbon and then subjecting them to two-step pressureless sintering process in argon atmosphere. First step involves the annealing of ball-milled mixture at 750 °C for 2 h followed by the second step in which the annealed mixture is subjected to heat-treatment at 1350 °C for 2 h. Analysis using XRD and Raman techniques revealed that the synthesized product consists of Cr2AlC phase, without any impurities. SEM studies confirmed that the Cr2AlC had a layered topography, while EPMA analysis indicated that the atomic percentage of Cr, Al, and C was consistent with the XRD phase analysis. XPS investigations confirmed the presence of Cr-C bonds representing Mn+1Xn of the MAX phase material. TG-DSC results showed an approximately 2% increase in weight. The Cr2AlC phase exhibited an endothermic pattern below 725 °C, an exothermic pattern above it, and did not decompose up to 1400 °C in vacuum environment. High-temperature XRD analysis at various temperatures also confirmed no formation of Al2O3 or CrO impurity compounds.

The designed high-performance PM Ti-6Al-4V alloy via two-step pressureless sintering

Sintering is a critical step to obtain the desired mechanical properties for titanium alloys. However, the residual porosity and the coarse microstructure caused by traditional pressureless sintering adversely affect the mechanical properties. Herein, we proposed a novel two-step pressureless sintering route to achieve sintering densification and grain refinement. The high density of 99.0% and the refined parent β grains with a size of 90.57 µm for Ti-6Al-4V alloy are acquired ultimately, which is among the best pressureless sintering practice reported in the literatures. The selection of two-sintering temperature (< 1250 °C) is confirmed to be the key to improving density and reducing grain size. Benefitting from grain refinement, the increased number of grain boundaries provides more channels for vacancy diffusion, and the formation of small α colonies with multiple orientations makes the Widmanstätten microstructure more uniform. During plastic deformation, the abundant dislocation cross slips and nanoscale twins of face-centered cubic phase within these colonies relieve the stress accumulation of phase interfaces, thereby delaying the formation of cracks and promoting the large plastic flow with the high-stress level. Compared with standard sintering, two-step sintered samples exhibit better mechanical properties (ultimate tensile strength of 968 MPa, yield strength of 871 MPa and elongation of 16.1%), achieving the strength-ductility synergy. This work casts light on developing high-performance and low-cost titanium and its alloys.

Two-step pressureless sintering of ultrafine powders for the preparation of high strength and transmittance AlON ceramics

AlON ceramics with high transmittance and strength are essential for transparent armor application. However, high-temperature sintering for long time would result in large grain sizes associated with low strength. In this paper, a low + high temperature two-step sintering method with ultrafine AlON powders was reported for the preparation of high-strength and high-transmittance AlON ceramics. Firstly, the high-purity AlON powders with high specific surface area of 29.77 m2/g was synthesized by solution combustion synthesis-nitridation method and subsequently ball-milling process. Thereafter a sample with 97% density and small grain size (less than 4 μm) can be obtained after sintered at 1750 °C for 3 h, in which the difference of kinetics between grain-boundary diffusion and grain-boundary migration was used to suppress the final stage of grain growth. Further increasing sintering temperature to 1850 °C resulted in the elimination of the residual porosity, and realizing transparent AlON ceramics with high optical transmission (85.4%) and high flexural strength (375.1 MPa) could be achieved.

Micro-structural and magnetic analysis of spinel high entropy oxides synthesized by two-step pressureless sintering provides insight into high entropy ceramics

High-entropy oxides are novel materials consisting of many metallic elements of equal proportions. Hence, the complex structure of high-entropy oxides leads to interesting properties. In this work, seven single-phase high entropy oxides were synthesized by two-step pressureless sintering. According to the experimental XRD data, all samples are found in the cubic crystal structure with the Fd-3m space group. This result shows that high entropy oxides have a high possibility of phase formation. For all samples, the EDS results show that all expected elements have similar elements content. The magnetization curves show that the magnetic properties change systematically with the changes in the high-entropy stable oxide composition. The seven different samples are divided into three groups for comparison. In group 1, with the substitution of iron for zinc, Hc increased from 19 Oe to 221 Oe, while Ms changed slightly. In group 2, the substitution of zinc for nickel leads to the rapid weakening of magnetism. In group 3, with nickel replacing cobalt, Ms increased from 15.060 emu/g to 19.801 emu/g, and Hc decreased from 28 Oe to 11 Oe. The complex element composition makes high entropy materials have great development space in the field of magnetic materials. Thus, the magnetic properties of the high-entropy spinel oxides can be adjusted according to this concept.

Characteristics of six layered Al/Si3N4 functionally graded materials prepared through two-step pressureless sintering process

The rapid advancement of the welding technology has simultaneously increased the demand for the online monitoring system in order to control the process. Among the methods that could be possibly used to assess the weld condition, an air-borne acoustic method grasps the attention from scholars due to its ability to provide a simple, non-contact, and low-cost measurement system. However, it is still lack of resources involving this subject in an attempt to deeply understand the emitted sound behaviour during welding especially when dealing with a complete deviation of a process parameter, welding types, workpiece material as well as the noise from the surrounding. This paper reviews the application of the acoustic method in monitoring the welding process. Specifically, this review emphasized the source of both structure-borne and air-borne acoustic during the welding process and the significance of applying the acoustic method in more detail. By focusing on the liquid state welding process, the scope of discussion converged on the arc and laser welding process. In the last part of this review, the potential future advancement of this method is pointed out before the overall conclusion is made.

Mechanical properties and microstructural evolution of pressureless sintered ceramics obtained from high-energy ball-milled TiB2–TiC powders

A series of TiB2–TiC composites were successfully fabricated through one-step and two-step pressureless sintering using powders obtained by high-energy ball-milling. Effects of ball-milling time on crystal size and morphology of powders as well as microstructural evolution and mechanical properties of TiB2–TiC composites were investigated. The crystal size of TiB2–TiC powders decreases to 14.7 nm after ball milling for 30 h. Two-step pressureless sintering is beneficial to the densification process without obvious grain coarsening. The core-rim structure of carbides in TiB2–TiC composites is formed by nonuniform diffusion of W atoms from the outside to the interior of TiC grains. The phenomenon of selective solution of WC into TiC grains was carried out through experiments and verification of first-principles DFT calculations. An in-situ TiB2 plate toughened microstructure is developed for TiB2-40 wt %TiC composites two-step sintered at 1500 °C for 4 h and 2000 °C for 1 h with excellent comprehensive performances.

Transparent polycrystalline γ-AlON fabricated using a hybrid sintering process

This paper reports the properties of polycrystalline transparent γ-AlON fabricated using a hybrid sintering process of hot pressing and pressureless sintering. The overall aim was to reduce the time taken for the two-step pressureless sintering process. Using Al2O3 and AlN as starting materials with 0.1 wt. % Pr2O3 added as a sintering additive, hot pressing was performed at 1500°C under 20 MPa for 1, 2, and 3 hr followed by pressureless sintering at 1940°C for 10 hr in an N2 atmosphere. The effects of the hot pressing time and Pr2O3 addition on the phase evolution, microstructure, mechanical properties, and optical transmittance of γ-AlON were examined. The γ-AlON fabricated using a hybrid process showed ≧ 80% transmittance, highlighting the feasibility of this process.

Effects of various rare-earth additives on the sintering and transmittance of γ-AlON

Several rare earth elements (Sc, La, Pr, Sm, Gd, Dy, Er, and Yb) were evaluated to develop a sintering additive for transparent γ-AlON. After adding 0.2 wt. % of each of the rare earth additives to the α-Al2O3 and AlN starting material, hot pressing was performed at 1850 °C for 1 h under 20 MPa as a screening test, whereby the γ-AlON with Pr-nitrate showed the highest transmittance of 60.4 % at 632 nm. Two-step pressureless sintering was conducted as a separate test for the same starting material after adjusting the amount of Pr-nitrate to 0, 0.1, or 0.2 wt. % to enhance transmittance. The γ-AlON with 0.1 wt. % Pr-nitrate showed the highest transmittance of 80.36 % after 1st and 2nd sintering steps at 1610 and 1940 °C, respectively, indicating that Pr can be an alternative sintering additive to conventional Y2O3, MgO, and La2O3 for the fabrication of transparent γ-AlON.

High thermoelectric performance of Ca3Co4O9 ceramics with duplex structure fabricated via two-step pressureless sintering

Two-step pressureless sintering processing was used to prepare high density (97.6%) Ca3Co4O9 thermoelectric ceramics. A duplex structure was observed and channel paths for charge carriers were formed between the grains. Oxygen vacancies were identified by fitting of XPS spectra, and the effect of the oxygen vacancies on thermoelectric properties was investigated. The electrical resistivity of samples decreased from 17.5 to 11.7 mΩ·cm and Seebeck coefficient decreased from 197.5 to 181.4 μV/K at 1073 K with increasing of the first-step sintering temperature. Furthermore, a low thermal conductivity of 0.94 W/(m·K) and the peak ZT of 0.30, which was 1.3 times larger than previous work, were obtained at 1073 K when the first-step sintering temperature was 1373 K.

Sintering kinetics and microstructure evolution in α-Al2O3 nanocrystalline ceramics: Insensitive to Fe impurity

Diffusion kinetics, processing, and microstructures of alumina ceramics have been of great scientific and technological interests. Here we reported the sintering kinetics and microstructure evolution in high-purity dispersed ultrafine α-Al2O3 nanoparticles with superior sinterability. An extremely fine grain size of 34 nm with 99.6 % theoretical density and a uniform microstructure can be produced by two-step pressureless sintering. Systematic kinetic analysis further points out to a surprising insensitivity of α-Al2O3 nanocrystalline ceramics to Fe and other impurities, which contrasts with prior reports in the literature. We propose this insensitivity is originated from extensive grain boundaries in nanocrystalline ceramics, which allows segregation of impurities and lowers their concentrations at grain boundaries.

Two-step sintering of TiB2–40wt%TiN composites

Two-step pressureless sintering was investigated for TiB2–40wt%TiN system. The calculation for phase equilibria was performed, which indicates that a significant nitrogen loss can occur at 1700 °C while ZrO2 readily dissolves into TiN lattice from 1600 °C. The XRD analysis revealed that nitrogen loss was greater during first step at low temperature but smaller for second step at high temperature. As a result, the high-temperature processing of the second step provided better densification and mechanical properties than those of one-step sintering.

Mechanical Properties of MgO-Doped Transparent Crystalline Alumina Fabricated by Two-Step Pressureless Sintering

Transparent polycrystalline alumina with high bending strength has been fabricated using two-step pressureless sintering. The microstructure and mechanical property has been investigated by varying the MgO doping concentration and sintering parameters. The results showed that the grain size markedly decreased with the increasing content of MgO addition. Although the density of alumina could be increased by the higher temperature for the first sintering step, the larger grain would depress the bending strength. Hence, the optimum bending strength was achieved with 0.5 wt% MgO with the two-step sintering of 1450°C for 20 min and 1400°C for 20 h. The Vickers hardness of the transparent PCA increased doping content, and the variation tendency was well accelerated with the relative density, indicated that the porosity plays a more important on hardness than the grain size.

The Preparation and Properties of Alumina Ceramics through a Two-Step Pressureless Sintering Process

In this paper, a high-dense alumina ceramics were prepared through the two-step pressureless sintering process with high-purity alumina powder as raw materials and high-purity MgO as sintering aids. The effects of the sintering temperature in the first-step (T1) and the soaking time (t) in the second sintering step (T2) on the density, microstructure and mechanical properties of the alumina ceramics were studied. The results indicated that the relative density increased with the increase of T1 temperature whereas it increased and then decreased with the increase of MgO content. Higher T1 temperature and extended soaking time caused larger grain size, which accompanied with the Ostwald ripening of the grain and led to non-uniformity of grain size distribution. The addition of MgO was beneficial to the decrease in grain size due to pinning effect of the second phase. For samples with shorter soaking time, sintering with higher T1 temperature led to better mechanical properties because of its high density. However, for the long soaking time, all samples after sintering at different T1 temperature were fully-densified, so the grain size become to the dominant factor of strength, thus samples with lower T1 temperature exhibited better mechanical properties due to the refinement grain. Excessive addition of MgO resulted in defects, by which the strength increased firstly and then decreased slightly with the increased MgO content. For the samples with 2.5wt.% MgO, the optimum condition for the two-step pressureless sintering was T1=1450°C and T2=1400°C for 20h, and the obtained sample achieved the relative density of 96% and the strength of 507±32MPa.

Microstructure and properties of aluminium-aluminium oxide graded composite materials

In this research works, four-layered aluminium-aluminium oxide (Al-Al2O3) graded composite materials were fabricated using powder metallurgy (PM) method. In processing, metal-ceramic graded composite materials of 0%, 10%, 20% and 30% weight percentage of ceramic concentration were prepared under 30 ton compaction load using a cylindrical die-punch set made of steel. After that, two-step pressureless sintering was carried out at sintering temperature and time 600°C and 3 hours respectively. It was observed that the sintered cylindrical specimens of 30 mm diameter were prepared successfully. The graded composite specimens were analysed and the properties such as density, microstructure and hardness were measured. It was found that after sintering process, the diameter of the graded cylindrical structure was decreased. Using both Archimedes method and rule of mixture (ROM), he density of structure was measured. The obtained results revealed that the microvickers hardness was increased as the ceramic component increases in the graded layer. Moreover, it was observed that the interface of the graded structure is clearly distinguished within the multilayer stack and the ceramic particles are almost uniformly distributed in the Al matrix.

Bi-colored zirconia as dental restoration ceramics

Machinable zirconia blocks with gradient colors are fundamental for fabrication of full-contour esthetic dental restorations. The aim of this study was to process and evaluate bi-colored zirconia ceramics as a pilot dental material by using well-established techniques. Two commercially available partially stabilized zirconia granules, one undoped and one doped with 0.202 wt% Fe2O3, resulted in white and yellow colors after sintering, respectively. Bi-colored zirconia was fabricated by two-step dry pressing of both zirconia granules one above the other to form green bodies, followed by cold isostatic pressing (CIP) and, a two-step pressureless sintering finally at 1450 °C. The dilatometer results showed that the Fe2O3 doped zirconia sintered slightly rapid, but the difference of shrinkage between two powders was <1%. Sintered bars achieved full density, 6.018 g/cm3 (∼99%TD), without cracks in the ∼1 mm color gradient zone. The microstructures were characterized by scanning electron microscopy (SEM) and careful observation of both surface and interior provided no obvious structural difference of either grains or pores among the three distinct regions, comprising white, yellow and color gradient zone. Vickers hardness of bi-colored zirconia was ∼13.1 GPa, with no obvious difference in the three regions. The four-point bending strength of the bi-colored zirconia bars was 745.5±159.6 MPa, which appeared noticeably lower than that of the single-colored references being above 1000 MPa. Fractographic analysis revealed that in most of the cases (60%) the fracture was initiated at the color gradient zone, where large voids with high coordination numbers, agglomerates with critical size and concentration of irregular grains with porous surfaces were observed. Above all, bi-colored zirconia ceramics prepared by the improved technique could meet the basic requirements of dental materials. The ways of minimizing the defects within bi-colored blocks should be developed for the production of esthetic zirconia ceramics of high strength and reliability.

Preparation and properties of nano-SiC-based ceramic composites containing nano-TiN

Nano-SiC-based ceramic composites containing 5 wt.% nano-TiN were prepared by a two-step pressureless sintering method, and the effect of sintering temperature and nano-TiN on the microstructure and properties of the resultant nanocomposites were investigated. The addition of nano-TiN improved the sintering and mechanical properties, restrained the growth of nano-SiC grains and resulted in uniform microstructure with a grain size of 200–400 nm. The reinforcing mechanism of nano-SiC-based composites is a combination of the strengthening and toughening mechanism of nano-TiN and Y3Al5O12 grains.

Two-step sintering and electrical properties of sol–gel derived 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 lead-free ceramics

Two-step pressureless sintering of sol–gel derived 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 (BNT-BT) lead-free piezoelectric ceramics were investigated in comparison with conventional sintering. The effect of sintering regimes on the densification, grain growth behavior and electrical properties was discussed in detail. The results indicated that BNT-BT ceramics with a density of 95%, a relatively fine grain size of 850 nm and comparable piezoelectric properties (d33 ~170 pC/N, kp ~0.26, Qm ~102) had been achieved by pre-sintering at 1,150 °C to reach a critical density of 78%, and then cooling to a lower temperature of 1,050 °C for 20 h. The critical density value proves important at which the grain boundary diffusion could be maintained but the grain boundary migration suppressed at the same time. Moreover, the volatilization loss of Bi and Na elements could be inhibited by two-step sintering. Both the reduction of the grain size and the inhibition of the stoichiometry deviation together account for the variation of various electrical properties.

Synthesis of silicon nitride-barium aluminosilicate self-reinforced ceramic composite by a two-step pressureless sintering

Dense 40wt%BAS/Si 3N 4 self-reinforced composite was synthesized by a two-step pressureless sintering process. The results showed that the BAS glass–ceramic not only served as an effective liquid phase sintering aid for attaining full densification and completing the α- to β-Si 3N 4 phase transformation, but also remained as a structural matrix. A bimodal microstructure could be obtained via this two-step process without the addition of special β-Si 3N 4 seeds. It is due that the first step could suppress the α- to β-Si 3N 4 phase transformation while allowing densification, and then at the higher temperature second step, the continued transformation could be used to facilitate the growth of the β-nuclei formed in the first step. The obtained composite exhibits excellent mechanical properties compared to unreinforced BAS matrix. The flexural strength and fracture toughness could reach 565 MPa and 7.4 MPa m 1/2, respectively. The crack deflection, grain bridging and pullout are considered as major toughening mechanisms in this composite.

The Preparation of TiAl-Based Intermetallics from Elemental Powders through a Two-Step Pressureless Sintering Process

Pressureless sintering is not considered feasible for the preparation of TiAl-based intermetallics from elemental powders due to the Kirkendall effect. When the compacts are sintered without pressure, swelling and surface cracking occur in the billets. Porous billets are obtained, which cannot be further used. Nevertheless, pressureless sintering is still worth investigating because of lower cost and easier operation. In this study, a two-step pressureless sintering process was developed to prepare the TiAl-based intermetallic preforms. The first step was to eliminate the aluminum particles in the billets through a solid-state sintering procedure to prevent the aluminum from melting and to suppress the formation of the Kirkendall pores by sintering with a constrained mold. The second step was to make the billets homogenized and densified by a high-temperature sintering procedure. Results show that the billets sintered with constrained molds are dense and crack free after the first step. Then, the billets are densified in the second step. Therefore, the application of constrained molds makes pressureless sintering feasible in the preparation of the TiAl-based intermetallics from the elemental powders. The microstructure is fully transformed from the aluminum and titanium compacts to the TiAl-based intermetallics after this two-step pressureless sintering process.

Microstructure and high-temperature strength of pressureless-sintered silicon carbide

Silicon carbide (SIC) cermic has been recognized as a prime candidate material for structural applications such as heat engine components, heat exchangers etc., due to its good strength, excellent resistance to corrosion and oxidation, and excellent thermomechanical as well as thermophysical properties. The densification of SiC via pressureless sintering has been an important subject for years, and the first work was pioneered by Prochazka [1] who obtained a dense SiC with the addition of boron and carbon as sintering aids by means of a liquid-phase sintering mechanism. In spite of sufficient flexural strength, SiC ceramics in the SiC + B + C system showed no improvement in the resistance to fracture [2, 3]. A number of investigations indicate the importance of sintering aids, which play a crucial role in changing the mechanical as welt as high-temperature properties of the SiC ceramic [3-6]. Omori and Takei [7], who used alumina and yttria as sintering aids, obtained a dense SiC ceramic with an improved mechanical strength of greater than 6 5 0 MPa. More recently, Lee et al. [3] obtained a dense SiC ceramic with a fracture toughness as high as -8 .3 MPam 1/2, by allowing exaggerated growth of the SiC grains at 2000 °C to form a microstructure containing considerable amounts of plate-like a-SiC grains. Unfommately, the fracture strength of their SiC ceramics decreased to -450 MPa. A recent investigation by the present authors has demonstrated that dense SiC ceramics with a four-point flexural strength above 600 MPa and a fracture toughness as good as 6 . 0 M P a m 1/2 are achievable through the control of microstructure evolution in monolithic SiC ceramic, by starting with a mixture of ozand /3-SIC powders via a two-step pressureless sintering procedure [8]. The requirement of high-temperature strength is a prime consideration in the development of SiC ceramics, since one of their principal uses is for heat-engine applications where the high-tempera~tre properties are critically important. Although studies on the high-temperature strength of SiC ceramics are extensive, most of the investigations involving sintering aids were focused on boron and carbon systems. For employing A1203 and 57203 as sintering aids, the available data on the strength at elevated temperatures are not extensive. In a number of patent references [6,9] the data for high-temperature strength of SiC ceramics, in spite of their high room-temperature strength, showed a considerable reduction in strength by over 25% at 1200 °C.