Oscillatory Pressure Sintering Research Articles

Ultrafine‐grained WC‐based ceramics prepared via two‐step oscillatory pressure sintering

AbstractGrain growth in ceramic‐based composites during sintering has always been an unavoidable problem. Based on the theory of fine‐grained reinforcement, the concept of rapid densification was coupled with the inhibition of grain growth to fabricate ultrafine‐grained WC‐based ceramics. Herein, by combining two‐step sintering with oscillatory pressure sintering (OPS), a new sintering process—two‐step oscillatory pressure sintering (TSOPS), was used for the preparation of ultrafine‐grained WC‐based ceramics. The comparison of the OPS with the TSOPS validated the feasibility of the strategy in achieving homogeneous and fine‐grained microstructures. The optimum mechanical properties of Vickers hardness, fracture toughness, and flexural strength reached up to 29.56 GPa, 7.97 MPa m1/2 and 1591 MPa, respectively. A developed TSOPS process was proposed and proved to dominate the grain size distribution homogenizing, which contributed to the simultaneous enhancement of mechanical properties of the ceramics.

Microstructure and mechanical properties of SiC-GNPS-SiCW ceramics by oscillatory pressure sintering

The preparation of SiC-based ceramics with high densification, high strength and exceptional toughness is still a severe challenge. In the present work, an advanced sintering technique namely oscillatory pressure sintering (OPS) was applied to prepare SiC ceramics with one-dimensional silicon carbide whiskers (SiCw) and two-dimensional graphene nanoplatelets (GNPs) as reinforcing phases, and the microstructure and mechanical properties were comprehensively investigated. When the contents of GNPs and SiCw were 1 wt% and 5 wt% respectively, the relative density of SiC-based ceramics reached 99.86%, while the Vickers hardness, flexure strength and fracture toughness were 31.55 GPa, 742 MPa and 7.32 MPa m1/2, respectively. The interface was strengthened, meanwhile the grain size was decreased, both of which enhanced the flexural strength of the composites. Results also proved that crack deflection and bridging as well as the bridging of GNPs and SiCw toughened SiC-based ceramics. Furthermore, the coherent interfaces of SiC/GNPS and SiC/SiCw with good match were observed, leading to an enhancement of mechanical properties of such ceramics.

Enhancing thermoelectric properties of Ca3Co4O9 ceramics through oscillatory pressure sintering

Oscillatory Pressure Sintering (OPS) has been demonstrated to be a favorable manufacturing approach for the control of fine and uniform distribution of grains in various ceramic systems, which can lead to enhanced densification and thus improved physical and mechanical properties. In this study, OPS was utilized to prepare pure Ca3Co4O9 thermoelectric material. The results show that the electrical properties and thermoelectric properties of Ca3Co4O9 prepared by OPS are superior to those of samples sintered by conventional hot pressing (HP) without oscillation. This may be attributed to the better alignment of plate-like grains in the OPS samples, which increase electrical conductivity, and also the higher density of grain boundaries that enhance the phonon scattering and, thus, the reduction of thermal conductivity. This study suggests that a straightforward oscillatory pressure sintering approach can be utilized to obtain high-quality and high-performance thermoelectric materials without the need for doping, templating, or high-temperature sintering.

Microstructure and mechanical properties of Ti6Al4V alloy prepared by oscillatory pressure sintering

In this study, a novel oscillatory pressure sintering (OPS) was explored for the preparation of Ti6Al4V alloy, and the microstructure characteristics and mechanical properties of OPS-Ti6Al4V were investigated subsequently. As a comparison, the spark plasma sintering (SPS) was also utilized to fabricate Ti6Al4V alloy. It was found that OPS-Ti6Al4V had an improved ductility of 16.6 % in elongation, which was 21.2 % higher than SPS-Ti6Al4V, while its strength was slightly reduced. The improved ductility of OPS-Ti6Al4V is attributed to the transition from Widmanstätten microstructure to bi-modal microstructure under the effort of oscillatory pressure. This study suggests that OPS could provide a promising way to fabricate titanium alloys with tailored strength and ductility.

Influence of TiC contents on the microstructure, mechanical properties, and oxidation behavior of Mo–10Si–8B alloys

The microstructure, mechanical properties, and oxidation behavior of Mo–10Si–8B-xTiC (at.%, x = 0,5,10,15) alloys fabricated via oscillatory pressure sintering are studied. The phase constitution of three TiC-added alloys is α-Mo + Mo3Si + Mo5SiB2 + Mo2C + TiC; in 15TiC alloy α-Mo phase no longer maintains its contiguity as in the other three alloys. With further TiC addition, the microhardness improves by 60–90 % (up to 1500 HV), Young's elastic modulus increases from 328 to 368 GPa, and the fracture toughness decreases from 14.23 to 10.8 MPa m1/2, accounting for the less-ductile phase and newly introduced brittle phases. Thermogravimetric analysis indicates that the addition of TiC reduces the oxidation resistance. For isothermal oxidation at 800, 1000, and 1200 °C, the serious oxidation occurs for all TiC-added alloys except for 15TiC alloy at 1200 °C because the oxidation sequence changes to TiC → Mo5SiB2 → Mo3Si → α-Mo. The anomalous phenomenon for 15TiC alloy at 1200 °C could be attributed to the rapid formation and cover of protective SiO2–TiO2 layer on alloy surface after the transient MoO3 volatilization.

Defects control of silicon nitride ceramics by oscillatory pressure sintering and consequent hot isotropic pressing

Micro-pores and micro-cracks inhibit the enhancement in strength and toughness of structural ceramics. Among various processing procedures, sintering is the key stage to control microstructure and then enhance mechanical performances of ceramics. Here, we reported the consolation of silicon nitride ceramics with high strength and toughness by defects control, using oscillatory pressure sintering (OPS) followed by hot isostatic pressing (HIP) techniques. The introduction of oscillatory pressure and isostatic pressure may enhance grain sliding and grains deformation of during sintering, thus removed the trapped pores at grain boundaries and accelerated growth of β-Si3N4 grains. Because of the microstructure evolution, the Si3N4 ceramics exhibited flexural strength of 1630 ± 21 MPa and fracture toughness of 7.1 ± 0.6 MPa m1/2 when sintered at 1750 °C. Thus, this work provides a path to prepare ultra-strong and tough ceramics by controlling various micro-defects.

Synergistic toughening effect of SiC whiskers and particles in ZrO2–Al2O3–SiC ceramics

Zirconia ceramics have a good application prospect in the high-tech fields, whereas their potential utilization is highly limited by their mechanical properties. To fabricate zirconia based ceramics with superior mechanical properties, SiC whiskers (SiCw) and SiC particles (SiCp) were selected as the toughening phases for the preparation of ZrO2–Al2O3–SiC ceramics, and an advanced sintering technique called as oscillatory pressure sintering (OPS) was applied in the present work. The mechanical properties of such ceramics with 5 vol% SiCw and 10 vol% SiCp were significantly improved. The Vickers hardness increased to 15.26 GPa while the bending strength reached 917 MPa, and the fracture toughness rose to 6.83 MPa·m1/2. The toughness of the ceramics was improved by synergistic toughening effects including whisker toughening, stress-induced phase transformation toughening and particle dispersion toughening. Furthermore, SiCp and ZrO2, Al2O3 and SiCp, ZrO2 and SiCw as well as Al2O3 and SiCw formed a well-matched coherent phase boundary. The semi-coherent relationship was also generated at the phase boundary of SiCp and SiCw.

Hot oscillatory pressure preparing low-cost FGH4096 superalloys with high performance

The low-cost and high-performance FGH4096 superalloy was prepared by replacing the high-cost fine powder particles (<51 μm) with coarse particles (>120 μm) using the hot oscillatory pressure (HOP) sintering method. The effects of powder particle size on the grain size and mechanical properties of the superalloy were investigated. The results showed that the tensile strength is 1350 MPa and the elongation is 30.4% for the HOP samples of coarse powder, the mechanical properties are higher than that of the samples prepared by the hot-press (HP) with finer powder. In addition, HOP sintering shows good inhibitory effect for the prior particle boundaries (PPB) in the powder metallurgy superalloys, for the HOP sintering providing the great driving force of particles plastic deformation. The results suggest that the coarse powder particle size with HOP sintering can achieve the better mechanical properties for the FGH4096 superalloys, thus provides a strategy for the low-cost and high-performance powder metallurgy superalloys.

Mechanical and tribological properties of SiC whisker‐reinforced SiC composites via oscillatory pressure sintering

AbstractSiC whisker (SiCw)‐reinforced SiC composites were prepared by an oscillatory pressure sintering (OPS) process, and the effects of SiCw content on the microstructure and mechanical and tribological properties of such composites were investigated. The addition of SiCw could promote the formation of long columnar α‐SiC, and the aspect ratio of α‐SiC grains first increased and then decreased with the increase of SiCw content. When the SiCw content was 5.42 wt%, the relative density of the SiC–SiCw composite reached up to 99.45%. The SiC–5.42 wt% SiCw composite possessed the highest Vickers hardness, fracture toughness, and flexural strength of 30.68 GPa, 6.66 MPa·m1/2, and 733 MPa, respectively. In addition, the SiC–5.42 wt% SiCw composite exhibited the excellent wear resistance when rubbed with GCr15 steel balls, with a friction coefficient of .76 and a wear rate of 4.12 × 10−7 mm3·N−1·m−1. This could be ascribed to the improved mechanical properties of SiC–SiCw composites, which enhanced the ability to resist peeling and micro‐cutting, thereby enhancing the tribological properties of the composites.

Improved comprehensive mechanical properties of oscillatory pressure sintered WC–ZrO2–Al2O3 ceramics with VC/Cr3C2 addition

To optimize the microstructure and comprehensive mechanical properties of WC-based ceramics, the addition of grain growth inhibitors was an important and effective approach. The effect of VC/Cr3C2 addition on the densification behavior, microstructural evolution, comprehensive mechanical properties of WC-based ceramics fabricated by oscillatory pressure sintering was investigated in this work. The introduction of VC/Cr3C2 significantly controlled the size of WC grains and regulated the distributed uniformity of ZrO2 and Al2O3 grains in the WC matrix. When the added contents of VC and Cr3C2 were 0.2 wt% and 0.8 wt% respectively, WC-based ceramics with the best comprehensive mechanical properties were achieved, in which their Vickers hardness, fracture toughness and bending strength were up to 29.15 GPa, 8.37 MPa m1/2 and 1513 MPa, respectively. Furthermore, the enhancement of mechanical properties was mainly attributed to the grain refining strengthening, the transformation of fracture modes and dislocation toughening.

Mechanical and tribological properties of SiC-GNPs composites prepared by oscillatory pressure sintering

SiC-based ceramics, as structural parts and wear-resistant parts, are required to possess outstanding mechanical properties and wear resistant. Herein, to prepare SiC-based ceramics with desirable properties, the effects of graphene nanoplatlets (GNPs) content on the densification, microstructure, mechanical and tribological properties of SiC-GNPs composites by using an oscillatory pressure sintering (OPS) process were investigated. The addition of GNPs could inhibit the growth of SiC grains, and the resultant mechanisms such as crack bridging, deflection, and GNPs pull-out also promoted the strengthening and toughening of such composites. The flexural strength and fracture toughness of the SiC-1.0 wt% GNPs composites attained the maximum values of 515 MPa (∼27% increase) and 7.11 MPa·m1/2 (∼26% increase), respectively. In addition, the friction behavior of SiC-GNPs composites was analyzed by selecting GCr15 steel ball as friction side under drying friction conditions. Compared with pure SiC ceramics, the friction coefficient and wear rate of SiC-1.0 wt% GNPs ceramics were as low as 0.65 (reduced by ∼25%) and 5.17 × 10−7 mm3·N−1·m−1 (reduced by ∼41%), respectively. This phenomenon can mainly be attributed to the lubricating film formed by GNPs during sliding and excellent mechanical properties.

Synergistic strengthening and toughening of oscillatory pressure sintered WC-ZrO2-Al2O3 ceramics

WC-ZrO2-Al2O3 ceramics with high performance were prepared by oscillatory pressure sintering, and their densification behavior, microstructure and mechanical properties were investigated. The results showed that addition of ZrO2-Al2O3 composite powder can reduce the sintering temperature (from 1900 ℃ to 1760 ℃) and promote densification of the ceramics. When the ZrO2-Al2O3 composite powder content was 8 wt%, it exhibited the best mechanical properties including Vickers hardness of 24.95 GPa, fracture toughness of 7.74 MPa·m1/2 and flexural strength of 1329 MPa. The main strengthening and toughening mechanisms included the phase transformation of zirconia, crack deflection, crack bridging and the presence of many dislocation defects. Also, the formation of well-matched coherent interfaces of WC/ZrO2 and WC/Al2O3 contributed to a synergistic improvement in strength and toughness of WC-ZrO2-Al2O3 ceramics.

Enhanced mechanical and tribological properties of oscillatory pressure sintered WC–GNPs composites

AbstractIt remains as a challenge to develop binderless WC ceramics that integrate high mechanical properties and low friction wear. Here, we report the preparation of strong and tough WC ceramics with low wear rate by adding graphene nanoplatelets (GNPs) and using oscillatory pressure sintering (OPS) process. The introduced GNPs lead to the formation of nearly fully dense composites with the aid of an oscillatory pressure. The OPS‐prepared WC–0.3‐wt% GNPs composites reached a high flexural strength, hardness, and fracture toughness, being up to 1420 MPa, 24.9 GPa, and 6.89 MPa m1/2, respectively. Moreover, a low friction wear rate of 3.17 × 10−7 mm3 N−1 m−1 is achieved for such composites, which can be ascribed to the formation of a friction lubrication film during dry sliding friction process and their higher mechanical properties.

Preparation of graphene nanoplatelets reinforced SiC composites by oscillatory pressure sintering

Preparation of SiC-based composites with higher relative density and mechanical properties remains challenging. Herein, we report the fabrication of a category of high performance SiC-based composites using oscillatory pressure assisted sintering (OPS) and 0.5 wt% graphene nanoplatelets (GNPs) as reinforced agents. As the sintering temperature changed from 1860 to 1950 °C, the relative density and mean grain size of the composites increased from 98.79% to 99.95% and 1.02 μm to 1.98 μm, respectively. The SiC-GNPs composites fabricated by OPS possessed much better mechanical properties in comparison with some published data obtained by other sintering methods. The optimal OPS-prepared temperature of the composites was 1920 °C; correspondingly, the relative density, flexural strength, hardness and fracture toughness were 99.72%, 488 MPa, 28.94 GPa and 6.73 MPam1/2, respectively. The crucial toughening mechanisms of crack deflection, crack branching, crack bridging and GNPs bridging were also confirmed.

Plasma Oscillatory Pressure Sintering of Mo-9Si-8B Alloy with ZrB2 Addition.

Oscillatory pressure sintering is a novel crystal refinement technology. The doping of different concentrations of ZrB2 under oscillatory sintering technology (9 Hz) is discussed here, focusing on its macroscopic mechanics and oxidation resistance. In particular, doping 2.5 wt% ZrB2 can effectively increase the hardness of the alloy, slightly increase the fracture toughness of the alloy and have an outstanding effect on the oxidation resistance of the alloy at 1300 °C, achieving the effect of reducing mass loss by 80.3%.

Microstructure and properties of CoCrNiFeMnAl0.5 high-entropy alloys prepared by gas atomization combined with oscillatory pressure sintering

In order to improve the mechanical strength of Cantor alloy and obtain the uniform microstructure, about 9.1 at.% of Al element was introduced and a new CoCrNiFeMnAl0.5 HEAs was prepared by gas atomization combined with oscillatory pressure sintering. The effects of powder sizes on the microstructure and mechanical properties of the as-sintered alloys were investigated and the corresponding strengthening mechanism was discussed. The results show that the CoCrNiFeMnAl0.5 HEAs were composed of FCC phase with good plasticity and BCC phase with high strength. The as-sintered alloys have higher relative density and uniform microstructure without the defects of composition segregation and coarse structure like the as-cast alloy. The yield strength and microhardness of the as-sintered alloys increased to 775 MPa and 369.3 HV, which were 173% and 104% higher than that of the as-cast alloy, respectively. It is indicated that the strength and microhardness of the as-sintered alloys depend on the powder sizes. The strengthening mechanisms of the as-sintered alloys were mainly attributed to the second phase strengthening and the grain boundary strengthening.

Optimization of microstructure and mechanical properties of oscillatory pressure sintered ZrO2–Al2O3-SiCp ceramics

Zirconia ceramic is a significant structural material, but its use under some extreme circumstances is limited by its mechanical properties. In this work, SiC particles (SiCp) were added into alumina toughened zirconia ceramics to prepare ZrO2–Al2O3-SiCp ceramics with high performance by using oscillatory pressure sintering (OPS). Results showed that the best OPS temperature of 1600 °C was obtained, and the optimal SiCp particle size and content were 200 nm and 10 vol% respectively. Under these conditions, the specimen exhibited higher mechanical properties including Vickers hardness of 15.43 GPa, bending strength of 1162 MPa and fracture toughness of 6.36 MPa m1/2. Moreover, it was found that the atomic matching between ZrO2/SiCp, Al2O3/SiCp, and ZrO2/Al2O3 was much higher, showing the coherent interface relationship. Therefore, it was favorable for enhanced mechanical properties of as-prepared ZrO2–Al2O3-SiCp ceramics.

Ultrastrong tough zirconia ceramics by defects‐engineering

AbstractWe report the preparation of a category of ultrastrong tough zirconia ceramics by engineering defects using an oscillatory pressure during pressure assisted sintering. The introduced oscillatory pressure enhances the dynamic grain rearrangement, plastic deformation, mass transportation, and pore removal, leading to the formation of pore‐free ceramics characterized by the rich coherent grain boundaries among individual mesocrystalline grains with intragranular quasi‐interfaces. As a proof of concept, the pressure required by oscillatory pressure sintering (OPS) for preparing fully dense 3Y‐TZP ceramics is significantly reduced, which is just ∼1/5 of that required by hot isostatic pressing. The OPS‐prepared 3Y‐TZP ceramics reached a record‐breaking high bending strength and fracture toughness, being up to 1.8 GPa and 16 MPa·m1/2, respectively. This success illustrates a universal principle of engineering defects for making breakthrough in exploring other ultrastrong tough ceramics.

Study on the Pre-Oxidation and Resulting Oxidation Mechanism and Kinetics of Mo-9Si-8B Alloy.

Molybdenum silicon boron alloy is regarded as the next generation of superalloy that is expected to replace nickel-based superalloys. However, the high-temperature oxidation resistance of Mo-Si-B alloy has always been an issue worth studying. In this study, Mo-9Si-8B alloy was prepared via a plasma oscillatory pressure sintering process and pre-oxidized at 1300 °C while maintaining a certain balance of mechanical and oxidation properties. The influence of the oxide protective layer on its performance at high temperature of 1150 °C was explored, the micro-mechanism of its performance and its failure mode of the hinge-locking mechanism was illustrated, and finally, its oxidation kinetics was inferred. In conclusion, pre-oxidized Mo-9Si-8B (at.%) alloy did play a role in delaying the oxidation process during the initial period of cyclic oxidation. However, with the increase of cyclic oxidation time, the improvement of high-temperature oxidation resistance was limited.

Microstructure and properties of Mo-Si-B alloy fabricated via novel technology: Plasma Oscillatory Pressure Sintering

Molybdenum silicon boron alloy is regarded as a new generation of ultra-high temperature alloys and is expected to replace the existing nickel-based alloys. However, the molybdenum silicon boron alloys prepared by powder metallurgy exhibited low density and had many pore defects, which were left hanging in the air. Here, a novel oscillatory sintering method is used to replace the traditional atmospheric sintering. It is found that the addition of the oscillatory frequency improved the density and mechanical properties of the alloy, and made the crystal grains refined to a large extent. It did have certain reference value for the preparation and development of molybdenum silicon boron alloy.