Effect of reinforcement NbC phase on the mechanical properties of Al2O3-NbC nanocomposites obtained by spark plasma sintering

The sintering behavior of Al₂O₃-NbC nanocomposites fabricated via conventional and spark plasma sintering (SPS) was investigated. The nanometric powders of NbC were prepared by reactive high-energy milling, deagglomerated, leached with acid, added to the Al₂O₃ matrix in the proportion of 5 vol% and dried under airflow. Then, the nanocomposite powders were densified at different temperatures, 1450–1600 °C. Effect of sintering temperature on the microstructure and mechanical properties such as hardness, toughness and bending strength were analyzed. The Al₂O₃-NbC nanocomposites obtained by SPS show full density and maximum hardness value > 25 GPa and bending strength of 532 MPa at 1500 °C. Microstructure observations indicate that NbC nanoparticles are dispersed homogeneously within Al₂O₃ matrix and limit their grain growth. Scanning electron microscopy examination of the fracture surfaces of dense samples obtained at 1600 °C by SPS revealed partial melting of the particle surfaces due to the discharge effect.     

Effect of Al addition on SiC–B4C cermet prepared by pressureless sintering and spark plasma sintering methods

SiC–B₄C–Al cermets containing 5, 10 and 20 wt.% of Al were fabricated by high-energy planetary milling followed by conventional sintering and spark plasma sintering (SPS) techniques separately. The average particle size reduced to ~ 3 μm from an initial size of 45 μm after 10 h of milling. The as-milled powders were conventionally sintered at 1950 °C for 30 min under argon atmosphere and SPS was carried out at 1300 °C for 5 min under 50 MPa applied pressure. The formation of Al₈B₄C₇ and AlB₁₂ phases during conventional sintering and SPS were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. The formation of Al₈B₄C₇ at 700 °C and AlB₁₂ at 1000 °C was well supported by XRD and differential scanning calorimetry (DSC). The maximum relative density, microhardness and indentation fracture resistance of SiC–B₄C–10Al consolidated by SPS are 97%, 23.80 GPa and 3.28 MPa·m^1/2, respectively.     

Oxidation-induced strength behavior of Ti3SiC2

Oxidation behavior and subsequent mechanical properties of Ti₃SiC₂ were studied. The oxide scale has significant effect on strength and hardness, which is mainly attributed due to the mismatch of coefficient of thermal expansion (CTE) of substrate and oxide phases. The improved flexural strength (～650 MPa) could be noticed at 1000 °C; however, at elevated temperature the ductility of Ti₃SiC₂ was predominant and reduced the strength.     

Spark plasma sintering of (Ti0.9W0.1) C powders prepared by mechanical alloying

Nanocrystalline (Ti_0.9W_0.1)C powder with a diffraction crystallite size of about 10 nm was synthesized by mechanical alloying. The formation of (Ti_0.9W_0.1)C carbide was detected by XRD measurements and microscopic observation. The sintering of these powders by a spark plasma sintering (SPS) at different temperatures were also studied. The results show that the maximum hardness was obtained for more relative density materials, meanwhile, the grain size is large. The micro-hardness and the relative density of the powder milled for 10 h and sintered at 1200 °C for 5 min under 100 MPa reach, respectively, 2978 HV and 98.35%.     

Sinterability studies of monolithic chromium diboride (CrB2) by spark plasma sintering

Spark plasma sintering (SPS) experiments were conducted to investigate the effect of the processing parameters such as temperature, mechanical pressure and dwell time on densification behavior of monolithic chromium diboride. The sintering experiments were performed at different temperatures ranging from 1100 °C to 1900 °C under the mechanical pressure of 30 MPa–70 MPa for 1 min–15 min duration. The onset temperature for the densification of CrB₂ is observed to be 1300 °C at 50 MPa. High dense chromium diboride (98.4%ρ_th) compact was obtained when processed at 1900 °C under a mechanical pressure of 70 MPa for 15 min duration. Hardness and fracture toughness of high density monolithic CrB₂ (98.4%ρ_th) sample were measured to be 15.89 ± 1.3 GPa and 1.8 ± 0.14 MPa·m^1/2 respectively.     

Nano-sized zirconium carbide powder: Synthesis and densification using a spark plasma sintering apparatus

Nano-sized zirconium carbide powder was synthesized at 1600 °C by the carbothermal reduction of ZrO₂ using a modified spark plasma sintering (SPS) apparatus. The synthesized ZrC powder had a fine particle size of approximately 189 nm and a low oxygen content of 0.88 wt%. The metal basis purity of the synthesized powder was 99.87%. The low synthesis temperature, fast heating/cooling rate and the effect of current during the modified SPS process effectively suppressed the particle growth. Using the synthesized powder, monolithic ZrC ceramics with high relative density (97.14%) were obtained after the densification at 2100 °C for 30 min at a pressure of 80 MPa by SPS. The average grain size of the densified ZrC ceramics was approximately 9.12 μm.     

Thermal stability of Ti3SiC2 thin films

The thermal stability of Ti₃SiC₂(0 0 0 1) thin films is studied by in situ X-ray diffraction analysis during vacuum furnace annealing in combination with X-ray photoelectron spectroscopy, transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray analysis. The films are found to be stable during annealing at temperatures up to ∼1000 °C for 25 h. Annealing at 1100–1200 °C results in the rapid decomposition of Ti₃SiC₂ by Si out-diffusion along the basal planes via domain boundaries to the free surface with subsequent evaporation. As a consequence, the material shrinks by the relaxation of the Ti₃C₂ slabs and, it is proposed, by an in-diffusion of O into the empty Si-mirror planes. The phase transformation process is followed by the detwinning of the as-relaxed Ti₃C₂ slabs into (1 1 1)-oriented TiC_0.67 layers, which begin recrystallizing at 1300 °C. Ab initio calculations are provided supporting the presented decomposition mechanisms.     

Microstructure and mechanical properties of ZrB2–SiC composites prepared by gelcasting and pressureless sintering

ZrB₂–SiC ceramic composites were prepared through water-based gelcasting and pressureless sintering. Effects of the pressureless sintering temperature (1500–2000 °C), heating rate (5–15 °C/min) and soaking time (0.5–2 h) on the relative density, microstructure and mechanical properties of the ZrB₂–SiC composites were investigated in detail. A sintering temperature of 2000 °C, a heating rate of 5 °C/min and a soaking time of 2 h were found to be the optimal pressureless sintering procedure. The relative density, flexural strength and fracture toughness of the ZrB₂–SiC composite prepared under the optimum condition were 97.8%, 403.1 ± 27.8 MPa and 4.05 ± 0.42 MPa·m^1/2, respectively.     

Spark plasma sintering of TiC particle-reinforced molybdenum composites

In order to improve the recrystallization resistance and the mechanical properties of molybdenum, TiC particle-reinforcement composites were sintered by SPS. Powders with TiC contents between 6 and 25 vol.% were prepared by high energy ball milling. All powders were sintered both at 1600 and 1800 °C, some of sintered composites were annealed in hydrogen for 10 h at 1100 up to 1500 °C. The powders and the composites were investigated by scanning electron microscopy and XRD. The microhardness and the density of composites were measured, and the densification behavior was investigated. It turns out that SPS produces Mo–TiC composites, with relative densities higher than 97%.The densification behavior and the microhardness of all bulk specimens depend on both the ball milling conditions of powder preparation and the TiC content. The highest microhardness was obtained in composites containing 25 vol.% TiC sintered from the strongest milled powders. The TiC particles prevent recrystallization and grain growth of molybdenum during sintering and also during annealing up to 10 h at 1300 °C. Interdiffusion between molybdenum and carbide particles leads to a solid solution transition zone consisting of (Ti_1 −x Mo_x)C_y carbide. This diffusion zone improves the bonding between molybdenum matrix and TiC particles. A new phase, the hexagonal Mo₂C carbide, was detected by XRD measurements after sintering. Obviously, this phase precipitates during cooling from sintering temperature, if (Ti_1 −x Mo_x)C_y or molybdenum, are supersaturated with carbon.     

Fabrication of W–Cu functionally graded material by spark plasma sintering method

Three-layered (W–25Cu/W–50Cu/W–75Cu, volume fraction) W/Cu functionally graded material (FGM) was synthesized by spark plasma sintering (SPS) at different temperatures for 5 min under a load of 40 MPa. The influences of different sintering processes on relative density, hardness, thermal conductivity and microstructure at various layers of sintered samples were investigated. The experimental results indicated that the graded structure of the composite could be well densified after the SPS process. The relative density increased with the increment of sintering temperature and it was up to 96.53% as sintered at 1050 °C. In addition, the thermal conductivity reached 140 W/m·K at room temperature and 151 W/m·K at 800 °C, which could be ascribed to the specific structure that W particles enwrapped by net-like Cu. And the Vickers hardness was converted from 4.11 to 4.68 GPa.     

Synthesis and properties of nanostructured dense LaB6 cathodes by arc plasma and reactive spark plasma sintering

Nanostructured polycrystalline LaB₆ ceramics were prepared by the reactive spark plasma sintering method, using boron nanopowders and LaH₂ powders with a particle size of about 30 nm synthesized by hydrogen dc arc plasma. The reaction mechanism of sintering, crystal structure, microstructure, grain orientations and properties of the materials were investigated using differential scanning calorimetry, X-ray diffraction, Neutron powder diffraction, Raman spectroscopy, transmission electron microscopy and electron backscattered diffraction. It is shown that nanostructured dense LaB₆ with a fibrous texture can be fabricated by SPS at a pressure of 80 MPa and temperature of 1300 °C for 5 min. Compared with the coarse polycrystalline LaB₆ prepared by traditional methods, the nanostructured LaB₆ bulk possesses both higher mechanical and higher thermionic emission properties. The Vickers hardness was 22.3 GPa, the flexural strength was 271.2 MPa and the maximum emission current density was 56.81 A cm^?2 at a cathode temperature of 1600 °C.     

Effect of silicidation pretreatment with gaseous SiO on sinterability of TiC powders

Silicidation pretreatment with gaseous SiO at 1350 °C for 30 min is employed for chemically modifying commercially available TiC powder. Phase composition and microstructural features of the pretreated powder are discussed. Densification behavior of the pretreated TiC powder during hot pressing is studied in comparison with that of non-pretreated one. Significantly improved densification behavior and sinterability of TiC powder after silicidation pretreatment are explained by the effect of Ti₃SiC₂ acting as a solid lubricant. Nearly fully dense TiC-based ceramics having flexural strength of 370 MPa, fracture toughness of 5.6 MPa m^½, and microhardness of 24 GPa is obtained by hot pressing under conditions as mild as 1600 °C and 20 MPa.     

In situ synthesis and sintering of ZrB2 porous ceramics by the spark plasma sintering–reactive synthesis (SPS–RS) method

Zirconium diborides (ZrB₂) porous ceramics were synthesized by the Spark Plasma Sintering-Reactive Synthesis (SPS–RS) technique using ZrO₂ and B₄C as precursors which undergo solid state reaction that lead to pore formation. Phase analysis of the products indicated that the reaction started between 1200 °C and 1300 °C and was carried out at 1600 °C within 10 min under SPS conditions, which was consistent with the thermodynamic calculations. The as-prepared ZrB₂ porous ceramics had a relatively smaller crystallite size (~ 1 μm), a lower oxygen content (~ 1.04 wt.%) and a relative density of 29.9%. The oxygen impurities decreased with the sintering temperature and holding time. In addition, the measured results showed that the reaction was carried out within 10 min holding time at the temperature of 1600 °C and the synthesized ZrB₂ products had high purity in comparison to commercial ZrB₂ powder product.     

Flexural strength behavior of a ZrB2–TaB2 composite consolidated by non-reactive spark plasma sintering at 2300 °C

A bulk zirconium–tantalum diboride ceramic composite was consolidated by non-reactive spark plasma sintering (SPS) at 2300 °C. In order to consolidate the ZrB₂–44 wt% TaB₂ composite and restrict grain-growth, a special loading procedure was used. Pressure was applied and released at 2150 °C and 1250 °C, respectively. These SPS conditions allowed us to obtain a crack-free bulk composite with a grain size of 4–8 μm. Flexural strength at temperatures up to 1800 °C was measured for the ZrB₂–TaB₂ composite. Importantly, at 1600 °C, the strength was 336 ± 23 MPa, which is superior to that of monolithic ZrB₂. Moreover, the ZrB₂–TaB₂ composite only showed plastic behavior at 1800 °C, a finding that is atypical for ZrB₂-based ceramics.     

Spark plasma sintering of a commercially available granulated zirconia powder: Comparison with hot-pressing

A commercially available granulated TZ3Y powder has been sintered by hot-pressing (HP). The “grain size/relative density” relationship, referred to here as the “sintering path”, has been established for a constant value of the heating rate (25 °C min^?1) and a constant value of the macroscopic applied pressure (100 MPa). It has then been compared to that obtained previously on the same powder but sintered by spark plasma sintering (SPS, heating rate of 50 °C min^?1, same applied macroscopic pressure). By coupling the analysis of a sintering law (derived from creep rate equations) and comparative observations of sintered samples using transmission electron microscopy, a hypothesis about the densification mechanism(s) involved in SPS and HP has been proposed. Slight differences in the densification mechanisms lead to scars in the microstructure that explain the higher total ionic conductivity measured, in the temperature range 300–550 °C, when SPS is used for sintering.     

Forming and sintering behaviors of commercial α-Al2O3 powders with different particle size distribution and agglomeration

The Al₂O₃ structure ceramics have been investigated extensively in previous studies. In order to compare micron- with submicron-scale powder on forming and sintering behaviors, three commercial α-Al₂O₃ powders were studied: 0.15 μm (denoted S as small in the paper) (granulating), 0.43 μm (denoted M as middle) (granulating), and 1.8 μm (denoted L as large) (granulate-free) at d₅₀ (median size). Although the (M) powder contains hard agglomerates, it forms more easily than the (S) powder. This is principally because the (M)'s soft agglomeration strength (0.03 MPa) is weaker than (S) (7 MPa). The (L) bulk formed easily with lower pressure 10 MPa because of wider starting-particle size distribution, 0.2–15 μm. The (S) primary particles rearranged before sintering, so it postponed its sintering onset temperature to about 1200 °C. Additionally, its shrinkage rate becomes maximal and concentrated at the 2nd stage of sintering from 1300 to 1400 °C. (M) bulk revealed the longest shrinkage range from 1000 to 1500 °C because the sintering occurred with its hard agglomerates at first. Although (L) powder formed rather easily, its sintering was impeded by a much wider particle size distribution.     

Synthesis of high purity titanium silicon carbide from elemental powders using arc melting method

The objective of this study is to investigate the formation of Ti₃SiC₂ from Ti/Si/C powders using the arc melting method. The results show that the sample sintered at 80 s produced a near single-phase of Ti₃SiC₂ (99.2 wt.%) with a relative density of 88.9%. These results were confirmed by phase determination using XRD analysis and were supported with micrographs from FESEM/EDX analyses. The relative density and porosity of all samples were dependent on the formation of macropores in bulk samples and micropores in TiC_x grains. The proposed reaction mechanisms for the synthesis of Ti₃SiC₂ by arc melting is that Ti₃SiC₂ might be formed from TiC_x + Si, Ti₅Si₃C_x + C, and Ti₅Si₃C_x + TiC_x at early arcing time (≤ 10 s), while TiC_x + TiSi₂ take place at 15 s to 80 s. After 80 s, decomposition of Ti₃SiC₂ into TiC_x, TiSi₂ and C was observed.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

Optimization of the Ti3SiC2 MAX phase synthesis

The synthesis of Ti₃SiC₂ MAX phase by self-propagating high-temperature synthesis (SHS) and pressureless argon shielding synthesis has been investigated following different pathways pertaining to the reactant systems Ti/Si/C, Ti/SiC/C and Ti/TiC/Si. Silicon in excess ranging from 10 to 50 mol% was employed to obtain powders mainly constituted by Ti₃SiC₂.Optimizing the excess of silicon and the pressing technique, the resultant powders with Ti₃SiC₂ content near to 100% were obtained. Result was consequent to the use of pressureless argon shielding synthesis obtained with 30 mol% of silicon excess in the examined different systems. The Ti₃SiC₂ was also obtained by SHS, but with lower proportion (88% and 86% from 3Ti + 1.2SiC + 0.8C and 3Ti + 1.3Si + 2C respectively). These results driving from XRD patterns were confirmed by FESEM observations and the EDAX analyses.     

Spark plasma sintering of pure TiCN: Densification mechanism,grain growth and mechanical properties

This study aims to disclose the densification mechanism and grain growth behaviors during the spark plasma sintering (SPS) of undoped TiCN powder. The SPS experiments were performed under temperatures ranging from 1600 °C to 2200 °C and a fixed pressure of 50 MPa. The sintering mechanisms were described in different models according to two grain growth behaviors: densification without grain growth at low temperatures (1600–1700 °C) and grain growth without apparent densification at higher temperatures (1800–2200 °C). At the constant grain stage, a creep model is applied to describe the densification process. In addition, the effective stress exponents, n, are calculated, indicating that the densification can be attributed to both grain boundary sliding (n = 1.5) and dislocation climbing (n = 3.13 or n = 4.29). During the second stage of sintering, the grain growth model reveals that the grain-growth is controlled by grain boundary diffusion. In addition, the Vickers hardness varies from 4326 Hv to 6762 Hv when the density ranges from 90% to 96.3%.