Improvement in microstructure and mechanical properties of Ti(C,N) cermet prepared by two-step spark plasma sintering

A fine grained Ti(C, N) cermet tool material was prepared by two-step spark plasma sintering. Microstructure evolution and densification mechanisms of Ti(C, N) during spark plasma sintering were studied. Effect of two-step sintering process and Ni content on microstructure and mechanical properties were also investigated. The critical activated densification temperature of Ti(C, N) is about 1300?℃, and the rapidest densification rate takes place at 1300?℃~1400?℃. Grains are in the size of 1?µm when the Ti(C, N) cermet was prepared by two-step spark plasma sintering. The optimal flexural strength, fracture toughness and Vickers hardness are 1094?±?42?MPa, 7.2?±?0.5?MPa?m^1/2 and 18.3?±?0.4?GPa, respectively. The Ti(C, N) cermets containing more content of Ni have higher toughness, which is due to the remarkable toughening effect of crack bridging by large grains.     

Densification mechanism,microstructure and mechanical properties of ZrC ceramics prepared by high-pressure spark plasma sintering

The traditional way of densifying high-melting-point ceramics at high temperatures with long soaking time leads to severe grain coarsening, which degrades the mechanical properties of ceramics. Here, highly dense (∼98%) zirconium carbide (ZrC) ceramics with limited grain growth were obtained by spark plasma sintering (SPS) at relatively low temperatures, 1900 ℃, with a high pressure up to 200 MPa in a reliable carbon-fiber-reinforced carbon composite (C_f/C) mold. Subgrains and high-density dislocations formed in the high-pressure sintered ceramics. The hardness and fracture toughness of the prepared highly dense ZrC ceramics reached 20.53 GPa and 2.70 MPa·m^1/2, respectively. The densification mechanism was mainly plastic deformation under high pressure. In addition, ZrC ceramics sintered at high pressure possessed a high dislocation density of 7.30 × 10¹² m⁻², which was suggested to contribute to the high hardness.     

Effects of high-energy ball milling and reactive spark plasma sintering on the densification of HfC-SiC composites

The combined effects of high-energy ball milling (HEBM) and reactive spark plasma sintering (R-SPS) of HfSi₂ and C powder mixture on the densification and microstructure of nanostructured HfC-SiC composites were investigated. HEBM significantly promoted the densification and improved the microstructure of the HfC-SiC composites. In contrast, the reactions between HfSi₂ and C did not directly promote the densification of the HfC-SiC composites. While the reaction was mostly completed at 1300 °C, the onset temperature of significant densification was 1610 °C. Fine and homogeneously distributed HfC and SiC particles formed by HEBM and R-SPS were the key factors for promoting the densification of the HfC-SiC composites. The fine particles had high surface energy, which provided enough driving force for densification. In addition, the homogeneously distributed SiC particles effectively suppressed the growth of HfC matrix grains during densification.     

Densification,microstructures, and mechanical properties of (Zr,Ti)(C,N) ceramics fabricated by spark plasma sintering

Dense (Zr, Ti) (C, N) ceramics were fabricated by spark plasma sintering (SPS) at 1900–2000 °C using ZrC, TiCN and ZrH₂ powders as raw materials. A single Zr-rich (Zr, Ti)(C, N) solid solution was formed in Zr_0.95Ti_0.05C_0.975N_0.025 and Zr_0.80Ti_0.20C_0.90N_0.10 ceramics (nominal composition). A Ti-rich solid solution appears in Zr_0.50Ti_0.50C_0.75N_0.25 ceramics. The coaddition of TiCN and ZrH₂ promoted the densification of (Zr, Ti) (C, N) ceramics by forming solid solutions and carbon vacancies, which could reduce critical resolved shear stress (CRSS) and promote carbon and metal atom diffusion. ZrC-45 mol% TiCN-10 mol% ZrH₂ (raw powder composition) possesses good comprehensive mechanical properties (Vickers hardness of 24.5 ± 0.9 GPa, flexural strength of 503 ± 51 MPa, and fracture toughness of 4.3 ± 0.2 MPa·m^1/2), which reach or exceed most ZrC-based (Zr, Ti) C and (Zr, Ti) (C, N) ceramics in previous reports.     

Thermoelectric properties of La7Mo7O30 sintered by reactive spark plasma sintering

In this work, spark plasma sintering of La₂Mo₂O₉ powder was used to achieve dense ceramics of La₇Mo₇O₃₀ and explore their thermoelectric properties. SPS sintering of La₂Mo₂O₉ powder at 973 K for 10 min under 90 MPa leads to a bicoloured sample with white and black faces. XRD patterns of white and black faces are attributed to La₂Mo₂O₉ and La₇Mo₇O₃₀ phases, respectively. These experimental conditions allow observing the in-situ reduction of La₂Mo₂O₉ during the SPS process. With a longer sintering time of 30 min, a ceramic of La₇Mo₇O₃₀ is obtained. Its electrical conductivity exhibits a semiconducting behaviour and reaches a value of 1000 Sm^-1 at 1000 K. The negative Seebeck coefficient show a n-type conduction in this phase. La₇Mo₇O₃₀ exhibits a very low thermal conductivity, less than 1 Wm⁻¹K⁻¹ from room temperature up to 1000 K, similar to the values reported for La₂Mo₂O₉. A figure of merit of 0.04 is reached at 1000 K.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

In-situ graphene platelets formation and its suppression during reactive spark plasma sintering of boron carbide/titanium diboride composites

Boron carbide composites with 10 vol.% TiB₂ were prepared by reactive sintering of B₄C, TiO₂, and carbon black powder mixture at the temperature of 1800 °C, under a pressure of 70 MPa in a vacuum. The combined effects of electric current and in-situ reactions led to a significant overheating of the central part of the sample, while no overheating was observed for hot press and non-reactive SPS processes. A lower electrical resistivity of TiB₂ produced a significant Joule heating of boron carbide, leading to its partial decomposition to form gaseous boron and graphene platelets. Homogenous, fully dense and graphene-free samples were obtained when employing an insulating Al₂O₃ paper during reactive SPS. A short dwell time (30 s after a degassing step of 6 min) and the uniform distribution of fine TiB₂ grains were the main advantages of isolated SPS over the reactive hot press and SPS processes, respectively.     

Microstructure and properties of NiAl/TiC composite synthesized by spark plasma sintering of mechanically activated elemental powders

In this study, NiAl/TiC_0.95 composite was synthesized by reactive spark plasma sintering of mechanically activated elemental powders. The microstructure and properties of activated powders and sintered samples were evaluated. The elemental powders were milled after different milling times and as-mixed and 10 h milled powder mixtures were sintered by the reactive spark plasma sintering method. The phase and the microstructure changes were evaluated by x-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy, respectively. The XRD pattern of 0 h milled powder after sintering showed that Ni₃Al, Ni₂Al₃ beside NiAl and TiC_0.75 formed. While after the sintering of 10 h mechanically activated powder, the Ni₃Al and Ni₂Al₃ were eliminated and NiAl remained with TiC_0.95. The nanoindentation result of the SPSed sample showed a hardness of 12.2 ± 0.1 GPa with an elastic modulus of 25.0 ± 0.5 GPa.     

Effect of solid solution formation on densification of spark plasma sintered ZrC ceramics with TiC as sintering aid

The densification of ZrC ceramics doped with different contents of TiC prepared by spark plasma sintering at the temperatures between 1750 and 1850°C has been investigated. The microstructure and mechanical properties of the ceramics have been characterised. It was shown that TiC additions effectively promoted the densification process by forming (Zr,Ti)C solid solution. The relative densities and mechanical properties of ZrC samples increased with the increasing of TiC content or the sintering temperature. Ceramic with the content of TiC up to 10 vol.-% sintering at 1850°C showed an excellent combination of properties including a relative density of 98.7%, hardness of 20.8?GPa and flexural strength of 605?MPa.     

Microstructure and mechanical properties of (Ti,W)C cermets prepared by ultrafast spark plasma sintering

To explore the impact of the sintering rate on the microstructure and mechanical properties of cermets, the preparation of (Ti,W)C cermets by ultrafast sintering via spark plasma sintering (SPS) is reported. Compared with a slow heating rate, the electric field produced by an ultrafast heating rate enhances the liquid phase mass transfer of the metal binder phase, thus achieving rapid densification of (Ti,W)C cermets and effectively inhibiting abnormal grain growth. However, an excessive heating rate will lead to an “overflow” phenomenon, which reduces the grain growth difficulty and the bonding strength between grains. The results show that when the heating rate is 500 °C/min, the liquid phase mass transfer is moderate, the densification degree is the highest and the mechanical properties are excellent. The flexural strength, Vickers hardness and fracture toughness are 1340.90 ± 23.55 MPa, 18.42 ± 0.46 GPa and 11.96 ± 0.23 MPa?m^1/2, respectively.     

Bulk boron carbide nanostructured ceramics by reactive spark plasma sintering

Bulk boron carbide (B₄C) ceramics was fabricated from a boron and carbon mixture by use of one-step reactive spark plasma sintering (RSPS). It was also demonstrated that preliminary high-energy ball milling (HEBM) of the B+C powder mixture leads to the formation of B/C composite particles with enhanced reactivity. Using these reactive composites in RSPS permits tuning of synthesized B₄C ceramic microstructure. Optimization of HEBM + RSPS conditions allows rapid (less than 30 min of SPS) fabrication of B₄C ceramics with porosity less than 2%, hardness of ~35 GPa and fracture toughness of ~ 4.5 MPa m ^1/2     

Densification behaviour and physico-mechanical properties of porcelains prepared using spark plasma sintering

Porcelain powder was consolidated using spark plasma sintering (SPS) at a constant heating rate of 100°C?min^?1 to peak temperatures ranging from 1000 to 1200°C and was observed to sinter at relatively low temperature ～920°C under the SPS conditions while conventional sintering requires ～1050°C. SPS produced densification rates about 10 times greater than conventional sintering. The dwelling step at the optimal peak temperature was negligible due to rapid flow of the molten glass assisted by applied pressure. SPSed samples exhibited denser microstructures, resulting in improved physico-mechanical properties compared with conventionally sintered samples such as apparent bulk density improved from 2.38 to 2.48?g?cm^?3, Vickers hardness improved from 3–5 to 6–7?GPa, and fracture toughness improved from 2–3 to 4–6?MPa?m^1/2.     

Novel single phase (Ti0.2W0.2Ta0.2Mo0.2V0.2)C0.8 high entropy carbide using ball milling followed by reactive spark plasma sintering

Single phase novel (Ti_0.2W_0.2Ta_0.2Mo_0.2V_0.2)C_0.8 high entropy carbide (HEC) compacts were successfully synthesized by reactive spark plasma sintering of ball milled metal-carbon elemental mixture at temperatures of 1400−1800 °C. X-ray diffraction and element distribution maps indicated single phase carbide formation with lattice parameter ranging from 4.307 Å to 4.312 Å with small amount of TiO₂. X-ray energy dispersive spectroscopy (EDS) mapping showed uniform distribution of the transition metals in the carbide phase. The microhardness, elastic modulus, fracture toughness, electrical resistivity and thermal expansion coefficient (25 °C–600 °C) of the compact sintered at 1800 °C were found to be 25.8 ± 2.8 GPa, 461 ± 36 GPa, 3.7 ± 0.4 MPa.m^1/2, 7 × 10⁻⁴ Ω/m² and 7 × 10^-6 K⁻¹ respectively.     

Pressure-enhanced densification of TaC ceramics during flash spark plasma sintering

Flash spark plasma sintering (FSPS) offers extremely high heating rates to consolidate ceramics at a short time. However, significant grain growth sometimes occurs accompanied by rapid densification. In this work, a FSPS apparatus available for applying pressure was used to sinter TaC ceramics from powder compacts without preheating. It is found that the use of a higher pressure can efficiently promote densification and retard significant grain growth. Dense bulk TaC ceramics (95.18%) with average grain size of 4.09 μm were obtained in 90 seconds under 80 MPa. Such a process should facilitate the fast preparation of refractory ceramics with fine-grained microstructure.     

Microstructure and properties of WC-8Co cemented carbides prepared by multiple spark plasma sintering

In this study, WC-8Co cemented carbides were prepared by spark plasma sintering. When the samples sintered at 1300℃ were cooled to room temperature, the samples were sintered multiple times at 1250℃. The changes in microstructure and mechanical properties of WC-8Co cemented carbides prepared by multiple spark plasma sintering were studied. The hardness of cemented carbides increased in the first two sintering, reaching 16.5 GPa. However, the hardness decreased seriously in the last two sintering. The attenuation rates of hardness were 6.2% and 2.5% due to the abnormal coarse grains. Furthermore, the crack path along the grain boundary was almost straight, causing a decrease in the indentation fracture toughness of cemented carbides. Additionally, the grains of cemented carbides were abnormally coarsened, and the morphologies of grains became unstable due to multiple sintering.     

Spark plasma sintering of WC-based cermets/titanium and vanadium added composites: A comparative study on the microstructure and mechanical properties

In an attempt to develop the composition and properties of W₂C-(W,Ti)C-TiC and WC-WC_1-x-VC-V super hardmetals, spark plasma sintering (SPS) method was implemented. WC powders were mixed separately with 10?wt% Ti and 10?wt% V in a high energy mixer mill and sintering processes were performed at temperatures of 2150 and 2000?°C, respectively. XRD investigations revealed the formations of TiC and (Ti,W)C as the reaction products in WC-10?wt% Ti composite. Moreover, the interfacial reaction between WC and V led to the formation of WC1-x and VC compounds. A higher bending strength (613?±?25?MPa) and fracture toughness (4.1?±?0.58?MPa?m^1/2) were obtained for WC-10?wt% V samples compared to WC-10?wt% Ti, While the WC-10?wt% Ti composite showed a higher value of hardness (3128?±?42 Vickers) in comparison to WC-10?wt% V (2632?±?39 Vickers), which can act as a super hard cermet.     

Effect of short carbon fiber content on the mechanical properties of TiB2-based composites prepared by spark plasma sintering

In this study, TiB₂-30 vol% SiC composites containing 0, 5, 10, and 15 vol% short carbon fibers (C_f) were produced by spark plasma sintering (SPS). The effect of carbon fiber content on microstructure, density, and mechanical properties (micro-hardness and flexural strength) of the fabricated composites was studied. Scanning electron microscopy (SEM) results indicated that the fibers were uniformly dispersed in the TiB₂–SiC matrix using wet ball milling before SPS process. Fully dense TiB₂–SiC–C_f composites were achieved by SPS process at 1900°C for 10 min under 30 MPa. With the addition of fibers, the relative density of the composites did not change considerably. Mechanical tests revealed that microhardness was reduced about 19% by the incorporation of carbon fibers, whereas the flexural strength improved significantly. However, the flexural strength diminished by adding carbon fibers above to critical value (5 vol%) due to residual thermal stresses, nonhomogeneous structure and graphitization of carbon fibers. It was found that the composite with 5 vol% C_f had the highest flexural strength (482 MPa), which was enhanced by 20% compared with the TiB₂–SiC composite.     

Densification, microstructure and mechanical properties of SiO2-cBN composites by spark plasma sintering

SiO₂-cBN composites were consolidated by spark plasma sintering at 1473-1973 K. The effects of cBN content and sintering temperature on the relative density, phase transformation, microstructure and mechanical properties of the SiO₂-cBN composites were investigated. The relative density of the SiO₂-cBN composites increased with increasing SiO₂ content. The phase transformation of cBN to hBN in SiO₂-cBN composites was identified at 1973 K, showing the highest transformation temperature in cBN-containing composites. The SiO₂-20 vol% cBN composites sintered at 1673 K showed the highest hardness and fracture toughness of 12.5 GPa and 1.5 MPa m^1/2, respectively.     

Joining of C/SiC composites by spark plasma sintering technique

CVD–SiC coated C/SiC composites (C/SiC) were joined by spark plasma sintering (SPS) by direct bonding with and without the aid of joining materials. A calcia-alumina based glass–ceramic (CA), a SiC + 5 wt% B₄C mixture and pure Ti foils were used as joining materials in the non-direct bonding processes. Morphological and compositional analyses were performed on each joined sample. The shear strength of joined C/SiC was measured by a single lap test and found comparable to that of C/SiC.     

Competition between densification and microstructure development during spark plasma sintering of B4C–Eu2O3

The densification of nonoxide ceramics has been a known challenge in the field of engineering ceramics. The amount and type of sinter‐aid together with sintering conditions significantly influence the densification behavior and microstructure in nonoxide ceramics. In this perspective, the present work reports the use of Eu₂O₃ sinter‐aid and spark plasma sintering towards the densification of B₄C. The densification is largely influenced by the solid‐state sintering reactions during heating to 1900°C. Based on the careful analysis of the heat‐treated powder mixture (B₄C–Eu₂O₃) and sintered compacts, the competitive reaction pathways are proposed to rationalize the formation of EuB₆ as dominant microstructural phase. An array of distinctive morphological features, including intragranular and intergranular EuB₆ phase as well as characteristic defect structures (asymmetric twins, stacking faults and threaded dislocations) are observed within dense B₄C matrix. An attempt has been made to explain the competition between microstructure development and densification.