Study of the Influence of Sintering Atmosphere and Mechanical Activation on the Synthesis of Bulk Ti2AlN MAX Phase Obtained by Spark Plasma Sintering

The influence of the mechanical activation process and sintering atmosphere on the microstructure and mechanical properties of bulk Ti₂AlN has been investigated. The mixture of Ti and AlN powders was prepared in a 1:2 molar ratio, and a part of this powder mixture was subjected to a mechanical activation process under an argon atmosphere for 10 h using agate jars and balls as milling media. Then, the sintering and production of the Ti₂AlN MAX phase were carried out by Spark Plasma Sintering under 30 MPa with vacuum or nitrogen atmospheres and at 1200 °C for 10 min. The crystal structure and microstructure of consolidated samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. The X-ray diffraction patterns were fitted using the Rietveld refinement for phase quantification and determined their most critical microstructural parameters. It was determined that by using nitrogen as a sintering atmosphere, Ti₄AlN₃ MAX phase and TiN were increased at the expense of the Ti₂AlN. In the samples prepared from the activated powders, secondary phases like Ti₅Si₃ and Al₂O₃ were formed. However, the higher densification level presented in the sample produced by using both nitrogen atmosphere and MAP powder mixture is remarkable. Moreover, the high-purity Ti₂AlN zone of the MAX-1200 presented a hardness of 4.3 GPa, and the rest of the samples exhibited slightly smaller hardness values (4.1, 4.0, and 4.2 GPa, respectively) which are matched with the higher porosity observed on the SEM images.     

Formation Mechanism of High-Purity Ti2AlN Powders under Microwave Sintering

In the present study, high-purity ternary-phase nitride (Ti₂AlN) powders were synthesized through microwave sintering using TiH₂, Al, and TiN powders as raw materials. X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were adopted to characterize the as-prepared powders. It was found that the Ti₂AlN powder prepared by the microwave sintering of the 1TiH₂/1.15Al/1TiN mixture at 1250 °C for 30 min manifested great purity (96.68%) with uniform grain size distribution. The formation mechanism of Ti₂AlN occurred in four stages. The solid-phase reaction of Ti/Al and Ti/TiN took place below the melting point of aluminum and formed Ti₂Al and TiN_0.5 phases, which were the main intermediates in Ti₂AlN formation. Therefore, the present work puts forward a favorable method for the preparation of high-purity Ti₂AlN powders.     

Synthesis and Tribological Characterization of Ti3SiC2/ZnO Composites

Dense Ti₃SiC₂/ZnO composites were sintered at different temperatures by spark plasma sintering (SPS). The effects of sintering temperature on composition and mechanical properties of Ti₃SiC₂/ZnO composites were studied. The tribological behaviors of Ti₃SiC₂/ZnO composites/Inconel 718 alloy tribo-pairs at elevated temperature from 25 °C to 800 °C were discussed. The experimental results showed that the initial decomposition temperature of the Ti₃SiC₂/ZnO composite was 1150 °C, and Ti₃SiC₂ decomposed into TiC. When the decomposition temperature was higher than 1150 °C, the compositions of the Ti₃SiC₂/ZnO composites were Ti₃SiC₂, ZnO, and TiC. It was found that Ti₃SiC₂/ZnO composites had better self-lubricating performance than Ti₃SiC₂ at elevated temperature from 600 °C to 800 °C, which was owing to material transfers of tribo-pairs and sheared oxides generated by tribo-oxidation reactions.     

Effect of Volume Fraction of Reinforcement on Microstructure and Mechanical Properties of In Situ (Ti,Nb)B/Ti2AlNb Composites with Tailored Three-Dimensional Network Architecture

The mechanical properties of (Ti, Nb)B/Ti₂AlNb composites were expected to improve further by utilizing spark plasma sintering (SPS) and inducing the novel three-dimensional network architecture. In this study, (Ti, Nb)B/Ti₂AlNb composites with the novel architecture were successfully fabricated by ball milling the LaB₆ and Ti₂AlNb mixed powders and subsequent SPS consolidation. The influence of the (Ti, Nb)B content on the microstructure and mechanical properties of the composites was revealed by using the scanning electron microscope (SEM), transmission electron microscopy (TEM) and electronic universal testing machine. The microstructural characterization demonstrated that the boride crystallized into a B27 structure and the α₂-precipitated amount increased with the (Ti, Nb)B increasing. When the (Ti, Nb)B content reached 4.9 vol%, both the α₂ and reinforcement exhibited a continuous distribution along the prior particle boundaries (PPBs). The tensile test displayed that the tensile strength of the composites presented an increasing trend with the increasing (Ti, Nb)B content followed by a decreasing trend. The composite with a 3.2 vol% reinforcement had the optimal mechanical properties; the yield strengths of the composite at 25 and 650 °C were 998.3 and 774.9 MPa, showing an 11.8% and 9.2% improvement when compared with the Ti₂AlNb-based alloy. Overall, (Ti, Nb)B possessed an excellent strengthening effect and inhibited the strength weakening of the PPBs area at high temperatures; the reinforcement content mainly affected the mechanical properties of the (Ti, Nb)B/Ti₂AlNb composites by altering the α₂-precipitated amount and the morphology of (Ti, Nb)B in the PPBs area. Both the continuous precipitation of the brittle α₂ phase and the agglomeration of the (Ti, Nb)B reinforcement dramatically deteriorated the mechanical properties.     

Sliding Wear Performance of AlCrN Coating on TiB2/Ti Composites at High Temperatures

The aim of the study was to investigate effect of Ti/TiB₂ composite composition and manufacturing technology parameters on the tribological behaviour of AlCrN coating-composite system. The AlCrN coating was deposited by PVD (Physical Vapour Deposition) method. The composites were manufactured by spark plasma sintering (SPS) from three variants of powders mixtures: Ti with TiB₂, Ti6Al4V with TiB₂ as well as Ti with B, using (five) different sintering temperatures. For each of the developed coating-composite systems, the wear resistance was evaluated using ball-on-disc SRV tester, at six temperatures (from room temperature up to 900 °C). The results confirmed that high-temperature wear resistance of the coating–substrate combination depends on Ti/TiB₂ composite composition and manufacturing technology parameters. In the case of uncoated composite, two processes manage the wear at high temperatures: cracking propagation and surface oxidation. The presence of AlCrN coating on the composite surface protects the surface against deep cracking and surface oxidation. The composites of Group I, sintered at 1250 °C from a mixture of pure Ti and TiB₂ (50/50 wt.% ratio) as well as Group III, sintered at 1350 °C from a mixture of pure Ti and B allow the achievement of a satisfactory surface quality, a high adhesion of the PVD coating and moderate wear at high temperatures. However, the composite made of pure Ti and B seems to be a better solution for temperatures exceeding 600 °C.     

Microstructure and Tribological Properties of Spark-Plasma-Sintered Ti3SiC2-Pb-Ag Composites at Elevated Temperatures

Ti₃SiC₂-PbO-Ag composites (TSC-PA) were successfully prepared using the spark plasma sintering (SPS) technique. The ingredient and morphology of the as-synthesized composites were elaborately investigated. The tribological properties of the TSC-PA pin sliding against Inconel 718 alloys disk at room temperature (RT) to 800 °C were examined in air. The wear mechanisms were argued elaborately. The results showed that the TSC-PA was mainly composed of Ti₃SiC₂, Pb, and Ag. The average friction coefficient of TSC-PA gradually decreased from 0.72 (RT) to 0.3 (800 °C), with the temperature increasing from RT to 800 °C. The wear rate of TSC-PA showed a decreasing trend, with the temperature rising from RT to 800 °C. The wear rate of Inconel 718 exhibited positive wear at RT and negative wear at elevated temperatures. The tribological property of TSC-PA was related to the tribo-chemistry, and the abrasive and adhesive wear.     

Novel Composite Nitride Nanoceramics from Reaction-Mixed Nanocrystalline Powders in the System Aluminum Nitride AlN/Gallium Nitride GaN/Titanium Nitride TiN (Al:Ga:Ti = 1:1:1)

A study is presented on the synthesis of reaction-mixed nitride nanopowders in the reference system of aluminium nitride AlN, gallium nitride GaN, and titanium nitride TiN (Al:Ga:Ti = 1:1:1) followed by their high-pressure and high-temperature sintering towards novel multi-nitride composite nanoceramics. The synthesis starts with a 4 h reflux in hexane of the mixture of the respective metal dimethylamides, which is followed by hexane evacuation, and reactions of the residue in liquid ammonia at −33 °C to afford a mixed metal amide/imide precursor. Plausible equilibration towards a bimetallic Al/Ga-dimethylamide compound upon mixing of the solutions of the individual metal-dimethylamide precursors containing dimeric {Al[N(CH₃)₂]₃}₂ and dimeric {Ga[N(CH₃)₂]₃}₂ is confirmed by ¹H- and ¹³C{H}-NMR spectroscopy in C₆D₆ solution. The precursor is pyrolyzed under ammonia at 800 and 950 °C yielding, respectively, two different reaction-mixed composite nitride nanopowders. The latter are subjected to no-additive high-pressure and high-temperature sintering under conditions either conservative for the initial powder nanocrystallinity (650 °C, 7.7 GPa) or promoting crystal growth/recrystallization and, possibly, solid solution formation via reactions of AlN and GaN towards Al_0.5Ga_0.5N (1000 and 1100 °C, 7.7 GPa). The sintered composite pellets show moderately high mechanical hardness as determined by the Vicker’s method. The starting nanopowders and resulting nanoceramics are characterized by powder XRD, Raman spectroscopy, and SEM/EDX. It is demonstrated that, in addition to the multi-nitride composite nanoceramics of hexagonal AlN/hexagonal GaN/cubic TiN, under specific conditions the novel composite nanoceramics made of hexagonal Al_0.5Ga_0.5N and cubic TiN can be prepared.     

Microstructure and Properties of TiB2 Composites Produced by Spark Plasma Sintering with the Addition of Ti5Si3

Titanium diboride (TiB₂) is a hard, refractory material, attractive for a number of applications, including wear-resistant machine parts and tools, but it is difficult to densify. The spark plasma sintering (SPS) method allows producing TiB₂-based composites of high density with different sintering aids, among them titanium silicides. In this paper, Ti₅Si₃ is used as a sintering aid for the sintering of TiB₂/10 wt % Ti₅Si₃ and TiB₂/20 wt % Ti₅Si₃ composites at 1600 °C and 1700 °C for 10 min. The phase composition of the initial powders and produced composites was analyzed by the X-ray diffraction method using CuK_α radiation. The microstructure was examined using scanning electron microscopy, accompanied by energy-dispersive spectroscopy (EDS). The hardness was determined using a diamond indenter of Vickers geometry loaded at 9.81 N. Friction–wear properties were tested in the dry sliding test in a ball-on-disc configuration, using WC as a counterpart material. The major phases present in the TiB₂/Ti₅Si₃ composites were TiB₂ and Ti₅Si_3. Traces of TiC were also identified. The hardness of the TiB₂/Ti₅Si₃ composites was in the range of 1860–2056 HV1 and decreased with Ti₅Si₃ content, as well as the specific wear rate W_v. The coefficient of friction for the composites was in the range of 0.5–0.54, almost the same as for TiB₂ sinters. The main mechanism of wear was abrasive.     

The Role of Sintering Temperature and Dual Metal Substitutions (Al3+, Ti4+) in the Development of NASICON-Structured Electrolyte

The aim of this study is to synthesize Li_1+xAl_xTi_xSn_2−2x(PO₄) sodium super ion conductor (NASICON) -based ceramic solid electrolyte and to study the effect of dual metal substitution on the electrical and structural properties of the electrolyte. The performance of the electrolyte is analyzed based on the sintering temperature (550 to 950 °C) as well as the composition. The trend of XRD results reveals the presence of impurities in the sample, and from Rietveld Refinement, the purest sample is achieved at a sintering temperature of 950 °C and when x = 0.6. The electrolytes obey Vegard′s Law as the addition of Al³⁺ and Ti⁴⁺ provide linear relation with cell volume, which signifies a random distribution. The different composition has a different optimum sintering temperature at which the highest conductivity is achieved when the sample is sintered at 650 °C and x = 0.4. Field emission scanning electron microscope (FESEM) analysis showed that higher sintering temperature promotes the increment of grain boundaries and size. Based on energy dispersive X-ray spectroscopy (EDX) analysis, x = 0.4 produced the closest atomic percentage ratio to the theoretical value. Electrode polarization is found to be at maximum when x = 0.4, which is determined from dielectric analysis. The electrolytes follow non-Debye behavior as it shows a variety of relaxation times.     

Preparation and Properties of (YCa)(TiMn)O3?δ Ceramics Interconnect of Solid Oxide Fuel Cells

(YCa)(TiMn)O_3–δ ceramics prepared using a reaction-sintering process were investigated. Without any calcination involved, the mixture of raw materials was pressed and sintered directly. Y₂Ti₂O₇ instead of YTiO₃ formed when a mixture of Y₂O₃ and TiO₂ with Y/Ti ratio 1/1 were sintered in air. Y₂Ti₂O₇, YTiO_2.085 and some unknown phases were detected in Y_0.6Ca_0.4Ti_0.6Mn_0.4O_3–δ. Monophasic Y_0.6Ca_0.4Ti_0.4Mn_0.6O_3–δ ceramics were obtained after 1400–1500 °C sintering. Dense Y_0.6Ca_0.4Ti_0.4Mn_0.6O_3–δ with a density 4.69 g/cm³ was observed after 1500 °C/4 h sintering. Log σ for Y_0.6Ca_0.4Ti_0.6Mn_0.4O_3–δ increased from –3.73 Scm^–1 at 350 °C to –2.14 Scm^–1 at 700 °C. Log σ for Y_0.6Ca_0.4Ti_0.4Mn_0.6O_3–δ increased from –2.1 Scm^–1 at 350 °C to –1.36 Scm^–1 at 700 °C. Increasing Mn content decreased activation energy E_a and increased electrical conductivity. Reaction-sintering process is proved to be a simple and effective method to obtain (YCa)(TiMn)O_3–δ ceramics for interconnects in solid oxide fuel cells.     

The Relationship between Microstructure and Fracture Behavior of TiAl/Ti2AlNb SPDB Joint with High Temperature Titanium Alloy Interlayers

In this paper, spark plasma diffusion bonding technology was employed to join TiAl and Ti₂AlNb with high temperature titanium alloy interlayer at 950 °C/10kN/60 min, then following furnace cooling at cooling rate up to 100 °C/min. After welding, the joint was aging heat-treated at 800 °C for 24 h. The microstructure and the elements diffusion of the TiAl/Ti₂AlNb joint was analyzed by field emission scanning electron microscopy (FESEM) with EDS. Moreover, the tensile properties of the joint were tested at room temperature, 650 °C, and 750 °C. The results show that the spark plasma diffusion bonding formed a high quality TiAl/Ti₂AlNb joint without microcracks or microvoids, while also effectively protecting the base metal. Significant differences in the microstructure of the joint appeared from TiAl side to Ti₂AlNb side: TiAl BM (Base Metal) → DP(Duplex) and NG (Near-Gamma) → α₂-phase matrix with needle-like α-phase → bulk α₂-phase → needle-like α-phase → metastable β-phase → Ti₂AlNb BM. After heat treatment at 800 °C for 24 h, the microstructure of the TiAl side and the interlayer region did not change, but the density and size of the needle-like α-phase in region 3 increased slightly. The microstructure of Ti₂AlNb near the weld changed obviously, and a large number of fine O phases are precipitated from the metastable β phase matrix after heat treatment. Except for the Ti₂AlN near-interface region, the effect of heat treatment on the microstructure of the joint is not significant. The microhardness of the joint is in the shape of a mountain peak. The maximum microhardness at the interface is above 500 HV, and it is significantly reduced to 400 HV after heat treatment. The fracture of the joint occurred at the interface at room temperature, 650 °C, and 750 °C. with the tensile strength 450 MPa, 540 MPa, and 471 Mpa, respectively, and mainly showing brittle fracture.     

Influence of MXene (Ti3C2) Phase Addition on the Microstructure and Mechanical Properties of Silicon Nitride Ceramics

This paper discusses the influence of Ti₃C₂ (MXene) addition on silicon nitride and its impact on the microstructure and mechanical properties of the latter. Composites were prepared through powder processing and sintered using the spark plasma sintering (SPS) technic. Relative density, hardness and fracture toughness, were analyzed. The highest fracture toughness at 5.3 MPa·m^1/2 and the highest hardness at HV5 2217 were achieved for 0.7 and 2 wt.% Ti₃C₂, respectively. Moreover, the formation of the Si₂N₂O phase was observed as a result of both the MXene addition and the preservation of the α-Si₃N₄→β-Si₃N₄ phase transformation during the sintering process.     

Sintering Temperature,Frequency, and Temperature Dependent Dielectric Properties of Na0.5Sm0.5Cu3Ti4O12 Ceramics

NSCTO (Na_0.5Sm_0.5Cu₃Ti₄O₁₂) ceramics have been prepared by reactive sintering solid-state reaction where the powder was prepared from the elemental oxides by mechanochemical milling followed by conventional sintering in the temperature range 1000–1100 °C. The influence of sintering temperature on the structural and dielectric properties was thoroughly studied. X-ray diffraction analysis (XRD) revealed the formation of the cubic NSCTO phase. By using the Williamson–Hall approach, the crystallite size and lattice strain were calculated. Scanning electron microscope (SEM) observations revealed that the grain size of NSCTO ceramics is slightly dependent on the sintering temperature where the average grain size increased from 1.91 ± 0.36 μm to 2.58 ± 0.89 μm with increasing sintering temperature from 1000 °C to 1100 °C. The ceramic sample sintered at 1025 °C showed the best compromise between colossal relative permittivity (ε′ = 1.34 × 10³) and low dielectric loss (tanδ = 0.043) values at 1.1 kHz and 300 K. The calculated activation energy for relaxation and conduction of NSCTO highlighted the important role of single and double ionized oxygen vacancies in these processes.     

Synthesis of Li2Ti3O7 Anode Materials by Ultrasonic Spray Pyrolysis and Their Electrochemical Properties

Ramsdellite-type lithium titanate (Li₂Ti₃O₇) powders were synthesized by performing ultrasonic spray pyrolysis, and their chemical and physical properties were characterized by performing Scanning Electron Microscope (SEM), powder X-ray Diffraction (XRD), and Inductively Coupled Plasma (ICP) analyses. The as-prepared Li₂Ti₃O₇ precursor powders had spherical morphologies with hollow microstructures, but an irregularly shaped morphology was obtained after calcination above 900 °C. The ramsdellite Li₂Ti₃O₇ crystal phase was obtained after the calcination at 1100 °C under an argon/hydrogen atmosphere. The first rechargeable capacity of the Li₂Ti₃O₇ anode material was 168 mAh/g at 0.1 C and 82 mAh/g at 20 C, and the discharge capacity retention ratio was 99% at 1 C after the 500th cycle. The cycle performance of the Li₂Ti₃O₇ anode was also highly stable at 50 °C, demonstrating the superiority of Li₂Ti₃O₇ anode materials reported previously.     

Microstructure and Properties of Densified Gd2O3 Bulk

In this work, Gd₂O₃ bulks were sintered at temperatures ranging from 1400 °C to 1600 °C for times from 6 h to 24 h, and their microstructure and properties were studied for a wider application of materials in thermal barrier coatings. The densification of the Gd₂O₃ bulk reached 96.16% when it was sintered at 1600 °C for 24 h. The elastic modulus, hardness, fracture toughness and thermal conductivity of the bulks all increased with the rise in sintering temperature and extension of sintering time, while the coefficient of thermal expansion decreased. When the Gd₂O₃ bulk was sintered at 1600 °C for 24 h, it had the greatest elastic modulus, hardness, fracture toughness and thermal conductivity of 201.15 GPa, 9.13 GPa, 15.03 MPa·m^0.5 and 2.75 W/(m·k) (at 1100 °C), respectively, as well as the smallest thermal expansion coefficients of 6.69 × 10⁻⁶/°C (at 1100 °C).     

Dielectric Properties of Bi2/3Cu3Ti4O12 Ceramics Prepared by Mechanical Ball Milling and Low Temperature Conventional Sintering

In the current study, Bi_2/3Cu₃Ti₄O₁₂ (BCTO) ceramics were prepared by mechanical ball mill of the elemental oxides followed by conventional sintering of the powder without any pre-sintering heat treatments. The sintering temperature was in the range 950–990 °C, which is 100–150 °C lower than the previous conventional sintering studies on BCTO ceramics. All the ceramic samples showed body-centered cubic phase and grain size ≈ 2–6 μm. Sintering temperature in the range 950–975 °C resulted in comparatively lower dielectric loss and lower thermal coefficient of permittivity in the temperature range from −50 to 120 °C. All the BCTO ceramics showed reasonably high relative permittivity. The behavior of BCTO ceramics was correlated with the change in oxygen content in the samples with sintering temperature. This interpretation was supported by the measurements of the energy dispersive x-ray spectroscopy (EDS) elemental analysis and activation energy for conduction and for relaxation in the ceramics.     

Fabrication of (SiC-AlN)/ZrB2 Composite with Nano-Micron Hybrid Microstructure via PCS-Derived Ceramics Route

In this work, a (SiC-AlN)/ZrB₂ composite with outstanding mechanical properties was prepared by using polymer-derived ceramics (PDCs) and hot-pressing technique. Flexural strength reached up to 460 ± 41 MPa, while AlN and ZrB₂ contents were 10 wt%, and 15 wt%, respectively, under a hot-pressing temperature of 2000 °C. XRD pattern-evidenced SiC generated by pyrolysis of polycarbosilane (PCS) was mainly composed by 2H-SiC and 4H-SiC, both belonging to α-SiC. Micron-level ZrB₂ secondary phase was observed inside the (SiC-AlN)/ZrB₂ composite, while the mean grain size (MGS) of SiC-AlN matrix was approximately 97 nm. This unique nano-micron hybrid microstructure enhanced the mechanical properties. The present investigation provided a feasible tactic for strengthening ceramics from PDCs raw materials.     

Injection Molding and Near-Complete Densification of Monolithic and Al2O3 Fiber-Reinforced Ti2AlC MAX Phase Composites

Near-net shape components composed of monolithic Ti₂AlC and composites thereof, containing up to 20 vol.% Al₂O₃ fibers, were fabricated by powder injection molding. Fibers were homogeneously dispersed and preferentially oriented, due to flow constriction and shear-induced velocity gradients. After a two-stage debinding procedure, the injection-molded parts were sintered by pressureless sintering at 1250 °C and 1400 °C under argon, leading to relative densities of up to 70% and 92%, respectively. In order to achieve near-complete densification, field assisted sintering technology/spark plasma sintering in a graphite powder bed was used, yielding final relative densities of up to 98.6% and 97.2% for monolithic and composite parts, respectively. While the monolithic parts shrank isotropically, composite assemblies underwent anisotropic densification due to constrained sintering, on account of the ceramic fibers and their specific orientation. No significant increase, either in hardness or in toughness, upon the incorporation of Al₂O₃ fibers was observed. The 20 vol.% Al₂O₃ fiber-reinforced specimen accommodated deformation by producing neat and well-defined pyramidal indents at every load up to a 30 kgf (~294 N).     

Effect of Cold Working on the Properties and Microstructure of Cu-3.5 wt% Ti Alloy

Cu-Ti alloys were strengthened by β’-Cu₄Ti metastable precipitation during aging. With the extension of the aging time, the β’-Cu₄Ti metastable phase transformed into the equilibrium β-Cu₄Ti phase. The Cu-3.5 wt% Ti(Cu-4.6 at% Ti) alloys with different processing were aged at different temperatures for various times after solution treatment at 880 °C for 1 h. The electrical conductivity of samples under different heat treatments had shown an upward trend as time increased during aging, but the hardness reached the peak value and then decreased. The hardness and electrical conductivity of the samples with 70% deformation after aging are higher tha n the samples without deformation. Deformation after aging would cause the metastable phase to dissolve into a matrix. The best combination value of conductivity and hardness is 13.88% IACS and 340.78 Hv, and the optimal heat treatment is 500 °C for 2 h + 70% deformation + 450 °C for 2 h.