The role of Al in the reactive synthesis of porous Mo2Ti2AlC3 ceramics

The mixed powders of TiH₂, molybdenum, aluminum and graphite with molar ratios of 2/2/n/2.85 (n ranges from 1.0 to 1.4 mol with an interval of 0.1 mol) were used as raw material powders for this work, and a novel porous Mo₂Ti₂AlC₃ was synthesized via reactive synthesis. Through systematic research on the pore structure parameters of porous Mo₂Ti₂AlC₃ prepared with different aluminum content, the results show that there is a clear correlation between the aluminum content and the pore structure parameters. With the aluminum content rising from 1.0 to 1.2 mol, the viscous permeability coefficient and pore size decreased, while the porosity increased; When the aluminum content increased from 1.2 to 1.4 mol, the pore structure parameters of porous Mo₂Ti₂AlC₃ displayed an opposite trend. The reasons for the evolution laws of these pore structure parameters were also discussed in depth. In addition, the pore structure forming mechanism of porous Mo₂Ti₂AlC₃ ceramics during the activation reaction sintering process has been explored. This work can provide an important reference for the subsequent preparation of quaternary porous MAX phase ceramics.     

Reactive synthesis for porous TiVAlC ceramics by TiH2,V,Al and graphite powders

Using the mixed powder of TiH₂, graphite, aluminum and vanadium as starting materials, porous TiVAlC ceramics were fabricated by the reactive synthesis technology at 1300 °C. The chemical steadiness of porous TiVAlC along with the effects of sintering temperature on the viscous permeability coefficient, strength, porosity, pore size and volume expansion rate of the porous TiVAlC were explored, and the mechanism of pore formation was also revealed. The preparation process includes five steps as follows: (i) the complete decomposition of stearic acid at 500 °C; (ii) the pyrolysis of TiH₂ at 700 °C, converting TiH₂ into hydrogen and titanium (iii) The solid-liquid chemical reaction of solid vanadium, titanium and molten aluminum at 700 °C, converting the mixture into V–Al and Ti–Al compounds; (iv) At 900–1100 °C, Surplus V and Ti interact with graphite to synthesize carbides of TiVC₂, VC, and TiC; (v) Reactive synthesized carbides (TiVC₂, VC, and TiC), Ti₂AlC, V–Al and Ti–Al compounds that yield porous TiVAlC at 1300 °C.     

Influence of Mo doping on the tribological behavior of Ti3AlC2 ceramic at different temperatures

(Ti_0·8Mo_0.2)₃AlC₂ solid solutions were successfully synthesized from Ti, Al, TiC, and Mo powders using the in situ hot-pressing sintering method. The tribological properties of (Ti_0·8Mo_0.2)₃AlC₂ and the reference Ti₃AlC₂ in the temperature range 25–800 °C were evaluated in ambient air with the counterpart of Al₂O₃ balls. The results show that (Ti_0·8Mo_0.2)₃AlC₂ has improved lubricating properties and wear resistance above 400 °C compared with Ti₃AlC₂. This can be contributed to the formation of tribo-oxidation films containing MoO₃ and MoO_3-x. Structural characterization of the tribo-oxidation films was conducted using SEM, EDS, Raman spectroscopy, and XPS to evaluate the effect of Mo doping on the wear mechanisms of Ti₃AlC₂ in detail.     

Low-temperature synthesis of high-purity Ti3AlC2 by MA-SPS technique

Ternary carbide Ti₃AlC₂ was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS) from elemental powder mixtures of Ti, Al and C, and the effect of Al content on formation of Ti₃AlC₂ during both processes was investigated. The results showed that adding proper Al content in the staring materials significantly increased the phase purity of Ti₃AlC₂ in the synthesized samples. Dense and high-purity Ti₃AlC₂ with <1 wt.% TiC could be successfully obtained by spark plasma sintering of powders mechanically alloyed for 9.5 h from a starting powder mixtures of 3Ti/1.1Al/2C at a lower sintering temperature of 1050 °C for 10–20 min.     

An experimental approach to delineate the reaction mechanism of Ti3AlC2 MAX-Phase formation

A comprehensive reaction mechanism of Ti₃AlC₂ MAX-phase formation from its elemental powders while spark plasma sintering has been proposed. Microstructural evaluation revealed that Al-rich TiAl₃ intermetallic forms at around 660 °C once Al melts. Gradual transition from TiAl₃ to Ti-rich TiAl and Ti₃Al intermetallic phases occurs between 700 °C and 1200 °C through formation of layered structure due to diffusion of Al from periphery toward the centre of Ti particles. Formation of TiC and Ti₃AlC transient carbide phases were observed to occur through two different reactions beyond 1000 °C. Initially, TiC forms due to interaction of Ti and C, which further reacts with TiAl and Ti and gives rise to Ti₃AlC. Later, Ti₃AlC also forms due to diffusion of C into Ti₃Al above 1200 °C. Above 1300 °C, Ti₃AlC phase decomposes into Ti₂AlC MAX-phase and TiC in presence of unreacted C. Finally, Ti₂AlC and TiC reacts together to from Ti₃AlC₂ MAX-phase above 1350 °C and completes at 1500 °C.     

Factors affecting the property of porous Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> metal ceramic fabricated through pressureless sintering

In this paper, micrometer-sized porous Ti₃SiC₂ metal ceramic was fabricated through a reactive synthesis method using titanium hydride, silicon and graphite elemental powders. The factors including raw powders, pressing pressure, sintering procedure affecting the purity and the pore property of porous Ti₃SiC₂ were systematically studied. The results indicate that the purity, pore property including maximum pore size and porosity can be effectively controlled by adjusting the synthesis parameters.     

Synthesis and formation mechanism of high-purity Ti3AlC2 powders by microwave sintering

High-purity titanium aluminum carbide (Ti₃AlC₂) powders were synthesized by a microwave sintering method using different titanium sources as raw materials. The prepared products were characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicated that the synthesized Ti₃AlC₂ powders have high purity (97.5%) and even distribution of the grain size when using a 3TiH₂/1.2Al/2C mixture as raw materials when the microwave sintering temperature and time were 1300°C and 30 minutes, respectively. The formation mechanism of the Ti₃AlC₂ is described as proceeding via four stages. The solid-phase reaction between titanium and aluminum occurs below the melting point of aluminum and the main product is a Ti₃Al phase, which is an observed intermediate compound for the formation of Ti₂AlC and Ti₃AlC₂. Thus, this study provides a beneficial approach to low-temperature synthesis of high-purity Ti₃AlC₂ materials.     

Molten salt synthesis of MAX phases in the Ti-Al-C system

The molten salt method was used to synthesise the MAX phase compounds Ti₂AlC and Ti₃AlC₂ from elemental powders. Between 900–1000?°C, Ti₂AlC was formed alongside ancillary phases TiC and TiAl, which decreased in abundance with increasing synthesis temperature. Changing the stoichiometry and increasing the synthesis temperature to 1300?°C resulted in formation of Ti₃AlC₂ alongside Ti₂AlC and TiC. The type of salt flux used had little effect on the product formation. The reaction pathway for Ti₂AlC was determined to be the initial formation of TiC_1-x templating on the graphite and titanium aluminides.     

Reactive synthesis for porous Ti3AlC2 ceramics through TiH2, Al and graphite powders

Porous Ti₃AlC₂ ceramics were fabricated by reactive synthesis. The process of fabrication involved five steps: (i) the pyrolysis of stearic acid at 450 °C; (ii) the decomposition of TiH₂ at 700 °C, which leads to the conversion of TiH₂ to Ti; (iii) the solid–liquid chemical reaction of solid Ti and molten Al at 800–1000 °C, which converts the mixture to Ti–Al compounds; (iv) the newly synthesized Ti–Al compounds that react with surplus Ti and graphite to form ternary carbides and TiC at 1100–1200 °C; and (v) reactive synthesized ternary carbides and TiC that yield porous Ti₃AlC₂ at 1300 °C.     

Synthesis of low-oxygen Ti3AlC2 powders by hydrogenation–dehydrogenation and deoxidation from titanium scraps

The MAX phase is a material with excellent electrical and thermal conductivity and thermal shock and oxidation resistance owing to its metallike bonding properties. The impurities in the Ti₃AlC₂ MAX phase must be controlled because the oxides and TiC derived from the synthesis process remain in MXene and markedly affect the electrical conductivity and chemical stability. This study investigated whether the Ti₃AlC₂ MAX phase can be synthesized from titanium powder prepared from low-cost titanium scrap by hydrogenation–dehydrogenation (HDH) and deoxidation in the solid-state (DOSS) processes. Almost single-phase Ti₃AlC₂ MAX phase was obtained by synthesis at 1450°C for 5 h. The oxygen concentrations of the HDH-MAX and DOSS-MAX powders (25–45 μm) were 7215 and 3875 ppm, respectively. Oxygen reduction of titanium powder through DOSS can help improve the purity of Ti₃AlC₂ MAX phase by minimizing the imbalance in the stoichiometric ratio during synthesis. The HDH-MAX and DOSS-MAX powders prepared from titanium scrap displayed a higher Ti₃AlC₂ phase fraction and lower oxygen concentration than those of commercial Ti₃AlC₂ MAX phase powders. This cost reduction and purity improvement will increase the accessibility of the Ti₃AlC₂ MAX phase, supporting further research into its applications.     

An optimized method for synthesizing phase-pure Ti3AlC2 MAX-phase through spark plasma sintering

So far many attempts have been made to synthesize phase-pure Ti₃AlC₂ MAX-phase. But still the challenge posed by the presence of TiC and Ti-Al based intermetallic transient impurity phases in the final product is a persisting problem. Spark plasma sintering (SPS) technique has been the most successful method to decrease the impurity content of the final product. Even so, synthesis of phase-pure Ti₃AlC₂ MAX-phase, without any TiC and Ti-Al based intermetallic impurities, has not been achieved and reported in literature with substantial evidences. Further, high purity Ti₃AlC₂ MAX-phase synthesized using SPS technique has shown lack of phase and microstructural stability above 1350°C temperature. In this work, we have reported an optimized method for producing phase-pure Ti₃AlC₂ MAX-phase (having more than 99 % purity) using commercial grade Ti, Al and C elemental powders through SPS technique. The final product also showed very good high temperature stability up to 1500°C under flowing Argon inert atmosphere.     

Enhanced mechanical properties of porous titanium implants via in-situ synthesized titanium carbide in lamellar pore walls

Directional lamellar porous titanium scaffolds are widely used as bone implant bearing materials because of their anisotropic pore structure. Their mechanical properties can be effectively improved by enhancing the strength of pore walls through the introduction of ceramics. In this work, porous titanium implants were prepared by freeze casting combined with TiH₂ decomposition. The graphene was introduced into the pore walls of porous titanium, which could transform into titanium carbide (TiC) in situ upon sintering. TiC was evenly distributed in the lamellar pore walls, and the interface was well bonded. The compression strength of the fabricated implants was up to 389.94 MPa when the graphene content was 3 wt%, which was 377.8% times as high as the porous titanium. The crack propagation was resisted by TiC because of the “pinning” effect on the pore wall. Some of TiC were pulled out from the matrix, and others were fractured. The strength of the fabricated implants was improved significantly by the large consumption of fracture energy. Also, fabricated porous titanium implants with TiC are suitable for bone implantation.     

Spark plasma sintering of TiB2-based ceramics with Ti3AlC2

A TiB₂–Ti₃AlC₂ ceramic was manufactured by spark plasma sintering at 1900 °C temperature for 7 min soaking time under 30 MPa biaxial pressure. The role of Ti₃AlC₂ additive on the microstructure development, densification behavior, phase evolution, and hardness of the ceramic composite were studied. The phase characterization and microstructural investigations unveiled that the Ti₃AlC₂ MAX phase decomposes at the initial stages of the sintering. The in-situ formed phases, induced by the decomposition of Ti₃AlC₂ additive, were identified and scrutinized by XRD and FESEM/EDS techniques as well as thermodynamics principles. The sintered TiB₂–Ti₃AlC₂ ceramic approached a near full density of ~99% and a hardness of ~28 GPa. The densification mechanism and sintering phenomena were discussed and graphically illustrated.     

Sintering,mechanical, electrical and oxidation properties of ceramic intermetallic TiC–Ti3Al composites obtained from nano-TiC particles

The paper discusses the development of a new material system for interconnect application in Solid Oxide Fuel Cells (SOFC) based on TiC–Ti₃Al. Nano-sized TiC powders utilized in this research were synthesized using carbon coated TiO₂ precursors from a patented process. The pressureless sintering of TiC–Ti₃Al in a vacuum was applied at temperatures between 1100 °C and 1500 °C and content of Ti₃Al was varied in the range of 10–40 wt%. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase evaluation and sintering behavior. Relative density increased markedly with increasing sintering temperature because of grain growth and formation of the Ti₃AlC₂ secondary phase. Dense products (>95% TD) were prepared from nanosized TiC powders with 10 and 20 wt% Ti₃Al, but with about 8 to 10% porosity for 30 and 40 wt% Ti₃Al. The mechanical properties were determined from Vickers hardness and fracture toughness calculations. Vickers hardness decreased and fracture toughness increased with increasing Ti₃Al content. The electrical conductivity and oxidation behavior of TiC–Ti₃Al composites were investigated to evaluate the feasibility for SOFC interconnect application. The electrical conductivity measurements in the air at 800 °C for 100 h were made using the Kelvin 4-wire method.     

Preparation and tribological behaviour of laminated Ti3AlC2 crystals as additive in base oil

Abstract

Laminated ternary compound Ti₃AlC₂ crystals were synthesised by pressureless sintering the mixture powders of 3Ti/1·1Al/1·8C, 3Ti/1Al/1·8C/0·2Sn, 1Ti/1·8TiC/1Al and 1Ti1·8TiC1Al0·1Sn at 1400°C with preliminary liquid magnetic stirring mixing. The X-ray diffraction results indicate that Ti₃AlC₂ prepared from 3Ti/1Al/1·8C/0·2Sn has the highest purity, and the addition of appropriate Sn favours the synthesis of high purity Ti₃AlC₂. Scanning electron microscopy images show that Ti₃AlC₂ samples exhibit lamellar-like microstructure with thickness of ～100 nm. The tribological properties of Ti₃AlC₂ as an additive in 100SN base oil were evaluated with a ball on disc tester. The results show that the Ti₃AlC₂ additives exhibited good friction reduction and wear resistance at 5 wt-% concentration. Under determinate conditions, the base oil containing 5 wt-% Ti₃AlC₂ samples presented good tribology performance under the load of 15 N. The improved tribological properties of the Ti₃AlC₂ samples could be attributed to the formation of tribofilm in friction process.     

Temperature-driven deintercalation and structure evolution of Ag/Ti3AlC2 composites

Ag/Ti₃AlC₂ composites are promising sliding contact material. Here, Ag/Ti₃AlC₂ composites were obtained via the hot pressing technique and their structure evolutions upon sintering temperature were investigated. Sintering temperature controlled the deintercalation of Al layers from Ti₃AlC₂, thus controlling the interfacial structure of Ag/Ti₃AlC₂ composites. Amorphous interface was found after sintering at 750?°C. TiC_x particles with a size of around 10?nm were found in the interfacial region after sintering at 800?°C. Increasing sintering temperature to 850?°C, stripy structures composed of alternately arranged silver-rich phase and TiC_x phase appeared around the edges of Ti₃AlC₂ particles. The over-saturated Al precipitates found in 850?°C sintered composites are cubic μ-Ag₃Al inter-metallic compounds, which have the coherent relationship with Ag matrix.     

High-temperature microstructural evolution of Ti3AlC2 ceramics in a graphite bed

Microstructural evolution and chemical reactions of Ti₃AlC₂ ceramics in the range from 1100 °C to 1500 °C in a graphite bed were investigated in the present work. The electron probe microanalysis (EPMA) results indicated that only a thin but unbroken reaction layer was formed on the surface of Ti₃AlC₂ ceramics at 1100 °C. It was further confirmed as aluminum oxide, while aluminum oxide fine grains were distributed separately over the surface layer along a dozen microns scale. At 1300 °C, a continuous and dense reaction layer which was mainly composed of aluminum oxide and titanium carbide was detected. After heat-treated at 1500 °C, an obvious difference in metallic luster contrast between the interior and exterior layer of cross-sections of the sample was observed. The Kirkendall effect was proposed to elucidate the above results. Titanium carbide, instead of TiO₂, became the main product phase in the reaction product layer. It was attributed to the weak oxidative condition in a graphite bed and low oxygen partial pressure, which was not considered in the previous study. Besides, thermodynamic calculations result was provided to elaborate on the reaction mechanism in detail.     

Pore structure of reactively synthesized nanolaminate Ti3SiC2 alloyed with Al

Ti₃SiC₂ has the unique properties integrating the advantages of metals and ceramics, and good open pore structure when alloyed with Al. In this work, porous Ti₃SiC₂ compounds with different Al/Si atom ratios were prepared through the reactive synthesis of elemental powders at 1300 °C. The results indicate that the phase compositions are determined by Al element mole number, and that the pore structure can be controlled through varying Ti particle size. The MAX phase transits from Ti₃SiC₂ with Al element mole number no more than 0.6 to Ti₃AlC₂ with Al element mole number in the range of 0.8–1.2. When Al element mole number is 0.6, the porous compound has a single MAX phase of Ti₃SiC₂ with uniform microporous structure and high bending strength. Porous Ti₃SiC₂ alloyed with 0.6Al has a slow linear increase rate of 0.0083%/μm in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum aperture and Ti particle size with the increase rate of 0.0342 μm/μm. The pore structure formed by the phase transition mechanism for porous MAX phase has the smallest tortuosity factor compared with that formed by the clearance mechanism and the Kirkendall effect.     

Mechanical properties and toughening mechanisms of highly textured Ti3AlC2 composite material

Composite material consisting of Al₂O₃ and TiC in a matrix of highly textured Ti₃AlC₂ was fabricated in a two-step fabrication process. The Lotgering orientation factor for {00 l} planes of Ti₃AlC₂ in the textured top surface plane reached 0.71. Texture analysis showed an orientation relationship among Ti₃AlC₂, Al₂O₃ and TiC grains of [110] Ti₃AlC₂ // [110] TiC, (001) Ti₃AlC₂ // (111) TiC, and [110] Ti₃AlC₂ // [120] Al₂O₃, (001) Ti₃AlC₂ // (001) Al₂O₃. The texture grained material exhibited excellent mechanical properties, with compressive and flexural strengths of more than 2.5 times those of conventional coarse grained Ti₃AlC₂, and fracture toughness and hardness were 50% higher than those of conventional coarse grained Ti₃AlC₂. The microstructures of textured Ti₃AlC₂ and reported textured Ti₂AlC were investigated and compared to interpret the differences in mechanical behavior of the two textured MAX phases.     

Reaction synthesis of layered ternary Ti2AlC ceramic

Reactive sintering of 8Ti:Al₄C₃:C powder mixtures to form the ternary carbide Ti₂AlC is studied in the temperature range 570–1400 °C. After sintering at 1400 °C for 1 h, only the MAX phase Ti₂AlC and some TiC are produced. A series of intermediate phases, such as TiC, Ti₃Al, Ti₃AlC are detected during the reactive sintering process. From X-ray diffraction (XRD) and scanning electron microscopy (SEM) characterizations, a reaction path is proposed for the intermediate phases and Ti₂AlC formation. Results show that reaction kinetics may play an important role in the understanding of the reaction mechanisms.