Phase Evolution of Ti3SiC2 Annealing in Vacuum at Elevated Temperatures

The thermal decomposition of Ti₃SiC₂ in vacuum furnace up to 1500°C has been investigated. The results show that the mild decomposition of Ti₃SiC₂ commences at 1300°C and the higher the holding temperature, the larger the volatilization of Si atoms. The Ti₃SiC₂ decomposition occurs simultaneously on the surface and in the bulk. Four phases coexist at 1400°C and 1450°C and the Ti₅Si₃C_x phase appears in the bulk and/or surface. Diffusion distance, rate, and volatilization of Si contribute to the porous structure and the presence of Ti₅Si₃C_x. The evolution of furnace pressure reflects the decomposition kinetics of Ti₃SiC₂.     

Intermediate phases in synthesis of Ti3SiC2 and Ti3Si(Al)C2 solid solutions from elemental powders

In this paper, we investigated the reaction path to synthesize Ti₃SiC₂ by the in situ hot pressing/solid–liquid reaction method. The effect of different content of Al addition on this process was also examined. Ti₃SiC₂ mainly formed from the reaction between Ti₅Si₃C_x, TiC_x, TiSi₂ and graphite at 1400–1500 °C. As an inescapable impurity in Ti₃SiC₂, TiC_x was removed by addition of small amounts of Al. This was owing to the fact that the addition of a minor quantity of Al increased the amount of “effective TiC_x” and relatively decreased that of “invalid TiC_x”. Further increasing Al content, however, resulted in the presence of TiC_x again in the final product. This was due to the fact when significant amounts of Al was added, the stoichiometric ratio of silicon and graphite has been deviated from that for Ti₃SiC₂. More Si and less graphite were needed to prepare a monolithic Ti₃Si(Al)C₂ solid solution with high Al content.     

Influence of Cu on the mechanical and tribological properties of Ti3SiC2

Ti₃SiC₂/Cu composites with different contents of Cu were fabricated by mechanical alloying and spark plasma sintering method. The phase composition and structure of the composites were analyzed by X-ray diffractometry and scanning electron microscopy equipped with energy dispersive spectroscopy. The mechanical and tribological properties of Ti₃SiC₂/Cu composites were tested and analyzed compared with monolithic Ti₃SiC₂ in details. The results show that the Cu leads to the decomposition of Ti₃SiC₂ to produce TiC_x, Ti₅Si₃C_y, Cu₃Si, and TiSi₂C_z. The friction coefficient and wear rate of the composites are lower than that of monolithic Ti₃SiC₂, which is ascribed to the fixing effect of hard TiC_x, Ti₅Si₃C_y, and Cu₃Si to inhibit the abrasive friction and wear. However, at elevated temperatures (ranging from room temperature to 600 °C) the friction and wear of the composites are higher than those at room temperature. Plastic flowing and tribo-oxidation wear accompanied by material transference contribute to the increased friction and wear at elevated temperatures.     

Near Surface Changes Due to 700 keV Si+ Irradiation of Titanium Silicon Carbide

The radiation damage response of Ti₃SiC₂ heated from 120°C to 850°C during 700 keV Si⁺ irradiation has been investigated. The samples were analyzed using glancing incidence X‐ray diffraction, Rutherford backscattering spectrometry, Raman spectroscopy, and scanning electron microscopy. For the sample at 120°C, irradiation results in a buildup of a heterogeneous surface and the formation of TiC_x. Irradiation at 200°C results in maximum microstrain, a maximum in the c lattice parameter, and the appearance of a β phase in addition to the normal α phase of Ti₃SiC_2. A minimum in the observed damage level near the surface was seen for irradiation at a sample temperature of 300°C but the damaged phase increases at higher temperatures. Differences between the present work and a previous C irradiation study have been ascribed to the enhanced Si defect transport at low temperatures.     

High temperature properties of the monolithic CVD β-SiC materials joined with a pre-sintered MAX phase Ti3SiC2 interlayer via solid-state diffusion bonding

Monolithic high purity CVD β-SiC materials were successfully joined with a pre-sintered Ti₃SiC₂ foil via solid-state diffusion bonding. The initial bending strength of the joints (∼ 220 MPa) did not deteriorate at 1000 °C in vacuum, and the joints retained ∼ 68 % of their initial strength at 1200 °C. Damage accumulation in the interlayer and some plastic deformation of the large Ti₃SiC₂ grains were found after testing. The activation energy of the creep deformation in the temperature range of 1000 – 1200 °C in vacuum was ∼ 521 kJmol⁻¹. During the creep, the linkage of a significant number of microcracks to form a major crack was observed in the interlayer. The Ti₃SiC₂ interlayer did not decompose up to 1300 °C in vacuum. A mild and well-localized decomposition of Ti₃SiC₂ to TiC_x was found on the top surface of the interlayer after the bending test at 1400 °C in vacuum, while the inner part remained intact.     

Machinability of Ti3SiC2 with layered structure synthesized by hot pressing mixture of TiCx and Si powder

Nanolaminate Ti₃SiC₂ was synthesized from a mixture of TiC_x (x = 0.67)/Si powder by hot pressing to increase machinability. Ti₃SiC₂ was synthesized at temperatures of 1360 °C and 1420 °C for 90 min under a pressure of 25 MPa. The X-ray diffraction results showed that while mainly Ti₃SiC₂ with some unreacted TiC_x were detected in the synthesized samples at 1360 °C, no phases except Ti₃SiC₂ phases remained in the synthesized samples at 1420 °C. The cutting resistance of Ti₃SiC₂ was measured in terms of the principle, feed, and thrust forces and was compared with that of middle-carbon steel, SM45C. The values of the principal force of the synthesized Ti₃SiC₂ were lower than those of SM45C. After machining, the roughness of the Ti₃SiC₂ was lower than those of SM45C; however, the damage to the tool bit used for the machining of SM45C was less than the damage to those used for the machining of the Ti₃SiC₂.     

Interface structure and wetting behaviour of Cu/Ti3SiC2 system

Wetting behaviour of a Cu/Ti₃SiC₂ system was investigated by the sessile drop technique under a vacuum atmosphere. Contact angles between Cu and Ti₃SiC₂ changed from 95 to 15° as temperatures increased from 1089 to 1270°C. Two distinct reaction layers consisting of different contents of Cu, TiC_x, Ti₃SiC₂ and Cu_xSi_y intermetallics were formed at the interface of Cu and Ti₃SiC₂. The formation of the interface layers contributes to the improvement of the wettability of the system. The dissolution of Si from theTi₃SiC₂ into the molten Cu at high temperature plays a dominant role in the wetting behaviour of Cu/Ti₃SiC₂ systems.     

Synthesis of Ti3SiC2 coatings onto SiC monoliths from molten salts

Coatings with composition close to Ti₃SiC₂ were obtained on SiC substrates from Ti and Si powders with the molten NaCl method. In this work, the growth of coatings by reaction in the salt between monolithic SiC substrates and titanium powder is obtained between 1000 and 1200 °C. At 1000 °C, a coating of 8 µm thickness is formed in 10 h whereas a thin coating of 0.5 µm has been grown in 2 h. A lack in silicon was first found in the coatings prepared at 1100 and 1200 °C. For these temperatures, the addition of silicon powder in the melt had a favorable effect on the final composition, which is found very close to the composition of Ti₃SiC₂. The reaction mechanism implies the formation of TiC_x layers in direct contact with the SiC substrate and the presence of more or less important quantities of Ti₃SiC₂ and Ti₅Si₃C_x in the upper layers.     

Microstructural evolution and growth kinetics of interfacial reaction layers in SUS430/Ti3SiC2 diffusion bonded joints using a Ni interlayer

Ti₃SiC₂ ceramic and SUS430 stainless steel (SS) were successfully joined by a solid diffusion bonding technique using Ni interlayers. Diffusion bonding was performed in the temperature range of 850 °C–1100 °C under vacuum. The interfacial reaction phase, morphology evolution, growth kinetics and tensile strength were systematically investigated. In all cases, the inter-diffusion and reaction between Ti₃SiC₂ and SS can be effectively prevented by Ni foil, and the good transition in the joint benefit to the sound joining. The interface in the joints adjacent to SS matrix was composed of γ solid solution and a small amount of σ intermetallic compound. The compounds in the Ni/Ti₃SiC₂ interface was Ni/Ni(Si)/Ni₃₁Si₁₂ + Ni₁₆Ti₆Si₇ + Ti₃SiC₂ + TiC_x/Ti₂Ni + Ti₃SiC₂ + TiC_x/Ti₃SiC₂, which formed by the inter-diffusion and chemical reactions between Si and Ni atoms. The diffusion mechanism and reaction mechanism were interrelated, and decided the width of each reaction zones. Furthermore, the diffusion activation energy was 113 kJ/mol. The tensile strength increases with increasing the bonding temperature. The minimum and maximum strength of 32.3 MPa and 88.8 MPa were obtained from SUS430/Ni/Ti₃SiC₂ joints, which bonding experiments were carried out at 850 °C and 1100 °C, respectively.     

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.     

Synthesis and characterization of quarternary Ti3Si(1−x)AlxC2 MAX phase materials

Ti₃Si_(1−x)Al_xC₂ (x=0–1) quarternary MAX phase materials were prepared by spark plasma sintering of TiC, Ti, Si and Al powder mixtures at 1200 °C. Effect of Al addition on lattice parameters, density and hardness were investigated. Impurities are limited to binary phases of TiC and Ti₅Si₃. No multinary compound other than Ti₃Si_(1−x)Al_xC₂ can be detected. TiC exists as impurity in all samples and trace amount of Ti₅Si₃ can be detected in Samples x=0.1–0.6. Oxidation of Al cannot be avoided although all sintering were performed under vacuum and trace amount of Al₂O₃ can be found in all samples with Al addition. Experimental results show that the lattice parameters a and c increase linearly with increasing Al content for x=0–1. The lattice variations are strongly anisotropic and follow Vegard׳s law. Both density and hardness decrease as Al content increases. The linear variation of lattice parameters, d spacings of crystalline faces and density against Al concentration suggest that continuous solid solutions of Ti₃Si_(1−x)Al_xC₂ (x=0–1) may have been formed between Ti₃SiC₂ and Ti₃AlC₂.     

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.     

Effect of Ge on microstructure and mechanical properties of Ti3SiC2/Al2O3 composites

Owing to the good physicochemical compatibility and complementary mechanical properties of Ti₃SiC₂ and Al₂O₃, Ti₃SiC₂/Al₂O₃ composites are considered as ideal structural materials. However, TiC and TiSi₂ typically coexist during the synthesis of Ti₃SiC₂/Al₂O₃ composites through an in-situ reaction, which adversely affects the mechanical properties of the resulting composites. In this study, Ti₃SiC₂/Al₂O₃ composites were prepared via in-situ hot pressing sintering at 1450 °C. Ge, which was used as a sintering aid, improved the purity and mechanical properties of the Ti₃SiC₂/Al₂O₃ composites. This is because Ge replaced some of the Si atoms to compensate the evaporation loss of Si to form Ti₃(Si_1-xGe_x)C₂, which showed a crystal structure similar to that of Ti₃SiC₂. Furthermore, the molten Ge accelerated the diffusion reaction of the raw materials, increasing the overall density of the Ti₃SiC₂/Al₂O₃ composites. The optimum Ge amount for improving the mechanical properties of the composites was found to be 0.3 mol. The flexural strength, fracture toughness, and microhardness of the composite with the optimum Ge amount were 640.2 MPa, 6.57 MPa m^1/2, and 16.21 GPa, respectively. The formation of Ti₃(Si_1-xGe_x)C₂ was confirmed by carrying out X-ray diffraction, energy dispersive spectroscopy, and transmission electron microscopy analyses. A model crystal structure of Ti₃(Si_1-xGe_x)C₂ doped with 0.3 mol Ge was established by calculating the solid solubility of Ge.     

Experimental synthesis and density functional theory investigation of radiation tolerance of Zr3(Al1‐xSix)C2 MAX phases

Synthesis, characterization and density functional theory calculations have been combined to examine the formation of the Zr₃(Al_1–xSi_x)C₂ quaternary MAX phases and the intrinsic defect processes in Zr₃AlC₂ and Zr₃SiC₂. The MAX phase family is extended by demonstrating that Zr₃(Al_1–xSi_x)C₂, and particularly compositions with x≈0.1, can be formed leading here to a yield of 59 wt%. It has been found that Zr₃AlC₂ ‐ and by extension Zr₃(Al_1–xSi_x)C₂ ‐ formation rates benefit from the presence of traces of Si in the reactant mix, presumably through the in situ formation of Zr_ySi_z phase(s) acting as a nucleation substrate for the MAX phase. To investigate the radiation tolerance of Zr₃(Al_1–xSi_x)C₂, we have also considered the intrinsic defect properties of the end‐members. A‐element Frenkel reaction for both Zr₃AlC₂ (1.71 eV) and Zr₃SiC₂ (1.41 eV) phases are the lowest energy defect reactions. For comparison we consider the defect processes in Ti₃AlC₂ and Ti₃SiC₂ phases. It is concluded that Zr₃AlC₂ and Ti₃AlC₂ MAX phases are more radiation tolerant than Zr₃SiC₂ and Ti₃SiC₂, respectively. Their applicability as cladding materials for nuclear fuel is discussed.     

Phase analysis and wear behavior of in-situ spark plasma sintered Ti3SiC2

In-situ synthesis of dense near-single phase Ti₃SiC₂ ceramics from 3Ti/SiC/C/0.15Al starting powder using spark plasma sintering (SPS) at 1250 °C is reported. Systematic analysis of the phase development over a range of sintering temperatures (1050–1450 °C) suggested that solid state reactions between intermediate TiC and Ti₅Si₃ phases lead to the formations of Ti₃SiC₂. The effect of starting powder composition on phase development after SPS at 1150 °C was also investigated using three distinct compositions (3Ti/SiC/C, 2Ti/SiC/TiC, and Ti/Si/2TiC). The results indicate that the starting powder compositions, with higher amounts of intermediate phase such as TiC, favor the formation of Ti₃SiC₂ at relatively lower sintering temperature. Detailed analysis of wear behavior indicated that samples with higher percentage of TiC, present either as an intermediate phase or a product of Ti₃SiC₂ decomposition, exhibited higher microhardness and better wear resistance compared to near single phase Ti₃SiC₂.     

Influence of small amounts of Fe and V on the synthesis and stability of Ti3SiC2

Polycrystalline bulk samples of (Ti_1-yMe_y)₃SiC₂, where Me=Fe or V and y=0.01 to 0.1, were fabricated by reactive hot isostatic pressing of a mixture of Ti, C (graphite), SiC and Fe or V at 1450°C for 4 h under a pressure of 60 MPa. X-ray diffraction and scanning electron microscopy of the fully dense samples have shown that small amounts of Fe and V interfere with the reaction between Ti, C and SiC leading to the presence of SiC, TiC_x, as well as different Fe and V-containing phases in the final microstructures. The presence of these impurity phases also reduces the temperature at which Ti₃SiC₂ decomposes. The decomposition is manifested by the formation of a network of pores when the samples are annealed at 1600°C, a temperature at which pure Ti₃SiC₂ is thermally stable. The concentration threshold for this decomposition is as low as 1 at%.     

Microstructure and mechanical strength of transient liquid phase bonded Ti3SiC2 joints using Al interlayer

Based on the structure characteristic of Ti₃SiC₂ and the easy formation of Ti₃Si_1−xAl_xC₂ solid solution, a transient liquid phase (TLP) bonding method was used for bonding layered ternary Ti₃SiC₂ ceramic via Al interlayer. Joining was performed at 1100–1500 °C for 120 min under a 5 MPa load in Ar atmosphere. SEM and XRD analyses revealed that Ti₃Si(Al)C₂ solid solution rather than intermetallic compounds formed at the interface. The mechanism of bonding is attributed to aluminum diffusing into the Ti₃SiC₂. The strength of joints was evaluated by three point bending test. The maximum flexural strength reaches a value of 263 ± 16 MPa, which is about 65% of that of Ti₃SiC₂; for the sample prepared under the joining condition of 1500 °C for 120 min under 5 MPa. This flexural strength of the joint is sustained up to 1000 °C.     

High-temperature hydrogenation behaviour of bulk titanium silicon carbide

Thermal stability of Ti₃SiC₂ was investigated at 1200–1400°C in hydrogen atmosphere for 3 hours. The hydrogenation mechanism was clarified by a combination of X-ray diffraction, scanning electron microscope, Raman spectroscopy and first principles calculation. At 1200°C, a dense and uniform TiSi₂ layer formed on the sample surface, which originated from both the preferable lose of silicon from the Ti₃SiC₂ substrate and the dissociation of Ti₃SiC₂. As temperature increased to 1300°C, TiSi₂ layer began to scale off and presented laminated Ti₃SiC₂ grains beneath this layer, which indicated preferential hydrogenation occurred along the basal planes. This phenomenon was ascribed to the fact that the introduction of H interstitial atom weakened the combination between titanium and silicon interface layer, which was confirmed by first principles calculations. In addition, the formation of TiSi₂ owing to the dissociation of Ti₃SiC₂ caused the volume expansion after hydrogenation, resulting in that majority of TiSi₂ layer spelled off at 1400°C.     

Fast seamless joining of SiCw/Ti3SiC2 composite using electric field-assisted sintering technique

A pair of Ti₃SiC₂ reinforced with SiC whiskers (SiC_w/Ti₃SiC₂) composites was successfully joined without any joining materials using electric field-assisted sintering technology at a temperature as low as 1090°C (T_i) and a short time of 30 s. The microstructure and mechanical properties of the obtained SiC_w/Ti₃SiC₂ joints were investigated. The solid-state diffusion was the main joining mechanism, which was facilitated by a relatively high current density (~586 A/cm²) at the joining interface. The shear strength of the sample joined at 1090°C was 51.8 ± 2.9 MPa. The sample joined at 1090°C failed in the matrix rather than at the interface, which confirmed that a sound inter-diffusion bonding was obtained. A rapid and high efficient self-joining process may find application in the case of SiC_w/Ti₃SiC₂ sealing cladding tube and end cap.     

Microstructure and microindentation of Ti3SiC2 – Titanium filler brazed joints by tungsten inert gas (TIG) process

Herein we study the joining of Ti₃SiC₂ - a MAX phase - with a Ti filler (Ti₃SiC₂/Ti-filler) using a TIG-brazing process. The microstructures of the interfaces were investigated by scanning electron microscopy and energy dispersive spectrometry. When Ti₃SiC₂ comes into contact with the molten Ti - filler during the TIG-brazing operation, it starts decomposing into TiC_x and a Si-rich liquid. Simultaneously, the molten Ti infiltrates into the Ti₃SiC₂ resulting in a 200 µm thick duplex region, comprised of TiC_x and a Ti-rich phase with some dissolved Si. Both Si and C are found in the solidified Ti; the Si source is from the Si-rich liquid, while the presence of C indicates that some of the C diffused into the Ti. Upon cooling, C- containing Ti- rich lamellae form the solidified Ti. Microindentation results of the decomposed Ti₃SiC₂ layer show an increase in hardness and a decrease in elastic modulus relative to T₃SiC₂. Notably, no cracks were observed.