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.     

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.     

Facile synthesis of Ti3SiC2 powder by high energy ball-milling and vacuum pressureless heat-treating process from Ti–TiC–SiC–Al powder mixtures

In this article, Ti/TiC/SiC/Al powder mixtures with molar ratios of 4:1:2:0.2 were high energy ball-milled, compacted, and heated in vacuum with various schedules, in order to reveal the effects of temperature, soaking time, thickness of the compacts, and carbon content on the purity of the sintered compacts. X-ray diffraction and scanning electron microscopy were employed to investigate the phase purity, particle size and morphology of the synthesized samples. It was found that the Ti₃SiC₂ content nearly reached 100 wt.% on the surface layer of the sintered compacts prepared in the temperature range from 1350 °C to 1400 °C for 1 h. Powder containing 91 wt.% Ti₃SiC₂ was successfully synthesized by heating 6 mm green compacts of 4Ti/1TiC/2SiC/0.2Al at 1380 °C for 1 h in vacuum. The excessive carbon content failed to improve the purity of Ti₃SiC₂ powder. TiC phase was the main impurity in the formation process of Ti₃SiC₂.     

Dependence of the microstructure and properties of TiC/Ti3SiC2 composites on extra C addition

TiC/Ti₃SiC₂ composites were synthesized with Ti/Si/C and Al (in which extra C addition ranges from 0 to 25 wt.%) as starting powders by hot-pressed sintering method at 1400 °C under 30 MPa. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the phase composition and the fracture surface. The results reveal that with the increase of extra C addition, the content of Ti₃SiC₂ phase decreases while the content of TiC phase increases. Graphite phase is detected in the samples with extra C addition of 20 wt.% and 25 wt.%. The bending strength decreases from 554.81 MPa to 57.44 MPa due to the decrease of the densification and Ti₃SiC₂ phase content. The electrical conductivity falls from 42,474.52 s/cm to 1524.95 s/cm, resulting from lower Ti₃SiC₂ phase content and higher contact resistance.     

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.     

High temperature interfacial phase stability of a Mo/Ti3SiC2 laminated composite

A Mo/Ti₃SiC₂ laminated composite is prepared by spark plasma sintering at 1300 °C under a pressure of 50 MPa. Al powder is used as sintering aid to assist the formation of Ti₃SiC₂. The fabricated composites were annealed at 800, 1000 and 1150 °C under vacuum for 5, 10, 20 and 40 h to study the composite's interfacial phase stability at high temperature. Three interfacial layers, namely Mo₂C layer, AlMoSi layer and Ti₅Si₃ solid solution layer are formed during sintering. Experimental results show that the Mo/Ti₃SiC₂ layered composite prepared in this study has good interfacial phase stability up to at least 1000 °C and the growth of the interfacial layer does not show strong dependence on annealing time. However, after being exposed to 1150 °C for 10 h, cracks formed at the interface.     

Surface strengthening of Ti3SiC2 through magnetron sputtering of Mo and Zr and subsequent annealing

Magnetron sputtering deposition of Mo and Zr and subsequent annealing were conducted with the motivation to modify the surface hardness of Ti₃SiC₂. For Mo-coated Ti₃SiC₂, Si diffused outward into the Mo layer and reacted with Mo to form molybdenum silicides in the temperature range of 1000–1100 °C. The MoSi₂ layer, however, cracked and easily spalled off. For Zr-coated Ti₃SiC₂, Si also diffused outward to form Zr–Si intermetallic compounds at 900–1100 °C. The Zr–Si compounds layer had good adhesion with Ti₃SiC₂ substrate, which resulted in the increased surface hardness.     

Effect of excess silicon content on the formation of nano-layered Ti3SiC2 ceramic via infiltration of TiC preforms

The aim of this work was to investigate the effect of silicon content on the formation and morphology of Ti₃SiC₂ based composite via infiltration of porous TiC preforms. The gelcasting process was used for fabrication of preforms. It was found that the infiltrated sample at 1500 °C for 90 min from a mixture of 3TiC/1.5Si containing 92 wt.% Ti₃SiC₂. With the increasing of TiC and SiC impurity phases, Vickers hardness was increased to the maximum value of 12.9 GPa in Ti₃SiC₂–39 wt.%TiC composite. Microscopic observations showed that the Ti₃SiC₂ matrix was composed of columnar, platelike and equiaxial grains with respect to silicon content.     

Pore accessibility of Ti3SiC2-derived carbons

We investigate the accessibility of Ti₃SiC₂-derived carbons (Ti₃SiC₂-DCs) synthesized non-isothermally using a temperature ramp. The microstructure of the Ti₃SiC₂-DCs is characterized using TEM, XRD, Raman spectroscopy and gas adsorption. For the characterization by gas adsorption, we adopt our Finite Wall Thickness (FWT) model to invert Ar adsorption isotherms at 87 K to obtain pore size and pore wall thickness distributions of the Ti₃SiC₂-DCs. Accordingly, we identify a pore accessibility problem in the Ti₃SiC₂-DCs, as reported for Ti₃SiC₂-DCs prepared at 1073 K in our previous work. A striking feature is that Ti₃SiC₂-DC prepared at the slowest ramping rate (2 K/min) has a very narrow pore size distribution, while the Ti₃SiC₂-DCs synthesized at higher ramping rates (5 and 15 K/min) have much broader pore size distributions centered around 5.2 Å. A significant amount of previously unreported ultra-microporosity is observed based on low pressure CO₂ adsorption at 273 K. Our results indicate that slow ramping rate could potentially be utilized for fine control of the ultra-microporosity of carbide-derived carbons. Finally, we have found that fast ramping rate above 5 K/min leads to subtle changes in microstructure, with long and periodic graphitic multilayers having some large pores formed in between.     

Tribological behavior of Ti3SiC2 and Ti3SiC2/Pb composites sliding against Ni-based alloys at elevated temperatures

The Ti₃SiC₂ and Ti₃SiC₂/Pb composites were tested under dry sliding conditions against Ni-based alloys (Inconel 718) at elevated temperatures up to 800 °C using a pin-on-disk tribometer. Detailed tribo-chemical changes of Pb on sliding surface were discussed. It was found that the tribological behavior were insensitive to the temperature from 25 °C (RT) to 600 °C (friction coefficient ≈0.61–0.72, wear rate ≈10⁻³ mm³ N m⁻¹). An amount of Pb in the composites played a key role in lubricating with the temperature below 800 °C. The friction coefficient (≈0.22) and wear rate (≈10⁻⁷ mm³ N m⁻¹) at elevated temperatures were both decreased by the added PbO. The wear mechanisms of Ti₃SiC₂/Pb-Inconel 718 tribo-pair at elevated temperatures were believed to be the combined effect of abrasive wear and tribo-oxidation wear. During the sliding, two oxidization reactions proceed, 2Pb+O₂=2PbO (below 600 °C) and 6PbO+O₂=2Pb₃O₄ (800 °C). The friction coefficient and wear rate of the composites were reduced due to the self-lubricating effect of the tribo-oxidation products.     

Energetics of point defects in TiC

Density functional theory was used to evaluate the energetics of point defects in TiC_x (x < 1): C vacancies and Al substitution at a C site. Our ambition is to contribute towards understanding the underlying atomic mechanisms enabling the Al intercalation into TiC_x and the subsequent formation of Ti₃AlC₂. The difference between the energy of formation for an Al substitution at a C site and a bulk C vacancy is 0.224 eV. Furthermore, only 49 meV/vacancy is required to order the existing bulk C vacancies. Surface effects were also considered: the energy of formation for Al on TiC(1 0 0) at a vacant surface C site is smaller by 2.779 eV than in the case of the C surface vacancy, indicating that Al is likely to be incorporated. Based on these energy differences, it is reasonable to assume that Ti₃AlC₂ is formed by Al surface ingress into TiC_x and that vacancy ordering takes place.     

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.     

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.     

Hot corrosion behavior of Ti3SiC2 in the mixture of Na2SO4–NaCl melts

The corrosion behavior of polycrystalline Ti₃SiC₂ was studied in the mixture of Na₂SO₄–NaCl melts with various mass ratios at 850 °C. The results demonstrated that Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of Na₂SO₄–NaCl melts when the concentration of Na₂SO₄ was higher than 35 wt.%. A large amount of corrosion products spalled from specimens during the tests and obvious mass loss was observed. Hot corrosion of Ti₃SiC₂ would become severe because NaCl had lower melting-point and caused Na₂SO₄–NaCl mixture melted below 850 °C. However, when the concentration of Na₂SO₄ was lower than 25 wt.% in the mixture, a protective oxide layer (SiO₂ + TiO₂) formed on the substrate, the corrosion rate of Ti₃SiC₂ became quite slow and slight mass gain was observed, the corrosion products did not spall from substrate at 850 °C. The microstructure and phase composition of the corroded samples were investigated by SEM/EDS and XRD.     

Synthesis and characterization of novel Ti3SiC2–cBN composites

In this paper, synthesis of novel super hard and high performance composites of titanium silicon carbide–cubic boron nitride (Ti₃SiC₂–cBN) was evaluated at three different conditions: (a) high pressure synthesis at ~ 4.5 GPa, (b) hot pressing at ~ 35 MPa, and (c) sintering under ambient pressure (0.1 MPa) in a tube furnace. From the analysis of experimental results, the authors report that the novel Ti₃SiC₂–cBN composites can be successfully fabricated at 1050 °C under a pressure of ~ 4.5 GPa from the mixture of Ti₃SiC₂ powders and cBN powders. The subsequent analysis of the microstructure and hardness studies indicates that these composites are promising candidates for super hard materials.     

Fabrication of Ti3SiC2-based composites via three-dimensional printing: Influence of processing on the final properties

In this work the influence of the processing routes on the microstructure and properties of Ti₃SiC₂-based composites was investigated. The three main processing steps are (i) three-dimensional printing of Ti₃SiC₂ powder blended with dextrin, (ii) pressing of printed samples (uniaxial or cold isostatic pressing), and (iii) sintering of pressed samples at 1600 °C for 2 h. The Ti₃SiC₂-based composites were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Young's Modulus and flexural strength were measured to examine the mechanical properties. Porosity, density, shrinkage, and mass change were measured at each processing step. Those samples uniaxially pressed at 726 MPa presented the highest density, shrinkage, and mass change. However, microstructural morphologies were crack-free and homogeneous for cold isostatic pressed Ti₃SiC₂-based composites as compared to uniaxially pressed samples. The highest values for Young's Modulus (~300 GPa) and flexural strength (~3 GPa) were observed with uniaxially pressed Ti₃SiC₂-based composites.     

Fabrication of Ti3SiC2 and Ti4SiC3 MAX phase ceramics through reduction of TiO2 with SiC

Ti₃SiC₂ and Ti₄SiC₃ MAX phase ceramics were fabricated through high-temperature vacuum reduction of TiO₂ using SiC as a reductant, followed by hot pressing of the products under 25 MPa of pressure at 1600 °C. It was found that both Ti₃SiC₂ and Ti₄SiC₃ may be obtained in good yields, depending on the annealing time during the reduction step. In addition to MAX phases, the products contained some amounts of TiC. The hot pressing step did not significantly affect the composition of the products, indicating good stability of Ti₃SiC₂ and Ti₄SiC₃ under these conditions. Analysis of the densification behavior of the samples revealed lower ductility in Ti₄SiC₃ compared to Ti₃SiC₂. The samples prepared herein exhibited the flexural strength, fracture toughness and microhardness typical of coarse-grained MAX-phase ceramics.     

Fabrication and thermal stability of two-dimensional carbide Ti3C2 nanosheets

Two-dimensional Ti₃C₂ nanosheets were obtained by exfoliation of synthesized Ti₃AlC₂ powders in 40% HF solution at room temperature. The influence of etching time on the synthesis, structural evolution during heating, and thermal stability of as-prepared Ti₃C₂ nanosheets were studied. The graphene-like structure of as-prepared Ti₃C₂ nanosheets was confirmed by XRD and SEM. TG–DTA and elemental analysis suggested that the OH and F groups attached on the surface of Ti₃C₂ nanosheets could be eliminated by heat treatment. It is noteworthy that Ti₃C₂ nanosheets are thermal stable up to 1200 °C in Ar atmosphere. In the heating process, a small quantity of TiO₂ anatase emerges at 500 °C and then reacts with Ti₃C₂ to form a solid solution of TiC_xO_1−x (0<x<1) with the increase of temperature.     

Effects of Xe+ irradiation on Ti3SiC2 at RT and 500 °C

In this study, effects of a 400 keV Xe⁺ irradiation on Ti₃SiC₂ were systematically investigated by transmission electron microscopy (TEM). At RT, results show that the Xe⁺ irradiation induced the dissociation of Ti₃SiC₂ to polycrystalline TiC first, and then the polycrystalline to TiC nanograins with the increasing fluence. However, there is no significant microstructure change observed on the sample irradiated at 500 °C. It is demonstrated that Ti₃SiC₂ had not been completely amorpherized even up to 116.9 displacements per atom (dpa).     

Microstructure evolution of Ti3SiC2 powder during high-energy ball milling

Ti₃SiC₂ powder was milled by high-energy ball milling under argon atmosphere and subsequently thermally annealed. The microstructure evolution of Ti₃SiC₂ after milling was investigated. It was found that 200 nm particle size Ti₃SiC₂ powder could be achieved by 9 h milling whereas a longer milling time would induce Ti₃SiC₂ decomposition. After 18 h milling, the particle size gradually decreased to 150 nm and TiC appeared in the XRD pattern. It is suggested that the collision of the milling balls triggered the formation of TiC from the amorphous phase which was generated in the milling process.