Polycrystalline Ti3SiC2 Research Articles

Electrochemical corrosion behaviors of Ti3SiC2/Cu composites in a 3.5% NaCl solution

AbstractThe electrochemical corrosion behaviors of Ti3SiC2/Cu composite and polycrystalline Ti3SiC2 in a 3.5% NaCl medium were investigated by dynamic potential polarization, potentiostat polarization, and electrochemical impedance spectroscopy. The polycrystalline Ti3SiC2 was tested on the identical condition as a control. The characterizations of XRD, X‐ray photoelectron spectroscopy, scanning electron microscope, and energy‐dispersive spectrometer were used to study the relevant passivation behavior and corrosive mechanism. The self‐corrosion current density of Ti3SiC2/Cu (6.46 × 10−6 A/cm2) was slightly higher than that of Ti3SiC2 (1.64 × 10−7 A/cm2). Under open circuit potential, the corrosion resistance of Ti3SiC2/Cu was better than that of Ti3SiC2. Ti3SiC2/Cu exhibited a typical passivation feature with a narrow passivation interval and a breakdown phenomenon. The better corrosion resistance of Ti3SiC2 was due to the more stable Si layer of the former. In comparison, for Ti3SiC2/Cu composites, Cu reacted with the reactive Si layers in Ti3SiC2 to form Cu–Si compounds and TiC, destroying the weak interaction between Si layers and Ti–C layers. In the other hand, the as‐formed Cu–Si compounds and TiC dissolved during the corrosion of Ti3SiC2/Cu in the 3.5% NaCl medium, causing to the destruction of the passivation film on its surface.

Kink Formation through Creep Deformation and Possibility of Kink Strengthening in Ti<sub>3</sub>SiC<sub>2</sub> MAX Phase

Kink formation and kink strengthening mechanisms were examined in the polycrystalline Ti3SiC2 MAX phase prepared by a reaction sintering process using a spark-plasma-sintering machine. The creep behavior tested by compression at 1200°C showed two deformation regions depending on the applied stresses; at stresses lower than 120 MPa, the stress exponent n exhibited ≈1.8, whereas at higher stresses, it exhibited n ≥ 6. The creep behavior can be ascribed to grain boundary sliding mechanisms for the lower stresses with n ≈ 1.8 and to dislocation-related creep mechanisms for the higher stresses with n ≥ 6. The kink bands were frequently observed to form in the grains deformed only at the higher stresses when its basal plane was inclined by about 10–20° against the compressive axis. This suggests that the kink bands might be formed only when two factors of the high stresses acting on the basal plane and the resultant dislocation activities were satisfied. Nanoindentation tests conducted around the formed kink bands showed that the nanohardness linearly changed with the distance from the kink bands and showed higher values around the kink bands. Since the kink bands blocked the slip line caused by the nanoindentation, those become obstacles against the dislocation motion caused by the indentation deformation. This suggests that the kink bands would contribute to improving the mechanical properties of the Ti3SiC2 MAX phase.

Microstructure and Mechanical Properties of Composites Obtained by Spark Plasma Sintering of Ti3SiC2-15 vol.%Cu Mixtures.

Method of soft metal (Cu) strengthening of Ti3SiC2 was conducted to increase the hardness and improve the wear resistance of Ti3SiC2. Ti3SiC2/Cu composites containing 15 vol.% Cu were fabricated by Spark Plasma Sintering (SPS) in a vacuum. The effect of the sintering temperature on the phase composition, microstructure and mechanical properties of the composites was investigated in detail. The as-synthesized composites were thoroughly characterized by scanning electron micrography (SEM), optical micrography (OM) and X-ray diffractometry (XRD), respectively. The results indicated that the constituent of the Ti3SiC2/Cu composites sintered at different temperatures included Ti3SiC2, Cu3Si and TiC. The formation of Cu3Si and TiC originated from the reaction between Ti3SiC2 and Cu, which was induced by the presence of Cu and the de-intercalation of Si atoms Ti3SiC2. OM analysis showed that with the increase in the sintering temperature, the reaction between Ti3SiC2 and Cu was severe, leading to the Ti3SiC2 getting smaller and smaller. SEM measurements illustrated that the uniformity of the microstructure distribution of the composites was restricted by the agglomeration of Cu, controlling the mechanical behaviors of the composites. At 1000 °C, the distribution of Cu in the composites was relatively even; thus, the composites exhibited the highest density, relatively high hardness and compressive strength. The relationships of the temperature, the current and the axial dimension with the time during the sintering process were further discussed. Additionally, a schematic illustration was proposed to explain the related sintering characteristic of the composites sintered by SPS. The as-synthesized Ti3SiC2/Cu composites were expected to improve the wear resistance of polycrystalline Ti3SiC2.

Self-lubricating behavior caused by tribo-oxidation of Ti3SiC2/Cu composites in a wide temperature range

Ti3SiC2/Cu composite, a new wide temperature range intelligent lubricating functional material, was fulfilled, for mechanical equipment components, by Spark Plasma Sintering process. The microstructure, composition and mechanical properties of the Ti3SiC2/Cu composites (TSC-Cu) were investigated. Additionally, the friction and wear behaviors of TSC-Cu sliding against Inconel 718 were conducted on a pin-on-disk configuration at a sliding speed of 0.5 m/s under a load of 5 N at 25–800 °C. For comparison, the tribological property of polycrystalline Ti3SiC2/Inconel 718 was measured in an identical condition. The worn surface of TSC-Cu was analyzed by SEM, EDS and XPS, respectively. The results indicated that TSC-Cu consisted of Ti3SiC2, TiC and Cu3Si. It was worth noting that the as-formed Cu3Si uniformly distributed along the grain boundary of Ti3SiC2. As for mechanical property, the addition of Cu increased the hardness, compressive strength of TSC-Cu but lowered its flexural strength. Compared with polycrystalline Ti3SiC2, the average friction coefficient of TSC-Cu was higher at 25–400 °C whereas it was lower at 600 °C and 800 °C. The lower friction coefficient was owing to the cooperative lubricating characteristic of tribo-oxidation films containing TiO2, SiO2 and CuO. Furthermore, the wear rate of TSC-Cu was absolutely lower than that of polycrystalline Ti3SiC2, which resulted from the effective surface strengthening effect of the as-formed hard TiC product. Moreover, the wear mechanism of the composite changed from three-body abrasion wear to adhesion wear and tribo-oxidation wear, with the temperature increasing from RT to 800 °C.

Infrared Transparency and Conductivity of Amorphous Ternary Ceramic Thin Films

In this paper, using polycrystalline Ti3SiC2 powder as target, for the first time, amorphous ternary ceramic (a-TSC) thin films are successfully fabricated on quartz and glass substrates at room temperature by direct-current (DC) sputtering. The microstructural and optoelectronic properties of the a-TSC thin films are investigated by means of XRD, Raman, EDS, four-point probe system and Vis-NIR spectrometer, respectively. It is found that despite of amorphous structure and having not so good conductivity compared with Ti3SiC2, the a-TSC thin films still have enough conductivity. Furthermore, compared with ITO, FTO and AZO thin films, the transmittance of a-TSC thin films is much higher in infrared region regardless of the film thickness. It is suggested for a-TSC thin films that the film thickness should be controlled less than 150nm in order to obtain a higher transparency with an appropriate conductivity. It is believed that a-TSC thin film will find its way to be applied in optoelectronic devices in the near future.

Determination of the elastic constants of oriented polycrystalline Ti3SiC2 via coherent inelastic neutron scattering and ab-initio Molecular Dynamics – Density Functional Theory calculations

Nanolaminates such as the Mn+1AXn (MAX) phases are a class of materials with hexagonal crystal structure for which ab-initio derived elasticity tensors have been published due to sizeable single-crystals not being available. Single crystal elastic constants, however, are fundamental to understanding phase transitions, and a range of mechanical, fracture, wear and electro-mechanical properties. Recent experiments using neutron powder diffraction indicated strong shear stiffness in case of Ti3SiC2 via a large value for c44. The data presented in this paper combine neutron spectroscopy and a detailed ab-initio Molecular Dynamics – Density Functional Theory calculation and confirm the magnitude of c44 without the need of a micromechanical model. Additionally, the calculations allow estimating the remaining cij using the experimental value of c44.

Effects of neutron irradiation of Ti3SiC2 and Ti3AlC2 in the 121–1085 °C temperature range

Herein we report on the formation of defects in response to neutron irradiation of polycrystalline Ti3SiC2 and Ti3AlC2 samples exposed to total fluences of ≈6 × 1020 n/m2, 5 × 1021 n/m2 and 1.7 × 1022 n/m2 at irradiation temperatures of 121(12), 735(6) and 1085(68)°C. These fluences correspond to 0.14, 1.6 and 3.4 dpa, respectively. After irradiation to 0.14 dpa at 121 °C and 735 °C, black spots are observed via transmission electron microscopy in both Ti3SiC2 and Ti3AlC2. After irradiation to 1.6 and 3.4 dpa at 735 °C, basal dislocation loops, with a Burgers vector of b = ½ [0001] are observed in Ti3SiC2, with loop diameters of 21(6) and 30(8) nm after 1.6 dpa and 3.4 dpa, respectively. In Ti3AlC2, larger dislocation loops, 75(34) nm in diameter are observed after 3.4 dpa at 735 °C, in addition to stacking faults. Impurity particles of TiC, as well as stacking fault TiC platelets in the MAX phases, are seen to form extensive dislocation loops under all conditions. Cavities were observed at grain boundaries and within stacking faults after 3.4 dpa irradiation, with extensive cavity formation in the TiC regions at 1085 °C. Remarkably, denuded zones on the order of 1 μm are observed in Ti3SiC2 after irradiation to 3.4 dpa at 735 °C. Small grains, 3–5 μm in diameter, are damage free after irradiation at 1085 °C at this dose. The results shown herein confirm once again that the presence of the A-layers in the MAX phases considerably enhance their irradiation tolerance. Based on these results, and up to 3.4 dpa, Ti3SiC2 remains a promising candidate for high temperature nuclear applications as long as the temperature remains >700 °C.

Effect of neutron irradiation on defect evolution in Ti3SiC2 and Ti2AlC

Herein we report on the characterization of defects formed in polycrystalline Ti3SiC2 and Ti2AlC samples exposed to neutron irradiation – up to 0.1 displacements per atom (dpa) at 350 ± 40 °C or 695 ± 25 °C, and up to 0.4 dpa at 350 ± 40 °C. Black spots are observed in both Ti3SiC2 and Ti2AlC after irradiation to both 0.1 and 0.4 dpa at 350 °C. After irradiation to 0.1 dpa at 695 °C, small basal dislocation loops, with a Burgers vector of b = 1/2 [0001] are observed in both materials. At 9 ± 3 and 10 ± 5 nm, the loop diameters in the Ti3SiC2 and Ti2AlC samples, respectively, were comparable. At 1 × 1023 loops/m3, the dislocation loop density in Ti2AlC was ≈1.5 orders of magnitude greater than in Ti3SiC2, at 3 × 1021 loops/m3. After irradiation at 350 °C, extensive microcracking was observed in Ti2AlC, but not in Ti3SiC2. The room temperature electrical resistivities increased as a function of neutron dose for all samples tested, and appear to saturate in the case of Ti3SiC2. The MAX phases are unequivocally more neutron radiation tolerant than the impurity phases TiC and Al2O3. Based on these results, Ti3SiC2 appears to be a more promising MAX phase candidate for high temperature nuclear applications than Ti2AlC.

Diffusion of Ag, Au and Cs implants in MAX phase Ti3SiC2

MAX phases (M: early transition metal; A: elements in group 13 or 14; X: C or N), such as titanium silicon carbide (Ti3SiC2), have a unique combination of both metallic and ceramic properties, which make them attractive for potential nuclear applications. Ti3SiC2 has been considered as a possible fuel cladding material. This study reports on the diffusivities of fission product surrogates (Ag and Cs) and a noble metal Au (with diffusion behavior similar to Ag) in this ternary compound at elevated temperatures, as well as in dual-phase nanocomposite of Ti3SiC2/3C-SiC and polycrystalline CVD 3C-SiC for behavior comparisons. Samples were implanted with Ag, Au or Cs ions and characterized with various methods, including X-ray diffraction, electron backscatter diffraction, energy dispersive X-ray spectroscopy, Rutherford backscattering spectrometry, helium ion microscopy, and transmission electron microscopy. The results show that in contrast to immobile Ag in 3C-SiC, there is a significant outward diffusion of Ag in Ti3SiC2 within the dual-phase nanocomposite during Ag ion implantation at 873K. Similar behavior of Au in polycrystalline Ti3SiC2 was also observed. Cs out-diffusion and release from Ti3SiC2 occurred during post-implantation thermal annealing at 973K. This study suggests caution and further studies in consideration of Ti3SiC2 as a fuel cladding material for advanced nuclear reactors operating at very high temperatures.

On the relevance of kinking to reversible hysteresis in MAX phases

This paper examines the idea that reversible hysteresis in MAX phases is caused by the formation, growth and collapse of unstable, or incipient, kink bands. In situ X-ray diffraction of polycrystalline Ti3SiC2 in compression showed that residual elastic lattice strains developed during the first loading cycle and remained approximately constant afterwards. These residual strains were compressive in grains with a low Schmid factor and tensile in grains with a high Schmid factor, consistent with previous observations of plastically deformed hexagonal metals. In contrast, incipient kink bands would be expected to collapse completely, without any residual strain. Elastoplastic self-consistent simulations showed that reversible hysteresis is predicted if some grains yield by slip on the basal plane, while others remain predominantly elastic, giving both the experimentally observed magnitude of the work dissipated and its dependence on the maximum applied stress. The reversible hysteresis in single crystals was studied by cyclically indenting thin films of Ti3SiC2 and Ti3SiC2/TiC multilayers on Al2O3 substrates. The work dissipated in the multilayer films was greater than in Ti3SiC2 alone, despite the reduction in volume fraction of Ti3SiC2. Reversible hysteresis was also observed during indentation of single-crystal cubic MgO, demonstrating that this behaviour can occur if there are insufficient slip systems to accommodate the strain around the indentation. These results show that reversible hysteresis is associated with conventional dislocation flow, without the need for unstable kinking.

Interfacial Nanostructure and Electrical Properties of Ti<sub>3</sub>SiC<sub>2</sub> Contact on p-Type Gallium Nitride

In the present study, the interfacial nanostructure and electrical properties of Ti3SiC2 formed by depositing a Ti­Si­C ternary film with a composition stoichiometrically equivalent to Ti3SiC2 on p-type GaN and subsequent annealing at 1073K were analyzed by X-ray diffraction, transmission electron microscopy and direct current conduction test. The results reveal that structural changes occur by the annealing. Polycrystalline Ti3SiC2 is formed at most of the contact interface area and single crystal Ti3SiC2 at small area of the contact interface. Furthermore, other than Ti3SiC2 phase, polycrystalline Ti5Si3 and TiSi2 with different grain sizes are also formed, resulting in a formation of three-layered film after the annealing. By all these structural change, the electric conduction profiles show that the Schottky barrier height (SBH) is reduced. The estimated SBH of the Ti3SiC2 contact on p-type GaN is 0.70eV, which is 1.73eV lower than the theoretically predicted value. [doi:10.2320/matertrans.MD201206]

Corrosion behaviour of nanolaminated polycrystalline Ti3SiC2 in acidic media

The corrosion characteristics of polycrystalline samples of Ti3SiC2, fabricated by two different powder production techniques, namely hot isostatic pressing (HIP) and hot pressing (HP), were investigated electrochemically by potentiodynamic testing. The corrosion behaviour of this material was studied in dilute HCl electrolytes. The effects of solution molarity and production technique were studied. The corrosion rate of HIP Ti3SiC2 was strongly affected by HCl concentration and the degree of attack reached a maximum at 2M HCl, which was the highest concentration used. Furthermore, HP samples showed less corrosion resistance to HCl than HIP samples. This is attributed to the finer grain size produced by HP. The corrosion rates of Ti3SiC2 and Hastelloy C are of the same order, whereas that of carbon steel is greater than that of HIP Ti3 SiC2 by ∼1000 times.

Effect of Y<sub>2</sub>O<sub>3</sub> on Properties of Al/Ti<sub>3</sub>SiC<sub>2</sub> Composite

Al/Ti3SiC2 composite samples were prepared by pressless-sintering route with high purity of polycrystalline Ti3SiC2 and aluminum powders. As yttria Y2O3 being additives during sintering process, the interesting change is that impurities Al4C3, Al4SiC4 and Al3Ti phase which are familiar in products of reactions between Ti3SiC2 and aluminum disappeared and that is valuable to stability of Al/Ti3SiC2 composite in atmosphere due to hydrolyzation of Al4C3. Then the tribological properties of 50Al/ 45Ti3SiC2/5Y2O3 and 50Al/50Ti3SiC2 were investigated by sliding the composites block dryly against low carbon steel disk for the sliding speed 20 m/s and the normal pressure of 0.2~0.8MPa. It was found that with load higher, the friction coefficient of 50Al/45Ti3SiC2/5Y2O3 increased from 0.21 to 0.57 and then reduced to 0.48, which is a little higher than 50Al/50Ti3SiC2 on large scale of pressure except under 0.2 ~ 0.3 MPa, but meanwhile it is remarkable that its rate of wear maintained a nearly steady value about 1.40 × 10-5 mm3/N·m comparing with 50Al/50Ti3SiC2, which shows a valuable tribological properties called non-pressure dependence to frictional materials.

Effect of Na2SO4 and Water Vapor on the Corrosion of Ti3SiC2

The corrosion behavior of polycrystalline Ti3SiC2 was studied in the presence of Na2SO4 deposit and water vapor at 900 degrees C and 1000 degrees C. The mass gain per unit area of the samples superficially coated with Na2SO4 exposed to water vapor was slightly lower than that of the samples corroded without water vapor. The microstructure and composition of the scales were investigated by SEM/EDS and XRD. Pores were observed in the corroded sample surfaces. The main corrosion phases oil the sample surface were identified by XRD as TiO2, Na2Si2O5 and Na2TiO3. After Ti3SiC2 corroded in the presence of the Na2SO4 deposit and water vapor, the scale had a three-layer microstructure, which was different from the duplex corrosion scale formed on Ti3SiC2 beneath the Na2SO4 film without water vapor. Because water vapor penetrated the Corrosion layer and then reacted with SiO2 to form volatile Si(OH)(4), all intermediate porous and TiO2-enriched layer formed in the corrosion layer.

Oxidation layer in sliding friction surface of high-purity Ti3SiC2

Oxidation layer in sliding friction surface of high-purity Ti₃SiC₂

Corrosion behavior and strength degradation of Ti3SiC2 exposed to a eutectic K2CO3 and Li2CO3 mixture

The corrosion of polycrystalline Ti3SiC2 was studied in the eutectic Li2CO3 (68 at.%) and K2CO3 (32 at.%) Mixture at 650-850degreesC. Ti3SiC2 exhibited better corrosion resistance at 650 degreesC. However, the mass loss was fast when temperature was above 700 degreesC. It was demonstrated that the surface chemical reaction-controlled shrinking core model could be applied to describe the relationship between the degree of the corrosion and reaction time for the corrosion of Ti3SiC2 in the 700-850 degreesC temperature range. The corresponding apparent activation energy was 206 kJ/mol. Corrosion resulted in roughness of specimen surface. The fracture strength of the corroded samples was evaluated by a three-point bending test. The results showed that the degradation of the fracture strength was about 25% of the original values for the corroded specimens tip to 10% weight loss. The mechanism of the strength degradation was discussed based on the analysis of the microstructure and composition of the corroded sample. (C) 2003 Elsevier Science Ltd. All rights reserved.

Dual Tribological Behavior of a Nanolayered Ceramic: Ti3SiC2

AbstractThe MAX phases are new, thermodynamically stable, nanolayered ternary carbides and nitrides. These materials have a big potential in tribological applications due to their structure, similar to graphite or molybdenum disulfide. For example, the friction coefficients of the basal planes of Ti3SiC2 have been shown to exhibit very low (<; 5 × 10-3) friction coefficients. The aim of this study is to better understand the tribological behavior of polycrystalline Ti3SiC2 against stainless steel. Experiments were conducted on a ball-on-flat tribometer (∼ 25°C and ∼ 30% relative humidity) that simultaneously measures friction coefficients and electrical contact resistance. Different stainless steel ball diameters and normal loads were used and resulted in contact pressures between 0.35 and 1.25 GPa. Two different tribological behaviors were observed, both with relatively low friction coefficients for ceramics. The first behavior, referred to as type I, is characterized by low friction coefficients (≍ 0.15); low wear and a transfer film containing titanium and carbon formed on the ball. The other behavior, type II, is characterized by friction coefficients that starts at ≍ 0.15, and then increases to about 0.4. At the end of the experiment, the ball is worn, and compacted wear debris containing iron can be found on the plane. The two behaviors seem to be independent of contact pressure, but are rather sensitive to normal applied load. The transition between these two regimes will be discussed.

Oxidation of Ti3SiC2 at 1000°C in air

Oxidation of polycrystalline Ti3SiC2 was investigated at 1000°C by XRD and SEM. Results show that a thin SiO2-rich scale forms after short-term oxidation, but it cannot prevent further oxidation effectively. The TiO2 content increased with increasing time and covered the whole surface eventually. For the long-term oxidation, the oxide scales are layered, the outer layer is comprised of pure TiO2, while the inner layer is a mixture of TiO2 and SiO2. The position of the pre-formed SiO2 layer indicates that it can serve as an initial marker for the subsequent oxidation.

Micro-scale plastic deformation of polycrystalline Ti3SiC2 under room-temperature compression

The micro-scale plastic deformation of Ti3SiC2 ceramic under compression test was investigated. The results demonstrate that Ti3SiC2 is a damage tolerant material and samples are shear fractured upon failure. Micro-scale plastic deformation was observed in the local deformed region on the fractured surfaces. Owing to its complex crystal structure and insufficient number of slip systems, the deformation modes for polycrystalline Ti3SiC2 at room temperature are a combination of delamination and kink-band formation of elongated grains, dislocation slip and intergranular fracture, which result in micro-scale plasticity in polycrystalline Ti3SiC2.

Temperature fluctuation/hot pressing synthesis of Ti3SiC2

A novel temperature fluctuation synthesis and simultaneous densification process for the preparation of Ti3SiC2 was developed. The advantages of this novel method include low synthesis temperature, short reaction time and simultaneous densification. The microstructure and room temperature mechanical properties of the Ti3SiC2 synthesized using this method were investigated. The result demonstrated that the Ti3SiC2 ceramic consisted of mainly laminated grains. It was found, with the aid of computer simulated crystallite shape, that the laminated Ti3SiC2 grains were composed of thin hexagonal plates. These laminated grains characterized the Ti3SiC2, and were responsible for the mechanical properties of the polycrystalline Ti3SiC2 ceramic. The measured flexural strength and the fracture toughness were 470 ± 26 MPa and 7.0 ± 0.2 MPa·m1/2, respectively. The high toughness was attributed to the contribution of crack deflection, crack bridging, delaminating and grain pull-out of laminated Ti3SiC2.