TaC Layers Research Articles

High Toughness Laminated Composites Fabricated from Ti3Al(Si)C2 Filled Preceramic Paper and Nb Foils: Formation Mechanism and Influence of Laminate Architecture

In this work high strength and tough metal‐ceramic laminated composites are fabricated by spark plasma sintering (SPS) of Ti3Al(Si)C2 MAX‐phase filled preceramic papers (TAC) and ductile Nb foils. The sintering is carried out at 1250 °C and 50 MPa for 5–20 min. Various stacking techniques are used to obtain Nb/TAC laminated composites with different architectures. SPS results in the formation of reaction layer (RL) with a complex composition, which changes the thickness insignificantly with increasing sintering time. The possible formation mechanism of RL is discussed. The bending strength of Nb/TAC composites is decreased from 410 to 350 MPa when lowering the thickness of ceramic layer. The maximum fracture toughness of 10.2 MPa·m1/2 is achieved for the composite with similar individual layers thickness. The toughening is explained by complex fracture mechanisms associated with deflection and branching of cracks at interfaces, delamination, plastic deformation of Nb layers, multiple cracking and crack deflection in ceramic TAC layers.

Enhanced corrosion resistance of graphite-like carbon coatings via dense ta-C interlayer encapsulating

Taking the combination of atomic bond density, in this work, the tetrahedral amorphous carbon (ta-C) layer with high sp3 bond were especially introduced into the sp2-riched graphite-like carbon (GLC) coatings, using a hybrid filtering cathodic arc and magnetron sputtering deposition technique. The focus was elaborated on the electrochemical behavior of coatings against the stimulated chloride solutions for marine applications. Results showed that inserting dense ta-C layer significantly encapsulated the intrinsic pore defects within GLC growth, thereafter enhancing the corrosion resistance remarkably. In particular, the coating porosity was reduced from 10.84 % for pristine GLC system to 3.19 % for GLC with ta-C sealing layer, accompanying with a reduction in corrosion current to 7.8 × 10−9 A/cm2 and an increment in protective efficiency to maximum value of 95.49 %. The small angle X-ray scattering (SAXS) analysis evidenced qualitatively that the coating porosity was suppressed greatly, which were well consisted with the enhanced corrosion resistance of GLC coating with ta-C sealing layer. Furthermore, the balance between pores sealing and interfacial matching could be tailored by controlling the thickness ratio of ta-C and GLC layers, enabling the achievement of both the compact structure and the superior corrosion resistance. These observations offer a new promising strategy to develop the protective coatings for harsh marine environment that required both excellent capability against corrosion and friction with long lifetime.

High temperature oxidation and thermal properties of laminated Ti3Al(Si)C2-TiC/Nb based composites obtained by spark plasma sintering

This paper describes high-temperature oxidation behavior and thermal properties of metal-ceramic laminated composites fabricated by spark plasma sintering of Ti3Al(Si)C2-filled preceramic papers (TAC) and Nb foils. The high-temperature oxidation tests were carried out in air at 800–1300 °C. The phase composition and microstructure of the oxide layers were analyzed by X-ray diffraction and scanning electron microscopy, respectively. The thermal properties were measured by the laser flash method in the temperature range of 30–800 °C. The evolution of the microstructure of the reaction layer (RL) formed at TAC/Nb interfaces was studied. The passive oxidation of the surface TAC layers results in the formation of outer TiO2 and inner continuous Al2O3 phase at temperatures up to 1100 °C, while local accelerated oxidation occurs above 1100 °C. As a result of breakaway oxidation, a multilayer oxide structure consisting of TiO2, Al2TiO5, Al2O3 and SiO2 phases were formed. The oxidation of inner Nb layers from the side surface can lead to high volume expansion and generation of stresses in the ceramic and metal layers due to the formation of Nb2O5 oxide phase. The stress relaxation occurred by complex mechanisms, including plastic deformation of the Nb layers, delamination at the interfaces, and formation of cracks in the reaction and TAC layers. The high temperature exposure results in slight increase in columnar grains in the reaction layers at 1200 °C, while the thickness of RL remains unchanged. The TAC/Nb composites exhibit higher thermal conductivity and diffusivity compared to monolithic TAC ceramic composites.

Diamond-like films of tetrahedral amorphous carbon deposited by anodic arc evaporation of graphite

A physical vapor deposition process using anodic arc evaporation in combination with a hollow cathode arc discharge was applied to the evaporation of graphite for deposition of hydrogen-free carbon layers. The diamond-like carbon (DLC) films deposited on 100Cr6 steel substrates were investigated by nanoindentation, Raman spectrometry, FE-SEM, AFM and spectroscopic ellipsometry. The relationships between the process parameters and the coating properties are discussed. Coatings deposited without bias voltage at substrate temperatures <200 °C are very hard (61–75 GPa) with also very high Young's modulus (588–685 GPa). The evaluation of the Raman spectra indicated a high proportion of tetrahedral sp3 bonds in the range of 70–88 %. Obviously, the vapor particle energies are high enough to achieve such high hardness values even without application of a bias voltage. The coatings proved to be completely droplet-free and have a very low surface roughness as confirmed by FE-SEM and AFM. The deposition rates in the range of 4–18 nm/s are exceptionally high for such tetrahedral amorphous carbon (ta-C) coatings, which is a good prerequisite for industrial applications. The ta-C layers with very high hardness and smooth surface are well suited as wear resistant layers.

Characteristics of physical properties of the sliding and its surrounding layers in landslides caused by the 2018 Hokkaido Eastern Iburi Earthquake

A 6.6-Mw earthquake struck the Iburi region of Hokkaido, Japan, in 2018, triggering massive landslides. Most of these landslides were shallow and occurred mostly in the Atsuma and Abira towns. Ta-c and Ta-d tephra layers have been found in the Towa landslide at Atsuma from the Tarumai volcano, while Ta-d, En-a, and Spfa-1 tephra layers have been found in the Mizuho landslide at Abira from the Tarumai and Eniwa volcanos, as well as the Shikotsu caldera. Field observations from previous studies revealed that the sliding layers were located in the Ta-d and En-a layers at the Towa and Mizuho landslides, respectively. Unlike previous research on earthquake-induced landslides, which were investigated using mechanical properties, this study investigates the characteristics of physical properties, saturated permeability properties, and content of clay minerals on sliding and surrounding tephra layers. Results from this study reveal that the physical properties of sliding layers from two landslides demonstrated the same characteristics: non-plastic soil with a low density of soil particles, void ratio, and dry density; these characteristics could influence earthquake-induced landslides. It also reveals a relationship between the plasticity chart and the age of tephra materials, including the relationship between the weathering process and density of soil particles and the dissimilarity in characteristics of saturated permeability properties in tephra materials.

Ta2C precipitation after low pressure carburizing of tantalum

Low pressure carburizing cycles were carried out on tantalum samples. A Ta2C precipitates layer is present under the TaC and Ta2C surface layers. As the TaC and Ta2C layers, the Ta2C precipitates layer (TPL) grows by carbon diffusion in tantalum matrix. Specific zones with low precipitates content were observed at the bulk surface and in the tantalum grain boundaries. Other zones with lower precipitates content are also present in carburized and annealed samples. All the zones with low precipitates content are induced by the Ta2C precipitation mechanism described by Dahmen et al. [1,2]. Indeed, Ta2C needs vacancies to precipitate. If vacancy sinks are present at the bulk surface, at a grain boundary or in a dislocation, then Ta2C precipitation is highly reduced. Another effect of the annealing treatment is an increase of the Ta2C precipitates layers growth rate.

Mechanical properties of tantalum carbide from high-pressure/high-temperature synthesis and first-principles calculations.

As a member of the refractory metal carbide family of materials, TaC is a promising candidate for ultra-high temperature ceramics (UHTC) with desirable mechanical strength. TaC sample quality and therefore mechanical properties are strongly dependent on synthesis method, and atomistic origins of mechanical failure are difficult to assign. Here, we have successfully synthesized high quality densified TaC samples at 5.5 GPa and 1400 °C using the high pressure and high temperature (HPHT) sintering method, with Vickers hardness determined to be 20.9 GPa. First-principles calculations based on the recently developed strain-stress method show that the ideal indentation strength of TaC is about 23.3 GPa in the (11[combining macron]0)[001] direction, in excellent agreement with experimental results. The detailed indentation shear deformation analysis and structural snapshots from the calculations indicate that the slip dislocations of TaC layers are the main structural deformation mode during the Vickers indentation process, and that the strong directional Ta-C bonds are responsible for the high mechanical strength of TaC. HPHT synthesis is shown to produce TaC samples with superior strength, and together with accurate first-principles calculations offers crucial insights for rational design and synthesis of novel and advanced UHTC materials.

In situ fabricated metal-carbide with core–shell structure for high impact-toughness iron-matrix composite

ABSTRACTA series of metal-carbide (Ta–TaC, Nb–NbC and W–WC) with core–shell structure for iron-matrix composites are fabricated by in situ solid-phase diffusion. Results show that the formation of metal-carbide with a rod-shaped core–shell structure, in which the metal-rod surface was covered with a carbide shell layer, in the iron- matrix after in situ solid-phase diffusion. The TaC, NbC, and WC shell layers are in situ synthesised by the diffusion of carbon atoms from the iron-matrix onto the surface of the Ta, Nb, and W rods, respectively. Metallurgical integration occurs between metal-carbide and iron-matrix. The metal-carbide-reinforced iron-matrix composites show excellent impact resistance, and the shell-layer hardness is extremely high.

The roles of Ag layers in regulating strengthening-toughening behavior and tribochemistry of the Ag/TaC nano-multilayer films

Though the high hardness nature of transition-metal carbides or nitrides TMC(N) has been extensively exploited in protective films for various tools and wear-resistant films for the machine parts, improving toughness and reducing friction continue to be immensely challenging. Herein, we prepared the Ag/TaC nano-multilayers with various Ag thickness (lAg) ranging from 2 nm to 14 nm by magnetron sputtering, successfully achieving enhanced hardness, toughness and wear-resistance ability, together with reduced friction at lAg = 2 nm. In addition, lAg-dominated growth, strengthening-toughening behavior, and tribochemistry of the Ag/TaC nano-multilayers were investigated. These studies revealed that as very thin Ag layers (=2 nm) are alternatively inserted into TaC layers, discontinuous Ag/TaC nano-multilayer structure constructs in term of poor wetting induced the growth of discontinuous Ag layers, remarkably enhancing the hardness to ∼42 GPa associated with the Orowan mechanism and improving toughness and wear-resistance ability. Together with all these remarkable properties, the coefficient of friction (CoF) rapidly drops to ∼0.227, because of the formation of lubricious Ag nanoclusters and AgTaO3 phase on the contact surface during sliding. However, at thicker lAg, continuous soft Ag layers would form, and these are easy to activate the slip in soft Ag layers, thereby decreasing the hardness and toughness as well as dramatically decreasing the wear-resistance, but accompanying with further declinein the CoF owing to the increment of lubricious silver tantalate (AgTaO3) and Ag nanoclusters. Obviously, the incorporation of very thin Ag layers into the TMC(N) layers to construct nano-multilayer structure may provide an effective strategy to boost the comprehensive properties including hardness, toughness, friction, and wear-resistance.

Tribochemistry dependent tribological behavior of superhard TaC/SiC multilayer films

TaC/SiC multilayer films with different thicknesses of SiC layer (tSiC) were deposited by alternating sputtering of TaC and SiC composite targets in the discharged Ar gas. The SiC layers were forced to crystallize to form a coherent interface with TaC layers at tSiC ≤ 0.8 nm, resulting in an enhancement in both hardness and fracture toughness. When tSiC reached 0.8 nm, the film exhibited the highest hardness (46 ± 2 GPa) and best toughness (4.2 ± 0.2 MPa m1/2). With further increase in tSiC, SiC lost crystallinity and transformed into amorphous state. The deterioration in coherent TaC/SiC interface resulted in the reduction of both hardness and toughness. The wear rate was also dependent on hardness and fracture toughness of multilayers films. The film with the lowest wear rate at tSiC = 0.8 nm was obtained. In addition, the alternating insertion of SiC layers into TaC layers tailored the optimization of film's tribological performance. Lubricious Ta silicate (TaSiOx) and amorphous carbon (a-C) on the surface of wear tracks were identified by SEM and XPS, and wear debris was identified by TEM. Consequently, it was believed that coefficient of friction (CoF) in multilayers was reduced when compared to the individual monolayers because of the synergistic effect of lubricious a-C and TaSiOx. Furthermore, steel and Al2O3 balls were also used as counterparts, and the same variation tendency was observed in both CoF and wear rate as when WC balls were used. Thus, improvement of wear-resistant ability and at the same time reduction of CoF can be simultaneously achieved in superhard TaC/SiC multilayers.

Crystallization of SiC and its effects on microstructure, hardness and toughness in TaC/SiC multilayer films

A series of TaC/SiC multilayer films with different SiC thicknesses (tSiC) have been prepared by magnetron sputtering and their microstructure, hardness and toughness investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning electron microscopy (SEM) and nanoindentation. Results show that SiC crystallized and grew coherently with TaC layers at low tSiC (≤ 0.8nm), resulting from the template effect of TaC layers. Maximum hardness and toughness of 46.06GPa and 4.21MPam1/2 were achieved at tSiC = 0.8nm with good coherent interface. With further increasing of tSiC, SiC layers partially transformed to an amorphous structure and gradually lost their coherent interface, leading to a rapid drop in hardness and toughness. The crystallization of SiC layers and the coherent growth are required to achieve superhardness and high toughness in the TaC/SiC multilayers.

Tantalum carbide coating via wet powder process: From slurry design to practical process tests

TaC coatings were deposited on graphite substrates via wet powder forming and sintering to potentially achieve both a highly reliable and low-cost process. Non-aqueous solvent mixtures of TaC slurries were optimized through characterization of raw materials and a novel design guide based on Hansen solubility parameters. The optimized TaC slurries enabled the formation of high-quality TaC powder compact films with ultrahigh powder packing densities of ≥70% (ca. 85% at maximum), which contributed to prevent defect formation in the TaC coatings after sintering. The TaC-coated components were tested in practical high-temperature processes, such as AlN and SiC bulk single crystal growth processes, and a SiC device fabrication process, which confirmed sufficiently low-levels of impurity incorporation and surface contamination from the TaC layers. The novel TaC-coated graphite components will contribute to higher quality and lower cost for bulk crystal growth, device fabrication, and epitaxial film growth processes of group-III nitrides and SiC.

Simple and quick enhancement of SiC bulk crystal growth using a newly developed crucible material

Newly developed highly reliable low-cost TaC-coated graphite materials prepared by a wet ceramic process were applied to SiC sublimation growth. We demonstrated an increased long-duration growth rate and a resultant increase in crystal size by a factor of ∼1.2 (experimental value) after 24 h of growth [and ∼1.5 (extrapolated value) after the optimum duration of 53.1 h] by simply and quickly replacing graphite crucibles with TaC-coated graphite crucibles. Growth with the TaC-coated graphite crucibles reduced source gas leakage and increased the material yield for single crystals because the TaC layers were gas-tight and had a low emissivity.

Ultrathin undoped tetrahedral amorphous carbon films: thickness dependence of the electronic structure and implications for their electrochemical behaviour.

In this paper we show that the electronic properties of ultrathin tetrahedral amorphous carbon (ta-C) films are heavily dependent on their thickness. By using scanning tunnelling spectroscopy, Raman spectroscopy, and conductive atomic force microscopy, it was found that a decrease of ta-C thickness from 30 to 7 nm leads to (i) the narrowing of the band gap; (ii) appearance of shallower monoenergetic traps as well as the increase of their concentration; (iii) the increase of the equilibrium concentration of free charge carriers and their mobility; which were caused by (iv) the increase in the sp(2) fraction. However, beyond a certain ta-C thickness (7 nm) the electronic properties of the studied samples start to deteriorate, which is highly likely related to titanium oxide formation at the Ti/ta-C interface. The same tendency is observed for the sample with beforehand air-formed native titanium oxide at the interface. With respect to the last point, it is suggested that the ta-C layer has no uniform coverage if its thickness is small enough (less than 7 nm). The experimental results were rationalized by detailed atomistic simulations. By using the so-called "Tauc plot" we introduce the possibility of the coexistence of bulk and surface band gaps originating from the large increase in sp(2) bonded carbon atoms in the surface region compared to that in the bulk ta-C. The results from the simulations were found to be consistent with the experimental measurements. The previously stated variation in the electronic properties of the layers as a function of their thickness was also exhibited in the electrochemical properties of the samples. It appears that the thinner ta-C layers had more facile electron transfer kinetics as determined with a ferrocenemethanol (FcMeOH) outer sphere redox system. However, if the ta-C layer thickness was reduced too much, the films were not stable anymore.

Deformation of Ultra-Thin Diamond-Like Carbon Coatings Under Combined Loading on a Magnetic Recording Head

Ultra-thin diamond-like carbon (DLC) coatings are used in precision engineering applications, such as magnetic storage devices, to protect intricate structures from wear and corrosion. A DLC coating typically consists of hard amorphous carbon in combination with an interlayer such as silicon (Si), to improve adhesion to the substrate material. Deformation and delamination of these coatings, even in part, could expose the substrate material and compromise its integrity and functionality. We have implemented a molecular dynamics model to quantify the strength of the interface between an ultra-thin tetrahedral amorphous carbon coating, a Si layer, and a permalloy (NiFe) substrate, under combined normal and tangential loading that mimics accidental contact between the recording head and the disk of a hard drive. We have evaluated the effect of the thickness of the different coating layers on deformation and interfacial strength of the coating during combined loading. The results indicate that deformation occurs primarily in the Si layer, and at the interface between the Ni–Si and the Si–C layers. Permanent separation of the Si and ta-C layers is observed, which gradually increases with multiple combined loading cycles. We find that increasing the Si and carbon layer thickness strengthens the DLC coating. However, increasing the carbon layer thickness has a larger effect on coating strength than increasing the Si layer thickness.

Stacked sputtering process for Ti, Ta, and W carbide formation for gate metal application

A sputtering process using multi-stacking of carbon and metal thin films with subsequent annealing process to reactively form metal carbides (TiC, TaC, and W2C) has been presented. Grain sizes of the carbides are as small as 3.9, 3.2, and 1.9 nm for TiC, TaC, and W2C, respectively. Work functions of TiC, TaC, and W2C layers have been extracted as 4.3, 4.7, and 4.9 eV, respectively, relatively high values due to oriented growth. W2C layer formed by the presented process gives high potential to form carbides with nano-sized grain and high work function for gate electrode application.

Investigation of nano structures on ta-C films made by gallium FIB lithography

Focussed ion beam (FIB) writing is used as a direct lithography process on tetrahedral amorphous carbon (ta-C) layers with high sp3 content. Ion induced conversion of sp3 to sp2 bonded carbon atoms by means of 30keV Ga+ irradiation takes place and conducting sp2 rich sites were formed on the nano scale. Investigations of the conductivity of graphitic layers embedded in thin ta-C films were done concerning the influence of Ga+ fluence and substrate temperature. Van-der-Pauw (VDP) structures were produced by Ga+-FIB lithography partly in combination with optical lithography. Sheet resistance decreases with increasing Ga+ fluence and the lowest RS was achieved with 290Ω sq−1 at 1.6×1017cm−2 ion fluence. Conductivity of sp2 structures could be further improved by rapid thermal annealing (RTA) without degradation of the ta-C insulation. Graphitic nano wires (NWs) were fabricated by FIB writing which dimensions and conductivity were characterised. Width and height of the NWs increase with increasing Ga+ fluence in a width range of 50 to 150nm and height range of 3 to 23nm.

Effect of the carbon ion energy on the microstructure of ta-C/Cr multilayers

The effect of the carbon ion energy applied in a pulsed arc deposition process on the morphology of the interface between Cr layers and subsequently deposited tetrahedral bonded carbon (ta-C) coatings was investigated. The carbon ion energy was varied through the bias voltage between ~25 and 500eV. Small-angle and wide-angle X-ray scattering (SAXS and WAXS), high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) were used as the central experimental methods for microstructure analysis. In order to improve the scattering power of the ta-C/Cr interfaces for SAXS, the samples were prepared in the form of ta-C/Cr multilayers. SAXS revealed density and thickness of individual layers in the multilayer stack, and the roughness and morphology of the ta-C/Cr interfaces. The density of ta-C layers ranged between 2.72 and 3.15g/cm3, which was related to the amount of sp3 bonds between 37% and 71%. The amount of the sp3 bonds calculated from the density of the ta-C layers agreed well with the amount of the sp3 bonds determined using EELS. HRTEM confirmed both the increase of the interface roughness with increasing carbon ion energy and the changes in the correlation of the interface corrugations observed by SAXS. Furthermore, HRTEM approved the crystallinity of Cr layers revealed by WAXS.

Direct Fabrication of Periodic Structures on Surfaces

AbstractThis report introduces a recently developed method for the direct fabrication of 2D and 3D structures. The technique involves the modulation of the laser beam intensity by generating interference patterns. By using high power lasers it is possible to directly modify the material's surface obtaining micro‐ and submicrometer periodic arrays with controlled spatial modulation. Some examples of fabricated structures on metals, polymers as well as ta‐C layers are discussed.

Successive superconducting transitions inTa2S2Cstudied by electrical resistivity and nonlinear ac magnetic susceptibility

${\mathrm{Ta}}_{2}{\mathrm{S}}_{2}\mathrm{C}$ compound undergoes superconducting transitions at ${T}_{\mathit{cl}}=3.60\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{\mathit{cu}}=9.0\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The nature of successive superconducting transitions has been studied from electrical resistivity and linear and nonlinear ac magnetic susceptibilities. The resistivity $\ensuremath{\rho}$ at $H=0$ shows a local maximum near ${T}_{\mathit{cu}}$, a kinklike behavior around ${T}_{\mathit{cl}}$, and reduces to zero at below ${T}_{0}=2.1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $\mathrm{ln}\phantom{\rule{0.2em}{0ex}}T$ dependence of $\ensuremath{\rho}$ is observed at $H=50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ at low temperatures, which is due to a two-dimensional weak-localization effect. Below ${T}_{\mathit{cu}}$, a two-dimensional superconducting phase occurs in each TaC layer. The linear and nonlinear susceptibilities ${\ensuremath{\chi}}_{1}^{\ensuremath{''}}$, ${\ensuremath{\chi}}_{3}^{\ensuremath{'}}$, ${\ensuremath{\chi}}_{5}^{\ensuremath{'}}$, and ${\ensuremath{\chi}}_{7}^{\ensuremath{'}}$ as well as the difference $\ensuremath{\delta}\ensuremath{\chi}$ $(={\ensuremath{\chi}}_{\mathit{FC}}\ensuremath{-}{\ensuremath{\chi}}_{\mathit{ZFC}})$ between the field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities start to appear below $6.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the onset temperature of irreversibility. A drastic growth of the in-plane superconducting coherence length below $6.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ gives rise to a three-dimensional superconducting phase below ${T}_{\mathit{cl}}$ through interplanar Josephson couplings between adjacent TaC layers. The oscillatory behavior of ${\ensuremath{\chi}}_{3}^{\ensuremath{''}}$, ${\ensuremath{\chi}}_{5}^{\ensuremath{''}}$, and ${\ensuremath{\chi}}_{7}^{\ensuremath{''}}$ below ${T}_{\mathit{cl}}$ is related to the nonlinear behavior arising from the thermally activated flux flow.