TiC Surface Research Articles

Investigation on Ti3AlC2/TiC interfacial properties: Experiment and first-principles calculation

The (Ti3AlC2+Al2O3)p/Al3Ti composite was fabricated through hot pressing. The Ti3AlC2(0001)/TiC(111) interface in the (Ti3AlC2+Al2O3)p/Al3Ti composite was clearly observed, and its interface properties were conducted using first-principles calculations. The results show that: for the TiC(111) surface, the surface energy of Ti-terminated TiC(111) is lower than that of C-terminated TiC(111), indicating that Ti-terminated TiC(111) is more stable. For the Ti3AlC2(0001) surface, surface energy from high to low is: C-terminated > Ti(Al)-terminated > Ti(C)-terminated > Al-terminated Ti3AlC2(0001) surfaces in environments of Ti-rich and Al-rich, while the stability is opposite. Among the 24 interface models, Ti(Al)-on-top-C1 has the highest work of adhesion (13.1552 J/m2) and the lowest interface energy (−3.3553 J/m2), indicating the best interface bonding strength and stability. During the relaxation process, the stacking sites of the three interface models, which were named C-hcp-hollow-C2, C-fcc-rollow-C3, and Ti(Al)-on-top-C1, changed. By analyzing the electronic structure of eight interface models, it was found that covalent and metal bonds form at the interface, such as Ti-C covalent bond, Al-C covalent bond, C-C covalent bond, and Ti-Ti metallic bond. This indicates that the type of chemical bonds at the Ti3AlC2(0001)/TiC(111) interface is determined by the TiC(111) surface termination and Ti3AlC2(0001) surface termination. In the first-principles tensile test, the critical strain and ultimate tensile strength of the Ti(Al)-on-top-C1 interface model are 8 % and 23.55 GPa, respectively.

Research on the electronic properties of TiC/γ-Fe and TiB2/γ-Fe interfaces based on first-principles calculations

Addressing the unclear understanding of the bonding mechanisms at the TiC/γ-Fe and TiB2/γ-Fe interfaces, the atomic structure, adhesive work, and electronic properties of the TiC (111)/Fe (111) and TiB2 (001)/Fe (111) interfaces were investigated using first-principles calculations. The results reveal that the C-terminated surface of TiC (111) and the B-terminated surface of TiB2 (001) exhibit high surface activity, facilitating their binding with the Fe (111) surface. The adhesive energy at the TiC (111)/Fe (111) interface is 13.04 J/m2, with a wetting angle of 41.32°, while at the TiB2 (001)/Fe (111) interface, the adhesive energy and wetting angle are 6.08 J/m2 and 74.69°, respectively. TiC demonstrates better wettability and stronger binding strength with γ-Fe surface. At the interface, the C-p orbitals and B-p orbitals hybridize with Fe-d orbitals to form bonds. The population of Fe-C bonds and Fe-B bonds is 0.46 and 0.02, respectively, indicating that Fe-C bonds exhibit covalent characteristics, while Fe-B bonds display ionic characteristics, with Fe-C bonds being stronger than Fe-B bonds. Statistical analysis of charge distribution and charge difference reveals electron accumulation near C and B atoms at the interface. Moreover, the electron transfer at the TiC (111)/Fe (111) interface is significantly greater than that at the TiB2 (001)/Fe (111) interface, which is the fundamental reason for the higher binding strength at the TiC (111)/Fe (111) interface.

Investigate the effect of C coating on the wetting and reaction mechanism of Sc2W3O12 and AgCuTi filler

Mitigating the interface reaction between negative thermal expansion materials and filler metals is crucial for enhancing the functionality of negative thermal expansion materials. However, the general approach to mitigate interface reaction, such as minimizing both reaction time and temperature, will result in insufficient metallurgical reactions necessary to establish reliable connections during the brazing process. Through wetting process analysis and first-principles calculations, the effect of C coating on the wetting and reaction mechanism of Sc2W3O12 and AgCuTi filler is revealed. The wetting experiment indicates that the C coating not only improves the wettability but also mitigates the interface reaction. According to the interfacial microstructure, 18 kinds of Ti/Ti3O5 (001) and 3 kinds of Ti/TiC (100) adsorption systems are constructed to calculate the adsorption characteristics. According to the calculations, the Ti/TiC (100) system has lower adsorption energy than Ti/Ti3O5 (001) system, indicating that Ti atoms are more easily adsorbed on the TiC surface. The charge density difference contour shows that the charge repulsion on the TiC surface restricts the entry of Ti atoms. This work provides new insights into the mechanism of how the C coating to reduce interfacial consumption of the negative thermal expansion materials.

Insight into the Grain Refining Mechanism of Al–Ti–C and the Interfacial Properties between the Inoculants and Aluminum

The grain refinement mechanism of Al–Ti–C grain refiners used in the casting procedure of aluminum has been studied by first‐principles calculation. Considering the controversy over Al3Ti, TiC, or Al3Ti–TiC as nucleation sites, the adsorption behavior of possible nucleation surfaces for Al atoms, including adsorption energies and adsorption sites, as well as the interfacial properties of Al/Al3Ti/TiC are calculated and compared. In the results, it is shown that the Al3Ti(112) surface has a stronger adsorption capacity for Al than TiC. By using the edge‐to‐edge matching crystallographic model and the calculation of the adsorption energy, it is found that the monolayer Al3Ti(112) can be stabilized on the TiC(100) surface, and the combination of TiC and monolayer Al3Ti can significantly improve the adsorption capacity of Al. Interfacial electronic structure analysis shows stronger chemical bonding between Al and Ti at the Al/Al3Ti/TiC interface compared to Al/Al3Ti, which explains that the monolayer Al3Ti covering the TiC surface has the strongest adsorption capacity for Al atoms. Therefore, it is speculated that TiC with a monolayer of Al3Ti covering the surface will serve as a potential nucleation site during grain refinement, which will provide a theoretical reference for future research work.

Complex material analysis of a TiC coating produced by hot pressing with optical light microscopy, EDS, XRD, GDOES and EBSD

The present study investigates the interface between carbon steel and titanium samples annealed at different temperatures (ϑ1 = ▪ and ϑ2 = ▪). In both cases, an observable layer forms at the interface, with its thickness increasing from tϑ1 = 2.75 ± ▪ at ▪ to tϑ2 = 8.86 ± ▪ at ▪. The layer's composition and thickness evolve with temperature. Analysis reveals approximately 40 at.-% carbon concentration in the exterior region, indicating likely titanium carbide creation. X-ray diffraction identifies titanium carbide peaks, while microscopy and elemental mapping confirm compositional gradients at the interface. Electron Backscatter Diffraction (EBSD) shows a gradient in grain size near the TiC surface, reflecting TiC nucleation rates. XRD data detect both titanium carbide and titanium phases, with TiC becoming more prominent at ▪. Rietveld analysis further confirms TiC formation. Notably, distinct diffraction patterns on the contact and rear sides suggest a Ti(C, O, N) presence. Depth profiles exhibit varying surface and depth carbon concentrations, attributed to temperature effects. The study successfully demonstrates TiC coating fabrication through hot pressing, wherein Ti(C, O, N) coatings arise from titanium's affinity for reacting with oxygen and nitrogen. This research contributes to the understanding of phase transformations and interfacial properties in titanium-carbon steel systems.

The adsorption behaviors of Cl2 on TiC (100) surface: A density functional theory study

It is important to study the adsorption behavior of Cl2 molecules on the TiC surface and the formation and transfer of reaction products to understand and to regulate the low-temperature chlorination reaction process of carbonized titanium-bearing slag. Based on the first-principles ab initio calculation method of density functional theory (DFT), the adsorption models of Cl2 molecules on the TiC (100) intact surface and the surface with carbon vacancy are established, and the adsorption structures, electronic properties, differential charge density and partial density of states (PDOS) are calculated and analyzed. The formation process and the optimal formation path of the adsorption reaction product are explored. The investigated results indicate that the Cl atoms in the system bonded to the surface Ti atoms are more stable than the structure formed by bonding to the surface C atoms. The electron flow direction is Ti → C and Ti → Cl. In the reaction of TiC with Cl2 molecules, the TiCl3 intermediate is easier to form than the TiCl2 intermediate. The energy barrier to be overcome to form the TiCl3 intermediate is lower and the energy to be supplied is easily met by external conditions. TiCl3 is then further chlorinated to form TiCl4.

C-Vacancy Mediated Methane Activation and C–C Coupling on TiC(001) Surfaces: A First-Principles Investigation

C-Vacancy Mediated Methane Activation and C–C Coupling on TiC(001) Surfaces: A First-Principles Investigation

Electronic characteristic, tensile cracking behavior and potential energy surface of TiC(111)/Ti(0001) interface: A first‑principles study

The preparation of TiC coating on Ti alloy surface to improve the wear resistance has attracted attention from researchers in aerospace field. The service life of TiC coating is related to the interfacial adhesion properties between TiC coating and Ti alloy substrate. However, it is difficult to explain its interfacial adhesion mechanism by experimental methods. Based on the termination of atoms on the TiC surface, two TiC/Ti interface models named as the C-terminated-TiC(111)/Ti(0001) and Ti-terminated-TiC(111)/Ti(0001) interface were constructed by first-principles. The interfacial electronic characteristic of C—Ti bond is a mixture of polar covalent and metal bonds, and that of Ti—Ti bond is the metal bond. The tensile strains of both C-terminated-TiC(111)/Ti(0001) and Ti-terminated-TiC(111)/Ti(0001) interface in the fracture stage are ranged from 12% to 14%. Their maximum tensile stresses are 16.201 GPa and 15.590 GPa, respectively. The sliding potential energy surface maximum of C-terminated-TiC(111)/Ti(0001) and Ti-terminated-TiC(111)/Ti(0001) interface are 5.387 J/m2 and 0.271 J/m2, respectively. And the sliding potential barriers on the minimum energy path are 2.094 J/m2 and 0.136 J/m2 with an ideal shear strength of 20.32 GPa and 1.61 GPa, respectively. In summary, the interfacial adhesion property of C-terminated-TiC(111)/Ti(0001) interface is better than that of Ti-terminated-TiC(111)/Ti(0001) interface.

Unveiling the role of adsorbed hydrogen in tuning the catalytic activity of CO2 conversion to methanol at Cu/TiC surfaces

First-principles calculations were performed to explore the detailed reaction mechanisms of CO2 conversion to methanol over two size-selected copper clusters supported on the TiC(001) surface, in which three potential routes including the formate, the reverse water-gas-shift (RWGS) + CO-hydrogenation, and the CO bond cleavage pathways were considered. Our findings show that the adsorption and migration of hydrogen atoms have obvious impact on the catalytic activity for CO2 conversion. The limited size of the active site of small Cu cluster with a planar configuration results in that the formate route is difficult to occur because the creation of H2COOH* intermediate requires the spillover of H atoms from the substrate to the active center by overcoming a high kinetic barrier. On the contrary, the polyhedron structure in the large Cu cluster can act as a reservoir for the hydrogen adsorption, making it possible to produce methanol via the formate pathway. Although the RWGS + CO-hydrogenation pathway is identified as the preferred reaction pathway on both surfaces, the relatively strong binding of hydrogen on the large copper cluster causes difficulty in the migration of H toward the reaction intermediate. The results of microkinetic simulations indicate that the rate-limiting steps are sensitive to cluster size, and small Cu cluster exhibits better catalytic activity for the conversion of CO2 to CH3OH.

New insights into the adsorption of Fe on Ti(CxN1−x) surface: First-principles study

The present study systematically investigates the adsorption behavior of Fe atoms on the titanium carbonitride (Ti(CxN1−x)) surface during the initial stage of austenite nucleation. The results indicate that the formation energy of TiN is lower than that of TiC, suggesting that TiN is more likely to form. However, it is observed that the TiN content in the steel is so little. The C-Top site of TiC exhibits the highest adsorption capacity for Fe atoms. For the various stacking models evaluated for Ti(CxN1−x), the CNNC model exhibits the most favorable adsorption performance for Fe atoms in Ti(C0.5N0.5). Furthermore, TiC exhibits superior adsorption performance as compared to TiN, which is attributed to its shorter bond length and higher electron orbital overlap. When the Fe atoms adsorb on the TiC(001) surface, covalent bonds are formed among C, Ti, and Fe atoms, revealing the presence of metal bonds.

In-situ preparation, modulation and mechanism of refractory metal carbide gradient coating

TiC, ZrC and TaC modified layers were in-situ prepared on graphite matrix by chemical vapor infiltration method with metal salts as the activator. Taking the TiC modified layer as an example, through thermodynamic calculation and experiment, the thermal decomposition process of raw materials (Ti/K2TiF6) was analyzed, the formation mechanism of TiC was determined, and the distribution of TiC modified layer was modulated. The results show that activator K2TiF6 has higher decomposition temperature than NH4Cl, which is conducive to improving the utilization rate of raw materials in the gas infiltration process. Increasing the content of Ti powder can increase the concentration of reaction gas and contribute to the formation of TiC modified layer. When the molar ratio of Ti to K2TiF6 is 3:1, the surface thickness and infiltration depth of TiC are 5.42 and 136.24 μm, respectively. Increasing the reaction temperature can improve the rate of in-situ reaction and the thickness of TiC surface layer. When the experimental temperature rises to 1600 °C, the TiC surface layer thickness increases to 20.27 μm.

Adhesion, tensile and shear properties of a-C/TiC interface: A first-principles study

TiC nanocrystals in amorphous carbon (a-C) films can improve the adhesion property between a-C films and substrate. The service life of a-C films is related to the interfacial adhesion property between a-C films and TiC. However, it is difficult to explain its interfacial adhesion mechanism by experimental methods. In this paper, based on the stacking of the interface models, two a-C/TiC interface models named as the C(110)/C-edge-TiC(110) and C(110)/Ti-edge-TiC(110) interfaces were constructed by first-principles method. After relaxation, their interfacial adhesion work are 3.624 and 3.622 J/m2, and interfacial energy are 6.831 and 6.833 J/m2, respectively. The interfacial electronic characteristic of CC bond at the interface is a mixture of non-polar covalent bond and metal bond, and the CTi bond is a mixture of polar covalent bond and metal bond. The tensile strains of both the C(110)/C-edge-TiC(110) and C(110)/Ti-edge-TiC(110) interfaces in the fracture stage are ranged from 9 % to 14 %, and the fracture location of the interface models all occur in the inner of the TiC surface model near the interface. The maximum tensile stresses of the C(110)/C-edge-TiC(110) and C(110)/Ti-edge-TiC(110) interfaces are 36.873 and 36.955 GPa, respectively. Therefore, the tensile strength of the C(110)/Ti-edge-TiC(110) interface is stronger than that of the C(110)/C-edge-TiC(110) interface. The sliding potential energy maximum of the C(110)/TiC (110) interface is 0.335 J/m2, and the sliding potential barrier on the minimum energy path is 0.090 J/m2 with an ideal shear strength (τMEP) of 0.030 GPa.

Investigation on TaC (1 1 1)/TiC (1 1 1) interface for coating with high-performancse corrosion resistance: First-Principle calculations and experiments

In this study, first-principle calculation is used to investigate the TiC (111)/TaC (111) interface in terms of surface energy, work function, adhesion energy, and electronic structure. Compared with TaC (111), the TiC (111) surface shows a higher surface energy and a lower electronic work function. The surface passivation of TiC can be achieved by covering the TaC layer. Besides, the interface bonding configuration of TiC (111)/TaC (111) that relates to corrosion resistance is studied in detail. In experimental verification, a set of TiC-based coatings are investigated by potentiodynamic polarization, potentiostatic polarization, and impedance. Compared with pure TiC and TiC/C coatings, the TaC/TiC/TaC/C structure exhibits the lowest corrosion current density of 0.28 μA cm−2 and the lowest corrosion tendency. SEM observation also reveals its minimal morphological damage after potentiostatic polarization. The modified multilayer coating presented in this work is successfully used as the protecting structure of stainless-steel bipolar plate, which may be of great significance to the light weight metallic bipolar plate in proton exchange membrane fuel cells.

Synthetization of TiC surface hardening using TIG melting technique - The effect of working distance

Surface hardening with and without hard reinforcing material can be achieved by melting technique on the top substrate area via high energy input fusion. The aim is to apply TIG torch heat input of 1344 J/mm at two different working distances such as 0.5 mm and 1.5 mm to incorporate the preplaced TiC particulates on the surface of AISI 4340 low alloy steel. Results pertaining to the microstructural features, melt pool sizes, defects, topographies and microhardness were investigated using the optical microscope, scanning electron microscope and Vickers microhardness tester. The results showed that the heat loss through the arc column at 1.5 mm of working distance resulted small melt pool geometry and poor dissolution of TiC particulates. Lower working distance increases heating value leading more precipitation of TiC whilst minimizing those the undissolved particulates. The surface submerged at working distance of 0.5 mm produced microhardness ranging from 800 HV to 1500 HV. The work suggested that heat fusion can be enhanced by marginal working distance whilst saving the energy to melt instead of increasing the used voltage, current, decreasing scanning speed, or adding fluxes or shielding with special gasses.

Catalytic Reduction of Carbon Dioxide on the (001), (011), and (111) Surfaces of TiC and ZrC: A Computational Study.

We present a computational study of the activity and selectivity of early transition-metal carbides as carbon dioxide reduction catalysts. We analyze the effects of the adsorption of CO2 and H2 on the (001), (011), and metal-terminated (111) surfaces of TiC and ZrC, as carbon dioxide undergoes either dissociation to CO or hydrogenation to COOH or HCOO. The relative stabilities of the three reduction intermediates and the activation energies for their formation allow the identification of favored pathways on each surface, which are examined as they lead to the release of CO, HCOOH, CH3OH, and CH4, thereby also characterizing the activity and selectivity of the two materials. Reaction energetics implicate HCO as the key common intermediate on all surfaces studied and rule out the release of formaldehyde. Surface hydroxylation is shown to be highly selective toward methane production as the formation of methanol is hindered on all surfaces by its barrierless conversion to CO.

Investigation on a Self-Breakdown Repetitive Gap Switch Based on the Graphite Electrodes With TiC Surface Modification

In this article, a self-breakdown repetitive gap switch based on the graphite electrodes with surface modified by TiC is investigated to improve the scattering of the breakdown voltage. The self-breakdown repetitive gap switch was experimentally studied with the graphite electrodes and two kinds of TiC modified graphite electrodes (hereinafter referred to as TiC-I electrodes and TiC-II electrodes). Results show that the scatterings of the breakdown voltages of the graphite, TiC-I and TiC-II electrodes were 2.49%, 1.80%, and 1.54%, respectively. Compared with the graphite electrodes, the mass losses of TiC-I and TiC-II electrodes are reduced by 50% and 68%. The influence of electrode surface morphology on the switch stability is analyzed based on the mechanism of switch conduction process. Compared with the graphite electrodes, the electric field amplification coefficients of TiC-I and TiC-II electrodes are more evenly distributed, which leads to the more evenly distributed field emission current density. Thus, the breakdown areas of TiC-I and TiC-II electrodes are smaller and the gas breakdown processes are more stable than those of the graphite electrodes, resulting in smaller scattering of the breakdown voltage. The method that decreases the scattering of the breakdown voltage and reduces the mass loss of the electrodes by modifying graphite electrodes surface can greatly improve the stability of the switch, which is of significance to the application of the switch in the compact, long time stable operation of high-power pulse power devices.

XPS studies of surface oxidation of metal carbides

Metal carbides MC (HfC, NbC, ZrC, TaC and TiC) that were exposed to air under the ambient temperature condition for a period of time were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and Raman. Fresh MC samples exposed to the air for different time (fresh, one day, one week and one quarter) illustrated unexpected oxidation degree of the surfaces. XRD tests provided very limited information, since the oxidation process occurred from the surface at the nanometer scale. However, the XPS tests provided more comprehensive information for the MOx (HfO2, ZrO2, Nb2O5, Ta2O5 and TiO2) film and MC (HfC, NbC, ZrC, TaC and TiC) interlayer. Meanwhile, the HRTEM and Raman tests could give some additional or supporting information for XPS tests, when the samples were exposed to the air for longer time (one week or one quarter). Meanwhile, this research is the first to show that the oxidation progress of the metal surfaces of HfC, NbC, ZrC, TaC and TiC can take place in air under the ambient temperature condition and the oxidation film is about 2–5 nm. Meanwhile, a new oxidation mechanism of metal carbides MC (HfC, NbC, ZrC, TaC and TiC) exposed to air under the ambient temperature condition is also proposed, considering the obtained Raman, TEM and XPS results that could clearly confirm the formation of HfO2 and free carbon.

Theoretical Insights into Synergistic Effects at Cu/TiC Interfaces for Promoting CO2 Activation.

The adsorption behaviors of CO2 at the Cun/TiC(001) interfaces (n = 1–8) have been investigated using the density functional theory method. Our results reveal that the introduction of copper clusters on a TiC surface can significantly improve the thermodynamic stability of CO2 chemisorption. However, the most stable adsorption site is sensitive to the size and morphology of Cun particles. The interfacial configuration is the most stable structure for copper clusters with small (n ≤ 2) and large (n ≥ 8) sizes, in which both Cu particles and TiC support are involved in CO2 activation. In such a case, the synergistic behavior is associated with the ligand effect introduced by directly forming adsorption bonds with CO2. For those Cun clusters with a medium size (n = 3–7), the configuration where CO2 adsorbs solely on the exposed hollow site constructed by Cu atoms at the interface shows the best stability, and the charger transfer becomes the primary origin of the synergistic effect in promoting CO2 activation. Since the most obvious deformation of CO2 is observed for the TiC(001)-surface-supported Cu4 and Cu7 particles, copper clusters with specific sizes of n = 4 and 7 exhibit the best ability for CO2 activation. Furthermore, the kinetic barriers for CO2 dissociation on Cu4- and Cu7-supported TiC surfaces are determined. The findings obtained in this work provide useful insights into optimizing the Cu/TiC interface with high catalytic activation of CO2 by precisely controlling the size and dispersion of copper particles.

Effect of TiC surface oxide overlayer on the control of LixOy behavior in lithium-oxygen batteries: Implications for cathode catalyst design

TiC has attracted tremendous recent attention in lithium-oxygen batteries as a means of reducing the irreversible accumulation of by-products (i.e., Li2CO3) on the air cathode. However, TiC is intrinsically reactive to oxygen under ambient conditions, and its stability is completely determined by the surface overlayer. Herein, the catalytic effects of oxide overlayer (TiO) on adsorption mechanism, reaction path, and growth morphology of lithium oxides were comprehensively elucidated using periodic density functional theory calculations. The results indicate that the TiO overlayer on the surface of TiC(100)-TiO enhances the adsorption of lithium oxides. As compared to the disproportionation reaction, the lithiation reaction is the primary contributor to the formation of Li2O2 and Li2O on TiC(100) surface. Interestingly, the oxidized TiC(100)-TiO performs better in inhibiting the generation of undesirable Li2O. Analysis demonstrates that the discharge performance is not only related to the widely known morphology and size of product but also to the adsorption of (Li2O2)n clusters on the TiC(100) surface. The electronic structures of (Li2O2)n clusters on TiC(100)-TiC and TiC(100)-TiO surfaces have been examined to study the influence of TiO overlayer on the conductivity of accumulated products, which helps explain the innovative discovery of the experiment that TiC has excellent performance in improving the cycle ability of lithium-oxygen batteries. Our results promote the understanding of the ORR mechanism on TiC surface and provide theoretical guidance for the design of efficient and stable air cathodes for lithium-oxygen batteries.

A model to partition the constituent element’s atomic energy in TiC crystal and to characterize the interface properties of α-Ti(0001)/TiC(111)

A periodic slab model was proposed to characterize the TiC(111)/α-Ti(0001) interface properties. The model provides an effective method to partition the total energy of TiC crystalline into its constituent element’s atomic energy by constructing a periodic TiC(111)/α-Ti(0001) interface structure with a specific atomic stacking sequence on the interface. The calculation result shows that Ti and C atom possess −116.65009 Ry and −12.06893 Ry in TiC crystal respectively, which is in good agreement with the TiC total energy of −128.71901 Ry, indicating a satisfactory accuracy better than ~10−4 Ry. The information of the constituent element’s atomic energy provides a direct solution to quantitatively characterize TiC(111) surface energy, or rather, the surface energy on an arbitrary crystallographic plane of TiC crystal. The calculated Ti- and C-terminated surface energies on TiC(111) surface are −0.154 J/m2 and 8.014 J/m2 respectively. Under the atomic configuration in the present periodic model, the Ti-Ti and Ti-C interface energies were calculated to be −1.705 J/m2 and −1.891 J/m2, respectively. The investigation on the ideal work of adhesion indicated that the Ti-Ti interface is the most vulnerable region in the TiC(111)/α-Ti(0001) interface structure on account of the weaker metallic bonding between the Ti-Ti atomic layers.