TiC Layer Research Articles

Multi-scale design of novel materials for emerging challenges in active thermal management: Open-pore magnesium-diamond composite foams with nano-engineered interfaces

Open-pore Mg foams, which have been traditionally discarded for heat dissipation applications given their low thermal conductivity values, can prove appealing materials for active thermal management if they incorporate diamond particles coated with a nano-dimensioned layer of TiC. These composite foam materials can be manufactured by the replication method, conveniently adapted to Mg, that requires a strict multi-scale control: correct distribution of structural constituents (pores, diamond and Mg) on the meso-/micro-scale ensures homogeneity and complete pore connectivity, while a proper nanoscale control of the TiC coating on diamond particles achieves high thermal conductance at the interface between diamond particles and Mg. The manufactured Mg-diamond foam materials attain outstanding thermal conductivity values (up to 82 W/m K) and maximum heat dissipation performance, tested on active convective cooling, almost two times higher than their equivalent magnesium foams and twenty per cent superior to that of conventional aluminium foams.

Brazing of C/C composite and Ti-6Al-4V with graphene strengthened AgCuTi filler: Effects of graphene on wettability, microstructure and mechanical properties

Graphene nanosheets (GNSs) strengthened AgCuTi composite filler (AgCuTiG) was used to braze C/C composite and Ti-6Al-4V. The effects of GNSs on the wettability of AgCuTiG filler on the C/C composite surface and the interfacial microstructure and mechanical properties of brazed joints were investigated. The results indicate that the addition of GNSs reduced the wettability of AgCuTiG. The interfacial microstructure of brazed joints evolved with the addition of GNSs, where Ti3Cu4 and TiCu4 were converted to TiCu and the thickness of the reaction layer adjacent to the base material decreased. The maximum shear strength of joints brazed at 0.3wt% GNSs was 23.3MPa (880°C/10min). Further adding GNSs deteriorated the shear strength of the joints. Fracture of the joints occurred in the C/C composite substrate and the TiC layer adjacent to C/C composite.

Vacuum brazing of Cf/β–spodumene composites and Ti–6Al–4 V alloy using Ag–Cu filler metal

Interfacial microstructure and mechanical properties of Ti–6Al–4V/Cf/β–spodumene composites joints brazed by Ag–Cu eutectic brazing filler were investigated. The effects of brazing temperature and holding time on the interfacial microstructure and mechanical properties of Ti–6Al–4V/Cf/β–spodumene composites joints were studied. The result indicated that the typical interfacial microstructure of the joint was Ti–6Al–4V/Ti2Cu+α-Ti (s,s)/Ti2Cu/TiCu/Ag (s,s)/Cu (s,s)/Ti2Cu3/TiSi2+TiC/Cf/β–spodumene composites. The thickness of the diffusion layer, Ti2Cu3 layer, and reaction layer on the interface of Cf/β–spodumene composites increased with the brazing temperature or holding time increasing, meanwhile, the thickness of Ti–Cu layers decreased. The diffusion of Ti in brazing seam, which was significantly determined by the brazing temperature and holding time, was a decisive factor on the interfacial microstructure and mechanical properties of brazed joints. The optimal shear strength of joint brazed at 880°C for 10min was 33.5MPa. Further raising the brazing temperature or extending the holding time would reduce the shear strength of joints. Brazing specimens brazed at 880°C and 910°C for 10min broke in form of cleavage fracture after shearing test. Moreover, the formation of excessive thickness of TiSi2+TiC layer or Ti3Cu4 reaction layer was harmful to the mechanical property of the joint.

Well-dispersion of CNTs and enhanced mechanical properties in CNTs/Cu-Ti composites fabricated by Molecular Level Mixing

Micron-sized CNTs/Cu2O composite powder prepared by Molecular Level Mixing (MLM) was mixed with flaked Cu-1.0 wt% Ti (Cu-1.0 Ti) matrix powders by low-energy ball milling. After reduction and hot pressing, CNTs/Cu-Ti composites were fabricated. Microstructure and mechanical properties of the composites were characterized by SEM, EDS, HRTEM, hardness and tensile tests, etc. The results showed that mechanical properties of the CNTs/Cu-Ti composites were enhanced compared with alloy matrix, which were ascribed to the well-distributed of CNTs and strong bonding interface with the matrix by the formation of a transition layer of TiC. Finally, strengthening mechanisms were discussed.

Optical simulation, corrosion behavior and long term thermal stability of TiC-based spectrally selective solar absorbers

This work is a continuation of earlier work where TiC/Al2O3 spectrally selective solar absorbers were deposited onto stainless steel (SS). In this study, the optical simulation, corrosion behavior and long term thermal stability are investigated in detail. A series of TiC and Al2O3 single layers are fabricated by magnetron sputtering using the same deposition parameters as described in the previous work. The optical constants and thicknesses of glass, stainless steel, TiC and Al2O3 layers are obtained by CODE software using the suitable dielectric function model. Then, the structure model of SS/TiC/Al2O3 is constructed and the reflectance spectra of the SS/TiC/Al2O3 solar absorber is successfully simulated using CODE software. The anti-corrosion performance of SS/TiC/Al2O3 solar absorbers is evaluated through electrochemical potentiodynamic polarization measurement and salt spray test. The SS/TiC/Al2O3 solar absorbers have a good thermal stability in vacuum at 500°C for 100h. Raman analysis indicates that the increased sp2 bonds of the coating is the main reason for the degradation of the optical properties.

Laser-induced exothermic bonding of carbon fiber/Al composites and TiAl alloys

In this paper, a highly-efficient method for bonding Carbon Fiber (Cf)/Al composites and TiAl alloys was developed by means of laser-induced exothermic reaction of a Ni-Al-Ti interlayer. To examine the interlayer exothermic performance, the adiabatic temperature was calculated and the reaction temperatures were measured. Based on SEM, XRD and TEM, the interfacial reactions between carbon fiber and interlayer products were investigated. Results show that the addition of Ti enhanced the interfacial reactions between carbon fibers and interlayer products. The resulting formation of a 300–400nm thin Ti-C layer at the NiAl3/carbon fiber interface was identified as a key player in improving the joining quality on the Cf/Al side. The typical interfacial structure of the joint is (Cf/Al)/Ti-C/NiAl3/Ni2Al3/(NiAl+Al3NiTi2)/Al3NiTi2/TiAl. The joint formation mechanism was discussed. In addition, the effect of Ti content on the microstructure and mechanical properties of joints were investigated in detail. The optimum value of Ti content was found to be 5wt.%, leading to a defect-free joint and a maximum shear strength of 45.8MPa.

Sputtered Titanium Carbide Thick Film for High Areal Energy on Chip Carbon‐Based Micro‐Supercapacitors

The areal energy density of on‐chip micro‐supercapacitors should be improved in order to obtain autonomous smart miniaturized sensors. To reach this goal, high surface capacitance electrode (>100 mF cm−2) has to be produced while keeping low the footprint area. For carbide‐derived carbon (CDC) micro‐supercapacitors, the properties of the metal carbide precursor have to be fine‐tuned to fabricate thick electrodes. The ad‐atoms diffusion process and atomic peening effect occurring during the titanium carbide sputtering process are shown to be the key parameters to produce low stress, highly conductive, and thick TiC films. The sputtered TiC at 10−3 mbar exhibits a high stress level, limiting the thickness of the TiC‐CDC electrode to 1.5 µm with an areal capacitance that is less than 55 mF cm−2 in aqueous electrolyte. The pressure increase up to 10−2 mbar induces a clear reduction of the stress level while the layer thickness increases without any degradation of the TiC electronic conductivity. The volumetric capacitance of the TiC‐CDC electrodes is equal to 350 F cm−3 regardless of the level of pressure. High values of areal capacitance (>100 mF cm−2) are achieved, whereas the TiC layer is relatively thick, which paves the way toward high‐performance micro‐supercapacitors.

Interfacial structure evolution of Ti-coated diamond particle reinforced Al matrix composite produced by gas pressure infiltration

Abstract The interfacial structure evolution of Ti-coated diamond particle reinforced Al matrix composite produced by gas pressure infiltration has been studied systematically, aimed to provide an insight into the optimization of interfacial structure and properties of the composite. The results show that the Ti coating layer on diamond particle consists of three layers, including fine TiC grains, TiC columnar grains and dispersed Ti particles. During the infiltration of molten Al, the dispersed Ti particles react with Al to form Al 3 Ti particles in Al matrix, while both TiC layers remain unchanged. However, in the subsequent cooling process, Al 4 C 3 particles are formed along the grain boundaries of TiC columnar grains as a result of Al diffusion and the subsequent Al/TiC reaction. By controlling the thickness of the TiC layers on diamond surface, an optimized thermal conductivity of 650 W m −1 K −1 is obtained for the composite. The finding is beneficial to the understanding of diamond coating engineering in the diamond particle reinforced Al matrix composite.

Interface and interaction of graphene layers on SiC(0001̄) covered with TiC(111) intercalation

It is important to understand the interface and interaction between the graphene layer, titanium carbide [TiC(111)] interlayer, and silicon carbide [SiC(0001[combining macron])] substrates in epitaxial growth of graphene on silicon carbide (SiC) substrates. In this study, the fully relaxed interfaces which consist of up to three layers of TiC(111) coatings on the SiC(0001[combining macron]) as well as the graphene layers interactions with these TiC(111)/SiC(0001[combining macron]) were systematically studied using the density functional theory-D2 (DFT-D2) method. The results showed that the two layers of TiC(111) coating with the C/C-terminated interfaces were thermodynamically more favorable than one or three layers of TiC(111) on the SiC(0001[combining macron]). Furthermore, the bonding of the Ti-hollow-site stacked interfaces would be a stronger link than that of the Ti-Fcc-site stacked interfaces. However, the formation of the C/Ti/C and Ti/C interfaces implied that the first upper carbon layer can be formed on TiC(111)/SiC(0001[combining macron]) using the decomposition of the weaker Ti-C and C-Si interfacial bonds. When growing graphene layers on these TiC(111)/SiC(0001[combining macron]) substrates, the results showed that the interaction energy depended not only on the thickness of the TiC(111) interlayer, but also on the number of graphene layers. Bilayer graphene on the two layer thick TiC(111)/SiC(0001[combining macron]) was thermodynamically more favorable than a monolayer or trilayer graphene on these TiC(111)/SiC(0001[combining macron]) substrates. The adsorption energies of the bottom graphene layers with the TiC(111)/SiC(0001[combining macron]) substrates increased with the decrease of the interface vertical distance. The interaction energies between the bottom, second and third layers of graphene on the TiC(111)/SiC(0001[combining macron]) were significantly higher than that of the freestanding graphene layers. All of these findings provided insight into the growth of epitaxial graphene on TiC(111)/SiC(0001[combining macron]) substrates and the design of graphene/TiC/SiC-based electronic devices.

On the mechanism of tool crater wear during titanium alloy machining

Tool crater wear is a major cause of accelerated tool failure during the machining of titanium alloy components. This paper presents a static diffusion couple method which replicates the complex reaction mechanisms occurring at the tool-chip interface during machining and is validated with dynamic turning trials. The diffusion couple tests are realised via the use of a bespoke vacuum compression rig. The rig consists of two graphite rods which secure the sample under a clamping load at the required temperature and pressure. Titanium alloys bearing a high molybdenum content are found to be less susceptible to TiC formation at the tool-workpiece interface. A TiC layer during machining coats and protects the tool from excessive crater wear therefore increasing tool life. Strong agreement was found between the diffusion couple technique and turning trial. This low cost test strategy will be used to design machinable titanium alloys and more compatible tools.

Structure and Properties of Duplex Layers Composed of Chromium and Titanium Carbides

Abstract The structures and properties of the CrC-TiC type duplex layers with an intermediate chromium carbide sublayer and an outer one of titanium carbide, as well as of the TiC-CrC type layers with an intermediate titanium carbide sublayer and an outer one of chromium carbide, produced on steels, were investigated. The structure of the duplex layers produced on plain carbon and alloy steel was compared with the single layers. The microstructure and chemical composition of these layers was determined by means of optical and transmission electron microscopy and X-ray phase analysis. A spectral linear analysis of the Cr, Fe, and Ti concentration in the diffusion zone was performed by means of an X-ray microanalyzer. Proof was presented that the thickness of the outer TiC layer depends on the thickness of the intermediate chromium carbide sublayer. This sublayer slows down the formation of the outer TiC layer. The structure of the chromium carbide sublayer undergoes changes during the process formation of the titanium carbide layer. The hardness and tribological properties of the layers have been determined. A comparison of CrC-TiC and TiC-CrC duplex layers with the CrC and TiC single carbide layers was performed. It was shown that the tribological properties of the CrC-TiC duplex layers are better than those of the TiC-CrC duplex layers, and those of the CrC and TiC type single carbide layers.

In Situ Synthesis of Ceramic Reinforcements for Carbon/CuCrZr Joints Brazed with Composite Fillers

Brazing of two kinds of carbon materials including graphite and carbon fiber-reinforced carbon composites to copper alloys has been realized with CuTiH2 + BN composite fillers. The microstructure characterization reveals that the ceramic reinforcements containing TiN particles and TiB whiskers have been synthesized by in situ reaction of BN additives with Ti discomposed from TiH2 in the composite filler. The filler layer of the joints is mainly composed of Cu-based solid solutions [Cu (ss)] and Ti-Cu intermetallics along with ceramic reinforcements. Furthermore, a continuous thin reaction layer mainly containing TiC is developed at the interface close to the carbon substrates. The growth of TiC layer is mainly controlled by the diffusion of carbon from the substrates into the liquid filler through the TiC layer formed. The interface evolution of the graphite/CuCrZr joints has been discussed. The electrical resistivity of the joining area is relatively low, which highly meets the requirement for the carbon commutator applications.

Tribological and Corrosion Properties of Nickel/TiC Bilayered Coatings Produced by Electroless Deposition and PACVD

Ni/TiC bilayered coatings are deposited on hot-working steel (H11) by plasma-assisted chemical vapor deposition and electroless technique. The TiC layer is deposited at 490 °C using a gas mixture of TiCl4, CH4, H2, and Ar, and a dense nanostructured TiC coating with minimum excessive carbon phases and low chlorine concentration is produced. The effects of the Ni intermediate layer on the microstructure, tribology, and corrosion behavior of the nanostructured TiC coating are investigated. The friction coefficient of the Ni/TiC bilayered coating (Ni thickness = 4 µm) at 500 cycles is much smaller than that of the coating without the Ni intermediate layer. The smallest friction coefficient is about 0.2, and the hardness values of the Ni/TiC bilayered samples with three different Ni layer thicknesses of 2, 4, and 6 µm are 2534, 3070, and 2008 Hv, respectively. The wear mechanism of the Ni/TiC bilayered coatings is abrasive induced by plastic deformation and fatigue during the sliding process. The smaller groove width on the 4-µm electroless nickel-Ni3P/TiC bilayered coating correlates with the larger H/E ratio and the 4-µm nickel/TiC bilayered sample shows the better wear resistance. The polarization resistance of the 6-µm electroless nickel-Ni3P/TiC coating in 0.05 M NaCl and 0.5 M H2SO4 increases by about 8 and 15 times, respectively. The Ni intermediate layer increases the toughness of the coating and adhesion between the hard coating and steel substrate thereby enhancing the tribological properties and corrosion resistance.

Effect of processing parameters on the formation of Cf/LAS composites/Ag[sbnd]Cu[sbnd]Ti/TC4 brazed joint

Cf/LAS composites were successfully jointed to TC4 alloy with AgCuTi filler by vacuum brazing. The interfacial microstructure of TC4/Cf/LAS composites joints was characterized by employing scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-diffraction (XRD) and transmission electron microscopy (TEM). The determination of the thin interfacial reaction layer (TiSi2+TiC layer) was realized by TEM. The effect of holding time on the interfacial microstructure and shear strength were investigated. With the increasing holding time, the thickness of diffusion layer, Ti3Cu4 layer, and TiSi2+TiC layer increased obviously, on the contrary, that of TiCu intermetallic compound layers decreased gradually. Besides, blocky Ti3Cu4 phase was coarsened when the joint was brazed at 890°C for 20min, which deteriorated the mechanical properties of the joint dramatically. The interfacial evolution of TC4/Cf/LAS composites joint and the formation of TiSi2, TiC, Ti3Cu4, TiCu and Ti2Cu phases were expounded. The maximum shear strength of 26.4MPa was obtained when brazed at 890°C for 10min.

Electrochemical Properties of Polycrystalline Phosphorus-Doped Diamond

Introduction Since a p-type boron-doped diamond (BDD) has excellent electrochemical properties, such as wide potential window, low background current, and so on, it has been investigated for both of fundamentals and electrochemical applications1. On the other hand, few studies have reported on the electrochemical properties of n-type diamond. The electrons in the conduction band of the diamond have the quite high reducing ability because it is situated at much higher energy level than that of other semiconductors. Hence, the n-type diamond electrode may present new electrochemical properties. In this work, we study on the preparation and the characterization of polycrystalline phosphorus-doped diamond (PDD), which is the most promising candidate for n-type diamond, and investigate the electrochemical properties of PDD. Experimental The polycrystalline PDD films were deposited on Si wafer substrates by a microwave plasma-assisted chemical vapor deposition system in following steps. Firstly, undoped diamond layer was deposited in CH4/H2 plasma for 4 hours. Subsequently, red phosphorus sublimed at 400ºC was introduced into CH4/H2 plasma for 1 hour in order to deposit PDD layer. The surface morphology was characterized by scanning electron microscopy (SEM), and structure was investigated by Raman spectroscopy and X-ray diffraction (XRD). Phosphorus doping level was evaluated by secondary ion mass spectroscopy (SIMS). The hall effect measurement was performed using van der Pauw method. Electrical contacts were made by sputtering Ti and Au on the PDD surface, followed by annealing in vacuum at 450ºC for 45 min to form the Ti-C layer. Electrochemical properties were characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) in an aqueous solution of 0.1 M H2SO4. Mott-Schottky plots were obtained by EIS measurements. Moreover, these results were compared with electrochemical properties of BDD electrode. Results and discussion The SEM images showed a polycrystalline nature with grain size of ca. 2 μm. Raman spectrum showed a sharp peak from center zone optical phonon of diamond at 1332 cm−1 and a wide band from sp2-bonded carbon at around 1500 cm−1. Phosphorus concentration in the synthesized film was 1.36×1018 cm−3. From Hall effect measurement, it was confirmed that the PDD film was n-type semiconductor because hall coefficient was negative. From the CV measurement, the cathodic current was much larger than the anodic current, which is a characteristic of the interface between n-type semiconductor and solution (Fig. 1). Since p-type BDD showed the reverse behavior, this characteristic is ascribed to the influence of phosphorus doping. In order to estimate the position of band edges of PDD and BDD, the space charge capacitance C was measured by EIS measurement. The obtained C as a function of potential E was fitted with Mott-Schottky equation (1): 1/C 2 = (±2/eε0εSCN)(E−Efb −kT/e) (1) where the plus and minus signs represent the type of semiconductors, ε0 and εSC are the permittivities of free space and diamond, respectively, e is the electron charge, and N is the donor or acceptor density. The Mott-Schottky plots obtained by EIS measurement (Fig. 2) revealed that the synthesized PDD film is n-type semiconductor because a positive slope was obtained. Moreover, from the intercept of the potential axis, flat band potential (Efb ) of the PDD and BDD was estimated because kT/e is almost zero at room temperature (298 K). These obtained Efb values were used to determine the band edge positions. The difference between Efb and EVB of BDD was assumed to be ~0.4 eV from the acceptor level of boron. On the other hand, the difference between Efb and ECB of PDD was assumed to be ~0.6 eV from the donor level of phosphorus. From the result of comparing PDD and BDD, the band edges of PDD were found to agree well with the ones of BDD and also the single-crystalline PDD reported on the previous research2. References (1) Y. Einaga, J. Appl. Electrochem. 40. 1807 (2010). (2) Y. Mukuda, T. Watanabe, A. Ueda, Y. Nishibayashi, and Y. Einaga, Electrochim. Acta 179. 599 (2015). Figure 1

Osseointegration is improved by coating titanium implants with a nanostructured thin film with titanium carbide and titanium oxides clustered around graphitic carbon

Titanium implants coated with a 500nm nanostructured layer, deposited by the Ion Plating Plasma Assisted (IPPA) technology, composed of 60% graphitic carbon, 25% titanium oxides and 15% titanium carbide were implanted into rabbit femurs whilst into the controlateral femurs uncoated titanium implants were inserted as control. At four time points the animals were injected with calcein green, xylenol orange, oxytetracycline and alizarin. After 2, 4 and 8weeks femurs were removed and processed for histology and static and dynamic histomorphometry for undecalcified bone processing into methylmethacrylate, sectioned, thinned, polished and stained with Toluidine blue and Fast green. The overall bone-implant contacts rate (percentage of bone-implant contacts/weeks) of the TiC coated implant was 1.6 fold than that of the uncoated titanium implant. The histomorphometric analyses confirmed the histological evaluations. More precisely, higher Mineral Apposition Rate (MAR, μm/day) (p<0.005) and Bone Formation Rate (BFR, μm2/μm/day) (p<0.0005) as well as Bone Implant Contact (Bic) and Bone Ingrowth values (p<0.0005) were observed for the TiC coated implants compared to uncoated implants.In conclusion the hard nanostructured TiC layer protects the bulk titanium implant against the harsh conditions of biological tissues and in the same time, stimulating adhesion, proliferation and activity of osteoblasts, induces a better bone-implant contacts of the implant compared to the uncoated titanium implant.

Evaluation of microstructure and mechanical properties of TiC/TiC-steel composite coating produced by gas tungsten arc (GTA) coating process

Abstract TiC/TiC–steel composite layer was produced on AISI 304 steel by gas tungsten arc (GTA) coating process to improve its hardness and wear resistance properties. Effect of current and scan speed on the coating geometry and corresponding microstructure was investigated from the SEM images. Pin-on-disc type abrasive wear test was performed to assess the tribological behavior of the coating. At low GTA current and high scan speed, a distinguished layer of TiC deposited that exhibits significantly higher hardness and high wear resistance than the steel substrate. Conversely, at high current and low scan speed produced TiC–steel composite layer offers relatively low hardness and lower wear resistance. The wear resistance improved five to fifty times depending on the coating microstructure.

Structure, optical properties and thermal stability of TiC-based tandem spectrally selective solar absorber coating

A new spectrally selective solar absorber coating of TiC/Al2O3 is prepared on stainless steel substrate using a magnetron sputtering method. The new coating exhibits a high absorptance of 0.92 and a low emittance of 0.13 (82°C). The tandem absorber is characterized using ultra-high resolution scanning electron microscope, UV–vis–NIR spectrophotometer, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The formation of amorphous carbon is due to the dissociation of TiC layer deposited under high substrate temperature (300°C). The diffused carbon at 600–800°C with mixed sp2 and sp3 bonds is further demonstrated by XPS and Raman spectra. With an increase in annealing temperature, the surface structure becomes more graphitic. The tandem absorber is stable in vacuum up to 650°C for 2h, indicating its importance for high temperature solar selective applications.

Effect of nanometer TiC coated diamond on the strength and thermal conductivity of diamond/Al composites

50 vol% TiC-coated diamond particles reinforced 6061Al (TiC@diamond/6061Al) composites were fabricated by using powder metallurgy method. The TiC coating 10 nm in thickness on the diamond particles could improve the wettability and increase the bonding between the diamond and Al matrix. The density of the TiC@diamond/6061Al composites reached 99.6% of the theoretical density, which was much higher than that (96.7%) of the composite using the diamond without the TiC layer (Diamond/6061Al). The bending strength of the TiC@diamond/6061Al was 396 MPa, 35% higher than that of the Diamond/6061Al. However, the thermal conductance of the TiC@diamond/6061Al was only 13.4% higher than that of the Diamond/6061Al. This indicates that the massive gap in phonon velocity between diamond and Al, not the pores in the composites, was the dominant factor in the thermal conductance of the diamond particles reinforced Al composites.

Corrosion resistance of self-growing TiC coating on SIMP steel in LBE at 600 °C

SIMP steel was newly developed as a candidate structural material for the accelerator driven subcritical system. The serious decarburization of SIMP steel because of the high Si content was used to successfully form a self-growing TiC coating on the surface, after the Ti deposition as a first step. This TiC layer can effectively protect the surface from the static liquid lead-bismuth eutectic (LBE) corrosion at 600 °C up to 2000 h in the low oxygen LBE. However, in the oxygen saturated LBE, the TiC coating is oxidized into porous TiO2 after only 500 h and fails to protect. Therefore, the self-growing TiC coating is desired only when the oxygen content of LBE is strictly controlled on a low level.