The effect of carbon content on the microstructure of hydrogen-free physical vapour deposited titanium carbide films

Titanium carbide (TiC) coatings for tribological applications were deposited on high speed steel. Several coatings with different titanium to carbon ratio were deposited by means of physical vapour deposition in which titanium was evaporated and carbon was sputtered. The coatings were characterised using analytical electron microscopy. It was observed that the change in titanium to carbon ratio significantly changed the microstructure of the coatings. The low carbon containing coatings consisted of columnar grains exhibiting a preferred crystallographic orientation whereas the coating with highest carbon content consisted of randomly ordered TiC grains in an amorphous carbon matrix. Energy filtered transmission electron microscopy revealed a change in Ti/C ratio as the distance from the substrate increased. The titanium to carbon ratio was observed to increase with distance from the substrate until a stable level was reached. This is due to a variation in the titanium evaporation during the early stages of film growth. This change of the titanium to carbon ratio affected the columnar growth in the initial stage of coating growth for the coatings with low carbon content.     

Microstructural study of oxidation of carbon-rich amorphous boron carbide coating

Carbon-rich amorphous boron carbide (B_xC) coatings were annealed at 400°C, 700°C, 1000°C and 1200°C for 2 h in air atmosphere. The microstructure and composition of the as-deposited and annealed coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy and energy dispersive X-ray spectroscopy (EDS). All of the post-anneal characterizations demonstrated the ability of carbon-rich B_xC coatings to protect the graphite substrate against oxidation. Different oxidation modes of the coatings were found at low temperature (400°C), moderate temperature (700°C) and high temperature (1000°C and 1200°C). Finally, the feasibility of the application of carbon-rich B_xC instead of pyrolytic carbon (PyC) as a fiber/matrix interlayer in ceramics-matrix composites (CMCs) is discussed here.     

Transmission electron microscopy studies of TiC and VCTiC deposits prepared by activated reactive evaporation

The activated reactive evaporation technique is used for processing carbide alloy coatings on tools. Improved properties of these films can be developed if the relationships between processing parameters, microstructural features and mechanical properties are well understood. The main aim of the present investigation was to characterize the microstructures of TiC and (V,Ti)C films prepared by the activated reactive evaporation process at varying deposition temperatures (550–1000°C) and to relate these features to the hardness. The transmission electron microscopy technique was used for this purpose. Two characteristic and completely different grain morphologies are produced in these films, one at low temperatures (extremely fine grained with grain diameters of about 10 nm) and one at high temperatures (of more normal grain structure with micronsized grains). These are all equilibrium single-phase structures of TiC and (V,Ti)C respectively except for the low temperature deposit of (V-23 at.% Ti)C prepared at 550°C; (V-23 at.% Ti)C is a mixture of TiC, VC and free graphite phases. The hardness is superior in the large-grained structures.     

Author index

Films of TiC, TiN and their composite were prepared on molybdenum by a reactive sputtering method with CH₄ and N₂ as the reactive gases and argon as the sputtering gas and applying bias potentials to the substrate material.The films were characterized by X-ray photoelectron spectroscopy and Auger electron spectroscopy. The quantitative chemical composition of the TiC and TiN coatings was determined as a function of the partial pressures of CH₄ (P_CH4) and N₂ (P_N2) during the reactive sputtering. For the TiC coating the most suitable P_CH4 range which gives the stoichiometric composition (carbon-to-titanium ratio, 0.8–1.0) without impurities was found to be (2–5) × 10^?4 Torr (substrate temperature, 300 °C; bias potential, ? 300 V). For the TiN coating the structure and composition of the films prepared by reactive sputtering were observed to depend greatly on the condition of applying the bias potential. The suitable P_N2 range which gives golden films of the stoichiometric composition was higher than 1 × 10^?4 Torr (substrate temperature, 200–300 °C; bias potential from ?75 to ?200 V).On the basis of these experimental studies of TiC and TiN coatings successive coatings of TiC and TiN were deposited onto a molybdenum substrate to achieve higher thermal stability and better adhesion to the substrate. The successive coating method is a promising technique for use in fusion reactors.     

Microstructural study of oxidation of carbon-rich amorphous boron carbide coating

Carbon-rich amorphous boron carbide (B _x C) coatings were annealed at 400°C, 700°C, 1000°C and 1200°C for 2 h in air atmosphere. The microstructure and composition of the as-deposited and annealed coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy and energy dispersive X-ray spectroscopy (EDS). All of the post-anneal characterizations demonstrated the ability of carbon-rich B _x C coatings to protect the graphite substrate against oxidation. Different oxidation modes of the coatings were found at low temperature (400°C), moderate temperature (700°C) and high temperature (1000°C and 1200°C). Finally, the feasibility of the application of carbon-rich BxC instead of pyrolytic carbon (PyC) as a fiber/matrix interlayer in ceramics-matrix composites (CMCs) is discussed here.     

Microstructure and rheological properties of titanium carbide-coated carbon black particles synthesised from molten salt

Homogeneous titanium carbide (TiC) coatings were prepared on carbon black (CB) particles by firing them with metallic titanium (Ti) powder in KCl or KCl–LiCl for 4 h at 750–850 °C. As-prepared TiC-coated CB retained the spherical shape of as-received CB, indicating that the ‘template-growth’ mechanism had dominated the reaction process. TiC coating thickness could be readily tailored by simply adjusting the Ti to CB ratio (Ti/C) of the initial starting mixture. The true density of TiC-coated CB particles increased with Ti/C, reducing the density difference between them and other aggregates used in castable systems and potentially improving the castables consistency. Zeta potential measurement, sedimentation comparison and rheology testing revealed that water-wettability and dispersivity of CB were improved significantly after TiC coating.     

Silver diffusion and high-temperature lubrication mechanisms of YSZ–Ag–Mo based nanocomposite coatings

Yttria-stabilized zirconia (YSZ) nanocomposite coatings consisting of silver and molybdenum were produced by a hybrid process of filtered vacuum arc, magnetron sputtering and pulsed laser depositions for tribological investigations at different temperatures. The coatings with 24 at.% Ag and 10 at.% Mo contents showed a friction coefficient of 0.4 or less for all temperatures from 25 to 700 °C. The wear scar surfaces and coating cross-sections were studied using scanning electron, transmission electron, scanning transmission electron and focused ion beam microscopes, which also provided the information on chemical composition distributions of silver and molybdenum along with microstructure features. It was demonstrated that silver diffusion and coalescence on surfaces played an important part in the high-temperature lubrication mechanism of the YSZ–Ag–Mo coatings. Silver was found to be an effective lubricant at temperatures below 500 °C and its coalescence on the surface isolated molybdenum inside coatings from ambient oxygen. Lubricious oxides of molybdenum were formed and lubricated at temperatures above 500 °C when the silver was worn off the contact surface. For silver containment inside the coating at high temperatures, a multilayer architecture was built by inserting a TiN diffusion barrier layer in the composite coatings. Microscopic observations showed that this barrier layer prevented silver exit to the coating surface. At the same time, this enabled a subsequent lateral lubricant supply toward a wear scar location where the diffusion barrier layer was worn through and/or for a next thermal cycle. The multilayer coating maintained a friction coefficient of 0.4 or less for more than 25,000 cycles, while the monolithic coating lasted less than 5000 cycles. In addition, a TiN surface barrier layer with pinholes was deposited on the YSZ–Ag–Mo composite surface to control vertical silver diffusion. With this coating design, the coating wear lifetime was significantly increased beyond 50,000 cycles.     

Improved adhesion between a sputtered alumina coating and a copper substrate

The influence of various metallic intermediate layers on the adhesion between a copper substrate and an alumina coating was studied. The alumina coatings and the intermediate layers were prepared by r.f. sputtering. Titanium and combined Ni/Ti intermediate layers were used. The adhesion properties of alumina with different combinations of coatings were compared by an interrupted tensile testing method and by thermal cycling from room temperature to 600°C.The adhesion of a sputtered alumina coating to a copper substrate without any intermediate layer appeared to be rather poor. The use of titanium as an intermediate layer enhanced the adhesion significantly. The adhesion was further increased when the deposition temperature of the titanium was increased from 200 to 350°C. The combined Ni/Ti intermediate layer led to better adhesion than the titanium layer alone did.     

In situ formation of TiC particulate composite layer on cast iron by laser alloying of thermal sprayed titanium coating

Commercial flake graphite cast iron substrate was coated with titanium powder by low pressure plasma spraying and was irradiated with a CO₂ laser to produce the wear resistant composite layer. The macro and microstructural changes of an alloyed layer with the traveling speeds of laser beam, the precipitate morphology of TiC particulate and the hardness profile of the alloyed layer was examined. From the results, it was possible to composite TiC particulate on the surface layer by direct reaction between carbon existed in the cast iron matrix and titanium with thermal sprayed coating by remelting and alloying them using laser irradiation. The cooling rate of the laser remelted cast iron substrate without a titanium coating was about 1 × 10⁴ K/s to 1 × 10⁵ K/s in the order under the condition of this study. The microstructure of the alloyed layer consisted of three zones; the TiC particulate precipitate zone (MHV 400–500), the mixed zone of TiC particulate + ledeburite (MHV 650–900) and the ledeburite zone (MHV 500–700). TiC particulates were precipitated as a typical dendritic morphology. The secondary TiC dendrite arms were grown to a polygonized shape and were necking. Then the separated arms became cubic crystal of TiC at the slowly solidified zone. In the rapidly solidified zone near the fusion boundary, however the fine granular TiC particulates were grouped like grapes.     

Preparation of layered bioceramic hydroxyapatite/sodium titanate coatings on titanium substrates using a hybrid technique of alkali-heat treatment and electrochemical deposition

Bioceramic hydroxyapatite/sodium titanate coating on sandblasted titanium substrate was fabricated by a three-step process. At first, the sandblasted titanium substrate was coated with a flake-like sodium titanate layer by alkali-heat treatment. In the second step, the alkali-heat treated titanium substrate was hydrothermal treated at 180 °C for 4 h with calcium solutions. In the third step, the hydroxyapatite (HA) coating was deposited onto the hydrothermal treated layer via electrochemical deposition method. The surface topography and roughness of the coatings were determined by field emission scanning electron microscope (FESEM) and a mechanical contact profilometer, respectively. The surface compositions were evaluated by X-ray diffraction (XRD), energy-dispersive X-ray spectrum (EDS), and X-ray photoelectron spectroscopy (XPS). The EDS, XPS, and XRD analysis confirm the presence of element Ca, Ca²⁺, and CaTiO₃ on sodium titanate layer after hydrothermal treatment with Ca(NO₃)₂ solution, respectively. FESEM micrograph shows the rod/needle-shaped crystallites are highly densely packed on the calcium-ion-containing layer with an average size of ~50 nm in diameter. The results indicate that the sodium titanate layer containing Ca²⁺ ions possesses higher ability to induce HA formation compared with the pure sodium titanate layer. It is revealed that surface composition plays an important role in the electrochemical deposition of HA. The calcium-ion-containing layer probably makes the nucleation of HA easy and effectively promotes orientated growth of HA on flake-like sodium titanate surface. The sodium titanate layer possesses a lower corrosion current density and a higher corrosion potential than sandblasted-Ti substrate. The sodium titanate layer should act as a barrier to the release of metal ions from metallic substrate to physiological solutions and thus reducing the electrochemical reaction rate.     

Transmission electron microscopy of the fibre-matrix interface in SiC-SCS-6-fibre-reinforced IMI834 alloys

In SiC-SCS-6-fibre-reinforced IMI834 alloys, a high thermal stability of tensile properties and of the fibre-matrix interface for temperatures up to 700 °C was reported. In the present paper, the interface of these composites is investigated by analytical transmission electron microscopy in the as-processed condition and after a thermal treatment at 700 °C for 2000 h and at 800 °C for 500 h. The interface in (SiC-SCS-6)-IMI834 composites consists of two layers: the TiC reaction zone with a thickness of about 0.4 μm and a layer of a (Ti, Zr)xSiy phase with a thickness of about ∼0.1 μm. The energy-dispersive electron beam analysis of the (Ti, Zr)xSiy layer results in a (Ti, Zr)2Si phase with a Ti-to-Zr ratio of approximately 1.4. Electron beam diffraction of the (Ti, Zr)xSiy layer identifies it as S2 silicides present in near-α alloys. The thermal stability of the interface in the (SiC-SCS-6)-IMI834 composites is ensured by the continuous coating of the (Ti, Zr)xSiy phase. This is the case for a thermal treatment at temperatures up to 700 °C for 2000 h. After the treatment at 800 °C for 500 h, the thickness of the TiC reaction zone is increased, gaps in the (Ti, Zr)xSiy layer appear, titanium carbide grows further into the titanium matrix and the thermal stability of the interface is lost. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sol-gel derived hard optical coatings via organic/inorganic composites

Hard optical coatings via TiO₂/organically modified silane composites have been prepared by the sol-gel technique using γ-Glycidoxypropyltrimethoxysilane (GLYMO, used as organically modified silane source) and tetrapropylorthotitanate (TPOT, used as TiO₂ source) as precursors. Scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy have been used to investigate the morphology and structural properties of the coatings. The hardness and Young's modulus of the coatings have been characterized by a Nanoindenter and found to depend on the heat-treatment temperature and titanium content. Hardness as high as 10 Gpa was achieved at a heat-treatment temperature of 1000 °C. It was proposed that the high hardness of the coating is related to the carbon and titanium content in the coating.     

Crystallography of epitaxially grown molybdenum on sapphire

Molybdenum thin films were deposited onto (0001), ($\text{1012}$) and (10$\text{12}$) sapphire substrates by electron-beam evaporation in ultrahigh vacuum. The surface morphology and crystallographic orientation of these films were characterized by electron microscope replication and reflection electron diffraction. The effect of the crystallographic plane of the sapphire substrates as well as that of the deposition rate on the crystallographic orientation of the deposited molybdenum films were examined in the temperature range 25°-1000°C. The epitaxial temperature range was 600°-900°C for basal plane substrates and 300°-1000°C for ($\text{1}$012) and (10$\text{1}$2) substrates. The orientation of the deposited film was strongly dependent on that of the sapphire substrate.     

Interfacial structure, residual stress and adhesion of diamond coatings deposited on titanium

The interfacial structures of diamond coatings deposited on pure titanium substrate were analyzed using scanning electron microscopy and grazing incidence X-ray diffraction. Results showed that beneath the diamond coating, there was one titanium carbide and hydride interlayer, followed by a heat-affected and carbon/hydrogen diffused Ti layer. Residual stress in the diamond coating and TiC interlayer under different process parameters were measured using Raman and X-ray diffraction (XRD) methods. Diamond coatings showed large compressive stress on the order of a few giga Pascal. XRD analysis also showed the presence of compressive stress in the TiC interlayer and tensile stress in the Ti substrate. With increasing deposition duration, or decreasing plasma power and concentration of CH₄ in gas mixture, the compressive residual stress in the diamond coating decreased. The large residual stress in the diamond coating resulted in poor adhesion of the coatings to substrate, but adhesion was also related to other factors, such as the thickness and nature of the TiC interlayer, etc. A graded interlayer design was proposed to lower the thermal stress, modify the interfacial structure and improve the adhesion strength.     

Fabrication and oxidation resistance of titanium carbide-coated carbon fibres by reacting titanium hydride with carbon fibres in molten salts

Using carbon fibres and titanium hydride as a reactive carbon source and a metal source, respectively, a protective titanium carbide (TiC) coating was formed on carbon fibres in molten salts, composed of LiCl-KCl-KF, at 750-950 °C. The structure and morphology of the TiC coatings were characterised by X-ray diffraction and scanning electron microscopy, respectively. The oxidation resistance of the TiC-coated carbon fibres was measured by thermogravimetric analysis. The results reveal that control of the coating thickness is very important for improvement of the oxidation resistance of TiC-coated carbon fibres. The oxidative weight loss initiation temperature for the TiC-coated carbon fibres increases significantly when an appropriate coating thickness is used. However, thicker coatings lead to a decrease of the carbon fibres' weight loss initiation temperature due to the formation of cracks in the coating. The TiC coating thickness on carbon fibres can be controlled by adjusting the reaction temperature and time of the molten salt synthesis.     

Oxidation resistance of TiN,CrN, TiAlN and CrAlN coatings deposited by lateral rotating cathode arc

In this paper, four kinds of hard coatings, TiN, CrN, TiAlN and CrAlN (with Al/Ti or Al/Cr atomic ratio around 1:1), were deposited on stainless steel substrates by a lateral rotating cathode arc technique. The as-deposited coatings were annealed in ambient atmosphere at different temperatures (500–1000 °C) for 1 h. The evolution of chemical composition, microstructure, and microhardness of these coatings after annealing at different temperatures was systematically analyzed by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and nanoindentation experiments. The oxidation behaviour and its influence on overall hardness of these four coatings were compared. It was found that the ternary TiAlN and CrAlN coatings have better oxidation resistance than their binary counterparts, TiN and CrN coatings. The Cr-based coatings (CrN and CrAlN) exhibited evidently better oxidation resistance than the Ti-based coatings (TiN and TiAlN). TiN coating started to oxidize at 500 °C. After annealing at 700 °C no N could be detected by EDX, indicating that the coating was almost fully oxidized. After annealed at 800 °C, the coating completely delaminated from the substrate. TiAlN started to oxidize at 600 °C. It was nearly fully oxidized (with little residual nitrogen detected in the coating by EDX) and partially delaminated at 1000 °C. Both CrN and CrAlN started to oxidize at 700 °C. CrN was almost fully oxidized (with little residual nitrogen detected in the coating by EDX) and partially delaminated at 900 °C. The oxidation rate of the CrAlN coating is quite slow. After annealing at 1000 °C, only about 19 at.% oxygen was detected and the coating showed no delamination. The Ti-based (TiN and TiAlN) coatings were not able to retain their hardness at higher temperatures (≥ 700 °C). On the other hand, the hardness of CrAlN was stable at a high level between 33 and 35 GPa up to an annealing temperature of 800 °C and still kept at a comparative high value of 18.7 GPa even after annealed at 1000 °C, indicating a very promising applicability of this coating for high speed dry machining and other applications under high temperature environments.     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

Effect of annealing on microstructural and optoelectronic properties of nanocrystalline TiO2 thin films

In this study, semi-transparent nanostructured titanium oxide (TiO₂) thin films have been prepared by sol–gel technique. The titanium isopropoxide was used as a source of TiO₂ and methanol as a solvent and heat treated at 60°C. The as prepared powder was sintered at various temperatures in the range of 400–700°C and has been deposited onto a glass substrate using spin coating technique. The effect of annealing temperature on structural, morphological, electrical and optical properties was studied by using X-ray diffraction (XRD), high resolution transmittance electron microscopy (HRTEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), dc resistivity measurement and optical absorption studies. The XRD measurements confirmed that the films grown by this technique have good crystalline nature with tetragonal-mixed anatase and rutile phases and a homogeneous surface. The HRTEM image of TiO₂ thin film (annealed at 700°C) showed grains of about 50–60?nm in size with aggregation of 10–15?nm crystallites. Electron diffraction pattern shows that the TiO₂ films exhibited a tetragonal structure. SEM images showed that the nanoparticles are fine and varies with annealing temperature. The optical band gap energy decreases with increasing annealing temperature. This means that the optical quality of TiO₂ films is improved by annealing. The dc electrical conductivity lies in the range of 10^?6 to 10^?5?Ω?cm^?1 and it decreases by the order of 10 with increase in annealing temperature from 400°C to 700°C. It is observed that the sample Ti700°C has a smooth and flat texture suitable for different optoelectronic applications.     

Pyrolytic deposition of nanostructured titanium carbide coatings on the surface of multiwalled carbon nanotubes

Nanostructured titanium carbide coatings have been deposited on the surface of multiwalled carbon nanotubes (MWCNTs) by the MOCVD method with bis(cyclopentadienyl)titanium dichloride precursor. The obtained TiC/MWCNT hybrid materials were characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. It is established that a TiC coating deposits onto the MWCNT surface with the formation of a core–shell (MWSNT–TiC) type structure.     

碳化钛/钛镍金属间化合物复合涂层相组织研究

利用钛粉、镍粉和胶体石墨,真空条件下通过反应钎涂技术在低碳钢基体上制备了与基体冶金结合的碳化钛/钛镍金属间化合物复合涂层。采用扫描电子显微镜、能谱仪、X射线衍射仪及硬度计,研究了涂层的相组成、组织结构和成分分布。涂层组织由NiTi2、NiTi、TiC和hcp Ti组成,而涂层界面由NiTi和少量的hcp Ti构成,并且TiC主要分布在涂层中层。涂层中的NiTi2、NiTi、TiC是在钎涂过程中原位反应合成的,而且TiC和NiTi的量随碳含量的增加而增加。涂层表面硬度达到85HR15N,但不随TiC和NiTi含量增加而增高。