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.     

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.     

Microstructural evolution in the combustion synthesis of titanium carbide

Microstructural evolution in the combustion synthesis of titanium carbide was investigated using the combustion front quenching method and scanning electron microscopy. The results showed that the combustion reaction between titanium and carbon began with reaction diffusion of carbon into the surface layer of titanium powder, leading to the formation of a TiC shell around the titanium powder. The titanium powder coated with the TiC shell melted and became a molten core, as the carbon atoms diffused into the core through the TiC shell, TiC grains gradually crystallized. In the final products, the morphology and size of the initial titanium powder were maintained in the form of the TiC granule which was composed of isometrical TiC grains. In addition, a lamellar eutectic was present at the centre of some TiC granules. The microstructural evolution in the combustion synthesis of TiC has been described with a shell-core model.     

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.     

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.     

Carbon fibres coated with titanium carbide using the liquid metal transfer agent technique

Protective coatings of titanium carbide were applied to PAN type carbon fibres by a liquid metal transfer agent (LMTA) technique using tin as a transfer agent. The effect of the coating on the strength of the fibres was evaluated by performing single fibre tensile tests. The coatings were examined metallographically, by X-ray diffractometry, and by scanning electron microscopy. Carbide coating thicknesses obtained ranged from approximately 0.05 to 0.5 μm and the coatings were found to be uniform and adherent to the fibres. It was found that wetting of the fibres by the tin alloy is associated with the spontaneous formation of a carbide layer with a thickness dependent upon the melt temperature, after which the carbide layer was found to grow parabolically with time and with an apparent activation energy of 187 kJ mol⁻¹. The strength of the carbon fibres decreased with increasing coating thickness according to a Griffith relation.     

Microstructural studies and wear assessments of Ti/TiC surface composite coatings on commercial pure Ti produced by titanium cored wires and TIG process

Tungsten Inert Gas (TIG) process and titanium cored wires filled with micro size TiC particles were employed to produce surface composite coatings on commercial pure Ti substrate for wear resistance improvement. Wire drawing process was utilized to produce several cored wires from titanium strips and titanium carbide powders. Subsequently, these cored wires were melted and coated on commercial pure Ti using TIG process. This procedure was repeated at different current intensities and welding travel speeds. Composite coating tracks were found to be affected by TIG heat input. The microstructural studies using optical and scanning electron microscopy supported by X-ray diffraction showed that the surface composite coatings consisted of α′-Ti, spherical and dendritic TiC particles. Also, greater volume fractions of TiC particles in the coatings were found at lower heat input. A maximum microhardness value of about 1100 HV was measured which is more than 7 times higher than the substrate material. Pin-on-disk wear tests exhibited a better performance of the surface composite coatings than the untreated material which was attributed to the presence of TiC particles in the microstructure.     

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.     

Effect of composition on mechanical behaviour of diamond-like carbon coatings modified with titanium

In this study, diamond-like carbon (DLC) films modified with titanium were deposited by plasma decomposition of metallorganic precursor, titanium isopropoxide in CH₄/H₂/Ar gas atmosphere. The obtained films were composed of amorphous titanium oxide and nanocrystalline titanium carbide, embedded in an amorphous hydrogenated (a-C:H) matrix. The TiC/TiO₂ ratio in the DLC matrix was found to be dependent on the deposition parameters. The dependence of the films chemical composition on gas mixture and substrate temperature was investigated by X-ray photoelectron spectroscopy, whereas the crystallinity of TiC nanoparticles and their dimension were evaluated by X-ray diffraction. The size of TiC crystallites varied from 10 to 35 nm, depending on the process parameters. The intrinsic hardness of 10-13 GPa, elastic modulus of 170-200 GPa and hardness-to-modulus ratio of obtained coatings were measured by the nanoindentation technique. Obtained results demonstrated a correlation of mechanical properties with the chemical composition and the ratio of amorphous/crystalline phases in the films. In particular, the formation of nanocrystalline TiC with atomic concentration not exceeding 10% and with grain size between 10 nm and 15 nm resulted in significantly enhanced mechanical properties of composite material in comparison with ordinary DLC films.     

TiC coatings deposited onto carbon and molybdenum surfaces by electron beam evaporation

TiC coatings were deposited onto graphite and molybdenum substrates by an electron beam evaporation method. A titanium film 1000–10000 Å thick was evaporated onto the graphite substrate which was then heated at 1000 °C for 5 min to form the TiC film by an interdiffusion process. In the case of the molybdenum substrate, a double-layer film consisting of titanium and carbon (Ti/C/Mo) was prepared by evaporation and the subsequent heat treatment was performed at 700 °C or at 1000 °C for 5 min. The properties of the coatings were examined by various surface analysis techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Rutherford backscattering (RBS). The atomic ratio of carbon to titanium in these coatings was found to be 0.9. The in-depth profiles obtained by XPS examination showed that the coating prepared at 700 °C had a carbon layer between the TiC layer and the molybdenum substrate, while that prepared at 1000 °C had an Mo₂C layer between the coating and the substrate.     

An auger analysis of substrate-layer interactions in the chemical vapor deposition and activated reactive evaporation of TiC

A formalism for quantitative Auger analysis is developed to account for “matrix effects” and peak shape changes in WCTiCTa(Nb)CCo materials by including shape factors and a linear term in the inverse sensitivity factors of carbide single crystals and binary carbide alloys. Atomic concentration profiles are then obtained for carbon, titanium, W+Ta(Nb) and cobalt across the interface between WCTiC Ta(Nb)CCo substrates and TiC coatings deposited by chemical vapor deposition (CVD) and activated reactive evaporation (ARE). Phase compositions of the near- surface region of the substrate estimated from the atomic concentrations indicate that the concentration of carbon dissolved in the binder phase drops from approximately 28 at.% prior to deposition to approximately 10 at.% after the deposition by both processes of TiC coatings about 5 μm thick. The amount of binder phase near the CVD substrate surface was found to be approximately twice as great as that near the ARE substrate surface, probably because of significant transport of cobalt at the higher temperatures used for CVD.     

Ion-plated titanium carbide coatings

A new development in the field of coatings is the reactive ion-plating process. In this process, metal evaporated from a source reacts with an atmosphere of mixed gases. The reaction can take place in the gas phase before the material deposits, or it can take place on the substrate. The stoichiometry of the reaction can be controlled by adjusting at least one of several process parameters. This technique allows the formation of carbides, nitrides, oxides and other materials and provides a method of controlling the stoichiometry of the deposited material. The ability of the process to provide graded stoichiometry through a coating layer has made possible the application of adherent coatings to difficult-to-coat substrates. This prevents, for instance, a sharp boundary zone between materials which have greatly differing thermal coefficients of expansion. The dependence of the coating composition upon the deposition parameters of gas pressure, substrate voltage and evaporation rates from the source is discussed. The application of the technique in the coating of titanium and mild-steel substrates with titanium carbide is discussed. Photomicrographs and hardness data for the deposited films are presented.     

Pulsed laser deposition of hydroxyapatite on titanium substrate with titania interlayer

Pulsed laser deposition (PLD) has been used to deposit hydroxyapatite (HA) ceramic over titanium substrate with an interlayer of titania. PLD has been identified as a potential candidate for bioceramic coatings over metallic substrates to be used as orthopedic and dental implants because of better process control and preservation of phase identity of the coating component. However, direct deposition of hydroxyapatite on titanium at elevated temperature results in the formation of natural oxide layer along with some perovskites like calcium titanate at the interface. This leads to easy debonding of ceramic layer from the metal and thereby affecting the adhesion strength. In the present study, adherent and stable HA coating over Ti6Al4V was achieved with the help of an interlayer of titania. The interlayer was made to a submicron level and HA was deposited consecutively to a thickness of around one micron by exposing to laser ablation at a substrate temperature of 400°C. The deposited phase was identified to be phase pure HA by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and inductively coupled plasma spectrometry. The mechanical behavior of coating evaluated by scratch test indicates that the adhesion strength of HA coating was improved with the presence of titania interlayer.     

Laser cladding nickel-titanium carbide composite coating on a 45 carbon steel: Preparation,microstructure and wear behavior

Almost fully dense nickel-titanium carbide composite coatings with varied titanium carbide content were deposited on 45 carbon steel by laser cladding. High content of titanium carbide particles up to 50 wt.% with bimodal microstructure could be homogeneously distributed in the nickel based matrix. Due to the presence of the harder nickel-titanium carbide composite coating on the 45 carbon steel, the surface hardness and wear properties were significantly improved. The Vickers hardness (HV 3) increased from about 260 HV 3 for the 45 carbon steel to 300 HV 3 – 360 HV 3 for nickel based composite coating containing 30 wt.% titanium carbide and 550 HV 3 – 680 HV 3 for nickel based composite coating containing 50 wt.% titanium carbide composite coating, respectively. The coefficient of friction and volume wear rate was reduced down to 0.41×10⁻⁶ mm³ N⁻¹ m⁻¹ and 9.3×10⁻⁶ mm³ N⁻¹ ⋅ m⁻¹ when a nickel based composite coating containing 50 wt.% titanium carbide was coated on the 45 carbon steel, respectively. The enhanced wear performance of the composite coating was due to presence of harder nickel-titanium carbide composite coating and formation of varied soft and lubricant metal oxides consisting of mainly titanium oxides and minor iron and nickel oxides.     

Investigation of the interlayers between cemented carbides and titanium carbide coatings obtained by chemical vapor deposition

Data are presented on the microstructure, microhardness and chemical composition of the interlayers between cemented carbides and chemically vapor-deposited TiC coatings. The carbon: metal ratios of the cubic carbides across the interlayers were obtained by Auger depth profile analysis. The mechanisms of nucleation and diffusional growth of the η-carbide skeleton at the upper layer of the substrate are discussed. It is proposed that a cubic carbide phase acts as a carbon donor and thus inhibits the localized formation of the η-carbide skeleton and retains the integrity of the surrounding network.     

Surface modification of titanium carbide with carbyne-containing nanocoatings

The aim of this research is to investigate a novel approach to surface engineering of biomaterials that are based on transition metals of the groups IVA-VIA. The approach taken relies on the fact that, during the electropolishing of TiC surfaces, the removal of Ti atoms from the TiC surface surpasses that of C atoms. This leads to enrichment of the TiC surface with carbon. Transmission electron microscopic investigation showed that carbon-based films contain carbynes in the form of nanorod-like clusters with lengths in the range of 5-100 nm. This carbyne-containing layer is 50-100 nm thick. It was generalized that carbyne-containing nanofilms are formed on the carbide surface of transition metals of groups IVA-VIA during electropolishing. Since carbynes, being one-dimensional chain-like structures [(-C identical to C-)n/(=C=C=)n] with sp1 carbon-carbon hybridization, have the highest degree of biocompatibility because of their biological activity, the development of such surface bioengineering with carbynes extends applications of biomaterials based on transition metals of the groups IVA-VIA.     

Comparison of the properties of ion-plated titanium carbide films prepared by different activation methods

A comparative study was made of titanium carbide films ion plated onto molybdenum by d.c. and r.f. discharge methods, from the point of view of the characteristics of the discharge and the quality of deposits made by each technique. A very high specimen current was achieved in the d.c. discharge method. Stoichiometric TiC deposits were easily obtained in a wide range of the pressure ratio of C₂H₂ gas to titanium vapour. In contrast, careful optimization of the deposition conditions was needed in the r.f. discharge method for controlling the stoichiometry of the deposits. Titanium carbide coatings deposited onto molybdenum by the d.c. discharge method showed excellent thermal stability compared with those prepared by the r.f. discharge method. These results indicate the strong influence of the ionization efficiency on the properties of ion-plated titanium carbide deposits.     

Microstructural study of titanium carbide coating on cemented carbide

Titanium carbide coating layers on cemented carbide substrates have been investigated by transmission electron microscopy. Microstructural variations within the typically 5m thick chemical vapour deposited TiC coatings were found to vary with deposit thickness such that a layer structure could be delineated. Close to the interface further microstructural inhomogeneities were obsered, there being a clear dependence of TiC deposition mechanism on the chemical and crystallographic nature of the upper layers of the multiphase substrate.     

Assessment of bioengineering ceramic coatings using the wetting method

The wettability of coatings, including ceramic ones, which show considerable promise for the use on bioengineering products, with physical solution (0.9% NaCl) have been studied. It has been found that the use of coatings of all types under study increases the wetting angles on the surface as compared with the initial metal materials (stainless steel of the 12X18H10T grade, titanium alloy of the BT6-grade, Co-Cr-Mo alloy), which serves as a prerequisite for an improvement in the biocompatibility of implants. The degree of the coating bioinertness increases in the following order: titanium nitride → diamond carbon films → aluminum nitride → titanium oxide (anodic oxide film) → nitride (oxidized) → oxide → titanium oxide (anodic-spark coating).     

Antibacterial hydroxyapatite coatings on titanium dental implants

Titanium and its alloys are often used as substrates for dental implants due to their excellent mechanical properties and good biocompatibility. However, their ability to bind to neighboring bone is limited due to the lack of biological activity. At the same time, they show poor antibacterial ability which can easily cause bacterial infection and chronic inflammation, eventually resulting in implant failure. The preparation of composite hydroxyapatite coatings with antibacterial ability can effectively figure out these concerns. In this review, the research status and development trends of antibacterial hydroxyapatite coatings constructed on titanium and its alloys are analyzed and reviewed. This review may provide valuable reference for the preparation and application of high-performance and multi-functional dental implant coatings in the future.