Tunable Nanostructures and Crystal Structures in Titanium Oxide Films

Controllable nanostructures in spin coated titanium oxide (TiO₂) films have been achieved by a very simple means, through change of post deposition annealing temperature. Electron beam imaging and reciprocal space analysis revealed as-deposited TiO₂ films to be characterized by a dominant anatase phase which converts to the rutile form at 600 °C and reverts to the anatase modification at 1,200 °C. The phase changes are also accompanied by changes in the film microstructure: from regular nanoparticles (as-deposited) to nanowires (600 °C) and finally to dendrite like shapes at 1,200 °C. Photoluminescence studies, Raman spectral results, and X-ray diffraction data also furnish evidence in support of the observed solid state phase transformations in TiO₂.     

Photocatalytic activities of N-doped nano-titanias and titanium nitride

TiO₂ doped with various loadings of nitrogen was prepared by nitridation of a nano-TiO₂ powder in an ammonia/argon atmosphere at a range of temperatures from 400 to 1100 °C. The nano-TiO₂ starting powder was produced in a continuous hydrothermal flow synthesis (CHFS) process involving reaction between a flow of supercritical water and an aqueous solution of a titanium salt. The structures of the resulting nanocatalysts were investigated using powder X-ray diffraction (XRD) and Raman spectroscopy. Products ranging from N-doped anatase TiO₂ to phase-pure titanium nitride (TiN) were obtained depending on post-synthesis heat-treatment temperature. The results suggest that TiN started forming when the TiO₂ was heat-treated at 800 °C, and that pure phase TiN was obtained at 1000 °C after 5 h nitridation. The amounts and nature of the Ti, O and N at the surface were determined by X-ray photoelectron spectroscopy (XPS). A shift of the band-gap to lower energy and increasing absorption in the visible light region, were observed by increasing the heat-treatment temperature from 400 to 700 °C.     

Comparison of the anti-coking performance of CVD TiN,TiO2 and TiC coatings for hydrocarbon fuel pyrolysis

Previous work has shown that both TiN and TiO₂ coatings can inhibit the metallic catalytic coking effectively, but both of them have their own shortage. In this work, TiC coating was prepared on the surface of SS304 tube using TiCl₄-CH₄-H₂ by CVD method. Its morphology, elemental composition, thickness and oxidation resistance were characterized by SEM, EDX, metalloscopy and TPO tests, respectively. The results show that CVD TiC coating is gray, homogeneous, and dense without cracks or holes. The TiC coating presents a cuboid particle structure with the sizes of about 1.0 µm for the cuboid crystals, and the Ti/C ratio close to 1:1, while the average thickness is about 11.62 µm. TPO results show that the TiC coating begins to react with O₂ and release CO₂ at about 810 °C. Compared with the TiN coating (The initial oxidation temperature of TiN is about 350 °C), the oxidation resistance of TiC coating is improved substantially. As a conclusion, the high oxidation resistance order is TiO₂ coating>TiC coating>TiN coating. Furthermore, the temperature programmed cracking of RP-3 Chinese jet fuel was employed to compare the anti-coking performance of TiN, TiO₂ and TiC coatings. The results show that each of TiN, TiO₂ and TiC coating has obvious anti-coking effect, and the anti-coking performance order is TiN coating=TiC coating>TiO₂ coating.     

Microstructure evolution and wear resistance of nitride/aluminide coatings on the surface of Ti-coated 2024 Al alloy during plasma nitriding

A duplex surface treatment consisting in depositing a Ti film followed by plasma nitriding was adopted to improve the wear resistance of 2024 Al alloys. Nano-grained Ti films were firstly deposited on the substrate surface by using magnetron sputtering, then plasma nitrided for 8 h at 400 °C, 430 °C, 460 °C and 490 °C, in a gas mixture of 40% N₂+60% H₂. Duplex coatings composed of three sublayers (i.e. the outmost TiN_0.3 layer, the intermediate Al₃Ti layer and the inside Al₁₈Ti₂Mg₃ layer) were obtained at nitriding temperature higher than 460 °C. The coatings obtained at 400 °C and 430 °C consisted of mainly α-TiN_0.3 with (002) preferred orientation. The surface hardness of the coatings increased at higher nitriding temperature, reaching the maximum of 500 HV at 490 °C, which was about 8 times higher than that of the uncoated alloy. The friction coefficients of 2024 Al alloy decreased in the coatings prepared at higher nitriding temperature, reaching the lowest values of 0.31 at 490 °C. The wear rate of the coated samples decreased by 56% compared with the uncoated ones. The analysis of worn surface indicated that the nitrided samples exhibited severe adhesive wear at 400 °C that changed to predominant abrasive wear at increased nitriding temperature.     

Phase transformation of nonstoichiometric cubic tungsten carbide on the surface of carbon nanotubes during high-temperature annealing of aluminum matrix composites

In this work, metal matrix composites based on 5049 aluminum alloy reinforced with multi-walled carbon nanotubes (CNTs) coated with nonstoichiometric cubic tungsten carbide were obtained by powder metallurgy. For the first time for this system, high-temperature annealing of the synthesized composites at 500–600 °C for 0.5 h was carried out. Effect of the annealing on the evolution of the structural-phase composition and the change in the microhardness and Young's modulus of the bulk composite was studied. Characterization of the structure shows that despite the growth of the matrix grains, the structural heterogeneity inherent for composites in the as-synthesized state is retained after heat treatment. Along with coarse grains, fine grains still remain, which indicates an increased resistance to recrystallization of the composite even at a temperature of ～0.9T_m. XRD analysis shows that annealing at temperatures above 525 °C leads to a solid-state interfacial reaction of the ceramic coating on the CNTs surface with the matrix material, resulting in the in-situ formation of the WAl₁₂ intermetallic compound around the reinforcing particles and Al₄C₃ nanorods. At the same time, the structural integrity of the CNTs is preserved. An increase in the annealing temperature contributed to an increase in the intensity of phase transformations and an increase in the fraction of the in-situ phases. The microhardness and Young's modulus of the composites decreased by ～20% and ～27%, respectively. Nevertheless, despite the increase in the grain size, the level of these properties remained quite high and equal to 141 HV and 80 GPa, correspondingly, due to the formation of a larger fraction of the in-situ WAl₁₂ and Al₄C₃ phases. The obtained results are applicable for varying the mechanical properties of the composite by controlling the degree of in-situ reaction between the matrix alloy and the ceramic coating on the CNTs.     

The effects of partial pressure of CO on the phase behaviour of synthesised Ti(C,N) through carbothermal reduction-nitridation at 1,380oC

The effects of partial pressure of CO (P_CO) on the product phases formed through titanium dioxide (TiO₂) carbothermal reduction and nitridation were studied. Electrode graphite and anatase TiO₂ were used as the raw materials, while the reaction was carried out under a flowing nitrogen gas atmosphere at 1,380°C. The effects of P_CO on the phase compositions, chemical compositions and microstructures of the nitridation products were investigated by adjusting the P_CO in the system. The chemical mineral compositions, and microstructures of the products were characterised via scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction. The results demonstrate that the product phases are mainly titanium nitride (TiN)_0.96 and residual graphite when the P_CO is at 0.003 atm. As the value of P_CO reached 0.08 atm in the furnace, a phase of Ti(O_0.91, C_0.53, N_0.32) gradually began to form, while when the CO content in the atmosphere was over 0.12 atm, a phase of Ti(C_0.2, N_0.8) could be observed in the products. With the increase of P_CO in the system, the evolution sequence of the reaction products was found to be TiO₂→TiN→Ti(O, C, N)→Ti(C, N). By controlling the partial pressure, the synthesised temperature of titanium carbonitride (Ti[C, N]) can be significantly reduced, and the manipulation of the TiN and Ti(C, N) can be effectively realised, which could provide new ideas for experiments closely related to the partial pressure of gases.     

Influence of annealing temperature on microstructure evolution of TiAlSiN coating and its tribological behavior against Ti6Al4V alloys

TiAlSiN multicomponent coating, owing to its high hardness and excellent high temperature resistance, was widely used in the cutting field of difficult-to-cut materials such as titanium alloys. For machining titanium alloys, high temperature is easy to gather on the tool chips and deteriorate the cutting tools. Moreover, high temperature will also promote the microstructure evolution and make the wear mechanism more complex. In this paper, TiAlSiN coatings were deposited on cemented carbides and annealed at 400 °C, 600 °C and 800 °C respectively for 60 min in air, followed by reciprocating friction tests against Ti6Al4V counterparts. AFM, SEM, EDS and XPS were applied to investigate the microstructure evolution and tribological behavior of TiAlSiN coating after high temperature annealing. The results demonstrated that the oxidation resistance of TiN phase in TiAlSiN coating was worse than Si₃N₄ and AlN phases. These nitrides can be oxidized to TiO₂, SiO_x and AlO_x under 600 °C, and the depth of oxide layer was increased with the rising annealing temperature, resulting in the coarsened microstructure. The wear mechanisms of as-deposited TiAlSiN coating were oxidation wear and adhesion wear. With the rising annealing temperature, abrasive wear was gradually enhanced. For the TiAlSiN coating annealed at 800 °C, abrasive wear became the dominant wear mechanism.     

Corrosion behavior of TiN and TiN/Ti composite films on Ti6Al4V alloy in Hank’s solution

Surface films of TiN and TiN/Ti were deposited on Ti6Al4V alloy by arc ion plating (AIP). Open-circuit potential, potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were employed to investigate the corrosion performance of TiN and TiN/Ti films in Hank’s simulated body fluid at 37 °C and pH 7.4. Scanning electron microscopy (SEM) was used to study the surface morphology of the corroded samples after the potentiodynamic polarization tests. The results show that the TiN and the TiN/Ti films can provide effective protection for the Ti6Al4V substrate in Hank’s fluid, and the TiN/Ti composite film showed a corrosion resistance superior to that of the TiN film. The outer TiN layer of the composite film mainly acted as an efficient barrier to corrosion during short-term experiments. In contrast to the bare Ti6Al4V, no pitting was observed on the surface of the TiN and TiN/Ti films deposited on the bare alloy after potentiodynamic polarization.     

Structural and mechanical investigations of magnetron sputtering TiO<Subscript>2</Subscript>/Ti/TiN multilayer films on Si(100) substrate

TiO₂/Ti/TiN multicompositional coating was prepared by DC reactive magnetron sputtering (MS) at a temperature of 150°C using a combination of a Ti metal target and a pure Ar, an Ar–O₂ mixture, or an Ar–N₂ mixture discharge gas, onto a Silicon(100) substrate. This system represents nano-structured multilayer substrate model for biomedical application as well as substrate models to reproduce the bulk titanium surface. This substrate model (Ti/TiN/Si(100)) makes possible and easy mechanical and microscopic characterization in particular for transmission electron microscopy after biocompatible test. The model multilayer TiO₂/Ti/TiN/(100) was obtained after preparation of two intermediate samples: TiN/Si(100) and Ti/TiN/Si(100). Structural (X-ray diffraction), morphological (scanning electron and atomic force microscopy) as well as mechanical (hardness and elastic modulus) studies of the MS films were performed.     

A new precursor for fabricating TiN coating on 316L stainless steel at low temperature without corrosive byproducts

High quality TiN coated 316L stainless steel can hardly be fabricated by the traditional chemical vapor deposition (CVD) methods because of the formation of HCl (g) corrosive byproduct and phase transformation at high deposition temperature. To address the problem, herein, for the first time a novel TiCl₂ precursor is proposed to fabricate TiN coating on 316L substrate in N₂ atmosphere at low temperature. The idea is based on that metastable TiCl₂ can release fresh Ti atoms by its disproportionation reaction, and then the fresh Ti atoms react easily with N₂ to form TiN coating at low temperature without HCl byproduct. The reaction path, interface reaction, and performance of the TiN coating were investigated. The optimized nanograin TiN (45 nm) coating was successfully fabricated on 316L at 750 °C and it exhibited excellent corrosion resistance.     

Effect of microstructural evolution on wettability and tribological behavior of TiO2 nanotubular arrays coated on Ti–6Al–4V

Self-organized TiO₂ nanotubular arrays were fabricated by electrochemical anodization of Ti–6Al–4V plates in an NH₄F/H₃PO₄ electrolyte. The effect of microstructural evolutions on the wettability and tribological behavior of the TiO₂ nanotubes was investigated. Based on the XRD profiles of the fabricated material, the characteristic TiO₂ peaks were not recognized after anodization; however, highly crystalline TiO₂ (anatase and rutile) was formed due to crystallization during annealing at 500 °C for 1.5 h. The nanotube arrays were converted entirely to rutile at 700 °C. From a microstructure point of view, a highly ordered nanotube structure was achieved when the specimen was annealed at 500 °C, with a length of 0.72 μm and a pore diameter of 72 nm. Further increasing the annealing temperature to 700 °C resulted in the complete collapse of the tubular structure. The results indicate that the improved wettability of the anodized specimens was due to the combination of the effects of both the surface oxide layer and the increased surface roughness achieved after anodization. Moreover, the wear resistance and wettability of the sample annealed 500 °C were improved due to the high hardness (435 HV) and low coefficient of friction (0.133–168) of the highly crystalline structure of the TiO₂ nanotubes.     

Chemical synthesis of TiO2 nanoparticles and their inclusion in Ni–P electroless coatings

The work addresses the preparation of Ni3P3TiO₂ nanocomposite coatings on mild steel substrate by the electroless technique. Nanosized TiO₂ particles were first synthesized by the precipitation method and then were codeposited (4 g/l) into the Ni3P matrix using alkaline hypophosphite reduced EL bath. The surface morphology, particle size, elemental composition and phase analysis of as-synthesized TiO₂ nanoparticles and the coatings were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD). Coatings with 20 µm thickness were heat treated at 400 °C for 1 h in argon atmosphere. The morphology, microhardness, wear resistance and friction coefficient characteristics (ball on disc) of electroless Ni3P3TiO₂ nanocomposite coatings were determined and compared with Ni3P coatings. The results show that as-synthesized TiO₂ nanoparticles are spherical in shape with a size of about12 nm. After heat treatment, the microhardness and wear resistance of the coatings are improved significantly. Superior microhardness and wear resistance are observed for Ni3P3TiO₂ nanocomposite coatings over Ni3P coatings.     

Fabrication of in-situ Ti(C,N) phase toughened Al2O3 based ceramics from natural bauxite

Al₂O₃–Ti(C,N) ceramics were fabricated via carbothermal reduction nitridation method with high-titania special-grade bauxite as the raw material. The formation mechanism of in-situ Ti(C,N) phase and its effect on the properties of materials are discussed. After nitrided at 1700 °C, Ti(C,N) phase could be formed in-situ with appropriate C/TiO₂ molar ratio. Due to the residual stress field formed by Ti(C,N) particles, the path of crack propagation is changed, leading to the crack deflection and pinning. Therefore, the mechanical properties of the materials are improved by forming in-situ Ti(C,N) phase. With a C/TiO₂ molar ratio of 2.2 and nitridation temperature of 1700 °C, Al₂O₃–Ti(C,N) ceramic with a hardness of 13.9 GPa, a fracture toughness of 8.28 MPa m^1/2 and a flexural strength of 387 MPa could be fabricated.     

Preparation and oxidation of composite TiN–AlN films from alkoxide solutions by plasma-enhanced CVD

Composite and compositionally graded (CGed) TiN–AlN films were deposited on Si wafers at 600 °C from Ti- and Al-alkoxide solutions by N₂ plasma-enhanced chemical vapor deposition (CVD). The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Vickers micro-hardness. In the composite TiN–AlN films, the Ti and Al contents varied linearly and complementarily with solution composition, the N content ranging from 35 to 40 at.%. In the CGed films, the Al component decreased complementarily with increasing Ti toward the substrate. Cross-sectional SEM observation showed both films to be about 1 μm thick with a columnar structure. Oxidation of the composite and CGed films was performed at 500, 700, and 900 °C in air for 1 h. The improvement of oxidation resistance in both composite and CGed films is discussed on the basis of the XRD and SEM observations, and the XPS analysis of the oxidized films.     

Characterization and mechanical properties of TiO2 nanotubes formed on titanium by anodic oxidation

The well-ordered titanium dioxide (TiO₂) nanotube array surfaces were formed at different voltages such as 20 V, 40 V, 60 V, 80 V and 100 V for 1 h on cp-Ti by anodic oxidation (AO) technique. And then, to improve crystallinity of the surface, heat treatment was applied at 450 °C for 1 h to all surfaces without any morphological changing. The surface and cross sectional morphology, elemental structure, phase composition, functional groups, roughness and thickness, wettability and mechanical results were investigated by SEM, EDX, XRD, FT-IR, AFM, contact angle measurement device and nanoindentation tester, respectively. Mainly, anatase- and rutile-TiO₂ phases were obtained at post-heat treatment whereas only, Ti phase was detected on AO surfaces at pre-heat treatment. All nanotube structures and the elements of Ti and O were uniformly distributed through the whole surface. The roughness and thickness of tube structures usually increased with increasing voltage values and measured. The roughness and thickness values were measured as 10.67–111.97 nm and 0.21–1.92 μm, respectively. TiO₂ nanotube surfaces exhibited hydrophobic behaviors with respect to plain Ti surface. Furthermore, mechanical properties such as hardness and elastic modulus of the coating produced at minimum voltage were great compared to ones at higher voltage and plain Ti surface under a Berkovich indenter due to phase structure, homogeneity and density of nanotube structures.     

Sintering behavior and thermal shock resistance of aluminum titanate (Al2TiO5)-toughened MgO-based ceramics

In order to improve the thermal shock resistance of MgO-based ceramics, aluminum titanate (Al₂TiO₅)-toughened MgO-based ceramics were successfully prepared by solid state sintering at 1450 °C and 1550 °C for 3 h starting from MgO and as-synthesized Al₂TiO₅ powders. The effects of various contents of Al₂TiO₅ second phase on the sintering behavior and thermal shock resistance of MgO-based ceramics were investigated. The sintering behavior of sintered samples was evaluated by comparing the relative density, apparent porosity, bending strength, phase composition as well as microstructure. The thermal shock resistance of sintered samples was characterized by using the residual bending strength after three thermal cycles and thermal expansion coefficient. The obtained samples with 10 wt% Al₂TiO₅, which were sintered at 1550 °C for 3 h, showed the highest relative density, lowest apparent porosity as well as optimum bending strength. In addition, the samples added 15 wt% Al₂TiO₅ at 1550 °C with a dwell time of 3 h were the highest residual bending strength and lowest thermal expansion coefficient. It revealed that the enhancement in thermal shock resistance was ascribed to the reduction of thermal expansion coefficient.     

Oxidation behaviour of Si3N4–TiN ceramics under dry and humid air at high temperature

The oxidation in air of Si₃N₄-based ceramics containing 35 vol.% of TiN secondary phase and different amounts of sintering additives has been studied at different temperatures up to 1400 °C in dry or humid environment. The oxidation starts by crystal growth of TiO₂ at the surface, then a multilayered scale develops under the rutile layer from 1000 °C. This subscale is composed of silicon nitride in which TiN particles are oxidized to agglomerates of rutile, glass and pores. The oxidation process is controlled by the matter transports, which take place in the intergranular phase. These transport phenomena are affected by the changes in distribution and composition of the glassy phase and by humidity which modifies the glass network structure and thus the in-diffusion rate. From 1200 °C, Si₃N₄ grains are also oxidized, the additional glass formed closes the residual porosities yielding scales more compact and developing an autoprotective behavior. At 1400 °C, glass phase crystallizes into cristobalite and the rutile top layer becomes discontinuous. Only composites with low amounts of sinter additives keep an autoprotective oxidation mode.     

Effect of Heat Treatment on Micromechanical Properties of Sprayed TiO2 Film on Ce-Doped Glass

Abstract

Anatase TiO₂ films were deposited on unheated Ce-doped soda–lime–silicate glass substrates by a spray technique from an anatase sol made in the laboratory. In order to investigate the micromechanical properties of TiO₂ films, the deposited films were heated treated at: 350, 500 and 550°C, each for 1 h. X-ray diffraction spectra revealed a crystalline structure with an anatase phase and the average diameter of the grains increased from 21.4 to 31.2 nm as the temperature in the heat treatment rose from 20 to 550°C. The films as-deposited and heat-treated at 350°C were found to be smooth and relatively dense. Cracks appeared in TiO₂ films when the heating temperature increased to 550°C. The results of nano-indentation test showed that when the heating temperature rose to 500°C, the TiO₂ films were found to have nano-hardness and elastic modulus values of 1.1 and 30.8 GPa, respectively. These were the highest values recorded in this work. When the temperature reached 550°C, the nano-hardness and elastic modulus decreased due to the presence of cracks in the films.     

Interfacial microstructure of the Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was jointed with itself by active brazing with a Cu–Pd–Ti filler alloy. Interfacial microstructure of the Si₃N₄/Si₃N₄ joint was analyzed by EPMA, TEM and X-ray diffract meter. The results indicate that a TiN reaction layer with a thickness about 5 μm is formed at the interface between Si₃N₄ ceramic and filler alloy. The TiN reaction layer is composed of two zones: one next to the Si₃N₄ ceramic with grains of 100 nm and the other zone that connects with the filler alloy and has grains of 1 μm. The microstructure of the joint can be described as: Si₃N₄ ceramic/TiN layer with fine grains/TiN layer with coarsen grains/Cu[Pd] solid solution. Some new phases, such as Pd₂Si, PdTiSi, Ti₅Si₃ and TiN, were formed in the Cu[Pd] solid solution interlayer. With increasing brazing temperature from 1100 °C to 1200 °C, the thickness of the TiN reaction layer is not changed. Meanwhile, the amount and size of the TiN and Pd₂Si phases in the Cu[Pd] solid solution increase, while, the amount of the PdTiSi phase decreases.     

A novel IrO<Subscript>2</Subscript> electrode with iridium–titanium oxide interlayers from a mixture of TiN nanoparticle and H<Subscript>2</Subscript>IrCl<Subscript>6</Subscript> solution

A novel IrO₂ anode on titanium substrate with iridium–titanium oxide interlayer (Ti/IrO _x –TiO₂/IrO₂) was prepared and investigated for oxygen evolution. IrO _x –TiO₂ interlayer was coated on titanium substrate by impregnation-thermal decomposition method from a mixture of TiN nanoparticles and H₂IrCl₆ solution at 500 °C. The results showed that the service life of Ti/IrO _x –TiO₂/IrO₂ was a factor of six times longer than that of Ti/IrO₂, which was attributed to the IrO _x –TiO₂ interlayer, it could form a metastable solid solution between IrO _x and thin titanium oxide layer on titanium substrate during calcination. The interlayer contributed to the decrease in migration rate of oxygen atom or molecule toward substrate and the increase in bonding force among IrO₂ layer, interlayer, and substrate. Therefore, besides keeping high electrocatalytic activity, the service life of Ti/IrO _x –TiO₂/IrO₂ electrode was greatly improved, and its overall electrocatalytic performance for oxygen evolution was increased as well.