Formation of titanium and niobium aluminides induced by hydrogen in a hydride cycle

The results of our study of formation of aluminides by the hydride cycle (HC) method in Ti–Al and Ti–Al–Nb systems and of their hydrides under the conditions of self-propagating high-temperature synthesis are presented. The HC method was developed at the High-Temperature Synthesis Laboratory of the Nalbandyan Institute of Chemical Physics, Armenian Academy of Sciences. The effect of various parameters: the ratio of titanium and niobium hydrides and aluminum powders in the reaction mixture, hydride powder grain size (micro and nano size), compacting pressure during compaction of hydrides, and conditions of dehydrogenation and sintering (heating temperature and rate) on the characteristics of the obtained aluminides: crystal structure, density, etc. was studied. As a result, single-phase aluminides α₂-Ti₃Al, Ti₂Al, γ-TiAl, TiAl₃, Ti_0.33Al_0.34Nb_0.33, Ti_0.5Al_0.23Nb_0.27, Ti_0.52Al_0.15Nb_0.33, TiAl₆Nb, etc., and hydrides were synthesized. The HC method has significant advantages over conventional procedures: low temperature (1000–1100°C) and processing time (30–60 min), one-stage formation of single-phase aluminides, etc.     

Comparative study of microstructural characteristics of electrospark and Nd:YAG laser epitaxially growing coatings

As low-heat input welding processes, electrospark deposition and pulsed Nd:YAG laser cladding can be commonly used to prepare epitaxially growing coatings. However, these two processes have quite different characteristics in the energy input, the amount of materials involved, and the temperature gradient, and hence might result in dissimilar microstructural characteristics. In this paper, a comparative study has been made between microstructural characteristics in epitaxial growth coatings prepared by electrospark deposition and pulsed Nd:YAG laser cladding. Some interesting results have been achieved. Firstly, epitaxial growth coatings can be commonly achieved by these two techniques. Secondly, microstructural morphologies of these two epitaxial growth coatings are obviously different, cellular columnar structure prevails in the electrospark coating while columnar dendritic structure occupies most of the laser coating thickness, more importantly, electrospark coating remains fully columnar in the whole layer whereas laser coating tends to change from columnar to equiaxed at the top of the layer. Thirdly, electrospark coating possesses finer and more homogeneous microstructure than laser coating.     

Study of coatings based on titanium oxides and oxynitrides using a set of methods

A set of methods has been applied to study the properties of titanium oxide and oxynitride coatings on steel. It is established that the coatings consist of two phases with a nanocrystalline and an amorphous structure with a high proportion (～50 %) of grain boundaries; anatase is the dominant crystalline phase. The obtained results can be applied in the development of coatings for stents as well as for manufacturing hydrogen accumulators.     

Investigation of the microstructure and properties of doped nanocomposite coatings based on titanium nitride

The special features of the elemental composition, structure-phase and elastically stressed states, and properties of coatings based on titanium nitride are investigated for different concentrations of Al, Si, Cu, Ni, Cr, and C doping elements using x-ray fluorescent analysis, x-ray microanalysis, dark-field electronmicroscopic analysis of the crystal lattice bending and torsion, microhardness measurements, and scratch tests. Influence of the structure and concentration of the doping elements on the relative fraction of nonmetallic atoms, crystal size, and phase composition of the coating is established. High values (several hundred degrees per micron) of the lattice bending-torsion with dipole configuration are established for nanocrystals with sizes smaller than 20 nm. Residual stresses in nanocrystals are estimated for the disclination model of the structural state. It is demonstrated that the increased degree of coating doping improves the thermal stability of their structure and properties.     

A fibre-optic thermometric sensor based on the thermo-optic effect of titanium dioxide coatings

An experimental determination of the thermo-optic effect of titanium dioxide is described in which the test element is a thin film deposited onto the end face of an optical fibre. The film acts as an etalon. The reflectivity of titanium dioxide films was measured as a function of temperature experimentally over the range 20 °C to 500 °C, and modelled theoretically to yield a value of the thermo-optic coefficient of − 1.49 × 10⁻⁴ K⁻¹. The use of the data in the design of a thin-film optical fibre temperature sensor is described.     

Surface analysis of biocompatible coatings on titanium

The coatings of hydroxyapatite, which is widely used for orthopaedic and dental prothesis, were deposited by using the dip-coating method. The layers of hydroxyapatite were grown on commercial Ti substrates. In order to improve the adhesion of hydroxyapatite, the substrate was a priori covered with titania or calcium titanate by using the sol-gel technique. For comparison, commercial samples of hydroxyapatite coating (manufactured by means of plasma-spray apparatus) were analysed. The chemical composition and the structure of the coatings (TiO₂, CaTiO₃ and hydroxyapatite) were studied by using X-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), X-ray diffraction (XRD) and secondary electron microscopy (SEM) techniques. The data of quantitative XPS analysis and the surface images (SAM and SEM) displayed the superior quality (cleanness, homogeneity, etc.) of hydroxyapatite deposited by sol-gel in comparison with commercial samples investigated.     

Formation of nanostructured weldments in the Al-Si system using electrospark welding

Electrospark welding (ESW) electrodes were manufactured from three binary aluminum-silicon alloys consisting of 12 and 17 wt% silicon, produced using chill and sand casting. The electrodes were used to assess the feasibility of producing aluminum-silicon weldments consisting of nano-sized silicon particles embedded in nanostructured aluminum matrix, using the ESW process. Line tests were performed to determine the optimal processing parameters resulting in a high quality deposit. X-ray diffraction (XRD) as well as optical and field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) was performed to determine the composition and microstructure of the depositions. It was determined that a capacitance of 110 μF and a voltage of 100 V resulted in the highest quality deposition. Furthermore it was determined that the ESW process was capable of producing a microstructure consisting of an extremely fine-grained silicon phase ranging from ∼6 to 50 nm for the eutectic composition, and 10-200 nm for the hypereutectic compositions. Finally it was determined that the functional thickness limit of the aluminum-silicon deposit produced under these process parameters was 120 μm.     

Formation and structure of sphene/titania composite coatings on titanium formed by a hybrid technique of microarc oxidation and heat-treatment

Sphene/titania composite coatings were fabricated on titanium by a hybrid technique of microarc oxidation (MAO) and heat treatment. The high-applied voltages promote the formation of sphene in the MAO coatings after heat-treatment. Heat treatment could change the surface morphology of the MAO coatings such as roughness, macropores size and density and the thickness of the MAO coatings. Increasing the heat-treatment temperature decreased the atomic concentration ratios of Ti/Si and Ti/Ca of the MAO coatings. The chemical states of Ti⁴⁺, Ca²⁺, Si²⁺ and O²⁻ were observed on all the coatings. Additionally, Ti²⁺ was detected in the MAO and heat-treated MAO coatings at 600 and 700 °C. The heat-treatment has obvious effect on the chemical states of Si, Ti and O elements due to the formation of sphene and oxidation of TiO phase of the MAO coating, but did not affect that of Ca. In the heat-treated MAO coatings at 800 °C (MAO-H8), the titanium surface shows a MAO top layer and oxidized interior layer. A concentration gradient in components in the MAO layer of the MAO-H8 coating was formed.     

Comparative study of titanium carbide and nitride coatings grown by cathodic vacuum arc technique

Titanium nitride (TiN), titanium carbide (TiC) thin films and TiC/TiN bilayers have been deposited on AISI 304 stainless steel substrates by plasma assisted physical vapor deposition technique—reactive pulsed vacuum arc method. The coatings were characterized in terms of crystalline structure, microstructure and chemical nature by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. Tribological behavior was investigated using ball on disc technique. The average coefficient of friction was measured, showing lower values for the TiN/TiC bilayer. Dynamic wear curves were performed for each coating, observing a better wear resistance for TiN/TiC bilayers, compared to TiN and TiC monolayers. On the other hand, the TiCN formation in the TiN/TiC bilayer was observed, being attributed to the interdiffusion between TiN and TiC at the interface. Moreover, the substrate temperature influence was analysing observing a good behavior at T_S = 115 °C.     

Oxidation of vanadium nitride and titanium nitride coatings

The oxidation of vanadium nitride (VN) and titanium nitride (TiN) coatings in ultra-high vacuum has been investigated in situ by X-ray photoelectron spectroscopy. On the VN coatings mixed oxide layers containing V³⁺ and V⁴⁺ species form at elevated temperatures (?600°C) and at high oxygen exposures, which cover completely the VN surface. Under similar oxidation conditions the TiN surface oxidises partially to a mixture of TiO₂ and Ti oxynitride (TiO_xN_y) phases. This oxidation behaviour has been correlated to the tribological properties of the VN and TiN coatings investigated recently.     

SEM and EDX studies of bioactive hydroxyapatite coatings on titanium implants

This work presents a study on an alternative coating method based on biomimetic techniques which are designed to form a crystalline hydroxyapatite layer very similar to the process corresponding to the formation of natural bone. The HA formation on the surface of titanium alloy pretreated with NaOH solution is investigated. Two types of solutions such as supersaturated calcification solution (SCS) and modified SCS (M-SCS) were used to investigate bone-like apatite formation on alkali-treated titanium. The hydroxyapatite deposits are investigated by means of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The data suggest that the method utilized in this work can be successfully applied to obtain deposition of uniform coatings of crystalline hydroxyapatite on titanium substrates.     

Improving the visible transmittance of low-e titanium nitride based coatings for solar thermal applications

Low-emissivity (low-e) coatings on glass are nowadays extensively used for energy saving applications in architectural windows and on solar thermal collectors. In this work the feasibility of TiN-based layers as low-e coatings has been studied. TiN samples were deposited by reactive magnetron sputtering and, in order to improve their optical properties (transmission in the visible range, T and emissivity, ?), we have investigated the changes in optical response following three different approaches: (i) post-deposition annealing treatments up to 500 °C, (ii) doping the TiN layers with aluminium (target composition: Ti/Al = 90/10, 75/25 and 50/50) and (iii) deposition of antireflective coatings of TiO₂ in multilayers structures. The crystalline structure and chemical composition of the multilayers were studied by X-Ray Diffraction and Rutherford Backscattering Spectroscopy, respectively. Spectroscopic Ellipsometry, Fourier Transform Infrared Spectroscopy and direct emissivity measurements were employed to determine the optical properties T and ?. We have concluded that the most noticeable improvement has been obtained by the deposition of multilayers structures increasing in a 30% the original transmittance of the single TiN coatings.     

Microhardness and refractive index of titanium dioxide-based binary coatings

Glass composites covered by sol–gel TiO₂–m Me _x O _y (Me _x O _y = ZnO, CdO, SnO, CuO, and Fe₂O₃ and m = 2, or 10 wt %) binary oxide coatings have been studied. The microhardness of the composites and glass substrate has been measured, and the microhardness of the coatings has been determined from these measurements. A correlation between the microhardness of the coatings, their refractive index, and packing density of disperse sol phase particles in the coating has been established.     

Comparison of calcium phosphate coatings formed on femtosecond laser-induced and sand-blasted titanium

High energy femtosecond laser process was employed to create regular surface patterning on titanium while sand blasting treatment made a coarse surface. Both laser-induced titanium and blasted titanium could promote the formation of calcium phosphate compounds after the acid and alkali treatment, but little crystallized hydroxyapatite was grown on the laser-induced titanium in 1.5SBF only for 6 h, whereas Ca₄P₆O₁₉ was formed on the sand-blasted titanium. The femtosecond laser process together with common acid and alkali treatment might provide potential choice to enhance the biocompatibility of titanium and its alloys.     

Formation of semiconductor titanium disilicide

Nanostructured titanium disilicide powders with semiconductor properties are synthesized and studied. The optical and electrophysical properties of TiSi₂ are found to be controlled by its crystallite size.     

Formation of Cr-modified silicide coatings on a Ti-Nb-Si based ultrahigh-temperature alloy by pack cementation process

Cr-modified silicide coatings were prepared on a Ti-Nb-Si based ultrahigh temperature alloy by Si-Cr co-deposition at 1250 °C, 1350 °C and 1400 °C for 5-20 h respectively. It was found that both coating structure and phase constituents changed significantly with increase in the co-deposition temperature and holding time. The outer layers in all coatings prepared at 1250 °C for 5-20 h consisted of (Ti,X)₅Si₃ (X represents Nb, Cr and Hf elements). (Ti,X)₅Si₄ was found as the only phase constituent in the intermediate layers in both coatings prepared at 1250 °C for 5 and 10 h, but the intermediate layers in the coatings prepared at 1250 °C for 15 and 20 h were mainly composed of (Ti,X)₅Si₃ phase that was derived from the decomposition of (Ti,X)₅Si₄ phase. In the coating prepared at 1350 °C for 5 h, single (Ti,X)₅Si₃ phase was found in its outmost layer, the same as that in the outer layers in the coatings prepared at 1250 °C; but in the coatings prepared at 1350 °C for 10-20 h, (Nb_1.95Cr_1.05)Cr₂Si₃ ternary phase was found in the outmost layers besides (Ti,X)₅Si₃ phase. In the coatings prepared at 1400 °C for 5-20 h, (Nb_1.95Cr_1.05)Cr₂Si₃ ternary phase was the single phase constituent in their outmost layers. The phase transformation (Ti,X)₅Si₄ → (Ti,X)₅Si₃ + Si occurred in the intermediate layers of the coatings prepared at 1350 and 1400 °C with prolonging co-deposition time, similar to the situation in the coatings prepared at 1250 °C for 15 and 20 h, but this transformation has been speeded up by increase in the co-deposition temperature. The transitional layers were mainly composed of (Ti,X)₅Si₃ phase in all coatings. The influence of co-deposition temperature on the diffusion ability of Cr atoms was greater than that of Si atoms in the Si-Cr co-deposition processes investigated. The growth of coatings obeyed inverse logarithmic laws at all three co-deposition temperatures. The Si-Cr co-deposition coating prepared at 1350 °C for 10 h showed a good oxidation resistance due to the formation of SiO₂ and Nb, Cr-doped TiO₂ scale after oxidation at 1250 °C for 10 h.     

Characteristics of grain growth of microarc oxidation coatings on pure titanium

Grainy titania coatings are prepared by microarc oxidation on pure titanium （TA2） substrates in a Na2SiO3NaF electrolytic solution. The coating thickness is measured by an optical microscope with a CCD camera. Scanning electron microscope （SEM） and x-ray diffraction （XRD） are employed to characterize the microstructure and phase composition of coatings. The results show that the coating thickness increases linearly as the treatment time increases. The coatings are mainly composed of anatase and rutile （TiO2）. With the increase of treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the oxidizing process. Meanwhile, the size of grains existing on the coating surface increases and thus the surface becomes much coarser.