Alumina Titania Research Articles

Sliding Wear Behavior of Al2O3–TiO2 Coatings Fabricated by the Suspension Plasma Spraying Technique

The friction and dry sliding wear behavior of alumina and alumina–titania near-nanometric coatings were examined. Coatings were obtained by the suspension plasma spraying technique. Dry sliding wear tests were performed on a ball-on-disk tribometer, with an Al2O3 ball as counterpart material, a normal load of 2 N, a sliding distance of 1200 m and a sliding speed of 0.1 m/s. The effect of including TiO2 in the fabricated coatings on friction coefficient behavior, wear rates and wear damage patterns was determined. The addition of TiO2 to the coatings was found to greatly increase wear resistance by, for example, 2.6-fold for 40 wt% of TiO2. The analysis of the wear surface was correlated with microstructural parameters, mechanical properties and wear rates.

Adhesion and residual stress of plasma sprayed Alumina–titania coatings

Plasma-sprayed coatings are formed by the impacting of particles onto a fixed substrate layer-by-layer. Residual stresses inside the coatings are essential for their influencing on the coatings’ performance and durability during service life. In the present work, heat transfer and elastic–plastic residual stresses generation during plasma spraying in Al2O3–13wt.%TiO2/NiCrAl (AT13) coating system were analyzed by finite element analysis (FEA). The sophisticated spraying process was simulated and the laminated structure of the coating was modeled under three-dimension. In this simulation, radial and axial compressive stresses were concentrated at the interfaces and inside the bond layer. Besides, at the specimen corner of the free edge, there were high tensile radial and axial stress concentrations. Such remarkable stresses, no matter tensile or compressive, may lead to the delamination and failure of coatings. Comparing with the numerical results, X-ray diffraction measurement was conducted on the AT13 coatings. As a result, the tested values matched well with the FEA simulated results.

Alumina–titania (Al2O3–TiO2) hollow fiber sorptive microextraction coupled to inductively coupled plasma mass spectrometry for determination of trace elements in diesel and gasoline samples

Application of alumina–titania hollow fiber sorptive microextraction coupled to ICP-MS for simultaneous preconcentration of trace elements in liquid fuel samples.

Deposition and characterization of alumina–titania coating by multi-chamber gas-dynamic sprayer

In this paper, alumina–titania coatings have been formed on aluminium substrate by multi-chamber detonation sprayer. The coatings were investigated using SEM, EDS, XRD and Vickers microhardness tester. The results show that the alumina–titania coatings consist of both fully melted regions and partially melted regions, and the fully melted region has a lamellar-like structure. The multi-chamber detonation sprayer produced the dense layers of coating with a high hardness.

Nanocomposite powder with three-dimensional network structure for preparing alumina–titania nanocomposite coating with advanced performance

Nanocomposite powder with three-dimensional network structure was prepared by adding nano-dopants (ZrO2 and CeO2) in Al2O3–TiO2 nanocomposite powder and employing plasma jet processing. Alumina–titania nanocomposite coating was fabricated through plasma spraying the nanocomposite powder. The formation mechanism of the nanocomposite powder was investigated, and the strengthening and toughening mechanisms of the nanocomposite coating was revealed. The internal microstructure of the powder particles experienced liquid phase sintering during plasma jet processing. The nanocomposite powder with three-dimensional network structure had a microstructure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the α-Al2O3 colonies. Microhardness, crack growth resistance and bonding strength of the nanocomposite coatings were improved compared with that of Metco 130 coating. The strengthening mechanism was mainly microstructure refinement strengthening. The toughening mechanisms included crack deflection and crack trapping. The three-dimensional network structure may work as reinforcing and toughening units in the nanocomposite coating.

Influence of composite powders’ microstructure on the microstructure and properties of Al2O3–TiO2 coatings fabricated by plasma spraying

Three kinds of alumina–titania composite powders with different microstructure were plasma sprayed to prepare alumina–titania coatings. The influence of composite powders’ microstructure on the microstructure and properties of alumina–titania coatings was investigated. There was typical lamellar structure and columnar grains formed in the conventional micro-composite coating, which were derived from the coarse-grained microstructure and segregated composition of the micro-composite powder feedstock. Comparing with the micro-composite coating, there was not typical lamellar structure and columnar grains in the coatings prepared from nanocomposite powders, which was mainly attributed to the fine-grained microstructure and homogeneous distribution of chemical composition in the nanocomposite powders. There were two types of microstructures formed in the two coatings prepared from nanocomposite powders. Fully-melted regions with splat structure consisting of γ-Al2O3 dissolved with titania and amorphous phase in two nanocomposite coatings were formed by the fully melted nanocomposite powders during plasma spraying. Partially-melted regions with particulate structure consisting of α-Al2O3 and amorphous phase in the nanocomposite coating was formed by the partially melted or unmelted nanocomposite powder with particulate structure, and partially-melted regions with three-dimensional network structure consisting of amorphous intergranular network thin film rich in Ti, Zr and Ce surrounding the α-Al2O3 colonies in the nanocomposite coating were formed by the partially melted or unmelted nanocomposite powder with three-dimensional network structure. The microhardness, crack growth resistance and sliding wear resistance of the coatings prepared from the nanocomposite powders, especially the nanocomposite coating prepared from the nanocomposite powder with three-dimensional network structure, were enhanced comparing with that of the conventional micro-composite coating. The superior properties of the nanocomposite coatings were attributed to the uniform composition distribution in their nanocomposite powders.

Microstructure and tribological properties of nanostructured and conventional plasma sprayed alumina–titania coatings

The nanostructured and two conventional alumina–titania coatings were deposited by atmospheric plasma spray. The presented studies show that nanoparticles are the predominant component of nanostructured Al2O3–13TiO2 powder grains. The microstructure of coating consisted of two distinct regions: fully melted and unmelted or partially melted nanostructured areas, which were comprised of components of a starting powder. Microhardness and modulus of these coatings were significantly lower than the values obtained for nanostructured coating. It was found that the plasma-sprayed nanostructured Al2O3–13TiO2 coating possessed better tribological properties than those of conventional alumina–titania coatings. The lower coefficient of friction and wear of the nanostructured Al2O3–13TiO2 coating is attributed to the bimodal microstructure and the enhanced mechanical properties in comparison to conventional alumina–titania coatings.

Effect of TiO 2 addition on the microstructure and nanomechanical properties of Al 2 O 3 Suspension Plasma Sprayed coatings

• Al 2 O 3 and Al 2 O 3 –13 wt% TiO 2 coatings were deposited by suspension plasma spraying. • Coating microstructure was formed by dense melted zones embedded in a softer matrix. • The addition of TiO 2 leads to a higher melting rate of the initial feedstock. • Al 2 O 3 –TiO 2 coating presents higher hardness and Young's moduli than pure Al 2 O 3 layer. Alumina–titania coatings are widely used in industry for wear, abrasion or corrosion protection components. Such layers are commonly deposited by atmospheric plasma spraying (APS) using powder as feedstock. In this study, both Al 2 O 3 and Al 2 O 3 –13 wt% TiO 2 coatings were deposited on austenitic stainless steel coupons by suspension plasma spraying (SPS). Two commercial suspensions of nanosized Al 2 O 3 and TiO 2 particles were used as starting materials. The coatings microstructure and phase composition were fully characterised using FEG-SEM and XRD techniques. Nanoindentation technique was used to determine the coatings hardness and elastic modulus properties. Results have shown that the addition of titania to alumina SPS coatings causes different crystalline phases and a higher powder melting rate is reached. The higher melted material achieved, when titania is added leads to higher hardness and elastic modulus when the same spraying parameters are used.

Microstructure and mechanical properties of plasma sprayed alumina-based coatings

Alumina-based coatings are employed in many industrial applications, in order to protect the surface of metal components against high temperature, wear, corrosion and erosion. In this work two different alumina-based coatings were fabricated by atmospheric plasma spraying (APS), starting from powder particles composed of pure Al2O3 and Al2O3-3wt% TiO2, respectively. Their phase composition was investigated by X-Ray Diffraction (XRD) and revealed that both the as-sprayed coatings were mainly composed of metastable γ- and α-Al2O3 phases. The γ phase recrystallized to α phase after heat treatment. The porous microstructure was analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Thin TiO2-rich splats were observed within the microstructure of alumina–titania coatings. The pure alumina coatings exhibited similar porosity and higher microhardness than the alumina–titania ones (12.8 against 9.9GPa). Both the coatings herein analyzed are particularly promising for high-temperature anti-wear applications, because of their enhanced mechanical properties.

Hydrogen production via methane decomposition over Al2O3–TiO2 binary oxides supported Ni catalysts: Effect of Ti content on the catalytic efficiency

Alumina–titania binary oxides with different TiO2 contents (15%, 25% and 50%) have been prepared via ammonia gas diffusion method. These oxides were applied as supports for 60wt% Ni which was also precipitated by the same method and tested for methane thermal decomposition to COx-free hydrogen production. Different analytical techniques such as XRD, BET, TPR, Raman spectra, TGA and TEM were performed for characterization the catalysts before and after the catalytic runs. Incorporation of TiO2 in the catalysts facilitates the reduction of the catalysts due to the low formation of NiAl2O4 spinel structure. The Ni/TiO2(25%)–Al2O3 exhibited better activity and stability for catalytic decomposition of methane where about 59% hydrogen yield is obtained at 390min. TEM images showed that the catalysts produced different types of carbon nanostructured materials such as carbon nanofiber, graphene nanosheets and carbon nanotubes depending on the concentration of TiO2 in the catalysts.

Influence of support and supported phases on catalytic functionalities of hydrotreating catalysts

Different mixed-oxide supports were synthesized using homogeneous co-precipitation methods. Alumina–silica, alumina–titania, and alumina–zeolite supports were prepared and then impregnated with Mo (or W) and Co (or Ni) in order to evaluate their behavior in the dibenzothiophene hydrodesulfurization. Supports and supported catalysts were characterized by atomic absorption and textural properties. The conversion of model compounds (tetralin, 1-methylnaphthalene, and decalin) was investigated with the aim of understanding ring opening reaction over the support in presence of hydrogen. The model test reactions for support as well supported sulfide catalysts were carried out in a batch reactor at 4MPa and 340°C. Conversion of cyclo-compounds showed that decalin had the highest conversion followed by 1-methylnaphthalene and tetralin when using a silica–alumina supported catalyst. The hydrodesulfurization results as a function of support variation indicated that high acidity of support has positive effect on the hydrogenolysis of CS bond breaking. Thus, it is inferred that a balance between metal sites of hydrogenation and cracking of the support is critical in order to synthesize a bifunctional catalyst for deep hydrodesulfurization where sulfur removal as well as cetane improvement are mandatory.

Microstructural and tribological characterization of air plasma sprayed nanostructured alumina–titania coatings deposited with nitrogen and argon as primary plasma gases

Air plasma sprayed nanostructured Al2O3–13wt%TiO2 coatings were deposited as a function of critical plasma spray parameter (CPSP), defined as the ratio of arc power to primary gas flow rate, using nitrogen and argon as the primary plasma gases. Microstructural features including percentage of α-Al2O3 phase, percentage of partially melted/unmelted regions, microhardness, and wear characteristics were evaluated for the deposited coatings. Effect of CPSP on microstructural and wear characteristics of coatings deposited with nitrogen was found to be relatively small. In contrast, significant effect of CPSP on coating characteristics was found for coatings deposited with argon. In wear tests, while strong effect of normal load on weight loss was observed for coatings deposited with nitrogen, weight loss for coatings deposited with argon was nearly independent of applied normal load, at least for coatings deposited at the highest CPSP.

Atmospheric plasma spraying coatings from alumina–titania feedstock comprising bimodal particle size distributions

In this work, Al2O3–13 wt% TiO2 submicron-nanostructured powders were deposited using atmospheric plasma spraying. The feedstocks were obtained by spray drying two starting suspensions of different solids content, prepared by adding nanosized TiO2 and submicron-sized Al2O3 powders to water. The spray-dried granules were heat-treated to reduce their porosity and the powders were fully characterised in both untreated and thermally treated state. Comparison with two commercial feedstocks was carried out. Characterisation allowed a temperature for the thermal treatment to be chosen on the basis of the sprayability of the feedstock and the preservation as much as possible of the submicron-sized structure of the unfired agglomerates. Optimisation of the deposition conditions enabled the reconstituted powders to be successfully deposited, yielding coatings that were well bonded to the substrate. The coating microstructure, characterised by SEM, was mostly formed by a matrix of fully molten particles where the presence of semi-molten feedstock agglomerates was also observed. Moreover, microhardness, toughness, adhesion and tribological behaviours were determined, and the impact of the granule characteristics on these properties was studied. It was found that changing the feedstock characteristics allows controlling the coating quality and properties. In general, good mechanical properties were obtained using a feedstock comprising a binary mixture of submicrometric Al2O3 and nanometric TiO2 particles in the spray-dried powder.

Effect of Microstructure of Composite Powders on Microstructure and Properties of Microwave Sintered Alumina Matrix Ceramics

Two kinds of different structured alumina–titania composite powders were used to prepare alumina matrix ceramics by microwave sintering. One was powder mixture of alumina and titania at a micron–submicron level, in which fused-and-crushed alumina particles (micrometers) was clad with submicron-sized titania. The other was powder mixture of alumina and titania at nanometer–nanometer level, in which nano-sized alumina and nano-sized titania particles were homogeneously mixed by ball-milling and spray dried to prepare spherical alumina–titania composite powders. The effect of the microstructure of composite powders on microstructure and properties of microwave sintered alumina matrix ceramics were investigated. Nano-sized composite (NC) powder showed enhanced sintering behavior compared with micro-sized composite (MC) powders. The as-prepared NC ceramic had much denser, finer and more homogenous microstructure than MC ceramic. The mechanical properties of NC ceramic were significantly higher than that of MC ceramic, e.g. the flexural strength, Vickers hardness and fracture toughness of NC ceramic were 85.3%, 130.3% and 25.7% higher than that of MC ceramic, respectively. The improved mechanical properties of NC ceramic compared with that of MC ceramic were attributed to the enhanced densification and the finer and more homogeneous microstructure through the use of the nanostructured composite powders.

Microstructure and dry sliding wear behaviour of titania and alumina–titania coatings

The microstructure, hardness and wear properties of atmospheric plasma spray coating of TiO2 and Al2O3–13TiO2 with incremental hydrogen fractions (11·1–23·8%) in primary gas have been investigated in this work. TiO2 coating shows a fine bead-like surface structure, while Al2O3–13TiO2 predominantly exhibits a splat-like morphology. The microstructure shows a slight refinement in the structure for the highest hydrogen content possibly due to the higher specific enthalpy of the plasma plume. Both coatings show the highest hardness for samples coated with the highest hydrogen content possibly due to the refined structure. The roughness shows a mild decreasing trend for TiO2 coating, while the reverse trend is observed for Al2O3–13TiO2 coating. The wear loss and friction coefficient show a flat trend (0·4–0·6) for all the hydrogen contents. The worn tracks show a compacted mixture of ceramic, graphite and metallic particles.

Permeation of saline solution in Al2O3–13 wt.% TiO2 coatings elaborated by atmospheric plasma spraying

The evolution of brine permeation in alumina–titania coatings elaborated by atmospheric plasma spray was studied using Electrochemical Impedance Spectroscopy (EIS). Two Al2O3–13wt.% TiO2 micrometer-sized (5–25 and 15–40μm) powders were sprayed onto aluminum substrates by a PTF4 torch 7mm in anode-nozzle internal diameter, using a mixture of Ar–H2 (45–15slpm) as plasma forming gas and applying a current intensity of 500A. The electrochemical results obtained were related to structural defects of coatings (pores and cracks) determined by image analyses of their polished cross section. During the first hours of immersion the chlorine solution permeates into the coatings. However, after about 340h the brine was stopped. The resistance to polarization (Rp) values' of coated samples are higher than those exhibited by the uncoated substrate, which indicates that these coatings could be potentially used to modify and protect the aluminum surface exposed to marine environment or other saline solutions. The protective capacity of coating obtained from finer feedstock powder is higher than that of coating sprayed from coarse powders, thanks to its lower porosity and crack content.

Synthesis of Nanomaterials Using Self-Assembled Nanotemplates

Nanoporous alumina templates and titania nanotube arrays have emerged as highly important materials due to their many potential applications in a variety of research areas. The templates are prepared using an electrochemical anodization process starting from metallic Al or Ti films. Several different synthesis routes for the deposition of such templates have been demonstrated in literature. This article provides an overview of the various synthesis routes that may be employed. The templates have diverse applications and may also be utilized in the electrodeposition of nanowire arrays of a variety of materials, such as metals, semiconductors, etc. A brief overview of these deposition methods is also provided.

Dispersion and reaction sintering of alumina–titania mixtures

Alumina–aluminium titanate (A–AT) composites are typically produced either by mixing the alumina matrix powder with already formed aluminium titanate or by reaction sintering of alumina and titania powders. Reaction-sintered materials usually exhibit limited final density and extensive microcracking. This paper describes the preparation of A–AT nanocomposites by slip casting and reaction sintering, using aqueous suspensions of submicrometre-sized alumina and nanometre-sized titania at a respective weight ratio of 87:13, which is typical for plasma-sprayed coatings. The colloidal stability of aqueous suspensions of the two individual powders and of their respective mixtures was determined first, measuring zeta potential and rheological behaviour as a function of deflocculant content and sonication time. The bimodal distribution yielded green relative densities of up to 70% of theoretical density. Dynamic and static sintering studies showed that aluminium titanate had already formed at 1400°C.

Synthesis, characterization and color performance of novel Co2+-doped alumina/titania nanoceramic pigments

Blue–green nanoceramic pigments from Co2+-doped alumina–titania (xCo2+–Al2O3/TiO2) have been synthesized by alkoxide sol–gel route from a mixture of titania and boehmite sols that derived from titanium isopropoxide and aluminum nitrate precursors. The composition of the alumina/titania (AT) matrix is 3:1M ratio. A series of xCo2+:3TiO2:1Al2O3 mixed oxides with different Co2+-dopant ion concentration has been prepared. The molar ratio of the Co2+-dopant ion in the Al2O3/TiO2 matrix is from x=0 to 0.5M ratio relative to the titania composition. Characterizations of Co2+-doped alumina/titania nanocomposites are carried out using reflectance spectroscopy, transmission electron microscope (TEM), scanning electron microscopy (SEM-EDS), thermo-gravimetric analysis (TGA) and X-ray diffractometer (XRD). The experimental results demonstrated that the prepared Co2+-doped alumina–titania nanocomposites fulfill the current technological requirements for ceramic pigment applications that exhibit a high physico-chemical and thermal stabilities at high firing temperatures.

Alumina–titania ceramics prepared by microwave sintering and conventional pressure-less sintering

Nanostructured alumina–titania composite powders were used to prepare alumina–titania ceramics by microwave sintering and conventional pressure-less sintering. The effect of microwave sintering and conventional pressure-less sintering on densification, microstructure and properties of alumina–titania ceramics were evaluated. The results showed that the alumina–titania ceramic prepared by microwave sintering exhibited shorter sintering time, significantly enhanced densification, higher density, finer and more homogenous microstructure, higher strength, higher hardness and higher fracture toughness when compared with conventionally processed counterpart.