Plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy

Abstract

The titanium affinity for oxygen is one of the main factors that limit the application of its alloys as structural materials at high temperatures. The objective of this work was to estimate the influence of the plasma-sprayed coatings for oxidation protection on creep of the Ti–6Al–4V alloy, focusing on the determination of the experimental parameters related to the creep stages. Yttria (8 wt.%) stabilized zirconia (YSZ) with a CoNiCrAlY bond coat was air plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on the Ti–6Al–4V alloy in air for coated and uncoated samples and in a nitrogen atmosphere for uncoated samples at 600°C to evaluate the oxidation protection on creep of the Ti–6Al–4V alloy. The steady-state creep rate of the coated alloy is smaller than that of the uncoated alloy in air and nitrogen atmosphere. Results about the activation energies and the stress exponent values indicate that the primary and stationary creep, for all test conditions, was probably controlled by dislocation climb. The plasma-sprayed coatings increased the time to rupture and the strain at rupture is smaller than for uncoated samples tested in air.     

Nanostructured Er3+-doped SiO2–TiO2 and SiO2–TiO2–Al2O3 sol–gel thin films for integrated optics

The nanostructured multilayer silica–titania or silica–titania–alumina films doped with Er³⁺ were prepared by sol–gel method. The sol–gel method is a flexible and convenient way to prepare oxide films on several types of substrates, and for this reason it was extensively investigated for optical waveguides fabrication. The selected molar composition was 90%SiO₂–10%TiO₂ or 85%SiO₂–10%TiO₂–5% Al₂O₃ and 0.5% Er₂O₃.The films were characterized by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Spectroellipsometry (SE), as well as by Atomic Force Microscopy (AFM) and photoluminescence (PL). The films deposited on Si/SiO₂ substrate by dip-coating or spin-coating, followed by annealing at 900 °C, presented homogenous and continuous surface and good adherence to the substrate. Differences were noticed in the structure and properties of the prepared films, depending on the composition and the number of deposited layers.Channel optical waveguides were obtained by patterning Er³⁺-doped SiO₂–TiO₂ and SiO₂–TiO₂–Al₂O₃ sol–gel layers deposited on oxidized silicon wafers.     

Electro co-deposition of Ni–Al2O3 composite coatings

Nickel–Al₂O₃ composite coatings have been successfully deposited galvanostatically on to stainless steel substrates by electro co-deposition from a Watts bath containing between 50 and 150?g/l of sub-micron or nano- sized alumina particles applying current density of ?10, ?20 and ?32?mA?cm^?2. The alumina distribution in the composite films on the two sides of the substrate was remarkably different due to solution hydrodynamics and electric field effects. The effect of current density, particle concentration in the bath and particle size are studied systematically producing a comprehensive set of data for better understanding the effects of these variables on the amount of particles co-deposited. The amount of Al₂O₃ co-deposited in the films increases with the particle concentration in the bath and strongly depends on the current density and on particle size. The effect of the current density and of the alumina inclusions on the crystallinity of the Ni matrix and on the Ni crystallites grain size has also been studied. The inclusions of nano or sub-micron-Al₂O₃ particles are found to strongly influence the metallic nickel microstructure.     

Properties of electrodeposited Ni–B–Al2O3 composite coatings

Ni–B coatings have gained a great deal of attraction due to their promising mechanical properties. Owing to tempting properties, Ni–B coatings have succeeded to find their applications in automotive, aerospace, petrochemical, plastic, optics, nuclear, electronics, computer, textile, paper, food and printing industries. Despite having promising properties, further improvement in their properties is essential so that more challenging requirements and new developments can be successfully addressed. In the present study, novel Ni–B–Al₂O₃ composite coatings have been synthesized through electrodeposition process by reinforcing Ni–B matrix with Al₂O₃ particles. A comparison of properties of Ni–B and Ni–B–Al₂O₃ coatings in their as deposited states is presented to elucidate the beneficial role of Al₂O₃ addition. The structural analyses indicate that Ni–B coatings exhibit a single broad peak indicative of an amorphous structure. However, the addition of Al₂O₃ into Ni–B matrix considerably improves the crystallinity of the deposit. The surface morphology study reveals the formation of uniform, dense and fine-grained deposit in both Ni–B and Ni–B–Al₂O₃ composite coatings. However, addition of Al₂O₃ particles into Ni–B coatings results in high surface roughness. The nanoindentation results demonstrates that the addition of Al₂O₃ into Ni–B matrix results in significant improvement in mechanical properties (hardness and modulus of elasticity) which may be attributed to dispersion hardening of Ni–B matrix by hard Al₂O₃ particles. The linear polarization tests confirm that the addition of Al₂O₃ improves the corrosion resistance of Ni–B coatings. This improvement in corrosion behavior may be attributed to the reduction in active area of Ni–B matrix by the presence of inactive Al₂O₃ particles.     

Phase Relations in the Systems Al2TiO5–Fe2O3 , Al2O3–TiO2–Fe2O3 , and Al2TiO5–Cr2O3

Phase relations in the systems Al₂TiO₅–Fe₂O₃, Al₂TiO₅–Cr₂O₃, and Al₂O₃–TiO₂–Fe₂O₃ are investigated, and the composition ranges of pseudobrookite Al_{2 – 2x} M_2x TiO₅ (M = Fe, Cr) solid solutions are determined.     

Amorphous phase formation of the pseudo-binary Al2O3–ZrO2 alloy during plasma spray processing

Thick deposits of the Al2O3–ZrO2 with near eutectic compositions were prepared by plasma-spray deposition and subjected to heat treatment to investigate the crystallization and phase transformation behaviors. The structures of as-sprayed deposits are mostly amorphous and a small amount of t-ZrO2 and -Al2O3 particles with a diameter of approximately 20 nm are also present. Simultaneous crystallization of t-ZrO2 and -Al2O3 from the glass occurs at 945 °C, followed by - and -Al2O3 above 1000 °C, and only -Al2O3 are observed above 1200 °C. Phase transformation of t-ZrO2 to m-ZrO2 occurs at 1213 °C. There is no appreciable difference in amorphous formation and subsequent crystallization and phase transformation behaviors with two different feedstock powder sizes. It is shown that it is feasible to produce the thick amorphous Al2O3–ZrO2 materials with proper control of plasma spraying process parameters.     

Nanostructured Cu–Al2O3 composite produced by thermochemical process for electrode application

Nanostructured Cu–Al₂O₃ composite powders were synthesized by thermochemical process. The synthesis procedures are (1) preparation of precursor powder by spray-drying of solution made from water-soluble copper and aluminum nitrates, (2) air heat treatments to evaporate volatile components in the precursor powder and synthesis of nanostructured CuO+Al₂O₃, and (3) CuO reduction by hydrogen into pure Cu. The suggested procedures stimulated the formation of the γ-Al₂O₃, and different alumina formation behaviors appeared with various heat-treating temperatures. The mean particle size of the final Cu–Al₂O₃ composite powders produced was 20 nm, and the electrical conductivity and hardness in the hot-extruded bulk were competitive with Cu–Al₂O₃ composite by the conventional internal oxidation process.     

High-pressure high-temperature transitions in nanocrystallineγ Al2O3,γ Fe2O3 and TiO2

We report first observation of new polymorphs of Al₂O₃ and Fe₂O₃ in specimens of xerogelγ Al₂O₃ andγ Fe₂O₃ quenched from high pressures and temperatures. At about 5 GPa and 1400°C, xerogel gamma alumina (XGA) transformed into a polymorphic mixture of phasesα Al₂O₃, B Al₂O₃ and C Al₂O₃, while XGA containing 1 wt% Cr₂O₃ transformed into a mixture of phasesαAl₂O₃, H Al₂O₃ and k′ Al₂O₃. The phases B Al₂O₃, C Al₂O₃ and H Al₂O₃ have the monoclinic-, cubic- and hexagonal-rare earth sequioxide (Ln₂O₃) type structure, respectively. At 5·2 GPa and 1450°C, XGA yielded a mixture ofα Al₂O₃ and hexagonalμ Al₂O₃. At STP, the phaseμ Al₂O₃ was found to transform to another hexagonal phaseλAl₂O₃ over a 10 week period. At 5·2 GPa and 900°C,γ Fe₂O₃ showed transition to a new phase H Fe₂O₃ which probably has an 8 layer close packed structure. In nanocrystalline TiO₂, only the anatase to rutile transition was found. The results are discussed using the free energy vs temperature diagram for xerogel and nanocrystalline materials.     

Formation of Al3Ti and Al2O3 from an Al–TiO2 system for preparing in-situ aluminium matrix composites

In-situ processing offers significant advantages over conventional processing from both technical and economic standpoints. Al–TiO₂ is one of the interesting systems that has recently been receiving some attention. In this paper, the formation mechanism of Al₃Ti and Al₂O₃ from an Al–TiO₂ system is investigated by using thermal analysis, XRD and microstructural characterisation. It is found that the in-situ processing involves three intermediate steps. In addition, TiO and γ-Al₂O₃ are transitional phases, which form during the reactive process.     

Nanostructured glass–ceramic coatings for orthopaedic applications

Glass–ceramics have attracted much attention in the biomedical field, as they provide great possibilities to manipulate their properties by post-treatments, including strength, degradation rate and coefficient of thermal expansion. In this work, hardystonite (HT; Ca₂ZnSi₂O₇) and sphene (SP; CaTiSiO₅) glass–ceramic coatings with nanostructures were prepared by a plasma spray technique using conventional powders. The bonding strength and Vickers hardness for HT and SP coatings are higher than the reported values for plasma-sprayed hydroxyapatite coatings. Both types of coatings release bioactive calcium (Ca) and silicon (Si) ions into the surrounding environment. Mineralization test in cell-free culture medium showed that many mushroom-like Ca and phosphorus compounds formed on the HT coatings after 5 h, suggesting its high acellular mineralization ability. Primary human osteoblasts attach, spread and proliferate well on both types of coatings. Higher proliferation rate was observed on the HT coatings compared with the SP coatings and uncoated Ti-6Al-4V alloy, probably due to the zinc ions released from the HT coatings. Higher expression levels of Runx2, osteopontin and type I collagen were observed on both types of coatings compared with Ti-6Al-4V alloy, possibly due to the Ca and Si released from the coatings. Results of this study point to the potential use of HT and SP coatings for orthopaedic applications.     

Study of the microstructure of plasma sprayed coatings obtained from Al2O3–13TiO2 nanostructured and conventional powders

The microstructure of coatings obtained from nanostructured or conventional Al₂O₃–13TiO₂ powders and deposited by plasma spraying technique on low-carbon steel was examined by transmission electron microscopy techniques. The dominating phase in both coatings was γ-Al₂O₃ phase. It has been observed that the grains of γ-Al₂O₃ grew in various shapes and sizes, that are particularly visible in the case of coating sprayed from nanostructured powder. The coatings obtained from the fully melted conventional powders exhibited a typical lamellar microstructure, into which the strips of TiO₂ phase were extended. The microstructure of coatings produced from agglomerates of nanostructured particles also revealed the regions consisting of partially melted α-Al₂O₃ powders surrounded by the net-like structure formed from fully melted oxides that improved the coating properties. Along with the observed morphology diversity some changes in the chemical composition on the cross sections of obtained coatings have been also noticed.     

Liquid-phase bonding of silicon nitride ceramics using Y2O3–Al2O3–SiO2–TiO2 mixtures

Hot-pressure sintered β-Si₃N₄ ceramic was bonded to itself using Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures. Reactive behavior at interface between Si₃N₄ and Y₂O₃–Al₂O₃–SiO₂–TiO₂ mixtures during silicon nitride ceramic joining was studied by means of scanning electron microscopy (SEM), electron probe microanalyses (EPMA), X-ray diffraction (XRD) and auger electron spectroscopy (AES). The joint strength under different bonding conditions was measured by four-point bending tests. The results of EPMA, AES and XRD analyses show that the liquid glass solder reacts with silicon nitride at interface, forming the Si₃N₄/Y–Si–Al–Ti–O–N glass/TiN/Y–Si–Al–O glass gradient interface. From the results of four-point bending tests, it is known that with increase of bonding temperature and holding time, the joint strength increased reaching a peak, and then decreased. The maximum joint strength of 200 MPa measured by the four-point bending tests is obtained for silicon nitride bonded at 1823 K for 30 min.     

Oxidation behaviour of nanocrystalline Fe–Ni–Cr–Al alloy coatings

Abstract

Nanocrystalline Fe–Ni–Cr–Al alloy coatings with ～4 wt-%Al were produced using the unbalanced magnetron sputter deposition technique with a composite 310S stainless steel target embedded with aluminium plugs. The oxidation behaviour of the coatings was studied, during which complete external α-Al₂O₃ scales were formed. During isothermal oxidation tests at 950, 1000, and 1050°C, the oxidation kinetics followed an essentially parabolic rate law, and the oxidation constants were measured to be 2·06 × 10^-3, 4·23 × 10^-3, and 1·14 × 10^-2 mg² cm^-4 h^-1 respectively. During a cyclic oxidation test at 1000°C the α-Al₂O₃ scale showed good scale spallation resistance. The surface hardness of the coatings was measured with a Knoop indentor before and after oxidation. After oxidation, the coating surface hardness was still significantly higher than that of the uncoated specimen, demonstrating the potential this coating has in the improvement of high temperature erosion resistance.     

Prediction of the mechanical properties of forged Ti–10V–2Fe–3Al titanium alloy using FNN

In this paper, a fuzzy neural network (FNN) prediction model has been employed to establish the relationship between processing parameters and mechanical properties of Ti–10V–2Fe–3Al titanium alloy. In establishing these relationships, deformation temperature, degree of deformation, solution temperature and aging temperature are entered as input variables while the ultimate tensile strength, yield strength, elongation and area reduction are used as outputs, respectively. After the training process of the network, the accuracy of fuzzy model was tested by the test samples and compared with regression method. The obtained results with fuzzy neural network show that the predicted results are much better agreement with the experimental results than regression method and the maximum relative error is less than 7%. And the optimum matching processing parameters can be quickly selected to achieve the desired mechanical property based on the fuzzy model. It proved that the model has a good precision and excellent ability of predicting.     

Sol–gel derived nickel-doped TiO2 films as wear protection coatings

Thin films of pure and Ni-doped TiO₂ were prepared on a glass substrate by sol–gel and spin-coating process from specially formulated ethanol sols. The morphologies of the films surface were observed with atomic force microscope (AFM). The tribological properties of the obtained thin films sliding against steel ball were evaluated on a one-way reciprocating friction tester. AFM results show that by the addition of the Ni in TiO₂, smooth surfaces were obtained. As a result, the Ni-doped TiO₂ films exhibit better wear protection properties than pure TiO₂. The best protection was observed for 5% Ni-doped TiO₂ films in this study.     

Al2TiO5–Al2O3–TiO2 nanocomposite: Structure,mechanical property and bioactivity studies

Novel biomaterials are of prime importance in tissue engineering. Here, we developed novel nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite as a biomaterial for bone repair. Initially, nanocrystalline Al₂O₃–TiO₂ composite powder was synthesized by a sol–gel process. The powder was cold compacted and sintered at 1300–1500 °C to develop nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite. Nano features were retained in the sintered structures while the grains showed irregular morphology. The grain-growth and microcracking were prominent at higher sintering temperatures. X-ray diffraction peak intensity of β-Al₂TiO₅ increased with increasing temperature. β-Al₂TiO₅ content increased from 91.67% at 1300 °C to 98.83% at 1500 °C, according to Rietveld refinement. The density of β-Al₂TiO₅ sintered at 1300 °C, 1400 °C and 1500 °C were computed to be 3.668 g cm^?3, 3.685 g cm^?3 and 3.664 g cm^?3, respectively.Nanocrystalline grains enhanced the flexural strength. The highest flexural strength of 43.2 MPa was achieved. Bioactivity and biomechanical properties were assessed in simulated body fluid. Electron microscopy confirmed the formation of apatite crystals on the surface of the nanocomposite. Spectroscopic analysis established the presence of Ca and P ions in the crystals. Results throw light on biocompatibility and bioactivity of β-Al₂TiO₅ phase, which has not been reported previously.     

Electrochemical Al2O3–ZrO2 composite coatings on non-oxide ceramic substrates

Aqueous solutions of xAl(NO3)3+(1–x)ZrO(NO3)2 were used for electrodeposition of ceramic Al2O3–ZrO2 composite on TiC, TiB2 and SiC sunstrates. The weight of the deposit was studied versus the duration of deposition, the current density and the temperature of the bath for Al-rich (x=0.9), Zr-rich (x=0.4) and eutectic (x=0.75) electrolyte compositions. Optimal current densities and durations of deposition were determined to obtain maxima weights of deposits. Amorphous deposits with thicknesses up to 10 m were formed. The microstructure and microchemical composition of the as-deposited and sintered deposits were characterized. Increase in the temperature of the bath inhibited microcracking due to shrinkage during drying. Coated TiC substrates exhibited enhanced oxidation resistance in air at 1100°C.     

Mechanical properties and microstructural evolution during multi-pass ECAR of Al 1100–O alloy

The main objective of this work is to achieve ultra-fine grained structures within the pure aluminum sheet via equal channel angular rolling (ECAR). An attempt has been made to investigate the microstructural evolution and mechanical properties of the processed specimens in terms of process pass numbers and routes. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) examination showed ultra-fine grains (UFGs) with the average grain size of 0.85, 0.34 μm for the seventh pass, respectively. Yield and tensile strengths and microhardness of specimens were significantly increased upon the first pass; however, elongation was dramatically reduced. Subsequent process cycles caused no considerable improvement on the mechanical properties.     

Dielectric resonators with complex crystal structures in the La2O3–Al2O3–TiO2 system for microwave applications

LaTi₂Al₉O₁₉ and La₃Ti₅Al₁₅O₃₇ ceramics in the La₂O₃–Al₂O₃–TiO₂ system have been prepared by conventional solid state ceramic route. The structure and microstructure of the compositions have been studied using powder X-ray diffraction and scanning electron microscopic techniques. Laser Raman studies have been employed to understand the complex crystal structure of these compositions in molecular level. The microwave dielectric properties of the sintered ceramic compacts were measured by Hakki and Colemann post resonator and TE_01δ cavity techniques using a vector network analyzer. LaTi₂Al₉O₁₉ and La₃Ti₅Al₁₅O₃₇ ceramics possess excellent microwave dielectric properties such as relatively high unloaded quality factors 7,762 and 7,415, low dielectric constant 15.7 and 22.1 and low temperature coefficient of resonant frequency values ?22 and +18.9 ppm/°C, respectively.     

Evaluation on tribological design coatings of Al2O3, Ni–P–PTFE and MoS2 on aluminium alloy 7075 under oil lubrication

The current work evaluated the friction and wear properties of tribological design surface coatings on aluminium alloy 7075 under various speed and nominal contact pressure. Hard-anodized Aluminium Oxide (Al₂O₃), burnished Refractory Metal Sulfide (MoS₂) and composite electroless nickel coatings with polytetrafluoroethylene (Ni–P–PTFE) particles were subjected to pin-on-disc sliding test against grey cast iron (GCI) under Mach 5 SL SAE 10 W-30 lubrication. The results indicated that Ni–P–PTFE composite coating possessed excellent friction–reduction capability but limited wear resistance due to low mechanical strength. Al₂O₃ coated sample showed outstanding wear resistance with high friction characteristic leading to high surface contact temperature. Furthermore, MoS₂ coating improved the wear resistance of the aluminium alloy.