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.     

On the “Anomalous” hardness of nanocrystalline materials

The present paper is a further contribution on a rule of mixtures approach for the prediction of hardness dependence on grain size for nanocrystalline metals and intermetallics. Different behavior for the “bulk” and the “grain boundary” regions is assumed in order to consider the special features of nanocrystalline materials, such as the arrangement of geometrically necessary dislocations and disclinations. The usual Hall-Peteh (HP) relationship is assumed to describe the hardness dependence for each one of the two regions. A “correction” term is introduced for the HP slope of the boundary region as compared to the grain interior HP slope. An integrated formula for the total hardness H is obtained involving the grain size, the grain boundary width, the hardness characteristics of coarse grain counterparts and an extra parameter describing the arrangement of triple junctions. This formula compares well with a large set of experimental data.     

Density variation and piezoelectric properties of Ba(Ti1 − x Sn x )O3 ceramics prepared from nanocrystalline powders

Nanocrystalline powders of tin-doped barium titanate with different concentrations of tin have been synthesized by a combination of solid state reaction and high-energy ball milling. The average particle size of the milled powders as determined from TEM analysis was about 5·96 nm. Analysis of all the milled powders using X-ray diffraction method showed single phase perovskite structure. The density variation of the ceramics with sintering temperature has been studied by sintering the samples at different temperatures. Density variation results show that 1350°C is the optimum sintering temperature for tin-doped barium titanate ceramics. SEM micrographs show high density and increasing trend of grain size with increasing content of Sn. The ferroelectricity decreases with increasing concentration of Sn. The electromechanical coupling coefficient also decreases with increasing Sn content corroborating decreasing trend of ferroelectricity. The bipolar strain curves show piezoelectric properties of the prepared ceramics.     

Structural and Electronic Properties of Zr x Al1−x N in Cubic Phase Using First Principles

We calculate the structural and electronic properties of the Zr _x Al_1?x N in cubic phase using first-principles total energy calculations. The study is based on the functional density theory, DFT, with Augmented Plane Wave Plus Local Orbitals method for Calculating Crystal Properties (FP-LAPW). Determination of the crystal lattice constant, the volume of the unit cell, the bulk modulus for concentrations x=0.25,0.50,0.75, is carried out. In addition, we also study the electronic properties for each x. The study consists of the analysis of the electrons energy versus the wave vector, in some directions of the first Brillouin zone, and the Density of States (DOS) as a function of the energy. The results show that the lattice constant increases with x, but the functional relations are not linear. On the other hand, the electronic properties vary substantially with x, and a possible semiconductor metal transition is predicted for any x<0.25.     

The dielectic response of K x Al x Ti8−x O16 and K x Mg x/2 Ti8−x/2 O16

Materials of the hollandite structure with the general formulae K_x Al_x Ti_8–x O₁₆ and K_x Mg_x/2 Ti_8–x/2 O₁₆ have been synthesized in the composition range 1.6x2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10^–3 to 10⁶ Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the universal dielectric relation in which the loss follows the law x() ^n–1, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.     

The effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system

In this work, the effect of Ti addition on alloying and formation of nanocrystalline structure in Fe–Al system was studied by utilizing mechanical alloying (MA) process. Structural and morphological evolutions of powder particles were studied by X-ray diffractometry, microhardness measurements, and scanning electron microscopy. In both Fe₇₅Al₂₅ and Fe₅₀Al₂₅Ti₂₅ systems MA led to the formation of Fe-based solid solution which transformed to the corresponding intermetallic compounds after longer milling times. The results indicated that the Ti addition in Fe–Al system affects the phase transition during mechanical alloying, the final crystallite size, the mean powder particle size, the hardness value and ordering of DO₃ structure after annealing. The crystallite size of Fe₃Al and (Fe,Ti)₃Al phases after 100 h of milling time were 35 and 12 nm, respectively. The Fe₃Al intermetallic compound exhibited the hardness value of 700 Hv which is significantly smaller than 1050 Hv obtained for (Fe,Ti)₃Al intermetallic compound.     

Combustion reaction of Ti–Al–C–N system

The combustion reaction of Ti–Al–C–N system was investigated by using Ti powders and one CNx precursor powder as reactant powder blends. The reactant powder blends ratio was adjusted to obtain different materials. The phase composition of the samples was investigated by X-ray diffraction (XRD). The microstructure of the samples was observed by scanning electron microscopy (SEM). The result showed that Ti₂Al(C, N)–TiAlx, AlN–Ti(C, N) and Ti₃Al(C, N)₂–TiC composites can be fabricated by changing the reactant powder blends ratio.     

Influence of ion energies on the structure,composition, and properties of multilayer Ti–Al–Si–N ion-plasma-deposited coatings

It is established that the energy of deposited particles influences the structure, composition, and properties of multilayer nitride coatings consisting of alternating layers of nanocrystalline TiN and amorphous Si₃N₄ phases with inclusions of nanocrystalline hexagonal AlN formed at energies of titanium, aluminum, and silicon ions exceeding ~317 × 10^–19, 267 × 10^–19, and 230 × 10^–19 J, respectively. As the energy of titanium ions bombarding the substrate increases above ~512 × 10^–19 J, the phase transition from disordered TiN _x to Ti₃N₂ and the appearance of 2- to 3-nm-thick sublayers in 15-nm-thick nanocrystalline TiN _x layers take place in the coating. The maximum hardness of such coatings reaches a level of ~54 GPa.

     

Time dependence of microstructure and hardness in plasma carbonized Ti–6Al–4V alloys

Ti–6Al–4V alloy was treated by plasma carburizing process at 950 °C for different durations of 1 h, 2 h, 3 h, and 4 h. Plasma carburizing was performed in pure Ar gas under 32 ± 2 Pa. Graphite rod was employed as carbon supplier. Optical microscope (OM) observations showed a carburizing layer formed after carburizing. Further FESEM examinations and XRD analysis confirmed that the carburizing layer consists of a TiC/α-Ti mixed layer and a thin compound (TiC) layer on the mixed layer. With increasing the carbonizing time, the thickness of the carburizing layer increased and the specimen treated at 950 °C for 3 h obtains maximum values of the hardness.     

Role of Al additions in wear control of nanocrystalline Mo(Si1−xAlx)2 coatings prepared by double cathode glow discharge technique

Abstract

Four kinds of nanocrystalline Mo(Si_1?xAl_x)₂ coatings with differing Al contents are prepared onto a Ti–6Al–4V substrates by a double cathode glow discharge apparatus. The microstructural features of the deposited coatings were characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These coatings are composed of the equiaxed C40–MoSi₂ grains with the average grain size of ～5 nm. Nano-indentation measurements indicated that the hardness H, elastic modulus E and the H/E or H³/E² ratio of the nanocrystalline Mo(Si_1?xAl_x)₂ coatings slightly increase with the increase in Al content. The tribological behaviour of the nanocrystalline Mo(Si_1?xAl_x)₂ coating sliding against a ZrO₂ ceramic ball at room temperature has been compared using a ball-on-disc type tribometer under unlubricated conditions. Experimental results showed that the specific wear rates of the nanocrystalline Mo(Si_1?xAl_x)₂ coatings decrease with increasing Al content and are dramatically reduced by more than 1–2 orders of magnitude over the uncoated Ti–6A1–4V. The enhancement of wear resistance of the nanocrystalline Mo(Si_1?xAl_x)₂ coatings by Al additions is correlated with the increased mechanical properties and the forming oxide layer by tribochemical reactions.     

Phase formation in the Ti–Al–Mo–N system during the growth of adaptive wear-resistant coatings by arc PVD

Ti–Al–Mo–N coatings have been grown by arc PVD at different bias voltages, V_b, applied to the substrate and partial pressures of nitrogen reaction gas, p(N₂), in the working chamber. The coatings have a nanocrystalline structure, with an average grain size on the order of 30–40 nm and a layered architecture made up of alternating layers based on a (Ti,Al)N nitride and Mo-containing phases of thickness comparable to the grain size. It has been shown that the phase composition of the coatings depends on V_b and p(N₂): raising the energy of deposited ions by increasing V_b from–120 to–140 V, as well as raising p(N₂) from 0.3 to 0.5 Pa, leads to a more complete molybdenum nitride formation during coating growth, which causes a transition from (Ti,Al)N–Mo–Mo₂N compositions to (Ti,Al)N–Mo₂N. Measurements of the binding energy of Mo 3d photoelectrons in metallic Mo and the Mo₂N nitride by X-ray photoelectron spectroscopy have shown that the transition from the former phase to the latter is accompanied by a negligible energy shift.     

Pressure Induced Transition from NaCl to Wurtzite Phases of the New Hard Coating Zr x Al1−x N

We have carried out first-principles calculations to investigate the structural stability of the Zr _x Al_1?x N for several x values in sodium chloride and wurtzite structures. The Density Functional Theory (DFT), such as is implemented in the wien2k code was used and its is found that for x<0.5, a transition phase induced by pressure exists. The pressures that induce the transition are about 13.3, 6.0, and 0.67 GPa for x=0, 1/4, and 1/2, respectively. However, for x>0.5, such transition does not exist. Graphs of Density of States (DOS) for pressures across the transition are obtained and relevant electronic features are discussed.     

Synthesis and characterization of nanocrystalline CoCr coatings by plasma spraying

The present paper describes the synthesis and characterization of nanocrystalline CoCr (ASTM F75) coating produced by plasma spraying for possible surgical implant applications. The feedstock powders were synthesized by mechanical milling to produce irregular agglomerates with an average grain size of less than 100 nm. The powders were then introduced into an argon plasma spray to successfully produce a nanocrystalline coating. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the resultant sprayed coatings. Microhardness and porosity measurements were performed on the conventional and the nanocrystalline coatings to characterize and compare the physical and mechanical properties.     

Microstructure and dielectric properties of BaZr x Ti1−x O3 ceramics

The crystalline structure and dielectric properties of BaZr _x Ti_1−x O₃ ceramics with x = 0.05, 0.10, 0.15, and 0.20 were investigated. As zirconium increased, the a-axis lattice constant gradually increased, however, the c-axis lattice constant and c/a ratio gradually decreased. When x = 0.20, the crystal structures of the BZT ceramics are very close to cubic, different from the tetragonal structure when x < 0.20. The temperature dependence of the dielectric constant was studied and an enhanced diffuse phase transition behavior is found to be caused by the increased Zr content. The decreases of coercive electric field and remanent polarization were the result of increase of Zr/Ti ratio in BaZr _x Ti_1−x O₃.     

Elastic behaviour of microwave hydrothermally synthesized nanocrystalline Mn1−x–Znx ferrites

Microwave-hydrothermal (M-H) method has been successfully used for synthesis of nanocrystalline Mn–Zn ferrites which are used for high frequency applications. The nanopowders were characterized using XRD and TEM. The particle size of the samples varies from ～20 nm to 25 nm. The powders were densified at 900 °C/30 min using microwave sintering method. The sintered ferrite samples were characterized using XRD, X-ray photoelectron spectroscopy (XPS), SEM and energy dispersive X-ray spectroscopy (EDS). The elastic behaviour and internal friction studies were carried out using composite piezoelectric oscillator method in the temperature range of 300–600 K. It was found that the anomalous behaviour observed in the temperature dependence of Young's modulus and internal friction disappears with the application of a magnetic field equal to the saturation field (900 mT) of the specimen under investigation. The observed anomalous behaviour in the vicinity of the Curie temperature was understood with the help of Landau's theory.     

Synthesis,corrosion and wear of anodic oxide coatings on Ti–6Al–4V

Electrodeposited anodic oxide coatings were produced on Ti–6Al–4V substrates using aqueous electrolytes containing dissolved calcium and phosphorus. Different coatings were produced by varying the time periods. The coatings were characterised by XRD technique and TEM. The coatings were exposed to Simulated Body Fluid (SBF). Electrochemical polarisation and ac impedance studies too were performed on the coatings in SBF. Pins were coated and run against wooden disc in pin-on-disc type of wear tests. Coatings produced from long time electrolysis showed very good resistance to the attack of SBF and less wear compared to those produced from short time exposure.     

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.