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In this research, the formation mechanism of platinum-modified (Pt-modified) aluminide coating structure applied by out-of-the-pack aluminizing is investigated. For this purpose, the samples of superalloy were coated with 7.5 µm platinum prepared by electroplating. Then, they were heat-treated at 1080°C for 1 h in the vacuum (10−4 torr). The platinum-coated samples were aluminized using an out-of-the-pack single-step aluminizing process at 1050°C under different times and with fixed powder composition. Afterwards, the microstructures of the coatings were evaluated using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and an X-ray diffraction (XRD). The results showed that Pt-modified aluminide coating was formed at first by the inward diffusion of Al and, then, by outward diffusion of Ni. The final coating microstructure consisted of three main layers. The outermost layer of PtAl2 precipitates in a β-Ni(Pt, Al) matrix, the middle layer of Ni-rich β-Ni(Pt, Al) and the innermost layer of interdiffusion zone. [ABSTRACT FROM AUTHOR]

In this study, Si-modified aluminide coating on nickel-base superalloy IN-738LC was prepared using a pack cementation method with various powder compositions at 1050 °C for 6 h. The cyclic oxidation test was conducted at 1000 °C followed by cooling at room temperature for 200 h and 20 cycles. The effect of powder composition and the way of cooling on the coatings microstructure and oxidation behavior were studied. Investigations carried out using a scanning electron microscope (SEM), EDS analysis, and XRD. Microstructural observations revealed that the coating thickness of 293 and 274 µm was achieved in the case of using pure Al and Si powder and alloyed Al-20wt.%Si one in the packed mixture, respectively. It was also found that utilizing pure Al and Si powder with NH4Cl as an activator in the pack led to the formation of silicide coating, owing to the higher diffusion of Si, which showed superior cyclic oxidation performance. [ABSTRACT FROM AUTHOR]

In the present study, zirconium modified aluminide coating on the nickel-base superalloy IN-738LC was first created by high activity high temperature aluminizing based on the out-of-pack cementation method. Then, Zr coatings were applied to simple aluminide coatings by sputtering and heat treatment in order to study the effect of Zr on the coating microstructure and oxide spallation. Microstructural studies were conducted by using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD) microanalysis. The results indicated that zirconium modified aluminide coating, like aluminide coating, has a two-layer structure including a uniform outer layer of NiAl and an interdiffusion layer in which zirconium is in a form of solid solution in the coating. Furthermore, the 300 nm Zr-coated NiAl demonstrated an excellent scale adhesion, a slow oxidation rate and lower amounts of other elements such as Ti and Cr in its oxide layer lead to a pure aluminide oxide layer. [ABSTRACT FROM AUTHOR]

In this study nanosized TiO2 coatings on the 316L stainless steel substrate were prepared by means of dip-coating technique in which thickness of the coating layer increased by repeating the coating cycles in two different routes: (I) dipping and drying, respectively, were repeated one, three and five times and finally the dried coated sample was heat treated (single); (II) multiple heat treatment performed after each dipping and drying cycle, respectively. The structural, morphological and optical characterizations of coatingsthe, as well as the thickness of coatings, were systematically studied. The photocatalytic activity of the various TiO2 coatings was investigated based on the degradation of an aqueous solution of Methyl orange. Moreover, the corrosion protective properties of coatings were evaluated in both dark and UV illumination conditions based on the obtained polarization curves. The results indicated 1.75 times improvement in photocatalytic reaction rate constant, a two orders of magnitude decrease in corrosion current density in dark condition and about 140 mV electrode potential reduction under UV illumination with optimum coating preparation procedure, repeating the cycle from dipping to heat treatment three times, than the sample prepared with one time coating and heat treatment since this procedure provided not only high thickness and defect-free coating but also transparent one. [ABSTRACT FROM AUTHOR]

Two γ-TiAl based alloys of Ti-48Al-2Nb-2Cr (at-%) and Ti-45Al-2Nb-2Mn-1B (at-%) were used as substrate materials. The coatings' deposition process was prepared using the slurry method. After the deposition of water based slurry containing Al and Si powders on the substrates, diffusion treatment has been carried out at 1000°C for 6 h in Ar atmosphere. The structure of the silicon modified aluminide coatings is as follows: (a) an outer layer consisting of a TiAl3 matrix and Ti7Al5Si12 grains dispersed within this matrix, (b) a middle layer composed of a TiAl3 matrix and columnar grains of Ti5Si3 and (c) an inner layer as TiAl2 phase. According to the energy dispersive spectroscopy analysis results, it can be concluded that Cr had a high tendency to move towards the Si enriched compounds. The results of isothermal oxidation test of 1000°C/200 h showed that the formation of Si modified aluminide coatings on γ-TiAl alloys by slurry method can greatly improve their high temperature oxidation resistance. [ABSTRACT FROM AUTHOR]

This study has been conducted to evaluate the influence of temperature and substrate roughness on microstructure and properties of electron beam physical vapour deposition NiCrAlY coating. The chemical composition was adjusted by continuous feeding in the molten pool. The results show that the coating roughness generally increased with an increase in the substrate roughness and decrease in its temperature. However, in rough substrates, coating roughness were lower than that of the substrates, and in samples with a lower substrate roughness, coatings had a greater roughness level in comparison with their corresponding substrates. The polished samples with the lowest level of substrate roughness had narrower columns, and by an increase in the roughness of the substrate, the columns became thicker. The optimised conditions in terms of microstructure and adhesion strength were observed in the sample with the roughness level of Ra = 0·02 μm and the homologous temperature of θ = 0·44. [ABSTRACT FROM AUTHOR]

MCrAlY coatings, as a group of high temperature coatings, have extensively been considered for protecting gas turbine blades against oxidation and hot corrosion. At temperatures of more than 1000°C, interdiffusion between superalloy substrate and coating is one of the most important factors for controlling the lifetime of these coatings. In this paper, thin layers of α-Al2O3 were used as a diffusion barrier between superalloy substrate and coating using electron beam physical vapour deposition method. These barrier layers successfully blocked the diffusion of substrate alloy elements such as tungsten, molybdenum and, in particular, titanium into the coating. Results showed that using these diffusion barrier layers between superalloy substrate and coating can considerably improve the oxidation resistance of NiCrAlY coatings so that the parabolic rate constant in the samples with micrometric diffusion barrier layer is about one-third of that in the samples without it. [ABSTRACT FROM AUTHOR]

One of the important factors affecting the microstructure of Ni base superalloys is the cooling rate from partial solution temperature; therefore, the prime purpose of the present research is to investigate the effects of this factor on some microstructural features of superalloy IN792. The results showed that by increasing the cooling rate from 5 to 160°C min-1 after partial solution treatment at 1120°C for 2 h, the size and volume fraction of primary γ' decreased, but during the subsequent aging the volume fraction of secondary γ' increased initially exponentially and later linearly. When high cooling rates >90°C min-1 were applied, the formation of secondary γ' was totally suppressed. However, after aging at 845°C for a period of 24 h, the amount of secondary γ'. increased with a decelerating rate up to 60°C min-1 and then with a linear rate up to 160°C min-1 The later observation has not been reported elsewhere. [ABSTRACT FROM AUTHOR]

Compared to metallic coating, composite coating has more significant mechanical and tribological properties. In this study, titania particles that were codeposited with zinc on a plain carbon steel substrate were used as reinforcements. In addition, the mechanical and tribological properties of these coatings were studied. The effects of bath temperature, stirring rate of electrodeposition solution and concentration of titania in bath on the morphological, mechanical and tribological properties were also investigated. The electrodeposited samples were observed with optical microscope, scanning electron microscope and atomic force microscope. Then, the microhardness and wear properties of the samples were measured. Finally, the optimum degrees of these parameters were achieved to attain maximum codeposition of reinforcement particles, hardness degrees and wear resistance of coatings. The optimised degrees of bath temperature, solution stirring rate and titania concentration in the bath were found to be 40°C, 300 rev min-1 and 5 g L-1 respectively. [ABSTRACT FROM AUTHOR]

In this paper, Zn-nano-TiO2 composite coatings were deposited on the mild steel substrate. Coatings were produced by electroplating method from sulphate bath in which nanosized TiO2 particles were suspended. The chemical composition and surface morphology of coatings were investigated using energy dispersive spectroscopy and scanning electron microscopy respectively. Corrosion behaviour of the coatings was studied in 3·5 wt-% NaCl solutions, and their microhardness was evaluated through Vickers microhardness test. According to the results, the parameters of current density, particle concentration in the solution and temperature were optimised with respect to TiO2 content in the composite coatings. The optimised current density, particle concentration and temperature of the bath were found to be 3 A dm-2, 5 g L-1 and 40°C respectively. Accordingly, an increase in the amount of TiO2 particles in the composite coatings increases the hardness of composite coatings but did not have harmful effects on corrosion resistance. [ABSTRACT FROM AUTHOR]

Full homogenisation behaviour of a Ni3Al based alloy containing Cr, Mo, Zr and B was investigated. At the as cast condition, the microstructure of this alloy consists of a γ solid solution γ'(Ni3Al) and a low melting point structure. At first, the low melting point structure, which is detrimental for full homogenisation process, was eliminated by annealing the specimens at 1100°C. After this prerequisite process, based on differential thermal analysis results, 1260°C was determined for the full homogenisation annealing temperature. The results showed that this alloy cannot be fully homogenised without the formation of locally melted regions. In order to explain the mechanism of this local melting phenomenon during full homogenisation treatment, some heating cycles were performed at 1200 and 1240°C. A scanning electron microscope equipped with energy dispersive spectroscopy and differential thermal analysis was used for introducing the local melting (incipient melting) mechanism. It was found that the difference in zirconium solubility limit in the γ and γ' phases is the main reason for the negative response to full homogenisation treatment. [ABSTRACT FROM AUTHOR]