فصلنامه علوم و مهندسی سطح ایران
پیاپی 42 (زمستان 1398)

One of the major concerns in the field of valves operating under severe conditions such as high temperature is their interaction with other surfaces. These valves are used to pass gas. These gasses are sometimes corrosive and can cause wear of the surface which in turn affect the geometry such as roughness etc. For precise utilities, even small change in the geometry highly affects its performance as well as their precision. Therefore, the ability to predict the wear in these valves is highly valuable. In this paper, a numerical model based on the solution of flow and concentration equations is developed to predict the wear in the surfaces which are adjacent to hot gases. The output of this model is used as input to another model to predict the wear. The predictions of this model are verified by comparing to other numerical models as well as experimental data. The predicted values are in accordance with experimental data with acceptable accuracy.

Nickel-copper nanoferrites with different weight ratios were synthesized by combustion method. Graphene was prepared from microporous graphite powder using liquid phase exfoliation. The obtained graphene was analysed by Raman spectra. Then graphene was combined with nickel-copper nanoferrites to prepare nickel-copper nanocomposites based on graphene. The optical, structural and magnetic properties of nanocomposites were investigated by analyses of XRD, FTIR, SEM, UV-VIS and VSM. The XRD diffraction patterns revealed the formation of nanoparticles with cubic spinel structure, and the formation of the ferrite phase can be confirmed according to the FTIR results of the vibrational mode of the Fe ions located at the quadrilateral position ofthe oxygenoxygen lattice. The SEM microscopic images showed more porosities in nanocomposites that indicate these structures have a large surface area that is useful in trapping viruses and bacterias. Also this study could be used in photothermal therapies research.

In this study, the effect of the wire cut machining parameters on the surface quality of superalloy718 was investigated. For the first time, the Inconel 718 superalloy machining was performed by wire cut method with spraying suds. This method of machining has had many advantages over the previous methods, which were immersed and cooled distilled water. The regulatory parameters were the intensity of the current, pulse lighting time, pulse power offset; forward speed and wire feed speed. Roughness and material removal rate were the Output characteristic of this process. Two methods of Taguchi design and experimental design (DOE) were used to determine the effect of the parameters on the performance of the wirecut output. Using the design of experiments in Taguchi method, the minimum roughness of the surface was 4.97 μm and the maximum roughness of the surface was 9.65 μm. The results showed that the parameters of the lighting time, the forward speed and the feeding rate of the wire increased the surface roughness and it decreased with increasing roughness time. With the analysis of the dispersion of the concentration of alloying elements on the surface, it was determined that the surface with maximum surface roughness, in addition to being sensitive to stresses and mechanical factors, has less resistance to chemical agents.

The wear and hardness properties of the protective aluminide coatings can be enhanced by reinforcing the surface, using hard materials such as diamonds. In this research, prior to the aluminizing process by pack cementation, an intermediate NickelNano diamond (Ni-ND) layer was fabricated on the surface of the Ni-based superalloy Hastelloy X with electroplating process. The electroplating of Ni-ND layer was accomplished by continuous direct current in nickel Watts electroplating bath containing 1 and 4 g/l ND particles. The coated and uncoated samples were aluminized by pack cementation method using aluminum, ammonium chloride and alumina powders. The aluminizing process was designed as a two-step procedure at 760 °C following the annealing process at 1080 °C. The cross-sections of the coated specimens were studied by Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) analysis. The results showed that during aluminizing process, the nano-diamond particles prolonged the nickel and aluminum diffusion path and as a consequence, the aluminum content of the coating layer was increased. This gave rise to the conversion of the Ni-based intermediate composite layer into an aluminum-rich NiAl layer. The hardness of the treated coating with ND particles was significantly higher than the untreated coating in a way that microhardness of the coating with no ND particles was 663 Vickers while the coating containing ND particles showed a value of about 968 Vickers.

In this research, formation and structure evaluation of optimized slurry aluminide coating by applying a pre-coating of Ni/Ni-Co electroplated layer on Ni-base superalloy were studied. First, some pure Ni and Ni–45Co (at. %) layers were deposited on Ni-base superalloy, Hastelloy-X, samples separately by electroplating process at the specified conditions and 22-23 μm thickness. Subsequently, the slurry aluminizing process was carried out on the pre-plated and a not-plated samples via the specified heat treatment. The obtained coatings with 142-148 μm thickness were investigated by X-ray diffractometer and scanning electron microscope equipped by energy-dispersive X-ray spectroscope, and elemental mapping. According to the obtained results, β-NiAl desirable phase in all three samples and β-CoAl in the Ni-Co pre-plated sample were formed, definitely. The pre-plated layer reduced the diffusion of Cr and Fe, up to 4.5 times of magnitude and Mo, up to 2.5 times of magnitude to the upper parts of coatings by forming a diffusion barrier state. Present Ni in the pre-plated layer decreased the consumption of present Ni in the substrate alloy, too. Accordingly, the composition of substrate alloy in the pre-plated samples was less affected by diffusion coating process. Also, the present Co in the pre-plated layer of the Ni-Co pre-plated sample increased its amount, up to 7 times of magnitude in upper parts of the coating. This subject improved the possibility of formation of cobalt aluminide phases which confirmed by the formation of β-CoAl in upper parts of the coating.