فصلنامه علوم و مهندسی سطح ایران
پیاپی 18 (تابستان 1392)

Thermal control coatings are used to protect spacecrafts from space environment and applied on their outer surfaces. In the present work, coatings were made by ZnO, TiO2 and produced pigments into potassium silicate binder and exposured to simulated space environment. Oxygen plasma exposure tests were conducted in a DC plasma chamber. Electron beam with 70 Gy absorption dose and thermal cycling in the region -75-100 °C were used to simulation of electron irradiation and thermal cycling. The results indicated that solar absorptance and thermal emittance were stable and observed no surface morphology change and mass lose. The produced pigment was stable as well as ZnO in this condition and well substitute of ZnO.

In this research Cu thin films were grown on n-type GaAs by electro and electroless deposition. Some Cu thin films were electrodeposited under constant current mode from 5 to 30 mA and some Cu thin films were grown by electroless at various temperatures from 25 to 77 oC. Their structures and morphology were studied by X-ray diffractometer (XRD) and scanning electron microscope (SEM). Roughness of both series of Cu films was examined by atomic force microscope (AFM).

A common problem in surface machining of gray cast iron parts is due to formation of carbide at the surface, which results in decreasing cutting tool life and increasing production cost. Therefore, detection of hardened layer and its hardness are key factors in quality control and inspection processes. In the present paper, nondestructive evaluation of metallurgical parameters (surface carbide, surface hardness and hardened depth) has been investigated using eddy current technique. The results show the high potential of the proposed method as a fast an accurate nondestructive technique in determination of metallurgical parameters affecting hardness of the surface and consequently separation of undesirable parts.

In this study microstructure and formation of NiAl phase using a modified (single-step) high activity aluminizing at 1080°C have been compared with those of common (two-step) high activity aluminizing at 850°C. The mechanism of this process has been investigated by carrying out the aluminizing process on electroplated Nickel on 316 stainless steel substrate at 1080°C in different durations of 0.25, 1, 2, 4 and 8 h (by investigation the Ni content in NiAl). Results showed that even in a short duration of 0.25 h the NiAl phase was formed. In 0.25 h NiAl phase with more than 50%at. of Al was formed while NiAl phase with less than 50%at. of Al was formed at 1, 2, 4 and 8 h. It is concluded that the mechanism of formation of NiAl coatings takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion and In the final stage, the outward diffusion of Ni dominates the coating formation process. The microstructure of the coatings sections were observed by optical microscopy and scanning electron microscopy. The phase analysis and the coating composition were investigated by XRD and EDS respectively.

In this study mechanical milling was employed to coat D2 steel with TiC. The steel sample, balls and the powder were placed within a milling vial. Milling treatments were carried out with annealed and quench-tempered samples using TiC powder having particle sizes of 40 and 200 µm for 5, 10, 15, 20, 50 and 100 h. During the milling treatment, the sample surface was exposed to high energy collisions and the powder particles trapped between balls and sample adhered to the surface through cold welding. It was shown that the thickness and the structure of the coating depend on powder particle size; hardness of steel and milling time. The thickness of the coating increased at first and decreased thereafter with milling time. The results showed that the substrate hardening decreased the thickness of the coating on milling for more that 20 h, but it increased the scratch resistance of the coatings. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were employed to investigate the structural and compositional characteristics of the coatings. X-ray Diffraction (XRD) analysis was also conducted to determine the existing phases in the coating. The SEM investigations showed that the greatest thickness of the coating was reached after 20 h of milling. Furthermore, no new phases were detected in the XRD results after 100 h. Scratch test was conducted to evaluate the adhesion of the films coated under different conditions.

In this study, 76Mo-14Si-10B and 30Mo-47Si-23B (at.%) powder compounds were milled for 20 hours using an attritor ball mill with milling speed of 365rpm under inert gas atmosphere. After degasing of As-milled powders at 450℃ temperature for 2 hours, they were pressed into cylindrical samples with 25mm diameter and were sintered at 1100℃-1400℃ temperature range by tube resistance furnace under Ar gas atmosphere. Wear behavior of Mo-Si-B alloys were investigated using a pin-on-disk wear test machine. The results of waer tests under 20N and 30N forces showed that sintered 30Mo-47Si-23B alloy at 1300℃ temperature for 3 hours has more wear resistance than 76Mo-14Si-10B alloy because of MoSi2 and MoB intermetallic particles fine and uniform distribution.

In the present study, the microstructural properties of the nanostructured surface layer of a low carbon steel (CK15) fabricated by ball milling technique was characterized using X-ray diffractometry, optical and scanning electron microscopies (SEM and FESEM). The results indicate that a nanostructured layer with the thickness of more than 30 m was formed on the surface of the steel by applying surface milling treatment. The minimum of the average grain size in top of surface layer was found to be about 15 nm after milling for 12 hours. Moreover, it was found that by applying the mentioned method, the average grain size and the hardness in the top of surface layer gradually vary as a function of depth below the surface.

In this study, La0.6Ca0.4Fe0.8Ni0.2O3 (LCFN oxide) were prepared by sol-gel method, to be used as a material in solid oxide fuel cell cathode. Also, Thin films of LCFN were prepared by pulsed laser deposition (PLD) on SrTiO3 (100) (STO) substrates in different pressures of oxygen. Crystal structure, morphology and electrical conductivity were studied by X-ray diffraction (XRD), AFM and four point probe method, respectively. Also the surface exchange coefficient (kchem) determined by electrical conductivity relaxation (ECR) technique.

In the present study NiCrAlY bond coating layer was produced by electroplating against common atmospheric plasma spraying (APS). Both types of the bond coats were applied on IN738LC base metal then, the YSZ (ZrO2-8% Y2O3) thermal barrier top layer was coated by atmospheric plasma spray technique. Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. In this investigation the hot corrosion behavior of coatings was tested in the furnace which was contain Na2SO4-55% V2O5 and mixed salts environment at 900°C up to 15 hr dwell time. Optical microscopy, scanning electron microscopy (SEM / EDS) and X-ray diffraction analysis (XRD) was used to determine the crystallographic structure and phase transformation of the coatings before and after the hot corrosion tests. The transformation of tetragonal Zirconia to monoclinic ZrO2 and formation of YVO4 crystals as hot corrosion products caused the degradation of mentioned TBCs. The results showed NiCrAlY coated by economical electroplating method a viable alternative for common thermals sprayed bond coats in hot corrosive environments with same corrosion behavior.