فصلنامه علوم و مهندسی سطح ایران
پیاپی 28 (تابستان 1395)

in this study, in situ surface composite based on Mg-Cu system produced on AZ91C alloy by friction stir processing. Microstructural studing in the 6 passes friction stirred zone, revealed the presence of Mg2Cu intermetallic phase in AZ91/Cu nanocomposite. after T6 heat treatment, microhardness value within the stirred zone increased due to increasing volume fraction of hard intermetallics and formation of MgCu2 phase in the composite. Wear surface and debris observation indicated that abrasion and delamination wear mechanisms occurred in AZ91C alloy. The results of EDS and microhardness test on AZ91/Cu sample revealed that hardness and durability of oxide film on wear surface was more than that of the base alloy, which led to decrease wear mass loss of the composite in comparison with base metal. the results of wear test revealed that T6 heat treatment weakened wear properties, compared with not heat treated composite. composite samples Wear rate increased because of microcrack formation during heat treatment which caused delamination occurrence in this wear sample.

Due to the significant effect of porosity on the abradability behavior of ceramic abradable coatings and dependence of porosity value on the thermal spraying parameters, the aim of this research is to develop an empirical relationship between the atmospheric plasma process parameters and porosity value of YSZ-10wt.%LaPO4 abradable coating by surface response methodology. Full factorial design was used to investigate the effects of three varying principal parameters at two levels, namely, the current (400 and 800 A), the primary gas flow rate (25 and 40 lit/min) and the secondary gas flow rate (0 and 8 lit/min), on porosity of coating. Porosity was measured using optical and SEM images with software image analyzer (ImageJ). Results revealed that value of the porosity constantly decreases with increasing of current and secondary gas flow rate and with decreasing of primary gas flow rate. Furthermore, current and primary gas flow rate have highest and lowest impact on the porosity level, respectively.

The FSP can be used as a generic process to modify the microstructure, refine the grains and change the composition, at selective locations. During FSP, the material that flows around the tool undergoes extreme levels of plastic deformation, that causes recrystallization and grain growth mechanism and is why the solid state processing technique that causes superplasticity. The nano grained alloy exhibited superplasticity at relatively lower temperatures and/or higher strain rates. In this research, FSP and cooling condition was used to create a microstructure with nano grains in a 7075 Aluminum alloy. Using cooling conditions don’t let grain growth, so can be achieved to nano grain structure. Cooling condition contains of alcohol, ice, dry ice and liquid nitrogen. Optimum ductility at an initial strain rate of 1 × 10−3 s−1 was obtained on the 500℃. Current results suggest that FSP and cooling system can be developed as a simple yet effective technique for producing microstructure amenable for superplasticity at high strain rates and/or lower temperatures.

To produce aluminum-matrix composite reinforced with graphite particles, mixing of pure aluminum and graphite powders by a new method, called homogenization in liquid phase, was used. In this procedure at first different amounts of graphite from zero to 4.5wt.% were poured into acetone, and mixed by ultrasonic. Aluminum powder was added to the solution and sonication was continued. Then the filtered mixture was dried in vacuum atmosphere at optimized temperature and time. Composite samples were produced using the mixed powder by way spark plasma sintering (SPS) under different pressure, temperature and time. The specimen microstructures were investigated by optical microscopy and scanning electron microscopy (SEM). To evaluate the mechanical properties of samples, hardness and wear tests were performed. According to the results, homogenization in liquid phase method caused significant improvement in distribution of graphite particles in the matrix and the addition of 3wt.% graphite in the matrix was gained. Improved mechanical properties were obtained at 3wt.% graphite. According to the results, SPS process decreased the wear rate of Al-3wt.% Gr composite up to 55%, compared to the pure aluminum.

Due to their high hardness, high wear resistance and self-lubricating capability, electroless nickel-boron coatings have attracted much attention in recent years. Many researchers have reported that the surface structure of this coating is cauliflower-like but few others have reported a nodular structure. There are two different models to describe each surface structure. In the present research, using 2-level factorial model, the effect of ethylenediamine, ammonia and stabilizer concentrations on the thickness and surface structure were researched. Investigating the effect of the variables, both cauliflower-like and granular structures were gained which the cauliflower-like structure and granular structure were contributed to lower and higher plating rates respectively after discussion. In addition, in condition of a stable bath and a pore-free coating where the plating rate was high enough to make surface structure granular, increase in thickness led to the coarsening of surface structure. The maximum thickness of 49 μm was attained when ethylenediamine concentration was at its low level (70 mL/L) and ammonia and stabilizer concentrations were at their high levels (46 and 3.33 mL/L respectively). Also minimum thickness of 23 μm was achieved when ethylenediamine, ammonia and stabilizer concentrations were at their high levels (130, 46, 3.33 mL/L respectively).

The wear behavior AISI 304 stainless steel was investigated after Friction Stir Processing (FSP). FSP was carried out using a WC pin less tool at tool rotation of 560 rpm and tool feed of 50 mm/min. Microstructure analysis was conducted by using optical and scanning electron microscopy evaluations. Wear resistance of the FS processed and base metal specimens were compared using pin-on-disk tests. The results demonstrated that FSP homogenized and refined the grain size of 304 stainless steel. A reduction in grain size throughout the stir zone (26±3) compared to that of the base metal (93±3) was achieved through FSP. These results were confirmed by microhardness evaluations. According to the wear results, wear resistance of 304 base material improved significantly (two orders of magnitude) after FSP. This behavior was attributed to the grain refinement obtained by FSP. According to SEM observations, the abrasive wear was the main mechanism of wear after FSP.

To improve mechanical properties of the 7075 Al alloy, alumina-zirconia nanostructured coatings were formed on it through Plasma Electrolytic Oxidation (PEO) operated in potentiostatic mode. The composite coatings were produced at 450V for 100-300s growth times in a stable electrolyte containing K2ZrF6. Phase evaluation of the layers coated at 200s and higher growth times proved alumina-zirconia layers comprising tetragonal zirconia (t-ZrO2) and  and -Al2O3 phases. Nanostructured alumina-zirconia coatings with 20-40 nm particles size, porous structure and increase in the porosity content by increasing growth times proved by microstructural investigation of the layers. The coating thickness was in the range of 12-19 m. The distribution of Al, Zr and O elements in the cross-section of the coatings was uniform. Significant enhancement in hardness for coated samples was achieved (about 10 times higher than bare samples).

Zn–Ni alloy coatings were electrodeposited from acidic sulphate bath. In this study effect of plating bath temperature on hardness, morphology, chemical composition of the coating and cathodic current efficiency was investigated. The morphology and chemical composition of the coatings were investigated using Scanning Electron Microscopy (SEM) equipped with EDX analyzer. X-ray diffraction (XRD) technique was used to analyze the structure of the coatings. To investigate the corrosion properties of the coatings, Tafel Polarization experiments and electrochemical impedance spectroscopy (EIS), were used. Results showed that the amount of nickel in the coating increased by increasing deposition temperature. The amount of nickel had a great effect on structure, morphology and hardness of the coatings. In addition, as deposition temperature increased cathodic current efficiency reduced which in turn led to reduction of hardness. SEM results showed that at low temperatures the structure had fine grains and with increasing temperature, the structure became coarser. Coatings with 14 wt% Ni, had maximum corrosion resistance among all the coatings with different chemical composition. This coating had a  phase hexagonal structure and this was the main reason for the increased corrosion resistance of the coatings.

In this study, Hardystonite powders were coated on Ti-6Al-4V alloy using electrophoretic deposition (EPD). Hardystonite was made by sol-gel technique. Structure, morphology and elemental analysis of produced Hardystonite were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Cytotoxicity of Hardystonite nanoparticles was investigated (MTT assay). Nano Hardystonite powder was electrophoretically deposited on titanium substrates. The coated samples were observed by SEM. In order to investigate bioactivity, coated substrates were immersed in simulated body fluid (SBF) for 7, 14 and 21 days. Scanning electron microscope (SEM) was used to study structure and morphology of layer before and after immersing in simulated body fluid (SBF). Tafel polarization test was used to evaluate the corrosion and protection properties of coatings. The bioactivity of the coating was confirmed using immersion test. The tafel results showed that the corrosion resistance of substrate increases by Hardystonite coating. The results of MTT assay confirmed the cell compatibility of hardystonite nanoparticles. The obtained results of this research showed that Hardystonite can be coated on Ti substrate using electrophoretic deposition. Also, corrosion behavior and biological properties of Ti substrate is improved and Hardystonite Coating can be used as a novel bioceramic coating.