فصلنامه علوم و مهندسی سطح ایران
پیاپی 26 (زمستان 1394)

The purpose of the current study is fabrication of germanium-carbon protective coating by plasma enhanced chemical vapor deposition (PECVD) using germane (GeH4) and methane (CH4) gases precursors, and then is its characterization. To this end, field emission scanning electron microscope (FESEM) equipped with energy dispersive spectroscope (EDS), X-ray diffractometer (XRD), fourier transform infrared spectrophotometer (FTIR), X-ray photoelectron spectrometer (XPS), laserinduced breakdown spectrometer (LIBS), nanoindentation test and adhesion test was used. Based on the structural evaluation, binding of germanium atoms with carbon atoms and the existence of a germanium carbon matrix in the coating was demonstrated. Also, the results indicated the formation of amorphous germanium-carbon coating as a homogeneous and dense layer free of pores and discontinuities with a very good adhesion to subst rate. Hardness and elastic modulus of germanium-carbon coating was obtained about 7.1 and 95.1 GPa, respectively.

In this research, the effect of different Immersion times in the 3.5% (w) NaCl solution on the rate of corrosion protection efficiency of silane coating were studied. The coating was composited of tetraethoxysilane (TEOS) and (3-mercaptopropyl) trimethoxysilane (MPTMS) and it was applied by a simple dip -coating method on the copper substrates. Fourier-transform infrared spectroscopy test showed that the TEOSヴ coating has connected Cu-S covalent bond with the copper surface by through the sequence of its sulfur. Atomic force microscopy images obtained after 120 hours of immersion, showed the little damage to the surface coating of TEOS MPTMS that corresponded to the image obtained by optical microscopy. The protective properties of covered and uncovered copper specimens were investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The results indicated that the highest corrosion protection of copper coating was achieved in the early hours of immersion and it decreased with time. But even after 168 hours, the silane coating has retained its inhibitory effect .

Optically active surface plasmon resonance modes in noble metals nanoparticles likes Au has figured out application potentials for them. Since various parameters such as particles shape, size and also dielectric constant of their surrounding affect the optical properties, investigations on them are of scientific importance. In this work, Au/Pd core-
shell nanoparticles were synthesized by laser (Nd:YAG pulsed laser, λ=1064 nm) ablation of gold target in PdCl2 solutions. X-ray diffraction (XRD) was used for crystals structure determination. X-ray photoelectron spectroscopy (XPS) determined the surface chemical composition of nanoparticles. Nanoparticles morphologies were observed on TEM. XRD showed the presence of metallic Au, PdCl2 and PdO phases. The effect of aging on nanoparticles was studied by time variations of surface plasmon resonance peaks. Finally, a physical description was given for the observed results.

In this research, the effect of adding chromium carbide to stellite-6 thermal spray coatings was investigated. For this reason,the various amount of Cr3C2 powder (10%, 20% and 30 wt . %) were mixed with stellite-6 powder. The mixed powders deposited on the Inconel-617 substrate with high velocity oxygen fuel (HVOF) process. The coatings properties were evaluated using optical microscopy, scanning electron microscopy equipped to Energy Dispersive Spectroscopy (EDS) analysis, microhardness and X-Ray Diffraction (XRD). XRD analysis results showed that no oxide phases formed in coatings. In order to evaluate tribological properties of the coatings ball on disk wear test was performed in 1000m with 40N load. Results show that hardness and wear resistance of the coatings increased with increasing of chromium carbide reinforcement. Stellite6-30 wt. % chromium carbide coating had the highest wear resistance than others coatings.

Fusion welding can deteriorate the corrosion feature of Ti-6Al-4V alloys due to the existence of some metallurgical defenses such as discontinuity, Oxygen and Nitrogen absorption and creation of pore holes. By employing friction stir welding, the appropriate corrosion resistance could be reachable. In current research, Ti-6Al-4V with thickness of 3mm was used. Structural characterize of welded zone were evaluated by scanning electron microscopy, equipped by EDS microanalyses XRay diffractometry (XRD). Corrosion behavior of fabricated specimen was investigated by means of potentiostat (EG & G 273A). The SEM micrographs and EDS results confirmed that ununiformity chemical composition and various types of metallurgical phase were formed, which caused the corrosion resistance of transition zone declined as compared to the stirzone and the base metal zone has the highest value of corrosion resistance. It was found that the stir zone contain the typical Alpha () phase plus recrystallization Beta () phase along with tungsten debris while transition zone contain equiaxed and bimodal lamellae structure. The corresponding current density for stir zone, transition zone and base metal were 3.126, 1.532, 0.530 μA respectively, while obtained potential for those mentioned zones were -240, -226 and -172 mV respectively.

Thermal barrier coatings are utilized in hot sections of gas turbines in order to isolate parts imposed to elevated working temperature and in consequent enhance efficiency. In the current research, ABAQUS commercial finite element package was used to analyze temperature distribution and imposed residual stress in thermal barrier coating with s inusoidal interfaces. Allphysical, thermal and mechanical properties need for simulation was introduced in temperature- dependency mode to the software. Results indicated that, significant thermal load, distortion and of course, induced stress were concentrated in the upper protective ceramic layer and functionally grading strategy cause to a 50% reduction in residual stress in the coating system. The sign of stress and also trends of stress variation in interface asperity posses a specific pattern both in heating and cooling to ambient temperature.

This work presents in-situ formation of Ti/TiC composite layer on Ti-6Al-4V via pre-placing mixture of Graphite-Nickel and subsequently surface melting by pulsed Nd:YAG laser. The phases and microstructures of the fabricated layers were characterized by Optical Microscope (OM); Field Emission Scanning Electron M icroscope (FESEM) equipped with Energy Dispersive Spectroscopy (EDS) and X-ray Diffractometry (XRD). The effect of laser parameters on the geometry of the melted pool and in-situ formation of titanium carbide was also investigated.The experiments revealed that there are increase in the depth, width composite layer and percent of titanium carbide as the energy density increases. I t was shown that the surface layers were mainly consisted of Martensite α΄- titanium as matrix and TiC, TiC2 dendrite. The hardness of composite layer fabricated by in-situ route reached to a maximum hardness value of 1400 HV0.3, more than 4 times of that of the base Ti-6Al-4V (300-350 HV0.3).

In this research work, mechnanism of SiC insertation into ASTM A106-Gr B steel surface and composite layer formation studied. This layers formed using the tungsten inert gas (TIG) arc welding process. Therefore, silicon carbide particles with different volume percentages were applied on the steel surface by changing the current density, melting and mixing process. Composite layer microstructures studied using optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) spot analysis. The results showed that surface layers have the dendritic structure with silicon carbides particles into ferite phase. Micro hardness tests results showed increasing of surface hardness about 650 to 1200 VHN compare to uncoated samples about 200 VHN. Studing of composite layers wear behavior showed significant improvements with added silicon carbide reinforcement. The dominant wear mechanism of uncoated samples were delamination wear and surface oxidation. But, results showed that coated samples had a mixture of delamination wear, surface oxidation and adhesive wear.

In this paper, the wear behavior of high velocity oxy fuel thermally sprayed WC-12Co coatings has been studied. The effect of the main process parameters of HVOF thermal spraying (independent parameters) on the temperature of particles and phase degradation (dependent parameters) of WC-12Co coatings on the wear behavior was studied. Four more effective parameters such as spray distance, oxygen to fuel ratio, powder feed rate and gun travel speed were selected as inputs. Taguchi approach was used for design of experiments. Spray Watch diagnostic system, scanning electron microscopy (SEM) and X-ray diffraction were used for this purpose. The pin on disc method was used to evaluate the wear behavior of coat ings. The statistical results showed that the temperature of particles has the most influence on the phase degradation as well as wear behavior. Therefore the more particle temperature the more degradation of WC and finally the more wear resistance.