IRANIAN JOURNAL OF SURFACE SCIENCE AND ENGINEERING
Year:2018 | Volume:14 | Issue:35|Papers Count:8

Yttria stabilized zirconia (7YSZ) is widely used as standard Thermal Barrier Coatings (TBCs). However, for temperature higher than 1200˚ c tetragonal phase may be transformed to the monoclinic phase, which results in the formation of cracks in the coating and accelerates the spallation failure of TBCs. In this study, nanostructured scandia-ceria doped zirconia (ScCeSZ) were synthesized via polymerized sol-gel method and the effect of different amounts of scandia and ceria stabilizers on the thermal phase stability at the temputre of 1400˚ c were investigated by slow scan XRD. By comparing the amounts of monoclinic, non-transformable tetragonal (t') and cubic phases, the amount of tetragonality was calculated and eventually the optimum combination in terms of thermal performance was determined (4. 78ScCeSZ). Nanostructured 4. 78ScCeSZ was deposited by atmospheric plasma spraying (APS) on NiCoCrAlYcoated Inconel 738 substrates and thermal phase stability of the coating was investigated after thermal shock test in 1000˚ c. With respect to enhancing thermal stability of nanostructured 4. 78ScCeSZ, it could be suggested that nanostructured 4. 78ScCeSZ are promising substitutes for conventional YSZ in gas turbine engine.

Enhancing the wear-resistance of mechanical elements is always of much interest for the end users as well as the manufacturers. One common method for surface treatment is nitriding. In this research, friction and wear of nitrided and unnitrided disks are experimentally and numerically studied. The experiments are conducted using pin-on-disk test rig on samples made from 4140 steel. A numerical model based on the load-sharing concept has been developed to predict the friction coefficient, surfaces' temperature, film thickness, and wear rate. The lubricant temperature along with the friction coefficient is used to predict the wear volume using the fractional film defect theory. The results for un-nitrided disks indicate that increasing the speed results in a higher lubricant film thickness and thus the wear volume decreases. Increasing the applied load, on the other hand, results in a decrease in the film thickness and as a result the wear volume increases. In the nitrided case, the wear volume is a function of load only and does not depend on the speed. The diagrams of wear volume versus sliding distance shows a running-in period approximately before 1500 m after which the wear rate stabilizes. Friction coefficient and wear volume results are shown to be in an acceptable agreement with the simulation results.

Increasing the temperature of combustion products of turbines in power plants for increasing efficiency and decreasing fuel costs resulted in using thermal barrier coating (TBC) to protect hot gas path components of the turbo engines. TBCs consist of a ceramic top coat, an intermediate thermally grown oxide layer, and a metallic bond coat. In this paper, the damage caused by thermal shock during trip shut down in power plants is investigated based on analytical and finite element calculations in plain stress condition. The used geometry is a thin disk under an axisymmetry condition. The results show a sudden jump in thermal strain and stress in the ceramic top layer of TBC due to fast and inhomogeneous temperature loss. The fact that leads to similar behavior in stress intensity factor of a preexisting surface crack as it approaches critical value of ceramic top coat. This condition has a significant effect on length and growth rate of the crack in comparison with heating and constant temperature period. In addition, analytical calculation and finite element results match together and obtained crack behavior is compatible with other researcher's output.

Silica based tetraethoxsilane (TEOS)-aminopropyltriethoxysilane coating have been developed for corrosion protection of IF steel by sol-gel technique. The coatings were characterized by fourier-transform infrared spectrometer (FT-IR) and SEM. The corrosion behavior of the coated specimens were investigated using polarization tests and electrochemical impedance spectroscopy (EIS) experiments in 0. 1 M H2So4. Fourier-transform infrared spectrometer (FT-IR) test showed that the coating was formed and bonded well with the substrate. The corrosion results indicated that the corrosion current density of the coated experiments was significantly decreased as compared to the substrate. Also, the electrochemical impedance test results showed that the silane coating good corrosion resistance. In fact, the silane coating acted as a barrier layer avoiding the release of metal ions and impeding the diffusion of sulfate ions and oxygen molecules to the surface substrate. Furthermore, these results were confirmed using the optical microscope images of the corroded specimens.

In the present research, a novel simple cost-effective powerless method for fabrication of micro-nano structured superhydrophobic surface on aluminu m alloy has been suggested. A contact angle of 167. 1 º was measured for water drop on superhydrophobic surface. Micro-nano roughness feature was observed by SEM images on the surface and chemical analysis of the surface revealed by EDS and showed the presence of carbon chains on the surface. The effect of etching time on water contact angle value was investigated an d the optimu m etching time of 4 min was reported. It was estimated that 97% of surface are contacted with air in aqueous solution. Potentiodynamic polarization test showed that the corrosion current density has been decreased on superhydrophobic surface fro m 1. 4 to 0. 7 µ A/cm 2 in co mpared with normal polished Al alloy surface. Th is indicates that the alloy lifetime has been increased two folds. Furthermore, self-clean ing character and repulsion capability of the surface against various common liquids was investigated experimentally.

Laser surface hardening is a promising technology used for surface modification to improve the tribological properties without affecting on the bulk properties of materials. This paper surveys the capability of laser surface hardening of AISI 410 martensitic stainless steel by using continues high power diode laser with a maximum power of 1600 W, experimentally. Laser power, scanning speed and focal plane position were variable parameters in this research. The influence of process parameters on depth and width of hardened layer and microhardness profile of laser treated areas were investigated. M icrostructure of the laser treated zone were studied and compared. Results show that by increasing laser p ower and decreasing scanning speed, higher hardness and depth achieved. Results also reveal that by decreasing in focal plane position higher depth and lower width achieved. Observations indicated that a hardened surface layer of about 1. 8 mm depth and 620 HV hardness is obtained. The surface hardnening of the 410 stainless steel by diode laser is about 1. 43 times the hardness of the furnace heat treatment.

In this research, a combination of Ferro-molybdenum powders (variable amount) and Ferro-chromium and graphite (constant amount) were coated on the st37 carbon steel substrate through Gas Tungsten Arc Welding (GTAW). In order to study properties of the layer established under the layer, scanning electron microscope (SEM), EDS element microanalysis and microhardness assessment have been used, and x-ray diffraction analysis (XRD) specified that microstructure of the samples consists of (Cr7C3), (CrC) chromium carbides and (MoC) molybdenum carbides. In addition, it was found that the austenite and ferrite in both samples is composed of molybdenum wire with an increase in base Fe-Cr-C carbide (CrC) was created less on coverage. Wear highest abrasion resistance test results to the sample 2 (69 Rockwell C) is shown. The samples were studied by electron microscopy in conjunction with rubbed surfaces, the samples showed that the mechanism of wear, for example, contains molybdenum, abrasive wear, sticky type and plowing type.

Considering the large functions of M g alloys in the various industries, their weak surface properties including resistance to oxidation and corrosion have been regarded as the primary challenge to commercializing them. Obtaining superhydrophobic surfaces have been an efficient way for enhancement of the corrosion resistance feature which has drawn much attention. In the present study, the formation of superhydrophobic structure (SHPs) on the M g alloy was investigated by immersion into the CuCl2 and NiSO4 solutions following by soaking in the stearic acid (SA) solution. The contact angle of the obtained surface after the process measured about 151. 5 ° . Fourier transform infrared spectroscopy (FTIR) spectra showed the presence of some bonds of SA on the surface of M g alloy. Besides, FESEM morphology of SHP M g alloy illustrated a flakelike morphology which could prevent corrosive electrolyte penetration toward M g surface. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests illustrated that the corrosion resistance of SHP M g was greater than M g alloys about 1000 times which could be due to the presence of rough morphology and adsorbed hydrophobic molecules.