فصلنامه علوم و مهندسی سطح ایران
پیاپی 45 (پاییز 1399)

In this study, three nickel-based superalloys consist of Colmonoy 6 ،Inconel 625 and Hastelloy C276 was coated by GTAW process method on 310 stainless steel. Evaluation of hardness, wear behavior and wear mechanism of samples was performed with Vickers, pin-on-disc method and using SEM images, respectively. The high temperature oxidation behavior was also studied at 1050°C in 8 cycles of 24hours. Also, to investigate corrosion behavior of samples, potentiodynamic polarization test was used in a solution of sulfuric acid and sodium chloride. The surface hardness results showed higher hardness of the Hastelloy C276 (727 HV) and Colmonoy 6 (612 HV) than the Inconel 625 (338 HV) and the steel substrate (217 HV). The best wear behavior was related to Hastelloy C276 with friction coefficient of 0.28. Surface morphology studies of the samples show a change in the wear-resistance mechanism from sliding to adhesive by applying nickel-base coatings on 310 stainless steel. In high temperature oxidation test, minimum weight gain was calculated for Hastelloy C276 (0.5mg/cm2). Corrosion test shows that fabrication of coatings improved 310 stainless steel corrosion resistance which maximum corrosion potential (-119/9 mV) and minimum corrosion current density (0.14 µA.cm-2) was owned by Hastelloy C276. Finally with considering the results, it seems that Hastelloy C276 be as the most desirable option with the aim of coating on 310 stainless steel in high-temperature conditions.

In this research, the erosion behavior of carbidic austempered ductile iron and alloyed ductile iron was studied and compared. For this purpose, cubic specimens with a size of 3*3*1 cm were prepared, and subsequently austempered. Microstructural characterization was carried out using optical microscopy and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Hardness and surface roughness were measured prior to the erosion test. Specimens underwent erosion test at the angles of 30, 45, 60, 75, and 90 degrees. Diagrams depicting weight loss for different angles denote that the carbidic specimens show higher erosion strength in all angels. Moreover, X-ray diffraction results from the surface of the carbidic specimens prior and after the erosion test demonstrated that there is 38 percent residual austenite on the surface of the carbidic specimens after the erosion test which provides evidence for the strain-induced transformation of austenite to martensite during erosion test. Micro-hardness tests on the surface of the carbidic specimens after the erosion test demonstrated an increase from 446 to 531 vickers due to the transformation. Moreover, as apparent from the SEM images, the dominant erosion mechanism is cutting and ploughing at low (30-60 degrees) and high angles (60-90 degrees) is crack and micro crack, respectively.

In this study, the mechanical properties and tribological behavior of galvanized and galvalume coatings produced by hot dip method have been evaluated and compared. In addition, due to the high application of these types of sheets at high temperatures, the effect of long-term thermal cycles on the properties of these sheets was studied. For this purpose, samples of carbon steel plate with galvanized coating and galvalume with the same thickness were prepared. Then, structural and surface morphology was studied by scanning electron microscope and Elemental analysis, phase composition and crystalline texture were determined by X-ray diffraction. The mechanical properties of the coatings were evaluated by using Vickers hardness test and tensile test. The tribological properties of the coatings were investigated by pins on the disk test. Laboratory findings showed that overall the strength of galvanized sheet are greater than the Galvalume sheet, while it has a relatively less long elongation. while Galvalume has a weaker abrasive behavior than galvanized. In the case of galvanized sheet heat up to 300 ° C, the iron penetration from the steel substrate to the surface of the coating occurs and it forms brittle intermetallic phases that have a negative effect on the galvanized abrasion behavior. In other side, oxide layers also form on the surface of the coating and due to a better abrasive behavior. By heating the galvalume sheet at temperatures up to 400 ° C, the surface of the cover is dark, and bulges appear on it which indicates the formation of intermetallic phases and the presence of oxide on the surface of the coating. According to the results of this research in applications that friction coefficient is important, it is not recommended to heat galvanized and galvalume coatings at temperatures, respectively, above 300 ° C and 400° C.

In this study, a fluorinated silicate film was produced by plasma polymerization using a radio frequency plasma system. In order to reduce the refractive index of the films, structured porosity was created in the film. The amount of impurities, optical absorption coefficient, surface roughness, the leakage current density and the threshold of the breakdown field were investigated. To investigate the effect of plasma polymerization process, fluoride gas type and working pressure, as well as placing the base for substrates were tested. The chemical bonding states of the films and the morphology of the films surface were investigated using the infrared Fourier transform spectrometer and atomic force microscope, respectively. In addition, the study of refractive index and absorption coefficient were also performed by spectroscopic ellipsometry. The results showed that the high purity fluorinated silicate film having structured porosity has ultra-low refractive index and the extinction coefficient lower than 10-4 , and increasing the pressure as well as placing the stand for silicon substrates causes to increase organic impurities, especially carbon bonds, thereby eliminating the structured porosity, which in total led to increase the refractive index (1.37 and 1.39) and the extinction coefficient (0.004 and 0.0005) of the films.

In this study, at first a precursor precipitate of La2(Zr0.7Ce0.3)2O7 was prepared by using ZrOCl2.8H2O, Ce(NO3)3.6H2O and La2O3 initial materials. The calcination process of this precipitate was performed at 400 -900 . The Phase constituents, particle size, and morphology of the calcined powders were investigated. The calcined precipitate agglomerates at 600 and 900 were used to apply by plasma spraying on the AMDRY 962 and METCO 204NS-G coated Hastelloy X substrates. The cyclic oxidation test at 1000 was done for two double ceramic layer coatings applied by agglomerates 600 and 900 and the conventional YSZ coating. According to the results of diffraction patterns and microstructural observations, the nanoparticles containing the main phases of LZ (lanthanum zirconate with low solubility of cerium cation) and LC (solid solution of cerium lanthanum zirconate) formed only in the La2(Zr0.7Ce0.3)2O7 precursor precipitate calcined at 900 because of relatively high calcination temperature. However, both double ceramic layer coatings were composed of LZ and LC phases, and theses coatings were as nano-TBCs with vertical cracks. The coating applied by agglomerates 600 had relatively less compaction compared to the coating applied by agglomerates 900 . The results of the cyclic oxidation test showed that only the coating applied by agglomerates 600 had a desirable performance. Proper microstructural compaction and the presence of vertical cracks in this coating caused this performance. Morphology of the Layers of this coating remained sound after 924 thermal cycles. While propagating crack was formed around the TGO/YSZ interface in the conventional YSZ coating after cyclic oxidation test.