IRANIAN JOURNAL OF SURFACE SCIENCE AND ENGINEERING
Year:2019 | Volume:- | Issue:41|Papers Count:5

In this research, novel hybrid coatings were fabricated with a gradient thickness of Ni-P by the electroless process on the steel substrate, so that the coating thickness along these coatings gradually changes. To fabricate coating with variable thickness, the method of taking out the sample from the electroless bath during the process was used. For purposive experiments, response surface methodology (RSM) was utilized. Optical microscopy, SEM, EDS and micro-hardness test have been used to characterize the fabricated coatings. The results of variance analysis of the models indicated that the models are highly valued. The sum of squares showed that pH is the most important factor in coating hardness. Also, the effect of pH on the gradient of coating thickness is about 1. 5 times the sample exit rate of the electroless bath and 2. 5 times the temperature. SEM images and EDS analysis results showed pH and temperature are two important factors in surface morphology and phosphorus content of coatings. The phosphorous content of coatings varies from 7. 8 to 10. 1 wt%. The coating fabricated in pH 5. 4 and 84℃ has the highest hardness about 650 HV. The coating fabricated in pH 5. 2, 86℃ and the sample exit rate from the electroless bath 0. 11mm/min has the highest gradient of thickness, about 3μ m/mm.

Improvement of hot corrosion resistance in calcium magnesium aluminosilicate environments is one of the new main goals in development of turbine industries especially thermal barrier coatings (TBCs). The purpose of this research is to improve the quality and efficiency of YSZ thermal barrier coating in calcium magnesium aluminosilicate environments through replacing YSZ/CSZ composite layer. For this purpose, single layer YSZ and dual layer YSZ/CSZ TBCs deposited with atmospheric plasma spray (APS) technique on Ni-based superalloy (IN738LC) substrates as a top coat with a CoNiCrAlY bond coat. Hot corrosion resistance of the samples was investigated in a cyclic mode in which after each 4 h exposure to calcium magnesium aluminosilicate salts at 1150° C. The results of microstructural and phase studies of coatings by FESEM/EDS and XRD analysis before and after hot corrosion tests showed better resistance to dual layer YSZ/CSZ than single layer YSZ coatings and could act as barrier for the corrosive calcium magnesium aluminosilicate salts penetration into the TBC. Also, destruction of TBCs in calcium magnesium aluminosilicate environments due to infiltration of molten salts into the TBCs during hot corrosion process and cause the transformation of tetragonal zirconia to monoclinic phase during cooling.

Reduction of friction coefficient is always of great importance in various mechanical systems. In this research, the effect of surface texturing and the lubricant volume on the friction coefficient is investigated. The experiments are conducted using a pin on disk test rig and a mechanical setup is used to texturize the surface of the disks. The results show that Stribeck curve can be obtained in the textured case. It is also observed that surface texture reduces the friction coefficient but this reduction is more pronounced under heavy loads. In the case when very low amount of lubricant is provided, an increase in the applied load results in an increase in the lift-off speed.

In this study, two processes of plasma electrolytic oxidation coating and gas nitriding have been investigated. In the first step, titanium oxide was coated by plasma electrolytic oxidation method in carbonate solution with current density of 1000 mA/cm2 for 10 min to form the first layer on the titanium substrate. In the second step, the second layer was formed by gas nitriding. Nitriding carried out in a tube furnace at 1000℃ for 6 hours on TiO2 coated titanium. The wear test, X-ray diffraction and scanning electron microscopy were performed on different coatings for determination of the properties. The results showed that, the initial coating mostly contains rutile. The morphology and the structure of the coating showed the presence of micro-cavities. Based on wear test results, the electrolytic plasma oxidation process, provides an ideal annealing condition with low friction coefficient of 0. 2. For the nitrided coating, some titanium oxides converted into nitrated phases. The tribological study showed that the wear resistance of nitrided titanium oxide increased significantly compared to titanium oxide coatings and uncoated samples which is, the friction coefficient from 0. 65 for the uncoated sample decreased to 0. 25 for the nitrided titanium oxide sample.

To improve the efficiency and power output of turbine engines, the turbine inlet temperature should be increased which requires higher oxidation resistance of MCrAlY coatings. Moreover, the oxidation resistance of the MCrAlY coating depends on the chemical composition and coating’ s microstructure, which can be improved by optimizing the parameters of coating process. In this study the parameters of LPPS process on the microstructure of NiCoCrAlY coating is evaluated and compared with the microstructure of NiCoCrAlY coating applied by the HVOF method. Thus, microstructure, phase composition and elemental analysis of the coatings are characterized by optical and field emission scanning electron microscopy (FESEM), X-ray diffractometer (XRD) and energy dispersive spectroscopy (EDS), respectively. The results reveal that the optimum microstructure of the coating with minimum porosity, non-melted particles and oxide phases was obtained at the highest temperature and particle’ s velocity impact to the substrate’ s surface. Hence, the optimized values for Ar and H2 flow rate (75 and 16 SLPM, respectively), and the current of 650 A at a spray distance of 12 cm has been optimized for for maximum amount of remaining particles at maximum speed in plasma jet. In addition, EDS and XRD analysis show that the NiCoCrAlY coating contains continuous γ nickel rich solid solution with dispersed β-(Ni, Co) Al phases and negligible ratio of oxide phase.