MCrAIY and YSZ coatings, and combinations of both, with voluminal percentages of 30%, 50%, and 70% in metallic phases, were prepared using APS and HVOF processes. In this study, the thermal expansion of the coatings was measured in a vacuum of 10-6mmHg ranging from room temperature to 1100°e. The results showed that, the expansion of each pure metal and ceramic specimen increased differently as the temperature increased. In fact, at high temperatures the difference was more than 70%. On the other hand, the porosity percentage decreased from 11.4 without metallic phase down to 4.3 in 70% of the content of the metallic phases. In contrast, the coefficient of the thermal expansion increased more than 68%. Therefore, it is recommended that, by adjusting the variables, coatings with appropriate thermal and expansion properties can be designed and employed for target working conditions.

Although the number and the severity of TBC applications on hot section components have dramatically increased in the past decade, premature spallation failure of TBCs, due to mismatch of thermal expansion at the metal/ceramic interface of the two coating layers, during service is still an overriding concern. Therefore, functionally graded materials (FGMs) with a gradual compositional variation have been introduced. In this study, comparison of properties of two different types of thermal barrier coatings (TBCs) was made to improve the surface characteristics on high temperature components. These TBCs consisted of a duplex TBC and a five layered functionally graded TBC. In both coatings, Yittria partially stabilized Zirconia topcoat (YSZ) was deposited by air plasma spraying (APS) and NiCrAlY bond coat was deposited by high velocity oxyfuel spraying (HVOF). In FGM coating, functionally graded layer was sprayed by air plasma process by varying the feeding ratio of YSZ/NiCrAlY powders using two separate powder feeders. Then, isothermal oxidation was carried out at 950oC in atmosphere to obtain the plot of mass change vs. time to study oxidation kinetic. Micro structural and compositional changes of coating, oxides formed during service were examined by optical microscope and SEM with EDS. FGM coating failed after 2100 h and duplex coating failed after 1700 h. Finally, it was found that FGM coating is more qualified than duplex TBC and stands for a longer time.

In this investigation, NiCrAlY coating was deposited on steel substrate using high velocity oxy-fuel (HVOF) process. Different spray parameters such as spray distance and powder feed rate were applied and the hot corrosion behaviour in molten salt (Na2SO4- 60%V2O5) environment at 900oC was studied. Optical microscope and scanning electron microscopy/energy dispersive spectroscopic analysis (SEM/EDS) techniques were used to analyse the corrosion products. The analysis of the results indicates that the spraying distance and the powder feed rate have a significant effect on the properties of NiCrAlY coatings such as oxygen content and porosity. The coatings prepared by 20 cm spraying distance and 32gr/min powder feed rate have the best corrosion resistance, because of the less oxides.

In this investigation, NiCrAlY coatings were deposited on steel substrate using the HVOF process. Different spray parameters were applied such as Propane flow rate, spray distance and powder feed rate. The hot corrosion behavior of resulted samples was studied in salt mixture Na2SO4-60%V2O5 at 900oC for 30 min to 22 h in an ambient atmosphere. After hot corrosion tests the microstructure and phase composition of these coatings were analysed using Optical microscope, Scanning electron microscopy/Energy dispersive spectroscopic analysis (SEM/ EDS), and X-ray Diffraction (XRD) techniques. The analysis of the results indicates that the parameters variation have a significant effect on the properties of NiCrAlY coatings, especially on the oxide phase content present in the coating, and therefore on the hot corrosion resistance of this coating. It was observed  that coating prepared by 60 lit/min Propane flow rate, 300 lit/min Oxygen flow rate, 20 cm spray distance and 32 gr/min powder feed rate has the best corrosion resistance, because of the smaller oxide contents inside the coating.

An attempt was made to investigate the thermal and residual stress distribution in a novel three layer (La2Zr2O7/8YSZ/NiCrAlY) during a real-like heating regime which includes heating, service time and final cooling. For achieving maximum accuracy and consistency in calculation of thermal and mechanical properties of hybrid coating system, all related and required properties were introduced to the software in temperature-dependent mode. Element modification approaches like mass scaling leads to a considerable reduction in running time while satisfying and not violating accuracy and converging criteria and constrains. Applying adaptive hybrid meshing techniques which applies both mesh part dependency and independency during numerical iterative solution avoids element distortion and diverging in coupled problem. Heat flux and nodal temperature contours indicated that, most of damaging and harmful thermal load and residual stresses concentrate on ceramic top coats and this may lead less harm and life time reduction in the substrate.