Seramik kaplamalarda ara malzeme ve porozitenin termal şok üzerine etkisi

Abstract

temperatures are usually unstable. Whereas ceramic coatings are thermodynamically stable. On the other hand they are easily damaged under the action of thermomechanical stresses. This is due to the mismatch of their thermal expansion coefficients with those of metallic substrates. Finite element technique is a computerized technique for engineering analysis. This technique is widely used for solutions to design challenges in the aerospace, automotive, power, consumer machinery, biomechanics and electrical/electronics industries. In this study, thermal shock analyses were carried out for MgO.Zr02 ceramic coating, NiAl, NiCrAlY and NiCoCrAlY bond coatings and spreodial cast iron substrate materials. In all models, the thickness of MgO.Zr02 coating and substrate were 0,4 and 4 mm, respectively. In the bond coats analysis, NiAL, NiCrAlY and NiCoCrAlY bond coatings were used. The thickness of bond coats was chosen as 0,1, 0,2 and 0,3 mm. Furthermore, the porosity in coating layer was also modelled. The results showed that the thermal stresses are highly influence by bond coating material, bond thickness and with percentage of porosity in coatings. xvi

SUMMARY INFLUENCE OF INTER LAYER AND POROSITY ON THERMAL SHOCK IN CERAMIC COATINGS Keywords: Finite element technique, thermal barrier coatings, thermal shock, inter layer, porosity, thermal stresses, Surface preparation techniques such as, plasma spraying, physical vapor deposition (PVD), chemical vapor deposition (CVD), detonation gun, flames spraying, have been used to make convenient material combinations in usage of high technological requirements. Coatings are used for many engineering applications in order to improve the surface properties of components or structures or to protect them against environmental degradation. Typical examples in high temperature technologies are coatings against thermal degradation, high temperature corrosion, erosion and wear. High temperature coatings are used for two main functions, either to protect a base metal against corrosion or erosion or to minimize wear. A third function is to reduce the base metal temperature in the case of thermal barrier coatings; however, resistance to hot corrosion and oxidation is again mandatory. Moreover, in case of gas turbine engines thermal barrier coats reduce substrate air cooling requirement. Because of its specific task the coating material differs from the substrate material in chemical composition, structure and physical as well as mechanical properties. Thus for high temperature applications good chemical and mechanical compatibility between the coating and the substrate material are a main design objective. However, it has to be taken into account that at high temperatures and after sufficient long service times changes occur because of aging, mterdiffusion or even environmental effects which additionally may modify specific properties. It is well known that metallic surfaces in aggressive environments (oxidation, carburization, nitration, sulphidation, attack by molten metals, etc.) and at elevated xvtemperatures are usually unstable. Whereas ceramic coatings are thermodynamically stable. On the other hand they are easily damaged under the action of thermomechanical stresses. This is due to the mismatch of their thermal expansion coefficients with those of metallic substrates. Finite element technique is a computerized technique for engineering analysis. This technique is widely used for solutions to design challenges in the aerospace, automotive, power, consumer machinery, biomechanics and electrical/electronics industries. In this study, thermal shock analyses were carried out for MgO.Zr02 ceramic coating, NiAl, NiCrAlY and NiCoCrAlY bond coatings and spreodial cast iron substrate materials. In all models, the thickness of MgO.Zr02 coating and substrate were 0,4 and 4 mm, respectively. In the bond coats analysis, NiAL, NiCrAlY and NiCoCrAlY bond coatings were used. The thickness of bond coats was chosen as 0,1, 0,2 and 0,3 mm. Furthermore, the porosity in coating layer was also modelled. The results showed that the thermal stresses are highly influence by bond coating material, bond thickness and with percentage of porosity in coatings. xvi