Numerical analysis of fracture in ceramic coatings subjected to thermal loading

In this paper the general purpose finite element code ANSYS has been employed to analyse fracture in ceramic coatings subjected to thermal loading. An approach is developed in which hypothetical material properties have been considered as material data for coupled (thermal and structure) finite element analysis. These properties were chosen by assumed changes in some functional properties of ZrO2-G.G. coatings. The aim was to evaluate the stress intensity factors in different coatings. Furthermore, to demonstrate the influence of crack length and coating geometry on the stress intensity in coatings, finite element analyses were carried out for various cases. The normalized stress intensity factors were obtained. The results showed that the shorter the crack length and the thinner the coating, the sounder the coatings. Furthermore, coatings representing a wide range of thermal and mechanical properties have a close normalized stress intensity factor values. It is also concluded that the finite element technique can be used to optimize the design and the processing of ceramic coatings. (C) 1997 Elsevier Science Ltd

Effects of some parameters on corrosion behaviour of plasma-sprayed coatings

The corrosion behaviour of ceramics and cermet coatings on AISI 304L steel substrates was investigated in several aqueous solutions. Plasma spray process was employed on substrates with the deposition of ceramic and cermet powders such as Al2O3, Al2O3 + TiO2 and Cr2C3 + NiCr. The porosity of coatings was measured by an electrochemical technique. Potentiodynamic polarization measurements and corrosion tests were employed to determine the corrosion behaviour of plasma-sprayed coatings. The microstructures of powders and coatings were investigated by means of an optical microscope, SEM, and X-ray diffractometry. The obtained results show that the protection against corrosion depends strongly on the porosity, thickness, surface roughness of the coatings, corrosion media and rotating speed of samples. (C) 1997 Elsevier Science S.A

Evaluation of functionally gradient coatings produced by plasma-spray technique

The results showed that there are no certain transition lines between the bond layer, the cermet layer, and the ceramic layer. The SEM results indicated the presence of porosity, oxide, inclusion and unmelted particles. Surface roughness values decreased with increasing MgZrO3 content. It was also found that the microhardness below a depth of 245 mu m from the coating surface decreased and followed first a linear function in the ceramic, cermet and bond coatings. Tn addition to this, the microhardness of steel substrate showed a parabolic function due to the blasting operation and heat effect during the coating process. (C) 1999 Elsevier Science S.A. All rights reserved

Oxidation behaviour of functionally gradient coatings including different composition of cermets

Functionally gradient coatings are coating systems which are used to increase performances of high temperature components in the hot gas turbine section of gas turbine engines, diesel engines, in aerospace and aircraft applications. These coatings consist of a transition from the metallic bond layer to cermet and from cermet to the ceramic layer. In this study, oxidation behaviour of functional gradient coatings produced by plasma-spray on AISI 304 L stainless steel substrates was investigated. The plasma spray process was employed to deposit metallic, cermet and ceramic powders such as NiCrAl, NiCrAl + MgZrO3 and MgZrO3 on the substrate. A certain cermet composition for each sample was selected. Oxidation tests were carried out at temperatures of 800 and 1000 degreesC. The coatings and oxidised coatings were examined by means of an optical microscope, SEM and X-ray diffractometry. The results showed that the oxide film thickness and oxidation rate at 1000 degreesC are considerably higher than at 800 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved

Effect of grit blasting of substrate on the corrosion behaviour of plasma-sprayed Al2O3 coatings

In this study, the corrosion behaviour of grit-blasted AISI 304L stainless steel substrates coated with Al2O3 was investigated in 1 N H2SO4 solution. Coatings were produced by employing a plasma-spray process. The phase analysis of powders and coatings was evaluated by means of optical microscopy and X-ray diffractometry. The microstructure of the coating was characterised with optical microscopy. The corrosion rates of these coatings were measured according to ASTM G 31-72 standard. The results indicated that the adherence of Al2O3 coatings on the substrate, the surface roughness and the corrosion rate depend on the parameters of grit blasting. It is also found that the corrosion resistance of plasma-sprayed coatings was reduced with increasing surface roughness and increased considerably with decreasing porosity and coating thickness. (C) 1999 Elsevier Science S.A. All rights reserved

Electrical resistivity of porcelain bodies with natural zeolite addition

In this study, the effect of natural zeolite addition on the electrical properties of porcelain bodies was investigated. Clinoptilolite, which is a type of natural zeolite, was added partially or fully in replacement of quartz in selected electro-porcelain compositions. Samples were fired in an electric furnace with a heating rate of 10 degrees C/min at 1200 and 1250 degrees C with a period of 60 min. The electrical resistance measurements of samples were performed at 50, 200, 400 and 600 degrees C. It was shown that the resistivity of samples increased at 50 degrees C temperature after zeolite addition, while it was decreasing after zeolite addition at higher temperatures. At the same time, it was recognized that the resistivity of samples depends on sintering temperature. The activation energy of electrical resistivity of samples was found to be in the range of 0.79-0.87 eV. (C) 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved

Effect of natural zeolite addition on sintering kinetics of porcelain bodies

The aim of this study was to investigate the effect of natural zeolite addition on the sintering kinetics. Clinoptilolite which is a type of natural zeolite was added partially or fully in replacement of quartz at selected electro-porcelain composition. Samples were fired in an electric furnace with a heating rate of 10 degrees C/min at 1150, 1200 and 1250 degrees C with a period of 5, 10, 20, 30, 60 and 90 min. The sintered samples were characterized by XRD and SEM. Sintering activation energies were determined according to the bulk density results. It was found that the sintering activation energy decreased with increasing zeolite addition. (C) 2007 Elsevier B.V. All rights reserved

Mıknatıs Teknolojisi İçin Ag Şeritler Üzerindeki Yüksek Sıcaklık Yb2O3- ZrO2 Yalıtkan Kaplamalar

In this study, synthesis and characterization of high temperature Yb2O3–ZrO2 insulation coatings on Ag substrate were investigated for magnet technologies. The produced powders and coatings were characterized by using DTA-TG, FTIR, XRD, SEM-EDS, refractometer, spectrophotometer and empendance machines. The DTA/TG result indicates three-stage decomposition for amorphous Yb2O3– ZrO2 composites. FTIR studies revealed that heat treated powders indicate several absoption bands corresponding to O-H, carboxyl and oxide groups. Though ZrO2 phase present in the structures with high intensity, Yb3O2 phase possessess low intensity peak. A regular surface morphology generally forms once Yb2O3 content increase in ZrO2 from 0 mole % to 12 mole %. The refractive indexes of Yb2O3–ZrO2 coatings were found to be in the range of 1.3539 and 1.3655. The film thicknesses of all coatings generally increased from 0.50 µm to 1.20 µm according to number of dipping. Optical band gap values of Yb2O3–ZrO2 coatings are approximately 3,014 eV. Empedance values of ZrO2 and Yb2O3- ZrO2 coatings were measured as 2.52 ohms and 2.51 ohms, respectively

Comparison of thermal stresses developed in Al2O3-SG, ZrO2-(12% Si+Al) and ZrO2-SG thermal barrier coating systems with NiAl, NiCrAlY and NiCoCrAlY interlayer materials subjected to thermal loading

In this investigation, thermal and structural finite element analysis has been employed to analyse the level of stresses developed in Al2O3-spherical cast iron (SG), ZrO2-(12% Si + Al) and ZrO2-SG coatings subjected to thermal loading. Coatings with a coating-to-substrate thickness ratio of 1/10 were modelled. ZrO2-SG coatings with an NiAl, NiCrAlY or NiCoCrAlY interlayer, and with different combinations of these interlayer materials, were also modelled. Nominal and shear stresses at the critical interface regions (film/interlayer/substrate) were obtained and compared. The results showed that the ZrO2-SG coatings have a higher thermal shock resistance than the Al2O3-SG and ZrO2-(12% Si + Al) coating systems. Furthermore, the interlayer thickness and material combinations have a significant influence on the level of thermal stresses developed. It is also concluded that the finite element technique can be used to optimise the design and processing of ceramic coatings. (C) 1999 Elsevier Science S.A. All rights reserved