E. Avci

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

Abstract In this study, the corrosion behaviour of grit-blasted AISI 304L stainless steel substrates coated with Al 2 O 3 was investigated in 1 N H 2 SO 4 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 Al 2 O 3 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.

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

Abstract 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 Al 2 O 3 , Al 2 O 3 +TiO 2 and Cr 2 C 3 +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.

High temperature sol–gel insulation coatings for HTS magnets and their adhesion properties

Abstract This paper describes the adhesion properties of sol–gel insulation coatings with silver tape substrates using a mini-tensile-testing for HTS magnets. The sol–gel coatings were produced using solutions of Zr, Mg, Y, Ce, In and Sn based organometallic compounds. The growth mechanism of these coatings on Ag and AgMg sheathed Bi-2212 superconducting tapes was characterized by scanning electron microscopy, energy dispersive spectroscopy, differential thermal analysis and X-ray diffraction. The lap joints were fabricated by laying fresh sol–gel coated silver tape samples over each other and then heat treated. These joints with varying heat treatment conditions were pulled to failure by using a mini-tensile-tester. The results of bonding strength and microstructure of the failure surface are presented. Oxides were formed between temperatures of 450°C and 550°C. The results also indicated that the failure was in the form of a mixed type as interfacial/cohesive defect. The average adhesive strength of ZrO 2 , MgO–ZrO 2 , Y 2 O 3 –ZrO 2 , CeO 2 –ZrO 2 , In 2 O 3 –ZrO 2 and SnO 2 –ZrO 2 were found to be 0.763, 1.300, 2.400, 2.087, 1.650 and 1.580 MPa, respectively. Y 2 O 3 and CeO 2 doped ZrO 2 showed the best adhesive properties.

Oxidation behaviour of functionally gradient coatings including different composition of cermets

Abstract 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°C. 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°C are considerably higher than at 800°C.

CeO/sub 2/ buffer layers for YBCO: growth and processing via sol-gel technique

In this study, the processing and growth mechanisms of CeO/sub 2/ buffer layers were evaluated on Ni substrates for YBCO superconductor. The sol-gel technique was utilized to produce buffer layers from six different Ce based precursor materials. The microstructure, phase composition and formation of buffer layers were characterized by means of ESEM, SEM, EDS, XRD, pole figure XRD and DTA techniques. The results indicated that the CeO/sub 2/ formation starts at temperature of 420/spl deg/C. It was found that the best crack free CeO/sub 2/ thin films were obtained by using Ce(NO/sub 3/)/sub 3//spl middot/6H/sub 2/O and colloidal CeO/sub 2/ based precursor materials. Different anions and their concentrations also influenced the structure of thin films.

Evaluation of functionally gradient coatings produced by plasma-spray technique

Abstract Functionally gradient coatings are widely used at high temperatures in order to improve performances of materials in automotive, aircraft and aerospace industries, etc. These coating systems contain a steel substrate, bonding layer, cermet layer and ceramic coating, respectively. Furthermore, the composition and morphology of the coatings change gradually from ceramic to metal. In the present work, 304 L stainless steel substrates were coated by functionally gradient coatings consisting of bond coat NiCrAl, 70% NiCrAl+30% MgZrO3, 50% NiCrAl+50% MgZrO3, 30% NiCrAl+70% MgZrO3 as the cermet layer and 100% MgZrO3 as the top coat. These coatings were produced using a plasma-spray technique. The coated samples were analysed using an optical microscope, scanning electron microscope and X-ray diffractometer. The surface roughness was measured via a Perthometer M4P, and the distribution of microhardness was determined on both the coating and the substrate. 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 μm from the coating surface decreased and followed first a linear function in the ceramic, cermet and bond coatings. In addition to this, the microhardness of steel substrate showed a parabolic function due to the blasting operation and heat effect during the coating process.

Evaluation of adhesion strength of sol-gel ceramic insulation for HTS magnets

Adhesion strength of high temperature insulation materials deposited on silver was investigated. CeO/sub 2/ and ZrO/sub 2/ based coatings on Ag substrates were prepared by the sol-gel technique. Experiments for determining the adhesion strength between insulations and silver were performed using a mini tensile testing machine. The microstructures of coatings and their interfaces were determined by means of SEM, EDS, and X-ray diffraction. The results indicated that the failure was in the form of a mixed type as interfacial/cohesive defect. The average adhesive strength of ZrO/sub 2/, CeO/sub 2/, ZrO/sub 2/+20 wt%MgO, Zr/sub O/2+8 wt%Y/sub 2/O/sub 3/, ZrO/sub 2/+8 wt%CeO/sub 2/, ZrO/sub 2/+15 wt%In/sub 2/O/sub 3/ and ZrO/sub 2/+15 wt%SnO/sub 2/ were found to be 0.763, 0.913, 1.045, 1.150, 0.915 0.833 and 0.797 MPa, respectively. Y/sub 2/O/sub 3/, MgO, and CeO/sub 2/ doped ZrO/sub 2/ showed the best adhesive properties.

Synthesis and characterization of high temperature compatible ZrO2 insulation coatings on Ag/AgMg sheathed Bi-2212 wires and tapes

Abstract High temperature compatible ZrO 2 insulation coatings have been successfully fabricated on Ag and AgMg sheathed Bi 2 Sr 2 Ca 1 Cu 2 O x (Bi-2212) superconducting wires and tapes for high magnetic field magnets. Solutions were prepared from a zirconium based organometallic compound, solvent and chelating agent. The ZrO 2 was coated on Ag and AgMg sheathed Bi-2212 wires and tapes at 500 °C for 2 min in air using a reel-to-reel, continuous sol–gel technique and these tapes were subsequently annealed at ∼860 °C in a flowing oxygen atmosphere. The relationship between the solutions and the coatings was evaluated. With reference to environment scanning electron microscope, energy dispersive spectroscopy, differential thermal analysis, thermogravimetric and X-ray diffraction studies, different steps in the process leading to the formation of ZrO 2 are discussed. These studies indicate that the formation of cubic ZrO 2 starts at approximately 450–550 °C and thin film thickness increases with number of dipping operations. The average adhesive strength of ZrO 2 was found to be 0.763 MPa.

MgO-ZrO2 Insulation Coatings on Ag/AgMg Sheathed Bi-2212 Superconducting Tapes By Sol-Gel Technique

The MgO-ZrO2 insulation coatings on Ag and AgMg sheathed Bi-2212 superconducting tapes were investigated by sol-gel technique. These coatings were prepared using solutions of Zr and Mg based organometallic compounds. These insulation coatings were characterized by SEM, EDS, DTA, TGA, and X-ray diffraction. The ceramic oxide structures were formed from amorphous states at temperatures between 450°C and 550°C. This insulation process has been used for a 3-T HTS coil insert proposed for a lGHz NMR program.

CeO/sub 2/-ZrO/sub 2/ insulation coatings on Ag/AgMg sheathed Bi-2212 superconducting tapes by sol-gel process for magnet technology

In this research, preparation, structure, morphology and thermal properties of 4, 6, 8, 9 and 12 mole% CeO/sub 2/-ZrO/sub 2/ insulation coatings on Ag and/or AgMg/Bi-2212 superconducting tapes were studied for the magnet technologies. These coatings were prepared by reel-to-reel sol-gel method using a solution of Zr and Ce based precursors. The coatings were annealed at 850/spl deg/C for 24 h under oxygen gas flowing. The structure of coatings was characterized by SEM, EDS, TGA and XRD. The investigation showed that dense, fully cubic phase CeO/sub 2/-ZrO/sub 2/ insulation coatings with 8-10 Fm were deposited on Ag and/or AgMg/Bi-2212 superconducting tapes.