Erdal Celik

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.

Development of 3 T class Bi-2212 insert coils for high field NMR

Based on a successful 1 T Class PIT insert coil, the authors are now pursuing a 3 T Class insert coil. This paper describes the design and the latest conductor and coil test results, as well as supporting experiments. The final product is envisioned to contain 3 concentric sections, requiring over one kilometer of conductor. This will be tested in a 20 T large bore resistive magnet at the NHMFL. Experimental work focuses on the use of conductor with a silver-alloy matrix in the outer sections, that are subject to the largest stresses when operated in a background field. Results from heat treatment optimization for wound coils, mechanical test of conductors and coil design studies are reported.

A new approach in biomimetic synthesis of calcium phosphate coatings using lactic acid–Na lactate buffered body fluid solution

The main objective of this study was to investigate calcium phosphate (CaP) coatings on Ti6Al4V substrates by using the biomimetic technique. To this purpose, a new solution was developed to coat CaP on Ti6Al4V alloy substrates. The newly formulated body fluid (Lac-SBF) contained appropriate amounts of sodium lactate (NaL) and lactic acid (HL), as well as all the other ionic constituents of the human blood plasma. The inorganic ion concentrations of the Lac-SBF solutions were identical with those of human blood plasma. The new Lac-SBF solution of this study eliminated the need for using Tris/HCl or Hepes/NaOH buffers. Prior to coating, Ti6Al4V substrates were chemically treated in NaOH and/or NaOH+H(2)O(2) solutions as an alternative route and then heated at 600 degrees C for 1h in air. In the previous applications, the Cl(-) ion concentration was found to be higher than blood plasma 103mM, which exists in human blood plasma as a result of Tris/HCl which are used to prevent precipitation and to keep the pH level at certain values. In this study, instead of using Tris/HCl, HL/NaL which are generated by human body and do not show any toxic behavior, are used and Cl(-) concentration was kept at 103mM value for the first time. The prepared Lac-SBF was shown to have similar concentration to human blood plasma in terms of all inorganic ions for the first time. Solution properties were evaluated by using turbidimeter, pH meter and rheometer. The coatings were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and a scratch tester. The obtained results are presented and discussed.

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 insulation coatings and their electrical properties for HTS/LTS conductors

Abstract The high temperature compatible insulation coatings were produced using solutions of Zr, Mg, Y, Ce, Er, Sm, In, Sn, Sr, Pb, Ba, Al, and La based organometallic precursors using a non-vacuum sol–gel technique. The growth mechanism and microstructure of these coatings was characterized using environmental scanning electron microscopy and X-ray diffraction. Dielectric constants and breakdown voltages of these coatings were also measured by using an insulation tester. The results of microstructure and electric properties of these coatings are presented. The oxide structures were formed between temperatures of 450 and 550 °C from amorphous coating. MgO–ZrO2 coatings on Ag and AgMg sheathed Bi-2212 tapes have been used as the insulation for the 1 T and 3 T HTS high field insert coil. These ceramic insulation coating can be built up to ∼10 μm by successive dipping. Crack-free, dense, very thin (submicron) coatings were also obtained using different solvent and acid with the same method with appropriate dilution of solutions.

Textured growth of multi-layered buffer layers on Ni tape by sol-gel process

Textured Cerium Oxide (CeO/sub 2/)/Yttrium-Stabilized Zirconia (YSZ)/CeO/sub 2/ buffer layers structure were grown by sol-gel dip coating process on bi-axially textured Ni tapes for processing of YBCO coated conductors. CeO/sub 2//YSZ/CeO/sub 2/ buffer layer structure has been demonstrated by vacuum techniques, but first time textured CeO/sub 2//YSZ/CeO/sub 2/ structure were grown by sol-gel on biaxially textured Ni tape. The buffer layer structure promoted c-axis oriented sol-gel YBCO films and prevented oxidation of nickel during YBCO processing. After each layer was coated, the layer was annealed. CeO/sub 2/ layers were annealed at 950 /spl deg/C for 30 min. and YSZ layers were annealed at 1150 /spl deg/C for 10 min. under 4% H/sub 2/ - Ar gas flow. Texture analysis of Ni substrates and bottom CeO/sub 2/ were done by Philips diffractometer. Sol-gel YBCO layers were coated on CeO/sub 2//YSZ/CeO/sub 2/ structure and critical current density was about 0.5 /spl times/ 10/sup 5/ A/cm/sup 2/. Microstructure of the buffer layer was investigated by Environmental Scanning Electron Microscope (ESEM).

Effects of porosity on thermal loadings of functionally graded Y2O3–ZrO2/NiCoCrAlY coatings

Abstract Plasma-sprayed thermal barrier coatings often have problems of spallation and cracking in service owing to their poor bond strength and high residual stresses. Functionally gradient coatings with a gradual composition variation from heat resistant ceramics to fracture-resistant metals are proposed to eliminate these problems. In this study, functionally gradient coatings obtained from five layers were prepared on Ni substrates from Y 2 O 3 stabilized ZrO 2 (YSZ) and NiCoCrAlY powders using an atmospheric plasma spray system. The residual stresses of the as-sprayed coatings with different graded layers and different porosities, as well as the changes of residual stresses during thermal cycling were simulated by finite element method (FEM). It was found that residual stresses decreased with an increase in porosity values of layers.

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.

NiCr coatings on stainless steel by HVOF technique

This paper demonstrates the successful application of NiCr coatings on stainless steel substrates using a high velocity oxy-fuel technique for corrosion applications. We present preliminary results of fabrication and microstructural characterization of NiCr coatings. These coatings were characterized by means of an optical microscope, image analyzer, scanning electron microscope and X-ray diffraction (XRD). A microhardness tester was used in order to determine the mechanical properties of the coating. Microstructural observations pointed out that the NiCr layers possessed porosity, oxidized, unmelted and semimelted particles, and inclusions. XRD results indicated that the outstanding phase of NiCr powder and coating was Ni. It was also found that the microhardness values decreased from the coating to substrate.

Friction and wear properties of Mo coatings on cast-iron substrates

In order to protect machining parts against wear and corrosion in automotive, aerospace, pulp and paper industries, they are coated with Mo-based materials. For these specific applications, Mo coatings were fabricated on cast-iron substrates using an atmospheric plasma-spray system and their friction and wear behavior were evaluated. The Mo coatings were subjected to sliding wear against AISI 303 counter bodies under dry and acid environments. A pin-on-plate type of apparatus was used with normal loads of 49, 89 and 129 N, and sliding speed of 1 Hz. In the steady state, it was demonstrated that the Mo-coated samples under dry conditions had slightly higher wear resistance than under acid conditions tested under a load of 129 N. Several wear failure mechanisms, such as local plastic deformation, cracks, pits, debris, grooves, scratches and tracks, were identified after the tests.