S. Ahmaniemi

Improved sealing treatments for thick thermal barrier coatings

Abstract Zirconia-based 8Y 2 O 3 –ZrO 2 , 22MgO–ZrO 2 and 25CeO 2 –2.5Y 2 O 3 –ZrO 2 thick thermal barrier coatings (1000 μm) were studied with different sealing methods for diesel engine and gas turbine applications. The aim of the sealing procedure was to improve the hot corrosion-resistance and mechanical properties of porous, thick thermal barrier coatings (TTBC). The surface of the coatings was sealed with three different methods: (1) laser glazing; (2) an aluminium phosphate sealing treatment; and (3) detonation gun spraying of a dense top coating on the TTBC. Sealant penetration into the coating and the coating microstructure were determined by scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) and optical microscopy. Coatings were characterised by X-ray diffraction (XRD), microhardness and porosity measurements. The thickness of the densified top layers in all cases was 50–400 μm. XRD analysis showed some minor phase changes and reaction products caused by the phosphate-based sealing treatment and some crystal orientation and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values.

Modified thick thermal barrier coatings: Microstructural characterization

Thick thermal barrier coatings were modified with laser glazing and phosphate based sealing treatments. Surface porosityof the sealed coatings decreased significantly in all cases. Structural analysis showed a strong preferred crystal orientation of the t 0 ZrO2 phase in direction [002] in laser-glazed 25CeO2–2.5Y2O3–ZrO2 coating. In laser-glazed 22MgO–ZrO2 coating the major phase was rhombohedral Mg2Zr5O12. In phosphate sealed 8Y2O3–ZrO2 coating the strengthening mechanism was identified as adhesive binding without chemical bonding. Coating microstructures were determined byscanning electron microscopy , energydispersive spectroscopy, transmission electron microscopy and optical microscopy. Coatings were also characterized by X-ray diffraction, microhardness and porosity. # 2003 Elsevier Ltd. All rights reserved.

Aluminum phosphate sealed alumina coating: characterization of microstructure

Abstract The microstructure of aluminum phosphate sealed plasma-sprayed alumina coating was characterized by X-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. Microstructural characterization was carried out to identify the phases of the coating and to understand better the strengthening effect of aluminum phosphate sealant in the coating. The main phases in the coating are metastable γ-Al 2 O 3 and stable α-Al 2 O 3 . The overall structure of the coating is lamellar with columnar γ-Al 2 O 3 grains. The aluminum phosphate sealant shows good penetration into the coating to the depth of about 300 μm filling the structural defects such as pores, cracks and gaps between the lamellae. The sealant in the coating has the relative composition of 26 at.% aluminum and 74 at.% phosphorus giving the molar ratio P:Al of 3, which refers to the metaphosphates Al(PO 3 ) 3 . There is also some crystalline aluminum phosphate in the coating, in the form of berlinite-type orthophosphate AlPO 4 , owing to the reaction between the sealant and the alumina coating. Thus, the phosphate bonding in the alumina coating is based both on chemical bonding resulting from the chemical reaction with the alumina coating and on adhesive binding resulting from the formation of the condensed phosphates in the structural defects of the coating.

Sealing procedures for thick thermal barrier coatings

Zirconia-based 8Y2O3-ZrO2 and 22MgO-ZrO2 thick thermal barrier coatings (TTBC, 1000 µm), were studied with different sealing methods for diesel engine applications. The aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of TTBCs was sealed with three different methods: (1) impregnation with phosphate-based sealant, (2) surface melting by laser glazing, and (3) spraying of dense top coating with a detonation gun. The thicknesses of the densified top layers were 50–400 µm, depending on the sealing procedure. X-ray diffraction (XRD) analysis showed some minor phase changes and reaction products caused by phosphate-based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na2SO4-40V2O5 deposit was determined in isothermal exposure at 650 °C for 200 h. Corrosion products and phase changes were studied with XRD after the test. A short-term engine test was performed for the reference coatings (8Y2O3-ZrO2 and 22MgO-ZrO2) and for the phosphate-sealed coatings. Engine tests, duration of 3 h, were performed at the maximum load of the engine and were intended to evaluate the thermal cycling resistance of the sealed coatings. All of the coatings passed the engine test, but some vertical cracks were detected in the phosphate-sealed coatings.

Corrosion Behavior of HVOF-Sprayed and Nd-YAG Laser-Remelted High-Chromium, Nickel-Chromium Coatings

Thermal spray processes are widely used to deposit high-chromium, nickel-chromium coatings to improve high temperature oxidation and corrosion behavior. However, despite the efforts made to improve the present spraying techniques, such as high-velocity oxyfuel (HVOF) and plasma spraying, these coatings may still exhibit certain defects, such as unmelted particles, oxide layers at splat boundaries, porosity, and cracks, which are detrimental to corrosion performance in severe operating conditions. Because of the process temperature, only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium, nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3% Cr, 42.5% Ni, 2.5% Mo, 0.5% B). The coatings were prepared by HVOF spraying onto mild steel substrates. A high-power, fiber-coupled, continuous-wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF-sprayed coating using different levels of scanning speed and beam width (10 or 20 mm). Coating that was remelted with the highest traverse speed suffered from cracking because of the rapid solidification inherent to laser processing. However, after the appropriate laser parameters were chosen, nonporous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of a dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as sprayed and after laser remelting were characterized by optical and electron microscopy (OM, SEM, respectively). Dilution between coating and substrate was studied with energy dispersive spectrometry (EDS). The properties of the laser-remelted coatings were directly compared with properties of as-sprayed HVOF coatings.

Residual stresses in aluminium phosphate sealed plasma sprayed oxide coatings and their effect on abrasive wear

Effect of residual stresses on plasma sprayed alumina and chromia coatings sealed with aluminium phosphate were studied as a function of the temperature of the sealing treatment. Stresses were measured by X-ray stress analysis and high-speed circular microhole drilling method. Residual stress states were correlated with other coating properties such as microhardness, porosity, microstructure and dry abrasion wear resistance. Correlations were found between sealing treatment temperature, residual stress state and wear resistance. Wear resistance of the oxide coatings was increased at all sealing temperatures. Sealing treatment affected coatings by two mechanisms. Aluminium phosphate sealing induced compressive stresses to coatings and simultaneously bonded coating lamellar structure.

Characterization of modified thick thermal barrier coatings

In gas turbines and diesel engines, there is a demand for thick thermal barrier coatings (TTBCs) due to the increased process combustion temperatures. Unfortunately, the increased thickness of plasma-sprayed thermal barrier coatings (TBCs) normally leads to a reduced coating lifetime. For that reason, the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally on elastic modulus reduction of the thick coating to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot-corrosion resistance or better mechanical properties are needed. In this article, several modified zirconia-based TTBC structures with specific microstructural properties are discussed. Coating surface sealing procedures such as phosphate sealing, laser glazing, and sol-gel impregnation were studied as potential methods for increasing the hot-corrosion and erosion resistance of TTBCs. Some microstructural modifications also were made by introducing segmentation cracks into the coating structures by laser glazing and by using special spraying parameters. These last two methods were studied to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, a scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The effect of sealing procedures on the basic thermal and mechanical properties of the coatings was studied. In addition, some correlations between the coating properties and microstructures are also presented, and the advantages and drawbacks of each modification procedure are discussed.

Wet abrasion and slurry erosion resistance of thermally sprayed oxide coatings

Abstract Wet abrasion and slurry erosion resistance of alumina and chromia coatings were studied and compared to their dry abrasion resistance. Bulk oxide ceramics and base metals were used for comparison. The coatings were produced by atmospherical plasma spraying. The effect of aluminum phosphate sealing treatment on the wear and corrosion behaviour of the coatings were also studied. In wet abrasion tests kaolin–water and silica–water mixtures were used as the abrasive media. Slurry erosion tests were performed using a slurry pot tester. Quartz sand, alumina and alumina–zirconia in water with various pH values were used as the erosive medium. Dry abrasion tests were performed using a rubber wheel abrasion equipment and quartz abrasive. The coatings and bulk ceramics were characterized for their phase structure, porosity and hardness. The results from wear tests are reported and correlated with coating and bulk ceramic properties. The influence of coating post-treatment to wear behaviour is also presented and discussed.

Microstructural study of aluminum phosphate-sealed, plasma-sprayed chromium oxide coating

Microstructural characterization of aluminum phosphate-sealed, plasma-sprayed chromium oxide coating was carried out in order to study the strengthening mechanisms of the aluminum phosphate sealant in the coating. Characterization was performed using x-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. The structure of the sealed coating was lamellar with columnar α-Cr2O3 grains extending through the lamella thickness. Amorphous aluminum phosphate sealant had penetrated into the structural defects of the coating such as cracks, gaps, and pores between the lamellae. The relative composition was 25 at.% aluminum and 75 at.% phosphorus for the sealant in the coating, giving the molar ratio P/Al of 3, which corresponds to that of metaphosphates Al(PO3)3. There is no indication of reaction products from the chemical reactions between the sealant and the coating. Thus, the aluminum phosphate sealing in the chromium oxide coatings can be explained mainly by adhesive binding resulting from the formation of the condensed phosphates with the appropriate adhesive properties to the coating, and not by chemical bonding resulting from the chemical reactions between the sealant and the coating.