Comparison of flame sprayed Al2O3/TiO2 coatings: Their microstructure, mechanical properties and tribology behavior
Introduction
Alumina–titania coatings are excellent candidates for providing protection against abrasive wear and resistance to galvanic and high temperature corrosion. Such coatings are desirable in electrical insulation and anti-wear applications where galvanic corrosion must be avoided, for example, in protective coatings for sleeve shafts, thermocouple jackets, electrical insulators, pump shafts, etc., and in any other application where it is necessary to combine high resistance to wear, a low friction coefficient and high service temperatures. These coatings are usually applied using the plasma spray (PS) process, because the high temperature of the plasma flame is considered to be necessary to melt the ceramic powder particles, whereas a lower proportion of melt particles is often found when the high-velocity oxygen fuel (HVOF) technique is used [1].
The flame spray deposition technique has a number of disadvantages compared to the PS or HVOF methods, including a bigger grain size microstructure, pore size and crack length, but it also has certain advantages, such as its being more economical, easier to handle and more adaptable to manufacturing processes with short series or recovery of pieces. Flame spray was the first thermal spray process developed (≌1910). Modern torches have changed and the high particle velocities are in the range of 200–300 m/s. Oxyacetylene torch are using acetylene as the main fuel in combination with oxygen to generate high combustion temperatures and particle temperature around 2600 °C [2].
This work investigated three ceramic coatings with different ratios of alumina–titania which were applied using the flame spray deposition technique under optimised work conditions. The properties that were determined are the hardness, indentation toughness and the resistance to abrasive wear offered by bonded SiC. The abrasive wear resistance was measured experimentally in terms of the hardness of the coatings.
Austenitic stainless steel AVESTA 253 MA, which offers excellent resistance to high temperature oxidation and good mechanical resistance, was selected as the base metal. Test samples had a sheet thickness of 2 mm and an 8 mm rod diameter; in both cases the base metal was austenitic in the tempering state (hypertempering).
Section snippets
The coatings
Abrasive particles consisting of angular corundum (500/700 μm) were blown with a compressed air pressure of 6 kg/cm2 for 1 min to prepare the surface of the base metal.
Two different powders were deposited: one consisted of Rototec 51000, which is a commercial powder made by Castolin with 95% Ni and 5% Al by weight and a grain size of 40/100 μm that operates as a coupling layer between the stainless steel base metal and the second is a ceramic finishing layer. Both layers were deposited by a
Hardness
The top surface and cross-section of all samples were polished using three sequential steps, namely 30, 6, 1 μm grade diamond lapping, before beginning each test. The hardness was measured by Vickers indentations with a 200 g load during 20 s on the top surface and cross-sections; applying higher loads made the corner indents difficult to observe due to the abundant coating porosity. Approximately 10 indentations were made for each hardness measurement and the results are shown in Table 2; the
Fracture toughness
There are several experimental methods to determine fracture toughness KIC. In this study, fracture toughness was measured by Vickers indentation method using 1 N load during 20 s; it was difficult to use higher loads due to abundant coatings porosity, in which radial cracks at the corner of the indent are well developed and reproducible. A number of semi-empirical formulas have been proposed on the basis of crack types and certain materials [15], [16], [17], [18], [19], [20]; in this work we
Abrasive wear
Abrasive wear tests were conducted on a TE79/P pin-on-disk multi-axis tribometer from Plint and Partners under ambient temperature and humidity. Ceramic-coated stainless steel pins with a diameter of 8 mm were slid under a load of 5 N against a disk consisting of SiC metallographic abrasive, Buehler 240 GRIT (100 μm), bonded to the paper.
A new abrasive disk was used for each 9 m test run in order to provide fresh abrasives over a sliding distance of 27 m. The loss of material was determined by
Conclusions
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Alumina–titania coatings deposited by flame spray have a low density.
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Three alumina–titania coatings deposited with flame spray techniques were studied and an inverse linear plot of porosity against the percentage of titania was found.
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The high capacity of the Al2O3–TiO2 system to produce different phases and oxide mixtures depends on the process variables.
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Increasing the titania content, diminishing the coating hardness, and the fracture toughness depends on diverse factors.
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The abrasive wear of
Acknowledgments
The authors gratefully acknowledge financial support provided by Caja Castellon—the Bancaja Foundation, under grants P1.B2002–28. Special thanks to Mr. José Ortega and Ms. Raquel Oliver, at the Laboratory of Engineering Materials, for their help during the experiments and also to Mr. Javier Gómez and Mr. Gabriel Perís, from the SCIC at the Universitat Jaume I in Castellón.
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