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1342 |
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Wednesday, June 04, 2008, Room 05 4:40 PM Polymers, Environmental-Friendly Coatings, Non Destructive Testing Testing coatings directly on coated parts without destroying any additionally coated samples, polymer-based coatings and environmental-friendly coatings |
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Residual stresses in HVOF- and APS-deposited Al2O3 and Cr2O3 coatings |
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Giovanni Bolelli* / Department of Engineering "Enzo Ferrari", University of Modena and Reggio Emilia, Italy Luca Lusvarghi/ Università di Modena e Reggio Emilia, Department of Materials and Environmental Engineering, Italy Erja Turunen/ VTT Technical Research Centre of Finland, Finland Tommi Varis/ VTT Technical Research Centre of Finland, Finland Cristy Leanor Azanza-Ricardo/ Università di Trento, Department of Materials Engineering and Industrial Technologies, Italy Matteo Leoni/ Università di Trento, Department of Materials Engineering and Industrial Technologies, Italy Paolo Scardi/ Università di Trento, Department of Materials Engineering and Industrial Technologies, Italy |
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HVOF-sprayed dense ceramic coatings have been shown to provide superior mechanical and tribological properties than plasma-sprayed ones. Knowledge of coating residual stresses is also very important. Therefore, residual stresses in three HVOF-sprayed ceramic coatings, manufactured using conventional Al2O3 powder, nanostructured-Al2O3 powder and Cr2O3 powder (respectively), have been measured by X-ray diffraction, substrate chemical removal technique and analytical computation (Tsui-Clyne model). They have also been compared to residual stresses in plasma-sprayed Al2O3 and Cr2O3. The in-situ curvature technique has been employed for HVOF-sprayed Al2O3, for further verification. Both HVOF-sprayed Al2O3-base coatings possess a similar, tensile residual stress state, with a moderate through-thickness gradient. The gradient is more pronounced than in plasma-sprayed Al2O3 coating, because, in the latter, cracks, pores and weak lamella boundaries can accommodate the deformations induced by bending moments arising during coating deposition and cooling. It is also noted that higher quenching stress exists in plasma sprayed Al2O3. The model-derived quenching stress values of HVOF-sprayed alumina coatings also allow reliable analytical prediction of the measured average coating stress from in-situ curvature measurement, confirming the reliability of the above results. Cr2O3-base coatings are significantly different. They display a lower surface residual stress; the HVOF one also exhibits a very large stress gradient. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones. |