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Tuesday, June 03, 2008, Auditorium 2 4:50 PM Junior Session This session will deal with different topics, most promising young talents, both from industry and universities will report shortly about their work. This session is accompanied by a special poster exposition. All visitors of this session are allowed to elect their favorite contribution, the winner will be anounced on the awards banquet |
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Characterization of thermally sprayed near net shape oxide ceramic and cermet coatings by acoustic emission analysis
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Martin Erne* / Leibniz Universität Hannover,
Institute of Materials Science
, Germany Friedrich-Wilhelm Bach / Institute of Materials Science, Leibniz Universität Hannover, Germany Kai Möhwald/ Institute of Materials Science, Leibniz Universität Hannover, Germany Tim Bause/ Institute of Materials Science, Leibniz Universität Hannover, Germany Christian Scheer/ Institute of Materials Science, Leibniz Universität Hannover, Germany |
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Near net shape coating is being aimed at for years in order to reduce process costs concerning the required feedstock, process duration and after-treatment. Standardized tensile adhesive strength tests are not suitable for the characterization of coatings of about 100 µm thickness, as adhesion significantly outweighs cohesion since the amount of coating defects is reduced with decreasing thickness. Stressing of the coating composite by compressive forces - as it is often the case in operation - however leads to damages, which are controlled by the cracking propagation behavior within the coatings. Three- and four-point bending tests are appropriate for the simulation of these operational demands and have also been utilized in the field of thermal spraying over a longer period of time.
Thin coatings exhibit a good bending behavior and therefore hardly show catastrophic failure in the bending test. In these cases, ultrasonic signals being emitted from cracks during their propagation can be recorded and analyzed in terms of feedstock material, applied force and test duration, which can be related to coatings quality.
In addition to previous work, which focused on oxide ceramics or cermet coating composites, three coating systems (Cr2O3, WC/10Co4Cr and Cr3C2/Ni20Cr 75/25) were analyzed and compared relating to their behavior during three-point-bending tests under tensile conditions. The composites significantly showed different behavior during the tests concerning their modulus of elasticity, residual stresses and cohesion. A correlation of the recorded ultrasonic events regarding quantity, time of occurrence, duration and released energy permitted an optimization of the coatings in terms of operation behavior under tensile stress. The results of these analyzes are useful for giving an estimation of the extent by which the fracture toughness is to be increased if the coating thickness is decreased to avoid precipitate failure of the coatings in operation.
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