Research project PA/28 (Research action PA)
The development of reliable methods for the quantitative measurement of adhesion and of damage resistance of thick coatings on metals is an area where considerable innovation is required. The prevention of damage formation, such as through thickness cracking and interfacial debonding leading to delamination or spallation is a key aspect in the development and design of coatings. Since the performance of coatings often depends on the bond strength between coating and substrate material, it is essential to quantify the adhesive strength of a coating so as to guarantee the overall quality and reliability of the final product.
Most existing adhesion test methods however are at best semi-quantitative assessments of coating/substrate adhesion and are not standardised. A few tests, such as the direct pull test normal to the coating which makes use of adhesives, are suitable for thick coatings, but are able to measure adhesion only of weakly adhered coatings. Modern anti-wear coatings have much higher adhesion levels than can be probed by this method.
Here it is proposed to validate simple and cost effective controlled straining tests with acoustical emission and optical detection of the progressive coating cracking during the test. The ultimate goal of the project is to attain a quantitative, accessible and standardized method for the determination of damage resistance and adhesion of thick hard coatings on metals. The project is focused on laser claddings, which is an emerging coating technology, but the method will be generally applicable on other thick coatings as thermal spray or thermal barrier coatings. Laser claddings have superior adhesion due to the metallurgical nature of the binding. It is an attractive, cost effective method since it is sufficient to apply only one layer or process step to obtain the desired composition of the coating and since it can be deposited very locally even on complex components.
Examples of the successful use of laser cladding are wear resistant parts on motor components for the automotive and the aerospace. The application of laser claddings with hard surfaces leads to longer component life times and hence of saving of materials.
In view of the objective of the proposed project, several laser claddings are deposited with different adhesion, different substrates and different composition, as well as thermal spray coatings for comparison. Characterization is substantial for the correct implementation of the straining tests. The determination of the internal stress state, together with the measurement of the mechanical properties of the coating are essential. The controlled straining test itself is implemented in different ways: 4 point bending and uniaxial tensile testing. Test parameters will be varied in order to know the sensitivity of the methods. The cracking of the coating will be observed optically and with acoustical emission. Statistical treatment of the obtained data are needed as input for the adhesion and fracture toughness calculation. Finally, post mortem studies ensure the observed phenomena are correctly interpreted. The validity of different models is evaluated by theoretical considerations and finite element modeling.
The recommended procedure following from the proposed research is further validated by an interlaboratory exercise. The results is compared with other emerging test methods, at least for the qualitative adhesion level of the different coatings. The procedures for the controlled straining tests is prepared for standardization. The technical work lasts three years and is performed in five interlinked work packages.
This project falls under the standardization domain of the technological attraction poles programme, but also has links to the aerospace sector through the increasing use of thermal spray coatings and laser claddings for various components.
Controlled straining tests for the adhesion and damage resistance of laser claddings (COSTA) : final report
Brussels : Federal Science Policy, 2007 (SP1832)
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