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| Abstract No.: |
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Scheduled at:
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Wednesday, May 05, 2010, Moor Room 2:40 PM
Properties 3
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| Title: |
Characterization of cohesion of selected HVOF coatings by scratch testing on cross section
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| Authors: |
Jiri Nohava* / CSM Instruments SA, Switzerland
Sarka Houdkova / Skoda Vyzkum, Czech Republic
Radek Enzl/ Flame Spray Technologies, Netherlands
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| Abstract: |
Cohesion and adhesion testing of thermal sprayed coatings has always been challenging task. Apart from tensile testing also shear testing was proposed but none of these methods has gained wide popularity. This paper focuses on the possibilities of evaluation of cohesion and adhesion of HVOF coatings by scratch testing. Selected HVOF coatings (Cr3C2-NiCr, WC-Co, (Ti, Mo)(C,N)-NiCo) were embedded all in the same resin, cross-sectioned and polished. The scratch test with constant load was done on the cross-section in the direction from the substrate through the coating into the resin. Five different constant loads were used for the tests. During the scratch a cone fracture occurs in the coating where the indenter is leaving the coating. Several results can be obtained from this test: depth and width of the cone, cone angle and projected cone area. Among these ones the projected area of the extracted cone (triangle) was found to be the only factor with monotonic relation to the load used during the scratch. The results of the scratch test were then evaluated using a plot of extracted cone area versus scratching load. It was found that all the coatings behaved in a similar manner at low loads while at higher loads the WC-Co showed the smallest extracted cone area and (Ti,Mo)(C,N)-NiCo showed the largest extracted cone area. Further tests performed on similar coatings deposited by different HVOF system showed also that at high loads interfacial (substrate-coating) cracks can be observed. These cracks indicate that the adhesion of the coating is rather poor and their length can be used for characterization of adhesion in such case. The constant load scratch results showed also good agreement with erosion tests performed on the same coatings and can therefore be used for fast and effective evaluation of cohesion and erosion resistance.
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