Discussion Topics and Threads on Thermal Spray
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Discussion Topics and Threads on Thermal Spray Compiled and edited by Dr. R.S. Lima, National Research Council of Canada (NRC). These questions and answers were extracted from the e-mail discussion group of the Thermal Spray Society of ASM International. The content has been edited for form and content. Note that the comments have not been reviewed. To sign up to the e-mail discussion group, previous and new subscribers will have to follow the instructions listed below: To subscribe: 1. 2. 3. 4. 5.
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chrome being porous. The idea was that we would replace the hard chrome with an HVOF-applied tungsten/cobalt/ chrome coating. We tried two pistons, and they have been in service for 1 year. When they were removed for evaluation, we noticed a coating failure down the very front of the legs. The failure is in small ‘‘spots’’ approximately 3 mm across, but all appear to have a ‘‘crack’’ perpendicular to the axis of the piston. Is it possible that the ‘‘cracks’’ were the result of stress in the coating? These planes normally operate out of grass runways that are no better than an ordinary paddock. Therefore, it must place tremendous loads on the pistons when the plane is taking off fully loaded, or normally overloaded. Has anyone experienced this issue? I believe this situation occurs when the coating is more rigid than the hard chrome.
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Answer 1.1: It is not a secret that HVOF WC coatings are inferior to chrome for crack resistance (fatigue, bending, etc.). There are many reasons for it, but density and hardness are evidently of the greatest importance. When we learned how to vary hardness/ density of our HVOF-sprayed WC coatings (new hardware, technology developments, use of different powders, etc.), we learned that harder is not always better. Now we analyze working conditions of the coating before making a choice of how ‘‘hard’’ it should be. The failure of your coating does not mean that HVOF cannot be used in this application.
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Question 1 edited WC-based coatings versus hard chrome. We we
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