fretting
Posted: Wed Nov 22, 2023 12:58 pm
The fan blades were clamped to aluminium support columns. The vibration causes fretting between the clamped surfaces, which quickly cause failure in the fan. I have attached a micrograph of the fretting of the clamped fan surface after 24 hours of operation. Six months of research was required to solve this problem. I soon realized that an interface material is required between the fan blade and the support. I tested pure aluminium, tin, zinc, copper, chromium, lead, superelastic nitinol, brass, titanium, nickel, silver, gold, molybdenum disulfide, and boron nitride. The final two materials in this list are solid lubricants and were excellent at preventing fretting, but are mobile, so they cannot be guaranteed to maintain the separation between the steel and the support under a continuous vibrational load. Copper and silver represented an improvement compared to aluminium, but still damaged the steel surface. Gold proved to yield excellent anti-fretting behaviour, due to its unreactive nature. However, gold is prohibitively expensive and is soft, so there is a risk that it will creep in high-stress locations.
The most suitable material that eliminated fretting was found to be a 100 micron thick sheet of pyrolytic graphite sourced from Panasonic. These sheets are flexible, soft, and compliant and prevented any sign of fretting for the duration of our testing. I speculate that the sheet prevents chemical reactions at the metal surface, as well as lubricates and that the anisotropic graphite structure resists microscopic horizontal movements in the plane of the sheet. Commercially available pyrolytic graphite sheets are predominantly used for their good heat transfer properties. However, i am unaware of this material being used to prevent fretting, so my findings might have value beyond cantilever fans. I am not an expert in tribology, so was reluctant to attempt to publish these findings in a tribology journal. It would be satisfying if others could pursue this research.
The most suitable material that eliminated fretting was found to be a 100 micron thick sheet of pyrolytic graphite sourced from Panasonic. These sheets are flexible, soft, and compliant and prevented any sign of fretting for the duration of our testing. I speculate that the sheet prevents chemical reactions at the metal surface, as well as lubricates and that the anisotropic graphite structure resists microscopic horizontal movements in the plane of the sheet. Commercially available pyrolytic graphite sheets are predominantly used for their good heat transfer properties. However, i am unaware of this material being used to prevent fretting, so my findings might have value beyond cantilever fans. I am not an expert in tribology, so was reluctant to attempt to publish these findings in a tribology journal. It would be satisfying if others could pursue this research.