Flexible Bristle-HVOF Coating Interface Wear and Tribological Oxidation Behavior at Different Temperature
Brush seal is a dynamical flexible contact structure with a tribo-pair in dynamic seal. The flexible bristle and the HVOF coating are used as the tribo-pair that effects on the seal-ability of the brush seal. In order to investigate the tribology behavior of the flexible bristle contact interface, the normal load of 2 N and the rotational speed of 900 r/min were used in the L605 bristle material friction tests by the pin-on-disc friction tester at different temperature.Scanning electron microscope is used to analyze the worn morphology of the bristle tips. Energy dispersive spectrometer and X-ray photoelectron spectroscopy were applied to analyze the chemical component of the worn surface, especially chemical states of oxygen element that could be used to character the oxidation phenomenon of the bristles tribological behavior.Two bristle tips connected in the shape of "8" could be observed on the worn bristles at 600 ℃, the hexagonal type bristle tips appeared under the condition of 400 ℃. In addition, the XPS results showed that the oxygen chemical state of the tribological interface varied as the temperature ranged from 200 ℃ to 600 ℃. Especially, as the temperature increased over 400 ℃, the O 1s binding energy peaks of worn surface existed in several different chemical states. The atom weight percentage of the oxygen elements was 34.15％, 27.48％ and 14.88％ at the temperature of 600 ℃, 400 ℃ and 200 ℃, respectively. As the temperature increased from 400 ℃ to 600 ℃, the O 1s peak was split into two different peaks which were 529.8 eV and 531.3 eV binding energy. When the environment temperature was 200 ℃, the peak site of O 1s binding energy was at 530.6 eV.As the temperature increased from 200 ℃ to 600 ℃, the friction coefficient value of the flexible bristle contact interface was declined from 0.9 to 0.5, while the variation of the friction coefficient presented strong wave. The temperature conditions could affect the steady state of the worn morphological contact interface. As the temperature increased, the flexible bristle frictional system from a third-body tribological system transferred into a two-body tribological system.The bristle cylinder contacting with the cylinder was the line contact mode and the circle usually changed into the polygonal shape after the friction tests. Specifically, the top tips were shown in the "8" shaped morphology with two bristle cylinder glued together after friction tests at 600 ℃. The flexible bristle contact interface state changed as the temperature increased and it was like that the bristle tips contacted with the flat coating sample under the flat contact condition, the tips of bristle changed from the coarse surface to the plough surface, and then to the smooth surface with the local oxidative layer spalling.The wear particles changed from the tiny nano-size particles, to common micron-size particles and then the thin slices as the temperature increased from 200 ℃ to 600 ℃.The wear mechanism of the flexible bristle frictional pair presented oxidative wear mixed with abrasive wear, and then transferred into oxidative wear with adhesion wear.
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