Q:
What are the definitions of wear categories?




A:

    ABRASION

  • Hard particles or protuberances (e.g., grind marks)
    pressed against a surface.

  • They only have to be harder than the surface receiving
    wear damage.

  • Produces cutting or shearing actions.

    EROSION

  • Loss of material from a surface to impinging fluid
    stream, or a fluid stream with solid particles

  • If fluids react with surface, erosion/corrosion
    = accelerated process.

  • Can be intentional erosion = sand blasting, grit
    blast cleaning.

  • Can be unintentional erosion = ash and flue gas exiting
    a combustor or passing over heat exchanger tubes.

  • Cavitation - imploding bubbles.

  • Low stress, high speed particles impinging on a
    surface create wear chips.

  • Can be minimized by increasing hardness of the
    surface through a wear resistant coating.

    ADHESION

  • Loss of material from surface asperities in
    relative motion, started by localized bonding
    due to very high local stresses (and possibly
    temperatures
    ).

  • Transfer of material may be permanent.

  • Asperities may fracture, forming wear debris
    -- which may oxidize -- changing the wear category
    back to abrasion.

    SURFACE FATIGUE

  • Fracture of material from a surface caused by cyclic
    stresses produced by repeated rolling or sliding over
    that surface.

  • Examples of surface fatigue would be bearing tracks
    and reciprocating shafts in sleeves.