LEFT: These photographs were taken with an electron microscope at 500x magnification. It shows the surface of a raw unfinished “as cast” turbine blade. The rough initial surface finish as measured by profilometer was in the 75 – 90 Ra (μin) range. As is typical of most cast, ground, turned, milled, EDM and forged surfaces this surface shows a positive Rsk [Rsk – skewness – the measure of surface symmetry about the mean line of a profilometer graph. Unfinished parts usually display a heavy concentration of surface peaks above this mean line, generally considered to be an undesirable surface finish characteristic from a functional viewpoint.] RIGHT (After) This SEM photomicrograph (500X magnification) was taken after processing the same turbine blade in a multi-step procedure utilizing orbital pressure methods with both grinding and polishing free abrasive materials in sequence. The surface profile has been reduced from the original 75 – 90 Ra (μin.) to a 5-9 Ra (μin.) range. Additionally, there has been a plateauing of the surface and the resultant smoother surface manifests a negative skew (Rsk) instead of a positive skew. This type of surface is considered to be very “functional” in both the fluid and aerodynamic sense. The smooth, less turbulent flow created by this type of surface is preferred in many aerodynamic applications. Another important consideration the photomicrographs indicate is that surface and subsurface fractures seem to have been removed. Observations with backscatter emission with a scanning electron microscope (SEM) gave no indication of residual fractures. PHOTOS courtesy Jack Clark, Surface Analytics


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