Figure 2-28.—HP turbine blade impact damage.life of an LM2500 GTE. The combined effect of filmcooling and protective coating will extend the servicelife. Coatings are thinly and uniformly applied by avacuum film deposition process. Coatings do notusually cause problems by chipping, peeling, or flaking.The normal failure mode is usually by pitting, rub off,or nicks and scratches. Occasionally a bubble will occurin the surface coating during the coating process. If abubble occurs, it will be tested at the coating facility toensure that it cannot be rubbed off the surface. Thesebubble imperfections pose no problem to the engine. Ifthe bubble area of the coating fails, you should monitorthat area to determine any further deterioration.Development and testing of new coatings that are highlyresistant to corrosion and erosion are in progress. Thepresent blade coating for single shank HP turbines isdesignated BC23. However, twin shank HP turbineFigure 2-30.—HP turbine rotor stage 1 blade—areas of severecorrosion after extensive operating time.blading presently have blade coating BC21. As theybecome serviceable by an area Naval Aviation Depot(NADEP) these blades will be replaced with bladescoated with BC23. Use of these newer blade coatingscan significantly extend blade service life.HP TURBINE BLADE FAILURE MODES.—Failures that you may observe during a borescopeinspection include the following types:Corrosion of the coating. This appears as pittingof the coating primarily in the 80-percent spanmidchord region of the concave airfoil(thumbprint) side and the 20-percent spanmidchord region (root print) (fig. 2-30). Thiscorrosion/erosion has not been found on bladescoated with BC23.Figure 2-29.—HP turbine leading edge impact damage.2-23
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