HP Turbine Blade Failure Modes

Figure 2-28.—HP turbine blade impact damage. life of an LM2500 GTE. The combined effect of film cooling and protective coating will extend the service life. Coatings are thinly and uniformly applied by a vacuum film deposition process. Coatings do not usually 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 occur in the surface coating during the coating process. If a bubble occurs, it will be tested at the coating facility to ensure that it cannot be rubbed off the surface. These bubble imperfections pose no problem to the engine. If the bubble area of the coating fails, you should monitor that area to determine any further deterioration. Development and testing of new coatings that are highly resistant to corrosion and erosion are in progress. The present blade coating for single shank HP turbines is designated BC23. However, twin shank HP turbine Figure 2-30.—HP turbine rotor stage 1 blade—areas of severe corrosion after extensive operating time. blading presently have blade coating BC21. As they become serviceable by an area Naval Aviation Depot (NADEP) these blades will be replaced with blades coated with BC23. Use of these newer blade coatings can significantly extend blade service life. HP TURBINE BLADE FAILURE MODES.— Failures that you may observe during a borescope inspection include the following types: Corrosion of the coating. This appears as pitting of the coating primarily in the 80-percent span midchord region of the concave airfoil (thumbprint) side and the 20-percent span midchord region (root print) (fig. 2-30). This corrosion/erosion has not been found on blades coated with BC23. Figure 2-29.—HP turbine leading edge impact damage. 2-23