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Coatings for High-Temperature Structural Materials Trends and Opportunities (1996) / Chapter Skim
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6 REFURBISHMENT OF COATED STRUCTURE
Pages 39-42

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From page 39...
... FACTORS AFFECTING COMPONENT LIFE For all gas-turbine applications, the minimum total service life as well as the life until an inspection interval must be determined by engineering analyses, component testing, and any justifiable combination of verifiable methods. Determin ing life expectancy often requires complex analysis of independent and interacting factors relating, not only to operating conditions but also to the reliability of the system and the quality of the finished components.
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From page 40...
... The main disadvantage of the acid baths is that they contain toxic chemicals that must be carefully handled and disposed; manual belting is costly, labor-intensive, and difficult to control precisely. The repair procedure must remove all of the coating and deteriorated base metal; incomplete removal can seriously shorten the service life of the new coating.3 Heat-tinting procedures work reasonably well for detecting remnants of the old coating.
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From page 41...
... Recently, this goal has been extended to include: · assessing the after-fabrication condition of the coating, system · aiding the characterization of advanced materials · developing process control methods to minimize the variation in quality of a manufactured part · ensuring safe operation by assessing the in-service condition of coated components and remaining life · evaluating the components after repair All NDE methods use some external source to produce a measurable response from the sample without causing a permanent change in the specimen. The information extracted by the measurement is determined both by the initial source/specimen interaction and the detection method.
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From page 42...
... Metallography, grains, surface breaking cracks Spectroscopy Wavelength variations in reflection/transmission Contaminants, oxide layers, composition Ellipsome~y Polanzabon sate of reflected light Thickness of surface films dielectric properties Infrared Reflected/transmitted light Surface/bulk characterization Imaging spectroscopy Spatial variations of reflection/tranmnission wavelength Spallation of TBC; structure of metallics; dependence of optical properties composition of coating; impurity type Emission spectroscopy Wavelength dependence of emission from heated materials Eminance versus wavelength for TBC and other ceramics X-RayTransrnitted Energy Attenuation/diffraction in bulk Radiography Integrated attenuation along path Diffraction Unit cell parameters, long-range lattice order Topography Single lattice reflection, spatial variations of lattice order Microwave Imaging Spatial changes in reflectivity of coating and substrate Voids; thickness changes in metal wall Plastic deformation; residual strain Spatial estimate of crystallinity of single-crystal blades/vanes TBC contaminants, coating thickness, substrate conductivity Spectroscopy Wavelength dependence of optical properties TBC coating thickness, rotational relaxation times Electron Source Characteristic X-rays Composition thickness Backscatter Energy selective detection of fluorescing X-ray photons Elemental composition, some lateral resolution, layer thickness Electromagnetic Eddy current Photoinductive imaging Magneto-optic imaging Magnetic fields from induced/injected currents TBC coating thickness; lateral resolution - 250 An Lateral resolution - 10 ,um; spatial variations in magnetic field of induced currents as modified by cracks, holes, etc. Thermal with Particle or Bulk and surface properties affected by temperature Electromagnetic Source Imaging Spatial vanahons temperature < 10 mK, e~russivin,r > 1 In Spectroscopy b(1)
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Key Terms

  • service life
  • aircraft engine
  • industrial gas
  • nde methods
  • base metal

  • nondestructive evaluation
  • spatial variations
  • gas turbines
  • engine components
  • lateral resolution

  • wavelength dependence
  • recoating operations
  • coatings applied
  • component life
  • overlay coatings

  • turbine blades
  • lattice order
  • aircraft turbine
  • original coating
  • acid baths

  • fuel costs
  • tbc coating
  • braze repairs
  • maintenance costs
  • standard designation

  • hot corrosion
  • optical properties
  • manual belting
  • repair procedures
  • life expectancy


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