Nondestructive measurement of porosity in thermal barrier coatings

Porosity is an integral part of thermal barrier coatings (TBC) and is required to provide thermal insulation and to accommodate operational thermal stresses. The effective use of TBC in hot-section components of aircraft engines requires nondestructive testing (NDT) methods to detect porosity variations and measure thickness changes to reduce the risk of damage to the coating due to such variations. The eddy current method has been used to measure the thickness of a plasma-sprayed TBC coating and either ultrasonic or capacitance techniques have been applied to assess porosity content based on thickness values obtained using the eddy current tests. The porosity values estimated by the NDT methods have been confirmed by destructive testing, which included metallography and vacuum volumetric measurement using nitrogen absorption.