You are here

Interface Inspection Method for Thermal Spray Coatings



OBJECTIVE: Develop a production-friendly non-destructive evaluation technique to inspect the interface between a thermal spray coating and the gas turbine component. 

DESCRIPTION: Surface preparation prior to thermal spray coating application is critical to the coating’s adhesion. Dirt, oil, and grit contamination on the surface of the turbine part can lead to coating failure and engine failure. Standard practice is to clean the part with alcohol to remove dirt and oil, then grit blast the part to remove oxide scale and roughen the surface (Ref 1.), then apply the thermal spray coating, then visually inspect the coating for cracks and flaking. Only large defects near the edge of the coating can be detected by visual inspection. Often a witness coupon is also blasted and sprayed with the part to evaluate the interface and microstructure of the coating. However, this flat coupon has not been exposed to the dirt and oils that the turbine part has, so it is not an adequate representation of the part. A non-destructive evaluation (NDE) technique is needed to inspect the interface between the thermal spray coating and the engine part itself following the application of the coating. NDE techniques such as Infrared Flash Thermography or Eddy Current may be suitable if tailored specifically for this application. (Ref 2, Ref 3.) Innovation is encouraged, so any NDE method that can ultimately prove successful is acceptable. This solicitation requests the design, construction, delivery, and demonstration of a production-friendly, lightweight, reliable, maintainable NDE solution to detect surface and subsurface defects such as coating delamination and excessive grit contamination. The evaluation surface area must be no smaller than .25”x.25”, but a preferable evaluation surface area is 2”x2”. The defect detection may either be an image or quantitative. A capability must be demonstrated for metallic, ceramic, and dual-layer coatings of thickness ranging between .003-.030”. A capability must be demonstrated on both flat and irregular (knife-edge) geometries. Routine setup, usage, and evaluation must be achievable in 15 minutes or less. This capability can be demonstrated at Tinker AFB with guidance from 76 Propulsion Maintenance Group engineers. The thermal spray process is used extensively to apply protective hard or wear-resistant coatings on a variety of parts in commercial markets to include but not limited to, automotive and civilian aerospace both new and for repair, as well as in military application for new and for repair throughout the DoD. An NDE approach to assure quality control has dual-use applicability. 

PHASE I: Design and construct a laboratory-scale concept for the interface evaluation of both metallic and ceramic coatings. Small coupons with varying geometries and defects will be provided by 76 PMXG. Typical defect size range is .001-.003” for grit contamination and .005-.020” for coating separation. The method should be able to estimate the size of the defect to within 25% of actual, to be verified using destructive characterization. 

PHASE II: Design and construct a full-scale NDE system based upon the lessons learned in Phase I. Develop necessary software and demonstrate the system at Tinker AFB on as-sprayed engine parts. 

PHASE III: Make necessary modifications to full-scale NDE system to make it production-friendly, fast, flexible, and reliable based upon lessons learned in Phase II. Demonstrate Probability of Detection (POD) according MIL-HDBK-1823A. Work with 76 PMXG to develop pass/fail criteria using this NDE system based upon correlated results from tensile testing and metallography. 


1: Tucker, Robert C. "Thermal Spray Coatings" ASM International, Volume 5, 1994

2:  Infrared Contrast Analysis Technique for Flash Thermography Nondestructive Evaluation, NASA Johnson Space Center, 2014

3:  Nondestructive Evaluation of Thermal Spray Coating Interface Quality by Eddy Current Method, DARPA, 2006

KEYWORDS: Thermal Spray, Plasma Spray, Coating, Non-destructive Inspection, Non-destructive Evaluation, Surface Preparation, Aerospace, Gas Turbine 


Siamack Mazdiyasni 

(937) 656-9100 

US Flag An Official Website of the United States Government