Infrared Methods for In-Process Inspection of Thermal Spray Coatings

Thermal spray coatings are widely used to rebuild worn parts to original dimensions, and to restore or modify surface properties. Twin wire arc spray (TWA) is among the most common methods used to spray metallic coatings. In TWA, an electric arc is used to melt a metal feedstock, while pressurized gas accelerates the molten droplets toward the surface to be coated. The coating performance is dependent upon the presence of intrinsic (pores, oxide) and extrinsic (damage, contamination) defects that can occur during the coating process. Unfortunately, there are currently no practical, in-process, nondestructive methods to inspect for these defects. Prior attempts to develop in-process inspection methods have been hampered by the complexity of the microstructure and its nonlinear effect on performance. Therefore, the present industry standard is off-line destructive testing of witness coupons coated in-line with parts. This approach is slow to provide results, and it cannot identify localized defects in the coating, which creates a need for in-line, real-time, inspection that can provide 100% coverage, and that can be correlated to the coating performance.

Two infrared (IR) methods, flash thermography and thermal wave imaging, were evaluated for detection of porosity and disbonds in TWA coatings. Test samples were flat coupons of grey iron coated with 12 or 24 layers (1mm or 2mm total thickness) of AISI420 stainless steel. Porosity was introduced by spraying graphite powder between the layers, and disbonds by locally oxidizing a single layer of coating near the mid-depth. The coatings were inspected in the as-sprayed condition, without removing the inherent surface roughness. The data will show that both IR methods detect the defects. Strategies for data analysis of the thermal response, and correlation to the adhesive strength of the coating will be explored. Finally, the merits and shortcomings of the two methods will be discussed.