Non-Destructive Evaluation of Thermal Spray Coatings using Infrared Thermography
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. 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.
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