DOE OSTI2018
This paper describes a technology review of non-destructive examination (NDE) methods that can be applied to cold spray coatings. Cold spray is a process for depositing metal powder at high velocity so that it bonds to the substrate metal without significant heating that would be likely to cause additional residual tensile stresses. Coatings in the range from millimeters to centimeters are possible at relatively high deposition rates. Cold spray coatings that may be used for hydroelectric components that are subject to erosion, corrosion, wear, and cavitation damage are of interest. The topic of cold spray NDE is treated generally however and may be considered applicable to virtually any cold spray application except where there are constraints of the hydroelectric component application that bear special consideration. Hydroelectric turbine components are primarily made from low carbon and austenitic stainless steels. They are typically very large components that are partially assembled on-site. Transport of these components within the hydroelectric facility and away from the hydroelectric facility is challenging due to the component’s size and weight, high cost of disassembly, and the confined space of the dam. One method for prolonging component life is application of cold spray and thermal powder metal coatings to the components most susceptible to erosion, corrosion, cracking, and mechanical wear. Process controls and visual inspections have historically been applied to cold spray application; however, quantitative NDE could detect anomalies not readily apparent from such a visual examination. Optical profilometry, Eddy current, ultrasound, and hardness tests are shown for one set of good, fair, and poor Nickle-Chrome (NiCr) on 304 stainless steel (304SS) cold spray samples to demonstrate inspection possibilities. The primary indicator of cold spray quality is the cold spray porosity that is most directly measured with witness-sample destructive examinations (DE) – mostly photo-micrographs. These DE generated porosity values are correlated with optical profilometry, eddy current, ultrasound, and hardness test NDE methods to infer the porosity and other Information of interest. These parameters of interest primarily include: • Porosity primarily caused by improper process conditions (temperature, gas velocity, spray standoff, spray angle, powder size, condition, surface cleanliness, surface oxide, etc.). • Presence/absence of the cold spray coating including possible over-sprayed voids • Coating thickness Optical profilometry measurements of surface roughness trended with porosity plus if compared with a reference measurement or reference drawing would provide information on the coating thickness. Ultrasound could provide similar surface profile information plus attenuation measurements trended with porosity. The ultrasound measurements however may be limited to geometries where the substrate back-wall is normal to the cold spray surface and not too thick. Eddy current showed a strong correlation with porosity. Eddy currents can also be sensitive to cracks and do not need fluid coupling to make measurements. Eddy currents however are not sensitive to coating thicknesses in most cases. Vickers hardness measurements also tracked well with porosity. These types of hardness measurements however are also not sensitive to coating thickness. An NDE program may include multiple measurements. Acknowledgement: This work is sponsored by Bonneville Power Administration (BPA) under their technology innovation project program. Such support does not constitute BPA endorsement of views expressed in this work.