Scanning acoustic microscopy characterization of cold sprayed coatings deposited on grooved substrates

TL;DR

This study uses scanning acoustic microscopy to evaluate the microstructural quality of cold sprayed Fe coatings on grooved Al substrates, revealing hidden heterogeneities and imperfections not visible with SEM.

Contribution

It demonstrates the effectiveness of SAM in detecting microstructural heterogeneities and coating imperfections on non-planar substrates, providing insights beyond SEM observations.

Findings

SAM detects larger coating imperfections around grooves than SEM.

Regions appearing homogeneous in SEM show microstructural heterogeneity in SAM.

Coating microstructure varies with the number of nozzle passes.

Abstract

The effect of non-planar substrate surface on homogeneity and quality of cold sprayed (CS) deposits was studied by scanning acoustic microscopy (SAM). Fe coatings were cold sprayed onto Al substrates containing artificially introduced grooves of square- and trapezoid-shaped geometries, with flat or cylindrical bottoms. The Al substrates were either wrought or cold sprayed, to comprehend their prospective influence on the Fe coatings build-up. SAM was then used to assess morphological properties of the materials from the cross-view and top-view directions. The microstructure below the surface of the studied samples was visualized by measuring the amplitudes of the reflection echoes and the velocity of the ultrasonic waves. The SAM analysis revealed that the regions of coating imperfections around the grooves are larger than what is suggested by standard scanning electron microscopy (SEM) observations. Furthermore, we found that the seemingly non-influenced coating regions that appear perfectly homogeneous and dense in SEM do, in fact, possess heterogeneous microstructure associated with the individual CS nozzle passes.