Thermal wave interferometry measurements and microstructural analysis of twin wire arc spray cylinder coatings for passenger car engines
With intent to increase the efficiency of combustion engines in passenger cars, thermally sprayed cylinder bore coatings replaced cast iron liners as cylinder running surfaces over the past years. Since the functionality of an engine depends on the life-long integrity of these coatings only a few 100 μm thick, monitoring their physical properties in series production is most important. This contribution focuses on the non-destructive testing of thermally sprayed coatings by thermal wave interferometry (TWI), investigating the relationship between thermal and microstructural properties. Microstructural characteristics such as laterally expanded splat interfaces or spherical pores correlate with macroscopic quality criteria such as adhesion, cohesion, or corrosion resistance. Further on, especially splat interfaces act as thermal barriers within the coatings. Hence, TWI thermal diffusivity measurements were compared to optical microscopy images of polished cross sections and three-dimensional X-ray computed tomography data at multiple positions in a crankcase. Both, microscopy images as well as computed tomography data were analyzed quantitatively by image processing algorithms. Due to this, local variations in thermal diffusivity were found, which revealed a significant correlation with microstructural characteristics. Further on, a significant influence of the amount, size, and shape of microstructural volume defects on the local thermal diffusivity of thermally sprayed coatings is carried out in this contribution.