Thermal wave interferometry measurements correlated to microstructural properties and bonding behavior of thermally sprayed coatings

Abstract

In order to guarantee the life-long functionality of modern combustion engines, the bond strength of thermally sprayed cylinder coatings is one crucial property monitored during the large-scale production of modern power trains. However, since the bond strength between cylinder coating and crankcase substrate can only be measured by destructive off-site testing, a fast and reliable non-destructive method characterizing the bonding behavior of cylinder coatings is highly desired. As the presence of defects in the microstructure lowers the bond strength of cylinder coatings, assessing the defect morphology by non-destructive thermal diffusivity measurements shows promising correlation between thermal and mechanical behavior. Laser-excited lock-in thermography is used to apply thermal wave interferometry (TWI) measurements on wire arc sprayed cylinder coatings of passenger car engines. Measured thermal diffusivity values of these coatings show significant variations along the investigated liners. Further, destructive bonding testing as well as microstructural analysis of the investigated cylinder coatings are acquired to evaluate mechanical and microstructural properties. Investigation of the relationship between thermal diffusivity, bonding behavior and microstructure disclose major correlations between the observed quantities. The application of thermal diffusivity measurements as a nondestructive testing method to evaluate the bond strength of thermally sprayed cylinder coatings is discussed in this work.