The Effect of Heat Treatment on the Microstructure and Properties of Detached SPS and APS Thermal Barrier Coatings

Partially Stabilised Zirconia (PSZ) coatings were produced by Suspension Plasma Spraying (SPS) and Air Plasma Spraying (APS) onto steel substrates, from which they were subsequently detached.  It was noted that the SPS coatings incorporated segmentation cracks and a fine globular pore architecture, while the APS coatings exhibited conventional overlapping splat microstructures.  Specimens were heat treated for up to 100 h at 1225¢ªC, with periodic monitoring of the pore architecture (using mercury porosimetry and BET surface area analysis), the through-thickness thermal conductivity (using laser flash and a steady state method) and the in-plane Young’s modulus (using four-point bending and an impulse excitation method).  The effect of periodic quenching to room temperature was also studied.  It was observed that the SPS coatings were more resistant to sintering than those produced by APS.  This was reflected in slower changes in both the microstructural parameters and the measured properties.  This effect is correlated with the different pre architectures in the two types of coating, particularly the more open structure of the SPS, leading to longer diffusion paths.  It is also noted that the segmentation cracks probably have an effect on the sintering behaviour.  Nanoindentation was employed to measure the (through-thickness) Young’s modulus of the regions between these cracks and these results were consistent with microstructural observations and the other property measurements.  Conclusions are drawn about strategies for optimising the thermo-mechanical stability of SPS coatings.