The effects of the bond coat structure on the microstructure evolution and lifetime performance of thermal barrier coatings (TBCs) were investigated through the cyclic thermal fatigue (CTE) and thermal-shock (TS) tests. The single layer and the first layer in the layered bond coat were prepared by high velocity oxy-fuel (HVOF) spray process using nickel-based feedstock. The second layer in the layered bond coat and the top coat were formed by air plasma spray (APS) process using nickel-based metallic feedstock and 8 wt % yttria-stabilized zirconia, respectively. The CTF tests were performed till 872 cycles with a dwell time of 60 min at a surface temperature of 1100°C and a bottom temperature of 950°C. Also, the TS tests were conducted until 300 cycles with a dwell time of 60 min at 1100°C. After the CTF and TS tests, the TBC system with the layered bond coat showed a better thermal durability than that with the single layer. The hardness value of the bond coat by HOVF process was dramatically decreased after the both tests, without much change in the bond coat by APS process. The diffusion trends of elements were similar with each other, less dependent on the bond coat structure, increasing amounts of cobalt and aluminum and decreasing those of nickel and tungsten. The microstructure evolution of the top coat, the growth behavior of thermally grown oxide layer, and the thermal durability were strongly affected by the thermal exposure condition and the bond coat structure.

For an engineering feasibility study, we studied a simple design to improve NCSS (nanocarbon composite strain sensor) sensitivity by using its geometric pattern at a macro scale. We fabricated bulk- and grid-type sensors with different filler content weights (wt.%) and different sensor lengths and investigated their sensitivity characteristics. We also proposed a unit gauge factor model of NCSS to find a correlation between sensor length and its sensitivity. NCSS sensitivity was improved proportional to its length incremental ratio and we were able to achieve better linear and consistent data from the grid type than the bulk type one. We conclude that the longer sensor length results in a larger change of resistance due to its piezoresistive unit summation and that sensor geometric pattern design is one of the important issues for axial load and deformation measurement.

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