Stresses developed within a thermal barrier coating (TBC) due to the mismatch in thermal expansion of different coating components can cause coating failure. Nanostructured materials have an increased volume fraction of grain boundaries and this microstructural attribute may allow coatings to relieve the strain in the coating structure thereby improving the effectiveness and the lifetime of the TBC. Multi – layered TBCs were prepared using two techniques; atmospheric pressure plasma spray (APS) using a commercial system, and reduced pressure plasma spray using the Triple Torch Plasma Reactor (TTPR). The coatings were deposited on mullite and on NiCrAlY-coated steel substrates, and consisted of an inter – layer of nano-phase partially stabilized zirconia (n – PSZ) and a layer of conventional partially yttria stabilized zirconia coating (c – YSZ) as the top thermal barrier coat. The coatings were heat treated at various temperatures and the microstructural changes analyzed using scanning electron microscopy (SEM) images. Mechanical properties of the coating were studied using four point bend testing to better understand the effect of the n-PSZ inter-layer on the strain relief mechanisms that may be operative within the TBC.

Applying an environmental barrier coating (EBC) and a thermal barrier coating (TBC) on the next generation gas turbine structural materials such as silicon carbide matrix composites will lead to large stresses due to thermal expansion mismatch; thereby limiting the coating's effectiveness and lifetime. Nanostructured materials possess a large volume fraction of grain boundaries and are conjectured to partially relieve the strain in the coating structure. A Triple Torch Plasma Reactor (TTPR) was used to spray multi-layered TBCs consisting of a mullite EBC deposited either on a silicon carbide or a mullite substrate, a nano-phase partially stabilized zirconia coating (n- PSZ), and a yttria stabilized zirconia coating (YSZ) as the TBC. The nanostructure of the n-PSZ could be maintained during the deposition process. The coatings were heat treated at 1300°C and the change in microstructure and mechanical properties were analyzed using scanning electron microscopy (SEM), micro-indentation and scratch testing applied to the coating cross section. While a change in the microstructure was observed, in particular grain growth, the hardness and elastic modulus appeared to be little affected by the heat treatment giving a preliminary validation of the multilayer concept.