The solution precursor plasma spray (SPPS) process is a relatively new and flexible thermal spray process that can produce a wide variety of novel materials, including some with superior properties. The SPPS process involves injecting atomized droplets of a precursor solution into the plasma. The properties of resultant deposits depend on the time-temperature history of the droplets in the plasma, ranging from ultra-fine splats to unmelted crystalline particles to unpyrolized particles. By controlling the volume fraction of these three different constituents, a variety of coatings can be produced, all with a nanograin size. In this article, we will be reviewing research related to thermal barrier coatings, emphasizing the processing conditions necessary to obtain a range of microstructures and associated properties. The SPPS process produces a unique strain-tolerant, low-thermal conductivity microstructure consisting of (i) three-dimensional micrometer and nanometer pores, (ii) through-coating thickness (vertical) cracks, (iii) ultra-fine splats, and (iv) inter-pass boundaries. Both thin (0.12 mm) and thick (4 mm) coatings have been fabricated. The volume fraction of porosity can be varied from 10% to 40% while retaining the characteristic microstructure of vertical cracks and ultra-fine splats. The mechanism of vertical crack formation will be described.

A combination of two optical methods — Fourier-domain optical coherence tomography (FD-OCT) and photo-stimulated luminescence piezo-spectroscopy (PLPS) is used as a non-destructive evaluation (NDE) tool for thermal barrier coatings (TBC). This research is focused on NDE of electron beam physical vapor deposition (EB-PVD) TBC’s. FD-OCT is an interferometric technique, which uses spectrally broadband visible or infrared light to obtain spectrally resolved interferograms of the light that is back-scattered from subsurface structures and defects (e.g., interfaces, cracks, voids) in optically translucent material. When the Fourier transform is applied to the interferogram, a depth-resolved image of the back-scattering sites is obtained. FD-OCT is shown to be a useful NDE tool that can profile the top coat-metal substrate interface and measure the top coat thickness. Also, it has the potential of assessing microcracking and spallation damage. PLPS provides quantitative information on stress in the thermally grown oxide (TGO) by measuring the spectral shifts in the laser-induced luminescence spectra of the Cr3+ ions present in the TGO. When combined, the PLPS and FD-OCT methods can provide a set of important input parameters for the TBC remaining life predicting model. Ultimately they will collect spatially resolved data on matching spatial domains. The two optical methods are applied to thermally cycled EB-PVD TBC samples. The experimental results are compared to destructive inspection data.