Quantification of void network architectures of suspension plasma-sprayed (SPS) yttria-stabilized zirconia (YSZ) coatings using Ultra-small-angle X-ray scattering (USAXS)

Abstract

Suspension plasma spraying (SPS) is able to process a stabilized suspension of nanometer-sized feedstock particles to form thin (from 20 to 100 μm) coatings with unique microstructures. The void (pore) network structure of these ceramic coatings is challenging to characterize and quantify using commonly used techniques due to small sizes involved. Nevertheless, the discrimination of these pores in terms of their size and shape distribution, anisotropy, specific surface area, etc., is critical for the understanding of processing, microstructure, and properties relationships. We will show that one of suitable combinations of techniques providing sufficient detail is ultra-small-angle X-ray scattering (USAXS) and helium pycnometry, combined with scanning electron microscopy (SEM). Yttria-partially stabilized zirconia (YSZ) coatings were manufactured by plasma processing of suspension of particles with average diameter of ∼50 nm. Several sets of spray parameters (plasma gas mixture, spray distance, electric arc intensity, etc.) were used to generate plasma jets with different mass enthalpies and coefficients of thermal transfer and different heat fluxes transferred to the substrate. Free-standing coatings were studied as-sprayed and annealed at 800 and 1100 °C for 10 and 100 h (non-constrained sintering). Results indicate that the SPS coatings exhibit nanosized pore microstructure: average void size was about the same size scale as the feedstock size; i.e., nanometer sizes with multimodal void size distribution. About 80% of the pores (by number) exhibited characteristic dimensions smaller than 30 nm. Total void content of as-sprayed SPS coatings varies between 13% and 20%. Most of the voids were found to be opened with only between one-tenth to one-third of voids volume being inaccessible by intrusion (not connected to either surface). During annealing, even at temperatures as low than 800 °C, the microstructure transformed: while the total void content did not change significantly, the void size distribution evolved toward larger sizes. This unique void system, together with the nanometer scale of the particulate matrix itself, gave these coatings very low apparent thermal conductivity (in the order of 0.1 W m −1 K −1), as rarefaction effect and phonon scattering mechanisms are very likely emphasized.