An investigation into the deposition of nanocrystalline yttria-stabilized zirconia powders by the high velocity oxygen-fuel (HVOF) thermal spray process

Early research on thermal spray, particularly air plasma spray (APS), deposited coatings established the notion that complete particle melting must be achieved in order for viable coatings to be obtained. The necessity of complete particle melting makes the deposition of yttria-stabilized zirconia (YSZ) by high velocity oxygen fuel (HVOF) challenging because the adiabatic flame temperature is only about 315 K higher than the melting temperature of YSZ (T_m = 2923 K). When heat transfer and intra-particle heat conduction are accounted for, as previous modelers have shown, the efficiency of an HVOF flame at melting YSZ particles is low. Indeed, this research documents one of the earliest successes in YSZ coating deposition by HVOF.; This dissertation seeks to fully characterize the deposition of YSZ by HVOF by determining the mechanism for coating adhesion during deposition. Modeling of particle velocity and temperature at the time of impact is employed in order to meet this objective. Multiple regression analysis was performed for a variety of process parameters and particle conditions at impact. It was found that: (1) Adhering YSZ to the underlying Ni-22wt.%Cr-10wt.%Al-1wt.%Y bondcoat layer occurred readily due to melting of the bondcoat layer and (2) Adhering YSZ particles to previously deposited YSZ layers was more difficult. Only conditions for which the small particles (<16μm) were molten at impact achieved adherent coatings.; The results of a comparative study between APS and HVOF deposited YSZ coatings indicate that: (1) Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density (ρ), surface roughness (R _a) and cross-sectional microhardness (H).; The durability of HVOF deposited YSZ coatings was examined in cyclic furnace testing apparatus. The currently used APS deposited coatings delaminate due to the combined effect of coating oxidation and thermal shock stresses generated during cycling. It was observed that: (1) The HVOF deposited YSZ coatings failed before the APS coatings and that failure occurred in layer-by-layer delamination (i.e. in some regions, YSZ was observed to have spalled, exposing underlying YSZ rather than bond coat or bond coat oxidation products), and (2) Sintering and densification of the YSZ coatings during high temperature exposure was a primary factor in coating spallation.; Concurrent with other researchers, this work showed that the oxidation product that forms after short times (1 hour) at high temperature is a protective α-Al ₂O₃ scale. The present study, however, is the first to fully characterize the interfacial oxidation products of failed APS deposited coatings using transmission electron microscopy. Scanning electron microscopy confirmed that the HVOF deposited YSZ coatings displayed similar interfacial oxidation products after high temperature exposure. The characterization results showed that: (1) Non-protective discontinuous regions form at the interface between the protective α-Al₂O₃ layer and YSZ after long-term exposure. (2) Energy dispersive spectroscopy and electron diffraction performed within those discontinuous interfacial regions confirmed the presence of discrete grains of higher order oxides such as Ni(Cr,Al)₂O₄ spinel and ordered (Cr-rich,Al) ₂O₃. and (3) The discontinuous regions result from the oxidation of the NiCrAlY bond coat by inward oxygen penetration, often near topographical asperities. (Abstract shortened by UMI.)...