Effects of Impurity Content on the Sintering Characteristics of Plasma-Sprayed Zirconia

Abstract

Yttria-stabilized zirconia powders, containing different levels of SiO 2 and Al 2 O 3 , have been plasma sprayed onto metallic substrates. The coatings were detached from their substrates and a dilatometer was used to monitor the dimensional changes they exhibited during prolonged heat treatments. It was found that specimens containing higher levels of silica and alumina exhibited higher rates of linear contraction, in both in-plane and through-thickness directions. The in-plane stiffness and the through-thickness thermal conductivity were also measured after different heat treatments and these were found to increase at a greater rate for specimens with higher impurity (silica and alumina) levels. Changes in the pore architecture during heat treatments were studied using Mercury Intrusion Porosimetry (MIP). Fine scale porosity (<$50 nm) was found to be sharply reduced even by relatively short heat treatments. This is correlated with improvements in inter-splat bonding and partial healing of intra-splat microcracks, which are responsible for the observed changes in stiffness and conductivity, as well as the dimensional changes.

Heat Transfer Through Plasma-Sprayed Thermal Barrier Coatings in Gas Turbines: A Review of Recent Work

“…The spraying conditions, and coating microstructures, were similar to those reported elsewhere [31,[75][76][77]. The predictions shown were obtained using input data, and predictions from the sintering model, shown in Table I.…”

“…There has been extensive study [31,[48][49][50][51][52][53][54] of microstructural features exhibited by PS YSZ and of changes induced under service conditions [26,55,56]. The main features are a series of interfaces parallel to the plane of the coating (normal to the heat flow direction), representing boundaries between the splats formed from incident molten droplets.…”

“…In general, these changes tend to cause reduced thermal insulation capacity [16][17][18]78] and reduced strain tolerance [19-21, 26, 79, 80], both of which are highly undesirable. It's also clear that sintering can be accelerated by the presence of species that become concentrated in grain boundaries or at free surfaces, whether present as impurities in the coatings [31] or as material absorbed during turbine operation [27][28][29][30]. Species that dissolve in the lattice, on the other hand, such as alternative stabilisers to yttria, tend to have little effect on the sintering characteristics [78].…”

“…Increases in coating stiffness [19][20][21] also occur during sintering, as a consequence of intersplat locking and splat stiffening, leading to increased danger of spallation [18,20,[22][23][24][25][26]. This is particularly problematic [27][28][29][30][31] when it is accelerated by the presence of impurities, such as calciamagnesia-alumina-silica (CMAS), either from the original powder or deposited during service. This concern relates equally to both PS and PVD coatings.…”

A sintering model for plasma-sprayed zirconia thermal barrier coatings. Part II: Coatings bonded to a rigid substrate

“…The effect of different values of E av is investigated here, but its value is assumed to remain constant during sintering. This is a limitation of the current formulation, since it is known that the stiffness tends to increase during sintering [18][19][20][21][22][23].…”