High‐Temperature Performance of Alumina–Zirconia Composite Coatings Containing Amorphous Phases

Abstract

Amorphous phases are commonly found in nanostructured plasma‐sprayed coatings. Nonetheless, the role of these phases in the resulting coatings’ properties has remained uninvestigated until now. In the present work, pseudo‐eutectic coatings—based on alumina and 8 wt% yttria‐stabilized zirconia (YSZ)—containing amorphous phases are produced using a suspension‐plasma‐spray process. These composite materials are a potential choice for thermal‐barrier coating applications. The role of the amorphous phase on the performance of the coatings is investigated before and after heat treatment. Results show that, although the amorphous phases in untreated coatings reduce the thermal conductivity, they impair the mechanical properties. However, treatment above the crystallization temperature leads to better mechanical properties as well as enhanced high‐temperature stability of the resulting nanostructure. Moreover, the role of alumina as a stabilizer of high‐temperature YSZ phases (tetragonal and cubic) is confirmed and the high‐temperature phase stability of the alumina–YSZ composite is demonstrated. The amorphous phases are found to crystallize into their corresponding high‐temperature stable phases; i. e., α‐alumina and tetragonal zirconia.

Thermal Cycling of Suspension Plasma Sprayed Alumina‐YSZ Coatings Containing Amorphous Phases

“…In this coating, the γ‐alumina and the amorphous humps have disappeared and (α + δ)‐alumina and (cubic + tetragonal) YSZ are the present phases in the coating after thermal cycling. This is somewhat different from the phases formed during isothermal heat treatment of the coatings at about the same temperatures, where no δ‐alumina was observed after 10 h at 1000°C or higher temperatures at 1300°C for 24 h. In fact, the coatings showed no crystallographic changes after 10 h at 1000°C from the initial structure containing α‐ and/or γ‐alumina and cubic zirconia; and after 1300°C for 24 h, the phases included α ‐alumina and (cubic and/or tetragonal) YSZ . This shows that the phase transformations resulted from heat treatment can be different than thermal cycling at the same approximate temperatures.…”

“…In addition, the appearance of α‐alumina at such a low temperature is somewhat unexpected. It is in contrast with the results of heat treatment at 1000°C for 12 h, where no alumina transformation could happen and also contrary to the literature that predicts higher transformation temperatures of ~1200°C . Thus, formation of α‐alumina phases, upon heat treatment, at such a low temperature can most probably be the result of crystallization of amorphous alumina.…”

“…The cyclic heating with high heating and cooling rates of the alumina‐YSZ composite coating with high amorphous content resulted in an interesting microstructure, not found in isothermal heat treatment of this kind of coating . Figure exhibits the microstructural changes of the high amorphous coating after 500 thermal cycles.…”

Microstructure evolution and thermomechanical properties of plasma‐sprayed Yb2SiO5 coating during thermal aging

“…With regard to thermal properties, the thermal conductivity of the as‐sprayed coating exhibits an increasing tendency with temperature rising. The phonon transfer in amorphous‐containing ceramics is weakened because of the decreased mean free path of phonons . The amorphous phase in the coating recrystallized after temperature increasing above 1000°C, causing remarkable increase in thermal conductivity.…”

MICROSTRUCTURES, MICROHARDNESS, AND CRYSTALLIZATION BEHAVIORS OF AMORPHOUS Al2O3-YSZ COATINGS PREPARED BY AIR PLASMA SPRAYING

“…Furthermore, the composite Al 2 O 3 -YSZ has shown the potential for amorphous phase formation as well. [17][18][19][20][21] In this study, we fabricated amorphous coatings of the near-eutectic composite Al 2 O 3 -YSZ using the air plasma spraying (APS) method. The microstructural characteristics of the amorphous coatings obtained under di®erent spraying conditions were investigated.…”