COMPOSITE CERAMICS FOR THERMAL BARRIER COATINGS  PRODUCED FROM ZrO₂ DOPED WITH OXIDES OF RARE-EARTH METALS IN THE YTTRIUM SUBGROUP

Abstract

The use of ZrO₂-based complex composite ceramics doped with a mixture of yttrium-subgroup rare-earth metal (REM) oxides of the yttrium subgroup for deposition of thermal barrier coatings (TBCs) was studied. For the research, a heavy concentrate (HC) of yttrium-subgroup REM oxides, consisting of (wt.%) 13.3 Y₂O₃, 1.22 Tb₄O₇, 33.2 Dy₂O₃, 8.9 Ho₂O₃, 21.8 Er₂O₃, 1.86 Tm₂O₃, 12.5 Yb₂O₃, 0.57 Lu₂O₃, and 6.65 other oxides (including 3.2 Al₂O₃), and Y₂O₃ and M-ZrO₂ powders were chosen. The targets for depositing electron-beam ceramic TBC layers¾both standard and complex composite ones¾were made of ceramic mixtures including (wt.%) M-ZrO₂–7 Y₂O₃ and 90 M-ZrO₂–10 HC. The properties of the complex composite and standard yttria-stabilized ZrO₂-based ceramic layers in electron-beam TBCs applied in one process cycle were compared. Two-layer metal/ceramic TBCs were deposited onto model blades by directional crystallization from the ZhS-26VI alloy employing an UE-174 industrial electron-beam installation (ELTECHMACH, Vinnytsia). The ceramic М-ZrO₂–7 Y₂O₃ layer was denoted as YSZ and 90 М-ZrO₂–10 HC as HCSZ. The MZP-6 alloy (nickel–chromium–aluminum–yttrium) was used to form an inner heat-resistant layer. This resulted in rough dense glassy coatings differing by color: light-gray YSZ and dark-gray HCSZ. The coatings were 90–95 µm thick on the blade back side and 90 µm thick on the pressure side. Both coatings included the F-ZrO₂ phase. Pinnate-like microstructures emerged in the coatings. The YSZ layer contained two types of dense structures, represented by columns and structures, and the HCSZ layer was of irregular microstructure with wide pinnate formations growth together. The laminar microstructure of the ceramic layer was due to the process features peculiar to electron-beam deposition. The YSZ layer had the following microhardness: 3884 MPa on the back side and 6052 MPa on the pressure side. The HCSZ layer had much lower microhardness: 1381 MPa on the back side and 1679 MPa on the pressure side. The complex composite coating withstood 161 thermal cycles and the standard coating 138 thermal cycles. Previous studies showed that ZrO₂ stabilization with concentrates of yttrium-subgroup REM oxides was promising for the microstructural design of TBC ceramic layers.