Lifetime‐determining spalling mechanisms of NiCoCrAlRE / EB‐PVD zirconia TBC systems

Abstract

The mechanisms that control the lifetime of thermal barrier coating (TBC) systems have been traced by two particular overlay bondcoats serving as model systems: superalloy pins (IN100, CMSX-4) with two alternative NiCoCrAlRE (RE: Hf, Y) bond coat compositions (i) NiCoCrAlY without and (ii) with co-dopants of silicon and hafnium. On top an electron-beam physical-vapor deposited (EB-PVD) yttria partially stabilized zirconia (YPSZ) TBC commonly mixed with 2 wt.% hafnia, or, rarely with 10 wt.%, was applied. The test pins were thermo-cycled at 1100 and 1150 C until failure. Identical lifetimes in cyclic tests on YPSZ TBCs with 2 (relatively high sintering rate) and 10 wt.% hafnia (relatively low sintering rate) preclude an effect of diffusion mechanisms of the YPSZ TBC on lifetime. The fit of lifetimes and test temperatures to Arrhenius-type relationships gives activation energies for failure. These energies agree with the activation energies for anion and cation diffusion in alumina for the respective bondcoat variant:(i) for the NiCoCrAlY/TBC system for O 2-diffusion in alumina, (ii) for the NiCoCrAlYSiHf/TBC system for Al 3+ diffusion in alumina.SEM and EDS investigations of the thermally grown oxides (TGOs) confirm the mechanisms responsible for TBC failure as indicated by activation energies. Two categories of failure can be distinguished: (i) NiCoCrAlY coatings fail by an "adhesive mode of failure" along smooth bond coat/TGO interfaces driven by a critical TGO thickness. (ii) NiCoCrAlYSiHf coatings fail later and more reluctantly by a "cohesive" crack mode via de-cohesion at the TGO/TBC interface. In the latter case a quasi-integrity of the crack-affected TGO is lengthily maintained up to failure by a crack-pinning mechanism which runs via Al 3+ supply from the bondcoat.Keywords

Compiled furnace cyclic lives of EB-PVD thermal barrier coatings

“…1 and 2 that only a handful of coating lives lie appreciably above the projected 7 μm boundary. It has been discussed that the activation energy surmised from TBC life correlated with that obtained by oxidation and diffusion in alumina [34]. In that regard, the 382 kJ/mol value obtained in Eq.…”

“…One of these studies systematically varied the Co content of a 3rd generation (high Re) composition and found that a low 2 wt.% Co alloy provided the maximum TBC life compared to 8 or 12 wt.% Co [30]. Other studies employed NiCoCrAlY bond coats [32][33][34][35] (Kim, Nijdam, Schulz, and coworkers) or a Pt coating over NiCoCrAlY(Ta) bond coat [36][37][38] (Yanar, Vande Put, and coworkers).…”

Thermal Fatigue Life Of Heat-protective Coating On Single-crystal Blade After Airfoil Edge Restoration By Tig Surfacing

“…Гафний концентрируется преимущественно на поверхности покрытия в виде оксидов (светлых фаз), образующихся в процессе окисления. Структуры TGO на модифицированном покрытии подтверждают тот факт, что активные элементы (иттрий, гафний, цирконий) образуют сегрегации в виде соединений под пленкой TGO, а избирательное окисление их на внешней поверхности связующего слоя приводит к формированию локальных прикреплений, обеспечивающих механическое сцепление защитной пленки (и керамики) с подслоем, тем самым улучшая ее адгезионную прочность [11,14]. Формирование неравномерной границы раздела связующий слой/TGO может увеличивать сопротивление против распространения трещины при термоциклических испытаниях ТЗП.…”

“…8, а, в, д). Эта особенность рассмотрена в работах [12,14,15,18,19]. В связующем слое с повышенным (за счет модифицирования активным элементом гафнием) пределом текучести могут уменьшаться или предотвращаться амплитудные изменения морщинистости поверхности в процессе термоциклирования [20].…”