Step-by-step investigation of degradation mechanisms induced by CMAS attack on YSZ materials for TBC applications

Over the past decades, Thermal Barrier Coatings (TBCs) have become essential parts in gas turbine engines. In working conditions, TBCs are subject to many kinds of degradation (erosion, foreign object damage (F.O.D), oxidation, etc.) which deteriorate integrity and mechanical properties of thewhole system.Moreover, with the aim to increase the turbine inlet temperature, a new type of damage has been highlighted: corrosion by molten Calcium–Magnesium–Alumino Silicates, better known as CMAS. In this paper, interactions between yttriastabilized zirconia (YSZ) materials synthesized via sol–gel process and synthetic CMAS powder were investigated via a step-by-step methodology. The approach was conducted starting from the more severe conditions of interactions and then gradually gets closer to the interactions taking place in service. It was proved that CMAS can induce faster densification of the ceramic leading to a loss of strain tolerance of the protective coating. Besides, a dissolution/re-precipitationmechanismcan also take place between YSZ andCMAS leading to the transformation of the initial tetragonal yttria-stabilized zirconia into globular particles of monoclinic zirconia. CMAS were also found to infiltrate the entire thickness of both EBPVD and sol–gel YSZ coatings at 1250 °C for 1 h. Nevertheless, the original non-oriented microstructure provided by sol–gel route leads to a different way of interaction due to the high reactivity of sol–gel precursors and materials. The behaviors of EBPVD and sol–gel coatings under CMAS exposure are discussed in this paper

CMAS interaction with yttrium based systems: Towards a promising solution?

Anti-CMAS coatings are designed to protect thermal barrier coatings against degradations due to CMAS infiltration. They are dedicated to react as fast as possible with CMAS compounds in order to generate a new phase that will not let the CMAS infiltration going on. In addition to the fast kinetics, the preparation of large quantities of phases with as less as anti-CMAS material as possible is sought as well as the obtaining of a dense and impermeable phase. Reference anti-CMAS material is gadolinium zirconate, it has been demonstrated as efficient to block CMAS infiltration. The efficiency of yttria for the same application has been studied by previous works at the CIRIMAT laboratory [1]. The starting point of this work was first, to make a comparative study of the anti-CMAS properties of gadolinium zirconate and yttria based anti-CMAS compositions and secondly, to discuss on the behaviour of a mixed composition i.e. yttrium zirconate. This insight on the mechanism of interactions of CMAS with the yttrium based systems and gadolinium reference is obtained at the light of a large number of interactions experiments and characterizations. Interaction durations between 1h and 100h were investigated, with either 50/50 or 80/20 mass ratio of CMAS/anti-CMAS. Different temperatures of interaction between 1200°C to 1300°C were also scanned. The phases in presence were systematically characterized by XRD, SEM, EDX and Castaing microprobe local analyses and cartographies (Figure 1a)). In particular, the proportion and composition of phases are detailed as a function of the interaction time (Figure 1b)) for each anti-CMAS-CMAS interaction system. From these experiments, the mechanisms of interaction between CMAS and Y2O3, Y2Zr2O7 and Gd2Zr2O7 are evidenced. Whereas Y2O3 interaction leads to the higher proportion of reaction products, the products impermeability is also superior for this composition. The depth of infiltration of CMAS into a dense pellet anti-CMAS material for a given time is longer for Y2Zr2O7 as compared to the two others. However Y2Zr2O7 benefit is due to a higher Ca2+ trapping capability with the formation of Ca4Y6 like phase instead of Ca2Y8 for Y2O3 (Figure 1c)). In this respect, yttrium zirconate demonstrates a synergetic effect as compared to Y2O3 and Gd2Zr2O7. The origin of this synergy is interpreted as coming from the presence of both zirconium and yttrium. A large part of the discussion is based on the study of powders interactions, an insight into the interactions of CMAS with anti-CMAS pellets of the different compositions will also be presented and discussed. Fundamental and applicative aspects will be covered. Please click Additional Files below to see the full abstract

Synthesis of yttria by aqueous sol-gel route to develop anti-CMAS coatings for the protection of EBPVD thermal barriers

Anti-CMAS yttria coatings have been prepared by sol-gel routes. Yttria powders with controlled morphology are prepared via auto-combustion of yttrium precursors in a polymerized matrix. The influence of key parameters of the water-based sols is assessed. Indeed, the pH of the initial sol and the temperature of thermal treatment play a major role in the morphology and grain size of yttria powders. To prevent infiltration of CMAS, yttria powders are proposed to be synthesized at pH=1 of the aqueous sol, with drying of the sol and heating at 900 °C. After optimization of the synthesis and deposition conditions via sol-gel route, yttria-based coatings with high specific surface area are obtained. They promote the interaction with melt CMAS and consequently limit the degradation of the thermal barrier coatings situated underneath. It was proved that anti-CMAS yttria coating is effective against the infiltration of CMAS at 1250 °C for 15 min and even 1 h

High-temperature cyclic oxidation behaviour of Pt-rich γ-γ’ coatings. Part I: Oxidation kinetics of coated AM1 systems after very long-term exposure at 1100 °C

The cyclic oxidation behaviour of several compositions of Pt-rich γ-γ’ bond-coatings on AM1 superalloy was studied at 1100 °C and was compared to the β-(Ni,Pt)Al coated and uncoated superalloy. AM1 superalloy exhibited an outstanding performance due to an optimized Hf doping and a low sulfur content. The Pt-rich γ-γ’ bond-coatings showed a better cyclic oxidation resistance than the reference system with a β-(Ni,Pt)Al coating. Aluminium addition during fabrication was found to be beneficial to improve the oxidation behaviour of Pt-rich γ-γ’ bond-coatings. Their breakaway resulted from an insufficient aluminium content below the TGO whereas the reference system suffered from rumpling

Synthesis of yttria by aqueous sol-gel route to develop anti-CMAS coatings for the protection of EBPVD thermal barriers

Anti-CMAS yttria coatings have been prepared by sol-gel routes. Yttria powders with controlled morphology are prepared via auto-combustion of yttrium precursors in a polymerized matrix. The influence of key parameters of the water-based sols is assessed. Indeed, the pH of the initial sol and the temperature of thermal treatment play a major role in the morphology and grain size of yttria powders. To prevent infiltration of CMAS, yttria powders are proposed to be synthesized at pH=1 of the aqueous sol, with drying of the sol and heating at 900 °C. After optimization of the synthesis and deposition conditions via sol-gel route, yttria-based coatings with high specific surface area are obtained. They promote the interaction with melt CMAS and consequently limit the degradation of the thermal barrier coatings situated underneath. It was proved that anti-CMAS yttria coating is effective against the infiltration of CMAS at 1250 °C for 15 min and even 1 h

High-temperature cyclic oxidation of Pt-rich γ-γ’ bond-coatings. Part II: Effect of Pt and Al on TBC system lifetime

Three kinds of Pt-rich γ-γ’ bond-coating were processed with different contents in Pt and Al. The cyclic oxidation tests performed at 1100 °C on TBC systems showed the superiority of the Pt-rich γ-γ’ coatings when compared with the β-(Ni,Pt)Al reference system. TBCs with a Pt-only bond-coating provided the highest performance. Whatever the bond-coating, the failure occurred at the TGO/bond-coating interface which appeared to be the weak point of these γ-γ’ bond-coating based systems. Al addition during bond-coating fabrication did not improve the durability. A decrease of 2 μm of electroplated Pt thickness led to a higher performance than the reference systems

Observation and modeling of α-NiPtAl and Kirkendall void formations during interdiffusion of a Pt coating with a γ-(Ni-13Al) alloy at high temperature

During the last 15 years, Pt-rich γ–γ′ bond-coatings have been studied extensively for their corrosion and oxidation resistance, and as a lower cost alternative to β-(Ni,Pt)Al bond-coatings in thermal barrier coating systems. To optimize their fabrication and durability, it is essential to investigate their interdiffusion with Ni-based superalloys. This study reports on experimental results and modeling of the interdiffusion of the model Pt/γ-(Ni-13Al) alloy system. Pt coatings were deposited either by electroplating or by spark plasma sintering using a Pt foil. Heat treatments at 1100 °C for 15min to 10 hwere performed either in a high-temperature X-ray diffraction device under primary vacuum or in a furnace under argon secondary vacuum. The α-NiPtAl phase with L10 crystal structure formed very rapidly, implying fast uphill Al diffusion toward the surface. For Pt electroplating, α-phase transformed to γ′-(Ni,Pt)3Al after only 45 min–1 h at 1100 °C. The resulting two-phased γ–γ′ microstructure remained up to 10 h. When using a Pt foil coating, the continuous layer of α-NiPtAl phase disappeared after 10 h and the γ′-(Ni,Pt)3Al or γ-(Ni,Pt,Al) phase appeared, resulting in two different diffusion paths in the Ni–Pt–Al phase diagram. Voids also formed at the interdiffusion zone/substrate interface for both systems after 1 h or more. Composition analyses confirmed that voids were located at the Pt diffusion front corresponding to the Al-depleted zone. Experiments performed with the samples coated with a Pt foil confirmed that voids are due to a Kirkendall effect and not to the Pt deposition process. Numerical simulations including the cross-term diffusion coefficients in the diffusion flux equations reproduced the experimental concentration profiles for the γ-phased systems

Fibers and sol-gel matrix based thermal barrier coating systems for outstanding durability

Thermal barrier coatings (TBC) are critical elements of the turbomachines. On turbine blades for aircraft engines, their preparation is based on EB-PVD industrial process. Such TBCs on first generation AM1 superalloy with a beta-NiPtAl bond coating exhibit 20% of surface spallation after about 600 1h oxidation cycles at 1100°C. In this work, a new method of TBC preparation was proposed and high durability of such structures was obtained with more than 1000 1h cycles at 1100°C before 20% of spallation. More than 1400 1h cycles was even obtained with the most performing formulations. A key point was that the surface spallation was lower than 10 % after 1000 cycles for TBCs made with the 70% and 80% fiber mix (Figure 1a). In the same conditions, EB-PVD TBCs exhibit 50-80% of spallation. The preparation process relied on the addition of a high temperature binder, namely a zirconia sol, to a mix of zirconia powder and fibers. TBCs with equiaxed porosity were obtained (Figure 1b). After thermal treatments, ceramic sintering bridges between the powder, the fibers and the ceramic derived from the sol transformation formed (Figure 1c). Another benefit was obtained from the anchoring of the fibers in the thermally grown oxide (TGO), inducing a tougher TGO layer. The outstanding durability of these fibers and sol-gel matrix based thermal barrier coatings is believed to be the consequence of higher toughness of both the TBC coating and modified TGO. Indeed, crack deviations were observed in these two elements. Moreover, contrary to EB-PVD TBCs, the porosity is isotropically distributed, limiting heat diffusion towards the superalloys. Please click Additional Files below to see the full abstract

Transient and steady states of Gd2Zr2O7 and 2ZrO2∙Y2O3 (ss) interactions with calcium magnesium aluminium silicates

Reactions between calcium magnesium aluminium silicates (CMAS) and Gd2Zr2O7 or 2ZrO2∙Y2O3 (ss) are investigated within a temperature range of 1200–1300 °C and for durations of 1 h–100 h. The evolution of CMAS penetration depth in Gd2Zr2O7 and 2ZrO2∙Y2O3 (ss) pellets varies considerably depending on the interaction time. A quantitative analysis of the nature and composition of phases observed in stationary conditions (powder/powder interaction) is performed by SEM-FEG coupled with WDS analyses using micro-agglomerated nanoparticles of Gd2Zr2O7 and 2ZrO2∙Y2O3. Faster kinetics of the gadolinium-based system are illustrated through an analysis of the morphology of the reaction area and of the resulting CMAS tightness of reaction products. The compositions and quantities of reaction products observed at equilibrium are very similar for the two systems, but transient states are significantly different

Outstanding durability of sol-gel thermal barrier coatings reinforced by YSZ-fibers

Thermal barrier coatings (TBC) were fabricated with commercial powders of yttria stabilized zirconia with spherical and fiber-like morphologies. The influence of fiber percentage and sintering temperature on the thermomechanical behavior was studied. TBCs with 60%–80% fibers content had the best lifetime in cyclic oxidation with less than 10% of coating spallation after 1000 cycles, with very good reproducibility. They reached lifetimes higher than industrial TBCs made by EB-PVD. The enhancement of durability is believed to be due to an increase in the thermomechanical constraints accommodation thanks to higher porosity and higher tenacity due to the presence of well anchored fibers, indeed deviation of the cracks were observed. Moreover, the morphology of the thermally grown oxide (TGO) layer is also favorable as it includes anchorage points of the TGO with fibers. This increased the adherence at the substrate interface and improved lifetime