ICMCTF2012 Session A2-2: Thermal and Environmental Barrier Coatings

Hot section components in aero and power generation engines utilize advanced thermal barrier coating systems for life extension and better efficiency. Thermally-sprayed ceramic / bondcoat systems are extensively used for combustors and power generation blades and vanes whereas EBPVD TBC on Pt modified diffusion aluminide coating is the coating of choice for highly stressed airfoils in aero engines. New technologies such as the suspension plasma spray process (SPS) are finding more and more interest for applying TBC’s. In addition, challenges such as the trend to low thermal conductivity and CMAS resistant coatings require new compositions, and respective processing technology. The process and coating characteristics of 7wt% YSZ based APS low density and dense vertically cracked (DVC) Zircoat™ TBC as well as EBPVD coatings are described, highlighting recent advances with ultra pure Zirconia for improved sintering resistance. New coating compositions for low thermal conductivity TBC’s and CMAS resistant TBC’s are also addressed. Lastly, the properties of new coating processes such as SPS are compared with conventional coating processes.

Turbine airfoils in advanced aircraft turbine engines are protected from the aggressive environment by thermal barrier coating systems (TBCs) comprising an insulating oxide and a metallic bond coat. Environmental contaminants ingested with the intake air form deposits generically known as CMAS (calcium-magnesium alumino-silicates) on the protective coatings. As the deposits melt during the engine cycle a silicate glass forms that infiltrates the porous coating and crystallizes under the imposed thermal gradient, stiffening the TBC and compromising its strain tolerance. Delamination failures have been documented in the past. A new failure mechanism that involves the formation of cavities within the bond coat under regions of the TBC penetrated by CMAS has been identified recently. Examination suggests that cavity formation occurs in regions subject to lateral thermal gradients; channel cracking and scalloping of the TBC are also observed above the bond coat cavities. Once the voids grow large enough to compromise the bond coat, the TBC delaminates and eventually spalls, leaving behind a thermally unprotected airfoil with a residual bond coat. This presentation will discuss the characteristics of this failure mode, and the possible underlying mechanisms.

With rising operating temperature, the prevalence of calcium magnesium alumino-silicate (CMAS) deposits melting on the surface of thermal barrier coatings (TBCs) used in gas turbines has increased. These molten CMAS deposits infiltrate and crystallize within the pores of the structure, stiffening the penetrated layer and leading to a loss of strain tolerance. The loss of compliance promotes coating delamination when the strain energy generated from the thermal expansion mismatch during thermal cycling reaches a critical level. A laser thermal gradient test (LGT), in which the thermal gradient and cooling rate can be controlled, was used to assess the TBC durability by imposing a range of thermal stresses. Both 7YSZ and gadolinium zirconate (GZO) TBCs, with and without CMAS deposits, were subjected to the LGT. In the absence of CMAS, no microstructural degradation was observed for either composition. When loaded with CMAS, TBC degradation was found to increase with increased cooling rate, and was generally higher for GZO than for 7YSZ, both materials processed by EB-PVD. The CMAS penetration, phase evolution and crack morphology of the thermally cycled TBCs have been characterized as a function of the thermal history and will be analyzed in the context of current delamination models.

This investigation was sponsored by the Office of Naval Research under grant N00014-08-1-0522, monitored by Dr. David Shifler

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) are widely used to protect the components in the hot area of power generation turbines. One identified cause of TBC failure is the degradation by molten deposits, mostly calcium-magnesium-alumina-silicates (CMAS), which enter the turbine from the environment. It infiltrates the pores and cracks, reacts with the YSZ and leads to its destabilization. The main purpose of the current research is to investigate to which extent the attack by molten CMAS can be reduced by coating the TBC with alumina.

A model CMAS, composed of 38 mol% CaO, 6 mol% MgO, 5 mol%Al₂O₃, 50 mol% SiO₂ and 1 mol% Fe₂O₃, ultra-milled, molten two times for 4 h at 1400°C and milled again, was deposited on the surface of a free standing sample from a commercial APS TBC. The samples were exposed to 1100°C and 1240°C for 50, 100 and 200 hours in air and were analyzed by X-ray diffraction with micro-focus (µ-XRD) and by field emission SEM. Surface scans by µ-XRD stepwise from the unaffected area to the CMAS infiltrated surface area show at 1100°C considerable portions of the monoclinic phase from the first exposure time of 50h. The micrographs however reveal only superficial infiltration. At 1240°C again the phase decomposition is detected already after 50 h and a deep infiltration is observed in the micrographs, almost across the whole TBC.

The Solution Precursor Plasma Spray (SPPS) process has the potential of providing more durable and low thermal conductivity thermal barrier coatings (TBCs). The increased durability derives from a highly strain-tolerant microstructure consisting of fine, through-coating-thickness cracks and an increased inter-splat crack resistance associated with ultra-fine splats (<2 microns). Low thermal conductivity SPPS TBCs are associated with unique planar arrays of nano- and micro-porosity that are referred to as inter-pass boundaries (IPBs).

Previous University of Connecticut SPPS work has shown inconsistent spallation lives during thermal cycle testing of yttria stabilized zirconia (YSZ) TBCs. For a variety of bond coats, the spallation life of SPPS YSZ TBCs varied from 1.0 to 2.5X air plasma spray YSZ. The key appears to be the initial bond coat oxide formed and whether spallation occurs within the ceramic or at the bond coat to ceramic interface. The longer lives are associated with ceramic (white) failure.

This presentation will discuss the progress made in producing SPPS YSZ TBCs with improved durability. It will also describe SPPS process optimization trials to produce low thermal conductivity YSZ microstructures using IPBs. Success in this effort will extend the use of YSZ TBCs and minimize the use of rare-earth elements required in most alternate low thermal conductivity TBCs.

Deposition process for thermal barrier coatings (TBCs) exerts a critical influence on the determination of its thermomechanical properties such as elastic modulus, thermal conductivity, and coefficient of thermal expansions. Moreover, failure phenomena usually occur at the interface between the top and bond coats due to the mismatch of mechanical and thermal properties, as TBC system applies to a high operating temperature and cools down to the ambient. In order to reduce the risk of failure a graded layer was created at the interface between the top and bond coats, and thermoelastic behaviors were investigated through mathematical approach. The microstructure of top coat in TBC specimens prepared with TriplexPro^TM-200 system was controlled by changing the feedstock and using a multiple hopper system, showing the dense/intermediate/porous microstructures from surface to interface or the reverse microstructure. Thermoelastic theory was applied to derive a couple of governing partial differential equations. Since the governing equations are too involved to solve analytically, a finite volume method was developed to obtain approximations. The thermoelastic characteristics of the TBC with graded layer obtained through mathematical approaches coincided with experimental results, and the various analyses may be useful to discover technologies for enhancing the thermomechanical properties of TBCs.