ICMCTF2003 Session A3-2: Thermal Barrier Coatings
Tuesday, April 29, 2003 1:30 PM in Room Sunset
A3-2-1 Highly Durable Thermal Barrier Coatings Made By The Solution Precursor Plasma Spray Process
M. Gell, L.D. Xie (University of Connecticut); X.Q. Ma (Inframat Corporation); E.H. Jordan, N.P. Padture (University of Connecticut)
Thermal barrier coatings are used to protect metallic turbine components from high temperatures so as to prolong the lifetime of the components or improve engine efficiency by allowing increased operating temperatures. There are two main commercial processes for the production of TBCs: air plasma spray (APS) and electron-beam physical vapor deposition (EB-PVD). APS TBCs are relatively low-cost, have low thermal conductivity, but are less durable. While EB-PVD TBCs generally last longer, - cost more, and have relative higher thermal conductivity. The need for durable, low cost, and low thermal conductivity TBCs has led to the development of a new TBC deposition process: Solution Precursor Plasma Spray (SPPS).
In this plasma spray process, solution precursor is used as feedstock, instead of powder. TBCs produced using SPPS process have the following novel microstructural features: (i) no splat boundaries, (ii) through thickness vertical cracks that makes the coating strain-tolerant, (iii) uniformly distributed nano- and micrometer size porosity that reduces the thermal conductivity and elastic modulus of the ceramic coating. Recent thermal cycling tests have verified the superior durability of the TBCs made using SPPS over TBCs deposited using EB-PVD and conventional APS processes.
A3-2-3 Deposition Mechanisms For Thermal Barrier Coatings Made By The Solution Precursor Plasma Spray Process
L.D. Xie (University of Connecticut); X.Q. Ma (Inframet Corporation); E.H. Jordan, N.P. Padture, M. Gell (University of Connecticut)
Solution precursor plasma spray (SPPS) process is a promising process for the production of durable, low thermal conductivity and affordable thermal barrier coatings (TBCs). In this process, a solution precursor containing Zr and Y is used as feedstock, instead of powder. TBCs made using SPPS process exhibit the following unique microstructure: (i) no splat boundaries, (ii) through thickness vertical cracks that makes the coating strain-tolerant, (iii) uniformly distributed micrometer and nanometer size porosity that reduces the thermal conductivity and elastic modulus. In this study, the deposition mechanisms for TBCs made by the SPPS process were studied systematically. The thermal history of the solution precursor is critical to its deposition onto the substrate and the final morphology of the formed deposits. Based on a series of specialized spray trials, several possible deposition mechanisms have been defined. These deposition mechanisms will be discussed in detail.
A3-2-4 On the Microstructural Development in a Class of Bond Coats for Thermal Barrier Systems
A.M. Karlsson (University of Delaware)
Thermal barrier coatings are multilayer systems, consisting of a super alloy substrate, a bond coat, a thermally grown oxide (TGO), and a ceramic top coat. The super alloy sustains the mechanical load. The top coat is the thermal insulator. The TGO is a reaction product, formed by oxidation of the bond coat and consists of alpha-alumina. The primary function of the bond coat is to provide the system with aluminum to form the TGO, while protecting the super alloy from oxidation. In doing so, the bond coat is depleted of aluminum. Thus, the micro-structure of the bond coat evolves, resulting in a change of its thermo-mechanical properties. As a consequence of the TGO formation and the transformation of bond coat properties, the overall properties of the thermal barrier system evolve during use. This study investigates these changes and the influence they have on the failure mechanics of thermal barrier coatings. A majority of the dominant failure modes are associated with the mechanics, morphology and thermo-mechanical properties of the interface region consisting of bond coat, TGO and top coat. Typically, micro cracks initiate around morphological features, and during thermal cycling, these cracks grow, coalesce, and finally causing overall spallation of the coating. Numerical simulations and analytical expressions are used to monitor how changes in the micro-structure affect the stresses and morphology of the interfacial region, and how this influences the initiation and propagation of cracks. Particularly, results pertaining to martensitic phase transformation and to the formation and presence of different grains in the bond coat structure will be discussed.
A3-2-5 A Preliminary Study Of Magnetron DC Sputtered NiAl-Hf(0.5 at%) Films
B. Ning, M Shamsuzzoha, M.L. Weaver (University of Alabama)
Modern TBC's bond coat are typically aluminide (NiAl) coatings achieved by high purity, low activity chemical vapor deposition aluminizing. Small additions of Hf have been found to be effective in improving the high-temperature creep strength of NiAl crystals . In this paper, NiAl-Hf (0.5 at%) were successfully grown by DC magnetron sputtering on Si wafers and CMSX-4 super alloys. XRD, SEM, EDX and TEM were used to characterize the composition and microstructure of the films. It was found that either crystalline or amorphous films could be formed basically depending on the sputtering powers. Crystalline NiAl-Hf film has the B2 structured NiAl phase and Hf exists as solid solute. Meanwhile, these films showed various preferred growth orientation on different substrates. Nanoindentation was applied by Hysitron TriboscopeÂ® to get the hardness and reduced modulus data.
A3-2-8 Phase Transformation of Thermally Grown Oxide on (Ni,Pt)Al Bond Coat During Electron Beam Physical Vapor Deposition and Subsequent Oxidation
S. Laxman, B. Franke, L.A. Giannuzzi, Y.H. Sohn (The University of Central Florida); K.S. Murphy (Howmet Research Corporation)
Phase constituents and microstructure of thermally grown oxide (TGO) as a function of electron beam physical vapor deposition (EP-PVD) time, (Ni,Pt)Al bondcoat surface treatment and subsequent oxidation in air were examined for a series of thermal barrier coatings (TBCs). Photostimulated luminescence spectroscopy (PSLS) and transmission electron microscopy (TEM) were employed to characterize TGO. Photostimulated luminescence from a-Al2O3 was observed on grit-blasted bond coat regardless of deposition time, while luminescence from metastable gamma- and theta-Al2O3 was observed for TGO on as-coated (i.e., not grit blasted) bond coat. Luminescence from metastable Al2O3 decreased with increase in deposition time. On subsequent oxidation in air at 1000 and 1050 and 1100°C for 0.5, 10 and 50 hours, relative luminescence intensity from gamma- and theta-Al2O3 was observed to decrease. These observations from PSLS were validated by analytical TEM with specimen preparation by in-situ liftout focused ion beam (FIB) technique.
A3-2-9 Inter-diffusion between a Platinum Aluminide Bond Coat and Superalloy Substrate in EB-PVD TBCs during Thermal Cycling
M.W. Chen, K.J.T. Livi, M. Glynn, D. Pan, K.J. Hemker (Johns Hopkins University)
The recent discovery of a martensitic transformation in thermally cycled platinum modified diffusion aluminide bond coats suggests that phase transformation strain plays an important role in the development of stresses and strains in thermal barrier coating (TBC) systems. Quantitative electron micro-probe measurements have shown that the microstructural evolution is associated with changes of bond coat chemistry from an as-fabricated Al-rich alloy to a Ni-rich alloy during thermal cycling. Such changes of bond coat chemical composition have been attributed to the depletion of Al by the growth of alpha-alumina. However, in this paper it will be shown that inter-diffusion between the Ni-rich superalloy substrate and the Al-rich bond coat dominates the chemical composition evolution in bond coats. The chemical composition profiles of the bond coats with different thermal exposures have been quantified with electron micro-probe measurements. In addition, the bond coat/superalloy interfaces have been investigated by transmission electron microscopy (TEM), and the precipitates related to the composition change around the interfaces have been characterized. Combining with these experimental analyses, theoretical calculations have been performed in an attempt to clarify the inter-diffusion mechanism. In addition, the effects of platinum and grain boundaries on the inter-diffusion will be discussed.
A3-2-10 The Effects of Surface Condition of Bond Coats on the Failure of TBS Systems
N.M. Yanar, F.S. Pettit, G.H. Meier (University of Pittsburgh)
TBC failures are complicated due to numerous factors that play a role in their failures. Surface condition of the bond coat prior to TBC deposition is one of these factors and it is known to be very important for the durability of TBC systems. However, the optimum surface condition for best performance is still not clearly known.
In this study, EBPVD YSZ TBCs on platinum aluminide bond coats with different surface conditions (as aluminized, heavy grit blast, light grit blast+preoxidation, media finish+preoxidation) were subjected to cyclic oxidation testing at 1100°C. The results showed an improvement in life for specimens with a smoother interface, which were also given a preoxidation treatment prior to TBC deposition. In this presentation, the failure behavior of these TBC systems with various surface conditions as well as the possible reasons for the improvement of life for specimens with smoother interfaces will be given.
A3-2-11 Investigation on Bond Coat Surface Roughness On The Thermal Cyclic Behavior Of Thermal Barrier Coatings
D. Zhang, S. Gong (Beihang University, PR China); H. Xu (Beihang University, PR China)
It is well recognized that failure of thermal barrier coatings (TBCs) prepared by electron beam physical vapor deposition (EB-PVD) almost always occurred at the thermally grown oxides (TGO) layer between bond coat and top coat. Formation and growth controlling for TGO layer is a key point for the thermal cycling lifetime. It is considerable that surface roughness of bond coat will affect both the interface adhesion of bond coat and top coat and the formation of TGO.
In this study, conventional two-layered structure TBCs with different surface roughness of bond coat were produced by EB-PVD on Ni-base superalloy. The thickness of bond coat and top coat was approximately 60µm and 120µm, respectively. Before ceramic depositing, surface of bond coat was blasted by different sizes of Al2O3 sand to form different surface roughness, denoted by the value of Ra, of 0.82µm,1.57µm and 3.76µm, respectively. The thermal cyclic test was carried out by exposuring to air at 1323K for 0.5h, and then cooling to room temperature within 5min. by forced air-cooling. Knoop indentation test was used to evaluate the adhesion between bond coat and top coat and the sinter extent of ceramic top coat. Scanning electron microscopy (SEM) was carried out to study the microstructure of the coatings.
As the results, the roughness of bond coat significantly affects the lifetime of TBCs in the present roughness range. With the increase of roughness Ra on bond coat, the weight gain increased evidently during thermal cyclic and the lifetime decreased. The lifetime was 730hrs (1460 cyclic times) for the TBCs with surface roughness Ra of about 0.82µm, however, only 530hrs was counted for the TBCs with the surface roughness Ra of 3.76µm. The results showed that the adhesion state of coating can be reflected and sinter extent of ceramic top coat can be evaluated by Knoop indentation method in the TBCs system.