Fractographic study of adhesion tested thermal barrier coatings subjected to isothermal and cyclic heat treatments (original) (raw)
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Bonding mechanisms in the application of thermal barrier coatings to turbine blades
2004
Thermal barrier coatings (TBC's) are used to protect gas turbine blades from environmental degradation as well as to increase thermodynamic efficiency. Most TBC systems consist of a ceramic thermal barrier coating such as partially stabilized zirconia adhering to an oxidation resistant bond coat, which in turn is bonded to the turbine blade. This is required since partially stabilised zirconia will not readily bond to superalloys. However, the TBC can fail in service either by bond coat oxidation or thermal expansion mismatch between the bond coat and the TBC. A systematic literature survey has shown that the superalloy substrate material, type of bond coat selected, with the coating application techniques i.e. thermal spray or Electron Beam PVD (EBPVD) plays a fundamental role in determining the failure mechanisms involved. This program of work is concerned with the development of coatings with enhanced temperature capabilities for turbine blade applications by understanding th...
International Journal of Fracture, 2008
Thermal barrier coatings (TBCs) have been extensively used in aircraft engines for improved durability and performance for more than fifteen years. In this paper, thermal barrier coating system with plasma sprayed zirconia bonded by a MCrAlY layer to SUS304 stainless steel substrate was performed under tensile tests at 1000 • C. The crack nucleation, propagation behavior of the ceramic coatings in as received and oxidized conditions were observed by high-performance camera and discussed in detail. The relationship of the transverse crack numbers in the ceramic coating and tensile strain was recorded and used to describe crack propagation mechanism of thermal barrier coatings. It was found that the fracture/spallation locations of air plasma sprayed (APS) thermal barrier coating system mainly located within the ceramic coating close to the bond coat interface by scanning electron microscope (SEM) and energy dispersive X-Ray (EDX). The energy release rate and interface fracture toughness of APS TBCs system were evaluated by the aid of Suo-Hutchinson model. The calculations revealed that the energy release rate and fracture toughness ranged, respectively, from 22.15 J m −2 to 37.8 J m −2 and from 0.9 MPa m 1/2 to 1.5 MPa m 1/2 . The results agree well with other experimental results.
Tensile toughness test and high temperature fracture analysis of thermal barrier coatings
Acta Materialia, 1997
In this paper, an effective fracture toughness test which uses interface fracture mechanics theory is introduced. This method is ideally suited for determining fracture resistance of multilayered thermal barrier coatings (TBCs) consisting of ceramic and bond layers and, unlike other fracture experiments, requires minimal setup over a simple tensile adhesion test. Furthermore, while other test methods usually use edge cracked specimens, the present test models a crack embedded within the coatings, which is more consistent with actual TBCs where failure initiates from internal voids or defects. The results of combined computational and experimental analysis show that any defects located within the ceramic coating can significantly weaken a TBC, whereas the debonding resistances of the bond coating and its interfaces are found to be much higher. In a separate analysis, we have studied fracture behavior of TBCs subjected to thermal loading in a high temperature environment. The computed fracture parameters reveal that when the embedded crack size is on order of the coating thickness, the fracture driving force is comparable to the fracture resistance of the coating found in the toughness test. In addition, the major driving force for fracture derives from the thermal insulating effect across the crack faces rather than the mismatch in the coefficients of thermal expansion. We have also investigated the effects of functionally graded material (FGM) within TBCs and found its influences on the fracture parameters to be small. This result implies that the FGM may not contribute toward enhancing the fracture toughness of the TBCs considered here.
Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats
Surface and Coatings Technology, 2007
Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.
Fracture behaviour of thermal barrier coatings after high temperature exposure in air
Materials Science and Engineering: A, 2005
Four point bending tests have been carried out on thermal barrier coating (TBC) specimens. The system studied consists of a plasma sprayed top coat (TC) of ZrO 2 6-8 wt.% Y 2 O 3 (YSZ), an FeNiCrAlY bond coat (BC) and a mild steel substrate. To study the influence of temperature on this system, two heat treatments were adopted. The specimens were held in a furnace for 2 h at 580 and 900 • C, and cooled inside the furnace to room temperature (RT). In addition, microstructural analyses were carried out after each treatment by using different techniques energy dispersive X-ray analysis (EDX); X-ray diffraction (XRD); Raman spectroscopy, and scanning electron microscopy (SEM)). Four point bending tests were carried out on heat-treated specimens and the results were compared with those for the as-sprayed condition. In situ observation of crack propagation events were carried out during the tests and it was found that cracks propagated through the TC-BC interface in specimens heat-treated at 580 • C heat treatment and also in the as-sprayed reference specimens. In contrast, after the 900 • C heat treatment they propagated across the BC-substrate interface. The drop in BC-substrate adhesion due to heat treatment at 900 • C prevailed over the mechanical weakness induced at the TC-BC interface.
Influence on Thermal Barrier Coating Delamination Behaviour of Edge Geometry
Fracture of Nano and Engineering Materials and Structures
Ceramic thermal barrier coatings are commonly used in gas turbine hot components (e.g., combustor liners/buckets and guide vane platforms). In components that are only partially coated or have cooling-air outlets, coating-end stress singularities may lead to the spallation of the coating. Depending on the geometry of the transition from coated to uncoated material, the severity of the stress singularity will vary. One way of decreasing the severity of the stress singularity is by introducing a chamfer angle φ < 90° at the coating end. In the present study, a thin thermal barrier coating system has been studied. Bondand top coats have been sprayed to a thickness of 150µm and 350µm, respectively. Vacuum-plasma-spraying technology was used, and the test specimens were rectangular (30x50x5mm) coupons of a nickel-based superalloy, Haynes 230. A NiCrAlSiY bond coat and an YB 2 B OB 3 B partially stabilised ZrOB 2 B top coat were used. In order to achieve well-defined chamfers, sprayed coupons were ground on the edges with SiC grinding paper to desired geometry. By inspections of cross-sections that had not undergone thermal fatigue cycling, it was ensured that no damage was introduced into the system. Mechanical testing was done in a thermal cyclic test rig where specimens are heated in a furnace and cooled with compressed air. FE modelling of the system has been done, aiming to support the findings from thermal fatigue tests. A parametric study including variation of the chamfer angle φ has been made and the stress state near the chamfer evaluated. Evaluation of fatigue damage can be done visually for observation of coating failure (macroscopic observation on coating surface). 20% area with complete spallation was considered as thermal barrier coating failure. For evaluation of damage development, additional light microscopy investigations of cross-sections have been carried out. Results show that the fatigue life benefits from introduction of a chamfer angle at the coating end during thermal fatigue cycling.
Failure Mode of Thermal Barrier Coatings for Gas Turbine Vanes Under Bending
International Journal of Turbo and Jet Engines, 2000
Crack propagation studies under bending are described which were performed with plasma sprayed zirconia bonded by a MCrAlY layer to Ni-base superalloy. Such thermal barrier composites are currently considered as candidate materials for advanced stationary gas turbine components. The crack propagation behaviour of the ceramic thermal barrier coatings (TBCs) at room temperature, in as received and oxidised conditions reveals that cracks grow linearly in the TBC with increase in bending load until about the yield point of the superalloy is reached. Approaching the interface between the ceramic layer and the bond coat, a high threshold load is required to propagate the crack further into the bond coat. Once the threshold is surpassed, the crack grows rapidly into the brittle bond coat without an appreciable increase in the load. At a temperature of 800°C, the crack is found to propagate only in the TBC (ceramic layer), as the ductile bond coat offers an attractive sink for stress relaxation. Effects of bond coat oxidation on crack propagation in the interface regime have been examined and are discussed.
Thermal Shock Testing of Thermal Barrier Coating/Bondcoat Systems
Journal of Materials Engineering and Performance, 2004
Various methods of thermal shock testing are used by aircraft and industrial gas turbine engine (IGT) manufacturers to characterize new thermal barrier coating systems in the development stage as well as for quality control. The cyclic furnace oxidation test (FCT), widely used in aircraft applications, stresses the ceramic/bondcoat interface, predominantly through thermally grown oxide (TGO) growth stress. The jet engine thermal shock (JETS) test, derived from a burner rig test, creates a large thermal gradient across the thermal barrier coating (TBC), as well as thermomechanical stress at the interface. For IGT applications with long high-temperature exposure times, a combination of isothermal preoxidation and thermal shock testing in a fluidized bed reactor may better represent the actual engine conditions while both types of stress are present. A comparative evaluation of FCT, JETS, and a combined isothermal oxidation and fluidized bed thermal shock test has been conducted for selected ceramic/bondcoat systems. The results and the failure mechanisms as they relate to the TBC system are discussed. A recommendation on the test method of choice providing best discrimination between the thermal shock resistance of the ceramic layer, the ceramic/bondcoat interface, and even substrate related effects, is given.