Al Bond Coat Research Articles

The effect of bond coat grit blasting on the durability and thermally grown oxide stress in an electron beam physical vapor deposited thermal barrier coating

Photo-stimulated luminescence Piezo-spectroscopy (PLPS) is being developed as a non-destructive technique for thermal barrier coatings (TBC). In this study, the evolution of photo-stimulated luminescence with thermal cycling was systematically investigated from the thermally grown oxide (TGO) in a production TBC, which consists of an electron beam physical vapor deposited (EB-PVD) 7 wt.% Y 2O 3–ZrO 2 top coat, a grit blasted (Ni,Pt)Al bond coat and a CMSX-4 superalloy substrate. The change of compressive stress in the TGO layer on the bond coat with thermal cycling was calculated from the wavelength shift of the luminescence spectra. The compressive stress increased from 1.0–2.2 GPa in the as received state to 2.8–3.3 GPa at 10 cycles, then gradually decreased to 1.2–1.9 GPa until 500 cycles and remained at this level until TBC spallation. Other fluorescence spectra characteristics, such as the width of R1 and R2 peaks and their relative intensity, were also evaluated. These PLPS measurements on TBCs with grit blasted bond coats are compared with previous measurements on similar TBC system but with non-grit blasted bond coats. It is concluded that the initial increase in stress is associated with the formation of a continuous oxide layer. The lower stress of the specimens with the grit blasted bond coats compared to that of the as-coated bond coats is associated partly with the greater surface roughness. And the fast decline in compressive stress is the result of bond coat surface rumpling facilitated by the initially rougher surface. The lifetime of the TBCs with grit blasted bond coats varies over a narrow range, 600–750 cycles with an average of 675 cycles, which is related to their consistent bond coat surface roughness.

Martensitic transformation in CVD NiAl and (Ni,Pt)Al bond coatings

The martensitic phase transformation in single-phase β-NiAl and (Ni,Pt)Al coatings was investigated. After isothermal exposure to 1150 °C for 100 h, the β phase in both types of coatings was transformed to a martensite phase during cooling to room temperature. Martensitic transformation was also observed in the (Ni,Pt)Al bond coat with and without a YSZ top layer after thermal cycling at 1150 °C (700 1-h cycles). The transformation took place due to Al depletion in the coating from the formation of the Al2O3 scale and interdiffusion between the coating and superalloy substrate. The effects of the martensitic transformation on coating surface stability (‘rumpling’) via volume changes during the phase transformation are discussed with regard to TBC failure.

Analysis of the Cr 3+ luminescence spectra from thermally grown oxide in thermal barrier coatings

Photo-luminescence spectra from the alumina thermally grown oxide (TGO) in thermal barrier coating (TBC) systems (CMSX-4 substrates, Pt–Al or Pt diffusion bond coats (BCs), and electron-beam evaporation (EBPVD) YSZ) have been obtained after thermal exposure using a micro Raman instrument. The spectra were often composed of at least two contributing spectra corresponding to two different residual stress levels in the analysed volume, one from intact regions of the coating and the other from the regions where the stress has been relaxed by local damage. The composite spectrum was deconvoluted to give the two contributing spectra using a fitting procedure that preserves certain essential constraints in the parameters of the individual spectra. These constraints have been established as a function of stress level in the TGO. The deconvoluted spectra acquired from rectangular grids of analysis points have been used to map the distribution of the damaged regions. This approach can form the basis of a non-destructive technique for assessing the amount of accumulated damage and estimating the residual life of a TBC on an engine component.

Characterization of commercial EB-PVD TBC systems with CVD (Ni,Pt)Al bond coatings

Failure of electron-beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) with aluminide bond coats is strongly influenced by bond coat oxidation behavior. This study investigated oxide (Al 2O 3) formation during EB-PVD processing of TBCs with (Ni,Pt)Al bond coats. The effects of substrate composition, coating impurities and bond-coat grit-blasting on the oxide phases, residual stress and microstructure were evaluated. As-deposited, high-purity commercial bond coats contained high concentrations of sulfur and other impurities at their surfaces. Numerous small voids formed at the oxide–metal interface when as-deposited bond coats were oxidized during EB-PVD processing. Grit-blasting of (Ni,Pt)Al had a significant impact, since the formation of α-Al 2O 3 during EB-PVD processing was significantly enhanced and voids did not form beneath the scale. Preliminary cyclic oxidation testing suggested an influence of superalloy sulfur content on TBC durability.

Characterization of a cyclic displacement instability for a thermally grown oxide in a thermal barrier system

The mechanism responsible for the performance of a commercial thermal barrier system upon thermal cycling has been investigated. It comprises an electron beam physical vapor deposited (EB–PVD) yttria-stabilized zirconia thermal barrier coating (TBC) on a (Ni,Pt)Al bond coat. At periodic interfacial sites, the thermally grown oxide (TGO) that forms between the TBC and the bond coat at high temperature displaces into the bond coat with each thermal cycle. These displacements induce strains in the superposed TBC that cause it to crack. The cracks extend laterally as the TGO displaces, until those from neighboring sites coalesce. Once this happens, the system fails by large scale buckling. The displacements are accommodated by visco-plastic flow of the bond coat and “vectored” by a lateral component of the growth strain in the TGO. They depend upon the initial morphology of the metal/oxide interface. The observed responses are compared with the predictions of a ratcheting model.

Surface rumpling of a (Ni, Pt)Al bond coat induced by cyclic oxidation

The surface of an initially flat, platinum-modified nickel aluminide bond coat formed on a single crystal superalloy is shown to progressively roughen (“rumple”) with thermal cycling in air. Far less surface roughening occurs after isothermal oxidation or after the same number of thermal cycles but with a shorter high-temperature exposure in each cycle. Mechanisms of the observed rumpling and the implications of the bond coat surface evolution leading to the failure of thermal barrier coatings are discussed. It is concluded that local volume changes in the bond coat, caused by aluminum depletion and subsequent decomposition of the β-(Ni, Pt)Al phase, are responsible for the observed rumpling.

Measuring the Properties of Diffusion Aluminide Bond Coat Layers with Microsample Tensile Testing

ABSTRACTThe bond coat layer plays an important role in the TBC system by providing oxidation resistance and a foundation for the ceramic top coat. The thermal cyclic durability of a TBC is thought to be strongly dependent on the physical and mechanical properties of the bond coat layer. Attempts to measure these properties for as-deposited and thermally cycled diffusion aluminide bond coats have been greatly inhibited by the limited thickness (∼60 μm) of these coatings. In the present study, novel high temperature microsample tensile testing technique is employed to measure the Young's modulus (E), coefficient of thermal expansion (CTE), yield strength and stress relaxation behavior of an as-deposited (Ni, Pt)Al bond coat in the temperature range of 25 oC to 1150 °C. Preliminary results for these material parameters are reported here. They are being used as material inputs for an independent finite element analysis (FEA) study of the development of stresses in the TBC layers during thermal cycling.

Structural and chemical analyses of a thermally grown oxide scale in thermal barrier coatings containing a platinum-nickel-aluminide bondcoat

The structure and chemistry of the thermally grown oxide scale in a thermalbarrier coating having an electron beam-physical vapor depositedyttria-partially stabilized zirconia (YPSZ) topcoat and aplatinum–nickel–aluminide (Pt–Ni–Al) bondcoat werestudied using transmission electron microscopy. The scale consisted ofhexagonal aluminum oxide (α-Al2O3) in both as-coated and thermallycycled specimens; no metastable Al2O3 polymorphs were observed. In as-coatedspecimens, the scale's grains had a fine, columnarmorphology. ZrO2-rich dispersoids were observed both intra- andintergranularly throughout the scale. Thermally cycled specimens had aduplex scale structure: one band of grains adjacent to the YPSZ had anequiaxed morphology and contained ZrO2-rich dispersoids; a second band ofdispersoid-free grains adjacent to the bondcoat had a coarse, columnarmorphology. Porosity and cracks were associated with the interface betweenthe bands. The formation of the banded morphology and the cracking areproposed to be associated with the presence of the ZrO2-rich dispersoids.

Thermal/residual stress in an electron beam physical vapor deposited thermal barrier coating system

Elastic–plastic finite element models are used to define the thermal/residual stress state responsible for the observed failure behavior of an electron beam physical vapor deposited yttria stabilized zirconia thermal barrier coating on a Pt–Al bond coat. The failures were observed to start at grain boundary ridges, some of which evolved into oxide filled cavities. Finite element models are made of the actual interface geometries through the use of metallographic sectioning and image processing. There is a one to one correspondence of calculated tension in the oxide layer and the observed localized damage. Purely elastic analysis failed to show some important tensile regions associated with the observed failure.

TEM characterization of plasma-sprayed thermal barrier coatings and ceramic–metal interfaces after hot isostatic pressing

Plasma-sprayed thermal barrier coatings (TBCs) consisting of both a Ni–5%wt. Al bond coat (Ni–Al) and a thermally insulating 8%wt. Y 2O 3 partially stabilized ZrO 2 ceramic top coat were subjected to hot isostatic pressing (HIP). Transmission electron microscopy (TEM) with qualitative microanalysis was utilized to characterize the microstructural changes occurring during HIP treatment and to clarify the nature of the ceramic–metal interface [ZrO 2–(Ni–Al)] both in the as-sprayed and hipped states. During HIP treatment, the as-sprayed ZrO 2 experienced a certain degree of hot plastic deformation and recrystallization, forming dense and crackless fine-equiaxed tetragonal grains. However, due to nonuniformities of the as-sprayed structure, the degree of recrystallization of the original columnar grains changed from location to location, resulting in a recrystallized microstructure ranging from coarse equiaxed grains 1–2 μm in size to co-existing grains of equiaxed and columnar shapes, including still intact columnar grains. Through TEM observations of a plasma-sprayed ZrO 2–(Ni–Al) interface more than 20 μm long, it was found that the interface is amorphous in nature and unevenly thick and even discontinuous in some locations. The amorphous interfacial layer is composed largely of the Ni–Al bond coat materials together with some ZrO 2 ceramic material. After HIP treatment, the amorphous ZrO 2–(Ni–Al) interfacial layer is replaced by a continuous, dense and polycrystalline Al 2O 3 layer which can act as a diffusion barrier for elements both from the ZrO 2 coating to the bond coat and to the substrate, and also in the opposite direction.