Diffusion Bond Coat Research Articles

Erosion behaviour of platinum aluminide bond coat on directionally solidified CM247 and AM1 single crystal superalloys

Thermal barrier coatings system normally consists of substrate, a bond coat and a ceramic top coat. Platinum aluminide diffusion bond coat are extensively used in modern aero-engine. Sometimes bond coat is subjected to solid particle erosion. The objective of present work is to examine the influence of test conditions on the erosion response of platinum aluminide bond coat. The erosion rate of bond coat deposited on CM247 and AM1 alloy is evaluated as function of test temperature, impact velocity and impact angle in this work. The results show that coating on AM1 alloy exhibits better erosion resistance than that on CM247. In general, erosion response is brittle. At elevated temperatures, oxidation affected erosion dominates. The erosion rate of Pt Al coating on both the substrates increases with increase in test temperature, impact velocity and impact angle. • The solid particle erosion of platinum aluminide coatings different superalloys has been evaluated as function of eropsion condition. • Different materials removal mechanisms under different test conditions are identified.

Refurbishment of thermally degraded diffusion Pt-aluminide (PtAl) bond coat on a Ni-base superalloy

The Ni-base superalloy components operating in the hot sections of gas turbine engines are applied with diffusion Pt-aluminide (PtAl) bond coat for enhanced oxidation resistance. During service, the components experience several thermal cycles and gas temperatures as high as 1400 °C. As a consequence, the PtAl coating undergoes undesirable changes in its composition as well as phase constitution, i.e. B2-NiAl → reversible B2/L10 NiAl martensite + γ′-Ni3Al → γ-Ni + γ′-Ni3Al. The oxidation resistance of the coating decreases due to continuous depletion of Al during use at high temperatures. Beyond a certain level of degradation, the bond coat becomes ineffective in terms of providing oxidation resistance. For further use of the component, the bond coat needs to be stripped and reapplied. The selective stripping of the degraded diffusion bond coat without inducing damage to the component surface is a challenging task. The present study describes a method for controlled stripping of a degraded Pt-aluminide (PtAl) bond coat on directionally solidified Ni base superalloy CM-247LC and the subsequent reapplication of a fresh bond coat on the stripped alloy surface. The stripping parameters, i.e. composition of the acid bath and dipping time, have been optimized using intermittent observation of the surface morphology and corresponding cross-sectional microstructure of the stripped samples. The microstructure and oxidation behavior of the refurbished coating formed on the superalloy after controlled stripping were comparable to those of the virgin coating. Pitting and cavitation damage was inflicted on the superalloy surface during uncontrolled rapid leaching, and the bond coats formed on these substrates did not develop the desired microstructure and exhibited inferior oxidation resistance.

Influence of Bondcoat Spray Process on Lifetime of Suspension Plasma-Sprayed Thermal Barrier Coatings

Development of thermal barrier coatings (TBCs) manufactured by suspension plasma spraying (SPS) is of high commercial interest as SPS has been shown capable of producing highly porous columnar microstructures similar to the conventionally used electron beam–physical vapor deposition. However, lifetime of SPS coatings needs to be improved further to be used in commercial applications. The bondcoat microstructure as well as topcoat–bondcoat interface topography affects the TBC lifetime significantly. The objective of this work was to investigate the influence of different bondcoat deposition processes for SPS topcoats. In this work, a NiCoCrAlY bondcoat deposited by high velocity air fuel (HVAF) was compared to commercial vacuum plasma-sprayed NiCoCrAlY and PtAl diffusion bondcoats. All bondcoat variations were prepared with and without grit blasting the bondcoat surface. SPS was used to deposit the topcoats on all samples using the same spray parameters. Lifetime of these samples was examined by thermal cyclic fatigue testing. Isothermal heat treatment was performed to study bondcoat oxidation over time. The effect of bondcoat deposition process and interface topography on lifetime in each case has been discussed. The results show that HVAF could be a suitable process for bondcoat deposition in SPS TBCs.

ZrB2-SiC-WC coating with SiC diffusion bond coat on graphite by spark plasma sintering process

Abstract A dense, crack free and well bonded ZrB 2 -SiC-WC top coating with silicon carbide diffusion bond coat was successfully prepared on graphite substrate by spark plasma sintering (SPS) at 1900 °C under 20 MPa pressure for 5 min. The thickness of top coat and diffusion bond coat was about 800 µm and 3 mm, respectively. The bonding strength was evaluated by bending test. In the bending specimens the fracture occurred not at the interface, but at the graphite substrate which indicates that the bonding strength of coating and substrate is higher than strength of graphite substrate. Phase continuity of silicon carbide from top coat to interface and diffusion zone led to high adhesion of coating to graphite.

Influence of the superalloy substrate in the synthesis of the Pt-modified aluminide bond coat made by slurry

The influence of the superalloy composition and crystal orientation on the formation of an unconventional thermal barrier coating system (TBC) is reported. A 6μm-thick Pt layer was first electrodeposited onto a directionally solidified (DS) CM-247LC and a single crystal (SX) PWA-1483 substrate followed by annealing. A water-based slurry containing Al microparticles was then applied and annealed to form simultaneously a ceramic top coat and a diffusion bond coat. Irrespective of the annealing treatment (Pt, slurry), thinner diffused layers were obtained with the SX than with the DS material. Dual PtAl2 and (Ni,Pt)Al phases formed in both diffused layers but Cr segregation at the coating/substrate interface restricted further ingress of Pt and of Al into the SX superalloy compared to the DS one. This was related to the greater Cr content in the SX than in the DS material and to the absence of grain boundaries in the former. Nevertheless, a ceramic top coat composed of hollow alumina spheres formed over the thermally grown oxide and the diffused layer on both substrates.

Influence of Surface Finish on the Cyclic Oxidation Lifetime of an EB-PVD TBC, Deposited on PtAl and Pt-diffused Bondcoats

Thermal barrier coatings (TBCs) are well established as protective systems for gas turbine hot path components, due to their ability, with substrate cooling, to reduce the maximum surface temperature experienced by the metal component. However, when subject to high temperature oxidation, cyclic heating and cooling during service, TBCs degrade in both thermal protection capability and mechanical stability as a result of a combined thickening of the alumina-thermally grown oxide and sintering of the ceramic top coat. Eventually the ceramic top coat spalls from the metallic substrates. The detailed failure mechanisms for the TBC often are complicated, reflecting a balance between defects introduced into the TBC during manufacture and service and the stored energy generated in the TBC as a result of cyclic thermal exposure. It has been shown that the surface finish influences the residual stress in the thermally grown oxide and thus the stored energy. In this study, the influence of substrate surface finish, prior to bondcoat manufacture, on the cyclic oxidation lifetime is examined. Two EB-PVD TBC systems, a zirconia 8 wt% yttria topcoat on a platinum aluminide bondcoat and a zirconia 8 wt% yttria topcoat on a platinum diffused γ+γ′ bondcoat have been studied. For these two systems, various substrate surface finishes have been investigated, including ground, grit blasted and polished and grit blasted surfaces. The lifetime data for these cyclic oxidation tests of EB-PVD TBCs on these two diffusion bondcoats, platinum aluminide and platinum diffused, on CMSX4, have been analysed statistically for the various surface finishes. It is shown that the variability in measured lifetime can be modelled using Weibull statistics. The role of surface finish on the Weibull model parameters, characteristic life (η) and Weibull modulus (β), are discussed in this paper and hence the role surface finish plays on the likelihood of early, short life, TBC failure. Based on this analysis a more optimised surface finish is recommended to extend TBC lifetimes with diffusion based bondcoats. Further, the platinum diffusion bondcoat is shown to outperform the platinum aluminide system once the substrate surface finish has been optimised.

Moisture-Induced TBC Spallation on Turbine Blade Samples

Delayed failure of TBCs is a widely observed laboratory phenomenon, although many of the early observations went unreported. “The weekend effect” or “DeskTop Spallation” (DTS) is characterized by initial survival of a TBC after accelerated laboratory thermal cycling, followed by failure when exposed to ambient humidity or water. Once initiated, failure can progress to completion quite dramatically in less than a second. To this end, the water drop test and digital video recordings have become useful techniques in studies at NASA (Smialek, Zhu, Cuy), DECHMA (Rudolphi, Renusch, Schütze), and CNRS Toulouse/SNECMA (Déneux, Cadoret, Hervier, Monceau). In the present study the results for a commercial turbine blade, with a standard EB-PVD 7YSZ TBC top coat and Pt-aluminide diffusion bond coat are reported. Cut sections were intermittently oxidized at 1100°, 1150°, and 1200°C and monitored by weight change and visual appearance. Failures were distributed widely over a 5–100hr time range, decreasing with oxidation temperature. At some opportune times, failure was captured by video recording, documenting the appearance and speed of the moisture-induced spallation process. The failure interfaces exhibited, on the TBC side, alumina scale grains (decorated with Ta-rich oxide particles) and, on the exposed bare metal surface of the blade, imprints from the scale grains (with alumina islands and streamers). The phenomenon is based on moisture-induced delamination at the alumina scale - bond coat interface. Cycling damage is a contributing factor as cracking allows access of moisture to this interface, while high strain energy provides the driving force for spallation. It has been proposed that moisture reacts with aluminum in the bond coat and releases hydrogen atoms that ‘embrittle’ the interface. A modified chemical viewpoint of scale adhesion results, including a negative synergistic effect with interfacial sulfur.

On the interfacial degradation mechanisms of thermal barrier coating systems: Effects of bond coat composition

Thermal barrier coating (TBC) systems based on an electron beam physical vapour deposited, yttria-stabilized zirconia (YSZ) top coat and a substrate material of CMSX-4 superalloy were identically prepared to systematically study the behaviour of different bond coats. The three bond coat systems investigated included two β-structured Pt–Al types and a γ–γ′ type produced by Pt diffusion without aluminizing. Progressive evolution of stress in the thermally grown aluminium oxide (TGO) upon thermal cycling, and its relief by plastic deformation and fracture, were studied using luminescence spectroscopy. The TBCs with the LT Pt–Al bond coat failed by a rumpling mechanism that generated isolated cracks at the interface between the TGO and the YSZ. This reduced adhesion at this interface and the TBC delaminated when it could no longer resist the release of the stored elastic energy of the YSZ, which stiffened with time due to sintering. In contrast, the TBCs with Pt diffusion bond coats did not rumple, and the adhesion of interfaces in the coating did not obviously degrade. It is shown that the different failure mechanisms are strongly associated with differences in the high-temperature mechanical properties of the bond coats.

Characterisation of residual stress and interface degradation in TBCs by photo-luminescence piezo-spectroscopy

Residual stress in the TGO in two different TBC systems, one with a Pt aluminide (β structure) bond coat and another with a Pt diffusion (γγ′ structure) bond coat, were studied using photo-luminescence piezo-spectroscopy (PLPS). The luminescence spectra and TGO morphology were investigated progressively with thermal cycling at 1135 °C. The two TBC systems were found to have distinctly different TGO residual stresses and different failure modes. Several stress relaxation mechanisms were found to be operative in the Pt aluminide system, while no stress relaxation was evident in the Pt diffusion system until close to the end of life (spallation). Luminescence spectral shape evolution has been quantitatively analysed and correlated with TBC system degradation processes. Both TBC systems showed clear spectral shape changes as a result of the formation of interface cracks when they reached approximately 75% lifetime. Characteristic spectral shape changes in response to different types of interface crack were demonstrated experimentally. The correlation between spectral shape evolution and interface degradation opens a new avenue for studies of degradation and lifetime assessment of TBCs.

Study of fracture behaviour of bond coats on nickel superalloy by three-point bending of microbeams

A microbeam testing geometry is designed to study the variation in fracture toughness across a compositionally graded NiAl coating on a superalloy substrate. A bi-material analytical model of fracture is used to evaluate toughness by deconvoluting load–displacement data generated in a three-point bending test. It is shown that the surface layers of a diffusion bond coat can be much more brittle than the interior despite the fact that elastic modulus and hardness do not display significant variations. Such a gradient in toughness allows stable crack propagation in a test that would normally lead to unstable fracture in a homogeneous, brittle material. As the crack approaches the interface, plasticity due to the presence of Ni 3Al leads to gross bending and crack bifurcation.

The influence of bondcoat and topcoat mechanical properties on stress development in thermal barrier coating systems

A finite-element study has been undertaken to investigate the stress development within a TBC system consisting of an EB-PVD YSZ topcoat and a Pt-aluminized diffusion bondcoat. Particular attention has been paid to the role of variables such as the elastic anisotropy within the topcoat, interface roughness, variation in creep strength of the bondcoat and the volumetric strains associated with the formation of the thermally grown oxide (TGO). Bond coat oxidation and thermal loading during cooling give rise to significant tensile stresses within the topcoat and tensile tractions at the TGO interfaces. Bondcoat creep, as distinct from yield and plastic behaviour, was the dominant stress relaxation process, and strong bondcoats (in creep) tended to show higher tensile stress levels. Another important factor determining thermal barrier coating stress levels was the level of elastic anisotropy of the topcoat: an elastic isotropic yttria-stabilized zirconia gave rise to considerably higher stresses than a transversely isotropic topcoat.

Investigation of TBCs on turbine blades by photoluminescence piezospectroscopy

Thermal barrier coating (TBC) blade specimens with Pt diffusion bond coats were subjected to thermal cycling with periodic measurements of the residual stress in the thermally grown oxide (TGO) using photoluminescence piezospectroscopy. Two distinct stress levels were generally found to coexist in the probed volume, i.e. a high stress (∼4 GPa) and a low stress (∼500 MPa) level. Both the high and low stress levels were independent of the curvature of the blade surface, in agreement with numerical modelling based on a composite cylinder stress model. The relative contributions of the two stress levels appear to be correlated with the θ-Al 2O 3 content of the TGO, which was dependent on the position on the blade. The TBCs tended to fail along the TGO/bond coat interface in thermal cycling. This was modified by the presence of mixed transition metal oxides in the TGO. The results are interpreted in terms of a likely failure mechanism.

Effect of thermally grown oxide (TGO) microstructure on the durability of TBCs with PtNiAl diffusion bond coats

The role of pre-oxidation surface treatments on the oxide microstructure and the failure mechanism of multi-layer thermal barrier systems based on Pt-modified NiAl bond coats and electron beam deposited thermal barrier coatings (TBCs) have been studied. The primary pre-oxidation experimental variable was the partial pressure of oxygen in the pre-oxidizing atmosphere at constant temperature and bond coat composition. The durability of TBCs deposited on surfaces following different pre-oxidation treatments were measured and compared using furnace cycling tests. The oxide layers corresponding to different levels of TBC performance were characterized microstructurally, chemically, and compositionally using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques. TBC performance was enhanced by the formation of a surface oxide having a coarse-grained columnar structure during the pre-oxidation process. Increased TBC durability was consistent with a slower oxide growth rate during exposure of the TBC to high-temperature, cyclic conditions, as was observed for this particular pre-oxidation condition. An oxide microstructure having fewer through-thickness transport pathways (grain boundaries) should also result in slower lateral oxide growth rates, consistent with a slowed rate of ratcheting as was observed in the pre-oxidized samples that had the best TBC performance. The desired surface oxide grain structure was achieved by pre-oxidizing the bond coat prior to TBC deposition at an intermediate partial pressure of oxygen.

The evolution of residual stress in the thermally grown oxide on Pt diffusion bond coats in TBCs

The evolution of residual stress in the thermally grown oxide (TGO), formed on bond coats (BC) produced by diffusion of Pt into a Ni-based superalloy, has been studied as a function of thermal cycling using photo-luminescence piezo-spectroscopy through the yttria-stabilised zirconia (YSZ) thermal barrier coating. The luminescence spectra were analysed in terms of a high stress and a low stress contribution to the residual stress sampled at a single analysis point (approx. 20 μm in size); the low stress contribution being caused by either local TGO fractures or interfacial delamination The mean compressive residual stress increases during the first two thirds of the lifetime due to gradual stiffening of the non-planar TGO as it thickens and becomes more resistant to bending. In the last third of life the mean stress tends to decrease due to relaxation by increasingly numerous local damage events. This is more pronounced as a dramatic increase in the parameters that quantify the extent of the low stress contributions. Spatial mapping of the low stress contributions shows that they are isolated at first, but become more numerous towards the end of life and coalesce into larger regions of damage a few hundred μm in size. Eventual spallation is by unstable delamination at the BC/TGO interface and the stored elastic energy released is between 40 and 80 J m −2. The lifetime is governed by the growth of the local damage regions until one (probably near an edge) reaches a critical size above which it propagates in a highly unstable way over most of the interface.

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.

Isothermal and thermal cyclic oxidation behaviour of thermal barrier coatings: Pt aluminide bond coats

β-phase and γ–γ′-phase Pt aluminide, two diffusion bond coats, were applied respectively to CMSX4 and RR3000, two single-crystal Ni-base superalloys, with and without a yttria partially-stabilised zirconia, EB-PVD top coat. The oxide scales developed on RR3000/γ–γ′-phase Pt aluminide systems without YPSZ top coat were essentially α-Al2O3 at 1,200°C and α-Al2O3 plus γ-Al2O3 at 1,100°C. The oxide scales developed during thermal-cycling oxidation on RR3000/γ–γ′-phase, Pt aluminide/YPSZ were found to be more resistant to spallation than the scales grown on CMSX4/β-phase, Pt aluminide/YPSZ. Spallation of the oxide scale occurred mainly at the bond coat–oxide scale interface but also within the oxide and at the oxide–top coat interface.