PtAl Bond Coat Research Articles

Pt-Al bond coat dependence on the high-cycle fatigue rupture and deformation mechanisms of a fourth-generation single crystal superalloy at various temperatures

The impact of a Pt-Al bond coat on the high-cycle fatigue (HCF) behaviour of fourth-generation single crystal superalloys at 760℃ and 900℃ was investigated. The Pt-Al bond coat was found to be almost negligible under low stresses at 760℃; however, the bond coat effect was detrimental at 900℃ or under high stresses at 760℃. At 760 °C with increasing high-amplitude stress, the tip cracks preferentially nucleated from defects within coat and propagated inwards, accelerating the fracture. The fatigue life of the coated alloys was considerably decreased at 900 °C, which was attributed to the damage accumulated in the bond coating via oxidation, crack-induced oxide cracking and interfacial microstructure degradation. An Elevated temperature led to the crack-induced oxides shifting from a small fan-shape to a large umbrella-shape, increasing the density of slip bands and quantity of secondary cracks in the substrate. Furthermore, more Ru diffused outwards at 900℃ than that at 760℃, which caused the disappearance of L-C dislocation locks and tertiary γ′ phases and aggravated TCP phases precipitation in the substrate at 900℃. Ultimately, to assess the degradation of the HCF life, an empirical life prediction method was developed, and the calculations results well matched the test results.

Factors affecting cyclic lifetime of EB-PVD thermal barrier coatings with various bond coats

Abstract Standard EB-PVD (electron beam physical vapor deposition) thermal barrier coatings (TBCs) of partially yttria-stabilized zirconia were applied on Ni-base substrate alloys and thermally cycled with T max = 1100 °C. Two different bond coats were investigated: NiCoCrAlY and NiPtAl. The longest lifetimes have been achieved on the Hf-containing Rene142 substrate. A rough interface between TGO (thermally grown oxide) and bond coat with hafnia pegs is characteristic for these alloys. A bond coat heat treatment in Ar–H significantly improved the lifetime of the TBC system with IN100 + PtAl bond coats, whereas reduced lifetimes were found on IN100 + NiCoCrAlY. Spallation of the TBCs was correlated to TGO thickness and microstructure. The TGO growth is faster for NiCoCrAlY than for NiPtAl bond coats, while TBC lifetime is longer for the NiCoCrAlY systems. Tests with reduced cycle length yielded an increased number of cycles to failure for IN100+NiCoCrAlY, but left the time to failure at temperature unchanged. TBC thickness had only a limited influence on cyclic lifetime.

Effect of Pt-Al bond-coat on the tensile deformation and fracture behaviors of a second-generation SX Ni-based superalloy at elevated temperatures

An investigation was conducted into the influence of applying PtAl bond-coat on the tensile deformation and fracture behavior of second-generation single crystal superalloy substrate with the temperature ranging between room temperature RT and 1100 °C. In the meantime, the development of a two-layer PtAl bond-coat, consisting of an outer layer of single-phase β-(Ni,Pt)Al and an inter-diffusion zone of refractory metal precipitates, has also been discussed. As revealed by the results, the application of PtAl bond-coat contributed to a noticeable deterioration in both yield strength and ultimate tensile strength of the superalloy substrate at elevated temperatures. Nevertheless, the elongation of coated superalloy was shown to be 17.2% (at RT) and 10.4% (at 750 °C) lower and 9.6% (at 1100 °C) higher compared to uncoated samples. Following tensile test, the TEM analyses indicated that there were onsets of various slip systems, the traditional superlattice stacking faults were retarded at intermediate temperature and the dislocation debris could be increased to develop dislocation networks at higher temperature in the region of superalloy near the bond-coat. As the temperature was on the rise, the fracture mechanism of coated superalloy shifted from tearing ridges and dimples to quasi-cleavage and then to micro-void coalescence. While that of PtAl bond-coat was transformed from brittle to ductile rupture. Within the temperature range from RT up to 750 °C, a combination of long crack lengths and greater crack penetration depths into bond-coat caused more deterioration in the tensile properties of superalloy. When the temperature rose above 750 °C, the application of PtAl bond-coat improved the ductility of the substrate, as a result of the ductile failure in the bond-coat which hindered the development of through-thickness cracks.

Oxidation and concomitant effects on the microstructure and high temperature tensile properties of a DS Ni-base superalloy applied with different thicknesses of Pt-aluminide (PtAl) bond coat

The oxidation resistant diffusion PtAl coatings applied on the hot-section Ni-superalloy components of aero-engines exhibit phase transitions during thermal exposure in air: B2-NiAl➔B2-NiAl/L10-NiAl martensite+γ´-Ni3Al➔γ-Ni + γ´-Ni3Al. The present study correlates the oxidation-microstructure-tensile properties for DS CM-247LC superalloy coated with 60 and 90 μm thick inward grown PtAl bond coat. Cyclic oxidation of the superalloy, in the uncoated condition and after application of 60 and 90 μm thick PtAl coating, has been carried out at 1100 °C for various durations of 10, 100, 750 and 1000 h. The microstructure changes occurring in the coating and superalloy substrate during oxidation for various durations are examined. The concomitant tensile properties for the various oxidized samples are evaluated at 870 °C and their fracture behavior ascertained. Both coatings prevented surface degradation and imparted superior tensile properties, despite 750–1000 h of cyclic oxidation at 1100 °C. The morphology of the phases formed in the coatings affected the tensile properties of the coated superalloy. Cracks formed along the inter/intra-phase boundaries in the coatings. The predominant formation of martensitic L10 laths and films of single-phase γ´ at the prior B2/L10 phase boundaries in the thicker coating during intermediate-term oxidation, such as after 100 h, promoted the initiation of sharp cracks, which penetrated into the substrate and deteriorated the strength and ductility appreciably. On the other hand, if the B2/L10 martensitic phase was enveloped by the γ´ phase or the coating was comprised of γ´ surrounded by the γ phase, then the coating cracks had blunt tips and the tensile properties were not affected adversely. Albeit early phase transformations, the thin coating imparted superior tensile properties.

Burner Rig for Small Particle Erosion Testing of Thermal Barrier Coatings

Abstract The development of a high-temperature laboratory test to evaluate the erosion resistance of new turbine-blade thermal barrier coatings (TBC) for aircraft gas turbine engines is described. The focus is on improvements to a previously reported design, specifically: (1) larger duct and nozzle diameters leading to a larger and more uniform wear pattern; (2) incorporation of a new auger-style feeder for precise feeding of fine particles at low flow rates; (3) an aperture after the duct to limit the erosive damage at the edges of button specimens; (4) bag filters for particle feed rate measurements. The results of both extensive computational fluid dynamics (CFD) modeling and experiments concentrating on laboratory specimens fabricated from turbine-blade superalloys were used to demonstrate the validity of the test results to meaningfully evaluate the performance of new coating compositions in representative erosive environments, and to help understand damage mechanisms under such conditions. Two specimen geometries were tested in the improved rig using Mach 0.5 jets: “teardrop” burner bar and button specimens. Both types of specimens had electron beam–physical vapor deposition (EB-PVD) prepared ceramic layers of either ZrO2-Y2O3 (7YSZ) or ZrO2-Y2O3-Gd2O3-Yb2O3 (ZYGdYb) applied over PtAl bond coats using various processing parameters or interface treatments. Specimen temperatures for this study were either 980 °C (1800 °F) or 1090 °C (2000 °F).

Micromechanisms of fracture and strengthening in free-standing Pt-aluminide bond coats under tensile loading

Free-standing Pt-aluminide (PtAl) bond coats exhibit a linear stress–strain response under tensile loading and undergo brittle cleavage fracture at temperatures below the brittle-to-ductile transition temperature (BDTT). Above the BDTT, these coatings show yielding and fail in a ductile manner. In this paper, the various micromechanisms affecting the tensile fracture stress (FS) below the BDTT and yield strength (YS) above the BDTT in a PtAl bond coat have been ascertained and quantified at various temperatures. The micromechanisms have been identified by carrying out microtensile testing of stand-alone PtAl coating specimens containing different levels of Pt at temperatures between room temperature and 1100°C and correlation of the corresponding fracture mechanisms with the deformation substructure in the coating. An important aspect of the influence of Pt on the tensile behavior, slip characteristics, FS/YS and BDTT in the PtAl coating has also been examined. The addition of Pt enhances the FS of the coating by Pt solid solution strengthening and imparts a concomitant increase in fracture toughness and yet causes a significant increase in the BDTT of the coating.

Microtensile testing of a free-standing Pt-aluminide bond coat

A finite element method (FEM)-based study has been carried out for the design of flat microtensile samples to evaluate tensile properties of Pt-aluminide (PtAl) bond coats. The critical dimensions of the sample have been determined using a two-dimensional elastic stress analysis. In the present testing scheme, the ratio of the dimensions of the holding length to the fillet radius of the sample was found important to achieve failure within the gage length. The effect of gage length and grip head length also has been examined. The simulation predictions have been experimentally verified by conducting microtensile test of an actual PtAl bond coat at room temperature. The sample design and testing scheme suggested in this study have also been found suitable for evaluation of tensile properties at high temperature.

Erosion and High Temperature Oxidation Resistance of New Coatings Fabricated by a Sol-Gel Route for a TBC Application

This paper examines the erosion and cyclic oxidation performance of novel thermal barrier coatings produced via the sol-gel route. The ceramic top coat, with a thickness of 5-80 m, was deposited via a sol-gel route onto standard MCrAlY and PtAl bond coats. In both the erosion and the cyclic oxidation tests it was found that the bond coat had a profound affect on the results. The erosion of the sol-gel coatings were compared to standard EB PVD and PS TBCs and were found to be significantly higher. The effect of aging (100 h at 1100°C) on the erosion rates was also evaluated and was found to increase the erosion rates. The information obtained from the erosion and cyclic oxidation tests have highlighted the need to develop and optimise the parameters for producing thicker coatings

Factors affecting cyclic lifetime of EB-PVD thermal barrier coatings with various bond coats

Abstract Standard EB-PVD (electron beam physical vapor deposition) thermal barrier coatings (TBCs) of partially yttria-stabilized zirconia were applied on Ni-base substrate alloys and thermally cycled with Tmax = 1100°C. Two different bond coats were investigated: NiCoCrAlY and NiPtAl. The longest lifetimes have been achieved on the Hf-containing Rene142 substrate. A rough interface between TGO (thermally grown oxide) and bond coat with hafnia pegs is characteristic for these alloys. A bond coat heat treatment in Ar–H significantly improved the lifetime of the TBC system with IN100 + PtAl bond coats, whereas reduced lifetimes were found on IN100 + NiCoCrAlY. Spallation of the TBCs was correlated to TGO thickness and microstructure. The TGO growth is faster for NiCoCrAlY than for NiPtAl bond coats, while TBC lifetime is longer for the NiCoCrAlY systems. Tests with reduced cycle length yielded an increased number of cycles to failure for IN100+NiCoCrAlY, but left the time to failure at temperature unchanged...

Influence of bondcoat pre-treatment and surface topology on the lifetime of EB-PVD TBCs

In the present study two types of bondcoats (BC) were used, a NiCoCrAlY overlay coating produced by electron beam physical vapor deposition (EB-PVD) and a platinum aluminide diffusion coating (Ni,Pt)–Al. The coatings were deposited onto nickel-base IN100 and René142 substrates and were pre-treated in vacuum or Ar–H gas mixture prior to the deposition of an EB-PVD yttria partially stabilized zirconia top coat. The specimens were thermal cyclically tested in air at 1100 °C (50 min at 1100 °C/10 min forced air cooling). The microstructures of the thermally grown oxide (TGO) layers prior and after testing were examined by scanning electron microscopy and electron energy dispersive spectroscopy. Heat-treatment in Ar–H significantly improved the lifetime of the thermal barrier coatings (TBC) system with IN100+PtAl BC, whereas reduced lifetimes were found on IN100+NiCoCrAlY BC and René142+PtAl. High Y-contents of the NiCoCrAlY BC significantly reduced TBC lifetimes. Furthermore, the results for IN100+PtAl and René142+PtAl demonstrated that early TBC failure was associated with high TGO surface roughness of the metal/oxide interface, indicating that surface roughness has a significant effect on TBC system performance.

Damage Accumulation Mechanisms in Thermal Barrier Coatings

Progressive damage evolution leading to spallation was investigated in Electron Beam—Plasma Vapor Deposition (EB-PVD) partially stabilized zirconia thermal barrier coating (TBC) applied to Nickel-based single crystal superalloy, Rene N5 with PtAl bondcoat. Thermal cycles were between 200-1177C. Progressive damage evolution was monitored using microscopy on samples subjected to a series of thermal cycles. Fick’s law can describe the thermally grown oxide (TGO) thickness for early cycles. However, at higher number of thermal cycles, damage in the form of microcracks and their link-up results in the development of a larger delamination crack through the TGO layer and monitoring oxide thickness becomes difficult. Thus, both oxidation kinetics and damage appears to play significant roles as they relate to spallation. As the early microcracks coalesce to form a major delamination crack, the susceptibility for TBC buckling is increased. The damage thickness continues to increase with number of thermal cycles indicating a progressive buckling condition. Estimation shows that a delamination crack length of about sixteen times the TBC thickness is needed for the current material system to cause buckling. Progressive microcrack linking to form a large delamination crack followed by progressive buckling of the TBC layer appear to promote spallation. Physical evidence of microcrack link-up and progressive buckling was found in specimens prior to complete spallation.