Protect Turbine Blades Research Articles

Characterization of chemical and microstructural evolutions of a NiPtAl bondcoat during high temperature oxidation

Bondcoats used to protect turbine blades, such as platinum modified NiAl alloys, are designed to develop a protective alumina scale during exposure conditions at high temperatures. However, during high temperature oxidation, the system is subjected to chemical and microstructural changes that arise from the consumption of aluminium to ensure alumina growth and interdiffusion between the underlying nickel-based superalloy and the bondcoat. The aim of the present work is to report experimental results concerning the chemical and microstructural evolutions of a NiPtAl bondcoat, deposited on a single crystal nickel-based superalloy, during isothermal oxidation tests at 1100 °C, up to 50 h. Analytical STEM studies were carried out, in conjunction with Auger experiments, in order to follow the various changes that occur in the bondcoat and at the Al 2O 3/bondcoat interface. Efforts were concentrated on the effect of interfacial sulfur segregation (at both intact interface and void surface) as a function of the oxidation time, as well as its dependence on phase transformations in the external layer of the bondcoat. Strong S segregation at some bondcoat NiPtAl/Al 2O 3 interface, especially at γ′/Al 2O 3, was found in co-segregation with Cr, which has diffused from the substrate.

Combined coating for turbine blades of high-temperature gas turbine engines

A combined coating for protecting turbine blades of high-temperature gas turbine engines is studied. Comparative tests of coatings under laboratory conditions and of coated blades in engine operation are performed. The microstructure of the coating is studied and the concentration profiles of alloying elements are determined by the method of x-ray diffraction analysis. Tests for high-temperature strength are performed.

Effect of morphology on thermal conductivity of EB-PVD PYSZ TBCs

Partially yttria stabilized zirconia (PYSZ) based thermal barrier coatings (TBC) manufactured by electron beam-physical vapour deposition (EB-PVD) protect turbine blades, working under severe service conditions in aero engines and stationary turbines. These coatings show a high strain tolerance relying on their unique morphology which is comprised of weakly bonded, preferred-oriented columns, voids between feather-like sub-columns and, finally, of intra-columnar closed pores.The results obtained in this work demonstrate that variation of the EB-PVD process parameters alters the resulting columnar morphology and porosity of the coatings. The physical properties and, most importantly, thermal conductivity, are greatly affected by these morphological alterations. This study investigates three morphologically different EB-PVD PYSZ TBC top coats in terms of the spatial and geometrical characteristics of their porosity and correlates those with the thermal conductivity values measured in as-coated state and after heat treatment at 1100 °C for 1 h and 100 h. Changes in the open and closed porosity caused by heat-treatment are characterized by small-angle neutron scattering (SANS), Brunauer–Emmett–Teller Method (BET) and scanning electron microscope (SEM). Correlation of shape and surface-area changes in all porosity types of the analysed coatings revealed that the thermal conductivity of these coatings is influenced primarily by size and shape distribution of the pores and secondarily by the pore surface-area available at the cross section perpendicular to the heat flux.

Magnetron Sputtered Intermetallic Al2Au and Al-Zr-Y Coatings for the Oxidation Protection of γ-TiAl

AbstractAl-based intermetallic coatings are widely used as bond coats and for oxidation protection of turbine blades and engine components. Here we present and discuss the structural and thermal properties of novel unbalanced-magnetron sputtered Al-based coatings (single-phase intermetallic Al2Au, binary Al-Zr, and nano-structured Al-Zr-Y) developed to protect gamma-TiAl from environmental attack at elevated temperatures. Al-Zr films exhibit a coarse-grained dual-phase Al3Zr2-Al2Zr structure and are nano-structured by alloying with ~5, 10, and 14 at% Y.Combined dynamic differential-scanning calorimetry and thermogravimetric analyses up to a temperature of 1150 °C reveal that the Al2Au film is very stable with only marginal mass gain from oxidation found between 800 and 1000 °C. High temperature X-ray diffraction shows that this coating retains its (311) texture up to 900 °C where Al2O3 formation leads to the depletion of Al in Al2Au and subsequently the precipitation of intermetallic AlAu. When gamma-TiAl is coated with Al2Au and exposed to cyclic oxidation tests at 750 and 850 °C good oxidation resistance is obtained as a protective oxide layer is formed.Dual-phase Al3Zr2-Al2Zr coatings form ZrO2 and Al2O3 in oxidizing atmosphere. However, the phase transition from monoclinic (m-)ZrO2 to tetragonal (t-)ZrO2 with the accompanying volume change causes flaking of the oxide. Yttrium addition to the Al-Zr films stabilizes the cubic (c-) and t-ZrO2 and hence avoids the fatal tetragonal-monocline transformation. The thermally grown c-ZrO2 based oxides allow good adhesion to thermal barrier coatings which are themselves based on c-ZrO2.

Study of sulfur distribution in a NiPtAl bondcoat

The degradation of coatings used to protect turbine blades is closely linked to spalling resistance, which depends on the stability of the protective oxide scale produced by oxidation of the bond coat. In the present study, TEM microstructural observations associated with SIMS analyses were performed, according to different experimental conditions, to describe the microstructural and chemical changes occurring in a NiPtAl bond coat deposited on a nickel based superalloy, as well as elemental segregation at the Al2O3/NiAlPt interface.Interfacial sulfur segregation is well known to be responsible for alumina spallation during exposure at high temperature, this phenomenon often being linked to cavity formation and growth. Sulfur detection was achieved using the SIMS technique which enables S segregation to be detected at the oxide/BC interface or within the bond coat layer.The purpose of the present study was to compare the degree of S segregation, at the scale/BC interface and within the NiPtAl alloy, for different Ni based alloys (two S contents) and different oxidation conditions (isothermal and cyclic). The results obtained showed that, at the TGO/BC interface, the concentration of voids depends on the initial sulfur content in the superalloy for isothermal treatments. On the contrary, after cyclic tests, interfacial sulfur enrichment increases while the interfacial porosity fraction remains constant. These results agree with the proposal that sulfur segregation occurs at both cavity surfaces and intact interfaces.

Synthesis and Oxidation Behavior of Nanocrystalline MCrAlY Bond Coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al2O3 layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.

Studies on the transient stage of oxidation of VPS and HVOF sprayed MCrAlY coatings

Abstract Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion. They consist of a MCrAlY bond coating and a ceramic top layer. The oxidation resistance of MCrAlY coating is based on the formation of α-Al2O3 in the steady-state stage of oxidation. The α-Al2O3 grows very slowly between the metallic bond coating and the ceramic top coating and is thermodynamically stable. High-temperature oxidation experiments with vacuum-plasma-sprayed (VPS) and high-velocity-oxygen-fuel (HVOF)-sprayed MCrAlY coatings showed that the transient stage of oxidation for HVOF coatings is very short. Moreover, in contrast to the VPS coatings, where also Cr2O3, NiO or CoO are identified, on HVOF coatings, only metastable modifications of alumina were observed. The transformation of metastable alumina modifications to α-Al2O3 is very fast. Oxidation experiments in a high-temperature chamber attached to a X-ray diffractometer were carried out. The morphology and the structure of the formed oxide scale were characterized by X-ray diffraction and scanning electron microscopy (SEM).

Coatings for the Protection of Turbine Blades From Erosion

Many types of turbines, including aircraft gas turbines, steam turbines, and power recovery turbines, suffer from solid particle erosion caused by a variety of materials ingested into the machines. Utilization of various laboratory erosion tests tailored to the specific application by using various erodents, temperatures, velocities, and angles of impact, have been shown to be effective in the development and selection of coatings for the erosion protection of turbine blades and other components. Detonation gun coatings have demonstrated their efficacy in providing substantial protection in many situations. It has now been shown that several tungsten carbide and chromium carbide Super D-Gun™ coatings not only have better erosion resistance than their D-Gun analogs, but cause little or no degradation of the fatigue properties of the blade alloys. Nonetheless, caution should be employed in the application of any laboratory data to a specific situation and additional testing done as warranted by the turbine designer.

Application of the Analytical Electron Microscope to the Characterization of Protective Coatings on Nickel-Base Superalloys

The combined chemical and structural analysis capabilities of the analytical electron microscope (AEM) make it an important tool for understanding the nature of protective coatings on nickel-base superalloys. One of the important coating systems for protecting turbine blades from oxidation and hot corrosion is a MCrAlY alloy applied by electron beam physical vapor deposition (where M = Ni, Co, or Fe). In the as deposited condition this coating consists of a fairly open, generally columnar structure. A post-coating heat treatment on the order of 2 to 4 hours at approximately 1100°C is normally given to the coating. The major coating constituents are a β[(Ni,Co)Al] intermetallic phase and a γ[Co,Cr,Ni] solid solution phase. This heat treatment results in the formation of a ∼ 10 μm interdiffusion zone of phase transformation products at the coating/substrate interface and a thin protective layer (< 1 μm) of Al2O3 at the coating surface (Fig. l). While the coating contains just a few tenths of a percent yttrium, this yttrium plays an important role in improving the adherence of the protective Al2O3 layer. It has previously been proposed that the yttrium causes oxide pegs to develop at the Al2O3/coating alloy interface and that these pegs improve oxide adherence by mechanically keying the Al2O3 to the coating alloy.

The Problem of Burning Residual Oils in Gas Turbines

The burning of residual oil fuels in gas turbines raises some special problems: the efficient combustion of the solid particles to which these fuels give rise, the avoidance of ash deposits, and the protection of turbine blades and other components from corrosion effects peculiar to these fuels. This lecture discusses these problems and the methods which are being considered for solving them. It is shown that the burning of these heavy oils is not the greatest of the difficulties, since with suitably designed equipment this process can be quite efficiently performed. What is difficult is the avoidance of ash deposition and of the secondary effects, notably blade corrosion, to which this deposition gives rise. These effects are considered in some detail.