Richard D. Sisson

Microstructural characterization of thermal barrier coatings on high pressure turbine blades

Abstract Thermal barrier coated high pressure turbine blades were characterized before and after the service by microstructural analysis and Cr3+ photostimulated luminescence piezo-spectroscopy. Thermal barrier coatings, in this study, consisted of electron beam physical vapor deposited yttria partially stabilized zirconia (YSZ; ZrO2–8 wt.% Y2O3), vapor-deposited aluminide bond coat and Ni-base superalloy. Compressive residual stress in thermally grown oxide, measured by Cr3+ photostimulated luminescence piezo-spectroscopy, was observed to be in the order of 2.5∼3.0 GPa and varied slightly as a function of substrate geometry. X-Ray diffraction and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy were utilized to investigate the microstructural development of thermal barrier coatings. The as-deposited non-equilibrium tetragonal (t′) phase in the YSZ coatings was observed to decompose after the service, but the monoclinic (m) phase was only found in the YSZ coatings with concave substrate curvature on the pressure side of the HPT blade. Also, a significant sintering of ZrO2–8 wt.% Y2O3 coating after the service was observed in the microstructure. Localized spallation of YSZ occurred within the thermally grown oxide (mostly α-Al2O3) and within the ZrO2–8 wt.% Y2O3 coating for pressure and suction sides of the serviced high pressure turbine blade near the tip, respectively.

Microstructural development in physical vapour-deposited partially stabilized zirconia thermal barrier coatings

Abstract The effects of processing parameters of physical vapour deposition on the microstructure of partially stabilized zirconia (PSZ) thermal barrier coatings have been experimentally investigated. Emphasis has been placed on the crystallographic texture of the PSZ coatings and the microstructure of the top surface of the PSZ coatings as well as the metal-ceramic interface. The variations in the deposition chamber temperature, substrate thickness, substrate rotation and vapour incidence angle resulted in the observation of significant differences in the crystallographic texture and microstructure of the PSZ coatings.

Modelling the microstructural evolution and degradation of M-Cr-Al-Y coatings during high temperature oxidation☆

Abstract A finite difference computer program has been developed to model the microstructural changes that occur in overlay coatings during simultaneous oxidation and interdiffusion with the base alloy. The degradation of the coating is predicted by modelling the aluminum concentrations in the coating and calculating the phase distribution from the Co-Cr-Al phase diagram. The results of the model are compared with experimental results for the isothermal oxidation of a Co-Cr-Al-Y coating on MM509 and a Co-24Cr alloy. Excellent agreement is found for cases where the diffusion coefficients are known.

Ion-beam profiling of3He in tritium-exposed type 304l and type 21-6-9 stainless steels

The nuclear reaction3He (d, p)4He was used to determine3He depth profiles in high-energy-rate-forged Type 304L and Type 21-6-9 stainless steels following exposure to tritium. A sensitivity to3He better than 1 atomic ppm and a depth resolution of about 0.5 μm were achieved. The3He decay product provides an essentially immobile trace of the diffusing tritium, and as a result tritium diffusivity and solubility may be calculated from the measured3He distribution. Tritium exposure in these experiments occurred at 343 K under 45 MPa partial pressure. The diffusion coefficient obtained at 343 K was (1.6 ± 0.3) × 10−9 mm2 per second in both steels, while the solubility at 45 MPa partial pressure was 3700 ± 900 atomic ppm for Type 304L and 7500 ± 1900 atomic ppm for Type 21-6-9.

Diffusional interactions and reactions between a partially stabilized zirconia thermal barrier coating and the NiCrAlY bond coat

The diffusional interactions and reactions between a partially stabilized zirconia thermal barrier coating and the NiCrAlY bond coat have been experimentally investigated in a mixture of ZrO28wt.%Y2O3 and NiCrAlY alloy powders. The blended powders were cold isostatic pressed at 25 000 lb in.−2 (170 MPa) and heated in air at 1100°C and 1300°C for 2, 10 and 50 h. The microstructure of the heat-treated cermet compacts was experimentally investigated using X-ray diffraction and scanning electron microscopy with energy dispersive X-ray spectroscopy. It was found that Al2O3 formed first followed by Cr2O3 and the spinel (Ni(Al, Cr)2O4). The formation of the spinel may be related to the failure of zirconia thermal barrier coatings during thermal fatigue.

Diffusional and thermodynamic interactions in the Cu-Ni-Zn system at 775°C

Diffusion was investigated in both α(fcc) and β(bcc) phase regions of the Cu-Ni-Zn system at 775°C with solid-solid diffusion couples and interdiffusion coefficients were determined at several compositions. Intrinsic and interdiffusion coefficients were also estimated from available data on thermodynamic activities and tracer diffusivities for α Cu-Ni-Zn alloys; and the estimated coefficients were consistent with those experimentally determined. Large off-diagonal coefficients indicating strong interactions among the diffusing species were observed and could be appreciated in terms of the compositional dependence of the thermodynamic activities of the components.

Investigation of fatigue crack initiation in Ti-6Al-4V during tensile-tensile fatigue

Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy.

Isothermal Oxidation of Physical Vapor-Deposited Partially-Stabilized Zirconia Thermal Barrier Coatings

Thermal barrier coatings (TBCs), consisting of physical vapor deposited (PVD) partially stabilized zirconia (PSZ, 8 wt.%Y2O3) and a diffusion aluminide bond coat, were characterized as a function of time after oxidative isothermal heat treatment at 1373 K in air. The experimental characterizations was conducted by X-ray diffraction analysis and scanning electron microscopy (SEM) with energy-dispersive spectroscopy. During cooling to room temperature, spallation of the PSZ ceramic coatings occurred after 200 and 350 h of isothermal heat treatment. This failure was always sudden and violent, with the TBC popping from the substrate. The monoclinic phase of zirconia was first observed on the bottom surface of the PVD PSZ after 200 h of isothermal heat treatment. The failure of TBCs occurred either in the bond coat oxidation products of αAl2O3 and rutile TiO2 or at the interface between the oxidation products and the diffusion aluminide bond coat or the PSZ coating.

FEM Modeling of Induction Hardening Processes in Steel

A modeling system for analyzing the integrated induction hardening processes was developed based on a general-purpose finite element program, with the capability to analyze the whole process from electromagnetic-induced thermal heating to final hardening. A coupled electromagnetic-thermal model was applied to study the induction heating process, which includes consideration of nonlinear material characteristics on temperature. Also, arrangement of AC current density distribution was conducted to simulate practical induction coil structure and magnetic concentrator effects to achieve desired heating patterns for later quenching and hardening analysis. Quenching analysis can provide cooling curve at any location in a heat-treated workpiece based on heat transfer principles. In hardening analysis, phase transformation was studied and an algorithm was developed to determine volumetric content of micro-structural constituents formed from austenitized phase in quenching process, based on analysis of the interaction between cooling curve and material time-temperature-transformation (TTT) diagram. Finally, hardness value was converted from martensite content based on a developed formulation. Validation was preliminary conducted based on comparison of hardening pattern of induction hardening of an automotive spindle with complex surface.

Crystallographic texture of EB-PVD TBCs deposited on stationary flat surfaces in a multiple ingot coating chamber as a function of chamber position

Abstract The crystallographic texture of EB-PVD TBCs has been experimentally determined by pole figure analysis and X-ray diffraction. It was found that the TBC coating deposited on a stationary flat surface directly above an ingot or between the two ingots and off center exhibited single crystal texture. Coatings deposited at the far corners of the coating chamber revealed a fiber texture with 〈111〉 column growth direction. The Bragg–Brentano X-ray diffraction analysis revealed a variety of textures (i.e. crystallographic planes that are parallel to the substrate surface) with the most predominant planes being the (220), (311), (420) and (620).