Curtis Alan Johnson

Role of environment deposits and operating surface temperature in spallation of air plasma sprayed thermal barrier coatings

Spallation of air plasma sprayed (APS) thermal barrier coatings (TBCs) was investigated on power generation combustors, military turboshaft engines, and commercial turboprop engines. In each case, irrespective of operating conditions or geographic location, spallation was linked to the presence and infiltration of high temperature molten phases of similar composition. Electron microprobe analysis found that, from all the possible oxides available in the external environment, only CaO, MgO, Al2O3 and SiO2 (CMAS) are incorporated in the molten phase that infiltrates the TBC microstructure. Fe and Ni oxides from metallic components and zirconia and yttria from the TBC were also found in varying amounts in the molten phase. The melting and recrystallization behavior of CMAS deposits was carefully defined by differential thermal analysis.

Relationships between residual stress, microstructure and mechanical properties of electron beam–physical vapor deposition thermal barrier coatings

Abstract Residual stresses develop in coatings during deposition and can have a large impact on coating mechanical properties and durability. In this study, in-plane residual stresses in Electron beam–physical vapor deposited (EB–PVD) thermal barrier coatings (TBCs) were characterized by the change in substrate curvature upon coating removal, and in-plane elastic moduli were measured from the resonant frequency of the coating–substrate system. Variations in deposition conditions were observed to produce in PVD TBCs a wide range of stress levels, between −70 and 20 MPa. The residual stress was observed to be correlated strongly with the in-plane elastic modulus. A significant difference in the in-plane elastic modulus was measured along different directions of PVD TBC specimens fabricated by rotating the specimens over the evaporation source. The elastic modulus in the direction perpendicular to the axis of rotation was always significantly lower than the modulus measured along the axis of rotation. The elastic modulus measured perpendicular to the axis of rotation was associated with compliant microstructural features produced by the rotation of the substrate over the melt pool. Strain tolerance was measured directly by a new mechanical test that measured the strain at delamination of a coating from an edge-initiated crack from a substrate that was loaded in compression. The strain tolerance of the coating decreased with increasing residual stress.

Effect of gas pressure on thermal conductivity of zirconia thermal barrier coatings

Abstract The thermal diffusivity of thermal barrier coatings (zirconia stabilized with 7–8 wt% yttria) deposited by air plasma spraying has been measured by the laser flash method. Free-standing samples (0.5–0.6-mm thick) polished on both sides were coated with graphite. Deduced values of thermal conductivity (e.g., 1.5 W m −1 K −1 at 200 °C in 1 atm. of nitrogen) are in the range measured in other studies for this type of thermal barrier coating. The % increase in thermal conductivity in 1 atm. of gas compared to its value in vacuum was found to be about 10% in nitrogen at 500 °C, close to a calculated value of 7% using a model in the literature. A simple extension of the model to higher temperatures and gas pressures predicts a significant rise, so that by 10 atm. of gas pressure the % increase would be 30% (with little sensitivity to temperature).

Thermomechanical behavior and phase relationships of plasma-sprayed zirconia coatings

Abstract The performance of plasma-sprayed zirconia coatings, used as thermal barriers in gas turbine applications, is influenced by residual stresses. Coating stresses are induced by the plasma spray process and are affected by thermophysical properties of the system. Stress alteration as a function of temperature and time was evaluated using bimaterial dilatometry. Reversible changes in stress associated with ΔαΔT relationships and phase transformations were observed along with irreversible changes owing to oxidation, metastable phase transformations and sintering of the coating materials. Dilatometric studies of yttria (8 wt.% (-stabilized zirconia coatings which had been chemically removed from the substrate indicated that a stable phase transformation occurs at around 1200 °C. The presence of this phase transformation suggests that changes are required in the generally accepted phase diagram by Scott. The changes and supporting evidence will be presented.

Role of Environmental Deposits in Spallation of Thermal Barrier Coatings on Aeroengine and Land-Based Gas Turbine Hardware

Thermal barrier coating (TBC) spallation on power generation combustors was compared with TBC spallation observed both in military turboshaft engines, and in commercial turboprop engines. In each case, irrespective of operating conditions or geographic location, spallation was linked to the presence and infiltration of high temperature molten phases of similar composition. Electron microprobe analysis found that, from all the possible oxides available in the external environment, only CaO, MgO, Al2O3 and SiO2 (CMAS) are incorporated in the molten phase that infiltrates the TBC microstructure. Iron and nickel oxides from turbine components and zirconia and yttria from the TBC were also found in varying amounts in the molten phase.Melting of environmental deposits in conjunction with infiltration was found to result in: densification of the TBC, an increase in its Young’s modulus and an increase in the room temperature compressive stress in the TBC. Delamination of the TBC during thermal cycling is, thereby, attributed to changes in the mechanical properties and associated changes in the stress state of the coating due to infiltration of the environmental deposit.Copyright

Development of Advanced Thermal Barrier Coatings for Severe Environments

Air plasma sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs) have been successfully used to extend life of superalloy components in utility gas turbines. GE Power Generation has over ten years of field experience with TBCs on combustor hardware, and over 20,000 hours of field experience with TBCs on turbine nozzles. Despite this promising experience, the full advantage of TBCs can be achieved only when the reliability of the coating approaches that of the superalloy component substrate. Recent work at GE has emphasized characterization of mechanical properties and physical attributes of TBCs to understand better the causes of delamination crack growth and coating spallation. In addition, unique tests to examine the TBC response under conditions simulating severe gas turbine service environments have been developed. Through evaluation of the results from comparative TBC ranking tests, pre-production application experience and field test results, gas turbine design engineers and materials process engineers are rapidly gaining the practical knowledge needed to integrate the TBC into the component design.Copyright

Finite Element Analysis of Crack-Path Selection in a Brick and Mortar Structure

Many natural composite materials rely on organized architectures that span several length scales. The structures of natural shells such as nacre (mother-of-pearl) and conch are prominent examples of such organizations where the calcium carbonate platelets, the main constituent of natural shells, are held together in an organized fashion within an organic matrix. At one or multiple length scales, these organized arrangements often resemble a brick-and-mortar structure, with calcium carbonate platelets acting as bricks connected through the organic mortar phase.