A Review of the Erosion of Thermal Barrier Coatings.

The application of thermal barrier coatings (TBCs) to components with internal cooling in the hot gas stream of gas turbine engines has facilitated a steep increase in the turbine entry temperature and the associated increase in performance and efficiency of gas turbine engines. However, TBCs are susceptible to various life limiting issues associated with their operating environment including erosion, corrosion, oxidation, sintering and foreign object damage (FOD). This is a review paper that examines various degradation and erosion mechanisms of TBCs, especially those produced by electron beam physical vapour deposition (EB-PVD). The results from a number of laboratory tests under various impact conditions are discussed before the different erosion and FOD mechanisms are reviewed. The transitions between the various erosion mechanisms are discussed in terms of the D/d ratio (contact area diameter/column diameter), a relatively new concept that relates the impact size to the erosion mechanism. The effects of ageing, dopant additions and calciumâ  magnesiumâ  aluminaâ  silicates on the life of TBCs are examined. It is shown that while ageing increases the erosion rate of EB-PVD TBCs, ageing of plasma sprayed TBCs in fact lowers the erosion rate. Finally modelling of EB-PVD TBCs is briefly intr

Erosion, corrosion and erosion-corrosion of EB PVD thermal barrier coatings

Electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings (TBCs) have been used in gas turbine engines for a number of years. The primary mode of failure is attributed to oxidation of the bond coat and growth of the thermally grown oxide (TGO), the alumina scale that forms on the bond coat and to which the ceramic top coat adheres. Once the TGO reaches a critical thickness, the TBC tends to spall and expose the underlying substrate to the hot gases. Erosion is commonly accepted as a secondary failure mechanism, which thins the TBC thus reducing its insulation capability and increasing the TGO growth rate. In severe conditions, erosion can completely remove the TBC over time, again resulting in the exposure of the substrate, typically Ni-based superalloys. Since engine efficiency is related to turbine entry temperature (TET), there is a constant driving force to increase this temperature. With this drive for higher TETs comes corrosion problems for the yttria stabilised zirconia (YSZ) ceramic topcoat. YSZ is susceptible to attack from molten calciumâ  magnesiumâ  aluminaâ  silicates (CMAS) which degrades the YSZ both chemically and micro-structurally. CMAS has a melting point of around 1240 à °C and since it is common in atmospheric dust it is easily deposited onto gas turbine blades. If the CMAS then melts and penetrates into the ceramic, the life of the TBC can be significantly reduced. This paper discusses the various failure mechanisms associated with the erosion, corrosion and erosionâ  corrosion of EB PVD TBCs. The concept of a dimensionless ratio D/d, where D is the contact footprint diameter and d is the column diameter, as a means of determining the erosion mechanism is introduced and discussed for E

Nano and Micro indentation studies of bulk zirconia and EB PVD TBCs

In order to model the erosion of a material it is necessary to know the material properties of both the impacting particles as well as the target. In the case of electron beam (EB) physical vapour deposited(PVD) thermal barrier coatings (TBCs) the properties of the columns as opposed to the coating as a whole are important. This is due to the fact that discrete erosion events are on a similar scale as the size of the individual columns. Thus nano* and micro* indentation were used to determine the hardness and the Young"s modulus of the columns. However, care had to be taken to ensure that it was the hardness of the columns that was being measured and not the coating as a whole. This paper discusses the differences in the results obtained when using the two different tests and relates them to the interactions between the indent and the columns of the EB PVD TBC microstructure. It was found that individual columns had a hardness of 14 GPa measured using nano indentation, while the hardness of the coating, using micro indentation decreased from 13 to 2.4 GPa as the indentation load increased from 0.1 to 3N. This decrease in hardness was attributed to the interaction between the indenter and a number of adjacent columns and the ability of the columns to move laterally under indentation

Evaluation of a load cell model for dynamic calibration of the rotor systems research aircraft

The Rotor Systems Research Aircraft uses load cells to isolate the rotor/transmission system from the fuselage. An analytical model of the relationship between applied rotor loads and the resulting load cell measurements is derived by applying a force-and-moment balance to the isolated rotor/transmission system. The model is then used to estimate the applied loads from measured load cell data, as obtained from a ground-based shake test. Using nominal design values for the parameters, the estimation errors, for the case of lateral forcing, were shown to be on the order of the sensor measurement noise in all but the roll axis. An unmodeled external load appears to be the source of the error in this axis

Effect of microstructure and temperature on the erosion rates and mechanisms of modified EB PVD TBCs

Thermal barrier coatings (TBCs) have now been used in gas turbine engines for a number of decades and are now considered to be an accepted technology. As there is a constant drive to increase the turbine entry temperature, in order to increase engine efficiency, the coatings operate in increasingly hostile environments. Thus there is a constant drive to both increase the temperature capabilities of TBCs while at the same time reducing their thermal conductivities. The thermal conductivity of standard 7 wt% yttria stabilized zirconia (7YSZ) electron beam (EB) physical vapour deposited (PVD) TBCs can be reduced in two ways: the first by modification of the microstructure of the TBC and the second by addition of ternary oxides. By modifying the microstructure of the TBC such that there are more fine pores, more photon scattering centres are introduced into the coatings, which reduce the heat transfer by radiation. While ternary oxides will introduce lattice defects into the coating, which increases the phonon scattering, thus reducing the thermal conductivity via lattice vibrations. Unfortunately, both of these methods can have a negative effect on the erosion resistance of EB PVD TBCs. This paper compares the relative erosion rates of ten different EB PVD TBCs tested at 90à ° impact at room temperature and at high temperature and discusses the results in term of microstructural and temperature effects. It was found that by modifying the coating deposition, such that a low density coating with a highly â  featheredâ  microstructure formed, generally resulted in an increase in the erosion rate at room temperature. When there was a significant change between the room temperature and the high temperature erosion mechanism it was accompanied by a significant decrease in the erosion rate, while additions of dopents was found to significantly increase the erosion rate at room and high temperature. However, all the modified coatings still had a lower erosion rate than a plasma sprayed coatings. So, although, relative to a standard 7YSZ coating, the modified coatings have a lower erosion resistance, they still perform better than PS TBCs and their lower thermal conductivities could make them viable alternatives to 7YSZ for use in gas turbine en

Erosion of gadolinia doped EB-PVD TBCs

Gadolinia additions have been shown to significantly reduce the thermal conductivity of EB-PVD TBCs. The aim of this paper is to further the understanding on the effects of dopants on the erosion resistance of EB-PVD TBCs by studying the effects of 2 mol% Gd2O3 additions on the room and high temperature erosion resistance of as received and aged EB-PVD TBCs. Previously it has been reported that gadolinia additions increased the erosion rate of EB- PVD TBCs, this is indeed the case for room temperature erosion, however under high temperature (825 à °C) erosion conditions this is not the case and the doped TBCs have a slightly lower erosion rate than the standard YSZ EB-PVD TBCs. This has been attributed to a change in the erosion mechanisms that operate at the different temperatures. This change in mechanism was not expected under the impact conditions used and has been attributed to a change in the column diameter, and how this influences the dynamics of particle impactio

INSURANCE-DEATH OF INSURED RESULTING FROM CRIMINAL ABORTION- RIGHT OF BENEFICIARY

Insured died as the result of a criminal abortion to which she had voluntarily submitted. The policies issued on her life contained a provision to the effect that no benefits should be payable or recoverable should the insured die as a result of a violation of law. The insurer resisted the action brought by the named beneficiary on the policy on two grounds: (a) The insured\u27s death was caused by her violation of law; (b) Although the stated terms of the policy be held not to exclude the risk of death thus caused, it would be contrary to public policy to allow recovery. Held, for plaintiff. Under Louisiana law a woman who solicits and submits to an illegal operation is guilty of no crime, hence the policy covers death resulting from such an operation. Moreover, public policy does not stand in the way of enforcing such a contract when the beneficiary is an innocent third party with vested rights. Payne v. Louisiana Industrial Life Insurance Co., (La. 1948) 33 S. (2d) 444

EXECUTORS AND ADMINISTRATORS-POWERS OF EXECUTOR PRIOR TO THE GRANT OF LETTERS TESTAMENTARY

Testator\u27s will was probated solely for the purpose of passing title to the real estate involved. There was no request for letters testamentary by those named executors in the will, it being alleged that there was no personal estate necessitating administration. Six days prior to the expiration of the statutory period for commencing such an action, plaintiffs, creditors, started a suit against the persons named as executors for the purpose of extending the lien of their debt against the land in the estate. The defendants appeared specially to question the propriety of the action against them. On appeal from the lower court\u27s order requiring them to answer the suit, held, affirmed. Defendants are executors for purposes of this suit, and must either answer the complaint or renounce their right to take out letters testamentary. Cavanaugh v. Dore, 358 Pa. 183, 56 A. (2d) 92 (1948)

ASSOCIATIONS-TITLE TO LAND CONVEYED TO CHURCH AFTER EXPIRATION OF CHARTER

The X Church was incorporated by special charter for a period of fourteen years from 1814. Subsequent charters extended its corporate existence until 1871, since which time, though the members have remained associated under the same name, there has been no attempt to re-incorporate. In 1922, land was conveyed to X Church, its successors and assigns. Defendant contracted with the chairman and the warden of the Church to buy the land described in the deed of 1922. Subsequently, plaintiff, who had been elected minister of the Church in 1945, was authorized to execute and deliver the necessary deed of the land to defendant by a resolution unanimously adopted at a meeting of the members of the Church at which a quorum was present. Defendant declined to accept the proffered deed. due to the uncertainty of the title and power of plaintiff to execute a valid conveyance of the land. Plaintiff petitioned for a declaratory judgment. Held, the deed offered is sufficient. The conveyance of 1922 vested title in the members of the Church as such, and the present members will be bound by a deed executed by plaintiff as their duly appointed agent. Jeffery v. Ehrhardt, (S.C. 1947) 43 S.E. (2d) 483