J.R. Nicholls

Methods to reduce the thermal conductivity of EB-PVD TBCs

Abstract This paper reviews the advantages and disadvantages of various thermal barrier coating (TBC) systems, with the aim of custom designing a TBC system to be both strain tolerant and have a low thermal conductivity. Methods of heat transfer within zirconia based ceramics are discussed, including the influence of coating microstructure and ceramic composition. It is shown the addition of dopant atoms (colouring) is effective in reducing ‘phonon’ transport and that layered microstructures are effective in reducing ‘photon’ transport. Advanced processing, using EB-PVD coating methods has allowed both coloured and layered ceramic coatings to be produced. Measured thermal conductivities of 1.0 W mK −1 have been achieved using these methods, much lower than current commercial EB-PVD coatings at 1.5–1.9 W mK −1 .

Smart overlay coatings — concept and practice

Smart overlay coatings are a functionally gradient coating system designed to provide high temperature corrosion protection over a wide range of operating conditions. The SMARTCOAT design consists of a MCrAlY base, enriched first in chromium, then aluminium to provide a chemically graded structure. At elevated temperatures, above 900°C (1650°F), the coating oxidises to form a protective alumina scale. However, at lower temperatures this alumina scale does not reform rapidly enough to confer protection under type II hot corrosion conditions. The coating is therefore designed with an intermediate chromium-rich interlayer, which permits the rapid formation of chromia healing areas of type II corrosion damage. Laboratory and burner rig tests have been carried out on a series of developmental smart overlay coatings. These have shown that the development of an intermediate chromium-rich phase provides protection under low temperature hot corrosion conditions, while the aluminium-rich surface layer provides resistance to high temperature oxidation and type I hot corrosion. Thus, the single application of SMARTCOAT permits operation over a broad range of industrial and marine turbine conditions.

A comparison between the erosion behaviour of thermal spray and electron beam physical vapour deposition thermal barrier coatings

Thermal barrier coating (TBC) technology is used in the hot sections of gas turbines to extend component life. To maximise these benefits, the TBC has to remain intact through the life of the turbine. High velocity ballistic damage can lead to total thermal barrier removal, while erosion may lead to progressive loss of thickness during operation. This paper compares the erosion behaviour of thermally sprayed and electron beam physical vapour deposited (EB-PVD) TBCs. A unique high velocity gas gun has been developed with the capability of impacting TBCs at particle velocities up to 300 m/s at test temperature up to 920°C. Using this facility it was found, both at room temperature and 910°C, that EB-PVD ZrO2–8 wt.% Y2O3 thermal barriers are significantly more erosion-resistant than the equivalent plasma spray coating, when impacted with either alumina or silica in the particle size range 40–100 μm. Examination of tested hardware reveals that cracking occurs within the near surface region of the columns for EB-PVD ceramic and that erosion occurs by removal of these small blocks of material. In stark contrast, removal of material for plasma sprayed ceramic occurs through poorly bonded splat boundaries. This difference in material removal mechanism accounts for the seven- to 10-fold improvement, in erosion resistance observed for EB-PVD TBCs over those produced by plasma spraying. At both room temperature and 910°C, erosion rates are linear with velocity for both types of coating. When impacted with hard particles (alumina at room temperature and 910°C, silica at room temperature) peak erosion rates occur for normal impact, with impact angle dependence scaling similarly for the air plasma sprayed (APS) and EB-PVD coatings. At high temperatures, erosion rates are increased over those measured at room temperature, consistent with the higher test velocities achieved at 910°C.

Some observations on erosion mechanisms of EB PVD TBCS

Abstract Following the successful application of electron beam (EB) physical vapour deposition (PVD) thermal barrier coatings (TBCs) to moving parts of turbine engines, the erosion resistance of these coatings has been of interest among researchers. However, although there are a number of papers on the erosion rate of these coatings, little has been reported on their erosion mechanism. This paper provides observations on the erosion damage of EB PVD TBCs and discusses the type of damage caused by erosion, as well as proposing a possible mechanism of erosion. The aim of the project as a whole was to model the erosion of EB PVD TBCs, but before modelling could begin, it was necessary to determine the erosion mechanism of these coatings. It was found that in all cases examined, the erosion of the coatings proceeds through the accumulation of damage in the form of horizontal cracks in the columns of the coating and subsequent removal of the fractured sections. Since it appears as though the contact radius is important in the erosion process, the effect of varying the elastic properties of the erodent and the target on the contact radius was assessed.

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 calcium?magnesium?alumina?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 introduced.

Particle-surface interactions during the erosion of a gas turbine material (MarM002) by pyrolytic carbon particles

Abstract A single-impact technique was used to study particle-surf ace interactions during the erosion of a typical turbine blade material, MarM002, by pyrolytic carbon particles. It was shown that carbon particles as small as 50 μn can cause severe erosive damage and that the predominant mode of material loss is by the removal of surface oxide. The erosive response of MarM002 was considered at 700, 750, 850 and 950 °C and is shown to be a function of the temperature and the oxide thickness, with lower temperatures and thicker oxide scales favouring brittle erosion behaviour. This behaviour can account for the reported increase in corrosion of superalloys at lower temperatures, provided that an erosive component is present in the system, and suggests that optimum conditions for erosion-corrosion resistance are established at a temperature in the region of 850 °C for the conditions under investigation.

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

Abstract 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 1 and micro indentation were used to determine the hardness and the Youngs 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 3 N. 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.

THE PREDICTION OF CORROSION BY STATISTICAL ANALYSIS OF CORROSION PROFILES

Abstract The corrosion of carbon manganese steel in CO2-acidified sea water has been assessed by a statistical analysis of corrosion profiles. The total population of depth data was multi-modal and could not be fitted to any well known statistical models. However the deepest pits in the profile followed a type I extreme value distribution. The time dependence of the extreme value distribution parameters has been determined and used to predict the rate of penetration of steel components under pitting conditions.

Particle-surface interactions during the erosion of aluminide-coated MarM002

Abstract Particle-surface interactions during the erosion of a nickel aluminide coating were assessed using a single-impact technique. It is shown that the erosive response is a function of the surface scale thickness and the temperature, with the temperature not only influencing the surface scale plasticity but also determining the contribution of the coating substrate to the impact process. In this respect the ductile-to-brittle transition temperature of the coating is of particular importance. Under a wide range of conditions typical of those found in gas turbines the erosion of aluminide coatings is shown to be controlled by the formation and removal of surface scales. This implies that the use of aluminide coatings will increase the erosion resistance of typical turbine blade materials because of the superior oxidation and corrosion resistance of this coating. This increase in erosion resistance will be particularly significant at higher operating temperatures, above 900 °C.

Hardness and modulus measurements on oxide scales

This paper provides a comparison between hardness and elastic modulus data measured using a mechanical properties microprobe (MPM), the acoustic microscope and two techniques based on resonant frequency to measure the elastic moduli of oxide systems. Measured values for bulk oxides, namely AI2O3 and Cr2O3, have been used to compare the various measurement systems. The comparison is then extended to measurements on oxide scales. In general, hardness values measured using the MPM technique agree with reported bulk values, although differences between laboratories have been identified which may be attributable to the position of indentation within the scales. Hardness values for scales are found to be similar to hardness values for the bulk, lying in the range 21–30 GPa for A12O3 scales and 18–33 GPa for Cr2O3 scales. Youngs moduli for recrystallized AI2O3 have been measured using the mechanical properties microprobe, acoustic microscopy and resonance methods. Data determined using the MPM technique give th...