2,582 research outputs found
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
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
Solid particle erosion of ceramics
The zeolite ZSM-5 is well-known for its unique intersecting channel system. This channel system has a great bearing on the shape-selective properties and the long life-times of ZSM-5. In this study, ZSM-5 was modified in various ways to eliminate the external acidity of the catalyst to further improve these properties, and the success and effects of these modifications were investigated primarily using temperature programmed desorption techniques. The internal surface of ZSM-5, a medium pore zeolite, plays a major role in the shape selective properties of this catalyst, due to the diffusional restrictions imposed by the channel system on bulky molecules. Even though the number of acid sites on the external surface is small compared to the total number of acid sites, these easily accessible and non-shape selective acid sites may provide a high turnover rate for non-shape selective reactions. Furthermore, the main cause of deactivation of ZSM-5 is thought to be the formation of polyaromatic molecules on the external surface, which block access to the channels of the catalyst
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
Educating the educators: Graduate student involvement in teaching natural resources
Awareness of the need to prepare future faculty for the teaching challenges they will face is growing rapidly. In one response, North Carolina State University established the Preparing the Professoriate program in 1994. Doctoral students work with a faculty mentor over the course of 1-2 years, depending on the frequency with which the course is taught, with the students moving from the role of observer to that of instructor. In addition to working closely with a member of the faculty in designing and delivering an undergraduate course, graduate students in the program attend monthly meetings at which they hear presentations and share experiences, and they complete a teaching portfolio. Entry into the program is competitive, as only ten student/mentor teams are selected each year. Selected students receive a $2,000 stipend
An exploration of concepts of community through a case study of UK university web production
The paper explores the inter-relation and differences between the concepts of occupational community, community of practice, online community and social network. It uses as a case study illustration the domain of UK university web site production and specifically a listserv for those involved in it. Different latent occupational communities are explored, and the potential for the listserv to help realize these as an active sense of community is considered. The listserv is not (for most participants) a tight knit community of practice, indeed it fails many criteria for an online community. It is perhaps best conceived as a loose knit network of practice, valued for information, implicit support and for the maintenance of weak ties. Through the analysis the case for using strict definitions of the theoretical concepts is made
Structural changes of thermal sprayed graphene nano platelets film into amorphous carbon under sliding wear
© 2020 The Authors Graphene has become a promising candidate to protect surfaces against friction due to its strength and lubricating ability. In this study, graphene nano platelets (GNP) thin films have been deposited onto stainless steel substrates by axially injecting GNP suspension through high velocity oxy fuel thermal spray gun. The tribological performance of the films under dry sliding wear was investigated through unlubricated ball on disc sliding wear test against a sintered alumina counter body ball under 5 N load. The understanding of the behaviour of the GNPs under sliding wear will be useful for improving the performance of graphene-based coatings which are in demand for wear resistant applications. A film was deposited showing significant improvements in friction with coefficient of friction value reduced by 7 times compared to uncoated stainless steel, even for a discontinuous film. A morphological analysis shows sliding wear led to change in particle shape from angular flakes into randomly oriented circles. Interatomic bonding and structural analysis performed reveals oxidation defect formations during wear test. Structural degradation and oxidation of GNPs during the process led to formation of amorphous carbon from graphene. Amorphous carbon formation reduces the lubricating ability and strength of the film, leading to failure
Steam Degradation of Ytterbium Disilicate Environmental Barrier Coatings: Effect of Composition, Microstructure and Temperature
Recession of environmental barrier coatings (EBC) in environments containing
steam is a pressing concern that requires further research before their
implementation in gas turbine engines can be realized. In this work,
free-standing plasma sprayed Yb2Si2O7 coatings were exposed to flowing steam at
1350 {\deg}C and 1400 {\deg}C for 96 h. Three samples were investigated, one
coating with a low porosity level (< 3 %) and 1 wt.% Al2O3 representing
traditional EBCs; and two coatings with higher porosity levels (~20 %)
representing abradable EBCs. Phase composition and microstructural evolution
were studied in order to reveal the underlying mechanism for the interaction
between high temperature steam and ytterbium disilicate. The results show
depletion of Yb2SiO5 near the surface and formation of ytterbium garnet
(Yb3Al5O12) on top of all three coatings due to the reaction with gaseous
Al-containing impurities coming from the alumina furnace tubes. The 1 wt.%
Al2O3 added to the EBC sample exacerbated the formation of garnet at 1400
{\deg}C compared to the abradable samples, which presented lower quantities of
garnet. Additionally, inter-splat boundaries were visible after exposure,
indicating preferential ingress of gaseous Al-containing impurities through the
splat boundarie
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