10,995 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
Perceptions of coach-athlete relationship are more important to coaches than athletes in predicting dyadic coping and stress appraisals: An actor-partner independence mediation model
Most attempts to manage stress involve at least one other person, yet coping studies in sport tend to report an athlete’s individual coping strategies. There is a limited understanding of coping involving other people, particularly within sport, despite athletes potentially spending a lot of time with other people, such as their coach. Guided by the systemic-transactional model of stress and coping among couples (Bodenmann, 1995), from relationship psychology, we assessed dyadic coping, perceptions of relationship quality, and primary stress appraisals of challenge and threat among 158 coach–athlete dyads (n D 277 participants). The athletes competed at amateur (n D 123), semiprofessional (n D 31), or professional levels (n D 4). Coaches and athletes from the same dyad completed a measure of dyadic coping, coach–athlete relationship, and stress appraisals. We tested an Actor–Partner Interdependence Mediation Model to account for the non-independence of dyadic data. These actor–partner analyses revealed differences between athletes and coaches. Although the actor effects were relatively large compared to partner effects, perceptions of relationship quality demonstrated little impact on athletes. The mediating role of relationship quality was broadly as important as dyadic coping for coaches. These findings provide an insight in to how coach–athlete dyads interact to manage stress and indicate that relationship quality is of particular importance for coaches, but less important for athletes. In order to improve perceptions of relationship quality among coaches and athletes, interventions could be developed to foster positive dyadic coping among both coaches and athletes, which may also impact upon stress appraisals of challenge and threat
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
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
Carbon and titanium diboride (TiB2) multilayer coatings.
Titanium Diboride, (TiB2) is a metal-based refractory ceramic material that has
been investigated in industrial applications ranging from, cutting tools to wear
parts and for use in the aerospace industry. The unique properties which make
this material so fascinating are, its high hardness, high melting point and its
corrosion resistance. TiB2 is prevented from wider mainstream use because of its
inherent brittle nature. With a view to overcome this in coating form and with
the aim of providing in addition inherent lubricity, in this study 50 layer
TiB2/C multilayer stacks have been fabricated, with varying volume fractions of
ceramic, whereby the interfaces of the layers limit crack propagation in the
TiB2 ceramic. TiB2 has been multilayered with carbon, to make use of the unique
and hybrid nature of the bonding in carbon coatings. DC magnetron sputtering
with substrate bias was the preferred route for the fabrication of these
coatings. AISI tool steel has been used as the substrate material. By varying
the amount of TiB2 ceramic from 50% to 95%, the Hardness of the coating is seen
to increase from 5 GPa to 17GPa. The Hardness is observed to decrease as a
function of increasing carbon content, agreeing with other studies that the
carbon layers are not load-bearing. The graphitic nature of the sp2 bond,
however, acts as a lubricant layer
Subsonic sphere drag measurements at intermediate Reynolds numbers
Wind tunnel drag measurements on subsonic spher
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