operation of a burner rig for thermal gradient cycling of thermal barrier coatings

1 Copyright © 2014 by ASME

Proceedings of the ASME Turbo Expo 2014

GT2014

June 16-20, Düsseldorf, Germany

GT2014- 26325

OPERATION OF A BURNER RIG FOR THERMAL GRADIENT CYCLING OF THERMAL BARRIER COATINGS

J.P. Feist Sensor Coating Systems London, United Kingdom

P.Y. Sollazzo Sensor Coating Systems London, United Kingdom

C. Pilgrim Sensor Coating Systems London, United Kingdom

J.R. Nicholls Cranfield University

Bedford, United Kingdom

ABSTRACT Thermal barrier coatings (TBC), in combination with

sophisticated cooling systems are crucial for the operation of

highly efficient gas turbines. New generations of coatings will

need to show increased cycling capability as a future energy

mix will contain a high proportion of renewable energy which

will be subject to rapid changes in supply. This will require gas

turbines to be on stand-by to fill shortages in power supply with

short notice. Furthermore, higher operating temperatures are

sought to improve the efficiency of the engine. It is, therefore,

an aim of the industry to find a coating composition or structure

which will enable the operation at temperatures greater than

1250°C and with high cycling capability.

Test methods are required to meet these new operating

conditions to validate new coatings. The maximum temperature

limit of commonly used isothermal or cyclic oxidation tests is

usually the temperature at which the substrate will start to

significantly oxidise. However, there is the technical need to

test the ceramic top layer at elevated surface temperatures up to

1500°C while keeping the substrate ‘cool’. Such capability

would allow the effects of ceramic sintering, and deposit

induced damage to be assessed at the TBC surface. This only

can be performed on a complete coating system, when a

thermal gradient is established throughout the coating.

This paper reviews a burner test facility, designed and built

by Sensor Coating Systems Ltd. (SCS), which combines severe

and frequent cycling with the exposure of the coating to high

surface temperatures and active cooling of the substrate.

Further, this test can include thermal shock by active cooling of

the surface at the end of each cycle. The paper will consider

different operating conditions and will review experiences in

building and operating the rig, including results from thermal

barrier coating tests on electron beam physical vapour

deposition (EBPVD) and atmospheric plasma spray (APS)

samples. Further, the rig is capable of testing optical techniques

such as pyrometry and thermographic phosphor thermometry

for measuring surface temperature in controlled laboratory

conditions and example of this will be presented. The paper

also will reflect on the ISO 13123:201 standard for this type of

test.

INTRODUCTION Efficiency increases in turbine operation have been

achieved mainly through increasing firing temperatures. Apart

from advanced high temperature alloys and sophisticated

cooling methods, thermal barrier coatings (TBCs) have played

a major role in this development. Gas temperatures in today’s

turbines are above the melting point of the metal components –

vanes, blades - in the hot section, the most expensive part of the

turbine. TBCs in combination with internal cooling allow

components to survive higher temperatures while achieving

acceptable life times.

Future changes in the global energy market and in

particular the increasing share of renewable energies in the

energy mix will result in changing operating conditions of gas

turbines when used as back-up power supplies. Electricity

generation from renewable energy sources can change quickly

dependent on, for example weather conditions and hence will

require back-up solutions to avoid power outages. Gas turbines

are best suited to efficiently provide the energy at short notice

and, if necessary, quickly reduce power output when demand is

reduced. This will consequently lead to increased cycling and

hence an appropriate test platform is required.

The coating community has not yet developed a standard

methodology to test all relevant parameters simultaneously. The

main ideas range from isothermal or cyclic oxidation of

samples in a furnace to thermal mechanical fatigue tests.

Further, additional corrosion and erosion tests have been


developed and were appropriately reported in a comprehensive

overview about test methodologies, with an emphasis on

European activities provided by Vaßen et al [1].

A thermal gradient cycling test combines severe and

frequent cycling with exposure to high temperatures on the

surface of the coating and active cooling of the substrate (at the

back). Further, for a more aggressive test it is possible to

include a thermal shock by active cooling of the surface at the

end of each cycle. Thus, a thermal gradient cycling test should

be representative of current and future operating conditions in

power generation gas turbines, where high cycling frequency at

even higher temperatures will occur due to short-term demand

of energy and higher efficiency needs.

In this paper, the authors briefly review a range of existing

test rigs, review instrumentation related aspects and looking at

a range of applications that may benefit from thermal gradient

testing, where the maximum surface temperature can be as high

as 1500°C, well above the melting point of the substrate alloy,

but representative of service environments. This includes the

testing of APS and EBPVD TBCs and will demonstrate a

method to determine the emissivity of doped TBCs at elevated

temperatures (700°C).

THERMAL BARRIER COATINGS TBCs are multilayered systems which consist of a bond

coat applied to the substrate and a ceramic top coat as the

surface layer. In service, a thermally grown oxide (TGO) forms

between the bond coat and ceramic which acts as an oxidation

barrier. The term TBC usually refers to the ceramic top layer

and this terminology practice will be followed here, although it

should be remembered that it is the alloy/coating system that is

evaluated. For many years yttria partially stabilized zirconia

(YSZ) has been the prime choice for this application due to its

relatively high thermal expansion coefficient, therefore

reducing thermally induced stresses, whilst still having a low

thermal conductivity, that is approximately an order of

magnitude lower than the substrate Ni-base alloy [2]. Between

6 and 8 wt.% yttria is added to partially stabilise the zirconia,

so avoiding the tetragonal to monoclinic phase change, which

can be induced upon cooling from the elevated temperatures in

operation [3]. This phase change causes a 3% volume change,

which in turn leads to stresses and subsequent spallation of the

surface of the coating. Full stabilisation, however, can lead to

the formation of the cubic phase which can compromise cyclic

thermal fatigue life [4].

Two main deposition processes have developed for the

industrial production of TBCs; namely plasma spraying (PS)

and electron beam physical vapour deposition (EB-PVD). The

different processes are known to produce distinctive

microstructures in the coating resulting in different properties.

EB-PVD TBCs exhibit improved strain tolerance, surface finish

and erosion resistance; however, the thermal conductivity is

compromised compared to PS TBCs. As such, the PS technique

is typically used for stationery components or low velocity

components, while EB-PVD is used for rotating parts such as

turbine blades, which rotate at high velocity in the jet engine.

TBCs are complex systems in which the many modes of

failure are related to the operating conditions and the coating

microstructure. In a clean combustion environment the primary

reported failure mechanism of TBCs is caused by the growth of

the TGO layer to an extent which results in a large mismatch

between thermal expansion coefficients and hence large

induced stresses under thermal cycling [5]. The stresses initiate

cracks which cause spallation of the TBC either at the interface

between the TGO and TBC or TGO and bond coat, therefore

complete removal of the protective coating occurs. In higher

temperature regimes, typical surface temperatures >1150°C,

sintering of the TBC can occur causing an increase in thermal

conductivity and stiffness therefore resulting in premature

failure. Whereas at even higher temperatures, >1250°C, molten

glossy deposits may form calcium-magnesium-alumino-silicate

(CMAS) attack that infiltrate the TBC ceramic causing

premature failure [5].

Doping of YSZ with lanthanides such as dysprosium and

europium can give the coating sensing properties such as

temperature [6], but also thickness [7], erosion [8] or corrosion

[9]. When illuminated with short light pulses the coating starts

to phosphoresce and this phosphorescence can be correlated to

temperature.

REVIEW OF BURNER RIGS AND INTERNATIONAL TEST STANDARDS

Although there is a recognised need for thermal gradient

and burner rig testing, there are still a limited number of test

rigs with a wide variation between them and the routines

employed. In an attempt to standardise procedures, an

international standard (ISO 13123:2011) has been generated

[10]. The standard describes the test equipment, method and

reporting which should be used. Despite this standard, there

still remains no common test configuration.

Hollow cylindrical test specimens have been used with

internal cooling to force the thermal gradient through the

coating and enable investigations on the effect of curvature on

the life of the TBC, for example a rig developed at NASA [11]

and Turbomeca [12]. More recently, however, the typical test

specimens are 30mm diameter discs to test flat coated surfaces.

It is critical to accurately measure the temperature of the

sample during such testing. Different methods are employed to

do this. The temperature of the substrate can be monitored

using a thermocouple attached to the back surface [13] or a

pyrometer [14]; but the standard approach is to embed a

thermocouple into the substrate [10, 15, 16]. The surface

temperature is predominantly measured using a pyrometer.

However, the transparency of the YSZ in the infra-red range is

recognised to be a potential source for error for this approach.

This may be overcome by correction of the emissivity [14], the

use of two colour pyrometers [13] or long-wavelength

pyrometers [15]. At wavelengths above approximately 9µm

YSZ can be considered opaque, therefore using pyrometers

with sensitivity in this range ensures the technique measure the

surface temperature of the coating..

The temperature conditions, during the tests, are not


standardised, nor are the duration of the cycles. Usually tests

are conducted for a fixed period or until failure which,

according to the ISO standard (ISO 13123:2011) is when 20%

of the coated area has spalled judging by visual inspection. The

duration and nature of the cycles can vary significantly between

reported tests. Heating and cooling times of between 75

seconds [14] and 10 minutes [13] have been used. Furthermore,

heating times of up to 60 minutes have been employed with

structured thermal profiles to replicate particular phases within

an operating engine [12]. A heating and cooling time of five

and two minutes respectively has been used previously [15]

and this allows a high frequency of cycles, while achieving

steady state temperature conditions during the heating phase.

Innovative test rig The rig described in this paper is compliant with the

international standard (ISO 13123:2011) so that comparative

testing could be conducted. The laboratory rig configuration

simulates a realistic environment in regards to temperature

gradients, substrate material and coating system. Further, the

accelerated cycling capability allows the simulation of fast

start-and-stop scenarios, but also varying cooling and heating

intervals. This allows customisation to specific engine

operation regimes. Consequently, the authors believe that this

‘high-fidelity’ test is best suited for coating evaluation

achieving technology readiness levels of 5 to 6 [17].

INSTRUMENTATION The thermal gradient test rig can operate with up to 4

burners simultaneously. The test rig is controlled by a computer

and a National Instrument acquisition card. The temperature of

the sample surface is measured by a long wavelength pyrometer

while the sample substrate temperature is measured by a K-type

thermocouple. The gas flow for one burner of the thermal

gradient test rig is illustrated in Fig. 1. The front cooling nozzle

and the burner are mounted on a pneumatic rail. A sample is

tested by alternating heating and cooling. Typically, at Sensor

Coating Systems Ltd. (SCS), the heating phase lasts for five

minutes while the cooling phase lasts for two minutes. This 2-

minute cooling time allows the sample to cool down to almost

room temperature. This type of cycle was adopted from Träger

et al. [15]. However, both heating and cooling duration can be

customised to match envisaged engine operating conditions.

The change between heating and cooling phase is

performed using the pneumatic rail which removes the burner

and positions the cooling nozzle instead. During the heating

phase oxygen and methane gases are flowing to the burner to

generate temperatures up to 1500°C on the surface of the

sample, while the cooling nozzle at the back of the sample

maintains lower substrate temperatures. Micrometric taps are

used to set accurately the flows of the different gases. The

pressure regulators generate independent settings of every

burner by depressurizing the gases to below 10bars. The mass

flow readers (MFR) keep track of the test rig settings. When the

rig is in cooling phase the oxygen supply is stopped by the

oxygen valve, while the methane supply is bypassed through a

common methane micrometric tap to generate a pilot flame.

The sample is maintained in position on the test rig with a

holder designed to retain the sample thanks to a groove inserted

in the sample slot, which is visible in Fig. 4.

Figure 1: Sketch of the burner test rig for 1 sample. The test rig

has 4 burners.

The computer, connected to the acquisition card, runs a

LabVIEW program which controls the operation of the valves

and the rail, monitors and records the temperatures of the long

wavelength pyrometers and the K-type thermocouples. The

LabVIEW program automatically switches off the test rig in

case the monitored temperatures are outside a predefined safety

range. Additionally, a watchdog system automatically switches

off the test rig power supply in case the computer controlling

the rig becomes unresponsive. When the watchdog system

shuts down the power the flames are automatically

extinguished because all the valves are normally closed. With

these safety measures in place the rig can be operated 24/7 for

several hundred hours.

Pyrometry The pyrometer installed on the test rig, monitors the

surface temperature of the coatings. It is a Heitronics infrared

radiation pyrometer which observes radiation in the range of

9.6 to 11.5 μm with an integration time of 30ms. Its emissivity

can be set up with a precision of 0.01. In this wavelength range

the TBC is considered to be opaque [15]. Each pyrometer is

installed 40cm away from the sample surface. The pyrometer is

positioned accurately using a laser pointer and this assures

measurements being taken from the centre of the sample on a

circular area of 5mm in diameter.

Emissivity estimation The setup used to estimate the emissivity of the TBCs is

shown in Fig. 2. The sample was placed on a heating plate with

the TBC facing up. The plate was then heated to approximately

300°C and the substrate and TBC surface temperature was

monitored using a pyrometer and a K-type thermocouple

PC DAQ

Air CH4O2

Thermocouple

Pyrometer

CoolingNozzle

CoolingNozzle

Sample

Valve

Regulator

MFR

Micrometrictap

BurnerSlide

Pilot flameby-pass line


respectively. The sample was considered to be isothermal, as

such the temperature measured by the pyrometer and

thermocouple should be equal. The emissivity setting on the

pyrometer was adjusted until the pyrometer temperature

matched that of the thermocouple. The corresponding

emissivity setting was considered to be the emissivity of the

TBC, typical values for TBCs are 0.9 or above in the sensitivity

range of the pyrometer used [18].

Figure 2: The sample is positioned on a 300°C heating plate. The

temperature of the surface and the substrate are measured

simultaneously by a pyrometer and thermocouple respectively.

The computer is equipped with NI acquisition card.

The temperatures measured by the pyrometer and the

thermocouple are presented in Fig. 3. The temperature

sinusoidal oscillations are induced by the heating plate

temperature controller. There is no phase shift between the

temperatures measured by the pyrometer and the thermocouple

confirming that the sample is in thermal equilibrium. First the

pyrometer emissivity set to 1. In order to estimate the

emissivity of the sample at around 300°C the temperature

reading difference between the pyrometer and the thermocouple

must be approximately zero. The temperature measured by the

pyrometer follows the Stefan-Boltzmann law, see Eq. (1), and

hence is related to its emissivity settings. In Eq. (1), j* is the

thermal radiation, or irradiance, emitted by the sample, ε is the

emissivity setting of the pyrometer, σ is the Stefan-Boltzmann

constant and T is the temperature measured by the pyrometer.

(1)

The emissivity setting of the pyrometer is changed such that if

the difference between the pyrometer temperature and the

thermocouple temperature is positive the pyrometer emissivity

is decreased. If the difference is negative the emissivity setting

is increased. When the difference is zero the emissivity setting

of the pyrometer is considered to be the emissivity of the

sample. In Fig. 3 between 0 and 7 minutes the emissivity of the

pyrometer is at 1 and the temperature error is equal to about

15°C. The emissivity setting is changed until the sample

emissivity is estimated to be 0.93 and the difference between

the temperature measured by the thermocouple and the

pyrometer is approximately zero.

Figure 3: Recorded data for the thermocouple and the pyrometer

(left y-axis) while estimating the emissivity of the sample. The

difference between the thermocouple and the pyrometer measured

temperature is plotted as the bottom trace and corresponds to the

right y-axis.

Thermocouples and sample geometry The thermocouple installed on the test rig which measures

the substrate temperature is a 1mm diameter K-type

thermocouple. The international thermal gradient test rig

standard recommends a thermocouple size of around 1mm [10].

The voltage generated by the chromel-alumel couple is

measured and compensated with a cold junction temperature by

the NI acquisition card. The thermocouple is not grounded nor

coated.

Figure 4: Drawing of a sample substrate. The bondcoat and the

ceramic are deposited on the top of the substrate. The 1.2mm hole

in the middle is used to insert a 1mm thick thermocouple which

monitors the substrate temperature. Drawing adapted from [15].

The sample geometry and, the hole size and length for the

thermocouple from Fig. 4 is adopted from a previous paper by

Träger et al. [15]. The rounded edges of the substrate shall

reduce premature edge failure of the ceramic coating according

to the same paper.

Temperature measurement variations At the end of a test, the recorded data are processed to

compute the number of cycles the sample experienced and the

average surface and substrate temperature which were applied

during the hot phases.

Sample

Heating plate

Computer

Thermocouple(K-type)


Figure 5: The cycle experienced by the samples on the test rig is

divided in two phases, the hot phase which lasts 5 minutes and the

cold phase which lasts 2 minutes. The average temperatures

experienced by the sample are calculated by averaging the

measured data which are above 97% of the targeted temperatures.

The number of cycles is calculated by counting the number

of times the data crossed above the half of the test target surface

temperature. The average temperatures are calculated by

averaging the data which are above 97% of the target

temperature. For instance if the substrate target temperature is

900°C, the average temperature of the substrate for the hot

phase is the average of all the thermocouple data which are

above 873°C (900 x 0.97). Fig. 5 shows the data recorded

during 15 minutes. The targeted temperatures (1400°C and

900°C) and the 97% of the targeted temperatures are plotted in

vertical dash lines. From this figure it can be observed that 97%

is a correct approximation to compute the average temperature

because the temperatures of the substrate and the surface are

either already or almost stabilised at the target temperature. The

international standard [10] defines the hot cycle temperature as

±5% of the targeted temperature. For a targeted set-point of

1400°C this equates to keep the temperature between 1330°C

and 1470°C. Typically SCS always keeps the temperatures

within ±1.5% of the targeted temperature, except during the

setup phase where the range is increased to ±5%.

The voltage generated by the chromel-alumel couple of the

K-type thermocouple is digitalised by an 8 bit digital to

analogue converter (DAC). The DAC (NI 4350) is set to

measure temperature between 0 and 1300°C and hence data can

only be acquired every 5°C (1300 / 28). By plotting the

histogram with a bin size of 1°C of the temperature measured

by the thermocouple over a test of 1027 cycles (~120 hours),

peaks will appear every 5°C as shown in Fig. 6. The

intermediate values are coming from the averaging of the

discrete values, because the temperatures are measured as an

averaged temperature over 1 second.

In Fig. 6 the number of occurrences is equivalent to the

number of seconds the sample stayed at the particular

temperature because each measurement is recorded every one

second.

A faster acquisition rate is possible by using a higher speed

Figure 6: A typical histogram of thermocouple temperatures

measured over a test of 1027 cycles (~120 hours). The peaks

represent the digitalisation limit of the 8 bit ADC card (NI 4350).

acquisition card (ADC). However, a compromise must be

reached between accurately following temperature transients

and the size of the recorded temperature data. A sampling rate

of 1Hz was observed to be sufficient to accurately record the

temperature transients during heating and cooling while also

providing manageable dataset sizes.

In Fig. 6 the number of occurrences above 97% of the

target temperature is 185378, which equate to 437 cycles (~51

hours). This means that during a test of 1027 cycles (~120

hours) the sample is exposed for about 42.5% of the time above

97% of the targeted temperature. Assuming that the cooling

time is kept constant at 2 minutes the 42.5% could be increased

to 75% or 90% by increasing the time of the hot phase to 14

and 38 minutes respectively.

The flexibility in the operating cycles is a unique feature of

this type of test rig design which is not always the case for

other gradient cycling rigs [1].

Camera operation The test rig is equipped with web-cameras which take

pictures of each sample surface before and after each heating

phase. From these pictures the ceramic delamination process

can be observed, enabling the operator to determine the exact

number of cycles before TBC failure. Additionally, by collating

these pictures, accelerated movies can be produced to

understand formation of damages or delamination. An example

of a time lapse movie is available at the following reference

[19]. The international thermal gradient test rig standard [10]

recommends the installation of cameras for visual inspection of

the surface damages.

APPLICATIONS AND OPERATION REGIMES The testing conducted on the thermal gradient test rig can

be adapted to study different failure mechanisms of coating

systems. A range of different coating systems have been tested

and this section provides illustrative examples showing the

possible conditions and experience acquired.

0

200

400

600

800

1000

1200

1400

1600

17:42 17:49 17:57

Tem

pe

ratu

re [

°C]

Time [minutes]

Thermocouple Pyrometer

97 % TTarget Surface

Cold phase

Hot phase

TTarget Surface

TTarget Substrate

97% TTarget Substrate

0 7 14

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

800 850 900 950

# O

ccu

ren

ce

Temperature [°C] - Bin size 1°C

97% TTarget

TTarget

TAverage


APS coatings A sensible testing strategy is to define a standard type of

coating before testing a novel TBC system. This enables the

investigator to make direct and relative comparisons between

their new coating and the standard. In this way accidental small

changes in the operational setup can be eliminated between

different test campaigns. Further slight variations in test

conditions could occur from slot to slot and it is advised to take

this into consideration when setting-up a test programme.

YSZ (700µm)

YSZ composite

(100µm)

YSZ (600µm)

Bondcoat (150µm) Bondcoat (150µm)

Substrate

Substrate

Sample 1 Sample 2 Figure 7: Illustration of tested sample structure. Layer structure

of sample 1 and 2. Bondcoat and TBC are deposited with APS.

This section describes the behaviour of two samples with

different TBC structures tested under the same conditions for

857 cycles (~100 hours) on the same slot. The test objective

was to generate monoclinic phases in a production line coating

and compare this with a new YSZ composite (see Fig. 7). This

required a high surface temperature in combination with

frequent cycling to room temperature. Hence each sample has

its surface temperature exposed to 1400°C (surface target

temperature).

Figure 8: On rig pictures of the samples after 857 cycles (~100

hours) of testing. Sample 1 (right) and sample 2 (left) show signs of

slight edge failure at the top right corner. The thermocouple hole

is positioned at the bottom of the sample.

However, to avoid early coating failure by growth of the

TGO it was decided to keep the substrate temperature at 900°C,

at which temperature the TGO growth has reportedly negligible

effect on the coating life [20] (substrate target temperature).

The thickness of the bondcoat, Amdry 995, on the samples

is 150µm and the thickness of the ceramic layer, deposited with

the APS process, is 700µm. The composition of the TBC layer

of sample 1 is YSZ while the composition of the TBC of

sample 2 is divided into two different materials, a 600µm layer

of YSZ and a 100µm layer of a YSZ composite as shown in

Fig. 7. The thickness of 700µm was chosen to enable the

operator to achieve both target temperatures – surface and

substrate – without using extensive cooling rates and keeping

heating rates to a minimum, hence keeping fuel consumption to

a minimum. A typical gradient across the ceramic is 1K/µm or

lower.

Sample

# of

cycles

performed

Average

surface

temperature

(°C)

Average

substrate

temperature

(°C)

Calculated

temperature

at bond coat

(°C)

1 857 1398 896 930

2 857 1392 896 929

Table 1: Summary of the test conditions for sample 1 and 2.

Whereas the substrate temperatures are similar the surface

temperatures show a difference of 0.4% compared to the targeted

temperature (1400°C)

Table 1 summarises the test conditions of both samples

during the 857-cycle test (~100 hours). The substrate

temperatures are similar but the surface temperatures have a

difference of 6°C which is 0.4% compared to the 1400°C

targeted temperature. The hot phase lasted for 5 minutes and the

cold phase for 2 minutes. In Table 1 the bond coat temperature

is calculated using Eq. (2) where Tp is the pyrometer

temperature (surface) and TTC is the thermocouple temperature

(substrate). ts is the distance between the thermocouple and

substrate surface. k and t are thermal conductivity and thickness

respectively. The subscript letter c refers to the coating while s

refers to the substrate. The values chosen for thermal

conductivity are 30, 0.8 and 1.65 W/mK for the substrate, APS

TBC and EBPVD TBC respectively.

(2)

Fig. 8 shows the pictures of sample 1 and 2 after 857

cycles (~100 hours) of testing on the thermal gradient test rig.

These pictures show edge failure of the coating for both

samples at the top right corner. However, edge failures are

usually less relevant as these are failures introduced by the

specific geometry of the sample. Initial edge failures can grow

into the centre during cycling and this needs to be evaluated

more thoroughly using other characterisation techniques such

as micrographs to analyse microstructures. In this case the

conclusion was that both samples have shown equal durability

for the same number of cycles. The international test standard

ISO 13123:2011 [11] considers a TBC failed when 20% of the

coating is lost.

In the previous test the samples did not show any sign of

major damage after 857 cycles (~100 hours) of testing.

However, using more severe conditions, 1450°C surface and

1050°C substrate temperatures, on a thinner 400µm single layer

YSZ TBCs (APS deposited) damages appear earlier after 64

cycles (~7.5 hours) and progress though the test. The cycle

duration is the same as before. The test lasted for 221 cycles

(~26 hours) and was stopped because 20% of the coating

delaminated.


Cycle 0 Cycle 64 Cycle 221

Figure 9: On rig pictures. The left picture shows the circular

sample in the cylindrical holder before testing. After 64 cycles

(~7.5 hours) some damage is observed on the sample (middle). At

the end of test, 256 cycles (~30 hours), the damage on the sample

exceeds 20% of the coated area (right).

Fig. 9 shows the sample at three different stages of the test.

The picture on the left of Fig. 9 shows the undamaged sample

at the start of the test. The picture in the centre shows the

sample at cycle 64 when the first TBC losses are seen at the

bottom right. Finally, the picture on the right shows the sample

at the end of the test (cycle 221) when 20% of the coating is

lost.

Cycle 220 Cycle 221

Figure 10: On rig pictures. Damage does not necessarily appear

gradually. From cycle 220 (left) to 221 (right), approximately 15%

of the coated area delaminated.

Depending on the nature of the coating, TBC loss can be

gradual or rapid. Fig. 10 demonstrates the rapid loss of a TBC -

up to 15% of the surface - in only one cycle. These types of

failures can occur when damages are accumulated from the

previous cycles underneath the TBC (eg crack growth). But one

additional cycle is sufficient to trigger the complete

delamination of the damaged part of the coating. This

demonstrates that visual inspection of TBCs in engine

maintenance alone might be insufficient to estimate TBC’s

conditions.

Another test strategy was applied investigating the

extended failure behaviour beyond the 20% coating loss

criterion. Tests were carried out on 300µm APS deposited TBC

samples tested between 1300°C and 1400°C surface

temperature and 950°C and 1050°C substrate temperature. The

aim was to continue testing beyond accumulated loss of 50%.

This was achieved after 286 cycles (~33 hours). Table 2

summarises the results in percentage of coating loss versus the

number of cycles experienced by the sample. Table 2 shows

that the coating lost 10% of its original size at about every 80

cycles. However the last 10%, from 40% to 50%, were lost in

half of the time (~40 cycles) which suggests that the remaining

Coating loss

in %

Cycles

#

Picture Fig 11

As-sprayed 0 A

20 81 B

30 161 C

40 239 N/A

50 286 D

Table 2: Summary of TBC losses in percentage versus the number

of cycles undergone. Pictures of samples in Figure 11.

coatings accumulated damages underneath the TBC. Fig. 11

shows pictures of the sample at the different stages of the test.

On each picture the thermocouple hole is positioned on the

bottom for consistency. The damage started from the bottom

right corner and gradually increased in a crescent shape. The

conditions were so severe that the substrate started to suffer

significantly and even developed a crack.

EBPVD coatings

Coatings deposited by EBPVD have been tested on the

gradient rig. The testing was conducted under a project

investigating the behaviour of coatings which comprises

multiple doped layers for the detection of erosion. The tests and

results are described in further detail elsewhere [7]. Doping

YSZ with rare-earth ions has been shown to affect the

coefficient of thermal expansion (CTE) [21]. Although this

change in CTE is an order of magnitude less than the mismatch

between the YSZ and the substrate, the interface between doped

layers could be seen as potential weak points in the coating

system. The objective of the test was to demonstrate that the

interfaces between the doped layers did not introduce new

failure modes to the coating system. Using thermal gradient

cyclic testing it was possible to generate the stresses associated

Figure 11: Pictures of gradual delamination of TBCs. Also

see Table 2.

A B

C D


with thermal expansion mismatch.

Coating samples were produced at Cranfield University

using a jumping beam EBPVD coater. The samples comprised

Inconel 718 substrates with a platinum enriched nickel

aluminide bond coat and a 150µm thick ceramic overcoat. Two

types of multi-layered sensor coatings, labelled A and B, were

produced along with reference samples comprising only a

single YSZ layer. The tests included two samples of Coating A

and the reference YSZ coating and one sample of Coating B.

The test conditions and results of all the tested samples are

provided in Table 3. The target temperature settings were

1225°C and 1110°C at the surface and substrate respectively,

except for sample A2 where the target settings were 1095°C

and 1250°C for the surface and substrate respectively.

The test conditions were set to achieve a bond coat

temperature at the limit of its operation and a high thermal

gradient through the coating. This was to accelerate the TGO

growth failure to investigate whether failure due to a CTE

mismatch between the doped layers occurred before the

primary failure mode, therefore determining whether the CTE

mismatch was life limiting.

Table 3 shows that, in all samples, the average calculated

bond coat temperature was close to 1150°C. At this

temperature, diffusion of the bond coat becomes significant,

rapidly degrading the performance of the bond coat [22].

Accordingly, these accelerated tests were conducted until

failure of the coating, which was considered to be when 20% of

the ceramic had spalled.

The number of cycles to failure in the different samples

was within the scatter of this type of testing, except for the

sample A2, which was tested under more rigorous conditions. A

photograph of each of the coatings after testing is shown in Fig.

12. In all samples, the failure occurred by delamination which

initiated at the edges of the samples and grew towards the

centre of the sample. This failure is typical of burner rig testing

and is indicative of oxidation induced degradation. The failure

observed in Fig. 12(e) is more extensive than in other samples

because the photograph was taken 20 cycles after failure. The

failure was determined from a significant change in the

recorded temperature traces.

The samples shown in Fig. 12 all have the same

orientation. This is to show that the edge failure does not occur

at the same location on the sample and is not correlated to any

non-uniformity in the rig operation.

The main conclusion of the testing was that the

introduction of doped layers did not destabilise the coating by

inducing ‘white’ failure, i.e. failure within the ceramic overcoat.

Any variation in the CTE between the layers in the ceramic

overcoat was not sufficient to initiate failure before the failure

at the bond coat interface.

Studies on Sensor Thermal Barrier Coatings Sensor TBCs are TBC doped with rare earths elements

such as YSZ:Dy. Their phosphorescence decay time is related

to the temperature at which they are being exposed to and the

technique is also called phosphor thermometry.

Figure 12. Photographs of Sample YSZ (a), YSZ 2 (b), A (c), A 2

(d) and B (e) after thermal cycling testing showing the

delamination of the coating starting from the edge of the sample.

The temperature of the sensor TBC can be measured as long as

optical access is provided. These type of coatings were used to

detect temperatures on nozzle guide vanes, in the combustion

chamber, and rotating turbine blades on an operating Rolls-

Royce engine [23-25].

Table 3. A summary of the thermal gradient cycling test results of

the EBPVD samples

Emissivity is an important input parameter for pyrometer

measurements in an engine and can be affected by several

unknown parameters such as stain on the coating, pollution on

optics, changes in the surfaces morphology or chemistry etc.

This means that pyrometry or infrared detectors can show high

errors in accuracy of absolute temperature readings this is

explained in detail elsewhere [26]. Hence a better

understanding of the real value of the emissivity will result in a

significant improvement of engine pyrometer readings [27].

Emissivity measurements at elevated temperatures are

challenging as the surface of interest needs to be at the specific

temperature while the surroundings need to be at much lower

temperatures when the pyrometer observes the surface. This

usually cannot be achieved in a standard furnace (with optical

access holes) as the black body radiation from the inside of the

furnace would be detected by the pyrometer as a reflection on

the sample surface hence changing the recorded intensity and

the determined emissivity would be close to the one of a black

body.

Surroundings are at ambient temperature, the observed

surface is at high temperature and the pyrometer is insensitive

to the flame heating up the surface.

The set-up is shown in Fig. 13. This time, additional to the

Sample

Cycles

to

failure

Average

surface

temperature

(°C)

Average

substrate

temperature

(°C)

Calculated

temperature

at bond

coat (°C)

YSZ 166 1108 1219 1147.5

YSZ 2 162 1117 1207 1148.5

A 102 1103 1221 1145.5

A 2 135 1095 1239 1146.0

B 167 1105 1216 1148.0


Figure 13: Sketch of the setup used to estimate the sample

emissivity of the sample. A gradient of 20°C is applied to the

sample. The emissivity is estimated by matching the temperature

measured by the phosphor thermometry probe (OPETS) and the

pyrometer at different emissivity settings.

pyrometer, a phosphor thermometry system monitors the

surface temperature of the sample [28]. The phosphor

thermometry system uses a 355nm laser beam to excite the

phosphorescent material which is embedded inside the TBC.

The Nd:YAG laser (Quantel Brilliant B) generates 200mJ

pulses, which are 6ns long, at a frequency of 10Hz. The laser

beam is reflected with a high laser power mirror to the front of

the OPETS where a dichroic mirror is positioned. The laser

beam excites the location where the pyrometer measures the

sample surface temperature. A fibre bundle routes the

phosphorescence signal collected by the OPETS to a

photomultiplier tube (PMT: Hamamatsu). The thermocouple,

the PMT, and the pyrometer are connected to the computer for

data reading and recording.

The YSZ:Dy coating was heated up to >700°C with a small

temperature gradient (~20°C) across the sample using the

cooling air. The coating surface temperature was measured with

the pyrometer with an emissivity setting of 1 and the substrate

temperature is monitored with a K-type thermocouple. The

temperature graphs are shown in Fig. 14.

In contrast to the previous study when using the heating

plate (Fig. 3), the thermocouple temperature could not be

utilised as it did not reflect the coating temperature due to the

gradient. However, when assuming that the pyrometer and the

sensor coating system both measured the coating temperature

the emissivity can be determined.

Fig. 14 shows the data recorded over a time span of 6

minutes when the rig was operated in steady state conditions.

At first, the pyrometer emissivity setting was set to 1 and Fig.

14 shows a temperature difference between 10°C and 15°C

between the temperatures measured with the pyrometer and the

one measured through phosphorescence. Assuming that both

show temperatures measured at the same location, the

pyrometer’s emissivity setting was reduced from 1 to 0.97 so

that both temperatures matched and showed about 710°C.

This result does not match the expected emissivity of 0.90

to 0.93 and the authors conclude that the assumption that

pyrometer temperature and phosphorescence temperature match

did not hold in a thermal gradient. The phosphorescence signal

detected the temperature further inside the coating and not at its

surface. When assuming an emissivity value of 0.93 the

pyrometer would return a higher temperature value, placing the

phosphorescence temperature inside the thermal barrier coating.

A detailed description of this process can be found in [28].

Figure 14: Recorded data of the pyrometer, thermocouple and

phosphor thermometry probe when estimating the sample

emissivity at 700°C. The bottom trace is the measured

temperature difference between the pyrometer and the phosphor

thermometry probe.

SUMMARY & CONCLUSION This paper reviews some of the operational aspects of a

thermal gradient cycling burner test rig at SCS Ltd. The design

allows simultaneous sample testing under customised operating

conditions.

Temperature ranges are achieved which are in line with

current and future temperature requirements in operating power

generation and aero engine applications. Cycle frequency can

be varied and the durations for hot and cold cycles can be

independently varied, too. This allows maximum flexibility to

tailor tests to customer specific operating conditions.

The paper provides some examples of previous tests on

both APS and EBPVD coatings. Further the issue of changing

surface emissivity is being addressed which could affect

accurate pyrometer measurements. Two methods are introduced

using a hot plate and the gradient rig to demonstrate the

possibility of determining the emissivity of the coating. The

latter utilises a phosphorescence sensor TBC.

This type of test equipment is clearly suitable for

advanced coating testing for technology readiness levels 5 and

6 and simulates future and current operating regimes.

Note: a video demonstration is provided under [29].

ACKNOWLEDGMENTS The authors would like to thank Sensor Coatings Systems Ltd

for the permission to publish. Some aspects of this research

Thermocouple

CoolingNozzle

SampleNd

:YA

G L

aser

PMT

Computer + DAQ

Laser beam

Mirror

Fibre bundle

Burner

-20

0

20

40

60

80

100

650

660

670

680

690

700

710

720

730

00:00 02:00 04:00 06:00

Tem

ep

artu

re d

iffe

ren

ce [

°C]

Tem

pe

ratu

re [

°C]

Time [minutes]

Thermocouple

Difference

Phopshoresence

Target setting

= 1 = 0.97

0 2 4 6

Pyrometer

0 2 4 6


were supported by the Office of Naval Research and the

Technology Strategy Board.

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