Chapter 3
Experimental Work
3.1 Introduction
The experimental work carried out within this thesis aims to analyse the phase stability of Yttria partially stabilised Zirconia deposited by electron beam physical vapour deposition. The behaviour of this material and the effects on its stability of either Dysprosia or Gadolinia as co-dopants has been analysed in case of as-deposited coating and after heat-treatment.
Given the limited amount of data available about these systems in the literature, a parallel study on the TBCs in powder form has also been conducted in order to have a wider outlook on the structure of these materials.
The present chapter is divided in four parts: Part 1: Coating deposition;
Part 2: Phase stability analysis; Part 3: Ageing;
Part 4: Chemical composition analysis.
Each of these parts describes materials, procedure for samples preparation, pieces of equipment used and experimental procedure adopted.
Part 1:
Coating Deposition
3.2 The Substrates
As mentioned in section 1.3 the structure of a thermal barrier coated component consists of three layers, some deposited, some formed on the metal body of the blade, usually referred to as metallic substrate. Nevertheless, in this work, the EB-PVD TBCs have been deposited on ceramic instead of metallic substrates. The choice of a ceramic material as a substrate is not unusual for studies concerning TBCs and it depends on the fact that, during the phase stability investigation, a temperature, higher than the melting point of the metals which the turbine blades are usually made of, needs to be reached.
Between the possible alternatives, laboratory alumina was chosen because, in the real life applications, the TGO layer essentially consists of α-aluminium oxide. A different option could have been the adoption of a material more similar to the TBC itself, such as 6-8 wt% Yttria stabilized Zirconia but, the wish of investigating also the possible influence that both the microstructure and the impurities of the alumina substrate could have had on the TBC phase stability led to discard such alternative. Three different grades of alumina have been adopted: they are listed in Table 3.1 and depicted in Fig. 3.1
Fig. 3.1: Image of the three different substrates: sapphire on the top, polycrystal-99.5% alumina on the left, polycrystal-96% alumina on the right.
The study of two different alumina microstructures (polycrystal and single crystal) with a similar grade of purity (99.5% is the maximum purity available for polycrystal alumina tiles) allows comparing the effect of grain boundaries on the TBC behaviour. On the other hand two different purity grades in substrates with the same microstructure allow studying the influence of the impurities (Table 3.2 and Table 3.3).
Type of
substrate Material Orientation Size (mm) temperature (˚C) Max. working
Single crystal
(Sapphire) Alumina 99.99% Random 25.4 dia x 3 International, 2002b)1800 (Marketech Polycrystal Alumina 99.5% Random 25.4 x 25.4 x 4 International, 2002a)1700 (Marketech Polycrystal Alumina 96% Random 25.4 x 25.4 x 2 International, 2002a)1600 (Marketech
Table 3.1: Substrates description.
SiO2 2.4%
MgO 0.7%
CaO 0.25%
Fe2O3 0.1%
Other 0.01%
Table 3.2: Alumina 96% typical impurities.
Fe 8 ppm Ce 3 ppm Na 3 ppm K 3 ppm Cu 2 ppm Ca 1 ppm Others each < 1ppm
Table 3.3: Sapphire 99.99% typical impurities. All the substrates have been supplied by PI-KEM.
3.3 The EB-PVD substrates holders
The substrates holders for the EB-PVD chamber (Fig. 3.2) adopted in this study have been especially designed and commissioned (technical drawing in Appendix I). They must guarantee that, during the deposition, the samples are as centred as possible with respect to the melt ceramic pool and that the resulting deposition on all the specimens is as homogeneous as possible. Each holder consists of two parts assembled together with five bolts.
The holders and the bolts are manufactured using NIMONIC alloy 75: an 80/20 Nickel-Chromium alloy with controlled additions of Titanium and Carbon (Table 3.4). This material melts between 1340 and 1380°C (Special metals, 2005).
Fig. 3.2: Holders for alumina substrates. Element COMPOSITION,% Carbon 0.08-0.15 Chromium 18.0-21.0 Copper 0.5 max Iron 5.0 max Manganese 1.0 max Silicon 1.0 max Titanium 0.2-0.6 Nickel Balance
Table 3.4: NIMONIC alloy 75 chemical composition (wt%) (Special metals, 2005).
3.4 The ingots
The cylindrical ceramic ingots are 33 mm in diameter and 201 mm in length. They have been supplied by Phoenix Coating Resources and their chemical composition is reported in Table 3.5. In the same table the total amount of REO is also indicated. This value is given, for each rod, as RO1.5 mol %, where R is
the Rare Earth element in a 2:3 ratio to the Oxygen.
Ingot name Description RO1.5 mol %
Y-PSZ 7wt%Y2O3-Zr2O3 7.8
G2 7wt%Y2O3-2mol%Gd2O3-Zr2O3 11.8
G4 7wt%Y2O3-4mol%Gd2O3-Zr2O3 15.8
D1 7wt%Y2O3-1mol%Dy2O3-Zr2O3 9.8
D2 7wt%Y2O3-2mol%Dy2O3-Zr2O3 11.8
Table 3.5: Ingots description.
The rods composition has been chosen in order to guarantee, according to the phase diagrams study (see Chapter 2), a metastable single phase in all the coated samples. The percentage either of Gadolinia or Dysprosia never exceeds 50% of the total amount of stabilizer in each rod since experimental work,
conducted on pyrolysed samples of Gadolinia co-doped Y-PSZ, showed that the samples stability degrades rapidly as Gadolinia become the dominant species in co-doped formulations (Rebollo, Fabrichnaya, and Levi, 2003).
3.5 Coating equipment
The EB-PVD equipment available at Cranfield University consists of only one working chamber for samples loading, pre-heating, and deposition. The EB gun chamber is located underneath the working one. Each chamber has its own pumping-down system which allows the two units to work at different pressures during the deposition; in particular the working chamber pressure can be varied between 1Pa and 10-3Pa.
The substrates holders (or the blades) are fixed on the deposition chamber back wall, perpendicularly to the evaporation source as shown in Fig. 3.3. A rotation mechanism enables the holders rotation at 20 rpm. Moreover, during evaporation, an ingot feed mechanism pushes the ceramic rod from the bottom into the chamber ensuring continuous growth of coating layer on the substrates. The EB gun chamber accommodates an electron beam gun inclined of 270° with respect to the rod. The power source works at 40 kW with a voltage of 10 kV. A controlled blend of Oxygen (90%) and Argon (10%) is pumped into the deposition chamber to achieve the required Zirconia stoichiometry.
Other process parameters are the condensation rate of the vapours on the substrates, which typically is around 2-3 µm/min, and the average substrate temperature during deposition which varies between 900 and 1015°C.
The thickness of the TBCs analysed in the present study ranges between 30 and 100 µm.
As mentioned in section 1.5, the process parameters influence the TBCs microstructure and consequently the coating mechanical properties and its life time. In this work the deposition parameters have been chosen in accordance to the results gathered during studies previously conducted at Cranfield (Lawson, 2005).
In particular the adopted value for the rotation speed (20 rpm) has been chosen because it seems to be a good compromise between the need of a lower speed (for a longer coating lifetime) and of a higher speed (for a lower coating anisotropy).
3.6 Experimental procedure
The holder geometry (Fig. 3.2) makes sure the coating is deposited on both the sides of four substrates at the same time. On the other hand a single holder could carry eight substrates by means of an additional plate. In this case, though, only one side of each specimen would be coated. From now on the word ‘sample’ will refer to the single coated side of each substrate.
In Table 3.6 the number of deposition runs for each rod composition, the number of samples obtained, and the type of substrate utilized are listed (D is for deposition, DS is for double side, and SS is for single side). All the substrates have been cleaned with acetone before the deposition.
Run Number of samples substratesNumber of Single crystal(sapphire) AlPolycryst.
2o3 99.5%
Polycryst. Al2o3 96%
Y-PSZ D0 8 8 - - 8 single side
Y-PSZ D1 16 8 6 DS 2 DS - Y-PSZ D2 16 8 8 DS - - Y-PSZ D3 16 12 4 DS+8 SS - - Y-PSZ D4 12 8 1 DS+4 SS 3 DS - G2 D1 16 12 1 DS+8 SS 3DS - G4 D1 16 12 1 DS+8 SS 3DS - D1 D1 16 8 5DS 3DS - D2 D1 16 8 5DS 3DS -
Part 2:
Phase stability analysis
3.7 X-Ray Diffraction (XRD) analyse
The phenomenon of diffraction occurs when penetrating radiation, such as X-rays, enters a crystalline substance and is scattered. The direction and intensity of the scattered (diffracted) beams depends on the orientation of the crystal lattice with respect to the incident beam. Any face of a crystal lattice consists of parallel rows of atoms (Fig. 3.4) separated by a unique distance (d-spacing), which are capable of diffracting X-rays. In order for a beam to be 100% diffracted, the distance it travels between rows of atoms at the angle of incidence must be equal to an integral multiple of the wavelength of the incident beam. In this case diffraction satisfy Bragg's Law
nλ = 2d sinθ
where d is the distance between atomic layers in a crystal, theta (θ) and lambda (λ) are the angle of incidence and the wavelength of the incident X-ray beam, and n is an integer (Cullity, 1978).
Fig. 3.4: Schematic of an X-ray diffraction. In blue the atoms in a crystal lattice and in red the X-ray beams.
An X-ray diffractometer utilises the X-ray diffraction technique. It basically consists of a goniometer and a detector: the former provides a variety of angles of incidence, and the latter measures the intensity of the diffracted beam. The resulting analysis is described graphically as a set of peaks with intensity on the Y-axis and goniometer angle on the X-axis. The exact angle and intensity of a set of peaks is unique to the crystal structure being examined. In this thesis XRD analysis has been used to evaluate time and extent of changes in the TBCs structure, with eventual destabilization, high temperature exposure causes.
3.7.1 Samples preparation for XRD analysis
It is important for the specimen to be analysed with XRD to be flat; in fact, as the diffraction starts from small angles, a bent or uneven sample could hamper the beam from reaching the upper surface, as schematically represented in Fig. 3.5.
Fig. 3.5: Schematic drawing of a bent and uneven sample.
All the coated substrates did not require any special preparation. Although there is a difference in the thermal expansion coefficients of Zirconia and alumina, the specimens surface did not present any curvature, neither after deposition nor after heat treatment in disagreement with the findings of Wellman et al. (Wellman and Nicholls, 2004).
TBC powder was obtained by smashing by hand some TBC chips which were removed, by means of a small chisel, from the holder bolts in the deposition chamber. The powder was then analysed preparing XRD specimens with some bi-adhesive tape on a glass slide (Fig. 3.6).
Fig. 3.6: Powdered samples.
3.7.2 Experimental procedure
The TBC phase composition, both as-deposited and after heat treatment, was determined at room temperature by means of a Siemens D5000 X-Ray. The diffractometer (Fig. 3.7 and Fig. 3.8) used a Nichel-filtered CuKα radiation,
comprising Kα1 (λ=0.154056 nm) and Kα2 (λ=0.1544 nm); it operated at 40 kV and
40 mA. The angular step size was 0.04° with a step time equal to 1 second and a two-theta value ranging from 20° to 90°.
Fig. 3.7: XRD equipment: Siemens D5000.
Fig. 3.8: Siemens D5000 diffractometer chamber.
Due to the coating crystallographic texture preferentially orientated as {311} or {200}, the evaluation of monoclinic volume fraction, would be much easier if conducted on their respective diffraction lines. Nevertheless the extensive overlap of the tetragonal and of the monoclinic peaks which occurs in these regions excludes them from a meaningful quantitative analysis. Further studies were therefore restricted to the analysis of the {111} lines.
The monoclinic volume fraction (Xm) has been evaluated using the following
equation (Garvie and Nicholson, 1972):
c t m m m m m I I I I I X / ) 111 ( ) 111 ( ) 11 1 ( ) 111 ( ) 11 1 ( + + + = (1)
where (hkl) represents the Miller indices and I(hkl)i is the integrated intensity of
and m for monoclinic). Formation of the monoclinic phase is in fact identified by the emergence of its {111}m reflections, which are clearly separate from the
{111}t/c. In reality, Xm is not a precise measure of the amount of monoclinic phase
and the method of Toraya et al (Toraya, Yoshimura, and Somiya, 1984a), (Toraya, Yoshimura, and Somiya, 1984b) should give a better quantitative phase analysis of the monoclinic-tetragonal/cubic contents in Zirconia polymorphs. However, equation (1) does provide a consistent criterion for the purposes of ranking the behaviour of the compositions studied. A coating is considered de-stabilised when partitioning leads to Xm in equation (1) to be higher than 10%.
A ‘stability limit’ is defined as the time, given as sum of the time for cycles, for which Xm <0.1.
3.8 Raman spectroscopy
Raman spectra were obtained using a Dilor LabRam 1B Raman Microprobe (Instruments SA, UK) (Fig. 3.9). Laser excitation was provided with a 17mW He-Ne laser, polarised 500:1 at an excitation wavelength of 632.8 nm. The diameter of the laser beam in focus using the long range objective gives an analytical area of approximately 2 µm2. The detector is liquid nitrogen cooled, 1024 x 256 pixel,
16 bit, dynamic range CCD detector with pixel size of 27 µm. A high-resolution, 1800 lines/mm grating was used. The spectra were obtained from 100 to 1000 cm-1. This kind of analysis does not require any special sample preparation.
Part 3:
Ageing
3.9 Ageing equipment
For the specimens heat treatment (‘ageing’) a box furnace from Pyrotherm has been used (Fig. 3.10). This furnace can reach temperature up to 1600°C by means of 4 Kanthal Silicon Carbide elements, arranged in 2 parallel pairs, each pair in series, which can yield up to 7 KW power. The chamber has a capacity of 6 litres and is made of suspended ceramic fibre within a steel outer skin.
Fig. 3.10: Pyrothern Box furnace.
3.10 Experimental procedure
The specimens heat treatment consisted of 9 hours cycles, or their multiple, at three different temperature: 1450°C, 1500°C and 1550°C. In each cycle the samples were placed into the preheated furnace, held in isothermal conditions for the prescribed time, and then withdrawn to cool in air. In such conditions the cooling rate is higher than 50°Cs-1 (Brandon and Taylor, 1991). An XRD analysis was performed after each cycle. The treatment was usually stopped when significant evidence of monoclinic formation was detected, as described in paragraph 3.7.2. When, after the first cycle, the samples already showed monoclinic formation, shorter cycles of 1h or 2h were used for subsequent analogue samples.
Samples layout during ageing is showed in Fig. 3.11. The holders were realized using furnace bricks, already aged for volatilization at 1500°C for 24h, and high purity alumina (99.8% Al2O3) tubes (type C799 from Anderman ceramics). They
were designed in order to prevent any surface contact between samples and bricks and to expose both the substrate sides to the furnace environment.
TBC powder ageing was carried out in high purity alumina crucibles (Alsint 99.7: 99.7% Al2O3 from Morgan Advanced Ceramics) (Fig. 3.12).
Fig. 3.11: Holders for the furnace.
Part 4:
Chemical composition analysis
3.11 X-ray Fluorescence Spectroscopy
X-ray fluorescence (XRF) technology measures the chemical composition of many types of materials. It is non-destructive and reliable, requires no, or very little, sample preparation and is suitable for solid, liquid and powdered samples. It can be used for a wide range of elements, from beryllium (4) to uranium (92), and provides detection limits at the ppm level; it can also precisely measure concentrations of up to 100%. XRF working principle is based on the fact that when a primary X-ray from an X-ray tube or a radioactive source strikes a sample, it can either be absorbed by the atom or scattered through the material. The process in which an X-ray is absorbed by the atom by transferring all of its energy to an innermost electron is called the ‘photoelectric effect’. During this process, if the primary X-ray had sufficient energy, electrons are ejected from the inner shells, creating vacancies. These vacancies present an unstable condition for the atom. As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process give off a characteristic X-ray whose energy is the difference between the two binding energies of the corresponding shells. Because each element has a unique set of energy levels, each element produces X-rays at a unique set of energies, allowing a non-destructively measure of the sample elemental composition.
Cranfield University laboratory is equipped with a Bruxer axs S2 Ranger (Fig. 3.13) that allows to load and measure up to 28 different samples (Fig. 3.14). All the specimens were ultrasonically cleaned in isopropyl alcohol before the analysis.
Fig. 3.14: Samples loading in the Bruxer axs S2 Ranger equipment.
3.12 Environmental Scanning Electron Microscopy
Environmental Scanning Electron Microscopy (ESEM) allows investigating specimens in their natural state or under natural environmental conditions without the need for conventional preparation techniques that may produce unwanted artefacts in the sample. Although a certain degree of resolution has been compromised, ESEM can be used to examine a non-conductive sample without coating it with a conductive material.
The primary electron beam hits the specimen and causes the emission of secondary electrons (Fig. 3.15). These are attracted to the positively charged detector electrode. As they travel through the gaseous environment, collisions occur between an electron and a gas particle resulting in emission of more electrons and in ionization of the gas molecules. This increase in the amount of electrons effectively amplifies the original secondary electron signal. The positively charged gas ions are attracted to the negatively biased specimen and offset charging effects.
As the number of secondary electrons varies, the amplification effect of the gas varies. If a large number of electrons are emitted from a position on the specimen during a scan, there is a high signal. If only a small amount of electrons are emitted the signal is less intense. The difference in signal intensity from different locations on the specimen allows an image to be formed.
The machine adopted for ESEM analysis was a Philips XL30 (Fig. 3.16) which was also equipped with a quantitative Energy Dispersive Spectrometer capable of chemical analysis.
Fig. 3.16: ESEM microscope.
3.12.1 ESEM samples preparation
The specimens were cut in half by a MetPrep low concentration diamond wheel in a MetPrep Brilliant 220 cut off machine (Fig. 3.17). The wheel rotation rate was 1200 min-1, the speed of the sample movement into the wheel was 1.8 mm/min. After cutting the sectioned specimens were mounted in resin.
The resin task is to support the specimen during polishing, thus it is important for it to have rigidity similar to the material to be polished. Alumina sample are extremely stiff, in fact they are usually mounted in phenolic resin which is harder than the epoxy. However, during this study, the adoption of phenolic resin and, consequently, of the high pressure moulding machine, often caused damaging of the sample (Fig. 3.19). This is the reason why, in some cases, the use of epoxy resin was preferred.
Fig. 3.18: Samples mounted in black phenolic resin and in transparent epoxy resin.
When the black phenolic resin (Conducto-Mount from MetPrep) was adopted, the sample was clamped in a metal spring and mounted by means of a Buehler Simplimet 2000 automatic mounting press (Fig. 3.20) which works with a pressure of 200 bar.
When the transparent epoxy resin (Epofix from Struers) was used, the sample was clamped in a plastic spring, then put it in a plastic crucible-shaped mould, covered with resin and left to cure for 24 hours.
The mounted specimen where then machined and polished. A first manual operation intended to flatten both surfaces of the mounted sample was conducted with a coarse grinding paper (220 grit), followed by a polishing procedure with a polishing machine (Fig. 3.21) which used a series of successively finer abrasives. When a minimum of 5 samples were prepared, they were placed on a support disk (Fig. 3.22) which was eventually clamped on the polishing machine.
Fig. 3.21: Polishing machine.
Table 3.7 summarizes the entire polishing operation. During the first seven automatic steps, water was used as lubricant and the support disk was co-rotating at 100 rpm with the abrasive disk, which was silicon carbide grinding paper. The pressure applied by the machine was 8 pounds on each sample.
Abrasive Operation Lubricant Duration
1st step SiC paper 220 grits Manual Water Until flattened
2nd step SiC paper 1200 grits Automated Water 4 min
3rd step SiC paper 1200 grits Automated Water 4 min
4th step SiC paper 1200 grits Automated Water 4 min
5th step SiC paper 1200 grits Automated Water 4 min
6th step SiC paper 2500 grits Automated Water 4 min
7th step SiC paper 2500 grits Automated Water 4 min
8th step SiC paper 2500 grits Automated Water 4 min
9th step Diamond paste 6µm Automated Diamond lubric. 10 min
10th step Diamond paste 6µm Automated Diamond lubric. 6 min
11th step Diamond paste 3µm Automated Diamond lubric. 10 min
12th step Diamond paste 3µm Automated Diamond lubric. 6 min
Table 3.7: Polishing operations.
The 9th and 10th steps ware carried out using a woven nylon cloth disk impregnated with diamond paste with a particle size of 6 µm; a woven silk cloth disk impregnated with a diamond paste with a particle size of 3 µm was used in the last 2 steps.
