Acceptable limits of degradation of TBC for high-efficient turbines (HET TBC) Department Materials (ALSTOM) Lab of Crystallography (ETH Zürich) CTI project Nr. 7820.3 EPRP-IW Project start Nov. 1. 2005 Project duration 24 months Baden, 28.11.2005
Outline XRD measurement methods and instruments Strain measurement with XRD techniques In-situ XRD measurements Ex-situ XRD measurements
XRD measurement methods and instruments Powder diffractometers of Lab of Crystallography: PANalytical X’Pert Pro Scintag PAD X STOE STADI
XRD measurement methods and instruments PANalytical X’Pert Pro Detector: X’celerator: multistrip system, detector width 2º, allows high speed scan Detector Monochromator Sample holders: Spinner Eulerian-cradle (for or texture and strains measurements) Furnace (up to 1300ºC) Possibility to use a monochromator to suppress the Cu K2 line Furnace
XRD measurement methods and instruments Scintag PAD X Detector: Liquid nitrogen cooled Ge solid state detector, low peak to background ratio due to high energy resolution Sample holders: Spinner Detector X-Ray tube
Tetragonal YSZ Monoclinic YSZ XRD measurement methods and instruments Spectrum of the feedstock Sulzer W71040 measured with the PANalytical X’Pert Pro: We can observe peaks of the monoclinic phase Their heights are 2 bigger than the background level
Tetragonal YSZ Monoclinic YSZ XRD measurement methods and instruments Spectrum of the feedstock Sulzer W71040 measured with the Sintag PAD X: We can observe distinctly peaks of the monoclinic phase Their heights are ~10 bigger than the background level
XRD measurement methods and instruments Adavantages and disavandtages of each systems: PANalytical X’Pert Pro: Can conduct most of the experiments needed (stress/strains, -2 scans, high temperature) The X-Ray tube is fixed, the sample and the detector moves Scintag PAD X: High peak to background ratio allowing to observe secondary phases with amount <0.1% The sample is fixed, the X-Ray tube and the detector move Possibility to perform biaxial stress/strains measurements, not able to perform triaxial stress measurements No furnace
XRD measurement methods and instruments What can bring the Rietveld refinement method? The unit cell dimensions of each phases are fitted during the refinement The percentage of each phase can be calculated The peak shape of each phase is fitted by functions whose dependence give information on micro-strains and crystallites dimensions Instrumental errors can be refined and allow to minimize the error due to shift of the zero and in deviancy from the plane of diffraction Information about the texture of samples can be obtained The positions of atoms within the unit cell and site occupancy and substitutions by impurity atoms can be refined
XRD measurement methods and instruments For determination of the phase fraction in powder samples and coatings at room temperature we will use the Sintag PAD X Experimental data Rietveld refinement Tetragonal YSZ (t’) Cubic ZrO2 Monoclinic YSZ Y2O3 SiZrO4 Phases like Y2O3 present at a level of ~0.1% can be observed
XRD measurement methods and instruments Problems which can be encountered for the accurate phase determination of each phases in feedstock powders: The crystallinity of secondary phases is not very good, peaks at high angle are broad, their fitting could be difficult and they can be hidden in the background noise FWHM ~0.5°
XRD measurement methods and instruments Problems which can be encountered for the accurate phase determination of each phases in feedstock powders: The average grain size is ~50 m, the XRD do not probe the complete grain volume but only its surface. The solutions are: • Grind the powder before measurements in order to have an average grain size < 10 m. • Use sieved powder
XRD measurement methods and instruments Are the feedstock powder particles single crystals or polycrystalline? Single crystal diffractometer Oxford diffraction, CCD camera YSZ spherical particle, Ø 70 µm, 100 µm
XRD measurement methods and instruments Rotation diffraction patterns (=10o, 20 sec) Ø 100 µm Ø 70 µm The grain in the left picture is composed of multiple crystallites having a high crystallinity. The grain in the right picture is composed of multiple crystallites having a high mosaicity.
200 µm 50 µm XRD measurement methods and instruments Powder 204NS-AP, Batch W71040 (Sulzer Metco US) ZrO2 7.8Y2O3, -125+16 µm, spheroidal morphology
XRD measurement methods and instruments Influence of impurities on XRD spectra Contaminations from impurities leading to the apparition of secondary phases could be detected from XRD by the apparition of new peaks. Determination of unit cell constant, space groups and finally of phase composition will be difficult because of the low intensity of the peaks and the presence of other phases whose peaks could overlap. Exact phases composition would need the use of other techniques like SEM+EDX and WDX, XPS, EELS, AES and chemical analysis.
XRD measurement methods and instruments Rietveld refinement of the monoclinic phase is reliable In order to have a precise measurement of phase content, one needs a to observe multiple peaks of each phase to be quantified. The monoclic YSZ phase has two strong peaks ~28º and ~31.5º as well as smaller peaks ~17.5º ~35º ~41º and ~50º. Peaks which are fingerprints of the monoclinic phase
XRD measurement methods and instruments The observed apparition of the monoclinic phase will probably not exactly correspond with the onset of nucleation of monoclinic crystallites for the following reasons: The determination of exact monoclinic phase content requires extended refinement (high number of parameters: peak shape, temperature factor, background level, texture). The comparison of the monoclinic phase content between different samples can be more reliable because some errors will cancel. • When monoclinic crystallites nucleate their size is small and the diffraction peaks associated to those crystallites are broad and difficult to distinguish from the background • When crystallites growth bigger peaks are narrower and distinguishable from the background noise
XRD measurement methods and instruments • Samples already measured: • Sulzer W71040 feedstock powder: • Panalytical X’Pert Pro: raw powder • Sintag PAD X: raw powder, grinded powder, powder • after ex situ treatment of 24h at 1200ºC • Coatings: • Panalytical X’Pert Pro: D0419-4, D0419-5,D0419-10,D0419-11, • D0419-12 on both sides • Sintag PAD X: D0419-4, D0419-5,D0419-10,D0419-12 on • both sides
XRD measurement methods and instruments • First results: • Sulzer W71040 feedstock powder: • The intensity of the monoclinic peaks decrease after 24h at 1200ºC
Strain measurement with XRD techniques Two different kinds of strains can be measured with XRD Macro-strains can be determined from the evolution of the d-spacing of one peak for different sample orientations Micro-strains can be determined from the dependence of peaks width with diffraction angle
S3 S2 S1 Strain measurement with XRD techniques Measurement of macro-strains When macro-strains are presents in a sample, the distance between planes will be be modified by the strains, modifying the d-value of diffracted peaks. By measuring the d-value of a peak for different orientations of the sample, one can determine the strains. The simplest technique is the sin2() technique which works well for polycrystalline samples with homogeneous macro-strains. By measuring the slope of d vs sin2() one can obtain in the direction of the measurement.
Strain measurement with XRD techniques Measurement of micro-strains Micro-strains leads to a distributions of d-value around d0. Since when diffraction angle increases, corresponding d-value decreases, the ratio d/d2 increases and the diffraction peaks broadens. The dependence of d/d2 is given by: When fitting peaks profile with a Gaussian function with a having a dependence given by: The strain S (in %) is given by: Ui is the contribution from the instrument to the peak broadening.
Strain measurement with XRD techniques • For the measurements of strains we will use the PANalytical X’Pert Pro • Advantage: • Use of the Eulerian-cradle allow to determine triaxial macro-strains • Use of monochromator allow to have peaks whose shape is not influenced by the Cu K2 line • Possibility to use Soller slits in order to increase the parallelism of the beam and reduce instrument contribution to the peak profile The instrumental contribution to the peak profile can be determined by measuring a standard sample.
In situ measurements up to 1300ºC(start Dec.12.2005) Using the PANalytical X’Pert Pro A furnace working up to 1300ºC can be mounted in the PANalytical X’Pert Pro. Using batch programs, one can perform a whole cycle of heating-cooling or multiple cycles and measure XRD spectra at various temperatures. For the determination of cell constants, fast measurements (<1h) can be performed For the determination of phase content longer measurements are necessary (few hours). A possibility to shorten measurement time is to scan limited regions were important peaks should be present, the problem which such technique is that unexpected phases could be missed.
In situ measurements up to 1300ºC(start Dec.12.2005) • Other possibilities to perform in situ measurements • In situ measurements can be performed at various synchrotron and neutron facilities, for instance: • SLS in Villigen has one furnace which can be used up to 1400ºC • ESRF in Grenoble has one furnace which can be used up to 1500ºC • ILL in Grenoble has two furnaces which can be used up to 1700ºC. • SINQ in Villigen has a furnace which can be used up to 1500ºC
Ex situ measurements • Advantages: • Doesn’t require a specific diffraction equipement, could be used with the Scintag PAD X diffractometer which has a high peak to background ratio. • Inconvenients: • During cooling, phase transformations can occur (tetragonal YSZ monoclinic YSZ) • Thermal expansion of each phases cannot be measured
Ex situ measurements • Furnaces belonging to the Lab of Crystallography: • Heraus Kendro Tmax=1100ºC in air • Gallekamp Box Furnace Tmax=1200ºC in air • Nabertherm Tmax=1200ºC in air • Vacuum furnace: Tmax=1300ºC under vacuum or Ar (possibility to quench with a flow of Ar) • Linn high ThermTmax=1800ºC in oxidative medium, possibility with overpressure, cannot be opened at high temperature
Ex situ measurements A first experiment was made at 1200ºC with the Nabertherm. The powder containing crucible is taken out from the hot furnace and its temperature evolution is measured with a thermocouple After six minutes the temperature of the powder is ~200ºC Peaks of monoclinic YSZ are weaker than in the raw powder