zürich, 19.01.2006 acceptable limits of degradation of tbc for high-efficient turbines (het tbc)...

Zürich, 19.01.2006

Acceptable limits of degradation of TBC forhigh-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

Outline

1 Determination of monoclinic content

2 About estimation of YSZ monoclinic phase

content from XRD

3 SEM pictures

4 WDX results

5 In-situ XRD up to 1200ºC

Determination of monoclinic content

Raw powder loaded from the bottom of the sample holder

Amount of Monoclinic phase is : 6 wt%

Raw powder loaded by the top of the sample holder



After milling the powder



Equation used in Mannheim Report (MR):

and are peak maxima of the monoclinic and tetragonal phases.

Phase content is related to the area of peaks (integral peak intensity), and not to their height

In Sulzer powder, the peaks of monoclinic phase are ca. 1.5 broader than tetragonal phase the ratio of peaks area is 1.5 bigger than the ratio of peaks height.

cmon =Imonheight(111)

Imonheight(111) + I tetr

height(101)

Imonheight (111)Itetr

height (101)

About estimation of YSZ monoclinic phase content from XRD

The (101) peak of the tetragonal phase is equivalent to the (111) peak of the cubic phase or of the monoclinic phase (the tetragonal cell is rotated by 45º and has half of the volume of the cubic or monoclinic cell).

The (111) peak of the monoclinic phase is splitted into two peaks and the sum of their intensities should come close to the value of the intensity of the (101) peak of the tetragonal phase

111 plane of cubic/monoclinic unit cell 101 plane of tetragonal unit cell

Tetragonal unit cell Cubic or monoclinic unit

cell


- Tetragonal- Monoclinic

2

I

Simulation of the XRD pattern of a mixture of 50% of monoclinic + 50% tetragonal YSZ shows that is approximately 2 times smaller than Imon

height (111)Itetrheight (101)

The true monoclinic content should be approximately 3 times larger than what is obtained with the formula used in the MR


A more accurate estimation of the monoclinic phase content could be obtained from:

Xmon =Imonint (111) + Imon

int (11−1)Imonint (111) + Imon

int (11−1) + I tetrint (101)

cmon =PXmon

1+ (P−1)Xmon

and are to the area of the two peaks of the monoclinic and the peak tetragonal phases, P=1.340 is a scaling factor

- The peaks of the monoclinic phase are ≈ 1.5 times broader than tetragonal ones- The height of the second monoclinic peak 111 forms ≈ 0.7 times of one of the first peaks cmon ≅ 20 vol% which corresponds to cmon ≅ 19 wt%.

Using (experimentally determined) P=1.311, cmon ≅ 19.5 vol%.

The monoclinic content found in MR is in fact closer to 15-20% than to 5-6%.

Itetrint (101)Imon

int (11±1)


1 Depending on how the powder is loaded in the sample holder the amount of measured monoclinic YSZ phase vary by a factor 5

2 When loading by the top of the sample holder big and less dense grains are more likely to be present at the surface

3 When loading from the bottom of the sample holder, small and dense grains are more likely to be present at the surface

4 After milling it is expected that grains size is more homogeneous and measured monoclinic content is the correct average value

5 Large amounts of monoclinic phase are expected to be present in big grains having a low density (big and porous grains)

Summary of XRD results

SEM

200 µm

SE pattern, SEM LEO

Hollowparticle

Porousparticle

Bulk particle

SE Bs

Grains composition measured by WDX:90-90.5wt% of ZrO2 8-8.5wt% of Y2O3 and ~1.5wt% HfO2

SEM

20 µm 20 µm

SE BS

Sometimes crystallites are present in hollow grains, their composition is not yet determined

SEM

20 µm 20 µm

SE BS

Grains encircled in red were measured by WDX: 97.5-98wt% of ZrO2 0.3-0.7wt% of Y2O3 and ~1.5wt% HfO2

SEM

20 µm20 µm

SE BS

Grains encircled in red were measured by WDX: 97.5-98wt% of ZrO2 0.3-0.7wt% of Y2O3 and ~1.5wt% HfO2

SEM

20 µm 20 µm

WDX Results

Grain ZrO2 Y2O3 HfO2 SiO2

Hollow 90.485 8.009 1.506 -

Hollow 89.977 8.444 1.432 0.147

Hollow 90.572 7.938 1.490 -

Porous 97.524 0.334 1.525 0.617

Porous 97.556 0.386 1.668 0.390

Porous 97.745 0.461 1.577 0.217

Porous 97.457 0.728 1.472 0.343

Hollow average 90.345 8.130 1.476 0.049

Porous average 97.571 0.477 1.560 0.392

Porous Grains

1 Composed of two phases with different grain shape and composition

2 The inner part of the grains (encircled in red in SEM pictures) is composed of YSZ with 0.5-1 wt% Y2O3 (Monoclinic YSZ)

3 High SiO2 content 0.2-0.6 wt%

Hollow Grains

1 In some hollow grains there are big crystallites whose composition is not yet determined

2 Hollow grains are homogeneous in composition: YSZ with 8-8.5 wt% Y2O3 (tetragonal YSZ)

3 Crystallites sizes are of the order 1 m

Summary SEM and WDX results

In-situ XRD up to 1200 ºC

During first heating, the monoclinic content decreases rapidly at ~400 ºC and then remains constant up to 1000 ºC

During cooling no monoclinic phase is observed above 400 ºC

During the second heating, the monoclinic content decreases steadily

Monoclinic phase

The a axis vary linearly with temperature and is comparable during the two heating-cooling cycles

The c axis variation deviates from linearity at high temperature and the c-axis value is increased after 1st heating

This variation of c-axis value can be better seen when plotting the c/a ratio


Tetragonalphase

YO1.5 (mol%) =1.0225 −

ca 2

0.0016

Y2O3(wt%)=112.905 ⋅YO1.5 (mol%)

123.223−0.10318 ⋅YO1.5 (mol%)

[1] Ilavsky J. et al., Jour. of Therm. Spray Technol., 10 (3) , 2001, pp. 497

Y-content can be calculated from c/a [1]:

After 1st cycle Y2O3 content of tetragonal phase is lowered by 0.5 percent

Y2O3 content of tetragonal phase remains constant during 2nd cycle


Furnace used for XRD is limited to 1200ºC

ESRF in Grenoble has one furnace which can be used up to 1500ºC

Deadline for applications in ESRF is fixed to the 1st March 2006 (for beam time between August 2006 and February 2007)

Before applying we should decide precisely what we want to measure there since the bean time is limited


Conclusion

During 1st heating Y2O3 is transferred from tetragonal to monoclinic phase and Y2O3 content of tetragonal phase is lowered

Part of monoclinic phase retain enough Y2O3 to be stabilized in its tetragonal form up to room temperature (Martensitic transition temperature onset is lowered) and measured monoclinic content is lowered at room temperature

After 1st heating the FWHM of tetragonal peaks is lowered indicating microstress relaxation and/or grain growth (sintering)

During second heating, continuous variation of monoclinic phase content indicates a broad distribution of monoclinic phase composition

During second cooling, parameters of tetragonal phase are similar to obtained during first cooling, indicating no drastic evolution of tetragonal and monoclinic phases during second high temperature cycle


XRD of new powders

Largest amount of monoclinic phase found in Sulzer Metco powder

Smallest amount of monoclinic phase in Industriekeramik Hochrhein

Intermediate amount of monoclinic phase in HC Stark Engineered Materials Solutions

SEM HCST

300 µm

100 µm

SEM HCST

60 µm

SEM HCST

20 µm

SEM HCST

zürich, 19.01.2006 acceptable limits of degradation of tbc for high-efficient turbines (het tbc)...

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