ceramic coating in stationary gas turbines

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ELSEVIER

IFCHNOIDGY Surface and Coatings Technology 76-77 (1995) 86-94

Experience with metal/ceramic coating in stationary gas turbines AS. Osyka a, AI. Rybnikov b, S.A Leontiev b

c,

N.V. Nikitin d, I.S. Malashenko e

a State Area Power Station N 3 of Mosenergo (GRES-3), 142530 Electrogorsk, Russia Polzunov Central Boiler and Turbine Institute (NPO TsKTI), Polytechnicheskaya 24, 194021 St. Petersburg, Russia C Leningradsky Metallitchesky Zavod, (LMZ), Sverdlovskaya nab. 18, 195009 St. Petersburg, Russia d Ivanovo State Area Power Station, 155150 Komsomolsk, Russia e E.O. Paton Electric Welding Institute, Bozhenko st. 11, 252005 Kiev, Ukraine

Abstract The uncooled rotating blades of the high pressure turbine (HPT) and of the low pressure turbine (LPT) of the peak load power gas turbine GT-IOO are protected with electron-beam physically vapour deposited CoCrAlY coatings. Owing to low-temperature hot corrosion leakage, corrosion failure is observed. In order to improve the corrosion resistance, yttria stabilized zirconia (YSZ)jCoCrAlY coatings with a ceramic layer thickness of 50-70 um have been developed. The maximum service life of blades with the YSZ/CoCrAlY coatings has exceeded 939 h with 253 starts. Study of the YSZ/CoCrAlY coated blades after different service periods (from 306 to 939 h) showed different conditions of the ceramic layer: without any kind of damage, with small local damage, and with large-area spalling. Keywords: Metal/ceramic coatings; Gas turbine units; Properties

1. Introduction

The use of electron beam metal coatings of CoCrAIY type for blades of the peak stationary GT-100 gas turbine allowed the blade service to be extended by more than a factor of five [1]. Nevertheless such coatings

are not effective enough to protect the blade against erosion and low-temperature sulphide-oxide corrosion, which appears primarily at the blade trailing edge (Fig. 1). Ceramic layers of zirconium dioxide partly stabilized by yttrium oxide (YSZ) are used to improve the erosion and corrosion resistance of the turbine blade [2-4]. Work performed at LMZ, NPO TsKTI and E.O. Paton Electric Welding Institute [5-7] has included commercial testing of two- and three-layer coatings with a ceramic surface layer. In this article the first results are reported of investigation of blades with such coatings after two periods of operation of total duration 936 h at 253 starts.

2. Experimental details

The subjects of inquiry were large sized uncooled rotating blades of the first stages of the high pressure 0257-8972/95/$09.50 © 1995Elsevier ScienceSA All rights reserved SSDI 0257 -8972 (95) 02541-3

Fig. 1. Rotating blade damage of GT-I00 gas turbine, showing corrosion damage on the trailing edge of the first stage blade of a HPT after 1114 operating hours, x 100.

turbine (HPT) and low pressure turbine (LPT) of the peak load power gas turbine GT-100 produced by LMZ (Fig. 2). The blades were made of EI893 alloy (Ni base, Cr 15%-17%, W 8%-10%, Ti 1.2%-1.6%, Al 1.2%-1.6%, Mo 3.5%-4.5%). At the E.O. Paton Electric Welding Institute at installations UE-137 and UE-187 [8], over 68 rotating blades

A.S. Osyka et al./Surfaceand Coatings Technology 76-77 (1995) 86-94

25/15:J

87

25-3

25-2

21,-/

24-2

24-3

23-/

23-2

23-3

22-1 ~

22-3

-

8

2/-/

16

-

9

-

20-1

20-3

19-/

~

11,

I

15 l---

21-3

1 sf HPT 250 mm

1 sf LPT 420 mm

/9-3 ID

7

6

18-3 5 ---

___

-- --- --4

13

J

12

1

11

I

Fig. 2. Diagram of the first stage blade of a HPT of the GT-I00 gas turbine cut into test coupons for determination of the microstructure and material properties of the blade and the coating: 1, 9, 16, test coupons for tensile testing at 20°C; 4, 7, the same at 750°C; 2, 8, test coupons for impact testing at 20°C; 5, 10, test coupons for testing by constant speed deformation at 650°C; 3, 6, test coupons for long-term strength testing at 650°C or 750 °C; 11-25, test coupons for metallographic examination.

of the first stage HPT, protected with electron beam physical vapour deposition (EBPVD) coatings SDP8, were coated with the ceramic layer KDPI (Table 1). The two-layer SDP8/KDP1 was coated on two rotating blades of the first stage at one cycle. A peculiarity of deposition of the coating onto the large blades of the GT-100 gas turbine is that the coating is applied on one blade at a time only (in contrast to marine engine and aircraft engine where the coating is applied on numerous blades at a time). Before coating, the blades are subjected to a heat treatment according to the following regime: 1020 °C for 1 h+ 1160 °C for 3 h+ 1000 DC for 4 h+900 °C for Table 1 Chemical composition of evaporated alloys used in power engineering for producing corrosion- and heat-resistant coatings Alloy (coatings)

8 h + 820 DC for 15 h. Since coating deposition is carried out on a blade heated up to 850-900 DC, and then a two-stage homogenizing anneal is carried out at 1030 °C for 2 h at each stage (in the interval surface treatment by microballs is conducted), then the two-stage heat treatment in vacuum is applied at 900°C for 8 h + 820 DC for 15 h for restoration of metallic properties [9]. The first stage blades of the HPT were put through this heat treatment cycle and after deposition of the ceramic layer (by heating the blade up to 850-900°C) the two-stage recovery heat treatment (RHT) was carried out again. The first stage blades of the LPT were put through the RHT once since deposition of the ceramic layer was carried out after the second stage of homogenizing annealing. The maximum operating temperature of the blade metal is 700°C. The GT-100 gas turbine operates on liquid fuel (Table 2).

Mass fraction (%) Co

Ni

Cr

Al

Y

3. Results and discussion

SDP-3A SDP-8 SDP-llA

Balance Balance Balance

0-2 0-2

22-24 25-28 24-26

11-13 9-11 3-5

0.2-0.5 0.2-0.5 0.3-0.6

3.1. Inspection of blades with the metal/ceramic coating

KDP-l

z-o, + 8% YZ03

Blades with the metal/ceramic coating have a smooth surface (Ra = 111m) of dark grey colour. Examination of

A.S. Osykaet al.iSurface and Coatings Technology 76-77 (1995) 86-94

88

Table 2 Impurity content in liquid fuel for GT-lOO gas turbine (mass fraction %) Content

Na+K

Ca

S

Particles

Real Demanded by specifications 38.101-858-80b

~0.0006a

Satisfied ~0.0005

Satisfied on the whole ~ 1.88

Satisfied

~0.0002

~0.Q2

a In some cases. Vanadium and lead content are not given in the specifications.

b

68 blades of the first stage HPT revealed some sections with spalled ceramic layer on seven blades. The ceramic layer was removed using technology developed at the E.O. Paton Electric Welding Institute based on a hydroabrasive method. Four blades were chosen from the 61 available for quality control of the metal/ceramic coating; these were cut according to the diagram in Fig. 2. Quality control of the metal/ceramic coating redeposited onto seven blades was carried out using test coupons onto which the ceramic layer was deposited when it was deposited onto the blades. Quality control of the metal/ceramic coating of the first stage LPT blades was carried out on test coupons cut from the blade top near the trailing edge (Fig. 2).

Table 3 Thickness and microhardness of metal and ceramic layers on rotation blades of the GT-100 gas turbine Group, stage

Thickness (urn) SDP8

A, 1 stage, HPT

90,75 80, 65 90, 85 85,70

B, 1 stage, HPT

3.2. Metal/ceramic coating The microstructure of the metal layer on all SDP8 blades has the two-phase structure y + f3 characteristic of such coatings having a high degree of homogeneity over the layer thickness and over the blade airfoil (Fig. 2). The microstructure of the diffusion zone indicates a high annealing performance. The metal layer

C, 1 stage, LPT D, 1 stage, HPT

80,70 100, 80 65,55 80,70

Microhardness (MPa)

KDP1

SDP8

KDP1

Base metal

50,40 60,45 65,45 65,45 50 55 55 50 55 50 50 70,50 70,50 45,40 45,40

5400,5400 5600, 5600 6200, 5600 5600, 6150

3240,3480 2650, 3000 3000, 3480 3270, 4260 7900 8100 7740 8200 7500 7740 8100 6260, 6600 5280, 5600 6470, 6730 4440, 4800

3400 3260 3600 3490

5640, 5600 5670, 5640 6180, 6180 5600, 5600

3900 3920 3930 4030

The first value is for the convex surface, and the second value is for the concave surface.

EI893

(a)

(b)

Fig. 3. Microstricture of the two-layer coating SDP8/KDPI on the first stage blade HPT of the GT-I00 gas turbine: (a) convex side x 320, (b) concave side x 320.

89

A.S. Osyka et al./Surface and Coatings Technology 76-77 (1995) 86-94 Table 4 Structural and phase characteristics of the metal of rotating blades Heat treatment

Group

Standard 1020 DC, 1 h +1160 DC, 3h + 1000 DC, 4 h +900 DC, 8 h +820 DC, 15 h

Phase composition of the alloy'

Amount of carbide and boride phases (%)

Amount of y' phase (%)

Characteristics of the y'-phase Mean particle size (urn)

Mean particle distance (urn)

Distribution of particles (t-tm - 1 )

y,y', M 6C, M13C6,MC,MsB3,M3Bl

0.4...0.6

10...12

0.07...0.12

0.08...0.12

45...60

Standard + 1030 DC, 4h +900°C, 8 h +820 DC, 15 h

C

y,y', M 6C, M 3B1,MsB3,MC

0.5...0.8

10...12

0.07...0.12

0.08...0.12

45...60

Standard +1030 DC, 4h +900 DC, 8 h +820°C,15h + 1030 DC, 2 h +900°C, 8 h +820 DC, 15 h

A,D

y,y', M 6C, M 3B1,MsB3,MC

0.6...0.8

10...12

0.08...0.12

0.07...0.12

45-60

a

Phases are given in order of decreasing amount.

thickness is 80-90 urn on the convex surface of the airfoil and 65-75 urn on the concave surface (Table 3). The microhardness of the metal layer is 5400-6200 MPa and meets the limits given in the specifications (Table 4). The ceramic layer KPDl has different structures on the convex surface of the airfoil and on the concave side; a dense structure on the concave surface and a less dense structure on the convex surface (Fig. 3). The micro hardness of the ceramic layer on the concave surface is significantly higher than that of the layer on the convex surface (Table 4). It should be noted that there is a large spread in the micro hardness values. The maximum values of the ceramic layer were found for seven blades

after redeposition (group B) of 7500-8200 MPa. The range of micro hardness values was slightly lower for the first stage blade of LPT (group C) of 5260-6600 MPa. The micro hardness of the ceramic layer is much lower for blades of the main group A, 2650-4260 MPa, which is equal to the micro hardness of the base metal and is lower than the microhardness of the metal coating. 3.3. Structure and properties ofthe base metal

The typical appearance of the alloy microstructure, grain boundaries and main hardening y-phase (Fig. 4) and the quantitative characteristics of the metal micro-

Table 5 Mechanical properties of GT-I00 gas turbine blades made of EI 893 alloy Group

Test temperature 20°C

HB (MPa)

Uncoated (standard heat treatment) C (standard + RHT) A, D (standard + RHT + RHT) Specification

2360 2500 2360 2500 2300 2550 2100 2700

0'0.1 (MPa)

530 580 530 570 530 580 490 670

Test temperature 750 DC

O'B

,)

i/t

(MPa)

(%)

(%)

KCU (kJ m- 1)

960 1040 970 1040 960 1020

27 37 25 37 25 35

31 39 27 38 28 35

680 1120 680 1010 650 1010

~830

~20

~22

~590

The first value is the minimum and the second value is the maximum test characteristic.

0'0.2 (MPa)

450 490 430 490 450 480

Creep test, T=750 DC, 0'=270 MPa

O'B

s

i/t

(MPa)

(%)

(%)

735 860 740 840 740 840

24 37 26 36 27 35

26 48 30 36 28 37

~640

~20

~22

r

s

(h)

(%)

550 1250 608 1307 587 1105 >500

9 23 10 28 11 27

90

A.S. Osyka et al./Surfaceand Coatings Technology 76-77 ( 1995) 86-94

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.

.

./

"z

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~:~

r

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,

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(a)

Fig. 5. Appearance of rotating blades of the first stage HPT of the GT-100 gas turbine after 633 operating hours at 166 starts. (b)

structure and phase composition of groups A-C of the investigated blades correspond to those of the EI 893 alloy after the heat treatment (Table 4). The mechanical properties at 20°C and 700°C, the impact strength and long-term strength of groups A-C blade metal are at levels usual after the used heat treatment (Table 5). The results obtained testify the possibility of repeated application of the recovery heat treatment after coating deposition without loss of alloy properties.

3.4. Operation ofblades with the metal/ceramic coating The first period of blade operation was 633 h at 166 starts. Visual examination and a more careful later

\Cj

Fig. 4. Microstructure of EI 893 alloy after coating deposition and reheat treatment: (a) optical image x 100, (b) optical image x 1000, (c) electron microscope image x 10000.

A.S. Osyka et al.jSurface and Coatings Technology 76-77 (1995) 86-94

(a)

(b)

(c)

(d)

91

Fig. 6. Microstructure of two-layer SDP8/KDPI coating on the first stage blade HPT of the GT·lOO gas turbine after 306 operating hours at 87 starts: (a) area on the concave side with a retained ceramic layer, x 1000; (b) area on the convex side with a partially retained ceramic layer, x 1000; (c) area on the convex side with a peeled ceramic layer without corrosion damage to the metallic layer, x 1000; (d) area on the convex side with a peeled ceramic layer with corrosion damage to the metallic layer, x 1000.

examination indicated that all seven blades with repeated deposition of the ceramic layer (group B) had no damage along the airfoil surface. Only two blades of the main blade group (group A) maintained a satisfactory condition of the ceramic layer with minor damage due to erosion. On the other 55 blades the ceramic layer was spalled on the convex surface after washing by hot water (Fig. 5) and on some blades the layer was spalled on the concave surface. The metal layer has a light-coloured surface without corrosion damage and deposition at the spalling location of the ceramic layer. Practically all the blades of group A have erosion damage at the leading edge on the convex surface. The reason for the low erosion resistance and low

adhesion strength of the ceramic layer and the metal layer is probably surplus porosity of the ceramic layer (this is confirmed by the very low level of ceramic layer microhardness, especially on the convex surface of the group A blades); this is caused by the increase in pressure of residual gases in the plant chamber at the beginning of the ceramic layer deposition. Blades of group C were under operation on the other GT-IOO gas turbine for 222 h at 71 starts. No damage of the ceramic layers was discovered over the whole airfoil surface and these blades were installed in the turbine for further operation. Blades of group B were installed for following operation too. The ceramic layer was removed from 55 blades of group A and then was redeposited once more.

Fig. 7. X-ray elemental maps of the region of SDP8/KDPl coating on the first stage blade HPT of the GT-lOO gas turbine after 306 operating hours at 87 starts.

~

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~

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->,

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A.S. Osyka et al.jSurface and Coatings Technology 76-77 ( 1995) 86-94

convex sectionside

concave section side

«,

.'

,'

SOP ·11A "

I'

EI893 (a)

(b)

69 60 63

66

60 69

66 63 73

(c)

Fig. 8. Microstructure of the three-layer coating SDPllA/SDP3A/KDPl on the first stage blade HPT of the GT-lOO gas turbine: (a) concave side x 320, (b) convex side x 320, (c) thickness distribution of coating layers along the airfoil in the top part of the blade.

Investigation of two blades with redeposited ceramic layer (out of 55 blades of group D) indicated a difference in the ceramic layer microhardness between 4440 and 6730 MPa (Table 3). Other characteristics of the metal layer and the ceramic layer, microstructural characteristics and blade metal properties, were similar to those obtained for blades of groups A and C (Tables 3-5). The second period of operation of the first stage HPT blades was 306 h at 87 starts. All seven blades of group B with total operating time of 939 h at 253 starts showed

no damage of the ceramic layer along all the blade surface. Two blades of group A for which the ceramic layer had not been redeposited did not show new damage. 43 of 53 blades of group D had individual slight damage, mainly erosion on the leading edge near the convex side. There was no ceramic layer damage on the concave side of 43 blades. 10 blades of group D had significant spalling of the ceramic layer mainly on the convex side of the blade surface. Micro test coupons of five blades of this group with

94

A.S. Osyka et al.ISurface and Coatings Technology 76-77 (1995) 86-94

considerable spallation of the ceramic layer were cut on the top of blades near the trailing edge for investigation (Fig. 2). Investigation of test coupons by optical metallography and X-ray microanalysis revealed that the following blade surface conditions: (i) completely retained ceramic layer of initial thickness (40-50 urn), Fig.6(a); (ii) partly retained ceramic layer of 5-10 um thickness, Fig. 6(b); (iii) ceramic layer entirely absent, metallic layer surface has no corrosion damage, Fig. 6(c); (iv) ceramic layer is entirely absent, metallic layer surface has early corrosion damage, Fig.6(d). For five investigated test coupons the second surface condition (ii) prevailed. The fourth surface condition (iv) was found on separate areas of only one of five test coupons. X-ray spectrometry analysis revealed an aluminium oxide film 1 mk thick at the boundary of metal and ceramic layers (Fig. 7). The microhardness of these areas of the ceramic layer is 4200-4480 MPa. It is evident that the reduced layer microhardness corresponds to the elevated porosity and to the elevated oxygen penetrability to the metal surface. Therefore the aluminium oxide film is formed at the boundary of the metal and ceramic layers despite moderate temperatures of about 700 °C and the limited time of operation (about 300 h). Before operation, no aluminium oxide film at the boundary of the metal and ceramic layers was discovered by X-ray spectrometry analysis on any test coupons which were investigated. All 10 blades of group D were dismounted and replaced by blades with three-layer coatings SDPllA/ SDP3AjKDP1. The microstructure of this coating and the distribution of coating layer thickness along the blade airfoil are given in Fig. 8. The microhardness of the ceramic layer on the investigated blade is 5860 MPa on the convex side and 6950 MPa on the concave side.

4. Conclusion (1) The ceramic layer of ZrOz+ 8%Y Z0 3 deposited onto the metal layer ofCo-22%Cr-lO% AI-O.l%Y by

electron beam physical vapour deposition and vacuum condensation, which is 40-60 urn thick and has a microhardness of 7500-8200 MPa, increases the erosion and corrosion resistance of uncooled rotating blades of the peak load GT-I00 gas turbine. The ceramic layer is undamaged along the whole blade airfoil after operation for 939 h at 253 starts. (2) Disturbance of the ceramic layer deposition process followed by a decrease in its microhardness to 2650-4260 MPa causes a dramatic decrease in its erosion resistance and peeling of the ceramic layer during operation for 300-600 h. (3) Single and multiple use of the reheat treatment regime developed for the EI893 alloy, applied after coating deposition and homogenizing annealing, allows the level of alloy properties typical of those before coating deposition to be retained.

Acknowledgements The authors wish to thank G. Pigrova, N. Mogaiskaya, E. Ovchinnikova and 1. Pertsovskaya for experimental support and assistance in preparing the typescript.

References [1] A.I. Rybnikov et al., Surf. Coat. Technol., 68-69 (1994) 38-44. [2] T.E. Strangman and J.L. Schienle, J. Eng. Gas Turbines Power, 112 (10) (1990) 531-535. [3] B.A. Nagazaj et al., Development of corrosion resistant coatings for marine gas turbine applications, ASME Paper GT-200, 1990, p. 11. [4] M.V. Roode and 1. Aurrecoechea, Rainbow field test of coatings for hot corrosion protection of gas turbine blades and vanes: 1, blade coatings, ASME Paper GT-242, 1989, p. 23. [5] I.S. Malashenko et al., Probl. Spets. Elektrometal, (3) (1990) 53-59. [6] A.I. Rybnikov et aI., Strength Mater., (5) (1990) 35-41. [7] B.A. Movchan et al., Surf. Coat. Technol., 67 (1994) 55-63. [8] B.E. Paton (ed.), Creep Strength of Cast Nickel Alloys and Oxidation Protection of these Alloys, Naukova Dumka, Kiev, 1987,p. 256. [9] B.A. Movchan et aI., Probl. Spets. Electrometall.; (1) (1985) 34-41.