Plasma-sprayed multilayer protective coatings for gas turbine units

Surface and Coatings Technology, 64 (1994) 5-9

5

Plasma-sprayed multilayer protective coatings for gas turbine units P. YU. Pekshev and S. V. Tcherniakov* Institute of Metallurgy, Russian Academy ofScience, 117911 Moscow (Russian Federation)

N. A. Arzhakin and V. V. Rutskin PGPP "Motorostroitel", Perm (Russian Federation)

(Received October 26,1992; accepted in final form August 4,1993)

Abstract Results are presented of a study of low pressure plasma-sprayed thermal barrier coatings for gas turbine engine components. The thermal barrier coatings were of a three-layer construction comprising a metal layer (NiCrAlY), a cermet layer (NiCrAlY +ZrOfY203) and a ceramic layer (Zr0 2.y 203)' The objective of the work was the development of an economical, low energy preparation process for multilayer thermal barrier coatings by low pressure plasma spraying in a single technological cycle. The composite three-layer thermal barrier coatings sprayed by the developed technique are characterized by a homogeneous structure and high density of some layers, a high cohesive strength of the layers and good adhesion between the metal layer and the substrate. Analyses of the structure and properties of the three-layer thermal barrier coatings as well as preliminary thermocyclic and extensive bench testing have given results encouraging further investigations.

1. Introduction Thermal barrier coatings have many potential applications in the protection of gas turbine engine components, e.g. nozzle guide vanes and turbine blades. At present the following methods for depositing thermal barrier coatings on gas turbine blades are under discussion: electron beam spraying and plasma spraying-a combination of low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS). Electron beam spraying has been used in the aerospace industry for a number of years to deposit coatings of the MCrAIY type for the protection of gas turbine blades against high temperature gas corrosion [1, 2]. However, the method suffers from various technological and economical disadvantages, e.g. a low factor of material utilization, limited coating composition, high prices for industrial equipment and high electric power of the latter. In addition, various structural peculiarities of electron-beam-sprayed ceramic coatings, namely their columnar structure and in particular their channel porosity, are disadvantageous from the standpoint of effective protection of gas turbine blades. Current developments in thermal barrier coating by plasma spraying have their own disadvantages, because a two-stage process (consecutive application of LPPS and APS, the latter for the upper ceramic layer) is

*Author to

whom correspondence should be addressed.

SSDI 0257-8972(93)02207-8

utilized. Besides, available LPPS equipment is characterized by a multiplicity of spraying chambers, high power plasmatrons and metal-consuming, energyconsuming supplementary arrangements, all of which contribute to the processing costs. The aim of this investigation was to develop an effective, economical and low energy thermal barrier coating for turbine blades in a single technological cycle based on plasma spraying in a controlled rarefied atmosphere (LPPS), the principal features of which are: (1) the possibility to spray particles at high velocities (up to 600-800 m S-1); (2) the reduction or elimination from the spraying material of impurities resulting from the products of uncontrolled reactions with the environment (principally oxides and nitrides); (3) the possibility to deposit a coating at a variety of spraying distances (up to 0.3-0.5 m) and thus a low sensitivity of the process to deviation of the distance from a given value; (4) high adhesion strength of the metal layers, which could be more than 60-70 MPa; (5) the combination of spraying, electrical cleaning and activation into a single technological cycle; (6) a high coefficient of use of relatively fusible metals and alloys owing to the guaranteed melting and acceleration of the particles; (7) the possibility to control the spraying parameters accurately over a wide range and thus to control the

Elsevier Sequoia

P. Yu. Peksheo et al. / Protective coatings for gas tltrbine units

6

effective diameter of the spraying spot (10-20 to 200-300 mm); (8) the possibility to prepare MCrAIY metallic coatings (1 %-2 % porosity) and Al20 r and ZrOrbased ceramic coatings with controlled porosity level and structure.

2. Experimental details 2.1. Equipment for spray coating

A laboratory installation for plasma spraying in a controlled rarefied atmosphere (Fig. 1) was constructed along the following lines: (1) the development of plasma guns for LPPS with special shielding tubes which promote the melting of material of any particle size at any melting point as well as consuming less electricity and conferring a longer service life on the electrodes because of the relatively low operating currents; (2) the development of special working chambers and load locks which, in combination with the plasma gun shielding tubes and some other arrangements, result in a considerable decrease in the mass and size of the installation; (3) special blowing of the chambers before spray deposition, thus reducing the cost of the preparatory stage; (4) the selection of pumps which ensure a longer service life of the vacuum pump station and a simplified principle of control and pressure maintenance in the

working chamber meeting the requirements of the filter system; (5) the development of a base for the installation's construction with two pressure levels of the protective atmosphere resulting from the combination of the low mass of the installation and the relatively large sizes of the parts; (6) the development of a special plasma gun shielding tube and the auxiliary equipment required for its action, thus allowing spray deposition on to thin-walled constructions, foils and other parts where overcooling is undesirable or there is a danger of accumulation of residual stresses. 2.2. Construction of thermal barrier coatings

For the thermal barrier coatings we have chosen a three-layer construction comprising a metal layer, a cermet layer and a ceramic layer (Fig. 2). The metal layer protects the substrate against oxidation and high temperature corrosion as well as ensuring a high adhesive strength between the three-layer coating and the substrate. There are three functions of the cermet layer: (i) to improve the cohesive strength of the metal and ceramic layers, thus ensuring a fluent transition in the coefficient of thermal expansion; (ii) to improve the thermocyclic stability of the coating by providing a complementary temperature difference (compared with the ceramic layer only); (iii) to improve the overall coating efficiency by protecting against high temperature corrosion and oxidation. The ceramic layer functions simply as a thermal barrier to protect the blade against

3 Plasma gun

zrOZ"YZ03 NiCrAIY (NiCrAlY

+Zr02~Y203)

2

1

Furnace

Pumping

Transfer chamber Narrow work chamber

Fig. 1. Schematic diagram of the LPPS installation developed for deposition of multilayer thermal barrier coatings.

Fig. 2. Multilayer construction of thermal barrier coatings for turbine blades. (1) Protection from high temperature corrosion and oxidation, high adhesion with substrate; (2) improvement of the thermo-mechanical properties of the thermal barrier coating on the whole; (3) protection from high temperature and erosion.

P. Yu. Pekshe» et af.

! Protective coatings for gas turbine lIlIits

7

erosion under impact by foreign solid particles contained in the high velocity gas stream. This multilayer coating design ensures good performance in high temperature, corrosive and oxidative environments as well as under alternating loads coupled with large temperature differences resulting in thermal loading. 2.3. Materials for thermal barrier coatings The powders for spraying the metal and ceramic coatings were chosen on the basis of current best practice in plasma spraying multilayer thermal barrier coatings for gas turbine engine components. For spraying the metal and ceramic layers respectively, NiCrAIY powder (40-60 11m) and 2r02 powder (20-40 11m) partially stabilized with Y20 3 were used. Both conventional grinding, sieving and drying and supplementary procedures were used to activate these powders and to average their composition. For the cermet interlayer a special composite NiCrAIY + 2r0 2· Y203 powder was prepared by mechanical alloying. Figure 3 shows the shape and structure of the composite NiCrAIY + Zr02·Y203 particles, indicating a high compositional homogeneity and a fine dispersivity in particle volume. The size of the Zr0 2 • Y203 inclusions in the composite particles was 300-500 A; they were not visible by optical microscopy. In addition, composite NiCrAIY + 2rOfY203 particles formed by mechanical alloying are characterized as having a high level of accumulated energy, this being related to their structural and substructural peculiarities. This energy is utilized in the plasma-spraying process to improve the heating and melting of particles in the plasma jet, thus positively affecting the structure and properties of the sprayed cermet layer [3].

(a)

3. Results and discussion Using the equipment described in Section 2.1, we have developed an effective, low energy technique of LPPS for the deposition of three-layer thermal barrier coatings for the protection of gas turbine blades. These coatings have been achieved in a single technological cycle (using the same plasma gun and working chamber) without changing the spraying distance, the power consumption of the plasma gun or the chamber pressure and with only minor changes in the consumption of the auxiliary gas. The spray deposition of the NiCrAIY and NiCrAIY +Zr02·Y203 layers was carried out under identical conditions. The total technological cycle included the following procedures: blowing and abradant blast treatment by placing the part in the load lock; electrical cleaning by means of transferred discharge; first-layer (NiCrAIY) spraying; electrical cleaning; second-layer (NiCrAIY +

(b)

Fig. 3. (a) Shape and (b) internal structure of composite NiCrAIY +ZrOfY203 particles prepared by mechanical alloying.

Zr02'Y203) spraying; third-layer (2r0 2·Y203) spraying; cooling of the part in the load lock; removal of the finished part. After removal the coated blade was subjected to thermochemical and mechanical treatment. The fixed parameters during the spraying process were in the following range: the pressure in the working vacuum chamber, 30-36 mbar; the consumption of the plasma gas Ar + H 2, 40-50 1 min -1; the distance between the plasma gun nozzle outlet and the horizontal suction face of the blade, 480 mm; the number of degrees of

P. Yl!. Peksheo et al. I Protective coatings for gas turbine units

8

freedom of the blade and plasma gun mutual movement, two. The electrical cleaning was carried out within the power range 0.57-0.7 kW. For the spraying of the first layer (NiCrAIY) the electric power used by the plasma gun amounted to 11.7-12 kW. The coefficient of powder use with a dispersion of 40-60 urn was equal to 41 % at a coating growth rate of 150-180).lm min-to The second layer (NiCrAIY +ZrOz'Y Z03 ) was deposited under identical conditions to the first layer. The coefficient of material use was 43% at a layer growth rate of 120-150).lm min-t. The third layer (ZrO Z'Y20 3 ) was deposited with a gun power of 14.4-15 kW at the same spraying distance as for the first two layers. The coefficient of material 'use amounted to 19% at a layer growth rate of 45-60 urn min -t. The structure of the three-layer thermal barrier coating sprayed by the developed technique is shown in Fig. 4, The coating is seen to be characterized by a homogeneous structure and low porosity of some layers, a high cohesive strength of the layers and good adhesion between the metal layer and the substrate. Upon adhesion testing, failure of the three-layer coating takes place at the interface between the cermet and ceramic layers at 19-22 MPa. The adhesive strength of the metal layer to the substrate is as high as 67 MPa. The structure of the interlayer differs from that of the starting composite

NiCrAIY +ZrO Z-YZ0 3 owing to coagulation of melted ZrO Z'Y20 3 particles during plasma spraying. Figure 5 shows the structure of the same coating after 4 h of diffusion annealing at 1050 DC. The annealing gives rise to a reduction in the porosity of some layers, the formation of a diffusion zone between the first metal layer and the substrate and an improvement in the interlayer boundaries. The structure of the interlayer has changed, with the Zr02'Y203 inclusions becoming larger. Nevertheless, the structure of the interlayer is much more homogeneous than that of cermet coatings which have been sprayed using mechanical mixtures or composite powders prepared by other methods such as mechanical alloying. Utilizing the developed technique, an experimental series of blades with thermal barrier coatings have been prepared which have been tested under thermocycling as well as in extensive bench tests. In the thermo cycling tests the coated blades were in one case heated to 1100 DC using a gas burner and then cooled in air, and in another case heated in a furnace to the same temperature followed by cooling in air; 150 cycles were used in each series of tests. Subsequent studies of the coated

.," . 0."...,, " -",

.. r ;

, "

. ·.t .. '. ': (

,

\"

"

"

"....... I

·

"

._

· lo

,

.\

..... "

• .r

, Fig. 5. Structure of the three-layer thermal barrier coating deposited

Fig. 4, Structure of the three-layer thermal barrier coating deposited by the developed LPPS technique.

by the developed LPPS technique after 4 h of diffusion annealing at 1050"C.

'

P. Yu. Pekshe» et al. / Protective coatings for gas turbille units

blade structure and shape showed no defects in the coating or the substrate.

9

References

r. S. Malashenko, Zharostoikie Pokritija Osazltdaemie v VakilI/me, Naukova Dumka, Kiev, 1983.

1 B. A. Movchan and

4. Conclusions The development of an improved LPPS technique in terms of equipment design and powder materials has been successfully carried out. The new process allows for the deposition of three-layer thermal barrier coatings in a single cycle. Compared with the process developed in 1970-1973 by Muchlberger, our process may be regarded as economical, low energy plasma spraying under reduced pressure with controlled chemical and thermal effects of the plasma stream upon the spraying material and substrate.

2 W. Dietrich, A. Feuerstein, E. Muechlberger and Ph. Meyer, EB-, PVD- and LPPS-process for protective coatings against high temperature corrosion and erosion, Proc. Bth Int. Conf. on Vacuum Metallurgy (Coating in Vacul/lIl or Controlled Atmosphere), Lille, 1985, 1985, pp.40-62. 3 V. V. Kudinov, P. Yu. Pekshev, S. V. Tcherniakov and 1. K. Kondratenko, Peculiarities of composite powders plasma spraying prepared by mechanical alloying, PJ'oc. lilt. Workshop on Plasma Jets in the Development of Nell' Materials Technology, Frunze, 1990,

1990,pp.227-242.