# Modified zirconia abradable seal coating for high temperature gas turbine applications

E. Novinski, J. Harrington, J. Klein

► To cite this version:

E. Novinski, J. Harrington, J. Klein. Modified zirconia abradable seal coating for high temperature gas turbine applications. Thin Solid Films, 1982, 95 (3), pp.255-263. 10.1016/0040-6090(82)90018-9. hal-01553705

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# MODIFIED ZIRCONIA ABRADABLE SEAL COATING FOR HIGH TEMPERATURE GAS TURBINE APPLICATIONS*

E. NOVINSKI, J. HARRINGTON AND J. KLEIN

Research and Development Department, METCO Inc., 1101 Prospect Avenue, Westbury, NY 11590
(U.S.A.)

The results and development of a new full ceramic abradable turbine seal coating material prepared by thermal spraying are presented. The main objective was to achieve high temperature abradability and low mating part wear using an erosion-resistant coating with high temperature stability and thermal shock resistance. The new coating was successfully laboratory tested at temperatures of at least 1100 °C (2012 °F). Commercial metal-based abradable coatings which are currently available are limited to lower operating temperatures. Typical plasma-sprayed ceramic coatings, because of inherent high particle velocities, are normally too dense to permit abrading without experiencing high turbine blade tip wear damage. In contrast, lower velocity combustion-sprayed ceramic coatings frequently have lower toughness and cohesive particle strength for resistance to abrasive erosion. The new coating material is designed to react exothermically, during combustion spraying, to produce a coating both with high interparticle cohesive strength for resistance to abrasive particle erosion and with controlled porosity for low turbine blade tip wear and effective abradability. Adjustment of spraying conditions gives flexibility to alter the coating hardness and porosity and permits the tailoring of abradability and erosion resistance properties for specific operating requirements.

Based on specially developed test methods for high temperature abradability, high temperature particle erosion and thermal cycling, the modified zirconia coating showed superior performance to high performance baseline materials tested in the program. Industrial evaluation of this coating is presently being conducted.

## 1. INTRODUCTION

The maintenance of close clearances at the various rotating seals is essential to the economic operation of a gas turbine engine¹. Engines of the 1950 design era operated at relatively low pressure ratios, and clearances over the blade tips were large enough to avoid rubbing. Modern engines have pressure ratios in the range of
25:1. To preserve efficiency, the sealing clearances were reduced. These smaller clearances made rubbing contact at sealing locations inevitable and brought about the introduction of abradable materials which are designed to wear sacrificially². Turbine blade tip clearances have a significant effect on specific fuel consumption. On some small modern high pressure turbines an increase in the clearance over the blade shroud of 0.127 mm (0.005 in) may result in an increase in specific fuel consumption of 0.5%¹. Gas path sealing is a proven and accepted method of improving gas turbine engine performance and thereby reducing fuel consumption. Generally, the turbine section of the engine presents the greatest potential for efficiency gains through the use of gas path sealing. Turbine abradable seals serve two principal functions.

(1) They permit minimum clearance operation for high efficiency while reducing the potential of rotor damage.

(2) They protect critical turbine components from the effects of high temperature gas through thermal barrier properties³.

It is projected that turbine inlet temperatures for advanced 1980s engines will continue to be increased in an effort to produce engines of minimum size and weight for high efficiency.

A principal problem in developing an abradable gas path coating is in designing for the two conflicting requirements of erosion resistance and ease of abradability without blade tip damage. This problem is greatly compounded when developing turbine zone seals that must operate at sustained temperatures of 1100°C (2021°F) and above. These engine operating conditions dictate materials having high temperature abradability and low mating part wear using an erosion-resistant material with high temperature stability and thermal shock resistance. In this paper we present a new thermal spray coating material that may satisfy these requirements and was developed within METCO Inc. METCO currently provides nine combustion- and plasma-sprayed abradable coatings to the gas turbine industry.

## 2. COATING DEVELOPMENT

### 2.1. Material fabrication

Criteria for materials selection focused on three principal properties: high temperature stability, low microhardness to minimize blade tip wear, and the ability to be thermally sprayed to achieve a coating with both high erosion resistance and high abradability. Ceramic materials offer the high temperature and insulation capability required for turbine outer gas seal application.

The concept of sprayed seal coatings offers the most flexibility and is less costly than other coatings such as a sintered ceramic that is braze bonded to a metal substrate. An additional benefit is the possibility of repair and/or refurbishment directly at an overhaul facility using existing spray facilities³. Several coating approaches were investigated in the program and, of the candidate materials screened, zirconia-24%Magnesia was found to have the desired temperature stability and low hardness properties. Coatings of this material were sprayed with both the plasma and combustion processes. Plasma spraying produced dense coatings which had a high resistance to particle erosion but were low in abradability
because of the high temperatures and high particle velocities characteristic of the plasma process. Combustion-sprayed coatings of the zirconia-24% magnesia, as a result of the lower velocity and temperature conditions, exhibited increased coating porosity and improved abradability compared with coatings produced by plasma deposition. However, unsatisfactory particle melting and interparticle cohesion was obtained giving coatings of low resistance to abrasive particle erosion.

An exothermic aluminum-(zirconia-24% magnesia) composite (patent applied for) discovered during this study provides a combustion-sprayed coating with both good particle melting and controlled porosity. This is achieved by combining particles of zirconia-24% magnesia with fine aluminum metal such that the aluminum behaves exothermically. The released heat energy of the aluminum exothermic reaction during spraying leads to essentially complete melting of the zirconia-24% magnesia particles during flight. This effect was observed by collecting particles during spraying captured in free flight for both the zirconia-magnesia and the aluminum-zirconia-magnesia and evaluating the degree of particle spheridization and melting. In the case of the zirconia-magnesia without aluminum, the sprayed particles are only partially melted (Fig. 1), while the aluminum-containing material shows almost complete particle melting during spraying (Fig. 2).

No unreacted aluminum metal is observed in the coating when properly sprayed. Microstructural analysis shows the existence of aluminum oxide uniformly distributed as a small volume fraction within the coating and the possible formation of oxide complexes of aluminum, zirconium and magnesium. More extensive analysis of the coating structure is planned.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Combustion-sprayed zirconia-24% magnesia particles collected during free flight.</figcaption></figure>

<caption>Fig. 2. Combustion-sprayed aluminum-(zirconia-24% magnesia) particles collected during free flight.</caption>

## 2.2. Coating fabrication and structure

Coatings of the aluminum-(zirconia-24% magnesia) composite (modified zirconia) are thermal spray deposited with an oxygen-acetylene combustion gun such as the METCO type 6P. This device operates by internally feeding spray material into the core of the combustion flame using a carrier gas and permits close control over delivery rate. The morphology of the coating produced is shown in Fig. 3 and is
<figure><img src="image_3.png" /><figcaption>Fig. 3. Modified zirconia abradable coating.</figcaption></figure>

characterized by approximately 30 vol.% porosity and a Rockwell hardness of 15Y 70-80. Adjustment of spraying conditions gives flexibility to alter the coating hardness and porosity and permits the tailoring of abradability and erosion resistance properties for specific operating requirements.

## 2.3. Substrate and bond coats

For test purposes, a substrate of Inconel 600 was used in the program. Prior to application of the abradable coating, the substrate material was vapor degreased and grit blasted with 16-18 mesh aluminum oxide. Thermally sprayed bond coats of Ni-Cr-Al-Y (22% Cr, 12% Al, 0.5% Y, balance Ni; 44-88 µm) or Ni-Cr-Al composite (METCO 443) were used as an underlayment for the abradable coating.

<figure><img src="image_4.png" /><figcaption>Fig. 4. Graded abradable coating configuration.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 5. High temperature abradability test rig.</figcaption></figure>
The feasibility of applying the modified zirconia abradable coating as a graded structure was verified by spraying a mechanical blend of 50 vol.% Ni-Cr-Al-Y plus 50 vol.% abradable composite as an intermediate layer as illustrated in Fig. 4. It is anticipated that the commercial development of this coating may take this approach. The data presented in this paper are for an abradable coating system using an Inconel substrate and Ni-Cr-Al bond coating.

### 3. TEST APPARATUS

Special high temperature particle erosion and abradability test rigs were designed and constructed as part of the program for effective laboratory evaluation of the coating materials. Devising a satisfactory relatively simple rig test, for evaluating the abradability of coatings for engine use, is difficult. It is similar in many ways to the problems associated with evaluating the friction and wear characteristics of materials for use in unlubricated journal bearings and sliding joints¹. The problem is further enhanced by the temperatures required for proper coating testing.

#### 3.1. *High temperature abradability rig*

The abradability test was designed for constant coating-to-blade interaction (Fig. 5). Two René 80 turbine blade segments were mounted to a fixture on the shaft of a 1.5 h.p. 25 000 rev min⁻¹ electric motor. A test coating 1.3 mm (0.05 in) thick was sprayed onto a substrate of dimensions 6.4 cm × 6.4 cm × 0.40 cm (2.5 in × 2.5 in × 0.16 in) mounted to a fixture above the blades. The coating was heated with a propane-oxygen burner ring to approximately 1100 °C (2012 °F) and brought into contact while dead weight loaded against the rotating blades to create a circular rub track. The abradability test conditions are shown in Table I. Coating performance was determined by measuring the depth of cut into the coating and by the loss of length of the blade material due to wear. Results are expressed as a ratio of coating wear to blade wear.

<caption>TABLE I</caption>

HIGH TEMPERATURE ABRADABILITY TEST CONDITIONS

<table><tr><td>Test temperature</td><td>1100 °C (2012 °F)</td></tr><tr><td>Blade material</td><td>René 80</td></tr><tr><td>Number of blades</td><td>2</td></tr><tr><td>Blade tip velocity</td><td>42 m s<sup>-1</sup> (137 ft s<sup>-1</sup>)</td></tr><tr><td>Blade pass frequency</td><td>700 cycles s<sup>-1</sup></td></tr><tr><td>Applied load</td><td>2.3 kgf (5.0 lbf)</td></tr><tr><td>Incursion rate</td><td>Load dependent</td></tr><tr><td>Run time</td><td>5-60 s</td></tr></table>

#### 3.2. *High temperature particle erosion*

The erosion test was designed to operate by impinging a hot gas stream of aluminum oxide abrasive particles against a heated coating surface (Fig. 6). A propane-oxygen burner ring was mounted around an abrasive feed nozzle directed at a water-cooled sample holder. Coating surface temperatures of approximately 980 °C (1800 °F) were achieved and the feed tube diameter was selected to produce a
<figure><img src="image_6.png" /><figcaption>Fig. 6. High temperature particle erosion test rig.</figcaption></figure>

steady rate delivery of abrasive particles. The erosion test conditions are shown in Table II. Coating performance was determined by measuring the coating volume loss and the results are expressed as the reciprocal of volume loss (cubic centimeters) per minute.

<table><caption>TABLE II
HIGH TEMPERATURE PARTICLE EROSION TEST CONDITIONS</caption><tr><td>Test temperature</td><td>980 °C (1800 °F)</td></tr><tr><td>Abrasive</td><td>Al<sub>2</sub>O<sub>3</sub>, 15-53 µm</td></tr><tr><td>Abrasive feed rate</td><td>73 g min<sup>-1</sup></td></tr><tr><td>Air flow rate</td><td>11 m<sup>3</sup> h<sup>-1</sup> (387 ft<sup>3</sup> h<sup>-1</sup>)</td></tr><tr><td>Nozzle diameter</td><td>4.9 mm (0.193 in)</td></tr><tr><td>Standoff distance</td><td>10.2 cm (4 in)</td></tr><tr><td>Impingement angle</td><td>45°</td></tr><tr><td>Test time</td><td>15-60 s</td></tr></table>

### 3.3. Cyclic thermal shock testing

The effects of rapid temperature cycling upon coating integrity were evaluated by alternately heating and cooling the coating surface with an oxygen-acetylene flame and a cold air jet (Fig. 7). A coating 3.0 mm (0.120 in) thick was applied to a substrate of dimensions 2.5 cm × 3.8 cm × 0.4 cm (1.0 in × 1.5 in × 0.16 in). The sample was mounted in an insulated shield, heated to approximately 1150 °C (2100 °F) for 5.0 s and cooled to 130 °C (266 °F) for 5.0 s. The back-face temperature of the substrate was measured as 120 °C (248 °F) during testing. The test criterion for coating performance was the total number of thermal cycles before coating cracking or delamination.

## 4. TEST RESULTS

### 4.1. High temperature abradability

The modified zirconia abradable material was compared with coatings of plasma-sprayed zirconia-20% yttria and combustion-sprayed Ni-Cr-Al-BN com-
<figure><img src="image_7.png" /><figcaption>Fig. 7. Cyclic thermal shock test rig.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 8. Coating erosion-abradability performance.</figcaption></figure>

posite (METCO 301C-NS) for baseline abradability and erosion (Fig. 8). Coatings of plasma-sprayed zirconia-20% yttria produced no measurable rub depth under the test rig conditions and consequently coating-to-blade wear ratios were essentially zero. High blade wear was experienced with significant metallic transfer to the coating surface (Fig. 9).

The Ni-Cr-Al-BN composite coating is a commercially used turbine engine seal material and was tested at its maximum service temperature of 815 °C (1500 °F). This coating produced moderate abradability of 1.3 coating-to-blade wear and the surface after testing was characterized by plastic deformation and some metal transfer from the blade material.
<figure><img src="image_9.png" /><figcaption>Fig. 9. Zirconia-20% yttria coating after high temperature abradability testing.
Fig. 10. Modified zirconia coating after high temperature abradability testing.</figcaption></figure>

Coatings of the modified zirconia composite produced a coating-to-blade wear rate of 2.5 in $s^{-1}$ and the performance of this coating was characterized as follows.

(1) The mechanism of abrading was observed as a fracture of the coating material and removal of wear debris from the rub track.

(2) No metallic transfer of blade material to the coating surface was experienced (Fig. 10).

(3) Relatively high blade incursion rates of approximately $3 \times 10^{-3}$ in $s^{-1}$ were produced without high blade wear.

(4) No abrasive coatings were applied to the blade tips to enhance abradability.

(5) Blade tip cutting forces appeared to be very low.

It is recognized that because of the many test rig designs and conditions used by other researchers, direct comparison of results are difficult to establish. The abradability test used for this work was severe because of the use of only two blades and the constant blade tip contact with the coating. However, on the basis of our tests, we believe that the modified zirconia coating is an excellent candidate for use as a ceramic abradable seal.

## 4.2. *High temperature particle erosion*

The high temperature particle erosion resistance of the modified zirconia coating was indicative of good performance relative to ceramic abradable seal materials. The coating presented in this paper had an erosion resistance of $12.0 (cm^3 min^{-1})^{-1}$ using our test apparatus and conditions. As mentioned earlier, the erosion resistance can easily be raised above this value by changing spray conditions and sacrificing some abradability. In comparison, the dense plasma-sprayed zirconia-20% yttria coating exhibited higher erosion resistance and no abradability relative to the modified zirconia coating (Fig. 8). The Ni-Cr-Al-BN composite also used for comparison offers a good balance of erosion resistance and abradability because of its metallic nature. However, its maximum service temperature is limited to approximately $815^\circ C$ (1500 $^\circ F$).
### 4.3. Cyclic thermal shock

Under the test rig conditions, the modified zirconia coating was cycled 35 times before substrate delamination initiated. In comparison, a commercially used thermal barrier coating of magnesium zirconate showed delamination after 15 cycles under equivalent conditions. A graded coating configuration of the new abradable (Fig. 4) was thermally cycled 300 times with no evidence of delamination or cracking.

## 5. CONCLUSIONS

The novel exothermic ceramic composite material developed under this program was compared with known abradable seal and candidate abradable seal coatings in the laboratory. The new coating requires no abrasive blade tip overlay and has demonstrated a unique combination of abradability, particle erosion resistance and thermal shock properties showing potential application to engine turbine zone seals for at least 1100 °C (2012 °F) operation. It is recognized that laboratory testing cannot simulate all conditions experienced under actual engine operation. Industrial evaluation of this new coating is presently being conducted and results to date have supported the conclusions based on laboratory data.

## REFERENCES

1. J. G. Ferguson, Use of coatings in turbomachinery gas path seals, *Seal Technology in Gas Turbine Engines, Proc. Propulsion and Energetics Panel's 51st (B) Specialists' Meet.*, London, 1978, in AGARD Conf. Publ. CP-237, August 1978, pp. 2-1, 2-5 (Advisory Group for Aerospace Research and Development).

2. L. P. Ludwig, Gas path sealing in turbine engines, *Seal Technology in Gas Turbine Engines, Proc. Propulsion and Energetics Panel's 51st (B) Specialists' Meet.*, London, 1978, in AGARD Conf. Publ. CP-237, August 1978, p. 1-1 (Advisory Group for Aerospace Research and Development).

3. L. T. Shiembob, Development of a plasma sprayed ceramic gas path seal for high pressure turbine applications, *NASA Contract. Rep. CR-135183*, April 1977, pp. 6, 7 (National Aeronautics and Space Administration).