Sealing the gap: benefits of clearance control thermal spray coatings

Sealing the gap: benefits of clearance control thermal spray coatings

Keywords: Coatings, Gas turbines

The major benefits of clearance control in industrial gas turbine (IGT) coatings are increased engine efficiency and reduced fuel consumption. Although there are many types of coatings and methods to manufacture them, the operation of refurbishment of the seals must be simple, reliable and efficient. Focuses on Sulzer Metco, which has developed its line of thermal spray materials and coating systems for use in turbine engine applications.

In response to customers' concerns regarding increasing demands on turbine operating temperatures and output efficiencies, Sulzer Metco has further developed its line of reliable thermal spray materials and coating systems. One result of this work are abradable coatings that are being used successfully in turbine engine applications for clearance control seals of gas leakage over the tips of the blades. The major benefits of clearance control in industrial gas turbine (IGT) coatings are increased engine efficiency and reduced fuel consumption. Although there are many types of coatings and methods to manufacture them, the operation of refurbishment of the seals must be simple, reliable and efficient. Thermal spray coatings meet these requirements and are becoming the most popular choice for sealing applications. Thermal sprayed coatings are easily applied and can be removed by stripping, with comparative ease.

New high-temperature compressor abradable

Plasma spray applied abradables are superior to other processes because of lower blade wear, lower operating cost, extended temperature operating capability, greater spray process control, and more process reliability. This results in fewer part rejections during manufacturing, and less downtime during operation. A typical plasma gun for spraying coatings with powdered materials is seen in Plate 1.

Plate 1 Sulzer Metco 9MB, a typical plasma gun used for spraying abradable coatings.

A major application area of clearance control coatings in gas turbines is the high-pressure section of the compressor (Plate 2). Gas temperatures in this section can typically extend up to 750°C. A recent study carried out by Sulzer Metco using a newly developed and patented plasma sprayed Cobalt-Nickel-Chromium-Aluminum- Yttrium-hexagonal boronnitride 15-weight per cent polyester (CoNiCrAlY-hBN polyester) coating confirmed the suitability of this advanced material for use in compressors.

Plate 2 In stationary gas turbines, a heavily polluted environment places high demands on the sealing material. Abradable seals by Sulzer Metco withstand sulfuric acid and other corrosive compounds in the compressor part.

Erosion resistance and abradability

Clearance control material generally consists of three components: metal matrix, solid lubricant, and polyester. The importance of the metallic CoNiCrAlY matrix is to give the coating its oxidation and hot-corrosion resistance, while the polyester is used as a means of controlling the porosity within the coating. The size and volume percentage of this polyester component are critical for optimizing both the abradability and erosion resistance performance. Typically, the erosion resistance and interparticle cohesive strength is reduced with increasing polyester levels, while the abradability characteristics of the coating improves. The boron nitride addition is as an entrapped solid lubricant and helps to minimize the coating's transfer to the blade material. Particle debris, released from the coating into the engine, is kept small so as not to cause damage or affect the cooling passages. The type of engine and its specific operating conditions and performance requirements dictate the abradability and erosion resistance balance that is necessary for a successful coating application.

Tight quality control

The ability to produce coated components with reliable and consistent microstructure begins with the thermal spray powder. Abradable materials are manufactured to tight quality control specifications to ensure repeatable performance. Key specification requirements for powders and coatings are particle size, chemistry, as-sprayed and heat-treated hardness, and deposit efficiency. When these specifications are met, the coating applicator can be confident that, with calibrated equipment, he will achieve reproducible coating microstructure and hardness. Knowing the microstructural characteristics provides the end-user with a high level of confidence that the sprayed parts have acceptable abradability, erosion resistance, and hardness.

Measurements taken by Sulzer Metco on heat-treated production components using a plasma sprayed CoNiCrAlY-hBN-polyester powder showed the dramatic effects that optimization of plasma process settings have on the reproducibility of the hardness range (Plate 3): After having optimized the process, hardness values differed from lot to lot by only a few points on the HR15Y scale.

Plate 3 If the spray parameters are allowing too much polyester entrapment (top), the hardness values of abradables fall below the desired limit of 65 on the HR15Y scale. Once the parameters are optimized, the scatter is drastically reduced (bottom).

Sulzer Innotec testing

The simulation test rig in the Sulzer Innotec laboratory (Plate 4) used for the abradability investigation allows accurate settings of a variety of important variables, including blade tip incursion rate between 2 and 2000µm/s, blade tip velocity from 100 to 450m/s, and abradable coating temperature up to 1200°C.

Removal of the abradable coating by the blade tip is key to the proper functioning of the sealing system. This abrading, also known as rubbing, is a machining process, influenced by the blade tip material and shape as well as the tip speed, temperature, and incursion rate. Test conditions are chosen which simulate the environment typical for the compressor part of a modern IGT. During the test, austenitic and ferritic steel blades, with a tip thickness of 1 mm, rubbed against a CoNiCrAlY-hBN-polyester coating at tip velocities of 300 m/s and 400 m/s. The temperature was either ambient or 600°C, while the incursion rate was set at 2 or 20µm/s.

Plate 4 Sulzer Innotec abradability rig, used for simulating customer conditions, allows for a variety of variables. High-velocity flame generator Blade Abradable specimen

All coatings are heat-treated prior to testing. The purpose of the heat treatment is to burn away the polyester from the CoNiCrAlY-hBN matrix. The presence of a polyester would help to cushion or absorb the cutting force, making it less efficient. Heating the blades to 600°C for the very brief test time does not remove all of the polyester from the CoNiCrAlY-hBN matrix. The removal of the polyester by heat treatment prior to any possible blade incursion is beneficial to the cutting behavior of CoNiCrAlY-based abradables, because it creates voids within the coating. The presence of controlled pores or voids enables the energy from the blade incursion to be transferred to the metal matrix, fracturing the interparticle bonds. This micro-rupture results in "clean-cuts", with minimal transfer of wear to the blades.

Compatible with blade material

All coatings that were tested at 600°C exhibited good abradability and compatibility with austenitic and ferritic blades. Micro-rupture of the coating becomes more observable at room temperature, although abrasion under these conditions is usually not experienced in gas turbines.

Another interesting finding was that no blade wear was observed, even for the austenitic blading material, which is softer than ferritic blades.

The poorest conditions occurred at room temperature, with 2µm/s incursion and blade speeds of 450m/s (Plates 5–11). The entrapped polyester created localized heating of the blade tip, causing micro-welding blade tip removal. The polyester and lower incursion rates effectively cause localized melting and heat build-up in the blade tip. High incursion conditions were not as sensitive and resulted in less blade damage.

Plate 5 Porosity generated by burning the polyester helps to reduce the bond strength between spray particles. Heavy local blade wear occurs at ambient temperature, however, this is a condition never present in an actual gas turbine. In the Figure: Wear tracks in the coating and steel blade after rub at ambient temperature (top) and 600°C (bottom).

Confidence in the coating system

Regardless of room temperature abradability aspect, the results of these investigations by Sulzer Metco give additional confidence to this coating system. All heat-treated CoNiCrAlY-hBN-polyester coatings work at different abradability conditions, as long as blade tip thickness and environmental conditions are as specified in the high- pressure compressor of IGT engines.

Plate 6

The coating's chemistry, thermal conductivity, particle-to-particle cohesive strength, volume percent porosity, size of porosity, oxide level and distribution, oxidation properties, thermal expansion, and grain size are all very important features that need to be considered. Understanding the interaction of the blade and coating at service conditions is essential in developing a fundamental understanding of the wear behavior of clearance control coatings. Also, end-user reproducibility of clearance control coatings can be better guaranteed by spray testing production lots of material and using statistical process control (SPC) to characterize process variables, such as coating hardness and deposit efficiency.

Plate 7

Plate 8

In addition to being compatible with austenitic and ferritic blades, CoNiCrAlY-hBN-polyester coatings are compatible with Inconel blades. In fact, the engineers of Sulzer Metco, along with their customers, have confirmed the excellent tribological properties with different blading materials. Today, these coatings are being used successfully in production applications around the world.

Figure 9

Figure 10

Figure 11

For more details Sulzer Metco (US) Inc. Mitchell Dorfman 1101 Prospect Avenue Westbury, NY 11590 USA Telephone +1 (1)516-338 22 51 Fax +1 (1)516-338 24 88 E-mail mitch.dorfman@sulzer.com

Mitchell Dorfman, Ulrich Erning and James MallonSulzer Metco