Application of Thermal Spraying in Aviation Industry

Aeroengines have always been a competitive arena for new materials and new processes. Aviation gas turbine engines not only have harsh working conditions for parts: high operating temperature (the temperature before the turbine has exceeded 1300°C), high speed (the turbine speed is about 10,000 rpm), high load, and high reliability requirements. With its unique advantages, thermal spraying technology has become an indispensable process in the manufacture and repair of aero-engines. Thermal spraying technology is not only adopted early, but also has a wide range of applications, and has achieved significant economic and technological benefits. Thermal spraying technology supports the development of aero-engine technology to a large extent, and aero-engine technology also drives the development of thermal spraying technology. Nowadays, all units that manufacture and repair aero turbine engines use thermal spraying technology. In the 1970s, there were more than 500 thermal sprayed parts of an aero turbine engine, but now it has thousands of parts and more than 3000 parts.

Several typical application examples of thermal barrier, sealing and wear-resistant coating are introduced as follows.

thermal barrier coating

Referred to as "TBC" coating (Thermal Barrier Coating), usually composed of metal (alloy) bonding and corrosion-resistant bottom layer and oxide ceramic surface layer with low thermal conductivity. It has been increasingly widely used in engine combustion chambers, afterburner Chambers, turbine components and hot gas channels and other hot end parts that are subject to thermal stress and thermal corrosion. Thermal barrier coating, also known as "thermal insulation" coating, can reduce the working temperature of the material, improve the service level of the material, and reduce the cooling airflow requirements of the parts. In addition, it can effectively prevent high-temperature gas from corroding materials, thereby prolonging the service life of materials.

A. Heat insulation effect of "TBC" coating on the inner wall of the afterburner:
The plasma sprayed "TBC" coating on the inner wall of the afterburner is made of nickel-aluminum (or nickel-chromium) material as the bottom layer, with a thickness of 0.1-0.15 mm, and the surface layer is oxide ceramics, with a thickness of 0.2 mm-0.35 mm. This thermal barrier coating has been applied to two types of afterburners, and both have achieved good heat insulation effects, with the maximum heat insulation temperature difference reaching 120°C.

B. Application of TBC coating for aero engine flame tube:
The JT8D aircraft engine of Pratt & Whitney (PRATT & WHITNEY) of the United States has nine flame tubes each, and its inner surface is plasma-sprayed with NiCoCrAlY+ZrO2 thermal barrier coating. The JT8D engine is widely used and has been continuously improved. So far, more than 13,000 units have been used. There are many types of flame tubes. Its NiCoCrAlY+ZrO2 coating design is mature, and it was once a leading thermal barrier coating in the world. Since 1984, a domestic factory has processed and sprayed more than 10,000 flame tubes and delivered them to Pratt & Whitney. The FT8 gas turbine flame tube jointly developed by the plant and Pratt & Whitney of the United States also uses thermal barrier coatings.

The NiCoCrAlY+ZrO2 thermal barrier coating used in the flame tube adopts the plasma spraying process. The original bottom layer was nickel-cobalt-chromium-aluminum-yttrium with a thickness of 0.076-0.12mm, and the surface layer was zirconia stabilized with magnesium oxide, with a thickness of 0.25-0.38mm. Later, the bottom layer was thickened to 0.12-0.20mm. Optimization was carried out in the 1990s. The bottom layer of the coating is still NiCoCrAlY, the thickness of the coating is relaxed to 0.12-0.23mm, and the surface layer is changed to yttria-stabilized zirconia powder, and the thickness is relaxed to 0.25-0.43mm. Because the inner diameter of the flame tube is only φ130mm, it is necessary to use an inner hole plasma spray gun to spray on its inner surface.

Seal coating

Aviation gas turbine engines have nearly twenty stages of compressors and turbine blades, and there is a pressure difference between stages. The gap between the rotating parts and the stationary parts seriously affects the efficiency of the compressor and the turbine, so that the air leakage loss of the blade tip alone accounts for 10-40% of the loss of the whole engine. As engines increase in size and temperature, engine sealing becomes extremely important.

A. Abradable Coatings for Radial Clearance Reduction
There are two types of thermal spray sealing coatings commonly used to control gaps and reduce leakage losses: abradable sealing coatings and gas path sealing coatings. The abradable coating is to spray an abradable and anti-scouring soft coating on the stationary ring belt radially outside of the rotor blade tip to reduce the radial clearance at the blade tip. Commonly used materials and applicable temperatures are shown in Table 1. Commonly used coating materials are: nickel/diatomaceous earth, nickel-chromium/diatomaceous earth and nickel-chromium-aluminum/diatomaceous earth. The wearable sealing coating can reduce the radial clearance of the turbine without changing the original design of the engine, so that the thrust can be increased by 1-3%, and the fuel consumption can be reduced by 2-5%.

Table 1 Commonly used abradable coating materials and applicable temperatures

coating name
Spraying method
proper temperature
use
1
polyphenylene
Plasma
-18°C to 340°C
Air compressor and lubricating oil system sealed
2
Aluminum graphite
flame, plasma
Below 450℃
Compressor parts sealed
3
Nickel graphite
flame, plasma
Below 480℃
Compressor parts sealed
4
nickel diatomaceous earth
flame, plasma
Below 750℃
Turbine parts sealed
5
Nichrome diatomaceous earth
flame, plasma
Below 850℃
Turbine parts sealed
6
Nickel-chromium-aluminum diatomaceous earth
flame, plasma
Below 1000℃
Turbine parts sealed

B. The gas path sealing coating is actually a wear-resistant coating. Since the working temperature and rotational speed of the aviation gas turbine engine are dynamic, the abradable coating can reduce the radial clearance and generally only obtain "zero clearance" in a certain state, such as the highest temperature and the maximum rotational speed. In order to reduce air leakage loss more effectively under higher temperature conditions, a metal foil honeycomb is generally used to make a sealing ring of the stationary part, which forms a sealing effect with the side of the crowned vane's teeth. Here the honeycomb is equivalent to the first type of soft "coating" that can wear and resist erosion, and the wear-resistant coating is sprayed on the side of the corresponding blade teeth. Commonly used coating materials are alumina or zirconia containing titania to improve friction and life. The turbine blade shrouds of the FT8 power turbine are all made of plasma sprayed aluminum oxide wear-resistant coating containing titanium oxide.

The abradable coating can also be used to seal the engine lubricating oil system to reduce lubricating oil leakage. For example, the bearing support of the engine is coated with plasma sprayed aluminum polyphenylene, which has a good sealing effect and a long service life.

Fretting resistant coating

Fretting is a prominent wear failure in aero-engines. It is produced on two metal surfaces with a certain positive pressure and a very small relative movement (generally less than 25μm) during work, also known as vibration abrasion. The surface damage caused by fretting erosion will accelerate the occurrence of other forms of failure until the component fails. Due to the pursuit of light weight of aero-engines, a large number of aluminum, magnesium, titanium alloys and nickel-based alloys are used, and many flexible structures are used. In addition, the operating temperature fluctuates greatly, and fretting abrasion should be paid special attention to. Thermal spraying technology can provide a variety of anti-fretting coatings according to different working conditions and materials of fretting parts. In long-life aero-engine parts, thermal sprayed fretting-resistant coatings are widely used, ranging from a gasket and pin to a casing with a diameter of nearly two meters. Dozens of parts need to be sprayed on some parts. Commonly used thermal spraying fretting-resistant materials are: nickel-clad aluminum, copper-nickel-indium, tungsten carbide/cobalt, chromium carbide/nickel-chromium.

A. Wear-resistant coating on the quick release ring in the middle of the engine fuselage
the
After a service life, the quick release ring, the outer ring of the guide and the diffuser are worn out at the same time, and they were scrapped in the past to avoid failures. After testing, the three parts were repaired by plasma spraying nickel/aluminum coating 0.1-0.2mm, and used again. Because the nickel/aluminum coating itself has a good anti-fretting effect, it not only restores the geometric size of the part but also improves the performance of the part. After another service life, the coating is intact and can continue to be used.

B. Application of flame tube wear-resistant coating
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On a JT8D or FT8 flame tube, there are up to 6 plasma sprayed fretting-resistant coatings, 5 of which are plasma sprayed nickel-chromium/chromium carbide (25/75) coatings, and the tail section is detonation sprayed or high-speed Plasma spraying nickel-chromium/chromium carbide (7/93) coating, the thickness is 0.05 ~ 0.10mm. Use Praxair3620 system for high-speed plasma spraying (spray gun is SG100, MACH Ⅱ parameters), all coatings have been approved by Pratt & Whitney of the United States, and are used on more than ten parts of JT8D and FT8 instead of explosive spraying. Generally, cobalt/tungsten carbide is used below 480°C, and nickel-chromium/chromium carbide is used at 480°C-870°C. The coating properties obtained by using different spraying processes are different, see Table 2 and Table 3.

Table 2: Performance comparison of cobalt/tungsten carbide coatings under different spraying processes

conventional plasma spraying
*High speed plasma spraying
explosion spraying
coating material
WC-12Co
WC-12Co
WC-12Co
Bond strength/MPa
greater than 45
greater than 70
greater than 70
Oxide content/%
less than 10
less than 1
less than 1
Microhardness/Hv
greater than 700
900～1200
900～1200
Porosity/%
less than 3
less than 1
less than 1 
*High-speed plasma spraying refers to using Praxair's SG100 spray gun to work under MachⅡ parameters.
the
Table 3: Performance comparison of nickel-chromium/chromium carbide coatings under different spraying processes

conventional plasma spraying
*High speed plasma spraying
explosion spraying
coating material
Cr3C2-25NiCr
Cr3C2-7NiCr
Cr3C2-20NiCr
Bond strength/MPa
greater than 35
greater than 70
greater than 70
Oxide content/%
less than 10
less than 1
less than 1
Microhardness/Hv
450～850
600～950
600～900
Porosity/%
less than 3
less than 1
less than 1.5
*High-speed plasma spraying refers to using Praxair's SG100 spray gun to work under MachⅡ parameters.