Tribology and Its Applications on Aircraft StructuresJoseph Ryan

0729841 ME4328

Contents

1. Abstract…………………………………………………………………Pg.1

2. Introduction……………………………………………………………..Pg.1

3. Tribology Applied to Aircraft………………………………………….Pg.2

4. Solutions..………………………………………………………………..Pg.3

5. Practical Applications of Tribology…………………………………...Pg.4

6. Conclusions……………………………………………………………..Pg.5

7. References………………………………………………………………Pg.6

AbstractThis report examines tribology, the science of erosion and wear. More precisely focuses on the

effect of erosion and wear on aircraft structures, as it is an important and costly aspect of the

aeronautics industry. It looks at the three main areas that make up tribology: friction, wear, and

lubrication, and the effect these have on aircraft structures. It examines the mechanisms of

erosion and wear; that is the analytic science behind tribology. It looks at how tribology affects

the main parties involved the aeronautics industry. It examines some solutions used to prevent

erosion and wear. Finally there is a section on how the science of tribology can be used to our

advantage when dealing with aircraft, particularly with regard to paint stripping methods. Several

sources were used to compile this report including a talk from Paul Butler, which focused on

paint removal methods. It found that tribology is an important and costly factor in the aircraft

industry, and that great effort is expended to prevent wear and erosion on aircraft. It also found

that the science of tribology is used to some advantage in aircraft maintenance.

IntroductionTribology is the science of surfaces interacting with each other in relative motion. More exactly

it studies how this surface interaction affects the materials involved. It consists of three main

areas: the friction forces involved when the surfaces interact, the wear that is a result of this

friction, and the effect of lubrication on this interaction. Friction is the lateral force encountered

when two surfaces come into contact, caused by the unsmooth nature of their skins. The force is

determined by a number of factors including the weight acting on the surfaces, the kinetic force

involved and the friction coefficient of the surfaces. This friction force is often responsible for

wear of either of the surfaces involved. Wear is a process whereby material is lost during the

interaction of a surface with other surfaces or its environment. Wear can be due to a number of

factors including abrasion (two surfaces rubbing, scuffing or scratching), erosion (repeated

localized mechanical impact, for example, sand particles impacting on metals) or corrosion (wear

due to chemical interaction, for example, oxidation). Lubrication is the process of reducing

friction, and therefore wear, between two surfaces by the application of a lubricant. Lubricants

are usually in liquid form (hydrodynamic), but can be solids (graphite for example) or

occasionally gasses. Or methods include the introduction of a boundary.

Tribology In Aircraft

Aircraft operate under some of the most extreme conditions possible in any environment. An

average passenger aircraft might travel from sea level to 30,000ft and experience a temperature

range of over 90°C. This presents an opportunity for several types of wear on several areas of the

aircraft. One of the main areas of wear on an aircraft are the engines. This is because they

operate under the most extreme conditions of any part of the aircraft. A jet engine may

experience temperatures from subzero to in excess of 1000C. Jet engines also consist of many

different moving parts, some of which can reach rotational speeds of up to 15,000 revs/min. This

causes abrasive wear and also fretting. Fretting is when two surfaces rub together at high

frequency and low amplitude in the presence of a corrosive environment. Engines are also prone

to erosion from elements mixed in with the air intake. Particles such as sand and dirt, when

impacted at high velocity and dragged along the surface of the blades can cause significant

damage over time. Another area which suffers significantly from wear is the aircraft skin. Rain

erosion is a major factor, especially on the nose and leading edges. At the high speeds jet aircraft

fly at, the impact of raindrops sometimes containing corrosive chemicals can create wear called

pitting. This is where the impact of the rain drop creates a small indent on the surface of the

plane. This can be a problem on the control surfaces of the aircraft. Another problem

experienced on the skin is crevice corrosion, where a corrosive liquid gets between joints,

borderlines or under a coating. Both these types of corrosion can lead to exfoliation, where some

of the skin separates in a leaf-like fashion. Oxidation is another form of corrosion which affects

aircraft skin, however with materials like aluminum, which is used extensively in aircraft,

oxidation can form a hardened layer which actually protects the metal and prevents further

corrosion.

Fig 1. Pitting on Aluminum Wing Fig 2. Erosive Wear Damage on Turbine Blade

Another area where wear is prominent is the undercarriage/brakes. Aircraft brakes are much

more complicated than automotive brakes, containing several pistons and are designed to absorb

up to 135 MJ of energy. They convert a large amount of kinetic energy to heat energy in a short

period of time, involving large friction forces. As stated already, friction is one of the main

causes of wear so wear is an obvious consideration when designing aircraft landing gear.

Solutions

Over time, wear on an aircraft can reach a point where maintenance and/or repair are required.

This is regulated by the law and airline regulatory bodies and is also in the interests of

passengers and crew. Maintenance is a costly procedure because of the cost of labor,

equipment/materials and also due to the fact that when an aircraft is not in service, profits are

being lost. Therefore it is best to prevent as much wear as is possible from happening, rather than

repairing it when it becomes a problem. A number of solutions are employed by both the

manufacturer and the airline to deal with wear. On jet engines material selection is the main

factor in preventing wear. Given the wide range of components with different operating

conditions, a number of specialist materials are used. In the fan, materials with a high strength-

to-weight ratio such as aluminum and titanium alloys or graphite composites are used to increase

wear life. Superalloys of titanium and nickel, for example Inconel 718, are used in the HPC

(High Pressure Compressor). Heat resistant materials are used in the combustor to reduce heat

related wear and oxidation. Examples of these would be nickel or cobalt alloys like Hastelloy X

or Haynes 188. The most advanced materials are found in the turbines, with directionally

solidified and single-crystal cobalt and nickel superalloys like Inconel X750, MAR-M-509 and

René 125. Also, on many components, a coating or treatment is applied, such as aluminum

ceramic coatings or tungsten-cobalt-carbide coatings in order to reduce wear, maintain optimum

airflow and reduce friction. On aircraft skin, the most widely used material, aluminum alloys,

require treatment to prevent corrosion. Alloys are developed which are especially resistant to

exfoliation corrosion, such as the alloy 7055-T7751. Alloys are also tempered to improve wear

resistance and may be shot-peened, a process where small pellets are fired at the sheet metal to

create a compressive residual stress layer. Wear resistant titanium alloys are used in areas of high

stress, such as entrances to the aircraft. Increasingly, carbon-fibre composites are used as they

are resistant to most forms of corrosion. However, care must be taken as carbon fiber composites

can induce galvanic corrosion when they come into contact with aluminum structures. An

example of this is on the Boeing 777 where a carbon-fiber floor beam is separated from the

aluminum frame by an aluminium splice channel. The skin is also often painted with water-based

polyurethane paint and sealed with belly protection tape. Sealant is applied to joints and fasteners

to prevent crevice corrosion and fillet seals may be used in some cases for extra protection. On

the undercarriage a mixture of corrosion resistant materials are used. Materials are also chosen

with the enormous amounts of strain the undercarriage is under in mind as well. Therefore tough,

corrosion resistant materials such as tungsten carbide/cobalt with 4% chromium (WC-10Co-4Cr)

are used. Other corrosion resistant non-metals used include teflon and polyester films. On the

actual brakes, carbon/carbon composites (carbon fiber in a graphite matrix) are the most

commonly used material today. This is not only because of the high strength-to-weight ratio, but

also because of the extreme durability, resistance to thermal expansion, temperature gradients

and thermal cycling which make it an ideal brake pad material. Additionally, aircraft brakes are

regularly inspected as dirt can have an extremely adverse effect on the braking efficiency. The

hydraulic systems in the braking systems are regularly checked for dirt which can destroy seals

or erode moving parts and brake linings are replaced regularly.

Practical Applications of Tribology

Tribology can also be used to the aircraft owner’s advantage. The science of Tribology is used to

devise effective paint removal methods. Aircraft are frequently repainted because, as described

above, several factors can lead to erosion of the aircrafts skin, including its paint. When the

aircraft are being repainted, it is possible, but not economical to paint over the old layer. When it

is taken into account that just 1kg of extra weight can cost an airline up to $1000 in fuel costs a

year, the weight of several layers of paint can be significant. Therefore it is in the airlines best

interests to employ an efficient, cost effective method of paint removal. Chemical-based paint

strippers such as non-methylene chloride paint strippers can be used, however this risks

damaging some of the aircraft surface through corrosion. CO² pellet blasting is another option.

This process uses high-velocity dry ice blasting and can be used with or without the aid of a

flashlamp. However this process is expensive in terms of equipment, materials and energy and

also may cause some erosion to more delicate parts of the airframe. Another option available is

DMS (Dry Media Stripping). This has the advantage of having a relatively low cost, being

environmentally friendly and can be used where chemical stripping is not an option. DMS is

essentially the process of propelling an abrasive material, (usually some form of wheat kernel or

possibly beads of glass or sand), at high velocity through a nozzle onto the surface of the aircraft

in order to remove the layer of paint without damaging the actual aircraft surface. This process

can be optimized through the application of tribology by choosing the most effective nozzle

design, standoff, angle, pressure, shape and size of the particles and mass flow.

Fig 3. Effect of Hand Sanding on Aircraft Fig 4. Effect of DMS on Aircraft

Conclusions

From this report a number of conclusions can be drawn:

Tribology is the science of friction, erosion and wear.

Tribology has many applications in the aircraft industry. This is due to the enormous

importance of safety in the airline industry, the high cost of maintenance and repair, and

the huge potential for wear under the conditions aircraft operate.

The main areas susceptible to wear on aircraft are the engines, the aircraft skin and the

aircraft undercarriage.

The main types of wear experienced by aircraft are oxidation and corrosion due to an

aircrafts corrosive environment, abrasion due to moving parts and erosion due to dirt,

sand and miscellaneous foreign particles.

Corrosion is combated by a practical choice of materials, processes such as tempering

and applying sealant and efficient design.

Tribology can be used to an aircraft owner’s advantage, for example in the use of DMS.

References

- Sarker, A.D. (1980) Friction and Wear, London: Academic Press

- IAP International (1985) Aircraft Corrosion Control, Casper WY, USA: IAP

- Koch, Gerhardus H. (2006) ‘Corrosion Costs: Apendix P, Aircraft’, available:

http://www.corrosioncost.com/pdf/aircraft.pdf

- Lombardo, David A., (1998) Aircraft Systems, 2nd Edition, Boston: Mc-Graw Hill

- ASM International (2009) ‘Friction and Wear of Aircraft Brakes’, available:

http://asmcommunity.asminternational.org/portal/site/www/AsmStore/ProductDetails/?

vgnextoid=8ede7e0e64e18110VgnVCM100000701e010aRCRD&campaign=recommends-

personal

- ASM International (2009) ‘Wear of Jet engine Components’, available:

http://asmcommunity.asminternational.org/portal/site/www/menuitem.2b9d1953d012ee1480a3c

01026e110a0?

vgnextoid=a1ee7e0e64e18110VgnVCM100000701e010aRCRD&itemId=asmhba0002310

- Butler, Paul (2010) Tribology, ME4328: Aircraft Maintenance, Feb 12th 2010, University of

Limerick, unpublished.