Tribology materials (Surfaces) PDF

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Tribology materials (Surfaces) 11 February 2021

10:29 PM

Tribological compatibility

- Solid solubility and wear resistance of metal pairs

For conformal contacts, where the surface area is large dissimilar metals will be bad

- Elements solubility - how much the elements mix with each other, If they are soluble the 2 surfaces will merge together

Lubricating passive metals - Metals like stainless steel, chromium and titanium are difficult to lubricate due to their underactivity with boundary additives

- Oxides on surfaces are stable, hence harder to react with an additive

- The yellowish region shows the oxide layer

Materials requirements

- To meet the requirements a soft thin material layer like Tin or Lead is coated on a hard material - To avoid scuffing the soft alloy should also have low adhesion to steel

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- However, the coating thickness for the Tin decreases the fatigue strength exponentially Therefore, a triple layer can be used to give even better results - To improve the corrosion resistance Aluminium can be added, however it's compatibility is 'fair' as it is soluble with the stainless steel Silica (sand particles) are added to prevent the mixing as Al is softer it is abraded off

Main types of metallic plain bearing materials

Fatigue strength and operating temperature The operating temperature needs to be considered when choosing the materials, some may be great in other properties but if it doesn't perform as well in the temp range then sacrifices need to be made

Dry bearing materials

- Used for low cost, finite life applications at low speeds and loads - No lubrication needed/if it cannot be used - Usually plastics

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Porous materials - They look dry but are porous, the holes contain the lubrication

- The power is sintered - The lubricant is released at the contact allowing for low amounts of lubrication almost dry

Materials for concentrated contacts - Rolling element bearing - They undergo high contact pressure (1 - 4 GPa) and many loading cycles Therefore require high strength and fatigue properties - Normally operate in full film, so wear and friction is less important

- Normally no tensile stresses (mainly compression for rolling element bearing) - Dimensional stability needed

- Rolling contact fatigue resistance is the critical property - use hard, clean steel - Commonly through hardened, high carbon (0.8-1% C, 1.3-1.6% Cr) bearing steels – AISI 52100 (US), 100Cr6 (Europe), EN31 (BS970) – For aerospace applications (M50 is used, 4 - 4.5% Mo)

Materials for concentrated contacts - Gears

Causes crack growth

- Using carburised steels

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- However, griding is required to smooth out the surface, instead Nitrided steel can be used

- Using Nitrided steel

- The penetration is lower for Nitrided steels - Other common materials used for gears

- Other common materials used for bearings

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