The Why's of Polymer Coatings for Tribological Applications.

Pixiang Lan
The Why's of  Polymer Coatings for Tribological Applications.

Tribology is a science that deals with friction, wear and lubrication of interacting surfaces in relative motion. If you see any moving parts, tribological forces are at play!

Friction is the key parameter to be managed in a tribological situation. The frictional force between two solids can be attributed to adhesion and deformation effects. The adhesion force involves the shearing between the contact surfaces and deformation is due to the hard material’s asperities plowing on the softer material.

Lubricated metal surfaces are the most commonly used frictional surfaces. Recently, polymers with solids lubricants have been finding their way into a number of application.

Due to their low surface energy, low shear strength and low elastic modulus, polymers will usually have a low coefficient of friction (COF) when they are used in tribological applications. Understanding the tribological performance of the polymers is crucial for their successful applications. The tribology of polymers is very complicated: friction is affected by load, sliding velocity and temperature. In polymers, wear can arise from abrasion, adhesion and fatigue. Therefore, tribological experiments that simulate the real working conditions are the most effective way to understand if the material is suitable for the applications or not [1].

Polymers are typically in either a thick section (bulk) form or a deposited coating format when they are used for tribological applications. Polymers in bulk format have been widely used, such as the top layer for thrust tilting pad bearings, washers, bushing and so forth. Usually, polymers in pure form (referred to as unfilled polymers) may not satisfy the tribological application needs. Adding different fillers and reinforcements (carbon fiber, glass fiber, solid lubricants, etc.) in the polymers can significantly improve their mechanical, thermal, and tribological properties. Research has shown well designed bulk polymer composites can show low COF and low wear [1].

However, polymers in bulk form have some shortcomings such as low dimension precision because of high thermal expansion; and high surface temperature. Thick polymers work as thermal insulators, making the dissipation of heat evolved by friction difficult.

Thin polymer coatings provide an excellent solution with the optimum blend of performance – enjoy the advantages polymers’ while minimize the drawbacks of bulk polymers. The tens of micron thickness of polymer coating cannot expand/shrink much even when there is a wide temperature change. And the thin polymer coatings do not strongly insulate the heat and thus can operate under reduced surface temperatures. Moreover, thin polymer coatings are more cost-effective because of their easy depositions method and lower quantities of material needed.

Polymer coatings have been widely used as tribological protective coatings, such as erosion/corrosion resistance coatings for automobile [2], transparent protective coating for touch panel screens [3], hydrophobic coatings for moisture repellent  and dirt resistance [4], bio-implant materials like artificial joints [5], bearing materials for compressor bearings [6] or thrust pad bearings [7]. In addition to these moderate conditions, thin polymeric coatings can also work in extreme working environments including: extreme temperature range from cryogenic to high temperature dry sliding in space application; different lubrication conditions such as boundary/starved and dry sliding conditions; as well as the bearings in the electrical submersible pumps in oil wells and drilling string of oil and gas drilling application, where the relative surfaces need to endure high temperature, high load, high speed, high environment pressure, and extreme abrasion wear[1].

ATSP NOWETM products serve well for both bulk and coating format and have shown excellent tribological performance over wide temperature range (-160˚C to 300˚C) even under extreme conditions such as high load, high speed, abrasive conditions, and boundary lubrication.

[1] Lan P. Tribological Performance of Advanced Polymeric Coatings Under Extreme Operating Conditions 2017.

[2] Trezona R, Pickles M, Hutchings I. A full factorial investigation of the erosion durability of automotive clearcoats. Tribology international. 2000;33:559-71.

[3] Masuko M, Ikushima F, Aoki S, Suzuki A. Preliminary study on the tribology of an organic-molecule-coated touch panel display surface. Tribology International. 2013;65:314-25.

[4] Khanjani J, Pazokifard S, Zohuriaan-Mehr MJ. Improving dirt pickup resistance in waterborne coatings using latex blends of acrylic/PDMS polymers. Progress in Organic Coatings. 2016.

[5] Song J, Liu Y, Liao Z, Wang S, Tyagi R, Liu W. Wear studies on ZrO 2-filled PEEK as coating bearing materials for artificial cervical discs of Ti6Al4V. Materials Science and Engineering: C. 2016;69:985-94.

[6] Akram MW, Meyer JL, Polycarpou AA. Tribological interactions of advanced polymeric coatings with polyalkylene glycol lubricant and r1234yf refrigerant. Tribology International. 2016;97:200-11.

[7] Zhang D, Ho JK, Dong G, Zhang H, Hua M. Tribological properties of Tin-based Babbitt bearing alloy with polyurethane coating under dry and starved lubrication conditions. Tribology International. 2015;90:22-31.