Technical > Design Guide > The surprising success of high-performance coatings

The surprising success of high-performance coatings

A. New options for designers
High-performance fluoropolymer coatings are remarkable low-friction, dry-lubricant materials that combine the capabilities of two types of “engineering plastics.” Fluoropolymers, with the lowest coefficient of friction of any known solid, are combined with the newest high-temperature organic polymers to provide a unique combination of properties.

These tough lubricant coatings can operate successfully at temperature extremes which, at the low end, would render ordinary fluid lubricants as brittle as glass and, at the high end, would char them to ash.

Originally, low-friction solids were used for applications where sliding parts were heavily loaded, infrequently lubricated, or operated at high temperatures. This has been true since the inception of dry lubricants, when ancient mechanics used graphite, talc, mica and other “slick” powders to reduce sliding friction.

But, with the development of modern lubricating coatings, other major properties have been designed in — including outstanding corrosion resistance. Today, when mechanical parts operate under any of the above conditions, dry film lubricants are often the only sensible, safe and economical way to lubricate and protect them.

In the past, the only materials recognized as dry lubricants for engineering applications were graphite and molybdenum disulfide (MoS2 or Moly) or blends of both. While useful, these materials solved only a limited range of problems. Moly coatings were typically used in high-pressure applications; graphite coatings were generally used in wet service or at elevated temperatures.

Enter the matrix
Today’s fluoropolymer coatings are the result of design engineering done several decades ago.

The first fluoropolymer coatings were relatively soft films, the kind found on frying pans and a few release applications in industry.

Then, in 1969, a team of polymer chemists and engineers devised the matrix concept for coatings, “building” a coating to protect the soft fluoropolymer from wear while taking advantage of its low-friction property. On this concept, Whitford was founded and Xylan fluoropolymer coatings became a reality.

The first significant order for Xylan was for a brake adjustment mechanism for GM cars. This was followed by other applications: saw blades, viscous fan drives, journal bearings, switch detents, carburetor shafts, steam-valve plugs, oil rig fasteners. The list grew quickly.

NOTE: The full range of Xylan coatings has become so extensive that properties vary widely. The coatings referred to in this section are principally those designed for industrial applications, and the bulk of the data refers to them. See Whitford’s “Introduction to the Xylan 1000 Series.”

Over the years, these self-lubricating materials have been used to solve a growing range of engineering and design problems.

Today, Xylan is the largest, most complete line of fluoropolymer coatings in the world. As the materials have been tried on an ever-widening spectrum of applications, we have learned a few things. For instance:

• A bonded, self-lubricating coating can last longer than hard chrome plating in certain high-wear applications
• Xylan can cut the cost of some pistons, plungers and splines by eliminating polishing and lapping processes.
• Xylan can replace heavy metal platings on engine journal bearings.
• Xylan can replace plating on hydraulic pistons and extend their lives in corrosive atmospheres.
  

A matrix coating is one in which soft ingredients, e.g. lubricants such as PTFE, are enveloped in harder, more wear resistant binders, i.e. the matrix. If PTFE, for example, is used by itself (without the matrix), and if it is exposed to a wear surface, it quickly wears away. Combined with the matrix, however, the new structure solves the engineering problem as it protects itself. Matrix coatings are adjustable. The specific properties of Xylan formulations depend on the materials used to form the total coating. By judicious selection of compounds, we can formulate a coating to have superior release, wear resistance, chemical resistance, high conductivity, etc.

B. Components of Xylan

The Xylan coating matrix is composed of three basic ingredients:

• A polymer binder for film strength, adhesion and protection of the softer lubricating particles.
• A solid lubricant for low friction and release.
• Pigments and fillers for color and additional properties.

All materials are suspended in solvent or water, providing a compatible carrier so that, together, they may be applied by conventional painting techniques such as spraying, dip/spin, roller coating, etc.

The result is a thin, continuous, protective barrier which resists chipping, spalling, abrasion, cold flow, temperature extremes, weathering and a wide range of corrosive environments.