Will Parylene Pass a Taber Test?

Taber tests are designed to measure a material’s capacity to withstand abrasion and its effects during operation. Conformal coatings – liquids and Parylene – are used on printed circuit boards (PCBs) and related electrical assemblies to safeguard their function from hostile environments where operational conditions could lead to development of abrasion, arcing, electrical shorting, fungus, internal moisture and other incidents detrimental to function.

In the case of abrasion, coatings are subjected to Taber Abrasion testing to determine their resistance during prolonged use. As in most other cases of comparative performance, Parylene surpasses liquid coatings – acrylic, epoxy, silicone, urethane – in Taber scores.

Parylene synthesizes in-process, transforming from a solid (powdered dimer material) into a gas that deposits both into and onto substrates. This process is called chemical vapor deposition (CVD) and is a method of conformal film application unique to Parylene. By penetrating the substrate’s surface, as well as adhering externally, Parylene provides additional, pinhole-free, conformal protection, adapting to any topography. This property adds to Parylene’s ability to withstand operational abrasion. In contrast, pre-synthesized liquid coatings brush, dip or spray wet conformal material onto substrates, providing an often effective, but limited, protective covering, confined to external surfaces only.

In practice, Taber Rotary Abraser testing attaches a level, uniform, square or round substrate specimen, measuring about 100mm, to a vertically-axised turntable platform, rotating at a fixed speed. The Taber Abraser accommodates materials to a thickness of 6.35 mm, although optional accessories can expand the testing range from >6.35 mm to <40 mm. Two Taber abrasive wheels are lowered to the specimen, moving across the surface at a level of pressure specific to its abrasion-resistance requirements. While one abrading wheel rubs the specimen inward toward its center, the second scrapes outward, toward the specimen’s periphery. As the wheels complete a circle across the specimen surface, abrasion resistance is revealed at all angles, relative to the material’s grain or weave. Abrasion markings’ circular pattern of crossed arcs cover about 30 cm.² along the specimen.

Methods of assessing Taber Abraser results include:

• Taber Wear Index (TWI) – Calculated by measuring the loss in milligram-weight per thousand cycles of abrasion, providing the rate of surface wear of a conformal coating. Lower wear index values indicate better abrasion resistance.
• Mass (Weight) Loss – Typically reported in milligrams, measuring the quantity of coating material removed by abrasion. It is important to clean test specimens before measurement, to assure loose particulate do not adhere to their surface.
• Wear Cycles – Calculates per mil the number of abrasion cycles necessary to breach a coating of a known thickness.

Like other conformal coatings, Parylene’s abrasion rate is based on the coated item’s surface area and the force applied to it. Regardless of the material, abrasion rate of wear is a function of abrasion force – greater force causes greater wear and smaller areas abrade faster than larger. Parylene passes the Taber Abraser test, with exceptional results compared to liquid coatings.

In one study, TWI values were 22.5 for Parylene C and 8.8 for Parylene N. By comparison, Teflon^™ – polytetrafluoroethylene (PTFE), a synthetic fluoropolymer – was 8.4, high impact PVC was 24.4, epoxy 41.9 and urethane 59.5. Since lower TWI values indicate greater performance, Parylene scores exceeded all but PTFE.

In general, Parylene coated objects resist abrasion better than even exceptionally hard liquid coatings like epoxy.