Conformal coatings are surface treatments applied to a wide range of products and devices used for aerospace, automotive, biomedical, consumer, military and numerous other purposes. Their primary objective is providing a protective film that supports a selected device’s ease of use, operating function, and service life, through an exceptional variety of working environments. Liquid Teflon (PTFE) and parylene are two of the more widely used hydrophobic conformal coatings.
Liquid PTFE and Parylene Compared
Liquid Teflon’s corrosion resistant qualities are well documented; repelling nearly everything, the bond that exists between its carbon and fluorine atoms is so strong, the substance is nearly bullet resistant. Its lubricating properties greatly reduce friction and component wear during operation, particularly for machines with sliding parts, effectively lubricating bearings, gears, slide-plates and other moving components, more reliably than most competing covering materials. Amazingly, it also plays a role in surgical interventions as a grafting material.
Teflon displays resistance to chemical reaction, corrosion, and stress-cracking. In addition, PTFE coatings:
- demonstrate excellent electrically conductive capacities,
- repel virtually any other substance, and
- provide reliable internal covering for ducts, pipeline, and containers for chemicals that are highly corrosive or reactive.
PTFE hydrophobic coatings enhance lubricity, a quality desirable for many applications, ranging from medical implants to aerospace/automotive mechanics, where decreased friction between components improves function and prolongs operational life. PTFE also has a lower co efficient of friction than Parylene, which normally would make it the coating of choice for applications where greater levels of lubricity are essential to enhanced performance. However, in many cases application-specific considerations make parylene a better choice.
Despite their advantages, liquid Teflon cures at temperatures and according to schedules that interfere with the basic thermo-mechanical characteristics of many of the components they coat, such as those using nitinol or similar alloys. Minimizing unintended transformation of these alloys’ thermo-mechanical properties while retaining sufficient performance lubricity is a major challenge for future liquid PTFE applications.
PTFE coatings can also produce particulates during prolonged operation. When used for medical purposes, these minute particles of matter can become embedded in patients’ tissue or bloodstream, potentially endangering bodily function and health; particulates can interfere with the performance of non-medical components as well. In contrast, hydrophobic parylene seldom dissociates or separates from the substrate during use, making it a superior option in cases where particulate development is a concern. Parylene’s generally improved lubricity in these cases transforms “sticky” substrate surfaces into those that are smooth and non-resistant. These advantages in relation to PTFE are also transferred in many aerospace, automotive, consumer and military applications.
An industry standard conformal coating material, parylene is applied via a unique chemical vapor deposition (CVD) process, wherein the gaseous parylene penetrates deeply into the substrate surface, covering even the most isolated or obscured regions of its topography. This quality makes parylene very useful for adaptation to microelectricalmechanical systems (MEMS) and nanotechnology applications, as well as larger-scale product treatments. Like liquid PTFE, parylene is noted for its durable lubricity, generating long-lasting and dependable barriers to biological, chemical, electrical, and mechanical conditions that can endanger the performance of electronic components used both for specialized purposes and in everyday life.
Parylene also provides:
- Adhesion to an extensive range of substrate materials.
- Micron-thin coating.
- Minimal increase in weight and volume.
- Pinhole-free coating.
- Penetration of crevices and small nooks.
- Tin whisker mitigation
Both PTFE and parylene are adaptable to a wide range of product applications. Choosing between the two is always a function of the specific uses of the component and its operational context. For instance, although PTFE is a harder coating than parylene, and is often a better option for many mechanical uses, it does tend to crack and wear. Thus, parylene is a superior film choice in specific cases, such as laser welding. These considerations need to be analyzed in all cases before deciding between them.
Parylene and liquid PTFE each have their own performance benefits and disadvantages. Selecting the appropriate hydrophobic coating for biomedical, consumer or other products is essentially determined by the specific uses and operating conditions of each device. Quality control and cleanliness testing are supported by such methods and instruments as:
- gauges and microscopes,
- product sampling,
- statistical control processing, and
- thin film analysis,
- appropriately managed and implemented by professional personnel.
Comparative performance testing of the device within the context of its intended application helps render a reliable decision but, despite its somewhat higher consumer profile, liquid PTFE lacks the overall range of product adaptations available with parylene.
To this extent, parylene’s CVD process is often superior to Teflon’s liquid application methodologies for assuring reliable conformal coating. Typically applied by dipping or spraying, PTFE is prone to bridging, edge-effect, or pooling, developments that limit the true conformality of the resultant protective film. Parylene CVD generates a micro-thin, authentically conformal film. These conditions position parylene as a generally better coating solution where dimensional tolerances are physically constricted (MEMS/nano technologies), and for components with more complex topographies that require exceptionally reliable conformal treatment.
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