Deciding on the ideal coating for your engineering application is always a very difficult task, you need to decide on chemical, process, mechanical and component compatibility. Some coatings are not ideal for specific applications. There are many different types of coatings for a vast array of applications, however this post will focus on an advanced Parylene Coating. Its coating methods, advantages and applications will be discussed.
What is Parylene coating?
The name Parylene refers to a range of polymer coatings that primarily serve as moisture and dielectric barriers. These polymer coatings have either a polycrystalline or linear structure. The chemical structure of these polymers allows for exceptionally thin and uniform coatings. These coatings combine to the substrate at a molecular level, this means that the coating will be completely even and uniform across the entire substrate. Parylene has been used on stents, heart pumps and various electrical components that are exposed to harsh environments like those in the automotive and aerospace industries.
How is it applied?
A Parylene coating makes use of vapour deposition polymerisation. This process occurs in a vacuum at room temperature. Parylene coating equipment basically consists of two polymerisation chambers and a vacuum chamber where the coating is deposited. The coating effectively grows on the substrate one molecule at a time. The basic process is as follows:
- A fine granular powder called dimer is heated inside a vaporiser. The initial quantity of this powder will determine the final Parylene coating thickness. The resulting vapour is then further heated at a higher temperature to create a dimeric gas. A dimeric gas is basically a molecule that contains two monomers that are bonded either with strong or weak forces. This dimeric gas is then further heated (pyrolyzed) to break it up into two monomers.
- The monomers are then deposited onto the substrate in a room temperature vacuum chamber, the nature of the monomeric vapour results in a coating that uniformly covers even the smallest features. It does this by bonding to the substrate on a molecular level.
Parylene can be applied to almost any substrate that can withstand the vacuum. These materials can include, stainless steel, plastic, silicon, paper and ceramics to name a few.
Why use a Parylene Coating?
Parylene is the ideal coating for protecting various components in the electronics, medical and instrumentation industries. This is due to a wide range of benefits as listed below:
Benefits of Parylene
- Chemically & biologically inert – This allows Parylene coated components to be implanted with little risk of rejection or infection.
- Transparent – Parylene is a completely transparent coating which makes it ideal for coating optical components.
- Stress free – Due to the room temperature coating process, the coating does not contain any internal/surface stresses.
- Highly resistant to solvents – Components that are typically damaged by solvents can benefit from a Parylene coating.
- Relatively good thermal resistance – A Parylene coating can withstand consistent temperature of 80°C. Some types of Parylene can withstand temperatures up to 350°C.
- Hydrophobic – A Parylene coating actively repels water which is a major risk for electronic components.
- Excellent electrical properties – Parylene is a non-conductive coating that has an extremely high dielectric of 5kV per 0.025 mm.
Types of Parylene
There are various types of Parylene with the most common typically being Parylene C. The types of Parylene are listed below.
- Parylene C – Carbon hydrogen combination with a chlorine atom. Very moisture resistant.
- Parylene D – Like Parylene C but contains two chlorine atoms. Can withstand temperatures of 125°C but is not very biocompatible.
- Parylene HT – Contains one atom of fluorine. Can withstand temperatures of 350°C and is also UV stable. Strongest form of Parylene and is ideal for coatings.
- Parylene N – Simplest form of Parylene with a Carbon and Hydrogen structure.
The table below lists some key properties of common Parylene coatings.
Table 1 – Parylene Properties
| Unit | C | N | D | |
|---|---|---|---|---|
| Mechanical Properties | ||||
| Young’s Modulus | GPa | 2.8 | 2.4 | 2.62 |
| Tensile strength | MPa | 68.9 | 48.3 | 75.84 |
| Density | g/cm3 | 1.29 | 1.11 | 1.42 |
| Hardness | Rockwell | 80 | 85 | 80 |
| Coefficient of friction | Static | 0.29 | 0.25 | 0.33 |
| Dynamic | 0.29 | 0.25 | 0.31 | |
| Yield Strength | MPa | 55.2 | 42.1 | 62.05 |
| Electrical Properties | ||||
| Dielectric Strength | V/micron @ 25.4 microns | 220 | 276 | Not Listed |
| Volume Resistivity | V/mil @ 0.001” | 5600 | 7000 | |
| Surface Resistivity | Ohms | 1×1014 | 1×1013 | |
| Dielectric Constant | 60 Hz | 3.15 | 2.65 | |
| 1 kHz | 3.10 | 2.65 | ||
| 1 MHz | 2.95 | 2.65 | ||
| 6 GHz | 3.06 | 2.46 | ||
| Dissipation Factor | 60 Hz | 0.020 | 0.0002 | |
| 1 kHz | 0.019 | 0.0002 | ||
| 1 MHz | 0.013 | 0.0006 | ||
| 6 GHz | 0.0010 | 0.0028 | ||
