A CVD coating is a material applied to a surface by a method called chemical vapor deposition. Let's discuss how the coatings are made, coating benefits, and how to get the most out of your coating.
CVD coatings are prevalent throughout industry and can be found in many consumer products. They're known as being an environmentally friendly durable thin film surface. CVD products can be found in applications ranging from machine tools, wear components, analytical flow path components, instrumentation and many other areas demanding a high performance thin film. There are lots of thin films out there so what is it about the chemical deposition process that makes a superior coating for high performance precision applications?
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In this blog post you will learn:
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About CVD Coatings: What is Chemical Vapor Deposition?
Chemical vapor deposition is a process that involves the reaction of a volatile precursor which is injected into a chamber (typically under vacuum). The chamber is heated to a reaction temperature that causes the precursor gas to react or break down into the desired coating and bond to the material surface. Over time the coating material builds on the surface and creates a coating throughout the exposed part's surface. So for example if we wanted to bond silicon to a surface we may want to use a trichlorosilane precursor. When the trichlorosilane is heated in the coating chamber the decomposition and coating reaction may look like this.
SiHCl3 → Si + Cl2 + HCl
The silicon will bond to any exposed surfaces (both internal and external) while the chlorine and hydrochloric acid gas will be vented from the chamber and scrubbed according to appropriate regulatory requirements. The CVD coating system may look something like the diagram below. In this example the desired coating material is a silicon oxide base coating with a functionalized (R) surface which is intended to cap any remaining active sites on the coating surface. This makes for a superior non reactive surface. Want to learn more about our coating process? Go to our Coating Technology and Quality page.
Materials used in CVD coating systems range from silicon compounds to carbon, to fluorocarbons or organofluorine, and nitrides like titanium nitride. Some materials like silicon can be further enhanced by doping the surface with other materials to functionalize the surface for a specific performance goal (as noted above).
Need help selecting the right CVD coating for your application? Try our online coating selector guide.
Selecting the right coating for the application is important, but coating quality control, (including surface preparation, process control, coating precursor, and material substrate quality) play a major role in the coating quality. The info graphic below shows a typical CVD coating process.
Characteristics of a CVD coating process
Coating Benefits
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Coating Drawbacks
The CVD coating process and how it makes color.
Customers often make note of the multi-color rainbow appearance of our coatings. What makes the coated part so colorful and why does the color vary by part or lot?
We bond silicon to metals, glass, or ceramics, this process allows us to build a silicon layer on a surface one Angstrom (0.1 nanometer) at a time. A super thin layer of silicon is pretty clear, in fact you can see through a thin silicon layer. As the silicon thickness increases on say a stainless steel surface, light which travels through the silicon is bent. The light then reflects off the stainless surface and is bent again. All this light bending causes some wavelengths of light to cancel out each other while other wavelengths are reinforced. The canceled (or interference) wavelengths aren't seen by your eye while the reinforced wavelengths hit your cones and are seen as color. Changing the color of a thin silicon surface is easy, a few nanometer change in silicon thickness can have a big impact on the colors you see.
The same holds true with silicon. Small variations in thickness can make a big difference in the color of the coating. So the different colors observed on Silco'd treated parts indicate different layer thicknesses. A blue color corresponds to a 300 to 450 Angstrom layer while a rainbow color indicates a coating of at least 1200 Angstroms (120 nanometers).
Colors associated with layer thickness are:
Depositions used in our Silcolloy® and SilcoGuard® processes are up to 1µm (1000nm) and have a rainbow to silver/metallic gray appearance. The photos below show colors created by SilcoNert®, a 500nm** coating (left) and Silcolloy®, an 800nm** coating (right). So as you can see, a few nanometers change in thickness will result in a dramatic change in the color of the part.
Our coatings are colorful because they're thin. Why do we make the coating thin? Because the amorphous silicon coating when thin does not impact part tolerance significantly and because our coatings are highly durable so they don't need to be thick.
Have other questions? Try our Frequently Asked Questions page.
So why don't we make parts all the same color? We're an industrial coating company so we're focused on the ultimate performance of the coating, not appearance. We could in theory make parts with uniform color, but that would impact coating thickness and may result in a less robust coating. Plus rainbow is more fun!
*Image credit: https://www.pinterest.com/explore/oil-spill/