Surface Finish

Recently, my supplier asked me what surface finish or coating I would like to have on my product. I initially said matte and shiny, but she then explained that there are many different finish options, including deburring, polishing, polished smooth, polished, painting, high polished transparent, spray matte black, spray primer, passivation, glossy, matte, sandblasted, coating, and anodization. This left me feeling quite confused, as I wasn't sure which would be the best surface finish for my product, or the most cost-effective option. I decided to do some research to better understand the different surface finish and coating options and how to choose the most suitable one for my application.

Of course, I learned about this topic during my engineering studies, but it's easy to forget the details over time. Additionally, industrial designers often handle the surface finish, so I haven't had much hands-on experience with it. But now I'm digging deeper into the subject.

To start, let's review the basics. Surface finish and coating are related concepts in material science engineering, but they refer to different processes and outcomes.

Surface Finish

Surface finish refers to the texture of a material's surface. This texture includes the roughness, waviness, and lay of the surface. Surface finish is a result of the manufacturing process and can significantly affect the material's performance, including its friction, wear, and appearance.

Surface finish is typically measured in terms of roughness average (Ra), which quantifies the deviations in the surface profile and it's achieved through machining, grinding, polishing, or other material removal processes.

Types of Surface finish

Polished Finish:

• Description: Achieved through mechanical polishing processes to create a smooth, reflective surface.
• Applications: Jewelry, high-end electronics, decorative items.

Brushed Finish:

• Description: A surface with fine, parallel lines created by brushing the material with an abrasive belt or brush.
• Applications: Stainless steel appliances, watches, consumer electronics.

Matte Finish:

• Description: A non-reflective surface produced by bead blasting, chemical etching, or matte painting.
• Applications: Consumer electronics, automotive parts, appliances.

Textured Finish:

• Description: A surface with a specific texture pattern, achieved through processes like embossing, casting, or adding a textured coating.
• Applications: Grips on tools, automotive interiors, architectural elements.

Satin Finish:

• Description: A semi-glossy surface that lies between matte and polished finishes, often created by light brushing or sanding.
• Applications: Consumer electronics, jewelry, automotive trim.

Anodized Finish:

• Description: An electrochemical process that thickens the natural oxide layer on the surface of aluminum, creating a durable and often colored finish.
• Applications: Aluminum components, consumer electronics, architectural elements.

Coating

Coating, on the other hand, involves applying a layer of material to the surface of an object. This layer can serve various purposes such as protection from corrosion, enhancing appearance, reducing friction, improving hardness, or adding color.

Coatings can be made from a wide range of materials including paints, varnishes, electroplated metals, ceramics, and polymers. Measured in terms of thickness, often in micrometers or millimeters, and its adherence and performance characteristics.

Coating applied through processes such as painting, electroplating, anodizing, or thermal spraying.

Types of Coating:

Paint Coating:

• Description: A liquid pigment applied to a surface, which dries to form a protective and decorative layer.
• Applications: Automotive bodies, consumer goods, industrial equipment.

Powder Coating:

• Description: A dry powder applied electrostatically and then cured under heat to form a hard, durable finish.
• Applications: Appliances, automotive parts, outdoor furniture.

Electroplating:

• Description: A process that uses an electric current to deposit a thin layer of metal onto a conductive surface.
• Applications: Jewelry, electronics, automotive parts.

Physical Vapor Deposition (PVD):

• Description: A vacuum deposition process that produces a thin, hard coating by condensing vaporized material onto the surface.
• Applications: Cutting tools, medical devices, watches.

Anodizing:

• Description: An electrochemical process that thickens the oxide layer on the surface of aluminum, enhancing corrosion resistance and allowing for dyeing.
• Applications: Aluminum components, consumer electronics, architectural elements.

Electroless Plating:

• Description: A chemical process that deposits a layer of metal onto a surface without using an electric current.
• Applications: Printed circuit boards (PCBs), automotive components, industrial machinery.

Ceramic Coating:

• Description: A coating made from ceramic materials, offering high-temperature resistance and hardness.
• Applications: Automotive exhausts, industrial equipment, electronics.

Polymer Coating:

• Description: A coating made from polymer materials, providing flexibility, corrosion resistance, and various surface properties.
• Applications: Medical devices, electronics, consumer goods.

How to select the best surface finish and coating?

Surface Finish Selection

It's depends on the functionality, aesthetics requirements and manufacturing process.

Functionality:

• Comfort: The surface finish should be smooth to ensure comfort when worn. Rough or abrasive finishes could cause irritation.
• Durability: The finish should withstand daily wear and tear without degrading quickly.

Aesthetic Requirements:

• Appearance: A polished or brushed finish can give the smart ring a premium look. Consider the target audience's preferences.
• Color and Gloss: Decide whether you want a matte or glossy finish based on the design language of the product.

Manufacturing Process:

• Feasibility: Choose a finish that can be consistently achieved with the manufacturing processes available. Polishing, electroplating, or PVD (Physical Vapor Deposition) can be used for metals, while plastics can be polished or given a surface texture during molding.

Coating Selection

Depending factors to select the best coating:

Protection:

• Corrosion Resistance: Since wearable devices are exposed to sweat and environmental elements, coatings that prevent corrosion are crucial. For metal rings, options like anodizing (for aluminum) or stainless steel can be considered. For plastics, UV-resistant coatings can protect against discoloration.
• Scratch Resistance: A durable, scratch-resistant coating will keep the ring looking new despite frequent use. PVD coatings or diamond-like carbon (DLC) coatings are excellent choices for high durability.

Comfort and Safety:

• Hypoallergenic Properties: Ensure the coating is hypoallergenic to avoid skin reactions. Nickel-free coatings are preferable.
• Smoothness: The coating should be smooth to enhance comfort.

Environmental Conditions:

• Water and Sweat Resistance: The coating should protect the device from moisture and sweat, which are common when wearing a ring. Waterproof coatings or encapsulation techniques can be employed.

Substrate Compatibility:

• Material of the Ring: The chosen coating should adhere well to the ring material. For example, PVD works well with metals, while certain lacquers or polymers are better for plastic.

Below is the table to categories all the surface finish and coating details:

Example

Let's take a specific product example to help us find the best surface finish and coating. We'll use a Jewelry such as necklaces the primary considerations are aesthetics, durability, comfort, and skin safety.

High-End Necklace (e.g., Gold or Platinum)

Surface Finish: Polished Finish

• Description: Provides a smooth, shiny, and luxurious appearance.

Coating: Gold Plating or Rhodium Plating

• Description: Enhances the shine, durability, and value of the necklace.

Fashion Necklace (e.g., Stainless Steel or Silver)

Surface Finish: Brushed or Satin Finish

• Description: Offers a modern, stylish look with moderate shine and texture.

Coating: PVD Coating or Anti-tarnish Coating

• Description: Provides durability, scratch resistance, and prevents tarnishing.

Artistic Necklace (e.g., Mixed Materials or Custom Designs)

Surface Finish: Hammered or Matte Finish

• Description: Adds unique texture and artistic flair.

Coating: Enamel or Epoxy Coating

• Description: Adds color, protects the surface, and enhances durability.

In summary, when selecting the appropriate surface finish and coating for a product, there are several important factors to consider such as, corrosion Resistance, Scratch Resistance, Hypoallergenic, Material Selection, etc.

The main tradeoffs when making this decision are the quality, cost, and comfort/feel of the final product. A balance must be struck between these competing priorities to select the most suitable surface treatment.

Overall, a thorough evaluation of all these factors is necessary to determine the optimal surface finish and coating for a given product application.