The surface finish of your product components is just as important as any as any of the other design considerations.  Often, my clients aren’t aware of all of the options available to them, nor the resulting properties and benefits.  Surfacing is an industry all on its own, and there are far too many options to comprehensively treat in this page, so instead I will discuss the most common surface finishes that I typically use in product design, and describe their various attributes.

Anodizing
Function: Aesthetic, Corrosion Resistance, Wear Resistance
Materials: Aluminum
Aluminum naturally has excellent corrosion resistance, surprisingly because pure aluminum has an extreme affinity for oxygen.  What happens is an aluminum part rapidly forms a very thin oxide layer, which is so cohesive and tenacious that it prevents any further oxidation of the parent metal (this is the exact opposite behavior of steel when it rusts).  However, in certain marine and acidic environments, aluminum will experience additional oxidation.  The anodizing process builds up an artificially thicker oxide layer by suspending the part in an acidic bath and passing high current through it.  The resulting thick oxide layer is very hard and is excellent at preventing scratching and environmental corrosion.  The layer is also porous, and can accept dye to give it an array of colors.  There is a special process called ‘hard anodizing’ that builds an even thicker layer, and is suitable for the most demanding industrial wear applications.

Black Oxide
Function: Corrosion Resistance, Wear Resistance, Aesthetics
Materials: Steel
Steel, more correctly iron, has a great variety of oxide compounds that naturally form.  The most common and obvious is rust, which is destructive, but there are other oxide compounds that are protective.  The black oxide process builds up a specific oxide layer on a steel part, that acts much like the protective anodized layer in aluminum.  The result is a hard, black layer that prevents scratches and corrosion in the part.

Galvanizing
Function: Corrosion Resistance
Materials: Steel
Galvanizing is a process that applies a coating of zinc to a steel part by dipping it in a molten zinc bath.  The resulting layer protects the underlying steel from corrosion in extreme environments because of the specific atomic properties of zinc and iron.  When subjected to a corrosive environment, then zinc layer acts as a ‘sacrificial anode’, and will completely corrode away before the steel begins to rust, even if there is bare steel exposed to the elements.

Painting
Function: Aesthetic, Corrosion Resistance
Materials: Any
Painting as a process does not need a description, but it does deserve a spot on this list.  Keep in mind that clear-coat paint is an option for certain applications as well.  For instance, polished aluminum can sometimes pit or oxidize and get hazy over time; clear-coating the virgin surface will preserve the finish, and is a common technique for after market automotive wheels.  Also, there are metallic painting options for plastic parts that give the appearance of chrome and other metallic finishes.

Plating (Chrome, E-Nickel)
Function: Aesthetic, Corrosion Resistance, Scratch Resistance, Wear Resistance
Materials: Metals, some Plastics
There are many kinds of plating available, as well as many different methods.  The most common forms are decorative and industrial chrome plating.  Decorative chrome is common in car and motorcycle parts, and consists of a thick layer of copper plate, a thin layer of nickel plate, and a very thin layer of pure chrome plate.  Industrial ‘hard’ chrome plating utilizes a much thicker layer of chrome.  Both of these processes are applied by electrolysis.  An alternative process to chrome plating is electroless nickel plating, which applies a thin nickel coating without electrolysis; instead the nickel layer is created as a result of a chemical reaction.

Polishing
Function: Aesthetic, Reflectivity
Materials: Metals, Plastics
Technically, polishing is actually the process of refining a surface (or dimension) by slowly removing material through an abrasive process.  What people really mean when they want a ‘polished’  surface is buffing.  Buffing is the process of smoothing a surface with a very fine abrasive medium entrained in a fiber or cloth medium.  If a part needs a mirror finish, for aesthetic or reflective purposes, buffing must be employed.  Plastic parts can also have a polished finish, but the smoothness must be applied to the injection mold tool.

Powder Coat
Function: Aesthetic, Corrosion Resistance, Scratch Resistance
Materials: Metals, some Plastics
Powder Coating is a type of painting that produces a thick, durable finish.  The paint is applied electrostatically in powder form (a charge is applied to the part, and the paint powder is oppositely charged).  Then the coated part is then baked in an oven to melt and cure the paint.

Shot Peening
Function: Surface Hardness, Fatigue Resistance
Materials: Metals
The shot peening process uses steel balls that are shot at the part at high speed, which in turn makes thousands of tiny indentations on the part surface.  This imparts local work hardening to the material surface, which increases the surface hardness, and introduces local compressive stresses on the surface.  The compressive stress prevents minute cracks from propagating (fatigue cracking).

Surface Roughness Designation
Function: Part Fitment, Wear Resistance
Materials: Metals
Surface roughness designation is important for any machined part. If a finer surface roughness is needed for a part function, sometimes alternative machining methods, or additional processes must be used, which comes at a cost.  There are three main grades of roughness designation: N Grade, AA (English, μ-in), and Ra (metric, μm).  I commonly use the following designations for typical applications:
N8 (3.2Ra, 125AA):   The standard maximum surface roughness for all machined surfaces, unless otherwise specified. 
N7 (1.6Ra, 63AA):   Typical for close fitting, or piloting features (locational clearance fits).
N6 (0.8Ra, 32AA):  Typical for interference (press) fit surfaces, and running/sliding fit surfaces.
N4 (0.2Ra, 8AA): Typical for precision bearing surfaces, such as the race surface of a needle bearing.