Coating categories.
Type II / Type III
Type II (decorative): 5-25 µm thick, 200-250 HV, color available. Type III (hardcoat): 25-75 µm, 300-500 HV, wear-resistant, limited colors. Both increase corrosion resistance of aluminum.
Chrome / Nickel / Zinc
Hard chrome: wear + corrosion, 25-50 µm. Electroless nickel: uniform coating, 5-25 µm, 500+ HV. Zinc: corrosion protection for steel, sacrificial. Cadmium: aerospace spec (declining use).
TiN / AlTiN / DLC
Thin (2-5 µm) dense ceramic coatings applied by vapor deposition. 2000-3000+ HV hardness. Low friction. For cutting tools, wear surfaces, premium cosmetic.
Powder / E-coat / Paint
Powder coat: 50-150 µm thick, colors, decorative + corrosion. E-coat: 15-35 µm, cathodic, automotive standard. Wet paint: 10-50 µm, variable quality.
Alodine / Phosphate
Chromate conversion (Alodine): 0.5-2 µm, clear to yellow, paint prep. Phosphate: 5-25 µm, paint prep + mild corrosion resistance.
Teflon / Ceramic / Dry film
PTFE coating: low friction, release. Ceramic thermal barrier: turbine engines. Dry film lubricant: MoS2 for aerospace bearings.
Application → coating.
Wear resistance
Corrosion protection
Cosmetic appearance
Electrical
Coating thickness by type.
| Coating | Typical thickness | Growth / part tolerance | Life estimate |
|---|---|---|---|
| Anodize Type II | 5-25 µm | Half grows out, half grows in | 10-20 years |
| Anodize Type III | 25-75 µm | Significant dimensional change | 20+ years |
| Hard chrome | 25-50 µm | All growth outward | 10-20 years wear |
| Electroless nickel | 5-25 µm | Uniform outward | 10 years + corrosion |
| Zinc plating | 5-25 µm | Outward | 2-5 years outdoor |
| PVD TiN | 2-5 µm | Negligible | Cutting tool dependent |
| DLC | 1-5 µm | Negligible | Application dependent |
| Powder coat | 50-150 µm | Outward | 10-20 years outdoor |
| E-coat | 15-35 µm | Outward | Automotive 10+ year |
| Chromate | 0.5-2 µm | Negligible | Primer only, not standalone |
Design considerations.
Masking: Most coatings go everywhere unless masked. For internal threads, bearing bores, electrical contact surfaces that must not be coated, masking is required — added cost and potential for masking errors. Design: allow for masking access, consider tolerance implications if masking line isn't perfect.
Dimensional growth: Coatings add thickness. Anodize Type III adds 25-50 µm per surface — tight tolerance bores may go out of spec. For anodized parts, account for growth in dimensional tolerance. Sometimes masking is preferred over dimensional accommodation.
Drain holes: For plating and anodizing, parts are immersed — need drain holes to remove solution from interior cavities. Lack of drain holes causes: trapped plating solution, corrosion from retained chemistry, uneven coating due to air pockets. Specify drain holes in all interior cavities before sending for coating.
Electrical contact: For plating and anodizing, electrical contact required with the part. Hanging points or contact marks show on finished surface. For cosmetic parts, specify contact location (in non-visible area) or design with non-critical hanging points.
Hydrogen embrittlement: High-strength steel (60+ HRC) plating: hydrogen absorbs into metal during plating, causes delayed brittle fracture. Bake-out post-plating (at 190°C for 8-24 hours) drives hydrogen back out. Required for aerospace plating. Specify bake-out for hardened steel plating.
FAQ
Type II: decorative, thinner (5-25 µm), wider color range, lower cost. Good for: consumer electronics, architectural aluminum, general aluminum. Type III (hardcoat): functional, thicker (25-75 µm), limited colors (black, dark gray, bronze), higher cost. Good for: firearms, military, wear surfaces, premium consumer. For aluminum housings, Type II usually adequate. For wear-prone aluminum surfaces, Type III worth the premium.
Zinc: cheapest, sacrificial, 5-10 year outdoor protection. Cadmium: aerospace historical (declining due to toxicity), high corrosion resistance. Nickel: electroless for uniform coverage, chemical resistance. Chrome: wear + corrosion, thickness range 25-50 µm, most durable. For new designs, avoid cadmium. Zinc for economic, chrome for wear-critical.
PVD coatings: $20-200 per part depending on coating type and size. Premium over bare material 50-500%. Justified for: cutting tools (2-5× life), wear components (longer service), premium consumer products (aesthetic value), medical implants (biocompatibility). Not economical for: general industrial parts, parts where wear isn't primary failure mode, cost-sensitive applications.
Powder coat: no VOC (environmental), uniform coverage including corners, durable finish, wide color range, limited to items that can handle 180°C curing. Wet paint: can be applied to heat-sensitive substrates, faster for small batches, wider color and effect range, less durable. For most industrial applications, powder coat preferred. For specialty or small-batch work, wet paint still viable.
Yes, sometimes. Common combinations: (1) Electroless nickel + hard chrome — nickel for corrosion, chrome for wear. (2) Zinc plating + powder coat — zinc for galvanic protection, powder for UV and appearance. (3) Anodize + PTFE impregnation — anodize for wear, PTFE for low friction. Each combination has specific processing requirements. Specify coating sequence carefully.
Anodize: 3-5 days. Zinc plating: 2-3 days. Hard chrome: 5-7 days. Powder coat: 2-4 days. PVD: 5-10 days (specialty process). Heat treatment + coating (like stainless passivation): 7-10 days total. For tight schedules, these add significantly to delivery. Plan coatings into project timeline early.
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