Hard Anodizing for CNC Parts: Wear and Thickness
Short answer: hard anodizing (Type III) builds a dense aluminium oxide layer typically 25–50 µm thick, roughly 380–600 HV in hardness, that resists abrasion and wear far better than decorative anodizing. The coating grows partly into and partly out of the surface, so plan a dimensional allowance of about half the coating thickness per side. It suits 6061, 6063 and 5000-series aluminium; it performs poorly on high-copper alloys and porous castings. Indicative cost runs a few dollars per part at volume, more for masking and tight colour control.
Hard anodizing is the finish engineers reach for when an aluminium part has to survive sliding contact, abrasion or a harsh environment without wearing out. It is often confused with the decorative anodizing on consumer products, but the two are different processes with different thicknesses, hardnesses and design rules. Getting the specification right on the drawing saves both money and rework.
What Hard Anodizing Is, and How It Differs From Decorative Anodizing
Anodizing is an electrochemical process that converts the aluminium surface into an aluminium oxide layer. The oxide is grown from the base metal rather than deposited on top, which is why it bonds so well. Hard anodizing, also called Type III anodizing, runs at lower temperature and higher current density than the decorative Type II process, producing a thicker, denser and much harder coating.
| Property | Type II (decorative) | Type III (hard anodize) |
|---|---|---|
| Typical thickness | 5–15 µm | 25–50 µm, up to ~75 µm |
| Hardness | Softer, cosmetic | ~380–600 HV |
| Wear resistance | Moderate | High, suitable for sliding wear |
| Colour range | Wide, easy to dye | Limited, usually dark grey to black |
| Dimensional change | Small | Larger, must be planned |
| Typical spec | MIL-A-8625 Type II | MIL-A-8625 Type III |
The practical takeaway: if the part is cosmetic and the surface only needs colour and mild scratch resistance, decorative anodizing is the right and cheaper choice. If the part rubs, slides, or faces abrasive dust, hard anodizing is what protects it. Our overview of CNC anodizing finishes covers the decorative side in more detail.
Thickness, Growth and Tolerance Planning
This is where most hard-anodize mistakes happen. The oxide layer forms partly outward and partly inward, and the industry rule of thumb is roughly 50% growth out, 50% penetration in. So a 40 µm total coating adds about 20 µm to the outside of the part and consumes about 20 µm of base metal per surface.
| Nominal coating | Outward growth per side | Inward penetration per side |
|---|---|---|
| 25 µm (0.001 in) | ~12 µm | ~12 µm |
| 50 µm (0.002 in) | ~25 µm | ~25 µm |
| 75 µm (0.003 in) | ~37 µm | ~37 µm |
For a part with a ±0.02 mm fit, a 50 µm coating will move that surface by roughly 25 µm outward, which can blow the tolerance. The fix is to pre-machine the dimension smaller by the expected growth, or to mask surfaces that must stay bare. Tell the shop the final dimension you need after coating, not the dimension before, and let the process engineer back-calculate the pre-machined size. This is exactly the kind of tolerance-stack conversation that belongs in the CNC machining tolerances guide.
Also note that hard anodizing has a mild surface texture: the coating follows the machined surface and can slightly dull a polished finish. A mirror finish stays dimensional but loses some gloss. If appearance matters, sequence bead blasting or polish before coating with the coating step in mind.
Which Aluminum Alloys Anodize Well
Not all aluminium hard-anodizes equally. The alloy's copper and silicon content drives the result.
| Alloy | Hard-anodize behaviour | Notes |
|---|---|---|
| 6061 | Excellent | The standard choice, dense uniform coating |
| 6063 | Excellent | Common in extrusions and housings |
| 5052 / 5005 | Very good | Good corrosion resistance after coating |
| 2024 | Poor | High copper; coating is thin and brittle |
| 7075 | Fair to poor | High copper and zinc; coating quality suffers |
| Castings (porous) | Poor | Porosity traps solution and causes blotches |
If wear resistance is the goal and the alloy is still open, choose 6061. It takes a dense, hard coating, machines well, and is widely available. If a design is locked to 7075 for strength, expect a thinner and less wear-resistant oxide, and consider whether a different surface treatment or a steel insert serves better.
Wear, Hardness and Real-World Durability
Hard anodized 6061 reaches roughly 380–600 HV depending on process details, which is far harder than the base aluminium and comparable to some hardened tool steels at the surface. That is what makes it stand up to sliding wear, abrasive particle contact and repeated handling. The oxide is also electrically insulating and chemically inert, useful for parts exposed to mild chemicals.
Two caveats matter. First, the coating is hard but thin and brittle; it can crack under sharp impact or bending, and the crack exposes bare aluminium. So hard anodizing protects against abrasion and sliding wear, not against heavy impact or severe bending of the underlying part. Second, the anodizing process reduces fatigue strength of the part, because the brittle oxide layer can initiate cracks under cyclic load. For fatigue-critical aluminium parts, that trade needs to be understood before spec'ing the coating.
For most wear and cosmetic-wear applications, though, hard anodizing is the highest value-per-dollar surface on aluminium. It is why it appears on hydraulic components, sliding guides, valve bodies and tooling faces where bare aluminium would gall and wear within weeks.
Cost, Colour and Lead Time
Indicative hard-anodizing cost at a China source factory runs a few dollars per part at moderate volume for simple parts, rising with size, masking complexity and how tight the colour and thickness control must be. The two biggest cost drivers are fixture and masking. Every surface you want left bare has to be masked, and masking is hand labour. If you can let the coating cover an entire part, cost drops; if half the part must stay conductive and bare, expect a higher per-part price.
Colour is limited. Hard anodize naturally produces a dark grey to black, and the exact shade varies with alloy and thickness. If a precise colour match across batches matters, that raises cost and process control. Many buyers accept the natural dark grey and skip dyeing entirely to keep both cost and risk down.
Lead time for anodizing itself is usually a few days once parts are machined. What lengthens total lead time is the pre-machining adjustment for growth and any first-article review to confirm the coating thickness lands where you expect.
It is also worth comparing hard anodizing against the alternatives before you commit, because the cheapest overall solution is sometimes a different one. Electroless nickel plating gives a harder, more uniform coating and works on non-aluminium parts, but it is heavier and adds more dimensional growth. Powder coating is cheaper and gives great colour coverage but offers almost no wear resistance and builds thickness fast. Bare machining with a hardened steel insert at the wear point is often the most durable answer of all. Hard anodizing wins when the part is aluminium, the wear is abrasive or sliding rather than impact, and you want the coating integrated into the metal rather than sitting on top of it.
What to Put on the Drawing
Specify the standard and the thickness: "Hard anodize per MIL-A-8625 Type III, 25–50 µm" is a clear instruction. State which surfaces to mask. Give the final dimensions after coating, and flag any surface that must remain electrically conductive or dimensionally bare. If you need a specific colour, say so and accept that dark grey is the default. If your assembly can tolerate a coating crack, mention impact or fatigue loading so the shop can advise.
BQUQ machines aluminium parts on CNC turning and milling lines, manages anodizing through qualified finishing partners, and ships each batch with a dimensional inspection report so coating growth is verified, not assumed. Send the drawing with the coating callout and end-use, and we will quote within 12 working hours.
Frequently Asked Questions
Q: How thick is hard anodizing compared with regular anodizing?
A: Hard anodizing (Type III) typically runs 25–50 µm and can reach about 75 µm, while decorative Type II anodizing is usually 5–15 µm. The thicker coating is what gives hard anodize its wear resistance.
Q: Does hard anodizing change the size of my part?
A: Yes. The coating grows mostly outward by roughly half the coating thickness per side, with the other half penetrating into the base metal. A 50 µm coating adds about 25 µm per surface, so plan the pre-machined dimension accordingly.
Q: Can you hard anodize 7075 aluminium?
A: It can be done but the result is inferior. The high copper and zinc content in 7075 produces a thinner, more brittle coating than 6061. Where wear resistance matters, 6061 is the better alloy.
Q: How much does hard anodizing cost?
A: Indicatively a few dollars per part at moderate volume, rising with part size, masking and colour control. Masking bare surfaces by hand is the largest labour cost, so parts that can be fully coated cost less.
Q: Does hard anodizing affect fatigue strength?
A: Yes, it can reduce fatigue strength because the brittle oxide layer can initiate cracks under cyclic loading. For fatigue-critical parts, weigh that against the wear benefit before specifying hard anodize.
Related Resources
- CNC Anodizing Finishes: Type II, Type III and Colour — decorative versus hard anodizing, colours and process choices.
- CNC machining services — aluminium turning and milling with finishing planned into the quote.
- About BQUQ — ISO9001 source factory in Dongguan running CNC, stamping, springs and heat sinks.
- Contact — send your drawing with coating callouts for a quote within 12 hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
