Does micro-arc oxidation change dimensions? What’s typical coating thickness?
Micro-arc oxidation changes part dimensions because the process grows a ceramic oxide layer on aluminum, titanium, or magnesium, and typical coating thickness is often about 10-100 microns depending on material, voltage program, electrolyte, time, and performance target. For any precision machining service, the key RFQ question is not only the nominal coating thickness. The drawing should state whether the thickness is measured per surface, how much outward growth is allowed, which bores or threads are masked, which datums are inspected after coating, and whether post-coating grinding, honing, or lapping is allowed.
Dimensional Impact of Micro-Arc Oxidation
Yes, Micro-arc Oxidation, also called MAO or Plasma Electrolytic Oxidation, changes the dimensions of a machined part. The process uses high-voltage micro-discharges to convert part of the substrate surface into a hard ceramic oxide. Because the oxide forms from the substrate and also builds above the original surface, final size can increase on outside features while inside diameters can become smaller.
The important geometry issue is that the coating grows both inward and outward from the original component surface. A common estimating approach treats part of the total coating as inward conversion and part as outward build-up, but the exact split depends on alloy, process route, coating thickness, porosity, and finishing. Some early drawings use a rough allowance such as one-third outward growth, yet that number should be verified by coated coupons or first articles before production. This means:
Net Size Increase: Outside diameters, boss widths, pads, ribs, and sealing lands can become larger after coating. Holes, slots, grooves, and threaded features can lose clearance because coating grows from both sides of the feature.
Consideration for Tolerances: Tight features may need pre-MAO machining allowance, masking, or final grinding after coating. This matters in CNC machining prototyping because the prototype should prove the coating allowance, roughness change, and inspection method before the same dimensions are released for production.
Typical Micro-Arc Oxidation Coating Thickness
MAO coatings are usually thicker and rougher than color anodizing, so coating thickness must be tied to wear, corrosion, dielectric, thermal, and dimensional requirements. A thicker MAO layer can improve surface function, but it also increases roughness, edge build-up, cracking tendency, and tolerance risk. Thickness should therefore be selected by part function, not by asking for the maximum possible coating.
General Range: Typical MAO coating thicknesses range from 10 to 100 microns (µm) for many engineering parts. Some specialized routes can exceed 150 µm, but thick coatings need stronger allowance control, roughness review, fatigue review, and section-specific measurement.
By Material:
Aluminum Alloys: Common functional coatings are often around the 20-50 µm range when wear, insulation, or corrosion resistance is the goal. For aluminum CNC machining parts used in automotive or aerospace assemblies, thickness must be checked against edge radius, mating clearance, and fatigue-sensitive surfaces.
Titanium Alloys: MAO coatings on titanium parts often fall around 10-30 µm for wear, friction, color, or bio-interface functions. Thicker titanium MAO coatings may be considered for thermal barrier or dielectric needs, but surface roughness and fatigue behavior must be validated.
Magnesium Alloys: Magnesium is highly reactive, so MAO can be used as a protective oxide route. Thicknesses around 15-50 µm are common screening values, but porous layers usually need sealing or topcoat review when long-term corrosion resistance is required.
Engineering Considerations for Design and Application
Design for Coating: Sharp edges, thin ribs, blind holes, and deep pockets can create non-uniform discharge density. The drawing should define minimum radii, masked zones, contact points, and areas where coating color or thickness variation is acceptable. The process can work on complex parts from multi-axis machining, but complex geometry needs more inspection points.
Post-Processing: The as-coated surface is often rough and porous. Bearing lands, sliding faces, sealing surfaces, and gauge-critical bores may need grinding, honing, lapping, or sealing after MAO. If post-processing is not allowed, the machined blank must already account for coating thickness and roughness.
Performance vs. Thickness: A thicker ceramic oxide can support wear, corrosion, dielectric, or thermal requirements, but it can also introduce micro-cracks, higher roughness, and a notch effect at edges. Fatigue-sensitive titanium or aluminum parts should specify test coupons, representative first articles, or fatigue review when thick MAO is requested.
Application Selection: MAO is best considered when surface hardness, insulation, corrosion support, or dielectric behavior is more important than zero dimensional change. It is less suitable for ultra-tight bores, miniature threads, flexible thin walls, sealing faces without finishing allowance, or parts where coating thickness would consume a large share of the tolerance band. For first production, inspect at least one coated part or coupon for coating thickness, roughness, and mating-fit change before releasing the remaining lot. If the part has press fits, sealing grooves, or miniature threads, the acceptance plan should name the pre-coat dimension and the final coated dimension separately. That prevents a correct coating from becoming a dimensional nonconformance.
