What surface finishes can be achieved with MAO?

Surface Finishes Achievable with Micro-arc Oxidation (MAO)

The surface finishes achievable with Micro-arc Oxidation (MAO) are primarily functional and technical in appearance, stemming directly from the process's plasma-driven mechanism. While it offers limited decorative color options compared to traditional anodizing, it provides a unique set of aesthetic and tactile properties aligned with high-performance applications.

1. Inherent Color and Appearance

The color palette of MAO coatings is determined by the substrate alloy, electrolyte chemistry, and process parameters, not by dyes. The finishes are typically matte and fall within a specific range:

• Shades of Gray: This is the most common outcome, ranging from light gray to nearly black. The specific shade is influenced by the alloy's silicon content; for instance, a high-silicon alloy like A380 will produce a darker, more mottled gray compared to the more uniform gray achievable with A360.

• Bronze and Olive Tones: By adjusting the electrolyte composition and electrical parameters, it is possible to achieve bronze, dark bronze, or olive-green hues, particularly on titanium and some aluminum alloys.

• Limited Color Uniformity: Achieving a perfectly uniform, consistent color on a complex part can be challenging. The nature of the plasma discharges can create subtle variations in shade, which is considered part of the coating's characteristic "technical" look.

2. Surface Texture

The MAO process inherently creates a distinct micro-texture:

• Micro-Roughness ("Orange Peel"): The surface has a characteristic low-gloss, matte texture similar to an orange peel. This is a result of the countless micro-discharges and the sintering of the ceramic material.

• Controlled Porosity: While much denser than traditional anodizing, the surface can have a micro-porous structure. This texture can be beneficial for retaining lubricants or as a preparatory layer for subsequent coating adhesion, but it is not a smooth, glossy finish.

This texture can be modified through post-processing. For example, a MAO-coated part can be polished to reduce the roughness and achieve a smoother feel, though this will add to the cost. The as-coated texture, however, provides excellent anti-fingerprint and anti-glare properties.

3. Functional Finishes and Post-Treatment Options

The real value of an MAO finish lies in its functional properties, which are integral to the coating itself:

• Wear-Resistant Matte Finish: The finish is not just a visual layer; it is an extremely hard, abrasion-resistant surface ideal for components like power tool housings and hydraulic pistons.

• Corrosion-Resistant Barrier: The matte gray finish is a visual indicator of the thick, dense ceramic layer that provides superior corrosion protection, validated by extensive Post Process testing.

For purely decorative applications requiring vibrant colors, traditional anodizing remains the preferred choice. MAO is specified when the performance of the finish is as important as its appearance, making it a cornerstone of our die castings Engineering for demanding sectors.