Mahle Ferroprint vs Ferrostan Piston Coatings

Comparison of Mahle Ferroprint vs Ferrostan Piston Coatings for Alusil/Lokasil Bores

Introduction

High-silicon aluminum engine blocks, such as Alusil and Lokasil, rely on exposed silicon particles in the bore for wear resistance instead of iron liners. To prevent aluminum-on-aluminum galling between pistons and these uncoated bores, manufacturers developed special iron-based piston skirt coatings. Mahle pioneered two such coatings: Ferrostan and its successor Ferroprint, used in engines with Alusil or Lokasil bores. This report provides a technical comparison of Ferrostan and Ferroprint, covering their material composition, application methods, tribological behavior, durability, and usage in engines (e.g. Porsche and Mercedes-Benz). Key attributes are summarized in tables for clarity.

Coatings for Alusil and Lokasil Bores

Alusil (a hypereutectic Al-Si alloy) and Lokasil (a similar silicon-infused casting technique) cylinders have no iron liners – instead, the aluminum bore is etched to expose hard silicon crystals that support the piston and ring contact. Pistons running in these bores must have a ferrous skirt coating to avoid rapid wear or seizing.

In the 1990s, Mahle introduced Ferrostan, an electroplated iron coating (with a tin overlay), for pistons in Alusil engines. By 2000, due to environmental regulations banning certain plating processes in Germany, Mahle replaced Ferrostan with Ferroprint, a newer ferrous coating applied via a resin-based process. Other manufacturers like Kolbenschmidt (KS) offered similar “Ferrocoat” iron spray coatings for their pistons.

These coatings are critical in OEM engines (e.g. BMW, Mercedes, Porsche) that adopted linerless Al-Si blocks. For instance, BMW switched from Nikasil liners to Alusil in 1996, requiring iron-coated pistons, and Mercedes-Benz used Alusil (branded Silitec) in V6/V8 engines starting in 1996.

Material Composition and Structure

Ferrostan (Iron/Tin Plating)

Mahle’s Ferrostan is a dual-layer metallurgical coating: a base layer of electroplated iron (~10–13 μm thick) with a very thin tin plating on top. The iron layer provides a hard ferrous surface on the aluminum piston skirt, while the soft tin overlayer aids in initial break-in (conforming to bore micro-asperities and preventing scuffing during the first runs).

Once the engine is run-in, the tin film wears away, leaving the iron as the primary contact surface. Ferrostan’s iron is sometimes referred to by Mahle as FerroTec (and in older literature “Ferrostan I” and “II” denoted variants with or without the tin overlay). The overall coating is relatively thin and firmly bonded metallurgically to the piston skirt.

Ferroprint (Stainless-Steel/Resin Composite)

Mahle’s Ferroprint is a composite coating consisting of stainless steel micro-particles dispersed in a synthetic resin (polyamide) matrix. It is applied to a thickness of about 20 μm, thicker than Ferrostan’s iron plate.

The stainless particles (iron-based alloy) serve as the ferrous contact material, while the resin binder adheres them to the piston. This polymer-based coating was introduced as an environmentally friendly alternative to electroplating – it eliminates the hazardous plating baths and by-products associated with Ferrostan’s process.

The use of stainless steel ensures corrosion resistance and durability of the particles, and provides a ferrous surface for tribological compatibility with Al-Si bores. The resin gives the coating a slightly compliant, “screen-printed” texture. In essence, Ferroprint is akin to other modern dry-film skirt coatings (like Grafal®) but uniquely loaded with metal particles to retain the benefits of an iron interface.

Application Methods

Ferrostan – Electroplating

The Ferrostan coating is applied by an electroplating process. The aluminum piston skirt is first prepared (typically by a zincating and copper strike process) and then submerged in a ferrous electroplating bath. A layer of iron is electrodeposited onto the skirt to the specified thickness (10–13 μm). After the iron layer, a thin tin layer is electroplated on top.

This two-step plating creates a strongly adherent, metallic coating. The result is often called an “iron-clad” or iron-plated piston skirt. The process yields a uniform ferrous coating, but involves toxic chemicals (e.g. cyanide or acid baths for plating) and heavy metal waste.

By the late 1990s, tightening environmental laws in Europe (and EPA regulations in the US) deemed the Ferrostan plating process unsustainable. For example, iron/tin electroplating was effectively outlawed in 2000 due to its hazardous by-products. This drove the development of non-plating alternatives.

Ferroprint – Screen/Spray Deposition

Ferroprint is applied via a spray or screen-print deposition of a liquid resin/metal-powder mixture onto the piston skirt, followed by curing. Mahle’s process effectively “prints” or sprays an iron-rich resin film onto the skirt (hence the name Ferroprint). The coating is then oven-cured to harden the polymer matrix and lock the stainless steel particles in place.

This method is solvent- and resin-based rather than electrochemical. It allows precise control of coating placement – for instance, coating can be applied only where needed on the skirt and in a desired pattern or thickness.

The resulting cured coating has a matte gray to black appearance (distinct from the silvery look of tin-plated Ferrostan). The process is environmentally cleaner, avoiding electroplating effluent. It is also flexible – the formulation can be adjusted (particle size, resin type) for performance. Mahle introduced Ferroprint in the early 2000s to meet regulatory demands while maintaining compatibility with Alusil/Lokasil bores.