Advanced Solutions for Filling the Gaps
The body of a car that automatically repairs small dents. A metal door that makes nitrogen oxides harmless. A ship hull that removes pesticides from the water. And antibacterial implants. For now, this all still sounds like pie in the sky, but scientists around the world are investigating and developing new processes to enable the application of such intelligent materials. As a part of two EU projects and one bilateral collaboration with China, scientists from Helmholtz-Zentrum Geesthacht are working on the development of “smart” multifunctional coatings on magnesium and aluminium alloys.
Maria Serdechnova examines the bone screw in the electrolyte bath. Photo: HZG/Christian Schmid
These coatings can, for example, be photoactive and use sunlight to transform dangerous chemicals. Or they can provide thermal and/or electrical conductive properties. Simultaneously, the coatings protect the active metallic material against corrosion.
The principle of this surface treatment appears simple: a piece of a metal is immersed in a bath, containing electrolyte, and a current is applied. This causes the oxidation of the metal and a hard, ceramic layer is formed on the surface. With plasma electrolytic oxidation (PEO), the science is in the details. Plasma is using not only components from the bath in order to form a new layer, but it also involves the modification of the metallic surface itself. This leads to a hard, strongly adhesive layer, which can be endowed with multiple properties.
Plasma Electrolytic Oxidation
Dr Serdechnova, the HZG scientist from the “Corrosion and Magnesium Surface Technology” department, explains: “PEO layers are characterised by complex microstructures: the plasma oxidises the metallic surface and causes the oxide layer formation, with increased hardness. Since layer formation occurs in an aqueous alkaline solution, without the addition of toxic compounds such as chromium, this makes the process especially eco-friendly.” At the same time, PEO allows different nanoparticles such as layered double hydroxides (LDHs) to be introduced into the porosity of ceramic coatings, leading to the formation of coatings with improved functionality, such as photoactive or “smart” anticorrosion coatings. The latter is studied in another EU project, ACTICOAT: the goal of this research project is to fill the PEO pores with anticorrosive compounds to create environmentally friendly protective layers for the light metals, such as magnesium and aluminium.
Maria Serdechnova says: “To apply these sensitive particles in the created PEO based microcontainers, very specific changes in the voltage and the process design are required. Many of the resulting aspects are difficult to predict.” The projects are investigating multistage post-treatment of active metals. For an effective result, some of the particles should be introduced into closed pores, and some into open pores.
Releasing the inhibitors when the PEO layer is damaged allows for protection in a variable manner. Alongside the development of the functionalised materials, a further project goal is the transfer of the technology to key industries such as automotive manufacturing, medical technology, or the chemical industry. A major point of focus is therefore the development of an interdisciplinary partnership between participants from both science and the private sector.
Author: Heidrun Hillen (HZG)
Published in in2science #9 (April 2020)