A number of researchers worldwide have tried to develop hybrid approaches producing surface coatings which combine the thickness of a polymer-based layer with the hardness of ion-beam implanted metal. These have tended to have the metal (typically silver rather than copper) retained within a ceramic matrix rather than a polymeric one. However, hard coatings tend to be brittle and this makes them liable to flake off when applied in a discrete thick layer. Scouring obviously exacerbates this tendency.
Recently, a new approach has been developed by an inter-disciplinary team of scientists at working in the Colleges of Engineering & Physical Science and Medicine & Dentistry at the University of Birmingham in the U.K. The researchers use a technique called active screen plasma (ASP) alloying. The object to be coated is placed inside a low pressure chamber with a metal lining. An electrical potential is applied between the object and the lining which causes a plasma (a high energy “soup” of ions and electrons) to form within the chamber. The team at Birmingham was led by Prof. Hanshan Dong in the School of Metallurgy & Materials.
The distinctive feature of ASP is that a mesh screen is placed between the wall of the chamber and the object to be coated. In the proprietary process developed at Birmingham, this screen is made from a mixture of stainless steel and copper and/or silver wires. As the plasma passes through the screen on its way to strike the target object it detaches a mixture of atoms from the steel, copper, and silver components of the mesh. These travel to the target where they are co-deposited into a single layer of mixed composition. Because the layer is built up progressively it can be much thicker than the limited penetration depth possible with ion beam implantation. And because the plasma behaves like a fluid, the ions being deposited do not travel in straight lines, allowing the coating of complex shapes.
A second distinctive feature of the Birmingham process is that organic gases containing carbon and/or nitrogen atoms are introduced into the ASP chamber. The gas molecules are broken apart by the plasma and some of the carbon/nitrogen atoms become incorporated in the new surface layer formed on the target. These interstitial atoms promote the formation of a stable S-phase in the basic stainless steel alloy (S-phase is also called expanded austenite—it is one particular geometric arrangement of the atoms in the metal lattice). This makes the surface layer harder than the underlying material (a similar effect is well known from the “case hardening” of low alloy steels).
The result is that a stainless steel surface treated by the ASP process is able to offer a unique combination of good wear resistance (superior to the untreated metal) promoted by the S phase component and long lasting anti-bacterial properties due to the nano-crystals of silver and/or copper. Tests at the University show that a stainless steel surface alloyed with 50 to 60 percent copper is 99 percent effective against E. coli NCTC 10418 and S. epidermidis NCTC 11047 within a six-hour testing contact time. This property remains intact after test instruments have been cleaned 120 times.
Dr. Goddard qualified as a metallurgist with degrees from Cambridge University and Imperial College, London. Since 2005, he has worked as an independent consultant advising start-up companies and public sector bodies on the commercialisation of materials and low carbon technologies. He can be reached at [email protected]
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