Empa researchers discover why implant coatings detach – and a method to prevent it
Nanocorrosion causes implants to fail
Extra-hard coatings made from diamond-like carbon (DLC) extend the operating lifetime of tools and components. In artificial joints, however, these coatings often fail because they detach. Empa researchers found out why – and developed methods to both make the interface between the DLC layer and the metal underneath corrosion-resistant and to predict the lifetime of the implants.
![https://www.empa.ch/documents/56164/279891/a592-2010-08-10-b0s+stopper+Implantate.jpg/e1a6ab5b-3dc0-4fc2-85ce-b0d9a9fbf6de?t=1448298127000&doAsUserId=mLYofz1ofIc4z2qdMCjTcg%253D%253D%3FuserDisplayName%3Dhmf](https://www.empa.ch/documents/56164/279891/a592-2010-08-10-b0s+stopper+Implantate.jpg/e1a6ab5b-3dc0-4fc2-85ce-b0d9a9fbf6de?t=1448298127000&doAsUserId=mLYofz1ofIc4z2qdMCjTcg%253D%253D%3FuserDisplayName%3Dhmf)
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Whether on computer hard disks, saw blades, embossing tools, razor blades or fuel-injection nozzles, extremely hard coatings made of diamond-like carbon (DLC) have proven their value over and over again. They reduce wear and thereby give tools and components a longer operating lifetime. What could be more logical than to apply DLC to medical implants such as artificial joints, reasoned a number of implant manufacturers. After all, wear is a problem there, too. |
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DLC has subsequently withstood endless in vitro tests in several manufacturers’ laboratories and has shown itself to be well tolerated by human tissue, extremely hard wearing, and resistant to the relatively aggressive environment in the human body. Despite this, when DLC-coated joints were first implanted into human patients, serious problems arose after only a few years. The DLC coatings were not worn away, but rather they detached from the implant material for no apparent reason. |
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Taking aims at the interface His team showed that the so far barely considered reaction layer, which is not always completely corrosion resistant, is responsible for the detachment of the DLC layer. On the one hand, stress corrosion cracking occurred in the reaction layer. The mechanical load in conjunction with the penetration of body fluids led to slow-growing cracks, which in turn caused the DLC substrate to detach little by little. |
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Caption: Under physiological conditions, stress corrosion cracking leads to a slow-growing crack in the metal carbide reaction layer, which is barely 5 nanometers wide. This, in turn, causes the DLC layer to detach from the implant material (cobalt-chrome-molybdenum, CoCrMo), as seen in a transmission electron microscope image. |
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In other cases, crevice corrosion was responsible for the damage. Over time, an aggressive, acidic medium develops in fine crevices and slowly dissolves the reaction layer or the additional adhesive layer, likewise leading to detachment. |
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Methods to determine operating lifetime |
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