Why nanocorrosion causes coated implants to fail
17 August 2010
Extra-hard coatings made from diamond-like carbon (DLC) are
used to extend the operating lifetime of tools and components that
undergo wear. In artificial joints, however, these coatings often fail
Researchers at the Swiss organisation Empa have found out that the
coatings fail due to corrosion in the reaction layer under the
coating. They have 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.
Extremely hard coatings made of diamond-like carbon (DLC) have
proven their value on products such as computer hard disks, saw
blades, embossing tools, razor blades and fuel-injection nozzles.
They reduce wear and thereby give tools and components a longer
operating lifetime. A number of implant manufacturers reasoned that
it would be useful to apply DLC to medical implants such as
artificial joints, where wear is also a problem.
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.
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 the transmission
electron microscope image above.
Targeting the interface
In a project financed by the Swiss Innovation Promotion Agency
(CTI) together with the medical technology company Synthes and the
coating company Ionbond AG, Empa researchers sought out the cause of
this detachment. For this, the researchers conducted detailed
studies of the interface between the implant material and the
"When two materials are placed in contact with each other, the
result is a reaction layer at the interface between them, which is
only several atomic layers thick. Thus a new material is formed,
which we investigated now for the first time", explains Roland
Hauert of Empa's "Nanoscale Materials Science" laboratory.
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.
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.
Methods to determine operating lifetime
Together with their industry partners Synthes and Ionbond, Empa
developed a corrosion-resistant intermediate layer at the interface
to the DLC layer. What's more, the researchers also developed a
process that can determine a crack’s growth rate under conditions
similar to those experienced in the human body as well as the
dissolution rate of the reaction layer in cases of crevice
"This then allows us to calculate the expected operating lifetime
of the coated implant in the human body", says Roland Hauert.
Whether or not a DLC-coated implant will fail prematurely in vivo
can henceforth already be determined during the development of the