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Int. J. Mol. Sci. 2014, 15, 10527-10540; doi:10.3390/ijms150610527
International Journal of
Molecular Sciences
ISSN 1422-0067
www.mdpi.com/journal/ijms
Article
On Interlayer Stability and High-Cycle Simulator Performance
of Diamond-Like Carbon Layers for Articulating
Joint Replacements
Kerstin Thorwarth 1,†,*, Götz Thorwarth 2,†, Renato Figi 1, Bernhard Weisse 1, Michael Stiefel 1
and Roland Hauert 1
1 Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129,
8600 Dübendorf, Switzerland; E-Mails: renato.figi@empa.ch (R.F.);
bernhard.weisse@empa.ch (B.W.); michael.stiefel@empa.ch (M.S.);
roland.hauert@empa.ch (R.H.)
2 DePuy Synthes Companies of Johnson & Johnson, Luzernstrasse 21, 4528 Zuchwil, Switzerland;
E-Mail: thorwarth.goetz@synthes.com
† These authors contributed equally to this work.
* Author to whom correspondence should be addressed; E-Mail: Kerstin.thorwarth@empa.ch;
Tel.: +41-58-765-4547.
Received: 29 April 2014; in revised form: 20 May 2014 / Accepted: 28 April 2014 /
Published: 11 June 2014
Abstract: Diamond like carbon (DLC) coatings have been proven to be an excellent
choice for wear reduction in many technical applications. However, for successful adaption
to the orthopaedic field, layer performance, stability and adhesion in physiologically
relevant setups are crucial and not consistently investigated. In vitro wear testing as well as
adequate corrosion tests of interfaces and interlayers are of great importance to verify the
long term stability of DLC coated load bearing implants in the human body. DLC coatings
were deposited on articulating lumbar spinal disks made of CoCr28Mo6 biomedical
implant alloy using a plasma-activated chemical vapor deposition (PACVD) process. As an
adhesion promoting interlayer, tantalum films were deposited by magnetron sputtering.
Wear tests of coated and uncoated implants were performed in physiological solution up
to a maximum of 101 million articulation cycles with an amplitude of ±2° and −3/+6°
in successive intervals at a preload of 1200 N. The implants were characterized by
gravimetry, inductively coupled plasma optical emission spectrometry (ICP-OES) and
cross section scanning electron microscopy (SEM) analysis. It is shown that DLC coated
OPEN ACCESS
Int. J. Mol. Sci. 2014, 15 10528
surfaces with uncontaminated tantalum interlayers perform very well and no corrosive or
mechanical failure could be observed. This also holds true in tests featuring overload and
third-body wear by cortical bone chips present in the bearing pairs. Regarding the
interlayer tolerance towards interlayer contamination (oxygen), limits for initiation of
potential failure modes were established. It was found that mechanical failure is the most
critical aspect and this mode is hypothetically linked to the α-β tantalum phase switch
induced by increasing oxygen levels as observed by X-ray diffraction (XRD). It is
concluded that DLC coatings are a feasible candidate for near zero wear articulations on
implants, potentially even surpassing the performance of ceramic vs. ceramic.
Keywords: diamond-like carbon; biomedical implants; adhesion; simulator testing; wear;
coating; tantalum interlayer
1. Introduction
Owing to the aging process and related factors, some major joints of the human physiology are
subject to decay during their lifetime and eventual replacement by artificial solutions. This mostly
affects hip, knee, spine and shoulder, each of which generate more than one million replacement
procedures each year. Generally, a solution retaining the articulation is preferred over a stiffening or
fusion for the benefit of the patient and also for prevention of further decay of the neighboring
structures; for example, articulating spinal disks (Total Disc Replacements, TDR) are of great
advantage respective to the healing time, pain-management and less follow-up operations compared to
arthrodesis [1,2].
To ensure a long lifetime of an artificial joint replacement it is crucial to know all possible failure
mechanisms including wear mechanisms. These wear mechanisms differ depending on the joint
materials and on the geometry of the implant. Furthermore, the question is not just one of reducing the
amount of wear, but also how the size, shape, and surface chemistry of released wear particles differ
among bearing surface combinations, since these factors may ultimately influence the biologic reaction
and subsequent tendency for adverse body reactions (adverse local tissue response and systemic effects).
For metal-on-metal type designs, which possess a number of distinctive advantages over other
material combinations [3], many potential issues can be addressed using low friction coatings for the
bearing surfaces such as diamond like carbon (DLC). Aside from its success for wear reduction in the
technical field [4], DLC coatings seem to be a potential solution to ion release and wear problems
encountered with metallic articulating joint replacements [5]. DLC is well known for its chemical
inertness, high hardness, low friction and high wear resistance [6,7]; yet the first aspect to be
investigated when considering its use in vivo is the interaction between the DLC and its ambiance. For
implants, the environment differs from a typical technical application by the presence of body fluid,
which acts as a corrosive medium. This opens the possibility for corrosion-assisted failure mechanisms
as shown in previous publications [8,9]. Specifically, it was found that a crack can propagate along
the reactive DLC/metal interface in body-like environment and that this propagation can be explained
by the laws of stress-corrosion cracking (SCC) [10–12]. Furthermore, the potential of proteins to clog
Int. J. Mol. Sci. 2014, 15 10529
e.g., pinholes or small cracks can generate crevice conditions [13]. Another issue with application of
DLC coatings is securing adhesion to the substrate material owing to its high compressive stress; this
is typically facilitated by means of a reactive interlayer [14]. A detailed overview of DLC coatings in
the field of medical applications is given by several groups [4,15–17]. The aim of this work is to
investigate the qualification of DLC coatings for long-term application in bearing implants using the
example of lumbar spinal disks regarding mechanical stability, corrosion resistance as well as defect
tolerance and third body wear.
2. Results and Discussion
The weight loss curve of the convex part of a DLC coated cobalt chrome molybdenum (CCM)
spinal lumbar disk over the number of articulating cycles up to 101 million cycles of articulation is
given in Figure 1. For comparison the weight loss for uncoated CCM spinal disks is shown up to
10 million cycles. Calculating the weight of a 4 μm thick DLC layer with an estimated area of 1 cm2
gives a total mass loss of 1.2 × 10−3 g, corresponding to a loss of the total layer volume after 70 million
cycles. This contradicts the observation that the layer still looked largely undamaged after this cycle
number. In direct comparison to an uncoated metal-on-metal part, the generation of high wear on the
latter following a run-in phase up to 7 million cycles is evident. This effect has been linked to
roughening of the uncoated CCM surface [18] by third-body wear particles and consequent
deterioration of the lubrication regime, which also correlates to observations on explanted metal-on-metal
joints [19]. In contrast, the gravimetry results on the coated parts show that the surface integrity is
retained up to high cycle numbers as also visible to optical inspection. Also, static loads applied to the
DLC coated wear couple at 20 million cycles (4 kN, 10 repeats) did not affect or induce any significant
increase in wear.
Figure 1. Gravimetrically measured weight loss over 100 million cycles (simplified
simulator setup) with comparison to an uncoated (metal-on-metal) pair.
Int. J. Mol. Sci. 2014, 15 10530
To corroborate the gravimetry results, ICP-OES was performed to analyze the wear fluid ion
concentrations at selected intervals as presented in Figure 2. It is evident that large parts of the
detected metal debris stem from the simulator setup and sample fixture, which were stainless steel
clamps tightened around protrusions on the convex wear part backside, while the concave part was
permanently pressed and locked into the fixture. Comparing the specific wear to the gravimetrically
measured wear, the results are in good quantitative agreement especially for the mid-range DLC-on-DLC
couple. Noting that the gravimetrically determined weight loss only accounts for the convex part and
the optically intact coating, the preliminary hypothesis may be drawn that the observed wear largely
originates from fixture-sample backside articulation, and the gravimetry curve (Figure 1) may be
corrected accordingly.
Figure 2. ICP-OES (inductively coupled plasma optical emission spectrometry)
measurement results for the testing fluid in comparison to the gravimetry results.
In cross-section SEM view (Figure 3a), this hypothesis is solidified. Layer thickness measurements
taken on all 250 cross-section images indicate no large-area deviation from the part’s original 4 µm
layer thickness (Figure 3b), with only few individual instances of local thinning detected. Factoring in
the low specific weight of carbon wear, the weight loss measured by gravimetry cannot be explained
by wear on the articulating surfaces and must be attributed to wear of the sample backside with the
sample fixture instead. It can therefore be concluded that DLC coatings on matched and polished metal
articulations represent a very low wear tribocouple, which was also observed in the case of hip joints [20].
Int. J. Mol. Sci. 2014, 15 10531
Figure 3. (a) SEM (scanning electron microscopy) image of cross-section of the convex
implant after 100 million cycles; (b) Resulting layer thickness distribution derived from the
cross sections.
Another interesting aspect of high cycle number in vitro testing is the evolution of local defects. As
observed on several occasions and also shown elsewhere [9,12], growth of local defects can lead to an
avalanche-like delamination once a defect size threshold is exceeded and the defects start to interact
including third-body wear mechanisms. A successful coating thus has to tolerate local defects that may
arise even during the coating process by coverage with dust particles or pull-out of loose grains from
the substrate material.
Figure 4 shows the changes to an exemplary defect observed at 20, 50 and 101 million articulation
cycles on the convex part of a simulator articulation pair. This group of defects was originally
observed early after 3 million cycles of articulation. A focused ion beam (FIB) cut was placed through
the defect’s edge to gain insights on eventual interfacial cracking. However, no evolution of the defect
size or shape at the given cycle numbers was observed, meaning the selected interface is tolerant
towards small failures.
14602920438058407300876010,220
Int. J. Mol. Sci. 2014, 15 10532
Figure 4. Defect evolution on a selected defect site after 20, 50 and 100 million cycles.
Figure 5 further illustrates the layer behavior after high cycle testing (101 million cycles) in several FIB
cut cross sections (center region of the convex part). In contrast to a mechanically or chemically weak
interface, the present cracks do not propagate along the DLC-substrate interface, instead deviating into
the DLC layer. Although the general presence of cracks cannot be avoided due to the eggshell effect,
this behavior might help to keep the defects more localized and not lead to large-scale delamination.
Figure 5. Focused ion beam (FIB) cross section on a defect edge after 100 million cycles.
Red arrows: “Bubble-like” failures, green arrow: Crack exiting the interface.
Int. J. Mol. Sci. 2014, 15 10533
An additional feature found in Figure 5 is a “bubble-like” form of the cracks in cross-section. This
morphology has not been observed at low cycle numbers and bears resemblance to fish grate-like
surface defects occasionally observed on top-view images. Potential explanations may be a form of
fatigue of the DLC coating, however more detailed studies are required for further discussion.
At 28 million cycles of articulation of an analogous sample pair, cortical bone powder was added
into the tribocontact to simulate a third-body wear situation. The resultant wear values are illustrated in
Figure 6. Both optical inspection and gravimetric analysis did not indicate a significant wear increase
or roughening due to addition of third body wear particles. It was noted that both variants of wear
particles were rapidly expelled from the contact, with the larger particles being ground into smaller
fragments. Further investigations are needed to check for adverse effects of other potential third-body
wear materials like bone cement (e.g., Polymethylmethacrylate (PMMA)).
Figure 6. (a) Third-body wear particles and (b) resulting wear rate on DLC/DLC pairs.
For investigations of potential failures of the Ta interlayer due to oxygen contamination during
deposition, several possible failure mechanisms are to be considered:
,… (1)
where delamination speed (limit: lifetime requirement),
,… failure mechanisms,
interface energy. With examples
,… crevice
Int. J. Mol. Sci. 2014, 15 10534
corrosion-type failure,
,…
stress-corrosion cracking type failure,
,… 0,
≫0,
instant mechanical type failure.
Particularly, these mechanisms can be attributed to pure mechanical (static and fatigue-based),
stress-corrosion-based and pure corrosive effects (e.g., crevice corrosion), all of which were found
relevant in recent publications [8,9,12,13]. It should be noted that the failure mechanism can change
through the implant lifetime, i.e., one mechanism can act as a “lead-in” for the other. Depending on the
individual initiation thresholds, this can greatly complicate failure analysis for long-term implants
targeting to cover all possible failure paths.
As of today, not all failure mechanisms outlined above can be tested effectively. No accurate way
to accelerate the process is known especially for the corrosion-dominated mechanisms; for these, a
combination of long-term observation and microscopic analysis (FIB/SEM) is presently required.
For the present interlayer system (CCM/Ta/DLC), defined oxygen contamination levels were
chosen as detailed in Table 1 to obtain failure limits for purely mechanical failure and stress-corrosion
cracking. For the simulator tests, rapid failure even for low oxygen contamination (R ≥ 5 × 10
−4
)
resulted in cycle numbers <100,000, with worsening delamination pattern at higher concentrations
(Figure 7). In contrast, several uncontaminated articulation pairs did not show any signs of
delamination up to 20 and even 101 million cycles.
Table 1. Implants coated in this study and resultant interlayer stability. Only samples with
R = 0 were found to be stable up to high cycle numbers in the simulator tests, whereas only
samples with R = 6.4 × 10
−2
(red) exhibited stress-corrosion-cracking behavior.
R = O32/Ar40 0 5 × 10−4 5 × 10−3 1 × 10−23× 10−2 6.4 × 10−2
Ta interlayer
Sample No. cycles
P42 P37 P36 P35 P32
P03 33 million
P07 101 million
P43 29 million
P45 20 million
P33 25 million
P34 25 million
Figure 7. Delamination observed on coated articulation pairs (concave parts) with
increasing interlayer oxygen contamination. Left: R = 0, 80 million cycles; center:
R = 5 × 10
−4
, 100,000 cycles; right: R = 6.4 × 10
−2
, 100,000 cycles.
Int. J. Mol. Sci. 2014, 15 10535
Stress-corrosion cracking tests on the oxygen-contaminated interlayers were performed by 1500 N
Rockwell indentation, immersion in phosphate buffered saline solution and following the
time-dependent delamination radius as detailed earlier. The stress-delamination speed dependence
allows for modeling of the SCC process for a given materials system. In the present case, the onset of
SCC was found at much higher oxygen contamination levels compared to mechanical failure, leading
to very quick crack propagation and failure above this level (Figure 8).
Figure 8. SCC (stress-corrosion cracking) test (delamination around indents) placed on
DLC-coated reference samples with increasing oxygen interlayer contamination.
Finally, concerning the potential for purely corrosion based failure (crevice corrosion, CC), the FIB
cross sections on the 101 million cycle defect edges may be reviewed for indications of a dissolving
(blunt) crack. Since these samples were immersed into physiologically relevant testing fluid for more
than 3 years, an upper limit for CC based dissolution rate may be deducted. No indications of crack
propagation were found in these analyses. Hence it may be concluded that crevice corrosion does not
play a dominant role in the potential in vivo failure mechanism of the Ta interlayer system, in contrast
to Si based interlayers [12,13].
In summary, purely mechanical failure seems the predominant effect among the tested failure
modes for the presented interlayer system (Ta). To elucidate the source of this behavior, X-ray
diffraction was performed for the Ta interlayer structure on three distinct contamination levels (Figure 9).
Changes are visible in particular to the α-Ta (110) and β-Ta (002) peaks, with the α-Ta peak
disappearing at higher contamination levels. Considering that the structure factor of β-Ta (002)
exceeds the α-Ta (110) by a ratio of 7.01 [21], the proportion of the α-Ta phase is relatively minor.
However, it must be noted that the amorphous and nanocrystalline Ta content of the layer is unknown,
that the β phase is documented as brittle in literature [22], and that oxygen is known to stabilize the
β phase in Tantalum [23,24]. Judging from the increase in absolute intensity of the β phase peak
between the three contaminations investigated, the absolute amount of X-ray detectable β-Ta is found
to increase by a factor of 2.75 between the highest and lowest dataset. As a hypothesis, it might
Int. J. Mol. Sci. 2014, 15 10536
therefore be suggested that oxygen contamination leads to high β content of the Ta interlayer and thus
enables mechanical failure.
Figure 9. X-ray diffractograms of the Ta interlayer structure with respect to the oxygen
contamination level (O32/Ar40 current ratio R).
3. Experimental Section
Four µm thick DLC layers were deposited on mirror polished high-carbon biomedical CoCr28Mo6
(CCM) implant alloy using radio frequency (13.56 MHz) plasma activated chemical vapor deposition
(PACVD) with acetylene (C2H2) as a process gas. As adhesion promoting layer a 90 nm Tantalum
interlayer was deposited in situ by magnetron sputtering using a pure Tantalum target (5 N) without
interruption of the plasma discharge between process steps. Prior deposition the CCM substrates were
ultrasonically cleaned in an acetone-ethanol mixture and additionally presputtered in an argon
discharge (19 sccm Ar flow; −600 V RF bias). The gas flow for Acetylene was fixed at 24.0 sccm
(2.5 Pa) and for sputtering a gas flow of 2.1 sccm (0.5 Pa) Argon was used. The target was
presputtered during the last minutes of the precleaning process of the sample. For all experiments a
base pressure of <1 × 10−5 Pa was established before process initiation. A more detailed description of
the DLC deposition process can be found in [25].
The microstructure of the DLC layers was determined with a FEI NovaNanoSEM 230 and a Hitachi
S-4800 scanning electron microscope. For the cross-sections, a focused ion beam instrument
(FIB-Dual Beam FEI STRATA DB235) was used.
To analyze wear behavior, coatings were deposited on ball-on-socket type lumbar spinal disk
replacement implant prototypes made of CCM with a radius of 14.5 mm. A simplified spinal simulator
setup with reference to ASTM F2423-05 was chosen [18], featuring a constant perpendicular load of
1200 N and one degree of motion, which was applied alternatingly in human lumbar spine lateral
80
Int. J. Mol. Sci. 2014, 15 10537
(+/−2°) and flexion-extension (+6°/−3°) mode. These ranges were chosen in accordance with
ISO 18192-1. For the change of motion mode, samples were rotated +/−90° after 0.2, 0.5, 1, 2, 5 and
10 million total cycles and every 10 million cycles at successive intervals. The articulation frequency
was 3 Hz. The sample stage was kept immersed into 37 °C 30 g/L protein-containing wear testing fluid
(Hyclone®, Cat. No. SH30856.04, Thermo Fisher Scientific, Logan, UT, USA), which was stabilized
with anti-fouling agents (NaN3, protease inhibitors) and periodically exchanged. Ultrapure water was
refilled to compensate for the evaporation, preventing volume and concentration changes of the
lubricant during the tests. Running simulator tests in non-protein media like Phosphate Buffered Saline
(PBS) would lead to deviations both in lubrication mode and corrosion-assisted failure characteristics
and is hence not recommended [18,26].
In the course of dismounting the sample for a +/−90° rotation on the simulator stage the samples
were cleaned according to ISO 14242-2 and the weight loss was determined with weight
measurements (ES 225SM-DR, Precisa, Dietikon, Switzerland; AE-163, Mettler Toledo, Greifensee,
Switzerland; resolution 0.01 mg). On selected defect sites, Focused Ion Beam (FIB) cross sections
were cut on a FIB-Dual Beam instrument (FEI STRATADB235) using a gallium ion beam. Additional
optical investigations were performed with a Philips XL30 ESEM-FEG scanning electron microscope
(SEM) equipped with an EDX (energy dispersive X-ray) detector.
Exchanged simulated wear testing fluid was itemized by thermal cracking in a 65 vol. % nitric
acid/30% hydrogen peroxide mixture at a temperature of 190 °C. Following thermal cracking, all
samples were optically clear. The Co, Cr, Mo, Fe and Ni content of the samples was measured using
ICP-OES referencing certified standards.
Following 101 million cycles of testing, one convex sample part was cut into 90° sectors by a diamond
saw and analyzed for remaining coating thickness in cross-sections using SEM (FEI NovaNanoSEM
230). 250 cross-section images were taken to gain an overview over the thickness distribution.
To corroborate the high-cycle simulator tests, additional studies with hard third-body wear particles
were performed. For this, coarse and fine-grained cortical bone was machined from porcine ribs and
inserted into the articulation couple at 28 million cycles. Gravimetrical measurements and optical
inspection were performed to identify potential layer failures or increases in wear volume.
The tolerance of the tantalum interface towards contaminations during the deposition process was
investigated. For this, a defined oxygen amount with respect to the process gas pressure was admitted
to the chamber during the interlayer growth by means of an oxygen leak valve. The level of oxygen
was monitored and set via the m/q = 32 to m/q = 40 current signal ratio (R), i.e., versus the detected
argon current using a mass spectrometer (SPM 200, Pfeiffer Vacuum, Zurich, Switzerland)
immediately prior to starting the RF discharge for sputter cleaning. The targeted I (O2+/Ar+) current
ratios varied from 0 to 6.5 × 10−2. The structure of the adhesion promoting Tantalum interlayer was
determined by X-ray diffraction (Bruker D8, Bruker, Karlsruhe, Germany) using monochromated Cu Kα
radiation in Bragg-Brentano configuration. Stress corrosion cracking (SCC) tests of the interfaces
involved were determined as described in [11]. A standard Ernst NR 3R Rockwell indentation setup
was used to induce the delamination of the DLC coatings via the plastic deformation of the CCM
substrates. The diamond Rockwell tip with a 120° cone was pressed into DLC coated substrate with a
load of 1.5 kN for 10 s.
Int. J. Mol. Sci. 2014, 15 10538
After indentation the samples were immersed in 0.01 M phosphate buffered saline (PBS) solution
(Sigma Aldrich, Buchs, Switzerland). All fluids were maintained at a constant temperature of 37 °C.
A Müller (Mueller-Optronic, Erfurth, Germany) metallographic microscope equipped with a
Premiere® MA88-300 CCD camera was employed to investigate the time dependent delamination of
the thin films in corrosive media.
Finally, to test defined contamination interlayers for mechanical failure, a series of corresponding
articulation pairs was prepared and run in the spinal simulator setup described above until
delamination was observed.
4. Conclusions
In this work, investigations on the stability of Tantalum interlayers for DLC coated articulating
implants were presented. It was found that uncontaminated (with respect to oxygen) Tantalum
interlayers give excellent adhesion stability to DLC coatings on CCM articulations, with little to no
noticeable wear detected after 101 million cycles in a simplified lumbar spine simulator setup.
Furthermore, it was found that overloading and third-body wear situations with bone particles do not
effect significant coating failure, and that local defects do not exhibit growth even at prolonged in vitro
testing. Therefore, the wear characteristic of a DLC coated metal-on-metal articulation fulfils even
extended lifetime requirements for joint replacements.
Regarding the Ta interlayer stability against coating process contamination, mechanical failure was
analyzed to be predominant for the presented materials system, with very low contamination levels
leading to early cycle failure in the simulator. Adding structural characterization results, this failure
was preliminarily linked to β Ta evolution induced by oxygen doping during the interlayer growth
process and the corresponding embrittlement. Other failure modes (like SCC) were found to be present
but requiring much higher oxygen contamination levels to exceed the activation threshold. It is
therefore concluded that amorphous/α phase Ta interlayers represent a viable solution for securing
DLC adhesion on CCM orthopaedic implants, given that tight contamination control is maintained.
Acknowledgments
Special acknowledgement is given to Guenther Hobi and Hans Michel for technical support. The
authors further wish to thank Ulrich Mueller and Cyril Voisard for helpful discussions, and the Swiss
Competence Center for Materials Science and Technology (CCMX) as well as the Swiss Commission
for Technology and Innovation (CTI) for project support. Finally, DePuy Synthes Companies are
acknowledged for financial support.
Author Contributions
K.T.: Simulator operation, gravimetry, SEM, SCC tests, manuscript; G.T.: Simulator operation,
gravimetry, XRD, manuscript; R.F.: ICP-OES; B.W.: Simulator operation, manuscript revision;
M.S.: FIB; R.H.: Manuscript revision, discussions.
Int. J. Mol. Sci. 2014, 15 10539
Conflicts of Interest
The authors declare no conflict of interest.
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