- Email: [email protected]
Biotribological study of multilayer coated metal-on-metal hip prostheses in a hip joint simulator
Wear 301 (2013) 234–242
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Biotribological study of multilayer coated metal-on-metal hip prostheses in a hip joint simulator J.A. Ortega-Saenz, M. Alvarez-Vera, M.A.L. Hernandez-Rodriguez n Universidad Autonoma de Nuevo Leon, FIME, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2012 Received in revised form 9 January 2013 Accepted 10 January 2013 Available online 23 January 2013
Metal-on-metal (MOM) hip joint bearings have demonstrated low wear rates and nowadays this contact pair has being considered as an alternative to metal-on-polymer (MOP) joint replacements. However, wear of MOM joints is a concern due to the toxicity and biological reaction of wear debris and metallic corrosion. This has motivated to investigate the possibility to apply thin hard coatings on metallic heads to reduce the wear and metallic ion release. The aim of the present study was to investigate the wear properties of metal-on-metal hip prostheses with surface engineered femoral heads using a multilayer coating (TiN/CrN) 3, in comparison with metal-on-metal pairs in a hip joint simulator. Different surface PVD coatings were applied on surgical grade wrought cobalt–chromium alloy femoral heads: multilayer (TiN/CrN) 3, CrN single layer and diamond-like carbon (DLC). These femoral heads were tested against high carbon content cast cobalt–chromium alloy acetabular cups using a three-axial multi-station hip joint simulator (FIME II). During the wear tests three directions of motion were applied with the following amplitudes: flexion–extension (FE) 7 231, abduction– adduction (AA) 7 231 and internal–external (IER) 7 81. All components were tested at 1.2 Hz under a Paul-type loading curve and bovine calf serum solution as lubricant. Results showed that both; the PVD coatings protects the femoral heads reducing wear up to 5 times in the case of the DLC coating and 28 and 55 times in the case of (TiN/CrN) 3 and CrN respectively compared with the MOM femoral heads. & 2013 Elsevier B.V. All rights reserved.
Keywords: Biotribology Co-Cr alloy Metal-on-Metal PVD coatings Multilayer coatings Hip joint simulator
1. Introduction The total hip arthroplastic surgery was a major medical advance of the 20th century. The materials used in this medical application must possess satisfactory mechanical properties such as stiffness and fatigue strength, wear and corrosion resistance, and biocompatibility. The first metal-on-metal (MOM) total hip prostheses implanted during the 1960s decade presented unsatisfactory short-term performance due to geometrical inaccuracies which led to high frictional forces and increased wear [1–5]. However, in some cases the implants lasted at least for two decades without osteolysis [2,5–7] and negligible wear [2,8–11]. In recent years, the use of second generation Co–Cr alloy metal-on-metal bearing joints in total hip arthroplasty surgery represents an attractive alternative to the traditional metalon-polyethylene pairs [12]. Despite the triblological pair metal-on-metal has proven to be more wear resistant than metal-on-polyethylene couple, the toxicity of metallic ions of cobalt and chromium released from wear particles from metal-on-metal hip prostheses into the
n
Corresponding author. Tel.: þ52 8114920375x5770; Fax: þ 52 8110523321. E-mail address: [email protected] (M.A.L. Hernandez-Rodriguez).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.024
human body [13–16] is a concern which has motivated to look for alternatives to solve or diminish this problem. PVD coatings are well known for providing surfaces with improved tribological properties in terms of low coefficient of friction and high wear resistance. This technology has been brought to the field of surgical implants with promising results [17–19]. Nowadays, there has been great scientific and commercial interest in nanostructured coatings, like multilayer or nanolayered films [20]. The main idea is that the coating needs to be hard to avoid abrasive wear, but it also needs to be strain tolerant and tough, in order to prevent crack propagation, therefore avoiding fracture or delamination of the coating [21]. In consequence, multilayer films can be designed to show an improved wear resistance [22]. In a previous study, the authors have reported the improved wear resistance of a multilayer TiN/CrN coating deposited by plasma assisted phisycal vapor deposition on wrought Co–Cr substrates [23]. These films also exhibit improved corrosion resistance [23,24], making them good candidates for orthopedic applications. The main aim of the present study was to assess the tribological behavior of femoral heads surface modified with a multilayer TiN/CrN coating articulating against uncoated metallic acetabular cups in an anatomical hip joint simulator. Femoral heads coated with two of the most successful coatings in the
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
recent years: CrN and DLC, were also included in the study and compared with metal-on-metal Co–Cr alloy bearing couple.
2. Material and methods 2.1. Samples preparation For the present study, a total of eight prototype cups and heads implants with 31.5 mm diameter were manufactured. Femoral heads were manufactured from wrought Co–Cr alloy ASTM F1537-08 (BioDur CCM Plus) supplied by Carpenter Technology, whereas the acetabular cups were manufactured by the investment casting method complying with the chemical composition of the ASTM F75/98 [25]. The chemical composition of the alloys used in this investigation is shown in Table 1. Femoral heads were modified using three different coatings: a multilayer coating (TiN/CrN) 3, monolayer CrN and, DLC. The first two coatings were created by the plasma assisted physical deposition (PAPVD) method, whereas the DLC coating (a-C: H) was supplied by Balzers (BALINITs DLC). The thicknesses of the coatings were 3.25 mm, 3.75 mm and, 2 mm respectively. Multilayer (TiN/CrN) 3 and monolayer CrN coatings were deposited by means of an industrial multi-source vacuum device equipped with five sources. A double rotation planetarium fixture was used. The substrates were precleaned in an ultrasonic cleaning line with trichloroethylene, alkaline detergent and demineralized water. The final drying was performed in hot trichloroethylene vapor. Prior to deposition, the vacuum chamber was pumped down to 2 10 3 Pa to initiate the etching/heating stage. A current of 80 A, for each of the five cathodes in operation, and a negative substrate bias voltage of 950 V was used. No nitrogen was introduced into the chamber during this stage. The etching/heating stage was interrupted when a temperature of 400 1C, as measured with a pyrometer, was reached; whereupon the deposition stage was initiated. Coating deposition was performed with five sources in operation. Parameters used during the PAPVD process such as: nitrogen gas pressure, substrate bias voltage (UBias), cathode source current (ICathode), and deposition time are presented in Table 2. The total thickness of the coating was controlled by the deposition time. The samples were cooled down to less than 100 1C in an N2 atmosphere before venting the chamber. The components were machined and finished following controlled standard implant specifications. For this purpose a coordinate measurement machine (Mitutoyo QM-Measure 353) and surface profilometer (with 0.8 mm cut-off) were used to measure the diametral clearance (Cd), sphericity and roughness (Ra). These parameters for the eight bearings are given in Table 3. Table 1 Chemical composition (wt%) of the pair specimens used in this investigation.
Femoral heads Acetabular cups
Cr
Mo
Si
Ni
Fe
Ni
Mn
C
Co
27.13 27.9
5.53 6.51
0.63 0.7
0.13 0.24
0.21 0.7
0.13 0.24
0.79 0.39
0.05 0.31
65.40 63.01
Particular layer
Multilayer (TiN/ TiN CrN CrN) 3 Monolayer CrN CrN
Atmosphere Pressure (Pa) N2 N2 N2
2.2. Surface characterization Scanning Electron Microscopy observations of the cross-sections of the samples were carried out in a JEOL JSM-6510LV apparatus operated in backscattering mode and the thickness of the coatings was determined. Morphology of the treated surfaces was analyzed using the secondary electron mode. Surface roughness was determined by measuring the Ra value by means of AFM using a Quesant Instruments Corporation Q-Scope 250. The measurements were performed applying a square area of 50 mm using contact mode. The hardness and Young’s modulus of the samples were measured by means of the indentation method. To achieve this aim, the nanohardness tester (NHT) made by the CSEM was used. Measurements were carried out with the Berkovich indentor in a single cycle without stopping under the following conditions: max depth¼250 nm, loading rate¼60 mN/min and unloading rate¼60 mN/min. To eliminate the influence of the substrate material on the result of the measurement of Young’s modulus, the range of the indentor’s penetration depth (g) was limited to the value of gr0.1d (d-layer thickness). The hardness and Young’s modulus were determined by the Oliver and Pharr method [26]. 2.3. Scratch test The measurements of coating adhesion were carried out using the scratch-test method by means of a Revetest (CSEM) scratchtester. The scratch indenter was a diamond stylus with a spherical tip having a radius of 200 mm. For the 10 mm scratch length, the applied load was progressively increased from 0 to 100 N at a rate of 10 N/mm. Three scratch tests were performed for each sample and an average value of the critical loads was obtained. After the test, a critical load, where failure occurred, was determined by observation of the scratch track using an optical microscope Nikon MM40.The friction force and acoustic emission signals were recorded during the scratch tests and later compared with the results of microscope observations of the scratches. The scratch resistant properties of the nitride layer and the coatings were then quantified in terms of the critical loads corresponding to the failure modes as defined as follows: first crack (LC0), beginning of the material removal (LC1), first breakthrough or lose of adhesion (LC2) and total material removal or worn out (LC3) [27]. 2.4. Hip joint simulator test Wear testing was performed using the four-station FIME II hip wear simulation rig [28]. During tests, the implants specimens were mounted in the normal orientation with the acetabular cups above the femoral heads. Femoral heads were mounted in a base device with rotational movement at an angle of 231 to the horizontal plane and rotated about a vertical axis at a frequency of 1.2 Hz achieving 7231 for flexion–extension, 7231 for abduction–adduction and 77.51 for internal–external rotation. A single axis Paul-type loading pattern [29] with a maximum load peak of 2 kN was applied through the vertical axis of the simulator. Table 3 The average values of surface roughness (Ra) of the heads and diametral clearance of head and cup components.
Table 2 Parameters used in PAPVD coating process. Coating
235
1.2 3.5 3.5
UBias (V)
ICathode (A)
200 80 200 80 200 70
Sample
Mean Diametral Clearance cd (mm)
Average surface roughness Ra (nm)
MOM (n¼2) CrN–Metal (n¼2) (TiN/CrN) 3–Metal (n ¼2) DLC–Metal (n¼ 2)
50–89 65–49 33–39 96–64
10.07 2.52 51.07 2.14 47.07 13.68 20.57 6.18
Time (min) 30 30 120
236
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
Fetal bovine serum solution (25% 72%) diluted with deionized water was used as a lubricant, according to ISO 14242-1 standard [30]. The resulting protein content of the bovine serum was 15.2 g/l. No additives like antibacterial or antifungal agents were used in the solution. The volume of lubricant, in which each joint was immersed during the test, was 150 ml. An automatic deionized water replenishment system compensated for the evaporation, preventing volume and concentration changes of the lubricant during the tests. The tests were conducted at controlled room temperature in a range of 25 71 1C. The specimens were tested up to 2 106 cycles. Gravimetric measurement of wear and lubricant change was undertaken every 333,000 cycles during the first million cycles, and then every 500 000. Prior to gravimetric measurement of wear, specimens were brushed with a soft nylon and washed with detergent, then the specimens were ultrasonically cleaned [31]. Wear was determined gravimetrically using an analytical balance (accuracy of 0.1 mg). Weight loss was converted into volume loss using specific densities 8.33 g/cm3 for Co–Cr, 6.0 g/cm3 for CrN, 5.4 g/cm3 for multilayer coating (TiN/CrN) 3 and, 2.5 g/cm3 for DLC to compare the wear between different materials. The surface condition of the specimens was examined at 1 106 cycles by SEM in the secondary electron mode as well as qualitative elemental analysis using an energy dispersive spectrometer (EDS). Three-dimentional surface topography analysis of the eight MOM components were carried out at the end of test using a confocal microscopy 3D Axio CSM 700.
is shown in Fig. 1a. The configuration of this coating consisted of three TiN layers intercalated with three CrN layers with a thickness of 1 and 0.25 mm respectively, resulting in a 3.75 mm thickness coating. Fig. 1b shows the cross-sectional image of the CrN coating with a thickness of 3.25 mm deposited on Co–Cr alloy. Fig. 1c shows the cross-sectional image of the DLC coating with a thickness of 2 mm. The basic parameter and mechanical properties of the conditions mentioned above are summarized on in Table 4 and compared with the sample Co–Cr untreated. 3.2. Morphology analysis of the coated femoral heads The morphology of the coatings was analyzed by scanning electron microscopy. SEM micrographs of the surface modified femoral heads with CrN and (TiN/CrN) 3 are shown in Fig. 2a and b, respectively. Small rounded pits and droplets were found on these surfaces. Surface of the DLC coating (Fig. 2c) showed a smooth finish and just a few number of localized defects were found. 3.3. AFM analysis Topography of the coatings was analyzed by AFM. Fig. 3 shows the three-dimensional AFM images of the coated femoral heads.
Table 4 Parameters and properties of the different conditions of CoCrMo alloy samples.
3. Experimental results 3.1. Configuration and coating parameters Cross-sectional SEM images from all the surface conditions are shown on Fig. 1. Multilayer (TiN/CrN) 3 coating micrograph
Sample
Thickness (mm)
Hardness (GPa)
Young’s modulus (GPa)
Co–Cr untreated CrN (TiN/CrN) 3 DLC
— 3.25 3.75 2.00
8.31 70.21 19.40 71.62 26.62 71.69 14.10 70.94
267 7 11.80 283 7 28.32 384 7 36.42 142 7 7.35
Fig. 1. Cross-sectional SEM micrograph of: (a) multilayer coating (TiN/CrN) 3, (b) CrN coating, and (c) DLC coating.
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
237
Fig. 2. SEM micrographs of femoral heads coated with: (a) CrN, (b) multilayer (TiN/CrN) 3 and (c) DLC.
For comparison, a three-dimensional AFM image of an untreated Co–Cr head was included (Fig. 3a). The roughness values presented in Table 3 are supported by the AFM images in Fig. 3, where the uncoated femoral head (Fig. 3a) showed the smoother surface. Femoral head DLC-coated (Fig. 3d) presents a homogenous surface where some grooves can be detected. These grooves probably remain from the polishing process. Femoral heads coated with multilayer (TiN/CrN) 3 and CrN (Fig. 3b and c) present rough surfaces. Pits observed on the surface of the (TiN/CrN) 3 coating in Fig. 2a are also detected on the threedimensional AFM image (Fig. 3b). 3.4. Scratch test The scratch resistant properties of samples were quantified in terms of the critical loads corresponding to the failure modes mentioned before. The mean values of critical loads corresponding to the different failure modes are summarized in Table 5. According to the results, multilayer (TiN/CrN) 3 and CrN and coatings showed similar values for the critical loads during the scratch test. Worn out of the coating for CrN and multilayer (TiN/ CrN) 3 occurred at 60 and 58.50 N respectively. On the other hand, DLC coating showed very low adhesion to the Co–Cr substrate, where the first breakthrough occurred at 4.8 N and worn out of the coating at 18.79 N. 3.5. Tribological results The mean wear results for the half coated prosthesis together with the MOM reference prostheses were recorded as a function of the number of loading cycles. According to the results from the wear tests, wear results of the femoral heads and cups are presented separately in Figs. 4 and 5. Mean volumetric wear of the coated femoral heads are shown in Fig. 4. Femoral heads coated with the PVD multilayer system (TiN/CrN) 3 and CrN
single layer showed a very low wear rate, transition from running in to steady state was not identified. Femoral heads DLC-coated showed a running in wear rate very similar to the multilayer system (TiN/CrN) 3 and CrN-coated and heads. However, a transition on wear behavior was identified near to 0.6 106 cycles. In the case of the uncoated heads it was observed the typical trend of wear for metal-on-metal hip joint implant with a high running-in wear rate followed by a low steady state wear rate. Mean volumetric wear of the cups is shown in Fig. 5. Cups tested against heads coated with multilayer system (TiN/CrN) 3 and CrN single layer showed the highest wear rates. These cups presented a running in wear rate 85 times higher than the cups tested against heads coated with DLC and uncoated MOM heads. Cups tested against femoral heads coated with DLC and untreated showed the lowest wear rate. 3.6. Wear surfaces Scanning electron micrographs depicting the changes in surface condition of the femoral heads and acetabular cups specimens during the simulation tests are presented in Figs. 6 and 7. For the MOM prostheses (Figs. 6 and 7a) micropittings were observed in the surface of both components. In addition, evidence of carbide detachments was found in the femoral heads (Fig. 6a). Fig. 6b and c show SEM micrographs of the femoral heads surface modified with multilayer (TiN/CrN) 3 and CrN coatings, remaining undamaged after the wear tests supporting the low wear rate, almost negligible, presented on Fig. 3. On the other hand, the cups tested against these two conditions (Fig. 7b and c) presented severe damage showing evidence of carbide detachments and abrasion grooves. Damage of femoral head DLC-coated is presented on Fig. 6d showing mild abrasion grooves. On the other hand, slight adhesion in the cups (Fig. 7d). Adhesion of organic protein films was observed for all conditions, (Fig. 8).
238
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
Fig. 3. Three-dimensional AFM images of prototype femoral heads: (a) untreated; surface modified with, (b) multilayer (TiN/CrN) 3, (c) CrN and (d) DLC coating.
Table 5 Critical loads of the different Co–Cr surface conditions corresponding to the failure modes occurring during the scratch test. Sample
Critical loads LC0 (N) First crack
CrN (TiN/CrN) 3 DLC
LC1 (N) Beginning of material removal
5.50 70.57 22.48 70.61 7.96 70.68 12.90 71.25 3.30 70.39 4.80 70.41
LC2 (N) First breakthrough
LC3 (N) Worn out
57.40 71.11 55.00 71.06 4.80 71.04
60.00 71.25 58.50 71.20 18.79 71.17
Fig. 5. Mean volumetric wear of the Co–Cr acetabular cups.
3.7. Surface topography analysis
Fig. 4. Mean volumetric wear of the coated femoral heads.
Surface topography of the samples after wear test is shown in Fig. 9. Damage on the uncoated femoral head from the tribological pair MOM is shown in Fig. 9a. Deep scratches and part of a protein layer attached to the surface can be observed. The topography of the femoral heads coated with the multilayer system (TiN/ CrN) 3 and CrN single layer are shown in Fig. 9b and c. From these figures it can be seen that after 2 106 cycles of wear testing, the coatings remained undamaged. Surface topography of the DLC-coated femoral head is shown in Fig. 9d. From this figure some light scratches can be observed, some of them probably remaining from the polishing process before wear test.
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
239
Fig. 6. SEM micrographs of femoral heads worn surfaces: (a) uncoated, 800; (b) multilayer (TiN/CrN) 3 coated, 500; (c) CrN-coated and (d) DLC-coated.
4. Discussion In the present study, the tribological behavior of femoral heads surface modified with a multilayer coating system (TiN/CrN) 3 coating was investigated articulating against uncoated metallic acetabular cups using a hip joint simulator. For comparison, femoral heads coated with two of the most successful coatings in the recent years: CrN and DLC, were also included in the study and compared with a metal-on-metal bearing couple. In order to obtain similar diametral clearances in the all conditions, the MOM samples were manufactured following the same manufacture procedures and specifications; however changes in roughness were identified due to inherent properties of coating process, as can be seen on Table 3. In spite of these observations; this work is in accordance with the published significance of diametral clearance on the wear behavior of MOM hip bearings [32–34]. Femoral heads coated with the multilayer system (TiN/ CrN) 3 presented similar volumetric wear values than femoral heads CrN-coated (Fig. 4). In these two conditions it was not possible to identify the transition from the running in to the steady sate. For the CrN condition the results are in accordance with other authors [17,18]. In the case of the multilayer coating system (TiN/CrN) 3, surface damage was not identified; just slight scratches and protein films attached to the surface (Figs. 6 and 9b). Fisher et al. reported localized cohesive failures in femoral heads coated with TiN single layer [17–18]. However, in the present study cohesive failures were not detected on the multilayer system (TiN/CrN) 3. This could be due to the effect of the multilayers. According to the literature, some authors ascribe the enhancement of the tribological properties of the multilayer coatings to a
modification of their mechanical properties such as hardness or the H/E ratio [35,36]. To verify this statement, in the present study both hardness and elastic modulus were measured by nanoindentation. The investigated range of plastic depth was limited to 10% of the thickness of the coatings to reduce the influence of the substrate. Multilayer coating hardness was 26 GPa while for monolayer CrN coating was 19 GPa (Table 4). The hardening effect of the multilayer coatings can be attributed to the Hall– Petch effect, to the variations of shear modulus or on internal stresses [37–39]. From Table 4, it can be seen that the H/E ratio is almost the same and thus mechanical properties do not represent the key parameter to explain the enhanced tribological behavior. For other authors [40], the explanation lies in the cracks propagation. Mendebide et al. [35,36] found that monolayer coatings are subjected to decohesion at the grain boundaries, while multilayer structure induce a deviation of the cracks at the multiple interfaces, associated with a slight degradation. However, when the counter faces (cups) of the two conditions mentioned above were examined, it was observed severe abrasion and fatigue with presence of carbide detachments. This can be attributed to the high roughness (Ra value) presented by the PVD coatings, as shown in Table 3. These cups presented a running in wear rate 85 times higher than the cups tested against heads coated with DLC and uncoated heads with MOM contact. This suggests that these coatings can work better self mated. Further works will be necessary to study it. Femoral heads DLC-coated showed a running in wear rate very similar to the CrN and multilayer system (TiN/CrN) 3 coated heads. However near to 0.6 106 cycles, the wear rate increased considerably compared to the other two coatings. This behavior can be explained by de-adhesion coating which can be attributed to the poor coating adherence resistance showed in the scratch
240
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
Fig. 7. SEM micrographs of acetabular cups worn surfaces tested against femoral heads: (a) uncoated, 1000; (b) multilayer (TiN/CrN) 3 coated, 200; (c) CrN-coated, 200 and (d) DLC-coated.
Fig. 8. SEM micrographs of femoral heads worn surfaces showing organic layers due to protein precipitation: (a) multilayer system (TiN/CrN) 3 coating and (b) CrN coating.
test, Table 5. In spite of this phenomenon, in this condition the Co–Cr cups exhibited a low rate in contrast with the Co–Cr cups tested against CrN and multilayer system (TiN/CrN) 3. For the case of the uncoated heads it was observed the typical trend of wear for metal-on-metal hip joint implant with a high running-in wear rate followed by a low steady state wear rate [17]. From SEM analyses shown in Figs. 6 and 7a it can be observed abrasion, pullout of carbides and micropitting produced by fatigue wear. The deepness of the abrasion grooves can be observed on the surface topography analysis (Fig. 9a). For all conditions, adhesion of organic protein films was observed (Fig. 8); however the roll of this phenomenon in the wear behavior was not elucidated in this work.
5. Conclusions Based on the experimental results of this work where different coated femoral conditions were tested against high carbon Co–Cr cups in a hip simulator; the following main conclusions can be drawn:
(a) Femoral heads:
The multilayer system (TiN/CrN) 3 showed similar volumetric mass loss than CrN coatings. In addition, both coatings presented similar critical loads during the scratch test.
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
241
Fig. 9. Surface topography for the femoral heads: (a) uncoated, (b) multilayer (TiN/CrN) 3 coated, (c) CrN-coated and (d) DLC-coated.
For the MOM condition, the femoral head showed a high running-in wear rate with a transition to steady state. The wear mechanisms in this condition were abrasion and fatigue wear with presence of micropitting. (b) Cups:
The high carbon cups exhibited a severe damage by abrasion and fatigue wear when tested against multilayer system (TiN/CrN) 3 and CrN coatings. These cups presented a running in wear rate 85 times higher than the cups tested against heads coated with DLC and uncoated MOM heads. Multilayer (TiN/CrN) 3 coatings are a promising solution to improve the wear resistance of hip prostheses. These coatings are hard enough to avoid abrasive wear and strain tolerant, in order to prevent crack propagation, therefore avoiding fracture or delamination of the coating. Considering the total volumetric mass loss of the cup and head; the femoral head DLC-coated evaluated against high carbon cups showed the lowest volumetric wear.
References [1] S. Scholes, S. Green, A. Unsworth, The wear of metal-on-metal total hip prostheses measured in a hip simulator, Proceedings of the Institution of Mechanical Engineers Part H 215 (2001) 523–530. [2] H. Amstutz, P. Grigoris, Metal on metal bearing in hip arthroplasty, Clinical Orthopaedics 329 (1996) 11–34. [3] G. McKee, Total hip replacement—past, present and future, Biomaterials 3 (1982) 130–135. [4] P. Walker, B. Gold, The tribology (friction, lubrication and wear) of all metal artificial hip joints, Wear 17 (1971) 285–299. [5] S. Jacobsson, K. Djerf, O. Wahlstrom, 20 years result of McKee-Farrar vs. Charnley prosthesis, Clinical Orthopaedics 329 (1996) 60–68. [6] B. Weber, Experience with metasul total hip bearing system, Clinical Orthopaedics 329 (1996) 69–77. [7] B. Weightman, I. Paul, R. Rose, S. Simon, E. Radin, A comparative study of total hip replacement prostheses, Journal of Biomechanics 6 (1973) 299–311.
[8] R. McCalden, D. Howie, L. Ward, Observation of the long term behavior of retrieved McKee-Farrar total hip replacements implants, Transactions of the Orthopedic Research Society 20 (1995) 242. [9] H. McKellop, P. Campbell, S. Park, Wear of retrieved metal–metal implants after two decades of clinical use, Transactions of the Combined Orthopaedic Research Societies 2 (1995) 228. [10] H. McKellop, S. Park, R. Chiesa, In vivo wear of three types of metal on metal hip prostheses during two decades of use, Clinical Orthopaedics 329 (1996) 128–140. [11] M. Schmidth, H. Weber, R. Schon, Cobalt Chromium molybdenum metal combinations for modular hip prostheses, Clinical Orthopaedics and Related Research 329 (1996) 35–47. [12] K. Brummitt, C. Hardaker, Estimation of wear in total hip replacement using a ten station hip simulator, Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 210 (1996) 187–190. [13] J. Firkins, J. Tipper, M. Saadatzadeh, E. Ingham, M. Stone, R. Farrar, J. Fisher, Quantitative analysis of wear and wear debris from metal-on-metal hip prostheses tested in a physiological hip joint simulator, Bio-Medical Materials and Engineering 11 (2001) 143–157. [14] J. Jacobs, A. Skipor, P. Doorn, P. Campbel, T. Schmalzried, C. Amstutz, Cobalt and chromium concentrations in patients with metal on metal total hip replacements, Clinical Orthopaedics 329 (1996) S256–S263. [15] P. Doorn, J. Mirra, P. Campbell, H. Amstutz, Tissue reaction to metal on metal total hip prostheses, Clinical Orthopaedics 329 (1996) S187–S205. [16] M. Germain, J. Matthews, M. Stone, J. Fisher, E. Ingham, The effect of clinically relevant cobalt–chrome particles on the viability of human peripheral blood mononuclear cell and fibroblast in vitro, in: Proceedings of the Transactions of the 16th meeting of the European Society for Biomaterials , 2001, p. 76. [17] J. Fisher, X.Q. Hu, J.L. Tipper, T.D. Stewart, S. Williams, M.H. Stone, C. Davies, P. Hatto, J. Bolton, M. Riley, C. Hardaker, G.H. Isaac, G. Berry, E. Ingham, An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface engineered femoral heads, Proceedings of the Institution of Mechanical Engineers—Part H 216 (2002) 219–230. [18] J. Fisher, X.Q. Hu, T.D. Stewart, S. Williams, J.L. Tipper, E. Ingham, M.H. Stone, C. Davies, P. Hatto, J. Bolton, M. Riley, C. Herdaker, G.H. Isaac, G. Berry, Wear of surface engineered metal-on-metal hip prostheses, Journal of Materials Science: Materials in Medicine 15 (2004) 225–235. ¨ [19] G. Thorwarth, C.V. Falub, U. Muller, B. Weisse, C. Voisard, M. Tobler, R. Hauert, Tribological behavior of DLC-coated articulating joint implants, Acta Biomaterialia 6 (2010) 2335–2341. [20] J. Musil, Hard and superhard nanocomposite coatings, Surface & Coatings Technology 125 (2000) 322. ¨ [21] Q. Luo, W. Rainforth, W. Munz, TEM observations of wear mechanisms of TiAlCrN and TiAlN/CrN coatings grown by combined steered-arc/unbalanced magnetron deposition, Wear 225–229 (1999) 74. [22] A. Leyland, A. Matthews, Spatial characteristics of discharge phenomena in plasma electrolytic oxidation of aluminium alloy, Surface & Coatings Technology 177–178 (2004) 317–324.
242
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
[23] J. Ortega, M. Hernandez, V. Ventura, R. Michalczewski, J. Smolik, M. Szczerek, Tribological and corrosion testing of surface engineered surgical grade CoCrMo alloy, Wear 271 (2011) 2125–2131. [24] C. Mendibide, P. Steyer, J. Millet, Formation of a semiconductive surface film on nanomultilayered TiN/CrN coatings and its correlation with corrosion protection of steel, Surface and Coatings Technology 200 (2005) 109. [25] F75-98 Standard Specification for Cobalt-28 Chromium-6 Molybdenum Casting Alloy and Cast Products for Surgical Implants (UNSR30075), ASTM Book of standards, vol. 13.01, September 2001. [26] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, Journal of Materials Research 7 (1992) 1564–1583. [27] Y. He, I. Apachitei, J. Zhou, T. Walstock, J. Duszczyk, Effect of prior plasma nitriding applied to a hot-work tool steel on the scratch-resistant properties of PACVD TiBN and TiCN coatings, Surface & Coatings Technology 201 (2006) 2534–2539. [28] J. Ortega, M. Hernandez, A. Perez, R. Mercado, Development of a hip wear simulation rig including micro-separation, Wear 263 (2007) 1527–1532. [29] J. Paul, Force actions transmitted by joints in the human body, Proceedings of the Royal Society of London Series B 192 (1976) 163–172. [30] ISO/DIS 14242-1 Draft International Standard, 2001. Implants for surgery—wear of total hip joint prostheses—Part 1: loading and displacement parameters for wear-testing machines and corresponding environmental conditions for test. [31] M. Hernandez, R. Mercado, A. Perez, D. Martinez, M. Cantu, Wear of cast metal– metal pairs for total replacement hip prostheses, Wear 259 (2005) 958–963. [32] D. Dowson, C. Hardaker, M. Flett, G. Issac, A hip joint simulator study of the performance of metal-on-metal joints, The Journal of Arthroplasty 19 (2004) 118–123.
[33] G. Isaac, J. Thompson, S. Williams, J. Fisher, Metal-on-metal bearings surfaces: materials, manufacture, design, optimization, and alternatives, Proceedings of the Institution of Mechanical Engineers—Part H:Journal of Engineering in Medicine 220 (2006) 119–133. [34] F. Liu, Z. Jin, P. Roberts, P. Grigoris, Importance of head diameter, clearance, and cup wall thickness in elastohydrodynamic lubrication analysis of metalon-metal hip resurfacing prostheses, Proceedings of the Institution of Mechanical Engineers—Part H:Journal of Engineering in Medicine 220 (2006) 695–704. [35] C. Mendibide, J. Fontaine, P. Steyer, C. Esnouf, Dry sliding wear model of nanometer scale multilayered TiN/CrN PVD hard coatings, Tribology Letters 17 (2004) 779. [36] C. Mendibide, P. Steyer, J. Fontaine, P. Goudeau, Improvement of the tribological behavior of PVD nanostratified TiN/CrN coatings—an explanation, Surface and Coatings Technology 201 (2006) 4119–4124. [37] M. Shinn, S.A. Barnett, Effect of superlattice layer elastic moduli on hardness, Applied Physics Letters 64 (1994) 61. [38] X. Junhua, L. Geyang, G. Mingyan, The microstructure and mechanical properties of TaN/TiN and TaWN/TiN superlattice films, Thin Solid Films 370 (2000) 45. [39] Q. Yang, C. He, L.R. Zhao, J.-P. Immarigeon, Preferred orientation and hardness enhancement of TiN/CrN superlattice coatings deposited by reactive magnetron sputtering, Scripta Materialia 46 (2002) 293. ¨ [40] Q. Luo, W.M. Rainforth, W.-D. Munz, TEM observations of wear mechanisms of TiAlCrN and TiAlN/CrN coatings grown by combined steered-arc/unbalanced magnetron deposition, Wear 225–229 (1999) 74.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Biotribological study of multilayer coated metal-on-metal hip prostheses in a hip joint simulator J.A. Ortega-Saenz, M. Alvarez-Vera, M.A.L. Hernandez-Rodriguez n Universidad Autonoma de Nuevo Leon, FIME, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2012 Received in revised form 9 January 2013 Accepted 10 January 2013 Available online 23 January 2013
Metal-on-metal (MOM) hip joint bearings have demonstrated low wear rates and nowadays this contact pair has being considered as an alternative to metal-on-polymer (MOP) joint replacements. However, wear of MOM joints is a concern due to the toxicity and biological reaction of wear debris and metallic corrosion. This has motivated to investigate the possibility to apply thin hard coatings on metallic heads to reduce the wear and metallic ion release. The aim of the present study was to investigate the wear properties of metal-on-metal hip prostheses with surface engineered femoral heads using a multilayer coating (TiN/CrN) 3, in comparison with metal-on-metal pairs in a hip joint simulator. Different surface PVD coatings were applied on surgical grade wrought cobalt–chromium alloy femoral heads: multilayer (TiN/CrN) 3, CrN single layer and diamond-like carbon (DLC). These femoral heads were tested against high carbon content cast cobalt–chromium alloy acetabular cups using a three-axial multi-station hip joint simulator (FIME II). During the wear tests three directions of motion were applied with the following amplitudes: flexion–extension (FE) 7 231, abduction– adduction (AA) 7 231 and internal–external (IER) 7 81. All components were tested at 1.2 Hz under a Paul-type loading curve and bovine calf serum solution as lubricant. Results showed that both; the PVD coatings protects the femoral heads reducing wear up to 5 times in the case of the DLC coating and 28 and 55 times in the case of (TiN/CrN) 3 and CrN respectively compared with the MOM femoral heads. & 2013 Elsevier B.V. All rights reserved.
Keywords: Biotribology Co-Cr alloy Metal-on-Metal PVD coatings Multilayer coatings Hip joint simulator
1. Introduction The total hip arthroplastic surgery was a major medical advance of the 20th century. The materials used in this medical application must possess satisfactory mechanical properties such as stiffness and fatigue strength, wear and corrosion resistance, and biocompatibility. The first metal-on-metal (MOM) total hip prostheses implanted during the 1960s decade presented unsatisfactory short-term performance due to geometrical inaccuracies which led to high frictional forces and increased wear [1–5]. However, in some cases the implants lasted at least for two decades without osteolysis [2,5–7] and negligible wear [2,8–11]. In recent years, the use of second generation Co–Cr alloy metal-on-metal bearing joints in total hip arthroplasty surgery represents an attractive alternative to the traditional metalon-polyethylene pairs [12]. Despite the triblological pair metal-on-metal has proven to be more wear resistant than metal-on-polyethylene couple, the toxicity of metallic ions of cobalt and chromium released from wear particles from metal-on-metal hip prostheses into the
n
Corresponding author. Tel.: þ52 8114920375x5770; Fax: þ 52 8110523321. E-mail address: [email protected] (M.A.L. Hernandez-Rodriguez).
0043-1648/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2013.01.024
human body [13–16] is a concern which has motivated to look for alternatives to solve or diminish this problem. PVD coatings are well known for providing surfaces with improved tribological properties in terms of low coefficient of friction and high wear resistance. This technology has been brought to the field of surgical implants with promising results [17–19]. Nowadays, there has been great scientific and commercial interest in nanostructured coatings, like multilayer or nanolayered films [20]. The main idea is that the coating needs to be hard to avoid abrasive wear, but it also needs to be strain tolerant and tough, in order to prevent crack propagation, therefore avoiding fracture or delamination of the coating [21]. In consequence, multilayer films can be designed to show an improved wear resistance [22]. In a previous study, the authors have reported the improved wear resistance of a multilayer TiN/CrN coating deposited by plasma assisted phisycal vapor deposition on wrought Co–Cr substrates [23]. These films also exhibit improved corrosion resistance [23,24], making them good candidates for orthopedic applications. The main aim of the present study was to assess the tribological behavior of femoral heads surface modified with a multilayer TiN/CrN coating articulating against uncoated metallic acetabular cups in an anatomical hip joint simulator. Femoral heads coated with two of the most successful coatings in the
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
recent years: CrN and DLC, were also included in the study and compared with metal-on-metal Co–Cr alloy bearing couple.
2. Material and methods 2.1. Samples preparation For the present study, a total of eight prototype cups and heads implants with 31.5 mm diameter were manufactured. Femoral heads were manufactured from wrought Co–Cr alloy ASTM F1537-08 (BioDur CCM Plus) supplied by Carpenter Technology, whereas the acetabular cups were manufactured by the investment casting method complying with the chemical composition of the ASTM F75/98 [25]. The chemical composition of the alloys used in this investigation is shown in Table 1. Femoral heads were modified using three different coatings: a multilayer coating (TiN/CrN) 3, monolayer CrN and, DLC. The first two coatings were created by the plasma assisted physical deposition (PAPVD) method, whereas the DLC coating (a-C: H) was supplied by Balzers (BALINITs DLC). The thicknesses of the coatings were 3.25 mm, 3.75 mm and, 2 mm respectively. Multilayer (TiN/CrN) 3 and monolayer CrN coatings were deposited by means of an industrial multi-source vacuum device equipped with five sources. A double rotation planetarium fixture was used. The substrates were precleaned in an ultrasonic cleaning line with trichloroethylene, alkaline detergent and demineralized water. The final drying was performed in hot trichloroethylene vapor. Prior to deposition, the vacuum chamber was pumped down to 2 10 3 Pa to initiate the etching/heating stage. A current of 80 A, for each of the five cathodes in operation, and a negative substrate bias voltage of 950 V was used. No nitrogen was introduced into the chamber during this stage. The etching/heating stage was interrupted when a temperature of 400 1C, as measured with a pyrometer, was reached; whereupon the deposition stage was initiated. Coating deposition was performed with five sources in operation. Parameters used during the PAPVD process such as: nitrogen gas pressure, substrate bias voltage (UBias), cathode source current (ICathode), and deposition time are presented in Table 2. The total thickness of the coating was controlled by the deposition time. The samples were cooled down to less than 100 1C in an N2 atmosphere before venting the chamber. The components were machined and finished following controlled standard implant specifications. For this purpose a coordinate measurement machine (Mitutoyo QM-Measure 353) and surface profilometer (with 0.8 mm cut-off) were used to measure the diametral clearance (Cd), sphericity and roughness (Ra). These parameters for the eight bearings are given in Table 3. Table 1 Chemical composition (wt%) of the pair specimens used in this investigation.
Femoral heads Acetabular cups
Cr
Mo
Si
Ni
Fe
Ni
Mn
C
Co
27.13 27.9
5.53 6.51
0.63 0.7
0.13 0.24
0.21 0.7
0.13 0.24
0.79 0.39
0.05 0.31
65.40 63.01
Particular layer
Multilayer (TiN/ TiN CrN CrN) 3 Monolayer CrN CrN
Atmosphere Pressure (Pa) N2 N2 N2
2.2. Surface characterization Scanning Electron Microscopy observations of the cross-sections of the samples were carried out in a JEOL JSM-6510LV apparatus operated in backscattering mode and the thickness of the coatings was determined. Morphology of the treated surfaces was analyzed using the secondary electron mode. Surface roughness was determined by measuring the Ra value by means of AFM using a Quesant Instruments Corporation Q-Scope 250. The measurements were performed applying a square area of 50 mm using contact mode. The hardness and Young’s modulus of the samples were measured by means of the indentation method. To achieve this aim, the nanohardness tester (NHT) made by the CSEM was used. Measurements were carried out with the Berkovich indentor in a single cycle without stopping under the following conditions: max depth¼250 nm, loading rate¼60 mN/min and unloading rate¼60 mN/min. To eliminate the influence of the substrate material on the result of the measurement of Young’s modulus, the range of the indentor’s penetration depth (g) was limited to the value of gr0.1d (d-layer thickness). The hardness and Young’s modulus were determined by the Oliver and Pharr method [26]. 2.3. Scratch test The measurements of coating adhesion were carried out using the scratch-test method by means of a Revetest (CSEM) scratchtester. The scratch indenter was a diamond stylus with a spherical tip having a radius of 200 mm. For the 10 mm scratch length, the applied load was progressively increased from 0 to 100 N at a rate of 10 N/mm. Three scratch tests were performed for each sample and an average value of the critical loads was obtained. After the test, a critical load, where failure occurred, was determined by observation of the scratch track using an optical microscope Nikon MM40.The friction force and acoustic emission signals were recorded during the scratch tests and later compared with the results of microscope observations of the scratches. The scratch resistant properties of the nitride layer and the coatings were then quantified in terms of the critical loads corresponding to the failure modes as defined as follows: first crack (LC0), beginning of the material removal (LC1), first breakthrough or lose of adhesion (LC2) and total material removal or worn out (LC3) [27]. 2.4. Hip joint simulator test Wear testing was performed using the four-station FIME II hip wear simulation rig [28]. During tests, the implants specimens were mounted in the normal orientation with the acetabular cups above the femoral heads. Femoral heads were mounted in a base device with rotational movement at an angle of 231 to the horizontal plane and rotated about a vertical axis at a frequency of 1.2 Hz achieving 7231 for flexion–extension, 7231 for abduction–adduction and 77.51 for internal–external rotation. A single axis Paul-type loading pattern [29] with a maximum load peak of 2 kN was applied through the vertical axis of the simulator. Table 3 The average values of surface roughness (Ra) of the heads and diametral clearance of head and cup components.
Table 2 Parameters used in PAPVD coating process. Coating
235
1.2 3.5 3.5
UBias (V)
ICathode (A)
200 80 200 80 200 70
Sample
Mean Diametral Clearance cd (mm)
Average surface roughness Ra (nm)
MOM (n¼2) CrN–Metal (n¼2) (TiN/CrN) 3–Metal (n ¼2) DLC–Metal (n¼ 2)
50–89 65–49 33–39 96–64
10.07 2.52 51.07 2.14 47.07 13.68 20.57 6.18
Time (min) 30 30 120
236
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
Fetal bovine serum solution (25% 72%) diluted with deionized water was used as a lubricant, according to ISO 14242-1 standard [30]. The resulting protein content of the bovine serum was 15.2 g/l. No additives like antibacterial or antifungal agents were used in the solution. The volume of lubricant, in which each joint was immersed during the test, was 150 ml. An automatic deionized water replenishment system compensated for the evaporation, preventing volume and concentration changes of the lubricant during the tests. The tests were conducted at controlled room temperature in a range of 25 71 1C. The specimens were tested up to 2 106 cycles. Gravimetric measurement of wear and lubricant change was undertaken every 333,000 cycles during the first million cycles, and then every 500 000. Prior to gravimetric measurement of wear, specimens were brushed with a soft nylon and washed with detergent, then the specimens were ultrasonically cleaned [31]. Wear was determined gravimetrically using an analytical balance (accuracy of 0.1 mg). Weight loss was converted into volume loss using specific densities 8.33 g/cm3 for Co–Cr, 6.0 g/cm3 for CrN, 5.4 g/cm3 for multilayer coating (TiN/CrN) 3 and, 2.5 g/cm3 for DLC to compare the wear between different materials. The surface condition of the specimens was examined at 1 106 cycles by SEM in the secondary electron mode as well as qualitative elemental analysis using an energy dispersive spectrometer (EDS). Three-dimentional surface topography analysis of the eight MOM components were carried out at the end of test using a confocal microscopy 3D Axio CSM 700.
is shown in Fig. 1a. The configuration of this coating consisted of three TiN layers intercalated with three CrN layers with a thickness of 1 and 0.25 mm respectively, resulting in a 3.75 mm thickness coating. Fig. 1b shows the cross-sectional image of the CrN coating with a thickness of 3.25 mm deposited on Co–Cr alloy. Fig. 1c shows the cross-sectional image of the DLC coating with a thickness of 2 mm. The basic parameter and mechanical properties of the conditions mentioned above are summarized on in Table 4 and compared with the sample Co–Cr untreated. 3.2. Morphology analysis of the coated femoral heads The morphology of the coatings was analyzed by scanning electron microscopy. SEM micrographs of the surface modified femoral heads with CrN and (TiN/CrN) 3 are shown in Fig. 2a and b, respectively. Small rounded pits and droplets were found on these surfaces. Surface of the DLC coating (Fig. 2c) showed a smooth finish and just a few number of localized defects were found. 3.3. AFM analysis Topography of the coatings was analyzed by AFM. Fig. 3 shows the three-dimensional AFM images of the coated femoral heads.
Table 4 Parameters and properties of the different conditions of CoCrMo alloy samples.
3. Experimental results 3.1. Configuration and coating parameters Cross-sectional SEM images from all the surface conditions are shown on Fig. 1. Multilayer (TiN/CrN) 3 coating micrograph
Sample
Thickness (mm)
Hardness (GPa)
Young’s modulus (GPa)
Co–Cr untreated CrN (TiN/CrN) 3 DLC
— 3.25 3.75 2.00
8.31 70.21 19.40 71.62 26.62 71.69 14.10 70.94
267 7 11.80 283 7 28.32 384 7 36.42 142 7 7.35
Fig. 1. Cross-sectional SEM micrograph of: (a) multilayer coating (TiN/CrN) 3, (b) CrN coating, and (c) DLC coating.
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
237
Fig. 2. SEM micrographs of femoral heads coated with: (a) CrN, (b) multilayer (TiN/CrN) 3 and (c) DLC.
For comparison, a three-dimensional AFM image of an untreated Co–Cr head was included (Fig. 3a). The roughness values presented in Table 3 are supported by the AFM images in Fig. 3, where the uncoated femoral head (Fig. 3a) showed the smoother surface. Femoral head DLC-coated (Fig. 3d) presents a homogenous surface where some grooves can be detected. These grooves probably remain from the polishing process. Femoral heads coated with multilayer (TiN/CrN) 3 and CrN (Fig. 3b and c) present rough surfaces. Pits observed on the surface of the (TiN/CrN) 3 coating in Fig. 2a are also detected on the threedimensional AFM image (Fig. 3b). 3.4. Scratch test The scratch resistant properties of samples were quantified in terms of the critical loads corresponding to the failure modes mentioned before. The mean values of critical loads corresponding to the different failure modes are summarized in Table 5. According to the results, multilayer (TiN/CrN) 3 and CrN and coatings showed similar values for the critical loads during the scratch test. Worn out of the coating for CrN and multilayer (TiN/ CrN) 3 occurred at 60 and 58.50 N respectively. On the other hand, DLC coating showed very low adhesion to the Co–Cr substrate, where the first breakthrough occurred at 4.8 N and worn out of the coating at 18.79 N. 3.5. Tribological results The mean wear results for the half coated prosthesis together with the MOM reference prostheses were recorded as a function of the number of loading cycles. According to the results from the wear tests, wear results of the femoral heads and cups are presented separately in Figs. 4 and 5. Mean volumetric wear of the coated femoral heads are shown in Fig. 4. Femoral heads coated with the PVD multilayer system (TiN/CrN) 3 and CrN
single layer showed a very low wear rate, transition from running in to steady state was not identified. Femoral heads DLC-coated showed a running in wear rate very similar to the multilayer system (TiN/CrN) 3 and CrN-coated and heads. However, a transition on wear behavior was identified near to 0.6 106 cycles. In the case of the uncoated heads it was observed the typical trend of wear for metal-on-metal hip joint implant with a high running-in wear rate followed by a low steady state wear rate. Mean volumetric wear of the cups is shown in Fig. 5. Cups tested against heads coated with multilayer system (TiN/CrN) 3 and CrN single layer showed the highest wear rates. These cups presented a running in wear rate 85 times higher than the cups tested against heads coated with DLC and uncoated MOM heads. Cups tested against femoral heads coated with DLC and untreated showed the lowest wear rate. 3.6. Wear surfaces Scanning electron micrographs depicting the changes in surface condition of the femoral heads and acetabular cups specimens during the simulation tests are presented in Figs. 6 and 7. For the MOM prostheses (Figs. 6 and 7a) micropittings were observed in the surface of both components. In addition, evidence of carbide detachments was found in the femoral heads (Fig. 6a). Fig. 6b and c show SEM micrographs of the femoral heads surface modified with multilayer (TiN/CrN) 3 and CrN coatings, remaining undamaged after the wear tests supporting the low wear rate, almost negligible, presented on Fig. 3. On the other hand, the cups tested against these two conditions (Fig. 7b and c) presented severe damage showing evidence of carbide detachments and abrasion grooves. Damage of femoral head DLC-coated is presented on Fig. 6d showing mild abrasion grooves. On the other hand, slight adhesion in the cups (Fig. 7d). Adhesion of organic protein films was observed for all conditions, (Fig. 8).
238
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
Fig. 3. Three-dimensional AFM images of prototype femoral heads: (a) untreated; surface modified with, (b) multilayer (TiN/CrN) 3, (c) CrN and (d) DLC coating.
Table 5 Critical loads of the different Co–Cr surface conditions corresponding to the failure modes occurring during the scratch test. Sample
Critical loads LC0 (N) First crack
CrN (TiN/CrN) 3 DLC
LC1 (N) Beginning of material removal
5.50 70.57 22.48 70.61 7.96 70.68 12.90 71.25 3.30 70.39 4.80 70.41
LC2 (N) First breakthrough
LC3 (N) Worn out
57.40 71.11 55.00 71.06 4.80 71.04
60.00 71.25 58.50 71.20 18.79 71.17
Fig. 5. Mean volumetric wear of the Co–Cr acetabular cups.
3.7. Surface topography analysis
Fig. 4. Mean volumetric wear of the coated femoral heads.
Surface topography of the samples after wear test is shown in Fig. 9. Damage on the uncoated femoral head from the tribological pair MOM is shown in Fig. 9a. Deep scratches and part of a protein layer attached to the surface can be observed. The topography of the femoral heads coated with the multilayer system (TiN/ CrN) 3 and CrN single layer are shown in Fig. 9b and c. From these figures it can be seen that after 2 106 cycles of wear testing, the coatings remained undamaged. Surface topography of the DLC-coated femoral head is shown in Fig. 9d. From this figure some light scratches can be observed, some of them probably remaining from the polishing process before wear test.
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
239
Fig. 6. SEM micrographs of femoral heads worn surfaces: (a) uncoated, 800; (b) multilayer (TiN/CrN) 3 coated, 500; (c) CrN-coated and (d) DLC-coated.
4. Discussion In the present study, the tribological behavior of femoral heads surface modified with a multilayer coating system (TiN/CrN) 3 coating was investigated articulating against uncoated metallic acetabular cups using a hip joint simulator. For comparison, femoral heads coated with two of the most successful coatings in the recent years: CrN and DLC, were also included in the study and compared with a metal-on-metal bearing couple. In order to obtain similar diametral clearances in the all conditions, the MOM samples were manufactured following the same manufacture procedures and specifications; however changes in roughness were identified due to inherent properties of coating process, as can be seen on Table 3. In spite of these observations; this work is in accordance with the published significance of diametral clearance on the wear behavior of MOM hip bearings [32–34]. Femoral heads coated with the multilayer system (TiN/ CrN) 3 presented similar volumetric wear values than femoral heads CrN-coated (Fig. 4). In these two conditions it was not possible to identify the transition from the running in to the steady sate. For the CrN condition the results are in accordance with other authors [17,18]. In the case of the multilayer coating system (TiN/CrN) 3, surface damage was not identified; just slight scratches and protein films attached to the surface (Figs. 6 and 9b). Fisher et al. reported localized cohesive failures in femoral heads coated with TiN single layer [17–18]. However, in the present study cohesive failures were not detected on the multilayer system (TiN/CrN) 3. This could be due to the effect of the multilayers. According to the literature, some authors ascribe the enhancement of the tribological properties of the multilayer coatings to a
modification of their mechanical properties such as hardness or the H/E ratio [35,36]. To verify this statement, in the present study both hardness and elastic modulus were measured by nanoindentation. The investigated range of plastic depth was limited to 10% of the thickness of the coatings to reduce the influence of the substrate. Multilayer coating hardness was 26 GPa while for monolayer CrN coating was 19 GPa (Table 4). The hardening effect of the multilayer coatings can be attributed to the Hall– Petch effect, to the variations of shear modulus or on internal stresses [37–39]. From Table 4, it can be seen that the H/E ratio is almost the same and thus mechanical properties do not represent the key parameter to explain the enhanced tribological behavior. For other authors [40], the explanation lies in the cracks propagation. Mendebide et al. [35,36] found that monolayer coatings are subjected to decohesion at the grain boundaries, while multilayer structure induce a deviation of the cracks at the multiple interfaces, associated with a slight degradation. However, when the counter faces (cups) of the two conditions mentioned above were examined, it was observed severe abrasion and fatigue with presence of carbide detachments. This can be attributed to the high roughness (Ra value) presented by the PVD coatings, as shown in Table 3. These cups presented a running in wear rate 85 times higher than the cups tested against heads coated with DLC and uncoated heads with MOM contact. This suggests that these coatings can work better self mated. Further works will be necessary to study it. Femoral heads DLC-coated showed a running in wear rate very similar to the CrN and multilayer system (TiN/CrN) 3 coated heads. However near to 0.6 106 cycles, the wear rate increased considerably compared to the other two coatings. This behavior can be explained by de-adhesion coating which can be attributed to the poor coating adherence resistance showed in the scratch
240
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
Fig. 7. SEM micrographs of acetabular cups worn surfaces tested against femoral heads: (a) uncoated, 1000; (b) multilayer (TiN/CrN) 3 coated, 200; (c) CrN-coated, 200 and (d) DLC-coated.
Fig. 8. SEM micrographs of femoral heads worn surfaces showing organic layers due to protein precipitation: (a) multilayer system (TiN/CrN) 3 coating and (b) CrN coating.
test, Table 5. In spite of this phenomenon, in this condition the Co–Cr cups exhibited a low rate in contrast with the Co–Cr cups tested against CrN and multilayer system (TiN/CrN) 3. For the case of the uncoated heads it was observed the typical trend of wear for metal-on-metal hip joint implant with a high running-in wear rate followed by a low steady state wear rate [17]. From SEM analyses shown in Figs. 6 and 7a it can be observed abrasion, pullout of carbides and micropitting produced by fatigue wear. The deepness of the abrasion grooves can be observed on the surface topography analysis (Fig. 9a). For all conditions, adhesion of organic protein films was observed (Fig. 8); however the roll of this phenomenon in the wear behavior was not elucidated in this work.
5. Conclusions Based on the experimental results of this work where different coated femoral conditions were tested against high carbon Co–Cr cups in a hip simulator; the following main conclusions can be drawn:
(a) Femoral heads:
The multilayer system (TiN/CrN) 3 showed similar volumetric mass loss than CrN coatings. In addition, both coatings presented similar critical loads during the scratch test.
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
241
Fig. 9. Surface topography for the femoral heads: (a) uncoated, (b) multilayer (TiN/CrN) 3 coated, (c) CrN-coated and (d) DLC-coated.
For the MOM condition, the femoral head showed a high running-in wear rate with a transition to steady state. The wear mechanisms in this condition were abrasion and fatigue wear with presence of micropitting. (b) Cups:
The high carbon cups exhibited a severe damage by abrasion and fatigue wear when tested against multilayer system (TiN/CrN) 3 and CrN coatings. These cups presented a running in wear rate 85 times higher than the cups tested against heads coated with DLC and uncoated MOM heads. Multilayer (TiN/CrN) 3 coatings are a promising solution to improve the wear resistance of hip prostheses. These coatings are hard enough to avoid abrasive wear and strain tolerant, in order to prevent crack propagation, therefore avoiding fracture or delamination of the coating. Considering the total volumetric mass loss of the cup and head; the femoral head DLC-coated evaluated against high carbon cups showed the lowest volumetric wear.
References [1] S. Scholes, S. Green, A. Unsworth, The wear of metal-on-metal total hip prostheses measured in a hip simulator, Proceedings of the Institution of Mechanical Engineers Part H 215 (2001) 523–530. [2] H. Amstutz, P. Grigoris, Metal on metal bearing in hip arthroplasty, Clinical Orthopaedics 329 (1996) 11–34. [3] G. McKee, Total hip replacement—past, present and future, Biomaterials 3 (1982) 130–135. [4] P. Walker, B. Gold, The tribology (friction, lubrication and wear) of all metal artificial hip joints, Wear 17 (1971) 285–299. [5] S. Jacobsson, K. Djerf, O. Wahlstrom, 20 years result of McKee-Farrar vs. Charnley prosthesis, Clinical Orthopaedics 329 (1996) 60–68. [6] B. Weber, Experience with metasul total hip bearing system, Clinical Orthopaedics 329 (1996) 69–77. [7] B. Weightman, I. Paul, R. Rose, S. Simon, E. Radin, A comparative study of total hip replacement prostheses, Journal of Biomechanics 6 (1973) 299–311.
[8] R. McCalden, D. Howie, L. Ward, Observation of the long term behavior of retrieved McKee-Farrar total hip replacements implants, Transactions of the Orthopedic Research Society 20 (1995) 242. [9] H. McKellop, P. Campbell, S. Park, Wear of retrieved metal–metal implants after two decades of clinical use, Transactions of the Combined Orthopaedic Research Societies 2 (1995) 228. [10] H. McKellop, S. Park, R. Chiesa, In vivo wear of three types of metal on metal hip prostheses during two decades of use, Clinical Orthopaedics 329 (1996) 128–140. [11] M. Schmidth, H. Weber, R. Schon, Cobalt Chromium molybdenum metal combinations for modular hip prostheses, Clinical Orthopaedics and Related Research 329 (1996) 35–47. [12] K. Brummitt, C. Hardaker, Estimation of wear in total hip replacement using a ten station hip simulator, Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 210 (1996) 187–190. [13] J. Firkins, J. Tipper, M. Saadatzadeh, E. Ingham, M. Stone, R. Farrar, J. Fisher, Quantitative analysis of wear and wear debris from metal-on-metal hip prostheses tested in a physiological hip joint simulator, Bio-Medical Materials and Engineering 11 (2001) 143–157. [14] J. Jacobs, A. Skipor, P. Doorn, P. Campbel, T. Schmalzried, C. Amstutz, Cobalt and chromium concentrations in patients with metal on metal total hip replacements, Clinical Orthopaedics 329 (1996) S256–S263. [15] P. Doorn, J. Mirra, P. Campbell, H. Amstutz, Tissue reaction to metal on metal total hip prostheses, Clinical Orthopaedics 329 (1996) S187–S205. [16] M. Germain, J. Matthews, M. Stone, J. Fisher, E. Ingham, The effect of clinically relevant cobalt–chrome particles on the viability of human peripheral blood mononuclear cell and fibroblast in vitro, in: Proceedings of the Transactions of the 16th meeting of the European Society for Biomaterials , 2001, p. 76. [17] J. Fisher, X.Q. Hu, J.L. Tipper, T.D. Stewart, S. Williams, M.H. Stone, C. Davies, P. Hatto, J. Bolton, M. Riley, C. Hardaker, G.H. Isaac, G. Berry, E. Ingham, An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface engineered femoral heads, Proceedings of the Institution of Mechanical Engineers—Part H 216 (2002) 219–230. [18] J. Fisher, X.Q. Hu, T.D. Stewart, S. Williams, J.L. Tipper, E. Ingham, M.H. Stone, C. Davies, P. Hatto, J. Bolton, M. Riley, C. Herdaker, G.H. Isaac, G. Berry, Wear of surface engineered metal-on-metal hip prostheses, Journal of Materials Science: Materials in Medicine 15 (2004) 225–235. ¨ [19] G. Thorwarth, C.V. Falub, U. Muller, B. Weisse, C. Voisard, M. Tobler, R. Hauert, Tribological behavior of DLC-coated articulating joint implants, Acta Biomaterialia 6 (2010) 2335–2341. [20] J. Musil, Hard and superhard nanocomposite coatings, Surface & Coatings Technology 125 (2000) 322. ¨ [21] Q. Luo, W. Rainforth, W. Munz, TEM observations of wear mechanisms of TiAlCrN and TiAlN/CrN coatings grown by combined steered-arc/unbalanced magnetron deposition, Wear 225–229 (1999) 74. [22] A. Leyland, A. Matthews, Spatial characteristics of discharge phenomena in plasma electrolytic oxidation of aluminium alloy, Surface & Coatings Technology 177–178 (2004) 317–324.
242
J.A. Ortega-Saenz et al. / Wear 301 (2013) 234–242
[23] J. Ortega, M. Hernandez, V. Ventura, R. Michalczewski, J. Smolik, M. Szczerek, Tribological and corrosion testing of surface engineered surgical grade CoCrMo alloy, Wear 271 (2011) 2125–2131. [24] C. Mendibide, P. Steyer, J. Millet, Formation of a semiconductive surface film on nanomultilayered TiN/CrN coatings and its correlation with corrosion protection of steel, Surface and Coatings Technology 200 (2005) 109. [25] F75-98 Standard Specification for Cobalt-28 Chromium-6 Molybdenum Casting Alloy and Cast Products for Surgical Implants (UNSR30075), ASTM Book of standards, vol. 13.01, September 2001. [26] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, Journal of Materials Research 7 (1992) 1564–1583. [27] Y. He, I. Apachitei, J. Zhou, T. Walstock, J. Duszczyk, Effect of prior plasma nitriding applied to a hot-work tool steel on the scratch-resistant properties of PACVD TiBN and TiCN coatings, Surface & Coatings Technology 201 (2006) 2534–2539. [28] J. Ortega, M. Hernandez, A. Perez, R. Mercado, Development of a hip wear simulation rig including micro-separation, Wear 263 (2007) 1527–1532. [29] J. Paul, Force actions transmitted by joints in the human body, Proceedings of the Royal Society of London Series B 192 (1976) 163–172. [30] ISO/DIS 14242-1 Draft International Standard, 2001. Implants for surgery—wear of total hip joint prostheses—Part 1: loading and displacement parameters for wear-testing machines and corresponding environmental conditions for test. [31] M. Hernandez, R. Mercado, A. Perez, D. Martinez, M. Cantu, Wear of cast metal– metal pairs for total replacement hip prostheses, Wear 259 (2005) 958–963. [32] D. Dowson, C. Hardaker, M. Flett, G. Issac, A hip joint simulator study of the performance of metal-on-metal joints, The Journal of Arthroplasty 19 (2004) 118–123.
[33] G. Isaac, J. Thompson, S. Williams, J. Fisher, Metal-on-metal bearings surfaces: materials, manufacture, design, optimization, and alternatives, Proceedings of the Institution of Mechanical Engineers—Part H:Journal of Engineering in Medicine 220 (2006) 119–133. [34] F. Liu, Z. Jin, P. Roberts, P. Grigoris, Importance of head diameter, clearance, and cup wall thickness in elastohydrodynamic lubrication analysis of metalon-metal hip resurfacing prostheses, Proceedings of the Institution of Mechanical Engineers—Part H:Journal of Engineering in Medicine 220 (2006) 695–704. [35] C. Mendibide, J. Fontaine, P. Steyer, C. Esnouf, Dry sliding wear model of nanometer scale multilayered TiN/CrN PVD hard coatings, Tribology Letters 17 (2004) 779. [36] C. Mendibide, P. Steyer, J. Fontaine, P. Goudeau, Improvement of the tribological behavior of PVD nanostratified TiN/CrN coatings—an explanation, Surface and Coatings Technology 201 (2006) 4119–4124. [37] M. Shinn, S.A. Barnett, Effect of superlattice layer elastic moduli on hardness, Applied Physics Letters 64 (1994) 61. [38] X. Junhua, L. Geyang, G. Mingyan, The microstructure and mechanical properties of TaN/TiN and TaWN/TiN superlattice films, Thin Solid Films 370 (2000) 45. [39] Q. Yang, C. He, L.R. Zhao, J.-P. Immarigeon, Preferred orientation and hardness enhancement of TiN/CrN superlattice coatings deposited by reactive magnetron sputtering, Scripta Materialia 46 (2002) 293. ¨ [40] Q. Luo, W.M. Rainforth, W.-D. Munz, TEM observations of wear mechanisms of TiAlCrN and TiAlN/CrN coatings grown by combined steered-arc/unbalanced magnetron deposition, Wear 225–229 (1999) 74.