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A method for improving the anti seizure ability of self-made rolling body mom total hip prosthesis

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  • Save American Journal of Materials Science 2013, 3(6): 205-216 DOI: 10.5923/j.materials.20130306.03 A Solution to Improving Seizure Resistence in MOM Total Hip Prostheses with Self – Directed Rolling Bodies Dumitru Catalin Bursuc1, Lucian Capitanu2,*, Virgil Florescu3 1“Carol I” National Defance Academy, Bucharest, Romania 2Tribology Department, Institute of Solid Mechanics of the Romanian Academy, Bucharest, Romania 3Mechanical Departatment , Institute of Civil Engineering, Bucharest, Romania Abstract The Metal-On-Metal (MOM) Total Hip Prostheses (THP) with self directed balls has as its greatest advantage the replacement of the sliding movement between the femoral head and the acetabulum socket specific to regular hip prostheses with the rolling movement between the balls, the socket and the femoral head. The balls aren't fixed but rather are freely rolling between the femoral head and the acetabulum socket as a consequence of the movement of the femoral head and the frictional force, finding their own space to move in while imposing minimal resistance on rolling. The balls do not take up the entire space between the femoral head and the acetabulum socket. It is a well known fact that a rolling frictional coefficient is always smaller than a sliding frictional coefficient. The studied experimental models have allowed the measuring of values of this coefficient, of a maximum value of 0,05. Studies of lubrication with saline solution have indicated a lubrication schedule EHL, the minimum value for the thickness of the lubrication pellicle (hmin), measured through the contact resistance methodology, being 0,06 μm. Notwithstanding, when continuously benchmarking the frictional coefficient we detected seizure tendencies or even seizure. This paper presents the laboratory models of the ball-socket contact (taken as having an infinite radius, so on a flat surface), for different roughness values of the rust-free steel surface SS315L (Ra = 0.015 μm; Ra = 0.045 μm; Ra = 0.075 μm; and Ra = 0.190 μm), which allowed determination of frictional coefficients of μ = 0.038; μ = 0.042; μ = 0.078 and μ = 0.080, respectively. In order to increase the wear resistance of the tested samples and the acetabulum sockets we have used thin covering layers of Titanium Nitride (TiN) and Chromium Nitride (CrN). The best performance derived out of tests using a CrN layer has been achieved on an undercoat of SS316 L, which under a surface roughness Ra = 0.015 μm determined a friction coefficient of 0.035, without nearing seizure at any time under test conditions. Keywords MOM Total Hip Prosthesis, Self Directed Movement, Wear, Rolling Friction Coefficient, TiN and CrN Surface Coatings, EHL 1. Introduction The loss stability of the Total Hip Prosthesis (THP) depends on the mechanical stress, the movements and the heat[1] on the supports of the artificial hip joint, the bone-cement and cement-stem interface strength for cemented stems[2], on the growth of bone inside of uncemented stems[3], the efficiency of the porous coatings of the femoral stems[4] and acetabulum sockets and on the resistance to wear of the femoral stems, acetabulum sockets and femoral heads of the THP[5]. So far, the most successful orthopaedic device is a progressive bio-tribo-system due to the on-demand nature of the balls under the hostile environmental conditions specific to the human body. The Co-Cr-Mo alloy and the Ti-6Al-4V alloy are the materials most used in manufacturing prosthetic * Corresponding author: (Lucian Capitanu) Published online at Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved femoral heads. Prosthetic femoral head damage remains a major problem with THPs in spite of all the technological advances such as coatings with mono or multi-thin films. Titanium and its alloys are resistant to corrosion[6] and are therefore biocompatible with the manufacturing of orthopaedic implants, being lightweight and having low elastic modulus. However, the use of these materials in joint implants is limited by their high coefficient of friction and poor wear resistance. In order to improve their tribological properties, these materials have been subjected to surface modification treatments. Since TiN alloys are hard biocompatible materials[7, 8, 9] of excellent abrasion resistance, many advanced processing methods have been developed aiming at the production of a nitrided layer on the materials surface. In plasma nitriding[10], nitrogen atoms diffuse into the Ti matrix thus forming a top layer of TiN and Ti2N compounds followed by a deeper diffusion layer. This layered structure produces a continuous hardness profile thus providing an adequate support of the protective layer[8]. The physical properties of the treated surface, however, depend 206 Dumitru Catalin Bursuc et al.: A Solution to Improving Seizure Resistence in MOM Total Hip Prostheses with Self – Directed Rolling Bodies strongly on the plasma technique and processing parameters. The excellent corrosion resistance of the Ti alloys results from the formation of a very stable protective oxide film strongly adhered to the metal surfaces. See also L. Capitanu et al[10] for a report of controlling the wear for a hip joint. Notwithstanding these improvements, the conventional MOP THP is undergoing a progressive wear process. Following the metallurgic industry advances in the fields of manufacturing and processing biocompatible alloys, the last 10-15 years have seen a reconsidering of the Metal-On-Metal constructive method in order to better ensure EHL lubrication conditions of the femoral head – acetabulum socket device. There are several studies of this method. Leiming Gao et al[11] published a paper on the effect of the 3D physiological load and movement over the elasto-hydro-dynamic properties of the lubrication of the joints in a MOM THP. In this study they present a simulation of the EHL lubrication of a MOM THP taking into account both a static state as well as transitory movement during walking cycles in all three directions. Leiming Gao, Peiran Yang, Ian Dyamond, John Fisher, Zhongmin Jin[12] have publushed a paper on the effect of surface texturing on the analysis of the EHL lubrication for MOM THPs in which they have presented an advanced numerical model for calculating the EHL properties of a surface textured with cylindrical dimples. The dimpled structure has been numerically simulated to correspond to a static state as well as walking cycles. Qingen Meng, Leiming Gao, Feng Liu, Peiran Yang, John Fisher si Zhonming Jin[13] have released a paper on the mechanics of contact and the EHL lubrication in a new THP implant with an asferical surface. R. Pourzal, R. Theissmann, M. Morlock si A. Fischer[14] have analyzed micro-structural differences in various zones of the articulated surfaces of an MOM system with a reconditioned head demonstrating that wear-related particles at nano-scale are a limitation due to the theoretical damage to the human body. In order to investigate the release of wear-related particles in ultra-low wear conditions, one must first understand the micro-structural differences in the substrate. Previous studies on MOM THPs have shown the in situ formation of a nano-crystalline layer that, together with the mechanical mixing, allows the Co-Cr-Mo alloy to adapt to its current usage requirements and maintain a very low wear rate. However there is little information on wear resistance for hip surface replacements which are subject to different exertion / deformation conditions. Y. Yana, A. Neville et al[15] have published a paper on the effect of metallic nano-particles on bio-tribo-corosion of MOM THPs, centred around the particles in the third body and their effect on the tribology and the corrosion processes of the bearing surfaces. A hip movement simulator together with a electro-chemical cell have been used to study the bio-tribo-corrosion system. Pre-built Cobalt particles have been used (28 nm diameter). The diameter of all the femoral heads used in this study has been 36 mm. The Open Circuit Potential (OCP) has been monitored with and without these nano-scaled Cobalt particles. C.X. Li, A. Hussain, A. Kamali[16] have presented considerations on conditions for non-bearing surfaces and their potential effect on tests for wear simulation, in which they show that the potential mass loss of the non-bearing surfaces (specimens) and its contribution to the total results of gravimetric readings are rarely mentioned in scientific papers. This study involved testing Co-Cr-Mo alloy discs for up to 1200 hours and the recovery of socket surfaces under different surface conditions. For the new generation of MOM hip joints the surfaces of the bearing are generally polished with a great degree of precision following the introduction of the international ISO 7202 standard for orthopaedic hip prostheses. Even so, the materials used, the shape and the finish of the bearing surface can show wide differences. S. Kataria, N. Kumar, S. Dash, A.K. Tyagi[17] have reported on the tribologic behaviour and the deformation of the Titanium layer under different sliding conditions, trying to explore the tribological properties of a Titanium coating 1 μm thick on a D9 steel substrate. The friction tests have been undertaken using a tribometer with reciprocal linear movement, on steel spheres, alumina and silicon nitride. W. Osterle, D. Klaffke, M. Griepentrog, U. Gross, I. Kranz si Ch. Knabe[18] have published a paper on the wear resistance of Ti-6Al-4V coatings for the bearing surfaces of the artificial hip joint in which they talk about the tribological tests (sphere/plane) undertaken in order to identify the right coating for bearing surfaces in hip joints whose acetabulum socket and femoral heads are made out of Ti-6Al-4V alloys. This alloy is highly appreciated for its low weight, good compatibility and the elastic properties similar to natural bones. In parallel with reconsidering the potential for MOM prostheses there is ongoing research on new manufacturing and functional solutions. The current paper discusses the Metal-On-Metal total hip prosthesis with self directed balls[19, 20]. It has the great advantage of replacing the relative sliding motion between the femoral head and the acetabulum socket, specific to a regular THP, with the rolling motion generated by the balls, the socket and the femoral head. The balls aren't fixed in a cage but freely roll continuously between the femoral head and the acetabulum socket, as a consequence of the motion of the femoral head and the generated friction, thereby finding the space where minimum resistance to rolling is met. The balls do not occupy the entire space found between the head and the socket and some free space is technologically allowing the movement of the balls. It's a known fact that a rolling friction coefficient is always smaller than a sliding friction coefficient. As such, so should the wear. Several studies have been published on the load transfer in rolling balls THPs[21]; the wear and tear of the femoral head of a THP[22, 23] and the stability of hip endoprosthetics[24]. 2. Material and Methods American Journal of Materials Science 2013, 3(6): 205-216 207 Three THPs with self directed balls have been manufactured for testing (Fig. 1). For lab experiments three MOM THPs with self directed balls have been manufactured, using commercially available femoral heads and stems, having: 1 - Socket, balls and the closing/adjustment nut made entirely out of rustproof ASTM SS 316L steel having 150 HV 30 hardness (fig. 1, 2, 3 and 4); 2 - 316L stainless steel socket with a 3 μm Ti N coating, having 2000 HV 0.02 micro-hardness; 3 - 316L stainless steel socket with a Cr N coating, having 627 HV 5 micro-hardness. Fig. 1 comparatively shows the classic THP and the new self directed balls THP, the latter being dismantled, showing the stem, the femoral head, the socket including the balls and the closing-adjustment nut, its components. ensure the self directed motion of the balls. Figure 2. Overhead view of the acetabulum socket and the SS316L rustproof steel balls Figure 3. Transversal forces acting on the femoral head of a THP wit self directed balls[19] Figure 1. Comparison of (top) the classic THP (top) and (bottom) a dismantled self directed balls THP, showing its components - the stem, the femoral head, and the socket including the balls and the closing-adjustment nut For a clearer view Fig. 2 shows an overhead view of the acetabulum socket and the SS316L rustproof steel balls. The layer of balls is not complete, there being a free space by design, that allows the movement of the balls as a consequence of the motion of the femoral head and the friction between the balls, the head and the socket. The transversal forces acting on the femoral head are shown in Fig. 3. One can easily observe that overall, most of the friction components will cancel each other given an even number of balls. The initial consideration was that the absence of a ball from the total number allowed by the available space between the femoral head and the acetabulum socket would The lab experiments on a simplified testing bench with a 2D oscillating motion of ± 30° on a vertical plane (non-anatomical position), presented elsevhere[20], have demonstrated that the space created is insufficient. A complex geometrical analysis of the distribution of the balls as well as Fortran software for calculating the number of balls per row have been produced, obtaining: n0 = 37; n1 = 19; n2 = 19; n3 = 1 ; n4 = 19; n5 = 19; n6 = 19; n7 = 19; n8 = 14; n9 = 9 ; n10 = 5; n11 = 1. Figure 4 shows the way to arrange the balls from two different perspectives (- 15° and 15°). The main design and the design of the articulated body and the closing-adjustment nut are presented elsevhere. Measurements taken using the friction coefficient on the contact surface have revealed some seizure tendencies so two other SS316 L couples steel prostheses models have been manufactured, using Titanium Nitride (TiN, Fig. 5) or Chromium Nitride (CrN, Figure 6) as the interior coating. 208 Dumitru Catalin Bursuc et al.: A Solution to Improving Seizure Resistence in MOM Total Hip Prostheses with Self – Directed Rolling Bodies (a) (b) Figure 4. Lateral view adjusted (a) by β = 15°; and (b) lateral view adjusted by β = - 15°[20] The coatings have been created using the Pulse Laser Deposition method (PLD), with help from our colleagues at the National Laser and Plasma Physics Institute of Bucharest – Magurele and support from Prof. Ioan Mihailescu. Experiments have been made using a ball – plane device, presented elsewhere[25], assuming an infinite radius for the acetabulum socket (flat surface). The tests have shown the presence of seizure tendencies even when the acetabulum socket is coated with thin layers of TiN or CrN. material. For the sake of clarity Fig. 7 shows an image of the friction couples components with single dot contact (spherical nut on a flat disc) used in testing. Figure 7. The friction couple used in experimental modelling Figure 5. SS316L acetabulum socket and closing-adjustment nut with interior TiN coating Figure 6. SS316 L acetabulum socket and self directed balls with interior CrN coating The device can determine the seizure load, the thickness of the lubricating film and, through microscopy, photograph the wear marks and then calculate the volume of removed Under static conditions the compression efforts due to the single dot contact, pmax si pmed (maximum and average contact pressure) are given by: ( ) = pm3 ax 1.5PE2 / π r2 1− µ2 2 (1) pmed = P π a2 (2) while the radius a, of the circular contact surface, is: ( ) = a3 1.5 1− µ2 P r (3) E where P is the load; a the radius of the contact surface and r the radius of the sphere. For steel made friction couple components the values of the above equations become: pmax ≈ 58003 P (4) pmed ≈ 17003 P (5) a ≈ 0.093 P (6) Particular care has been given to finishing the friction surfaces of the samples. The state of the surface, being defined by its topography, the micro-structure of the American Journal of Materials Science 2013, 3(6): 205-216 209 superficial layer and its oxidation state, has a major contribution to the wear process. Due to the complexity of creating the surface through an abrasion process, the safest way to ensure a consistently reproducible surface is by strictly adhering to all the stages of the finishing process. The following stages have been established: the turning of the shape, the finishing turning, thermal treatment, rectification, fine adjustment and finally the super finishing of the work surface. While all the stages, up to the fine adjustment, are using traditional manufacturing techniques, it is worth mentioning that the overall intensity of the process is purposefully maintained at a low level in order to protect the superficial structure of the material. The super finishing stage uses the metallographic polishing techniques and includes: wet polishing using abrasive paper of 32 μm and 17 μm granulation, diamond paste polishing of 6 μm and 1 μm granulation and, finally, wet polishing using alumina suspension having a 2000 Ǻ granulation. The surfaces are then cleansed with alcohol and distilled water and then dried. The storage of the finished samples is done in sealed containers on a silica gel layer. Following the super finishing stage we have obtained four different roughness values for the SS316 L sample surfaces: Ra = 0.015 μm, Ra = 0.045, Ra = 0.075 μm si Ra = 0.190 μm. The roughness of the surfaces has been determined using a profilometer with parameter translator and graphic recording. The instrument allows not only the recording of the surface profiles but also the determination of the Ra and r.m.s. values, defined as: Ra = 1 l l ∫ 0 y dx (7) ∫ r.m.s = 1 l y2dx l0 (8) h0 r ≈ 0.84   auµ0 r 0.741    Er 2 P 0.074  (9) where: h0 - minimum thickness of lubricant film; r - radius of the sphere; α - pressure coefficient of vascosity; u - velocity sum; μ0 - dynamic viscosity at atmospfericic pressure; E reduced elasticity modulus; P - load. Relation (9) reproduces satisfacatory the dependence of h0 by the main quantities u, μ0, r and P. The exact determination of the minimum lubricant film thickness depends on the knowledge of pressure coefficient α for lubricant used and the accuracy of the numerical coefficient of relationship (9). In working conditions, using an estimated value α, resulted a minimum lubricant film thickness h ≈ 0.06 μm. Spatial form of lubricant film in the loaded area results from Fig. 8 (a) and (b), which represents two sections of lubricant film, longitudinal and cross sections (in relation to the movement) by the symetry axes. (a) 3. Results and Discussion 3.1. Evolution of Surface State in the Wear Process Experimental determinations were made under these test conditions: - Load: variable between P = 20 ÷ 300 N for determining of seizure limit. A load of 50 N was used for wear tests. - Sliding speed: main speed for determining the wear rate was u = 174 cm/s. To determine the influence of speed on the wear, the device allows achieving the following speeds: u1 = 60 cm/s; u2 = 18 cm/s and u3 = 3,2 cm/s. - Lubricant: BSF (Body Simulated Fluid) having a density of 1183 kg/m3 and a viscosity of 0.84 Pa∙s (HyClone, SH30212.03). Using the parameters above mentioned, the couple operates in elastohydrodynamic regime. For the minimum thickness of the lubricant film, Archard[8] proposed the relationship: (b) Figure 8. a) Variation of lubricant thickness in the x = 0 plane and (b) the z = 0 plane, function of load, at speed u = 8 cm/s. ▪ 0.7 N; ▼ 1.1 N; x 1.5 N; ◊ 3 N; ▪ 4.6 N; ▲ 7.8 N; ○ 10.8 N 210 Dumitru Catalin Bursuc et al.: A Solution to Improving Seizure Resistence in MOM Total Hip Prostheses with Self – Directed Rolling Bodies Curves were obtained experimentally, under close conditions to those used by Dowson[9]. Are noticed significantly higher values for minimum thickness h0, even at speed u = 23 cm/s. To watch in good conditions the wear of fixed surface, function of couple roughness, the following solution was used: roughness of the couple was focused on one of surfaces, in particular on themobile one. Fixed surface had always the minimum roughness achievable, meaning about Ra ≈ 0.015 μm. As it is known, the composed roughness of the couple, expressed as standard deviations, σ, is: σ=2 σ12 + σ 2 2 (10) where σ1, σ2 represent the standard deviations of the two surfaces. If one of the surfaces has small roughness, i.e., σ1 << σ2, then σ ≈ σ1. So it is possible to study the influence of roughness on the wear, just by changing the roughness of the of a single surface. Under these conditions and at a load P = 50 N, speed u = 1.74 m/s and volume temperature of the lubricant θ = 50℃, it vas determined the evolution of the surface wear function of the time, for the following roughness of the mobile surface: Ra = 0.015 μm; Ra = 0.045 μm; Ra = 0.075 μm and Ra = 0.19 μm. Figures 9 and 10 exemplify the cross-cut profile and the optical micro-photography of the seizure marks of the sample surfaces with Ra = 0.015 μm and Ra = 0.19 μm, for a 30 second test. The oscilloscope recordings of the contact resistance (the breakage of the lubricating film – BSF) have shown seizure tendencies while the wear volume of the flat sample has increased with its roughness as a consequence of the heavy wear of the tips present in the topography of the surface during the initial testing period. The recording is presented in Figure 11. Figure 12 shows the double logarithmic friction and wear evolution relationship as a function of the test duration and the roughness of the surface. Regarding wear tests it must be stated that at the same time as measuring surface wear the friction coefficient has also been measured. Surprisingly, the minimum friction coefficient doesn't coincide with minimum wear. Figure 13 shows the data points of a Vu/μ model for the four roughness values used. Figure 13 indicates the existence of a minimum wear volume for roughness Ra = 0.045 μm. At the same time the friction coefficient reaches a minimum at roughness Ra = 0.015 μm. The rapid decrease of the wear speed as a function of time is obvious, exception being the surface roughness value of Ra = 0.045 μm. 25% to 50% of the wear recorded in a 30 minute wear test is being produced during the first three seconds. Increasing the test time from 5 to 30 minutes only increases the wear by 10%. (a) (b) Figure 9. Central cross-cut profile (a) and (b) the image of the wear marks. Ra = 0.015 μm, t = 30 sec. Sample 835 (a) (b) Figure 10. Central cross-cut profile (a) and (b) the image of the wear marks. Ra = 0.19 μm, t = 30 sec. Sample 849 American Journal of Materials Science 2013, 3(6): 205-216 211 t (min) Figure 11. Oscilloscope recordings of the contact resistance (the breakage of the lubricating film – BSF) for SS 316L. Sample 849 Figure 12. Friction and wear evolution as a function of time for different roughness values of the flat surface 212 Dumitru Catalin Bursuc et al.: A Solution to Improving Seizure Resistence in MOM Total Hip Prostheses with Self – Directed Rolling Bodies Figure 13. The variation of used material as a function of the friction coefficient The wear evolution is explained by the conformation of the surfaces that results in the alteration of the lubricating conditions. It must be noted that the value of static wear is determined by the initial wear (during the first few seconds). This observation allows the usage of the wear value of t = 5 min. as representative for the usage conditions. Using this duration the spread of values is greatly decreased. The different roughness values used have resulted not only in differences in the used volume of material but also in differences in the shape (type) of the used material. For surfaces with Ra = 0.015 μm, Ra = 0.075 μm, (t =3 sec.) and Ra = 0.19 μm, (t =3 sec.), the wear is of an adhesive type (metallic aspect with emphasized fissures). For surfaces with Ra = 0.045 μm, the prevailing wear is of an oxidative type. Once wear speed is reduced, following the conformation of the surfaces, the Ra = 0.075 μm and Ra = 0.19 μm surfaces also enter an oxidative wear stage. For Ra = 0.045 μm the wear is reduced to such an extent that during the entire test time the lubrication conditions remain largely unchanged. The average values of volume of used material (calculated using the footprint method[11] that consists in considering the wear footprint as on a spherical surface, photographing it using optical microscopy, dividing it into ten equidistant spherical segments, calculating their volumes and summing them up using the approximate formulas (4), (5) and (6)) for American Journal of Materials Science 2013, 3(6): 205-216 213 the four roughness values are: Ra = 0.015 μm; → Vu = 7.0 x 10-5 mm3; → μ = 0.038; Ra = 0.045 μm; → Vu = 7.0 x 10-6 mm3; → μ = 0.042; Ra = 0.075 μm; → Vu = 3.7 x 10-5 mm3; → μ = 0.068; Ra = 0.190 μm; → Vu = 1.0 x 10-3 mm3; → μ = 0.080. For Ra = 0.015 μm, Ra = 0.075 μm and Ra = 0.19 μm roughness, the values for the volume of used materials vary according to the degree of stress for the contact determined by the h min / σ parameter of the lubricating film. For Ra = 0.045 μm roughness we have also observed an influence on the grind. After the first 5 minutes the entire contact surface is oxidized. The footprint obtained after another 5 minutes using the same axle-box has a particular shape, the oxidized area halving itself at the boundary of the load area. It follows that after grinding the lubrication conditions improve. For super finished surfaces there was no obvious grinding improvement. Regarding the influence of the initial roughness on the wear and the friction coefficient it must be said that the surface wear is a value determined by the fraction of the surface that stays in contact and should therefore be a monotonous function of the h / σ film parameter. The minimum wear value must coincide with the minimum surface roughness value. Time-observed wear evolution leads to the conclusion that, under the tested conditions, the minimum wear value is obtained for a particular (and not the minimum) roughness value. To verify this result a great number of determinations have been performed for the four different roughness values: Ra = 0.015 μm; Ra = 0.045 μm; Ra = 0.075 μm and Ra = 0.19 μm. The existence of the optimal roughness value may be explained either through the effect on the lubricating pellicle or through a modification of the mechanical properties of the surface. In the latter case, for the optimal roughness, the decrease of the h / σ ratio is cancelled by an increase in wear resistance. In order to limit the possibility of seizure we used thin coatings of TiN and CrN on the flat SS316 L surface (Figure 9), resulting in a micro-hardness value of 2000 HV 0.02 for the TiN coating and a 627 HV 5 value for the CrN coating. The PLD coating process that was used, is described elsevhere[25]. The structures of the pellicles have been analysed using X-Ray diffraction. An example of this analysis is shown in Fig. 15. Checking the adherence of the pellicle has been done using scratch tests and is presented in[10]. The sample used for the TiN coating has been corrosion tested by submerging in a 3% NaCl solution at 25℃ No corrosion was observed after three weeks. In conclusion these coatings have an excellent corrosion resistance as well as an increased hardness. The TiN coated surfaces have been polished for the same roughness states as the uncoated surfaces. The wear profiles and the seizure marks have been recorded. Figures 14 and 15 show the central cross-cut profiles and the micro photographs of the SS316 L sample surfaces with TiN coatings of 2000 HV 0.02 micro-hardness and roughness values of Ra = 0.015 μm and Ra = 0.045 μm after 5 minutes of testing. In the particular case of a TiN coated sample (1057), for Ra = 0.045, the variation of the contact surface resistance has been observed on the oscilloscope recording, such that after 48 minutes the breakage tendency of the lubricating pellicle was apparent and after 66 minute effective seizure took place for a load of 50 N at the relative speed v = 1.73 m/s and the contact temperature θ = 50℃ (Fig. 16). (a) (b) Figure 14. Central cross-cut profile (a) and (b) TiN wear mark photography. Ra = 0.015 μm, t = 5 min. Sample 1057 214 Dumitru Catalin Bursuc et al.: A Solution to Improving Seizure Resistence in MOM Total Hip Prostheses with Self – Directed Rolling Bodies (a) (b) Figure 15. Central cross-cut profile (a) and (b) TiN wear mark photography. Ra = 0.045 μm, t = 5 min. Sample 1058 t (min) Figure 16. Oscilloscope recordings of the contact resistance (the breakage of the lubricating film – BSF) for TiN coating on SS 316L. Sample 1057 (a) (b) Figure 17. Central cross-cut profile (a) and (b) CrN wear mark photography. Ra = 0.045 μm, t = 5 min., N = 50 N, v = 1.73 m/s, θ = 50℃. Sample 706

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