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Effect of bio-lubrication on the tribological behavior of UHMWPE against M30NW stainless steel
Tribology International 94 (2016) 550–559
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of bio-lubrication on the tribological behavior of UHMWPE against M30NW stainless steel M. Guezmil a, W. Bensalah b,n, S. Mezlini a a b
Laboratoire de Génie Mécanique (LGM), ENIM, Monastir University, Rue Ibn Eljazzar 5019, Tunisia Laboratoire de Génie des Matériaux et Environnement (LGME), ENIS, Sfax University, B.P.W. 1173-3038, Tunisia
art ic l e i nf o
a b s t r a c t
Article history: Received 26 July 2015 Received in revised form 14 October 2015 Accepted 19 October 2015 Available online 27 October 2015
The tribological behavior of M30NW stainless steel against UHMWPE was investigated. The wear tests were conducted on a reciprocating pin-on-disc tribometer under dry and lubricated conditions. Saline solution (NaCl 0.9%), sesame oil and nigella sativa oil were used as bio-lubricants. The friction coefficient and wear volume of UHMWPE were examined. It is found that under oil lubrication, the friction and wear behaviors of UHMWPE were the best. Morphological and chemical studies of the worn surfaces were conducted and wear mechanisms were proposed. The observed performance of natural oils was linked to their chemical composition and their adsorption ability to the stainless steel. & 2015 Elsevier Ltd. All rights reserved.
Keywords: UHMWPE Bio-lubricant Friction coefficient Wear mechanism
1. Introduction Nowadays, the most implanted type of hip joint consists of an ultra-high-molecular weight polyethylene (UHMWPE) acetabular cup and metallic or ceramic femoral head. During use, UHMWPE wear particles are released nearby tissue leading to aseptic loosening of the acetabular or femoral components [1,2]. The produced UHMWPE wear particles and the consequent tissue reactions have been known as one of the main sources of osteolysis and implant failure [3,4]. Therefore, in order to increase the longevity of total hip replacement joint, the amount of the generated UHMWPE wear debris must be reduced. Many researchers have developed some kinds of reinforcements such as hydroxyapatite [5], carbon nanotubes (CNT) [6], nacre [7] and talc [8]. An alternative to improve the wear behavior of the UHMWPE is by using lubricants such as distilled water, saline solution, polyethylene glycol (PEG), sodium hyaluronate, bovine and calf serum as well as hyaluronic acid [9–13]. Throughout attempts aimed to overcome this problem by developing new solutions, we have retained to try effective and adequate natural bio-oils, as lubricants, ensuring in the same time low wear rate and therapeutic effects. Knowing that there is a good affinity between fatty acids, the main components of bio oils, and the human serum [14,15], these bio-lubricants can be used as additive to the existing well known lubricants as bovine and calf n
Corresponding author. E-mail address: [email protected] (W. Bensalah).
http://dx.doi.org/10.1016/j.triboint.2015.10.022 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
serum without affecting their tribological performances and ensuring healing impacts. Since the beginning of the 20th century, pure natural oils have gained a great interest as lubricants in various industrial applications. In fact natural oils have demonstrated good results in lowering friction and wear when they are used in tribological purposes [16–19]. This choice is, also, strongly motivated by the environmentally-friendly character of natural oils. These oils are also known to be biodegradable, non-toxic, renewable and can be used in most cases as ingredient in many medicinal and cosmetic products. Among natural oils, we have focused our attention on sesame and nigella sativa oils. This choice is enthused by double great interests: good tribological performance and wide medical and pharmaceutical applications [16–27]. Many previous works have been undertaken in order to evaluate the tribological or the therapeutic potentials of sesame and nigella sativa oils [16–27]. Reeves et al. [18] have studied the friction and wear properties of eight oils in copper/2024 aluminum contact. They have demonstrated that sesame oil has promising friction of coefficient and wear volume. Mannekote et al. [16] have tested the aging effect on five oils using a standard four ball tester machine. They have shown that fresh sesame oil ensures very low coefficient of friction (o0.1). From therapeutic point of view, studies on sesame oil have shown that it can reduce the growth of human colon cancer in vitro [21], decreases blood pressure, lowers lipid peroxidation, and enhances the status of antioxidant in hypertensive patients [22]. Hsu et al. have demonstrated that the oral administration of sesame oil can reduce proinflammatory cytokine and the production of nitric oxide [23].
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Sesame oil helps joints keep their flexibility, helps to attenuate oxidative stress, ameliorate multiple organ failure, enhance survival rates [24], and to reduce atherosclerosis lesion formation [25]. Turning to the nigella sativa oil benefits, it is used as antihypertensive, anti-bacterial, anti-rheumatism, anti-tumor, antiinflammatory, anti-parasitic and anti-histamine [26,27]. It is found that Nigella sativa oil promotes the growth rate of bone marrow cells. Moreover, it strengthens the immune system and promotes lactation [26,27]. It remains that, to the author knowledge, the study of the tribological performance of nigella sativa oil has not yet been done. In the present paper, we compare the tribological behavior of UHMWPE against M30NW stainless steel pin using reciprocating pin-on-disc tribometer in dry and lubricated conditions. Saline solution (0.9% NaCl), sesame and nigella sativa oils were used as lubricants. The coefficient of friction (CoF) and wear volume of UHMWPE against stainless steel were investigated and compared. Morphological studies were, equally, conducted and wear mechanisms were proposed. The observed difference between oils performance was linked to their chemical composition and their adsorption ability to the stainless steel.
2. Experimental details 2.1. Materials During this study, M30NW stainless steel was used as the pin material. The pins have hemi-spherical shape with 10 mm diameter. The discs of UHMWPE were machined in cylindrical shape with 30 mm diameter and 10 mm thickness from a medical-grade bar. M30NW stainless steel and UHMWPE were supplied by C2F implants- France (Tunisian plant). Fig. 1 shows photographs of the UHMWPE disc and the stainless steel pin. The chemical composition of the stainless steel pin is given in Table 1. The microhardness of the M30NW stainless steel was 4407 20 HV0.5 and the shore hardness D of the UHMWPE was 65 SH D. Three lubricants were selected: saline solution 0.9% NaCl in deionized water, sesame oil, and nigella sativa oil. The saline solution, named equally physiological solution, is isotonic with blood plasma. The used oils were cold-press extracted by STVPAA industry (Tunisia) for cosmetic, pharmaceutical and nutritious domains. Cold pressing is a technology which does not requires heat or chemical treatment during the oil extraction process. The physiochemical properties of sesame and nigella sativa oil are presented in Table 2. It is to mention that the viscosity of oils was conducted using an open air viscometer.
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Fig. 1. Scheme of the wear device with the Stainless steel pin and UHMWPE disc images. Table 1 Chemical composition of M30NW stainless steel. Element
C
Cr
Ni
Mn
Mo
N
Weight (%)
r 0.06
21
9.5
4
2.2
0.40
Table 2 Physicochemical properties of sesame and nigella sativa oils. Property
Unit
Sesame oil
Nigella sativa oil
Density at 20 °C Viscosity at 20 °C Saponification value Water content Iodine value Refractive index at 40 °C
Kg/m3 mPa s mg KOH/g of oil % g/100 g
923 58 195 o 0.1 104 1.465
915 64.5 204 o 0.1 119 1.470
in vivo and corresponds to the contact pressure of 51 MPa. The stroke range was 30 mm and the sliding speed was 30 mm/s. This value corresponds to that usually found in hip joints (0–50 mm/s) [28]. The number of cycles was varied from 0 to 15,000 cycles which is equal to a total sliding distance of 450 m. The friction force was digitally recorded using a load transducer attached to the pin holder. 2.2.2. Morphological and chemical characterization techniques The worn surfaces of UHMWPE discs and the stainless steel pins were examined by Hitachi FEG 4800 Scanning Electron Microscope (SEM) coupled with an Energy Dispersive X-ray Spectroscopy (EDS) probe. Before carrying out SEM images, the UHMWPE were coated with sputtered gold layers. s The 3D profilometry was examined using a VEECO profils ometer and the obtained data were analyzed by the DEKTAK 3D software.
2.2. Methods 3. Results and discussion 2.2.1. Wear test Wear tests on UHMWPE have been performed with a reciprocating pin on disc tribometer at room temperature (Fig. 1). Before tests, samples were polished using polishing papers up to #4000 to obtain an adequate roughness and then they were cleaned by ethanol. The initial roughnesses of the stainless steel pins and the UHMWPE discs were respectively 0.06 70.01 mm and 0.31 70.05 mm. During the friction tests, the stainless steel pin was fixed to the specimen holder and the UHMWPE disc was implemented reciprocating motion continuously. The normal load during the friction test was set to 20 N. This value was chosen based on a simplification developed by Borruto [9]. The used loading condition is assumed to be equivalent of that
3.1. Friction behavior Fig. 2 shows the variation of the coefficient of friction (CoF) of stainless steel pin against UHMWPE disc under dry and lubrication conditions as a function the number of cycles. It can be seen that there is an increase of the coefficient of friction at the start when working under dry and saline solution. The COF under dry conditions and saline solution reaches steady-state at approximately 4000 and 12,000 cycles respectively. The use of sesame and nigella sativa oils results in a significant reduction of fluctuations and the steady-state is reached since the first 3000 cycles. In contrary to the dry and saline solution, the CoF decreases slowly at the beginning of tests when oils are used. Moreover, the examination
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Fig. 2. Coefficient of friction of UHMWPE against stainless steel pins under dry and lubrication conditions as a function of number of cycles.
of Fig. 2 shows that lubrication decreases the friction coefficient and the lowest values of the studied response are obtained with both sesame and nigella sativa oils. The CoF of oil lubricants was reduced two and seven times compared to saline solution and dry conditions respectively. The decrease of the CoF would be an important factor in decreasing UHMWPE wear. It is to be noted that the fluctuations observed in the dry test and with the use of saline solution are accentuated during the first thousands of cycle. The observed fact supposes the presence of morphological changes in the contact between the mating materials. Under dry condition, the energy and the work of friction force produced in contact are very important. These dissipate in the stainless steel pin and the UHMWPE disc, causing them to heat. Under the simultaneous action of overheating and local overstressing many changes will occur during sliding, the stainless steel ball and the UHMWPE disc will be subjected to geometric and chemical modifications. In fact, the rise of the CoF can be related to a severe plastic deformation and/or adherence of polymer in the contact. During these tests, wear particles can be removed easily from the UHMWPE disc surface. This action will increase the surface roughness of the polymer. In fact, the roughness of the UHMWPE before the wear tests was 0.31 mm. This roughness reached 1.27 mm, 0.67 mm, 0.49 mm and 0.45 mm for dry, saline solution, nigella sativa and sesame oil conditions at the completion of the test. Therefore, wear particles would react as third body in the friction process which can lead to high fluctuation. For a long duration of the friction test, the active chlorine ions (Cl ) of the saline solution could form with the disturbed surface of the stainless steel pin a chemical reaction layer [29,30]. According to Yan et al. [29], this layer, when formed, is essentially, composed of lepidocrocite (FeOOH), magnetite (Fe2O3), Cr2O3 and NiO with thickness of 15–20 nm. Turning to sesame and nigella sativa oils, the remarkable decrease of the CoF is mainly attributed to their chemical composition, more specifically their fatty acids composition [18]. Further discussion on the effect of the chemical composition on friction and wear of UHMWPE will be presented in Section 3.3. Finally, it is to mention that the COF can be related to the lubrication regime in the studied contact. A suitable method to evaluate this relation is the Stribeck curve, largely used for oils [31]. This curve relates the COF with the product of tangential velocity and dynamic viscosity divided by the normal contact load [32]. In our case, the normal load and the tangential velocity are maintained constant for all tests. The main difference between the studied lubricants is the viscosity. The viscosity of sesame and
Fig. 3. (a) Typical 3D wear track area on the UHMWPE and (b) typical 2D view of x– z plane from a wear track.
nigella sativa oil is higher than that of saline solution. The decrease of the COF when oils were used is probably due to a change in the hydrodynamic behavior. The long chain of molecules in sesame and nigella sativa oils seem to maintain a lubricating film between the stainless steel and the UHMWPE. Further study is needed in order to give the predominant regime of lubrication in the retained contact. 3.2. Wear behavior The coefficient of friction is not enough to characterize the tribological behavior of the studied couple of materials. In order to have more complete information, the wear volume was computed using 3D profilometric scans. Fig. 3a shows a typical 3D scan of the wear track on the UHMWPE and Fig. 3b presents a typical 2D view taken on a given position from the wear track. The 2D view can provide information about the wear surface, the depth and the width of the wear track. Fig. 4 depicts the variation of the UHMWPE wear volume after 15,000 cycles under dry and various lubricants. As can be concluded from this figure, the use of lubricants increases remarkably the wear resistance of the UHMWPE. The wear volume of the UHMWPE under dry conditions is 2 times higher than with oil lubricants. Among all conditions, under dry friction the UHMWPE had the highest wear volume (0.0511 mm3). Fig. 4 shows that sesame oil ensures the lowest wear volume. Moreover, nigella sativa oil has the closest wear volume to sesame oil and saline solution has a moderate wear volume. Compared to the dry test, the wear volume was reduced to 40%, 46% and 54% when using saline solution, nigella sativa oil and sesame oil respectively. These results are in accordance with those of the coefficient of friction, low CoF induces low wear volume. With regards to the obtained results, sesame and nigella sativa oils demonstrate good tribological performances. These benefits
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can be related to their chemical composition [16–19]. Furthermore, the observed wear difference between the oils seems to have direct relation with their composition, more particularly their fatty acids content [16–19]. Therefore, it is sensible to undergo a chemical analysis of the oils in order to show possible correlation between the tribological performance and the chemical composition.
Fig. 4. Variation of the wear volume measured on the UHMWPE at the completion of the test ( 15,000 cycles) under dry and lubricated conditions.
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3.3. Morphological analysis and wear mechanism After the wear tests, wear tracks were investigated using SEM analysis (Figs 5–8). These figures show SEM micrographs of wear tracks under dry, saline solution, sesame and nigella sativa oils. As observed, worn surfaces did not exhibit similar morphology. Fig. 5 shows the SEM images of the UHMWPE surface after dry wear tests under different test cycles. As can be seen the UHMWPE suffered significant damage: some grooves, roughness and delamination. As can be figured out from Fig. 5a and b, some grooves and detached polymer crushed into small particles located at the track extremities. This supposes the formation of adhesive joints and transfer phenomena at the interface of the pin. Wear debris are carried away by pins and ejected outside the wear track. Moreover, important plastic deformations occur at the boundary of the wear track (Fig. 5c) with clear appearance of delamination. The observed finding can be attributed to an increase of temperature at the interface. At 15,000 cycles (Fig. 5d), the UHMWPE shows a significant crushed polymer under the pin and seems to hide the wear grooves. The EDS analysis of UHMWPE disc at the completion of the test is shown in Fig. 5e. This figure demonstrates the transfer of Fe, Cr and Ni, components of the pin. Fig. 5f shows significant degradation of the stainless steel pin with some
Fig. 5. SEM images of UHMWPE after wear tests under dry conditions: (a) 2000 cycles; (b) 6000 cycles; (c) 10,000 cycles; (d) 15,000 cycles; (e) EDS analysis of UHMWPE after 15,000 cycles and (f) SEM image of the pin after 15,000 cycles. Red lines indicate the movement direction.
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Fig. 6. SEM images of UHMWPE after wear tests in presence of the saline solution: (a) 2000 cycles; (b) 6000 cycles; (c) 15,000 cycles and (d) Surface EDS analysis of UHMWPE after 15,000 cycles.
Fig. 7. SEM images of UHMWPE after wear tests in the presence of sesame oil: (a) 2000 cycles; (b) 6000 cycles; (c) 10,000 cycles and (d) 15,000 cycles.
transferred polymer film. A similar observation was made by Wilches et al. [33] in AISI 316LVM-UHMWPE couple. The obtained conclusions are strengthened by the ascertainment taken from the discussion of the friction coefficient curve of Fig. 2. With regards to the wear mechanism, Wilches et al. [33], Liu et al. [34] and Gongde et al. [35] demonstrated that the development of polymer adhesive joints and the failure of the cohesive sub-surface govern the
first stages of UHMWPE surface degradation. The results found in this work are in accordance with these conclusions. It remains that the scratches suggest the presence of some abrasive wear mode. This finding was observed by Kobayashi et al. [11] and Patten et al. [36]. Fig. 6 shows the worn surfaces of UHMWPE in the presence of saline solution. It can be retained as prior observation, that the
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Fig. 8. SEM images of UHMWPE after wear tests in the presence of nigella sativa oil: (a) 6000 cycles; (b) 10,000 cycles and (c) 15,000 cycles.
Fig. 9. EDS of stainless steel pin after 15,000 cycles of wear tests: (a, b) dry; (c,d) sesame oil and (e,f) nigella sativa oil.
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wear mechanism is different from that of the dry test. In fact, the worn surfaces tested under saline solution look to be smoother and some ploughing actions are observed. It is to mention that for the first 2000 cycles (Fig. 6a), the polymer surface does not show any remarkable wear event. When the number of cycles increases (Fig. 6b) some shallow grooves and debris are generated. At 15,000 cycles (Fig. 6c), the wear track appears to be very soft and it seems that the wear mechanism of UHMWPE under lubrication conditions was mainly adhesive wear. The observed change in the morphology, compared to dry test can be ascribed to the decrease of contact temperature due the presence of water. These results are in accordance with those of Marcus & Allen [37]. Fig. 6d shows the EDS analysis of the wear track of UHMWPE under saline solution after 15,000 cycles. The analysis reveals that the wear track was composed of low trace of Fe and Cr originated from the stainless steel pin and Cl and Na components of the test solution. The observed result explains that material transfer is occurred between the mating materials. Figs. 7 and 8 show that with the use of oil lubrication, the scratches cannot be observed and wear seems to decrease remarkably. The use of both of oils show close morphological appearances. Fig. 7 shows that, some debris are generated after 6000 cycles (Fig. 7b) and the surface becomes more tormented with the increase of the number of cycles (Fig. 7c and d). The surface of the UHMWPE at the completion of the wear test is smooth with some plastic deformation which supposes an adhesive wear mechanism. Fig. 8 shows that, plastic deformations appear during the first 6000 cycles (Fig. 8a) and then some debris are generated with approximately 2 mm of size (Fig. 8b). At the completion of the test the UHMWPE surface exhibits rough surface with some shallow grooves (Fig. 8c). As remarked with sesame oil, using nigella sativa oil seems to produce an adhesive wear mechanism. The examination of stainless steel pin after 15,000 cycles of wear tests shows remarkable degradation under dry conditions (Fig. 9a). Moreover the EDS spectrum reveals the presence of carbon (1.9%) which can confirm the transfer phenomenon of a polymeric film on the surface. The use of sesame and nigella sativa oils decreases significantly the degradation impact of the stainless steel pins (Fig. 9c–e). EDS analysis shows an appreciable percentage of carbon (11.9%) on the pin surface under sesame oil (Fig. 9d) compared to that of nigella sativa oil (3.2%). These results suggest that the chemical adsorption of oils on the stainless steel surface depends on the oil itself and it seems that sesame oil has more adsorption ability. The decrease of wear loss with oils might be attributed to the effect of their main components: fatty acids. It is well known that the fatty acids have very interesting tribological performances [16–19]. Generally, natural oils have high lubricity, high shear stability, low volatility and high viscosity index [18]. The used oils
Fig. 10. Representation of triglyceride structure with three fatty acids having different level of saturation.
Table 3 Fatty acids composition of sesame and nigella sativa oils (ISO 5508 COI/T.20/ DOCN°24). Fatty acid name
Sesame oil
Nigella sativa oil
Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic acid (C20:0) Behenic acid (C22:0) Others
0.20 10.70 0.20 5.20 41.50 40.30 0.50 0.60 0.40 Rest
0.30 17.20 1.20 2.80 25.00 50.30 0.34 0.10 2.00 Rest
have triglyceride structural molecules with long chains of polar fatty acids (Fig. 10) [17,38–40]. The fatty acid chains which contain 8–22 carbon atoms are advantageous in boundary lubrication due to their capacity to attach to metallic surfaces and to form a multimolecular layer on the surface conducting to less friction and wear [39,40]. The main difference among natural oils is the amount of unsaturated and saturated fatty acids. Table 3 presents the percentages of the main fatty acids of sesame and nigella sativa oils. Both acids are composed entirely of triacylglycerol with up of 95% of fatty acids including both saturated (palmitic and stearic acids) and unsaturated (oleic and linoleic acids). The examination of Table 3 shows that the high amount of fatty acids contained in sesame and nigella sativa oils are unsaturated: oleic and linoleic acids and the most important percentage of saturated fatty acids are palmitic and stearic acids. With regards to the composition analysis of sesame and nigella sativa oils, it seems that there is strong correlation between the tribological behavior in the contact stainless steel/UHMWPE and the amount of unsaturated fatty acids. These results are in agreement with those of Reeves et al. [18]. They demonstrated that natural oils having high oleic and low linoleic acids content tend to have low wear volume. As can be seen from Table 3, sesame oil has the higher oleic acid content and the lower wear volume. According to Reeves et al. [18], the observed effects are related to the structure of the fatty acids. The linoleic acid has two double bonds but oleic acid has one double bond [18].The oleic acid will forms on the stainless steel dense monolayer (Fig. 11) which will prevent wear and minimize the CoF. The unsaturation number (UN) of sesame and sativa oils can give the average number of double bonds contained by the triacylglycerol molecule of both oils. This number was found to be correlated with the tribological behavior of oil lubricant. The UN is computed from the fatty acid percentages as follows [18]: 1 h X UN ¼ 1x %monounsaturatedfattyacids 100 X þ 2x %diunsaturatedfattyacids X i þ 3x %triunsaturatedfattyacids Table 4 gives the calculated UN from values of Table 3 and the wear volume of nigella sativa and sesame oils after 15,000 cycles. The analysis of Table 4 shows that there is a correlation between the responses. In fact, the wear volume increases with the increase of the UN. These results suppose that a great number of double bonds prevents the fatty acid chains to form and adsorbed protective layer on the stainless steel pin. The obtained results are in accordance with those of Reeves et al.[18]. Based on some proposed adsorption mechanisms of fatty acids on steel existing in the literature [18,39–42], the presence of oxides and hydroxides favors the hydrogen bonding of the fatty acids
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Fig. 11. Schematic presentation of (a) monolayer adsorption; (b) multilayer adsorption and (c) formation of carboxylate structures. Table 4 Unsaturation number and wear volume of nigella sativa and sesame oils. Oil
Unsaturation number
Wear volume (mm3)
Nigella sativa Sesame
1.266 1.236
0.0275 0.0236
molecules to the steel surface. Moreover, due to the frictional heat in the contact the chemisorptions is improved and fatty acid molecules can dissociate to form an iron carboxylate [18,39–42]. Loehlé et al. [40] have also demonstrated that contact pressure and local temperature rise favor the chemisorption via the two oxygen atoms of the acid group with the formation of iron carboxylate. The carboxylate anions can adsorb via two different ways (Fig. 11c): (i) symmetric through two oxygen atoms and (ii) asymmetric through one oxygen atom, the C ¼O bond is kept intact [39]. In our case, the oxidized layer of stainless steel, mainly, contains FeOOH, NiO and Cr2O3. When these oxides are exposed to fatty acids, we suppose that iron, nickel and chromium carboxylate complexes are formed according to: FeOOHþRCOOH-RCOOFeO þH2O NiO þ2RCOOH-(RCOO)2Ni þH2O Cr2O3 þ 6RCOOH-2[(RCOO)3Cr] þ3H2O According to Sahoo et Biswas [39], fatty acids migrate to the contact, especially to the available vacant sites. They adsorb and form a protective layer till they are removed by wear and the process is repeated. Adsorption and desorption is a dynamic process. On the other side, some authors have proposed other different mechanisms of chemisorptions [43,44]. According to Sahoo and Biswas [39], the strong coupling between the high electronic charge density related with the
double bonds and one of the mating materials is possible when one of them is metallic. These authors have demonstrated that there is no difference in the frictional performances of stearic and linoleic acids when they are used with silicon as non-metallic substrate. Simic and Kalin [42] have established that the adsorption of fatty acid molecules on steel is much better than that on diamond-like-carbon (DLC). In an earlier work Kalin et al. [41] have shown a better wear behavior steel/steel contact than that of DLC/DLC contact when using synthetic (saturated and un-saturated) and natural (sunflower) biodegradable oils. From this we infer that, in our configuration there is no interaction between the chemical contents of sesame and nigella sativa oils and the UHMWPE. This suggests that the use of these bio-lubricants will be more efficient when they are used in metallic-on-metallic (MOM) biomaterials. It remains that, elaborating UHMWPE with filler as additives to activate the adsorption of fatty acids is a very advantageous solution to more improve the tribological behavior of the stainless steel/UHMWPE couple. On the other hand, the thermal–oxidative property of natural oil is a pertinent parameter not to neglect when characterizing the efficiency of a lubricant [16,18]. It is well known that the decomposition temperature depends on the chemical composition of the oil. The higher the content of fatty acids having 18 carbon atoms the higher the decomposition temperature is [18]. Accordingly, sesame oil seems to be slightly resistant to oxidation than nigella sativa oil. Many authors have demonstrated that natural oils still intact up to a temperature of about 370 °C [45] which is very high comparatively to the human body temperature. In the same context, it was well established that the natural oils reveal constant viscosity against shear rate, showing Newtonian behaviors [46]. Sesame and nigella sativa oils seem to be good bio-lubricant base stock candidates for hip or knee joints. Spector [14], Shanbhag and Johansson [15] have demonstrated in earlier works that, the affinity of saturated fatty acids to human serum albumin increases with the increase of their chain length and this between chain lengths of 8 and 18 carbons. In fact, they can be mixed with
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the existing lubricants as an intra-articular supplement as proposed by Kobayashi et al.[11]. In order to well utilize them, many other properties must be investigated such as thermal, oxidative, hydrolytic stabilities and the interaction with the human body. For more stable oils many researchers have successfully modified their composition by increasing the proportion of oleic acid [47] or by adding antioxidants [38] and antiwear additive [48].
4. Conclusion This study was focused on the friction and wear behaviors of M30NW stainless steel against UHMWPE used as biomaterials in hip and knee joints under dry and lubricating conditions. Saline solution (0.9% NaCl), sesame and nigella sativa oils were used as lubricants. The tribological tests were carried out with the aid of pin on disc tribometer. The current study revealed the following conclusions:
Sesame and nigella sativa oils were shown to have the best
tribological behaviors when compared to dry and saline solution. The wear volume of the UHMWPE under dry conditions is two times higher than oil lubricants. The CoF of oil lubricants was reduced two and seven times compared to saline solution and dry conditions respectively, Morphological and chemical analyses using SEM and EDS show that the wear mechanism is adhesive–abrasive under dry conditions with the highest wear volume; adhesive–oxidative under saline solution with a moderate wear volume and adhesive under oil lubrications with the lowest wear volume, The observed low coefficient of friction (CoF) and wear volume of natural oils was ascribed to the fatty acids adsorption on the stainless steel surface and the formation of carboxylate complexes soap. The relative difference in the tribological behavior between the both tested oils is related to the unsaturated fatty acids contents.
Acknowledgments The authors would like to thank Monastir University (LGM: LAB-MA-05) and the Ministry of Higher Education and Scientific Research-Tunisia for their support. We would like to thank Mr. Didier VOILLEMIN the manager of C2T implants industry in Tunisia for his collaboration. The authors are, also, grateful to Mr. Cyril GORNY and Mlle. Sarah BAIZ from PIMM Laboratory of ENSAM-ParisTech (France) for their support carrying out SEM and 3D profilometry.
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Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effect of bio-lubrication on the tribological behavior of UHMWPE against M30NW stainless steel M. Guezmil a, W. Bensalah b,n, S. Mezlini a a b
Laboratoire de Génie Mécanique (LGM), ENIM, Monastir University, Rue Ibn Eljazzar 5019, Tunisia Laboratoire de Génie des Matériaux et Environnement (LGME), ENIS, Sfax University, B.P.W. 1173-3038, Tunisia
art ic l e i nf o
a b s t r a c t
Article history: Received 26 July 2015 Received in revised form 14 October 2015 Accepted 19 October 2015 Available online 27 October 2015
The tribological behavior of M30NW stainless steel against UHMWPE was investigated. The wear tests were conducted on a reciprocating pin-on-disc tribometer under dry and lubricated conditions. Saline solution (NaCl 0.9%), sesame oil and nigella sativa oil were used as bio-lubricants. The friction coefficient and wear volume of UHMWPE were examined. It is found that under oil lubrication, the friction and wear behaviors of UHMWPE were the best. Morphological and chemical studies of the worn surfaces were conducted and wear mechanisms were proposed. The observed performance of natural oils was linked to their chemical composition and their adsorption ability to the stainless steel. & 2015 Elsevier Ltd. All rights reserved.
Keywords: UHMWPE Bio-lubricant Friction coefficient Wear mechanism
1. Introduction Nowadays, the most implanted type of hip joint consists of an ultra-high-molecular weight polyethylene (UHMWPE) acetabular cup and metallic or ceramic femoral head. During use, UHMWPE wear particles are released nearby tissue leading to aseptic loosening of the acetabular or femoral components [1,2]. The produced UHMWPE wear particles and the consequent tissue reactions have been known as one of the main sources of osteolysis and implant failure [3,4]. Therefore, in order to increase the longevity of total hip replacement joint, the amount of the generated UHMWPE wear debris must be reduced. Many researchers have developed some kinds of reinforcements such as hydroxyapatite [5], carbon nanotubes (CNT) [6], nacre [7] and talc [8]. An alternative to improve the wear behavior of the UHMWPE is by using lubricants such as distilled water, saline solution, polyethylene glycol (PEG), sodium hyaluronate, bovine and calf serum as well as hyaluronic acid [9–13]. Throughout attempts aimed to overcome this problem by developing new solutions, we have retained to try effective and adequate natural bio-oils, as lubricants, ensuring in the same time low wear rate and therapeutic effects. Knowing that there is a good affinity between fatty acids, the main components of bio oils, and the human serum [14,15], these bio-lubricants can be used as additive to the existing well known lubricants as bovine and calf n
Corresponding author. E-mail address: [email protected] (W. Bensalah).
http://dx.doi.org/10.1016/j.triboint.2015.10.022 0301-679X/& 2015 Elsevier Ltd. All rights reserved.
serum without affecting their tribological performances and ensuring healing impacts. Since the beginning of the 20th century, pure natural oils have gained a great interest as lubricants in various industrial applications. In fact natural oils have demonstrated good results in lowering friction and wear when they are used in tribological purposes [16–19]. This choice is, also, strongly motivated by the environmentally-friendly character of natural oils. These oils are also known to be biodegradable, non-toxic, renewable and can be used in most cases as ingredient in many medicinal and cosmetic products. Among natural oils, we have focused our attention on sesame and nigella sativa oils. This choice is enthused by double great interests: good tribological performance and wide medical and pharmaceutical applications [16–27]. Many previous works have been undertaken in order to evaluate the tribological or the therapeutic potentials of sesame and nigella sativa oils [16–27]. Reeves et al. [18] have studied the friction and wear properties of eight oils in copper/2024 aluminum contact. They have demonstrated that sesame oil has promising friction of coefficient and wear volume. Mannekote et al. [16] have tested the aging effect on five oils using a standard four ball tester machine. They have shown that fresh sesame oil ensures very low coefficient of friction (o0.1). From therapeutic point of view, studies on sesame oil have shown that it can reduce the growth of human colon cancer in vitro [21], decreases blood pressure, lowers lipid peroxidation, and enhances the status of antioxidant in hypertensive patients [22]. Hsu et al. have demonstrated that the oral administration of sesame oil can reduce proinflammatory cytokine and the production of nitric oxide [23].
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Sesame oil helps joints keep their flexibility, helps to attenuate oxidative stress, ameliorate multiple organ failure, enhance survival rates [24], and to reduce atherosclerosis lesion formation [25]. Turning to the nigella sativa oil benefits, it is used as antihypertensive, anti-bacterial, anti-rheumatism, anti-tumor, antiinflammatory, anti-parasitic and anti-histamine [26,27]. It is found that Nigella sativa oil promotes the growth rate of bone marrow cells. Moreover, it strengthens the immune system and promotes lactation [26,27]. It remains that, to the author knowledge, the study of the tribological performance of nigella sativa oil has not yet been done. In the present paper, we compare the tribological behavior of UHMWPE against M30NW stainless steel pin using reciprocating pin-on-disc tribometer in dry and lubricated conditions. Saline solution (0.9% NaCl), sesame and nigella sativa oils were used as lubricants. The coefficient of friction (CoF) and wear volume of UHMWPE against stainless steel were investigated and compared. Morphological studies were, equally, conducted and wear mechanisms were proposed. The observed difference between oils performance was linked to their chemical composition and their adsorption ability to the stainless steel.
2. Experimental details 2.1. Materials During this study, M30NW stainless steel was used as the pin material. The pins have hemi-spherical shape with 10 mm diameter. The discs of UHMWPE were machined in cylindrical shape with 30 mm diameter and 10 mm thickness from a medical-grade bar. M30NW stainless steel and UHMWPE were supplied by C2F implants- France (Tunisian plant). Fig. 1 shows photographs of the UHMWPE disc and the stainless steel pin. The chemical composition of the stainless steel pin is given in Table 1. The microhardness of the M30NW stainless steel was 4407 20 HV0.5 and the shore hardness D of the UHMWPE was 65 SH D. Three lubricants were selected: saline solution 0.9% NaCl in deionized water, sesame oil, and nigella sativa oil. The saline solution, named equally physiological solution, is isotonic with blood plasma. The used oils were cold-press extracted by STVPAA industry (Tunisia) for cosmetic, pharmaceutical and nutritious domains. Cold pressing is a technology which does not requires heat or chemical treatment during the oil extraction process. The physiochemical properties of sesame and nigella sativa oil are presented in Table 2. It is to mention that the viscosity of oils was conducted using an open air viscometer.
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Fig. 1. Scheme of the wear device with the Stainless steel pin and UHMWPE disc images. Table 1 Chemical composition of M30NW stainless steel. Element
C
Cr
Ni
Mn
Mo
N
Weight (%)
r 0.06
21
9.5
4
2.2
0.40
Table 2 Physicochemical properties of sesame and nigella sativa oils. Property
Unit
Sesame oil
Nigella sativa oil
Density at 20 °C Viscosity at 20 °C Saponification value Water content Iodine value Refractive index at 40 °C
Kg/m3 mPa s mg KOH/g of oil % g/100 g
923 58 195 o 0.1 104 1.465
915 64.5 204 o 0.1 119 1.470
in vivo and corresponds to the contact pressure of 51 MPa. The stroke range was 30 mm and the sliding speed was 30 mm/s. This value corresponds to that usually found in hip joints (0–50 mm/s) [28]. The number of cycles was varied from 0 to 15,000 cycles which is equal to a total sliding distance of 450 m. The friction force was digitally recorded using a load transducer attached to the pin holder. 2.2.2. Morphological and chemical characterization techniques The worn surfaces of UHMWPE discs and the stainless steel pins were examined by Hitachi FEG 4800 Scanning Electron Microscope (SEM) coupled with an Energy Dispersive X-ray Spectroscopy (EDS) probe. Before carrying out SEM images, the UHMWPE were coated with sputtered gold layers. s The 3D profilometry was examined using a VEECO profils ometer and the obtained data were analyzed by the DEKTAK 3D software.
2.2. Methods 3. Results and discussion 2.2.1. Wear test Wear tests on UHMWPE have been performed with a reciprocating pin on disc tribometer at room temperature (Fig. 1). Before tests, samples were polished using polishing papers up to #4000 to obtain an adequate roughness and then they were cleaned by ethanol. The initial roughnesses of the stainless steel pins and the UHMWPE discs were respectively 0.06 70.01 mm and 0.31 70.05 mm. During the friction tests, the stainless steel pin was fixed to the specimen holder and the UHMWPE disc was implemented reciprocating motion continuously. The normal load during the friction test was set to 20 N. This value was chosen based on a simplification developed by Borruto [9]. The used loading condition is assumed to be equivalent of that
3.1. Friction behavior Fig. 2 shows the variation of the coefficient of friction (CoF) of stainless steel pin against UHMWPE disc under dry and lubrication conditions as a function the number of cycles. It can be seen that there is an increase of the coefficient of friction at the start when working under dry and saline solution. The COF under dry conditions and saline solution reaches steady-state at approximately 4000 and 12,000 cycles respectively. The use of sesame and nigella sativa oils results in a significant reduction of fluctuations and the steady-state is reached since the first 3000 cycles. In contrary to the dry and saline solution, the CoF decreases slowly at the beginning of tests when oils are used. Moreover, the examination
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Fig. 2. Coefficient of friction of UHMWPE against stainless steel pins under dry and lubrication conditions as a function of number of cycles.
of Fig. 2 shows that lubrication decreases the friction coefficient and the lowest values of the studied response are obtained with both sesame and nigella sativa oils. The CoF of oil lubricants was reduced two and seven times compared to saline solution and dry conditions respectively. The decrease of the CoF would be an important factor in decreasing UHMWPE wear. It is to be noted that the fluctuations observed in the dry test and with the use of saline solution are accentuated during the first thousands of cycle. The observed fact supposes the presence of morphological changes in the contact between the mating materials. Under dry condition, the energy and the work of friction force produced in contact are very important. These dissipate in the stainless steel pin and the UHMWPE disc, causing them to heat. Under the simultaneous action of overheating and local overstressing many changes will occur during sliding, the stainless steel ball and the UHMWPE disc will be subjected to geometric and chemical modifications. In fact, the rise of the CoF can be related to a severe plastic deformation and/or adherence of polymer in the contact. During these tests, wear particles can be removed easily from the UHMWPE disc surface. This action will increase the surface roughness of the polymer. In fact, the roughness of the UHMWPE before the wear tests was 0.31 mm. This roughness reached 1.27 mm, 0.67 mm, 0.49 mm and 0.45 mm for dry, saline solution, nigella sativa and sesame oil conditions at the completion of the test. Therefore, wear particles would react as third body in the friction process which can lead to high fluctuation. For a long duration of the friction test, the active chlorine ions (Cl ) of the saline solution could form with the disturbed surface of the stainless steel pin a chemical reaction layer [29,30]. According to Yan et al. [29], this layer, when formed, is essentially, composed of lepidocrocite (FeOOH), magnetite (Fe2O3), Cr2O3 and NiO with thickness of 15–20 nm. Turning to sesame and nigella sativa oils, the remarkable decrease of the CoF is mainly attributed to their chemical composition, more specifically their fatty acids composition [18]. Further discussion on the effect of the chemical composition on friction and wear of UHMWPE will be presented in Section 3.3. Finally, it is to mention that the COF can be related to the lubrication regime in the studied contact. A suitable method to evaluate this relation is the Stribeck curve, largely used for oils [31]. This curve relates the COF with the product of tangential velocity and dynamic viscosity divided by the normal contact load [32]. In our case, the normal load and the tangential velocity are maintained constant for all tests. The main difference between the studied lubricants is the viscosity. The viscosity of sesame and
Fig. 3. (a) Typical 3D wear track area on the UHMWPE and (b) typical 2D view of x– z plane from a wear track.
nigella sativa oil is higher than that of saline solution. The decrease of the COF when oils were used is probably due to a change in the hydrodynamic behavior. The long chain of molecules in sesame and nigella sativa oils seem to maintain a lubricating film between the stainless steel and the UHMWPE. Further study is needed in order to give the predominant regime of lubrication in the retained contact. 3.2. Wear behavior The coefficient of friction is not enough to characterize the tribological behavior of the studied couple of materials. In order to have more complete information, the wear volume was computed using 3D profilometric scans. Fig. 3a shows a typical 3D scan of the wear track on the UHMWPE and Fig. 3b presents a typical 2D view taken on a given position from the wear track. The 2D view can provide information about the wear surface, the depth and the width of the wear track. Fig. 4 depicts the variation of the UHMWPE wear volume after 15,000 cycles under dry and various lubricants. As can be concluded from this figure, the use of lubricants increases remarkably the wear resistance of the UHMWPE. The wear volume of the UHMWPE under dry conditions is 2 times higher than with oil lubricants. Among all conditions, under dry friction the UHMWPE had the highest wear volume (0.0511 mm3). Fig. 4 shows that sesame oil ensures the lowest wear volume. Moreover, nigella sativa oil has the closest wear volume to sesame oil and saline solution has a moderate wear volume. Compared to the dry test, the wear volume was reduced to 40%, 46% and 54% when using saline solution, nigella sativa oil and sesame oil respectively. These results are in accordance with those of the coefficient of friction, low CoF induces low wear volume. With regards to the obtained results, sesame and nigella sativa oils demonstrate good tribological performances. These benefits
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can be related to their chemical composition [16–19]. Furthermore, the observed wear difference between the oils seems to have direct relation with their composition, more particularly their fatty acids content [16–19]. Therefore, it is sensible to undergo a chemical analysis of the oils in order to show possible correlation between the tribological performance and the chemical composition.
Fig. 4. Variation of the wear volume measured on the UHMWPE at the completion of the test ( 15,000 cycles) under dry and lubricated conditions.
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3.3. Morphological analysis and wear mechanism After the wear tests, wear tracks were investigated using SEM analysis (Figs 5–8). These figures show SEM micrographs of wear tracks under dry, saline solution, sesame and nigella sativa oils. As observed, worn surfaces did not exhibit similar morphology. Fig. 5 shows the SEM images of the UHMWPE surface after dry wear tests under different test cycles. As can be seen the UHMWPE suffered significant damage: some grooves, roughness and delamination. As can be figured out from Fig. 5a and b, some grooves and detached polymer crushed into small particles located at the track extremities. This supposes the formation of adhesive joints and transfer phenomena at the interface of the pin. Wear debris are carried away by pins and ejected outside the wear track. Moreover, important plastic deformations occur at the boundary of the wear track (Fig. 5c) with clear appearance of delamination. The observed finding can be attributed to an increase of temperature at the interface. At 15,000 cycles (Fig. 5d), the UHMWPE shows a significant crushed polymer under the pin and seems to hide the wear grooves. The EDS analysis of UHMWPE disc at the completion of the test is shown in Fig. 5e. This figure demonstrates the transfer of Fe, Cr and Ni, components of the pin. Fig. 5f shows significant degradation of the stainless steel pin with some
Fig. 5. SEM images of UHMWPE after wear tests under dry conditions: (a) 2000 cycles; (b) 6000 cycles; (c) 10,000 cycles; (d) 15,000 cycles; (e) EDS analysis of UHMWPE after 15,000 cycles and (f) SEM image of the pin after 15,000 cycles. Red lines indicate the movement direction.
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Fig. 6. SEM images of UHMWPE after wear tests in presence of the saline solution: (a) 2000 cycles; (b) 6000 cycles; (c) 15,000 cycles and (d) Surface EDS analysis of UHMWPE after 15,000 cycles.
Fig. 7. SEM images of UHMWPE after wear tests in the presence of sesame oil: (a) 2000 cycles; (b) 6000 cycles; (c) 10,000 cycles and (d) 15,000 cycles.
transferred polymer film. A similar observation was made by Wilches et al. [33] in AISI 316LVM-UHMWPE couple. The obtained conclusions are strengthened by the ascertainment taken from the discussion of the friction coefficient curve of Fig. 2. With regards to the wear mechanism, Wilches et al. [33], Liu et al. [34] and Gongde et al. [35] demonstrated that the development of polymer adhesive joints and the failure of the cohesive sub-surface govern the
first stages of UHMWPE surface degradation. The results found in this work are in accordance with these conclusions. It remains that the scratches suggest the presence of some abrasive wear mode. This finding was observed by Kobayashi et al. [11] and Patten et al. [36]. Fig. 6 shows the worn surfaces of UHMWPE in the presence of saline solution. It can be retained as prior observation, that the
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Fig. 8. SEM images of UHMWPE after wear tests in the presence of nigella sativa oil: (a) 6000 cycles; (b) 10,000 cycles and (c) 15,000 cycles.
Fig. 9. EDS of stainless steel pin after 15,000 cycles of wear tests: (a, b) dry; (c,d) sesame oil and (e,f) nigella sativa oil.
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wear mechanism is different from that of the dry test. In fact, the worn surfaces tested under saline solution look to be smoother and some ploughing actions are observed. It is to mention that for the first 2000 cycles (Fig. 6a), the polymer surface does not show any remarkable wear event. When the number of cycles increases (Fig. 6b) some shallow grooves and debris are generated. At 15,000 cycles (Fig. 6c), the wear track appears to be very soft and it seems that the wear mechanism of UHMWPE under lubrication conditions was mainly adhesive wear. The observed change in the morphology, compared to dry test can be ascribed to the decrease of contact temperature due the presence of water. These results are in accordance with those of Marcus & Allen [37]. Fig. 6d shows the EDS analysis of the wear track of UHMWPE under saline solution after 15,000 cycles. The analysis reveals that the wear track was composed of low trace of Fe and Cr originated from the stainless steel pin and Cl and Na components of the test solution. The observed result explains that material transfer is occurred between the mating materials. Figs. 7 and 8 show that with the use of oil lubrication, the scratches cannot be observed and wear seems to decrease remarkably. The use of both of oils show close morphological appearances. Fig. 7 shows that, some debris are generated after 6000 cycles (Fig. 7b) and the surface becomes more tormented with the increase of the number of cycles (Fig. 7c and d). The surface of the UHMWPE at the completion of the wear test is smooth with some plastic deformation which supposes an adhesive wear mechanism. Fig. 8 shows that, plastic deformations appear during the first 6000 cycles (Fig. 8a) and then some debris are generated with approximately 2 mm of size (Fig. 8b). At the completion of the test the UHMWPE surface exhibits rough surface with some shallow grooves (Fig. 8c). As remarked with sesame oil, using nigella sativa oil seems to produce an adhesive wear mechanism. The examination of stainless steel pin after 15,000 cycles of wear tests shows remarkable degradation under dry conditions (Fig. 9a). Moreover the EDS spectrum reveals the presence of carbon (1.9%) which can confirm the transfer phenomenon of a polymeric film on the surface. The use of sesame and nigella sativa oils decreases significantly the degradation impact of the stainless steel pins (Fig. 9c–e). EDS analysis shows an appreciable percentage of carbon (11.9%) on the pin surface under sesame oil (Fig. 9d) compared to that of nigella sativa oil (3.2%). These results suggest that the chemical adsorption of oils on the stainless steel surface depends on the oil itself and it seems that sesame oil has more adsorption ability. The decrease of wear loss with oils might be attributed to the effect of their main components: fatty acids. It is well known that the fatty acids have very interesting tribological performances [16–19]. Generally, natural oils have high lubricity, high shear stability, low volatility and high viscosity index [18]. The used oils
Fig. 10. Representation of triglyceride structure with three fatty acids having different level of saturation.
Table 3 Fatty acids composition of sesame and nigella sativa oils (ISO 5508 COI/T.20/ DOCN°24). Fatty acid name
Sesame oil
Nigella sativa oil
Myristic acid (C14:0) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Stearic acid (C18:0) Oleic acid (C18:1) Linoleic acid (C18:2) Linolenic acid (C18:3) Arachidic acid (C20:0) Behenic acid (C22:0) Others
0.20 10.70 0.20 5.20 41.50 40.30 0.50 0.60 0.40 Rest
0.30 17.20 1.20 2.80 25.00 50.30 0.34 0.10 2.00 Rest
have triglyceride structural molecules with long chains of polar fatty acids (Fig. 10) [17,38–40]. The fatty acid chains which contain 8–22 carbon atoms are advantageous in boundary lubrication due to their capacity to attach to metallic surfaces and to form a multimolecular layer on the surface conducting to less friction and wear [39,40]. The main difference among natural oils is the amount of unsaturated and saturated fatty acids. Table 3 presents the percentages of the main fatty acids of sesame and nigella sativa oils. Both acids are composed entirely of triacylglycerol with up of 95% of fatty acids including both saturated (palmitic and stearic acids) and unsaturated (oleic and linoleic acids). The examination of Table 3 shows that the high amount of fatty acids contained in sesame and nigella sativa oils are unsaturated: oleic and linoleic acids and the most important percentage of saturated fatty acids are palmitic and stearic acids. With regards to the composition analysis of sesame and nigella sativa oils, it seems that there is strong correlation between the tribological behavior in the contact stainless steel/UHMWPE and the amount of unsaturated fatty acids. These results are in agreement with those of Reeves et al. [18]. They demonstrated that natural oils having high oleic and low linoleic acids content tend to have low wear volume. As can be seen from Table 3, sesame oil has the higher oleic acid content and the lower wear volume. According to Reeves et al. [18], the observed effects are related to the structure of the fatty acids. The linoleic acid has two double bonds but oleic acid has one double bond [18].The oleic acid will forms on the stainless steel dense monolayer (Fig. 11) which will prevent wear and minimize the CoF. The unsaturation number (UN) of sesame and sativa oils can give the average number of double bonds contained by the triacylglycerol molecule of both oils. This number was found to be correlated with the tribological behavior of oil lubricant. The UN is computed from the fatty acid percentages as follows [18]: 1 h X UN ¼ 1x %monounsaturatedfattyacids 100 X þ 2x %diunsaturatedfattyacids X i þ 3x %triunsaturatedfattyacids Table 4 gives the calculated UN from values of Table 3 and the wear volume of nigella sativa and sesame oils after 15,000 cycles. The analysis of Table 4 shows that there is a correlation between the responses. In fact, the wear volume increases with the increase of the UN. These results suppose that a great number of double bonds prevents the fatty acid chains to form and adsorbed protective layer on the stainless steel pin. The obtained results are in accordance with those of Reeves et al.[18]. Based on some proposed adsorption mechanisms of fatty acids on steel existing in the literature [18,39–42], the presence of oxides and hydroxides favors the hydrogen bonding of the fatty acids
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Fig. 11. Schematic presentation of (a) monolayer adsorption; (b) multilayer adsorption and (c) formation of carboxylate structures. Table 4 Unsaturation number and wear volume of nigella sativa and sesame oils. Oil
Unsaturation number
Wear volume (mm3)
Nigella sativa Sesame
1.266 1.236
0.0275 0.0236
molecules to the steel surface. Moreover, due to the frictional heat in the contact the chemisorptions is improved and fatty acid molecules can dissociate to form an iron carboxylate [18,39–42]. Loehlé et al. [40] have also demonstrated that contact pressure and local temperature rise favor the chemisorption via the two oxygen atoms of the acid group with the formation of iron carboxylate. The carboxylate anions can adsorb via two different ways (Fig. 11c): (i) symmetric through two oxygen atoms and (ii) asymmetric through one oxygen atom, the C ¼O bond is kept intact [39]. In our case, the oxidized layer of stainless steel, mainly, contains FeOOH, NiO and Cr2O3. When these oxides are exposed to fatty acids, we suppose that iron, nickel and chromium carboxylate complexes are formed according to: FeOOHþRCOOH-RCOOFeO þH2O NiO þ2RCOOH-(RCOO)2Ni þH2O Cr2O3 þ 6RCOOH-2[(RCOO)3Cr] þ3H2O According to Sahoo et Biswas [39], fatty acids migrate to the contact, especially to the available vacant sites. They adsorb and form a protective layer till they are removed by wear and the process is repeated. Adsorption and desorption is a dynamic process. On the other side, some authors have proposed other different mechanisms of chemisorptions [43,44]. According to Sahoo and Biswas [39], the strong coupling between the high electronic charge density related with the
double bonds and one of the mating materials is possible when one of them is metallic. These authors have demonstrated that there is no difference in the frictional performances of stearic and linoleic acids when they are used with silicon as non-metallic substrate. Simic and Kalin [42] have established that the adsorption of fatty acid molecules on steel is much better than that on diamond-like-carbon (DLC). In an earlier work Kalin et al. [41] have shown a better wear behavior steel/steel contact than that of DLC/DLC contact when using synthetic (saturated and un-saturated) and natural (sunflower) biodegradable oils. From this we infer that, in our configuration there is no interaction between the chemical contents of sesame and nigella sativa oils and the UHMWPE. This suggests that the use of these bio-lubricants will be more efficient when they are used in metallic-on-metallic (MOM) biomaterials. It remains that, elaborating UHMWPE with filler as additives to activate the adsorption of fatty acids is a very advantageous solution to more improve the tribological behavior of the stainless steel/UHMWPE couple. On the other hand, the thermal–oxidative property of natural oil is a pertinent parameter not to neglect when characterizing the efficiency of a lubricant [16,18]. It is well known that the decomposition temperature depends on the chemical composition of the oil. The higher the content of fatty acids having 18 carbon atoms the higher the decomposition temperature is [18]. Accordingly, sesame oil seems to be slightly resistant to oxidation than nigella sativa oil. Many authors have demonstrated that natural oils still intact up to a temperature of about 370 °C [45] which is very high comparatively to the human body temperature. In the same context, it was well established that the natural oils reveal constant viscosity against shear rate, showing Newtonian behaviors [46]. Sesame and nigella sativa oils seem to be good bio-lubricant base stock candidates for hip or knee joints. Spector [14], Shanbhag and Johansson [15] have demonstrated in earlier works that, the affinity of saturated fatty acids to human serum albumin increases with the increase of their chain length and this between chain lengths of 8 and 18 carbons. In fact, they can be mixed with
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the existing lubricants as an intra-articular supplement as proposed by Kobayashi et al.[11]. In order to well utilize them, many other properties must be investigated such as thermal, oxidative, hydrolytic stabilities and the interaction with the human body. For more stable oils many researchers have successfully modified their composition by increasing the proportion of oleic acid [47] or by adding antioxidants [38] and antiwear additive [48].
4. Conclusion This study was focused on the friction and wear behaviors of M30NW stainless steel against UHMWPE used as biomaterials in hip and knee joints under dry and lubricating conditions. Saline solution (0.9% NaCl), sesame and nigella sativa oils were used as lubricants. The tribological tests were carried out with the aid of pin on disc tribometer. The current study revealed the following conclusions:
Sesame and nigella sativa oils were shown to have the best
tribological behaviors when compared to dry and saline solution. The wear volume of the UHMWPE under dry conditions is two times higher than oil lubricants. The CoF of oil lubricants was reduced two and seven times compared to saline solution and dry conditions respectively, Morphological and chemical analyses using SEM and EDS show that the wear mechanism is adhesive–abrasive under dry conditions with the highest wear volume; adhesive–oxidative under saline solution with a moderate wear volume and adhesive under oil lubrications with the lowest wear volume, The observed low coefficient of friction (CoF) and wear volume of natural oils was ascribed to the fatty acids adsorption on the stainless steel surface and the formation of carboxylate complexes soap. The relative difference in the tribological behavior between the both tested oils is related to the unsaturated fatty acids contents.
Acknowledgments The authors would like to thank Monastir University (LGM: LAB-MA-05) and the Ministry of Higher Education and Scientific Research-Tunisia for their support. We would like to thank Mr. Didier VOILLEMIN the manager of C2T implants industry in Tunisia for his collaboration. The authors are, also, grateful to Mr. Cyril GORNY and Mlle. Sarah BAIZ from PIMM Laboratory of ENSAM-ParisTech (France) for their support carrying out SEM and 3D profilometry.
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