Experimental and submodeling technique to investigate the wear of silicon nitride against Ti6Al4V alloy with bio-lubricants for various gait activities
Introduction
Implant failure occurs mainly due to the generation of wear particles which results in implant loosening leading to revision surgery. This forces surgeons and engineers to develop new biomaterials that could resist against not only wear and also should be highly biocompatible as well as biodegradable. Most commonly used polyethylene material UHMWPE (ultra high molecular weight polyethylene) against metallic and ceramic femur heads like CoCr (cobalt chromium), Alumina (Al2O3) and Zirconia (ZrO2) generated large amount of wear particles [1,2]. Ceramic-on-Metal (C-o-M) combination was proposed as an alternative to metal-on-polyethylene (M-o-P) and metal-on-metal (M-o-M) [3]. The titanium alloy (Ti6Al4V) known for its high biocompatibility and better corrosion resistance proposed as an alternative material to joint replacement. Similarly, silicon nitride (Si3N4) was found to be a better alternative over metallic implants due to its high fracture toughness, better bio-compatibility, no cytotoxic effects and resistance to crack propagation [4,5]. Moreover, the ion concentration of titanium and Si3N4 in cell culture medium were 0.11Ā Ā±Ā 0.05 and 0.27Ā Ā±Ā 0.04Ā mg/L and showed no cytotoxic effect [6]. Many experimental tests were conducted to analyze the wear of titanium alloy and silicon nitride for the purpose of artificial implantation using bio-lubricant. The vast adopted experimental test setups mainly used for wear analysis of biomaterials include hip simulator and pin-on-disc (POD) tribometer. The titanium alloy against Si3N4 ball was tested using ball-on-flat style wear test rig for different bio-lubricants for a constant load of 9.8Ā N. The titanium alloy exhibited lesser wear rate under lubrication conditions [7]. Though synovial fluid should be a preferred lubricant for tribological testing of biomaterials, many literatures focused on the effect of alternate bio-lubricants like distilled water, sesame oil, ringers solution, 0.9% NaCl (saline solution) and phosphate buffer saline(PBS) [[8], [9], [10], [11], [12]]. These bio-lubricants had significant reduction in friction and wear coefficient based on different parameters like load, sliding distance and velocity tested using POD tribometer and hip simulators. These bio-lubricants exhibited better tribological behavior by developing a tribofilm in reducing the wear and friction between the biomaterials. Though POD tribometer was used in estimating the friction and wear coefficient of bio-implants, several other parameters like radial clearance, gait activities, cup inclination angles and femur head diameter also influences the wear of bearings.
The role of finite element modeling in analyzing the stress and wear of bearing was a widely recognized technique by many researchers and also as a reliable tool to estimate the wear based on the above said parameters [[13], [14], [15], [16], [17]]. Normally the global modeling approach was used to predict the wear of biomaterials for various gait activities. This approach of predicting the wear of implants had finer elements at the contacting surfaces and coarser elements in remaining regions. This resulted in lesser accuracy in predicting the contact stress which is required to estimate the wear of bearings. The linear wear was highly influenced by contact stress and also had higher linear wear for larger radial clearance values [16]. Also the difficulty in modeling, choosing of finer and coarser meshing elements between the contact interface of the global model led to increase in computational time [18]. So, to eliminate those effects and to improve the accuracy of the wear modeling technique, submodeling technique was proposed [19]. This submodeling technique improved the accuracy of the finite element (FE) model in predicting the contact stress as well as wear of bio-implants to greater extent. This approach used a uniform element size in estimating the contact stress and wear as opposite to that in global modeling. Also time taken to solve the local model was greatly reduced in this approach [19].
The purpose of using bio-lubricant in this study was that it contains salt content similar to natural synovial fluid. Also natural oils like sesame oil exhibited better reduction in friction and wear. Sesame oil was used for medical purpose since it exhibited better bio-degradable and non-toxic properties [20]. The reason for choosing silicon nitride for the current study was that it exhibited better tribological behavior for higher load i.e. reduction in friction and wear rate was observed for maximum POD load. It also showed better reduction in wear for physically demanding gait activities when analyzed using FEA [21].
The present study aims at analyzing the friction and wear coefficient for five different bio-lubricants for Si3N4 against Ti6Al4V alloy using POD tribometer for a sliding distance of 20Ā km under 20Ā N load for first time. Later submodeling technique was adopted to predict the wear for this combination with four different gait activities which include normal walking, stairs down 25Ā kg, carrying load 25Ā kg and lifting load 40Ā kg for 2 million cycles and the wear comparison between these gait activities are analyzed to select better bio-lubricant.
Section snippets
Ball on disc tribological testing
Silicon Nitride ball of 10Ā mm diameter with surface roughness 0.13Ā Ī¼m (SunChengkai, China) and Ti6Al4V disc medical grade 5 (Mukesh steel, INDIA) having surface roughness of 0.09Ā Ī¼m of 4Ā mm thickness with 160Ā mm diameter were used for the current study. The track radius was kept at 40Ā mm with sliding velocity of 0.71Ā m/s [22]. Ball-On-Disc (BOD) computerized tribometer (MAGNUM, Bengaluru) with the necessary data acquisition system was used to extract the friction and wear coefficient for the
Friction and wear coefficient behavior
The friction and wear coefficient for five bio-lubricants are shown in Fig. 6. The CoF value obtained for these bio-lubricants range from 0.22 to 0.41. Among different bio-lubricants, sesame oil showed least friction coefficient of 0.22 and highest friction coefficient was observed for 0.9% NaCl saline solution. The physiochemical properties of sesame oil is listed in Table 4. The reason for which saline solution showing high friction coefficient was due to the corrosion effect of Clā ion
Conclusions
For the first time, the realistic wear coefficient under five different bio-lubricants combined with submodeling technique was used to estimate the wear of C-o-M combination (Si3N4 Vs Ti6Al4V) up to 2 million cycles.
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The coefficient of friction value was found to be low for sesame oil and high for 0.9% NaCl saline solution. At the same time, wear coefficient was found to be least for PBS solution while distilled water showed maximum wear coefficient for given load.
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The submodeling wear approach
Funding
The authors hereby thank Institute of Engineers (INDIA) for providing financial support through R&D scheme (PG2020006) to execute this work.
CRediT authorship contribution statement
S. Shankar: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition. R. Nithyaprakash: Conceptualization, Methodology, Software, Investigation, Writing - original draft. B.R. Santhosh: Software, Investigation, Funding acquisition. Ali Kaya Gur: Conceptualization, Writing - review & editing. Alokesh Pramanik: Conceptualization, Methodology, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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