Experimental and submodeling technique to investigate the wear of silicon nitride against Ti6Al4V alloy with bio-lubricants for various gait activities

Highlights

• Si₃N₄ Vs Ti6Al4V wear predicted using BOD 20 km run with bio-lubricants.

• Realistic friction and wear coefficients are used in FE model.

• Finite element submodeling technique is used to predict wear of hip prosthesis.

Abstract

The present work focused on analyzing the wear and friction coefficients of Si₃N₄ against titanium alloy grade 5 material using ball-on-disc tribometer for a sliding distance of 20 km with five different bio-lubricants. Later using finite element submodeling technique, post implantation wear is predicted for 2 years considering four different gait activities which includes normal walking, stairs down 25 kg, carrying 25 kg and lifting 40 kg. The friction coefficient value is found to be least for sesame oil. The increase in percentage difference in friction coefficients for phosphate buffer saline (PBS), distilled water, Ringer's solution and saline solution are 77.27%, 81.81%, 84.09% and 86.36% when compared with sesame oil. The percentage difference in reduction in wear coefficients for sesame oil, saline solution, Ringer's solution and PBS are 36.49%, 48.81%, 56.11% and 84.36%. The linear and volumetric wear is found to be quite low for all gait activities for PBS bio-lubricant. Overall the least linear and volumetric wear are obtained for normal walking gait activity followed by Stair down 25 kg, Carrying 25 kg and Lifting 40 kg gait activity.

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 0.11 ± 0.05 and 0.27 ± 0.04 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 9.8 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 20 km under 20 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 25 kg, carrying load 25 kg and lifting load 40 kg for 2 million cycles and the wear comparison between these gait activities are analyzed to select better bio-lubricant.