Fretting wear behaviour of titanium modified by heat treatmentby B. Feng, X. Lu, J., M. Chen, S., X. Qu, M., H. Zhu, X., H. Yao, J. Weng

Surface Eng.

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Fretting wear behaviour of titanium modified by heat treatment

B. Feng*1, X. Lu1, J. M. Chen2, S. X. Qu1, M. H. Zhu1, X. H. Yao1 and J. Weng1

Fretting damage is one of the factors leading to failure of orthopaedic implants. The present work studied the fretting behaviour of heat treated titanium in air and water vapour respectively. The tribological experiments were performed with a fretting tester under dry friction and lubrication conditions with the simulated body fluid (SBF). The worn surfaces of the specimens were analysed by scanning electron microscopy with X-ray energy dispersion spectroscope, confocal laser microscope and X-ray diffractometer. Heat treatments enhanced fretting wear resistance and decreased friction coefficients of titanium under dry friction and SBF lubrication conditions respectively. The effect of heat treatment in water vapour was superior to that in air. SBF as lubricant could decrease the wear and friction coefficients of non-heat treated and heat treated titanium.

Keywords: Heat treatment, Fretting wear, Titanium, Oxide titanium, Implant

Introduction

It has become recognised that wear is a major cause of long term failure of joint replacements. The accumulation of wear debris may produce an adverse cellular response resulting in inflammation, release of damaging enzymes, osteolysis, infection, pain and implant loosening.1,2

Titanium based implants are used extensively as orthopaedic implants because of their good biocompatibility, high strength and corrosion resistance. However, they are characterised by low wear resistance. Surface modifications are effective to improve their wear resistance. Various methods of surface treatments have been studied, for example, carbon/nitrogen/oxygen ion implantation, glow discharge, plasma nitriding and oxidising, and anodic oxidation.3–10 But these techniques generally need large apparatus and high cost. Heat treatment is a simple method. In the authors’ previous work, titanium was surface modified by heat treatments in air and water vapour respectively. The pin on disc test confirmed that the heat treated titanium showed longer wear life and lower friction coefficients.11

However, fretting damage is also inevitable in orthopaedic reconstruction. Shearing micromovements may often appear at the interface between the implant and bone due to the large differences in their elastic moduli. Insufficient initial fixing, or movement of the limb, which sustains a large number of stress reversals in the course of 1 day, can also cause micromovement. The oscillatory micromovements at the contact induce fretting wear and sometimes, fatigue cracks, causing the early failure of joint prosthesis.12–15 There have been some investigations on how to inhibit fretting damage of orthopaedic implants made from titanium alloys and stainless steels.16–19 The present work studied the fretting behaviour of heat treated titanium specimens in air and water vapour respectively. Surface analyses were carried out using scanning electron microscope attached with an X-ray energy dispersion spectroscope, confocal laser microscope and X-ray diffractometer. The fretting mechanism was preliminarily analysed.

Materials and methods

Materials

Commercially, pure titanium blocks (Poisson’s ratio m50?33, elastic modulus E 5116 GPa) of 10610615 mm in size were wetly ground down with 800 grit alumina paper, and then washed ultrasonically in turn with actone, ethyl alcohol and deionised water, and dried at ambient temperature. Among them, two groups of five specimens were heat treated at 550–600uC for 30 min in air and water vapour with 1?136105– 1?156105 Pa respectively, followed by cooling to ambient temperature in heating devices. Three groups of specimens were obtained: (i) S: non-heat treated (ii) H: heat treated in air (iii) W: heat treated in water vapour.

Fretting test

Tribological properties were investigated with a servohydraulic dynamic test system, as shown in Fig. 1. The moving specimen was made of silicon nitride (Si3N4) ceramic ball (m50?26, E5300–320 GPa) with a diameter of 12 mm. Si3N4 ceramic as friction mate is high wear resistant compared with titanium. The flat specimens of titanium (S, H and W) were mounted on a table. The 1Key Lab of Advanced Technologies of Materials, Ministry of Education,

School of Materials Science and Engineering, Southwest Jiaotong

University, Chengdu 610031, China 2State Key Lab of Solid Lubrication, Lanzhou Institute of Chemical

Physics, Chinese Academy of Science, Lanzhou 730000, China *Corresponding author, email fengbo@swjtu.edu.cn  2011 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 22 October 2008; accepted 2 November 2008 246 Surface Engineering 2011 VOL 27 NO 4 DOI 10.1179/174329409X397769 oscillatory amplitude was 60 mm with a frequency of 5 Hz. The number of cycles was 20 000 cycles under a load of 50 N.

Fretting tests were conducted under dry friction and lubrication conditions respectively. The lubricant was

Kokubo’s stimulated body fluid (SBF) with ion concentrations (Naz 142?0 mM, Kz 5?0 mM, Mg2z 1?5 mM, Ca2z 2?5 mM, Cl2 148?8 mM, HCO3 2 4?2 mM, HPO4 22 1?0 mM, SO4 22 0?5 mM) nearly equal to those of human blood plasma at 36?5uC. The SBF solution, prepared by dissolving reagent grade NaCl,

NaHCO3, KCl, K2HPO4.3H2O, MgCl2.6H2O, CaCl2 and Na2SO4 in distilled water, was buffered at pH 7?40 with tris-(hydroxymethyl)-aminomethane [(CH2OH)3

CNH3] and hydrochloric acid at 36?5uC. 20

Surface characterisation

The microhardness of the specimens was assessed using a microhardness apparatus (HXD-1000, China), and the data came from the average of measurement at eight areas on each group titanium plate. The wear volume was calculated based on measurement with a confocal laser scanning microscope (OLS1100, Olympus Corp.,

Japan). For this test, at least three specimens from each group material were used. The surface morphology of the specimens was observed with a scanning electron microscope (SEM; Quanta 200, FEI, The Netherlands).