Adhesion of Sol-Gel Derived Zirconia Nano-Coatings on Surface Treated Titanium

The morphology , adhesion and tribological properties of the zirconi a sol-gel coatings on phosphate treated, anodized and un-treated titanium surfaces were investigated. The anodization of titanium involves the formation of a thin, compact, oxide layer, which improves the wettability for further coating. This process involves the conversi on of the rutile structure of the original titanium oxide into a mostly crystalline anatase structure. T he samples were anodized in sulphuric and phosphoric acid at varying concentrations. The samp les were anodized at differing currents and differing time periods ranging from 10 to 30 minute s. Phosphate adsorption treatment involves soaking samples in 10% H 3PO4 solution for 10 minutes. These samples were spin coated with zirconia, yield ing 100 nm thick films. The nanocoatings were prepared by alkoxide sol-gel chemistry, using techni ques and protocols developed in an earlier work and were examined with x-ray diffraction, and scannin g electron microscopy. Interfacial and adhesion properties were measured u sing a micromechanical tensile test. The tribological properties were investigated using an Orthopod machine, with commercial grade ultrahigh molecular weight polyethylene (UHMWPE) pins (3 /8 inch diameter) that can articulate in number of different combinations against opposing c oated and control specimens. The UHMWPE pins were used in a bovine serum environment. The a mount of the wear was measured gravimetrically and wear features were observed using SEM. Introduction Surface coatings offer the possibility of modifying the surface pr operties of an surgical component and thereby achieving improvements in performance, reliability and biocompatibi lity. Zirconia films (both nano and macro) produced by different methods are of great interest for variety of biomedical and engineering applications. As a result, a wide range of different d position techniques for zirconia have been proposed. Many of the common thermal barrier coatings and corrosion resistant coatings use plasma or thermal spraying, while several more rec ent techniques have also been used including physical vapour deposition, sputtering, thermal and electron beam e vaporation, plasma MOCVD, electrochemical vapour deposition, and sol-gel processing [1]. The term sol-gel is currently used to describe any chemical procedure or process capable of producing ceramic oxides, non-oxides and mixed oxides from solutions. In this current work we aimed to modify pure titanium metal surfac es by first anodizing and then phosphotazing with specific phosphate adsorption and photocatalysis treatment s o improve the wettability. The specimens were then coated with a sol-gel der ive alkoxide based zirconia for increased biocompatibility and adhesion. After characterization, in thi s preliminary work, tensile Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 455-458 doi:10.4028/www.scientific.net/KEM.254-256.455 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,19:47:14) Title of Publication (to be inserted by the publisher) adhesion and tribological pin-on-disc wear tests were carried out for their assessment of interfacial adhesion properties. Materials and Methods The titanium samples , both CP Ti and Ti alloy , were machined from a titanium rod without lubricant and with a HSS tool, then polished on a Leco Auto polisher to a 1 micron finish, before the final polish in a vibratory polisher with an alpha alumina to 0.5 mic ron finish. Samples were then degreased in MEK and finally cathodically cleaned in an 10% sodium hydroxide solution for 1 minute. The micro-adhesion and tribology samples were punched out using a f lywheel punch and die from a 1mm thin CP Ti sheet and then put through the same polishing processes as the other titanium samples. The titanium alloy samples were anodized in a phosp ric and sulphuric solution at varying concentrations. The samples were anodized at differ ing currents and differing time periods ranging from 10 mins to 30 mins based on an earlier work [2 ]. Phosphate adsorption treatment involves soaking samples in 10% H 3PO4 solution for 10 minutes then rinsing immediately i n pure distilled water then drying at 70 0 C. Photocatalysis treatment consists of the treatm nt of the anatase layer with a UV wavelength of approximately 380nm for 1 hour. E xposing the catalyst to UV generates an excited state on the surface, which is able to initi ate subsequent processes like redox reactions and molecular transformations. These samples were then coated with alkoxide-derived zirconia. These coatings were applied by sol-gel spin coating methods, usi ng techniques and protocols developed in an earlier work and were examined with x-ray diffraction and scanning electron microscopy [3,4]. Tensile tests on titanium samples were conducted using a speciall y designed micromechanical tensile tester equipped with a 2500N capacity load cell. The flat " dog-bone-shaped" converted and coated titanium samples were tested in tension at a rate of 0.005 m m/s using a specially designed high-stiffness micromechanical testing device positioned directly under the objective lens of an optical microscope (Zeiss Axioplan) at a fixed magnification of 200x (Fig.1). This allowed direct observation of crack initiation, and evolution and debonding of the thin films on t he titanium specimens. The applied load and the imposed displacement were recorded dur ing the tests (every 2 s). Simultaneously, optical images of the coated surface were ca ptured every 2 s using a MTI analog camera with image analysis software (Scion Image, NIH). Af ter testing, the samples were imaged with a LEO SUPRA55VP, scanning electron microscope (SEM). The Ortho-POD (AMTI, Watertown, MA) was programmed carefully not to touch the edges of the disks. A square wear path was chosen for tests. A cycle time of 666ms (1.5 Hz) was used. During testing, the materials were immersed in diluted bov ine serum (50% serum, 50% deionized water). The tests were run for 500,000 cycles, with the serum c hanged every 125,000 cycles. (The control pins were soaked in bovine serum while each test was run ning). Cobalt chromium was the metallic chosen for the first wear tes t. Of the four disks, two disks had a zirconia coating and two disks had a zirconia coating combined with a phos phate adsoption treatment. A titanium substrate was chosen for the second wear t est. Of the four disks, two disks had a zirconia coating and two disks had a zirconia coating combined wit h phosphate a UHMWPE sorption treatment. All of the polyethylene samples were each wei ghed three times using an AG245 scale (Mettler Toledo, 0.00001 gm resolution), and were averaged before and after testi ng. 456 Bioceramics 16