Date

October 1996

Document Type

Thesis

Degree Name

M.S.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Institute of Science & Technology

Abstract

Due to the constantly growing number of total joint arthroplasty patients, the demand for improvements in the design of implant fixation has become more apparent. One method of implant fixation involves the use of a porous metal prosthetic that has been coated with a bioactive ceramic such as Bioglass®. This ceramic coating acts to initiate early bone infiltration into the porous surface provided by the implant. Once osseointegration has occurred, the implant is securely fixed within the host bone of the patient. Analysis of the integrity of the plasma-sprayed bioceramic on a titanium alloy (Ti-6AI-4V) was the focus of this study. Both numerical and experimental components were used to investigate the bond strength of Bioglass®-coatings on Ti-6Al-4V. The physical specimen consisted of a Ti-6Al-4V cylinder which was plasma-sprayed with Bioglass® (BG) on one of its flat surfaces. This BG surface was adhered to another Co-Cr-Mo mushroom cap with an epoxy adhesive. The titanium alloy cylinder was 25.4mm in diameter and 12.7mm in length. A 150µm thick coating was applied onto the flat surface of the titanium cylinder. The test samples were pulled in tension at a rate of 0.1 in/min until complete failure occurred. Finite element analysis (FEA), being a widely accepted method of analyzing or simulating a variety of engineering stress, strain, and failure problems, was used in this study. Specifically, two-dimensional (2D) and three-dimensional (3D) FEA models were developed to evaluate the stress/strain distributions at a bimaterial interface of a tensile test specimen. Dimensions of the FEA models (AnsysS.2 [1]) were identical to those of the experimental specimens. The heights of the Ti-6A1-4V pieces, however, were reduced strategically to minimize the overall problem size in order to focus on detail analysis of the domain in the neighborhood of the interface. Elements capable of linear strain were used in the models. A linear-elastic solution was chosen with the knowledge that BG does not exhibit plastic deformation. The material properties of interest were Young's modulus (E) and Poisson's ratio (υ) [2-9]. Necessary and sufficient boundary conditions were applied in the FEA model so as to accurately represent the physical problem. Specifically, the boundary conditions would simulate appropriate lateral shrinkage of the specimen as it elongates under the action of the applied tensile load. The tensile load was applied as a uniform tensile pressure. While various pressure loads were applied, only results obtained from a pressure level of 30MPa will be presented. This is for purposes of consistency with data reported in the literature. The tensile tests showed that weaker bond strengths exist with the porous specimens. Bond strengths for both porous and smooth samples were inversely proportional to the BG coating. The maximum bond strength was observed with the smooth coupon group having the lowest BG thickness. The FEA models yielded a singular stress field at the free-edge interface corners of the biaterial. These high stresses suggest that the corners could be the location of failure initiation. That the singular stress fields were found to be at the free edge interface comers was consistent with expectation. The equivalent von Mises stress was used in assessing the stress-strain response of the specimen since it gives an overall more realistic picture of the state of deformation [10]. The von Mises stresses for the FEA study revealed slightly higher stresses along the interface in comparison to the rest of the model. In addition, the equivalent stresses resulted in having 45° "wings" of high stresses which protrude from the free edge into the titanium blocks. These high stress wings were oriented along the slip directions for metal [10, 11] subjected to a tensile load. The highest strains appeared in the center region of the BG layer which was expected since BG has the lower value of Young's modulus. The deformation fields observed were in accordance with expectations, namely the longitudinal elongation and lateral shrinkage of the specimen. Chern Lin et al [6] performed a similar study where plasma-sprayed BG coatings were subjected to tension for delamination. Fractures were observed to occur within the coatings and only some of the coating was separated from the substrate while the majority still remained tightly bonded to the substrate. Since most of the failures occurred within the coatings themselves, the actual bond strengths should be higher than those measured. The von Mises stresses show that failure would occur initially at the free-edge interface and propagate along the Ti-6Al-4V/BG interface. This differs from the experimental observations in that the BG did not completely delaminate from the substrate. Many areas on the substrate surface were still covered by BG. The FEA evaluation was made on the assumptions that the materials were isotropic, homogeneous, and free of microvoids. The 45° stress zones are generally observed in metals due to their ductility and corresponding slip system characteristics. These oblique stress zones may be interpreted as resolved shear stresses which developed during the lateral contraction of the specimen experienced under tensile loading [11].

Identifier

doi:10.6083/M408638P

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