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Significance Statement
Ultra-high molecular weight polyethylene is one of the few structural materials used for osteo-articular prosthetics. Nevertheless, a poor integration with surrounding biological tissues often remains a major limitation when a strong anchoring to the bone is required. Coating prosthetics with porous titanium layers could provide an immediate solution to enhance the osseointegration and anchoring of orthopaedic implants. However, the fabrication of such coatings having the appropriate mechanical and adhesion strengths remains highly challenging. This work reports an original mechanical study of polyethylene substrates coated with porous titanium layers. It shows that the plasma spraying process did not cause any major degradation of the substrate. Microscopic observations combined to bending and laser shock testing captured the existence of a cracking threshold in the coating under tension and a decohesion threshold under laser shock. Besides the results on this system, the proposed approach can easily be applied to other metallic or ceramic coatings on polymeric substrates, which are of outmost interest for numerous industrial applications.
Figure legend: Scanning electron micrograph of a cross-section of a polyethylene piece coated with vacuum plasma sprayed titanium. Image is taken at an angle so that the titanium coating can be seen at the top and the polyethylene substrate at the bottom. (Scale bar is 50µm)
Journal Reference
Journal of Thermal Spray Technology, 2015, Volume 24, Issue 1, pp 206-214
Géraldine Wolinne 1,Céline Harnisch 1,2,Eva Héripré 3,Sylvie Ruch 2, Armando Salito2, Michel Jeandin 1,Laurent Corté 1
[expand title=”Show Affiliations”]- Centre des matériaux, UMR 7633 Mines-ParisTech, BP 87, 91003, Evry, France
- Alhenia AG, Baden Dättwil, Switzerland
- Laboratoire de Mécanique des Solides, UMR 7649 Ecole Polytechnique, 91128, Palaiseau, France
Abstract
Thick metallic or ceramic functional coatings onto polymers are of great interest for different domains such as the aerospace and medical industries. A vacuum plasma spray process has been developed to produce coatings on high- and low-temperature melting polymers including PEEK and polyethylene. This study reports the first experimental characterization of the strength and adherence of such titanium coatings on medical grade polyethylene substrates. Four-point bending coupled to microscopic observations show the existence of a critical tensile strain of 1% corresponding to the onset of cracking in the coating. For strains up to 6%, the crack density increases without any noticeable debonding. Fatigue tests over 106 cycles reveal that under this critical strain the coating remains uncracked while above it, the cracks number and size remain stable with no noticeable coating detachment. A protocol for laser shock adhesion testing (LASAT®) was developed to characterize the coating-substrate adhesion and captured the existence of a debonding threshold. These results provide quantitative guides for the design of orthopedic implants for which such a titanium coating is used to enhance anchorage to bone tissues. More generally, they open the way for systematic measurements quantifying the adhesion of metallic coating onto polymer substrates.
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