R TCP Laser Processed CoCrMo-Calcium Phosphate Composites for Articulating Surfaces Himanshu Sahasrabudhe, Thomas Gualtieri, Susmita Bose and Amit Bandyopadhyay W. M. Keck Biomedical Materials Research Laboratory, School of Mechanical and Materials Engineering Washington State University, Pullman, WA 99164 USA. E-mail: [email protected] Laser Engineered Net Shaping Laser Engineered Net Shaping (LENS TM ) is a additive manufacturing technique in which a powdered material is simultaneously melted and deposited by a high power Nd:YAG laser. The powder is carried through Argon carried gas. Successive layer wise deposition of the material finally yields a complete 3D part. The design is specified through a CAD software. CoCrMo is a widely used material for implants. CoCrMo is hard and shows good wear and corrosion resistance. However, release of Co and Cr ions in vivo is a serious concern. Various attempts have been made to reduce metal ion release from CoCrMo. In this research, we have used Laser Engineered Net Shaping (LENS TM ) to form a meal-ceramic composite layer by the addition of calcium phosphate to CoCrMo. The objective of our study is to measure the effect of CaP on the tribological properties of CoCrMo. Our hypothesis is that CaP will act as a solid lubricant and reduce metal ion release from CoCrMo during articulation. If successful, these composites hold the potential to considerably improve the life of metal-on-metal implants without altering the material significantly. Acknowledgement References Authors would like to acknowledge the financial support from the Life Sciences Discovery Fund (LSDF) Authority, Washington. Processing of CoCrMo-CaP by LENS TM Three different compositions: CoCrMo-0%, 1% & 3% by weight of Hydroxyapatite was premixed. Processed at a power of 425W on SS410 base plate Each sample was ~15mm thick with layers each 0.15mm thick Cross section SEM and top surface XRD analysis was done to verify phases. 50 μm 50 μm CoCrMo-3%CaP structure appears more discontinuous as compared to the CoCrMo-0%CaP structure. Structure of CoCrM-0% shows more equiaxed grained with random orientation. Both 1% and 3% composites do not show CaP particles. 50 μm Addition of CaP to CoCrMo alloy stabilizes the ε phase. Γphase of CoCrMo is lost after CaP addition and LENS TM processing. HAP though not seen from SEM images, is prominent in the diffraction pattern. XRD Analysis of CoCrMo-CaP Composites Hardness Testing of CoCrMo-CaP Composites Sample Vickers Hardness (HV 0.1 ) CoCrMo-0%CaP 564±10 CoCrMo-1%CaP 568±7 CoCrMo-3%CaP 570±8 Addition of CaP to CoCrMo does not change the hardness of the CoCrMo alloy. Wear Testing of CoCrMo-CaP Composites Linear Reciprocating Wear Tests performed in DI Water Medium Short distance wear test for 1km and long distance wear test for 3km Constant Load of 5N, constant speed of 1200mm/min and 10mm long wear track CoCrMo-0%CaP CoCrMo-3%CaP 500 μm 500 μm Wear Track Wear track in DI medium for CoCrMo-3%CaP sample is not as wide and as deep as the wear track for CoCrMo-0%CaP tested under the same conditions. Wear track is difficult to locate on SEM because of its small depth and width. Sample Wear Rate x 10 -5 (mm 3 /Nm) CoCrMo-0%CaP 3.02±0.32 CoCrMo-1%CaP 1.71±0.29 CoCrMo-3%CaP 1.02±0.37 Wear rate decreases by almost 3 times by the addition of 3%CaP to CoCrMo alloy and LENS TM processing. Addition of CaP to CoCrMo alloy does not reduce the coefficient of friction; it slightly increases it. All samples achieve a steady state at a distance of ~100m, after which the COF is more or less the same. CaP does not reduce friction but reduces surface wear. CoCrMo-3%CaP CoCrMo-0%CaP Formation of Tribofilm During Wear High magnification SEM reveals the formation of a tribofilm in the CoCrMo alloy samples’ wear tracks with CaP addition. It was not seen on the wear track of CoCrMo-0%CaP. 50 μm Tribofilm CoCrMo alloy underneath Tribofilm was discontinuous for 1km wear track of CoCrMo-3%CaP samples. It was relatively more continuous towards the center of the wear track than towards the edges. CoCrMo-3%CaP- 1km Wear Distance 20 μm CoCrMo-3%CaP- 3km Wear Distance There was full coverage of the tribofilm after longer wear distance of 3000m. Development of tribofilm during wear is time dependent. It may be absent or highly scattered during initial phases of wear. It becomes more continuous with time. Thus wear rate reduces with distance. Tribofilm After Etching The Wear Tracks 50 μm 40 μm Partially Etched Wear Track Deep Etched Wear Track Partial etching of the wear track removed some of the tribofilm from the surface. Complete etching shows the microstructure of the wear track, completely etching out the tribofilm. Summary 1. Addition of CaP to CoCrMo alloy has found to reduce the wear in DI medium. 2. Addition of CaP does not however decrease the coefficient of friction of CoCrMo alloy. CaP seems to be the preferentially wearing phase that prevents the wear of the alloy. 3. A distinct tribofilm forms during wear. It is discontinuous in shorter distances of wear and more continuous for longer distances of wear . 1. R. Pourzal et al., Wear (2011), 271-9-10, pp 1658-1666. 2. S.A. Dittrick et al., MSE ‘C’(2011), 31-4, pp 809-814. 2014 Annual Meeting in Denver, CO.