Materials Science and Engineering A 472 (2008) 338–346 Sintering of biocompatible P/M Co–Cr–Mo alloy (F-75) for fabrication of porosity-graded composite structures M. Dourandish a , D. Godlinski b , A. Simchi a,c,∗ , V. Firouzdor a a Department of Material Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran b Fraunhofer Institute for Manufacturing Technology and Applied Materials Research (IFAM), Wiener Str. 12, 28359 Bremen, Germany c Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran Received 11 January 2007; received in revised form 9 March 2007; accepted 13 March 2007 Abstract Manufacturing of complex-shaped bimetals utilizing two-color powder injection molding (2C-PIM) and three-dimensional printing (3DP) processes, which basically involve sintering of a powder/binder mixture, has been attracted a great interest. This article addresses sintering of biocompatible Co–Cr–Mo alloy for manufacturing stepwise porosity-graded composite structures. Such composite structures provide strength at the core and a porous layer for the tissue growth. To evaluate the process, two grades of gas atomized Co–Cr–Mo powder with an average particle size of 19 and 63 m were used. Isothermal and non-isothermal sintering behavior of the loose powders under hydrogen and argon atmospheres, which is a simulated condition of 2C-PIM and 3DP processes after de-binding, was studied. Microstructural characteristics of the sintered specimens were evaluated. It was found that an intermediate sintering temperature of 1280 ◦ C in argon can be used for manufacturing of the porosity-graded composite layers, i.e., a relatively dense core (5% porosity) with a porous layer (33% porosity) can be produced. A hip-joint with a core/shell structure was produced as a case study. © 2007 Elsevier B.V. All rights reserved. Keywords: Co–Cr–Mo alloy; Sintering; Porosity-graded composite; Densification; Microstructure 1. Introduction Among all implant materials, Co–Cr–Mo alloys demonstrate the most useful balance in strength, fatigue and wear [1,2] along with resistance to corrosion [3–6]. The cast alloy containing 28 wt%Cr and 6 wt%Mo (balance Co) has been used for many years to produce medical implants such as hips, knees, ankles and bone plates [7]. The wrought Co–Cr–Mo alloys exhibit superior mechanical and chemical properties compared with the cast alloys due to a finer grain size and more homogenous microstructure [8,9]. Although fabrication of surgical implants by conventional methods are common, powder metallurgy (P/M) route offers additional advantages. Pressing and sintering [10], hot isostatic pressing [11,12] and powder injection molding [13,14] are being used. It has been shown [15,16] that the prop- erties of P/M implants are comparable with those of the wrought ∗ Corresponding author at: Department of Material Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Azadi Avenue, 14588 Tehran, Iran. Tel.: +98 21 6616 5262; fax: +98 21 6616 5261. E-mail address: simchi@sharif.edu (A. Simchi). alloys. Furthermore, through P/M route one can increase certain properties by micro alloying [8] and producing porous coat- ing [17–19]. It also offers the possibility of fabricating near-net shape parts with a porous surface, providing suitable location sites for bone growth [17]. The level of porosity can be graded from a highly porous to a dense core, giving the component suitable strength to withstand the physiological loadings while having a porous surface layer [18]. In order to fabricate porosity-graded composite structures, a few procedures including centrifugal spray, two-color pow- der injection molding, and three-dimensional printing might be usable. Ozols et al. [17] have shown that the centrifugal spray process is a promising method for manufacturing porous coat- ings with microstructures that favor the mechanical anchorage of cementless joint prostheses. Tandon [13] has shown the viability of the metal injection molding (MIM) process for manufacturing Co–Cr–Mo (F-75) implants. He has found that the static mechan- ical properties of MIM F-75 are superior to the cast alloy while the fatigue properties are comparable. Becker and Bolton [20] have studied the effects of powder morphology and sintering atmosphere on the structure-property relationships in P/M pro- cessed F-75 alloys for surgical implants. They have shown that 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.03.043