An Investigation of LENS®-Deposited Medical-Grade CoCrMo Alloys

A series of deposition experiments using CoCrMo were performed using an Optomec LENS® machine. An analysis of hardness, microstructure, and wear trends of the deposited alloys was undertaken in an attempt to determine the applicability of using LENS® to create better materials for orthopedic implants. It was found that LENS®-deposited CoCrMo alloys were harder than wrought materials, however initial wear tests indicated that LENS®-deposited alloys were less resistant to abrasive wear than wrought alloys. INTRODUCTION Many knee and hip total joint replacements performed in the United States use polyethylene-on-CoCrMo bearing surfaces due to the forgiving nature of the combination to the individual biomechanical nuances of joint recipients. Current studies show that, although polyethylene is biologically inert in the body as a whole, microscopic particles of polyethylene that result from years of wear can possibly be toxic if encountered in large amounts and lead to osteolysis (Ratner et al. 1996; Amstutz et al. 1992; Hamilton and Gorczyca 1995). This has led to a growing interest in metal-on-metal bearing surfaces for implants (Walker and Gold 1971; Medley et al. 1996). A number of metal-on-metal hip implants survived more than 25 years with low wear rates and minimal osteolysis (Schmalzried et al. 1996). Surgeons will likely prefer metal-on-metal joints over polyethylene-on-metal when scientists can mitigate the causes for early failure of metal-on-metal implants. Cobalt-based alloys are frequently used in applications that require wear and/or corrosion resistance. CoCrMo is the hardest known biocompatible metal alloy. This makes it an ideal candidate for metal-on-metal bearing surfaces in orthopedic implants. Several microstructural characteristics determine the material properties and thus the life of a CoCrMo implant. First, a finer grain size can contribute to higher tensile strengths and improved fatigue strengths. Second, carbide precipitates are the main contributors to the wear protection for metal-on-metal interfaces. Carbide volume fraction, shape, morphology, and the strength of the carbide/matrix interface play a significant role in determining the amount of wear resistance of a particular alloy (Steen 1985). Laser Engineered Net ShapingTM (LENS®), a technology commercialized in the 1990s, allows for microstructural control during part fabrication. The LENS® process melts a powdered metal onto a substrate using a high-powered laser and has the advantage of directly controlling the heating properties within a localized area, thus affecting the grain structure and morphology. The LENS® process can control several parameters that affect cooling rate, which alter the microstructure of a deposited material. Recent research has focused mainly on the effect of laser power and laser traverse speed on microstructure (Hofmeister et al. 2001; Kobryn et al. 2000). In addition, other parameters such as powder feed rate, hatch spacing, and layer thickness 1 LENS® is the registered trademark and service mark of Sandia National Laboratories and Sandia Corporation.