The wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) is critical to the success of total joint replacements. Recent attempts to modify the wear behavior of UHMWPE by processing, in particular, crosslinking UHMWPE have shown promise to increase wear resistance, but concerns persist regarding other mechanical properties. It is also unclear what specific surface mechanical properties govern the wear resistance seen in these materials. The goal of this study was to demonstrate a custom-built surface mechanical test system and method that measures the micromechanical response of microtomed UHMWPE surfaces to depth-sensing microindentation tests. The surface structure of these UHMWPE materials was also examined using scanning electron microscopy and atomic force microscopy. A custom designed microindentation test system assessed the microindentation behavior of three UHMWPE resins: 1. Hylamertrade mark, 2. GUR-1020 CMS, and 3. Marathontrade mark-a lightly crosslinked material. The effects of material and indentation depth were studied. Microindentation tests were performed with indentation depths ranging from 2 to 45 microm. Four different measurements of surface micromechanical behavior were obtained including the surface modulus, microhardness, hysteresis energy (irreversible work done to the sample per unit cycle) and its associated energy dissipation factor, and loading slope. Statistically significant differences in each of these parameters were found for each material. Generally, Hylamer had the largest values for these parameters, followed by the GUR resin and then the Marathon. Surface modulus was independent of depth of testing and found to be 651 MPa for Marathon, 738 MPa for GUR, and 1015 MPa for Hylamer (Modulus for bulk UHMWPE is 540 MPa for Hylamer, 620 for GUR, and 1380 for Hylamer). The microhardness varied between 67 and 162 MPa depending on material and depth of testing. Surface structural characterization shows that the microtoming process for surface preparation generated distinct surface features that varied between materials. Intermittent drawn ribbons of polymer with oriented crystals were observed in both scanning electron microscopy and atomic force microscopy. The surface density and size of these features were characteristic of the materials with the Hylamer having the fewest, but largest ribbons, followed by GUR and then Marathon.

In this work, we expand a grain boundary (GB) pinning model that considers a range of different spatial distributions of particles to also account for a distribution of particle sizes. We begin by developing a phase field model that describes GB and pore interactions and verify it by comparing to molecular dynamics simulations. We then develop an analytical pinning model that considers the impact of the particle size distribution, in terms of the mean and standard deviation of the particle radius. The analytical model is verified by comparing to simulation results of our phase field model and those of a simple Monte Carlo model. A significant finding from the model is that the mean value of the resistive pressure decreases with increasing standard deviation of the particle radius.

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Using molecular dynamic (MD) calculations, meso-scale phase field simulations behave differently by including anisotropic grain boundary energy in the model for transient energy minimization in uranium-dioxide. As a preliminary step, MD will be used to find face centered cubic (FCC) copper grain boundary energies and to explore its anisotropic behavior in phase field simulations. MARMOT is a C++ code that uses object-oriented programming to implement the Allen-Cahn and Cahn-Hilliard equations for phase field modeling. MARMOT uses constants for mobility and grain boundary energy for the entire domain of the simulation. For MARMOT to reach its full capability, these constants need to vary within the domain. This paper explores the expansion of MARMOT to include an anisotropic view of grain boundary energy in transient phase field simulations.