Effect of Crystallinity on the Friction Behavior of Ultra-high-molecular-weight-polyethylene
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Effect of Crystallinity on the Friction Behavior of Ultra-high-molecular-weightpolyethylene Kanaga Karuppiah K.S. 1, Angela L Bruck1, Sriram Sundararajan1, and Zhiqun Lin2 1 Mechanical Engineering, Iowa State University, Ames, IA, 50011 2 Materials Science & Engineering, Iowa State University, Ames, IA, 50011 ABSTRACT In this study we evaluate the interfacial shear strength and scratch resistance of medical grade ultra-high molecular weight polyethylene (UHMWPE) (GUR 1050 resin) as a function of polymer crystallinity. Crystallinity was controlled by heating UHMWPE samples to a temperature above its melting point and varying the hold time and cooling rates. Degree of crystallinity of the samples was evaluated using differential scanning calorimetry (DSC). Quantitative nanoscale friction experiments were conducted using an atomic force microscope with commercially available Si3N4 probes under dry conditions. A higher crystallinity resulted in lower friction force and lower interfacial shear strength as well as increased scratch resistance. The trend in friction response was observed in microscale friction measurements. INTRODUCTION Tribological properties of the articulating surfaces in total joint replacement (TJR) are critical factors affecting their durability and reliability [1]. For example, in a hip joint, the interface between the femoral head (usually a hard material) and acetabular cup lining (usually a softer material) is critical in determining the useful life of the implant. Today, the combination of a UHMWPE liner and metallic or ceramic (cobalt chromium, titanium, alumina and zirconia based materials) femoral heads are extensively used [2]. One factor affecting the tribological performance of UHMWPE in implants is its processing. Medical grade UHMWPE stock material may undergo a variety of processing techniques during implant manufacturing [3], including molding, extrusion and milling or turning. These manufacturing processes can affect the surface morphology, crystallinity and mechanical properties of the polymer [4, 5], which in turn can affect its friction and wear performance. The objective of this study is to measure the friction and scratch response of medical grade UHMWPE as a function of crystallinity using atomic force/friction force microscopy (AFM/FFM). FFM techniques are used because accurate determination of the probe and contact dimensions is possible, thereby allowing calculation of interfacial shear strength for the material pairs used. EXPERIMENTAL DETAILS Materials Commercially available, ram extruded GUR 1050, rod-stock; medical grade UHMWPE (Poly Hi Solidur, Fort Wayne, Indiana) was cut into small square (30 mm) pieces. The melting
point was established using differential scanning calorimetry (DSC) to be 125 0C. Samples were heated to 200 0C and held for 3 hours after which one sample was held at 110 0C for 48 hours in order to allow recrystallization while another sample was rapidly cooled by a liquid nitrogen quench. Both samples were clamped with minimal loa
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