Influence of Phase Morphology on the Sliding Wear of Polyethylene Blends Filled with Carbon Nanofibers Weston Wood, Bin Li, Wei-Hong Zhong School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164 In this work, phase separation in carbon nanofiber (CNF) composites with a blend of ultrahigh molecular weight polyethylene (UHMWPE)/high-density polyethyl- ene (HDPE) was revealed, and its effects on tribological properties were investigated. Results from morphologi- cal analysis by optical and scanning electron micros- copy indicated two distinct microstructures: a dis- persed UHMWPE phase and a continuous microstruc- ture containing HDPE and CNFs. The addition of CNFs into the UHMWPE/HDPE blend induced a decreased steady-state torque indicative of a decreased dissolu- tion and improved processability. Because CNFs pre- dominantly resided into the HDPE phase, neat HDPE, a HDPE/CNF composite, and neat UHMWPE samples were also prepared for comparison. Wear results, determined by a pin-on-disk apparatus, showed that both initial run-in and steady-state wear rates of the UHMWPE/HDPE/CNF nanocomposites were reduced with an increasing concentration of CNFs. The wear re- sistance of the UHMWPE/HDPE blend was more strongly influenced than neat HDPE by the addition of CNFs, which may have been affected by a reduced dis- solution and improved interfacial interaction between the two phases. Results from this study suggested that HDPE may not be appropriate for processing UHMWPE composites, as CNFs reside in the HDPE phase, and HDPE diminishes the wear resistance of the mate- rial. POLYM. ENG. SCI., 50:613–623, 2010. ª 2009 Society of Plastics Engineers INTRODUCTION Ultrahigh molecular weight polyethylene (UHMWPE) has outstanding mechanical and tribological properties that make it a top material for the acetabular cup in total hip replacement [1–3]. Despite its superior properties, UHMWPE is the limiting factor in the lifetime of the total joint prosthesis. The wear debris of this material pro- duces a negative biological response, resulting in osteoly- sis and loosening of the implant [4, 5]. Many researchers have attempted to improve the wear resistance and mechanical properties of UHMWPE through crosslinking [6–8], increasing crystallinity [9–12], and incorporating fillers [13–23]. Although crosslinking has improved wear rates dramatically in vitro, in vivo case studies show con- trasting results, with an increase in crosslinking density resulting in reduced crack propagation resistance [7]. Additionally, the size and number of wear debris particles produced from highly cross-linked UHMWPE may have more of an influence on the development of osteolysis than the wear debris of conventional UHMWPE [8]. Simi- larly, highly crystalline UHMWPE processed by exceed- ingly high pressure has shown poor properties in vivo, attributed to a change in the crystal structure of UHMWPE [12]. Because of the unsatisfactory results in vivo for cross-linked and highly crystalline UHMWPE, then it is worthwhile to investigate alternative methods of improving UHMWPE properties through the incorporation fillers. Various fillers incorporated into UHMWPE for improv- ing wear properties have included: kaolin [13], quartz [14], hydroxyapitite [15], Al-Cu-Fe powder [16], short carbon fibers [17–19], carbon nanotubes [20–22], and car- bon nanofibers (CNF) [23]. Although the incorporation of some ceramic and metal fillers have shown improved wear resistance in preliminary studies, harsh abrasive third body wear caused by both hard fillers and counter- face wear debris have been a major concern [16]. Addi- tionally, hard fillers induce a dramatic increase in the elastic modulus of the polymer composite, resulting in elevated contact stresses. Carbon-based fillers appear to be more promising for UHMWPE composites, as these fillers induce a lower modulus of the overall composite compared with the effects of harder ceramic and metal fillers, while improving strength. Furthermore, graphitic materials are biocompatible, display enhanced solid lubri- cation, and have good thermal conductivity and thus can reduce the wear caused by heat of friction [24]. The addition of carbon-based fillers into UHMWPE for improved properties is not a new idea by any means. In the 1970s, randomly oriented short carbon fibers have been incorporated into UHMWPE as a reinforcement for total joint replacement components, and was commer- cially available for several years under the name of Poly- II TM . The use of carbon fiber-reinforced polyethylene in Correspondence to: Dr. Wei-Hong Zhong; e-mail: katie_zhong@wsu.edu DOI 10.1002/pen.21549 Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2009 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2010