Wear 270 (2011) 550–554 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Influence of crosslinking density on the tribological behavior of norbornene-based polymeric materials S. Bhuyan a,∗ , S. Sundararajan a , X. Sheng b , M. Kessler b a Department of Mechanical Engineering, Iowa State University, Ames, IA 50011, United States b Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, United States article info Article history: Received 30 July 2010 Received in revised form 22 December 2010 Accepted 21 January 2011 Available online 26 January 2011 Keywords: ROMP Polymer Scanning electron microscopy Abrasive wear Adhesive wear abstract In this study, the friction and the wear behavior of norbornene-based polymers prepared by ring-opening metathesis polymerization (ROMP) reaction are evaluated as a function of the crosslinking density. Tri- bological measurements were performed using a ball-on-flat reciprocating microtribometer. Friction and wear experiments under dry sliding were evaluated using a 1.2 mm radius Si 3 N 4 spherical probe as well as a 100 m radius conical diamond probe with a 90 ◦ cone angle. Wear experiments were conducted at constant normal loads for 100–500 reciprocating cycles. Wear depths were estimated from wear grooves using a contact profilometer, and scanning electron microscopy was used to elucidate deformation mech- anisms in the various samples. Correlation between the crosslinking density and the wear behavior were observed. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction Past studies have shown that the norbornene-based monomer endo-dicyclopentadiene (endo-DCPD) has been used effectively as the healing agent in self-healing composites [1–8]. A unique feature of the healing concept is the use of the unterminated chain-end polymerization catalysts, thus enabling multiple heal- ing events. During damage, microcracks rupture the embedded capsules and release the healing agent into the crack plane. The released healing agent comes in contact with an embedded catalyst and polymerizes, thereby, bonding the crack faces back together [8,9]. Researchers have used Grubbs’ first generation catalyst as the embedded trigger to set off the ring-opening metathesis poly- merized (ROMP) reaction [9,10]. The end product is a crosslinked structure with high toughness and strength. In the work done by Kessler and coworkers, endo-DCPD was polymerized in presence of ruthenium-based Grubbs catalyst to form crosslinked polymers of high toughness [11,12]. They have found that the addition of a norbornene-based crosslinker to the monomer at different con- centrations can alter the crosslinking density of the polymerized system resulting in controlled properties for various potential applications including tribology. ∗ Corresponding author. Tel.: +1 515 294 0349; fax: +1 515 294 3261. E-mail address: satyam@iastate.edu (S. Bhuyan). Crosslinking increases the tightness of the polymer network and reduces the molecular mobility of chains between the junctions. As a result, the number of conformations that a polymer can adopt decreases as the crosslinking density increases making it resistant to bending and penetration [13] Thus, crosslinking increases the stiffness and the hardness of the polymers, which should in turn affect the tribological properties. In this paper, the friction and the wear behavior of norbornene-based polymers under dry sliding have been reported as a function of the crosslinking density. 2. Materials 2.1. Norbornene-based polymers The endo-DCPD (95% purity, Acros Organics, Belgium) was used as received without further purification. The crosslink- ing agent used in this study is a mixture of two isomers – 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,-endo-dimethan onaphtha- lene (isomer-a) and 1,4,4a,5,8,8a-hexahydro-1,4,5,8-endo,-endo- dimethanonaphthalene (isomer-b). It was prepared by adding bicyclo[2.2.1]hepta-2,5-diene (8.6 g, 93.3 mmol) and hydroquinone (13 mg, 0.12 mmol) to endo-DCPD (6.2 g, 46.9 mmol). The reaction was carried out in hard-glass pressure vessel at 190 ◦ C for 18 h. The resulting mixture was subjected to reduced pressure frac- tional distillation and gave crosslinking agent CL (7.8 g, 52.9 mol%) which is the mixture of isomer-a and isomer-b approximately in 5:1 ratio. Fig. 1 shows the chemical structures of the endo-DCPD 0043-1648/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.01.023