Friction and multiple scratch behavior of polymerCmonomer liquid crystal systems Marı ´a Dolores Bermu ´dez a , Witold Brostow b,c, * , Francisco Jose ´ Carrio ´n-Vilches a,b , Juan Jose ´ Cervantes a , Dorota Pietkiewicz b a Grupo de Ciencia de Materiales e Ingenierı ´a Metalu ´rgica, Departamento de Ingenierı ´a de Materiales y Fabricacio ´n, Universidad Polite ´cnica de Cartagena, C/Doctor Fleming s/n, 30202 Cartagena, Spain b Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science and Engineering, University of North Texas 1 , Denton, TX 76203-5308, USA c Centro de Fisica Aplicada y Tecnologia Avanzada (CFATA), Universidad Nacional Auto ´noma de Mexico, A.P. 1-1010, Quere ´taro, Qro. 76001, Mexico Received 9 July 2004; received in revised form 27 October 2004; accepted 2 November 2004 Available online 26 November 2004 Abstract We have studied in turn: polystyrene (PS), styrene/acrylonitrile (SAN) and Polyamide 6 (PA6), adding each time to the polymer 1, 3, 5, 7 or 10 wt% of 4,4 0 -dibutylazobenzene (LC1) which is a monomer liquid crystal (MLC). LC1 reduces both static and dynamic friction of PS and SAN against stainless steels or polytetrafluoroethylene (PTFE). By contrast, friction values are lower for pure PA6 than for PA6 modified with various MLCs or with MoS 2 . Multiple scratching tests were carried out with a micro scratch tester on every system between 2.5 and 15 N. The presence of LC1 in PS reduces penetration depth and residual depth and increases the viscoelastic recovery. So far PS was the only polymer, which does not show strain hardening in multiple scratching. The present results confirms this, but it also shows that only 1 wt% of LC reduces the brittleness of PS so that strain hardening appears. This effect is maintained at all higher concentrations of LC1 investigated as well. For SAN or PA6, additions of LC1 reduce penetration depth values with respect to pure polymers, but do not have a significant effect on viscoelastic recovery. Scanning electron microscopy (SEM) was used to study the deformation and wear mechanisms, and to relate the data obtained in multiple scratch sliding wear tests. For PS we see in SEM that increasing the LC1 concentration causes a more ductile behavior, with less crack nucleation. For SAN the debris accumulation in sliding wear is mitigated by the presence of the liquid crystalline lubricant. No debris formation is observed in PA6, with or without a lubricant. q 2005 Elsevier Ltd. All rights reserved. Keywords: Scratch resistance; Sliding wear; Multiple scratching 1. Introduction and scope Tribology is still much better developed for metals than it is for polymers. An exhaustive book on tribology by Rabinowicz [1] deals with metals almost exclusively. The ongoing process in several industries of replacement of metal parts and components by polymeric ones is slowed down since polymeric surfaces undergo scratching and wear much more easily than metal surfaces. As pointed out by Rabinowicz [1], wear is a very serious economical problem, and thus even more acute for relatively ‘weak’ polymer surfaces. Some progress in polymer tribology has been made, as reviewed in Ref. [2]. Let us try to make a list of existing options. Lowering friction and/or increasing scratch resis- tance of polymeric surfaces can reduce wear. The options are: 1 Using fillers [3]—which is a two-edged sword: in certain 0032-3861/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.11.003 Polymer 46 (2005) 347–362 www.elsevier.com/locate/polymer * Corresponding author. Address: Laboratory of Advanced Polymers and Optimized Materials (LAPOM), Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203-5308, USA. Tel.: C1 9405654358; fax: C1 9405654824. E-mail addresses: mdolores.bermudez@upct.es (M.D. Bermu ´dez), brostow@unt.edu (W. Brostow), fjc.vilches@upct.es (F.J. Carrio ´n- Vilches). 1 http://www.unt.edu/LAPOM