The beneficial effect of graphene nanofillers on the tribological performance of ceramics Manuel Belmonte a, * , Cristina Ramı´rez a , Jesu ´ s Gonza ´ lez-Julia ´n b , Johannes Schneider c , Pilar Miranzo a , Marı´a Isabel Osendi a a Institute of Ceramics and Glass (ICV-CSIC), Kelsen 5, 28049 Madrid, Spain b Institute of Materials Science and Technology (IMT), Friedrich Schiller University, 07743 Jena, Germany c Karlsruhe Institute of Technology-IAM-ZBS, 76344 Eggenstein-Leopoldshafen, Germany ARTICLE INFO Article history: Received 10 March 2013 Accepted 27 April 2013 Available online 21 May 2013 ABSTRACT The tribological properties of graphene nanoplatelets (GNPs)/Si 3 N 4 composites are investi- gated for the first time using a reciprocating ball-on-plate configuration under isooctane lubrication. The role of these carbon nanostructures is studied through the analysis of the debris and wear tracks by micro-Raman spectroscopy. GNPs are excellent nanofillers for enhancing the tribological performance of ceramics. Under high contact pressures, GNPs are able to reduce friction and, especially, to increase the wear resistance up to 56% due to the exfoliation of the nanoplatelets that creates an adhered protective tribofilm. These composites are promising for their use in gasoline direct injection systems. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The recent burst into the research scene of graphene, the 2- dimensional carbon allotrope with comparable or even supe- rior electronic and physicochemical properties than CNTs [1,2], has created a great expectation about their possible ben- efits on the properties of composite materials, namely on polymer-based ones [3] and, in a lesser extent, also on cera- mic nanocomposites [4–7]. At present, and focussing on the latter materials, the addition of graphene nanosheets or nanoplatelets (GNPs) to alumina (Al 2 O 3 ) or silicon nitride (Si 3 N 4 ) ceramics allows reaching maximum electrical conduc- tivity values of 5700 S m 1 [4] and 4000 S m 1 [5], respec- tively, which are 170% and 5 times higher than the best results previously reported for the equivalent CNTs/ceramic composites [8,9]. The trend is not the same for mechanical properties and, until now, it seems that the results depend on the kind of ceramic matrix. In this way, nearly three times higher fracture toughness (K IC ) is reported for CNTs/Al 2 O 3 composites (9.7 MPa m 1/2 ) compared to Al 2 O 3 monolithics [10] whereas the composite containing GNPs just gets an in- crease of 50% [6]. Conversely, GNPs/Si 3 N 4 composites are reportedly tougher (up to 235%) than the monolithic [7], but CNTs lead to modest K IC increments for this ceramics [11]. Nowadays, the research for new materials with improved tribological properties is widely justified by the targets of higher efficiency and environmental protection that the man- ufacturing, power generation, and transportation industries are compelled to comply [12]. As illustrative example, one- third of the fuel energy produced in a passenger car is used to overcome friction in the engine, transmission, tires, and brakes [13]. The excellent lubricating properties of graphite are well known [14] and atomic scale studies also point out the low friction attained by CNTs [15,16] and graphene [17,18]. For the latter, it has been reported that friction mono- tonically decreases as the number of graphene layers in- creases, four layers showing similar behaviour to bulk graphite [17]. Besides, graphene considerable reduces the 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.04.102 * Corresponding author: Fax: +34 917355843. E-mail address: mbelmonte@icv.csic.es (M. Belmonte). CARBON 61 (2013) 431 – 435 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon