Wear 267 (2009) 718–725 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear Influence of pH values and aging time on the tribological behaviour of posterior restorative materials P. Vale Antunes a,b,∗ , A. Ramalho a a CEMUC, University of Coimbra, Portugal b EST - Instituto Politecnico de Castelo Branco, Portugal article info Article history: Received 22 September 2008 Received in revised form 19 December 2008 Accepted 19 December 2008 Keywords: Dental composites Wear pH effect abstract Besides the complexity of the contact between teeth, the mouth is also a very specific service environment. Dental restorative materials, and teeth, are subjected to very specific service conditions, wide-ranging temperatures, contact loading, loading rates, sliding conditions and pH variation. Several studies have established that an exposure of resin composites to aqueous environments lowers their mechanical prop- erties. Due to changes in the structure of these materials, softening of the resin matrix and/or bond failure in the outer layer of the filler and in the filler–matrix interface region reduce the mechanical and tribo- logical performance. During the day, the mouth presents different values of pH, and thus it is important to determine the effect of pH on the composites’ tribological behaviour. The main objective of this study is the determination of the nature of the dependency between the tribological behaviour of commercial composites and the influence of values of pH and aging time. Seven commercial resin composites, con- densable and suitable for posterior restoration, have been characterized. Each material composite was aged during a period of 3, 6 and 22 months at a temperature of 37 ◦ C, and considering three values of pH: 3, 7 and 9. The tribological behaviour was evaluated in a reciprocating contact against glass spheres. At the end of the tests, the wear was assessed on the composite and also on the counterface. The removal mech- anisms enclosed in the wear process are discussed, taking into account the systematic SEM observations to evaluate the failure modes. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The warm, wet nature of the mouth is a very specific environ- ment that demands a great deal from resin restorative materials, especially in order for these materials to maintain reasonable sta- bility in terms of mechanical and tribological behaviours. Bathed in saliva with an average pH near 7, humidity of 100% and constantly altered environmental conditions, namely temperature and pH, due to the introduction of food and even due to dental treatments, these resin restorative materials have to show their operating capabilities in such an environment. Within the last few years, several new types of resin composites have been introduced to the market. The new classes are packable (condensable) composites, universal composites, reinforced micro- fills and nano-filled composites. The restorative materials and teeth are subjected to a high range of physical and chemical conditions. Some of these variables were already objects of study but their influence on the mechanical ∗ Corresponding author at: EST - Instituto Politecnico de Castelo Branco, Portugal. Tel.: +351 272339300; fax: +351 272339399. E-mail address: pedrov@est.ipcb.pt (P.V. Antunes). and tribological behaviour is not well known. The warm, aqueous, chemical environment is one aspect of the oral environment that has an appreciable influence on the in vivo degradation of resin composites. The in vivo evaluation of restorative materials is gen- erally, besides ethical reasons, very complicated, time-consuming and expensive. Thus, in vitro aging studies are more often used and performed in various aging media. Among these, the most frequently used are water and ethanol/water solutions [1,2]. It is well established that the exposure of resin composites to an aque- ous environment lowers their mechanical properties [3,4]. When stresses are applied to composites, these are absorbed by the resin matrix and transferred through the coupling agent to the stiffer, inorganic filler particles. Several mechanisms have been suggested to explain the decrease of mechanical properties after aging in various liquids, mostly softening of the resin matrix and/or bond failures occurring in the outer layer of the filler, in the silane coat- ing, at the silane–matrix interfacial region or within the matrix. It is not known where such “interphase” failures are induced, and which of the above regions can be regarded as the weakest links of a dental composite [5]. Nowadays, composite materials show a good clinical longevity [6], although problems due to fractures, increased surface rough- ness and microleakage have been reported. Failures due to 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.12.054