Effect of grain fractions of crushed carbon foam on morphology and thermomechanical and tribological properties of random epoxy-carbon composites

Highlights

• Carbon foam grains as reinforcement of epoxy matrix were used.

• Carbon foam fraction had an effect on thermomechanical properties of composites.

• Dependence of composites friction coefficients on foam grain fractions was found.

• Epoxy composites with various foam fractions showed different wear characteristics.

Abstract

The effect of dimension of carbon foam (CF) grains used as a filler of epoxy matrix on the morphology and thermomechanical and tribological properties of final composites was described. The carbon foam proposed as particle reinforcement of composites was prepared from epoxy resin of diglycidyl ether of bisphenol A type cured with phenol-formaldehyde resin (novolac) in a self-foaming process followed by carbonization. Structures of the carbon foam filler and resultant composites were studied by microscopic and spectroscopic methods. Three different carbon foam grain fractions, below 200 μm, 200–315 μm and below 315 μm, keeping CF porous structure and specific properties, were used to obtain new composite materials. There were observed good quality dispersion of CF grains in epoxy matrix and excellent adhesion at interfacial areas, regardless of carbon foam fraction. In the effect, these composites have enhanced thermomechanical and tribological properties and relatively low density compared to carbon-polymer composites produced so far. It was found that all CF fractions used reduced friction coefficient of resultant composites compared to pure epoxy matrix, however the influence of individual filler fraction on composite's COF was different. The morphology of worn surfaces of epoxy composites after friction tests showed that the effect depends on grain fraction of CF filler as well as on the load used.

Introduction

Despite intensive research on polymer composite materials since the middle of the last century, there is still a need for new high-performance materials. An effective way to achieve this goal seems to be choosing reinforcing constituents (fillers) for polymer matrices with a specific structure and properties, which allows modeling the final composite's parameters. An important group of composite materials is polymer-carbon composites composed of a polymer matrix and various forms of carbon, such as carbon fibers, carbon black, and graphite, have been widely used as fillers for a large spectrum of polymers [1,2]. Recently, carbon nanomaterials, i.e. nanofibers, nanotubes, graphene materials, and other forms of nanocarbons [[3], [4], [5]], have received widespread interest. The specific type of carbon materials researched in this article is carbon foams (CFs), new generation materials characterized by a very high degree of porosity, even up to ~99%. CF morphology is described, on the one hand, by a system of porosity, i.e. total porosity, character of pores (open, closed), average pore size, and distribution of pores in the carbon matrix, and, on the other hand, by the solid matrix structure (ordered, unordered). There are numerous examples of applying various precursors and the production methods for carbon foams in the literature [[5], [6], [7], [8], [9], [10]] and other papers cited there. The porosity and structure of CFs, in turn, define their properties. The most important properties of CFs are low density, high thermal stability, high sound wave, and electromagnetic wave absorption, etc. The structure, surface properties, and features of CFs differ from the previously mentioned carbon fillers, which creates opportunities for developing novel polymer-carbon composites with unique properties. Additionally, the presence of various functional moieties on the surface of carbon foams results in a good affinity for various polymer matrices. The properties of CFs may be tailored and improved by adding external substances, e.g. carbon [11,12] or mineral [13,14] fillers into precursors before the foaming process. Adding carbon fillers to foams affects, among other properties, their bulk density, thermal conductivity, and mechanical properties and resistance to abrasion, crushing, and vibrations.

According to the authors' knowledge, CFs have been used and studied to a limited extent as polymer composite reinforcements. Szeluga et al. [15] investigated the influence of the structure and content of the crushed CF from epoxy-novolac precursor on the viscoelastic and friction properties of its epoxy composites. The application of other porous carbon materials, namely, glassy carbon particles, to produce epoxy-carbon composites [16] was studied, obtaining positive effects on the mechanical strength and electrical behavior of composites. Myalski et al. [17] described the results of research concerned with the influence of particulate glassy carbon on the tribological properties of polymeric matrix composites compared with metallic matrix composites. The presence of glassy carbon in composites with a polymeric matrix influenced friction coefficient stability at an elevated temperature. Myalski et al. [18] also studied the effect of CFs applied as skeleton reinforcements in epoxy resin on the hardness and compressive strength of polymer matrix composites. Glassy CFs were produced by pyrolysis of polyurethane foam with a layer of phenol-formaldehyde resin. Chen et al. [19] studied epoxy composites filled with graphite foam as a new material with good heat transfer properties. The effects of the mass fraction of EP and the network structure of graphene foams (GFs) on the thermal diffusivity and the compression strength of the composites were investigated.

However, there are many reports related to the application of GFs in polymer composites described in the literature. Ni et al. [20] proposed using the three-dimensional graphene skeleton obtained in the self-assembly process and reducing graphene oxide with poly(amidoamine) dendrimers in the form of the continuous reinforcement of epoxy composites leading to exceptionally good mechanical and thermal properties for their epoxy composites. Li et al. [21] fabricated new composites by assembling vertically aligned and interconnected graphene nanoplatelets in a bisphenol F-based epoxy resin. A similar preparation procedure using the water dispersion of GO functionalized with silane coupling agent was developed by Ming et al. [22].

The purpose of this work was to investigate the effect of the particle size distribution of CF used as particulate fillers on the properties of their epoxy composites. The problem of the influence of particles size distribution of porous carbon materials on mechanical, thermal and tribological parameters of polymer-carbon composites has not been studied so far, although the dimensions of particles of crushed carbon foam is extremely important for composite quality. On the one hand, foam particles should be small enough not to affect negatively the mechanical strength of the composite, and on the other hand they should be large enough to preserve the specific porous structure and properties of the original foam crucial for thermal properties and thermal resistance. The CF was obtained in the self-foaming process of a highly cross-linked epoxy-novolac system by the volatile products of its thermal degradation released at elevated temperature. This CF preparation method was applied and described for the first time by the authors of this paper [15]. This process was coupled with the formation of a stiff carbon skeleton with a high true density (~ 1.41 g/cm³). Most attention was focused on basic research related to interactions between porous carbon filler particles and the polymer matrix, e.g. the adhesion of foam grains to the matrix and the effect of these relationships on the curing process; morphology; and viscoelastic, thermal, and friction properties of the final composites. The lack of deeper knowledge about interphase phenomena between filler particles and polymer matrices is a serious obstacle to obtain polymer-carbon composites with required properties. The results of this work might be used to develop a method for producing a new group of light and easily processed polymer composites with crushed porous carbon materials, to replace conventional polymer in parts used as sliding elements and elements of high resistant to abrasion as well as to high-temperature friction. Replacing metal alloys matrix, mainly based on aluminium, with epoxy matrix will allow for energy savings in manufacturing process and, additionally, for significant reduction of the density of final products.