11-08-2012, 04:40 PM
Polymeric materials
INTRODUCTION
Polymer based composites have been increasingly used for numerous tribological applications such as seals, gears, bearings, cams, wheels, brakes and clutches, for they have the ability to provide light weight alternatives to conventional materials. Polymer composites are promising in tribological applications due to the possibility of tailoring their properties with special fillers such as MoS2, CuO, CuS, Al2O3, graphite, tungsten carbide, tantalum, niobium, bronze and silicon carbide (SiC) [1-4]. Thermoset epoxy resins are extensively studied as a matrix material for composite structures as well as adhesives for space and aerospace applications because they exhibits low shrinkage, higher mechanical properties, easy fabrication, excellent chemical and moisture resistance, good wet ability and good electrical characteristics.
Wear is defined as damage to a solid surface, generally involving progressive loss of material, due to relative motion between the contacting surfaces. Abrasive wear is caused due to hard particles or hard protuberances that are forced against and move along a solid surface [7-8]. Abrasive wear is usually divided into two types: two-body and three-body abrasion. The situation where exactly two bodies are involved in the interaction is known as two-body
abrasion. Two-body abrasive wear is caused by the displacement of material from a solid surface due to hard particles sliding along the surface or when rigidly held grits pass over the surface like a cutting tool. In three body abrasive wear the grits are free to roll as well as slide over the surface. Two-body abrasive wear is a complex process often involving high strain and plastic deformation and fracture of micro volumes of the material, which might be described as the removal of discrete surface by a harder substance which tends to gauge, score, or scratch [9]. Significant differences between two-body and three-body abrasive wear have been revealed by Emory et al. [10] and Sasada et al. [11]. They have found the three-body abrasive wear to be ten times lower than the two-body abrasive wear since it has to compete with other mechanisms such as adhesive wear. The two-body abrasive wear is undesirable due to high wear rates, dramatic surface damage, and activation of other wear mechanisms. Two-body abrasion test was conducted with abrasive papers or rough metal counter faces where abrasive is fixed. In fact, in practical applications, many components, for example, conveyor belt, shuttle, tillage tools and wind blades are subjected to two-body abrasive wear. The studies covering eighteen types of polymers, low-density polyethylene (LDPE) exhibited the lowest wear rate in abrasion against rough mild steel but the highest wear rate in abrasion with coarse silicon carbide (SiC) abrasive paper [12-15]. Many researchers have studied the abrasive wear behavior of polymer composites [16-20]. Suresha et al. [21] investigated the three-body abrasive wear behavior of SiC and graphite fillers filled glass fabric-vinyl ester composites. They concluded that SiC filled composite showed excellent abrasion resistance.
Polymers and their composites form a very important class of tribo-engineering materials and were invariably used in mechanical components such as gears, cams, bearings, bushes, bearing cages, etc. where wear performance in non-lubricated condition was a key parameter for the material selection [3]. Polymer composites were subjected to abrasive wear in many applications [4]. Abrasive wear was one type of wear where hard asperities on one surface move across a softer surface under load, penetrate and remove material from the softer surface, leaving grooves [5]. Most of the abrasive wear problems arise in chute liners in power plants, mining and earth moving equipments. Three-body abrasive wear was often of considerable practical importance, for example in coal handling equipments in power plants, gear pumps handling industrial fluids and agricultural machine components, but appears to have received much less attention than a two-body abrasion. Very little has been reported on the effect of fiber/filler reinforcement on three-body abrasive wear behavior of polymer composites [6-8]. How such composites perform in abrasive wear situations, needs a proper understanding. In recent years, much research has been devoted to exploring the potential advantage of thermoset matrix for composite applications [9,10]. One such matrix was vinyl ester, which has found a place in the family comprising the thermoset engineering polymers due to it’s the excellent mechanical properties with good chemical/ corrosion resistance. Vinyl ester resins were stronger than polyester resins and cheaper than epoxy resins. Vinyl ester resins offer better resistance to moisture absorption than polyester resins. It was also known that vinyl ester resins bond very well to fiberglass. A notable advance in the polymer industry has been the use of fiber and particulate fillers as reinforcements in polymer matrix [11, 12] .Particulate fillers were of considerable interest, not only from an economic viewpoint, but as modifiers especially the physical properties of the polymer. It was well documented in the literature that majority fillers have a positive influence on mechanical properties.
However, the three-body abrasive wear and their mechanical properties of glass fiber reinforced epoxy composites filled with natural fiber dust as filler were not studied. In this research article an attempt has been made to understand the three-body abrasive wear behavior of natural fiber dust filled with glass fiber reinforced epoxy composites and compared with conventional filler materials like Al2O3 and SiC matrix composites at 50N load and for five different abrading distances.
Polymeric materials are being increasingly used in a wide number of applications where resistance to wear is important. These range from its use as a bearing material (in applications such as machinery parts and biomedical joint replacements) to its use as a glazing material where damage results in loss of optical properties. Polymers are ideal materials for bearing applications due to their general resistance to corrosion, galling and seizure, their tolerance to small misalignments and shock loading and their low coefficients of friction; as glazing materials, their low density and high toughness (compared to traditional glass) along with high transparency are desirable properties. In many applications, polymers may be subjected to abrasive wear, often due to contaminants within a system, and such abrasion may result in loss of function. The abrasive wear of polymers and polymer-based composites is the subject of a large body of literature. A number of test methods have been employed; both two-body and three-body abrasion have been examined, the former with both abrasive papers and rough metal counter faces.