Volumen II - SAM

Congreso SAM/CONAMET 2009 Buenos Aires, 19 al 23 de Octubre de 2009
ELUCIDATION OF FAILURE MICROMECHANISMS INVOLVED IN WEAR AND
FRACTURE OF POLYETHYLENE COMPOSITES BY MEANS OF MULTIPLE
MICROSCOPY TECHNIQUES
V. Pettarin y P.M. Frontini
Instituto de Investigaciones en Ciencia y Tecnología de Materiales
Universidad Nacional de Mar del Plata - CONICET
Av. Juan B. Justo 4302 (B7608FDQ) Mar del Plata, Argentina.
e-mail: pettarin@fi.mdp.edu.ar
SUMMARY
In order to understand material’s behavior structural observations must be correlated with material’s
properties. In this work a combination of microscopy techniques – transmition optical microscopy (TOM),
scanning electron microscopy (SEM) complemented with backscatterd electron images (BEI) and electron
probe microanalyses (EPMA) – was used to elucidate wear and fracture failure mechanisms of
thermoplastic composites. High density polyethylene/molybdenum disulphide composites were investigated.
Different features were elucidated: composites morphology, relationship between MoS2 content and
microstructure, and the correlation structure/damage mechanism/property. Results demonstrate that wear
and fracture failure mechanisms strongly depends on composites’ microstructure and morphology.
Key words: polymer, composites, microstructure, failure mechanisms, microscopy
1. INTRODUCTION
Plastics and polymeric composites are being increasingly used for many structural applications due to their
advantageous material properties, such as high strength and high stiffness to weight ratio as well as increased
design flexibility, reduced manufacturing costs and excellent life expectancy. Materials science, the study of
the structure and properties of materials, is applied to polymers in much the same way as it is to metals and
ceramics: to understand the relationships between the manufacturing process, the structures produced and the
resulting physical and mechanical properties.
There is been an increasing use of optical and electron microscopy as applied to polymer research due to the
widespread acceptance of the techniques and extended property requirements of the polymer materials. The
structures present in a polymer reflect the process variables, and they greatly influence the physical and
mechanical properties. In polymer science, the term ‘morphology’ generally refers to form and organization
on a size scale above the atomic arrangement but smaller than the size and shape of the whole sample. The
term ‘structure’ refers more to the local atomic and molecular details. The characterization techniques used
to determine structure differs somewhat from those used to determine morphology. X-ray, electron and
optical scattering techniques and a range of other analytical tools are commonly applied to determine the
structure of polymers. The morphology of polymers is determined by a wide range of optical and electron
microscopy techniques [1]. However, they are complementary to one another and both types are needed to
determine fully the morphology and microstructure and to develop structure-property relationships.
Structural observations must be correlated with properties to understand material’s behavior.
Polymer composites, especially advanced thermoplastic composites, have been replacing metals, with
increasing proliferation, in a variety of tribological applications [2] which include both sliding (bush
bearings, bearing cages, slides, gear seals) and abrasive (conveyor and conveyor aids, piping, duct work,
wear strips, sleeve bearings, liners) wear conditions. In particular, high molecular weight high density
polyethylene (HMW-HDPE) is widely used in a variety of bearing applications where temperatures are low
and good chemical resistance is required [3]. Tribological behavior of polymers can be improved by filling
them with inorganic compounds such as molybdenum disulphide (MoS2) [4]. In previous works we assessed
the effect of MoS2 on the tribological behavior of HMW-HDPE [5]. Composites with different filler content
were studied (results are summarized in Table 1). MoS2 effectively improved sliding and abrasive wear
performance of HMW-HDPE with an optimum content for minimum wear rate near 10%. It is known that
the development of composites based on solid lubricants with enhanced tribological properties often conflicts
with the simultaneous achievement of superior mechanical strength [6]. Therefore, we evaluated mechanical
1046