Volumen II - SAM

and fracture behavior of composite with best tribological performance. We found that the introduction of
10% of MoS2 particles into HMW-HDPE deteriorated its integral mechanical behavior, resulting in a
noticeable change of fracture mode from stable to unstable under quasi-static conditions, and from semiductile
to brittle under dynamic conditions; with a concomitant abrupt reduction of toughness values.
A major use of microstructural analysis on multiphase polymers is failure analysis, i.e. determination of the
mode or cause of failure. In this work the use of microscopy techniques – transmition optical microscopy
(TOM), scanning electron microscopy (SEM) and electron probe microanalyses (EPMA) – to elucidate wear
and fracture failure mechanisms and their relationship with composite’s morphology and microstructure is
described. Their usefulness is exemplified by applying them to previously studied HMW-HDPE/MoS2
composites.
Table 1. Physical, mechanical and wear properties of HMW-HDPE/MoS2 composites.
Material Specific wear rate
(μm 2 /N)
Mechanical
properties (3)
MoS2
(%w)
Density
(gr/cm
Fracture toughness
3 )
Sliding (1) Abrasive (2)
σy
(MPa)
E
(GPa)
εb
(%)
Quasi-static (4)
KIC (MPa.m 1/2 )
Dynamic (5)
GIC (kJ/m 3 )
PE 0 2,86 34,77 0,99 26 1,50 >190 6,01 19,80
PE/07 6,86 2,78 3,13 0,77 -- -- -- -- --
PE/10 9,00 2,53 0,50 0,72 23 1,49 4 1,85 1,98
PE/20 17,31 2,71 18,12 0,76 -- -- -- -- --
PE/25 24,93 2,61 68,62 0,95 -- -- -- -- --
(1)
roller(steel)-on-plate(composites) tests
(2)
Dry Sand/Rubber Wheel Technique (ASTM G 65-91, procedure B)
(3)
Uniaxial stress-strain tests at 10 mm/min on machined dumbbell-shaped specimens (ASTM D 638 type I)
(4)
Single edge notched SE(B) specimens tested at 10 mm/min (ASTM D5045-93 and ESIS Protocol [7])
(5)
Single edge notched SE(B) specimens tested at 1 m/s (ISO/DIS 17281)
2. EXPERIMENTAL PROCEDURE
Composites morphology was studied by transmition optical microscopy (TOM) with a Leica DMLB
microscope. Samples were also seen under cross polarizers (POM).
Materials structure was evaluated by elemental composition of samples in a scanning electron microscope
(SEM, JEOL JSM-6460LV) by means of electron probe microanalyses (EPMA). EPMA was carried out with
an energy dispersive X-ray spectroscopy analyzer (EDAX) attached to the microscope (Genesis XM4—Sys
60, equipped with Sapphire Si(Li) detector and Super Ultra Thin Window of Be). Prior to the observations,
samples were coated with a thin gold layer.
Abraded and fracture surfaces were examined by SEM and EPMA after coating them with a thin gold layer.
Some samples were examined by TOM previous to fracture.
3. RESULTS AND DISCUSSION
TOM demonstrates that composites’ morphology consists of two phases. It seems that there are HMW-
HDPE rich domains surrounded by MoS2 rich zones (Figure 1a). POM reveals a very fine spherultic texture
in PE that is practically not altered by MoS2 (Figure 1b). This is confirmed by thermal analysis (differential
scanning calorimetry DSC, according to ASTM D 3417-83). Only a limited increase in crystallinity was
promoted by MoS2. Crystallinity increased from 60% to 66.5% with the addition of 7% of MoS2, reaching a
plateau with further incorporation of MoS2. Melting temperature (≈136ºC) resulted practically unaltered
consistent with no changes in crystal structure.
(a) (b) PE PE-10
Figure 1. TOM picture of composite with 20% MoS2 (a) transmition (b) with cross polarizers.
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