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Tribological Behaviour of Hemp, Glass and Carbon Fibre Composites
Journal Pre-proof Tribological Behaviour of Hemp, Glass and Carbon Fibre Composites
Dario De Fazio, Luca Boccarusso, Massimo Durante PII:
S2352-5738(19)30044-7
DOI:
https://doi.org/10.1016/j.biotri.2019.100113
Reference:
BIOTRI 100113
To appear in:
Biotribology
Received date:
2 August 2019
Revised date:
20 November 2019
Accepted date:
30 November 2019
Please cite this article as: D. De Fazio, L. Boccarusso and M. Durante, Tribological Behaviour of Hemp, Glass and Carbon Fibre Composites, Biotribology(2019), https://doi.org/10.1016/j.biotri.2019.100113
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© 2019 Published by Elsevier.
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Tribological behaviour of hemp, glass and carbon fibre composites
Dario De Fazio* , Luca Boccarusso and Massimo Durante Department of Chemical, Materials and Production Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125, Naples, Italy.
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*Corresponding author. Tel.: +390817682370. E-mail address: [email protected]
Abstract Lightweight composite materials are frequently used for transportation, or the interiors of furniture and boxes. Wear of the surfaces of these materials is a potential health risk affecting the respiratory system or skin. The latter can frequently occur due to human touch of
Journal Pre-proof uncovered synthetic fibres after wear causing dermatitis, or inflammation of the skin. Therefore, composite materials made of natural fibres as reinforcement are an interesting alternative to synthetic fibres, because they are usually less dangerous to human health. Therefore, the goal of this research is to highlight the wear resistance of hemp fibres and compare it with glass and carbon fibre composites. In this work, hemp, glass and carbon fibres in form of woven fabrics were impregnated with epoxy resin through vacuum infusion process. In order to compare the tribological behaviour of the manufactured composites, a detailed experimental campaign, including tribologica l tests, microgeometrical measurements and indentation tests, was carried out. The tribological
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behaviour was studied through the pin-on-disk tests under different conditions that mainly differ in the applied load and both the composite and the single un-impregnated fabrics were tested.
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The results demonstrate good wear behaviour of the laminates reinforced by hemp fibres
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emphasising a better wear resistance at prolonged time and under high load conditions.
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Keywords Tribological properties; Hemp composites; Natural fibers; Pin-on-disk; Wear
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resistance.
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1 Introduction
Glass, carbon and aramid fibres composites have excellent mechanical properties indeed their use for different applications is well known. Structural and interior components for automotive and aeronautic fields, sporting goods, building furniture and biomedical devices are typical examples. If on one hand these applications can appear completely different and unlinked, they are characterized by an important common keypoint: the possibility to being in contact with the biological human system such as skin, eyes and respiratory airway. This
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aspect together with the low fracture toughness, the weak interface strength and the low wear resistance that characterize the abovementioned composites can represent a risk for human health.
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The wear or the damage of the manufactured composites surface could represent a risk for the
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human health when synthetic fibres are used. In fact, the damage of the surface and then of the polymeric matrix causes fibre exposition that can be subjected to further wear, rubbing or
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degradation. Synthetic fibres like glass and carbon can be reduced in micrometric fragments able to pollute the environment [1] and to penetrate in skin of the hands for instance.
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For example, fiberglass dermatitis (FGD) is an occupational irritant contact dermatitis resulting from mechanical irritation due to the penetration into the skin of these fragmented
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fibres through the stratum corneum [2].
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In addition the use of these fibres presents some industrial and environmental problems such as the high quantity of energy used for their production, they are too expensive and in particular, at the end of their life, these materials have known disposal problems [3,4] In this contest it could be interesting to avoid, where it is possible, the use of synthetic fibres for example by replacing them with natural fibres. Indeed, during the last years, due to the abovementioned considerations together with the increasing environmental awareness and the waste materials recycling problems, the interest in the use of more eco-friendly materials is increasing more and more [5–7]. For these reasons the use of vegetable fibres as reinforcement is reaching an increasing attention and a lot of researches have aimed on the study of a range of recyclable materials based on natural fibres in order to study their possible use as good alternative to the conventional ones [8–10]. The use of vegetable fibres implies several advantages, such as: abundance, non-corrosive property, non- irritation of the skin, eyes or respiratory system, non-toxicity [8,9,11], they have lower cost and required of less energy than glass and carbon [12,13]. Additionally, the
Journal Pre-proof lower weight (20 -30 wt.%) of natural fibres compared to synthetic fibres, can improve the fuel efficiency and reduce emission for example in automotive applications [14–16]. For these reasons, in recent years these materials have been drawn considerable attention in numerous application fields, e.g. furniture, packing, automobiles and construction [16–22]. For example, it is largely found in literature that they are used as interior and insulation components in the automotive, aeronautic and building sectors [23–26]. Among different types of natural fibres used as reinforcement for bio-composite materials, hemp is one of the most attractive fibres due to its interesting characteristics, e.g. low cost, low density, high specific strength, etc… The hemp fibre had an important history in terms of
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providing high tensile strength, especially in the use for roping, and in being part of a large productive system[27]. After a few decades of oblivion, due to drug production-related issues, the availability of varieties with low tetrahydro-cannabinoids (THCs) content allowed the
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hemp production to resume [28]. Therefore, it is necessary to raise up the profile of the use of
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this fibre towards more engineered components, also considering that the hemp plant is available as removable resource, can easily be grown around the world and has the ability to
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extract heavy metals from the soil makes.
Regarding the matrix coupled with natural fibres, its selection is limited by the temperature at
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which natural fibres degrade, and it depends on the final properties required from the products.
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Thermoplastics are preferable when the main aim is to aspire to the green production because they can be repeatedly softened by the application of heat and hardened by cooling and have
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the potential to be easily recycled and completely eco-friendly (e.g. PLA), whereas a better emphasis of the fibres mechanical properties is generally achieved by using thermosets as matrix. Among these, the epoxy resin is one of the most used, it is known that this resin is able to form covalent cross-links with plant cell walls via -OH groups[29]. In addition, epoxy resin does not produce volatile products during curing which is very desirable in production of void free composites. If on one hand the improvement of the flame resistance of natural fibres/epoxy bio-composite [30,31], their thermal stability [32], as well as the study of the fibre-matrix compatibility[33], have generated many research papers, on the others, to authors best knowledge, there are few works dealing with the study of their wear behaviour. Indeed, the literature review shows that the wear behaviour of these materials has not been comprehensively examined [11,34]. Chin and Yousif [20] used Kenaf fibres reinforced composite for bearing application and concluded that the composites show a better wear behaviour when the fibres orientation is normal to the
Journal Pre-proof sliding direction. Xsin et al. [35] studied the tribological performance of Sisal fibre reinforced resin brake composites with different fibre contents. At the end of this study, they conclude that the Sisal fibres are a good substitute of asbestos fibres in brake pads applications. Yousif and El-Tayeb [36] studied the tribological properties of oil palm fibres based polymer composite. They used a block-on-ring machine to carry out the tests and the results indicated that the applied load, the sliding velocity and the sliding distance influenced the composite wear behaviour. Chand and Dwivedi [37] studied the tribological performance of sisal fibres polyester composites. The results showed an improving of wear behaviour when fibres oriented normal to the specimen sliding direction. In addition, it was also highlighted that the
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coefficient of friction increased with the increasing of fibres content.
Most of the industrial and manufacturing parts, composite products are exposed to tribological loadings such as adhesive, abrasive, etc. during their service or life. Therefore,
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tribological performance of materials becomes an essential element to be considered in the
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design of components [38] also considering that the worn surfaces are more prone causing injuries in that broken fibres penetrate the skin.
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On the basis of these considerations, this work is focused on the study of the wear behaviour of hemp/epoxy composites by comparing it with epoxy composites using carbon and glass
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2.1 Materials
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2 Materials and Methods
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fibres.
Three different typologies of reinforcement impregnated with epoxy resin SX10 were used for the composites under investigation. The choice of epoxy resin is justified considering its interesting characteristics that allow a large use as matrix for glass and carbon fibres to high performance composites. This type of resin is able to cure at room temperature and it is characterized by a value of glass transition temperature, Tg , of 80°C. For this purpose, hemp, glass and carbon woven fabric are used to obtain an orthotropic behaviour on the plane in which tribological tests are executed. Their main characteristics are listed in Table 1.
Fabric
Fabric type
Areal density
Fibre density
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[g/m2 ]
[g/cm3 ]
Hemp
Woven
160
1.4
Glass
Woven
290
2.5
Carbon
Woven
280
1.8
Table 1 Main properties of the fabrics used as reinforcements.
2.2 Specimen production The sample types under investigation are listed in Table 2. All laminates from which the
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specimens were obtained, had the same stratification sequence and the same number of plies
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equal to four. The laminates were produced through the vacuum infusion process technique by using a close mould constituted by a glass plane and a polymeric bag. Therefore, three
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specimens 200 x 100 mm2 were obtained from each sample type. All the below described
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tests were carried out on three specimens for each sample type.
C G
Epoxy
Fibre volume fraction [%]
4
1,52
30,0
Carbon Epoxy
4
1,56
29,7
Glass
4
1,80
34,2
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Hemp
Epoxy
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H
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Sample Type Fabric Matrix Plies Number Thickness [mm]
Table 2 Sample type characteristics.
Before the impregnation step, the hemp fabrics were soaked in 2% of NaOH solution at room temperature for 30 min in order to improve the fibres-matrix adhesion. After treatment, fibres were copiously washed with water to remove any traces of alkali on the fibres surface and subsequently neutralized with 1% acetic acid solution. Then, the treated fibres were dried in an oven at 60 °C for 12 h. From the SEM observation, see Fig. 1, it is possible to observe a single hemp tow before and after the chemical treatment.
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Fig. 1 Hemp tow surface before (a) and after (b) the chemical treatment.
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2.3 Experimental Set-up
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For each sample typology, indentation tests were preliminary carried out in order to evaluate the elastic behaviour of the sample surfaces. In these tests, a martensitic stainless-steel AISI
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440C ball, 8 mm in diameter, (its properties are listed in table 3) was pressed against the sample surface with a crosshead speed of 0.5 mm/min at room temperature by means of a universal testing machine (MTS alliance RT/50). Each specimen was supported from a rigid
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were manufactured.
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steel plate end each test was repeated three times. So three laminates for type of composite
Stainless steel AISI 440C
Density [g/cm3 ]
7,70
Tensile Strength Young Module
Rockwell Roughness Ra
[MPa]
[GPa]
Hardness
[μm]
1900 – 2000
210
57 – 60
0,01 – 0,15
Table 3 Indenter Properties.
The choice in the use of a ball (8 mm in diameter) as indenter tool was to ensure the same contact conditions that characterised the tribological tests carried out in this experimental campaign.
Journal Pre-proof Indeed, the tribological behaviour were evaluated by using a pin-on-disk apparatus (Ducom TR20-LE) at room temperature according to ASTM G99–95a standard. The apparatus consists of a rotating disk, a pin holder, a loading rig and a measuring equipment for friction force and wear depth[39]. During the test, a fixed steel ball (8 mm in diameter) was used as pin and pressed against the rotating specimen under a fixed normal load of 10, 20, 50 and 70 N. The friction force and the wear depth were continuously measured during the sliding time by the measuring equipment of the Ducom TR20-LE. All tribological tests were conducted at established constant peripheral speed of 210 mm/s by varying the applied load and the track radius (20, 24, 28, 32 mm), as listed in Table 4. The
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sliding speed value was chosen considering that the application of carbon or glass and especially hemp composites is not for kinematic couples (as bearing),
so it was chosen a
value of the sliding speed adequate to simulate the velocity of an accidental contact with a
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rigid body as the rubbing at which an interior component made in composite material, for
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example, can be subjected. In addition, by using a pyrometer to measure the temperature in the contact point between pin and laminate, it was revealed that the sliding speed of 210 mm/s
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allowed the reaching of a temperature always lower than 50°C (Tg of resin is 80°C), therefore the effect of the temperature on the matrix properties can be considered negligible. The
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rotation speed of the disk and the test time was chosen to ensure that for each track radius the
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total number of revolutions is the same.
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Experimental Conditions
Sample Type H Type
C Type
G Type
Applied Load [N]
10 20
50
70 10 20
50
70 10 20
50
70
Wear track radius [mm]
20 24
28
32 20 24
28
32 20 24
28
32
Time [min]
96 115 134 153 96 115 134 153 96 115 134 153
Table 4 Pin-on-disk test conditions.
At the end of tribological tests, each wear track was observed by means of a stereomicroscope (OLYMPUS SZ60 - PT) to have its global view and an optical microscope with a greater magnification (ZEISS AxiosKop 40) in order to obtain a more detailed track visual. In addition, the wear track surfaces were acquired by means of a confocal microscope (Leica
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shown in Fig.2 [40].
Fig. 2 Diametric sample areas on the wear tracks acquired via confocal microscope (a) and an example of one of their 3D representation (b) from which single profiles are extracted (c).
For this purpose, for each wear track and for each specimen, eight areas (4 x 20 mm2 ), as the ones indicated with rectangular areas in Fig. 2a, were acquired (Fig. 2b) and five profiles (Fig. 2c) were extracted from each area. Therefore, the depth and the width of the wear tracks were measured for each single profile and their mean values were evaluated. The wear behaviour was evaluated by comparing the curves of the wear depth over the number of revolutions and the loss of volume (∆V) at the end of the tests. Each acquired surface (Fig. 2b) was analysed using a routine written in MATLAB code in order to evaluate the volume of the tracks and then the final loss of volume (∆V).
Journal Pre-proof At the end of this experimental campaign carried out on the composites under investigation, other tests conducted on un-impregnated fabrics were also performed in order to understand the fibres wear behaviour without consider the matrix influence.
3 Results and discussion
3.1 Indentation tests No significative differences between experimental indentation curves of the specimens of the same sample type were observed, this validate the reproducibility of test results. Therefore,
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Fig. 3 presents the typical load versus displacement of indentation curves for each sample type. For each sample type, the typical curve plotted in Fig. 3 represents the intermediate
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curve between three obtained from three specimens of the same type.
Fig. 3 Typical indentation load versus displacement curves of each sample type.
From the results, it is clear that the hemp type is characterized by a softer behaviour than the others. The hemp fabric shows a lower out of plane rigidity (due to the hemp fibres properties) and this involves in a greater tendency to accommodate the deformation induced by the pin. This consideration will be useful for the next sections, when the results of the tribological tests will be illustrated.
3.2 Pin-on-disk tests results Figs. 4-7 show typical curves of the wear depth and the dynamic coefficient of friction versus the number of revolutions for all the load conditions under investigation and for each sample type. The dynamic coefficient of friction was determined as the ratio between the measured
Journal Pre-proof friction force and the applied normal load. Also for this test, for each sample type, the typical curves plotted in Figs. 4-7 represent the intermediate curve between three obtained from three
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specimens of the same type.
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Fig. 4 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 10
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N.
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Concerning the wear depth trend, all the curves of the test carried out with a load of 10 N (Fig. 4a), show a similar behaviour especially in the initial tract (short-time). Under this low
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load and short sliding time condition the wear response was given from the resin, as confirmed from the similar trends of the coefficients of friction showed Fig. 4b. This means
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that under a load of 10 N the tribological behaviour was mainly given by the matrix that was the same for each typology.
However, looking at Fig. 4a the hemp composite type possess a slightly worse wear behaviour than the others due to a lower rigidity that characterised this sample type, as evidenced in Fig.3.The upper surface of each specimen is mainly constituted by resin and then only after the removal of the resin layer, the wear behaviour change because the fibres begin to manifest its effects on the sliding contact conditions with the pin. Indeed, the final slope of the curves is positive for both glass and carbon composite instead of the one observed for the hemp composite type that is almost zero; this might suggest that as the sliding time or the test load increases the trends of the wear depth could be inverted. Indeed, looking at Fig. 5a, when a higher load was used (20 N) and the sample started to show a major wear depth damage, the fibre type begins to manifest its tribological effect. In
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coefficient of friction curve become to differ from the others (Fig. 5b).
Fig. 5 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 20
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N.
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Looking at Fig. 5a, when a normal load equal to 20 N was used, the H type starts to show its interesting behaviour over the G type and its curve tends to the one of the C type. In
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particular, under a load of 20 N, it is possible to observe that the carbon and natural type curves show a similar trend instead of the one of the glass type. Therefore, respect to the
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previous conditions (10 N) it is possible to highlight two main curve behaviour blocks: one
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showed from the glass type and another showed from the carbon and hemp composite type. This difference, is more evident as the normal load increases, as shown in Figs. 6 and 7. In both cases the glass types showed the worst wear behaviour and its curves are characterised by the higher wear depth and coefficient of friction values. It is also interesting to note that despite the previous load conditions, as the load increases (see Figs. 6a and 7a) the wear depth curves of the carbon and hemp composites type are perfectly superimposed up to around 1200 revolutions (211,0 and 241,1 m for 50 N and 70 N respectively), after this point the curve of the H type emphasised its better wear behaviour.
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Fig. 6 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 50 N.
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The advantage in the use of hemp composites in long time conditions is also clear looking at
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Fig. 7, where the curves concerning the tribological tests carried out with a normal load of 70 N are reported. Indeed, both the glass and carbon type curves show a final positive slope
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instead of the curves of the H type that has got a zero slope in the final tract. This means that predictably as the test time will increase, the wear depth will further increase for the glass and
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value.
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carbon composite type instead of the one of the hemp type that tends to keep its constant
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Fig. 7 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 70 N.
Journal Pre-proof By comparing Figs. 6 and 7, it is also possible to observe that the final wear depth of the H type under normal loads of 50 and 70 N (high load) is quite similar instead of the ones showed from the other composite types. In these load conditions, as the tests time increases (long time conditions) the H type starts to emphasise its interesting wear properties, indeed in both cases it showed the best wear resistance. To corroborate the results of the wear curves, it is also interesting to observe same typical damaged area at the end of the tests carried out at 70 N. Fig. 8 shows comparative microscope images for each sample type.
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From these images, it is clear the presence of broken fibres for the specimen having glass and carbon fabric as reinforcement (Fig. 8b and c), this has a double effect: on the tribological point of view the pin affects the fabric involving in its damage that generates a three bodies
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wear mechanism[41] and on the practical point of view, the presence of synthetic broken
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fibres can lead health problems and environmental pollution. Therefore, when the test time increases the top matrix layer was completely removed and the underlying fabric fibres stated
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to show their wear resistance behaviour that reached its maximum when the hemp fabric was used. In this case, due to the no-brittles behaviour of the natural fibres, there are evidences of
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shown in Fig. 8a.
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any broken fibres but just a fabric distortion along the sliding direction was observed, as
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Fig. 8 Wear track details of hemp (a), carbon (b) and glass (c) composite specimens.
Finally, Fig. 9 summarises, for each sample type, the mean value (obtained as mean of three values shown by three specimens of the same sample type) of the coefficient of friction measured at the end of the tests versus the applied normal load. From this, it is possible to observe that as the normal load increases, the coefficient of friction values decrease. This can be explained considering that under low load conditions (10 N) the wear mechanism is mainly adhesive and then the presence of resin on the ball surface could involve in the highest values of the coefficient of friction. On the other hand, when the applied load increases the ball affects the fabric and the abrasive wear mechanism begins to be prevalent, this could justify the decreasing of the coefficient of friction versus the load. In particular, after long sliding time the wear damage can generate a three-body mechanism due to the presence of broken
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rotating specimen; this involved in a further decreasing in the coefficient of friction.
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Fig. 9 Values of coefficient of friction versus the applied normal load for each composite type.
On the basis on these considerations it could be interesting to better understand the wear
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behaviour of the fibres type under investigation without consider the matrix contribute to the wear resistance. To do this, additionally tribological tests on un-impregnated single fabrics
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were also carried out by using the same testing equipment used before. Under a constant
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peripheral speed of 210 mm/s a normal load of 10 N was applied until an evident fabric damage or a constant wear depth was achieved.
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Fig. 10 shows the typical trend of the percentage dimensionless wear depth versus the number of revolutions curves for each sample type; the dimensionless wear depth was evaluated as the ratio between the wear depth and the specimen thickness. For each sample type, the typical curve plotted in Fig10 represent the intermediate curve between three obtained from three specimens of the same type.
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Fig. 10 Dimensionless wear depth curves versus the sliding time for each un-impregnated fabric type.
The curves confirm what was above explained: the hemp fabric possesses a better wear
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resistance than glass and carbon one indeed only the 10 % of the total thickness results to be
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affected by the wear. In the other cases, the pin quickly destroyed the fabrics, as showed in Fig. 11. In addition, in the case of the hemp fabric more than broken fibres it is evident a
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fabric distortion along the sliding direction.
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Fig. 11 Un-impregnated samples at the end of the test: glass (a), carbon (b) and hemp (c) fabric.
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In order to determine the loss of volume, the depth and the width of each wear track were evaluated by means the confocal microscope by using 10X as magnification (see Fig. 2). The
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results in terms of mean and standard deviation values for each specimen are listed in Tables 5-7. The values of depth and width reported in each table represent the mean and the standard
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deviation obtained from 40 profiles at different locations of the same track, i.e. of the same specimen as showed in Fig. 2. Looking at these values, it is possible to observe that in terms
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of width and depth the track is almost regular.
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Comparing the values of Tables 5-7, that represent the same measurement on different specimens, i.e. specimen 1, 2 and 3, it is worth to note that the variability of the measurement and then of the test is very small.
Sample H Applie
Depth [μm]
d Load [N]
Mean
St. Dev.
Sample G
Width [mm] Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Sample C
Width [mm] Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Width [mm] Mean
St. Dev.
10
26,11
1,48
0,870
0,02
22,99
1,65
0,83
0,01
16,60
0,47
0,72
0,03
20
34,60
1,73
1,050
0,03
87,80
0,73
1,66
0,03
31,60
1,50
1,00
0,04
50
35,90
1,79
1,070
0,01
102,60
1,47
1,80
0,03
53,20
1,44
1,30
0,01
70
36,60
0,86
1,065
0,13
129,00
1,68
2,01
0,01
66,59
1,28
1,30
0,03
Journal Pre-proof Table 5 Depth and width of the wear tracks for each sample type of the specimen n°1.
Sample H Depth [μm]
Applie d Load
Mean
[N]
St. Dev.
Sample G
Width [mm] St.
Mean
Dev.
Depth [μm] Mean
St. Dev.
Sample C
Width [mm] Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Width [mm] Mean
St. Dev.
27,10
1,38
0,890
0,03
23,10
1,55
0,84
0,02
16,90
0,48
0,75
0,02
20
35,50
1,64
1,060
0,02
87,90
0,44
1,76
0,02
32,30
1,35
1,08
0,03
50
36,80
1,56
1,073
0,01
103,60
1,48
1,88
0,03
54,10
1,48
1,38
0,02
70
37,10
1,11
1,076
0,11
129,48
1,56
2,31
67,49
1,32
1,40
0,03
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10
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0,02
d Load [N]
Mean
St. Dev.
Width [mm] St.
Mean
25,92
1,45
0,831
20
33,49
1,78
1,042
50
34,84
1,72
1,066
70
35,98
1,055
St.
Dev.
Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Width [mm] Mean
St. Dev.
21,78
1,60
0,79
0,02
16,36
0,51
0,70
0,02
0,04
86,52
0,79
1,58
0,04
30,48
1,56
0,96
0,03
0,02
100,96
1,40
1,73
0,02
51,89
1,41
1,25
0,04
126,15
1,62
1,94
0,03
65,42
1,35
1,28
0,05
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Mean
Width [mm]
0,01
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10
Dev.
Depth [μm]
Sample C
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Depth [μm]
Applie
Sample G
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Sample H
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Table 6 Depth and width of the wear tracks for each sample type of the specimen n°2.
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Table 7 Depth and width of the wear tracks for each sample type of the specimen n°3.
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Fig. 12. Volume variation for each load conditions.
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From Fig. 12, it is possible to observe that the loss of volume (ΔV) versus the applied load increases in a different way for each sample type, showing that for the hemp one, its increases
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in the slightest way reaching the lowest values. This result further highlighted the interesting
4 Conclusions
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wear behaviour of the natural fibre composite under investigation.
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Wear tests on different composite types having hemp, glass or carbon fabric as reinforcement
composites.
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were carried out and the results highlighted the interesting wear behaviour of the hemp/epoxy
From this study the better wear behaviour of natural fibres with respect to other common synthetic fibres used as reinforcement is evident; furthermore uncovered natural fibres that can appear on the composite surface after wear or scratch damage of the external surface of a composite laminate, are not as dangerous for human health as some synthetic materials like glass or carbon fibres. This aspect makes the hemp fiber composite an interesting candidate material for all applications in which a composite product is in continuous contact with the human body, i.e. automotive or aircraft interior components, building furniture, medical devices, sporting goods and so on. From the pin on disc test results, it is clear that under low load condition (10 N) the composites constituted by hemp fabrics are not able to show their beneficial wear behaviour because the load conditions are not sufficient to excessively stress the fibres. In this case the
Journal Pre-proof wear resistance is mostly given by the matrix and the wear depth is mostly affected by the stiffness possessed by the composite laminates that showed the lowest values for the natural type. The advantage in the use of the hemp composites was evident under higher load conditions (50 and 70 N) during prolonged time, and due to the no-brittle behaviour of natural fibres. There was no evidence of any broken fibres; just a fabric distortion along the sliding direction was observed. This behaviour was also confirmed by the pin-on-disc tests carried out on the single un-impregnated fabrics. Conflict of interest: None
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Journal Pre-proof Highlights Wear resistance comparison between natural and synthetic fibre composites
The tests highlight the interesting wear resistance of hemp/epoxy composite
Improved wear resistance of hemp composite under high load conditions
Adhesive wear is detected for Bio-composite instead of abrasive for synthetic ones
No broken fibres but just a fabric distortion at the end of the tribological test
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Dario De Fazio, Luca Boccarusso, Massimo Durante PII:
S2352-5738(19)30044-7
DOI:
https://doi.org/10.1016/j.biotri.2019.100113
Reference:
BIOTRI 100113
To appear in:
Biotribology
Received date:
2 August 2019
Revised date:
20 November 2019
Accepted date:
30 November 2019
Please cite this article as: D. De Fazio, L. Boccarusso and M. Durante, Tribological Behaviour of Hemp, Glass and Carbon Fibre Composites, Biotribology(2019), https://doi.org/10.1016/j.biotri.2019.100113
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© 2019 Published by Elsevier.
Journal Pre-proof
Tribological behaviour of hemp, glass and carbon fibre composites
Dario De Fazio* , Luca Boccarusso and Massimo Durante Department of Chemical, Materials and Production Engineering, University of Naples “Federico II”, P.le Tecchio 80, 80125, Naples, Italy.
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*Corresponding author. Tel.: +390817682370. E-mail address: [email protected]
Abstract Lightweight composite materials are frequently used for transportation, or the interiors of furniture and boxes. Wear of the surfaces of these materials is a potential health risk affecting the respiratory system or skin. The latter can frequently occur due to human touch of
Journal Pre-proof uncovered synthetic fibres after wear causing dermatitis, or inflammation of the skin. Therefore, composite materials made of natural fibres as reinforcement are an interesting alternative to synthetic fibres, because they are usually less dangerous to human health. Therefore, the goal of this research is to highlight the wear resistance of hemp fibres and compare it with glass and carbon fibre composites. In this work, hemp, glass and carbon fibres in form of woven fabrics were impregnated with epoxy resin through vacuum infusion process. In order to compare the tribological behaviour of the manufactured composites, a detailed experimental campaign, including tribologica l tests, microgeometrical measurements and indentation tests, was carried out. The tribological
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behaviour was studied through the pin-on-disk tests under different conditions that mainly differ in the applied load and both the composite and the single un-impregnated fabrics were tested.
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The results demonstrate good wear behaviour of the laminates reinforced by hemp fibres
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emphasising a better wear resistance at prolonged time and under high load conditions.
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Keywords Tribological properties; Hemp composites; Natural fibers; Pin-on-disk; Wear
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resistance.
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1 Introduction
Glass, carbon and aramid fibres composites have excellent mechanical properties indeed their use for different applications is well known. Structural and interior components for automotive and aeronautic fields, sporting goods, building furniture and biomedical devices are typical examples. If on one hand these applications can appear completely different and unlinked, they are characterized by an important common keypoint: the possibility to being in contact with the biological human system such as skin, eyes and respiratory airway. This
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aspect together with the low fracture toughness, the weak interface strength and the low wear resistance that characterize the abovementioned composites can represent a risk for human health.
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The wear or the damage of the manufactured composites surface could represent a risk for the
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human health when synthetic fibres are used. In fact, the damage of the surface and then of the polymeric matrix causes fibre exposition that can be subjected to further wear, rubbing or
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degradation. Synthetic fibres like glass and carbon can be reduced in micrometric fragments able to pollute the environment [1] and to penetrate in skin of the hands for instance.
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For example, fiberglass dermatitis (FGD) is an occupational irritant contact dermatitis resulting from mechanical irritation due to the penetration into the skin of these fragmented
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fibres through the stratum corneum [2].
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In addition the use of these fibres presents some industrial and environmental problems such as the high quantity of energy used for their production, they are too expensive and in particular, at the end of their life, these materials have known disposal problems [3,4] In this contest it could be interesting to avoid, where it is possible, the use of synthetic fibres for example by replacing them with natural fibres. Indeed, during the last years, due to the abovementioned considerations together with the increasing environmental awareness and the waste materials recycling problems, the interest in the use of more eco-friendly materials is increasing more and more [5–7]. For these reasons the use of vegetable fibres as reinforcement is reaching an increasing attention and a lot of researches have aimed on the study of a range of recyclable materials based on natural fibres in order to study their possible use as good alternative to the conventional ones [8–10]. The use of vegetable fibres implies several advantages, such as: abundance, non-corrosive property, non- irritation of the skin, eyes or respiratory system, non-toxicity [8,9,11], they have lower cost and required of less energy than glass and carbon [12,13]. Additionally, the
Journal Pre-proof lower weight (20 -30 wt.%) of natural fibres compared to synthetic fibres, can improve the fuel efficiency and reduce emission for example in automotive applications [14–16]. For these reasons, in recent years these materials have been drawn considerable attention in numerous application fields, e.g. furniture, packing, automobiles and construction [16–22]. For example, it is largely found in literature that they are used as interior and insulation components in the automotive, aeronautic and building sectors [23–26]. Among different types of natural fibres used as reinforcement for bio-composite materials, hemp is one of the most attractive fibres due to its interesting characteristics, e.g. low cost, low density, high specific strength, etc… The hemp fibre had an important history in terms of
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providing high tensile strength, especially in the use for roping, and in being part of a large productive system[27]. After a few decades of oblivion, due to drug production-related issues, the availability of varieties with low tetrahydro-cannabinoids (THCs) content allowed the
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hemp production to resume [28]. Therefore, it is necessary to raise up the profile of the use of
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this fibre towards more engineered components, also considering that the hemp plant is available as removable resource, can easily be grown around the world and has the ability to
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extract heavy metals from the soil makes.
Regarding the matrix coupled with natural fibres, its selection is limited by the temperature at
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which natural fibres degrade, and it depends on the final properties required from the products.
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Thermoplastics are preferable when the main aim is to aspire to the green production because they can be repeatedly softened by the application of heat and hardened by cooling and have
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the potential to be easily recycled and completely eco-friendly (e.g. PLA), whereas a better emphasis of the fibres mechanical properties is generally achieved by using thermosets as matrix. Among these, the epoxy resin is one of the most used, it is known that this resin is able to form covalent cross-links with plant cell walls via -OH groups[29]. In addition, epoxy resin does not produce volatile products during curing which is very desirable in production of void free composites. If on one hand the improvement of the flame resistance of natural fibres/epoxy bio-composite [30,31], their thermal stability [32], as well as the study of the fibre-matrix compatibility[33], have generated many research papers, on the others, to authors best knowledge, there are few works dealing with the study of their wear behaviour. Indeed, the literature review shows that the wear behaviour of these materials has not been comprehensively examined [11,34]. Chin and Yousif [20] used Kenaf fibres reinforced composite for bearing application and concluded that the composites show a better wear behaviour when the fibres orientation is normal to the
Journal Pre-proof sliding direction. Xsin et al. [35] studied the tribological performance of Sisal fibre reinforced resin brake composites with different fibre contents. At the end of this study, they conclude that the Sisal fibres are a good substitute of asbestos fibres in brake pads applications. Yousif and El-Tayeb [36] studied the tribological properties of oil palm fibres based polymer composite. They used a block-on-ring machine to carry out the tests and the results indicated that the applied load, the sliding velocity and the sliding distance influenced the composite wear behaviour. Chand and Dwivedi [37] studied the tribological performance of sisal fibres polyester composites. The results showed an improving of wear behaviour when fibres oriented normal to the specimen sliding direction. In addition, it was also highlighted that the
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coefficient of friction increased with the increasing of fibres content.
Most of the industrial and manufacturing parts, composite products are exposed to tribological loadings such as adhesive, abrasive, etc. during their service or life. Therefore,
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tribological performance of materials becomes an essential element to be considered in the
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design of components [38] also considering that the worn surfaces are more prone causing injuries in that broken fibres penetrate the skin.
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On the basis of these considerations, this work is focused on the study of the wear behaviour of hemp/epoxy composites by comparing it with epoxy composites using carbon and glass
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2.1 Materials
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2 Materials and Methods
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fibres.
Three different typologies of reinforcement impregnated with epoxy resin SX10 were used for the composites under investigation. The choice of epoxy resin is justified considering its interesting characteristics that allow a large use as matrix for glass and carbon fibres to high performance composites. This type of resin is able to cure at room temperature and it is characterized by a value of glass transition temperature, Tg , of 80°C. For this purpose, hemp, glass and carbon woven fabric are used to obtain an orthotropic behaviour on the plane in which tribological tests are executed. Their main characteristics are listed in Table 1.
Fabric
Fabric type
Areal density
Fibre density
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[g/m2 ]
[g/cm3 ]
Hemp
Woven
160
1.4
Glass
Woven
290
2.5
Carbon
Woven
280
1.8
Table 1 Main properties of the fabrics used as reinforcements.
2.2 Specimen production The sample types under investigation are listed in Table 2. All laminates from which the
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specimens were obtained, had the same stratification sequence and the same number of plies
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equal to four. The laminates were produced through the vacuum infusion process technique by using a close mould constituted by a glass plane and a polymeric bag. Therefore, three
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specimens 200 x 100 mm2 were obtained from each sample type. All the below described
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tests were carried out on three specimens for each sample type.
C G
Epoxy
Fibre volume fraction [%]
4
1,52
30,0
Carbon Epoxy
4
1,56
29,7
Glass
4
1,80
34,2
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Hemp
Epoxy
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H
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Sample Type Fabric Matrix Plies Number Thickness [mm]
Table 2 Sample type characteristics.
Before the impregnation step, the hemp fabrics were soaked in 2% of NaOH solution at room temperature for 30 min in order to improve the fibres-matrix adhesion. After treatment, fibres were copiously washed with water to remove any traces of alkali on the fibres surface and subsequently neutralized with 1% acetic acid solution. Then, the treated fibres were dried in an oven at 60 °C for 12 h. From the SEM observation, see Fig. 1, it is possible to observe a single hemp tow before and after the chemical treatment.
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Fig. 1 Hemp tow surface before (a) and after (b) the chemical treatment.
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2.3 Experimental Set-up
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For each sample typology, indentation tests were preliminary carried out in order to evaluate the elastic behaviour of the sample surfaces. In these tests, a martensitic stainless-steel AISI
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440C ball, 8 mm in diameter, (its properties are listed in table 3) was pressed against the sample surface with a crosshead speed of 0.5 mm/min at room temperature by means of a universal testing machine (MTS alliance RT/50). Each specimen was supported from a rigid
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were manufactured.
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steel plate end each test was repeated three times. So three laminates for type of composite
Stainless steel AISI 440C
Density [g/cm3 ]
7,70
Tensile Strength Young Module
Rockwell Roughness Ra
[MPa]
[GPa]
Hardness
[μm]
1900 – 2000
210
57 – 60
0,01 – 0,15
Table 3 Indenter Properties.
The choice in the use of a ball (8 mm in diameter) as indenter tool was to ensure the same contact conditions that characterised the tribological tests carried out in this experimental campaign.
Journal Pre-proof Indeed, the tribological behaviour were evaluated by using a pin-on-disk apparatus (Ducom TR20-LE) at room temperature according to ASTM G99–95a standard. The apparatus consists of a rotating disk, a pin holder, a loading rig and a measuring equipment for friction force and wear depth[39]. During the test, a fixed steel ball (8 mm in diameter) was used as pin and pressed against the rotating specimen under a fixed normal load of 10, 20, 50 and 70 N. The friction force and the wear depth were continuously measured during the sliding time by the measuring equipment of the Ducom TR20-LE. All tribological tests were conducted at established constant peripheral speed of 210 mm/s by varying the applied load and the track radius (20, 24, 28, 32 mm), as listed in Table 4. The
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sliding speed value was chosen considering that the application of carbon or glass and especially hemp composites is not for kinematic couples (as bearing),
so it was chosen a
value of the sliding speed adequate to simulate the velocity of an accidental contact with a
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rigid body as the rubbing at which an interior component made in composite material, for
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example, can be subjected. In addition, by using a pyrometer to measure the temperature in the contact point between pin and laminate, it was revealed that the sliding speed of 210 mm/s
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allowed the reaching of a temperature always lower than 50°C (Tg of resin is 80°C), therefore the effect of the temperature on the matrix properties can be considered negligible. The
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rotation speed of the disk and the test time was chosen to ensure that for each track radius the
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total number of revolutions is the same.
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Experimental Conditions
Sample Type H Type
C Type
G Type
Applied Load [N]
10 20
50
70 10 20
50
70 10 20
50
70
Wear track radius [mm]
20 24
28
32 20 24
28
32 20 24
28
32
Time [min]
96 115 134 153 96 115 134 153 96 115 134 153
Table 4 Pin-on-disk test conditions.
At the end of tribological tests, each wear track was observed by means of a stereomicroscope (OLYMPUS SZ60 - PT) to have its global view and an optical microscope with a greater magnification (ZEISS AxiosKop 40) in order to obtain a more detailed track visual. In addition, the wear track surfaces were acquired by means of a confocal microscope (Leica
Journal Pre-proof DCM 3D) in order to scan the wear tracks and to evaluate its depth (d) and its width (w), as
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c)
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shown in Fig.2 [40].
Fig. 2 Diametric sample areas on the wear tracks acquired via confocal microscope (a) and an example of one of their 3D representation (b) from which single profiles are extracted (c).
For this purpose, for each wear track and for each specimen, eight areas (4 x 20 mm2 ), as the ones indicated with rectangular areas in Fig. 2a, were acquired (Fig. 2b) and five profiles (Fig. 2c) were extracted from each area. Therefore, the depth and the width of the wear tracks were measured for each single profile and their mean values were evaluated. The wear behaviour was evaluated by comparing the curves of the wear depth over the number of revolutions and the loss of volume (∆V) at the end of the tests. Each acquired surface (Fig. 2b) was analysed using a routine written in MATLAB code in order to evaluate the volume of the tracks and then the final loss of volume (∆V).
Journal Pre-proof At the end of this experimental campaign carried out on the composites under investigation, other tests conducted on un-impregnated fabrics were also performed in order to understand the fibres wear behaviour without consider the matrix influence.
3 Results and discussion
3.1 Indentation tests No significative differences between experimental indentation curves of the specimens of the same sample type were observed, this validate the reproducibility of test results. Therefore,
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Fig. 3 presents the typical load versus displacement of indentation curves for each sample type. For each sample type, the typical curve plotted in Fig. 3 represents the intermediate
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al
Pr
e-
pr
curve between three obtained from three specimens of the same type.
Fig. 3 Typical indentation load versus displacement curves of each sample type.
From the results, it is clear that the hemp type is characterized by a softer behaviour than the others. The hemp fabric shows a lower out of plane rigidity (due to the hemp fibres properties) and this involves in a greater tendency to accommodate the deformation induced by the pin. This consideration will be useful for the next sections, when the results of the tribological tests will be illustrated.
3.2 Pin-on-disk tests results Figs. 4-7 show typical curves of the wear depth and the dynamic coefficient of friction versus the number of revolutions for all the load conditions under investigation and for each sample type. The dynamic coefficient of friction was determined as the ratio between the measured
Journal Pre-proof friction force and the applied normal load. Also for this test, for each sample type, the typical curves plotted in Figs. 4-7 represent the intermediate curve between three obtained from three
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specimens of the same type.
b)
e-
a)
Fig. 4 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 10
Pr
N.
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Concerning the wear depth trend, all the curves of the test carried out with a load of 10 N (Fig. 4a), show a similar behaviour especially in the initial tract (short-time). Under this low
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load and short sliding time condition the wear response was given from the resin, as confirmed from the similar trends of the coefficients of friction showed Fig. 4b. This means
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that under a load of 10 N the tribological behaviour was mainly given by the matrix that was the same for each typology.
However, looking at Fig. 4a the hemp composite type possess a slightly worse wear behaviour than the others due to a lower rigidity that characterised this sample type, as evidenced in Fig.3.The upper surface of each specimen is mainly constituted by resin and then only after the removal of the resin layer, the wear behaviour change because the fibres begin to manifest its effects on the sliding contact conditions with the pin. Indeed, the final slope of the curves is positive for both glass and carbon composite instead of the one observed for the hemp composite type that is almost zero; this might suggest that as the sliding time or the test load increases the trends of the wear depth could be inverted. Indeed, looking at Fig. 5a, when a higher load was used (20 N) and the sample started to show a major wear depth damage, the fibre type begins to manifest its tribological effect. In
Journal Pre-proof particular, the glass fibres highlighted a very lower wear resistance than the others and its
b)
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a)
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coefficient of friction curve become to differ from the others (Fig. 5b).
Fig. 5 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 20
e-
N.
Pr
Looking at Fig. 5a, when a normal load equal to 20 N was used, the H type starts to show its interesting behaviour over the G type and its curve tends to the one of the C type. In
al
particular, under a load of 20 N, it is possible to observe that the carbon and natural type curves show a similar trend instead of the one of the glass type. Therefore, respect to the
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previous conditions (10 N) it is possible to highlight two main curve behaviour blocks: one
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showed from the glass type and another showed from the carbon and hemp composite type. This difference, is more evident as the normal load increases, as shown in Figs. 6 and 7. In both cases the glass types showed the worst wear behaviour and its curves are characterised by the higher wear depth and coefficient of friction values. It is also interesting to note that despite the previous load conditions, as the load increases (see Figs. 6a and 7a) the wear depth curves of the carbon and hemp composites type are perfectly superimposed up to around 1200 revolutions (211,0 and 241,1 m for 50 N and 70 N respectively), after this point the curve of the H type emphasised its better wear behaviour.
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a)
b)
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Fig. 6 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 50 N.
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The advantage in the use of hemp composites in long time conditions is also clear looking at
e-
Fig. 7, where the curves concerning the tribological tests carried out with a normal load of 70 N are reported. Indeed, both the glass and carbon type curves show a final positive slope
Pr
instead of the curves of the H type that has got a zero slope in the final tract. This means that predictably as the test time will increase, the wear depth will further increase for the glass and
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value.
al
carbon composite type instead of the one of the hemp type that tends to keep its constant
a)
b)
Fig. 7 Wear depth (a) and friction coefficient (b) vs number of revolutions under a normal load of 70 N.
Journal Pre-proof By comparing Figs. 6 and 7, it is also possible to observe that the final wear depth of the H type under normal loads of 50 and 70 N (high load) is quite similar instead of the ones showed from the other composite types. In these load conditions, as the tests time increases (long time conditions) the H type starts to emphasise its interesting wear properties, indeed in both cases it showed the best wear resistance. To corroborate the results of the wear curves, it is also interesting to observe same typical damaged area at the end of the tests carried out at 70 N. Fig. 8 shows comparative microscope images for each sample type.
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From these images, it is clear the presence of broken fibres for the specimen having glass and carbon fabric as reinforcement (Fig. 8b and c), this has a double effect: on the tribological point of view the pin affects the fabric involving in its damage that generates a three bodies
pr
wear mechanism[41] and on the practical point of view, the presence of synthetic broken
e-
fibres can lead health problems and environmental pollution. Therefore, when the test time increases the top matrix layer was completely removed and the underlying fabric fibres stated
Pr
to show their wear resistance behaviour that reached its maximum when the hemp fabric was used. In this case, due to the no-brittles behaviour of the natural fibres, there are evidences of
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rn
shown in Fig. 8a.
al
any broken fibres but just a fabric distortion along the sliding direction was observed, as
a)
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c)
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Pr
e-
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b)
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Fig. 8 Wear track details of hemp (a), carbon (b) and glass (c) composite specimens.
Finally, Fig. 9 summarises, for each sample type, the mean value (obtained as mean of three values shown by three specimens of the same sample type) of the coefficient of friction measured at the end of the tests versus the applied normal load. From this, it is possible to observe that as the normal load increases, the coefficient of friction values decrease. This can be explained considering that under low load conditions (10 N) the wear mechanism is mainly adhesive and then the presence of resin on the ball surface could involve in the highest values of the coefficient of friction. On the other hand, when the applied load increases the ball affects the fabric and the abrasive wear mechanism begins to be prevalent, this could justify the decreasing of the coefficient of friction versus the load. In particular, after long sliding time the wear damage can generate a three-body mechanism due to the presence of broken
Journal Pre-proof fibres that, as in the case of C and G types, it could facilitate the sliding of the ball on the
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rotating specimen; this involved in a further decreasing in the coefficient of friction.
e-
Fig. 9 Values of coefficient of friction versus the applied normal load for each composite type.
On the basis on these considerations it could be interesting to better understand the wear
Pr
behaviour of the fibres type under investigation without consider the matrix contribute to the wear resistance. To do this, additionally tribological tests on un-impregnated single fabrics
al
were also carried out by using the same testing equipment used before. Under a constant
rn
peripheral speed of 210 mm/s a normal load of 10 N was applied until an evident fabric damage or a constant wear depth was achieved.
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Fig. 10 shows the typical trend of the percentage dimensionless wear depth versus the number of revolutions curves for each sample type; the dimensionless wear depth was evaluated as the ratio between the wear depth and the specimen thickness. For each sample type, the typical curve plotted in Fig10 represent the intermediate curve between three obtained from three specimens of the same type.
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Fig. 10 Dimensionless wear depth curves versus the sliding time for each un-impregnated fabric type.
The curves confirm what was above explained: the hemp fabric possesses a better wear
pr
resistance than glass and carbon one indeed only the 10 % of the total thickness results to be
e-
affected by the wear. In the other cases, the pin quickly destroyed the fabrics, as showed in Fig. 11. In addition, in the case of the hemp fabric more than broken fibres it is evident a
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al
Pr
fabric distortion along the sliding direction.
a)
b)
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c)
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Fig. 11 Un-impregnated samples at the end of the test: glass (a), carbon (b) and hemp (c) fabric.
e-
In order to determine the loss of volume, the depth and the width of each wear track were evaluated by means the confocal microscope by using 10X as magnification (see Fig. 2). The
Pr
results in terms of mean and standard deviation values for each specimen are listed in Tables 5-7. The values of depth and width reported in each table represent the mean and the standard
al
deviation obtained from 40 profiles at different locations of the same track, i.e. of the same specimen as showed in Fig. 2. Looking at these values, it is possible to observe that in terms
rn
of width and depth the track is almost regular.
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Comparing the values of Tables 5-7, that represent the same measurement on different specimens, i.e. specimen 1, 2 and 3, it is worth to note that the variability of the measurement and then of the test is very small.
Sample H Applie
Depth [μm]
d Load [N]
Mean
St. Dev.
Sample G
Width [mm] Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Sample C
Width [mm] Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Width [mm] Mean
St. Dev.
10
26,11
1,48
0,870
0,02
22,99
1,65
0,83
0,01
16,60
0,47
0,72
0,03
20
34,60
1,73
1,050
0,03
87,80
0,73
1,66
0,03
31,60
1,50
1,00
0,04
50
35,90
1,79
1,070
0,01
102,60
1,47
1,80
0,03
53,20
1,44
1,30
0,01
70
36,60
0,86
1,065
0,13
129,00
1,68
2,01
0,01
66,59
1,28
1,30
0,03
Journal Pre-proof Table 5 Depth and width of the wear tracks for each sample type of the specimen n°1.
Sample H Depth [μm]
Applie d Load
Mean
[N]
St. Dev.
Sample G
Width [mm] St.
Mean
Dev.
Depth [μm] Mean
St. Dev.
Sample C
Width [mm] Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Width [mm] Mean
St. Dev.
27,10
1,38
0,890
0,03
23,10
1,55
0,84
0,02
16,90
0,48
0,75
0,02
20
35,50
1,64
1,060
0,02
87,90
0,44
1,76
0,02
32,30
1,35
1,08
0,03
50
36,80
1,56
1,073
0,01
103,60
1,48
1,88
0,03
54,10
1,48
1,38
0,02
70
37,10
1,11
1,076
0,11
129,48
1,56
2,31
67,49
1,32
1,40
0,03
f
10
oo
0,02
d Load [N]
Mean
St. Dev.
Width [mm] St.
Mean
25,92
1,45
0,831
20
33,49
1,78
1,042
50
34,84
1,72
1,066
70
35,98
1,055
St.
Dev.
Mean
St. Dev.
Depth [μm] Mean
St. Dev.
Width [mm] Mean
St. Dev.
21,78
1,60
0,79
0,02
16,36
0,51
0,70
0,02
0,04
86,52
0,79
1,58
0,04
30,48
1,56
0,96
0,03
0,02
100,96
1,40
1,73
0,02
51,89
1,41
1,25
0,04
126,15
1,62
1,94
0,03
65,42
1,35
1,28
0,05
Jo u 0,83
Mean
Width [mm]
0,01
rn
10
Dev.
Depth [μm]
Sample C
al
Depth [μm]
Applie
Sample G
Pr
Sample H
e-
pr
Table 6 Depth and width of the wear tracks for each sample type of the specimen n°2.
0,12
Table 7 Depth and width of the wear tracks for each sample type of the specimen n°3.
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Journal Pre-proof
pr
Fig. 12. Volume variation for each load conditions.
e-
From Fig. 12, it is possible to observe that the loss of volume (ΔV) versus the applied load increases in a different way for each sample type, showing that for the hemp one, its increases
Pr
in the slightest way reaching the lowest values. This result further highlighted the interesting
4 Conclusions
al
wear behaviour of the natural fibre composite under investigation.
rn
Wear tests on different composite types having hemp, glass or carbon fabric as reinforcement
composites.
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were carried out and the results highlighted the interesting wear behaviour of the hemp/epoxy
From this study the better wear behaviour of natural fibres with respect to other common synthetic fibres used as reinforcement is evident; furthermore uncovered natural fibres that can appear on the composite surface after wear or scratch damage of the external surface of a composite laminate, are not as dangerous for human health as some synthetic materials like glass or carbon fibres. This aspect makes the hemp fiber composite an interesting candidate material for all applications in which a composite product is in continuous contact with the human body, i.e. automotive or aircraft interior components, building furniture, medical devices, sporting goods and so on. From the pin on disc test results, it is clear that under low load condition (10 N) the composites constituted by hemp fabrics are not able to show their beneficial wear behaviour because the load conditions are not sufficient to excessively stress the fibres. In this case the
Journal Pre-proof wear resistance is mostly given by the matrix and the wear depth is mostly affected by the stiffness possessed by the composite laminates that showed the lowest values for the natural type. The advantage in the use of the hemp composites was evident under higher load conditions (50 and 70 N) during prolonged time, and due to the no-brittle behaviour of natural fibres. There was no evidence of any broken fibres; just a fabric distortion along the sliding direction was observed. This behaviour was also confirmed by the pin-on-disc tests carried out on the single un-impregnated fabrics. Conflict of interest: None
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[1]
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Journal Pre-proof Highlights Wear resistance comparison between natural and synthetic fibre composites
The tests highlight the interesting wear resistance of hemp/epoxy composite
Improved wear resistance of hemp composite under high load conditions
Adhesive wear is detected for Bio-composite instead of abrasive for synthetic ones
No broken fibres but just a fabric distortion at the end of the tribological test
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Pr
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