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high density polyethylene composites: Morphological study and quantitative analysis
Wear 294–295 (2012) 326–335
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Effect of reinforcement on wear debris of carbon nanofiber/high density polyethylene composites: Morphological study and quantitative analysis Tian Liu, Weston Wood, Bin Li, Brooks Lively, Wei-Hong Zhong n School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2012 Received in revised form 12 June 2012 Accepted 13 July 2012 Available online 23 July 2012
Polymeric nanocomposites are promising tribological materials. However, the wear debris generated on the sliding surface of composite materials is highly affected by the nanofillers used (types, surfaces, etc.). In this study, based on the high density polyethylene (HDPE) composites with various carbon nanofibers (CNFs) prepared, the effects of CNF concentration and surface modification on morphology and size distribution of wear debris were examined. The individual wear particle morphology was observed with a scanning electron microscope (SEM). The particle size distributions of the wear debris were statistically analyzed based on the optical microscope imaging for all the debris collected after 24 h of wear testing. In addition, the influence of various CNFs (untreated and organosilane treated) on debris components was analyzed by both differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) measurements. The results revealed that there were less mechanical damage and a lower proportion of large particles in the wear debris for the nanocomposites with heavily organosilane coated CNFs. Published by Elsevier B.V.
Keywords: Nanocomposite Wear debris Morphology Quantitative analysis
1. Introduction Polymers have attracted increasing attention in the field of tribology due to their general resistance to corrosion and low coefficients of friction [1]. Nowadays, polymer tribological applications cover a wide field, notably in machinery parts of automotive and aeronautical manufacturing, medical devices, artificial joint bearing surfaces, etc. Different polymers, such as polyethylene (PE) [2,3], polyamides (PA) [4], poly(ether ether)ketones (PEEK) [5], can be applied for various tribological systems in terms of their properties. Research on the tribological performance of these polymers mainly focuses on wear resistance, wear surfaces, and wear debris. The characterization of wear debris generated from sliding process is particularly important. The existence of debris with different morphologies and sizes may have a great impact on the wear surfaces between sliding pairs. In this case, the tribological behavior of a material could be affected dramatically by debris during the continued wear process. Therefore, it is quite essential to analyze the wear debris. Because of its outstanding tribological properties, high resistance to moisture permeation, excellent chemical resistance performance, and moldability, polyethylene (PE) is one of the most generally used polymers for tribology, particularly in biomedical and military
n
Corresponding author. Tel.: þ1 509 335 7658; fax: þ1 509 335 4662. E-mail address: [email protected] (W.-H. Zhong).
0043-1648/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.wear.2012.07.010
applications [6,7]. In spite of this, the quality of PE bearings would not satisfy the tribological requirements completely, owing to the increasingly high demand for improving the wear resistance of PE materials [8,9]. Making polymers into polymeric nanocomposites by adding appropriate nanofillers is a common technique to suit some particular tribological application. In the past few years, many studies on PE composites reinforced by various nanofillers have been carried out, for example with hydroxyapatite nanorods (nHA) [10], carbon nanotubes (CNTs) [11,12] and carbon nanofibers (CNFs) [13]. The reported work showed that the sliding wear resistance of PE nanocomposites was improved significantly compared with the pure polymers. Although there has been much work on the tribology of nanocomposites in general, very few studies on the characterization of wear debris of nanocomposites have been carried out [14]. Almost all the studies on the wear debris of polyethylene (PE) related materials produced during sliding process were limited to the analysis for pure PE [15–22]. The influence of the addition of nanofillers on morphology and particle size distribution of the PE nanocomposite wear debris has not been previously reported to the best of our knowledge. In the nanofiller/polymer multicomponent system, both the fillers themselves and the large interfacial areas between the matrix and the reinforcement can affect the tribological performance of the nanocomposites directly. This influence could be reflected by the wear debris of the composites via morphology, particle size and composition. Since the generated debris can significantly affect mechanical
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systems with the composites, the wear debris analysis is particularly important to such multi-component composite materials. The prerequisite for producing nanocomposites with superior performance includes the following two aspects: homogenous filler dispersion and optimal interaction/adhesion between fillers and polymer matrix. Numerous attempts for enhancing the load transfer from filler to matrix have been made [23–27]. Generally, the surface modification for nanofillers has been used. According to the different polarities of various polymer matrixes, different modification approaches can be developed. Hydrocarbon polymers, such as PE, as the matrix materials, which are non-polar polymers, functionalizing fillers via various hydrophobic moieties similar to the matrix is the most feasible method [23,24]. In our previous study [28], octadecyltrimethoxysilane (ODMS), an organosilane coupling agent with long non-polar hydrocarbon chains, was selected to treat the preoxidized carbon nanofibers (ox-CNFs) resulting in the preferable compatibility with high density polyethylene (HDPE). These hydrocarbon chains of the ODMS were coated onto the CNF surface to form a non-polar layer. The dynamic mechanical analysis and wear testing results indicated that CNFs treated with such silane coatings are effective nanofillers for enhancing mechanical and tribological properties of polyethylene, indicating improved interaction and adhesion between the nanofibers and the matrix [29,30]. In this paper, our main objective is to examine the influence of different nanofiller surface on the wear debris morphologies, particle size distributions and compositions of HDPE nanocomposites during the continued wear process. We prepared HDPE and its nanocomposites with different filler loadings (0.5 wt.% and 3 wt.%) for analysis. The results indicated that polymeric composite system with nanofiller reinforcements exhibited improved tribological behavior with respect to two aspects observed from debris analysis: less mechanical damage and a reduction amount of long PE fibrils. In order to reveal the influence of different fiber–matrix interaction on the wear debris of nanocomposites, pristine CNFs and our previously silane treated
Disk
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CNFs (thin and thick coatings) were selected as the reinforcements of HDPE [28]. Study results suggested that the debris size distribution and the morphology of the wear debris are related to the interactions between the nanofibers and the polymer matrix. The silane coatings can contribute to the enhanced interaction between the two phases, especially for the thicker coating. The experimental results revealed that heavily silanized CNF composites yielded smooth wear surfaces and a very small proportion of large wear debris.
2. Experimental procedures 2.1. Sample preparation High density polyethylene (HP54–60 Flake) was supplied by Bamberger Polymers Inc. with density of 0.954 g cm 3. The pristine CNFs (fiber type: PR-24-HHT) and pretreated oxidized CNFs (fiber type: PR-24-HHT-OX) were purchased from Applied Sciences Inc., which are approximately 60–150 nm in diameter and 30–100 mm in length. Octadecyltrimethoxysilane (ODMS) (90% technical grade) as organosilane coupling agent was purchased from Sigma-Aldrich. Ethanol was obtained from Decon Laboratories Inc. The preoxidized CNFs (oxCNFs) were modified under subsequent ODMS (90% technical grade) treatment in boiling ethanol solution. Then, the condensation reaction of organosilane coupling agent onto oxCNFs occurred due to the reactive hydroxyl groups on the surface of organosilane after hydrolysis, forming a silane coating to cover the fiber surface. The degree of hydrolysis and the thickness of silane-coating can be controlled by changing the ratio of oxCNFs, ODMS, ethanol, water and the addition of NaOH as catalyst in this reaction. The coating thicknesses were determined using TGA data, which are heavily silanized CNF-h (about 46 nm) and lightly silanized CNF-l (about 2.8 nm). Details of the treatment are provided in our previous study [28]. The 0.5 wt.% and 3 wt.% fiber contents for both pristine CNFs (Comp-p) and silanized CNFs (Comp-l with light coating; Comp-h with heavy coating) were mixed with HDPE by a Haake Torque Rheometer. Mixing was set at 180 1C with a rotator speed of 70 rpm for 15 min. A neat HDPE reference was also prepared under identical conditions. Thereafter, the processed HDPE and mixed polymeric nanocomposites were hot-pressed at 180 1C for 10 min via a hydraulic presser. Samples were allowed to cool down to room temperature naturally after turning off the heat. Each sample was cut for wear testing with the size of 10 mm 10 mm and similar average thickness around 2.5 mm. 2.2. Debris collection by wear testing
Rotation Disk
Normal Force
Sample
Sample Holder
Fig. 1. Schematic diagram of the components part of pin-on disk wear-testing apparatus.
The wear tests were performed in a custom build rig for the samples with a vertical 1020 carbon steel disk (HITACHI L200 Series, supplied by Hitachi Industrial Equipment system Co., Ltd), which was used in our pervious study [29]. The schematic diagram of the components part of the equipment was shown in Fig. 1. The effective radius of the disk was 65 mm, and testing was performed at 180 rpm. Also a normal force of 36 N was applied to the sample
Table 1 Detailed descriptions for each sample. Samples
Reinforcement
Fiber treatment
Coating average thickness (nm)
Fiber content
Debris collection
Pure HDPE Comp-p Comp-l Comp-h
– Pristine CNFs Silanized CNFs Silanized CNFs
– – Lightly silane treated Heavily silane treated
– – 2.8 46
– 0.5 wt.% & 3 wt.%
24 h-Wear Testing
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specimen with the wear surface of 10 mm 10 mm. All the tests were conducted at room temperature. Thereafter, all debris generated from the wear tests for each specimen were collected from the surface of the wear tester disk after 24 h. The detailed descriptions of each sample were shown in Table 1. In order to avoid the influence of previous tests, the disc was cleaned thoroughly with ethanol and dried, then polished by fine sand paper of the type Silicon Carbide 220b (220 grit) prior to next use.
2.3. Wear debris treatment for separation The debris particles tended to be gathered through electrostatic attraction produced by the wear process. To separate the aggregated wear debris, the Nine-step Debris Treatment (NDT) was applied. All collected wear debris of each sample was put into clean test tubes and mixed with NaOH solution. The mixtures were bath sonicated at room temperature for 10 h. Glycerol was added to each sample. Then, wear debris were centrifuged 8 h at 3000 rpm (Multi-RF Series Multipurpose Centrifuge OM 8464, Thermo Electron Co.). The top layer of each sample was siphoned and placed in new test tubes. After adding ethanol, debris was bath sonicated for 1 h, centrifuged for 1 h, and siphoned again. All specimens were washed several times with ethanol and deionized water. Finally, the treated samples
were extracted randomly (15 drops for each type of sample) and put on different microscope glass slides (one drop on each slide) for the particle length measurement and then quantitative analysis of individual wear debris. 2.4. Characterizations 2.4.1. Optical microscope The particle sizes of separated wear debris after NDT were determined by using an optical microscope (Olympus BX51TRF) equipped with a camera (Olympus U-CMAD 3, Portland, OR, USA). 10 Amount of Wear Debris (mg)
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8 6 4 2 0 pure HDPE
comp-p comp-p (0.5 wt%) (3 wt%)
comp-l comp-l (0.5 wt%) (3 wt%)
comp-h comp-h (0.5 wt%) (3 wt%)
Fig. 3. Amount of wear debris of pure HDPE and HDPE nanocomposites with different fiber loadings collected after 24 hwear testing (error bar: standard deviation).
Fig. 2. FESEM micrographs (scale bar: 20 mm; inset: 1 mm) of the fracture surfaces of HDPE nanocomposites reinforced by pristine CNFs (A) and (B), lightly silanized CNFs (C) and (D), heavily silanized CNFs (E) and (F). The fiber concentrations of all nanocomposites were 0.5 wt.% (left column) and 3 wt.% (right column).
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The quantitative analysis of wear debris size distribution was carried out through the length of each separated debris particle by optical image analysis. The experimentally determined particle number percentage provided the number proportion in each size range, and then two-dimensional particle size distributions of the wear debris were plotted.
2.4.2. FESEM (field emission scanning electron microscope) To investigate the interaction between nanofibers and polymer matrix, the microstructure features on the fracture interface of HDPE
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nanocomposites before wear testing were characterized with field emission scanning electron microscope (FESEM type Quanta 200 F). The overall fiber dispersion status in the nanocomposites was observed. FESEM images for fractured surfaces were prepared by freezing in liquid nitrogen for 10 min prior to fracturing. Additionally, the morphologies of separated wear debris of all specimens after NDT were also observed by FESEM. The images for microcosmic morphology features of the samples were prepared by desiccating in vacuum oven for 30 min (vacuum state). The surfaces of all samples were sputter coated with gold to improve electrical conductivity.
Fig. 4. FESEM micrographs examples (scale bar: 10 mm for A–C; 300 mm for D) of separated pure HDPE wear debris.
Fig. 5. FESEM micrographs (scale bar: 10 mm; inset: 50 mm) of examples for the separated wear debris of HDPE nanocomposites with different fiber concentrations: 0.5 wt.% for first row; 3 wt.% for second row. (A) and (D) Comp-p; (B) and (E) Comp-l; (C) and (F) Comp-h.
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2.4.3. DSC (differential scanning calorimetry) The melting behaviors of pure HDPE, HDPE nanocomposites and their wear debris without NDT under consideration were performed in a TA instrument TA DSC 822 (Mettler-Toledo, Inc.). The bulk samples and their wear debris were sealed under aluminum pans were scanned under a nitrogen atmosphere with a heating rate of 10 1C/min from 25 1C to 160 1C.
2.4.4. TGA (thermogravimetric analysis) A Thermogravimetric Analysizer (SDT Q600) was applied to determine the weight loss of the bulk samples and their wear debris for each nanocomposite. The wear debris was collected after 24 h wear testing. The temperature range for all TGA tests were from ambient temperature to 700 1C, with a heating rate of 10 1C/min in an N2 environment and flow rate of around 100 mL/min.
3. Results and discussion 3.1. Morphology and wear performance of nanocomposites The typical fracture surface morphology of all nanocomposites was observed through a field emission scanning electron microscope (FESEM). From Fig. 2A–F, the SEM images with different magnifications are shown as representatives for the dispersion and distribution status of the three types of CNFs in HDPE. No obvious larger agglomerates were found in all nanocomposites, suggesting that the nanofibers were well dispersed and uniformly distributed by means of the melt mixing process. From Fig. 2A and B, several long pristine CNFs were exposed on the surface of the PE matrix, and some of them were pulled out. Additionally, there were many gaps between the nanofibers and the polymer matrix, indicating the interaction and the adhesion between the two
Fig. 6. Examples of optical images (scale bar: 50 mm) for the separated wear debris of all types of samples.
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phases were not satisfactory (Fig. 2B). In comparison, large amounts of short CNFs were observed on the fracture surface of Comp-l (Fig. 2C and D) and Comp-h (Fig. 2E and F) even with the high fiber concentration of 3 wt.% (Fig. 2D and F). Moreover, very few fibers were pulled out and there were few voids between the two phases (Fig. 2D and F). These phenomena are more obvious in Comp-h (Fig. 2E and F), revealing that heavy silane coating may induce an increased interaction and load transfer between filler and matrix [28]. Therefore, these results demonstrate that the silane coating enhanced interfacial interaction and adhesion in the HDPE/CNF system. The debris amount was measured after 24 h of wear testing. It can be seen from Fig. 3 that all the nanocomposites exhibited lower wear debris amount than pure HDPE, indicating improved wear resistance of the nanocomposites. The improvement of wear resistance in nanocomposites may be attributed to load transfer from the polymer matrix to the nanofibers [31]. With higher loadings of CNFs (3 wt.%), the amounts of wear debris increased for all nanocomposites. Compared to the composites with 0.5 wt.% CNF loading, the larger interfacial area exists in the 3 wt.% CNF composites. The interface is very critical for the wear performance of the composites, as the wear debris can be formed at this region. Hence, the influence of shear stress caused by the wear process on the interfacial zone in the composites with higher fiber loading can be larger. This phenomenon in the Comp-h sample was not as obvious as that of the other two composites owing to the strengthened interfacial zone. The heavily crosslinked silane coatings covered the fiber surface, potentially reducing the interaction force between the fibers. Additionally, Comp-h displayed the least amount of wear debris,
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reflecting the best wear properties, especially for the 0.5 wt.% CNF composites. Hence, the obtained results revealed that the organosilane coatings, particularly with the thicker coating, enhanced wear resistance of HDPE/CNF system effectively.
3.2. Morphology of wear debris treated by Nine-step Debris Treatment To date, various descriptive studies on the morphology of treated (e.g. g radiation treatment) or non-treated wear debris for pure PE materials have been carried out [16,18–22,32]. These investigations mainly focused on wear debris features, including shape and size. The researchers observed many micro-wear features of debris, such as ripples, tufts, shreds, granules, nodules, fibrils, etc. In our study, some of these configurations were found. Fig. 4A–D displays the examples of the separated pure HDPE wear debris after Nine-step Debris Treatment (NDT). The granule, PE fibril and other irregular submicron size particles can be seen. Additionally, some corrugated structures were observed on the particle surface, as shown in Fig. 4B. Moreover, there existed some cracks, ripples and micro-voids on debris surface (Fig. 4A). We attributed these defects to the mechanical damage caused by the wear process. For thermoplastic materials, the mechanical damage is relevant to the mechanism of ductile deformation, mainly including the yielding under shear stress. In Fig. 5A–F, the differences in the wear debris morphologies among HDPE nanocomposites with various CNFs and different fiber concentrations can be observed. Compared with pure HDPE wear debris, as shown in Fig. 4, there was less damage found in
Fig. 7. Particle size distributions (particle lengths were below 1200 mm) for the wear debris of pure HDPE and HDPE nanocomposites specimens with 0.5 wt.% concentration of fibers. (A) pure HDPE; (B) Comp-p; (C) Comp-l; (D) Comp-h. Two inset bar charts in each figure were the corresponding detailed particle size distributions for small debris with the sizes below 300 mm and large debris with the size range of 300–1200 mm, respectively.
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the debris of composite materials, indicating the improved wear resistance in the nanocomposites resulting from the fibers. The addition of nanofibers could have effectively acted as reinforcement in the matrix, exhibiting enhanced tribological characteristics of the composite system during the wear process. This was more obviously seen in the two types of silanized CNF composites (Fig. 5B, C, E and F), especially for the samples of Comp-h reinforced by heavily silanized CNFs (Fig. 5C and F). The wear debris surfaces of Comp-h were quite smooth. For Comp-l, there were several smaller wear debris on the bigger portion of debris with a smooth surface (Fig. 5E). In comparison, some surface damage still can be seen in the wear debris of Comp-p, such as cracks and granules (Fig. 5A and D). The distinction of the
morphology suggests that the sufficient silane coating on the fiber surface could induce an increased strengthening in the interfacial region between HDPE and CNFs, which was attributed to the enhanced interaction and adhesion. Therefore, the strengthened composite system of Comp-h can be also reflected by the morphology of debris generated from the wear test. 3.3. Quantitative analysis of the wear debris The individual wear debris treated by Nine-step Debris Treatment (NDT) was observed from optical images. Some typical examples of the obtained optical images for the debris observation were shown in Fig. 6. Because the morphologies of most separated debris were nearly disk-shaped, the maximum length of each particle was selected to represent the size of wear debris. The measured length of every debris particle was recorded for the further quantitative particle size distribution analysis. The distribution of the debris lengths was found to be in a broad range from several nanometers to 1200 mm for pure HDPE and its nanocomposites with 0.5 wt.% fiber content (Fig. 7A–D). The proportions of the particles with the lengths below 40 mm were 10% less for all composites’ debris (Fig. 7B–D) compared to pure HDPE wear debris (Fig. 7A). Additionally, particles with the length range from 750 mm to 1200 mm were present in the pure HDPE debris specimen. The proportions of the large debris particles with such size range were much lower in the three types of nanocomposites than of that in pure HDPE. These results can be explained by the existence of fiber reinforcement in the polymer matrix, which played the role of carrying load during sliding wear process. Also, pure HDPE is able to be drawn out during wear, whereas CNFs limit the ability for the chains to be drawn out in the direction of shear [33,34]. The detailed states of small (particle lengths below 300 mm) and large (particle length range: 300–1200 mm) debris particle size distributions for each specimen are more clearly seen in the inset charts of Fig.7. For the particle size below 30 mm, similar length distributions were observed for pure HDPE, Comp-p (0.5 wt.%) and Comp-l (0.5 wt.%). For the Comp-h (0.5 wt.%), there was a higher proportion of wear debris below 10 mm. For the large debris inset, particles with the length above 1050 mm were attributed to the existence of long PE fibrils. Among the debris of three nanocomposites, there were no such large particles. In particular, the size ranges of large debris particles in Comp-h (0.5 wt.%) were found in 400–600 mm range and the proportion was fairly small (around 0.2%). Thus, the wear debris of Comp-h (0.5 wt.%) displayed not only the high percentage of very small debris particles, but also the few large ones. The percentage distributions of debris lengths for HDPE nanocomposites with 3 wt.% fiber content were displayed in Table 2 Melting temperatures (Tm) of the nanocomposites and their wear debris. Sample list
Fig. 8. Particle size distributions (particle lengths were below 1200 mm) for the wear debris of HDPE nanocomposites specimens with 3 wt.% concentration of fiber. (A) Comp-p; (B) Comp-l; (C) Comp-h. Two inset bar charts in each figure were the corresponding detailed particle size distributions for small debris with the sizes below 300 mm and large debris with the size range of 300–1200 mm, respectively.
Pure HDPE Comp-p 0.5 wt.% 3 wt.% Comp-l 0.5 wt.% 3 wt.% Comp-h 0.5 wt.% 3 wt.%
Melting temperature Tm (1C) Bulk sample
Wear debris
134.5 71.2
135.1 7 1.3
135.2 71.6 135.7 71.5
133.2 7 1.0 133.8 7 1.7
135.0 71.1 137.3 72.0
134.8 7 0.8 131.8 7 1.9
134.7 71.3 136.9 71.2
134.5 7 1.2 132.3 7 1.6
p, Pristine CNFs; l, lightly silanized CNFs; h, heavily silanized CNFs.
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Fig. 8A–C. It is clear that there were only very little differences among them. The proportions of particles below 40 mm in wear debris of 3 wt.% composites were on average 20% higher than that of 0.5 wt.% CNF composites. Furthermore, the percentages of large wear debris (length above 300 mm) decreased dramatically. The detailed states of small (particle lengths below 300 mm) and large (particle length range: 300–1200 mm) debris particle size distributions for the 3 wt.% CNF composites can be seen clearly in the inset charts of Fig. 8. According to the results of small particle size distribution, most of debris lengths were below 30 mm, which was similar with that of the composite materials at 0.5 wt.% loading level (Fig. 7). For two types of silanized CNF composites (Fig. 8B and C), a higher frequency of small particles with the lengths below 10 mm were seen. From the other inset chart in Fig. 8 (large debris size distribution), it should be noted that the reductions in large particle proportions and lengths existed in the wear debris of composites with 3 wt.% fiber content compared with that of 0.5 wt.% CNF composites. However, it was not very obvious in the sample of Comp-h. In this case, we may conclude that the sufficient silane coating onto fiber surface could limit the production of large wear debris of the nanocomposites [28]. Combined with the wear testing results (Fig. 3), we can obtain such conclusions: (1) the debris particle sizes were reduced for the nanocomposites with 3 wt.% CNFs (higher percentage of small debris particles, as shown in Fig. 8). During the wear process, the polymer chains can be drawn out to form the long PE fibrils (Fig. 4D) owing to the plastic deformation [31,33,34]. The addition
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of CNFs can contribute to the limitation of this plastic deformation of PE matrix, thus there were no long fibrils found in the wear debris of composite systems. (2) The wear debris amounts for the nanocomposites with 3 wt.% CNFs were increased (as shown in Fig. 3). It is due to the larger interfacial region between filler and matrix in the composites at a higher fiber loading. The interface is very critical for the wear performance of nanocomposites, as the wear debris can be formed at this region. This phenomenon in the samples of Comp-p and Comp-l was obvious owing to their weak interfacial zones. In comparison, Comp-h with 3 wt.% heavily silanized CNFs showed less amount of increased wear debris, which can be attributed to the strengthened interface. Therefore, the limited deformation of PE and the existence of larger interfacial region were the main contributions of the decreased particle size and increased amount in the wear debris of nanocomposites with a high fiber loading.
3.4. Differential scanning calorimetry The melting behaviors of pure HDPE and its nanocomposite with 0.5 wt.% and 3 wt.% fiber loadings as well as their wear debris were studied by differential scanning calorimetry (DSC). Table 2 presented the melting temperatures (Tm) of each sample during the heating cycle, showing similar Tm values between pure HDPE and all composites before wearing. This result is coincident with our previous studies [28]. Additionally, it is hard to see
Fig. 9. TGA thermograms for the bulk materials (a and b) and the wear debris (c and d) of Comp-p, Comp-l and Comp-h with the fiber concentrations of (a) and (c) 0.5 wt.%; (b) and (d) 3 wt.%.
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obvious changes in Tm between the nanocomposites and their wear debris (slight reductions of melting temperatures of wear debris for the composites with 3 wt.% fiber content), indicating the similar influences of varying CNFs (untreated and organosilane treated) on debris components for the nanocomposites. 3.5. Thermogravimetric analysis In order to perform the compositional analysis of HDPE/CNFs multi-component systems, thermogravimetric analysis (TGA) was applied to measure the weight loss of the materials based on different thermal stabilities of each constituent in the nanocomposites. Fig. 9a–d exhibited the typical TGA curves for the bulk material and the wear debris (WD) of three types of composites at 0.5 wt.% and 3 wt.% fiber loadings, respectively. As we can see, there are no obvious differences between the bulk sample and its wear debris, revealing that the components of wear debris were similar with that of corresponding bulk material. For the samples with 0.5 wt.% CNF concentration, owing to the fact of relatively small amount of fibers, very similar thermal degradation procedures of all specimens were described in Fig. 9a and c. Nearly complete decomposition occurred between 400 1C and 500 1C, representing the degradation of HDPE matrix. At a higher fiber loading, we found the following differences in degradation temperature and weight loss among these samples, as shown in Fig. 9b and d. First, the degradation of bulk material and wear debris in Comp-l (3 wt.%) and Comp-h (3 wt.%) occurred sooner than Comp-p (3 wt.%) due to the relatively low degradation temperature of organic coatings. Our previous TGA results for the treated nanofibers have demonstrated that the organosilane coupling agent had a low degradation temperature, resulting in about 300 1C mass loss initiation [28]. Hence, the degradation of silane coated CNFs composite samples initiated at a slight low temperature. This phenomenon occurred in the wear debris as well, indicating the existence of silane coatings in Comp-l-WD and Comp-h-WD. Second, the specimens of Comp-h with 3 wt.% heavy silanized CNFs yielded approximately 85 wt.% loss, compared to nearly 100% for other samples. The siloxane group of the coatings with a glass-like structure has high thermal stability, and thus this part will not be burned off at such temperatures. In the specimens of Comp-h (3 wt.%), there might be some composite materials that wrapped and/or reacted with the siloxane groups in the coatings. Hence, the 15 wt.% mass residues were attributed to both silane coatings and materials trapped by the siloxane portion of the coating. Further characterizations on these unusual results are under way.
4. Conclusions The effects of various CNFs (treated and untreated) and their concentrations on the wear debris of HDPE/CNF composites were studied in this work. Morphological characterization and quantitative analysis for the wear debris of the nanocomposites were reported. The results revealed that less wear debris was generated during the wear process for the organosilane treated CNF composites with the thicker coating on the fiber surface, especially at 0.5 wt.% fiber loading, indicating it has the best wear resistance. The morphological studies for the wear debris showed smooth surfaces with less mechanical damage existing in the silanemodified CNF composites. For the quantitative analysis of the collected debris after 24 h wear testing, pure HDPE displayed the highest percentage in large particle size ranges. Among the specimens of nanocomposites, the proportions of small particles (less than 40 mm) in each type at 3 wt.% fiber concentration level were on average 20% higher than that of the samples with
0.5 wt.% nanofibers, and the percentage of large debris decreased dramatically. Compared with the other two types of nanocomposites, the wear debris of the heavily silanized CNF composites exhibited a lower percentage of small particles (length below 40 mm) and the lowest fraction of large particles (length above 300 mm). According to all the obtained results, we believe the silanized CNFs with the thicker coating are most promising for superior tribological performance in HDPE/CNF nanocomposites.
Acknowledgments The authors gratefully acknowledge the support from National Science Foundation (Civil, Mechanical and Manufacturing Innovation 0856510). The authors are also grateful to Ms. Jianying Ji for the helpful discussion and suggestions.
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Wear journal homepage: www.elsevier.com/locate/wear
Effect of reinforcement on wear debris of carbon nanofiber/high density polyethylene composites: Morphological study and quantitative analysis Tian Liu, Weston Wood, Bin Li, Brooks Lively, Wei-Hong Zhong n School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2012 Received in revised form 12 June 2012 Accepted 13 July 2012 Available online 23 July 2012
Polymeric nanocomposites are promising tribological materials. However, the wear debris generated on the sliding surface of composite materials is highly affected by the nanofillers used (types, surfaces, etc.). In this study, based on the high density polyethylene (HDPE) composites with various carbon nanofibers (CNFs) prepared, the effects of CNF concentration and surface modification on morphology and size distribution of wear debris were examined. The individual wear particle morphology was observed with a scanning electron microscope (SEM). The particle size distributions of the wear debris were statistically analyzed based on the optical microscope imaging for all the debris collected after 24 h of wear testing. In addition, the influence of various CNFs (untreated and organosilane treated) on debris components was analyzed by both differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) measurements. The results revealed that there were less mechanical damage and a lower proportion of large particles in the wear debris for the nanocomposites with heavily organosilane coated CNFs. Published by Elsevier B.V.
Keywords: Nanocomposite Wear debris Morphology Quantitative analysis
1. Introduction Polymers have attracted increasing attention in the field of tribology due to their general resistance to corrosion and low coefficients of friction [1]. Nowadays, polymer tribological applications cover a wide field, notably in machinery parts of automotive and aeronautical manufacturing, medical devices, artificial joint bearing surfaces, etc. Different polymers, such as polyethylene (PE) [2,3], polyamides (PA) [4], poly(ether ether)ketones (PEEK) [5], can be applied for various tribological systems in terms of their properties. Research on the tribological performance of these polymers mainly focuses on wear resistance, wear surfaces, and wear debris. The characterization of wear debris generated from sliding process is particularly important. The existence of debris with different morphologies and sizes may have a great impact on the wear surfaces between sliding pairs. In this case, the tribological behavior of a material could be affected dramatically by debris during the continued wear process. Therefore, it is quite essential to analyze the wear debris. Because of its outstanding tribological properties, high resistance to moisture permeation, excellent chemical resistance performance, and moldability, polyethylene (PE) is one of the most generally used polymers for tribology, particularly in biomedical and military
n
Corresponding author. Tel.: þ1 509 335 7658; fax: þ1 509 335 4662. E-mail address: [email protected] (W.-H. Zhong).
0043-1648/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.wear.2012.07.010
applications [6,7]. In spite of this, the quality of PE bearings would not satisfy the tribological requirements completely, owing to the increasingly high demand for improving the wear resistance of PE materials [8,9]. Making polymers into polymeric nanocomposites by adding appropriate nanofillers is a common technique to suit some particular tribological application. In the past few years, many studies on PE composites reinforced by various nanofillers have been carried out, for example with hydroxyapatite nanorods (nHA) [10], carbon nanotubes (CNTs) [11,12] and carbon nanofibers (CNFs) [13]. The reported work showed that the sliding wear resistance of PE nanocomposites was improved significantly compared with the pure polymers. Although there has been much work on the tribology of nanocomposites in general, very few studies on the characterization of wear debris of nanocomposites have been carried out [14]. Almost all the studies on the wear debris of polyethylene (PE) related materials produced during sliding process were limited to the analysis for pure PE [15–22]. The influence of the addition of nanofillers on morphology and particle size distribution of the PE nanocomposite wear debris has not been previously reported to the best of our knowledge. In the nanofiller/polymer multicomponent system, both the fillers themselves and the large interfacial areas between the matrix and the reinforcement can affect the tribological performance of the nanocomposites directly. This influence could be reflected by the wear debris of the composites via morphology, particle size and composition. Since the generated debris can significantly affect mechanical
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systems with the composites, the wear debris analysis is particularly important to such multi-component composite materials. The prerequisite for producing nanocomposites with superior performance includes the following two aspects: homogenous filler dispersion and optimal interaction/adhesion between fillers and polymer matrix. Numerous attempts for enhancing the load transfer from filler to matrix have been made [23–27]. Generally, the surface modification for nanofillers has been used. According to the different polarities of various polymer matrixes, different modification approaches can be developed. Hydrocarbon polymers, such as PE, as the matrix materials, which are non-polar polymers, functionalizing fillers via various hydrophobic moieties similar to the matrix is the most feasible method [23,24]. In our previous study [28], octadecyltrimethoxysilane (ODMS), an organosilane coupling agent with long non-polar hydrocarbon chains, was selected to treat the preoxidized carbon nanofibers (ox-CNFs) resulting in the preferable compatibility with high density polyethylene (HDPE). These hydrocarbon chains of the ODMS were coated onto the CNF surface to form a non-polar layer. The dynamic mechanical analysis and wear testing results indicated that CNFs treated with such silane coatings are effective nanofillers for enhancing mechanical and tribological properties of polyethylene, indicating improved interaction and adhesion between the nanofibers and the matrix [29,30]. In this paper, our main objective is to examine the influence of different nanofiller surface on the wear debris morphologies, particle size distributions and compositions of HDPE nanocomposites during the continued wear process. We prepared HDPE and its nanocomposites with different filler loadings (0.5 wt.% and 3 wt.%) for analysis. The results indicated that polymeric composite system with nanofiller reinforcements exhibited improved tribological behavior with respect to two aspects observed from debris analysis: less mechanical damage and a reduction amount of long PE fibrils. In order to reveal the influence of different fiber–matrix interaction on the wear debris of nanocomposites, pristine CNFs and our previously silane treated
Disk
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CNFs (thin and thick coatings) were selected as the reinforcements of HDPE [28]. Study results suggested that the debris size distribution and the morphology of the wear debris are related to the interactions between the nanofibers and the polymer matrix. The silane coatings can contribute to the enhanced interaction between the two phases, especially for the thicker coating. The experimental results revealed that heavily silanized CNF composites yielded smooth wear surfaces and a very small proportion of large wear debris.
2. Experimental procedures 2.1. Sample preparation High density polyethylene (HP54–60 Flake) was supplied by Bamberger Polymers Inc. with density of 0.954 g cm 3. The pristine CNFs (fiber type: PR-24-HHT) and pretreated oxidized CNFs (fiber type: PR-24-HHT-OX) were purchased from Applied Sciences Inc., which are approximately 60–150 nm in diameter and 30–100 mm in length. Octadecyltrimethoxysilane (ODMS) (90% technical grade) as organosilane coupling agent was purchased from Sigma-Aldrich. Ethanol was obtained from Decon Laboratories Inc. The preoxidized CNFs (oxCNFs) were modified under subsequent ODMS (90% technical grade) treatment in boiling ethanol solution. Then, the condensation reaction of organosilane coupling agent onto oxCNFs occurred due to the reactive hydroxyl groups on the surface of organosilane after hydrolysis, forming a silane coating to cover the fiber surface. The degree of hydrolysis and the thickness of silane-coating can be controlled by changing the ratio of oxCNFs, ODMS, ethanol, water and the addition of NaOH as catalyst in this reaction. The coating thicknesses were determined using TGA data, which are heavily silanized CNF-h (about 46 nm) and lightly silanized CNF-l (about 2.8 nm). Details of the treatment are provided in our previous study [28]. The 0.5 wt.% and 3 wt.% fiber contents for both pristine CNFs (Comp-p) and silanized CNFs (Comp-l with light coating; Comp-h with heavy coating) were mixed with HDPE by a Haake Torque Rheometer. Mixing was set at 180 1C with a rotator speed of 70 rpm for 15 min. A neat HDPE reference was also prepared under identical conditions. Thereafter, the processed HDPE and mixed polymeric nanocomposites were hot-pressed at 180 1C for 10 min via a hydraulic presser. Samples were allowed to cool down to room temperature naturally after turning off the heat. Each sample was cut for wear testing with the size of 10 mm 10 mm and similar average thickness around 2.5 mm. 2.2. Debris collection by wear testing
Rotation Disk
Normal Force
Sample
Sample Holder
Fig. 1. Schematic diagram of the components part of pin-on disk wear-testing apparatus.
The wear tests were performed in a custom build rig for the samples with a vertical 1020 carbon steel disk (HITACHI L200 Series, supplied by Hitachi Industrial Equipment system Co., Ltd), which was used in our pervious study [29]. The schematic diagram of the components part of the equipment was shown in Fig. 1. The effective radius of the disk was 65 mm, and testing was performed at 180 rpm. Also a normal force of 36 N was applied to the sample
Table 1 Detailed descriptions for each sample. Samples
Reinforcement
Fiber treatment
Coating average thickness (nm)
Fiber content
Debris collection
Pure HDPE Comp-p Comp-l Comp-h
– Pristine CNFs Silanized CNFs Silanized CNFs
– – Lightly silane treated Heavily silane treated
– – 2.8 46
– 0.5 wt.% & 3 wt.%
24 h-Wear Testing
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specimen with the wear surface of 10 mm 10 mm. All the tests were conducted at room temperature. Thereafter, all debris generated from the wear tests for each specimen were collected from the surface of the wear tester disk after 24 h. The detailed descriptions of each sample were shown in Table 1. In order to avoid the influence of previous tests, the disc was cleaned thoroughly with ethanol and dried, then polished by fine sand paper of the type Silicon Carbide 220b (220 grit) prior to next use.
2.3. Wear debris treatment for separation The debris particles tended to be gathered through electrostatic attraction produced by the wear process. To separate the aggregated wear debris, the Nine-step Debris Treatment (NDT) was applied. All collected wear debris of each sample was put into clean test tubes and mixed with NaOH solution. The mixtures were bath sonicated at room temperature for 10 h. Glycerol was added to each sample. Then, wear debris were centrifuged 8 h at 3000 rpm (Multi-RF Series Multipurpose Centrifuge OM 8464, Thermo Electron Co.). The top layer of each sample was siphoned and placed in new test tubes. After adding ethanol, debris was bath sonicated for 1 h, centrifuged for 1 h, and siphoned again. All specimens were washed several times with ethanol and deionized water. Finally, the treated samples
were extracted randomly (15 drops for each type of sample) and put on different microscope glass slides (one drop on each slide) for the particle length measurement and then quantitative analysis of individual wear debris. 2.4. Characterizations 2.4.1. Optical microscope The particle sizes of separated wear debris after NDT were determined by using an optical microscope (Olympus BX51TRF) equipped with a camera (Olympus U-CMAD 3, Portland, OR, USA). 10 Amount of Wear Debris (mg)
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8 6 4 2 0 pure HDPE
comp-p comp-p (0.5 wt%) (3 wt%)
comp-l comp-l (0.5 wt%) (3 wt%)
comp-h comp-h (0.5 wt%) (3 wt%)
Fig. 3. Amount of wear debris of pure HDPE and HDPE nanocomposites with different fiber loadings collected after 24 hwear testing (error bar: standard deviation).
Fig. 2. FESEM micrographs (scale bar: 20 mm; inset: 1 mm) of the fracture surfaces of HDPE nanocomposites reinforced by pristine CNFs (A) and (B), lightly silanized CNFs (C) and (D), heavily silanized CNFs (E) and (F). The fiber concentrations of all nanocomposites were 0.5 wt.% (left column) and 3 wt.% (right column).
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The quantitative analysis of wear debris size distribution was carried out through the length of each separated debris particle by optical image analysis. The experimentally determined particle number percentage provided the number proportion in each size range, and then two-dimensional particle size distributions of the wear debris were plotted.
2.4.2. FESEM (field emission scanning electron microscope) To investigate the interaction between nanofibers and polymer matrix, the microstructure features on the fracture interface of HDPE
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nanocomposites before wear testing were characterized with field emission scanning electron microscope (FESEM type Quanta 200 F). The overall fiber dispersion status in the nanocomposites was observed. FESEM images for fractured surfaces were prepared by freezing in liquid nitrogen for 10 min prior to fracturing. Additionally, the morphologies of separated wear debris of all specimens after NDT were also observed by FESEM. The images for microcosmic morphology features of the samples were prepared by desiccating in vacuum oven for 30 min (vacuum state). The surfaces of all samples were sputter coated with gold to improve electrical conductivity.
Fig. 4. FESEM micrographs examples (scale bar: 10 mm for A–C; 300 mm for D) of separated pure HDPE wear debris.
Fig. 5. FESEM micrographs (scale bar: 10 mm; inset: 50 mm) of examples for the separated wear debris of HDPE nanocomposites with different fiber concentrations: 0.5 wt.% for first row; 3 wt.% for second row. (A) and (D) Comp-p; (B) and (E) Comp-l; (C) and (F) Comp-h.
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2.4.3. DSC (differential scanning calorimetry) The melting behaviors of pure HDPE, HDPE nanocomposites and their wear debris without NDT under consideration were performed in a TA instrument TA DSC 822 (Mettler-Toledo, Inc.). The bulk samples and their wear debris were sealed under aluminum pans were scanned under a nitrogen atmosphere with a heating rate of 10 1C/min from 25 1C to 160 1C.
2.4.4. TGA (thermogravimetric analysis) A Thermogravimetric Analysizer (SDT Q600) was applied to determine the weight loss of the bulk samples and their wear debris for each nanocomposite. The wear debris was collected after 24 h wear testing. The temperature range for all TGA tests were from ambient temperature to 700 1C, with a heating rate of 10 1C/min in an N2 environment and flow rate of around 100 mL/min.
3. Results and discussion 3.1. Morphology and wear performance of nanocomposites The typical fracture surface morphology of all nanocomposites was observed through a field emission scanning electron microscope (FESEM). From Fig. 2A–F, the SEM images with different magnifications are shown as representatives for the dispersion and distribution status of the three types of CNFs in HDPE. No obvious larger agglomerates were found in all nanocomposites, suggesting that the nanofibers were well dispersed and uniformly distributed by means of the melt mixing process. From Fig. 2A and B, several long pristine CNFs were exposed on the surface of the PE matrix, and some of them were pulled out. Additionally, there were many gaps between the nanofibers and the polymer matrix, indicating the interaction and the adhesion between the two
Fig. 6. Examples of optical images (scale bar: 50 mm) for the separated wear debris of all types of samples.
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phases were not satisfactory (Fig. 2B). In comparison, large amounts of short CNFs were observed on the fracture surface of Comp-l (Fig. 2C and D) and Comp-h (Fig. 2E and F) even with the high fiber concentration of 3 wt.% (Fig. 2D and F). Moreover, very few fibers were pulled out and there were few voids between the two phases (Fig. 2D and F). These phenomena are more obvious in Comp-h (Fig. 2E and F), revealing that heavy silane coating may induce an increased interaction and load transfer between filler and matrix [28]. Therefore, these results demonstrate that the silane coating enhanced interfacial interaction and adhesion in the HDPE/CNF system. The debris amount was measured after 24 h of wear testing. It can be seen from Fig. 3 that all the nanocomposites exhibited lower wear debris amount than pure HDPE, indicating improved wear resistance of the nanocomposites. The improvement of wear resistance in nanocomposites may be attributed to load transfer from the polymer matrix to the nanofibers [31]. With higher loadings of CNFs (3 wt.%), the amounts of wear debris increased for all nanocomposites. Compared to the composites with 0.5 wt.% CNF loading, the larger interfacial area exists in the 3 wt.% CNF composites. The interface is very critical for the wear performance of the composites, as the wear debris can be formed at this region. Hence, the influence of shear stress caused by the wear process on the interfacial zone in the composites with higher fiber loading can be larger. This phenomenon in the Comp-h sample was not as obvious as that of the other two composites owing to the strengthened interfacial zone. The heavily crosslinked silane coatings covered the fiber surface, potentially reducing the interaction force between the fibers. Additionally, Comp-h displayed the least amount of wear debris,
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reflecting the best wear properties, especially for the 0.5 wt.% CNF composites. Hence, the obtained results revealed that the organosilane coatings, particularly with the thicker coating, enhanced wear resistance of HDPE/CNF system effectively.
3.2. Morphology of wear debris treated by Nine-step Debris Treatment To date, various descriptive studies on the morphology of treated (e.g. g radiation treatment) or non-treated wear debris for pure PE materials have been carried out [16,18–22,32]. These investigations mainly focused on wear debris features, including shape and size. The researchers observed many micro-wear features of debris, such as ripples, tufts, shreds, granules, nodules, fibrils, etc. In our study, some of these configurations were found. Fig. 4A–D displays the examples of the separated pure HDPE wear debris after Nine-step Debris Treatment (NDT). The granule, PE fibril and other irregular submicron size particles can be seen. Additionally, some corrugated structures were observed on the particle surface, as shown in Fig. 4B. Moreover, there existed some cracks, ripples and micro-voids on debris surface (Fig. 4A). We attributed these defects to the mechanical damage caused by the wear process. For thermoplastic materials, the mechanical damage is relevant to the mechanism of ductile deformation, mainly including the yielding under shear stress. In Fig. 5A–F, the differences in the wear debris morphologies among HDPE nanocomposites with various CNFs and different fiber concentrations can be observed. Compared with pure HDPE wear debris, as shown in Fig. 4, there was less damage found in
Fig. 7. Particle size distributions (particle lengths were below 1200 mm) for the wear debris of pure HDPE and HDPE nanocomposites specimens with 0.5 wt.% concentration of fibers. (A) pure HDPE; (B) Comp-p; (C) Comp-l; (D) Comp-h. Two inset bar charts in each figure were the corresponding detailed particle size distributions for small debris with the sizes below 300 mm and large debris with the size range of 300–1200 mm, respectively.
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the debris of composite materials, indicating the improved wear resistance in the nanocomposites resulting from the fibers. The addition of nanofibers could have effectively acted as reinforcement in the matrix, exhibiting enhanced tribological characteristics of the composite system during the wear process. This was more obviously seen in the two types of silanized CNF composites (Fig. 5B, C, E and F), especially for the samples of Comp-h reinforced by heavily silanized CNFs (Fig. 5C and F). The wear debris surfaces of Comp-h were quite smooth. For Comp-l, there were several smaller wear debris on the bigger portion of debris with a smooth surface (Fig. 5E). In comparison, some surface damage still can be seen in the wear debris of Comp-p, such as cracks and granules (Fig. 5A and D). The distinction of the
morphology suggests that the sufficient silane coating on the fiber surface could induce an increased strengthening in the interfacial region between HDPE and CNFs, which was attributed to the enhanced interaction and adhesion. Therefore, the strengthened composite system of Comp-h can be also reflected by the morphology of debris generated from the wear test. 3.3. Quantitative analysis of the wear debris The individual wear debris treated by Nine-step Debris Treatment (NDT) was observed from optical images. Some typical examples of the obtained optical images for the debris observation were shown in Fig. 6. Because the morphologies of most separated debris were nearly disk-shaped, the maximum length of each particle was selected to represent the size of wear debris. The measured length of every debris particle was recorded for the further quantitative particle size distribution analysis. The distribution of the debris lengths was found to be in a broad range from several nanometers to 1200 mm for pure HDPE and its nanocomposites with 0.5 wt.% fiber content (Fig. 7A–D). The proportions of the particles with the lengths below 40 mm were 10% less for all composites’ debris (Fig. 7B–D) compared to pure HDPE wear debris (Fig. 7A). Additionally, particles with the length range from 750 mm to 1200 mm were present in the pure HDPE debris specimen. The proportions of the large debris particles with such size range were much lower in the three types of nanocomposites than of that in pure HDPE. These results can be explained by the existence of fiber reinforcement in the polymer matrix, which played the role of carrying load during sliding wear process. Also, pure HDPE is able to be drawn out during wear, whereas CNFs limit the ability for the chains to be drawn out in the direction of shear [33,34]. The detailed states of small (particle lengths below 300 mm) and large (particle length range: 300–1200 mm) debris particle size distributions for each specimen are more clearly seen in the inset charts of Fig.7. For the particle size below 30 mm, similar length distributions were observed for pure HDPE, Comp-p (0.5 wt.%) and Comp-l (0.5 wt.%). For the Comp-h (0.5 wt.%), there was a higher proportion of wear debris below 10 mm. For the large debris inset, particles with the length above 1050 mm were attributed to the existence of long PE fibrils. Among the debris of three nanocomposites, there were no such large particles. In particular, the size ranges of large debris particles in Comp-h (0.5 wt.%) were found in 400–600 mm range and the proportion was fairly small (around 0.2%). Thus, the wear debris of Comp-h (0.5 wt.%) displayed not only the high percentage of very small debris particles, but also the few large ones. The percentage distributions of debris lengths for HDPE nanocomposites with 3 wt.% fiber content were displayed in Table 2 Melting temperatures (Tm) of the nanocomposites and their wear debris. Sample list
Fig. 8. Particle size distributions (particle lengths were below 1200 mm) for the wear debris of HDPE nanocomposites specimens with 3 wt.% concentration of fiber. (A) Comp-p; (B) Comp-l; (C) Comp-h. Two inset bar charts in each figure were the corresponding detailed particle size distributions for small debris with the sizes below 300 mm and large debris with the size range of 300–1200 mm, respectively.
Pure HDPE Comp-p 0.5 wt.% 3 wt.% Comp-l 0.5 wt.% 3 wt.% Comp-h 0.5 wt.% 3 wt.%
Melting temperature Tm (1C) Bulk sample
Wear debris
134.5 71.2
135.1 7 1.3
135.2 71.6 135.7 71.5
133.2 7 1.0 133.8 7 1.7
135.0 71.1 137.3 72.0
134.8 7 0.8 131.8 7 1.9
134.7 71.3 136.9 71.2
134.5 7 1.2 132.3 7 1.6
p, Pristine CNFs; l, lightly silanized CNFs; h, heavily silanized CNFs.
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Fig. 8A–C. It is clear that there were only very little differences among them. The proportions of particles below 40 mm in wear debris of 3 wt.% composites were on average 20% higher than that of 0.5 wt.% CNF composites. Furthermore, the percentages of large wear debris (length above 300 mm) decreased dramatically. The detailed states of small (particle lengths below 300 mm) and large (particle length range: 300–1200 mm) debris particle size distributions for the 3 wt.% CNF composites can be seen clearly in the inset charts of Fig. 8. According to the results of small particle size distribution, most of debris lengths were below 30 mm, which was similar with that of the composite materials at 0.5 wt.% loading level (Fig. 7). For two types of silanized CNF composites (Fig. 8B and C), a higher frequency of small particles with the lengths below 10 mm were seen. From the other inset chart in Fig. 8 (large debris size distribution), it should be noted that the reductions in large particle proportions and lengths existed in the wear debris of composites with 3 wt.% fiber content compared with that of 0.5 wt.% CNF composites. However, it was not very obvious in the sample of Comp-h. In this case, we may conclude that the sufficient silane coating onto fiber surface could limit the production of large wear debris of the nanocomposites [28]. Combined with the wear testing results (Fig. 3), we can obtain such conclusions: (1) the debris particle sizes were reduced for the nanocomposites with 3 wt.% CNFs (higher percentage of small debris particles, as shown in Fig. 8). During the wear process, the polymer chains can be drawn out to form the long PE fibrils (Fig. 4D) owing to the plastic deformation [31,33,34]. The addition
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of CNFs can contribute to the limitation of this plastic deformation of PE matrix, thus there were no long fibrils found in the wear debris of composite systems. (2) The wear debris amounts for the nanocomposites with 3 wt.% CNFs were increased (as shown in Fig. 3). It is due to the larger interfacial region between filler and matrix in the composites at a higher fiber loading. The interface is very critical for the wear performance of nanocomposites, as the wear debris can be formed at this region. This phenomenon in the samples of Comp-p and Comp-l was obvious owing to their weak interfacial zones. In comparison, Comp-h with 3 wt.% heavily silanized CNFs showed less amount of increased wear debris, which can be attributed to the strengthened interface. Therefore, the limited deformation of PE and the existence of larger interfacial region were the main contributions of the decreased particle size and increased amount in the wear debris of nanocomposites with a high fiber loading.
3.4. Differential scanning calorimetry The melting behaviors of pure HDPE and its nanocomposite with 0.5 wt.% and 3 wt.% fiber loadings as well as their wear debris were studied by differential scanning calorimetry (DSC). Table 2 presented the melting temperatures (Tm) of each sample during the heating cycle, showing similar Tm values between pure HDPE and all composites before wearing. This result is coincident with our previous studies [28]. Additionally, it is hard to see
Fig. 9. TGA thermograms for the bulk materials (a and b) and the wear debris (c and d) of Comp-p, Comp-l and Comp-h with the fiber concentrations of (a) and (c) 0.5 wt.%; (b) and (d) 3 wt.%.
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obvious changes in Tm between the nanocomposites and their wear debris (slight reductions of melting temperatures of wear debris for the composites with 3 wt.% fiber content), indicating the similar influences of varying CNFs (untreated and organosilane treated) on debris components for the nanocomposites. 3.5. Thermogravimetric analysis In order to perform the compositional analysis of HDPE/CNFs multi-component systems, thermogravimetric analysis (TGA) was applied to measure the weight loss of the materials based on different thermal stabilities of each constituent in the nanocomposites. Fig. 9a–d exhibited the typical TGA curves for the bulk material and the wear debris (WD) of three types of composites at 0.5 wt.% and 3 wt.% fiber loadings, respectively. As we can see, there are no obvious differences between the bulk sample and its wear debris, revealing that the components of wear debris were similar with that of corresponding bulk material. For the samples with 0.5 wt.% CNF concentration, owing to the fact of relatively small amount of fibers, very similar thermal degradation procedures of all specimens were described in Fig. 9a and c. Nearly complete decomposition occurred between 400 1C and 500 1C, representing the degradation of HDPE matrix. At a higher fiber loading, we found the following differences in degradation temperature and weight loss among these samples, as shown in Fig. 9b and d. First, the degradation of bulk material and wear debris in Comp-l (3 wt.%) and Comp-h (3 wt.%) occurred sooner than Comp-p (3 wt.%) due to the relatively low degradation temperature of organic coatings. Our previous TGA results for the treated nanofibers have demonstrated that the organosilane coupling agent had a low degradation temperature, resulting in about 300 1C mass loss initiation [28]. Hence, the degradation of silane coated CNFs composite samples initiated at a slight low temperature. This phenomenon occurred in the wear debris as well, indicating the existence of silane coatings in Comp-l-WD and Comp-h-WD. Second, the specimens of Comp-h with 3 wt.% heavy silanized CNFs yielded approximately 85 wt.% loss, compared to nearly 100% for other samples. The siloxane group of the coatings with a glass-like structure has high thermal stability, and thus this part will not be burned off at such temperatures. In the specimens of Comp-h (3 wt.%), there might be some composite materials that wrapped and/or reacted with the siloxane groups in the coatings. Hence, the 15 wt.% mass residues were attributed to both silane coatings and materials trapped by the siloxane portion of the coating. Further characterizations on these unusual results are under way.
4. Conclusions The effects of various CNFs (treated and untreated) and their concentrations on the wear debris of HDPE/CNF composites were studied in this work. Morphological characterization and quantitative analysis for the wear debris of the nanocomposites were reported. The results revealed that less wear debris was generated during the wear process for the organosilane treated CNF composites with the thicker coating on the fiber surface, especially at 0.5 wt.% fiber loading, indicating it has the best wear resistance. The morphological studies for the wear debris showed smooth surfaces with less mechanical damage existing in the silanemodified CNF composites. For the quantitative analysis of the collected debris after 24 h wear testing, pure HDPE displayed the highest percentage in large particle size ranges. Among the specimens of nanocomposites, the proportions of small particles (less than 40 mm) in each type at 3 wt.% fiber concentration level were on average 20% higher than that of the samples with
0.5 wt.% nanofibers, and the percentage of large debris decreased dramatically. Compared with the other two types of nanocomposites, the wear debris of the heavily silanized CNF composites exhibited a lower percentage of small particles (length below 40 mm) and the lowest fraction of large particles (length above 300 mm). According to all the obtained results, we believe the silanized CNFs with the thicker coating are most promising for superior tribological performance in HDPE/CNF nanocomposites.
Acknowledgments The authors gratefully acknowledge the support from National Science Foundation (Civil, Mechanical and Manufacturing Innovation 0856510). The authors are also grateful to Ms. Jianying Ji for the helpful discussion and suggestions.
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