- Email: [email protected]
Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures
+Model
ARTICLE IN PRESS
JECS-9554; No. of Pages 9
Available online at www.sciencedirect.com
ScienceDirect Journal of the European Ceramic Society xxx (2014) xxx–xxx
Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures Ján Balko a , Pavol Hvizdoˇs a,∗ , Ján Dusza a , Csaba Balázsi b , Jana Gamcová c a
b
Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 04353 Koˇsice, Slovakia Bay Zoltán Non-profit Ltd. for Applied Research, Institute for Materials Science and Technology, Fehérvári út 130, H-1116 Budapest, Hungary c Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, D-22603 Hamburg, Germany
Abstract Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt.% of multilayered graphene (MLG) platelets were studied and compared to monolithic Si3 N4 . The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at temperatures 25 ◦ C, 300 ◦ C, 500 ◦ C, and 700 ◦ C in dry sliding. Addition of such amounts of MLG did not lower the coefficient of friction. Graphene platelets were integrated into the matrix very strongly and they did not participate in lubricating processes. The best performance at room temperature offers material with 3 wt.% graphene, which has the highest wear resistance. At medium temperatures (300 ◦ C and 500 ◦ C) coefficient of friction of monolithic Si3 N4 and composite with 1%MLG reduced due to oxidation. Wear resistance at high temperatures significantly decreased, at 700 ◦ C differences between the experimental materials disappeared and severe wear regime dominated in all cases. © 2014 Elsevier Ltd. All rights reserved. Keywords: Si3 N4 ; Graphene; Friction; Wear
1. Introduction Shortly after their discovery carbon nanomaterials such as nanofibers, nanotubes1 and graphene2,3 were brought to the attention of material scientists who sought to utilize their unique properties for structural applications1–4 as they hold great promise as reinforcing nanofillers. In previous years silicon nitride based composites reinforced by carbon nanophases have been introduced and studied not only with respect to their improved mechanical and functional properties but also for their tribological performance.5–9 After extensive studies of composites with carbon nanofibers and carbon nanotubes, graphene, particularly in the form of multilayered platelets, came into focus as a reinforcing additive, thanks to its lower price, easier preparation and handling.10 Its potential for enhancing fracture resistance has been recognized and demonstrated.11–16 Walker et al.17 reported an improvement in fracture toughness
∗
Corresponding author. Tel.: +421 55 792 2464; fax: +421 55 792 2408. E-mail address: [email protected] (P. Hvizdoˇs).
in graphene containing ceramic nanocomposites where they identified novel toughening micromechanisms. Ramírez et al.18,19 explored the enhancement of electrical properties of graphene/silicon nitride composites. Following this, the potential of silicon nitride/graphene composites for the improvement of wear behavior has just started to be explored. It was found that coefficient of friction (COF) was reduced only under certain higher loads (≥200 N),6 wear resistance however was quite successfully improved.5–7 It has been observed that in Si3 N4 addition of MLG lead to better coefficient of friction than addition of CNTs and to much lower wear rates.7 Because of typical applications of structural ceramics as materials with exceptional thermal stability, the performance of the newly developed composites at elevated temperatures is also of a great importance. To our knowledge there is a lack of literature on this topic until now. The aim of the present work, as an extension of our previous studies,7 was to investigate the influence of the addition of multilayered graphene nanoplatelets on the tribological properties of Si3 N4 based composites (COF and wear) in the temperature range from room temperature (RT) up to 700 ◦ C, to observe
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model
ARTICLE IN PRESS
JECS-9554; No. of Pages 9 2
J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Table 1 Notation, composition and properties of experimental materials. Material
C, wt.%
Porosity, %
Average pore size, m
Roughness Ra, m
HV5, GPa
KIC , MPa m1/2
SN SN-1G SN-3G
0 1 3
<0.5 5.5 20.1
– 4.1 13.2
0.05 ± 0.01 0.07 ± 0.01 0.15 ± 0.02
15.4 ± 0.4 14.6 ± 0.2 7.7 ± 0.1
6.9 ± 0.4 7.8 ± 0.4 -
and identify wear mechanisms and their changes with changing temperature. 2. Materials and methods The experiments were carried out on three different materials. The first one was a reference silicon nitride based dense ceramic made of Si3 N4 (Ube, SN-ESP) with 6% Y2 O3 (H.C. Starck, grade C) and 4% Al2 O3 (Alcoa, A16) as sintering additives. The powder mixture was milled in attritor mill in distilled water using a zirconia tank, zirconia media (balls with 1 mm diameter) and zirconia agitator delta disks for 5 h, 4000 rpm. As a consequence, each material contained also approximately 3.6 wt.% zirconia from the milling media.10,12 The reference material is henceforth denoted as SN. For preparation of the graphene containing composites commercially available graphene platelets were admixed as reinforcement materials. Based on previous studies7 of various types of MLG, the most promising type was selected for conducting further high temperature experiments. They were exfoliated graphene nanoplatelets (xGnP-M-5 – particle size 5 m)20 produced by XG Sciences. In the case of graphene added powder mixtures, after finishing the first milling step, which was identical to the reference Si3 N4 , 1 or 3 wt.% of graphene nanoplatelets were admixed into the silicon nitride-based powder mixture in the attrition mill with low rotational speed, 600 rpm for 30 min. The mixtures then were dried and sieved with a filter with mesh size of 150 m. Green samples were obtained by dry pressing at 220 MPa. Samples prepared for HIP were oxidized at 400 ◦ C to eliminate the PEG. Hot isostatic pressing (HIP) was performed at 1700 ◦ C in high purity nitrogen by a two-step sinter-HIP method using BN embedding powder at 20 MPa, with 3 h holding time. The heating rate did not exceed 25 ◦ C/min. The microstructure and basic mechanical properties of the experimental materials were described and characterized in previous studies.7,11,12,21 The initial wear testing at room temperature was performed, and results were reported in Ref. 7. Based on the obtained results the most promising materials were selected for the present study which focuses on high temperature wear testing. The sample notation, carbon content and basic microstructural and mechanical characteristics are summarized in Table 1. The wear behavior of the experimental materials was studied in dry sliding in air. The surfaces were carefully prepared by polishing down to a surface roughness Ra below 0.05 m for the reference material. For the composites, particularly for that with higher volume fraction (3 wt.%) of carbon additives, this was not entirely possible because of residual porosity. The roughness
in terms of Ra was measured by optical interferometry and it is given in Table 1. The wear testing was carried out on the tribometer HTT by CSM Instruments in dry conditions using the pin-on-disk technique, where the tribological partner was a highly polished (roughness Ra < 0.20 m) Si3 N4 ball with 6 mm diameter. The applied load was 5 N, the sliding speed 0.1 m/s and the sliding distance was 300 m. The testing temperatures were 25 ◦ C, 300 ◦ C, 500 ◦ C, and 700 ◦ C. The tangential forces during the test were measured and friction coefficients calculated. The worn surfaces were subsequently observed and the wear regimes, damage type and micromechanisms were identified. The material losses (volume of the wear tracks) due to wear were measured by a high precision confocal microscope PLu neox 3D Optical Profiler, by SENSOFAR, and then specific wear rates (W) were calculated in terms of the volume loss (V) per distance (L) and applied load (Fp ) according to the standard ISO 2080822 : W (mm3 /m N) =
V L · Fp
At the same time, the worn cap on the counter body was measured by a light microscope, its volume and then its wear rate calculated, too. The microstructure and the wear tracks created at the surfaces of the investigated materials were observed using scanning electron microscopy (JEOL JSM 7000). Porosity and average pore size was found using image analysis of micrographs of polished ceramographic sections. At least 1 mm2 of surface was documented and then treated using ImageJ 1.47e software. Raman study of the tested specimens was carried out using a Raman spectroscope XploRA by HORIBA Jobin Yvon. Green laser with wavelength λ = 532 nm was used and Raman shift from 100 to 3000 cm−1 was observed. The phase composition of the wear debris was studied using synchrotron X-ray diffraction. XRD measurements were realized at the high X-ray diffraction endstation P02.1 situated at the storage ring PETRA III at DESY at Hamburg. The P02.1 operates with photons of fixed energy ∼ 60 keV ˚ selected by Laue monochromator crystals of (λ = 0.20727 A) diamond (1 1 1) and silicon (1 1 1) from synchrotron radiation produced at the high energy undulator. Collection of X-ray diffraction patterns was performed in transmission mode with a two-dimensional Perkin Elmer 1621 detector (2048 × 2048, the pixel size 200 m × 200 m). The reading time of the used detector was about 1 ms. The illumination time of the sample was 180 s with a photon beam of size 0.5 mm × 0.5 mm. The sample–detector distance was 1024 mm. Two dimensional XRD patterns were integrated to the 2-theta space using software package FIT2D.23 The phase analysis was done via PCW software.24
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
3. Results
3
1.0 0.9 0.8 0.7
COF [-]
0.6 0.5 0.4 0.3
SN SN-1G SN-3G
0.2 0.1 0.0 0
100
200
300
400
500
600
700
Temperature [°C] Fig. 1. Average values of coefficient of friction for the experimental materials at all experimental temperatures.
SN SN-1G SN-3G
Wear rate [mm /N.m]
1E-3
3
1E-4
1E-5
1E-6
1E-7 0
100
200
300
400
500
600
700
800
Temperature [°C]
Fig. 2. Specific wear rates of the experimental materials vs temperature.
1E-3
SN SN-1G SN-3G 1E-4
3
Ball wear rate [mm /N.m]
The microstructures of the experimental materials were studied in detail previously in Refs. 21, 25 and the relevant results were concisely summarized in Ref. 7. It was found that in all cases good mixing and homogeneous distribution of graphene additives was achieved21 and the graphene platelets were well embedded within the matrix. However, the graphene platelets induced porosity in the matrix and reduced the size of the Si3 N4 grains in the resulting composites. Dusza et al.25 performed a statistical analysis of the grain sizes and found that while in the monolithic Si3 N4 the diameter of the matrix grains was typically 0.4 m, the average matrix grains in the composites had narrow size distribution with maximum at approximately 0.2 m. The results of X-ray investigations showed only -Si3 N4 diffraction peaks, suggesting that the ␣ →  transformation was complete.21,25 The monolithic silicon nitride was fully dense. Carbon filler phases made densification of the composites more difficult, and the pores were always associated with graphene platelets. That lead to porosity increasing with increasing volume fraction of carbon phases, Table 1. They do not tend to form extensive clustering as it is typical for CNTs,8 but occasional stacks of several graphene platelets were present.7,25 Morphology studies of the graphene platelets25 showed that the MLG had a size distribution corresponding to the values given by the producer,20 i.e. 5 ± 1 m. The pores present in the composites were always associated with graphene platelets. Values of porosity and average pore size found by image analysis of ceramographic sections are also shown in Table 1. The tests showed that the friction behavior of all materials under the testing conditions was stable during the tests. In all cases the friction force stabilized over a short run-up phase, usually shorter than 1 m. Then, depending on material, the friction remained more or less stable with small oscillations but in no case did it exhibit any significant change that would suggest some important change in character of wear damage mechanisms over the 300 m sliding distance. Figs. 1–3 summarize results of the tribological testing, namely coefficient of friction (COF), wear rates (W) of the experimental materials and counterbodies, respectively, as functions of testing temperature. Fig. 1 shows the comparison of average COF for the experimental materials at all experimental temperatures. It can be seen that at room temperature the COF of the composites was similar, or slightly higher than that of the reference SN. At medium temperatures, COF values reduced for SN and SN-1G and at 700 ◦ C they were high again. The COF of SN-3G was 0.7 at lower temperatures (RT, 300 ◦ C), at higher ones (500 ◦ C, 700 ◦ C) it increased to approximately 0.8. The wear resistance results are summarized in Fig. 2. At room temperature the MLG had beneficial effect. In both types of composites a significant improvement of wear resistance was recorded. The wear rate in SN-3G was about 60% lower than that for the reference Si3 N4 which was discussed in greater detail in Ref. 7.
1E-5
1E-6 0
100
200
300
400
500
600
700
Temperature [°C]
Fig. 3. Specific wear rates of counterbodies (silicon nitride balls).
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9 4
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
At 300 ◦ C the wear rates of the materials with low carbon content (SN, SN-1G) decreased by about an order of magnitude. At 500 ◦ C the wear rate of SN improved a little whereas SN-1G showed a reduction of wear resistance. The wear rates of SN-3G had a clear tendency to increase with increasing temperature. Wear rates at 700 ◦ C were similar for all three materials and were one to two orders of magnitudes higher than those at room temperature. Fig. 3 shows the wear rate of the counterbodies (silicon nitride balls). It completes the information about the wear damage of the tribological pairs. It shows a general tendency of the wear rates of the counterbodies to increase with increasing amount of MLG in the paired material. They are generally stable up to 300 ◦ C, then at 500 ◦ C for the pairs with highly reduced COF (SN, SN-1G) they decrease. At 700 ◦ C all increase dramatically along with increase of wear rates of the samples. 4. Discussion 4.1. Wear mechanisms The wear tracks of the monolithic SN, SN-1G, and SN-3G samples worn at the experimental temperatures are shown in Figs. 4–6, respectively. The COF values obtained at room temperature were reported also in Ref. 7. The presence of MLG in composites did not lower the COF, which is similar to the results of the study of Si3 N4 -CNT composites.8 This suggests that the graphene platelets do not have a lubricating effect. It is in agreement with findings of Belmonte et al.6 for low loads, up to 100 N. They argue that higher friction for the composites is connected to the nanoplatelets producing continuous surface changes between graphene and silicon nitride grains which leads to always higher roughness for the composites than for the monolithic material. This is consistent with our observation where the COF is connected to higher roughness, which increases with the carbon content (Table 1) – see also Figs. 4a,b, 5a,b and 6a. Only at much higher loads, 200 N according to Ref. 6, the composite shows reduced friction and graphene nanoplatelets do lubricate the tribosystem. Such loads, however, were not reached in our tests. At higher temperatures (300 and 500 ◦ C) the reference SN had a significantly lower COF, dropping from 0.6 down to about 0.2. This behavior is consistent with findings reported in the literature,26,27 where at low loads selective oxidation was reported to take place between 400 and 700 ◦ C. In the present study an oxide rich layer on the surfaces of the wear tracks was identified (Figs. 4c–f and 5c,d). The inset in Fig. 4f shows a typical EDX spectrum of this layer. At 700 ◦ C the integrity of this layer was lower and surface microcracking and break-up accompanied by much higher production of debris became the dominant mechanism which consequently lead to a significant increase of wear rate in SN (Fig. 2). In SN-1G a higher temperature (500 ◦ C) was necessary to activate oxidation of the degree similar to that of the monolith material. Then the COF dropped to the same values as for SN
(Fig. 1). The tribochemical reaction produced typical thin and long cylindrical oxide particles (Fig. 5c and d), sometimes they were embedded back into the tribofilm. Dong and Jahanmir26 reported formation of needle shaped oxides and explained it as oxidation of silicon nitride, with a possible contribution from  to ␣-Si3 N4 phase transformation. Here, knowledge of bulk and flash temperatures could be of interest. Their calculation is subject to many simplifying assumptions about the details of the contact between the surfaces, and a large number of models exist which differ substantially in their quantitative predictions of temperatures. The simple model of Ashby et al.28 uses energy input/output balance to estimate the maximum temperature difference that can be sustained by a sample of given dimensions. Using thermal conductivity of silicon nitride as 15–30 W/mK this T under present conditions would be only between 20 and 40 K. However, the localized flash temperatures in points of contact could be much higher. Although the real contact area can be estimated only with difficulty, according to Gee and Butterfield29 they can be high enough at any speed above about 0.1 m/s. It is also significant that a layer of oxide on the surface of the silicon nitride increases the flash temperature by a factor of about 2, so that if increasing the temperature of a reaction leads to higher rates of oxidation, then any incipient oxide film formation is likely to increase the temperatures even more. Therefore, in order to decide whether such transformation takes place independent experiments are necessary, as it is shown later in Section 4.3. The composite with 3 wt.% of MLG did not exhibit a tendency to lower the COF at 300 ◦ C and/or 500 ◦ C, it stayed at high values. Above 500 ◦ C it even increased (Fig. 1). At RT the wear track surface showed differences from the other two materials. It was much rougher, and even though it had lower wear rate it exhibited more plastic deformation (Fig. 6a). At 300 ◦ C most of the wear track suffered not only local plastic deformation but also surface microcracking (Fig. 6b). At even higher temperatures extensive surface cracking took place. Its high intensity was probably connected to much higher surface roughness. The asperities in the contact zone were constantly being created and broken up, producing a lot of debris in the process. The MLG inside the wear tracks usually remain in their sites even at 500 ◦ C (Fig. 6c and d) so at these conditions the fact that they had lead to porosity was more important than their role as a reinforcing filler. At 700 ◦ C then all materials entered different wear regime and all had a coefficient of friction higher than at room temperature, in the case of SN-3G it could be up to 0.9. The wear rates of all three materials were much higher than 10−5 mm3 /Nm, which indicates that here clearly severe wear prevailed.30,31 The damage had similar character since the carbon phases had decomposed and all three materials were compositionally identical. In all cases the dominant damage mechanism was the surface cracking, -silicon nitride grain breaking and debris production (Figs. 4g,h, 5e and 6e,f). 4.2. Stability of MLG For a better understanding of MLG and its changes during wear testing a Raman study of the tested specimens was
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
5
Fig. 4. Wear tracks of SN. Smooth wear surface at room temperature (a and b), tribofilm and selective oxidation at 300 ◦ C (c and d) and 500 ◦ C (e and f) with EDX spectrum of the surface layer. Surface break-up and debris production at 700 ◦ C (g and h).
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9 6
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Fig. 5. Wear of SN-1G, RT (a and b), 300 ◦ C (c), 500 ◦ C (d), 700 ◦ C (e). Effect of selective oxidation at medium temperatures and formation of needle shaped oxides, particularly at 500 ◦ C.
performed. The results are illustrated by an example in Fig. 7 where measurements on the material SN-3G worn at 500 ◦ C are shown. The spectra were collected from a typical MLG (site 1), and from polished and worn surfaces (sites 2 and 3). The topmost spectrum comes from a basically undisturbed MLG located inside a large pore (similar to that pictured in Fig. 6d). The two very prominent peaks are called G and G and are typical for graphite or graphene. Spectra of these two
materials are, of course, very similar, since graphite is just stacked graphene. The difference is in relative intensity of the two peaks, where G is much more intense than the G in graphite compared to graphene.32 This means that in our case the large MLG is really a flake of graphite containing a number of layers. There is also a prominent band around 1350 cm−1 . It is small for the spectrum from the site 1 but very intense for the other two. This band is known as the D band and it is typically present in
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
7
Fig. 6. Wear of SN-3G at various temperatures –RT (a), 300 ◦ C (b), 500 ◦ C (c and d), and 700 ◦ C (e and f).
signal by nanotubes. The D band originates from a hybridized vibrational mode associated with graphene edges and it indicates the presence of some disorder to the graphene structure. This band is often referred to as the disorder band or the defect band and its intensity relative to that of the G band is often used as a measure of the quality of nanotubes.32 In the present case it indicates many edges and defects in MLGs at the polished and worn
surfaces, due to polishing and wear, respectively. High G with respect to G means that these processes also disintegrate MLGs so that they have fewer layers and become more graphene-like. What we can see, however, is that the two spectra are basically identical, which means that the process of polishing and that of high temperature wear in areas of abrasion, without much microcracking, had the same effect on the surface of the specimen.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model
ARTICLE IN PRESS
JECS-9554; No. of Pages 9 8
J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Fig. 7. Raman study of MLGs, SN-3G after wear testing at 500 ◦ C, site 1 – MLG inside a pore; site 2 – polished surface; site 3 – worn surface.
4.3. Stability of β-silicon nitride In order to identify possible -to-␣ Si3 N4 transformation during the high temperature wear testing, as suggested by Dong and Jahanmir,26 analyses of phases in the wear track and debris were carried out. For the wear track analysis Raman spectroscopy was used as in Section 4.2. In Fig. 7 it is shown that the signal from the polished surface (site 2) and the deformed worn surface (site 3) are basically identical. In both locations there are all main peaks typical for -silicon nitride (183, 205, 226, 452, 620, 733, 866, 930, and 1048 cm−1 )33,34 clearly present. On the other hand, no signal that would correspond to ␣-phase is visible (259, 365, 515, 668, 868, 915, 976, and 1034 cm−1 ).35 This result was the same for all specimens. Beside the worn surface analysis, the debris particles were collected, too. Because of their small size and very low quantities their phase composition was studied using synchrotron X-ray diffraction. The results found for materials tested at 700 ◦ C (the highest quantities of debris collected, i.e. the best spectra available) are given in Fig. 8, together with positions of expected peaks for ␣ 500
-Si3N4 -Si3N4
Intensity [a.u.]
400
300
200
SN-3G
SN-1G
100
SN
0 3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
2-theta [deg.]
Fig. 8. X-ray diffractograms of the debris particles produced at 700 ◦ C. No ␣, only -Si3 N4 was identified.
and  silicon nitride phases. Again, only -silicon nitride was identified, in no case any ␣-Si3 N4 peaks were found.
5. Conclusions A series of silicon nitride based composites containing 1 and 3 wt.% of multilayered graphene platelets were prepared and subjected to wear testing at temperatures 25 ◦ C, 300 ◦ C, 500 ◦ C, and 700 ◦ C. The results allow the following conclusions: • MLG makes densification more difficult which leads to higher porosity. • At room temperature the monolithic Si3 N4 had a high coefficient of friction, and its wear tracks had a smooth self polished look. The composites behaved similarly, with increasing amount of MLG the wear resistance increased. On the other hand, the composites suffered more surface microcracking and debris production. • At 300 ◦ C and 500 ◦ C monolithic Si3 N4 had reduced COF due to formation of a tribofilm, where a tribochemical reaction created oxide particles, which reduced the wear rate. • 1% MLG containing composite had damage mechanism similar to that of the monolith. Wear rate improved at 300 ◦ C. Reduction of COF was found at 500 ◦ C. • 3% MLG containing composite had high COF at all testing temperatures and its wear rate increased with increasing temperature. Surface break-up due to high porosity and surface roughness was typical. • Wear resistance at the highest temperature (700 ◦ C) significantly decreased and a severe wear regime (W > 10−5 ) prevailed in all materials. • -Silicon nitride was stable during wear testing and did not transform to create ␣-Si3 N4 phase. • Generally, MLGs did not participate in a lubricating process. Significant graphite transfer film was not observed. Improved fracture toughness by graphene lead to better wear resistance at lower testing temperatures up to 300 ◦ C.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Acknowledgement This work was supported by the Slovak Grant Agency, projects VEGA 2/0075/13 and APVV-0108-12. References 1. Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. 2. Novoselov KS, Falko VI, Colombo L, Gellert PR, Schwab MG, Kim K. A roadmap for graphene. Nature 2012;490:192–200. 3. Lee SH, Lee DH, Lee WJ, Kim SO. Tailored assembly of carbon nanotubes and graphene. Adv Funct Mater 2011;21:1338–54. 4. Peigney A, Laurent Ch, Flahaut E, Rousset A. Carbon nanotubes in novel ceramic matrix nanocomposites. Ceram Int 2000;26:677–83. 5. González-Julián J, Schneider J, Miranzo P, Osendi MI, Belmonte M. Enhanced tribological performance of silicon nitride-based materials by adding carbon nanotubes. J Am Ceram Soc 2011;94:2542–8. 6. Belmonte M, Ramírez C, González-Julián J, Schneider J, Miranzo P, Osendi MI. The beneficial effect of graphene nanofillers on the tribological performance of ceramics. Carbon 2013;61:431–5. 7. Hvizdoˇs P, Dusza J, Balázsi C. Tribological properties of Si3 N4 -graphene nanocomposites. J Eur Ceram Soc 2013;33:2359–64. 8. Hvizdoˇs P, Puch´y V, Duszová A, Dusza J, Balázsi C. Tribological and electrical properties of ceramic matrix composites with carbon nanotubes. Ceram Int 2012;38:5669–76. 9. Pfeifer J, Sáfrán G, Wéber F, Zsigmond V, Koszor O, Arató P, et al. Tribology study of silicon nitride-based nanocomposites with carbon additions. Mater Sci Forum 2010;659:235–8. 10. Kun P, Wéber F, Balázsi C. Preparation and examination of multilayer graphene nanosheets by exfoliation of graphite in high efficient attritor mill. Cent Eur J Chem 2011;9:47–51. 11. Kvetková L, Duszová A, Hvizdoˇs P, Dusza J, Kun P, Balázsi C. Fracture toughness and toughening mechanisms in graphene platelet reinforced Si3 N4 composites. Scr Mater 2012;66:793–6. 12. Balázsi C. Silicon nitride composites with different nanocarbon additives. J Korean Ceram Soc 2012;49:352–62. 13. Balázsi C, Fényi B, Hegman N, Kovér Z, Wéber F, Vértesy Z, et al. Development of CNT/Si3 N4 composites with improved mechanical and electrical properties. Compos Part B Eng 2006;37:418–24. 14. Porwal H, Grasso S, Reece MJ. Review of graphene–ceramic matrix composites. Adv Appl Ceram 2013;112:443–54. 15. Porwal H, Tatarko P, Grasso S, Khaliq J, Dlouh´y I, Reece MJ. Graphene reinforced alumina nano-composites. Carbon 2013;64:359–69. 16. Porwal H, Tatarko P, Grasso S, Hu C, Boccaccini AR, Dlouh´y I, et al. Toughened and machinable glass matrix composites reinforced with graphene and graphene-oxide nano platelets. Sci Technol Adv Mater 2013;14:055007.
9
17. Walker LS, Marotto VR, Rafiee MA, Koratkar N, Corral EL. Toughening in graphene ceramic composites. ACS Nano 2011;5:3182–90. 18. Ramírez C, Figueiredo FM, Miranzo P, Poza P, Osendi MI. Graphene nanoplatelet/silicon nitride composites with high electrical conductivity. Carbon 2012;50:3607–15. 19. Ramírez C, Garzon L, Miranzo P, Osendi MI, Ocal C. Electrical conductivity maps in graphene nanoplatelet/silicon nitride composites using conducting scanning force microscopy. Carbon 2011;49:3873–80. 20. http://www.xgsciences.com 21. Kun P, Tapasztó O, Wéber F, Balázsi C. Determination of structural and mechanical properties of multilayer graphene added silicon nitride-based composites. Ceram Int 2012;38:211–6. 22. ISO 20808:2004(E). Fine ceramics—determination of friction and wear characteristics of monolithic ceramics by ball-on-disc method; 2004. 23. Hammersley AP, Svensson SO, Hanfland M, Fitch AN, Häusermann D. Two dimensional detector software: from real detector to idealized image or two-theta scan. High Pressure Res 1996;14:235–48. 24. Kraus W, Nolzeb G. POWDER CELL—a program for the representation and manipulation of crystal structures and calculation of the resulting X-ray powder patterns. J Appl Crystallogr 1996;29:301–3. 25. Dusza J, Morgiel J, Duszová A, Kvetková L, Nosko M, Kun P, et al. Microstructure and fracture toughness of Si3 N4 + graphene platelet composites. J Eur Ceram Soc 2012;32:3389–97. 26. Dong X, Jahanmir S. Wear transition diagram for silicon nitride. Wear 1993;165:169–80. 27. Skopp A, Woydt M, Habig KH. Unlubricated sliding friction and wear of various Si3 N4 pairs between 22 ◦ C and 1000 ◦ C. Tribol Int 1990;23: 189–99. 28. Ashby MF, Abulawi J, Kong HS. Temperature maps for frictional heating in dry sliding. Tribol Trans 1991;34:577–87. 29. Gee MG, Butterfield D. The combined effect of speed and humidity on the wear and friction of silicon nitride. Wear 1993;162(164):234–45. 30. Kato K, Adachi K. Wear of advanced ceramics. Wear 2002;253: 1097–104. 31. Adachi K, Kato K, Chen N. Wear map of ceramics. Wear 1997;203–204:291–301. 32. Hodkiewicz J. Characterizing carbon materials with Raman spectroscopy. Thermo Fisher Scientific, Application Note: 51901; 2010. 33. Honda K, Yokoyama S, Tanaka S. Assignment of the Raman active vibration modes of -Si3 N4 using micro-Raman scattering. J Appl Phys 1999;85:7380–4. 34. Muraki N, Katagiri G, Sergo V, Pezzotti G, Nishida T. Mapping of residual stresses around an indentation in -Si3 N4 using Raman spectroscopy. J Mater Sci 1997;32:5419–23. 35. Kuzuba T, Kijima K, Bando Y. Raman active modes of alpha silicon nitride. J Chem Phys 1978;69:40–2.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
ARTICLE IN PRESS
JECS-9554; No. of Pages 9
Available online at www.sciencedirect.com
ScienceDirect Journal of the European Ceramic Society xxx (2014) xxx–xxx
Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures Ján Balko a , Pavol Hvizdoˇs a,∗ , Ján Dusza a , Csaba Balázsi b , Jana Gamcová c a
b
Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 04353 Koˇsice, Slovakia Bay Zoltán Non-profit Ltd. for Applied Research, Institute for Materials Science and Technology, Fehérvári út 130, H-1116 Budapest, Hungary c Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, D-22603 Hamburg, Germany
Abstract Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt.% of multilayered graphene (MLG) platelets were studied and compared to monolithic Si3 N4 . The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at temperatures 25 ◦ C, 300 ◦ C, 500 ◦ C, and 700 ◦ C in dry sliding. Addition of such amounts of MLG did not lower the coefficient of friction. Graphene platelets were integrated into the matrix very strongly and they did not participate in lubricating processes. The best performance at room temperature offers material with 3 wt.% graphene, which has the highest wear resistance. At medium temperatures (300 ◦ C and 500 ◦ C) coefficient of friction of monolithic Si3 N4 and composite with 1%MLG reduced due to oxidation. Wear resistance at high temperatures significantly decreased, at 700 ◦ C differences between the experimental materials disappeared and severe wear regime dominated in all cases. © 2014 Elsevier Ltd. All rights reserved. Keywords: Si3 N4 ; Graphene; Friction; Wear
1. Introduction Shortly after their discovery carbon nanomaterials such as nanofibers, nanotubes1 and graphene2,3 were brought to the attention of material scientists who sought to utilize their unique properties for structural applications1–4 as they hold great promise as reinforcing nanofillers. In previous years silicon nitride based composites reinforced by carbon nanophases have been introduced and studied not only with respect to their improved mechanical and functional properties but also for their tribological performance.5–9 After extensive studies of composites with carbon nanofibers and carbon nanotubes, graphene, particularly in the form of multilayered platelets, came into focus as a reinforcing additive, thanks to its lower price, easier preparation and handling.10 Its potential for enhancing fracture resistance has been recognized and demonstrated.11–16 Walker et al.17 reported an improvement in fracture toughness
∗
Corresponding author. Tel.: +421 55 792 2464; fax: +421 55 792 2408. E-mail address: [email protected] (P. Hvizdoˇs).
in graphene containing ceramic nanocomposites where they identified novel toughening micromechanisms. Ramírez et al.18,19 explored the enhancement of electrical properties of graphene/silicon nitride composites. Following this, the potential of silicon nitride/graphene composites for the improvement of wear behavior has just started to be explored. It was found that coefficient of friction (COF) was reduced only under certain higher loads (≥200 N),6 wear resistance however was quite successfully improved.5–7 It has been observed that in Si3 N4 addition of MLG lead to better coefficient of friction than addition of CNTs and to much lower wear rates.7 Because of typical applications of structural ceramics as materials with exceptional thermal stability, the performance of the newly developed composites at elevated temperatures is also of a great importance. To our knowledge there is a lack of literature on this topic until now. The aim of the present work, as an extension of our previous studies,7 was to investigate the influence of the addition of multilayered graphene nanoplatelets on the tribological properties of Si3 N4 based composites (COF and wear) in the temperature range from room temperature (RT) up to 700 ◦ C, to observe
http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025 0955-2219/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model
ARTICLE IN PRESS
JECS-9554; No. of Pages 9 2
J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Table 1 Notation, composition and properties of experimental materials. Material
C, wt.%
Porosity, %
Average pore size, m
Roughness Ra, m
HV5, GPa
KIC , MPa m1/2
SN SN-1G SN-3G
0 1 3
<0.5 5.5 20.1
– 4.1 13.2
0.05 ± 0.01 0.07 ± 0.01 0.15 ± 0.02
15.4 ± 0.4 14.6 ± 0.2 7.7 ± 0.1
6.9 ± 0.4 7.8 ± 0.4 -
and identify wear mechanisms and their changes with changing temperature. 2. Materials and methods The experiments were carried out on three different materials. The first one was a reference silicon nitride based dense ceramic made of Si3 N4 (Ube, SN-ESP) with 6% Y2 O3 (H.C. Starck, grade C) and 4% Al2 O3 (Alcoa, A16) as sintering additives. The powder mixture was milled in attritor mill in distilled water using a zirconia tank, zirconia media (balls with 1 mm diameter) and zirconia agitator delta disks for 5 h, 4000 rpm. As a consequence, each material contained also approximately 3.6 wt.% zirconia from the milling media.10,12 The reference material is henceforth denoted as SN. For preparation of the graphene containing composites commercially available graphene platelets were admixed as reinforcement materials. Based on previous studies7 of various types of MLG, the most promising type was selected for conducting further high temperature experiments. They were exfoliated graphene nanoplatelets (xGnP-M-5 – particle size 5 m)20 produced by XG Sciences. In the case of graphene added powder mixtures, after finishing the first milling step, which was identical to the reference Si3 N4 , 1 or 3 wt.% of graphene nanoplatelets were admixed into the silicon nitride-based powder mixture in the attrition mill with low rotational speed, 600 rpm for 30 min. The mixtures then were dried and sieved with a filter with mesh size of 150 m. Green samples were obtained by dry pressing at 220 MPa. Samples prepared for HIP were oxidized at 400 ◦ C to eliminate the PEG. Hot isostatic pressing (HIP) was performed at 1700 ◦ C in high purity nitrogen by a two-step sinter-HIP method using BN embedding powder at 20 MPa, with 3 h holding time. The heating rate did not exceed 25 ◦ C/min. The microstructure and basic mechanical properties of the experimental materials were described and characterized in previous studies.7,11,12,21 The initial wear testing at room temperature was performed, and results were reported in Ref. 7. Based on the obtained results the most promising materials were selected for the present study which focuses on high temperature wear testing. The sample notation, carbon content and basic microstructural and mechanical characteristics are summarized in Table 1. The wear behavior of the experimental materials was studied in dry sliding in air. The surfaces were carefully prepared by polishing down to a surface roughness Ra below 0.05 m for the reference material. For the composites, particularly for that with higher volume fraction (3 wt.%) of carbon additives, this was not entirely possible because of residual porosity. The roughness
in terms of Ra was measured by optical interferometry and it is given in Table 1. The wear testing was carried out on the tribometer HTT by CSM Instruments in dry conditions using the pin-on-disk technique, where the tribological partner was a highly polished (roughness Ra < 0.20 m) Si3 N4 ball with 6 mm diameter. The applied load was 5 N, the sliding speed 0.1 m/s and the sliding distance was 300 m. The testing temperatures were 25 ◦ C, 300 ◦ C, 500 ◦ C, and 700 ◦ C. The tangential forces during the test were measured and friction coefficients calculated. The worn surfaces were subsequently observed and the wear regimes, damage type and micromechanisms were identified. The material losses (volume of the wear tracks) due to wear were measured by a high precision confocal microscope PLu neox 3D Optical Profiler, by SENSOFAR, and then specific wear rates (W) were calculated in terms of the volume loss (V) per distance (L) and applied load (Fp ) according to the standard ISO 2080822 : W (mm3 /m N) =
V L · Fp
At the same time, the worn cap on the counter body was measured by a light microscope, its volume and then its wear rate calculated, too. The microstructure and the wear tracks created at the surfaces of the investigated materials were observed using scanning electron microscopy (JEOL JSM 7000). Porosity and average pore size was found using image analysis of micrographs of polished ceramographic sections. At least 1 mm2 of surface was documented and then treated using ImageJ 1.47e software. Raman study of the tested specimens was carried out using a Raman spectroscope XploRA by HORIBA Jobin Yvon. Green laser with wavelength λ = 532 nm was used and Raman shift from 100 to 3000 cm−1 was observed. The phase composition of the wear debris was studied using synchrotron X-ray diffraction. XRD measurements were realized at the high X-ray diffraction endstation P02.1 situated at the storage ring PETRA III at DESY at Hamburg. The P02.1 operates with photons of fixed energy ∼ 60 keV ˚ selected by Laue monochromator crystals of (λ = 0.20727 A) diamond (1 1 1) and silicon (1 1 1) from synchrotron radiation produced at the high energy undulator. Collection of X-ray diffraction patterns was performed in transmission mode with a two-dimensional Perkin Elmer 1621 detector (2048 × 2048, the pixel size 200 m × 200 m). The reading time of the used detector was about 1 ms. The illumination time of the sample was 180 s with a photon beam of size 0.5 mm × 0.5 mm. The sample–detector distance was 1024 mm. Two dimensional XRD patterns were integrated to the 2-theta space using software package FIT2D.23 The phase analysis was done via PCW software.24
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
3. Results
3
1.0 0.9 0.8 0.7
COF [-]
0.6 0.5 0.4 0.3
SN SN-1G SN-3G
0.2 0.1 0.0 0
100
200
300
400
500
600
700
Temperature [°C] Fig. 1. Average values of coefficient of friction for the experimental materials at all experimental temperatures.
SN SN-1G SN-3G
Wear rate [mm /N.m]
1E-3
3
1E-4
1E-5
1E-6
1E-7 0
100
200
300
400
500
600
700
800
Temperature [°C]
Fig. 2. Specific wear rates of the experimental materials vs temperature.
1E-3
SN SN-1G SN-3G 1E-4
3
Ball wear rate [mm /N.m]
The microstructures of the experimental materials were studied in detail previously in Refs. 21, 25 and the relevant results were concisely summarized in Ref. 7. It was found that in all cases good mixing and homogeneous distribution of graphene additives was achieved21 and the graphene platelets were well embedded within the matrix. However, the graphene platelets induced porosity in the matrix and reduced the size of the Si3 N4 grains in the resulting composites. Dusza et al.25 performed a statistical analysis of the grain sizes and found that while in the monolithic Si3 N4 the diameter of the matrix grains was typically 0.4 m, the average matrix grains in the composites had narrow size distribution with maximum at approximately 0.2 m. The results of X-ray investigations showed only -Si3 N4 diffraction peaks, suggesting that the ␣ →  transformation was complete.21,25 The monolithic silicon nitride was fully dense. Carbon filler phases made densification of the composites more difficult, and the pores were always associated with graphene platelets. That lead to porosity increasing with increasing volume fraction of carbon phases, Table 1. They do not tend to form extensive clustering as it is typical for CNTs,8 but occasional stacks of several graphene platelets were present.7,25 Morphology studies of the graphene platelets25 showed that the MLG had a size distribution corresponding to the values given by the producer,20 i.e. 5 ± 1 m. The pores present in the composites were always associated with graphene platelets. Values of porosity and average pore size found by image analysis of ceramographic sections are also shown in Table 1. The tests showed that the friction behavior of all materials under the testing conditions was stable during the tests. In all cases the friction force stabilized over a short run-up phase, usually shorter than 1 m. Then, depending on material, the friction remained more or less stable with small oscillations but in no case did it exhibit any significant change that would suggest some important change in character of wear damage mechanisms over the 300 m sliding distance. Figs. 1–3 summarize results of the tribological testing, namely coefficient of friction (COF), wear rates (W) of the experimental materials and counterbodies, respectively, as functions of testing temperature. Fig. 1 shows the comparison of average COF for the experimental materials at all experimental temperatures. It can be seen that at room temperature the COF of the composites was similar, or slightly higher than that of the reference SN. At medium temperatures, COF values reduced for SN and SN-1G and at 700 ◦ C they were high again. The COF of SN-3G was 0.7 at lower temperatures (RT, 300 ◦ C), at higher ones (500 ◦ C, 700 ◦ C) it increased to approximately 0.8. The wear resistance results are summarized in Fig. 2. At room temperature the MLG had beneficial effect. In both types of composites a significant improvement of wear resistance was recorded. The wear rate in SN-3G was about 60% lower than that for the reference Si3 N4 which was discussed in greater detail in Ref. 7.
1E-5
1E-6 0
100
200
300
400
500
600
700
Temperature [°C]
Fig. 3. Specific wear rates of counterbodies (silicon nitride balls).
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9 4
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
At 300 ◦ C the wear rates of the materials with low carbon content (SN, SN-1G) decreased by about an order of magnitude. At 500 ◦ C the wear rate of SN improved a little whereas SN-1G showed a reduction of wear resistance. The wear rates of SN-3G had a clear tendency to increase with increasing temperature. Wear rates at 700 ◦ C were similar for all three materials and were one to two orders of magnitudes higher than those at room temperature. Fig. 3 shows the wear rate of the counterbodies (silicon nitride balls). It completes the information about the wear damage of the tribological pairs. It shows a general tendency of the wear rates of the counterbodies to increase with increasing amount of MLG in the paired material. They are generally stable up to 300 ◦ C, then at 500 ◦ C for the pairs with highly reduced COF (SN, SN-1G) they decrease. At 700 ◦ C all increase dramatically along with increase of wear rates of the samples. 4. Discussion 4.1. Wear mechanisms The wear tracks of the monolithic SN, SN-1G, and SN-3G samples worn at the experimental temperatures are shown in Figs. 4–6, respectively. The COF values obtained at room temperature were reported also in Ref. 7. The presence of MLG in composites did not lower the COF, which is similar to the results of the study of Si3 N4 -CNT composites.8 This suggests that the graphene platelets do not have a lubricating effect. It is in agreement with findings of Belmonte et al.6 for low loads, up to 100 N. They argue that higher friction for the composites is connected to the nanoplatelets producing continuous surface changes between graphene and silicon nitride grains which leads to always higher roughness for the composites than for the monolithic material. This is consistent with our observation where the COF is connected to higher roughness, which increases with the carbon content (Table 1) – see also Figs. 4a,b, 5a,b and 6a. Only at much higher loads, 200 N according to Ref. 6, the composite shows reduced friction and graphene nanoplatelets do lubricate the tribosystem. Such loads, however, were not reached in our tests. At higher temperatures (300 and 500 ◦ C) the reference SN had a significantly lower COF, dropping from 0.6 down to about 0.2. This behavior is consistent with findings reported in the literature,26,27 where at low loads selective oxidation was reported to take place between 400 and 700 ◦ C. In the present study an oxide rich layer on the surfaces of the wear tracks was identified (Figs. 4c–f and 5c,d). The inset in Fig. 4f shows a typical EDX spectrum of this layer. At 700 ◦ C the integrity of this layer was lower and surface microcracking and break-up accompanied by much higher production of debris became the dominant mechanism which consequently lead to a significant increase of wear rate in SN (Fig. 2). In SN-1G a higher temperature (500 ◦ C) was necessary to activate oxidation of the degree similar to that of the monolith material. Then the COF dropped to the same values as for SN
(Fig. 1). The tribochemical reaction produced typical thin and long cylindrical oxide particles (Fig. 5c and d), sometimes they were embedded back into the tribofilm. Dong and Jahanmir26 reported formation of needle shaped oxides and explained it as oxidation of silicon nitride, with a possible contribution from  to ␣-Si3 N4 phase transformation. Here, knowledge of bulk and flash temperatures could be of interest. Their calculation is subject to many simplifying assumptions about the details of the contact between the surfaces, and a large number of models exist which differ substantially in their quantitative predictions of temperatures. The simple model of Ashby et al.28 uses energy input/output balance to estimate the maximum temperature difference that can be sustained by a sample of given dimensions. Using thermal conductivity of silicon nitride as 15–30 W/mK this T under present conditions would be only between 20 and 40 K. However, the localized flash temperatures in points of contact could be much higher. Although the real contact area can be estimated only with difficulty, according to Gee and Butterfield29 they can be high enough at any speed above about 0.1 m/s. It is also significant that a layer of oxide on the surface of the silicon nitride increases the flash temperature by a factor of about 2, so that if increasing the temperature of a reaction leads to higher rates of oxidation, then any incipient oxide film formation is likely to increase the temperatures even more. Therefore, in order to decide whether such transformation takes place independent experiments are necessary, as it is shown later in Section 4.3. The composite with 3 wt.% of MLG did not exhibit a tendency to lower the COF at 300 ◦ C and/or 500 ◦ C, it stayed at high values. Above 500 ◦ C it even increased (Fig. 1). At RT the wear track surface showed differences from the other two materials. It was much rougher, and even though it had lower wear rate it exhibited more plastic deformation (Fig. 6a). At 300 ◦ C most of the wear track suffered not only local plastic deformation but also surface microcracking (Fig. 6b). At even higher temperatures extensive surface cracking took place. Its high intensity was probably connected to much higher surface roughness. The asperities in the contact zone were constantly being created and broken up, producing a lot of debris in the process. The MLG inside the wear tracks usually remain in their sites even at 500 ◦ C (Fig. 6c and d) so at these conditions the fact that they had lead to porosity was more important than their role as a reinforcing filler. At 700 ◦ C then all materials entered different wear regime and all had a coefficient of friction higher than at room temperature, in the case of SN-3G it could be up to 0.9. The wear rates of all three materials were much higher than 10−5 mm3 /Nm, which indicates that here clearly severe wear prevailed.30,31 The damage had similar character since the carbon phases had decomposed and all three materials were compositionally identical. In all cases the dominant damage mechanism was the surface cracking, -silicon nitride grain breaking and debris production (Figs. 4g,h, 5e and 6e,f). 4.2. Stability of MLG For a better understanding of MLG and its changes during wear testing a Raman study of the tested specimens was
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
5
Fig. 4. Wear tracks of SN. Smooth wear surface at room temperature (a and b), tribofilm and selective oxidation at 300 ◦ C (c and d) and 500 ◦ C (e and f) with EDX spectrum of the surface layer. Surface break-up and debris production at 700 ◦ C (g and h).
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9 6
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Fig. 5. Wear of SN-1G, RT (a and b), 300 ◦ C (c), 500 ◦ C (d), 700 ◦ C (e). Effect of selective oxidation at medium temperatures and formation of needle shaped oxides, particularly at 500 ◦ C.
performed. The results are illustrated by an example in Fig. 7 where measurements on the material SN-3G worn at 500 ◦ C are shown. The spectra were collected from a typical MLG (site 1), and from polished and worn surfaces (sites 2 and 3). The topmost spectrum comes from a basically undisturbed MLG located inside a large pore (similar to that pictured in Fig. 6d). The two very prominent peaks are called G and G and are typical for graphite or graphene. Spectra of these two
materials are, of course, very similar, since graphite is just stacked graphene. The difference is in relative intensity of the two peaks, where G is much more intense than the G in graphite compared to graphene.32 This means that in our case the large MLG is really a flake of graphite containing a number of layers. There is also a prominent band around 1350 cm−1 . It is small for the spectrum from the site 1 but very intense for the other two. This band is known as the D band and it is typically present in
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
7
Fig. 6. Wear of SN-3G at various temperatures –RT (a), 300 ◦ C (b), 500 ◦ C (c and d), and 700 ◦ C (e and f).
signal by nanotubes. The D band originates from a hybridized vibrational mode associated with graphene edges and it indicates the presence of some disorder to the graphene structure. This band is often referred to as the disorder band or the defect band and its intensity relative to that of the G band is often used as a measure of the quality of nanotubes.32 In the present case it indicates many edges and defects in MLGs at the polished and worn
surfaces, due to polishing and wear, respectively. High G with respect to G means that these processes also disintegrate MLGs so that they have fewer layers and become more graphene-like. What we can see, however, is that the two spectra are basically identical, which means that the process of polishing and that of high temperature wear in areas of abrasion, without much microcracking, had the same effect on the surface of the specimen.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model
ARTICLE IN PRESS
JECS-9554; No. of Pages 9 8
J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Fig. 7. Raman study of MLGs, SN-3G after wear testing at 500 ◦ C, site 1 – MLG inside a pore; site 2 – polished surface; site 3 – worn surface.
4.3. Stability of β-silicon nitride In order to identify possible -to-␣ Si3 N4 transformation during the high temperature wear testing, as suggested by Dong and Jahanmir,26 analyses of phases in the wear track and debris were carried out. For the wear track analysis Raman spectroscopy was used as in Section 4.2. In Fig. 7 it is shown that the signal from the polished surface (site 2) and the deformed worn surface (site 3) are basically identical. In both locations there are all main peaks typical for -silicon nitride (183, 205, 226, 452, 620, 733, 866, 930, and 1048 cm−1 )33,34 clearly present. On the other hand, no signal that would correspond to ␣-phase is visible (259, 365, 515, 668, 868, 915, 976, and 1034 cm−1 ).35 This result was the same for all specimens. Beside the worn surface analysis, the debris particles were collected, too. Because of their small size and very low quantities their phase composition was studied using synchrotron X-ray diffraction. The results found for materials tested at 700 ◦ C (the highest quantities of debris collected, i.e. the best spectra available) are given in Fig. 8, together with positions of expected peaks for ␣ 500
-Si3N4 -Si3N4
Intensity [a.u.]
400
300
200
SN-3G
SN-1G
100
SN
0 3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
5.0
2-theta [deg.]
Fig. 8. X-ray diffractograms of the debris particles produced at 700 ◦ C. No ␣, only -Si3 N4 was identified.
and  silicon nitride phases. Again, only -silicon nitride was identified, in no case any ␣-Si3 N4 peaks were found.
5. Conclusions A series of silicon nitride based composites containing 1 and 3 wt.% of multilayered graphene platelets were prepared and subjected to wear testing at temperatures 25 ◦ C, 300 ◦ C, 500 ◦ C, and 700 ◦ C. The results allow the following conclusions: • MLG makes densification more difficult which leads to higher porosity. • At room temperature the monolithic Si3 N4 had a high coefficient of friction, and its wear tracks had a smooth self polished look. The composites behaved similarly, with increasing amount of MLG the wear resistance increased. On the other hand, the composites suffered more surface microcracking and debris production. • At 300 ◦ C and 500 ◦ C monolithic Si3 N4 had reduced COF due to formation of a tribofilm, where a tribochemical reaction created oxide particles, which reduced the wear rate. • 1% MLG containing composite had damage mechanism similar to that of the monolith. Wear rate improved at 300 ◦ C. Reduction of COF was found at 500 ◦ C. • 3% MLG containing composite had high COF at all testing temperatures and its wear rate increased with increasing temperature. Surface break-up due to high porosity and surface roughness was typical. • Wear resistance at the highest temperature (700 ◦ C) significantly decreased and a severe wear regime (W > 10−5 ) prevailed in all materials. • -Silicon nitride was stable during wear testing and did not transform to create ␣-Si3 N4 phase. • Generally, MLGs did not participate in a lubricating process. Significant graphite transfer film was not observed. Improved fracture toughness by graphene lead to better wear resistance at lower testing temperatures up to 300 ◦ C.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025
+Model JECS-9554; No. of Pages 9
ARTICLE IN PRESS J. Balko et al. / Journal of the European Ceramic Society xxx (2014) xxx–xxx
Acknowledgement This work was supported by the Slovak Grant Agency, projects VEGA 2/0075/13 and APVV-0108-12. References 1. Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. 2. Novoselov KS, Falko VI, Colombo L, Gellert PR, Schwab MG, Kim K. A roadmap for graphene. Nature 2012;490:192–200. 3. Lee SH, Lee DH, Lee WJ, Kim SO. Tailored assembly of carbon nanotubes and graphene. Adv Funct Mater 2011;21:1338–54. 4. Peigney A, Laurent Ch, Flahaut E, Rousset A. Carbon nanotubes in novel ceramic matrix nanocomposites. Ceram Int 2000;26:677–83. 5. González-Julián J, Schneider J, Miranzo P, Osendi MI, Belmonte M. Enhanced tribological performance of silicon nitride-based materials by adding carbon nanotubes. J Am Ceram Soc 2011;94:2542–8. 6. Belmonte M, Ramírez C, González-Julián J, Schneider J, Miranzo P, Osendi MI. The beneficial effect of graphene nanofillers on the tribological performance of ceramics. Carbon 2013;61:431–5. 7. Hvizdoˇs P, Dusza J, Balázsi C. Tribological properties of Si3 N4 -graphene nanocomposites. J Eur Ceram Soc 2013;33:2359–64. 8. Hvizdoˇs P, Puch´y V, Duszová A, Dusza J, Balázsi C. Tribological and electrical properties of ceramic matrix composites with carbon nanotubes. Ceram Int 2012;38:5669–76. 9. Pfeifer J, Sáfrán G, Wéber F, Zsigmond V, Koszor O, Arató P, et al. Tribology study of silicon nitride-based nanocomposites with carbon additions. Mater Sci Forum 2010;659:235–8. 10. Kun P, Wéber F, Balázsi C. Preparation and examination of multilayer graphene nanosheets by exfoliation of graphite in high efficient attritor mill. Cent Eur J Chem 2011;9:47–51. 11. Kvetková L, Duszová A, Hvizdoˇs P, Dusza J, Kun P, Balázsi C. Fracture toughness and toughening mechanisms in graphene platelet reinforced Si3 N4 composites. Scr Mater 2012;66:793–6. 12. Balázsi C. Silicon nitride composites with different nanocarbon additives. J Korean Ceram Soc 2012;49:352–62. 13. Balázsi C, Fényi B, Hegman N, Kovér Z, Wéber F, Vértesy Z, et al. Development of CNT/Si3 N4 composites with improved mechanical and electrical properties. Compos Part B Eng 2006;37:418–24. 14. Porwal H, Grasso S, Reece MJ. Review of graphene–ceramic matrix composites. Adv Appl Ceram 2013;112:443–54. 15. Porwal H, Tatarko P, Grasso S, Khaliq J, Dlouh´y I, Reece MJ. Graphene reinforced alumina nano-composites. Carbon 2013;64:359–69. 16. Porwal H, Tatarko P, Grasso S, Hu C, Boccaccini AR, Dlouh´y I, et al. Toughened and machinable glass matrix composites reinforced with graphene and graphene-oxide nano platelets. Sci Technol Adv Mater 2013;14:055007.
9
17. Walker LS, Marotto VR, Rafiee MA, Koratkar N, Corral EL. Toughening in graphene ceramic composites. ACS Nano 2011;5:3182–90. 18. Ramírez C, Figueiredo FM, Miranzo P, Poza P, Osendi MI. Graphene nanoplatelet/silicon nitride composites with high electrical conductivity. Carbon 2012;50:3607–15. 19. Ramírez C, Garzon L, Miranzo P, Osendi MI, Ocal C. Electrical conductivity maps in graphene nanoplatelet/silicon nitride composites using conducting scanning force microscopy. Carbon 2011;49:3873–80. 20. http://www.xgsciences.com 21. Kun P, Tapasztó O, Wéber F, Balázsi C. Determination of structural and mechanical properties of multilayer graphene added silicon nitride-based composites. Ceram Int 2012;38:211–6. 22. ISO 20808:2004(E). Fine ceramics—determination of friction and wear characteristics of monolithic ceramics by ball-on-disc method; 2004. 23. Hammersley AP, Svensson SO, Hanfland M, Fitch AN, Häusermann D. Two dimensional detector software: from real detector to idealized image or two-theta scan. High Pressure Res 1996;14:235–48. 24. Kraus W, Nolzeb G. POWDER CELL—a program for the representation and manipulation of crystal structures and calculation of the resulting X-ray powder patterns. J Appl Crystallogr 1996;29:301–3. 25. Dusza J, Morgiel J, Duszová A, Kvetková L, Nosko M, Kun P, et al. Microstructure and fracture toughness of Si3 N4 + graphene platelet composites. J Eur Ceram Soc 2012;32:3389–97. 26. Dong X, Jahanmir S. Wear transition diagram for silicon nitride. Wear 1993;165:169–80. 27. Skopp A, Woydt M, Habig KH. Unlubricated sliding friction and wear of various Si3 N4 pairs between 22 ◦ C and 1000 ◦ C. Tribol Int 1990;23: 189–99. 28. Ashby MF, Abulawi J, Kong HS. Temperature maps for frictional heating in dry sliding. Tribol Trans 1991;34:577–87. 29. Gee MG, Butterfield D. The combined effect of speed and humidity on the wear and friction of silicon nitride. Wear 1993;162(164):234–45. 30. Kato K, Adachi K. Wear of advanced ceramics. Wear 2002;253: 1097–104. 31. Adachi K, Kato K, Chen N. Wear map of ceramics. Wear 1997;203–204:291–301. 32. Hodkiewicz J. Characterizing carbon materials with Raman spectroscopy. Thermo Fisher Scientific, Application Note: 51901; 2010. 33. Honda K, Yokoyama S, Tanaka S. Assignment of the Raman active vibration modes of -Si3 N4 using micro-Raman scattering. J Appl Phys 1999;85:7380–4. 34. Muraki N, Katagiri G, Sergo V, Pezzotti G, Nishida T. Mapping of residual stresses around an indentation in -Si3 N4 using Raman spectroscopy. J Mater Sci 1997;32:5419–23. 35. Kuzuba T, Kijima K, Bando Y. Raman active modes of alpha silicon nitride. J Chem Phys 1978;69:40–2.
Please cite this article in press as: Balko J, et al. Wear damage of Si3 N4 -graphene nanocomposites at room and elevated temperatures. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.02.025