SCI SINTERING 57 1 2025 01pdf
SCI SINTERING 57 1 2025 01pdf
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  1. Science of Sintering, 57 (2025) 1-15________________________________________________________________________ _____________________________ *) Corresponding author:pelincagim.tokat@dpu.edu.tr https://doi.org/10.2298/SOS240306016CUDK: 692.233.1; 676.017.2 Synthesis of Y2O3 and Graphite-Added Copper-Based Hybrid Composites by Powder Metallurgy: Mechanical Properties and Tribological Behavior Pelin Çağım Tokat Birgin1*), Hediye Aydin1, Esad Kaya21Department of Metallurgy and Material Engineering, Kütahya Dumlupınar University, 43100 Kütahya, Turkey. 2Department of Mechanical Engineering, Eskişehir Osmangazi University, 26480, Eskişehir, Turkey. Abstract: In the presented study, Y2O3 and graphite-reinforced Cu matrix hybrid composite materials were prepared using the powder metallurgy method. The aim is to compare the microstructure, density, hardness, and wear behavior of composites with different yttriacontents (5, 10, and 15 wt%) and graphite (2 wt%) reinforcements. An increase in the Y2O3ratio, along with the addition of graphite, resulted in an observed increase in the hardness values of the composites. Additionally, it was observed that Y2O3 and graphite are homogeneously distributed in the copper matrix in the SEM-EDX spectra. Upon examining the microstructures of the composites, it was determined that with the increase in yttrium oxide content, partially yttrium oxide agglomerations formed in some regions. The addition of Y2O3 resulted in a copper sample that is 1.34 times harder than pure copper. Addition of Y2O3-graphite, samples with 3.301–22.3 % higher wear resistance were obtained. Keywords: Powder Metallurgy (PM); Graphite; Y2O3; Copper-based metal matrix composites. 1. Introduction With good strength and ductility, ease of processing, and availability, copper has been used for thousands of years in objects, jewelry, and daily-use tools. It has been an excellentmaterial for creating components with technological content. Today, copper, while it is mainly used in its pure form in applications requiring high thermal or electrical conductivity, its alloys (bronzes and brasses) are widely used in many areas due to their excellent corrosion and wear resistance [1]. Copper and its alloys have been and will be a tremendous natural resource for the future of humanity. Despite these unique properties of copper, its relatively low strength and hardness limited their applications as structural materials. A composite material is a heterogeneous material composed of theoretically immiscible two or more materials that are different from each other in crystal structure, physical properties, and/or chemical composition. Copper matrix composites have become more popular in recent years within metal matrix composite materials. Pure copper, due to its low strength and less-than-ideal physical properties, cannot be used alone in certain applications. Current efforts are ongoing to
  2. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________2 develop copper matrix composites with an optimal combination of high mechanical strength, electrical conductivity, and improved softening resistance [2]. Many studies are conducted on combinations containing a suitable copper matrix and reinforcement phase to achieve thedesired strength at high temperatures [3-5]. Sometimes, using a single type of reinforcement in copper matrix composites canlead to undesirable changes in some physical and mechanical properties of the matrix. To overcome these undesirable changes, copper matrix hybrid composites are produced by adding two or more different types of reinforcements. In other words, by incorporating one or more reinforcement materials into pure copper, significant improvements in mechanical and physical properties can be achieved even at high temperatures, without causing substantial alterations in the thermal and conductivity characteristics of the main matrix. Hybrid composites have been developed as an alternative to single-reinforcement composite materials. Hybrid composites possess unique features that can more effectively meet different design requirements compared to traditional composites. Significant efforts have been made towards developing Cu matrix composites containing ceramic-based reinforcement particles with various properties such as TiC, TiB, ZrC, Al2O3, and Y2O3 [6]. Yttrium oxide (Y2O3) and many rare earth oxides are intriguing due to their excellent dispersion properties within copper alloys. Thanks to its superior mechanical, electrical, and thermal characteristics, yttrium oxide is utilized in copper matrices [7-8]. The larger atomic radius of the elements composing this oxide results in low solubility and, consequently, lowdiffusion in metal matrices. This situation is undesirable and increases microstructural stability by preventing coarsening. When yttrium oxide is dispersed within or between copper grains, it leads to the hindrance of dislocation motion. This result in a noticeably higher hardness of the composite with Y2O3 compared to pure copper. In other words, when yttrium oxide is effectively distributed within or between the copper matrix, it plays a role in increasing the material's hardness by restricting dislocation movement [9]. Also, graphite plays a vital role among high-tech materials with excellent tribological properties [10]. Researchers claim that the addition of graphite can yield an optimal combination of both electrical and thermal conductivity, along with the best tribological properties for copper-based composites. In a conducted study, it was determined that the addition of graphite up to 5% by weight improves the wear resistance of Cu matrix composites [11-12]. Upon reviewing the literature, numerous studies on particle-reinforced copper matrix composites, along with composites obtained using in situ synthesis methods, stand out [9]. Other commonly used methods include powder metallurgy, mechanical alloying, composite electrodeposition [13], and composite casting [14]. Powder metallurgy stands out as one of the most preferred methods for producing Cu matrix composites due to its economic nature, low agglomeration, weathering, and favorable particle-to-particle interactions [15-16]. In this article, Cu–C-Y2O3 hybrid composites have been fabricated through the utilization of powder metallurgy. The effects of 2% by weight graphite and different yttrium oxide contents (5, 10, and 15% by weight) on the properties of copper, such as density, hardness, and wear, were also investigated. 2. Materials and Experimental Procedures 2.1 Raw materials Preparation of Cu–C-Y2O3 hybrid composites, Cu(NaNokar, 99.9%, 44 μm,), Y2O3(Bostonchem, 99.99% purity, 50 nm), and graphite (Alfa Easer, 99.5 % purity, 44 μm) were used at the stoichiometric ratios in Table I. For homogenization, these raw materials were conducted by ball milling using a Retsch PM 400 planetary mill machine. Ball milling proceeded for 12h at 150 rpm. A Calver brand manual press shaped the powders at 6 MPa
  3. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________3 pressure for 20 seconds. The pellets were pressed for the 2nd time with a cold isostatic press (MSE-CIP) at a pressure of 200 MPa. The shaped samples were heated from room temperature up to 900 oC for 2h in an argon atmosphere, and cooled back to room temperature. Tab. I Mass percentage ratios and codes of the samples produce. Sample Code Copper (% vol.) Y2O3 (%vol.) Graphite (%vol.) S-0 100 - - S-1 Balance - 2 S-2 Balance 5 - S-3 Balance 10 - S-4 Balance 15 - S-5 Balance 5 2 S-6 Balance 10 2 S-7 Balance 15 2 2.2 Characterization of the samples A PANalytical X-ray diffractometer (Cu Ka radiation and Ni filter) at Kutahya Dumlupınar University Advanced Technology Center was used for phase analysis of the sintered samples. The rule of mixtures was used for the theoretical densities of the samples, and Archimedes' method was used for experimental density calculations. Relative density was calculated from the ratio of these two values. The microstructures, phase distributions, elemental ratios, and the distribution of reinforcements in the matrix were examined with the help of a scanning electron microscope (FEI Nova NanoSEM 650) at the same center. Three hardness tests were performed on the specimens on a Future Tech FM-800-type machine. Average hardness values were taken as the average of the three indentations. Standard deviations were calculated and shown. The parameters of 25 gf load and 10 s dwell time were considered appropriate for the hardness tests. A CSM tribometer test device was used to determine the tribological performance of the produced samples. With a 3 mm diameter 100Cr6 hardened bearing steel ball used as an abrasive counter body, 5N was selected as normal wear load, 196 RPM rotation speed (linear speed of ~3 cm/s), and test distance determined as 100 meters. The cross-section of the worn surface was measured with a Mitutoyo SJ-400 surface roughness profilometer. The worn surface's roughness was measured using the Gaussian filtering technique. The COF values were recorded throughout the test. SEM and EDS tests were assessed on the worn surfaces todetermine the tribological characteristics of the produced samples. The wear mechanism is characterized based on the obtained results. 3. Results and Discussion XRD patterns of Y2O3 and Y2O3-graphite doped copper matrix composites are given in Fig. Y in (a) and (b). It is seen that the main phase is copper for all samples. The peaks of the copper phase are seen at 2theta = 43.4°, 50.5° and 74.9° for composites. The peak
  4. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________4 intensity of the Y2O3 phase, which appears minimally in samples S-2 and S-5 containing 5% Y2O3, increases in samples S-3 and S-6 containing 10% Y2O3, while it is maximum for S-4 and S-7 having 15% Y2O3. The peak phase was detected at 29.4 in all samples containing Y2O3. In the study by Chen et al. Y2O3 was added at different ratios in copper, and the peak of the Y2O3 phase was obtained between 29-29.5 in the samples [17]. In another study, Huang et al. detected a minimal peak between 33-34 in addition to this peak [18]. No graphite phase peak is observed in S-1, S-5, S-6, and S-7 samples containing 2% graphite. A similar situation was observed in the results of the study by Tokat-Birgin. In the brass matrix composite with 5% graphite, although it was proved to be graphite in SEM-EDX analysis, the graphite phase could not be detected in XRD analysis. In the same study, peaks belonging to the graphite phase were seen in the XRD pattern of the brass sample containing 10% graphite [19]. Fig. 1. XRD images of S-0, S-1, S,2, S-3, S-4, S,5, S-6 and S-7. Recently, production under atmospheric control has become widespread because it is economical. The shaped samples were pressed for the second time using an isostatic press to obtain a denser product. Thus, we expected to improve the density-related properties of the samples (strength, hardness, etc.). The theoretical density for the specimens, the experimental density using the Archimedes method, and the relative density values calculated based on these two density values are calculated and given in Fig. 2. In this study, to eliminate the disadvantage of the density of the samples produced using the tube furnace compared to those produced in other furnaces; it was aimed to produce samples with high relative density by double pressing. The relative density of the sample s-0 without additives (containing only copper) was maximum. The relative density of the sample s-1 with graphite addition was relatively lower than s-0. Kumar et al. also found that the density of copper samples with different ratios of graphite content was lower than that of pure copper [20]. As the amount of
  5. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________5 Y2O3 addition increased (S-2, S-3, and S-4, respectively), the relative density value decreased linearly to a very small extent, although they were very close to each other. The decrease in the relative density as the ceramic addition increases may be related to the increase in the metal-ceramic contact area. The same situation was observed in the samples with graphite addition. As the amount of Y2O3 addition increased (S-1, S-5, S-6, and S-7, respectively), the relative densities of the graphite-added samples decreased slightly. The theoretical density decreases when lower-density additions such as graphite and/or Y2O3 are added. There is a linear relationship between the calculated experimental density and the theoretical density in the samples produced. The theoretical density decreased as the addition increased for S-0, S-2, S-3, and S-4, respectively. With decreasing theoretical density, the experimental density also decreased as expected. The graph shows that this situation is similar for samples S-1, S-5, S-6, and S-7, respectively. Fig. 2. Theoretical, experimental and relative densities of samples. The FESEM micrographs of all composites are presented in Fig. 3(a–h). When the images were evaluated, it was seen that Y2O3 particles were discontinuously distributed in the copper matrix. However, yttrium oxide started agglomerating when the content was further increased by weight. It formed a network-like second phase at the copper grain boundaries. This may be due to insufficient ball milling and insufficient refining of Y2O3 powder. As mentioned in related studies, due to the low solid solubility of Y in the Cu matrix, it may cluster as compounds at the grain boundaries of the matrix when the solubility limit is exceeded [17]. Regarding mechanical properties, the second phase, yttrium oxide, is dispersed within the copper grains or in the intergranular space, thus providing a robust inhibitory effect on the dislocation motion and significantly increasing the hardness of the copper alloy compared to pure copper [9]. Fig. 4 shows the SEM/EDX analysis of the Cu-%15 Y2O3(S-4), and Cu-%15 Y2O3-%2C (S-7) composite surface. The C and Y2O3 particles in the matrix were determined, and the ratios indicated in the analysis from three different areas are given in Fig 4a and Fig 4b. Spot 3 shows the analysis results obtained from the whole area. Y2O3 and C looking at the Y and O elements distribution, 7.61 wt% yttrium oxide has a relatively uniform distribution on copper (Fig. 5).
  6. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________6 Fig. 3. SEM images of (a) pure copper, (b) %5 Y2O3-Cu, (c) %10 Y2O3-Cu, (d) %15 Y2O3-Cu, (e) %5 Y2O3-%2C-Cu, (f) %10 Y2O3-%2C-Cu and (g) %15 Y2O3-%2C-Cu.Fig. 4. SEM-EDX analysis of (a)Cu- %15 Y2O3 and (b)Cu-%15 Y2O3-%2C.
  7. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________7 Fig. 5. (a) SEM image of the sample S-7, (b) elemental mapping of sample S-7, (c) the element mapping of Cu in (a), (d) the element mapping of Y in (a), (e) the element mapping of C in (a), (f) the element mapping spectrum of sample S-7. Fig. 6. Microhardness of samples.
  8. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________8 It is seen in Fig. 6 that the hardness value decreases when graphite (S-1) is added to the copper sample without doped (S-0). It can be observed that with increasing Y2O3 content in composite, the hardness change increases significantly. In the study by Qin et al, a relative density of 100% was achieved with a different production method. In the study, although the relative density value did not reach 100%, the hardness value of the sample containing 5% Y2O3 was similar to the study of Qin et al. [9]. Studies in the literature have shown that the hardness of Y2O3 increases compared to pure copper, as in this study [17-18]. Also, similar to this study, as the amount of Y2O3 is increased, the hardness increases [9, 17]. The hardness value of the graphite/Y2O3 doped samples (S-5, S-6 and S-7) is lower than that of the doped samples containing only Y2O3(S-2, S-3 and S-4). It is higher than the copper composite sample containing only graphite (S-1). Fig. 7. Wear rate and average COF diagram of the samples. The wear rate-average coefficient of friction (COF) diagram of the samples is displayed in Fig. 7. There is a correlation between the wear and friction characteristics. It is seen that the addition of graphite to the alloy design has a beneficial effect on the friction behavior. The COF decreased approximately twofold in all groups compared to the pure-produced Cu sample (S-0). The highest wear rate was seen in the S-1 group non-Y2O3ceramic and lean graphite content (73.27x10-5mm3/Nm). Wear resistance was improved between 3.301 and 15.239 times in all groups containing different amounts of ceramic and graphite content (S2-S7) and in groups containing lean graphite additive content samples (S-1). It is seen that the wear resistance of Cu alloys increases with the Y2O3 ceramic additive. The lowest wear rates were seen in the groups containing the highest amount of ceramic reinforcement (S-4, 5, 6, and 7). Incorporating graphite into ceramic reinforcing groups enhanced wear performance and improved friction behavior. It has been shown that adding the graphite phase enhanced the wear resistance of ceramic-reinforced samples by 21% to 46%. The COF of the samples' instantaneous alteration graph based on the distance is displayed in Fig. 8. Upon a closer examination of the figures, it becomes evident that the friction behavior remains stable across all samples with varying concentrations of graphite additive and Y2O3 ceramic particle reinforcement. The lean Cu sample (S-0) performed the highest COF regime. Examining the first 60 meters of this sample's friction behavior reveals itis in a high regime with oscillations. Tribochemical wear residues from contact remain at the interface and intensify the wear and friction regime. As a result, this sample had the highest average COF (~0.58). It is observed that the samples with varying concentrations of Y2O3
  9. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________9 ceramic additions exhibit a slight reduction in friction (S-2,3 and 4). Graphite-containing samples show an improvement in friction compared to the lean ceramic additive (S-5, 6, and 7). This situation indicates that the graphite content creates a layer on the surfaces and has a lubricating effect. As the amount of Y2O3 ceramic content in the samples increases, fluctuations in the friction behavior are visible after 70 meters of the test. This phenomenon is particularly evident in the samples (S-4 and S-7) that had the highest concentration of Y2O3ceramic additions. An increased ceramic reinforcement ratio is likely to contact the hard abrasive counter object (100Cr6) on the surface. Contact between the steel counter body and the rigid Y2O3 ceramic reinforcement within the soft Cu matrix caused surface ripples. Fig. 8. COF Variation diagram of the of the samples based on the test distance. Fig. 9. The behavior of the graphite additive and Y2O3 ceramic on the surface at the point of contact. The contact mechanism that occurs on the sample surfaces at the location of friction is illustrated in Fig. 9. Fig. 9a shows the interaction of the graphite additive in the microstructure at the moment of contact. As it is known, graphite is a structure in two-dimensional layers atomically. Layered carbon atoms are in the form of flat sheets stacked on each other. These carbon atom layers are connected in two dimensions. Under the influence of the applied force and increasing distance during the wear test, the graphite in the structure was attached to the interfaces. It also prevented pure steel-MMC substrate contact by forming tribochemical films in the contact interface. It resulted in an overall reduction in the friction behavior of the graphite content samples. Fig. 9b summarizes the situation that occurs in contact with the hard Y2O3 ceramic additive in the sample structure and the opposite object. The hard oxide ceramic reinforcement body comes into contact with the counter body without wearing, while the intermediate soft matrix tears apart and decreases the contact area. Hard-to-hard phase contact
  10. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________10occurs. As a result of this situation, it is seen as a fluctuation in the COF-distance diagram. As the wear distance increases, the intermediate soft matrix suffers a material loss due to the hard counter object, and the friction passes entirely under the contact of two hard objects. Fig. 10. General SEM image of the worn surface of the samples. Following the wear test, the general SEM images of the worn surfaces are displayed in Fig. 10. Every sample has a uniform channel broadness in the worn track. The conductedtest was reliable, and the produced MMC material's overall rigidity was stable. The worn track broadness was measured from three different spots. As can be seen, the broadest worn tracks were measured as 864.303 μm in the S-0 sample and 834.320 μm in the S-1 sample, respectively. It is known that these two samples do not contain any hard Y2O3 reinforcing oxide phase. Examining the worn surfaces reveals noticeable frictions, partial cracks, and
  11. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________11grooves in the sliding direction. It is observed that as the graphite phase is present and the reinforcement Y2O3 ratio in the microstructure is increased, the wear symptoms remain at a minimal level. The worn track widths for the Y2O3 reinforced and graphite content samples vary between 300 μm and 400 μm. These results appear to be consistent when compared with COF data. Fig. 11 shows the EDS analysis of the worn surfaces of the samples. As can be seen from the EDS analysis, Cu, Y, O, Fe, Cr, and C elements were detected on the surfaces. It is an indicator that presents the Fe, Cr, and C atoms in the MMC substrate surfaces transferred from the counterbody object. It shows material transfer from the counter body steel in all samples. Large regions of oxide layer formation are typically seen on the S-0 sample's worn surface EDS analysis (Point 1, Area 2-3). These oxide layers subsequently formed at the initial distances and constantly broke over time due to contact with the opposite object, causing fluctuations in the COF change graph. These oxides, which formed and broke repeatedly in the first 60 meters of the experiment, aggravated the wear. Ruptured particles increase the wear rate of this sample. With increasing test distance, these tribochemical formations remained outside the contact surface, and friction evolved into a regular regime at values of ≈0.5. Sample S-1 was produced using an additive of lean graphite and Cu. The friction behavior is in a significantly reduced regime, as is evident. The graphite in the microstructure is thought to be crushed upon contact and transferred to the surfaces. It was covered onto both the steel counterbody object and the MMC surface. This situation prevents the direct metal contact, thus reducing the friction. Fig. 9a provides a schematic description of the friction-reducing mechanism. It is well-known in the literature that graphite functions as a solid lubricant in friction-based surface couplings [21]. Worn surface EDS analyses of samples containing different amounts of Y2O3reinforcement non-graphite additives are shown in Fig. 11 (S-2, 3, and 4 samples). Upon examining the surfaces of these samples, it is clear that the increased rigid oxide reinforcements within the structure have resulted in a significant increase in abrasive wear resistance. Examining the sample with the lowest content reinforcement phase (S-2 sample), weak Cu-Cr-C-O structures were partially formed on the observed surface. Despite providing some surface protection, the formed oxide layer broke away from the surface as the wear test distance increased, remained at the interface, and caused surface damage as three body abrasive damage. As the amount of reinforcement phase in the samples increased, it was noted that these formations were not visible. The uniform distribution of the reinforcement phase in the microstructure significantly improved the load-carrying ability. As the amount of homogeneously distributed hard Y2O3 oxide phase in the microstructure increases, the probability of encountering an abrasive counterobject at the contact points increases. Due to this situation, the damage caused by abrasive effects on the surface has decreased. Fig. 9b illustrates the contact mechanism that was mentioned above. As a result, the wear rate has decreased, and wear resistance has improved. This situation is mainly observed in samples S-3 and S-4, which have a high reinforcement phase ratio. The chemical affinity of the surfaces decreased with an increase in the hard ceramic reinforcement phase involvement. Since the wear mechanism changed from oxidative to abrasive, there was a decrease in the thin oxide layer formations on the surfaces. These similar results were also seen in samples containing graphite-doped rigid Y2O3 oxide reinforcement. Examining the wear EDS analysis results of the S-5, S-6, and S-7 samples reveals thin oxide layer structures containing Cu-Y-Cr-C on the surfaces and abrasive marks. Samples with low reinforcement ratios also exhibit partial oxidative wear, but samples with high Y2O3 ratios exhibit a primarily abrasive mechanism. The Y2O3 reinforcement ratio correlates with wear resistance in these samples. Compared to the S-2, 3, and 4 samples, the friction regime resulting from the small quantity of graphite present in these samples was in a low steady profile.
  12. P.C.T.Birgin et al.,/Science of Sintering, 57(2025)1-15___________________________________________________________________________12Fig. 11. General EDS analysis image of the worn surface of the samples. The graphite in the structure was plastically deformed by being crushed by a steel counterbody in the contact areas. It was observed that graphite particles broke away from the sample surfaces and were included in the contact interface. Oxides containing graphite moved
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