Abstract:
The aim of this study was to investigate the effect of hexagonal boron nitride (h-BN) addition on copper based alloy system. Varying amounts of h-BN were added to the prepared metal matrix composition as a reinforcing material. The powder mixture was subjected to high-energy mechanical alloying and compacted under 400 N/mm2 and were sintered at 820°C for 2 hours. Following the sintering of the samples characterization procedures were performed after wear and friction testing. The addition of h-BN led to a decrease in dimensional shrinkage after sintering, while the difference between the achieved density and theoretical density increased. XRD results revealed the presence of characteristic peaks attributed to the reinforcement and metallic matrix. The wear tests revealed a significant increase in wear with higher h-BN content under different applied loads. The initial decline in friction coefficient with h-BN addition was followed by an increase as the h-BN content increased.
Keywords: Cu based sliding materials; Powder metallurgy; Mechanical alloying; Wear; Sintering
Сажетак: Циљ овог рада био је да се испита ефекат додавања хексагоналног бор нитрида (h-BN) на систем легура на бази бакра. У припремљену смешу металне матрице додаване су различите количине h-BN као ојачавајућег материјала. Прашкаста смеша је подвргнута високоенергетском механичком легирању и пресована под 400 N/mm2 и синтерована на 820°С током 2 сата. Након синтеровања узорака, поступци карактеризације узорака спроведени су након испитивања хабања и трења. Додатак h-BN довео је до смањења димензионалног скупљања након синтеровања, док је разлика између постигнуте густине и теоријске густине повећана. Резултати XRD открили су присуство карактеристичних пикова који се приписују ојачању и металној матрици. Тестови на хабање су открили значајно повећање хабања са већим садржајем h-BN под различитим примењеним оптерећењима. Почетно смањење коефицијента трења са додатком h-BN праћено је повећањем како се садржај h-BN повећавао.
Кључне речи: клизећи материјали на бази Cu, металургија праха, механичко легирање, хабање, синтеровање.
1. Introduction
Bearing or journal bearing-like parts are widely used with the industrial revolution in the moving parts of most machines and equipment designed and used. The performance, service life and operating characteristics of these parts are affected by their mechanical, tribological, chemical and physical properties as well as the working environments and conditions in which they were used. Many studies have been carried out to improve the performance of these parts and increase their service life [1-3]. Metals such as Cu, Sn, Zn, Al or their alloys used as especially journal bearings materials have good electrical and thermal conductivity. In addition to ease of alloying and having relatively high mechanical strengths, studies on the improvement of wear resistance with reinforcement materials on these materials have been carried out [4,5]. Copper-based metallic plain bearings are generally produced by casting or alloying with powder metallurgy using Sn, Zn or Al elements [6-12]. Ceramic particles such as SiC, Al2O3, BN4, Si3N4, ZrB2, B4C are also added to copper and aluminiumbased alloys in order to improve the wear characteristics and increase their load carrying capacity. As a result of these modifications made with ceramic particles to the alloys aforementioned, it has been reported that the hardness, toughness, fatigue resistance and % elongation are improved. Therefore, as a result of the improvement of these properties the service life of these plain bearings increased [13-17].
Solid lubricating additives are commonly used in sliding bearings to achieve low friction coefficients and, in addition to expected low wear, to ensure smooth operation in both lubricated and dry environments if necessary. The most used materials as solid lubricants are MoS2, WS2, H3BO, graphite, and h-BN, which have a lamellar structure. Graphite, particularly in conjunction with MoS2, is one of the most employed lubricants [18-25]. The very weak Van der Waals bonds between the lamellae in the structure of these solid lubricants break under the influence of friction and wear, creating a sliding effect between two surfaces and contributing to a reduction in the friction coefficient [26].
Solid lubricants are utilized on friction surfaces either in their pure form or by incorporating an additional additive through various coating processes such as PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposition), electroplating, thermal diffusion, or painting [27-31]. In some studies, it has been reported that sulfur-containing compounds (MoS2, CuS2, ZnS) or graphite compounds within the solid lubricant group undergo partial degradation and a decrease in lubrication effectiveness when they react with the matrix material, particularly during sintering or at operating temperatures exceeding 530°C [32].
Furthermore, unlike solid lubricants such as graphite, or sulfur-based compounds, hBN exhibits remarkable thermal stability even at significantly elevated temperatures (up to 900°C) [33]. This exceptional feature of h-BN to retain its structural integrity without undergoing degradation at such high temperatures confers a notable advantage.
The incorporation of h-BN to CuSn10 alloy used in sliding bearings has been reported to reduce the friction coefficient and enhance wear resistance and strength [34,35]. In another study, it was emphasized that in addition to h-BN coating with Mn or Ag additives in a similar alloy, the pre-sintering process increased the metal/lubricant contact interface, reduced the porosity, and increased toughness [36-38].
Moreover, studies on CuZn30 alloy have also been conducted for similar applications alongside CuSn10 alloy as a sliding bearing material [10].
No existing literature has reported a study investigating the combined use of CuSn10 and CuZn30 alloys as sliding bearing materials. Furthermore, there is a lack of research specifically investigating the addition of h-BN to an alloy mixture consisting of equal proportions of Sn and Zn. Additionally, no study has been found that explores the addition of h-BN to an alloy mixture containing equal amounts of Sn and Zn.
In order to address the existing research gap, the present study adopted the mechanical alloying technique to fabricate samples using CuSn10 and CuZn30 metallic powders. The primary objective of the mechanical alloying process was to ensure a homogeneous distribution of Sn and Zn within the final alloy composition. Subsequently, different quantities of h-BN were introduced as a solid lubricant into the samples. Through subsequent high-temperature sintering, a composite sliding bearing alloy was successfully synthesized, featuring a metal matrix reinforced with h-BN ceramic particles. Post-testing, comprehensive characterization procedures were conducted to evaluate the wear and friction properties of the produced samples. This investigation aimed to shed light on the impact of incorporating h-BN into the CuSn10 and CuZn30 alloy system, thereby contributing to the development of advanced sliding bearing materials with enhanced functionality.
2. Materials and Experimental Procedures
The raw materials used in this study, including CuSn10 and CuZn30 alloy powders, as well as Sn and Zn metal powders, were obtained from Nanokar company. The purity of the alloy powders is 99.97%, with an average particle size of approximately 25 µm. The purity of Sn and Zn metal powders is also 99.97%, with an average particle size of around 45 µm. The reinforcing material h-BN, obtained from Chempounds company, has an average particle size of approximately 15 µm. In this study, the samples were prepared using CuZn30, CuSn10, Sn, Zn alloy powders, and h-BN as a reinforcing material. The powders were subjected to a mechanical alloying process for 3 hours at a speed of 1000 rpm. A high-energy ball mill (Retsch) with a water-cooled chamber made of 50 cm3 wolfram carbide (WC) was used for mechanical alloying. The chamber contained 8 WC balls with a diameter of 10 mm. After mechanical alloying, the powder mixture samples were compacted under a pressure of 400 N/mm2 using a cylindrical mold with a diameter of 20 mm. The obtained samples were then sintered at 820°C for 2 hours in a controlled atmosphere furnace under argon gas.
After sintering, dimensional changes on the samples were measured using a digital micrometer with an accuracy of ±1 µm. The density of the sintered samples was determined using an analytical balance (Radwag AS220.R2 model) based on the Archimedean principle. Theoretical densities of the composition components were considered, and the theoretical density of the final composition was calculated and compared with the measured density. Additionally, the volume of the porosity within the samples was calculated.
The hardness of the sintered samples was measured using the Brinell method with a diameter 2.5 mm WC indenter and a 62.5 kg load. For the analysis of porosity, microstructure, phase distribution, and chemical composition of the samples, a scanning electron microscope (SEM) with an integrated energy dispersive spectrometer (EDS) unit (Philips XL30/SFEG) was used. Chemical analyses were conducted in both mapping and spot modes by SEM-EDS.
XRD analysis was conducted using a Bruker D8 Advance XRD instrument. The samples were exposed to X-rays in the range of 20°-100° with a Cu Kα (graphite monochromator) radiation source. The instrument operated at 40 kV and 40 mA, with a scanning speed of 100 /min and a step size of 0.2 degrees per step.
Wear and friction tests were performed using a Bruker UMT2 wear testing machine. The tests were conducted under 1 N and 3 N loads, with a sliding speed of 5 mm/s over a distance of 10 m. A WC ball with a diameter of 5 mm was used during the tests. Following the wear test, the amount of wear and the topographic analysis of the worn surfaces were examined using a Zeiss Smartproof 5 optical profilometer.
3. Results and Discussion
The SEM images in Fig. 1 depict the metallic powders (CuSn10, CuZn30, Zn, Sn) and h-BN powder used in the study. The CuSn10 and CuZn30 alloys, as well as the Zn and Sn metals, exhibit similar morphologies characterized by spherical and ligament shapes, with the presence of some satellite particles. However, h-BN displays a flake-like morphology, with flakes measuring less than 20 µm in size, along with some agglomeration.
3.1. Dimensional Change
Fig. 2 depicts the dimensional change graph of samples with varying concentrations of h-BN after the sintering process. The sample without h-BN exhibited a significant shrinkage of approximately 7%. However, with the incorporation of 0.125 wt.% h-BN, the shrinkage reduced to approximately 4%. Notably, the dimensional change exhibited a linear decrease with an increase in the h-BN content. In the sample containing 3 wt.% h-BN a distinct difference was found compared to other samples: it exhibited a positive (expansion) dimensional change of 4%. This behavior was attributed to the lamellar and planar plate-like structure of the h-BN particles, which hindered the flow of the metallic phase during sintering. Furthermore, it has been reported that one of the reasons for this phenomenon is the variation in relative densities of the leaf-like h-BN particles, which is dependent on the compression pressure and powder size. It has been documented that at similar pressing pressures (400 MPa), the relative density can decrease to as low as 80% [39]. This change in relative density indicates a rearrangement or compaction of the h-BN particles, affecting their ability to impede the flow of metallic components during sintering. Thus, the observed correlation between the h-BN content and dimensional change suggests the potential of h-BN as a controlling factor for the final dimensions and properties of the sintered materials. This phenomenon underscores the significance of understanding the influence of h-BN morphology and content on the overall dimensional stability of the sintered structures. Further investigations are warranted to explore the underlying mechanisms and optimize the h-BN content for desired dimensional characteristics in sintered materials.
3.2. Hardness Variation
The average hardness variation in the samples after sintering, depending on the h-BN content, is shown in Fig. 3. It was observed that the hardness of the samples increased linearly to some extent with the addition of 0.125 wt.% and 0.25 wt.% h-BN, and then decreased linearly with further increase in h-BN content. The hardness value for the sample without hBN was measured at 18 HB, while the maximum hardness value of 23 HB was obtained for the sample containing 0.25 % h-BN, and the minimum hardness value of 9 HB was measured for the sample with 3 % h-BN. Additionally, higher h-BN content in the samples resulted in increased scattering of hardness values. This is believed to be due to the variation in the amount of low-strength lamellar h-BN incorporated into the structure. Considering this in conjunction with the dimensional changes in the samples after sintering (Fig. 2), it can be inferred that the addition of h-BN beyond a certain threshold prevents the material from reaching the desired density. Similar observations have been reported in literature studies where copper matrix materials with Mo2S, which has a similar crystal structure, were added, showing an initial increase in hardness followed by a decrease with increasing Mo2S content [24]. Hence it's not difficult to conclude that the inclusion of solid lubricants such as h-BN and Mo2S in copper matrix materials initially increases hardness, further addition of these lubricants decreases the hardness of the samples.
3.3. Variation in Density
The density variation of samples containing h-BN after sintering, experimentally measured and theoretically calculated according to the linear mixture rule, is presented as a function of h-BN content in Fig. 4. The change in total porosity obtained by calculating the difference between these two densities is also shown as a function of h-BN content in the same graph (Fig. 4).
It can be observed that the theoretical density decreases linearly with increasing h-BN content. The experimentally measured density values, on the other hand, initially increase with the addition of h-BN and then decrease in a linear-like manner with further increase in hBN content beyond 0.75%. The calculated total porosity decreases with the increase in h-BN content initially, but for samples containing more than 0.75 % h-BN, it increases with the increase in h-BN content. The difference between the theoretically calculated density values and the experimentally measured density values is associated with the sintering behavior of the samples and the resulting porosity. Overall, the density and porosity variations observed in the samples, depending on the h-BN content, are consistent with the changes in density (Fig. 2) and hardness (Fig. 3) observed in response to h-BN addition. These findings highlight the interplay between h-BN content, densification, porosity formation, and resulting material properties in the sintered samples.
3.4. XRD Analysis
Fig. 5 displays the XRD patterns obtained from samples with varying amounts of hBN after sintering, along with the corresponding phase identifications based on matching the observed peaks with the relevant database. The intensity of the characteristic peak at approximately 26°, which is attributed to h-BN, was found to increase with an increase in the h-BN content in the samples. These characteristic XRD peaks corresponding to h-BN particles have also been reported in similar studies involving h-BN additions [23,40]. In the XRD patterns of all samples, a peak observed at approximately 39° was determined to be related to the presence of Zn metal in the samples. The characteristic peaks at approximately 43°, 50°and 74° were identified to be associated with the main component of Cu in the samples. Additionally, the peaks at approximately 89° and 94° were attributed to the phases Cu5Zn8 and δ-Cu41Sn11, respectively.
3.5. SEM and SEM-EDS Analysis
Fig. 6 presents the SEM images of samples with varying amounts of h-BN after the sintering process. In the h-BN-free sample (OD-1), the structure reveals the presence of isolated pores, suggesting a generally high-density sintering. It is observed that the number of pores in the structure initially decreases with the increase in h-BN content, and then increases with the further addition of h-BN. The calculated porosity values (Fig. 4) and the observed pore quantities in the SEM images of the samples exhibit a similar trend. It can be stated that the addition of h-BN prevents excessive pore growth. Additionally, the highest content of hBN hinders the sintering of metallic powders. The pores/h-BN network formed among the metallic matrix particles creates a continuous network, which negatively affects both the density increase and the mechanical properties.
In the sample containing 0.125 wt.% of h-BN (OD-2), it can be observed that the porosity is reduced compared to the OD-1 sample. The obtained SEM-EDS results (Fig. 7 and Fig. 8) reveal the presence of h-BN particles in some of the regions considered as pores. This observation becomes more evident upon closer examination in Fig. 8. The SEM-EDS analysis result of the region marked as A1 in the SEM image (Fig. 8) is determined to be rich in h-BN, while the region marked as A2 is identified as a copper-rich matrix. This indicates that the hBN phase is present only in the pores or between the grains and does not dissolve in the metallic matrix (Fig. 8). Additionally, it is speculated that in some of the regions considered as entirely porous, the h-BN particles may have been dislodged or removed during sample preparation due to their soft nature.
The SEM image and SEM-EDS analysis results obtained from the sample with 3 wt.% h-BN (OD-6) are presented in Fig. 9. Upon examination of these results, it can be observed that hBN particles are distributed in a three-dimensional network within the structure. The presence of a partially porous structure and the partial removal of h-BN from some of the voids indicate that there is not a strong chemical bond between the metallic powders and h-BN during the sintering process.
3.6. Wear Test and Coefficient of Friction (COF)
Fig. 10 illustrates the variation in wear volume of samples containing different weight percentages of h-BN after undergoing a wear test at 1N and 3N loads, with a sliding speed of 5 m/s over a distance of 10 meters.
The wear test results revealed that the wear characteristics were similar for both loads of 1N and 3N, indicating that the presence of h-BN had a consistent effect on the wear behavior of the samples. However, higher wear volumes were obtained in the tests conducted under 3N load. Additionally, for samples with h-BN content up to 0.75 wt.%, the variation in wear rate was not significant under both loads. However, in samples with higher h-BN content, the wear volume increased dramatically, and this increase was more pronounced under the 3N load. This suggests that a higher content of h-BN led to a decrease in wear resistance, possibly due to the formation of a more porous and less mechanically stable structure. When the wear volume results are evaluated in conjunction with the hardness variation results (Fig. 3), it can be seen that as the hardness decreases, especially in samples with high h-BN content, the wear volume increases. In conclusion, the incorporation of h-BN in the samples affected their wear behavior, with higher h-BN content leading to increased wear volumes. The applied load and the hardness of the material were found to be important factors influencing the wear resistance. These findings provide valuable insights for the development and optimization of materials with improved wear properties, particularly in applications where high loads and abrasive conditions are involved.
The surface images obtained by optical microscopy of the wear tracks after the wear tests are shown in Fig. 11 and Fig. 12. Under a 1N load, the highest wear track width is observed in the sample with the highest h-BN content (OD-6), measuring approximately 220 µm, while the sample with the lowest h-BN content (OD-2) has a wear track width of approximately 100 µm. Under a 3N load, the widest wear track width is found in the sample with the highest h-BN content (OD-6), measuring approximately 370 µm, while the sample with the lowest h-BN content (OD-2) has a wear track width of approximately 250 µm.
Considering the wear track results of the OD-1 sample in conjunction with the density and porosity variations after sintering (Fig. 4), it can be inferred that the higher presence of wear tracks in the OD-2 and OD-3 samples compared to OD-1 is associated with the abrasive ball contact surface. The low density and coarse pore size of the OD-1 sample have led to non-uniform contact and increased contact area. However, the total wear volume remains relatively unchanged among all three samples (Fig. 10). Furthermore, it is observed that as the h-BN content increases in the samples, the width and continuity of the wear tracks increase during the wear tests. This can be attributed to the abundant presence of h-BN, which results in both the wear from the h-BN-rich regions and the smearing of the metallic matrix onto the sample surface during the wear test. In studies where h-BN is used as a solid lubricant with different metallic phases, it has been reported that layered-type wear, characterized by the formation of distinct layers, can occur after wear tests [25]. Additionally, an increase in the hBN content leads to an increase in both the width and depth of the wear tracks, consequently resulting in an overall increase in the wear volume. This observation is also evident in the 3D optical profilometer results shown in Fig. 13.
When comparing the results of wear tests conducted under 1N and 3N loads, it is evident that the increase in load leads to a significant increase in the width of the wear tracks and, consequently, an increase in the wear volume (Fig. 11 and Fig. 12). The optical profilometer results also confirm this trend (Fig. 13). So, the results indicate that the h-BN content, density, porosity, and applied load all have significant effects on the wear behavior of the samples. Understanding these relationships is crucial for optimizing material formulations and processing parameters to achieve desired wear resistance properties in practical applications.
The variation of the COF for samples with different weight percentages of h-BN under 1 N and 3N loads during a 4000-second duration of the wear test is shown in Fig. 14 and Fig. 15. The results indicate that the COF varied depending on the h-BN content. Under a 1N load, the COF ranged approximately from 0.50 to 0.85, while under a 3N load, it ranged approximately from 0.40 to 0.75. It is possible to divide the COF variation graph based on time into two regions regardless of the load. In the first region observed for all samples (1N for 1000 sec, 3N for 500 sec), the COF appears to increase linearly regardless of the h-BN content. In this region, it can be inferred that there is an adhesive wear mechanism independent of the applied load. After the linear behavior, it is possible to observe a second region in the graphs. In this region, the COF-duration graphs show a different variation depending on the h-BN content. This is attributed to a change in the wear mode between the abrasive ball and the samples.
As the test duration increases, a change in the wear mode occurs, and in the second region, abrasive wear is observed. In this region, the lubricating effect of h-BN is particularly evident, especially under 3N load (Fig. 15). Comparing the samples without h-BN and the samples with h-BN content up to 0.75 wt.%, it can be observed that the COF decreases due to the lubricating nature of h-BN that is smeared onto the surface by the abrasive ball. However, when the h-BN content exceeds 0.75 wt.%, the weakening mechanical properties of the samples lead to increased deformation under load, resulting in increased interaction area/depth with the abrasive ball and an increase in the COF.
In a similar study conducted with the addition of h-BN as a solid lubricant in a different matrix composition, it has been reported that the COF initially decreases with h-BN addition but increases with further increase in h-BN content [41,42]. Similar observations have been made in studies using Mo2S as a solid lubricant, where the wear volume initially decreases with the addition of the lubricant, but increases with higher additive content [43].
4. Conclusion
A metal matrix composition was prepared using CuSn10 and CuZn30 alloy powders, along with Sn-Zn metallic powders, and varying amounts of h-BN as a reinforcing material. The powder mixture underwent a high-energy mechanical alloying process and was then compressed under a pressure of 400 N/mm2 . After the mechanical alloying and sintering process at 820°C for 2 hours in a controlled atmosphere furnace, the samples were subjected to characterization procedures and wear and friction tests.
The conclusions of this study can be summarized as follows:
* The dimensional shrinkage decreases with an increase in h-BN content after sintering in Cu-Sn-Zn metallic matrix sliding bearing alloys. However, samples containing 3 wt.% h-BN showed an increase in sample dimensions (expansion).
* The density of sintered samples increased up to 0.75 wt.% h-BN, but decreased with further addition of h-BN.
* The average hardness values of the samples increased slightly with the addition of 0.125 wt.% and 0.25 wt.% h-BN, but decreased approxiamately linearly with further addition of h-BN.
* During sintering, the formation of Cu5Zn8 and δ-Cu41Sn11 phases was identified, but no reaction was observed between the Cu-Sn-Zn metallic phase and the h-BN phase, according to the XRD results.
* The wear tests showed that both an increase in load and an increase in h-BN content lead to an increase in wear volume.
* The COF decreased in samples with h-BN content up to 0.75 wt. %, but increased with higher h-BN additions.
* The COF -wear duration graphs clearly indicate two distinct regions, suggesting the presence of two different wear mechanisms.
Overall, the addition of h-BN affects the properties and wear behavior of the Cu-Sn-Zn alloys, with optimal h-BN content resulting in improved performance. The presence of h-BN in the metal matrix contributed to the formation of a protective tribolayer on the sliding surfaces, reducing direct metal-to-metal contact and minimizing wear.
Acknowledgments
This study was supported by Hema Industry R&D Center, Gebze Technical University and Yıldız Technical University, where laboratory and infrastructure equipment are used. We would also like to thank Prof. Mehmet Tarakçı and Mr.Cantekin Kaykılarlı for his help and support within the scope of the study.
*) Corresponding author: [email protected]
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Abstract
The aim of this study was to investigate the effect of hexagonal boron nitride (h-BN) addition on copper based alloy system. Varying amounts of h-BN were added to the prepared metal matrix composition as a reinforcing material. The powder mixture was subjected to high-energy mechanical alloying and compacted under 400 N/mm2 and were sintered at 820°C for 2 hours. Following the sintering of the samples characterization procedures were performed after wear and friction testing. The addition of h-BN led to a decrease in dimensional shrinkage after sintering, while the difference between the achieved density and theoretical density increased. XRD results revealed the presence of characteristic peaks attributed to the reinforcement and metallic matrix. The wear tests revealed a significant increase in wear with higher h-BN content under different applied loads. The initial decline in friction coefficient with h-BN addition was followed by an increase as the h-BN content increased.