- Email: [email protected]
Copper-alumina nanocomposite coating on copper substrate through solution combustion
Author’s Accepted Manuscript Copper-Alumina Nanocomposite Coating on Copper Substrate through Solution Combustion E. Mohammadi, H. Nasiri, J. Vahdati Khaki, S.M. Zebarjad www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(17)32549-X https://doi.org/10.1016/j.ceramint.2017.11.094 CERI16749
To appear in: Ceramics International Received date: 23 October 2017 Revised date: 13 November 2017 Accepted date: 13 November 2017 Cite this article as: E. Mohammadi, H. Nasiri, J. Vahdati Khaki and S.M. Zebarjad, Copper-Alumina Nanocomposite Coating on Copper Substrate through Solution Combustion, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.11.094 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Copper-Alumina Nanocomposite Coating on Copper Substrate through Solution Combustion E. Mohammadia, H. Nasirib*, J. Vahdati Khakia, S.M. Zebarjadc a: Department of Metallurgy and Materials Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran. b: Department of Materials Engineering,, Birjand University of Technology. Birjand. Iran. c: Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran. * Corresponding author. E-mail address: [email protected], [email protected]. Tel/Fax: +985632252181
Abstract The main objective of the present research is to investigate the production of Cu-Al2O3 nanocomposite coating on a copper substrate using solution combustion synthesis. Solution combustion synthesis is mainly used to produce nanocomposite powders; however, in this study it is applied to produce nanocomposite coat. For this purpose, both copper and aluminum nitrates (Cu (NO3)2.3H2O and Al (NO3)3.9H2O) are used as oxidizers. Also, urea and graphite are respectively used as fuel to synthesize the Cu-Al2O3 nanocomposite and as inhibitor to prevent the oxidation of the synthesized copper. The microstructure and morphology of the nanocomposite coating, which includes 25wt.% alumina as the reinforcing phase, was studied using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy at different fuel/oxidizer ratios ranging from 0.9 to 2. The temperature variation during the process was measured as a function of time using a precise thermocouple. Finally, micro-hardness and wear tests were conducted on the nanocomposite coating. The results verified the formation of Cu-Al2O3 nanocomposite coating. Time-temperature curve illustrated that the highest temperature was achieved at the fuel/oxidizer ratio of 1.25. The results of the microhardness and wear resistance test showed that these properties depend heavily on the fuel/oxidizer ratio, with the best condition attained at the ratio of 1.25. Keywords: B. Nanocomposites, C. Hardness, C. Wear resistance, D. Al2O3. 1.
Introduction
Owing to its high electrical and thermal conductivity, copper is perhaps the most beneficial of all well-known metals. However, one of the most significant problems associated with copper is its low surface mechanical properties such as hardness, wear resistance, and yield strength. Many studies have investigated methods to improve
1
the mechanical behaviors of copper without compromising any of its intrinsic advantages [1-3]. Towards this end, the application of nanocomposite coatings is shown to be a promising solution [4]. In this type of coating, a metal acts as a matrix and a second compound, such as oxides or carbides, is used as the reinforcing phase, e.g. Cu-TiO2, Cu-SiC, Cu-Al2O3 [5-6]. Among different options for use as a reinforcing phase, alumina is a strong and viable option owing to its reasonable price and stability at high temperatures [7]. There are numerous industrial methods to coat such nanocomposite films, such as electro plating or spraying. However, the application of such methods is limited by the expensive, complex and time-consuming processes. Several research studies have focused on approaches to overcome such limitations [8-11]. As a combination of self-sustained combustion and reactive solution, solution combustion synthesis (SCS) is a type of self-propagating high-temperature synthesis method that can produce homogenous and specific nanomaterials [12, 13]. This method is based on oxidation-reduction reactions, metal nitrate(s) act as oxidizer and fuel(s) such as: urea, glycine, hydrazine and etc. act as reducing agent. The reaction between fuel and oxygen species results in a rapid synthesis. The following advantages render SCS a unique method among other alternatives: (a) all reactants are solved homogenously as a result of which a precise formulation is achieved in the final product; (b) the process can be completed in one step, making extra processes such as heat treatments unnecessary; and (c) the essential energy needed for the synthesis is extracted from the internal energy of the materials [14-18]. Briefly, SCS is an economically viable method which does not require complex facilities [19]. The fabrication of copper-alumina nanocomposite powder has conventionally required an additional step, e.g., reduction in hydrogen atmosphere, to reduce the copper oxide into pure copper. Nevertheless, in recent years, researchers were able to manufacture copper-alumina nanocomposite powder in only one step, using carbon to prevent copper oxidation [7]. The carbon approach is applied for in-situ production of the copper-alumina nanocomposite coating. Overall, SCS is potentially an easy, cost-effective, and fast method for the manufacturing of copper-alumina nanocomposite coatings. The present work aims to demonstrate that SCS can be effectively used to coat Cu-Al2O3 nanocomposite on copper in a single step. This coating can noticeably improve the hardness and wear resistance of the substrate surface. 2.
Materials and methods
Copper nitrate (Cu (NO3)2.3H2O>99.5%), aluminum nitrate (Al (NO3)2.9H2O>99.5%) are used as oxidizer, and urea ((NH2)2CO>98.5%) as fuel to produce Cu-Al2O3 nanocomposite coating. Also, as an auxiliary material, graphite is
2
mixed with the nitrates and urea before adding water to the mixture. The amounts of copper and aluminum nitrates for the coating are calculated so that 25wt.% of reinforcing phase is achieved according to Eq. 1. 4.4Al (NO3)3 + 10.5Cu (NO3)2 + 32CO(NH2)2→ 10.5Cu + 2.2Al2O3 +32CO2 + 64H2O+49.1N2
Eq. 1
Graphite is added to the solution as an inhibitor to stop the copper oxidation. According to Nasiri et al. [7], the optimum amount of graphite for the production of the exact nanocomposite powder is achieved at 25% of stoichiometric value. Also, the fuel to oxidizer (F/O) ratio can be measured using the method proposed by Jain and Adiga [20]. All reactants are mixed in the minimum amount of distilled water, i.e., 2CC. A copper plate of 10 × 10 × 3 mm dimensions is used as substrate. Prior to the coating, the plate is polished using 1200 grit silicon carbide grinding paper. The plate is then washed with distilled water and soap and finally rinsed with acetone in an ultrasonic bath for 30 minutes. The solution with reactants is placed on a hotplate at 300 °C. The gel combustion begins once the water is fully vaporized. X-ray diffraction (XRD) was used to identify the phases. Patterns were obtained using a Philips X`perts diffract meter with Cu Kα radiation (λ=0.15406 nm) in the range of 2θ= 4-90°. An Advantech USB 4718 data acquisition board was used to plot the behavior of the solution and to record ignition and combustion temperatures at different F/O ratios. This device is connected to a computer on one end and to the solution on the other, using a USB cable and a K-type thermocouple, respectively. The data is transferred from the solution to the computer with the ratio of 10 data/sec and the time-temperature diagrams (TTDs) are plotted in real time. The surface morphology and microstructure of the coating were investigated using a LEOVP1450 scanning electron microscope (SEM) and a PhilipsCM120 transmission electron microscope (TEM). Prior to TEM analysis, the surface of the coated sample (with the F/O ratio 1.25) was first cut and then treated with a solution of nitric acid and distilled water (with the ratio of 1:1) to dissolve the copper. Subsequently, the residue was washed in distilled water. Next, the resulting materials were placed in an oven to dry at 70 °C for 1 hour. The microhardness of the samples was determined using a Buehler-1600-6125 Vickers micro-hardness tester. Ten hardness measurements were taken using a 25gr load for each sample. The wear test was conducted using the pin-on-disc method. For this test, every cubically shaped specimen, i.e., 10×10×3 mm, had a contact area of 100 mm2. Samples were loaded against a disc which rotated at a sliding velocity of 0.12 m/s. An 800-grit silicon carbide grinding paper was attached to the disk and was replaced after each test. Three levels of loads were applied, namely 1, 3, and 5N. The sliding distance was
3
kept constant at 50 meters for all samples. The abrasive wear rate was calculated by dividing the difference in the weight of specimens before and after the tests (in milligram) by the sliding distance (in meter). The weight was measured with the accuracy of 0.001 milligrams. 3.
Result and Discussion
Fig 1 illustrates the XRD patterns of the coated sample at the F/O ratio of 1.25. Fig.1a belongs to graphite free sample. As it can be seen, there are Cu2O peaks with considerable intensity that shows coating prepared by synthesis without graphite has considerable oxides. Fig.2b illustrates the result of synthesis in the same conditions plus 25wt.% graphite as inhibitor to prevent the oxidation of the synthesized metallic copper. It shows the metallic copper peaks increased meanwhile the copper oxide peaks decreased.
Fig. 1. XRD patterns of synthesis products with F/O=1.25 a: without graphite and b: 25wt.% calculated graphite. The mechanism of metallic copper formation during SCS is as follows: after reaching the ignition temperature (Tig), i.e., temperature required for starting the combustion reaction, urea decomposed to CO2 and NH3 (Eq. 2) and it is not only a source of fuel, but it also acts as a reducing agent [21]. CO (NH2)2 + H2O → CO2 + 2NH3
Eq. 2
As the decomposition of the fuel continues, copper and aluminum nitrates also begin to decompose to alumina and copper oxide, as shown in Eq. 3 and Eq. 4. After full decomposition of copper and aluminum nitrates, NO2 and NH3 start to react according to Eq. 5 [7]. 2Cu (NO3)2→ 2CuO + 4NO2 + O2
Eq. 3
2Al (NO3)3 → Al2O3 + 6NO2 + 1.5 O2
Eq. 4
NH3 + 10NO2→ 9N2 + 12H2O + 4O2
ΔH= -2865.4 KJ
Eq. 5
As shown in Eq. 5, this reaction releases a considerable amount of energy, which implies the sudden increase in temperature during the coating process. The product of the fuel decomposition reduces copper oxide to metal
4
copper. The presence of graphite significantly prevents any oxidation of copper. Eq. 6 demonstrates the role of graphite in reduction of copper oxide, and prevention of copper oxidation [7]. CuO + C = Cu + CO
Eq. 6
Unlike copper oxide, alumina has a stable composition in nature, which cannot be reduced by the amount of urea present at the recorded temperature. In order to precisely detect the alumina phase, the residue of the copper solution is analyzed by XRD. As shown in Fig. 2, the formation of alumina phase can be observed in the XRD pattern. TTDs of the solutions with different F/O ratios (0.9~2) and with 25wt.% of alumina are shown in Fig. 3. According to Fig. 3, the combustion temperature for the sample with F/O=1.25 is about 1000°C and hence, the synthesized alumina is alpha-phase. Alpha alumina is produced at temperatures greater than 800°C [22].
Fig. 2. XRD pattern for synthesized residue products (F/O=1.25) after solving in nitric acid.
Fig. 3. TTDs for F/O ratios, a: 0.9, b: 1.0, c: 1.25, d: 1.5, e: 1.75 and f: 2.0. As shown in this figure, the first step, which lasted about 3 minutes, consisted of water vaporization at 100°C. Next, the water molecules started to vaporize and the temperature slowly increased to 250°C. The gradual rise of temperature stopped as it drew nearer to the ignition temperature. Once the ignition temperature was reached, the synthesis occurred rapidly. Note that all the otherwise overlapping diagrams with different F/O ratios are either shifted to the right or to the left in Fig. 3. Therefore, there was no correlation between amount of fuel and the ignition delay time. As shown in diagrams 3a and 3b, the recorded combustion temperature was low due to
5
insufficient amount of fuel. It can be attributed to the fact that some part of the fuel started to decompose before reaching the reaction temperature because the hydrolyzation of urea starts at 80 °C which is less than the ignition temperature (200°C) [23]. Consequently, the insufficient amount of fuel at the ignition time, at both F/O ratios of 0.9 and 1, prevented the reaction from taking place fully, as shown in Fig. 3-b [21]. The highest temperature was obtained at the F/O ratio of 1.25. The excessive weight of fuel in the solution at the F/O ratio of 1.25 compensated for the amount of hydrolyzed urea, allowing the synthesis to come to a full finish. Also, the combustion temperature steeply increased to 975°C at this ratio, as shown in Fig. 3-c. But, at F/O ratio of 1.5, the high volume of CO2, H2O, and N2 gases led to loss of heat and, in turn, to the decrease of the combustion temperature, as shown in Fig. 3-d. Another reason for the decrease in the combustion temperature was the extra fuel, which acted as a diluent in the solution [24], especially at the beginning of the synthesis. This led to an increase in heat dissipation. By increasing the amount of fuel, the effect of fuel as a diluent rose remarkably for the F/O ratio of 1.75, and therefore the reaction became unstable. This phenomenon can be observed in Fig. 3-e, where the peak appearing after the combustion temperature indicates that the decomposition reaction together the combustion reaction contributes in this sample. The overwhelming amount of fuel at F/O ratio of 2 further corroborates the diluting effect of the fuel and how this prevented the reaction from happening, as shown in Fig. 3-f. Figs. 4-a and 4-b show micrographs of the coated sample at F/O ratios of 1.25 and 1.5, respectively.
Fig. 4. SEM images for samples coated at: a: F/O=1.25, b: F/O=1.5. As shown in these figures, some porosity can be discerned in the micrographs, which is due to the intrinsic behavior of SCS [23]. It is clear that the amount of porosity depends heavily on the gas content generated during SCS, and this is why the sample at F/O ratio of 1.5 has more porosity than the one with F/O ratio of 1.25. Fig. 5 indicates the EDS pattern of the coated sample at F/O ratio of 1.25.
6
Fig. 5. SEM image and EDS analyses for sample coated with F/O=1.25. The detected peaks represent aluminum, copper and oxygen, verifying the formation of alumina and copper phases. The undetected peak at the right side of the copper peak belongs to platinum as a platinum coating was applied on the sample to improve the quality of the SEM images. The results of Vickers hardness test for different F/O ratios are presented in Fig. 6.
Fig. 6. Vickers hardness results for different ratios of F/O. At first glance, it is obvious that the hardness of the coated samples is considerably higher than the ones of uncoated samples. At the first two ratios, i.e., 0.9 and 1.0, such an improvement can be attributed to the moderate amount of rough alumina phase with which the synthesis reaction was performed. Fig. 3 indicates that for samples with F/O ratios of 0.9 and 1.0, the combustion temperature slightly increased, which was not enough to complete the reaction. Consequently, compared to the uncoated sample, the hardness of the coated samples only increased marginally. Nevertheless, by increasing the F/O ratio to 1.25, the temperature rose remarkably, reaching approximately 1000°C, and the reaction fully completed. At this temperature, crystalline alpha alumina was produced, which led to a significant increase in hardness. However, as shown in Fig. 4, further increase of fuel, i.e., F/O ratios of 1.5 and 1.75, produced more gases, as suggested by Eq. 1, which in turn resulted in increased porosity and decreased hardness. Fig. 7 shows the results of wear test for 1, 3 and 5 N loads at different F/O ratios.
7
Fig. 7. Results of wear test for 1, 3 and 5 N loads for different F/O ratios. As the results in Fig. 7 depict, the wear rate was higher for the coated samples at all fuel ratios. However, the wear resistance was smaller for the samples with the F/O ratios of 0.9 and 1 than the sample with F/O ratio of 1.25, mainly because of their lower hardness. By increasing the F/O ratio to 1.25, the wear resistance reached a maximum value because the reaction was completely performed. However, as mentioned before, by further increasing the amount of fuel to F/O ratios of 1.5 and 1.75, the excess fuel acted as a diluent and caused the combustion temperature to decrease. Also, the large amount of gases produced during the synthesis increased the porosity, which eventually led to decrease in the wear resistance, although it still remains higher than the resistance of uncoated samples. Despite the fact that it is argued otherwise in some literature [6], in the light of the results of the present study, it is firmly believed that there is a direct correlation between the wear resistance and hardness in nanocomposite coatings. As shown in Fig. 3, the entire process lasted almost 7 minutes and it only needed such sources of energy as hotplate or hot wire to start the reaction. This suggests a remarkable improvement over electroplating-based coatings, where it takes nearly 1 hour to achieve the same hardness and wear resistance (at 17.8 wt.% alumina the micro hardness increased from 65 to 157 Vickers and at 4N load the wear rate decreased from 0.0004 to 0.0001 gr/m). Moreover, electroplating-based coating method is inherently more complicated as it requires additional pieces of equipment for checking the process and there are numerous parameters that play a role in the reaction [6]. Figs. 8a and 8b show SEM images of the surface of uncoated and coated samples, respectively.
Fig. 8. SEM image after wear test for (a): uncoated substrate and (b): coated substrate with F/O ratio=1.25, 5000X.
8
As shown in Fig. 8a, sever plastic deformation in the uncoated sample, with much wider and deeper scratches, confirms the trends observed in Figs. 6 and 7, i.e., the alumina-copper nanocomposite coating improves the hardness and wear resistance. Fig. 9 shows a TEM image of the alumina phase after ensuring that the alumina is formed based on Fig. 2.
Fig. 9. TEM image of Al2O3 particles. Based on the TEM images, the size of the obtained particles was in the range of 20-50 nanometers. TEM image proves that the nanocomposite coating was formed in a single step. 4.
Conclusions
The produced Cu-Al2O3 nanocomposite coating in this study improved the mechanical behavior of the surface of copper substrate. The findings of this study are summarized as below: 1- Copper-alumina composite coating was successfully synthesized using solution combustion synthesis (SCS) method; 2- The Cu-Al2O3 coating increased the wear resistances and hardness of the substrate up to three times more than the uncoated copper substrate; 3- The amount of porosity on the surface increased in line with the rise in the F/O ratio; 4- Optimum hardness and wear resistance was achieved at F/O ratio of 1.25; 5- There was a direct correlation between the hardness and wear resistance of the SCS-based coatings. References [1] A. Liu, H. Zhu, Z. Guo, Y. Meng, G. Liu, E. Fortunato, R.o Martins and F. Shan, Solution Combustion Synthesis: Low-Temperature Processing for p-Type Cu:NiO Thin Films for Transparent Electronics. Adv. Mater. 15 (2017) 1-8
9
[2] M. Lekka, G. Zendron, C. Zanella, A. Lanzutti, L. Fedrizzi, P.L. Bonora, Corrosion properties of micro- and nanocomposite copper matrix coatings produced from a copper pyrophosphate bath under pulse current, Surf. Coat. Technol. 205 (2011) 3438-3447. [3] F. Chen, J. Ying, Y. Wang, S. Du, Z. Liu, Q. Huang. Effects of graphene content on the microstructure and properties of copper matrix composites. Carbon. 96 (2016) 836-842. [4] A. Arman, C. Luna, M. Mardani, F. Hafezi, A. Achour, A. Ahmadpourian. Magnetoresistance of nanocomposite copper/carbon thin films. J. Mater. Sci. - Mater. Electron. .6 (2017) 4713–4718. [5] A. Fathy, A. A. Megahed, Prediction of abrasive wear rate of in situ Cu – Al2O3 nanocomposite using artificial neural networks, Int J AdvManufTechnol. 62 (2012) 953 – 963. [6] S.R.Allahkaram, S.Golroh, M.Mohammadalipour, Properties of Al2O3nano-particle reinforced copper matrix composite coatings prepared by pulse and direct current electroplating, Mater. Des. 32 (2011) 4478–4484. [7] H. Nasiri, J. Vahdati Khaki, S.M. Zebarjad, One-step fabrication of Cu–Al2O3nanocomposite via solution combustion synthesis route, J. Alloys Compd. 509 (2011) 5305–5308. [8] A. Sova, R. Maestracci, M. Jeandin, Ph. Bertrand, I. Smurov. Kinetics of composite coating formation process in cold spray: Modelling and experimental validation. Surf. Coat. Technol. 318 (2017) 309-314. [9] A. Amri, Xi. Duan, C. Yin, Z. Jiang, M. Rahman, T. Pryor. Solar absorptance of copper–cobalt oxide thin film coatings with nano-size, grain-like morphology: Optimization and synchrotron radiation XPS studies. Appl. Surf. Sci. 275 (2013) 127-135. [10] Y. Wen, W. Huang, B. Wang, A novel method for the preparation of Cu/Al2O3nanocomposite, Appl. Surf. Sci. 258 (2012) 2935–2938. [11] Z. Arabgol, H. Assadi, T. Schmidt, F. Gärtner, T. Klassen. Analysis of Thermal History and Residual Stress in Cold-Sprayed Coatings. J. Therm. Spray Technol. 23 (2014) 84-90. [12] T. Shi, L. Guo, J. Hao, C. Chen, J. Luo, Z. Guo. Microstructure and wear resistance of in-situ TiC surface composite coating on copper matrix synthesized by SHS and Vacuum-Expendable Pattern Casting. Surf. Coat. Technol. 324 (2017) 288-297.
10
[13] S. Aruna, C. Sanjeeviraja, N. Balaji, N. Manikandanath. Properties of plasma sprayed La2Zr2O7 coating fabricated from powder synthesized by a single-step solution combustion method. Surf. Coat. Technol. 219 (2013) 131-138. [14] S.T. Aruna, A.S. Mukasyan, Combustion synthesis and nanomaterials, Curr. Opin. Solid State Mater. Sci. 10 (2008) 44-50. [15] A. S. Mukasyan and P. Dinka, Novel Approaches to Solution–Combustion Synthesis of Nanomaterials, ISSN 1061-3862, Int. J. Self Propag. High Temp. Synth. 16 (2007) 23–35. [16] A. S. Mukasyan, P.Dinka, Novel Method for Synthesis of Nano-Materials: Combustion of Active Impregnated Layers, Adv. Eng. Mater. 9 (2007) 653–657. [17] K.Deshpande, A. Mukasyan, A.Varma, Direct Synthesis of Iron Oxide Nanopowders by the Combustion Approach: Reaction Mechanism and Properties, Chem. Mater. 16(2004)4896 – 4904. [18] S. L. González-Cortés, F. E. Imbert, Fundamentals, properties and applications of solid catalysts prepared by solution combustion synthesis (SCS), Appl. Catal. A: Gen, 452 (2013) 117-131. [19] B. Roy, K. Loganathan, H.N. Pham, A.K. Datye. C.A. Leclerc, Surface modification of solution combustion synthesized Ni/Al2O3 catalyst for aqueous-phase reforming of ethanol, Int. J. Hydrogen Energy.35 (2010) 11700-11708. [20] S.R.Jain, K.C.Adiga, A New Approach to Thermochemical Calculations of Condensed Fuel-Oxidizer Mixtures,Combust. Flame.40 (1981)71-79. [21] Choong-Hwan Jung, SahilJalota, Sarit B. Bhaduri, Quantitative effects of fuel on the synthesis of Ni/NiO particles using a microwave-induced solution combustion synthesis in air atmosphere, Mater. Lett.59 (2005) 2426 – 2432. [22] H. Nasiri, J. Vahdati Khaki, N. Shahtahmassebi, Effect of alumina percentage on size and superparamagnetic properties of Ni-Al2O3 nanocomposite synthesized by solution combustion, Mater. Des. 109 (2016) 476–484.
[23] K. Tahmasebi, M.H. Paaydar, Microwave assisted solution combustion synthesis of alumina-zirconia, ZTA, nanocomposite powder, J. Alloys Compd.509 (2011) 1192-1196.
11
[24] H.Nasiri, E.BahramiMotlagh, J.VahdatiKhaki,S.M.Zebarjad, Role of fuel/oxidizer ratio on the synthesis conditions of Cu-Al2O3nanocomposite prepared through solution combustion synthesis, Mater. Res. Bull. 47 (2012) 3676–3680
12
PII: DOI: Reference:
S0272-8842(17)32549-X https://doi.org/10.1016/j.ceramint.2017.11.094 CERI16749
To appear in: Ceramics International Received date: 23 October 2017 Revised date: 13 November 2017 Accepted date: 13 November 2017 Cite this article as: E. Mohammadi, H. Nasiri, J. Vahdati Khaki and S.M. Zebarjad, Copper-Alumina Nanocomposite Coating on Copper Substrate through Solution Combustion, Ceramics International, https://doi.org/10.1016/j.ceramint.2017.11.094 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Copper-Alumina Nanocomposite Coating on Copper Substrate through Solution Combustion E. Mohammadia, H. Nasirib*, J. Vahdati Khakia, S.M. Zebarjadc a: Department of Metallurgy and Materials Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran. b: Department of Materials Engineering,, Birjand University of Technology. Birjand. Iran. c: Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran. * Corresponding author. E-mail address: [email protected], [email protected]. Tel/Fax: +985632252181
Abstract The main objective of the present research is to investigate the production of Cu-Al2O3 nanocomposite coating on a copper substrate using solution combustion synthesis. Solution combustion synthesis is mainly used to produce nanocomposite powders; however, in this study it is applied to produce nanocomposite coat. For this purpose, both copper and aluminum nitrates (Cu (NO3)2.3H2O and Al (NO3)3.9H2O) are used as oxidizers. Also, urea and graphite are respectively used as fuel to synthesize the Cu-Al2O3 nanocomposite and as inhibitor to prevent the oxidation of the synthesized copper. The microstructure and morphology of the nanocomposite coating, which includes 25wt.% alumina as the reinforcing phase, was studied using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy at different fuel/oxidizer ratios ranging from 0.9 to 2. The temperature variation during the process was measured as a function of time using a precise thermocouple. Finally, micro-hardness and wear tests were conducted on the nanocomposite coating. The results verified the formation of Cu-Al2O3 nanocomposite coating. Time-temperature curve illustrated that the highest temperature was achieved at the fuel/oxidizer ratio of 1.25. The results of the microhardness and wear resistance test showed that these properties depend heavily on the fuel/oxidizer ratio, with the best condition attained at the ratio of 1.25. Keywords: B. Nanocomposites, C. Hardness, C. Wear resistance, D. Al2O3. 1.
Introduction
Owing to its high electrical and thermal conductivity, copper is perhaps the most beneficial of all well-known metals. However, one of the most significant problems associated with copper is its low surface mechanical properties such as hardness, wear resistance, and yield strength. Many studies have investigated methods to improve
1
the mechanical behaviors of copper without compromising any of its intrinsic advantages [1-3]. Towards this end, the application of nanocomposite coatings is shown to be a promising solution [4]. In this type of coating, a metal acts as a matrix and a second compound, such as oxides or carbides, is used as the reinforcing phase, e.g. Cu-TiO2, Cu-SiC, Cu-Al2O3 [5-6]. Among different options for use as a reinforcing phase, alumina is a strong and viable option owing to its reasonable price and stability at high temperatures [7]. There are numerous industrial methods to coat such nanocomposite films, such as electro plating or spraying. However, the application of such methods is limited by the expensive, complex and time-consuming processes. Several research studies have focused on approaches to overcome such limitations [8-11]. As a combination of self-sustained combustion and reactive solution, solution combustion synthesis (SCS) is a type of self-propagating high-temperature synthesis method that can produce homogenous and specific nanomaterials [12, 13]. This method is based on oxidation-reduction reactions, metal nitrate(s) act as oxidizer and fuel(s) such as: urea, glycine, hydrazine and etc. act as reducing agent. The reaction between fuel and oxygen species results in a rapid synthesis. The following advantages render SCS a unique method among other alternatives: (a) all reactants are solved homogenously as a result of which a precise formulation is achieved in the final product; (b) the process can be completed in one step, making extra processes such as heat treatments unnecessary; and (c) the essential energy needed for the synthesis is extracted from the internal energy of the materials [14-18]. Briefly, SCS is an economically viable method which does not require complex facilities [19]. The fabrication of copper-alumina nanocomposite powder has conventionally required an additional step, e.g., reduction in hydrogen atmosphere, to reduce the copper oxide into pure copper. Nevertheless, in recent years, researchers were able to manufacture copper-alumina nanocomposite powder in only one step, using carbon to prevent copper oxidation [7]. The carbon approach is applied for in-situ production of the copper-alumina nanocomposite coating. Overall, SCS is potentially an easy, cost-effective, and fast method for the manufacturing of copper-alumina nanocomposite coatings. The present work aims to demonstrate that SCS can be effectively used to coat Cu-Al2O3 nanocomposite on copper in a single step. This coating can noticeably improve the hardness and wear resistance of the substrate surface. 2.
Materials and methods
Copper nitrate (Cu (NO3)2.3H2O>99.5%), aluminum nitrate (Al (NO3)2.9H2O>99.5%) are used as oxidizer, and urea ((NH2)2CO>98.5%) as fuel to produce Cu-Al2O3 nanocomposite coating. Also, as an auxiliary material, graphite is
2
mixed with the nitrates and urea before adding water to the mixture. The amounts of copper and aluminum nitrates for the coating are calculated so that 25wt.% of reinforcing phase is achieved according to Eq. 1. 4.4Al (NO3)3 + 10.5Cu (NO3)2 + 32CO(NH2)2→ 10.5Cu + 2.2Al2O3 +32CO2 + 64H2O+49.1N2
Eq. 1
Graphite is added to the solution as an inhibitor to stop the copper oxidation. According to Nasiri et al. [7], the optimum amount of graphite for the production of the exact nanocomposite powder is achieved at 25% of stoichiometric value. Also, the fuel to oxidizer (F/O) ratio can be measured using the method proposed by Jain and Adiga [20]. All reactants are mixed in the minimum amount of distilled water, i.e., 2CC. A copper plate of 10 × 10 × 3 mm dimensions is used as substrate. Prior to the coating, the plate is polished using 1200 grit silicon carbide grinding paper. The plate is then washed with distilled water and soap and finally rinsed with acetone in an ultrasonic bath for 30 minutes. The solution with reactants is placed on a hotplate at 300 °C. The gel combustion begins once the water is fully vaporized. X-ray diffraction (XRD) was used to identify the phases. Patterns were obtained using a Philips X`perts diffract meter with Cu Kα radiation (λ=0.15406 nm) in the range of 2θ= 4-90°. An Advantech USB 4718 data acquisition board was used to plot the behavior of the solution and to record ignition and combustion temperatures at different F/O ratios. This device is connected to a computer on one end and to the solution on the other, using a USB cable and a K-type thermocouple, respectively. The data is transferred from the solution to the computer with the ratio of 10 data/sec and the time-temperature diagrams (TTDs) are plotted in real time. The surface morphology and microstructure of the coating were investigated using a LEOVP1450 scanning electron microscope (SEM) and a PhilipsCM120 transmission electron microscope (TEM). Prior to TEM analysis, the surface of the coated sample (with the F/O ratio 1.25) was first cut and then treated with a solution of nitric acid and distilled water (with the ratio of 1:1) to dissolve the copper. Subsequently, the residue was washed in distilled water. Next, the resulting materials were placed in an oven to dry at 70 °C for 1 hour. The microhardness of the samples was determined using a Buehler-1600-6125 Vickers micro-hardness tester. Ten hardness measurements were taken using a 25gr load for each sample. The wear test was conducted using the pin-on-disc method. For this test, every cubically shaped specimen, i.e., 10×10×3 mm, had a contact area of 100 mm2. Samples were loaded against a disc which rotated at a sliding velocity of 0.12 m/s. An 800-grit silicon carbide grinding paper was attached to the disk and was replaced after each test. Three levels of loads were applied, namely 1, 3, and 5N. The sliding distance was
3
kept constant at 50 meters for all samples. The abrasive wear rate was calculated by dividing the difference in the weight of specimens before and after the tests (in milligram) by the sliding distance (in meter). The weight was measured with the accuracy of 0.001 milligrams. 3.
Result and Discussion
Fig 1 illustrates the XRD patterns of the coated sample at the F/O ratio of 1.25. Fig.1a belongs to graphite free sample. As it can be seen, there are Cu2O peaks with considerable intensity that shows coating prepared by synthesis without graphite has considerable oxides. Fig.2b illustrates the result of synthesis in the same conditions plus 25wt.% graphite as inhibitor to prevent the oxidation of the synthesized metallic copper. It shows the metallic copper peaks increased meanwhile the copper oxide peaks decreased.
Fig. 1. XRD patterns of synthesis products with F/O=1.25 a: without graphite and b: 25wt.% calculated graphite. The mechanism of metallic copper formation during SCS is as follows: after reaching the ignition temperature (Tig), i.e., temperature required for starting the combustion reaction, urea decomposed to CO2 and NH3 (Eq. 2) and it is not only a source of fuel, but it also acts as a reducing agent [21]. CO (NH2)2 + H2O → CO2 + 2NH3
Eq. 2
As the decomposition of the fuel continues, copper and aluminum nitrates also begin to decompose to alumina and copper oxide, as shown in Eq. 3 and Eq. 4. After full decomposition of copper and aluminum nitrates, NO2 and NH3 start to react according to Eq. 5 [7]. 2Cu (NO3)2→ 2CuO + 4NO2 + O2
Eq. 3
2Al (NO3)3 → Al2O3 + 6NO2 + 1.5 O2
Eq. 4
NH3 + 10NO2→ 9N2 + 12H2O + 4O2
ΔH= -2865.4 KJ
Eq. 5
As shown in Eq. 5, this reaction releases a considerable amount of energy, which implies the sudden increase in temperature during the coating process. The product of the fuel decomposition reduces copper oxide to metal
4
copper. The presence of graphite significantly prevents any oxidation of copper. Eq. 6 demonstrates the role of graphite in reduction of copper oxide, and prevention of copper oxidation [7]. CuO + C = Cu + CO
Eq. 6
Unlike copper oxide, alumina has a stable composition in nature, which cannot be reduced by the amount of urea present at the recorded temperature. In order to precisely detect the alumina phase, the residue of the copper solution is analyzed by XRD. As shown in Fig. 2, the formation of alumina phase can be observed in the XRD pattern. TTDs of the solutions with different F/O ratios (0.9~2) and with 25wt.% of alumina are shown in Fig. 3. According to Fig. 3, the combustion temperature for the sample with F/O=1.25 is about 1000°C and hence, the synthesized alumina is alpha-phase. Alpha alumina is produced at temperatures greater than 800°C [22].
Fig. 2. XRD pattern for synthesized residue products (F/O=1.25) after solving in nitric acid.
Fig. 3. TTDs for F/O ratios, a: 0.9, b: 1.0, c: 1.25, d: 1.5, e: 1.75 and f: 2.0. As shown in this figure, the first step, which lasted about 3 minutes, consisted of water vaporization at 100°C. Next, the water molecules started to vaporize and the temperature slowly increased to 250°C. The gradual rise of temperature stopped as it drew nearer to the ignition temperature. Once the ignition temperature was reached, the synthesis occurred rapidly. Note that all the otherwise overlapping diagrams with different F/O ratios are either shifted to the right or to the left in Fig. 3. Therefore, there was no correlation between amount of fuel and the ignition delay time. As shown in diagrams 3a and 3b, the recorded combustion temperature was low due to
5
insufficient amount of fuel. It can be attributed to the fact that some part of the fuel started to decompose before reaching the reaction temperature because the hydrolyzation of urea starts at 80 °C which is less than the ignition temperature (200°C) [23]. Consequently, the insufficient amount of fuel at the ignition time, at both F/O ratios of 0.9 and 1, prevented the reaction from taking place fully, as shown in Fig. 3-b [21]. The highest temperature was obtained at the F/O ratio of 1.25. The excessive weight of fuel in the solution at the F/O ratio of 1.25 compensated for the amount of hydrolyzed urea, allowing the synthesis to come to a full finish. Also, the combustion temperature steeply increased to 975°C at this ratio, as shown in Fig. 3-c. But, at F/O ratio of 1.5, the high volume of CO2, H2O, and N2 gases led to loss of heat and, in turn, to the decrease of the combustion temperature, as shown in Fig. 3-d. Another reason for the decrease in the combustion temperature was the extra fuel, which acted as a diluent in the solution [24], especially at the beginning of the synthesis. This led to an increase in heat dissipation. By increasing the amount of fuel, the effect of fuel as a diluent rose remarkably for the F/O ratio of 1.75, and therefore the reaction became unstable. This phenomenon can be observed in Fig. 3-e, where the peak appearing after the combustion temperature indicates that the decomposition reaction together the combustion reaction contributes in this sample. The overwhelming amount of fuel at F/O ratio of 2 further corroborates the diluting effect of the fuel and how this prevented the reaction from happening, as shown in Fig. 3-f. Figs. 4-a and 4-b show micrographs of the coated sample at F/O ratios of 1.25 and 1.5, respectively.
Fig. 4. SEM images for samples coated at: a: F/O=1.25, b: F/O=1.5. As shown in these figures, some porosity can be discerned in the micrographs, which is due to the intrinsic behavior of SCS [23]. It is clear that the amount of porosity depends heavily on the gas content generated during SCS, and this is why the sample at F/O ratio of 1.5 has more porosity than the one with F/O ratio of 1.25. Fig. 5 indicates the EDS pattern of the coated sample at F/O ratio of 1.25.
6
Fig. 5. SEM image and EDS analyses for sample coated with F/O=1.25. The detected peaks represent aluminum, copper and oxygen, verifying the formation of alumina and copper phases. The undetected peak at the right side of the copper peak belongs to platinum as a platinum coating was applied on the sample to improve the quality of the SEM images. The results of Vickers hardness test for different F/O ratios are presented in Fig. 6.
Fig. 6. Vickers hardness results for different ratios of F/O. At first glance, it is obvious that the hardness of the coated samples is considerably higher than the ones of uncoated samples. At the first two ratios, i.e., 0.9 and 1.0, such an improvement can be attributed to the moderate amount of rough alumina phase with which the synthesis reaction was performed. Fig. 3 indicates that for samples with F/O ratios of 0.9 and 1.0, the combustion temperature slightly increased, which was not enough to complete the reaction. Consequently, compared to the uncoated sample, the hardness of the coated samples only increased marginally. Nevertheless, by increasing the F/O ratio to 1.25, the temperature rose remarkably, reaching approximately 1000°C, and the reaction fully completed. At this temperature, crystalline alpha alumina was produced, which led to a significant increase in hardness. However, as shown in Fig. 4, further increase of fuel, i.e., F/O ratios of 1.5 and 1.75, produced more gases, as suggested by Eq. 1, which in turn resulted in increased porosity and decreased hardness. Fig. 7 shows the results of wear test for 1, 3 and 5 N loads at different F/O ratios.
7
Fig. 7. Results of wear test for 1, 3 and 5 N loads for different F/O ratios. As the results in Fig. 7 depict, the wear rate was higher for the coated samples at all fuel ratios. However, the wear resistance was smaller for the samples with the F/O ratios of 0.9 and 1 than the sample with F/O ratio of 1.25, mainly because of their lower hardness. By increasing the F/O ratio to 1.25, the wear resistance reached a maximum value because the reaction was completely performed. However, as mentioned before, by further increasing the amount of fuel to F/O ratios of 1.5 and 1.75, the excess fuel acted as a diluent and caused the combustion temperature to decrease. Also, the large amount of gases produced during the synthesis increased the porosity, which eventually led to decrease in the wear resistance, although it still remains higher than the resistance of uncoated samples. Despite the fact that it is argued otherwise in some literature [6], in the light of the results of the present study, it is firmly believed that there is a direct correlation between the wear resistance and hardness in nanocomposite coatings. As shown in Fig. 3, the entire process lasted almost 7 minutes and it only needed such sources of energy as hotplate or hot wire to start the reaction. This suggests a remarkable improvement over electroplating-based coatings, where it takes nearly 1 hour to achieve the same hardness and wear resistance (at 17.8 wt.% alumina the micro hardness increased from 65 to 157 Vickers and at 4N load the wear rate decreased from 0.0004 to 0.0001 gr/m). Moreover, electroplating-based coating method is inherently more complicated as it requires additional pieces of equipment for checking the process and there are numerous parameters that play a role in the reaction [6]. Figs. 8a and 8b show SEM images of the surface of uncoated and coated samples, respectively.
Fig. 8. SEM image after wear test for (a): uncoated substrate and (b): coated substrate with F/O ratio=1.25, 5000X.
8
As shown in Fig. 8a, sever plastic deformation in the uncoated sample, with much wider and deeper scratches, confirms the trends observed in Figs. 6 and 7, i.e., the alumina-copper nanocomposite coating improves the hardness and wear resistance. Fig. 9 shows a TEM image of the alumina phase after ensuring that the alumina is formed based on Fig. 2.
Fig. 9. TEM image of Al2O3 particles. Based on the TEM images, the size of the obtained particles was in the range of 20-50 nanometers. TEM image proves that the nanocomposite coating was formed in a single step. 4.
Conclusions
The produced Cu-Al2O3 nanocomposite coating in this study improved the mechanical behavior of the surface of copper substrate. The findings of this study are summarized as below: 1- Copper-alumina composite coating was successfully synthesized using solution combustion synthesis (SCS) method; 2- The Cu-Al2O3 coating increased the wear resistances and hardness of the substrate up to three times more than the uncoated copper substrate; 3- The amount of porosity on the surface increased in line with the rise in the F/O ratio; 4- Optimum hardness and wear resistance was achieved at F/O ratio of 1.25; 5- There was a direct correlation between the hardness and wear resistance of the SCS-based coatings. References [1] A. Liu, H. Zhu, Z. Guo, Y. Meng, G. Liu, E. Fortunato, R.o Martins and F. Shan, Solution Combustion Synthesis: Low-Temperature Processing for p-Type Cu:NiO Thin Films for Transparent Electronics. Adv. Mater. 15 (2017) 1-8
9
[2] M. Lekka, G. Zendron, C. Zanella, A. Lanzutti, L. Fedrizzi, P.L. Bonora, Corrosion properties of micro- and nanocomposite copper matrix coatings produced from a copper pyrophosphate bath under pulse current, Surf. Coat. Technol. 205 (2011) 3438-3447. [3] F. Chen, J. Ying, Y. Wang, S. Du, Z. Liu, Q. Huang. Effects of graphene content on the microstructure and properties of copper matrix composites. Carbon. 96 (2016) 836-842. [4] A. Arman, C. Luna, M. Mardani, F. Hafezi, A. Achour, A. Ahmadpourian. Magnetoresistance of nanocomposite copper/carbon thin films. J. Mater. Sci. - Mater. Electron. .6 (2017) 4713–4718. [5] A. Fathy, A. A. Megahed, Prediction of abrasive wear rate of in situ Cu – Al2O3 nanocomposite using artificial neural networks, Int J AdvManufTechnol. 62 (2012) 953 – 963. [6] S.R.Allahkaram, S.Golroh, M.Mohammadalipour, Properties of Al2O3nano-particle reinforced copper matrix composite coatings prepared by pulse and direct current electroplating, Mater. Des. 32 (2011) 4478–4484. [7] H. Nasiri, J. Vahdati Khaki, S.M. Zebarjad, One-step fabrication of Cu–Al2O3nanocomposite via solution combustion synthesis route, J. Alloys Compd. 509 (2011) 5305–5308. [8] A. Sova, R. Maestracci, M. Jeandin, Ph. Bertrand, I. Smurov. Kinetics of composite coating formation process in cold spray: Modelling and experimental validation. Surf. Coat. Technol. 318 (2017) 309-314. [9] A. Amri, Xi. Duan, C. Yin, Z. Jiang, M. Rahman, T. Pryor. Solar absorptance of copper–cobalt oxide thin film coatings with nano-size, grain-like morphology: Optimization and synchrotron radiation XPS studies. Appl. Surf. Sci. 275 (2013) 127-135. [10] Y. Wen, W. Huang, B. Wang, A novel method for the preparation of Cu/Al2O3nanocomposite, Appl. Surf. Sci. 258 (2012) 2935–2938. [11] Z. Arabgol, H. Assadi, T. Schmidt, F. Gärtner, T. Klassen. Analysis of Thermal History and Residual Stress in Cold-Sprayed Coatings. J. Therm. Spray Technol. 23 (2014) 84-90. [12] T. Shi, L. Guo, J. Hao, C. Chen, J. Luo, Z. Guo. Microstructure and wear resistance of in-situ TiC surface composite coating on copper matrix synthesized by SHS and Vacuum-Expendable Pattern Casting. Surf. Coat. Technol. 324 (2017) 288-297.
10
[13] S. Aruna, C. Sanjeeviraja, N. Balaji, N. Manikandanath. Properties of plasma sprayed La2Zr2O7 coating fabricated from powder synthesized by a single-step solution combustion method. Surf. Coat. Technol. 219 (2013) 131-138. [14] S.T. Aruna, A.S. Mukasyan, Combustion synthesis and nanomaterials, Curr. Opin. Solid State Mater. Sci. 10 (2008) 44-50. [15] A. S. Mukasyan and P. Dinka, Novel Approaches to Solution–Combustion Synthesis of Nanomaterials, ISSN 1061-3862, Int. J. Self Propag. High Temp. Synth. 16 (2007) 23–35. [16] A. S. Mukasyan, P.Dinka, Novel Method for Synthesis of Nano-Materials: Combustion of Active Impregnated Layers, Adv. Eng. Mater. 9 (2007) 653–657. [17] K.Deshpande, A. Mukasyan, A.Varma, Direct Synthesis of Iron Oxide Nanopowders by the Combustion Approach: Reaction Mechanism and Properties, Chem. Mater. 16(2004)4896 – 4904. [18] S. L. González-Cortés, F. E. Imbert, Fundamentals, properties and applications of solid catalysts prepared by solution combustion synthesis (SCS), Appl. Catal. A: Gen, 452 (2013) 117-131. [19] B. Roy, K. Loganathan, H.N. Pham, A.K. Datye. C.A. Leclerc, Surface modification of solution combustion synthesized Ni/Al2O3 catalyst for aqueous-phase reforming of ethanol, Int. J. Hydrogen Energy.35 (2010) 11700-11708. [20] S.R.Jain, K.C.Adiga, A New Approach to Thermochemical Calculations of Condensed Fuel-Oxidizer Mixtures,Combust. Flame.40 (1981)71-79. [21] Choong-Hwan Jung, SahilJalota, Sarit B. Bhaduri, Quantitative effects of fuel on the synthesis of Ni/NiO particles using a microwave-induced solution combustion synthesis in air atmosphere, Mater. Lett.59 (2005) 2426 – 2432. [22] H. Nasiri, J. Vahdati Khaki, N. Shahtahmassebi, Effect of alumina percentage on size and superparamagnetic properties of Ni-Al2O3 nanocomposite synthesized by solution combustion, Mater. Des. 109 (2016) 476–484.
[23] K. Tahmasebi, M.H. Paaydar, Microwave assisted solution combustion synthesis of alumina-zirconia, ZTA, nanocomposite powder, J. Alloys Compd.509 (2011) 1192-1196.
11
[24] H.Nasiri, E.BahramiMotlagh, J.VahdatiKhaki,S.M.Zebarjad, Role of fuel/oxidizer ratio on the synthesis conditions of Cu-Al2O3nanocomposite prepared through solution combustion synthesis, Mater. Res. Bull. 47 (2012) 3676–3680
12