1. INTRODUCTION
⌅There have been significant developments in materials used for engineering applications in recent years. Traditional engineering materials are not suitable for many applications as they are unmodified monolithic materials, while composite materials are useful in meeting the requirements of such applications. Composite materials differ from traditional engineering materials. Much research focuses on metal matrix composites (MMCs) today due to the advantages they provide in scientific and technical engineering applications (Madhusudhan et al., 2017Madhusudhan, M., Naveen, G.J., Mahesha, K. (2017). Mechanical Characterization of AA7068-ZrO2 reinforced Metal Matrix Composites. Mater. Today Proc. 4 (2), 3122-3130. https://doi.org/10.1016/j.matpr.2017.02.196.). Almost 69% of the MMCs used annually for industrial purposes are Aluminum Metal Matrix composites (AlMMCs) (Idusuyi et al., 2019Idusuyi, N., Olayinka, John, I. (2019). Dry sliding wear characteristics of aluminium metal matrix composites: A brief overview. J. Mater. Res. Technol. 8 (3), 3338-3346. https://doi.org/10.1016/j.jmrt.2019.04.017.). Aluminum-based metal matrix composites are manufactured by various techniques such as liquid metallurgy, powder metallurgy, diffusion bonding, and spray forming. The liquid metallurgy techniques are more economical and more suitable for mass production than the powder metallurgy method (Aruna et al., 2018Aruna, K., Diwakar, K., Bhargav Kumar, K. (2018). Development and Characterization of AL6061-ZrO2 Reinforced Metal Matrix Composites. IJARCSSE 8 (4), 270-275.). The liquid metallurgy method consists of such techniques as hot pressing, stir casting, pressure casting, centrifugal casting, pressure infiltration, non-pressure infiltration, and vacuum infiltration. Vacuum infiltration is a new liquid phase technique used in the production of composite materials, which is relatively less reported in the literature. Ceramic based materials such as SiC, Al2O3, TiC, B4C, MgO, TiB2, ZrO2 are used as reinforcing elements in the production of AlMMCs. ZrO2 (Zirconia), with its high strength capacity and excellent mechanical and wear properties at high temperatures, is one of the cheapest and most available (Udayashankar and Ramamurthy, 2018Udayashankar, S., Ramamurthy, V.S. (2018). Development and Characterization of Al6061-Zirconium Dioxide Reinforced Particulate Composites. Int. J. Res. Eng. Technol. 7 (12), 128-132. ). Zirconium oxide (ZrO2) is highly used in fuel cell technology, catalyst or catalyst support, protective coating for optical mirrors and filters, ceramic biomaterial production and thermal barrier coating (Şimşek, 2019Şimşek, İ. (2019). Investigation of Wear Performance of Different Amounts ZrO2 Reinforced Al-2Gr Matrix Composite Materials Produced by Mechanical Alloying Method. El-Cezerî J. Sci. Eng. 6 (3), 594-605. https://doi.org/10.31202/ecjse.560741.). There are studies on the aluminum matrix-ZrO2 particle-reinforced composites in the literature. Some of these studies employ the mixing casting technique, while others use the powder metallurgy technique (Abdizadeh and Baghchesara, 2013Abdizadeh, H., Baghchesara, M.A. (2013). Investigation Into The Mechanical Properties And Fracture Behavior Of A356 Aluminum Alloy-Based ZrO2-Particle-Reinforced Metal-Matrix Composites. Mech. Compos. Mater. 49 (5), 571-576. https://doi.org/10.1007/s11029-013-9373-z.; Karthikeyan and Jinu, 2015aKarthikeyan, G., Jinu, G.R. (2015a). Experimental investigation on mechanical and wear Behaviour of Aluminium LM6/ZrO2 composites fabricated by stir casting method. Journal of the Balkan Tribological Association 21 (3), 539-556. ; Ramachandra et al., 2015Ramachandra, M., Abhishek, A., Siddeshwar, P., Bharathi, V. (2015). Hardness and Wear Resistance of ZrO2 Nano Particle Reinforced Al Nanocomposites Produced by Powder Metallurgy. Proc. Mat. Sci. 10, 212-219. https://doi.org/10.1016/j.mspro.2015.06.043.; Prasad and Rao, 2016Prasad, C., Rao, K.M. (2016). A Study on Effect of Mechanical Properties Of Al-ZrO2 Composite by Liquid Routing. International Journal of Science Engineering and Advance Technology 4 (4), 189-192. ; Rao et al., 2017Rao, P.C.S., Prasad, T., Harish, M. (2017). Evaluation of Mechanical Properties of Al 7075-ZrO2 Metal Matrix Composite by using Stir Casting Technique. International Journal of Scientific Research Engineering & Technology 6 (4), 377-381. ; Radhika and Venkata Priyanka, 2017Radhika, N., Venkata Priyanka, M.L. (2017). Investigation of Adhesive Wear Behaviour Of Zirconia Reinforced Aluminium Metal Matrix Composite. Int. J. Eng. Sci. Technol. 12 (6), 1685-1696. ).
In this study, the vacuum infiltration technique was used in the production of Al 2024 composite materials reinforced with 5%, 10%, 15% and 20% of ZrO2 ratios. Certain mechanical properties of the composites such as microstructure, hardness, fracture strength, and wear behavior were investigated.
2. MATERIALS AND METHODS
⌅In the production of composite materials, ZrO2 and Al 2024 powders sized ≤105μm were used in composite compacts in the infiltration tubes while the alloy Al 2024 was employed as the molten matrix material. Table 1 shows the technical properties of Al 2024 and the reinforcing element ZrO2.
| ZrO2 | |||||||
| ZrO2 % | SiO2 % | TiO2 % | Fe2O3 % | Density | Hardness | Tensile Strength | Melting point |
| 99.5 | 0.10 | 0.007 | 0.002 | 5.0-6.15 g.cm-3 | 1350 HV | 711 MPa | 2823 °C |
| Al 2024 | |||||||
| Al % | Cu % | Mg % | Mn % | Density | Hardness | Tensile Strength | Melting point |
| 93.5 | 4.4 | 1.5 | 0.6 | 2.77 g.cm-3 | 55 HB | 186 MPa | 638 °C |
In vacuum infiltration tests, Al 2024 was used as molten matrix material and ZrO2 powders sized ≤ 105 m as the reinforcing element. The following steps were followed in the calculation of the reinforcement-volume ratios (R-V) for the reinforcing element and matrix material to be filled into the infiltration tube (Eq. 1).
In other words, the ratio of the actual weight of the reinforcing powders in the infiltration tube to the theoretical weight gives the R-V ratio. Al 2024 and ZrO2 powders with 5% wt, 10% wt, 15 % wt, 20% wt ratios were mixed in the rotary drum mixer for 120 min at 300 rpm. Composite mixtures were calculated according to (Eq. 2).
Where (Wc) is the weight of the composite, (Wr) is reinforcement weight, (Vr) is reinforcement volume, (Wm) is matrix weight, (Vm) is matrix volume. Then, the mixed Al 2024 and ZrO2 powders were filled into steel infiltration tubes and compressed. The stainless filter elements were placed at the top and bottom of the composite compacts compressed in infiltration tubes. With the filters installed, Al 2024 and ZrO2 powders in the tube were prevented from mixing into the molten metal and leaking into the vacuum line during the vacuum process. Finally, the infiltration process was initiated by immersing the test tubes into the molten Al 2024 in 750 +/-10 ºC. The tubes were preheated at 500 ºC before immersion. This process has been done in order to remove the moisture and oxidized structures in the composite mixture and to better wet the reinforcing particles of the matrix material. Additionally, pure magnesium weighing 0.5% was added to molten aluminum to aid wetting. Infiltration procedures, which lasted for 10 min, were carried out at 650 mm Hg. The test setup used in the vacuum infiltration process is shown in Fig. 1.
The manufactured composite materials were removed from the steel test tubes to prepare the test samples. Composite materials measuring 20 mm in diameter and 60 mm in length were removed from the test tubes. Then the composite materials were machined to the sample size required for each test. First, images of the composite materials were taken under an optical microscope. Then, density values were measured according to Archimedes Principle in order to determine the density and porosity of the composite structure. Density measurements were repeated three times on each composite sample. Arithmetic means of the values found were calculated. Finally, the tests to determine such mechanical properties of the composite materials as hardness, cross-breaking strength, and abrasive wear were conducted. While the Brinell Method was employed for hardness measurements, the three-point bending technique for cross-breaking tests, pin-on-disk method for wear tests were used. The hardness measurements were made from 5 different regions of composite samples. The arithmetic average of the obtained values are calculated and transferred to the graphics. Cross-breaking tests were performed on three samples of composite in each reinforcement volume ratio. The arithmetic average of the calculated values are calculated and transferred to the graphics. The wear tests were carried out with 1200 mesh, 600 mesh, and 280 mesh grain-sized abrasives. The tests were conducted under 10 N, 20 N and 40 N loads, with a sliding speed of 0.5 m·s-1 and a wear distance of 40 meters. The data obtained from the tests were evaluated in graphs. The wear test set up is shown in Fig. 2.
3. RESULTS
⌅3.1. Evaluation of microstructures
⌅Optical microscope images of vacuum-infiltrated Al 2024 composites reinforced with 5%, 10%, 15%, and 20% of ZrO2 ratios are given in Fig. 3.
The microstructure images in Fig. 3 shows the distribution of ZrO2 reinforcement particles in the composite structure. The images suggest that the increase in the ZrO2 reinforcement ratio improved the homogeneity in the reinforcement distribution. However, increasing the ZrO2 ratio caused agglomeration of the reinforcing element. Similar results were reported in several studies in which ZrO2-reinforced composites were produced by the mixing casting method (Jebaraj and Chennakesava Reddy, 2000Jebaraj, P.M., Chennakesava Reddy, A. (2000). Simulation and Microstructural Characterization of Zirconia/AA7020 Alloy Particle-Reinforced Metal Matrix Composites. 2nd National Conference on Materials and Manufacturing Processes. Hyderabad, India, pp. 134-140. http://jntuhceh.org/web/tutorials/faculty/1144_ZrO-7020.pdf.; Hajizamani and Baharvandi, 2011Hajizamani, M., Baharvandi, H. (2011). Fabrication and Studying the Mechanical Properties of A356 Alloy Reinforced with Al2O3-10% Vol. ZrO2 Nanoparticles through Stir Casting. AMPC 1 (2), 26-30. https://doi.org/10.4236/ampc.2011.12005.; Ramachandra et al., 2015Ramachandra, M., Abhishek, A., Siddeshwar, P., Bharathi, V. (2015). Hardness and Wear Resistance of ZrO2 Nano Particle Reinforced Al Nanocomposites Produced by Powder Metallurgy. Proc. Mat. Sci. 10, 212-219. https://doi.org/10.1016/j.mspro.2015.06.043.; Udayashankar and Ramamurthy, 2018Udayashankar, S., Ramamurthy, V.S. (2018). Development and Characterization of Al6061-Zirconium Dioxide Reinforced Particulate Composites. Int. J. Res. Eng. Technol. 7 (12), 128-132. ). It appears that the displacement of the ZrO2 particles during the vacuum infiltration process gives way to agglomeration in certain regions in the molten Al 2024 matrix material. Figure 3, displaying the microstructure of composite material reinforced with 20% ZrO2, suggests that melting the matrix material in the infiltration tube caused the ZrO2 particles to move towards the top of the tube in the vacuum direction.
It is frequently reported in the literature that sufficient wetting between the matrix and reinforcing particles in such composite structures would not occur. However, this study, in which the vacuum infiltration method was used, indicated that the wetting between the matrix material and the reinforcement was sufficient and there was no significant porosity in the composite structure. Addition of the 0.5% of magnesium to the molten aluminum during the infiltration process might also have contributed to wetting. The pre-heating process before immersion of the infiltration tubes in the molten metal could also have affected the wetting positively. A similar result is reported in a study in the literature (Hemanth, 2011Hemanth, J. (2011). Fracture behavior of cryogenically solidified aluminum alloy reinforced with Nano-ZrO2 metal matrix composites (CNMMCs). JCEM 2 (8), 110-121. https://academicjournals.org/article/article1379497880_Hemanth.pdf.). Therefore, it can be concluded that the parameters for the infiltration process used in this study (750 ºC temperature of the molten metal matrix, 10 min of infiltration time and 650 mm Hg of vacuum value) were suitable. A major problem encountered in the production of such particle-reinforced aluminum composites is the formation of the porous composite structure. It is reported in the literature that the porosity increases with the increase of the reinforcing particles in the composite structure (Harish et al., 2016Harish, B.R., Shaik Dawood, A.K., Nagabhushan, A., Pimpale, S., Raja Reddy, C.V. (2016). Comparative Study On Individual And Combined Effects Of Zirconium Dioxide And Graphite Reinforcements On Mechanical Properties Of Al 6061 Composites. Int. J. Res. Eng. Technol. 5 (4), 412-416. https://ijret.org/volumes/2016v05/i16/IJRET20160516090.pdf.). The vacuum infiltration technique used in this study seems to have led to a more successful result in matrix-reinforcement compatibility than other production methods. Figure 3 (20% ZrO2-enlarged image) shows more clearly the interface of the ZrO2 reinforcing element and the Al 2024 matrix material. This microstructure images indicate that there is no porous structure between the aluminum matrix (Al 2024) and the ZrO2 reinforcing particle and the bonding of the two phases is good. This suggests that this good wetting will increase the mechanical strength of the composite material, which also suggests that the vacuum infiltration technique used in the production of composites in this study is successful.
3.2. Effect of ZrO2 ratio on the density
⌅Figure 4 shows the density values according to Archimedes Principle of Al 2024 composites reinforced with 5%, 10%, 15% and 20% of ZrO2 produced by vacuum infiltration. Figure 4 reveals that the density gradually increases due to the increase in the ZrO2 reinforcement ratio. The increase in density can be attributed to the density of the reinforcing element, which is higher than the density of the matrix material. Similar results are reported in the literature (Ravi Kumar et al., 2018Ravi Kumar, K., Pridhar, T., Sree Balaji, V.S. (2018). Mechanical properties and characterization of zirconium oxide (ZrO2) and coconut shell ash(CSA) reinforced aluminium (Al 6082) matrix hybrid composite. J. Alloys Compd. 765, 171-179. https://doi.org/10.1016/j.jallcom.2018.06.177.; Govindan and Gowthami, 2019Govindan, K., Gowthami, T.R.J. (2019). Mechanical Properties and Metallurgical Characterization of LM25/ZrO2 Composites Fabricated by Stir Casting Method. Revista Matéria 24 (3), e12439. https://doi.org/10.1590/S1517-707620190003.0753.; Parveen et al., 2019Parveen, A., Chauhan, N.R., Suhaib, M. (2019). Mechanical and Tribological Behaviour of Al-ZrO2 Composites: A Review. In: Advances in Engineering Design. Lecture Notes in Mechanical Engineering. Prasad A., Gupta S., Tyagi R. (Eds). Springer, Singapore. https://doi.org/10.1007/978-981-13-6469-3_20.). However, the increase in density was not as high as the increase in the ZrO2 reinforcement ratio. The increase in the ZrO2 reinforcement ratio caused reinforcement agglomeration in the composite structure. Porous regions caused by the agglomeration in the composite led to a decrease in the density value. Theoretical densities were calculated mathematically by considering the matrix and reinforcement material densities. It is reported in the literature that the density values measured in different studies are slightly lower than the theoretical density values (Govindan et al., 2017Govindan, K., Raghuvaran, J.G.T., Pandian, V. (2017). Weldability Study of LM25/ZrO2 Composites by Using Friction Welding. Revista Matéria 22 (3), e11855. https://doi.org/10.1590/S1517-707620170003.0189.).
3.3. Effect of ZrO2 ratio on the hardness
⌅Figure 5 shows the hardness values of the vacuum-infiltrated Al 2024 composites reinforced with 5 %, 10 %, 15 %, and 20 % of ZrO2. Figure 5 reveals that the hardness value gradually increases due to the increase in the ZrO2 reinforcement ratio. Similar results are reported in the literature (Karthikeyan and Jinu, 2015bKarthikeyan, G., Jinu, G.R. (2015b). Dry Sliding Wear Behaviour of Stir Cast LM 25/ZrO2 Metal Matrix Composites. Trans. Famena 39 (4), 89-98. https://hrcak.srce.hr/index.php?show=clanak&id_clanak_jezik=223897.; Madhusudhan et al., 2016Madhusudhan, M., Vikram, K.V., Mahesha, K., Chandra Babu, C.K. (2016). Evaluation Of Microstructure And Mechanical Properties Of As Cast Aluminium Alloy 7075 and ZRO2 Dispersed Metal Matrix Composites. International Journal of Mechanical and Production Engineering. Special Issue, 93-99. ; Madhusudhan et al., 2017Madhusudhan, M., Naveen, G.J., Mahesha, K. (2017). Mechanical Characterization of AA7068-ZrO2 reinforced Metal Matrix Composites. Mater. Today Proc. 4 (2), 3122-3130. https://doi.org/10.1016/j.matpr.2017.02.196.; Rao et al., 2017Rao, P.C.S., Prasad, T., Harish, M. (2017). Evaluation of Mechanical Properties of Al 7075-ZrO2 Metal Matrix Composite by using Stir Casting Technique. International Journal of Scientific Research Engineering & Technology 6 (4), 377-381. ; Mirjavadi et al., 2017Mirjavadi, S.S., Alipour, M., Hamouda, A.M.S., Matin, A., Kord, S., Afshari, B.M., Koppad, P.G. (2017). Effect of multi-pass friction stir processing on the microstructure, mechanical and wear properties of AA5083/ZrO2 nanocomposites. J. Alloys Compd. 726, 1262-1273. https://doi.org/10.1016/j.jallcom.2017.08.084.; Pandiyarajan et al., 2017Pandiyarajan, R., Maran, P., Marimuthu S., Ganesh, K.C. (2017). Mechanical and tribological behavior of the metal matrix composite AA6061/ZrO2/C. J. Mech. Sci. Technol. 31 (10), 4711-4717. https://doi.org/10.1007/s12206-017-0917-3.; Aruna et al., 2018Aruna, K., Diwakar, K., Bhargav Kumar, K. (2018). Development and Characterization of AL6061-ZrO2 Reinforced Metal Matrix Composites. IJARCSSE 8 (4), 270-275.). The major reason for this increase in the hardness is the hard phase ZrO2 reinforcement particles in the composite structure. It can be stated that good wetting between the matrix material Al 2024 of the composite and the ZrO2 reinforcing particles increases the mechanical strength of the structure as well as the hardness. The 55 HB hardness value of pure Al 2024 alloy without ZrO2 reinforcement gradually increased depending on the ZrO2 reinforcement ratio.The highest hardness value was found to be 71.7 HB in the composite material with 20% of ZrO2 reinforcement, which increased the hardness of the Al 2024 alloy by 30%. Another study in the literature reported that the increase of hard reinforcement particles, in addition to increasing the hardness of the composites, caused an increase in the formation of dislocation, which prevents plastic deformation. It can be stated that the fluidity of the molten matrix, the density of the reinforcing particles, the rate of solidification and the distribution of the reinforcing particles are the main factors affecting the hardness of the composites (Ravi et al., 2018Ravi Kumar, K., Pridhar, T., Sree Balaji, V.S. (2018). Mechanical properties and characterization of zirconium oxide (ZrO2) and coconut shell ash(CSA) reinforced aluminium (Al 6082) matrix hybrid composite. J. Alloys Compd. 765, 171-179. https://doi.org/10.1016/j.jallcom.2018.06.177.). As a result, it was observed that there was a significant increase in the hardness values of ZrO2 reinforced composites compared to pure Al 2024 alloy.
3.4. Effect of ZrO2 ratio on the cross-breaking strength
⌅Figure 6 shows the cross-breaking strength values of the vacuum-infiltrated Al 2024 composites reinforced with 5%, 10%, 15% and 20% of ZrO2 obtained from the three-point bending tests. Figure 6 reveals that an increase in the ZrO2 ratio leads to lower breaking strength and lower breaking force. The reason for this might be that the hard phase ZrO2 reinforcing particles increased the brittleness of the composite structure by reducing its ductility. Similar results were reported in a study in the literature (Hemanth, 2011Hemanth, J. (2011). Fracture behavior of cryogenically solidified aluminum alloy reinforced with Nano-ZrO2 metal matrix composites (CNMMCs). JCEM 2 (8), 110-121. https://academicjournals.org/article/article1379497880_Hemanth.pdf.). The ZrO2 reinforcement particles also might have caused the notch effect in the composite structure. The sharp-cornered ZrO2 reinforcing particles caused the notch effect that initiated the breaking during the bending test. These notches eased the breaking. It is generally thought that the increase in hardness causes a decrease in breaking strength. Hardness means reduced flexibility, which naturally causes direct breakings of the composite materials during the three-point bending test. Therefore, higher hardness increased the brittleness of the composite structure and reduced the breaking strength. This generally occurs in the bending tests of ceramic-based particle-reinforced aluminum matrix composites. Figure 7 shows the SEM and BSE (Back-Scattered Electron microscopy) images of the broken surfaces in detail for the evaluation of the breaking behavior of composite materials.
Figure 7 shows the effect of the brittle breaking mechanism on the broken surfaces. The ZrO2 reinforcing particles increased the hardness of the composite structure as well as its brittleness. Figure 7a reflects the SEM images taken from the broken sample surface and the BSE images of the same surface. The BES images reflect the ZrO2 particles in the matrix more clearly. The BSE images indicate that partial agglomeration occurred with the increase in the ZrO2 ratio, causing a porous structure. During cross-breaking tests, the mechanical strength of the composite structure might have decreased and this might have facilitated the breaking in areas where agglomeration and pores occurred. There are studies in the literature reporting similar results (Chong et al., 1993Chong, S.Y., Atkinson, H.V., Jones, H. (1993). Effect of ceramic particle size, melt superheat, impurites and alloy conditions on threshold pressure for infiltration on SiC powder compacts by aluminium-based melts. Mat. Sci. Eng. A 173 (1-2), 233-237. https://doi.org/10.1016/0921-5093(93)90221-Y. ; Pul, 2019Pul, M. (2019). Effect of sintering on mechanical property of SiC/B4C reinforced aluminum. Mater. Res. Express. 6 (1), 016541. https://doi.org/10.1088/2053-1591/aacee1.). It was previously stated that the ZrO2 reinforcing element increased the mechanical strength of the composite structure under normal conditions. Fig. 7f also gives an enlarged view showing the wetting on the interface of Al 2024 and ZrO2 particles better. The image shows that wetting is good and a strong bond is formed between the matrix and reinforcement. However, as the ZrO2 ratio increases in the composite structure, agglomeration occurs, leading to some porosity. It is also known that the angular and amorphous structure of ZrO2 particles creates a notch effect. Therefore, the increase of the ZrO2 ratio in the composite structure helped the breaking factors be more effective. Lowered breaking strength is a common result of increasing the reinforcing element ratios in the composite structure. A similar result has been reported in the literature (Chong et al.,1993Chong, S.Y., Atkinson, H.V., Jones, H. (1993). Effect of ceramic particle size, melt superheat, impurites and alloy conditions on threshold pressure for infiltration on SiC powder compacts by aluminium-based melts. Mat. Sci. Eng. A 173 (1-2), 233-237. https://doi.org/10.1016/0921-5093(93)90221-Y. ).
The images in Fig. 7 (b,c,d,e) also reveal that the breaking of the composite samples did not deform the ZrO2 particles in the structure so badly. The breakings generally occurred within the Al 2024 matrix material and the ZrO2 particles remained embedded in the Al 2024 matrix material. Pores expected to have been caused by the breaking of ZrO2 particles were not available, which suggests that the bond between the matrix and reinforcement was strong due to successful wetting. All in all, the results suggest that the most important parameter affecting the cross-breaking test was the amount of reinforcing elements in the composite structure.
3.5. Effect of ZrO2 ratio on the wear
⌅Figure 8 shows the wear losses obtained from the abrasive wear tests of the vacuum-infiltrated Al 2024 composites reinforced with 5%, 10%, 15%, and 20% of ZrO2 using the pin-on-disc method. Figure 8 shows that the wear resistance of the composite material increased while the wear losses decreased due to the increase in the ZrO2 reinforcement ratio in the composite structure. The highest wear losses occurred at 40 N with the highest abrasion load. The findings also suggest that the abrasive paper was influential on the wear loss The highest wear losses in all composite samples occurred in tests with 280 mesh abrasive paper. Abrasive Al2O3 particles behaved as a multiple cutting tool. Therefore, the highest material loss was caused by 280 mesh abrasive with the largest particle size.
I already mentioned in the evaluation of Fig. 5 that the structure was getting harder with the increase of the ZrO2 reinforcement ratio. The increase in the hardness in the composite material was the major effect on the reduction of material losses in abrasive wear tests. In addition, the slippery ZrO2 reinforcing particles helped the composite test samples slide more easily on the abrasive paper, which caused a slight reduction in the friction coefficient. Therefore, the increase in the ZrO2 reinforcement ratio in the composite structure led to gradual decrease in the wear losses. Similar results are reported in the studies in the literature (Ramachandra et al., 2015Ramachandra, M., Abhishek, A., Siddeshwar, P., Bharathi, V. (2015). Hardness and Wear Resistance of ZrO2 Nano Particle Reinforced Al Nanocomposites Produced by Powder Metallurgy. Proc. Mat. Sci. 10, 212-219. https://doi.org/10.1016/j.mspro.2015.06.043.; Karthikeyan and Jinu, 2015aKarthikeyan, G., Jinu, G.R. (2015a). Experimental investigation on mechanical and wear Behaviour of Aluminium LM6/ZrO2 composites fabricated by stir casting method. Journal of the Balkan Tribological Association 21 (3), 539-556. ; Karthikeyan and Jinu, 2016Karthikeyan G., Jinu, G.R. (2016). Dry sliding wear behavior optimization of stir cast LM6 /ZrO2 composites by response surface methodology analysis. Trans. Can. Soc. Mech. Eng. 40 (3), 351-369. https://doi.org/10.1139/tcsme-2016-0026.; Madhusudhan et al., 2016Madhusudhan, M., Vikram, K.V., Mahesha, K., Chandra Babu, C.K. (2016). Evaluation Of Microstructure And Mechanical Properties Of As Cast Aluminium Alloy 7075 and ZRO2 Dispersed Metal Matrix Composites. International Journal of Mechanical and Production Engineering. Special Issue, 93-99. ; Pandiyarajan et al., 2017Pandiyarajan, R., Maran, P., Marimuthu S., Ganesh, K.C. (2017). Mechanical and tribological behavior of the metal matrix composite AA6061/ZrO2/C. J. Mech. Sci. Technol. 31 (10), 4711-4717. https://doi.org/10.1007/s12206-017-0917-3.; Veeresh Kumar et al., 2019Veeresh Kumar, G.B., Pramod, R., Guna Sekhar, Ch., Pradeep Kumar, G., Bhanumurthy, T. (2019). Investigation of physical, mechanical and tribological properties of Al6061-ZrO2 nano-composites. Heliyon 5 (11), e02858. https://doi.org/10.1016/j.heliyon.2019.e02858.). In another study in the literature, it was stated that the oxide formed at the matrix-reinforcing interface plays an important role in reducing both the friction coefficient and the amount of wear (Karthikeyan and Jinu, 2015bKarthikeyan, G., Jinu, G.R. (2015b). Dry Sliding Wear Behaviour of Stir Cast LM 25/ZrO2 Metal Matrix Composites. Trans. Famena 39 (4), 89-98. https://hrcak.srce.hr/index.php?show=clanak&id_clanak_jezik=223897.). It is possible to relate the wear behavior of composite materials with the hardness of the composite structure. According to the Archard equation, the abrasion resistance of the composites is directly related to their hardness (Eq. 3).
where Q is the total volume of wear, K is a dimensionless constant, W is the total normal load, L is the sliding distance, and H is the hardness of the softest contacting surfaces. In addition, the slippery ZrO2 reinforcing particles helped the composite test samples slide more easily onthe abrasive paper, which caused a slight reduction in the friction coefficient. Therefore, the increase in the ZrO2 reinforcement ratio in the composite structure led to gradual decrease in the wear losses. Similar results are mentioned in the studies in the literature (Ramachandra et al., 2015Ramachandra, M., Abhishek, A., Siddeshwar, P., Bharathi, V. (2015). Hardness and Wear Resistance of ZrO2 Nano Particle Reinforced Al Nanocomposites Produced by Powder Metallurgy. Proc. Mat. Sci. 10, 212-219. https://doi.org/10.1016/j.mspro.2015.06.043.; Madhusudhan et al., 2016Madhusudhan, M., Vikram, K.V., Mahesha, K., Chandra Babu, C.K. (2016). Evaluation Of Microstructure And Mechanical Properties Of As Cast Aluminium Alloy 7075 and ZRO2 Dispersed Metal Matrix Composites. International Journal of Mechanical and Production Engineering. Special Issue, 93-99. ; Karthikeyan and Jinu, 2016Karthikeyan G., Jinu, G.R. (2016). Dry sliding wear behavior optimization of stir cast LM6 /ZrO2 composites by response surface methodology analysis. Trans. Can. Soc. Mech. Eng. 40 (3), 351-369. https://doi.org/10.1139/tcsme-2016-0026.; Pandiyarajan et al., 2017Pandiyarajan, R., Maran, P., Marimuthu S., Ganesh, K.C. (2017). Mechanical and tribological behavior of the metal matrix composite AA6061/ZrO2/C. J. Mech. Sci. Technol. 31 (10), 4711-4717. https://doi.org/10.1007/s12206-017-0917-3.; Veeresh Kumar et al., 2019Veeresh Kumar, G.B., Pramod, R., Guna Sekhar, Ch., Pradeep Kumar, G., Bhanumurthy, T. (2019). Investigation of physical, mechanical and tribological properties of Al6061-ZrO2 nano-composites. Heliyon 5 (11), e02858. https://doi.org/10.1016/j.heliyon.2019.e02858.). In another study in the literature, it was stated that the oxide formed at the matrix-reinforcing interface plays an important role in reducing both the friction coefficient and the amount of wear (Karthikeyan and Jinu, 2015bKarthikeyan, G., Jinu, G.R. (2015b). Dry Sliding Wear Behaviour of Stir Cast LM 25/ZrO2 Metal Matrix Composites. Trans. Famena 39 (4), 89-98. https://hrcak.srce.hr/index.php?show=clanak&id_clanak_jezik=223897.). As a result of good wetting between matrix and reinforcing element ZrO2, the strength of the composite structure, as well as the wear resistance, increased. In a study in the literature, it was stated that the wettability and interfacial strength, microhardness value and friction coefficient of the reinforcement in the matrix were related to the wear property of the metal matrix composite (Veeresh Kumar et al., 2011Veeresh Kumar, G.B., Rao, C.S.P., Selvaraj, N. (2011). Mechanical and Tribological Behavior of Particulate Reinforced Aluminum Metal Matrix Composites -a review. JMMCE 10 (1), 59-91. https://doi.org/10.4236/jmmce.2011.101005.).
There was an increase in the wear loss with the increase of the applied load, which is an expected result. However, with the applied load increasing from 10 N to 20 N, the wear losses also increased on average by 2 times, while the load increased from 20 N to 40 N, the wear losses increased by 2.5 times on average. Although the load applied was doubled each time, the wear losses more than doubled. With the increase of the load to 40 N, the temperature of the composite sample surface in contact with the abrasive paper increases more. The increasing temperature decreases the hardness of the composite structure, making it ductile and less resistant to wear. The increase in temperature on the wear surface might have helped to break the ZrO2 particles from the structure by weakening the binding in the matrix-reinforcing interface. In this case, the ZrO2 particles removed from the composite structure might have shown an abrasive effect like abrasive grains and increased the wear losses. Therefore, wear losses under 40 N load were more higher.
In a study in the literature, it is stated that the wear mechanism changes to the load and the loss of material at low loads occurs by the rolling of the abrasive, and at high loads it is caused by the shearing effect. Again, the same authors determined that the increase in the applied load decreased the surface hardening (Singh et al., 2006Singh, M., Mondal, D.P., Das, S. (2006). Abrasive wear response of aluminium alloy-sillimanite particle reinforced composite under low stress condition. Mat. Sci. Eng. A 419 (1-2), 59-68. https://doi.org/10.1016/j.msea.2005.11.056.). In another study, it was found that wear with increasing load decreased proportionally. It has been stated that wear at low loads occurs mainly through the nucleation and increase of microcracks. In the case of high load, it is stated that the abrasion depth is higher and the metallic matrix is plastically deformed (Sawla and Das, 2004Sawla, S., Das, S. (2004). Combined effect of reinforcement and heat treatment on the two body abrasive wear of aluminum alloy and aluminum particle composites. Wear 257 (5-6), 555-561. https://doi.org/10.1016/j.wear.2004.02.001.). One of the important parameters in wear tests is abrasive paper dimensions. Fig. 9 shows the graphs created by abrasive size and ZrO2 reinforcement rate.
Given the particle size of the abrasive papers, the highest wear losses occurred in tests with 280 mesh abrasives. When the abrasive grain sizes are converted to microns, 1200 mesh corresponds to 12 μm, 600 mesh corresponds to 19 μm and 280 mesh corresponds to 52 μm. Therefore, the amount of the material removed by 280 mesh abrasive paper was directly proportional to the grain size. Figure 9 clearly shows this increase in the amount of wear loss. Therefore, the fact that the abrasive particle size has a significant effect on the amount of wear loss is clear. In a study in the literature, it was reported that the composite material had more resistance to 20 µm and 35 µm abrasive grain size than to 100 µm (Modi, 2001Modi, O.P. (2001). Two-body abrasion of a cast Al-Cu (2014 Al) alloy-Al2O3 particle composite: influence of heat treatment and abrasion test parameters. Wear 248 (1-2), 100-111. https://doi.org/10.1016/S0043-1648(00)00534-2.). In order to study the wear behavior of composites in more detail, the SEM images of the worn surfaces are given collectively in Fig. 10. The images were taken from tests where 40 N load and 280 mesh abrasive papers were used with the highest wear losses.
The microstructure images in Fig. 10 suggest that the abrasive wear mechanism is effective in all reinforcement ratios. The Al2O3 abrasives used in the tests could be the major factor in the wear loss. The hard phase ZrO2 particles, which were removed from the composite structure, might also have contributed to the wear loss by friction onto the surface. A similar result was reported in the literature (Yılmaz and Buytoz, 2011Yılmaz, O., Buytoz, S. (2011). Abrasive wear of Al2O3-reinforced aluminium-based MMCs. Compos. Sci. Technol. 61 (16), 2381-2392. https://doi.org/10.1016/S0266-3538(01)00131-2.). The SEM images in Fig. 10 displays the wear cavities formed by the abrasive particles and the ZrO2 particles remaining on the surface. The SEM images also show the noticeable Al 2024 matrix smearing on the wear surfaces due to the increase in the ZrO2 ratio. The ZrO2 reinforcement material has a slippery structure as said above. Therefore, the increase in the ZrO2 ratio caused an increase in the slipperiness on the surface and the Al 2024 matrix material in the soft phase was smeared due to the friction. As explained before, increases in the ZrO2 reinforcement ratio cause agglomerations. The reinforcing elements in the agglomerated regions might have been removed from the composite structure in larger masses during the abrasion tests and formed large pores on the surface. Most probably, these pores were filled and smeared with the Al 2024 matrix material. Therefore, in the composite samples with higher ZrO2 ratios, lower Al 2024 losses led to lower wear losses. The SEM images in Fig. 11 show porous regions formed by agglomerated ZrO2 particles and removed ZrO2 particles.
The surface images also indicate that the cavities are irregular, that is, some are larger or deeper than others. The size of ZrO2 particles in the composite structure ranged between 20 μm and 105 μm . The ZrO2 particles removed from the composite structure by breaking must have been rather small, as particles with smaller surfaces have lower binding between the matrix-reinforcing interface. Therefore, the fine cavities on the worn surfaces must have been formed by smaller ZrO2 particles. Different studies in the literature report that abrasive wear behavior is associated with the shape, distribution, and condition of the reinforcing element in the composite material (Berger et al., 1999Berger, M., Wiklund, U., Eriksson, M., Engqvist, H., Jacobson, S. (1999). The multilayer effect in abrasion-optimising the combination of hard and tough phases. Surf. Coat. Technol. 116-119 (1138-1144). https://doi.org/10.1016/S0257-8972(99)00151-6.; Candan et al., 2001Candan, E., Ahlatci, H., Çı̈menoğlu, H. (2001). Abrasive wear behaviour of Al-SiC composites produced by pressure infiltration technique. Wear 247 (2), 133-138. https://doi.org/10.1016/S0043-1648(00)00499-3.). Looking at the graphs in Fig. 8, it is understood that the most abrasion is in pure 100% Al 2024 samples. When the worn surface image of 100% Al 2024 sample in Fig. 10a is examined, a surface with continuous scratches appears. This is a result of the micro-cutting mechanism in abrasive wear. These continuous lines indicate that the greatest wear is in the pure 100% Al 2024 sample. This supports the graphic data in Fig. 8. In a study in the literature, it is stated that, in soft materials, in abrasive wear, besides micro cutting, abrasion occurs due to high deformation, and there are continuous and wide scratches. If a general evaluation is made, it can be thought that with the increase in the reinforcement elements in the hard phase in the composite structure, the wear resistance will increase and the hard materials will wear less (Hasirci and Gül, 2010Hasirci, H., Gül, F. (2010). Investigation of abrasive wear behaviours in B4C /Al composites depending on reinforcement volume fraction. SDU Int. Technol. Sci. 2 (1), 15-21. https://acikerisim.isparta.edu.tr/xmlui/handle/123456789/3336.). However, the wear properties of composites should not only be associated with hardness but should be handled from different directions (Saheb et al., 2001Saheb, N., Laoui, T., Daud, A.R., Harun, M., Radiman, S., Yahaya, R. (2001). Influence of Ti addition on wear properties of Al-Si eutectic alloys. Wear 249 (8), 656-662. https://doi.org/10.1016/S0043-1648(01)00687-1.). In addition, the physical conditions of the matrix and reinforcing elements in the composite structure, technical properties, heat treatments and production parameters of the composite have been demonstrated by some studies that affect the wear (Sun et al.,1999Sun, Y., Baydoğan, M., Çimenoğlu, H. (1999). The effect of deformation before ageing on the wear resistance of an aluminum alloy. Mater. Lett. 38 (3), 221-226. https://doi.org/10.1016/S0167-577X(98)00162-1.; Lasa and Rodriguez-Ibabe et al., 2002Lasa, L., Rodriguez-Ibabe, J.M. (2002). Effect of composition and processing route on the wear behaviour of Al-Si alloys. Scripta Mater. 46 (6), 477-481. https://doi.org/10.1016/S1359-6462(02)00020-9.; Prasad and Rao, 2016Prasad, C., Rao, K.M. (2016). A Study on Effect of Mechanical Properties Of Al-ZrO2 Composite by Liquid Routing. International Journal of Science Engineering and Advance Technology 4 (4), 189-192. ).
4. CONCLUSIONS
⌅Having evaluated the microstructures and mechanical properties of the vacuum infiltrated Al 2024 composites reinforced with 5%, 10%, 15% and 20% of ZrO2, I can summarize the results as follows:
Al 2024 matrix composites reinforced with 5%, 10%, 15% and 20 of % ZrO2 have been successfully produced by vacuum infiltration technique with the determined manufacturing parameters
ZrO2 reinforcement distribution in the composite structure was partially homogeneous. Strong bond formation occurred as a result of the good wetting between the matrix-reinforcement.
The ZrO2 reinforcing elements increased the density, hardness and wear resistance of the composite structure while reducing the breaking strength.
Breakings generally occurred within the matrix material as a brittle fracture mechanism.
It was found that the ZrO2 reinforcement in the composite structure generally increased the mechanical strength values except for the breaking strength.
It was found that the wear resistance of the composite material could be associated not only with hardness but also the amount and size of the matrix phase and reinforcing elements in the composite structure. The abrasive grain size was also found to be effective on the wear loss.
The results obtained from this study suggest that the strong bonding formed by good wetting between the matrix and the reinforcing phases increases the mechanical properties of the composite.