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Vol. 9. Issue 1.
Pages 641-653 (January - February 2020)
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Vol. 9. Issue 1.
Pages 641-653 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.11.005
Open Access
Dry sliding friction and wear characterization of in situ TiC/Al-Cu3.7-Mg1.3 nanocomposites with nacre-like structures
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Lei Wanga,b,c, Baixin Donga,b, Feng Qiua,b,f,
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qiufeng@jlu.edu.cn

Corresponding author.
, Run Genga,b, Qian Zoud, Hongyu Yanga,e, Qingyuan Lib, Zihan Xub, Qinglong Zhaoa,b, Qichuan Jianga,b,*
a State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun, Jilin, PR China
b Key Laboratory of Automobile Materials, Ministry of Education and Department of Materials Science and Engineering, Jilin University, Renmin Street NO. 5988, Changchun, Jilin 130025, PR China
c State Key Laboratory of Metal Matrix Composites, School of Materials Science & Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
d Department of Mechanical Engineering, Oakland University, Rochester, MI 48309, United States
e School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, Jiangsu, PR China
f Qingdao Automotive Research Institute of Jilin University, Qingdao, 266000 Shandong, PR China
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Abstract

In situ TiC/Al-Cu3.7-Mg1.3 nanocomposites containing 5–20vol. % nano-TiC with nacre-like structures were prepared via combustion synthesis and hot pressing assisted by hot extrusion. Dry sliding friction and wear tests of the prepared in situ TiC/Al-Cu3.7-Mg1.3 nanocomposites and Al-Cu3.7-Mg1.3 alloy was carried out by a pin-on-disc apparatus under various applied loads and at different sliding velocities at room temperature. The nanocomposites showed much better friction and wear performance than the Al-Cu3.7-Mg1.3 alloy. SEM and EDS analyses were conducted to identify the wear mechanisms. The enhanced wear resistance of the nanocomposites was attributed to the formation of a protective mechanically mixed layer reinforced by nano-TiC particles.

Keywords:
Nanocomposites
Nano-sized TiC particles
Nacre-like structures
Wear resistance
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1Introduction

Recently, the performances and applications of nanomaterials have attracted more and more attentions of the researchers. Due to their special characteristics in structure and size, their potentials in the engineering and light-weight manufacture, microwave absorption, electromagnetic protection, energy conversion, biology detection and photocatalytic, etc. areas has been widely studied [1–9]. Among them, the engineering light-weight productions are the basis applications of the nanostructure materials during the industrial manufacture and have a broad development prospects, especially in the application of aluminum alloys.

However, because the wear resistance of Al and its alloys are not sufficiently good to meet requirements associated with tribological applications, massive efforts pertaining to their improvement in mechanical and wear resistance have been made [10,11]. Incorporating tough ceramic particles, especially nanoparticles into Al matrix to produce Al matrix composites (AMCs) has been found to be a valid approach to overcome such shortcomings. For instance, Al matrix composites reinforced with 20wt.% SiCp exhibited better wear resistance than did ausferritic ductile iron under the same test conditions [12]. Tee et al. [13] discovered that the wear resistance of Al-15vol. % TiB2 composite was nearly identical to that of plain carbon steel. Actually, the improvement of wear resistance of aluminum alloys will contribute to the replacement of nodular cast iron engine blocks with aluminum alloy engine blocks (without cylinder liner or with cylinder liner made by aluminum alloys), which can reduce the total weight, enhance the fuel efficiency and save the cost of production. Moreover, some friction and wear parts in the vehicle, including brake discs and friction plates, etc. are also rely on the remarkable wear resistance. Therefore, to extend the applications of such aluminum alloys materials in the aerospace and automobile industries, in which strong tribological properties are desired, the study of the tribological behavior of AMCs has become significantly important.

The preparation of particle-reinforced AMCs is commonly performed by in situ or ex situ methods. The in situ methods take advantage of cleaner particle-matrix interfaces, finer sizes and more thermodynamically stable reinforcements when compared with conventional ex situ methods [14,15], leading to better mechanical properties. Various particles, such as TiB2[13,16–18], SiC [19,20], B4C [21] ZrB2[22] and TiC [23–25], have been used in AMCs, and some studies on the tribological performance of these particles have been reported in the literature. Niranjan et al. [17] investigated the tribological behavior of in situ TiB2/Al composites and found that their enhanced tribological behavior was attributed to fine-sized TiB2 particles, uniform particle dispersion and strong particle-matrix bonding that enabled the in situ TiB2 particles in the wear surface throughout the entire sliding process. Ramesh et al. [16] developed in situ TiB2/Al6063 composites with a smaller friction coefficient and lower wear rates compared with those of Al6063 alloy over a load range of 10 to 50N and at sliding velocities varying from 0.209m/s to 1.256m/s. The authors suggested that fine TiB2 particles form uniformly within the Al6063 matrix, leading to their excellent wear resistance. Therefore, a cleaner particle-matrix interface and finer size particles are essential to obtain high tribological performance. In fact, most of the work has focused on in situ micro- or submicro-sized particle-reinforced AMCs and their tribological behaviors. However, the addition of micro- or submicro-sized particles will inevitably decrease the plasticity and may do harm to the processable and tribological behaviors. In contrast, the specials characteristics of nanosized particles or nanostructures will provide positive effects to the comprehensive properties of the aluminum and other alloys or compounds [3]. In addition, there are very limited available information with respect to the tribological properties of in situ nano-sized particle reinforced AMCs. It can be predicted that such a new nano-structure used in materials will have an extraordinary effect on the wear resistance and will expand the application of nano-sized particles reinforced AMCs, meanwhile, the service life will enhance and production cost can be controlled.

As reported previously by the present authors, in situ TiC/Al-Cu3.7-Mg1.3 nanocomposites with high strength and elevated temperature tensile properties were produced by combustion synthesis in the Al-Ti-C system and hot pressing assisted by hot extrusion [26–28]. Compared with the microstructures of reported AMCs containing in situ nano-sized particles [14,29], our present nanocomposites possess nacre-like structures that will significantly enhance their mechanical properties. The purpose of the present study was to investigate the tribological behavior of in situ TiC/Al-Cu3.7-Mg1.3 nanocomposites. Furthermore, the effects of nano-TiC content, applied load and sliding velocity, as well as the nacre-like structures, on the dry sliding wear behavior of the developed in situ TiC/Al-Cu3.7-Mg1.3 nanocomposites were analyzed. The results will offer useful guidance for the applications and enhance the service life of such in situ nanocomposites in environments where high wear resistance is desired.

2Experimental procedure

Raw materials including Al-Cu3.7-Mg1.3 alloy powders (∼75μm in diameter, containing 3.7Cu, 1.3Mg, 0.25 Si and 0.05 Fe in wt.%), Ti powders (>99.5% purity, ∼48μm in diameter) and carbon nanotubes (CNTs, 10–20nm in diameter and 20–100μm in length) were used. A 1:1 molar ratio of Ti and CNT powders was firstly mixed with Al-Cu3.7-Mg1.3 alloy powders for 48h. Then the hybrid powders were compressed coldly under axial force to form cylindrical compacts with diameters of 45mm and heights of 35mm. Subsequently, combustion synthesis of the compacts assisted with hot press experiments was carried out in a vacuum furnace, where a W5-Re26 thermocouple was preformed to measure the variation of temperature. A rapid increase in temperature indicated the occurrence of the desired reaction, and pressure was applied immediately while the reactants were still soft. Cooling the reactants to room temperature yielded nanocomposites containing 5, 7, 9, 15 and 20vol. % nano-TiC particles. In addition, Al-Cu3.7-Mg1.3 alloy prepared by powder metallurgy (PM) under identical conditions was selected as a sample for comparison. All samples containing nanocomposites and Al-Cu3.7-Mg1.3 alloy were extruded at an extrusion ratio of 19:1 at 773K. Then, a T4 treatment (solution treatment at 783K for 1h, water quenching and aging for 96h at room temperature) was applied to those samples.

As shown in Fig. 1, a pin-on-disc apparatus (MG-2000, China) was used to perform room-temperature dry sliding wear tests. The specimens of the nanocomposites and their matrix alloy were made into pins with dimensions of 5mm×5mm×15mm (Fig. 1). Those pins were loaded against an H13 steel disk; the hardness of the disc was 50 HRC. The wear tests were conducted with applied loads of 20N, 30N, 40N and 50N at sliding velocities of 0.63m/s, 0.94m/s and 1.26m/s for 10min. Before the wear tests, the pins and discs were prepared by mechanical polishing with 1000 mesh SiC (13 micron) abrasive paper. The wear rates depended on the volumetric wear loss per unit sliding distance.

Fig. 1.

Schematic image of the pin-on-disc wear test.

(0.16MB).

To survey the loss of weight, an electronic balance with a resolution of 0.1mg was employed. The densities of the Al-Cu3.7-Mg1.3 alloy and its nanocomposites were obtained by Archimedes’ water-immersion method. Moreover, the microstructure and worn surface were observed by an evolution SEM (Tescan vega3 XM, Czech Republic) equipped with an EDS detector and TEM (JEM 2100F, Japan). In addition, the micro-hardness was measured by a Vickers hardness apparatus (Model 1600–5122 VD, Chicago, USA) with a static load of 3N and a dwell time of 10s. An MTS (servo hydraulic materials testing system, MTS 810, Minneapolis, USA) was used to test the tensile properties of the samples, for which the strain rate was 3×10−4s−1.

3Results and discussion

Fig. 2 shows the XRD analysis of the reaction products in Al-Cu3.7-Mg1.3 alloy powders, Ti powders and CNTs reaction system. The TiC/Al-Cu3.7-Mg1.3 nanocomposites with different nano-TiC particles contents (5, 7, 9, 15 and 20vol. %) are detected and shown in Fig. 2(a)–(e) with the comparison of the standard TiC, Al and Al3Ti peaks. It can be seen that the reaction products were mainly TiC and the residual Al, while the Al3Ti phase can only be detected in the sample of 5vol. %TiC/Al-Cu3.7-Mg1.3.

Fig. 2.

The XRD analysis of the reaction products with different contents of TiC nanoparticles: (a) 5vol. % nano-TiC/Al-Cu3.7-Mg1.3; (b) 7vol. % nano-TiC/Al-Cu3.7-Mg1.3; (c) 9vol. % nano-TiC/Al-Cu3.7-Mg1.3; (d) 15vol. % nano-TiC/Al-Cu3.7-Mg1.3; (e) 20vol. % nano-TiC/Al-Cu3.7-Mg1.3.

(0.56MB).

Fig. 3(a–d) show the SEM images of the transverse sections (perpendicular to the extrusion direction) and flank sections (parallel to the extrusion direction) of the Al-Cu3.7-Mg1.3 alloy and TiC/Al-Cu3.7-Mg1.3 nanocomposite samples. It can be seen from Fig. 3(a) and (b) that both the transverse and flank sections of the Al-Cu3.7-Mg1.3 alloy exhibit equiaxed grain structures with a mean size of approximately 20μm. The full recrystallization observed can be explained by the equiaxed grain structure. In the transverse section for the TiC/Al-Cu3.7-Mg1.3 nanocomposite samples, an equiaxed grain structure can be observed as well, but those grains are wrapped by nano-TiC particles (Fig. 3(c)). Elongated grains wrapped by nano-TiC particles can be clearly observed in the flank section in Fig. 3(d). As reported in our previous studies [26–28], nano-TiC particles were located on grain boundaries and in grain interiors. Therefore, the recrystallization of the nanocomposites was hindered by the nano-TiC particles. Then, the present nanocomposite microstructures consist of two regions, one is a nano-TiC lean region, which corresponds to a ‘soft’ nanocomposite phase and the other is nano-TiC rich region, corresponding to a ‘hard/strong’ nanocomposite phase (as Fig. 3(e) and (f) exhibited).

Fig. 3.

SEM micrographs of the transverse and flank section for samples of Al-Cu3.7-Mg1.3 alloy and TiC/Al-Cu3.7-Mg1.3 nanocomposites.

(1.62MB).

The nano-TiC/Al interfaces in the sample of 5 and 20vol. % nano-TiC/Al-Cu3.7-Mg1.3 are shown in Fig. 4. It can be seen that at different addition levels, the interfaces between nano-TiC particles and the α-Al matrix are always clean. When the addition lever of nano-TiC was 5vol. %, the lattice mismatch between α-Al(111) and TiC(111) was approximately 5.6%, while the lattice mismatch between α-Al(200) and TiC(200) was even 2.4% when the addition lever of nano-TiC was 20vol. %. It was suggested that good nano-TiC/Al interfacial bonding can be obtained in the as-prepared TiC/Al-Cu3.7-Mg1.3 nanocomposites with different contents of nano-TiC particles.

Fig. 4.

High resolution image for the interface of nano-TiC and Al matrix (a) the interface in 5vol. % TiC/Al-Cu3.7-Mg1.3 composite; (b) the interface in 20vol. % TiC/Al-Cu3.7-Mg1.3 composite.

(0.68MB).

The micro-hardness values at the grain boundary and in the grain interior of the Al-Cu3.7-Mg1.3 alloy and the nanocomposites are shown in Fig. 5(a). The micro-hardness values at the grain boundary of the Al-Cu3.7-Mg1.3 alloy and the nanocomposites are much higher than those in the grain interior. With increasing nano-TiC content, the hardness at both the grain boundary and in the grain interior of the Al-Cu3.7-Mg1.3 alloy and the nanocomposites increased. Fig. 5(b) shows the variation of the tensile properties as a function of nano-TiC content. Additionally, as the content of nano-TiC increased, the tensile strength (σb) and yield strength (σ0.2) of the described nanocomposites increased first and then decreased, while their fracture strain (εf) continued to decline. The 9vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposite showed the highest σb and σ0.2 (601MPa and 404MPa). In contrast to the reported materials [30–32], the nanocomposites in this study possess a nacre-like lamellar structure that enhances their mechanical properties. As indicated in Fig. 5(c), the wear volume loss varied as a function of nano-TiC content perpendicular and parallel to the extrusion direction of the matrix alloy and the nanocomposites at 0.94m/s and 40N. The samples tested perpendicular to the extrusion direction showed lower wear volume loss than those tested parallel to the extrusion direction. The reason is that the microstructures of those nanocomposite samples tested perpendicular to the extrusion direction are more uniform. Thus, the samples tested perpendicular to the extrusion direction were mainly studied.

Fig. 5.

(a) Micro-hardness of Al-Cu3.7-Mg1.3 alloy and the nanocomposites at grain boundary and in grain interior, (b) variation of tensile properties (σb, σ0.2, εf) as a function of nano-TiC content, and (c) variation of wear volume loss as a function of nano-TiC content for the matrix alloy and the nanocomposites perpendicular and parallel to the extrusion direction at 0.94m/s and 40N.

(0.25MB).

Fig. 6(a–c) display the variation of wear volume loss with nano-TiC content at sliding velocities of 0.63m/s, 0.94m/s and 1.26m/s, respectively. It can be observed that the nano-TiC particle content increases the wear volume loss decreases. The wear volume loss reaches a minimum at a nanoparticle content of 15vol. %. When the nano-TiC particle content exceeds 15vol. %, the wear volume loss starts to increase. The applied load for all sliding velocities increases, and the wear volume loss increases simultaneously. Clearly, the 15vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites exhibit the best wear resistance under the current test conditions.

Fig. 6.

Variation of wear volume loss as a function of nano-TiC content at sliding velocities of (a) 0.63m/s, (b) 0.94m/s and (c) 1.26m/s. (d), (e) and (f) The corresponding coefficients of friction at the three sliding velocities.

(0.59MB).

Fig. 6(d–f) present the coefficients of friction for the Al-Cu3.7-Mg1.3 alloy and the nanocomposites with 5, 15 and 20vol. % nano-TiC as a function of the applied load at sliding velocities of 0.63m/s, 0.94m/s and 1.26m/s, respectively. The friction coefficients for both the Al-Cu3.7-Mg1.3 alloy and its nanocomposites decrease as the applied load increases, which may be due to the variation in contact between the sample and disc. The coefficient of friction for the nanocomposites decreases as the nano-TiC content increases at all three sliding velocities. Compared with the Al-Cu3.7-Mg1.3 alloy, the 20vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites showed the lowest friction coefficient. The low coefficient of friction values implies particle-ceramic contact with each other. It is suggested that the contact between the counter material and nano-TiC particles increases as more nano-TiC particles are added to the Al-Cu3.7-Mg1.3 alloy, leading to significant enhancements in the wear resistance of the TiC/Al-Cu3.7-Mg1.3 nanocomposites.

Fig. 7(a–c) illustrate the effect of the sliding velocity on the wear volume loss of the Al-Cu3.7-Mg1.3 alloy and the nanocomposites with 5, 15 and 20vol. % nano-TiC under applied loads of 20N to 50N, respectively. The values of the wear volume loss decrease with increasing nano-TiC content up to 15vol. %, after which those values slightly increase. The 15vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites exhibit the lowest wear volume loss. As indicated in Fig. 7(a and b), when the applied load is less than 30N, the wear volume losses for the Al-Cu3.7-Mg1.3 alloy and its nanocomposites are notably invariant with the increase in sliding velocity. This finding suggests that the Al-Cu3.7-Mg1.3 alloy and the nanocomposites are not sensitive to the sliding velocity at low applied loads. As the applied load increases from 40N to 50N, the wear volume loss value for the Al-Cu3.7-Mg1.3 alloy increases, while those for the nanocomposites decrease with increasing sliding velocity, as shown in Fig. 7(c) and (d).

Fig. 7.

Variation of wear volume loss for Al-Cu3.7-Mg1.3 alloy and TiC/Al-Cu3.7-Mg1.3 nanocomposites containing 5, 15 and 20vol. % nano-TiC as a function of sliding velocity at loads of (a) 20N, (b) 30N, (c) 40N and (d) 50N.

(0.6MB).

SEM images of the worn surfaces of Al-Cu3.7-Mg1.3 alloy and its nanocomposites containing 5, 15 and 20vol. % nano-TiC under applied loads of 30N and 50N at a sliding velocity of 0.94m/s are shown in Fig. 8. Under an applied load of 30N, the Al-Cu3.7-Mg1.3 alloy shows relatively wide wear grooves and delaminated layers on the surface along the sliding direction, and this worn surface has a mean surface roughness (Ra) of 0.506μm. In addition, Fig. 8(a) displays agglomerated wear debris and larger flake-like debris on the worn surfaces. These features suggest the presence of an oxide layer on the worn surface. The fracture and formation of the oxide layer can be observed, indicating a mechanism of fracture and oxidative layer formation. The worn surface morphology of the nanocomposites is different from that of the Al-Cu3.7-Mg1.3 alloy. With the nano-TiC content varying from 5vol. % to 20vol. %, as implied in Fig. 8(b–d), the worn surfaces of the nanocomposites reveal narrower grooves and nearly flat surfaces. The values of the mean surface roughness of the worn surfaces for the three described nanocomposites are 0.484μm, 0.365μm and 0.348μm, respectively. These results indicate features of a mild wear mechanism for the nanocomposites and support the lower wear volume loss of the nanocomposites compared with that of the Al-Cu3.7-Mg1.3 alloy at an applied load less than 30N (Fig. 6(b)). As the applied load increases to 50N, the worn surface of the Al-Cu3.7-Mg1.3 alloy exhibits enlarged grooves. Moreover, as shown in Fig. 8(e), the amount of agglomerated wear debris on the worn surface apparently decreases, and the localized plastic deformation becomes severe, which is mainly caused by the increased load. The mean surface roughness of the worn surface of the Al-Cu3.7-Mg1.3 alloy is 0.637μm. The nanocomposites show shallow ploughs and some delaminated layers on the worn surfaces, as shown in Fig. 8(f–h), indicating characteristics of abrasive and delamination wear mechanisms. The nanocomposites containing 5, 15 and 20vol. % nano-TiC show mean surface roughnesses of 0.54μm, 0.387μm and 0.42μm, respectively.

Fig. 8.

SEM micrographs of the worn surfaces of (a, e) Al-Cu3.7-Mg1.3 alloy and its nanocomposites containing (b, f) 5vol. %, (c, g) 15vol. % and (d, h) 20vol. % nano-TiC under applied loads of 30N and 50N at a sliding velocity of 0.94m/s.

(1.11MB).

To further analyze the wear mechanisms, SEM images of the wear debris morphology and EDS analysis for Al-Cu3.7-Mg1.3 alloy and its nanocomposites containing 5, 15 and 20vol. % nano-TiC tested at 50N and 0.94m/s were obtained, as shown in Fig. 9(a–d). Fig. 9(a) shows that the Al-Cu3.7-Mg1.3 alloy possesses flake-like wear debris of non-uniform dimensions. The surface of the partial platelets presents flow lines, indicating that the Al-Cu3.7-Mg1.3 alloy underwent plastic deformation during the dry sliding process. Moreover, the EDS spectra of the wear debris at point A show the existence of oxygen and iron, which implies that oxidation and iron transfer from the steel disc to the worn surface occurred between the two sliding surfaces. A severe adhesion mechanism has been previously reported to occur on the worn surface of Al-Cu3.7-Mg1.3 alloy [33]. The wear debris of 5vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites comprise a hybrid of large flake-like platelets and fine particles. This feature is observed because the enhanced strength and hardness of the nanocomposites hamper the delamination process. Moreover, the EDS spectra of the fine wear debris at point B show the existences of oxygen, titanium and iron, indicating that the fine wear debris contains oxygen, iron and nano-TiC particles. These findings suggest a combined delamination and abrasive wear mechanism for the nanocomposite containing 5vol. % nano-TiC.

Fig. 9.

SEM images of wear debris morphology and their EDS analysis for (a) Al-Cu3.7-Mg1.3 alloy and the nanocomposites containing (b) 5vol. %, (c) 15vol. % and (d) 20vol. % nano-TiC tested at 50N and 0.94m/s.

(1.65MB).

As the nano-TiC content increased to 15vol. %, the collected wear debris showed a spherical morphology, and the large flake-like platelets mostly disappeared, as shown in Fig. 9(c). Apparently, adding nano-TiC particles to the Al-Cu3.7-Mg1.3 alloy plays a crucial role in reducing the size of the wear debris. Similarly, as shown in Fig. 9(d), spherical wear debris collected from the 20vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposite can be observed as well, but the size of the wear debris appears to be coarser. The increase in the size of wear debris enhances the ploughing effect on the nanocomposites. Furthermore, the spherical debris between the two sliding surfaces can form a three-body tribological system, leading to the lower coefficient of friction shown in Fig. 6(d–f). This result indicates that the predominant wear mechanism of the nanocomposites containing 15 and 20vol. % nano-TiC is abrasive wear.

Fig. 10(a–c) shows the worn surfaces of the Al-Cu3.7-Mg1.3 alloy and its nanocomposites containing 15 and 20vol. % nano-TiC tested under an applied load of 50N and a sliding velocity of 1.26m/s. As shown in Fig. 10(a), the Al-Cu3.7-Mg1.3 alloy shows severe plastic deformation on the worn surface, where material flows along the sliding direction and large flake-like wear debris can be observed. EDS linear scanning analysis cross the flow of the metal, shown in Fig. 10(d), implies the existence of oxygen and iron on the worn surface as well. As previously mentioned, the occurrence of oxidation and the transfer of iron on the worn surface indicates an adhesion wear mechanism. In Fig. 10(b), the 15vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites contain a shallower plough and delaminated layer on the worn surface, and the EDS linear scanning analysis shown in Fig. 10(e) also indicates the presence of oxygen and iron on the worn surface. Similarly, the worn surface of the nanocomposites containing 20vol. % nano-TiC shows a delaminated layer and deep ploughs. These features may be mostly attributed to the enhanced ploughing effect of the large and rigid debris (Fig. 10(c)). In turn, these findings indicate that the predominant wear mechanism of the nanocomposites is oxidation wear assisted by an abrasive wear mechanism.

Fig. 10.

SEM micrographs of the worn surfaces of (a) Al-Cu3.7-Mg1.3 alloy and its nanocomposites containing (b) 15 and (c) 20vol. % nano-TiC tested at an applied load of 50N and sliding velocity of 1.26m/s; (d) and (e) EDS linear scanning analysis of lines A and B in Fig. 8(a) and (b).

(0.79MB).

Worn samples of Al-Cu3.7-Mg1.3 alloy and 15vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites tested at 50N and 1.26m/s are shown in Fig. 11(a) and (b), in which the sections are perpendicular to the extrusion direction, namely, the longitudinal cross section. Three regions can be well distinguished by red dotted lines in Al-Cu3.7-Mg1.3 alloy and its nanocomposite: a mechanically mixed layer (MML), a plastically deformed region and a non-affected region, or normal region. The mechanically mixed layer can enhance the wear resistance, in many cases, during the wear process of AMCs against steel [34–37]. However, the formation of the mechanically mixed layer requires mechanically mixed material to deposit. When the deposition rate equals the wear loss rate of the material, a steady layer will be formed. At higher loads, if the wear loss rate exceeds the deposition rate, it will be difficult to form a mechanically mixed layer. In this study, the Al-Cu3.7-Mg1.3 alloy possessed a thin mechanically mixed layer, as shown in Fig. 11(a), mainly because the wear loss rate increased with the applied load. Furthermore, the 15vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites exhibit a mechanically mixed layer with a thickness of approximately 10μm, and their wear resistance is therefore enhanced. Moreover, the compositions of the mechanically mixed layer of the nanocomposites and their matrix alloy are different. The mechanically mixed layer of Al-Cu3.7-Mg1.3 alloy contains oxygen, iron and Al, as shown in Fig. 11(d) and Fig. 10 (d), while the mechanically mixed layer of the nanocomposites contains not only oxygen, iron and Al but also nano-TiC particles. Obviously, the significant increase in the wear resistance of the nanocomposites can be attributed to the nano-TiC, which can enhance the mechanically mixed layer of the nanocomposites.

Fig. 11.

Longitudinal section of (a) Al-Cu3.7-Mg1.3 alloy and (b) 15vol. % TiC/Al-Cu3.7-Mg1.3 nanocomposites tested at 50N and 1.26m/s; (c) and (d) EDS linear scanning analysis of lines A and B in Fig. 9(a) and (b).

(0.88MB).

To further understand the wear mechanisms observed during the dry sliding of Al-Cu3.7-Mg1.3 alloy and the TiC/Al-Cu3.7-Mg1.3 nanocomposites, a schematic description of the wear process is presented in Fig. 12. Fig. 12(a) and (b) show the Al-Cu3.7-Mg1.3 alloy and the nanocomposites before testing. During the dry sliding wear of the Al-Cu3.7-Mg1.3 alloy, the generation and agglomeration of wear debris can be observed in Fig. 12(c). Fig. 12(e) shows the compaction of agglomerated wear debris particles and the development of a protective oxide layer in some regions. During the dry sliding process, some wear protective layers are broken down, and new protective layers are generated in other regions, as shown in Fig. 12(g). However, if the breakdown rate of protective layers exceeds the generation rate, the wear resistance of materials will decrease. For the wear process of the nanocomposites, much smaller debris particles and a protective layer containing oxide, iron and nano-TiC are generated, as shown in Fig. 12(d) and (f). Such layers reinforced by nano-TiC can prevent the nanocomposites from being worn. Moreover, the breakdown and generation of protective layers also occur during the wear process of the nanocomposites. The cracking of a brittle surface layer can result in loose debris particles, which later act as abrasive particles [38].

Fig. 12.

Schematic description of the wear processes during dry sliding wear of Al-Cu3.7-Mg1.3 alloy and its nanocomposites.

(0.85MB).
4Conclusions

Nacre-like structured Al-Cu3.7-Mg1.3 nanocomposites containing 5–20vol. % in situ nano-TiC particles were fabricated via combustion synthesis and hot pressing assisted by hot extrusion processes. The following conclusions can be drawn under the test conditions of the study:

  • 1.

    Compared with the Al-Cu3.7-Mg1.3 alloy, the nanocomposites possess lower coefficients of friction. The friction coefficient decreases as the applied load increases and as the nano-TiC content increases.

  • 2.

    The wear volume loss of the nanocomposites decreases as the nano-TiC content increases up to 15vol. %. Beyond that content, the wear volume starts to increase slightly.

  • 3.

    Under a low applied load, the wear volume loss is not sensitive to the sliding velocity. When the applied load exceeds than 40N, the wear volume of the Al-Cu3.7-Mg1.3 alloy increases with the sliding velocity, while the wear volume of the nanocomposites decreases with the sliding velocity.

  • 4.

    During the dry sliding process, protective mechanically mixed layers are formed on the worn surface of both Al-Cu3.7-Mg1.3 alloy and the nanocomposites. The generation rate of the protective layer is equal to the wear rate when the applied load is less than 30N, which prevents the materials from being worn. When the tested load exceeds 40N, the wear rate of the Al-Cu3.7-Mg1.3 alloy increases, and only a thin protective layer is formed, while the protective mechanically mixed layers of the nanocomposites are enhanced by the nano-TiC and lead to excellent wear resistance for the nanocomposites.

Conflicts of interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgement

This work is supported by the National Natural Science Foundation of China (NNSFC, No. 51790483, and No. 51971101), the Science and Technology Development Program of Jilin Province, China (20190302004GX), the Source Innovation Plan of Qingdao City, China (No. 18-2-2-1-jch), and the College Student Innovation Plan of Jilin University (No. 2018A1603).

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Journal of Materials Research and Technology

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