materials Article

The Dry Sliding Wear Properties of Nano-Sized TiCp/Al-Cu Composites at Elevated Temperatures Wei-Si Tian 1,2 , Qing-Long Zhao 1, * and Qi-Chuan Jiang 1,2, * 1

2

*

ID

, Chuan-Jiang Zhao 1 , Feng Qiu 1,2

ID

Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun 130025, China; [email protected] (W.-S.T.); [email protected] (C.-J.Z.); [email protected] (F.Q.) State Key Laboratory of Automotive Simulation and Control, Jilin University, No. 5988 Renmin Street, Changchun 130025, China Correspondence: [email protected] (Q.-L.Z.); [email protected] (Q.-C.J.); Tel./Fax: +86-431-8509-4699 (Q.-L.Z.)

Received: 10 July 2017; Accepted: 8 August 2017; Published: 11 August 2017

Abstract: Nano-sized ceramic particle reinforced aluminum composites exhibit excellent room-temperature mechanical properties. However, there is limited research on the dry sliding wear behavior of those composites at elevated temperatures, which should be one of the major concerns on elevated temperature applications. Here the Al-Cu composites reinforced with nano-sized TiCp were fabricated. The dry sliding wear behaviors of the nano-sized TiCp /Al-Cu composites at various temperatures (140–220 ◦ C) and loads (10–40 N) with different TiCp contents were studied, and the results showed that the nanocomposites exhibited superior wear resistance. For instance, the relative wear resistance of the 0.5 wt.% nano-sized TiCp /Al-Cu composite was 83.5% higher than that of the Al-Cu matrix alloy at 180 ◦ C under 20 N, and was also 16.5% higher than that of the 5 wt.% micro-sized TiCp /Al-Cu composite, attributed to the pronounced Orowan strengthening effect of nanoparticles. The wear rates of the nanocomposites were always lower than those of the Al-Cu matrix alloy under the same test condition, which increased with the increase in temperature and load and with the decrease in TiCp content. Keywords: metal matrix composites; nano-sized TiCp ; elevated temperature dry sliding wear

1. Introduction Aluminum alloys have relatively high room-temperature strength. However, the reduced mechanical properties at elevated temperatures limit their applications. For example, some high-speed reciprocating or rotating components (e.g., pistons, drive shafts and brake rotors) often operate at temperature higher than 140 ◦ C, where dry sliding would happen if oil starvation or adverse operating conditions arise [1]. One way to improve the elevate temperature mechanical properties of Al alloys is to introduce various particles into the Al matrix alloys. The particle reinforcements (such as TiC [1–3], ZrSiO4 [4], TiB2 [5,6], ZrB2 [7], B4 C [8] and SiC [9,10]) provide isotropic strengthening effect and cost-effectiveness. The wear behavior of the particle reinforced Al matrix composites at elevated temperatures is one of the major concerns for elevated temperature applications. The high-temperature wear behaviors of micro-sized particle reinforced metal matrix composites (microcomposites) have been reported in the literature [5,11,12]. Natarajan et al. [11] showed that the relative wear resistance of the 5.0 micro-sized TiB2 /6063 composite fabricated by the flux assisted synthesis approach was 9.0% higher than that of the matrix alloy in dry sliding wear tests at 200 ◦ C under 9.8 N applied load. However, there is a lack of published work on the dry sliding wear behavior of nanocomposites at elevated temperatures and comparison the effect of nano-sized particles with micro-sized particles Materials 2017, 10, 939; doi:10.3390/ma10080939

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on the elevated temperature wear behavior of Al matrix composites. The effect of the particle size on the room-temperature dry sliding wear behavior of the particle reinforced metal matrix composites against the steel counterface has been reported [3,9,13]. Mahdavi et al. [13] found that the increased SiC particle size from 19 µm to 146 µm resulted in higher wear resistance of the SiCp /Al6061 composites at room temperature. However, if the particle size is reduced to nanometer scale, the nano-sized particle reinforced metal matrix composites (nanocomposites) exhibit better room-temperature tribological properties than the microcomposites. Nemati et al. [3] found that the relative wear resistance of the 5.0 wt.% nano-sized TiCp /Al-4.5Cu composite produced by the powder metallurgy technique was 88.7% and 41.1% higher than that of the matrix alloy and the 5.0 wt.% micro-sized TiCp /Al-4.5Cu composite in the room-temperature dry sliding wear tests under 20 N applied load, respectively. Moazami-Goudarzi et al. [9] reported that the 5.0 wt.% nano-sized SiCp /Al5252 composite which were produced using powder metallurgy showed 211.8% and 120.4% higher relative wear resistance than the matrix alloy and the 10 wt.% micro-sized SiCp /Al5252 composite in the room-temperature dry sliding wear tests under 45 N load. Although studies exploring the room temperature wear behaviors of Al matrix nanocomposites are numerous, research on their elevated temperature wear behaviors is very limited. Therefore, in this paper, an effort has been made to bridge this gap. Among the various ceramic reinforcement particles, TiC is attractive due to its outstanding properties, such as high hardness (2895–3200 HV [14]), low density (4.93 g/cm3 [2]), high melting temperature (3250 ◦ C [14]), high modulus (269 GPa [2]), excellent wear resistance and good wettability with molten aluminum [15]. Casting is a popular method to fabricate metal matrix composites, owing to cost effectiveness and near net shaping [4]. In our previous work [16,17], the addition of the 0.5 wt.% nano-sized TiCp by casing method could significantly improve the strength and ductility of an Al-Cu alloy at room temperature. It seems promising that the addition of nano-sized TiCp would improve its elevated temperature wear properties. However, to our knowledge, the elevated temperature wear resistance of nano-sized TiCp reinforced Al-Cu matrix composites has not been reported. Therefore, in the present study the elevated temperature dry sliding wear properties of the nano-sized TiCp /Al-Cu composites at different temperatures under various applied loads were investigated and compared with the micro-sized TiCp /Al-Cu composites at 180 ◦ C under 20 N load. The wear mechanisms and the strengthening mechanisms were also investigated. This work aims to provide a new approach to increase the elevated temperature wear resistance of Al matrix composites. 2. Experimental Procedures The Al-Cu alloy with a chemical composition in mass% of Al-5Cu-0.45Mn-0.3Ti-0.2Cd-0.2V0.15Zr-0.04B was chosen as the matrix alloy of the composites. 70Al-Ti-carbon nanotubes/ 30Al-Ti-carbon black (in wt.%) at a ratio corresponding to that of stoichiometric TiC was used as the self-propagation high-temperature synthesis (SHS) reaction system to produce nano-sized/micro-sized TiCp -Al master alloy in a self-made vacuum thermal explosion furnace at 900 ◦ C, respectively [18,19]. The composites containing 0.1–1.0 wt.% nano-sized TiCp and 1.0–5.0 wt.% micro-sized TiCp were casted by adding different master alloys into molten Al-Cu alloys followed by mechanical stirring. For more details of the fabrication process, readers can refer to [16]. The master alloys were dissolved in an 18 vol.% HCl water solution and were dried in air to observe the morphologies and sizes of the nano-sized and micro-sized TiCp . The microstructures were investigated by optical microscopy (Axio Imager A2m, Zeiss, Oberkochen, Germany), field emission scanning electron microscope (FESEM, JSM 6700F, Tokyo, Japan), transmission electron microscope (TEM, JEM 2100F, Tokyo, Japan) and scanning electron microscopy (SEM, Evo18, Zeiss, Oberkochen, Germany). The phase compositions in both the nano-sized and micro-sized TiCp -Al master alloys were identified by X-ray diffraction (XRD, Rigaku D/Max 2500PC, Tokyo, Japan) with CuKα radiation using a scanning speed of 4 ◦ /min. After T6 heat treatment (solution treatment at 538 ◦ C for 12 h and aging at 165 ◦ C for 10 h), the composites were machined to cylindrical pins with a diameter of 6 mm and a height of 12 mm.

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Vickers hardness measurements were performed at room temperature with a load of 1 kg and a dwell time of 15 s. Dry sliding wear tests were carried out using a pin-on-disc wear apparatus (MG2000, Zhangjiakou, China). A steel disc (H13 steel) with the hardness of 50 HRC and a diameter of 70 mm was used as the counter Materials 2017, 10, 939 disc. Both the samples and disc were polished mechanically using 3 ofthe 13 emery paper with an average grit size of 5 µm before each test, and were placed inside a furnace chamber. hardness measurements were performed at room temperature with a load of 1 kg and a dwell time The sliding velocity and test time were kept constant in all the wear tests at 200 r/min and 10 min, of 15 s. Dry sliding wear tests were carried out using a pin-on-disc wear apparatus (MG2000, ◦ C) and loads respectively. The wear testsA were carried various temperatures (140, and 220 Zhangjiakou, China). steel disc (H13 out steel)atwith the hardness of 50 HRC and180 a diameter of 70 mm (10, 20, 30 40asN). testdisc. temperature was measured by apolished thermocouple located inside the furnace wasand used theThe counter Both the samples and disc were mechanically using the emery chamber. Each heated to the setting temperature in the furnace chamber and held paper withspecimen an averagewas grit size of 5 μm before each test, and were placed inside a furnace chamber. The at sliding and test time were kept constant in all the The wear weight tests at 200 r/min 10 min, using for 15 min the velocity isothermal condition before the wear test. loss wasand measured respectively. Thean wear tests were various and 220 °C) per and loads a microbalance with accuracy ofcarried 0.0001out g at and thentemperatures converted (140, into 180 volume loss unit sliding (10, 20, 30 and 40 N). The test temperature was measured by a thermocouple located inside the distance to calculate the wear rate. Relative wear resistance, i.e. the ratio of the wear rate of the Al-Cu furnace chamber. Each specimen was heated to the setting temperature in the furnace chamber and matrix alloy to15 that composite, was used tothe evaluate the wear resistance of the materials and held for minof at the the isothermal condition before wear test. The weight loss was measured using the relative wear resistance the Al-Cu matrix alloy was taken asinto 1.000. Theloss worn and wear a microbalance with anofaccuracy of 0.0001 g and then converted volume per surfaces unit sliding distance to calculate wearand rate.the Relative wear resistance, i.e. the ratio of surfaces the wear rate of the Aldebris were examined bythe SEM surface roughness of the worn were examined by Cu matrix alloy to that of the composite, was used to evaluate the wear resistance of the materials stylus profiler (XP-100, Ambios, Santa Cruz, CA, USA). and the relative wear resistance of the Al-Cu matrix alloy was taken as 1.000. The worn surfaces and

wear debris were examined by SEM and the surface roughness of the worn surfaces were examined 3. Results and Discussion by stylus profiler (XP-100, Ambios, Santa Cruz, CA, USA).

3.1. Microstructure

3. Results and Discussion

Figure 1a,b show the FESEM images of the TiCp extracted from the nano-sized and micro-sized Microstructure TiCp -Al3.1. master alloys, respectively. It is evident that the in-situ nano-sized and micro-sized TiCp were Figure 1a and 1b showwith the FESEM of about the TiC97 p extracted nano-sized and micronear-spherical in morphology a meanimages size of nm andfrom 1.88the µm (Figure 1c,d, respectively. sizedshow TiCp-Al respectively. It is evidentTiC that-Al the master in-situ nano-sized and micro-sizedTiC -Al Figure 1c,d themaster XRDalloys, patterns of the nano-sized alloy and micro-sized p p TiCp were near-spherical in morphology with a mean size of about 97 nm and 1.88 μm (Figure 1c,d, master alloy, respectively. As indicated, the master alloys contained only Al and TiC phases without respectively. Figure 1c,d show the XRD patterns of the nano-sized TiCp-Al master alloy and microphases such or Al4alloy, C3 . Figure 2 shows the optical microstructures of Al theand Al-Cu 3 Timaster sized as TiCAl p-Al respectively. As indicated, the as-cast master alloys contained only TiC matrix alloy and the composites. The mean size of α-Al grains in the Al-Cu matrix alloy was about phases without phases such as Al3Ti or Al4C3. Figure 2 shows the optical as-cast microstructures of160 µm matrix alloy composites. mean size α-Algrains, grains in the Al-Cu matrixabout alloy 65 µm (Figure the 2a).Al-Cu The addition of and TiCthe the The mean size of of α-Al which achieved p reduced was about 160 μm (Figure The addition of TiC(Figure p reduced theand mean in the 0.5 wt.% nano-sized TiCp2a). /Al-Cu composites 2b) 75size µmofinα-Al the 5grains, wt.%which micro-sized achieved about 65 μm in the 2d). 0.5 wt.% TiCp/Al-Cu (FigureTiC 2b) and 75 μm in TiCp /Al-Cu composites (Figure Thenano-sized TEM micrograph ofcomposites the nano-sized p /Al-Cu composite the 5 wt.% micro-sized TiCp/Al-Cu composites (Figure 2d). The TEM micrograph of the nano-sized shows that the nano-sized TiCp were distributed in the grain interior (Figure 2c). The SEM observation TiCp/Al-Cu composite shows that the nano-sized TiCp were distributed in the grain interior (Figure of the 52c). wt.% TiC /Al-Cu 2e)composite shows many micro-sized The micro-sized SEM observation ofpthe 5 wt.%composite micro-sized (Figure TiCp/Al-Cu (Figurespherical 2e) shows many particlesspherical embedded inside the matrix alloy. The Energy-dispersive spectroscopy (EDS) micro-sized particles embedded inside the matrix alloy.X-ray The Energy-dispersive X-rayanalysis (EDS) analysis that those particles TiCpto . The p were found verifiedspectroscopy that those particles wereverified micro-sized TiC TiCpmicro-sized were found be TiC distributed relatively p . Thewere to bewith distributed relatively uniformly with limited agglomerations andα-Al situated insideinstead the α-Al grains uniformly limited agglomerations and situated inside the grains of the grain instead of the grain boundaries, due to the good wettability between the in situ TiCp and the boundaries, due to the good wettability between the in situ TiCp and the aluminum matrix [16]. aluminum matrix [16].

Figure 1. Morphologies and sizes of the TiCp formed in the SHS reaction systems of (a) 70Al-Ti-carbon

Figure 1. Morphologies and sizes of the TiC(c,d) in the SHSstatistical reactionresults systems of diameters (a) 70Al-Ti-carbon p formed nanotubes and (b) 30Al-Ti-carbon black, the corresponding of the of nanotubes and (b) 30Al-Ti-carbon black, (c,d) the corresponding statistical results diameters andthe microthe TiCp in (a,b), respectively, (e,f) the XRD patterns of the nano-sized TiCp-Al master alloy of sized TiC p-Al master alloy, respectively. of the TiC (a,b), respectively, (e,f) the XRD patterns of the nano-sized TiCp -Al master alloy and p in micro-sized TiCp -Al master alloy, respectively.

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Figure 2. The typical optical microstructures of the as-cast (a) Al-Cu matrix alloy, (b) 0.5 wt.% nanoFigure 2. The typical optical microstructures of the as-cast (a) Al-Cu matrix alloy, (b) 0.5 wt.% sized TiCp/Al-Cu composite and (d) 5.0 wt.% micro-sized TiCp/Al-Cu composite; (c) the TEM nano-sized TiCp /Al-Cu composite and (d) 5.0 wt.% micro-sized TiCp /Al-Cu composite; (c) the TEM micrograph of the 0.5 wt.% nano-sized TiCp/Al-Cu composite; (e) the SEM micrograph of the 5.0 wt.% micrograph of the 0.5 wt.% nano-sized TiCp /Al-Cu composite; (e) the SEM micrograph of the 5.0 wt.% micro-sized TiCp/Al-Cu composite. micro-sized TiCp /Al-Cu composite.

3.2. Dry Sliding Wear Behavior of the Nano-sized TiCp/Al-Cu Composite 3.2. Dry Sliding Wear Behavior of the Nano-sized TiCp /Al-Cu Composite 3.2.1. Effect of Temperature 3.2.1. Effect of Temperature Figure 3 shows the wear rates of the Al-Cu matrix alloy and 0.5 wt.% nano-sized TiCp/Al-Cu Figure 3inshows the wear rates the 140 Al-Cu matrix 0.5 wt.% nano-sized composite the temperature rangeoffrom °C to 220 °Calloy underand a constant load of 20 N. ItTiC is obvious p /Al-Cu ◦ ◦ that the wear of the nanocomposite to that aofconstant Al-Cu matrix alloy at all composite in theresistance temperature range from 140 was C tosuperior 220 C under load of 20 N. It is temperatures. wearofresistance of the nanocomposite was 51.7% higher obvious that the The wearrelative resistance the nanocomposite was superior to72.7%, that of83.5% Al-Cuand matrix alloy at all than that of the matrix at 140 °C, 180 nanocomposite °C and 220 °C, respectively. the51.7% wear rates temperatures. TheAl-Cu relative wearalloy resistance of the was 72.7%, Besides, 83.5% and higher ◦ ◦ ◦ of both the matrix alloy and nanocomposite tended to increase with increasing test temperature, and of than that of the Al-Cu matrix alloy at 140 C, 180 C and 220 C, respectively. Besides, the wear rates the wear rate increment was small from 140 °C to 180 °C, but a steep increase was observed from 180the both the matrix alloy and nanocomposite tended to increase with increasing test temperature, and °C rate to 220 °C. Figurewas 4 shows worn of both theaAl-Cu alloy nanocomposite ◦ C to 180 ◦ C, but wear increment smallthe from 140surfaces steep matrix increase wasand observed from 180at◦ C different temperatures. When the sliding temperature was 140 °C, the worn surface of the matrix at ◦ to 220 C. Figure 4 shows the worn surfaces of both the Al-Cu matrix alloy and nanocomposite alloy was comprised of distinct parallel grooves and minor delamination, while that of the ◦ different temperatures. When the sliding temperature was 140 C, the worn surface of the matrix alloy nanocomposite showed only shallower grooves. It indicates that the wear mode at 140 °C for the Alwas comprised of distinct parallel grooves and minor delamination, while that of the nanocomposite Cu matrix alloy was a combination of ploughing and delamination while ploughing was dominant showed only shallower grooves. It indicates that the wear mode at 140 ◦ C for the Al-Cu matrix for the nanocomposite (Figure 4a,d). With the increase of temperature, the grooves of both matrix alloy was a combination of ploughing and delamination while ploughing was dominant for the alloy and nanocomposite became shallower. The grooves were formed on the soft materials (Al-Cu nanocomposite (Figure 4a,d). With the increase of temperature, the grooves of both matrix alloy alloy and composites) by the ploughing of hard asperities. It is reported that the ductility of the Aland nanocomposite shallower. The grooves formed on the materials (Al-Cu alloy Cu alloy increased became with increasing temperatures [20].were During the wear testsoft at elevated temperature, and by the ploughing hard to asperities. is reported that the ductility ofatthe Al-Cu thecomposites) soft metal which was ploughedof tended flow back,Itresulting in the shallower grooves higher alloy increased with increasing temperatures [20]. During the wear test at elevated temperature, temperatures [5]. This result is consistent with other works [5,21]. At 220 °C, the worn surfaces ofthe softboth metal tended to exhibited flow back,obvious resulting in the shallower at wear higher the which matrix was alloyploughed and nanocomposite delamination, leadinggrooves to higher ◦ temperatures [5].delaminated This result is consistent withalloy other works At 220 worn surfaces of rates, while the area of the matrix was much[5,21]. larger than thatC, of the nanocomposite, both the matrix alloy nanocomposite exhibited obvious delamination, leading to higher wear as seen in Figure 4c,f.and Therefore, in this work, the onset temperature of obvious delamination of the nanocomposite (220 °C) was higher than that of the was matrix alloylarger (not higher than indicating rates, while the delaminated area of the matrix alloy much than that of140 the°C), nanocomposite, wear 4c,f. resistance of the in nanocomposite. two contrary effects on of thethe as the seenbetter in Figure Therefore, this work, theSliding onset temperature temperaturehad of obvious delamination ◦ C), wear behavior(220 of Al◦ C) alloys. the one hand, at elevated temperatures led140 to the formation nanocomposite was On higher than that oxidation of the matrix alloy (not higher than indicating a thick protective oxide layer on the worn surface, which could reduce the wear rate effects [1]. Onon thethe theofbetter wear resistance of the nanocomposite. Sliding temperature had two contrary other hand, the softening of the matrix alloy with the increase of sliding temperature could increase wear behavior of Al alloys. On the one hand, oxidation at elevated temperatures led to the formation wearprotective rate [5]. Inoxide the range of on 140–180 °C, the protective layer couldreduce be strong to counteract of athe thick layer the worn surface, which could theenough wear rate [1]. On the the thermal softening effect of the matrix alloy, which led to little increase of wear rate. At °C, other hand, the softening of the matrix alloy with the increase of sliding temperature could220 increase however, the worn surface softened so severely that the oxide layer was detached and the bulk metal ◦ the wear rate [5]. In the range of 140–180 C, the protective layer could be strong enough to counteract was transferred from the pin to the counterface (see Figure 4c,f), giving rise to the significant increase the thermal softening effect of the matrix alloy, which led to little increase of wear rate. At 220 ◦ C, in wear rate. Besides, the presence of nano-sized TiCp in the nanocomposite was expected to improve however, the worn surface softened so severely that the oxide layer was detached and the bulk metal the softening resistance of the Al-Cu alloy, resulting in the better elevated temperature wear was transferred from the pin to the counterface (see Figure 4c,f), giving rise to the significant increase in resistance [21]. wear rate. Besides, the presence of nano-sized TiCp in the nanocomposite was expected to improve the softening resistance of the Al-Cu alloy, resulting in the better elevated temperature wear resistance [21].

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Figure The wear composite at Figure 3. 3. The The wear wear rates rates of of the the Al-Cu Al-Cumatrix matrixalloy alloyand and0.5 0.5wt.% wt.%nano-sized nano-sizedTiC TiCppp/Al-Cu /Al-Cu composite composite at at Figure 3. rates of the Al-Cu matrix alloy and 0.5 wt.% nano-sized TiC /Al-Cu different temperatures. different temperatures. different temperatures.

Figure 4. 4. The The worn worn surfaces surfaces of of the the Al-Cu Al-Cu matrix matrix alloy alloy and and 0.5 0.5 wt.% wt.% nano-sized nano-sized TiC TiCpp/Al-Cu /Al-Cu composite composite Figure Figure 4. The worn surfaces of the Al-Cu matrix alloy and 0.5 wt.% nano-sized TiCp /Al-Cu composite at different temperatures under 20 N load: the Al-Cu matrix alloy at (a) 140 °C, (b) 180 °C and (c) 220 220 at different temperatures temperaturesunder under2020NNload: load:the theAl-Cu Al-Cu matrix alloy at (a) (c) ◦ C,°C, ◦ C °C ◦ C; at different matrix alloy at (a) 140140 (b)(b) 180180 andand (c) 220 °C; the nanocomposite at (d) 140 °C, (e) 180 °C and (f) 220 °C. °C; the nanocomposite at (d) 140 °C, (e) 180 °C and (f) 220 °C. ◦ ◦ ◦ the nanocomposite at (d) 140 C, (e) 180 C and (f) 220 C.

EDS analysis analysis of of the the worn worn surfaces surfaces confirmed confirmed the the presence presence of of Fe Fe and and O O in in both both the the Al-Cu Al-Cu matrix matrix EDS EDS analysis of the worn surfaces confirmed the presence of Fe and O in both the Al-Cu matrix alloy and and the the nanocomposite. nanocomposite. The The existence existence of of Fe Fe on on the the worn worn surface surface indicated indicated the the transfer transfer of of Fe Fe alloy alloy and the nanocomposite. The existence ofcould Fe onbe theattributed worn surface indicated the transfer of Fe from the steel disc to the worn surface, which to the abrasive wear mechanism from the steel disc to the worn surface, which could be attributed to the abrasive wear mechanism from the disc to the worn surface, with whichthe could be attributed to the abrasive mechanism [22]. The Thesteel abrasive wear was reduced reduced increase of temperature, temperature, leadingwear to the the decreased[22]. Fe [22]. abrasive wear was with the increase of leading to decreased Fe The abrasive wear was reduced with the increase of temperature, leading to the decreased Fetested content. content. The Fe contents on the worn surfaces of the Al-Cu matrix alloy and nanocomposite at content. The Fe contents on the worn surfaces of the Al-Cu matrix alloy and nanocomposite tested at The Fe contents on the worn20surfaces of the5.41, Al-Cu matrix alloy nanocomposite tested at 140, 180 140, 180 and 220 °C under N load were 3.31, 1.81 at.% and 5.80, 4.23, 2.58 at.%, respectively. 140, 180 ◦and 220 °C under 20 N load were 5.41, 3.31, 1.81 at.% and 5.80, 4.23, 2.58 at.%, respectively. and 220 C under 20 N were 5.41, 3.31, 1.81 at.% 5.80,nanocomposite 4.23, 2.58 at.%, and respectively. higher The higher higher hardness ofload the nanocomposite nanocomposite (173.5 Hvand for the the 158.2 Hv HvThe for the the AlThe hardness of the (173.5 Hv for nanocomposite and 158.2 for Alhardness ofalloy the nanocomposite (173.5 Hv for the nanocomposite and 158.2of Hv for the action Al-Cu of matrix Cu matrix at room temperature) resulted in the increased possibility abrasive steel Cu matrix alloy at room temperature) resulted in the increased possibility of abrasive action of steel alloy at room temperature) resulted in the actionThe of steel asperities asperities by ploughing ploughing into into the steel steel discincreased and thus thuspossibility increased of theabrasive Fe transfer. transfer. existence of O O asperities by the disc and increased the Fe The existence of by ploughing into the steel disc and thus increased the Fe transfer. The existence of O indicated the indicated the the oxidation oxidation reaction. reaction. During During the the sliding sliding process, process, aa protective protective layer layer called called mechanically mechanically indicated oxidation reaction. During the sliding process, a protective layer called mechanically mixedmatrix layer mixed layer (MML) was formed on the worn surface, which was much harder than the Al-Cu mixed layer (MML) was formed on the worn surface, which was much harder than the Al-Cu matrix (MML) was formed onwear the worn surface, Figure which was much harder than the Al-Cu matrixsurfaces and thus and thus reduced the rate [9,23,24]. 5 shows that MML existed in the worn of and thus reduced the wear rate [9,23,24]. Figure 5 shows that MML existed in the worn surfaces of reduced the wear rate alloy [9,23,24]. Figure 5 shows tested that MML in the worncrack surfaces of both the both the the Al-Cu Al-Cu matrix and nanocomposite nanocomposite at 180 180existed °C. A A subsurface subsurface was observed observed in both matrix alloy and tested at °C. crack was in ◦ C. A subsurface crack was observed in the Al-Cu matrix alloy and nanocomposite tested at 180 the Al-Cu matrix alloy (Figure 5a), which would grow and lead to the removal of a layer of metal by the Al-Cu matrix alloy (Figure 5a), which would grow and lead to the removal of a layer of metal by Al-Cu matrix alloy 5a), which would grow and subsurface lead to the removal of a observed layer of metal by delamination wear.(Figure However, no evidence evidence of the the subsurface crack was was observed in the the delamination wear. However, no of crack in delamination wear. no evidence of the subsurface crack was observed in the nanocomposite nanocomposite (seeHowever, Figure 5b). 5b). nanocomposite (see Figure (see Figure 5b).

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Figure Cross sections normal worn surface ofof(a) matrix and (b) 0.5 Figure 5.5.5.Cross normal toto the worn surface of (a) Al-Cu matrix alloyalloy and 0.5 wt.% Figure Crosssections sections normal tothe the worn surface (a)Al-Cu Al-Cu matrix alloy(b) and (b) 0.5 wt.% nano-sized TiC p/Al-Cu composite at 180 °C under 20 N load. ◦ nano-sized TiCp /Al-Cu at 180 C under N under load. 20 N load. wt.% nano-sized TiCcomposite p/Al-Cu composite at 18020°C 3.2.2. Load 3.2.2. Effect ofofApplied Applied 3.2.2.Effect Effectof AppliedLoad Load The wear rate atat180 180 toto40 4040N NNis isisshown shown ininFigure Figure 6.6.The The wear The C under the load range from 1010N NNto Thewear wearrate rateat 180◦°C °Cunder underthe theload loadrange rangefrom from10 shownin Figure6. Thewear wear rates of the Al-Cu matrix alloy and 0.5 wt.% nano-sized TiC p/Al-Cu composite increased with the rates wt.% nano-sized TiC ratesofofthe theAl-Cu Al-Cumatrix matrixalloy alloyand and0.50.5 wt.% nano-sized TiC p/Al-Cucomposite compositeincreased increasedwith withthe the p /Al-Cu increase resistance ininin the load range ofof increase ofofapplied applied load. The nanocompositeexhibited exhibitedsuperior superiorwear wear resistance the load range increaseof appliedload. load.The Thenanocomposite exhibited superior wear resistance the load range 101010 N–40 N, ofofwhich the relative wear resistance was improved by compared with the of N–40 ofwhich which the relative wear resistance was improved by 59.6%–83.5%, compared with N–40 N,N, the relative wear resistance was improved by59.6%–83.5%, 59.6%–83.5%, compared with the Al-Cu matrix alloy. Figure 7 shows the worn surfaces of both the Al-Cu matrix alloy and the the Al-Cu matrix alloy.Figure Figure7 7shows showsthe theworn wornsurfaces surfaces of of both both the Al-Cu Al-Cu matrix alloy. Al-Cu matrix matrix alloy alloyand andthe the nanocomposite atatdifferent different loads. At 1010N NNload, load, the worn exhibited nanocomposite surface ofofthe the Al-Cu matrix alloy nanocompositeat differentloads. loads.At At10 load,the theworn wornsurface surfaceof theAl-Cu Al-Cumatrix matrixalloy alloyexhibited exhibited relatively smooth surface with shallow parallel grooves, asasshown shown ininFigure Figure 7a. relatively As the load increased relativelysmooth smoothsurface surfacewith withshallow shallowparallel parallelgrooves, grooves,as shownin Figure7a. 7a.As Asthe theload loadincreased increased toto20 2020N NNand and above, the surface morphology ofofthe the worn surface changed from distinct grooves toto to andabove, above,the thesurface surfacemorphology morphologyof theworn wornsurface surfacechanged changedfrom fromdistinct distinctgrooves groovesto heavy heavy delamination, leading totoaaarapid rapid increase ininthe the wear rate (see Figures and 7b,d). However, heavydelamination, delamination,leading leadingto rapidincrease increasein thewear wearrate rate(see (seeFigures Figures666and and7b,d). 7b,d).However, However, the worn surface of the nanocomposite showed grooves with relatively small width and depth and the theworn wornsurface surfaceofofthe thenanocomposite nanocompositeshowed showedgrooves grooveswith withrelatively relativelysmall smallwidth widthand anddepth depthand and little delamination at 10–30 N load, compared with the matrix alloy, as seen in Figure 7e–g. At 40 NN little littledelamination delaminationatat10–30 10–30NNload, load,compared comparedwith withthe thematrix matrixalloy, alloy,asasseen seenininFigure Figure7e–g. 7e–g.At At4040N load, load, delamination was observed in the worn surface thenanocomposite nanocomposite(Figure 7h),leading leadingtoto toa a load,delamination delaminationwas wasobserved observedin inthe theworn wornsurface surfaceofof ofthe (Figure7h), 7h), leading rapid increase in the wear rate (Figure 6), but the delaminated area of the nanocomposite was smaller a rapid rapidincrease increaseininthe the wear rate (Figure 6), but the delaminated area the nanocomposite was wear rate (Figure 6), but the delaminated area of theof nanocomposite was smaller than that of the matrix alloy. Increasing the applied load could result in higher extent of plastic smaller thanofthat the matrix alloy. Increasing applied loadcould couldresult resultin in higher higher extent than that theofmatrix alloy. Increasing thethe applied load extent of ofplastic plastic deformation larger grooves inin deformation ofofthe the soft matrix and the matrix would be ploughed deeper totocreate create deformationof thesoft softmatrix matrixand andthe thematrix matrixwould wouldbe beploughed plougheddeeper deeperto createlarger largergrooves groovesin the worn surface. Furthermore, large load would lead to the onset of delamination and larger material the theworn wornsurface. surface.Furthermore, Furthermore,large largeload loadwould wouldlead leadtotothe theonset onsetofofdelamination delaminationand andlarger largermaterial material was removed from the surface [25], but the onset load of delamination of the nanocomposite was (40 N) wasremoved removedfrom fromthe thesurface surface[25], [25],but butthe theonset onsetload loadofofdelamination delaminationofofthe thenanocomposite nanocomposite(40 (40N) N) was higher than that of the Al-Cu matrix alloy (20 N), indicating the better high temperature wear was washigher higherthan thanthat thatofofthe theAl-Cu Al-Cumatrix matrixalloy alloy(20 (20N), N),indicating indicatingthe thebetter betterhigh hightemperature temperaturewear wear resistance Therefore, the wear rates both the matrix alloy resistance nanocomposite. Therefore, the wear rates of both theofof matrix nanocomposite resistanceofofthe of the the nanocomposite. nanocomposite. Therefore, the wear rates bothalloy theand matrix alloy and and nanocomposite were higher at larger load. nanocomposite wereload. higher at larger load. were higher at larger

Figure The wear rates ofofAl-Cu the alloy TiC p/Al-Cu Figure 6.6.6. The wear rates of the matrixmatrix alloy and 0.5and wt.% nano-sized TiCp /Al-Cu composite Figure The wear rates theAl-Cu Al-Cu matrix alloy and0.5 0.5wt.% wt.%nano-sized nano-sized TiC p/Al-Cu ◦ composite under different loads at 180 °C. under different loadsdifferent at 180 C.loads at 180 °C. composite under

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Figure 7.7.The surfaces ofofthe Al-Cu matrix alloy and 0.5 wt.% nano-sized TiC /Al-Cu Figure7. Theworn wornsurfaces surfacesof theAl-Cu Al-Cumatrix matrixalloy alloyand and0.5 0.5wt.% wt.%nano-sized nano-sized TiCp/Al-Cu p/Al-Cucomposite composite Figure The worn the TiC composite p under different loads at 180 °C: the Al-Cu matrix alloy (a–d): 10 N, 20 N, 30 N and 40 N; the under different loads at 180 °C: the Al-Cu matrix alloy (a–d): 10 N, 20 N, 30 N and 40 N; thenanonano◦ under different loads at 180 C: the Al-Cu matrix alloy (a–d): 10 N, 20 N, 30 N and 40 N; the nano-sized p/Al-Cu (e–h): 10 N, 20 N, 30 NNand 40 N. sized TiC p/Al-Cucomposite composite (e–h): 10 N, 20 N, 30 and 40 N. sized TiC TiCp /Al-Cu composite (e–h): 10 N, 20 N, 30 N and 40 N.

3.2.3. Effect ofofTiC 3.2.3.Effect Effectof TiCp pContents Contents 3.2.3. TiC p Contents Figure exhibits the variation ofofwear rates with the ofofnano-sized TiC 20 Figure888exhibits exhibits variation wear rates thecontents contents nano-sized TiCp punder under 20NN Figure thethe variation of wear rates withwith the contents of nano-sized TiCp under 20 N load −13 3/m) −13 3 load at 180 °C. The wear rate (10 m of the Al-Cu matrix alloy was 18.9. By adding the 0.1 wt.% 180 °C.wear The wear rate (10 m /m) of the Al-Cu matrix alloy was 18.9.By Byadding addingthe the0.1 0.1wt.% wt.% ◦ C. −13 atload 180at The rate (10 m3 /m) of the Al-Cu matrix alloy was 18.9. nano-sized TiC p,p,the wear rate decreased toto16.2, and the relative wear resistance was improved by nano-sized TiC the wear rate decreased 16.2, and the relative wear resistance was improved by nano-sized TiCp , the wear rate decreased to 16.2, and the relative wear resistance was improved by 17.0%. With the increase ininthe content ofofnano-sized TiC p,p,the wear rate tended totodecrease and the 17.0%. With the increase the content nano-sized TiC the wear rate tended decrease and the 17.0%. With the increase in the content of nano-sized TiC−13 wear rate tended to decrease and p , the 3/m) −13m 3/m)and relative wear resistance tend totoincrease. The wear rate (10 the wear resistance relative wear resistance tend increase. The wear rate (10 m therelative relative resistance the relative wear resistance tend to increase. The wear rate (10−13 and m3 /m) and thewear relative wear ofof the 1.0 wt.% nano-sized TiC p/Al-Cu were 7.8 and 2.42, respectively, which were the 1.0 wt.% nano-sized TiC p/Al-Cu composite composite were 7.8 and 2.42, respectively, which were resistance of the 1.0 wt.% nano-sized TiCp /Al-Cu composite were 7.8 and 2.42, respectively, which decreased by 58.7% and increased by 242.3%, compared with the Al-Cu matrix alloy. Figure 99shows decreased by 58.7% and increased by 242.3%, compared with the Al-Cu matrix alloy. Figure shows were decreased by 58.7% and increased by 242.3%, compared with the Al-Cu matrix alloy. Figure 9 the surface under NNload at 180 °C of the matrix alloy and nano-sized TiC theworn worn under20 20 load theAl-Cu matrix alloy andthe the nano-sized TiCp/Al-Cu p/Al-Cu ◦Al-Cu shows the surface worn surface under 20 at N 180 load°Catof180 C of the Al-Cu matrix alloy and the nano-sized composite reinforced by different contents ofofnano-sized TiC p.p.As seen from Figure 9a, the worn composite reinforced by different contents nano-sized TiC As seen from Figure 9a, the worn TiCp /Al-Cu composite reinforced by different contents of nano-sized TiCp . As seen from Figure 9a, surface of Al-Cu matrix alloy was comprised of parallel grooves and delaminated areas. However, surface of Al-Cu matrix alloy was comprised of parallel grooves and delaminated areas. However, the worn surface of Al-Cu matrix alloy was comprised of parallel grooves and delaminated areas. with of nano-sized TiC of material removal became smaller and shallower, withthe theaddition addition nano-sized TiCp,p,the thepatches patches materialof removal smaller and shallower, However, with the of addition of nano-sized TiCp , theofpatches materialbecame removal became smaller and as shown ininFigure 9b–f. With the increase ofofthe content ofofnano-sized TiC p,p,the width and depth ofof as shown Figure 9b–f. With the increase the content nano-sized TiC the width and shallower, as shown in Figure 9b–f. With the increase of the content of nano-sized TiCp , thedepth width the grooves tended totodecrease and the were smooth. Besides, ininthe theparallel parallel tended decrease theworn wornsurfaces surfaces werebecoming becoming smooth. Besides, the and depth ofgrooves the parallel grooves tendedand to decrease and the worn surfaces were becoming smooth. nano-sized TiC p/Al-Cu little or none delamination was observed ininthe worn surfaces, nano-sized TiC p/Al-Cucomposites, composites, little or none delamination was observed the worn surfaces, Besides, in the nano-sized TiCp /Al-Cu composites, little or none delamination was observed in the indicating the resistance ofofthe nanocomposites. indicating thebetter betterwear wear resistance the nanocomposites. worn surfaces, indicating the better wear resistance of the nanocomposites.

Figure 8.8.The wear rates ofofthe Al-Cu matrix alloy and nano-sized TiC p/Al-Cu Figure8. Thewear wearrates ratesof theAl-Cu Al-Cumatrix matrixalloy alloyand andnano-sized nano-sizedTiC TiC p/Al-Cucomposites compositesreinforced reinforced Figure The the p /Al-Cu composites reinforced p punder 20 NNload atat180 °C. by different contents ofofnano-sized TiC ◦ under 20 load 180 °C. by different contents nano-sized TiC by different contents of nano-sized TiCp under 20 N load at 180 C.

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Figure 9. The worn surfaces of (a) the Al-Cu matrix alloy and nano-sized TiCp/Al-Cu composite

Figure 9. The worn surfaces of (a) the Al-Cu matrix alloy and nano-sized TiCp /Al-Cu composite reinforced by different contents of nano-sized TiCp under 20 N load at 180 °C: (b) 0.1 wt.%, (c) 0.3 reinforced by different contents of nano-sized TiC under 20 N load at 180 ◦ C: (b) 0.1 wt.%, (c) 0.3 wt.%, wt.%, (d) 0.5 wt.%, (e) 0.7 wt.% and (f) 1.0 wt.%.p (d) 0.5 wt.%, (e) 0.7 wt.% and (f) 1.0 wt.%.

3.3. Comparing with the Micro-Sized TiCp/Al-Cu Composite

3.3. Comparing with the Micro-Sized TiCp /Al-Cu Composite Table 1 lists the wear rates of the Al-Cu matrix alloy, 1.0–5.0 wt.% micro-sized TiCp/Al-Cu Table 1 lists the wear rates of the Al-Cu matrix alloy, 1.0–5.0 wt.% micro-sized TiC /Al-Cu composite and 0.5 wt.% nano-sized TiCp/Al-Cu composite at 180 °C under a constant load of p20 N. ◦ C under a constant load of 20 N. composite and wt.% nano-sized TiCp /Al-Cu composite was at 180 As shown in0.5 Table 1, the wear resistance of the composites much higher in comparison to the As shown in Table 1, the wear resistance composites higher in comparison to the unreinforced Al-Cu matrix alloy. Besides,of thethe wear resistance was of themuch microcomposites increased with unreinforced Al-Cu matrix alloy. resistance of the microcomposites with the increasing micro-sized TiCpBesides, content. the Thewear relative wear resistance of the 5.0 wt.% increased micro-sized the TiC increasing micro-sized The relative wear the The 5.0 wt.% micro-sized p/Al-Cu composite wasTiC improved by 57.5% compared withresistance the Al-Cuof alloy. nanocomposite p content. the highest was relative wear resistance, improved 83.5% andalloy. 16.5%, compared with TiCpexhibited /Al-Cu composite improved by 57.5%which compared withby the Al-Cu The nanocomposite the Al-Cu alloy and 5.0 wt.% micro-sized TiC p /Al-Cu composite, respectively, although the mass exhibited the highest relative wear resistance, which improved by 83.5% and 16.5%, compared with the fraction the5.0 nano-sized TiCp (0.5%) much composite, lower than that of the micro-sized (5%).fraction The Al-Cu alloyof and wt.% micro-sized TiCwas respectively, although TiC thepmass p /Al-Cu improvement wear resistance of the composites compared with the matrix alloy was more of the nano-sized in TiCthe (0.5%) was much lower than that of the micro-sized TiC (5%). The improvement p p significant than that of of the the composites 5.0 micro-sized TiB2/6063 composite (9% at 200was °C under N applied in the wear resistance compared with the matrix alloy more 9.8 significant than load) fabricated by the flux assisted synthesis approach reported by Natarajan et al. [11]. It indicates ◦ that of the 5.0 micro-sized TiB2 /6063 composite (9% at 200 C under 9.8 N applied load) fabricated by that the nano-sized reinforcement TiCp was very effective to improve the elevated temperature wear the flux assisted synthesis approach reported by Natarajan et al. [11]. It indicates that the nano-sized resistance of the Al matrix composites. The Vickers hardness of the Al-Cu matrix alloy (158.2 Hv), 5.0 reinforcement TiCp was very effective to improve the elevated temperature wear resistance of the Al wt.% micro-sized TiCp/Al-Cu composite (166.6 Hv) and 0.5 wt.% nano-sized TiCp/Al-Cu composite matrix composites. The Vickers hardness of the Al-Cu matrix alloy (158.2 Hv), 5.0 wt.% micro-sized (173.5 Hv) were measured. It is obvious that as the hardness increased, the wear rates decreased. This TiCpresult /Al-Cu composite (166.6 Hv) and 0.5Archard’s wt.% nano-sized composite (173.5 Hv) were p /Al-Cu is consistent with the well-known equation TiC in which the wear volume is inversely measured. It is obvious that as the hardness increased, the wear rates decreased. This result is consistent proportional to the hardness of the worn material [26,27]. In the alloys with lower hardness, larger withplastic the well-known equation in whichduring the wear volume is inversely proportionaland to the deformationArchard’s of surface and subsurface sliding caused more delamination hardness of theofworn material [26,27]. Indebris, the alloys withtolower hardness, larger plastic deformation production a higher volume of wear leading the increased surface roughness and higher of wearand ratesubsurface [27]. surface during sliding caused more delamination and production of a higher volume

of wear debris, leading to the increased surface roughness and higher wear rate [27]. Table 1. The high-temperature wear properties and the theoretical values of ΔσOro and ΔσLoad at 180 °C of the Al-Cu alloy and TiCp/Al-Cu compositesand withthe different sizesvalues and contents Table 1. The high-temperature wear properties theoretical of ∆σ of TiC andp.∆σ at 180 ◦ C Oro

Load

of the Al-Cu alloy and TiCp /Al-Cu composites with different sizesRoughness and contents ofΔσ TiC Wear Rate Orop+. ΔσLoad Relative Wear Surface Samples (wt.%)

(10−13 m3/m)

Wear Rate + 1.7 18.9 Samples (wt.%) Al-Cu alloy (M) (10−13 m3-/m) 1.9

M+1.0 micro-TiCp

Al-Cu alloy (M)

M+3.0 micro-TiCp

M+1.0 micro-TiCp

M+5.0 micro-TiCp M+3.0 micro-TiCp

M+0.5 nano-TiCp

M+5.0 micro-TiCp M+0.5 nano-TiCp

+ 1.7 14.1 +1.7 - 1.4 18.9 −1.9 + 1.8 13.3 +1.7 - 1.6 14.1 −1.4 + 1.9 12.0 +1.8 - 1.5 13.3 + 1.7 −1.6 10.3 -1.5 +1.9 12.0

−1.5 +1.7 10.3 −1.5

Resistance Relative Wear 1.000 Resistance

Ra (μm) Surface Roughness 1.918 Ra (µm)

(MPa) ∆σ Oro +0.0 ∆σ Load (MPa)

1.340

1.812

2.5 + 0.6

1.421

1.706

4.7 + 1.8

1.575

1.640

6.4 + 3.1

1.000 1.340 1.421

1.918 1.812 1.706

0.0

2.5 + 0.6 4.7 + 1.8

1.835

1.501

21.8 + 0.3

1.575

1.640

6.4 + 3.1

1.835

1.501

21.8 + 0.3

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Figure 10a–e show the SEM micrographs of worn surfaces of the Al-Cu matrix alloy, microcomposites and Materials 2017, 10, 939nanocomposite. Figure 10f–j show the surface roughness profiles of the 9 of samples 13 after wear tests, and Table 1 presents the arithmetic average height (Ra ) of the profiles, which is defined Figure 10a–e show the SEM micrographs of worn surfaces of the Al-Cu matrix alloy, as the average absolute deviation of the roughness irregularities from the mean line over a sampling microcomposites and nanocomposite. Figure 10f–j show the surface roughness profiles of the samples length [28]. As seen from Figure 10a, the worn surface of the Al-Cu matrix alloy was comprised of parallel after wear tests, and Table 1 presents the arithmetic average height (Ra) of the profiles, which is grooves and large areas. The worn profile of the Al-Cu matrix undulated defined as thedelaminated average absolute deviation of thesurface roughness irregularities from the mean alloy line over a greatly,sampling and the grooves were relatively large in width and depth (see Figure 10f). Figure 11a shows the length [28]. As seen from Figure 10a, the worn surface of the Al-Cu matrix alloy was comprised of parallel grooves and large delaminated wornhad surface profile of shape the Al-Cu wear debris generated from the Al-Cu matrix alloy. Theareas. wearThe debris an irregular and were alloy undulated greatly, and was the grooves werefrom relatively large delaminated in width and depth Figure with large inmatrix size (more than 50 µm), which ploughed the large areas.(see However, 10f). Figure shows theTiC wear content, debris generated from the Al-Cu matrix alloy. Theand wear debris hadand the the increase of the11a micro-sized the delaminated area became smaller shallower, p an irregular shape and were large in size (more than 50 μm), which was ploughed from the large roughness profile became smoother and the Ra value decreased, as shown in Figure 10b–d,g–i. This led delaminated areas. However, with the increase of the micro-sized TiCp content, the delaminated area to the creation of smaller wear debris, as shown in Figure 11b–d. In the nanocomposite, no delaminated became smaller and shallower, and the roughness profile became smoother and the Ra value area was observed. The worn surface was relatively andofthe depth of parallel grooves decreased, as shown in Figure 10b–d,g–i. This led tosmooth the creation smaller wear debris, asnarrow shown in was shallow, roughness profile showedno a smoothest appearance with the lowest Ra value 1.501 µm, Figure the 11b–d. In the nanocomposite, delaminated area was observed. The worn surfaceofwas relatively smooth and the depth of parallel grooves shallow, roughness profile decreased by 21.7% and 8.5%, compared with narrow the matrix alloywas and 5 wt.%the micro-sized TiCp /Al-Cu showed a smoothest indicating appearance with Ra value of 1.501 μm, decreased bywear 21.7%resistance. and 8.5%, The composite, respectively, that the thelowest nanocomposite exhibited the best compared with the matrix alloy and 5 wt.% micro-sized TiC p/Al-Cu composite, respectively, absence of delamination wear in nanocomposite could be due to the stronger interfacial bond between indicating that the nanocomposite exhibited the best wear resistance. The absence of delamination nano-sized particles and Al matrix than that between micro-sized particles and Al matrix. It is reported wear in nanocomposite could be due to the stronger interfacial bond between nano-sized particles that in and the composites reinforced with micro-sized micro-sizedparticles particles larger than 1Itµm, cracks are more likely to Al matrix than that between and Al matrix. is reported that in the be nucleated at the particle/matrix interface during sliding wear than in the nanocomposites [9]. composites reinforced with micro-sized particles larger than 1 μm, cracks are more likely to be As a result, nucleated the micro-sized TiCp /Al-Cu interface composites exhibited morethan wear loss due to[9]. delamination at the particle/matrix during sliding wear in volume the nanocomposites As a result,the the nanocomposite micro-sized TiCp/Al-Cu composites exhibited more wear volume dueresistance. to delamination wear while resisted delamination and showed better loss wear Figure 11e while the nanocomposite delamination and showed better wear resistance. Figure 11e from shows wear that the wear debris createdresisted from the nanocomposite was much smaller than that created shows that the wear debris created from the nanocomposite was much smaller than that created from the Al-Cu matrix alloy and microcomposites, leading to the less volume loss. the Al-Cu matrix alloy and microcomposites, leading to the less volume loss.

Figure 10. The worn surfaces of (a) the Al-Cu matrix alloy and Al-Cu matrix composites reinforced

Figurewith 10. The worn surfaces of (a) the Al-Cu matrix alloy and Al-Cu matrix composites reinforced with different contents of micro-sized TiCp ((b) 1.0 wt.%, (c) 3.0 wt.%, (d) 5.0 wt.%) and (e) 0.5 wt.% different contents of micro-sized TiCp ((b) 1.0 wt.%, (c) 3.0 wt.%, (d) 5.0 wt.%) and (e) 0.5 wt.% nano-sized TiCp at 180 ◦ C under a constant load of 20 N; (f–j) corresponding roughness curves of the worn surface.

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worn surface.

Figure 11. The wear debris morphologies of (a) the Al-Cu matrix alloy and Al-Cu matrix composites Figure 11. The wear debris morphologies of (a) the Al-Cu matrix alloy and Al-Cu matrix composites reinforced with different contents of micro-sized TiCp: ((b) 1.0 wt.%, (c) 3.0 wt.%, (d) 5.0 wt.%) and (e) reinforced with different contents of micro-sized TiCp : ((b) 1.0 wt.%, (c) 3.0 wt.%, (d) 5.0 wt.%) and 0.5 wt.% nano-sized TiCp at 180 °C under a constant load of 20 N. (e) 0.5 wt.% nano-sized TiCp at 180 ◦ C under a constant load of 20 N.

TiCp have two effects on the elevated temperature wear behavior, that is, strengthening effect TiC effects the elevated behavior, that is, strengthening and protective effect. It is on reported that thetemperature wear rate of awear material was inversely proportional toeffect its p have two yield strengtheffect. [29]. Therefore, the improvement the of strength can also theproportional wear resistance. and protective It is reported that the wearinrate a material wasenhance inversely to its Firstly, it is [29]. reported that Orowan strengthening of ceramic much more significant in yield strength Therefore, the improvement in the strengthparticles can alsoisenhance the wear resistance. nanocomposites than in microcomposites [30]. The increment of yield strength at 180 °C caused by Firstly, it is reported that Orowan strengthening of ceramic particles is much more significant in ◦ C caused by Orowan strengthening (ΔσOro) [30]. can be calculated by the following equation [31]: nanocomposites than in mechanism microcomposites The increment of yield strength at 180

Orowan strengthening mechanism (∆σOro ) can be MAGb calculatedπd by the following equation [31]: (1) ln ( ) ΔσOro=0.81 2πλ  4b  πd MAGb = 0.81 ln of aluminum (G = 24.2 GPa at 180 °C(1) where M = 3 is the Taylor factor, ∆σ G Oro is the shear modulus 2πλ 4b according to [32]), b = 0.286 nm is the Burgers vector, λ = 0.4d( π/fV - 2) is the interparticle spacing, ◦ C according d and V are and G volume fraction of the TiC p, respectively. The GPa valueatof180 constant A is where M f= 3 is the the diameter Taylor factor, is the shear modulus of aluminum (G = 24.2 p
estimated to be 1.8 [31]. The theoretical value of Δσ Oro in the nanocomposite is 21.8 MPa, much higher to [32]), b = 0.286 nm is the Burgers vector, λ = 0.4d π/fV − 2 is the interparticle spacing, d and fV that of theand microcomposite (2.5–6.4 efficient transfer of load from the Al-Cu to arethan the diameter volume fraction of theMPa). TiCp , Secondly, respectively. The value of constant A is estimated matrix to the stiffer TiC p can also contribute to the increase of yield strength. Compared with the Albe 1.8 [31]. The theoretical value of ∆σOro in the nanocomposite is 21.8 MPa, much higher than that of matrix alloy, the(2.5–6.4 increaseMPa). in theSecondly, yield strength of thetransfer composites caused bythe load transfer effecttoatthe theCu microcomposite efficient of load from Al-Cu matrix 180 °C can be estimated using the following equation [33]: stiffer TiCp can also contribute to the increase of yield strength. Compared with the Al-Cu matrix alloy, ◦ ΔσLoad = 0.5f Vσym by load transfer effect at 180 C can (2) be the increase in the yield strength of the composites caused estimated using the following equation [33]: ∆σLoad = 0.5fV σym

(2)

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where fV is the volume fraction of the TiCp and σym is the yield strength of the Al-Cu matrix alloy at 180 ◦ C, which equals 221 MPa according to [20]. The calculated ∆σLoad in the 5 wt.% micro-sized TiCp /Al-Cu composite is 3.1 MPa, while in the nanocomposite the result is 0.3 MPa. The calculated results of ∆σOro and ∆σLoad are listed in Table 1. It is obvious that the contribution of the nano-sized TiCp to yield strength at 180 ◦ C is more significant than that of the micro-sized TiCp , attributed to the pronounced Orowan strengthening effect of nanoparticles. Thirdly, it is found that the wear resistance and grain size of pure aluminum follow a relationship identical to Hall-Petch effect [34]. The grain refinement contributes to the improvement of wear resistance of both the nanocomposite and the microcomposite compared with the Al-Cu matrix alloy. However, the grain refinement effect is negligible for the comparison between the nanocomposite and microcomposite owing to the little difference in grain size. Besides, grain refinement of the Al-Cu matrix alloy leads to a larger number and finer θ0 precipitates after aging, resulting in an enhanced precipitation strengthening effect [16]. Similarly, the precipitation strengthening effect is negligible for the comparison between the nanocomposite and microcomposite due to their similar grain sizes and the rapid coarsening of θ0 precipitates at elevated temperatures [20]. Therefore, the yield strength of the nanocomposite was much higher than that of the microcomposite due to the pronounced Orowan strengthening effect of nanoparticles, leading to higher wear resistance at high temperature. Meanwhile, our previous study found that the nano-sized TiCp bonded well with the Al matrix [16]. During sliding, the Al matrix surrounding the nano-sized particles was worn away and the contact was provided between the nano-sized particles and the steel counter face. Therefore, the presence of the nano-sized hard ceramic particles could protect the Al-Cu matrix from direct contact with the counter face, resulting in the better wear resistance of the nanocomposite [35]. 4. Conclusions 1.

2.

3.

The relative wear resistance of the 0.5 wt.% nano-sized TiCp /Al-Cu composite was 72.7%, 83.5% and 51.7% higher than that of the Al-Cu matrix alloy at 140 ◦ C, 180 ◦ C and 220 ◦ C under 20 N load, respectively. The wear rate of the nanocomposite increased with the increase in temperature, but it was still lower than that of the Al-Cu matrix alloy at the same temperature. The worn surfaces indicated that the dominant wear mode for the nanocomposite was ploughing at 140 ◦ C and 180 ◦ C, and a combination of ploughing and delamination at 220 ◦ C, while for the matrix alloy a combination of ploughing and delamination was dominant at all sliding temperatures studied. The nanocomposite exhibited superior wear resistance in the load range of 10 N–40 N at 180 ◦ C, of which the relative wear resistance was improved by 59.6–83.5%, compared with the Al-Cu matrix alloy. The wear rate of the nanocomposite increased with the increase in the load, similarly to the Al-Cu matrix alloy, while it was lower than that of the matrix alloy under the same load. The worn surfaces indicated that the onset load of obvious delamination of the nanocomposite (40 N) was higher than that of the Al-Cu matrix alloy (20 N). Besides, with the increase in the content of nano-sized TiCp , the relative wear resistance tended to increase. The 0.5 wt.% nano-sized TiCp /Al-Cu composite exhibited superior high-temperature dry sliding wear resistance to the 5 wt.% micro-sized TiCp /Al-Cu composite at 180 ◦ C under a constant load of 20 N, of which the relative wear resistance was 16.5% higher, attributed to the pronounced Orowan strengthening effect of nanoparticles in the nanocomposite.

Acknowledgments: This work is supported by the National Natural Science Foundation of China (NNSFC, No. 51571101 and No. 51601066), the Science and Technology Development Program of Jilin Province, China (Grant No. 20160520116JH and 20170101215JC), “thirteenth five-year plan” Science & Technology Research Foundation of Education Bureau of Jilin Province, China (Grant No. 2015–479), and the Project 985–High Properties Materials of Jilin University. Author Contributions: Wei-Si Tian participated in the design of the work, contributed substantially to the experimental work, the analysis and interpretation of data, and wrote the paper. Qi-Chuan Jiang contributed substantially to the design of the work, supervised the work and wrote the paper. Qing-Long Zhao participated in the design of the work, contributed substantially to the paper writing. Chuan-Jiang Zhao participated in the

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design of the work, and contributed substantially to the experimental. Feng Qiu contributed to the experimental work, the analysis and interpretation of data and the writing of the paper.

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