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Sep 6, 2017 - characterized by lower and more stable coefficients of friction (µ), as well ... and structure of a composite material intended for tribological ... and Sr in area of reinforcement; (b) SEM image of interpenetrating .... obtained by alumina foam addition will have a positive effect on the operation of the tribological.
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Tribological Properties of AlSi12-Al2O3 Interpenetrating Composite Layers in Comparison with Unreinforced Matrix Alloy Anna Janina Dolata Faculty of Materials Engineering and Metallurgy, Silesian University of Technology, Krasinskiego ´ 8, 40-019 Katowice, Poland; [email protected]; Tel.: +48-32-603-4426 Received: 3 August 2017; Accepted: 4 September 2017; Published: 6 September 2017

Abstract: Alumina–Aluminum composites with interpenetrating network structures are a new class of advanced materials with potentially better properties than composites reinforced by particles or fibers. Local casting reinforcement was proposed to take into account problems with the machinability of this type of materials and the shaping of the finished products. The centrifugal infiltration process fabricated composite castings in the form of locally reinforced shafts. The main objective of the research presented in this work was to compare the tribological properties (friction coefficient, wear resistance) of AlSi12/Al2 O3 interpenetrating composite layers with unreinforced AlSi12 matrix areas. Profilometric tests enabled both quantitative and qualitative analyses of the wear trace that formed on investigated surfaces. It has been shown that interpenetrating composite layers are characterized by lower and more stable coefficients of friction (µ), as well as higher wear resistance than unreinforced matrix areas. At the present stage, the study confirmed that the tribological properties of the composite layers depend on the spatial structure of the ceramic reinforcement, and primarily the volume and size of alumina foam cells. Keywords: aluminum matrix composites (AMCs); interpenetrating composites (IPCs); alumina foams; centrifugal infiltration; tribological properties

1. Introduction The new directions of research in the area of metal matrix composites (MMCs) include the activities aimed at the development of effective methods for manufacturing these materials. At present, the composites with layered or gradient structures [1–6], multiphase composites (hybrid, heterophase) [7–11], and particularly ceramic–metal interpenetrating composites (IPCs) are investigated [12–17]. Special emphasis is placed on the development of “net shape” or “near net shape” technologies, which to a large extent allow the elimination or reduction of the machining of composite products, and thus reduce both wastes and production costs. Casting aluminum alloys are most often used for matrix composites due to their advantageous properties, low price, and density [18]. In turn, by the proper selection of the type, size, volume fraction, and morphology of the reinforcing ceramic components, as well as the composite manufacturing method, it is possible to produce aluminum matrix products with special properties. It has been noted that the interpenetrating composites (IPCs), which consist of 3-dimensionally continuous matrices of two different phases (ceramic and metallic), are interesting materials with potentially superior properties when compared with traditional composites containing discontinuous particles or whiskers as well as continuous or short fibers [14,19–21]. For example, Peng et al. [19] reported that alumina–aluminum interpenetrating phase composites show a higher modulus of elasticity compared with the traditional AA6061/Al2 O3 Duralcan composites. Similarly, the results of our own previous studies [22,23] showed that the hardness, compressive strength, and Young’s Materials 2017, 10, 1045; doi:10.3390/ma10091045

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modulus in such composites increase, while decreasing the pores’ size in alumina foam. It has been clearly demonstrated that the level of any IPC’s properties depends on the degree of filling empty ceramic spaces by the liquid Al alloy. The favorable mechanical characteristics are the result of a specific composite macrostructure with percolation of ceramic and metal phases. Due to the potential applications for IPCs in the machine, construction, and automotive industries (disc brake, pistons, cylinder sleeve, etc.), good mechanical properties such as thermal and dimensional stability and proper wear resistance are essential. The research conducted by Binner et al. [20,21] showed that the Al(Mg)/Al2 O3 interpenetrating composites obtained by pressureless infiltration have significantly better wear resistance than matrix alloy, and moreover in comparison to conventional AA6061/Al2 O3 and AA2014/Al2 O3 Duralcan composites. The authors found that the composite made from the lowest foam density exhibited a ‘ploughing’ wear throughout the process, whilst the composites with the higher foam density, and hence a higher hardness and load-bearing capability, exhibited a transition from ‘ploughing’ to ‘protective’ wear [20]. The obtained results are promising for the potential use of these new engineering materials in areas requiring wear resistance coupled with lightweight applications. However, it should be noted that in the process of designing the composition and structure of a composite material intended for tribological cooperation, external factors enforcing a certain set of the material reaction are taken into consideration. These include: load, operational temperature, lubrication type, speed of movement, presence of vibration. They also include a broad range of structural properties of the material, such as the type of matrix and the reinforcing phase, the fraction and size of the reinforcing phase, and its morphology [24–26]. Each of these factors has a direct influence on the durability and reliability of a tribological pair. Changing any from these factors can give different results, which justifies further research in this area. In our own previous papers [27–30], the theoretical background, results of experiments and structure of AlSi12/Al2 O3 interpenetrating composite layers obtained by the centrifugal infiltration method, have been described in detail. The aim of research presented in this work was to compare the tribological properties (wear resistance, friction coefficient) of these composites with unreinforced AlSi12 matrix alloy. Profilometric tests enabled quantitative and qualitative analyses of the wear trace that formed on investigated surfaces. 2. Materials and Methods The alumina oxide foams (Al2 O3 ) with various total porosity and different cell size (Figures 1 and 2) resulting from the applied manufacturing method (replacement of porous polymer matrix) were used as reinforcing [28,30]. In the first foam, which was designated as Al2 O3 _1, slight differences of cell size were observed (Figures 1a and 2a). Over 60% were in the range from 350 to 550 µm. The second foam, Al2 O3 _2, is characterized by larger pore diameters and a much greater dispersion of their size from 300 to 1150 µm, where over 50% of cell sizes were in the range of 800–1150 µm (Figures 1b and 2b). To shape the castings, locally reinforced via ceramic skeletons with known spatial structures, and the centrifugal infiltration process were used [27–30]. As a result of the centrifugal force acting on the liquid AlSi12 alloy surface, castings containing a composite layer with percolation structure were obtained (Figure 3). The residual porosity of the reinforced areas, measured by computer-assisted tomography, did not exceed 1% [30]. Moreover, the complete filling of the cells, and the absence of structure defects and discontinuities at the metal–ceramics interface, have been confirmed by detailed examinations using scanning electron microscopy and energy dispersive spectroscopy (SEM, EDS), as described in previous works [27,30].

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Figure 1. Scanning electron microscopy (SEM) micrographs of alumina oxide preforms used for Figure 1. 1. Scanning Scanningelectron electron microscopy(SEM) (SEM) micrographs micrographs of of alumina alumina oxide oxide preforms used used for for (a) (b) preforms Figure microscopy 2O3_1; (b) Al2O3_2. centrifugal infiltration process by liquid AlSi alloy: (a) Al centrifugal infiltration process by liquid AlSi alloy: (a) Al O _1; (b) Al O _2. 2 3_2. 3 (b)alumina Al2O centrifugal process microscopy by liquid AlSi alloy: (a) Al22O33_1;of Figure 1. infiltration Scanning electron (SEM) micrographs oxide preforms used for centrifugal infiltration process by liquid AlSi alloy: (a) Al2O3_1; (b) Al2O3_2.

(a) (b) (a) (b) (a) (b)infiltration process by Figure 2. SEM micrographs of alumina oxide preforms used for centrifugal

Figure 2. micrographs of alumina oxide preforms FigureAlSi 2. SEM SEM alumina preforms used used for for centrifugal centrifugal infiltration infiltration process process by by (b) Al2O3_2.oxide liquid alloy:micrographs (a) Al2O3_1; of Figure 2. SEM micrographs of alumina oxide preforms used for centrifugal infiltration process by 2O3_1; (b) Al2O3_2. liquid AlSi alloy: (a) Al liquid AlSi alloy: (a) Al2 O3 _1; (b) Al2 O3 _2. liquid AlSi alloy: (a) Al2O3_1; (b) Al2O3_2.

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Figure 3. The AlSi12/Al2O3_2 composite cast obtained by centrifugal infiltration: (a) view of the Figure The AlSi12/Al 3_2 composite cast obtained by centrifugal infiltration: infiltration: (a) (a) view viewofofthe the Figure3. Thecentrifugal AlSi12/Al2O 2O cast obtained by representative the form of locally reinforced shaft, and infiltration: X-ray mapping of Al,of O,the Si Figure 3.3.The AlSi12/Al O33_2 _2incomposite composite cast obtained bycentrifugal centrifugal (a) view 2cast representative centrifugal cast in the form of locally reinforced shaft, and X-ray mapping of Al, O, Si representative centrifugal cast in in the form of locally reinforced and mapping ofofAl, representative centrifugal cast the form of locally reinforced shaft, andX-ray X-ray(IPCs) mapping Al,O,O,SiSi and Sr in area of reinforcement; (b) SEM image of interpenetrating composites layer. and SrSrininarea reinforcement; composites(IPCs) (IPCs) layer. and areaof reinforcement;(b) (b)SEM SEMimage image of of interpenetrating interpenetrating composites and Sr in area ofofreinforcement; (b) SEM image of interpenetrating composites (IPCs)layer. layer.

Tribological studies (wear resistance, coefficient of friction) were carried out in cross-sections of Tribological were carried carriedout outin incross-sections cross-sectionsofof Tribologicalstudies studies(wear (wearresistance, resistance,coefficient coefficient of friction) were the composite layer formed by the centrifugal infiltration process and compared with the thecomposite composite layer layer formed formed by by the the centrifugal centrifugal infiltration process the process and and compared compared with with the the unreinforced area. The samples used to determine the tribological properties (Table 1), sized 30 × 15 unreinforcedarea. area.The Thesamples samplesused usedto todetermine determine the the tribological properties unreinforced properties (Table (Table1), 1),sized sized30 30××1515

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Tribological studies (wear resistance, coefficient of friction) were carried out in cross-sections of the composite layer formed by the centrifugal infiltration process and compared with the unreinforced area. The samples used to determine the tribological properties (Table 1), sized 30 × 15 × 10 mm, were cut out from composite shafts and polished before testing. Thus, a prepared composite and matrix surface was subjected to the abrasion test under dry sliding conditions using tribology pin-on-block tester [31]. A normal load of 15 N (unit pressure of 2 MPa) and a sliding speed of 0.1 m/s were applied throughout the tests. The counter-pin material, φ = 3 mm and 20 mm in length, was made of EN-GJ250 cast iron. The tests were carried out with the 9-mm stroke length over a distance of 1000 m at air temperature (20 ◦ C). During tests, the friction coefficient was measured. The obtained results were presented in the form of graphs as a function of the sliding distance. Identical load conditions and abrasion speed allowed the comparison of friction coefficient values and the wear of composites with those of the unreinforced matrix. Table 1. Designation of samples used to determine tribological properties. Designation P0 P1 P2

Material AlSi12 matrix AlSi12/Al2 O3 _1 AlSi12/Al2 O3 _2

Volume of Al2 O3 [%]

Pore Size [µm]

22 18

350–550 300–1100

The wear trace that appeared on the surface of composite and matrix samples was subjected to profilometric analyses using the MicroProf 3000, FRT optical profilometer (FRT GmbH, Germany). The study of the wear trace geometry was carried out immediately after the friction process. Only an ultrasonic scrubber was used to clean the surface of the tested samples. The basic features of the surface, such as the depth of the wear trace and roughness, were assessed. 2D and three-dimensional images were used in the analysis. The wear resistance of the tested samples (AlSi12 matrix and AlSi12/Al2 O3 composite layers) was determined based on volume loss measurements of the wear traces formed on their surfaces. The research was carried out based on 3D image analysis with 0.1 µm accuracy in X- and Y-axes, and with 0.01 µm in Z-axis. 3. Results and Discussion The results of the friction coefficient measurement as a function of the sliding distance for the matrix area (without reinforcement), and comparisons with areas reinforced by two different alumina foams, are shown in Figure 4a. It was observed that in the case of unreinforced areas (AlSi12 matrix alloy), the course of changes in the coefficient of friction is unstable, with variations of 0.1. Such sudden changes in friction coefficient values are characteristic for the adhesive wear mechanism, which is also confirmed by profilometric observations of the wear track surface (Figure 5). In turn for both composite layers with a percolation structure (red and green lines in Figure 4a), which differ in porosity, and cell size in alumina foam, a similar character of friction coefficient change was recorded. At the initial stage of friction (500 m), the coefficient of friction is significantly higher than its value recorded in the second half of the sliding distance. For the composite layer marked P1, the value of the friction coefficient during the sliding distance changes from µ = 0.3 to µ = 0.23, while for composite layer signed P2, this change is µ = 0.4 in the first part, and µ = 0.27 at the final stage of friction, respectively. The 2D and 3D images of the wear track of the composite layers have been shown in Figures 6 and 7.

The results of the friction coefficient measurement as a function of the sliding distance for the matrix area (without reinforcement), and comparisons with areas reinforced by two different alumina foams, are shown in Figure 4a. It was observed that in the case of unreinforced areas (AlSi12 matrix alloy), the course of changes in the coefficient of friction is unstable, with variations of 0.1. Such sudden changes in friction coefficient values are characteristic for the adhesive wear mechanism, Materials 2017, 10, 1045 5 of 10 which is also confirmed by profilometric observations of the wear track surface (Figure 5).

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composite layers with a percolation structure (red and green lines in Figure 4a), 5 of 10 which differ in porosity, and cell size in alumina foam, a similar character of friction coefficient In turn for both composite layers with a percolation structure (red and green lines in Figure 4a), In turn both composite layers withofafriction percolation structure (red and of green linesisinsignificantly Figure 4a), change was for recorded. At the initial stage (500 m), the coefficient friction which differ in porosity, and cell size in alumina foam, a similar character of friction coefficient which in value porosity, and cell size in alumina foam, a similar character of friction coefficient higher differ than its recorded in the second half of the sliding distance. For the composite layer change was recorded. At (a) the initial stage of friction (500 m), the coefficient (b) of friction is significantly change of friction (500the m),sliding the coefficient friction from is significantly markedwas P1, recorded. the value At of the initial frictionstage coefficient during distanceofchanges μ = 0.3 to higher than4.its valuecoefficient recorded(μ) in versus the second half of the sliding distance. the matrix composite Figure Friction sliding distance for un-reinforced areaFor (AlSi12 alloy)layer Figure 4. its Friction coefficient (µ) sliding distance forissliding un-reinforced (AlSi12 matrix higher than value recorded inversus the second half of the theand composite layer μ = 0.23, while for composite layer signed P2, this change μ = 0.4distance. in thearea firstFor part, μ = alloy) 0.27 at the marked the value of the friction coefficient during the sliding distance changes from μ = 0.3 to 2O3 foam composite layers (a); view of AlSi12/Al AlSi12/Al 2O3_1 surface after and P1, AlSi12/Al AlSi12/Al layers of _1composite composite surface afterμdry dry 2 O3 foam 2 Owear 3distance marked P1, value of thecomposite frictionThe coefficient during the of sliding from = layers 0.3 to finaland stage ofthe friction, respectively. 2D (a); andview 3D images the trackchanges of the composite μ = 0.23, while for composite layer signed P2, this change is μ = 0.4 in the first part, and μ = 0.27 at the sliding wear test (b). sliding wear test μ = 0.23, while for composite P2, this change is μ = 0.4 in the first part, and μ = 0.27 at the have been shown in(b). Figures 6layer and signed 7. final stage of friction, respectively. The 2D and 3D images of the wear track of the composite layers final stage of friction, respectively. The 2D and 3D images of the wear track of the composite layers have been shown in Figures 6 and 7. have been shown in Figures 6 and 7.

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(a) track (b) Figure5.5.View View of wear in unreinforced area (AlSi12 matrix) after dry sliding (a) condition: Figure of wear track condition: digital (a) in unreinforced area (AlSi12 matrix) after dry sliding(b) (a) digital image; (b) cross-section through 3D view. image; cross-section through 3D unreinforced view. Figure (b) 5. View of wear track in area (AlSi12 matrix) after dry sliding condition: Figure 5. View of wear track in unreinforced area (AlSi12 matrix) after dry sliding condition: (a) digital image; (b) cross-section through 3D view. (a) digital image; (b) cross-section through 3D view.

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(a)track in AlSi12/Al2O3_1 composite layer (P1) after dry (b) sliding condition: Figure 6. View of wear (a) (b) (a) digital image; (b) cross-section through 3D view. Figure 6. View of wear track in AlSi12/Al 2O3_1 composite layer (P1) after dry sliding condition: Figure O33_1 _1 composite composite layer layer (P1) (P1) after after dry Figure 6. 6. View View of of wear wear track track in inAlSi12/Al AlSi12/Al22O dry sliding sliding condition: condition: (a) digital digital image; image; (b) (b) cross-section cross-section through through 3D 3D view. view. (a) (a) digital image; (b) cross-section through 3D view.

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dry sliding condition: Figure 7. View of wear (a) track in AlSi12/Al2O3_2 composite layer (P2) after (b) (a) (b) (a) digital image; (b) cross-section through 3D view. Figure 7. View of wear track in AlSi12/Al2O3_2 composite layer (P2) after dry sliding condition: 3_2 composite layer (P2) after dry sliding condition: Figure 7. 7. View View of wear wear track track in inAlSi12/Al AlSi12/Al2O Figure (a) digital image;of(b) cross-section through 3D 2 Oview. 3 _2 composite layer (P2) after dry sliding condition: Asdigital can be seen,(b)the friction coefficient ofview. the tested composite layers decreases as the pores (a) image; cross-section through 3D (a) digital image; (b) cross-section through 3D view.

diameter in the alumina foam decreases. In both cases, following the running-in phase (about 500 m), As can be seen, the friction coefficient of the tested composite layers decreases as the pores As can coefficient be seen, the friction and coefficient of the tested decreases as the pores the friction stabilized, remained stable untilcomposite the end oflayers the test. Such a characteristic diameter in the alumina foam decreases. In both cases, following the running-in phase (about 500 m), diameter the alumina decreases. In both cases, running-in phase (about 500from m), course of in changes in the foam coefficient of friction could be following related to the change of wear mechanism the friction coefficient stabilized, and remained stable until the end of the test. Such a characteristic the friction coefficienttostabilized, and remained the endthe of the test. Such aofcharacteristic adhesive—abrasive the abrasive only. As stable can beuntil expected, stabilization the friction course of changes in the coefficient of friction could be related to the change of wear mechanism from course of changes in the of friction could behave related to the change wear from coefficient obtained by coefficient alumina foam addition will a positive effectofon the mechanism operation of the adhesive—abrasive to the abrasive only. As can be expected, the stabilization of the friction adhesive—abrasive the abrasive only.layer. As can be expected, the stabilization of the friction tribological system’sto cast iron—composite

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As can be seen, the friction coefficient of the tested composite layers decreases as the pores diameter in the alumina foam decreases. In both cases, following the running-in phase (about 500 m), the friction coefficient stabilized, and remained stable until the end of the test. Such a characteristic course of changes in the coefficient of friction could be related to the change of wear mechanism from adhesive—abrasive to the abrasive only. As can be expected, the stabilization of the friction coefficient obtained by alumina foam addition will have a positive effect on the operation of the tribological system’s cast iron—composite layer. In addition, a detailed analysis of the tested materials surface both before and after friction were performed based on the profilometric measurements. The initial state of the surface geometry of the unreinforced area (matrix AlSi12 alloy) and AlSi12/Al2 O3 _2 composite layer (P2) is shown in the Figures 8–10, respectively. As can be seen from the presented images, the surface of the composite layers has different a geometry in comparison with unreinforced areas. The distribution of the ceramic cells is regular, and their shape is clearly extended (Figure 9a). In this case, as well as for the second tested composite layer (P1), the ceramic reinforcement protruding above the matrix at a height of about 5 µm was observed (Figure 9b—exemplary line marked on Figure 9a). Future profilometric tests enabled quantitative and qualitative analyses of the wear trace that formed on investigated surfaces (Figures 10–12). The profilometric analysis of the AlSi12 matrix wear track images confirmed the specific, plastic material deformation, 0.4 mm in depth, formed at the edges (Figure 10). The quantitative analysis performed for the AlSi12 matrix wear track surface after dry friction conditions showed intensive wear. On the basis of qualitative observation, the wear trace can be divided into two zones (Figure 10a). In the first area, marked A, the deep wear with abrasive phenomenon can be observed. In the second zone, determined as B, the wear has an adhesive character. In turn, the depth of the wear track calculated as the maximum difference of elevation of the wavy line and its lowest position, in the case of AlSi12 matrix area after cooperation with cast iron pin, is 0.25 mm. The use of local reinforcement in the form of ceramic alumina foams has resulted in a significant increase in the wear resistance under technically dry friction conditions. In both cases, the consumption was based on abrasive wear, irrespective of the cell size and the volume fraction of the ceramic phase (Figures 11 and 12). For the P1 composite layer (Figure 11), the depth of the wear track did not exceed 100 µm. In turn, the P2 composite (Figure 12) was characterized by a twice as large depth of the wear Materials 2017, 10, 1045 6 of 10 track, which reaches 220 µm. However, in both materials, plastic deformation was not observed, and matrix elevation at the wear edges is did not exceed 20 µm in depth. ceramic cells is regular, and track their shape clearly extended (Figure 9a). In this case, as well as for the Furthermore, the volume loss of the wear traces was measured inabove orderthe to compare wear second tested composite layer (P1), the ceramic reinforcement protruding matrix at athe height resistance thewas examined reinforced and unreinforced Theon obtained results (Figure 13) clearly of about 5ofμm observed (Figure 9b—exemplary lineareas. marked Figure 9a). Future profilometric showed a 10 times smaller volume loss of composite layers compared with the unreinforced matrix tests enabled quantitative and qualitative analyses of the wear trace that formed on investigated area in dry(Figures friction10–12). conditions. surfaces

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Figure8.8.Initial Initial state of the surface geometry the unreinforced area: (a) 3D(b)view; (b)difference height Figure state of the surface geometry of theofunreinforced area: (a) 3D view; height difference on line (1). on line (1).

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(a) surface geometry of the unreinforced area:(b)(a) 3D view; (b) height Figure 8. Initial state of the difference onFigure line (1). 8. Initial state of the surface geometry of the unreinforced area: (a) 3D view; (b) height

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difference on line (1).

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(a) Figure 9. Initial state of surface geometry of the AlSi12/Al2O3_2 composite layer(b) (P2): (a) 3D view; (b) height difference on line (1). Initial state of of surface geometry of of the AlSi12/Al 2O3_2 composite layer (P2): (a) 3D view; 9. Initial state surface geometry the AlSi12/Al 2 O3 _2 composite layer (P2): (a) 3D view;

Figure 9. Figure (b)(b) height difference onon line (1). The profilometric analysis height difference line (1). of the AlSi12 matrix wear track images confirmed the specific, plastic material deformation, 0.4 mm in depth, formed at the edges (Figure 10).

The profilometric analysis of the AlSi12 matrix wear track images confirmed the specific, plastic material deformation, 0.4 mm in depth, formed at the edges (Figure 10).

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The quantitative analysis performed for the AlSi12 matrix wear track surface after dry friction conditions showed intensive wear. On the basis of qualitative observation, the wear trace can be divided into two zones (Figure 10a). In the (a) first area, marked A, the deep wear with abrasive phenomenon can be observed. In the second zone, determined as B, the wear has an adhesive character. In turn, the depth of the wear track calculated as the maximum difference of elevation of the wavy line and its lowest position, in the case of AlSi12 matrix area after cooperation with cast iron pin, is 0.25 mm. The use of local reinforcement in the form (a) of ceramic alumina foams has resulted in a significant increase in the wear (b) resistance under technically dry friction conditions. In both cases, the (c) consumption was based on abrasive wear, irrespective of the cell size and the volume fraction of the Figure 10. Surface geometry of the AlSi12 unreinforced after working with ironof pin: (a)wear viewtrack ceramic phase (Figures 11 of andthe 12). For theunreinforced P1 compositearea layer (Figure 11), thecast depth the Figure 10.ofSurface geometry AlSi12 area after working with cast iron pin: (a) view the wear 100 track; (b) roughness distribution across(Figure to the friction direction on line (1); did not exceed μm. In turn, the P2 composite 12) was characterized by(c)a roughness twice as large of the wear track; (b) roughness distribution across to the friction direction on line (1); (c) roughness along thewhich frictionreaches direction220 on μm. line (2). depthdistribution of the wear track, However, in both materials, plastic deformation was distribution along and the matrix frictionelevation direction on wear line (2). not observed, at the track edges did not exceed 20 μm in depth.

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Figure 10. Surface geometry of the AlSi12 unreinforced area after working with cast iron pin: (a) view of the wear track; (b) roughness distribution across to the friction direction on line (1); (c) roughness distribution along the friction direction on line (2).

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Figure 11. The surface geometry of the AlSi12/Al2O3_1 composite layer (P1) after working with cast

Figure 11.iron Thepin: surface of the AlSi12/Al afteron working 2 O3 _1 composite (a) viewgeometry of wear track; (b) roughness distribution across to thelayer friction(P1) direction line (1); with cast iron pin: (a) view of wear track;along (b) the roughness distribution across to the friction direction on line (1); (c) roughness distribution friction direction on line (2). (c) roughness distribution along the friction direction on line (2).

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Figure 11. The surface geometry of the AlSi12/Al2O3_1 composite layer (P1) after working with cast iron pin: (a) view of wear track; (b) roughness distribution across to the friction direction on line (1); 8 of 10 Materials 2017, 10, 1045 (c) roughness distribution along the friction direction on line (2).

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Figure 12. Surface geometry of AlSi12/Al2O3_2 composite layer (P2) area after working with the cast iron pin: (a) view of the wear track; (b) roughness distribution across to the friction direction on line (1); (c) roughness distribution along the friction direction on line (2). (b) (c) composite layer (P2) after working with cast Figure AlSi12/Al Furthermore, thegeometry volume of loss of the2O was measured order to compare Figure 12. 12. Surface Surface geometry of AlSi12/Al O3_2 composite layer (P2)area areain after working withthe thethe castwear 2wear 3 _2 traces iron pin: (a) view of the wear track; (b) roughness distribution across to the friction direction on line resistance of the examined reinforced and unreinforced areas. The to obtained results (Figure 13)(1); clearly iron pin: (a) view of the wear track; (b) roughness distribution across the friction direction on line (1); (c) roughness distribution along the friction direction on line (2). (c) roughness distribution along the friction direction on line (2). showed a 10 times smaller volume loss of composite layers compared with the unreinforced matrix

area in dry friction conditions. Furthermore, the volume loss of the wear traces was measured in order to compare the wear resistance of the examined reinforced and unreinforced areas. The obtained results (Figure 13) clearly showed a 10 times smaller volume loss of composite layers compared with the unreinforced matrix area in dry friction conditions.

Figure 13. Volume loss of investigated interpenetrating composite layers (P1 and P2) in comparison Figure 13. Volume loss of investigated interpenetrating composite layers (P1 and P2) in comparison with the unreinforced matrix area (AlSi12) in dry sliding conditions. with the unreinforced matrix area (AlSi12) in dry sliding conditions.

The lower wear in composite layers compared with the unreinforced matrix can be attributed to The lower wear inloss composite layersinterpenetrating compared withcomposite the unreinforced be attributed to Figure 13. Volume of investigated layers (P1matrix and P2)can in comparison 2O3 network protruding out of the worn surface, which protects the direct wear of the AlSi12 an Al an Al network out the worn surface, protects the direct wear of the AlSi12 with unreinforced matrix areaof (AlSi12) in dry sliding which conditions. 2O 3the matrix alloy by the protruding cast iron pin. matrix alloy by the cast iron pin. The lower wear in composite layers compared with the unreinforced matrix can be attributed to 4. Conclusions 4. Conclusions an Al2O3 network protruding out of the worn surface, which protects the direct wear of the AlSi12 The interpenetrating composite layers obtained in the centrifugal infiltration process have a matrix alloy by the cast iron pin. The interpenetrating composite layers obtained in the centrifugal infiltration process have a good good connection at the interface between alumina preforms and the AlSi12 matrix. The strong connection at the interface between alumina preforms and the AlSi12 matrix. The strong boundary and boundary and the characteristic interpenetrating structure of the composite layers influence 4. Conclusions the characteristic interpenetrating structure of the composite layers influence tribological properties tribological properties (wear resistance, friction coefficient). The investigation results proved that the (wear resistance, friction coefficient). The investigation results provedinfiltration that the composite layers The interpenetrating composite layers obtained in the centrifugal process have a composite layers with the Al2O3 foams are characterized by a lower friction coefficient of about 30% with the Al2 O3 foams areinterface characterized by aalumina lower friction coefficient of AlSi12 about 30% for P1 foam and good connection at the between preforms and the matrix. The strong for P1 foam and near 25% for P2 in comparison with the unreinforced area in the cast. Moreover, it near 25% for P2 the in comparison withinterpenetrating the unreinforcedstructure area in the has been shown boundary and characteristic of cast. the Moreover, composite itlayers influence has been shown that the friction coefficient of composite layers decreases as the pores’ diameters that the friction coefficient composite layerscoefficient). decreases asThe theinvestigation pores’ diameters decrease. In turn, tribological properties (wearofresistance, friction results proved that the decrease. In turn, the composite layers’ higher wear resistance in comparison with matrix areas is composite layers with the Al2O3 foams are characterized by a lower friction coefficient of about 30% related to the change of wear mechanism from adhesive—abrasive to the abrasive only. for P1 foam and near 25% for P2 in comparison with the unreinforced area in the cast. Moreover, it In addition, it has been proved that local reinforcement of castings improves their properties has been shown that the friction coefficient of composite layers decreases as the pores’ diameters only in areas that are highly exposed to wear, and it allows to maintain the initial mechanical and decrease. In turn, the composite layers’ higher wear resistance in comparison with matrix areas is plastic properties in unreinforced areas. This solution is favorable, particularly from the point of view related to the change of wear mechanism from adhesive—abrasive to the abrasive only.

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the composite layers’ higher wear resistance in comparison with matrix areas is related to the change of wear mechanism from adhesive—abrasive to the abrasive only. In addition, it has been proved that local reinforcement of castings improves their properties only in areas that are highly exposed to wear, and it allows to maintain the initial mechanical and plastic properties in unreinforced areas. This solution is favorable, particularly from the point of view of the finishing problems of composite products. Further studies will concern a wider description of the wear mechanism of IPCs in various friction coupling. Acknowledgments: Publication supported under the Rector’s Habilitation Grant. Silesian University of Technology, 11/030/RGH16/0098. Conflicts of Interest: The authors declare no conflict of interest.

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