Introduction

Development of strong lightweight materials is of interest for the automobile industry. The main aim of using lightweight materials includes a reduction in fuel requirements and also as a consequence saving energy and reducing greenhouse gases. In order to produce these materials composites with a matrix of aluminum metal (AMMCs) are promising since they exhibit mechanical and tribological strength compared with normal aluminum alloys. Therefore, their splendid properties are used in aerospace and automobile industries [1,2,3,4,5,6,7,8,9]. With dry friction such forged materials as aluminum alloys normally demonstrate significant wear [10,11,12,13,14,15,16,17,18,19]. During AMMCs production various strengthening additions are made to their composition in order to increase wear resistance and improve mechanical properties [20,21,22,23]. It is noted that the wear resistance of Al–Si alloys is also affected by such production factors as loading, rate, lubrication, surface morphology, microstructure, and volume fraction of silica [24,25,26,27,28]. Natarazhan and others [19] point to significant improvement of the wear resistance of A356/125% SiC composite under action of dry friction. Also well known is the reduction in wear rate for Al?Si alloy with addition of 15 – 20 wt.% SiC [29]. A considerable part of the research of wear of AMMCs composites has been performed using iron contact surfaces (ferrous counterfaces). Significant binding reaction is observed between AMMCs and ferrous materials that is seen in the formation in the course of wear of a layer at an AMMCs surface [6, 8]. In addition, wear characteristics for composites under similar conditions are more favorable than with annealing or use of unstrengthened alloy [30,31,32]. In order to strengthen AMMCs composites there is extensive use of SiC ceramic particles since they exhibit good strength and resistance to mechanical action, significant wear resistance, and corrosion resistance, low LTEC, and also low cost [33,34,35,36].

On the other hand Si3N4-ceramic exhibits high strength and wear resistance. In addition it has low density and LTEC. Due to the splendid mechanical properties of this ceramic it may be used for example in producing pistons, brake rotors, and drums, and also refractories. There is information [37,38,39] about of loose Si3N4-ceramic, its behavior during wetting, mechanical properties and microstructure. It may be confirmed that AMMCs/Si3N4 composites may be used in very different areas.

Currently AMMCs composites are prepared by a solid-phase or liquid-phase method [40, 41]. However, due to the clear advantage of production by a liquid phase method the greatest part of AMMCs composites used in industry is in fact produced by this technology. In addition, liquid metal technology of less expensive. It is simpler to operate with liquid metal than with powder. It is possible to prepare composites of different shapes from liquid metal [42]. During AMMCs production there is extensive use of mixing [32, 43]. Composites prepared by mixing technology exhibit greater porosity than composites prepared by squeeze casting [43]. Squeeze casting has a favorable effect on solidification time, and compaction in a permanent mold and forging makes it possible to increase wear resistance and to improve the composite mechanical properties, and also to strengthen the phase interface due to a reduction in porosity between matrix alloy and strengthening additives, as a result of which more reliable mechanical adhesion is obtained [44].

The authors of present work have introduced different amounts of SiC and Si3N4 into matrix alloy A359 as strengthening from mixtures of their particles and with use of squeeze casting. The effect of additions on thermal conductivity, wear resistance, and friction resistance with different loads has been studied.

Experimental Method

Composite treatment

The matrix alloy A359 used was of the following composition, wt.%: 9.1 Si, 0.58 Mg, balance Al. SiC and Si3N4 particles were added in the form of strengthening particles to alloy A359 in an amount of 5, 10, and 15 wt.% in accordance with the ratio 2:1. The grain size composition of the additive was correspondingly 40 and 3 μm. At first composites were prepared by mixing technology, and then by squeeze casting. Alloy A359 was melted at 1350°C. In order to remove impurities from the let surface layer was collected before introducing strengthening additives. The frequency of melt rotation gradually increased as there was an increase in rotation frequency of a titanium stirrer to 850 rpm. This was required in order to create turbulence during introduction of additives. Mixing lasted for 1 min after introduction of dry substances in order to guarantee uniform distribution of additions to the matrix. Then composites atomized at 680°C were placed in a previously heated (200°C) steel mold in the form of a cylinder with an inner diameter of 50 mm. The temperature of the plunger and piston was 200°C and the pressure was 100 MPa. Holding duration in the course melt compaction was 1 min; then solidified specimens were extracted from the mold.

Measurement of thermal conductivity

Cylindrical disks 2.7 mm in diameter and 2 mm thick were prepared from A359 composites. Thermal conductivity of specimens in the form of disks was determined in an LFA (Netzsch) instrument for performing thermal analysis. An average value was obtained from measurements for four specimens. Specific heat content was A359 specimens was measured in a Setaram Labsys differential scanning calorimeter in argon in the range 25 – 300°C, and density was determined by Archimedes rule Composite thermal conductivity was evaluated by an equation

$$ \uplambda ={c}_p\uprho \upalpha, $$

where λ is thermal conductivity, cp is specific heat content; ρ is density; α is thermal conductivity.

Wear measurement

Tests were conducted in an instrument for testing friction and wear by a pin and disc scheme. In this case pins of composite 8 mm in diameter and 11 mm long slid over a rotating steel disk 200 mm in diameter and 3 mm thick, and with hardness HRC 62. ASurftest SJ-201P instrument was used in order to measure roughness Ra; Ra of control specimens and a steel disk before testing was correspondingly 1.55 and 0.35 μm. The track radius was maintained at a constant level (42 mm) and the disk rotation frequency 655 rpm, which provided a constant sliding rate of 2.75 m/sec. Applied test loads were increased from 20 to 60 N, which corresponded to load levels of 0.40 and 1.19 MPa. Sliding tests were performed continuously for 30 min. Temperature at the start of a test was 20°C. After several minutes the track was contaminated with wear particles. The temperature of the pin surface and force of friction was recorded for each 5 min. Temperature was recorded with an accuracy of 0.1 °C, and friction force with precision of 0.1 N. Data were used to determine friction coefficient. By means of electronic scales with sensitivity of 0.0001 g pin specimen weight loss was evaluated. The wear rate if a pin R, mm3/m, was determined by an equation [20]

$$ R=\frac{\Delta W}{d\uprho}, $$

where ΔW is weight loss; d is sliding distance.

After completion of a test worn pin surfaces were studied in a Vega 3 LMH (Tescan) scanning electron microscope (SEM). Surface morphology and chemical composition of wear particles were studied using and energy-dissipation analyzer (EDXA).

Results and Discussion

Thermal conductivity

The dependence of A359/(SiC + Si3N4) thermal conductivity with a different amount of additive is shown in Fig. 1. It was detected that the thermal conductivity of an A359/(SiC + Si3N4) composite decreases linearly from 158.2 to 130.7 W/(m·K) as there is an increase in the amount of (SIC + Si3N4) to 15%. This feature is explained by the difference in the thermal conductivity of (SiC + Si3N4) particles and matrix alloy A359. However, it should be noted that the form of loose additive, its amount, apparent density, and porosity, and also treatment technology, are significant factors affecting composite thermal conductivity [24, 25].

Fig. 1.
figure 1

Effect of amount of (SiC + Si3N4) strengthening phase on composite λ.

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Use of squeeze casting technology plays an important role in increasing the thermal conductivity of A359/(SIC + Si3N4) composite, guaranteeing preparation of strong composites with low porosity and high density. Therefore, dense contact between components of a composite improves its thermal conductivity. In addition, the thermal conductivity of a A359/5 wt.% (SiC + Si3N4) composite is 151.62 W/(m·K) with porosity of 0.85%, and for a composite A359/15 wt.%(SiC + Si3N4) it is 130.72 W/(m·K) with porosity of 1.95%. This signifies that a reduction in composite A359/(SiC + Si3N4) thermal conductivity proceeds as a result of a change in porosity. Therefore, the amount of strengthening additive and porosity have a strong effect on composite A359 thermal conductivity.

The change in composite A359/(SiC + Si3N4) thermal conductivity in relation to temperature is shown in Fig. 2. As there is an increase in temperature thermal conductivity gradually decreases. However, thermal conductivity is affected not only by temperature, but also the form of strengthening additive and also its composition: in the heating range (25 – 300°C) there is a significant reduction in A359/(SiC + Si3N4) composite thermal conductivity. In order to determine the thermal conductivity of different materials, including composites with a metal matrix, Maxwell and Eucken [46] proposed a simplified form assuming and ideal bond between loose material and matrix in the composite obtained. In the present work the Maxwell – Eucken equation is used for theoretical evaluation of composite thermal conductivity

Fig. 2.
figure 2

Dependence of composite λ on temperature.

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$$ \uplambda ={\uplambda}_m\left(\frac{2\left(\frac{\uplambda_p}{\uplambda_m}-1\right){V}_p+\frac{\uplambda_p}{\uplambda_m}+2}{\left(1-\frac{\uplambda_p}{\uplambda_m}\right){V}_p+\frac{\uplambda_p}{\uplambda_p}+2}\right), $$

where λ is composite thermal conductivity; λp is loose material thermal conductivity (in this study it is 77 and 35 W/(m·K) for SiC and Si3N4 respectively); λm is alloy A350 thermal conductivity (161 W/(m·K)); Vp is volume fraction of added particles.

Comparison of experimental and theoretical values of A359 composite thermal conductivity with a different content of strengthening additives is provided below:

Composite . . . . . . . .

A359

(matrix)

C 5 wt.%

(SiC + Si3N4)

C 10 wt.%

(SiC + Si3N4)

C 15 wt.%

(SiC + Si3N4)

Thermal

 conductivity, W/(m·K):

 theoretical. . . . . . .

161.00

152.62

144.53

136.72

 experimental . . . . .

158.20

148.62

138.03

130.72

As is seen, experimental results almost coincide with those calculated the Maxwell–Eucken equation, which confirms the strong connection between ceramic loose material and a matrix of alloy A359. This especially concerns a A359/(SiC + Si3N4) composite. In addition, this confirms presence of small or insignificant damage around additive particles. Therefore, use of pressure during employment of squeeze casting technology may be considered as valid. In addition, important factors should also be considered such as matrix alloy grade, type and grain size of composition of loose additive [24, 37,38,39].

Two types of particles (SiC + Si3N4) have been used in the present work with different grain size composition. Finer particles of Si3N4 and intended for improving mechanical properties, and the relatively coarser particles of SiC are intended for increasing wear resistance. The additive grain size composition not only affects composite thermal conductivity, but also facilitates an increase in wear resistance. It has been established that additive with a coarser grain size (as in this study) will be fastened with the matrix during a longer time than additive with a finer grain size.

Dependence of wear rate on load

The effect of load on A359/(SiC + Si3N4) composite wear rate is shown in Fig. 3. As is seen, composites demonstrate significantly better wear resistance than alloy A359. A matrix of A359 is a material subject to maximum wear rate with all of the loads used. Wear rate decreases with all loads as there is an increase in amount of strengthening additive (SiC + Si3N4). With a load of 20 N the A359/(SiC + Si3N4) composite wear rate is a little lower than for alloy A359, but with a load of 60 N it is significantly lower than for alloy A359, i.e., the differences between wear rates for composites and A359 matrix is very marked.

Fig. 3.
figure 3

Effect of load on composite wear rate R.

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In addition, the wear rate for a composite based on A359 is considerably reduced as there is an increase in amount of strengthening additive. A possible explanation for this includes the following: as there is an increase in additive content there is an increase in surface area between additive and a steel disk, and additive hinders direct participation of matrix in the wear process. If the amount of additive large, it covers a whole specimen surface. Therefore, the contact area is only between additive and a steel disk. As a consequence, an increase in (SiC + Si3N4) content in composites based on A359 reduces wear rate. In addition, with relatively high loads (60 N) for A359/(SiC + Si3N4) composites there is a slower wear rate than for the A359 matrix. This occurs due to the enormous superiority of ceramic material (SiC + Si3N4) particle properties; low LTEC, significant strength splendid hardness and wear resistance [37,38,39] The superb metallic properties of ceramic material (SiC + Si3N4) and its wear resistance facilitate a reduction in wear rate.

Effect of load and friction path on friction coefficient

The friction coefficient μ for A359/(SiC + Si3N4) composite increases as there is an increase in load in the range 20 – 60 N (Fig. 4). Normally μ for composites and the A359 matrix increases with an increase in load from 20 to 60 N, and for A359/(SiC + Si3N4) composites it is higher than for an alloy A359 matrix with an identical load. This is explained by the significant action of (SiC + Si3N4) material high hardness, due to which greater friction forces arise in the course of the wear process.

Fig. 4.
figure 4

Dependence of composite friction coefficient μ on load.

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The change in friction coefficient is shown in Fig. 5 in relation to friction path with a load of 60 N for a matrix of A359 and a A359/(SiC + Si3N4) composite. It is seen that values of friction coefficient increase in the course of movement with sliding. A logical explanation suggests itself: in the course of wear the formation of products in the form of powder, which enters into contact with the friction surface. Then there is an increase in friction area between the two surfaces, which leads to additional wear between wear products and the contact surface. Then plastic deformation of friction increases the amount of deformation strengthening, and as a consequence of this an increase in friction coefficient. On the other hand, Si phases break down and are distributed between composite and the contact surface, which leads to an increase in friction coefficient as there is an increase in friction path.

Fig. 5.
figure 5

Dependence of composite μ on friction path.

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Increase in composite underlayer temperature in relation to load

The effect of load on the temperature of a A359/(SiC + Si3N4) composite underlayer after a friction path of 5241 m is shown in Fig. 6. It is seen that the temperature of the underlayer region increases almost linearly in relation to applied load. A359/(SiC + Si3N4) composites demonstrate higher underlayer temperature than the matrix alloy A359. This is explained by the greater friction forces for a A359/(SiC + Si3N4) composite during wear. In addition, the greater the amount of SiC in the additive, the higher the underlayer temperature is raised. Due to an increase in contact area and presence of wear products between friction surface the friction force increases and as a consequence there is an increase in surface temperature [47].

Fig. 6.
figure 6

Effect of load and amount of strengthening additive on composite pin temperature.

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Worn surface morphology

SEM microphotographs are shown in Fig. 7 of a worn surface of matrix alloy A359 and A359/(SiC + Si3N4) composite tested under a load of 60 N [48]. Presence at the alloy A359 surface (Fig. 7a) of troughs, and cavitation projections in the wear direction point to significant ductility, especially with a load of 60 N. For an A359/(SiC + Si3N4) composite in wear surfaces fewer troughs or projections are observed than for alloy A359. This is especially noticeable in Fig. 7c, d for A359/(SiC + Si3N4) composites with a higher content of compacting additives. Wear testing is accompanied by plastic deformation of a pin surface (see Fig. 7b). This plastic flow of metal in the surface layer of a pin covers (SiC + Si3N4) particles and prevents their advance towards the contact surface until a metal layer covering the ceramic material is partly worn away. Then as a result of action of a load a mixture of particles of additive is refined into a finely ground fraction and distributed over the wear surface.

Fig. 7.
figure 7

SEM-microphotographs of A359 composite worn surfaces with a load of 60 N: a) A359 alloy: b) A359/5 wt.% (SiC + Si3N4); c) A359/10 wt.% (SiC + Si3N4); d) A359/15 wt.% (SiC + Si3N4).

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Refined particles are distributed over the pin surface, broken apart and joined forming a continuous film of hard substance (SiC + Si3N4) on a worn surface. At the same time, a solid hybrid layer develops from broken (SiC + Si3N4) particles. As there is wear it spreads at the composite pin surface. Then a film of (SiC + Si3N4) is transferred to the contact surface. In the course of this process (SiC + Si3N4) material is fed continuously, which acts as a solid layer between the two friction surfaces. That is, the composite becomes more wear resistant due to transfer of ground loose material of additive to the rubbing surfaces and formation ion them of a thin film. Therefore, improvement of composite wear characteristics depends on the capacity of (SiC + Si3N4) material particles to emerge from the “built-in” position in a matrix and to be distributed uniformly in the form of a solid film between tribological (rubbing) surfaces [20, 47]. This adhesive wear is expressed in the form of adhesive “ploughing” of the pin surface. Wear of A359 depends to a considerable degree on crack generation and propagation over rubbing surfaces. A source of crack generation in Al–Si alloy is the Al/Si interface.

In the initial period of the wear process silicon particles prevent breakdown and protect the surface. Then cracks are generated, and they join each other forming wear products. Hard silicon particles break into fine lumps separating from the matrix. Crushing of silicon particles proceeds when the load on an Si particles appears to be greater than the silica failure resistance [48]. Worn surfaces of A359/5 wt.% (SiC + Si3N4) composites may differ with respect to plastic deformation and short cracks in the sliding direction, These cracks probably arise under the wear surface of the pin, and propagate towards the wear surface. This points to local plastic deformation in the underlayer region, adjacent to a contact surface. Deformation caused by shear leads to occurrence of considerable tension in a worn pin layer. Localization of this tension leads to crack generation and propagation over tracks under action of shear. Considerable plastic deformation normally accelerates generation of cavities and their growth in particles present within underlayer regions. These cavities then join and form a crack and the alloy wear products separate into layers [49].

Wear products of A359/15 wt.% (SiC + Si3N4) composite are shown in Fig. 8 under a load of 60 N, having the form of thin platelets and fine particles. They may be considered as wear products over the steel disk and composite pin. The composition of wear products mainly includes Al, Si, Fe, C, O, and Cr (Fig. 9) wear products formed with high loads are a mechanical mixture of particles separating both from specimens and from the contact surface. There are mechanisms of wear product formation including separation at the surface and abrasion. Presence of Fe may be considered as a sign of abrasive action of the projecting ends of (SiC + Si3N4) and Si particles on a worn steel contact surface. Broken fragments of ceramic particles are mixed with loose wear products and act as a third abrasive factor, having an effect both on the composite surface, and on the steel surface. Iron oxides, covering wear surfaces, act as a lubricant and provide a smooth form of worn surface finally increasing is wear resistance.

Fig. 8.
figure 8

SEM-microphotographs of A359/15 wt.% (SiC + Si3N4) composite wear products tested with a load of 60 N.

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Fig. 9.
figure 9

EDXA-spectrum of A359/15 wt.% (SiC + Si3N4) composite wear products tested with a load of 60 N.

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