Tribological characterization of aluminum-based composites with reinforcement of submicro SiC or Al 2O3 particles with glassy carbon addition

XX International Student’s Day of Metallurgy

Cracow, 14-16 March 2013

Tribological characterization of aluminum-based composites with reinforcement of submicro SiC or Al2O3 particles with glassy carbon

addition

Bartosz Hekner

Silesian University of Technology, Department of Material Science and Metallurgy, Krasińskiego 8, 40-019 Katowice, Poland

bartoszhekner@gmail.com

1. ABSTRACT The effect of a type of reinforcing submicro-particles in aluminum matrix composites on their tribological properties is presented in this article. The resultant materials were reinforced via addition of 20 mass % of the ultra fine ceramic particles with simultaneous addition of 5 mass % of glassy carbon particles. Two types of ceramic particles were chosen: α- Al2O3 as the chemically inert ones in the contact with the Al matrix or SiC with possibility of a displacement reaction in the contact zone. The modification of composition by glassy carbon particles was used in order to create the lubricant function in the working regime and as a consequence to change the composite wear mechanism and to reduce their mass losses. The application of nano- or submicrosized ceramic particles could provide higher adhesive stresses around the particles in the metal matrix and to limit their pulling-out during wear process. An average size of reinforcing particles was reduced to the nanometer size using high-energy ball milling. A permanent connection of the metal – ceramic interphase was obtained as a result of a mechanical alloying technique with subsequent hot-pressing and sintering in the next step. The resultant composites were examined by tribological measurements (mass losses, coefficient of friction), microstructure properties (SEM with EDS technique) and Vickers hardness. It has been shown that the type of ceramic particles considerably changes the wear behavior of the tested samples.

Keywords: aluminum matrix composites (AMC), Al2O3 / SiC reinforcements, wear mechanism, tribological properties

2. INTRODUCTION A proper coefficient of friction with fine stability and low mass losses, which ensure the long life-time, are the most important requirements for tribological materials. Nowadays, among materials for the brake system, the metal-matrix composites reinforced with ceramic particles play a predominant role. Because of high hardness and low ability for abrasion, Al2O3 and SiC particles are the most popular reinforcement for aluminum based composites and several papers have been published every year [1-4]. The properties of this materials are dependent on the material characteristic features and the production method. Literature data suggested strong influence of grain dimension on the coefficient of friction [5]. The most popular production method for Al2O3 and SiC reinforcement is casting vibration. Because of micro- dimension of grains and the production method the resultant materials

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show friction coefficient above 0,3 [5]. Although casting is the cheapest method of aluminum matrix composites (AMC) manufacturing, it introduces several technical problems if the submicro-sized ceramic particles are considered for reinforcement. Apart from the huge gap in the thermal expansion coefficient between ceramics and aluminum, poor wettability and high probability of Al4C3 formation during casting, agglomeration of the fine ceramic particles during casting is extremely difficult to be overcome. Thus the solid-state reaction as it occurs during mechanochemical alloying of the constituents followed by the relevant densification could be the competitive method of AMC’s manufacturing [6]. Abrasion is a dominant wear mechanism in this type of materials. Application of the fine ceramic particles to AMCs could change that mechanism. Consequently, a change of wear mechanism could provide elongated life-time of composites [7]. The present paper deals with fragmentation of ceramic particles to the sub-micron size and glassy carbon addition during mechanochemical alloying actions followed by subsequent consolidation to the bulk composites. It was expected that plastic deformation of working surfaces as a wear mechanism would provide lower mass losses. Two types of ceramic particles SiC and Al2O3 were tried in order to compare the effect of the matrix-particles bonding on the wear behavior, as some type of the displacement reactions could occur with SiC particles, while the Al 2O3 ones should be inert in contact with aluminum matrix.

3. EXPERIMENTAL SETUP AND CHARACTERIZATION Presented powders were produced by metallurgy powder method with mechano-chemical processing of ceramic particles and aluminum particles. Composition of materials was set by experimental method and consisted of 73 mass % of Al, 20 mass% of Al 2O3/ SiC, 5 mass % of glassy carbon and 2 mass % of stearic acid. Applied aluminum powder with a grain size under 100 µm was designed to form a matrix in the final composite. Two types of reinforcing particles were applied. As a first type of reinforcement was alpha Al2O3 – Martoxid 70 (Al2O3 content over 99.8 mass%, primary particles diameter d90=3 µm), while the second one was SiC – of high purity and primary particle size of 5- 10 µm. Additionally, glassy carbon particles were introduces, in order to serve as lubricant for diminishing the wear effects. SEM studies (Fig. 1-4) showed significant agglomeration of the initial ceramic powders and irregular large particles of glassy carbon.

Fig. 1. Aluminum powder, SEM

Fig. 2. Glassy carbon powder, SEM

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Fig. 3. Alpha Al 2O3 powder, SEM Fig. 4. SiC powder, SEM

3.1 BALL MILLING High energy ball milling was used as the first step of production of composite materials. Milling process was performed in the planetary mill- Fritsch Pulverisette Premium Line 7. In order to prevent temperature increase higher than Al melting point, the intervals during milling were applied. The total operation consisted of 12 cycles with 5 minutes milling time followed by 30 minutes breaks. Because of high aluminum affinity for oxidation, the protective atmosphere in the form of pure argon was used throughout preparation, milling and storing . The milling process created the composite particles of diameter in the range of 5-10 µm: aluminum particles were reinforced with alumina or SiC fine particles disintegrated to the size of 0,1-3 µm [Fig. 5, 6]. Because of that the reinforcing ceramic particles could be regarded as submicro- sized particles. Thus high-energy ball milling made the mechanical alloying effect- it put in hard ceramic particles into ductile matrix particles.

Fig. 5. The composite powder of Al-Al2O3+C, SEM Fig. 6. The composite powder of Al-SiC+C, SEM

3.2 PRESSING AND SINTERING

Hot pressing under 15 MPa was used for consolidation of the composite particles. This process was performed in Deguss press. Melting of contact areas during hot pressing at 700 °C was used for making the permanent matrix connection to ceramic particles, giving the bulk composites with improved mechanical properties. In consequence the proper density and reduce porosity was obtained.

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Fig. 7. Microstructure of the Al-Al2O3+C composite,

SEM Fig. 8. Microstructure of the Al-SiC+C composite,

SEM A fine homogeneity of the composite was observed in Scanning Electron Microscope shown in Fig. 7-8. The composite reinforced with Al2O3 particles showed a microstructure with randomly distributed glassy carbon particles with diameter below 5 µm as it could be visible in the backscattered electron mode (Fig. 7). On the other hand, the Al-SiC+C composite exhibited grain boundaries enriched with carbon (grey colour) apart from glassy carbon particles of similar shape and size as in the case of the composite with alumina (Fig. 8).

3.3 CHARACTERIZATION The initial stage of evaluation of material properties was measurement od density and opened porosity in order to identify packing density of the resultant bulk samples. Density was measured by the Archimedes method, while the porosity was calculated from the weight fraction and density of the composite components. The produced materials were valuated for tribological applications. The most important measurement was tribological tests. During this assessment friction coefficient as a function of sliding distance and mass losses were measured. Tribological test included cooperation of both: composite specimen and cast iron (GJL 300) pin. Tribological test was performed under the following conditions: load 35 [N], speed 0,05 [m/s], distance 50 [m]. The surfaces areas after wear were observed for both composites at Scanning Electron Microscope, with EDS technique and mapping function. Those measurements were done to define dominating wear mechanism. An evaluation of hardness by Vickers method was the last step of material properties evaluation. The measurement was conducted on a transverse section of composites with application of the HV10 technique.

4. RESULTS Characterization of materials density is given in Table 1.The higher porosity was observed for composite reinforced with SiC (Tab. 1). Accordingly, the higher surface porosity was confirmed by the microscopic studies (Fig. 7-8). The results variability of friction coefficient for both materials are showed in Fig. 9.

Tab. 1. Basic characteristic of composites

Composite Density [g/cm^3]

Porosity [%]

Al-Al 2O3+C 2,64 6,7

Al-SiC+C 2,25 16,9

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Fig. 9. Friction coefficient of tested materials as a function of

sliding distance Fig. 10. The composites mass losses

Despite significant changes during the initial stage of the tribological test, less fluctuations in the value of friction coefficient during the course were observed for Al-SiC+C composite. The changes in friction coefficient were caused by lapping the friction couple, and thus the sudden changes of wear mechanism were observed. For Al- Al2O3+C composite unstable work conditions were observed after lapping the materials, which is a very negative phenomenon. An average value of coefficient of friction was similar for both composites- 0,12 for Al- Al 2O3+C composite and 0,10 for Al-SiC+C composite. The value is significantly different if compared to the previous data in the literature [5], where composites reinforced with Al2O3 particles and grain size of 15-30 µm showed the friction coefficient in the range of 0,35-0,50. A different value of mass losses was observed for both, a disc and a pin after the tribological test. More than doubled mass losses were detected for composite reinforced with Al2O3 in contrary to SiC reinforcement [Fig.10]. This indicates the possibility of the longer life-time of Al-SiC+C material under high abrasion. Also, lesser mass losses were obtained for the friction counterpart- the cast iron pin. Thus we could expect that composites reinforced with SiC give much more possibilities in the future technological applications.

Fig. 11. Surface area after the wear test of the Al–

Al 2O3+C composite, SEM Fig. 12. Surface area after the wear test of in the Al–

SiC+C composite, SEM

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Fig. 13. Mapping of elements in the area after the wear test in the Al–Al 2O3+C composite, EDS

Fig. 14. Mapping of elements in the area after the wear test in the Al–SiC+C composite, EDS

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The different wear mechanism was observed in the analysed areas in Scanning Electron Microscope. after the friction tests. In the case of Al–Al 2O3+C composite a dominant wear mechanism was mechanically abrasive action through spacing of irregularities friction partner. Plastic deformation with ridging mechanisms was observed in the Al-SiC+C composite sample. A discrepancy of wear mechanism in the both tested composites was the main reason of different values of mass losses after the wear test. Additionally, iron inclusions and iron oxide layer were found on the samples surfaces if studied by the EDS technique (Fig.13-14). In both cases they formed weak adhesive connection which made protection against further degradation. However, iron debris from the pin were observed only in the friction area in composite reinforced with SiC. A reason of this phenomena could be related to the higher porosity of this material. The presence of iron inclusions contributed in some extent to a decreased value of mass losses in examined materials. Homogeneity of the glassy carbon distribution

in both composites was found at analysed surfaces after the friction test. This is the cause of proper protection between friction surfaces. Significant differences of hardness were observed in the tested materials. Composite reinforced with Al2O3 particles had significantly higher hardness (140 HV) than composite reinforced with SiC particles (98 HV). However, too high hardness could be the reason of higher wear, as a result of hardness dissimilarity between the disc and the pin.

5. SUMMARY

A change of friction behaviour was achieved as a result of reinforcing particles size reduction by using high-energy ball milling technology. The decrease of coefficient of friction and high stability of work conditions were achieved. The most important effect was a change of wear mechanism for Al–SiC+C composite to plastic deformation and ridging, in comparison to the abrasive action which occurred in the Al– Al2O3+C composite. Thus application of the fine SiC particles as a AMCs reinforcement was more effective if compared to Al2O3 reinforcement.

LITERATURE [1] Olszówka-Myalska A. Szala J., Śleziona J., Formanek B., Myalski J. Influence of Al–Al2O3 composite powder on the

matrix microstructure in composite casts. Mat. Charact. v. 49 (2003), pp 165-169 [2] Song M, Huang B. Effects of particle size on the fracture toughness of SiCp/Al alloy metal matrix composites. Mat. Sci.

Eng., v. A488 (2008), pp. 601-607 [3] Boz. M., Kurt A. The effect of Al2O3 on the friction performance of automotive brake friction materials. Tribology

Internat., v. 40 (2007), pp. 1161-1169 [4] Sedlacek J., Svancarek P., RAtkinson Riedel R., Atkinson A., Wang X: Abrasive wear of Al2O3–SiC and Al2O3–(SiC)–C

composites with micrometer- and submicrometer-sized alumina matrix grains. J. Eur. Ceram. Soc., v.28 (2008), pp. 2983-2993

[5] Wieczorek J., Dolata-Grosz A., Dyzia M., Śleziona J., Właściwości tribologiczne kompozytowych materiałów o osnowie stopu aluminium AK12 zbrojonych cząstkami ceramicznymi, Kompozyty (Composites) 1(2001)2

[6] Ahamed H., Senthilkumar V. Experimental investigation on newly developed ultrafine-grained aluminium based nano-composites with improved mechanical properties. Mat. Design, v. 37 (2012), pp. 182-192

[7] Lawrowski Z., Tribologia. Tarcie, zużywanie i smarowanie, Oficyna Wydawnicza Politechniki Wrocławskiej, Wrocław 2008

[8] Śleziona J., Hyla I., Myalski J., Formation of the layer structure in Al - ceramic particle composites, Science and Engineering of Composite Materials, v.7, no 4. 1998, p. 287-291

[9] Posmyk A., Myalski J: Producing of composite materiale with aluminium alloy matrix containing solid libricants, Solid State Phenomena, 2012, vol. 19176, p.67-74

Fig. 15. Evaluation of hardness of composites

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