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M iron based composites reinforced with ultrafine particulates
Composites: Part B 58 (2014) 16–24
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
On the properties and tribological behaviors of P/M iron based composites reinforced with ultrafine particulates Eugene E. Feldshtein a,⇑, Larisa N. Dyachkova b a b
Department of Mechanical Engineering, University of Zielona Góra, Prof. Z. Szafrana 4, 65-516 Zielona Góra, Poland Institute of Powder Metallurgy, Belarusian National Academy of Sciences, ul. Platonova 41, Minsk 220005, Belarus
a r t i c l e
i n f o
Article history: Received 9 April 2013 Received in revised form 13 September 2013 Accepted 21 October 2013 Available online 30 October 2013 Keywords: A. Metal Matrix Composites (MMCs) B. Microstructures B. Strength D. Surface analysis
a b s t r a c t This paper describes the changes of structure, some mechanical and tribological properties of P/M iron based composites reinforced with ultrafine additives. Nanocrystalline additives of oxides, borides and diamond in the base material allow increasing the compressive strength and the tensile strength 1.5–3 times. An introduction of 0.2–0.3 wt% of ultrafine-grained diamonds, 0.5 wt% of chromium borides and 0.2–0.5 wt% of alumina or oxides mixture provides the best results. The coefficients of friction of MMCs containing nanocrystalline particulates are reduced 2–3 times compared to the base P/M material while the critical seizure pressure is enhanced 2–5 times. The wear resistance of the MMCs increases 2–4 times. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The use of powder metallurgy ferrous-based materials increases permanently because of their low cost and no or a minimum need for metalworking processes such as machining, stamping and forging. Powder metallurgy (P/M) has continued to displace traditional technologies in different mechanical engineering applications. This is due to one of the main advantages of the P/M technology, i.e., the possibility of composing new materials with unique properties that cannot be obtained by standard melting-casting processes. Different compositions of ferrous powder P/M materials are used in industry, in particular [1]: P/M iron and carbon steels, P/ M iron-copper and copper steels, P/M iron-nickel and nickel steels, P/M low alloy steels, P/M sinter-hardened steels and so on. In the first group of P/M materials the following types can be identified: unalloyed iron materials used for lightly loaded or self lubrication structural applications; the combined carbon content of these materials can be from 0 to 0.3 wt%, carbon P/M materials with 0.3–0.6 wt% combined carbon characterized by moderate strength and apparent hardness, used where such properties combined with machinability are desired,
⇑ Corresponding author. Tel.: +48 68 3282617. E-mail address: [email protected] (E.E. Feldshtein). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.10.015
carbon P/M materials with 0.8–1.5 wt% combined carbon which have a moderately higher strength compared with other carbon materials, but are more difficult to machine. One of the major disadvantages of ferrous powder P/M materials is their low strength. The introduction of additional reinforcing components is a modern method for increasing the material strength. In this case, we deal with Metal Matrix Composites (MMCs). Their features make MMCs an attractive option for the replacement of conventional materials in many engineering applications. The use of P/M MMCs in the automotive and aerospace industries depends on achieving the mechanical and operation properties of materials. The conventional P/M technology can be used in production (mixing of starting powders, pressing and sintering) and also in different novel technologies, for example, as described in [2,3] or in [4]. It was reported in [5] that MMCs produced by P/M methods are usually used for small objects made of high strength and heat resistance materials. MMCs reinforced by different particulates and produced by P/M methods were described in many investigations. It has been emphasized that the introduction of ceramic particulates into metallic powders, on the one hand, decreased the compressibility and formability of the mixed powder due to the physical and mechanical properties of the hard particulates. But on the other, the incorporation of particulate ceramics into iron matrices significantly improves material properties: it offers higher hardness, higher strength at elevated temperatures, and wear resistance compared to monolithic iron [6].
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MMCs with a wide range of matrix materials (including aluminum, titanium, copper and other nonferrous metals as well as iron and different steels) and second-phase particulates are produced [1,7,8]. SiC or Al2O3 are most commonly used particulate materials, but many other materials such as TiB2, B4C, SiO2, TiC, WC, BN, and ZrO2 have also been used, Mixtures of these materials can be used too [9–11]. Additives to the metal matrix can be introduced directly while sintering P/M MMCs [1,7,9 and many others], or by impregnation of the metal matrix with the use a liquid metal (infiltration) [12]. The effect of reinforcing particulates size and matrix microstructure on the properties of MMCs is very important. In [13] the influence of Al2O3 particulate size (0.053–0.063 and 0.090– 0.105 mm) on Al–Al2O3 composite properties is presented. The positive effect of 5 vol% alumina additive on improving the properties of Al–Al2O3 composites is demonstrated. The properties of the nano-sized powder of Cu–Al2O3 nanocomposites with various alumina contents are described in [14]. Finer alumina particulates of 30 nm compared to 50 nm improve composite properties in terms of density, microhardness and abrasive wear resistance. In [15], particulates of alumina and a mixture of alumina and mullite of the size of (0.005–0.03) mm were used to investigate creep behavior of Al-6061 MMC. According to [16], the base metal of engine bearings may be reinforced with dispersed alumina particulates (0.5–2 vol% and the size of 0.01–0.02 mm) to ensure better material operating properties. The amount of reinforcing additives can be varied, reaching 20– 30 vol% in the total volume of the material [15,17]. There are two trends for improving the properties of metals: (1) adding alloying elements to the base metal that enhance its consumer properties (mechanical, thermal, etc.), the volume of these elements may be equal to even 20–30%, and (2) microalloying. When microalloying, additives affect the state of the metal, changing in particular: the conditions of formation of interstitial solid solutions or replacement solid solutions, the size of secondary grains, the dispersity, shape and distribution of inclusions, the structure of boundaries, fine grain structure, etc. The effect of microalloying on the properties of steels and alloys is explained by the theory of internal absorption in metals [18]. According to this theory, there are two types of microalloying additives: horophile impurities and horophobe impurities. The first type of additives reduces the free energy of the grain boundaries of polycrystalline alloys; the additives are distributed unevenly throughout the volume of the metal and are concentrated mainly on the grain boundaries. The other type of additives increases the free energy of the grain boundaries in polycrystalline alloys; they are distributed unevenly in the metal and are located in the body of the grain. This results in the formation of areas with a low concentration of impurities near grain boundaries. The adsorption of horophile impurities at the grain boundaries affects the recrystallization process of polycrystalline alloys and, therefore, influences their strength and plastic properties [18]. Microalloying efficiency increases in the case of using nanocrystalline particulates [19]. The presence of such particulates in the steel contributes to the intensification of diffusion at grain boundaries [20]. The introduction of 0.5 wt% of Y2O3 oxides (20 nm in size) in ferritic oxide dispersion strengthened alloys increases their hot hardness behavior [21]. The addition of ultrafine grained SiC
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particulates with the size of 0.001–0.005 mm has a tendency for crumbling the eutectic Si phase in Al–12Si alloy [22]. Recently, ultrafine-grained diamond (UFGD) particulates have been introduced into the structure of MMCs as reinforcing additives. The UFGD has typical properties of a nanometer material, as well as inherent characteristics of diamond such as high hardness, good thermal conductivity, good wear resistance and chemical resistance. It can be used to improve lubrication properties [23] as well as thermal conductivity and mechanical properties [9,24] of composite materials. In this paper, the effect of nanocrystalline additives to carbon P/ M materials on their structure, mechanical and tribological properties is studied. 2. Experimental procedure 2.1. Base powders used and preparation of reinforcing particulates Ready-made powders of iron and graphite were used. The particulates of base atomized iron powder were of the size less than 200 lm. Its ability to compaction under 600 MPa pressure was 6.5–6.6 g/cm3. Graphite with particulates of the size less than 40 lm was used as carbon. Powders of oxides and borides were ground in an attritor with ceramic balls of 0.9–1.2 mm in diameter. Ultrafine powders of diamond grains (UFGD) were obtained when explosives with negative oxygen balance were detonated in an inert medium, as it was described in [25,26]. Single UFGD particulates of the size of 10–20 nm form conglomerates whose size is 100–500 nm (Fig. 1). In order to fragmentize the particulates before their introduction into the base material, UFGD conglomerates were sonicated in alcohol, then mixed with iron powder in the ratio 1:1 and dried in air. 2.2. Preparation of composite samples Powdered iron (Fe) and iron with 1 wt% graphite (Fe + 1 wt% Gr) were used as base materials in the tests. Nanocrystalline
Fig. 1. The shape of UFGD conglomerates.
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Fig. 4. Volume changes of the samples after sintering depending on the sintering temperature.
Fig. 2. The shape of powder particles of complicated iron–nickel–zinc oxides.
Fig. 3. The influence of the pressure value on the density of MMCs with additives of nanocrystalline alumina and UFGD.
particulates were added into the base materials: UFGD additives in the quantity of 0.1%, 0.3% and 0.5%, alumina powders, a mixture of alumina and zirconium oxides, composite iron–nickel–zinc oxides (75% of an iron + 15% of a nickel + 10% of a zinc) in the quantity of 0.5%, as well as chromium borides in the quantity of 0.2%, 0.5%, 1% and 2%. The powder components were mixed in a mixer of ‘‘drunken barrel’’ type for 1.5 h. Then the samples were pressed using a hydraulic press with the pressure 500 MPa and sintered in vacuum or in the atmosphere of an endothermic gas at temperatures of 900–1100 °C for 1 h. 2.3. Particulates measurement The size of the UFGD, oxide particulates and mixtures of oxides were measured using the ‘‘MIRA’’ Scanning Electron Microscope.
Fig. 5. Microstructures: (a) sintered iron, and (b) iron with 0.5% of the UFGD.
2.4. Microstructure analysis The microstructure of cross-sections of the samples was analyzed using the optical microscope ‘‘MeF-3’’ and ‘‘MIRA’’ Scanning Electron Microscope. 2.5. Strength testing The ‘‘Instron-1195’’ testing machine was used for strength testing with the cross head speed of 2 mm/min. Compression tests were performed using samples with a diameter of 10 mm, height 12 mm; bending tests – using samples 5 10 55 mm. The
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Fig. 7. The effect of sintering atmosphere on the compressive strength of sintered iron with UFGD additives.
Fig. 8. The effect of the UFGD content on the compressive strength of sintered iron.
Fig. 6. Microstructures of composites with CrB2 additives: (a) base material Fe + 1% Gr, (b) 0.2% CrB2, (c) 0.5% CrB2, (d) 1.0% CrB2, and (e) 2.0% CrB2.
Fig. 9. The effect of the composition and concentration of additives on the compressive strength of MMCs.
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Fig. 10. The effect of the chromium boride content on the strength parameters of MMCs.
compressibility of MMCs was investigated for samples 10 mm in diameter and 10 mm in height with different pressures. The microhardness HV0.05 of the samples was determined using the ‘‘Micromet-II’’ digital microhardness tester. The fractures of tested materials were observed using the ‘‘MIRA’’ Scanning Electron Microscope.
Fig. 11. The fracture surface of the composite with 0.5% chromium boride additive.
The magnitude of the linear wear was registered using an optimeter with an accuracy of 0.001 mm.
2.6. Tribological testing
3. Results and discussion
Tribological tests were performed in conditions of distributed contact using the MT-2 tester of ‘‘pin-on-disc’’ type [27]. Rotating counter-bodies were made of C45 steel and had a disc form 110 mm in diameter and hardness 42–45 HRC. They were in contact with the flat surfaces of the three pin samples 10 mm in diameter. Tests were carried out at the sliding speed of 4 m/s and different pressures. The test time was equal to 1 h. I-20 industrial oil (analog to ISO 6743/4-82 HH-32 oil or Vitrea 32 Shell oil) was used as the lubricant with the flow rate of 4 drops per minute. The oil was fed to the center of the lower rotating disc and spread over the entire surface of the counter-body under centrifugal forces.
3.1. Ultra- and nanocrystalline particulates sizes and shapes The size of alumina particulates after breakage was 0.5–0.8 lm and chromium boride particulates were of the size of 3–5 lm. In a dispersed mixture of alumina and zirconium oxide, the size of alumina particulates was 0.5–0.8 lm and the size of zirconium oxide was 100–200 nm, because zirconium oxides are less prone to conglomeration. The average size of the iron oxide in the mixture of iron, nickel and zirconium oxides is 0.3–0.5 lm. Nanocrystalline particulates of harder nickel and zirconium oxides of the size of 30–100 nm were observed on the surface of iron particulates (Fig. 2).
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Fig. 13. The morphology of the worn surface of the basic material.
UFGD, when the sintering temperature is 1100 °C, can be explained, apparently, by the activation of recrystallization processes. 3.4. Microstructures of composites
Fig. 12. The effect of pressure on the friction coefficient of MMCs with ultrafine additives.
3.2. Materials compressibility On the basis of the study of compressibility of the initial mixture with the addition of nanoparticulates it was found (Fig. 3) that the 0.5% UFGD additive renders it possible to achieve a higher density of the samples at a low (200 MPa) pressure of molding. At higher pressures, the density of the composite material with the UFGD additive is slightly lower than that of the materials without additives and the materials with the addition of ultrafine alumina powder. This is perhaps due to an increase in friction of base material particulates with the UFGD additive. 3.3. Samples deformations after sintering When sintering, depending on the material composition and sintering temperature an increase in the size of the samples as well as their shrinkage were observed (Fig. 4). The shrinkage of the material is increased when increasing the sintering temperature. An increased shrinkage of the material with the addition of the
An analysis of the microstructure of samples sintered in vacuum at 1100 °C, has revealed two types of grains of non-uniform size in the structure of the base material: (1) small recrystallized grains and (2) large ones, formed due to the grain growth (Fig. 5a). Irregular grain sizes are observed in the structure of a material without UFGD additives: small recrystallized and large ones formed as a result of grain growth (Fig. 5a). In the material with an addition of 0.5% UFGD the structure consists of dispersed grains of uniform size, approximately 5–10 lm, grain boundaries are fine (Fig. 5b). This is due to a large number of nucleation sites in the form of nanosized impurities of silicon, calcium, titanium and chromium oxides which are present in UFGD particulates. They are located at the grain boundaries and impede grain growth [28–30]. The introduction of chromium borides in the quantity of 0.1– 0.5% to the Fe + 1% Gr material leads to a significant breakage of its structure (Fig. 6a–c). This is due to the increase in the number of recrystallization centers. When the content of chromium boride is increased up to 1% differences in the grain size appear, and for the 2% content, these differences increase. This phenomenon is due to the formation of liquid phase at the locations of borides. Areas with dendritic structure are formed in this case with grains of different sizes (Fig. 6d). Furthermore, complex doped borides of (FeCr)B type and chromium carbide–borides are formed (Fig. 6e). 3.5. Strength of the tested materials The correct choice of the sintering atmosphere increases the compressive strength of the MMC with the addition of the UFGD
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Fig. 15. The morphology of the worn surface of the MMC with 0.5% additive of a mixture of alumina and zirconium oxides: (a) 25, (b) 2000, and (c) 10,000.
Fig. 14. The morphology of the worn surface of the MMC with 0.2% UFGD additive: (a) 25, (b) 100, (c) 2000, and (d) 10,000.
1.5–3 times (Fig. 7). When sintering, the carbon from UFGD particulates dissolves in the iron, strengthening it, so the strength of the iron sintered in an endothermic gas is higher than that of the iron sintered in vacuum, where carbon is removed from the material. Furthermore, as a result of sintering in vacuum the boundaries of iron grains are refined and the grains grow. However, the stability of the strength characteristics of MMCs sintered in vacuum is much higher compared to MMMs sintered in an endothermic gas (spread: 2–3% and more than 10%, respectively).
The relation between the compressive strength of the MMCs and content of the UFGD additives is parabolic (Fig. 8), and the optimal content of additives is 0.2–0.3%. The influence of various additives on the compressive strength of the MMCs is shown in Fig. 9. The strengthening effect of ultrafine additives is manifested when sintering in the endothermic gas atmosphere already at 900 °C, and it is much stronger at 1100 °C. The greatest strengthening occurs with the introduction of 0.5% of a mixture of alumina and zirconium oxides due to the presence of nanosized particulates with a stronger, defect-free connection with an iron base. Due to strengthening of the grain boundaries, the hardness of the composite material increases too. The microhardness of the ferrite part of the structure increases from 680–965 MPa to 1100–1230 MPa. Chromium boride additives have a positive effect on the strength of the MMCs too; the value of its increment depends on the sintering temperature, because the borides alloy the base material. The introduction of 0.2% chromium boride allows increasing the flexural strength of the composite to 60–65 MPa. The maximum value of the flexural strength is attained when the chromium boride content reaches 0.5% (Fig. 10a). The same
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dependence is observed for the compressive strength (Fig. 10b), the strength increases almost by half. An increase in the chromium boride content to 1% leads to a sharp decrease in the strength of the composite. When the content of chromium boride is 2%, the compressive strength remains unchanged and the flexural strength is reduced, because this characteristic is more sensitive to changes in the material structure. In this case, the strength of the composite with 2% of chromium boride is higher in comparison with a material without the additive. The decrease of the MMC strength with increasing the chromium boride content may be explained by material embrittlement due to the formation of complex doped borides of (FeCr)B type and chromium carbide–borides in the Fe-base material containing graphite. This was confirmed by research on the morphology of fracture surfaces after embrittlement of the material in liquid nitrogen. In the fractures, the above mentioned compounds are located in the pits of destruction, most of chromium boride inclusions are located along the grain boundaries (Fig. 11). An optimal amount of oxide additives is 0.2–0.5 wt%. With an increase in their content up to 1 wt% or more, regardless of the additive type, the strength of composites decreases. The reason is that the input additives have low density and an increase of their content increases the unevenness of their distribution in the base material. In addition, the additives act as horophile impurities, located mainly along the grain boundaries of the iron base, and an increase in their concentration leads to weakening of the grain boundaries and a reduction of strength. 3.6. Friction coefficient values Tests revealed (Fig. 12a) that the introduction of 0.2% UFGD leads to some reduction of the friction coefficient of the base material and an increase of the critical seizure pressure from 2.8 to 3.7 MPa. The addition of 0.5% of ultrafine alumina reduces the coefficient of friction to 0.025–0.027, and more than twofold increases the critical seizure pressure (up to 6 MPa). The introduction of the mixture of 0.4% alumina with 0.1% zirconium oxide to the base material yields a decrease in the friction coefficient of 0.021–0.023 and increases the critical seizure pressure up to 7– 7.5 MPa.
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The addition of 0.2–0.5% chromium borides reduces the coefficient of friction of the base material more than 2 times and increases the critical seizure pressure from 3.0 to 8.6 MPa (Fig. 12b). By increasing the quantity of chromium borides, the coefficient of friction is increased slightly and the critical pressure seizure is reduced up to 4.8–6 MPa. 3.7. Wear behavior Extensive areas of seizure and tearing out of the material are observed on the wear surface of the base material without additives even with a small (25-time) magnification. A greater magnification shows the formation of cracks, mainly on the grain boundaries (Fig. 13). When introducing UFGD additives, the character of wear is changed: the wear surface is smooth and, in contrast to the material without additives, when observed with a slight magnification, the texture of the wear surface is very homogeneous (Fig. 14a). Only at 100-time magnification shallow grooves of wear have been revealed (Fig. 14b). Traces of a seizure on the wear surface are practically absent. This indicates a good running-in of the material, modified by the UFGD. In [23,31,32] the possibility of using the UFGD for fine polishing is mentioned. Impurities of the UFGD initiate branching of formed fracture cracks (Fig. 14c), the destruction occurs not in the individual grains, but in the material body, and as a result the tendency of the material destruction is reduced. A study of the wear surface at high magnifications revealed that the UFGD particulates are located mainly along the grain boundaries, but there is a small amount of them in the grain body (Fig. 14d). The SEM analysis confirmed the presence of fine calcium compounds particulates on grain boundaries. Most probably, these are calcium oxides or calcium carbonates. Additives of calcium compounds are used for the synthesis of the UFGD. It can be assumed that the carbon from UFGD particulates partially diffuses into the iron matrix, and calcium oxides or carbonates that are contained in the UFGD, remain at places of their location. These particulates strengthen the material, preventing the movement of dislocations. Wear surfaces of the MMC with the addition of a mixture of ultrafine alumina and zirconium oxides are characterized by an extremely fine texture; there are weak abrasion grooves left by grinding initial surfaces of samples and wear tracks are thin and smooth (Fig. 15a). Together with a microfailure and pitting of ultrafine particulates, traces of plastic deformation are observed (Fig. 15b). Branching of microcracks occurs in the same way as in the material with the addition of the UFGD. Due to a more dispersed structure, this phenomenon is more marked (Fig. 15c). Ultra-fine oxide particulates are located mainly along the grain boundaries. In the process of wear mainly pitting of alumina occurs. The SEM analysis of wear surfaces identified zirconium oxides, which may melt when sintering, decompose and diffuse into the iron matrix. The wear resistance of the materials with nanocrystalline additives increases significantly (Fig. 16).
4. Conclusions In the present investigation the structure, mechanical and tribological properties of MMCs based on carbon P/M materials reinforced with additives of nanocrystalline particulates of oxides, borides and diamond were addressed. The results of the investigation reveal the following:
Fig. 16. Wear rates of the composites with different additives.
1. In comparison with other additives, the introduction of 0.3% UFGD makes it possible to obtain a higher density at lower pressures of pressing. At higher pressures, the density of the
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material with the UFGD is slightly lower than in the materials without additives and with the addition of ultrafine alumina powder. 2. The addition of up to 0.5% UFGD, oxides or borides to the composite facilitates breakage of the material structure. An increase in the content of additives to 1% or more leads to a heterogeneity of the structure. 3. The introduction of additives increases the compressive strength and the flexural strength of composites. The optimum contents of the additives are as follows: the UFGD content 0.2– 0.3%, the alumina content 0.2–0.5% and the chromium boride content 0.5%. In the case of using the optimum contents, the strength of composites with UFGD additives increases 1.5–3 times, whereas the strength of composites containing oxides or borides increases up to 2 times in comparison with the sintered carbon P/M materials. A further increase of the content of additives causes a sharp decrease in the MMC strength. 4. The coefficients of friction on the surfaces of MMCs containing nanocrystalline particulates, are reduced by 2–3 times compared to the base material, while the critical seizure pressure is increased from 2 to 5 times. The presence of nanocrystalline particulates hinders the appearance of fracture micro-cracks on the surfaces of friction, preventing the movement of dislocations. The wear resistance of MMCs increases by 2–4 times compared to the base material. The use of the UFGD and the mixture of alumina and zirconium oxides is the most effective.
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
On the properties and tribological behaviors of P/M iron based composites reinforced with ultrafine particulates Eugene E. Feldshtein a,⇑, Larisa N. Dyachkova b a b
Department of Mechanical Engineering, University of Zielona Góra, Prof. Z. Szafrana 4, 65-516 Zielona Góra, Poland Institute of Powder Metallurgy, Belarusian National Academy of Sciences, ul. Platonova 41, Minsk 220005, Belarus
a r t i c l e
i n f o
Article history: Received 9 April 2013 Received in revised form 13 September 2013 Accepted 21 October 2013 Available online 30 October 2013 Keywords: A. Metal Matrix Composites (MMCs) B. Microstructures B. Strength D. Surface analysis
a b s t r a c t This paper describes the changes of structure, some mechanical and tribological properties of P/M iron based composites reinforced with ultrafine additives. Nanocrystalline additives of oxides, borides and diamond in the base material allow increasing the compressive strength and the tensile strength 1.5–3 times. An introduction of 0.2–0.3 wt% of ultrafine-grained diamonds, 0.5 wt% of chromium borides and 0.2–0.5 wt% of alumina or oxides mixture provides the best results. The coefficients of friction of MMCs containing nanocrystalline particulates are reduced 2–3 times compared to the base P/M material while the critical seizure pressure is enhanced 2–5 times. The wear resistance of the MMCs increases 2–4 times. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The use of powder metallurgy ferrous-based materials increases permanently because of their low cost and no or a minimum need for metalworking processes such as machining, stamping and forging. Powder metallurgy (P/M) has continued to displace traditional technologies in different mechanical engineering applications. This is due to one of the main advantages of the P/M technology, i.e., the possibility of composing new materials with unique properties that cannot be obtained by standard melting-casting processes. Different compositions of ferrous powder P/M materials are used in industry, in particular [1]: P/M iron and carbon steels, P/ M iron-copper and copper steels, P/M iron-nickel and nickel steels, P/M low alloy steels, P/M sinter-hardened steels and so on. In the first group of P/M materials the following types can be identified: unalloyed iron materials used for lightly loaded or self lubrication structural applications; the combined carbon content of these materials can be from 0 to 0.3 wt%, carbon P/M materials with 0.3–0.6 wt% combined carbon characterized by moderate strength and apparent hardness, used where such properties combined with machinability are desired,
⇑ Corresponding author. Tel.: +48 68 3282617. E-mail address: [email protected] (E.E. Feldshtein). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.10.015
carbon P/M materials with 0.8–1.5 wt% combined carbon which have a moderately higher strength compared with other carbon materials, but are more difficult to machine. One of the major disadvantages of ferrous powder P/M materials is their low strength. The introduction of additional reinforcing components is a modern method for increasing the material strength. In this case, we deal with Metal Matrix Composites (MMCs). Their features make MMCs an attractive option for the replacement of conventional materials in many engineering applications. The use of P/M MMCs in the automotive and aerospace industries depends on achieving the mechanical and operation properties of materials. The conventional P/M technology can be used in production (mixing of starting powders, pressing and sintering) and also in different novel technologies, for example, as described in [2,3] or in [4]. It was reported in [5] that MMCs produced by P/M methods are usually used for small objects made of high strength and heat resistance materials. MMCs reinforced by different particulates and produced by P/M methods were described in many investigations. It has been emphasized that the introduction of ceramic particulates into metallic powders, on the one hand, decreased the compressibility and formability of the mixed powder due to the physical and mechanical properties of the hard particulates. But on the other, the incorporation of particulate ceramics into iron matrices significantly improves material properties: it offers higher hardness, higher strength at elevated temperatures, and wear resistance compared to monolithic iron [6].
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MMCs with a wide range of matrix materials (including aluminum, titanium, copper and other nonferrous metals as well as iron and different steels) and second-phase particulates are produced [1,7,8]. SiC or Al2O3 are most commonly used particulate materials, but many other materials such as TiB2, B4C, SiO2, TiC, WC, BN, and ZrO2 have also been used, Mixtures of these materials can be used too [9–11]. Additives to the metal matrix can be introduced directly while sintering P/M MMCs [1,7,9 and many others], or by impregnation of the metal matrix with the use a liquid metal (infiltration) [12]. The effect of reinforcing particulates size and matrix microstructure on the properties of MMCs is very important. In [13] the influence of Al2O3 particulate size (0.053–0.063 and 0.090– 0.105 mm) on Al–Al2O3 composite properties is presented. The positive effect of 5 vol% alumina additive on improving the properties of Al–Al2O3 composites is demonstrated. The properties of the nano-sized powder of Cu–Al2O3 nanocomposites with various alumina contents are described in [14]. Finer alumina particulates of 30 nm compared to 50 nm improve composite properties in terms of density, microhardness and abrasive wear resistance. In [15], particulates of alumina and a mixture of alumina and mullite of the size of (0.005–0.03) mm were used to investigate creep behavior of Al-6061 MMC. According to [16], the base metal of engine bearings may be reinforced with dispersed alumina particulates (0.5–2 vol% and the size of 0.01–0.02 mm) to ensure better material operating properties. The amount of reinforcing additives can be varied, reaching 20– 30 vol% in the total volume of the material [15,17]. There are two trends for improving the properties of metals: (1) adding alloying elements to the base metal that enhance its consumer properties (mechanical, thermal, etc.), the volume of these elements may be equal to even 20–30%, and (2) microalloying. When microalloying, additives affect the state of the metal, changing in particular: the conditions of formation of interstitial solid solutions or replacement solid solutions, the size of secondary grains, the dispersity, shape and distribution of inclusions, the structure of boundaries, fine grain structure, etc. The effect of microalloying on the properties of steels and alloys is explained by the theory of internal absorption in metals [18]. According to this theory, there are two types of microalloying additives: horophile impurities and horophobe impurities. The first type of additives reduces the free energy of the grain boundaries of polycrystalline alloys; the additives are distributed unevenly throughout the volume of the metal and are concentrated mainly on the grain boundaries. The other type of additives increases the free energy of the grain boundaries in polycrystalline alloys; they are distributed unevenly in the metal and are located in the body of the grain. This results in the formation of areas with a low concentration of impurities near grain boundaries. The adsorption of horophile impurities at the grain boundaries affects the recrystallization process of polycrystalline alloys and, therefore, influences their strength and plastic properties [18]. Microalloying efficiency increases in the case of using nanocrystalline particulates [19]. The presence of such particulates in the steel contributes to the intensification of diffusion at grain boundaries [20]. The introduction of 0.5 wt% of Y2O3 oxides (20 nm in size) in ferritic oxide dispersion strengthened alloys increases their hot hardness behavior [21]. The addition of ultrafine grained SiC
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particulates with the size of 0.001–0.005 mm has a tendency for crumbling the eutectic Si phase in Al–12Si alloy [22]. Recently, ultrafine-grained diamond (UFGD) particulates have been introduced into the structure of MMCs as reinforcing additives. The UFGD has typical properties of a nanometer material, as well as inherent characteristics of diamond such as high hardness, good thermal conductivity, good wear resistance and chemical resistance. It can be used to improve lubrication properties [23] as well as thermal conductivity and mechanical properties [9,24] of composite materials. In this paper, the effect of nanocrystalline additives to carbon P/ M materials on their structure, mechanical and tribological properties is studied. 2. Experimental procedure 2.1. Base powders used and preparation of reinforcing particulates Ready-made powders of iron and graphite were used. The particulates of base atomized iron powder were of the size less than 200 lm. Its ability to compaction under 600 MPa pressure was 6.5–6.6 g/cm3. Graphite with particulates of the size less than 40 lm was used as carbon. Powders of oxides and borides were ground in an attritor with ceramic balls of 0.9–1.2 mm in diameter. Ultrafine powders of diamond grains (UFGD) were obtained when explosives with negative oxygen balance were detonated in an inert medium, as it was described in [25,26]. Single UFGD particulates of the size of 10–20 nm form conglomerates whose size is 100–500 nm (Fig. 1). In order to fragmentize the particulates before their introduction into the base material, UFGD conglomerates were sonicated in alcohol, then mixed with iron powder in the ratio 1:1 and dried in air. 2.2. Preparation of composite samples Powdered iron (Fe) and iron with 1 wt% graphite (Fe + 1 wt% Gr) were used as base materials in the tests. Nanocrystalline
Fig. 1. The shape of UFGD conglomerates.
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Fig. 4. Volume changes of the samples after sintering depending on the sintering temperature.
Fig. 2. The shape of powder particles of complicated iron–nickel–zinc oxides.
Fig. 3. The influence of the pressure value on the density of MMCs with additives of nanocrystalline alumina and UFGD.
particulates were added into the base materials: UFGD additives in the quantity of 0.1%, 0.3% and 0.5%, alumina powders, a mixture of alumina and zirconium oxides, composite iron–nickel–zinc oxides (75% of an iron + 15% of a nickel + 10% of a zinc) in the quantity of 0.5%, as well as chromium borides in the quantity of 0.2%, 0.5%, 1% and 2%. The powder components were mixed in a mixer of ‘‘drunken barrel’’ type for 1.5 h. Then the samples were pressed using a hydraulic press with the pressure 500 MPa and sintered in vacuum or in the atmosphere of an endothermic gas at temperatures of 900–1100 °C for 1 h. 2.3. Particulates measurement The size of the UFGD, oxide particulates and mixtures of oxides were measured using the ‘‘MIRA’’ Scanning Electron Microscope.
Fig. 5. Microstructures: (a) sintered iron, and (b) iron with 0.5% of the UFGD.
2.4. Microstructure analysis The microstructure of cross-sections of the samples was analyzed using the optical microscope ‘‘MeF-3’’ and ‘‘MIRA’’ Scanning Electron Microscope. 2.5. Strength testing The ‘‘Instron-1195’’ testing machine was used for strength testing with the cross head speed of 2 mm/min. Compression tests were performed using samples with a diameter of 10 mm, height 12 mm; bending tests – using samples 5 10 55 mm. The
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Fig. 7. The effect of sintering atmosphere on the compressive strength of sintered iron with UFGD additives.
Fig. 8. The effect of the UFGD content on the compressive strength of sintered iron.
Fig. 6. Microstructures of composites with CrB2 additives: (a) base material Fe + 1% Gr, (b) 0.2% CrB2, (c) 0.5% CrB2, (d) 1.0% CrB2, and (e) 2.0% CrB2.
Fig. 9. The effect of the composition and concentration of additives on the compressive strength of MMCs.
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Fig. 10. The effect of the chromium boride content on the strength parameters of MMCs.
compressibility of MMCs was investigated for samples 10 mm in diameter and 10 mm in height with different pressures. The microhardness HV0.05 of the samples was determined using the ‘‘Micromet-II’’ digital microhardness tester. The fractures of tested materials were observed using the ‘‘MIRA’’ Scanning Electron Microscope.
Fig. 11. The fracture surface of the composite with 0.5% chromium boride additive.
The magnitude of the linear wear was registered using an optimeter with an accuracy of 0.001 mm.
2.6. Tribological testing
3. Results and discussion
Tribological tests were performed in conditions of distributed contact using the MT-2 tester of ‘‘pin-on-disc’’ type [27]. Rotating counter-bodies were made of C45 steel and had a disc form 110 mm in diameter and hardness 42–45 HRC. They were in contact with the flat surfaces of the three pin samples 10 mm in diameter. Tests were carried out at the sliding speed of 4 m/s and different pressures. The test time was equal to 1 h. I-20 industrial oil (analog to ISO 6743/4-82 HH-32 oil or Vitrea 32 Shell oil) was used as the lubricant with the flow rate of 4 drops per minute. The oil was fed to the center of the lower rotating disc and spread over the entire surface of the counter-body under centrifugal forces.
3.1. Ultra- and nanocrystalline particulates sizes and shapes The size of alumina particulates after breakage was 0.5–0.8 lm and chromium boride particulates were of the size of 3–5 lm. In a dispersed mixture of alumina and zirconium oxide, the size of alumina particulates was 0.5–0.8 lm and the size of zirconium oxide was 100–200 nm, because zirconium oxides are less prone to conglomeration. The average size of the iron oxide in the mixture of iron, nickel and zirconium oxides is 0.3–0.5 lm. Nanocrystalline particulates of harder nickel and zirconium oxides of the size of 30–100 nm were observed on the surface of iron particulates (Fig. 2).
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Fig. 13. The morphology of the worn surface of the basic material.
UFGD, when the sintering temperature is 1100 °C, can be explained, apparently, by the activation of recrystallization processes. 3.4. Microstructures of composites
Fig. 12. The effect of pressure on the friction coefficient of MMCs with ultrafine additives.
3.2. Materials compressibility On the basis of the study of compressibility of the initial mixture with the addition of nanoparticulates it was found (Fig. 3) that the 0.5% UFGD additive renders it possible to achieve a higher density of the samples at a low (200 MPa) pressure of molding. At higher pressures, the density of the composite material with the UFGD additive is slightly lower than that of the materials without additives and the materials with the addition of ultrafine alumina powder. This is perhaps due to an increase in friction of base material particulates with the UFGD additive. 3.3. Samples deformations after sintering When sintering, depending on the material composition and sintering temperature an increase in the size of the samples as well as their shrinkage were observed (Fig. 4). The shrinkage of the material is increased when increasing the sintering temperature. An increased shrinkage of the material with the addition of the
An analysis of the microstructure of samples sintered in vacuum at 1100 °C, has revealed two types of grains of non-uniform size in the structure of the base material: (1) small recrystallized grains and (2) large ones, formed due to the grain growth (Fig. 5a). Irregular grain sizes are observed in the structure of a material without UFGD additives: small recrystallized and large ones formed as a result of grain growth (Fig. 5a). In the material with an addition of 0.5% UFGD the structure consists of dispersed grains of uniform size, approximately 5–10 lm, grain boundaries are fine (Fig. 5b). This is due to a large number of nucleation sites in the form of nanosized impurities of silicon, calcium, titanium and chromium oxides which are present in UFGD particulates. They are located at the grain boundaries and impede grain growth [28–30]. The introduction of chromium borides in the quantity of 0.1– 0.5% to the Fe + 1% Gr material leads to a significant breakage of its structure (Fig. 6a–c). This is due to the increase in the number of recrystallization centers. When the content of chromium boride is increased up to 1% differences in the grain size appear, and for the 2% content, these differences increase. This phenomenon is due to the formation of liquid phase at the locations of borides. Areas with dendritic structure are formed in this case with grains of different sizes (Fig. 6d). Furthermore, complex doped borides of (FeCr)B type and chromium carbide–borides are formed (Fig. 6e). 3.5. Strength of the tested materials The correct choice of the sintering atmosphere increases the compressive strength of the MMC with the addition of the UFGD
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Fig. 15. The morphology of the worn surface of the MMC with 0.5% additive of a mixture of alumina and zirconium oxides: (a) 25, (b) 2000, and (c) 10,000.
Fig. 14. The morphology of the worn surface of the MMC with 0.2% UFGD additive: (a) 25, (b) 100, (c) 2000, and (d) 10,000.
1.5–3 times (Fig. 7). When sintering, the carbon from UFGD particulates dissolves in the iron, strengthening it, so the strength of the iron sintered in an endothermic gas is higher than that of the iron sintered in vacuum, where carbon is removed from the material. Furthermore, as a result of sintering in vacuum the boundaries of iron grains are refined and the grains grow. However, the stability of the strength characteristics of MMCs sintered in vacuum is much higher compared to MMMs sintered in an endothermic gas (spread: 2–3% and more than 10%, respectively).
The relation between the compressive strength of the MMCs and content of the UFGD additives is parabolic (Fig. 8), and the optimal content of additives is 0.2–0.3%. The influence of various additives on the compressive strength of the MMCs is shown in Fig. 9. The strengthening effect of ultrafine additives is manifested when sintering in the endothermic gas atmosphere already at 900 °C, and it is much stronger at 1100 °C. The greatest strengthening occurs with the introduction of 0.5% of a mixture of alumina and zirconium oxides due to the presence of nanosized particulates with a stronger, defect-free connection with an iron base. Due to strengthening of the grain boundaries, the hardness of the composite material increases too. The microhardness of the ferrite part of the structure increases from 680–965 MPa to 1100–1230 MPa. Chromium boride additives have a positive effect on the strength of the MMCs too; the value of its increment depends on the sintering temperature, because the borides alloy the base material. The introduction of 0.2% chromium boride allows increasing the flexural strength of the composite to 60–65 MPa. The maximum value of the flexural strength is attained when the chromium boride content reaches 0.5% (Fig. 10a). The same
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dependence is observed for the compressive strength (Fig. 10b), the strength increases almost by half. An increase in the chromium boride content to 1% leads to a sharp decrease in the strength of the composite. When the content of chromium boride is 2%, the compressive strength remains unchanged and the flexural strength is reduced, because this characteristic is more sensitive to changes in the material structure. In this case, the strength of the composite with 2% of chromium boride is higher in comparison with a material without the additive. The decrease of the MMC strength with increasing the chromium boride content may be explained by material embrittlement due to the formation of complex doped borides of (FeCr)B type and chromium carbide–borides in the Fe-base material containing graphite. This was confirmed by research on the morphology of fracture surfaces after embrittlement of the material in liquid nitrogen. In the fractures, the above mentioned compounds are located in the pits of destruction, most of chromium boride inclusions are located along the grain boundaries (Fig. 11). An optimal amount of oxide additives is 0.2–0.5 wt%. With an increase in their content up to 1 wt% or more, regardless of the additive type, the strength of composites decreases. The reason is that the input additives have low density and an increase of their content increases the unevenness of their distribution in the base material. In addition, the additives act as horophile impurities, located mainly along the grain boundaries of the iron base, and an increase in their concentration leads to weakening of the grain boundaries and a reduction of strength. 3.6. Friction coefficient values Tests revealed (Fig. 12a) that the introduction of 0.2% UFGD leads to some reduction of the friction coefficient of the base material and an increase of the critical seizure pressure from 2.8 to 3.7 MPa. The addition of 0.5% of ultrafine alumina reduces the coefficient of friction to 0.025–0.027, and more than twofold increases the critical seizure pressure (up to 6 MPa). The introduction of the mixture of 0.4% alumina with 0.1% zirconium oxide to the base material yields a decrease in the friction coefficient of 0.021–0.023 and increases the critical seizure pressure up to 7– 7.5 MPa.
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The addition of 0.2–0.5% chromium borides reduces the coefficient of friction of the base material more than 2 times and increases the critical seizure pressure from 3.0 to 8.6 MPa (Fig. 12b). By increasing the quantity of chromium borides, the coefficient of friction is increased slightly and the critical pressure seizure is reduced up to 4.8–6 MPa. 3.7. Wear behavior Extensive areas of seizure and tearing out of the material are observed on the wear surface of the base material without additives even with a small (25-time) magnification. A greater magnification shows the formation of cracks, mainly on the grain boundaries (Fig. 13). When introducing UFGD additives, the character of wear is changed: the wear surface is smooth and, in contrast to the material without additives, when observed with a slight magnification, the texture of the wear surface is very homogeneous (Fig. 14a). Only at 100-time magnification shallow grooves of wear have been revealed (Fig. 14b). Traces of a seizure on the wear surface are practically absent. This indicates a good running-in of the material, modified by the UFGD. In [23,31,32] the possibility of using the UFGD for fine polishing is mentioned. Impurities of the UFGD initiate branching of formed fracture cracks (Fig. 14c), the destruction occurs not in the individual grains, but in the material body, and as a result the tendency of the material destruction is reduced. A study of the wear surface at high magnifications revealed that the UFGD particulates are located mainly along the grain boundaries, but there is a small amount of them in the grain body (Fig. 14d). The SEM analysis confirmed the presence of fine calcium compounds particulates on grain boundaries. Most probably, these are calcium oxides or calcium carbonates. Additives of calcium compounds are used for the synthesis of the UFGD. It can be assumed that the carbon from UFGD particulates partially diffuses into the iron matrix, and calcium oxides or carbonates that are contained in the UFGD, remain at places of their location. These particulates strengthen the material, preventing the movement of dislocations. Wear surfaces of the MMC with the addition of a mixture of ultrafine alumina and zirconium oxides are characterized by an extremely fine texture; there are weak abrasion grooves left by grinding initial surfaces of samples and wear tracks are thin and smooth (Fig. 15a). Together with a microfailure and pitting of ultrafine particulates, traces of plastic deformation are observed (Fig. 15b). Branching of microcracks occurs in the same way as in the material with the addition of the UFGD. Due to a more dispersed structure, this phenomenon is more marked (Fig. 15c). Ultra-fine oxide particulates are located mainly along the grain boundaries. In the process of wear mainly pitting of alumina occurs. The SEM analysis of wear surfaces identified zirconium oxides, which may melt when sintering, decompose and diffuse into the iron matrix. The wear resistance of the materials with nanocrystalline additives increases significantly (Fig. 16).
4. Conclusions In the present investigation the structure, mechanical and tribological properties of MMCs based on carbon P/M materials reinforced with additives of nanocrystalline particulates of oxides, borides and diamond were addressed. The results of the investigation reveal the following:
Fig. 16. Wear rates of the composites with different additives.
1. In comparison with other additives, the introduction of 0.3% UFGD makes it possible to obtain a higher density at lower pressures of pressing. At higher pressures, the density of the
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material with the UFGD is slightly lower than in the materials without additives and with the addition of ultrafine alumina powder. 2. The addition of up to 0.5% UFGD, oxides or borides to the composite facilitates breakage of the material structure. An increase in the content of additives to 1% or more leads to a heterogeneity of the structure. 3. The introduction of additives increases the compressive strength and the flexural strength of composites. The optimum contents of the additives are as follows: the UFGD content 0.2– 0.3%, the alumina content 0.2–0.5% and the chromium boride content 0.5%. In the case of using the optimum contents, the strength of composites with UFGD additives increases 1.5–3 times, whereas the strength of composites containing oxides or borides increases up to 2 times in comparison with the sintered carbon P/M materials. A further increase of the content of additives causes a sharp decrease in the MMC strength. 4. The coefficients of friction on the surfaces of MMCs containing nanocrystalline particulates, are reduced by 2–3 times compared to the base material, while the critical seizure pressure is increased from 2 to 5 times. The presence of nanocrystalline particulates hinders the appearance of fracture micro-cracks on the surfaces of friction, preventing the movement of dislocations. The wear resistance of MMCs increases by 2–4 times compared to the base material. The use of the UFGD and the mixture of alumina and zirconium oxides is the most effective.
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