- Email: [email protected]
Fabrication and abrasive wear properties of metal matrix composites reinforced with three-dimensional network structure
RARE METALS Vol. 25, No. 6, Dee 2006, p . 671
Fabrication and abrasive wear properties of metal matrix composites reinforced with three-dimensionalnetwork structure WANG Shouren””, GENG Haoran3),LI Kunshan”, SONG Bo’), WANG Yingzi3),and HUI Linhai3’ 1) School of MechanicalEngineering,Jinan University,Jinan 250022, China
2) The Key Laboratory of Liquid Structureand Heredity of Materials, Shandong University,Jinan 250014, China 3) School of Materials Science,Jinan University,Jinan 250022, China
(Received2005-07-07)
Abstract: Reticulated polyurethane was chosen as the preceramic material for preparing the porous preform using the replication process. The immersing and sintering processes were each performed twice for fabricating a high-porosity and super-strong skeleton. The aluminum magnesium matrix composites reinforced with three-dimensional network structure were prepared using the infiltration technique by pressure assisting and vacuum driving. Light interfacial reactions have played a profitable role in most of the ceramic-metal system. The metal matrix composites interpenetrated with the ceramic phase have a higher wear resistance than the metal matrix phase. The volume fraction of ceramic reinforcement has a significant effect on the abrasive wear, and the wear rate can be decreased with the increase of the volume fraction of reinforcement.
Key words: metal matrix composites; infiltration; friction and wear; three dimensional network structure; microstructure [Thiswork was$nanciully supported by the Natural Science Foundation of Shandong Province, China (Y2a)6FO3).]
1. Introduction Metal matrix composites (TVWlCs) have some reinforcements such as particle, whisker, and fiber [l-31 as well as the three-dimensional network structure [4]. In recent years, metal matrix composites reinforced with three-dimensional network structure (3DNRMMCs) have been paid considerable attention, since they possess a rather high specific strength, stiffness, and wear-resistance and can be attractive candidates for structural and functional materials [5]. There are several methods available for fabricating 3DNRMMCs including stir casting (SC) [6-71, mechanical alloying (MA) [8], powder metallurgy (PM) [S], squeeze casting (SQC) [lo], molten metal infiltration (MMI) [ll], and self-propagating high-temperature synthesis (SHS) [12-131. Amongst all techniques the infiltration method is the only technique suitable for fabricating the high volume fraction (>50%) of MMCs [14] and thus, has become the most attractive process for fabCorresponding author: WANG Shouren
ricating 3DNRMMCs. Based on the source of the driving forces, the infiltration technique can be classified into three categories: pressure assisting, vacuum driving, and nonpressure or capillarity driving ~141. The wear of the material is one of the tremendous and unavoidable losses in engineering. Thus, the development of novel wear-resistance materials has been an important task, which is of interest to many researchers. Amongst these the research of metal matrix composites reinforced with the ceramic phase is a hotspot [15-201. However, little research has been done on the fabrication and the abrasive wear properties of 3DNRMMCs. In this article, it is seen that the fabrication technique by pressure assisting and vacuum driving (PAVD) is at the forefront in the fabrication of 3DNRMMCs. The friction and wear behaviors of 3DNRMMCs have been investigated. The purpose of this study is to reveal the characteristics of the fabrication process and the properties of friction and wear of 3DNRMMCs.
E-mail: [email protected]
672
2. Experimental 2.1. Fabrication of the porous ceramic skeleton High-purity p-Si3N4 powders (Si3N42 97%, diameter I100 pm, Shanghai Silicon Materials Plant, China) were used as the starting material, and their microstructure is shown in Fig. 1. The sintering additives were composed of 5 wt.% A1203and 5 wt. % Al fine powders, which were mixed with the starting material and then ball-milled for 4 h using A1203 balls. The silica sol was added in the powder mixture as the cohesive agent. The mixed slurry was stirred for 2 h using a glass rocker. Its viscosity had a decisive effect on the immersion of the slurry. In these experiments, the mass proportion between the powder mixture and the silica sol (Si02, 30.0%3 1.O%; pH = 8.5-10.0) was chosen as 2: 1.
Fig. 1. SEM mimastructureof the PSiaA material.
Reticulated polyurethane (PU) with interconnected pores was chosen as the template for preparing the porous preform using the replication process. The pore sizes of the PU were about 1-5 mm. The framework of the PU must be strong and needs to be cleaned with deionized water. The cleaned PU was cut into a standard form of $30 mm x 100 mm and was immersed in the homogeneous slurry for about 10 min. The samples were extracted from the slurry repeatedly till the slurry had covered the PU. The samples were then added into a rotating cylinder and were rotated within it. The superfluous slurry that jammed the mesh of the
RARE METALS, VoL 25, No. 6,Dec 2006 PU could be thrown off by the centrifugal force. The rotating process was necessary for achieving the porous ceramic with open cells and was an essential procedure for fabricating the porous Si& ceramic reinforcement. The samples were placed in a Wig stove for 20 h at 160°C to remove most of the moisture. Two different sintering processes, a single sintering process and a twice sintering process, were used and compared in the sintering experiment. During the single sintering process, the samples were first heated to 400°C at a heating rate of 50"C/h and the PU framework was burnt away in this stage; the samples were then heated to the desired tempemture at 1200"c/h and were maintained at the highest temperature for 60 min. The twice sintering process was as follows: in the first sintering stage, the samples were heated to 400°C at 50"C/h and then to 800°C at 200"C/h. The samples were then immersed in a slurry for 2 h and dried for 20 h at 160°C. To prevent the samples from crashing, the slurry used in the second immersion must have a lower concentration than that of the slurry used in the first immersion. The mass ratio of the second slurry between the powder mixture and the silica sol was chosen as 1:2. In the second sintering stage, the samples were heated from room temperature to 1600°C at a rate of 200"Ch and were maintained for 60 min. The morphology of the reticulated Si3N4ceramic preform after twice immersing and twice sintering is shown in Fig. 2(a).
2.2. Fabrication of 3DNRMMCs The raw materials used in this experiment were Al ingot (99.7 wt.%), Zn ingot (99.99 wt.%), and Mg ingot (99.95 wt.%), which were melted in a furnace and then prepared for infiltratingthe preform. The porous Si3N4ceramic skeletons were heated in a furnace under nitrogen atmosphere. After reaching the required temperam, the liquid metal was infiitrated into the preform with pressure assisting and vacuum driving. The porosity (@ and the compressive strength (a,)of the reinforcement and the composition of the matrix alloy are shown in Table 1.
...
Wung S.R. et ah, Fabrication and abrasive wear properties of metal matrix composites
673
in Fig. 3. The chemical element distributions examined using the energy dispersive spectrometer (EDS, OXFOED INCA) are also shown in Fig. 3.
"0
200
400 600 Distance / brn
SO0
0
200
400 600 Distance / pm
SO0
Fig. 2. Morphologies of the Si& reticulated ceramics preform (a) and 3DNRMMCs (b). l-Si&ceramic skeleton; M e t a l matrix. Table 1. Characteristic of reinforcement and chemical compositionof the alloys
S2
Si3N41 wt.% 0 12
S3 Sq
Sample S1
Ss
0 1 a,l % MPa - -
Mgl
Znl
All
wt.%
wt.%
wt.%
4.5
bal. bal.
88
9
4.5
1.7 1.7
18
82
4.5
1.7
bal.
25 18
75 72
13 19 13
4.5 0
1.7
bal.
1.7
bal.
The micro-structural construction of the 3DNRMMCs was viewed using scanning electron microscopy (S-2500), which is presented in Fig. 2@). The phase marked as 1 indicates the Si3N4ceramic skeleton, and the phase marked as 2 indicates the metal matrix. High magnification of SEM is shown
"
I I
~
0
200
400 600 Distance / pm
OK,,
I
SO0
Fig. 3. SEM micrograph of 3DNRMMCs under high magnification(a) and EDS analyses (b-d).
674
RARE METALS, VoL 25, No. 6, Dec 2006
23. Friction and wear experiment The wear specimens were tested under dry conditions using a simple and self-made experimental apparatus, which is illustrated schematically in Fig. 4. The test samples were machined to dimensions of 15 mm x 15 mm x 10 mm. The applied load was 20 N, the rotating velocity of the steel disk was 1400 r/min, and the testing time periods were 5, 10, 15, and 20 min, respectively. Under dry conditions, the samples were degreased and cleaned using an alcohol solution. An alumina sand paper (360') was extended on the friction disk. Each test result was achieved at the end of the sand paper life. All conditions of the tribological tests are shown in Table 2. Applied load Sand paper
'1
Specimen I
Fig. 4. Schematic diagram of the wear machine.
Table 2. Parameters for friction and wear experiment Sand paper disk, diameter 5 mm Number of cycles / (rmin-') 1400 Relative humidity J % 30-50 Counter body
Temperature / "C
20 22
Motion type
Rotational sliding
Surrounding environment
Air 15mmx15mmx10mm
Normal force / N
Sample size Lubricant Sliding time / min
shown in Fig. 5. The D T m G analysis of the sponge was performed under air atmosphere at a heating rate of lO"C/min to determine the thermal decomposition range. The PU framework was burned out gradually when the temperature reached 372.7"C. An exothermic peak was observed at 632.9"C owing to the first strong oxidation. An endothermic peak was observed at 660.2"C, which was the melting point of the aluminum metal powder. Further oxidation took place at 959.1"C. When the temperature reached 1261.5"C, the glass phase occurred and acted as a medium for mass transport during densification. Some cavities that were formerly occupied by the sponge were maintained. Low heating rate was adopted to avoid cracking during the volatilization of the polymeric sponge framework. The twice immersing and sintering processes were carried out to ensure the high quality of the porous ceramic framework. The inner cavities of the ceramic framework were padded after the twice sintering process, whereas they could still remain after the single sintering process, as shown in Figs. 6(a) and 6@). From the sintering results, it is shown that the single sintering process could not ensure the high quality of the porous S i a 4 ceramic, and hence, it is not acceptable. During the second immersion process, the slurry was immersed into the cavities by capillarity, and the cavities were padded effectively. Thus, the relative density increased from 2.9 to 3.3 g/cm3.The twice immersing and sintering processes were adopted as the standard experiment method, and all samples used in the compressive strength test were sintered using the twice immersing and sinteringprocesses.
None
5, 10, 15,u)
3. Results and discussions 3.1. Fabrication of the porous skeleton The D T m G analysis curve of the skeleton is
Temperature / "c
Fig. 5. D T m G curve of the Si& ceramic.
...
Wung S.R. et aZ., Fabrication and abrasive wear properties of metal matrix composites
675
strength can be decreased according to the increase of porosity. With increased temperature, sufficient reaction and diffusion of the additives can accelerate the densification of the framework and increase the compressive strength. However, excess flow of the liquid phase during sintering may destroy the structure of the porous ceramic and jam the porous mesh.
3.2. Fabrication of 3DNRMMCs To fabricate the excellent composites with good interface characteristics and proprties, the fabrication conditions, such as pressure, temperature, time, atmosphere, and the chemical composition of the matrix, should be considered. The P A W (pressure assisting and vacuum driving) process is an innovative technique for fabricating 3DNRMMCs, and the parameters are shown in Table 3. In this study, composites were fabricated using pressure-assisted infiltration, vacuum-driving infiltration, and P A W infiltration, respectively, and their interface microstructures are shown in Fig. 7. Using pressure assisting and vacuum driving, a kind of excellent composite interface has been obtained (Fig. 7(c)). Table 3. Parameters of the P A W process
Fig. 6. SEM cross-sectional micrographs of the porous Si& ceramic: (a) single sintering process; (b) twice sintering process.
Various factors such as sintering temperature, porous ceramic framework, uniformity of porous structure, and particle size are decisive for the compressive strength of the porous ceramic, amongst which the porosity is the primary factor. A higher compressive strength can be achieved by decreasing the porosity, appropriately raising the sintering temperature, fabricating the regular ceramic framework, and by using pure precursor powders with small particle size. The SiSN4 ceramic with different porosities can be fabricated to meet the reinforcement requirement of the aluminum magnesium matrix composite according to the application requirements. The compressive strength with different porosities of the skeleton is shown in Table 1. The compressive
Infiltration pressure / Mpa Infiltration temperature / "C Protection atmosphere Warming-up temperature of mould / "C Heat preservation time / min
3-5 800 Nitrogen 400 45
Light interfacial reactions play a profitable role in most of the ceramic-metal systems [14]. It has been reported that the existence of Mg content plays a useful role in infiltration [21]. In this study, two kinds of compositions of the alloy are designed, one including 4.5 wt.% Mg (Sample Sj) and the other without it (Sample S,). The interface microstructures of the composites are shown in Fig. 8. It has been obviously demonstrated that the existence of Mg results in a good coalescence of the metal with the ceramic. With the increase of Mg content, the interface improves; however, an Mg content of more than 10 wt.% has little effect [21].
RARE METALS, Vol. 25, No. 6,Dec 2006
676
Fig. 7. Interface microstructures of 3DNRMMCs fabricated Using vacuum driven htlltration (a), pres~~re-assisted infiltration (b), and the P A W process infiltration (c).
Fig. 8. Interface microstructures of 3DNRMMCs fabricated under the same a t r a t i o n condition but with different compositions of the matrix (a) without Mg; @) with 4.5 wt.% Mg.
33.
and wear properties Of 3DNRMMCs
The abrasive wear resistance of 3DNRMMCs significantly exceeds that of a monolithic material. The wear process in 3DNRMMCs is more complex
than that in metal. The reinforcement volume frattion has a large effect on the abrasive wear, which is indicated in Fig. 9. The wear loss of the alloy sample without reinforcement increases rapidly with the increase in sliding time at the initial friction. The wear
Wang S.R. et d,Fabrication and abrasive wear properties of metal matrix composites...
rate becomes slow when the friction is going on, owing to the serious wear and then the decrease of the cutting ability of the A203 sand paper on the friction disk. For 3DNRMMCs, the wear loss also increases with the increase of friction time; 1.4 ^p.
C
*g
1.2 1.0
3 -.0.8
-
a,
2 0.6 kl 0.4 .a, *2 e0.0 O 4
6
8
10 12 14 16 18 20 22
Wear time / min Fig. 9. Wear rate of the composifes as a function of wear time at different contents of Si& ceramic phase.
677
moreover, the wear rate turns even slower. The wear losses of 3DNRMMCs are considerably smaller than that of the alloy. The wear losses decrease with the increase in the content of S i g 4 ceramic phase. The wear loss of the alloy is 0.51 g during the test time period of 5 min, whereas it is 1.37 g during the test time period of 20 min, while the wear loss of 3DNRMMCs is 0.02 g during the test time period of 5 min and 0.05 g during the test time period of 20 min. With the decrease of the volume fraction of reinforcement, the morphology of the worn surface changes gradually from fine scratches to distinct grooves and to further severe deformation and fracture.The worn surface micrographs of the alloy and 3DNRMMCs are shown in Fig. 10. The degree of wear and abrasion decreases with the increase of the volume fraction of reinforcement with the same test time and test load. It is indicated that abrasion is
Fig. 10. Surface morphologies of the worn samples: (a) Al alloy; (b) Sz; (c) S,; (d) S, (20 N load, 20 min friction time).
RARE METALS, Vol. 25, No. 6,Dee 2006
678
evident on the worn surfaces of the samples. The local-damaged and even-fractured flakes are observed on the worn surface of the Samples S1 and S2. The abrasive surface of Sample Sz (Fig. 10(b)) shows a distinct abrasion-plastic deformation. The abrasions microstructures of Samples S3 (Fig. lO(c)) and S4 (Fig. 10(d)) are tiny, which may testify the beneficial effect of the Si3N4network structure. In Fig. lO(a), the alloy has a distinct abrasion microstructure that has a deep and symmetrical furrow. It is indicated that the surface of the sample was peeled and broken off. However, the abrasion microstructures of Samples S3 and S4 differ considerably from the alloy. There are no distinct and coherent furrows but some scratching injuries caused by the large grains breaking off from the Si3N4 ceramic reinforcement phase and the sand paper. The Si3N4 ceramic skeleton with a three-dimensional network structure has high stiffness and strength, thereby forming a strong sustaining skeleton for the matrix alloy and reducing the contact area of the matrix alloy and the sand paper. The friction pair of 3DNRMMCs versus sand paper is hard-hard materials. And hence, the mechanism of tribology is quite different from the matrix alloys which are hard-soft contact.
4.
Conclusions
(1) A series of metal matrix composites with different volume fraction reinforcements have been prepared using the pressure assisting and vacuum driving infiltration technology. A preform skeleton has been fabricated through the replication process. (2) The sintering temperature of the preform is 1200°C and the preform skeleton exhibits high compressive strength. The compressive strength of 12% Si3N4is 9 m a , that of 18% Si3N4is 13 MPa, and that of 25% Si3N4is 19 MPa. The sintering additives of 5 wt.% A1203and 5 wt.% A1 can jam the pore of the struts and accelerate the densification of the framework, and thus increase the compressive strength. The P A W infiltration technology ensures good coalescence of the metal with the ceramic, and with the increase of the Mg content (1 wt.%-10
wt.%), the interface becomes excellent. (3) 3DNRMMCs have better wear-resistance than the matrix phase alloy. The wear rate decreases with the increase in the volume fraction of reinforcement of the composites. The ceramic that interpenetrated into the matrix can reduce the wear rate of the materials and slow the transformation from mild-wear situation to severe-wear situation.
References [l] Wang H.Y., Jiang Q.C., and Zhao Y.Q., Fabrication of TiB2and TiB2-TiCparticulates reinforced magnesium matrix composites, Muter. Sci. Eng. A, 2004, 372 109. [2] Trojanova Z., Lukac P., Riehemann W., and Mordike B.L., Study of relaxation of residual internal stress in Mg composites by internal friction, Muter. Sci. Eng. A , 2002,324: 122. [3] Xu Z.R., Chawla K.K.. Neuman A., Liggett G.M., Wolfenden A., and Chawla N., Stiffness loss and density decrease due to thermal cycling in an alumina fiber/magnesium alloy composites, Muter. Sci. Eng. A, 1995,203 75. [4] Konopka K., Olszbwka-Myalska A., and Szafran M., Ceramic-metal composites with an interpenetrating network, Muter. Chem. Phys., 2003,81: 329. [5] Lu L., Lai M.O., and Froyen L., Structure and properties of Mg metal-metal composite, Key Eng. Muter., 2002,230 (2):287. [6] Jiang Q.C., Wang H.Y., Wang J.G., Guan Q.F. and Xu C. L., Fabrication of TiCp/Mg composites by the thermal explosion synthesis reaction in molten magnesium, Muter. Lett, 2003,57: 2580. [7] Saravanan R.A. and Surappa M.K., Fabrication and characterization of pure magnesium 30~01%SiCp composites, Muter. Sci. Eng. A., 2000,276 108. [8] Lu L., Thong K.K., and Gupta M., Mg-based composite reinforced by Mg2Si, Compos. Sci. T e c h l . , 2003,63:627. [9] Ferkel H. and Mordike B.L., A-structure materials propedes microstructure and processing, Muter. Sci. Eng. A., 2001,298: 193. [lo] Filho A.L., Atkinson H., and Jones H., Hot isostatic pressing of metal reinforced metal matrix composites, . I Muter. . Sci., 1998,33:5517. [ll] Thakur S.K. and Dhindaw B.K., The influence of interfacial characteristics between SiCp and Mg/Al
Wung S.R. et aL, Fabrication and abrasive wear properties of metal matrix composites... metal matrix on wear, coefficient of friction and microhardness, Wear, 2001,247:191. [12] Jiang Q.C., Wang H.Y., Guan Q.F. and Li X. L., Effect of the temperature of molten magnesium on the thermal explosion synthesis reaction of AI-Ti-C system for fabricating TiC/Mg composite, A&. Eng. Mater., 2003,s:722. [13] Wang H.Y., Jiang Q.C., Li X.L. and Wang J.G., Fabrication of Tic particulate reinforced magnesium matrix composites, Scrpta. Muter., 2003,48:1349. [14] Srinivasa Rao B. and Jayaram V., Pressureless infiitration of Al-Mg based alloys into A1203preforms: mechanisms and phenomenology, Acta Muter., 2001,
49 2373. [15] Sannino A.P., Rack H.J., Dry sliding wear of discontinuously reinforced duminium composites: review and discussion, Wear, 1995,189:1. [16] Alpas A.T. and Zhang J. Effect of microstructure and counterface material on the sliding wear resistance of particulate reinforced aluminium matrix
679
composites, Met. Trans.A, 1994,25:969. [17] Subramanian C. On mechanical mixing during dry sliding of Aluminium-12.3wt% Silicon alloy against copper, Wear, 1993,161:53. [18] Iwai Y.,Yoneda H., and Honda T. Sliding wear behavior of Sic whisker-reinforced aluminium com594. posite, Wear, 1995,181-183 [19] Antoniou R.A., Brown L.R., and Cashion J.D., The unlubricated sliding of AI-Si alloys against steel: Mossbauer spectroscopy and X-ray diffraction of wear debris, Acta Metall. Muter., 1994,423545. [20] Zhao H.F. and Wang L., Casting Zn Based Alloy and Composites, China Standard Press, Beijing,
2002. [21] Zufia A. and Hand R.J., Role of Mg and Mg+Si as external dopants in production of pure Al-Sic metal matrix composites by pressureless infiltration, Muter. Sci. Technol.,2000,16(7):867. [22] Pepper S.V.,Shear strength of metal-sapphire contacts, J. Appl. Phys., 1976,47(3): 801.
Fabrication and abrasive wear properties of metal matrix composites reinforced with three-dimensionalnetwork structure WANG Shouren””, GENG Haoran3),LI Kunshan”, SONG Bo’), WANG Yingzi3),and HUI Linhai3’ 1) School of MechanicalEngineering,Jinan University,Jinan 250022, China
2) The Key Laboratory of Liquid Structureand Heredity of Materials, Shandong University,Jinan 250014, China 3) School of Materials Science,Jinan University,Jinan 250022, China
(Received2005-07-07)
Abstract: Reticulated polyurethane was chosen as the preceramic material for preparing the porous preform using the replication process. The immersing and sintering processes were each performed twice for fabricating a high-porosity and super-strong skeleton. The aluminum magnesium matrix composites reinforced with three-dimensional network structure were prepared using the infiltration technique by pressure assisting and vacuum driving. Light interfacial reactions have played a profitable role in most of the ceramic-metal system. The metal matrix composites interpenetrated with the ceramic phase have a higher wear resistance than the metal matrix phase. The volume fraction of ceramic reinforcement has a significant effect on the abrasive wear, and the wear rate can be decreased with the increase of the volume fraction of reinforcement.
Key words: metal matrix composites; infiltration; friction and wear; three dimensional network structure; microstructure [Thiswork was$nanciully supported by the Natural Science Foundation of Shandong Province, China (Y2a)6FO3).]
1. Introduction Metal matrix composites (TVWlCs) have some reinforcements such as particle, whisker, and fiber [l-31 as well as the three-dimensional network structure [4]. In recent years, metal matrix composites reinforced with three-dimensional network structure (3DNRMMCs) have been paid considerable attention, since they possess a rather high specific strength, stiffness, and wear-resistance and can be attractive candidates for structural and functional materials [5]. There are several methods available for fabricating 3DNRMMCs including stir casting (SC) [6-71, mechanical alloying (MA) [8], powder metallurgy (PM) [S], squeeze casting (SQC) [lo], molten metal infiltration (MMI) [ll], and self-propagating high-temperature synthesis (SHS) [12-131. Amongst all techniques the infiltration method is the only technique suitable for fabricating the high volume fraction (>50%) of MMCs [14] and thus, has become the most attractive process for fabCorresponding author: WANG Shouren
ricating 3DNRMMCs. Based on the source of the driving forces, the infiltration technique can be classified into three categories: pressure assisting, vacuum driving, and nonpressure or capillarity driving ~141. The wear of the material is one of the tremendous and unavoidable losses in engineering. Thus, the development of novel wear-resistance materials has been an important task, which is of interest to many researchers. Amongst these the research of metal matrix composites reinforced with the ceramic phase is a hotspot [15-201. However, little research has been done on the fabrication and the abrasive wear properties of 3DNRMMCs. In this article, it is seen that the fabrication technique by pressure assisting and vacuum driving (PAVD) is at the forefront in the fabrication of 3DNRMMCs. The friction and wear behaviors of 3DNRMMCs have been investigated. The purpose of this study is to reveal the characteristics of the fabrication process and the properties of friction and wear of 3DNRMMCs.
E-mail: [email protected]
672
2. Experimental 2.1. Fabrication of the porous ceramic skeleton High-purity p-Si3N4 powders (Si3N42 97%, diameter I100 pm, Shanghai Silicon Materials Plant, China) were used as the starting material, and their microstructure is shown in Fig. 1. The sintering additives were composed of 5 wt.% A1203and 5 wt. % Al fine powders, which were mixed with the starting material and then ball-milled for 4 h using A1203 balls. The silica sol was added in the powder mixture as the cohesive agent. The mixed slurry was stirred for 2 h using a glass rocker. Its viscosity had a decisive effect on the immersion of the slurry. In these experiments, the mass proportion between the powder mixture and the silica sol (Si02, 30.0%3 1.O%; pH = 8.5-10.0) was chosen as 2: 1.
Fig. 1. SEM mimastructureof the PSiaA material.
Reticulated polyurethane (PU) with interconnected pores was chosen as the template for preparing the porous preform using the replication process. The pore sizes of the PU were about 1-5 mm. The framework of the PU must be strong and needs to be cleaned with deionized water. The cleaned PU was cut into a standard form of $30 mm x 100 mm and was immersed in the homogeneous slurry for about 10 min. The samples were extracted from the slurry repeatedly till the slurry had covered the PU. The samples were then added into a rotating cylinder and were rotated within it. The superfluous slurry that jammed the mesh of the
RARE METALS, VoL 25, No. 6,Dec 2006 PU could be thrown off by the centrifugal force. The rotating process was necessary for achieving the porous ceramic with open cells and was an essential procedure for fabricating the porous Si& ceramic reinforcement. The samples were placed in a Wig stove for 20 h at 160°C to remove most of the moisture. Two different sintering processes, a single sintering process and a twice sintering process, were used and compared in the sintering experiment. During the single sintering process, the samples were first heated to 400°C at a heating rate of 50"C/h and the PU framework was burnt away in this stage; the samples were then heated to the desired tempemture at 1200"c/h and were maintained at the highest temperature for 60 min. The twice sintering process was as follows: in the first sintering stage, the samples were heated to 400°C at 50"C/h and then to 800°C at 200"C/h. The samples were then immersed in a slurry for 2 h and dried for 20 h at 160°C. To prevent the samples from crashing, the slurry used in the second immersion must have a lower concentration than that of the slurry used in the first immersion. The mass ratio of the second slurry between the powder mixture and the silica sol was chosen as 1:2. In the second sintering stage, the samples were heated from room temperature to 1600°C at a rate of 200"Ch and were maintained for 60 min. The morphology of the reticulated Si3N4ceramic preform after twice immersing and twice sintering is shown in Fig. 2(a).
2.2. Fabrication of 3DNRMMCs The raw materials used in this experiment were Al ingot (99.7 wt.%), Zn ingot (99.99 wt.%), and Mg ingot (99.95 wt.%), which were melted in a furnace and then prepared for infiltratingthe preform. The porous Si3N4ceramic skeletons were heated in a furnace under nitrogen atmosphere. After reaching the required temperam, the liquid metal was infiitrated into the preform with pressure assisting and vacuum driving. The porosity (@ and the compressive strength (a,)of the reinforcement and the composition of the matrix alloy are shown in Table 1.
...
Wung S.R. et ah, Fabrication and abrasive wear properties of metal matrix composites
673
in Fig. 3. The chemical element distributions examined using the energy dispersive spectrometer (EDS, OXFOED INCA) are also shown in Fig. 3.
"0
200
400 600 Distance / brn
SO0
0
200
400 600 Distance / pm
SO0
Fig. 2. Morphologies of the Si& reticulated ceramics preform (a) and 3DNRMMCs (b). l-Si&ceramic skeleton; M e t a l matrix. Table 1. Characteristic of reinforcement and chemical compositionof the alloys
S2
Si3N41 wt.% 0 12
S3 Sq
Sample S1
Ss
0 1 a,l % MPa - -
Mgl
Znl
All
wt.%
wt.%
wt.%
4.5
bal. bal.
88
9
4.5
1.7 1.7
18
82
4.5
1.7
bal.
25 18
75 72
13 19 13
4.5 0
1.7
bal.
1.7
bal.
The micro-structural construction of the 3DNRMMCs was viewed using scanning electron microscopy (S-2500), which is presented in Fig. 2@). The phase marked as 1 indicates the Si3N4ceramic skeleton, and the phase marked as 2 indicates the metal matrix. High magnification of SEM is shown
"
I I
~
0
200
400 600 Distance / pm
OK,,
I
SO0
Fig. 3. SEM micrograph of 3DNRMMCs under high magnification(a) and EDS analyses (b-d).
674
RARE METALS, VoL 25, No. 6, Dec 2006
23. Friction and wear experiment The wear specimens were tested under dry conditions using a simple and self-made experimental apparatus, which is illustrated schematically in Fig. 4. The test samples were machined to dimensions of 15 mm x 15 mm x 10 mm. The applied load was 20 N, the rotating velocity of the steel disk was 1400 r/min, and the testing time periods were 5, 10, 15, and 20 min, respectively. Under dry conditions, the samples were degreased and cleaned using an alcohol solution. An alumina sand paper (360') was extended on the friction disk. Each test result was achieved at the end of the sand paper life. All conditions of the tribological tests are shown in Table 2. Applied load Sand paper
'1
Specimen I
Fig. 4. Schematic diagram of the wear machine.
Table 2. Parameters for friction and wear experiment Sand paper disk, diameter 5 mm Number of cycles / (rmin-') 1400 Relative humidity J % 30-50 Counter body
Temperature / "C
20 22
Motion type
Rotational sliding
Surrounding environment
Air 15mmx15mmx10mm
Normal force / N
Sample size Lubricant Sliding time / min
shown in Fig. 5. The D T m G analysis of the sponge was performed under air atmosphere at a heating rate of lO"C/min to determine the thermal decomposition range. The PU framework was burned out gradually when the temperature reached 372.7"C. An exothermic peak was observed at 632.9"C owing to the first strong oxidation. An endothermic peak was observed at 660.2"C, which was the melting point of the aluminum metal powder. Further oxidation took place at 959.1"C. When the temperature reached 1261.5"C, the glass phase occurred and acted as a medium for mass transport during densification. Some cavities that were formerly occupied by the sponge were maintained. Low heating rate was adopted to avoid cracking during the volatilization of the polymeric sponge framework. The twice immersing and sintering processes were carried out to ensure the high quality of the porous ceramic framework. The inner cavities of the ceramic framework were padded after the twice sintering process, whereas they could still remain after the single sintering process, as shown in Figs. 6(a) and 6@). From the sintering results, it is shown that the single sintering process could not ensure the high quality of the porous S i a 4 ceramic, and hence, it is not acceptable. During the second immersion process, the slurry was immersed into the cavities by capillarity, and the cavities were padded effectively. Thus, the relative density increased from 2.9 to 3.3 g/cm3.The twice immersing and sintering processes were adopted as the standard experiment method, and all samples used in the compressive strength test were sintered using the twice immersing and sinteringprocesses.
None
5, 10, 15,u)
3. Results and discussions 3.1. Fabrication of the porous skeleton The D T m G analysis curve of the skeleton is
Temperature / "c
Fig. 5. D T m G curve of the Si& ceramic.
...
Wung S.R. et aZ., Fabrication and abrasive wear properties of metal matrix composites
675
strength can be decreased according to the increase of porosity. With increased temperature, sufficient reaction and diffusion of the additives can accelerate the densification of the framework and increase the compressive strength. However, excess flow of the liquid phase during sintering may destroy the structure of the porous ceramic and jam the porous mesh.
3.2. Fabrication of 3DNRMMCs To fabricate the excellent composites with good interface characteristics and proprties, the fabrication conditions, such as pressure, temperature, time, atmosphere, and the chemical composition of the matrix, should be considered. The P A W (pressure assisting and vacuum driving) process is an innovative technique for fabricating 3DNRMMCs, and the parameters are shown in Table 3. In this study, composites were fabricated using pressure-assisted infiltration, vacuum-driving infiltration, and P A W infiltration, respectively, and their interface microstructures are shown in Fig. 7. Using pressure assisting and vacuum driving, a kind of excellent composite interface has been obtained (Fig. 7(c)). Table 3. Parameters of the P A W process
Fig. 6. SEM cross-sectional micrographs of the porous Si& ceramic: (a) single sintering process; (b) twice sintering process.
Various factors such as sintering temperature, porous ceramic framework, uniformity of porous structure, and particle size are decisive for the compressive strength of the porous ceramic, amongst which the porosity is the primary factor. A higher compressive strength can be achieved by decreasing the porosity, appropriately raising the sintering temperature, fabricating the regular ceramic framework, and by using pure precursor powders with small particle size. The SiSN4 ceramic with different porosities can be fabricated to meet the reinforcement requirement of the aluminum magnesium matrix composite according to the application requirements. The compressive strength with different porosities of the skeleton is shown in Table 1. The compressive
Infiltration pressure / Mpa Infiltration temperature / "C Protection atmosphere Warming-up temperature of mould / "C Heat preservation time / min
3-5 800 Nitrogen 400 45
Light interfacial reactions play a profitable role in most of the ceramic-metal systems [14]. It has been reported that the existence of Mg content plays a useful role in infiltration [21]. In this study, two kinds of compositions of the alloy are designed, one including 4.5 wt.% Mg (Sample Sj) and the other without it (Sample S,). The interface microstructures of the composites are shown in Fig. 8. It has been obviously demonstrated that the existence of Mg results in a good coalescence of the metal with the ceramic. With the increase of Mg content, the interface improves; however, an Mg content of more than 10 wt.% has little effect [21].
RARE METALS, Vol. 25, No. 6,Dec 2006
676
Fig. 7. Interface microstructures of 3DNRMMCs fabricated Using vacuum driven htlltration (a), pres~~re-assisted infiltration (b), and the P A W process infiltration (c).
Fig. 8. Interface microstructures of 3DNRMMCs fabricated under the same a t r a t i o n condition but with different compositions of the matrix (a) without Mg; @) with 4.5 wt.% Mg.
33.
and wear properties Of 3DNRMMCs
The abrasive wear resistance of 3DNRMMCs significantly exceeds that of a monolithic material. The wear process in 3DNRMMCs is more complex
than that in metal. The reinforcement volume frattion has a large effect on the abrasive wear, which is indicated in Fig. 9. The wear loss of the alloy sample without reinforcement increases rapidly with the increase in sliding time at the initial friction. The wear
Wang S.R. et d,Fabrication and abrasive wear properties of metal matrix composites...
rate becomes slow when the friction is going on, owing to the serious wear and then the decrease of the cutting ability of the A203 sand paper on the friction disk. For 3DNRMMCs, the wear loss also increases with the increase of friction time; 1.4 ^p.
C
*g
1.2 1.0
3 -.0.8
-
a,
2 0.6 kl 0.4 .a, *2 e0.0 O 4
6
8
10 12 14 16 18 20 22
Wear time / min Fig. 9. Wear rate of the composifes as a function of wear time at different contents of Si& ceramic phase.
677
moreover, the wear rate turns even slower. The wear losses of 3DNRMMCs are considerably smaller than that of the alloy. The wear losses decrease with the increase in the content of S i g 4 ceramic phase. The wear loss of the alloy is 0.51 g during the test time period of 5 min, whereas it is 1.37 g during the test time period of 20 min, while the wear loss of 3DNRMMCs is 0.02 g during the test time period of 5 min and 0.05 g during the test time period of 20 min. With the decrease of the volume fraction of reinforcement, the morphology of the worn surface changes gradually from fine scratches to distinct grooves and to further severe deformation and fracture.The worn surface micrographs of the alloy and 3DNRMMCs are shown in Fig. 10. The degree of wear and abrasion decreases with the increase of the volume fraction of reinforcement with the same test time and test load. It is indicated that abrasion is
Fig. 10. Surface morphologies of the worn samples: (a) Al alloy; (b) Sz; (c) S,; (d) S, (20 N load, 20 min friction time).
RARE METALS, Vol. 25, No. 6,Dee 2006
678
evident on the worn surfaces of the samples. The local-damaged and even-fractured flakes are observed on the worn surface of the Samples S1 and S2. The abrasive surface of Sample Sz (Fig. 10(b)) shows a distinct abrasion-plastic deformation. The abrasions microstructures of Samples S3 (Fig. lO(c)) and S4 (Fig. 10(d)) are tiny, which may testify the beneficial effect of the Si3N4network structure. In Fig. lO(a), the alloy has a distinct abrasion microstructure that has a deep and symmetrical furrow. It is indicated that the surface of the sample was peeled and broken off. However, the abrasion microstructures of Samples S3 and S4 differ considerably from the alloy. There are no distinct and coherent furrows but some scratching injuries caused by the large grains breaking off from the Si3N4 ceramic reinforcement phase and the sand paper. The Si3N4 ceramic skeleton with a three-dimensional network structure has high stiffness and strength, thereby forming a strong sustaining skeleton for the matrix alloy and reducing the contact area of the matrix alloy and the sand paper. The friction pair of 3DNRMMCs versus sand paper is hard-hard materials. And hence, the mechanism of tribology is quite different from the matrix alloys which are hard-soft contact.
4.
Conclusions
(1) A series of metal matrix composites with different volume fraction reinforcements have been prepared using the pressure assisting and vacuum driving infiltration technology. A preform skeleton has been fabricated through the replication process. (2) The sintering temperature of the preform is 1200°C and the preform skeleton exhibits high compressive strength. The compressive strength of 12% Si3N4is 9 m a , that of 18% Si3N4is 13 MPa, and that of 25% Si3N4is 19 MPa. The sintering additives of 5 wt.% A1203and 5 wt.% A1 can jam the pore of the struts and accelerate the densification of the framework, and thus increase the compressive strength. The P A W infiltration technology ensures good coalescence of the metal with the ceramic, and with the increase of the Mg content (1 wt.%-10
wt.%), the interface becomes excellent. (3) 3DNRMMCs have better wear-resistance than the matrix phase alloy. The wear rate decreases with the increase in the volume fraction of reinforcement of the composites. The ceramic that interpenetrated into the matrix can reduce the wear rate of the materials and slow the transformation from mild-wear situation to severe-wear situation.
References [l] Wang H.Y., Jiang Q.C., and Zhao Y.Q., Fabrication of TiB2and TiB2-TiCparticulates reinforced magnesium matrix composites, Muter. Sci. Eng. A, 2004, 372 109. [2] Trojanova Z., Lukac P., Riehemann W., and Mordike B.L., Study of relaxation of residual internal stress in Mg composites by internal friction, Muter. Sci. Eng. A , 2002,324: 122. [3] Xu Z.R., Chawla K.K.. Neuman A., Liggett G.M., Wolfenden A., and Chawla N., Stiffness loss and density decrease due to thermal cycling in an alumina fiber/magnesium alloy composites, Muter. Sci. Eng. A, 1995,203 75. [4] Konopka K., Olszbwka-Myalska A., and Szafran M., Ceramic-metal composites with an interpenetrating network, Muter. Chem. Phys., 2003,81: 329. [5] Lu L., Lai M.O., and Froyen L., Structure and properties of Mg metal-metal composite, Key Eng. Muter., 2002,230 (2):287. [6] Jiang Q.C., Wang H.Y., Wang J.G., Guan Q.F. and Xu C. L., Fabrication of TiCp/Mg composites by the thermal explosion synthesis reaction in molten magnesium, Muter. Lett, 2003,57: 2580. [7] Saravanan R.A. and Surappa M.K., Fabrication and characterization of pure magnesium 30~01%SiCp composites, Muter. Sci. Eng. A., 2000,276 108. [8] Lu L., Thong K.K., and Gupta M., Mg-based composite reinforced by Mg2Si, Compos. Sci. T e c h l . , 2003,63:627. [9] Ferkel H. and Mordike B.L., A-structure materials propedes microstructure and processing, Muter. Sci. Eng. A., 2001,298: 193. [lo] Filho A.L., Atkinson H., and Jones H., Hot isostatic pressing of metal reinforced metal matrix composites, . I Muter. . Sci., 1998,33:5517. [ll] Thakur S.K. and Dhindaw B.K., The influence of interfacial characteristics between SiCp and Mg/Al
Wung S.R. et aL, Fabrication and abrasive wear properties of metal matrix composites... metal matrix on wear, coefficient of friction and microhardness, Wear, 2001,247:191. [12] Jiang Q.C., Wang H.Y., Guan Q.F. and Li X. L., Effect of the temperature of molten magnesium on the thermal explosion synthesis reaction of AI-Ti-C system for fabricating TiC/Mg composite, A&. Eng. Mater., 2003,s:722. [13] Wang H.Y., Jiang Q.C., Li X.L. and Wang J.G., Fabrication of Tic particulate reinforced magnesium matrix composites, Scrpta. Muter., 2003,48:1349. [14] Srinivasa Rao B. and Jayaram V., Pressureless infiitration of Al-Mg based alloys into A1203preforms: mechanisms and phenomenology, Acta Muter., 2001,
49 2373. [15] Sannino A.P., Rack H.J., Dry sliding wear of discontinuously reinforced duminium composites: review and discussion, Wear, 1995,189:1. [16] Alpas A.T. and Zhang J. Effect of microstructure and counterface material on the sliding wear resistance of particulate reinforced aluminium matrix
679
composites, Met. Trans.A, 1994,25:969. [17] Subramanian C. On mechanical mixing during dry sliding of Aluminium-12.3wt% Silicon alloy against copper, Wear, 1993,161:53. [18] Iwai Y.,Yoneda H., and Honda T. Sliding wear behavior of Sic whisker-reinforced aluminium com594. posite, Wear, 1995,181-183 [19] Antoniou R.A., Brown L.R., and Cashion J.D., The unlubricated sliding of AI-Si alloys against steel: Mossbauer spectroscopy and X-ray diffraction of wear debris, Acta Metall. Muter., 1994,423545. [20] Zhao H.F. and Wang L., Casting Zn Based Alloy and Composites, China Standard Press, Beijing,
2002. [21] Zufia A. and Hand R.J., Role of Mg and Mg+Si as external dopants in production of pure Al-Sic metal matrix composites by pressureless infiltration, Muter. Sci. Technol.,2000,16(7):867. [22] Pepper S.V.,Shear strength of metal-sapphire contacts, J. Appl. Phys., 1976,47(3): 801.