Effect of fly ash and its particle size on
properties of sintered Cu-5%Sn-fly ash
particulate composites
N.Vijaya Sai*, P.Nanda Kishore, D. Suresh
Dept. of Mechanical Engineering, V.R.Siddhartha Engineering College, Vijayawada -520007, India
* e-mail: [email protected] Abstract
Copper-5% tin-fly ash powder mixtures containing 0-16 wt% fly ash with different particle sizes were prepared. Small cylindrical specimens of 9 mm diameter and 10.5 mm length were fabricated at 300 MPa using single action die compaction at ambient temperature. These compacts were sintered in argon atmosphere at 7800C. The physical, mechanical and electrical properties of composite specimens were determined as a function of particle size of fly ash and its weight percent. The density of the specimens was found to decrease with increasing weight percent of fly ash. It was also observed that addition of fly ash resulted in increase in porosity and hardness and decrease in compressive yield strength of composite specimens. Dry sliding wear behaviour of the composite specimens was carried out at room temperature using a pin-on-disc machine. Copper-5%tin-fly ash compacts exhibited better wear resistance than the matrix within the range investigated. The electrical conductivity of the composite specimens was also determined using a digital conductivity meter. The electrical conductivity decreased with increase in fly ash content. It was further observed that with decrease in particle size of fly ash, the density, hardness, compressive yield strength, wear resistance and electrical conductivity of specimens gradually increased, while porosity of the composites decreased.
Key words: Density, Porosity, Hardness, Compressive Yield Strength, Wear Resistance, Electrical conductivity 1. Introduction
Copper and copper alloy powders have been used in industrial applications for many years. Probably the best known of these is the self lubricating bearing which was the first major application and still accounts for about 70% of all granular copper powder [1] used.With new frontiers being opened in space, and with the increasing occurrence of specialized working conditions and environments, the self-lubricating bearing industry is under pressure to develop newer and superior bearings. Fedorchenko [2] proposed that powder metallurgy composite antifriction materials offer a way by which complicated compositions can be manufactured. It was suggested that improvements would result from the use of either (i) a strong matrix material in which pores were filled with a soft material or with an alloy capable of playing the role of a hard lubricant, or (ii) a matrix of soft material in which hard inclusions raise bearing capacity.
The extensive use of MMCs in aerospace, automotive industries and in structural applications has increased over the past two decades due to the availability of cheaper reinforcements and cost effective processing routes. Copper composites have become popular because of their good mechanical, thermal and tribological properties. Copper based sintered composites produced by powder metallurgy processes are now widely used in tribological engine parts, e.g., bearings and bushes due to their improved wear resistance. Also, composites based on copper-tin alloys have been developed as self-lubricating bearings under extreme conditions of load, atmosphere and temperature [3]. The effects of hard inclusions and additional production treatment on the strength properties of a bronze-based antifriction material were studied by I.I.Beloborodov et al. It was reported that the highest strength was exhibited by bronze containing powders of stainless steel and an alloy based on iron and cobalt [4]. Y.Z.Zhan et al. have investigated the friction and wear behaviour of copper matrix hybrid composites reinforced with silicon carbide and graphite particles [5]. B.S.Unlu & E.Atik have studied the effect of alloying elements in copper based cusn10 and cuzn30 bearings on tribological and mechanical properties [6]. They have reported that the highest frictional coefficient occurred in CuSn10 and pure Cu bearings, whereas the lowest frictional coefficients occurred in pure Sn and pure Zn bearings. P. Senthil Kumar et al. have studied the effect of molybdenum disulphide on wear behaviour of sintered Cu-Sn composites[7].
Fly ash is one of the cheapest and low-density reinforcements obtained in large quantities as a waste by-product during combustion of coal in thermal power plants [8-9]. Fly ash, compared with other ceramic dispersoids commonly used in MMCs, such as SiC and Al2O3, demonstrates that many of the constituents of fly
studied the feasibility of preparing copper-fly ash composites using powder metallurgy [20]. The powder metallurgy technique is capable of producing metal matrix composites with a wider range of reinforcement distribution. It has been reported by many researchers that the addition of finer particles of the reinforcement significantly improves the physical and mechanical properties of the composite [21-22]. Hence, in the present study, it is proposed to disperse low cost and low density fly ash reinforcement of different particle sizes in a soft matrix of copper-5% tin alloy for possible bearing applications. Thus, the effect of fly ash and its particle size on properties of copper-5% tin- fly ash sintered particulate composites constitute the present study.
2. Experimental procedure
Copper powder (99.5% pure) and tin powder (99.9% pure) were procured from M/s Loba Chemie Pvt. Ltd., Mumbai and the fly ash powder was obtained from Vijayawada Thermal Power Station, Vijayawada. The average particle size of copper and tin powders was 44 µm. The chemical analysis of the fly ash was carried out by M/s Natural Resource Development Co-operative Society Ltd., Hyderabad, India. The chemical composition of fly ash is shown in table 1.
The coarse particles of fly ash obtained from the power plant were pulverized into finer particles manually. In order to study the effect of fly ash and its particle size on the physical, mechanical and electrical properties, namely, density, porosity, hardness, compressive yield strength, wear resistance and electrical conductivity of copper-5% tin-fly ash composites, three different sizes of fly ash powders, namely 75, 53 and 38 μm were prepared using sieve analysis. The densities of these fly ash powders were measured using Archimedes principle and are represented in table 2. Mixtures of copper-5% tin-fly ash powders containing 0-16 wt % fly ash with 75, 53 and 38 μm particle sizes were prepared. In order to obtain uniform distribution of copper, tin and fly ash powders, they were mixed mechanically using a rotating rectangular container for a period of one hour. Cylindrical compacts were obtained at 300 MPa using single action die compaction at ambient temperature. The specimens were compacted at a uniform load rate of 5 kN/min for a period of 3.8 min. Silicone spray was used as the die wall lubricant. The compact dimensions were 9 mm diameter and 10.5 mm length. The above compacts were sealed in transparent silica tube under argon atmosphere and sintered at 7800C in a tubular furnace for a period of 45 min.
Table-1: Chemical Composition of Fly ash (wt%)
Constituents Al2O3 SiO2 Fe2O3 TiO2 CaO Na2O P K2O SO4 MgO Mn LOI
Wt% 27.79 61.75 1.06 0.95 4.36 0.15 0.83 0.64 0.98 0.73 0.14 0.52
Table-2: Density of Fly ash
Scanning electron micrographs were used to study the structural details of the particles. Metallographic examination of sintered compacts was carried out using optical microscopy. Density of the specimens was determined by physical measurements. The porosity of compacts was determined by taking theoretical density of the specimen into consideration. Vickers hardness measurements were obtained using TIME TH130 Integrated Micro Hardness Tester. Compression testing was conducted using an electronic UTM at a crosshead speed of 0.2 mm/min. The dry sliding wear behaviour of composite specimens was studied using a pin-on-disc machine at room temperature. The test specimens were cylindrical powder metallurgical compacts of 9 mm diameter and 10.5 mm length. All tests were performed against hardened and ground EN31 steel discs of diameter 100 mm and hardness of 62 HRC. The specimens and counter disc were cleaned using hexane to remove any contamination before conducting each wear test. All the wear tests were conducted at a load of 78.4 N (8 kgf) with a surface speed of 0.25 m/sec for 30 min duration. The linear wear rate of the cylindrical specimen was measured during the wear test using a gap sensor fitted on the machine. Wear resistance was characterized by wear volume loss. Wear volumes were evaluated from linear wear rates. The electrical conductivity of the composite specimens was measured in % IACS using a digital electrical conductivity meter.
Particle size of Fly ash Density (kN/m3)
0-75 microns 20.52
0-53 microns 20.99
3. Results and Discussion
3.1 Powder characteristics
The scanning electron micrographs of copper, tin and fly ash particles(38 μm) as shown in figures 1-3 indicate their size, shape, size distribution and structure. Figs.1 & 2 reveal the flaky, dendritic structure of copper powder and the spherical shape of tin powder, while Fig.3 indicates the spherical and angular shape fly ash particles. Copper particles are flaky in nature and have the diameter in the range of 2-3 microns while the length is around 20 microns. They also indicate dendritic shape of the copper particles which are partly porous. The particle size of tin varies from 10 -100 µm and the distribution is more uniform. Scanning electron micrograph of 38 μm fly ash particles indicates that a large number of particles are spherical and there exists a wide range of particle size right from as small as 1-2 microns to as high as 38 microns.
Fig.1: SEM of Copper powder
Fig.2: SEM of Tin powder
17 18 19 20 21 22 23 24 25 26
0 2 4 6 8 10 12 14 16
Po
ros
it
y (%
)
Fly ash content (wt%) 38 Microns
53 Microns
75 Microns 3.2 Physical and Mechanical Characteristics
Dimensional changes (volume changes) always occur during sintering of a green compact due to solid state diffusional processes or liquid phase sintering in a multi-component system with widely different melting points of constituents. Since the sintering temperature employed in the present investigation (7800C) is more than the melting point of tin (231.9 0C), copper-5% tin-fly ash composites undergo liquid phase sintering.
45 50 55 60 65 70 75 80
0 2 4 6 8 10 12 14 16
De
ns
it
y (
kN/
m
3)
Fly ash content (wt%)
38 Microns 53 Microns 75 Microns
Fig.4: Effect of Fly ash content on Density
Fig.5: Effect of Fly ash content on Porosity
The effect of fly ash weight percent and its particle size on sintered density and porosity are shown in figures 4 & 5 respectively. They show that sintered density for the range of particle sizes investigated decreases where as porosity increases with increase in fly ash weight percent. The sintered compacts of 0% and 16 % fly ash (38µm) composites have exhibited densities of 77.97 and 49.35 kN/m3 respectively. The addition of 38 µm fly ash powder to copper increased the porosity of pure copper compacts from 17.88% to 19.87%. The decrease in density of the composite with increase in fly ash weight percent can be attributed to the lower density of fly ash powder as compared to that of copper alloy matrix materials. Since the density of fly ash is very low, for a given weight percentage, significant volume of matrix phase is replaced. With increase in fly ash content, the proportion of direct fly ash-fly ash contacts increases. The direct fly ash-fly ash contacts degrade the quality of sintering at the processing temperature, because the fly ash has a melting point (>13000C) higher than the sintering temperature. It results in reduction of sintered density with a corresponding increase in sintered porosity of the composites.
Fig.6: Microstructure of Cu-5% Sn-4% Fly ash (38 µm) Sintered compact
Fig.7: Microstructure of Cu-5% Sn-12% Fly ash (38 µm) Sintered compact
30 35 40 45 50 55 60 65 70 75 80
0 2 4 6 8 10 12 14 16
H
ardn
es
s (H
V
)
Fly ash content (wt%)
38 Microns
53 Microns
75 Microns
Fig.8: Effect of Fly ash content on Hardness
Fly ash
30 35 40 45 50 55 60 65 70 75 80
0 2 4 6 8 10 12 14 16
Com pre ss ive Y ie ld S tre ngt h (M P a)
Fly ash content (wt%)
38 Microns
53 Microns
75 Microns
Fig.9: Effect of Fly ash content on Compressive Yield Strength
Figures 8 & 9 represent the variation of hardness and compressive yield strength of the copper-5% tin-fly ash composites as a function of tin-fly ash weight percent and its particle size. The figures show that hardness increases, while compressive yield strength decreases with increase in fly ash content. The increase in hardness with increase in fly ash is due to the presence of hard alumina silicates in fly ash. The sintered compressive yield strength is particularly low for composites containing more than 10 wt% fly ash. It indicates that the useful range of fly ash that can be added to copper-5% tin matrix lies below 10%. The decrease in yield strength is due to the high porosity and ineffective sintering between fly ash particles and the copper-5% tin alloy matrix in the composites. The decrease in strength is an indication of poor diffusion and interfacial bonding between fly ash particles and the matrix in sintered copper-5% tin-fly ash composites. These figures also show that hardness and yield strength increase with decrease in particle size of fly ash. This is because of the high density and lower porosity of the compacts.
3.3 Wear Characteristics
Fig.10 shows the relation between wear volume and fly ash content and its particle size at 8 kgf load for copper-5%tin- fly ash composites. The results of the wear test reveal that wear resistance increases with the addition of fly ash to the copper-tin alloy. It can be further observed that the wear resistance increases up to 8 wt% fly ash addition, beyond which a marginal decrease is observed for the fly ash composites. It is also observed that with decrease in particle size of fly ash, wear resistance of the composites gradually increased.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0 2 4 6 8 10 12 14 16
We ar V ol um e (m m 3)
Fly ash content (wt%)
38 Microns 53 Microns 75 Microns
10 20 30 40 50 60 70 80 90 100
0 2 4 6 8 10 12 14 16
E
le
ct
ri
ca
l Co
nduc
ti
vi
ty (%
IA
C
S)
Fly ash content (wt%)
38 Microns
53 Microns
75 Microns
Fig.11: Effect of Fly ash content on Electrical Conductivity
3.4 Electrical conductivity
The effect of fly ash and its particle size on electrical conductivity is shown in fig.11. Electrical conductivity gradually decreased with increase in fly ash content and increased with decrease in particle size of fly ash. The decrease in electrical conductivity with increase in fly ash content may be due to the poor conductivity of the constituents of fly ash and high porosity of the compacts. The increase in electrical conductivity with decrease in particle size of fly ash can be attributed to the high density and low porosity of the composite specimens.
4. Conclusions
From the data obtained in this investigation, the following conclusions are arrived.
1. The copper-5% tin-fly ash composites with uniform dispersion of fly ash can be fabricated by powder metallurgy processing route.
2. Incorporation of fly ash particles modified the physical, mechanical and electrical conductivity properties of the pure copper compacts.
3. The density of sintered composites decreased with increasing fly ash content. Porosity and hardness increased with increasing fly ash weight percent.
4. Compressive yield strength of sintered copper-5%tin-fly ash composites decreased, while wear resistance increased with increasing fly ash weight percent.
5. Sintered density, sintered hardness and sintered compressive yield strength gradually increased with decrease in particle size of fly ash. Sintered porosity decreased with decrease in particle size of fly ash.
6. The addition of fly ash improved the wear resistance of the copper-5%tin-fly ash composites. The wear resistance of the composites further improved with decrease in particle size of fly ash.
7. The electrical conductivity of the sintered copper compacts decreased with addition of fly ash and increased with decrease in particle size of fly ash.
8. These results suggest that useful range of fly ash that can be added to copper lies below 10% and finer particles of fly ash result in improved physical, mechanical and electrical conductivity properties of the copper- 5%tin-fly ash composites.
Acknowledgements
The authors thank All India Council for Technical Education, New Delhi, India for the financial grant sanctioned under AICTE-RPS (F.No 8023/RID/RPS-154(pvt.)/2011-2012) to carry out this work.
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