development and investigations of copper metal matrix composites

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056

Volume: 03 Issue: 05 | May-2016 www.irjet.net p-ISSN: 2395-0072

© 2016, IRJET | Impact Factor value: 4.45 | ISO 9001:2008 Certified Journal | Page 557

Development and Investigations of Copper Metal Matrix Composites

Reinforced with Graphite

Jitendra Kumar1, Vicky Kumar2, Sunil Kumar3, Sandeep3

1,2PG student, Dept. of Mechanical Engineering, MITM, Jevra, Hisar, Haryana, India. 3,4Assistant Professor, Dept. of Mechanical Engineering, MITM, Jevra, Hisar, Haryana, India.

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Abstract - Sliding wear is a key determinant of the performance of electrical sliding contacts used in electrical machines. The behaviour of the contacts in sliding couple is controlled by the mutual metal transfer, friction and wear. The present work focuses on evaluation of tribological performance of copper-graphite composites. Copper-graphite composites of graphite contents of 5 wt. %, 10 wt. % and 15 wt. % made by stir casting route. The wear testing was carried out using pin-on-disk tribometer. . Extensive metallographic investigations were carried out using Scanning Electron Microscope to study the structural changes at the surface and sub-surface of each tested pin. It was found that the investigated materials showed three distinct wear rate regimes: low, mild and severe. The addition of graphite particles simultaneously decreased the wear rates of the composite and the counterpart disk. The wear mechanisms of pure copper at each wear rate regime were believed to be oxidation-delamination, delamination, and seizure wear mechanism, whereas the involved wear mechanisms of either composites were the same, and they were: oxidative-dominant, strain-induced delamination, and sub-surface delamination.

Key Words: Copper, Graphite, stir casting, Scanning Electron Microscope, Wear test, etc.

1. INTRODUCTION

Composite materials are emerging chiefly in response to unprecedented demands from technology due to rapidly advancing activities in aircrafts, aerospace and automotive industries. These materials have low specific gravity that makes their properties particularly superior in strength and modulus to many traditional engineering materials such as metals. As a result of intensive studies into the fundamental nature of materials and better understanding of their structure property relationship, it has become possible to develop new composite materials with improved physical and mechanical properties. These new materials include high performance composites such as Polymer matrix composites [1, 2], Ceramic matrix composites [3, 4] and Metal matrix composites [5] etc. Continuous advancements have led to the use of composite materials in more and more diversified applications.

1.1 Composites

Generally, a composite material is composed of reinforcement (fibers, particles, flakes, and/or fillers) embedded in a matrix (polymers, metals, or ceramics). The matrix holds the reinforcement to form the desired shape while the reinforcement improves the overall mechanical properties of the matrix. When designed properly, the new combined material exhibits better strength than would each individual material. Composites consist of one or more discontinuous phases embedded in a continuous phase. The discontinuous phase is usually harder and stronger than the continuous phase and is called the ‘reinforcement‘ or ‘reinforcing material’, whereas the continuous phase is termed as the ‘ matrix’.

Properties of composites are strongly dependent on the properties of their constituent materials, their distribution and the interaction among them. The composite properties may be the volume fraction sum of the properties of the constituents or the constituents may interact in a synergistic way resulting in improved or better properties. Apart from the nature of the constituent materials, the geometry of the reinforcement (shape, size and size distribution) influences the properties of the composite to a great extent. The concentration distribution and orientation of the reinforcement also affect the properties.

1.2 Copper-Graphite Metal Matrix Composite

Metal matrix composites in general, consist of at least two components, one is the metal matrix and the second component is reinforcement. The matrix is defined as a metal in all cases, but a pure metal is rarely used as the matrix. It is generally an alloy. In the productivity of the composite the matrix and the reinforcement are mixed together. Copper-graphite composites are an example of metal matrix composites. Basically, there is dispersion of graphite in pure copper matrix. The composite that has been studied has been fabricated by stir casting route. They exhibit excellent lubricating and anti-seizing properties due to the presence of graphite and good electrical conductivity due to the pure copper. But there is also the problem of poor interfacial bonding between copper and graphite. The properties of the copper-graphite composites are a function of the type and amount of graphite fiber incorporated in the composite, as well as the orientation of that fiber.




1. Cu-Graphite composites typically have a coefficient of

thermal expansion between 4-6 ppm/oC (depends on the temperature).

2. Cu-Graphite has a density that ranges from 7.0 to 7.5 grams/cubic centimeter (20% less than copper). Switching from molybdenum or copper-tungsten to copper-graphite can save significant weight while providing better thermal performance.

3. Cu/G composites also have high resistance to thermal shock.

2. METHODOLOGY

In recent years the potential of metal-matrix composite (MMC) materials for significant improvement in performance over conventional alloys has been recognized widely. However, their manufacturing costs are still relatively high. There are several fabrication techniques available to manufacture the MMC materials: there is no unique route in this respect. Due to the choice of material and reinforcement and of the types of reinforcement, the fabrication techniques can vary considerably. The processing methods used to manufacture particulate reinforced MMCs can be grouped as follows. Solid-Phase Fabrication Methods: Diffusion bonding, Hot rolling, Extrusion, Drawing, Explosive welding, P/M route, Pneumatic impaction, etc. [6,7]. Liquid-Phase Fabrication Methods: Liquid-metal infiltration, Squeeze casting, pressure casting, stir casting etc. [6,7]. Two Phase (solid/liquid) Processes: Which include Rheo-casting [7] and Spray atomization [8].

Normally the liquid-phase fabrication method is more efficient [9] than the solid-phase fabrication method because solid-phase processing requires a longer time. The matrix metal is used in various forms in different fabrication methods. Generally powder is used in pneumatic impaction and the powder metallurgy technique, and a liquid matrix is used in liquid-metal infiltration, plasma spray, spray casting, squeeze casting, pressure casting, gravity casting, stir casting, investment casting, etc. A molecular form of the matrix is used in electroforming; vapour deposition and metal foils are used in diffusion bonding, rolling, extrusion, etc.

In this work, Stir Casting method is used for making samples of Metal Matrix composite. Stir-casting techniques shown in Figure 1 is currently the simplest and most commercial method of production of MMCs. This approach involves mechanical mixing of the reinforcement particulate into a molten metal bath and transferred the mixture directly to a shaped mould prior to complete solidification. The next step is the solidification of the melt

containing suspended dispersoids under selected conditions to obtain the desired distribution of the dispersed phase in the cast matrix. In preparing metal matrix composites by the stir casting method, there are several factors that need considerable attention, including The difficulty in achieving a uniform distribution of the

reinforcement material. Wettability between the two main substances. Porosity in the cast metal matrix composites. Chemical reactions between the reinforcement material

and the matrix alloy

In order to achieve the optimum properties of the metal matrix composite, the distribution of the reinforcement material in the matrix alloy must be uniform, and the wettability or bonding between these substances should be optimized. The porosity levels need to be minimized.

Microstructural in-homogeneities can cause notably particle agglomeration and sedimentation in the melt and subsequently during solidification. Inhomogeneity in reinforcement distribution in these cast composites could also be a problem as a result of interaction between suspended ceramic particles and moving solid-liquid interface during solidification. This process has major advantage that the production costs of MMCs are very low.

Fig - 1: MMC by casting route through Stir Casting method

2.1 Wear Test Measurement:

The experiments were carried out on pin-on-disc apparatus from which wear volume and coefficient of friction can be obtained. The diameter of the bar is 8 mm and length is 30 mm. Dry sliding wear tests were conducted on a pin-on-disc friction and wear monitoring test rig as shown in Figure 2. The pin was held against the counter-face of a rotating disc of hardened ground steel (EN 50) with wear track diameter 80 mm. The cylindrical pin of diameter 8 mm and length 30 mm was loaded against the disc through a dead weight loading system. The specimen was held stationary and the disc was rotated while a normal force was applied through a lever mechanism.




Fig - 2: Friction and wear test rig

The wear test for all specimens was conducted under the normal load (10 N, 20 N & 30 N), sliding distance (1000 m, 1500 m, 2000 m), and sliding velocity (1.5 m/s, 2.5 m/s and 3.5 m/s). The samples were weighed (using electronic microbalance up to an accuracy of ±0.0001 g) before and after each test and weight loss was calculated as the difference between these two data. The wear of the composite was studied as a function of the sliding distance, applied load and the sliding velocity. The surface of the pin samples rubbed using emery paper of Silicon Carbide (1000 grit size) prior to test in ordered to ensure effective contact of fresh and flat surface with the steel disc.

For finding Specific Wear Rate, the weight loss method was used for calculating specific wear rate during the experiments. Before experiment performing on the pin-on-disc apparatus, initial weight of specimen is measured and after the completion of experiment again final weight of specimen is measured. Then weight loss is calculated by subtracting initial and final weight of specimen. Then Specific Wear Rate (SWR) can be found by the following:

Where Ks is Specific Wear Rate (mm3/N-m), Δm is the mass loss in the test duration (g), ρ is the density of the composite (g/cm3) and F is the normal load in newton (N), L is the sliding distance in meters (m) and V is the sliding velocity in m/s.

2.2 Design of Experiments Taguchi Design of Experiments (DOE) is a powerful analysis tool which is adopted for optimizing design parameters. Taguchi method provides the designer with a systematic and efficient approach for experimentation to determine near optimum settings of design parameters for performance, quality, and cost. The most important stage in the design of experiment lies in the selection of the control factors. In the present work, the impact of the five such factors were studied using L27 orthogonal array which has 27 rows corresponding to the number of tests. In conventional full factorial experimental design, it would require 34 = 81 runs to study five factors each at three levels, whereas, Taguchi’s factorial experiment approach reduces it to only 27 runs offering a

great advantage in terms of experimental time and cost. The operating conditions under which sliding wear tests were carried out is given in the Table 1. Table - 1: Different sets of combination using Taguchi approach

Mess

Size

Nominal

Load Wt. %

Sliding

Velocity

Sliding

Distance RPM Time

200 10 5 1.5 1000 358.1 11.06

200 10 10 2.5 1500 596.83 10

200 10 15 3.5 2000 835.56 9.31

200 20 5 2.5 2000 596.83 13.2

200 20 10 3.5 1000 835.56 4.45

200 20 15 1.5 1500 358.1 16.4

200 30 5 3.5 1500 835.56 7.08

200 30 10 1.5 2000 358.1 22.13

200 30 15 2.5 1000 596.83 6.4

400 10 5 1.5 1000 358.1 11.06

400 10 10 2.5 1500 596.83 10

400 10 15 3.5 2000 835.56 9.31

400 20 5 2.5 2000 596.83 13.2

400 20 10 3.5 1000 835.56 4.45

400 20 15 1.5 1500 358.1 16.4

400 30 5 3.5 1500 835.56 7.08

400 30 10 1.5 2000 358.1 22.13

400 30 15 2.5 1000 596.83 6.4

600 10 5 1.5 1000 358.1 11.06

600 10 10 2.5 1500 596.83 10

600 10 15 3.5 2000 835.56 9.31

600 20 5 2.5 2000 596.83 13.2

600 20 10 3.5 1000 835.56 4.45

600 20 15 1.5 1500 358.1 16.4

600 30 5 3.5 1500 835.56 7.08

600 30 10 1.5 2000 358.1 22.13

600 30 15 2.5 1000 596.83 6.4

The experimental observations were transformed into a Signal-to-Noise (S/N) ratio. There are three S/ N ratios available depending upon the type of characteristics (smaller-the-better, larger-the-better, and nominal-the-better). The S/N ratio for minimum (friction and wear rate) coming under smaller is better characteristic, which can be calculated as logarithmic transformation of the loss function as shown below:




Where ‘n’ is the repeated number trial conditions and y1, y2 ……yn are the response of the friction and sliding wear characteristics. “Smaller is better” characteristic, with the above S/N ratio transformation is suitable for minimizations of coefficient of friction and specific wear rate. The standard linear graph is used to assign the factors and interactions to various columns of the orthogonal array (OA).

3. RESULTS

The wear test was performed using a pin-on-disk apparatus under dry sliding conditions to monitor Specific Wear Rate (SWR). The tested samples were in the form of cylindrical pins of 8 mm diameter, and 30 ± 2 mm in length. These pins were rubbed against a hardened steel disk (EN 50) of surface hardness of 60 HRC. The applied loads varied from 10 N, 20 N & 30 N at linear sliding speed at the friction surface of 1.5, 2.5 and 3.5 m/s, and for a variable sliding distance of 1000 m, 1500 m and 2000 m for various test. The wear track was cleaned with acetone before performing every experiment. A microbalance of accuracy ±0.0001 g was used for weighting of specimens. After that the specimen is mounted in the jaws of wear machine which held the specimen against the disc. For all experiments, the track diameter is kept 80 mm and the experiment have been performed according Taguchi design using L27 array. After getting different sets of combinations we begin the experimental procedure. After every run specimen is weighed to get weight difference and hence specific wear rate was calculated. Taguchi analysis was done for copper-graphite composite samples by varying different parameters as per the Taguchi table.

600400200

86.0

85.5

85.0

84.5

84.0

302010 15105

3.52.51.5

86.0

85.5

85.0

84.5

84.0

200015001000

MESS SIZE

Me

an

of

SN

ra

tio

s

LOAD WT%

VELOCITY DISTANCE

Main Effects Plot for SN ratiosData Means

Signal-to-noise: Smaller is better

Fig - 3: Effects plot for S/N ratio Figure 3 shows that there is increase in wear as load increases. Same has been observed in case of sliding distance, which was well predicted from literature review. As weight percentage increased wear decreased because graphite has self-lubricating properties.

600400200

0.000065

0.000060

0.000055

0.000050

302010 15105

3.52.51.5

0.000065

0.000060

0.000055

0.000050

200015001000

MESS SIZE

Me

an

of

Me

an

s

LOAD WT%

VELOCITY DISTANCE

Main Effects Plot for MeansData Means

Fig - 4: Effects plot for Means Figure 3 and 4 shows that the variation of wear rate of copper–graphite composites with the normal load. Copper 10 wt. % and 15 wt. % graphite composites exhibited the lowest wear rate for all the range of normal loads. It was observed from the figure 4 that copper 10 wt. % graphite composites was showing higher wear rate than copper 15 wt. % graphite composite. This increased wear rate was attributed not only to the lower order interfacial strength between the copper and uncoated graphite but also to the relatively higher hardness, lower porosity and finer microstructure were attributed to improved wear resistance of graphite reinforced composites. It was also observed that the wear rate of copper-graphite composites decreases with increase in the graphite content. The wear behavior of composites was mainly influenced by the volume content of graphite, as observed from the figure 3 and 4. In the 10 wt. % and 15 wt. % graphite composites, wear rate with normal load was not following the similar trend. This happens due to the nature of self-lubrication at the contact surface and relative densification of graphite composites. Self-lubrication was mainly affected by volume content, spatial distribution and size of graphite particles. When the graphite was at the lowest percentage (5 wt. %), the ability of forming graphite layer was inadequate at the contact zone which increased the metal to metal contact. The possibility of more metal to metal contact was also increased with increase in normal load. Consequently, hard asperities of counter surface material tend to plow in the surface of the composite pin. This action was severed with the increase in normal load. It results in the steady increase of wear rate of composites with normal load. Hence the wear rate was in the highest order for 5 wt. % graphite composites when compared to 10 wt. % and 15 wt. % graphite reinforced composites.

3.1 Wear Mechanism Scanning Electron Microscope (SEM) micrograph of worn surface of copper–graphite composite shows a distinct characteristic of wear scar, as shown in figure 5. SEM image of copper–graphite worn surface at lower load shows many slim and deep wear grooves along the sliding direction due to



© 2016, IRJET | Impact Factor value: 4.45 | ISO 9001:2008 Certified Journal | 61

mild plastic deformation occurred during sliding, as shown in figure 6

. Fig - 5: SEM of MMC with 5% graphite of 600 mesh size at 20 N load (500x)

Fig - 6: SEM of MMC with 10% graphite of 400 mesh size at 20 N load (500x) These grooved lines on the worn surface indicate the occurrence of plastic deformation. During sliding, considerable amount of plastic deformation occurred in the matrix which results in the exposure of the graphite particles to the contact surface. These graphite particles smeared at the sliding surface under the normal load along the sliding direction form a thin graphite layer. This thin graphite layer covered at the worn surface prevents further plastic deformation of the matrix. Plastic deformation was observed to be increased with the increase in normal load. It leads to the formation of many wider wear grooves on the worn surface at higher load, as seen from figure. The grooves observed in figure is magnified and observed to have clear understanding. The wear track and ploughing of materials

were observed from figure 7. Due to increased localized pressure, the formed thin graphite layer ruptured during sliding and again fresh exposure of graphite particles at the contact surface was observed. Formation and rupture of graphite layer are repeated during the entire wear test.

Fig - 7: SEM of MMC with 15% graphite of 200 mesh size at 30 N load (500x)

3. CONCLUSIONS The significant conclusions of the studies carried out on copper-graphite composite are as follows: 1. The Liquid metallurgy technique (stir casting route)

was successfully adopted in the preparation of copper-graphite composites containing the filler contents of 5 wt. %, 10 wt. % and 15 wt. % with varying mesh size of graphite particles.

2. The microstructural studies revealed the uniform distribution of the particulates in the matrix system.

3. The wear resistance of the composites is higher than that of copper metal.

4. Increased applied load and sliding distances resulted in higher volumetric wear loss. Further, the graphite reinforcement contributed significantly in improving the wear resistance of copper-graphite composites.

5. It has been observed that copper-15 wt. % of graphite of 600 mesh size exhibits superior tribological properties compared with copper, copper-5 wt. % graphite and copper-10 wt. % graphite composite.

6. Impact strength of the composites was found to decrease with increased filler content as the graphite is a soft material and has low strength.

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development and investigations of copper metal matrix composites

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