Development of Aluminium-Nickel Coated Short

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ScienceDirect Materials Today: Proceedings 5 (2018) 11336–11345

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ICMMM - 2017

Development of Aluminium-Nickel Coated Short Carbon Fiber Metal Matrix Composites Nithin Kumara, H. C. Chittappab, S. Ezhil Vannanc* Research scholar, Department of Mechanical Engineering, UVCE, Bangalore-560001, Indiaa Associate Professor, Department of Mechanical Engineering, UVCE, Bangalore-560001, Indiab Assistant Professor, Department of Mechanical Engineering, SJCE, Mysuru-570006, Indiac

Abstract Fiber reinforced composites have become a great boon ever since matrix and fibre are combined in macroscopic manner. Among them the metal matrix composites are unique because of their superior properties .The manufacturing of carbon fiber reinforced Aluminum Metal Matrix Composites [AMMCs] involves the formation of a brittle intermetallic phase due to the reaction taking place at the fiber-matrix interface at temperatures exceeding 700°C. To avoid this undesired chemical reaction that ultimately results in the degradation of composite properties, the reinforcement is coated with nickel by electro less process. This paper describes the Electroless Nickel [EN] coating process carried out on PAN based short carbon fibers. The resulting fibers are investigated for their metallographic characteristics by microscopy. These coated fibers were mixed with aluminium alloy by stir casting technique. The resulting composite’s hardness and tensile strengths were studied © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).

Keywords: AMMCs; carbon fibers; EN coating

1. Introduction Here introduce the Composite materials provide the flexibility to modify its material properties to meet different requirements. Researchers and design experts these days are showing keen interest to find low-cost, lightweight, environmental friendly, high quality and good performance materials. With this emerging trend, MMCs prove to be a suitable choice among the experts. MMCs have the advantages of altering its tensile and compressive properties, creep, notch resistance, tribological properties as well as density, thermal expansion and diffusivity. The focus of research hence has been on Aluminium matrix composites because of its light weight and excellent specific

* Corresponding author. Tel.:+91 9845575450; E-mail address: [email protected]

2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Materials Manufacturing and Modelling (ICMMM - 2017).

Nithin kumar et al./ Materials Today: Proceedings 5 (2018) 11336–11345

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Nomenclature MMC EN PAN

Metal Matrix Composite Electroless Nickel Polyacrylonitrile

properties at room or higher temperatures [2] which makes them obvious choice in automobile, aerospace and defense industries applications [3]. When compared with conventional alloys [4] composites are more preferred because of their high specific strength, chemical stability, excellent creep resistance, high specific modulus and high resistance to thermal expansion. In fibre reinforced MMC’s, the original physical and chemical identities of fibre and matrix are retained. The interface between fibre and matrix does not provide material with higher mechanical properties. This can be overcome when the two are combined together. The study of interfaces is important to know what actually happens transfer between the matrix and reinforcement and hence to analyze the mechanical behavior of MMCs. Proper transfer of load from the matrix to the fibre depends on the distribution of the fibre within a composite. Matrix composition, type of the reinforcement, fabrication technique and post treatment methods of the composite are the factors affecting the load transfer. Wettability between the two phases in the composite is the major governing factor that decides uniformity in the distribution of the fibres in the matrix and also properties of the composites. This hinders the spontaneous flow of the liquid metal within the interstices of the fibres. Also, it is reported that aluminium and carbon chemically react with each other at the interface to produce the undesirable aluminium carbide [5], which leads to detrimental products on the interface behaving as damage nucleation sites under stress. This leads to embrittlement of the matrix, degradation of the fibers and appearance of interfacial brittle phases [6]. Generally short fibers of length less than 3 mm are referred to as short fibers whose diameter ranges from 6-8 microns. Translation of mechanical and electrical properties and greater processing ability are the advantages of these fibers. Their compounds have higher heat distortion temperatures and excellent thermal conductivity. Critical length lc of short fibres is given by lc = d*Sf / Sm. (1) Where, d →fibre diameter Sf→ reinforcement strength Sm →matrix strength. Aspect ratio here is greater than 5. In applications such as electromagnetic interference shielding, antistatic and refractory insulation, these are used as electrically conductive filler for polymers. However, they possess low compressive strength compared to tensile strength and have a tendency to become oxidized above 400° C. There are many coating techniques available. Among them cementation and electro less coating usually employed coating methods for obtaining homogenous and continuous coating of metallic film over fibres. Results revealed that, coating deposition was continuous for thickness above 0.2 microns and was discontinuous at below 0.2 microns. Cementation coating technique leads to fibres with lower ultimate tensile strength indicating that the coating was defective [5, 13]. Stir casting and subjecting composites to age-hardening treatment can be employed in characterization of Al 6061, copper coated carbon fibres by electroless method and also to produce different quantities of carbon fibres. The studies revealed that the tensile strength increased up to 4 wt.% of CF [7]. Wettability of fibres and formation of brittle Al4C3 can be prevented by coating. Firstly, structure of the composite and role of nickel coating on them was studied. Then composite system was simulated and temperature distribution and stresses were obtained. It was concluded that thermal stresses leads to the failure of nickel coating [8]. Performance test by electroplating technique in order to coat the carbon fibers with nickel, before plating, the fibers were pre-treated to improve the wettability in the bath. The results indicated that the nickel-coated carbon fibers exhibit a good bonding strength as well as excellent oxidation resistance at high temperature. Also the wettability with aluminium was seen to have improved [9].

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Investigation was carried out by studying the influence of coating parameters such as time of sensitization, activation etc. on the thickness of metallic layer deposited on fiber by electroless method,. During and after the coating process [10], resultant composite fibre was characterized by SEM/EDX. Here, production of aluminium alloy/nickel coated carbon fibre MMCs using low cost stir casting technique and technical difficulties in obtaining a homogeneity in the distribution of reinforcement, wetting ability between phases are presented and discussed [11]. PAN based CF reinforced with Al 2024 composite by squeeze casting method, the fibres was provided with a 0.5 microns thick layer of nickel by electroless method. Finally, inference was drawn that the tensile strength and wear resistance of the composites [12, 14] was improved by good bonding of the metallic coat of nickel on the fibres. Inferences drawn from survey are metallic coating on the reinforcement improves the wettability in between the constituents and during stages of electro less coating, temperature and pH should be maintained as per the standards for the uniform metallic coating to be deposited on the carbon fibers.

2. Experimental Procedure 2.1 Electroless Nickel Coating of Carbon Fibers The reinforcement used for this work is in the form of long polyacrylonitrile based carbon fibers. About 1/2 kg of fiber is used for this work. Fibers of average diameter 7μm are used in this study. They were chopped cut to length about 1-1.5 mm. The fibers were coated by electro less nickel coating technique. The electro less coating procedure involves pre cleaning, sensitizing, activating and metal depositing along with swilling and drying in the intermediate stages. Resulting coated fiber were mixed with Aluminium matrix with reinforcement weight percentage of 0, 2, 4, 6 and 8. The EN coating bath contains nickel sulphate which is a source of nickel ions. It supplies the nickel ions (Ni+2) that are chemically reduced to become the EN coating. (2) 3NaH2PO2 + 3H2O + NiSO4 3NaH2PO3 + H2SO4 + 2H2 + Ni Ni++ + H2PO2- + H2O Ni0 + H2PO3- + 2H+

(3)

Reduction takes place by means of reducing agent sodium hypophosphite NaH2PO2 which supplies electrons for reducing the nickel ion compound to metallic nickel. H2PO2- + H2O H+ + HPO32- +2Habs

(4)

where, H2PO2- is the hypophosphite ion, HPO3-2 is the orthophosphate ion and Habs is the absorbed atomic hydrogen. The above reaction takes place in the presence of heat from the catalyst. Ni+2 + 2Habs Ni0 + 2H+

(5)

Where, Ni0 is the elemental nickel being deposited on the fibers. H2PO2- + Habs H2O + OH- +P

(6)

Where, OH- is the hydroxyl ion and P is phosphorus. Phosphorus is co-deposited with nickel to form a nickelphosphorus alloy that serves as a natural lubricant to the EN coatings. Typical phosphorus content in the coating ranges from 6 to 12 percentage. Higher phosphorus content results in a continuous coating, but with decreased hardness, wear resistance and corrosion resistance in alkaline environments. M2+ +2e (supplied by reducing agent) M0

(7)


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This cathodic partial reaction takes place in the presence of a catalyst. Heat energy also affects the rate of deposition. Complexing agents (chelates) play the role of reducing the free Ni+ ions availability for the reaction. They are a class of additives which help to maintain a stable pH level and prevent the precipitation of nickel salts. After the reduction reaction, the quality of coating surface degrades resulting in rough and dull coating. Sodium citrate checks coating from begin porous and dull. Additional requirements are buffers, stabilizers and accelerators. Stabilizers or inhibitors adsorb to impurities in the solution and prevent unwanted nickel precipitation and decomposition of the entire bath. Accelerators are added to the bath to increase the plating rate by driving the oxidation of the reducing agent. In general, deposition requires cleaning, sensitization, catalysing, activation (acceleration) and metallization. Rinsing or swilling is required between the steps. Prior to coating, fibres are treated with acetone and rinsed in distilled water to ensure that fibres are free from any impure substances. This helps to achieve effective coating. The coating process begins with the sensitization process where the removal of pyrolytic coatings around the asreceived fibers is done in an oven for a small duration. It is followed by the immersion of fibers in a solution of 20 g/l stannous chloride and continuously stirred. The process time is then checked. The fibers will be isolated from the above sensitizer solution and cleaned in distilled water. This process is called as swilling and is done multiple times to ensure that the fibers are free of chemicals. It is followed by neutralization stage where 40 ml of hydrochloric acid is used. It is swilled to clean the fibers in distilled water. This removes any acid traces. In order to have activated surface, the sensitized fibers are dipped in an aqueous solution constituting of 5 g/l of palladium chloride under ultrasonic agitation. This process is called as activation produces the formation of palladium pores on the surface of fiber which allows the continuous deposition of nickel ions. This is followed by swilling to make the parts free from palladium chloride and cleaned in distilled water. After this, neutralization process is done with 2.5 ml of hydrochloric acid. It is again swilled to clean the fibers in distilled water. This removes further acid traces. Next is the metallization stage. It involves the immersion of activated fibers in a solution containing 15 g/l of nickel sulphate which is a metallic ion source. The process time, chemical concentration and temperature is checked. Liquid ammonia is used in order to increase the pH from 10 to 12 since at this value the coating will be accurate. Also, sulphuric acid controls the pH value if it goes beyond it and the bath temperature is in the range of 40°C to 50°C. It is subjected to swilling the parts in order to free it from plating chemicals. Then they are cleaned in distilled water. The submerged fibers are then removed from the chemical bath and subjected to drying at a temperature of about 70°C. The temperature and the part surface are checked. Finally, the parts are examined visually for the required thickness. Table 1 shows the different chemicals used for coatings at various stages. Table1. Chemicals used for Coating for Various stages Stage and conditions

Concentration of chemicals

Sensitization at room temp

12 g/l SnCl2 - 2H2O 40 ml/l HCl 40 ml/l HCl

Activation at room temp

0.2 g/l PdCl2 2.5 ml/l HCl

Metallization at different conditions tested 40⁰ C and 50⁰ C pH 12 and pH 13

20 g/l NaH2PO2 - H2O 40 g/l NiSO4 - 2H2O 100 g/l Na3C6H5O7 - 2H2O NH3 to control the pH

Fig 1. Short uncoated PAN carbon fibers

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Fig 2. SEM image of uncoated fibers

2.2 Mixture of Composites by Liquid Stir Casting Technique The challenging factor in the preparation of composite is to maintain the proper dispersion of reinforcement to achieve a defect free microstructure or the matrix and fiber interface. The liquid state stir casting is preferred rather than any other process because of its ease in distributing fibers within the matrix during fabrication. If alternative process such as powder metallurgy is carried out, the fibers break or crack during the process The AA7075 has a melting temperature of 750 0C, many non-conventional process has been carried out using this metal such as rheo-casting, stir casting, compo casting,etc. For every case the different microstructures were found in different cases. In this study the AA7075 and the coated carbon fibers are mixed using stir casting method. This involves high melting temperature and constant stirring. The carbon fiber used at the different weight percentage of 0, 2, 4, 6 and 8%.The long Al ingots are placed in the crucible which has a capacity of 1 Kg. The crucible is then place in furnace and constantly heated till the Al melts i.e. up to 7000C. The coated carbon fibers are preheated about 2000C. When the Al melts at 7000C the Magnesium metal powder with weight percentage of 1% of total weight (10 grams) is poured into the melt. This reduces the surface tension of the Al and encourages the uniform mixture of the carbon fiber in the Al melt. The stirrer is placed one third to the height of the crucible which will be partly immersed on the Al melt. The stirrer is rotated at speed of 500 RPM which forms the vortex in the melt. The stir casting setup is as shown in the figure 4 and 5. The heated weighed carbon fibers are poured into the crucible for mixture with the Al melt. When the carbon fibers are poured simultaneously the stirrer has to be stirred at the constant speed, this helps the fiber to be immersed in the Al melt and allow the fibers to spread wide over inside the crucible. Stirring is continued till the matrix and the reinforcement interacts between each other and promoted wet ability. Then the melt with the crucible is removed from the heater and poured into the heated mould. After the melt is poured, it is then left out for cooling for around 1 hour and the mould specimen is separated out for any testing process. 2.3 Material Characterization Grinding of the composite was carried out make it 2 mm thickness and later polished. Finally, etching is done by Nital solution. Then it is examined by scanning electron microscopy (SEM) for microstructure studies.


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3. Result and Discussion 3.1 Characterization of Coating The obtained photographic images of uncoated and nickel coated short PAN based carbon fibers and the surface characteristics of the nickel coated fibers as seen under SEM are shown in Figure 3 (a) & (b)

a

b

Fig. 3. (a) (b) Nickel coated carbon fibres Metals that accelerate the electrochemical reaction (i.e, nickel), shows a linear trend between thickness of the coat and time is obtained as shown in Table 2. The rate of nickel deposition is proportional to the rate of dissociation of nickel complex to form free nickel ions. The coating thickness is uniform and homogeneous throughout the fiber circumference regardless of the shape or surface irregularities of the part being coated because the coating is applied without the use of electric current Table2. Time limits for each stage Sensitization(min)

Activation(min)

Metallization(min)

Coat thickness(micron)

5

5

3

1.8

10

10

6

3.0

15

15

9

5.8

20

20

12

7.4

25

25

15

11.2

30

30

18

13

3.2 Characterization of Composite Fabrication was done for 0, 2, 4, 6 and 8 weight percentages of carbon fiber. Specimens were examined for their Microstructure, hardness and tensile properties, the images obtained from microscope for unreinforced alloy and of the composites with 2%, 4% 6%and 8% of reinforcements are shown in figure 6 and figure 7. The microstructure analysis of following specimens shows that the reinforcing particles are uniformly distributed in matrix. The uniform and random distribution of the reinforcing particles is attributed to the good wettability of Al matrix.

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Optical microscopic of the composites revealed that the distribution of the fibers in the aluminium is dependent on the temperature of the aluminium melt. Observations were made on the fibre distribution in composites fabricated. Then ingots were cut into 4 pieces along the length and the fibre distributions were studied. It was found that the microstructures at the bottom, middle and top portion of casting have shown better uniformity of fibre distribution.

Fig 4. SEM image for as-cast Aluminium 7075.

Fig 5. SEM image for Aluminium 7075 reinforced 2% CF


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Fractured Fibers


The above table 2 shows the effect of presence of short coated carbon fiber reinforcement on the hardness of composite. From the test, it is observed that as the weight percentage of short coated carbon fiber increases, the hardness values are also increases. In fact as the percentage of reinforcement content raised by 0 to 2 percentages, hardness increased by 10.6%, and 0 to 4, 0 to 6 and 0 to 8 percentage of short coated carbon fiber has lead the increase in hardness by 18.18%, 30.30% and 36.36% respectively, which is shown in Figure 8. Hence it is clear that due to the increase in weight percentage of reinforcement, composite will have significantly higher value of hardness than the monolithic alloy to certain level. The reason behind increase in hardness number may be the presence of short coated carbon fiber, which acts as barrier to indentation movement. The tensile test was conducted for different weight percentage of short coated carbon fiber. The plot for ultimate tensile strength (UTS) with the variation in the dispersed of coated carbon fiber is shown in the table 3. It is observed that there is an increase in ultimate tensile strength (UTS) over the variation of dispersed fibers. The variation of the UTS value from as cast Al7075 alloy and the different percentage weight of reinforced carbon fibers are shown in the table 3. It has been observed that the variation in the fibers from 0 to 2% there was increase in the UTS value of 5.88%. Further increase up to 8% has shown the UTS value increment to 35.29%. It is evident that UTS value increases with increase in weight percentages of carbon fiber in composite which is shown in figure 9. The presence of carbon fibers has attributed the increase in the UTS value which contributes to the increase in the strength of overall Al metal matrix composite.

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Tear ridges


Fig 9. Material properties vs weight % of carbon fiber.

4. Conclusions From the results, it is clear that EN provides a coating that has more uniform thickness. This makes the film an effective corrosion protecting agent. A very bright and smooth electroless deposit of nickel is obtained. From the metallographic study, it is confirmed that the nickel coatings are bonded firmly to the carbon fibers along the length as well as the diameter of the filaments. The images also reveal that the thickness and morphology of the nickel layers is invariably dependent on the metallization conditions in terms of time, bath temperature and pH. Though it is a controlled chemical reaction, the cost of the chemicals required for the process is high. The coating tends to be brittle at times and the coating rate is a bit slow. From the experimental investigation, it is concluded that maximum coating thickness of 13 microns was observed for 30 minutes of sensitization time and activation time and 18 minutes of metallization time. It is also concluded that in order to retain the properties the coating thickness has to be maintained within 3 microns and it is confirmed that properties are enhanced by hardness and tensile studies.


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