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Before the carburised samples can be mounted for micro-hardness testing or optical microscopy analysis, they have to be nickel plated. This is to protect the surface edges of the samples as the carburised layer is micrometres thin. As the tribilogical properties of the polymer mount are weaker than the metal alloys, the mount wears down quicker when being ground. This creates a “step” between the alloy and the mount (Figure 1) and therefore the edge of the alloy starts being ground into a chamfer; destroying the carburised layer that is of interest (Figure 2). By nickel plating sample, it is this nickel layer that takes the damage whilst preserving the carburised layer (Figure 3).

 

 

 

 

 

 

 

 

 

 

For the best adhesion between the nickel and the alloy, the sample has to be as clean as possible. This meant that each sample had to be placed in an ultrasonic bath for ten minutes in isopropanol. After the bath the sample cannot be touched by bare human hands and minimum contact is advised.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1. Electrolyte Solution (20-25% NiSO4 and 5-10% NiCl2).

2. Nickel Strip (Anode)

3. Metal Alloy (Cathode)

4. Digital Thermometer (Feedback)

5. Heating Plate

6. Voltage Generator

7. Magnetic Stirrer

 

The principle behind coating the samples in Nickel is electroplating, where the alloy is the cathode and a strip of Nickel is the anode, immersed in an electrolyte solution. In this particular investigation the electrolyte solution contained 20-25% NiSO4 and 5-10% NiCl2.

 

First a deep beaker with a large bore is filled about halfway with the electrolyte solution. A magnetic stirrer is then submerged in the centre of the solution and the beaker placed upon a special heating plate that controls the temperature and the revolution speed of the stirrer. A digital thermometer is then placed in the solution and acts as feedback to the heating plate. The optimum temperature for this particular procedure is 47°C and the magnetic stirrer is set at a rate of 120RPM.

 

The strip of nickel is connected by a crocodile clip to a voltage generator, the metal alloy is also connected to a different crocodile clip and they are placed in the electrolyte solution to complete the circuit. Making sure that the nickel strip and metal are not in contact with each other, the voltage is then set to 2.9V. Care is taken on which way the current is set as the metal should be the positive terminal and the nickel strip should be the negative terminal.

 

The alloy is then left for 30 minutes to be adequately plated. The nickel plated alloy is then ready to be mounted. The mounting procedure can be tricky as it the cross section that is of interest. This means standing the sample up on its smallest face and having a high centre of gravity. This has increased risk of tipping over whilst pouring the polymer mounting powder in. Small clips can be used to prevent this, however they have to be plastic in this investigation as later on the samples will undergo SEM analysis, in which no other metals can be present.

 

After the samples are mounted, they undergo the standard grinding and polishing procedure as described earlier, taking strict care that there are no scratches remaining.