Advanced Testing and Characterisation

Artefacts study

Introduction

The common DVD player works in much the same way as a CD player. Just like its sister product, the information on a DVD is coded as a series of pits on the disks surface. These pits are arranged in a spiral structure in order to be read off by a laser within the DVD player. This laser is the key reason why DVDs have superseded CD’s in the world of data storage. DVD’s use a shorter wavelength for the laser that enables it to place the pits in a much denser arrangement, therefore enabling the same area of disk to hold much more data.

Within a DVD player, its main components consist of a disk drive mechanism, which uses a spindle to hold the disk and a motor to spin it, a printed circuit board, which contains all of the electrical components to convert the data being read into a usable format, and an optical system assembly, which is the laser that reads the information off the DVD. All of these parts can be broken down into their component pieces, which will consist of a range of metals and polymers that have been specifically chosen for their use by their physical and sometimes electrical properties.

There are two types of plastics in production today, the first of which are classified as thermoplastics, they fall under this classification if they possess the properties of being remouldable under heat and pressure. Thermoplastics, such as polyethylene and polypropylene are used in production in low stress applications where they are not put under any load as thermoplastics have a tendency to display creep when under extreme loads.(1) The other category is Thermosetting plastics, these are polymers which form a highly crosslinked molecule upon heating. The reaction is irreversible but the highly crosslinked structure adds properties of heat and chemical resistance, and also a high dimensional stability due to the chains being unable to slip therefore reducing ductility.

In this experiment, 9 parts have been taken from the internals of a DVD player to be analysed with respect to what material they are made from and how they were processed during production. The metallic parts of the DVD player are to be mounted in Bakelite on a metallographers mounting press, whilst the plastic elements can be Identified using the process of Simple Identification, whilst any hard to identify metals can be analysed using the Transmission Electron Microscope. The TEM works on the same basic principle as a light microscope but instead of light, it uses electrons. A TEM uses the much lower wavelength of electrons to get a resolution nearly a thousand times better than a conventional microscope. It uses electromagnetic lenses to focus a beam of overhead electrons into a very thin beam.(3) This beam then travels through the specimen you want to study and then any unscattered electrons hit a fluorescent screen which shows the image as dark and light density patches.Four metallic samples and five polymeric samples were chosen to give a wide range of materials, giving a large spectrum of possible results.

Experimental Procedure

The testing of the polymeric samples was done using a process called Simple Identification. This involves a number of simple and conclusive tests that, when referenced against a table of results, a clear material can easily be seen.

The first test was a density test to measure whether it floated or not. Care was taken to ensure that surface tension was broken and that no air was trapped inside the specimen. The next test for the polymers was a test called the Beilstein test. This involved heating the end of a copper wire in a Bunsen flame until the flame turned colourless, using this hot wire to burn through the polymer on test left an amount of residue on the wire. When put back in the flame, the presence of halogen within the polymer could be seen if the flame turned green/blue. Testing after this was more of a visual test rather than chemical, firstly the specimens were tested to see if they could be torn or cut easily, this gave another property that could be cross referenced to eliminate some polymers. The heating test was also used, this was useful to gauge whether or not the polymer was a thermoplastic i.e. whether it melted or not, and also, using litmus paper, the pH of the reaction in the escaping vapours could be measured. Lastly the sample was left in the Bunsen flame to burn, this allowed us to see whether the sample was self extinguishing, what colour the smoke that was given off was, whether it gave off particles, and also what kind of smell the smoke had as this is an easy way to detect some polymers, such as PE smelling like candle wax.

An alternative and altogether more accurate way of testing a polymer to determine its molecular make up is by means of Infra Red Spectroscopy.  The use of infrared rays is based on the principle first given by the Beer-Lambert law.(2) This states that the amount of rays absorbed is wholly dependant upon the chemical composition of a given material. When the material is hit with the rays, the outer electrons get excited. This only occurs at differing energy levels. When testing polymers, infrared is used at a frequency of 0.8-25 µm, which correlates with the frequency required to excite the outer electrons. This leads to the infrared light getting absorbed by the outer electrons, which creates a lower density of that frequency which can be detected and set on a graph.(3) This printout can then be compared to past printouts to determine by which peaks and troughs are evident, which material it is. This process was needed for a sample that was an elastomer band used for driving a pulley system within the DVD drive.

In order for the metallic samples to be properly analysed under the microscope to determine structure and composition, they had to be mounted and polished. The samples were first cut into small pieces, mounted in Bakelite using the metallographers mounting press, and finely polished. The polishing process was time consuming but necessary, starting off with coarse silicon carbide paper and working onto finer and finer grades, the scratches were finally rubbed out of the surface of the metal using the polishing wheels. Care was taken to ensure the correct polishing wheel was used depending on whether or not the metal was ferrous or not. This was first tested using a magnet. Using initial estimates as to what the metal was, the correct etching liquid was used to finely etch the metal to bring out the internal structure more clearly. The samples were then viewed and photographed under a microscope to determine what the metals were by their structure.

One metallic sample that appeared at first glance to be a steel of some kind did not respond to the etching agent of 2% Nital, so the agent was changed for 10% Nital. Again, the metal remained unchanged and the structure did not appear, at this point the metal could’ve been a number of things including a type of Stainless Steel or possibly a Chromium alloy, so to determine exactly what it was it was sent to the Transmission Electron Microscope.

Results

On looking at the first of the metallic samples, which was a screw threaded piece that rotated to move the laser reader, it was noticed that where the sample had snapped off of the original piece, the break revealed a structure not dissimilar to that of a ceramic which led us to believe that the process by which it was made was that of a powdered metal possibly by injection forming. It was not until the molecular structure was revealed after the etching in Carapellas Reagent , which is made up for ferric chloride, hydrochloric acid and alcohol, that this was found not to be the case. The screw was found to be a Brass of composition 60% Copper, 40% Zinc.

                         (1)                                                          (2)

The second piece that was also suspected to be brass was an element of the electrical side of the DVD player, during the process of mounting it in Bakelite and polishing it, a coating was rubbed off leaving a brass coloured metal, the coating can be seen on the side of figure (2), but due to the extremely thin section of metal that was left on the side, the composition of the metal coating could not be deduced. Looking at the molecular structure when comparing it to the other brass section we can see there is only one phase, leading us to the conclusion that it is a brass of composition 70% Copper, 30% Zinc.

Brass in its standard form is mostly a solid solution of zinc, dissolved into copper. It can be seen from the microstructures of the second brass sample that around the 30% zinc mark, the zinc atoms can fit in amongst the copper atoms without disrupting the crystallographic arrangement of the pre-existing copper atoms. As the percentage of zinc increases within the brass, the mass density, colour and average molecular spacing gradually change, saying this, it is worth noting that the shape of the actual structure and how the atoms are positioned stays the same. Looking at the first brass sample, where the amount of copper gets to around the 40% mark a new phase starts within the copper-zinc solid solution. The first phase no longer increases their zinc content as the percentage of zinc increases, it merely makes the molecules in the second phase larger and more numerous

An interesting thing to notice on the 70:30 copper in figure (1) is the twinning that has occurred on some of the grain boundaries. This is when a part of the atomic lattice is deformed so that it forms a mirror image of the undeformed lattice next to it. This process leaves small but well-defined regions of the crystal deformed. This does not distort the properties of the metal by too large an amount but in the process of forming twins within the structure it places some slip systems in a better direction for shear stress to affect the metal, so can indirectly increase the chances for additional slip within the structure. This can create a more ductile metal which is not always wanted.(4)

The first of the steels is the outer casing for the internal motor that was used to drive the wheel that moved the laser along a screw thread. This was found to be magnetic and upon inspection the results pointed to low carbon steel. Looking closer at the shape of the grains, they are not elongated or equiaxed, which leads us to believe that the steel has been annealed. Annealing has the effect of getting rid of any internal stresses in a metal, creating a metal with better stress resistance properties. You can see on the structure photograph the sample contains both pearlitic and ferritic regions, although being a low carbon steel, there is a lot more ferrite. The darker areas of the picture are the pearlite regions, and the lighter areas, the ferrite.

The second steel that was tested had to be sent to the TEM to get analysed due to the fact that none of the etching techniques managed to bring out a usable structure. The results were as follows:      

The results of the scan show the metal to be a ferritic stainless steel. Stainless steel is defined to be an iron-carbon alloy with at least 10.5% Chromium content, it becomes highly resistant to corrosion after 12% due to a protective film of chromium oxide that forms on the metal surface.

For the polymeric samples, most were tested using the process of simple identification, but due to the nature of the elastomer band, Infra red spectroscopy was used to gain a better understanding of what the material was. Below are the results for the simple identification tests that were carried out on the other polymer samples.

For the first sample that was tested you could tell from the marks made by the ejector sprues that the piece was injection moulded, so immediately this suggests a thermoplastic. When flexed, the material showed signs of whitening, again indicating a thermoplastic. Another indication of this is that it softened when heated, indicating that is was a semi crystalline thermoplastic. When put into water, the polymer showed signs of having higher density than water so we can rule out it being a polyolefin. From this information and the fact that it was brittle this sample is simply Polystyrene.

The second sample is slightly anomalous, the signs point to a polyolefin, but this clashes with the fact that it is self-extinguishing. On closer inspection it was found to be a three part lamination which incorporated a film of aluminium foil which would explain the polymers ability to self extinguish.

The third sample, the fan casing was similar to the first sample in the fact that it was injection moulded. This therefore indicates a thermoplastic. Also, looking at the fact that the polymer could be cut relatively easily points us to a polymer which is highly amorphous in its composition. The red litmus paper is a sign that the fumes given off are of an acidic nature, this combined with the green flame in the Beilstein test leads us to the conclusion that it must be Polyvinyl Chloride.

The fourth sample was a surround for a small electric motor, again it was injection moulded and showed signs of being a semi crystalline thermoplastic. On the same premise as the first sample I would be confident to say this sample was Polystyrene as well.

The fifth polymer to be tested was tested using Infra Red Spectroscopy. It is a highly cross linked elastomer which proved very difficult to determine what it was. Below is the graph and results of the IR test.

Initially it had a definite resemblance to raw butadiene, but due to the fact this material was not used in these kinds of applications, that was ignored. Nitrile rubber was another possibility that would be very suitable to the application, but there was no peak representing that. There is a large peak at 1000 suggesting some kind of silicon rubber, but this had to be discounted for being too far out of the plausibility range. The only plausible material it could be is a highly crosslinked butadiene polymer.

Discussion

When designing a product to go into production, many different things have to be looked at when trying to pick the correct material for your application. What stresses are going to be applied to it, how the component might fail, how it will be made to a high level of quality but with the lowest level of cost, and many other factors. In this experiment just 9 pieces were taken from the hundreds that were in the DVD player to analyse.

Starting with the first piece, the 60/40 brass was most likely rolled on the production like due to the fact that when you look at the grain structure inside, all the grains are in 1 direction. This creates a strong and standardised structure within the screw that has high tensile properties and as they will be put under constant torsional load as they turn. This piece is a key part to the DVD player and so has to be designed so it doesn’t corrode or break. Brass is a good choice as it is non corrosive and yet still cheap to manufacture.

The second piece of Brass was part of an electrical connection which had a metallic coating to it which was most likely applied by the process of electroplating. Making the brass element the cathode in the reaction allows it to be evenly coated with whichever metal the producer deems necessary.

The steel casing for the motor is a very simple press forming involving probably thousands of identical parts being punched out of a large sheet of thin steel sheet at the same time. Low carbon steel was used for this application due to its low cost and ductile structure being easy to form into different shapes.

The stainless steel element was part of a runner that the laser moved upon inside the DVD reader. This is a vital component that needs to be smooth at all times, even if some moisture were to get into the DVD player, if this part were to corrode, the player would stop working. Stainless was used to stop any corrosion, and although it is slightly more expensive, the cost is worth the peace of mind. The stainless steel would be made through a process of drawing.

Both of the Polystyrene pieces will have been made in the same way. Injection moulding, although the initial cost is high for the mould tool, is a very quick and effective way of producing many identical and intricate parts. Injection moulding works best with thermoplastics of a lower molar mass than average, ensuring a steady flow is maintained.

Conclusion

Upon breaking down a DVD player into its smaller component parts you can tell by very simple tests what the material is and why the manufacturer chose it. The process of Simple ID and IR can tell most polymers apart, but not always conclusively, additives can not always be recognised.

References

1. William D Callister Jr – Materials science and engineering, an introduction – ISBN 0471320137

2. Vernon John – Introduction to engineering materials – ISBN 033394917x

3. Wiliam F Smith – Foundations of materials science and engineering –

ISBN 0070592920

4. Chapman and Hall – Materials Science – ISBN 0412341506

5. Dietrich Braun – Simple methods for identification of plastics –

         ISBN 1569902801