PRECIOUS METALS
QUALITY CONTROL

Obtaining an inspection surface requires a succession of operations, each as important as the next, regardless of the material. These steps are in the following order:

• The removal of the product to be examined (if necessary), called “CUTTING”.
• Standardisation of the geometry of the sample taken (if necessary), called “MOUNTING”.
• Improvement of the surface condition of this sample, called “POLISHING”.
• Characterisation of the sample: revealing the microstructure of the sample by an etching reagent (if necessary) called “METALLOGRAPHIC ETCHING” and microscopic observation (optical or electronic).

=> Each of these steps must be carried out rigorously, otherwise the following steps will not be pos sible.

The purpose of cutting is to remove a precise section of a product, in order to obtain a suitable surface for inspection, without altering its physico-chemical properties.

In other words, it is essential to avoid heating or any deformation of the metal that could lead to strain hardening. Cutting is a fundamental step which conditions the further preparation and inspection of parts.

PRESI’s wide range of medium and large capacity cutting and micro cutting machines can be adapted to any need with regard to cutting precision, sizing or quantity of products to be cut:

For cutting precious metals, it is generally advisable to use micro-cutting machines, which are adapted to the dimensions of the precious metal samples. The Mecatome T205 or even the Mecatome T215 are both suitable for cutting jewellery pieces for example.

Each of the cutting machines in the range is equipped with the appropriate consumables and acces sories. The clamping system and the choice of these consumables are always essential elements for the success of a metallographic cut.

=> Clamping, i.e. holding the workpiece, is also essential. Indeed, if the workpiece is not well held, the cut can present risks for the consumable, the workpiece and the machine. Figures 3 to 6 show examples of clamping for jewellery parts.

Fig. 3-4-5-6: Jewellery clamping

All cutting machines are used with a lubricating/cooling liquid composed of a mixture of water and anti-rust additive in order to obtain a clean cut without overheating. The additive also protects the sample and the machine from corrosion.

=> The choice of the cut-off wheel type has to be adequate, in order to avoid cutting failure, or excessive cut-off wheel wear or even breakage. The hardness of the workpiece determines the wheel selection.

Samples can be difficult to handle due to their complex shape, fragility or small size. Mounting makes them easier to handle by standardising their geometry and dimensions.

=> Achieving good-quality mounting is essential to protect fragile materials and also to achieve good preparation results for polishing and future analysis.

Before mounting, the specimen should be deburred with coarse abrasive paper, for example, to re- move any cutting burrs. Cleaning with ethanol (in an ultrasonic tank for even greater efficiency) is also possible. This allows the resin to adhere as well as possible to the sample and thus limits shrinkage (space between the resin and the sample).

If shrinkage persists, it can lead to problems during polishing. Abrasive grains may become lodged in this space and then be released at a later stage, thus creating a risk of pollution for the sample and the polishing surface. In this case, cleaning with an ultrasonic cleaner between each step is recommended.

There are two mounting options:

Is to be preferred for edge inspection purposes or if the metallographic preparation is carried out in preparation for hardness testing. This option requires a hot-mounting machine.

is to be preferred:
• If the parts to be examined are fragile/sensitive to pressure
• If they have a complex geometry such as a honeycomb structure.
• If a large number of parts are to be mounted in series.

Cold resins do not always provide a flat mounting “back” because of the meniscus of the liquid resin. Before any polishing operation, a brief step using abrasive paper will remove this meniscus. The important thing is to ensure that this operation renders the two sides of the mounting parallel.

To meet user needs, PRESI offers a full range of cold mounting moulds. The cold process has different mounting moulds with diameters from 20-50mm. These are divided into several types: optimised moulds called “KM2.0”, rubber, Teflon or polyethylene moulds. Cold mounting is also more flexible, hence the existence of rectangular moulds for more specific needs.

In the case of a coating inspection, for example, it is advisable to favour the use of a resin with low shrinkage. For a general inspection, a slightly lower quality resin will be sufficient. The choice of resin should be adapted according to the final objective of the desired observations.

The last and crucial phase in the sample preparation process is polishing. The principle is simple, each step uses a finer abrasive than the previous one. The aim is to obtain a flat surface and to eliminate scratches and residual defects that would hinder the performance of metallographic control examinations such as microscopic analysis, hardness tests, microstructure or dimensional inspections.

PRESI offers a wide range of manual and automatic polishing machines, with a wide choice of accessories, to cover all needs, from pre-polishing to super-finishing and polishing of single or series samples.

The MINITECH range of manual polishers incorporates the most advanced technologies. User-friendly, reliable and robust, they provide a simple answer to all needs.

The MECATECH range of automatic polishers allows both manual and automatic polishing. With its advanced technologies, motor power from 750-1500 W, all the PRESI experience is concentrated in this very complete range. Whatever the sample number or size, MECATECH guarantees optimal polishing.

All the polishing ranges below are given for automatic sample preparation (for manual polishing: do not take into account the parameters at the top). They are the most commonly used and are given for information and advice.

All the first steps of each range are called “levelling” and consist of removing material quickly to level the surface of the sample (and resin). Those given below are standard and can therefore be modified as required.

Applied pressures vary according to sample size, but in general the following applies: 1daN per 10mm mounting diameter for the pre-polishing steps (ex: Ø40mm = 4 daN) then reduce force by 0.5daN at each polishing step with an abrasive suspension.

The first range is suitable for silver polishing:

Silver polishing is performed applying a rather atypical range. The first steps are carried out using medium to fine grit abrasive papers. The speeds of rotation are also particular in order to limit the incrustation of abrasives in the material.

Steps 3 and 4 are made using ADRII cloth. This cloth has flexible fibres, which polish silver best.

Finally, the last step is an RFI cloth which is used with a suspension of Alumina Presi n°2. During this step, it is advisable to moisten the cloth for a few seconds beforehand. A rinse at the end of the step is preferable in order to clean the cloth and the sample from the Alumina suspension. The rotation of the head is reversed in relation to the platen in order to keep the suspension on the cloth as much as possible.

The range below was developed for a copper sample with silver deposit:

The first step is again carried out using fine abrasive paper. Here, the range is oriented for copper polishing: we find the use of monocrystalline diamond suspension less aggressive for copper. The last step is carried out using a colloidal silica suspension (SPM). Implementation is identical to that of Alumina.

The range below is suitable for polishing gold (in the form of a deposit):

In this case, the gold is in the form of a deposit on an electronic sample. The polishing range begins with the use of abrasive papers.

It continues with polishing using Gel 2+ polycrystalline diamond suspensions on polishing cloths (TOP, ADRII and NT). These suspensions contain the diamond abrasive and the lubricant, a 2-in-1 product. Their “gel” formula allows the product to remain on the polishing cloth longer.

The last step is also done with the SPM colloidal silica suspension.

Gold in alloyed form can also be polished with the following range:

This polishing range is quite similar to that used for copper with silver plating. The first step is done with a P1200 abrasive paper.
LDM 9μm, 3μm and 1μm monocrystalline diamond suspensions are used with TOP, RAM and NT
polishing cloths.
The last step is carried out using colloidal silica suspension (SPM). This superfinishing step improves the surface finish for final observation.