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Plating Specialists
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Non Destructive Testing of Electroless Nickel Plating
Non destructive testing methods are required in a number of areas where electroless nickel plating is used. The uniformity of the electroless nickel plating mean the coating is ideal for many engineering applications. Threaded articles can be plated without deformation of the integrity of the thread (see Technical Information). Hydraulic components, shafts, tube bores and cogs can all be accurately electroless nickel plated, while maintaining the machined tolerances. Electroless nickel plating can also be used to repair over machined articles, by accurately building up the surface with the plating. Many of these types of components will have tight tolerances of only a few microns in thickness. The thickness of electroless nickel deposited therefore needs to be carefully monitored.
To measure a deposit thickness, a widely used non destructive testing method is to use an eddy current gauge. This can be used to measure the thickness of paints or plated coatings on steel substrates. However, this method relies on the substrate being magnetic. Non destructive testing in this way can not be carried out if the deposit itself is magnetic. Electroless nickel plating forms a magnetic deposit, when the phosphorus content of the deposit is less than 10%. Increased phosphorus contents reduce the magnetism to negligible levels.
Thickness can be determined very accurately by microsection of the component and the coating. This however, is destructive and time consuming. An alternative method of non destructive testing is required when depositing electroless nickel plating that is quick and efficient. NiTEC employ two methods of non destructive testing for controlling the deposit thickness to high levels of accuracy.
Engineers regularly use callipers and micrometers as a form of non destructive testing, to determine the correct machining tolerances, when manufacturing components. NiTEC also so use callipers and micrometers on components with large diameters, as a non destructive testing method for thickness control. Where a component is too large to use callipers, a control shim is plated with the component, having been measured with a micrometer both before and after plating, to determine the coating thickness. The use of shims for thickness control and micrometers as a method of non destructive testing, can be done with electroless nickel plating, as opposed to electrolytic plating, as the uniformity of the coating does not vary relative to size, shape, or position in the plating tank of either the component or the shim. This way a quick and reliable non destructive testing method will give the deposit thickness, within a few minutes, ensuring accurate control of the deposit thickness achieved on the component. This method is important where the component to be electroless nickel plated, has an aluminium substrate. The pre-treatment procedure for processing aluminium, involves the removal of a small amount of base material. The more accurate way to determine the actual deposit thickness is to plate an unaffected steel shim alongside the aluminium component and determine the coating thickness on this.
As technology has moved forward and NiTEC has invested in it, an alternative non destructive testing method for deposit thickness is also available. This is known as XRF measurement. XRF is an abbreviation for X-ray Fluorescence. It can be used to determine the thickness of different metallic coatings on different metallic substrates. It is based on the effect of the interaction of x-rays with matter, which causes excitation of certain electrons which, when relaxing to their ground state, emit x-rays of a characteristic wavelength, peculiar to that element. Although it can be used to measure the first few hundred microns of the surface, there can be limitations to the maximum deposit thickness it can determine, dependant on the coating or substrate material and the calibration standards available. However, it is a quick and reliable alternative method of non destructive testing, to the use of a micrometer. A drawback with this system is the size of component that can be measured, before it has to be cut into smaller pieces. This obviously moves away from non destructive testing. Here, the use of test shims is still the best alternative.
Deposit thickness is usually a customers' main requirement. However, electroless nickel plating is used in many cases for the other properties the deposit exhibits, mainly the deposit hardness. Electroless nickel deposits can have 'as deposited' hardness values of 450-750 VHN. By carrying out heat treatment procedures, the hardness of the deposit can be increased. to the region of 1100 VHN. The most accurate way to determine the hardness of a deposit was to impress a square cut diamond onto the deposit, using a known weight loading. If this was done directly onto the surface of the deposit, the hardness result could be affected by the action on the substrate, as the depth of penetration was neither known, nor controlled. The more accurate method was to cut a cross section through the coating and the substrate and make the impression on the coating, at right angles to the substrate. This is a destructive and time consuming method.
NiTEC have recently invested in a non destructive testing method for hardness determination, which can give a quick result on a variety of component sizes, without requiring the part to be destroyed. The measurement is carried out in the same way, by making a diamond imprint. The same loadings can also be applied to the diamond. However, its descent into the electroless nickel plating is controlled and can be graphed and monitored by a computer. The programme allows the depth of the imprint to be monitored, ensuring that the substrate has no affect on the final hardness result. The instrument is adjustable so can, within reason be applied to an actual component rather than a reference tag. This method of non destructive testing can then provide the customer with a print out of the hardness result achieved on their component. The computer programme can also provide the result in either Vickers Hardness, or Rockwell figures. The speed of the non destructive testing in this particular application means that results can be obtained before and after heat treatment, to ensure that the heat treatment profile was successful in increasing the hardness by the desired degree.
