Details of ICP-AES Method VPN

where U is the entrainment speed in m/s and η is the dynamic viscosity in Pa s (assuming a lubricant pressure viscosity coefficient of 15 GPa^sup -1^) .To ensure mixed lubrication conditions, an entramment speed of 0.05 m/s was employed for most of the tests, to provide a theoretical initial lambda ratio (ratio of EHD film thickness to composite root mean square roughness) of approximately 0.5 The speed conditions used to obtain the results reported in this paper are summarised in Table I. It can be seen that contra-rotation of the ball and disc enabled a sliding speed of 0.25 m/s to be combined with the required entrainment speed of 0.05 m/s.

Details of ICP-AES Method

The ICP-AES is very widely used to measure the concentration of metals in solution Its key advantages are (1.) it can measure very low concentrations (down to ppb levels) and (2.) it has a multi-element capability, being able of measure up to 40 metals simultaneously. In ICP-AES, a small quantity of metal-containing solution is injected into an argon plasma to reach a temperature of ca. 10000 °K The intensity of light emitted at characteristic wavelengths by the metal atoms at this temperature is then used to quantify the concentration of the metal present.

ICP-AES is routinely used in tribology to measure the metals content of lubricants in operating machinery, both for routine condition monitoring and in post-failure analysis. These are essentially two techniques: direct and indirect In direct methods, the oil sample is diluted with an organic solvent such as methyl-iso-butyl-ketone (MIBK). white spirit or xylene. and then injected into ICP-AES without the other treatment. In indirect methods, the metal is transferred from the oil to an aqueous phase and the latter is analyzed There are various ways of accomplishing this transfer, with different levels of seventy depending on how strongly the metal is held in the oil phase and its particle size These include extraction into acid, microwave heating, high-pressure bomb digestion, and even dry-ashing of the lubricant followed by solution of the ash in water.

A direct measurement from the oil phase is simple and rapid but is generally less sensitive to very low metal levels and also more difficult to calibrate. It is also not suitable for analyzing oils containing large wear particles. Transference of metal from the oil to aqueous phase is time consuming and carries more risk of loss of material or contamination during digestion, but results in a product which can be very reliably analyzed with ICP-AES down to extremely low levels.