There are currently two main limitations of the optical interferometry method, Ultra Thin Film Interferometry (UTFI) as shown in Figure 90.
One limitation is that it cannot measure very thin films when sliding is employed. Under such conditions the spacer layer, made of silica, is destroyed due to the high frictional forces present in the contact caused by a local rise in temperature of the contact region when sliding is introduced [101].
This means that very thin films can only be measured in pure rolling conditions. Several lubricant additives have been used over the years by different researchers and it was found that only boundary films are formed between the two solid surfaces in contact. There is thus a need to overcome this problem to be able to measure very thin films, characteristic of boundary and mixed lubrication regimes in sliding contacts.
The other major limitation of the UTFI technique is that one of the two contacting surfaces is not steel but coated glass or sapphire.
For glass this leads to a much lower contact pressure than for steel on steel but the main concern is that lubricant additives will behave differently in a glass on steel tribopair than a steel–on–steel one, as is present in real machine elements.
Figure 90: Experimental setup for the classical Ultra Thin Film Interferometry method One of the proposed solutions in this project is to eliminate the fragile silica layer currently employed by the classical UTFI method, and use a higher Chromium layer thickness sputtered on the glass disc, as showed in Figure 91. A thicker Cr layer is believed to provide a higher phase shift corresponding to the spacer layer used in the classical UTFI method. This, together with a spectral analysis of the resulting interferograms, will facilitate measurements of very thin lubricant films. It is supposed that a thicker Cr is much tougher and more resilient than silica, thus tolerating various sliding conditions, allowing thin films to be measured accurately.
Lubricant Spectrometer White light source Silica layer Thin semi–reflective Cr layer Glass/sapphire disc Steel ball
The optimum Cr thickness that will deliver the necessary phase change is unknown, but this will be established by experimenting with several different thicknesses, finding the equilibrium between the layer transmittance and its durability.
Figure 91: Experimental setup for the modified Ultra Thin Film Interferometry method As explained in previous chapter (3.2) capacitive methods are quite often used to measure film thickness in lubrication research but these are confined to applications where the lubricant film thickness is relatively large, such as hydrodynamic bearings, or some piston–ring contacts [47, 58–61].
The electrical capacitance of EHD contacts has also been used in the past to estimate the film thickness in elastohydrodynamically lubricated contacts [47]. However, those studies have been largely abandoned with the development of optical interferometry and the success the method had in accurately measuring film thickness down to a few nanometres. A comprehensive, comparative study of film thickness by electrical capacitance and optical interferometry, the method which has proven to be the most accurate in determining film thickness, has never been attempted.
There appear to be two main problems with applying the capacitance technique when looking at very thin lubricant films.
Film thickness determination depends on the dielectric constant of the film, which can vary considerably depending on the chemical nature of the film used, unlike optical interference which depends on the refractive index, which varies relatively little with chemical composition.
For very thin films in the nanometre range it is not known if the conventional inverse relationship between capacitance and separation would supply good values.
The main issue this project will overcome are the two limitations summarized above. This will be done by using thin film optical interferometry to calibrate capacitance measurements. In the ball–on–disc optical interference method, the conducting chromium surface can be used as one
Lubricant film Spectrometer White light source Thick semi- reflective Cr layer Glass/sapphire disc Steel ball
plate of a capacitor and the contacting ball as the other. If the other armature is the Hertzian flat on the ball surface, the capacitance of this system can be measured and correlated to the film thickness determined by optical interferometry in the same experiment. The lubricated Hertzian contact of interest is several hundred microns across and less than one micron thick and thus can be approximated to a parallel plate capacitor. Part of the project investigations were conducted to estimate the influence of the lubricant composition upon the capacitance of the contact. Tests were carried out with base oil and two solutions: one organic friction modifier and one a viscosity index improver polymer, both of which are known to form boundary films [41, 42]. Once the correlation between the capacitance of the contact and the EHD film thickness was established, the next phase of the project consists in measuring film thickness in steel–on–steel contacts. This was accomplished by replacing the glass/sapphire disc with a steel one and evaluating the influence the Cr/steel had on the resulting values.