Chapter 4. Experimental methods
4.3 Film formation experiments
Friction in grease-lubricated contacts should be evaluated in light of the sources that generate it. Firstly, it is essential to understand if friction is entirely due to the shearing resistance of the lubricant, or if asperity contacts are also contributing. Secondly, it is important to understand how the shearing resistance of this lubricant film correlates to its thickness. Finally, due to the multiphase composition of greases, the film nature (i.e. its composition and rheological behaviour) determining friction is, in principle, unknown, and is likely to vary
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depending on the operating conditions. For these reasons the mechanism of grease film formation was investigated in order to complement the friction results and help establish the friction mechanisms.
4.3.1 Description of the film thickness test rig
Optical interferometry is extensively applied to the study of lubricated EHL contacts, and its validity and versatility in this respect have been fully verified. An extensive review of the principles of this technique as applied to film thickness measurements can be found in a paper by Spikes and Cann140.
All the film thickness measurements carried out on the commercial greases were taken using the PCS Instruments EHD rig, illustrated schematically in Figure 4.3. This machine also employs a ball-on-disc geometry. The ball and disc specimens are end mounted on shafts which are independently driven by electric motors to allow for any sliding/rolling conditions. The load is applied by a stepper motor moving the ball upwards, against the disc. The ball is a standard 19.05 mm diameter AISI 52100 steel ball, with roughness Rq ≈ 15 nm, i.e. the same ball specimen as used in the MTM friction tests. The disc is made of glass, with a coating of approximately 20 nm of chromium (CrSiO3), overlaid by 500 nm of silica (SiO2) on its lower surface. White light is shone vertically through the disc, focussed into the centre of the contact. Part of this light is reflected by the chromium layer, the remaining portion by the steel ball. These two beams recombine at wavelength values that depend on the difference in their path length, which in turn is determined by the thickness of the film separating the ball from the disc. This interference light is then passed into a spectrometer slit where it is dispersed to produce a set of interference fringes. The resulting spectral image is then detected by a black-and-white CCD camera and captured by a frame grabber. The wavelength corresponding to the maximum constructive interference in the central region of the contact is then processed by the software to calculate the central film thickness.
One of the limitations of this technique, commonly known as ‘spacer layer optical inteferometry’141, is that it can only provide one film thickness measurement over a specific area of the contact. In oil lubrication the film formation mechanism and shape are generally known, and the film in the central area of contact is relatively flat. However, in the case of grease lubrication, depending on the operating conditions the typical EHL horseshoe shape can appear highly disturbed142, and the description of film formation in terms of a single absolute film thickness value provides very limited information. In order to improve this aspect, a different rig was used in the second experimental part of this research work. All the measurements carried out on the custom greases were taken with the Wedeven Associates Inc. WAM5 rig, with nearly
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identical settings to the EHD rig. The main difference between the standard setups present in the two rigs consists in the method used to process the interference image of the contact. Instead of using a spectrometer to determine the wavelength of maximum interference, a high resolution CCD colour camera is employed on the WAM5 to grab an image of the whole contact. This technique is known as ‘spacer layer imaging method’ (SLIM)143. With this setup the whole film thickness map across the contact can be dynamically observed, and the formation of films with different shapes than classical EHL films can be identified. In this way the film thickness measurements, also taken with the SLIM technique in the tests performed with the custom greases, could also be accompanied by a dynamic visual image of the film shape inside the contact. In this case the film thickness was calculated as an average of the predominant colours in the central area of the contact, for each picture taken at the same speed.
Figure 4.3 (a) Schematic representation of the interferometry film thickness test rig; (b) detail of transparent disc
4.3.2 Film thickness test procedure and conditions
All the tests were performed in fully flooded conditions, hence a grease scoop similar to the one described in Section 4.2.3.1 was used. Pure rolling conditions were chosen to limit scratching of the glass disc. The film thickness testing procedure consisted of 3 steps. The first step is equivalent to the preliminary step used in the fully flooded friction test. A low load was applied to ‘work’ the grease and ensure equal grease distribution and pre-shearing conditions before commencing each test. The film thickness measurements were taken during the following two speed-sweep steps, where the speed was first increased from 5 mm s-1 to 1000 mm s-1, and then decreased back to 5 mm s-1. Five measurements were taken during each step at the same speed. The reported film thickness results are an average of the speed decrease steps in each test and its
Spectrometer image
Colour camera image
Grease scoop Light source
Speed Load
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repeat, i.e. an average of ten values. The tests were run at 40 ˚C and ambient temperature (22 ± 1 ˚C), depending on the lubricant samples. The test conditions are summarised in Table 4.4. Prior to each test, ball and disc specimens were cleaned using the same procedure employed for the friction test specimens.
Table 4.4 Conditions for the film thickness measurements Maximum Hertzian pressure [GPa] 0.52
Temperatures [˚C] 40, ambient (22 ± 1 ˚C)
Slide/roll ratio [%] 0
Entrainment speed range [mm s-1] 5 – 1000
4.4 Conclusions
In this chapter the experimental methods used for the tribological investigation of the test greases have been described. Two ball-on-disc tribometers have been employed to study the grease friction and film thickness behaviour, due to their suitability in simulating rolling bearings- like concentrated contacts, whilst allowing for carefully controlled test conditions. Their principle of operation has been briefly explained. The material and surface characteristics of the specimens have been described. The test profiles and test conditions have been specified. The next two chapters report the experimental results obtained with these techniques.
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