3.1. Materials and methods
Film thickness measurements were performed on an EHD2 ball-on-disc test apparatus from PCS Instruments equipped with optical interferometry as presented in Fig. 2. The machine measures the lubricant film thickness properties in the contact formed between a 3=4 in diameter steel ball and a rotating glass disk by optical interferometry. The lubricant film thickness at any point in the image can be accurately calculated by measuring the wavelength of light at that point. Normally the system measures the wavelength of the light returned from the central plateau of the contact and hence calculates the central film thickness.
Optical interferometry measurements of lubricant film thickness have already been described by several authors. Details of this technique have been reported elsewhere [20–22] and only a brief description will be given here.
The lubricated contact is formed by the reflective steel ball and the flat surface of the glass disc. The glass disc is coated with a semi-reflecting chromium coating on top of which a spacer layer of SiO
is deposited. The load is applied by moving the ball upwards toward the disc. The disc is mounted on a shaft driven by an electric motor. The steel ball is also controlled by an electric motor, making it possible to run the tests under rolling/sliding conditions.
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The EHD2 method to perform film thickness measurements is described in 4 steps (see Fig. 3):
1. The contact is illuminated by a white light source directed down a microscope through a glass disc on to the contact.
2. Part of the light is reflected from the Cr layer and part travels through the SiO
layer and fluid film and is reflected back from the steel ball.
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3. Recombining the two light paths forms an interference image which is passed into a spectrometer and high resolution black and white CCD camera.
4. The camera image is captured by a video frame grabber and analysed by the control software to determine the film thickness.
The method used for translating the optical phase difference map into film thickness has been described by several authors [22–24].