A tri-pin-on-disk apparatus for in situ X-ray photoelectron spectroscopy of worn surfaces

Chemical analysis of the interface between two contacting materials. before and after a wear test, is essential for a complete understanding of the tr ibological process. X-ray photoelectron spectroscopy (XPS) is a powerful te chnique for the analysis of surfaces and for this reason, we have developed a tri-pin-on-disk apparatus to run within the controlled environment of an XPS spectrometer. The rig can run at variable speeds, with a normal force between 6 and 42 N on each 5-mm diameter pin, giving an apparent maximum pr essure of 2 MPa, and, therefore, a wide range of conditions. The normal for ce is measured before the disk is lowered on to the pins and the friction f orce can be recorded throughout a wear test. At the end of a wear test, wor n pins are transported to the analysis chamber of the XPS spectrometer with out contamination from the atmosphere. Experiments have been carried out to investigate the extent and chemical composition of transfer films after te sts with PEEK (poly-ether-ether-ketone) and APC2 (a PEEK-bonded carbon fibr e composite) sliding against 316 stainless steel. No chemical change was fo und for PEEK pins sliding against fine ground stainless steel disks (Ra 0.3 +/- 0.02 mu m [optical]). For steel pins sliding against an APC2 disc in v acuum (10(-8) mbar), relatively thick layers of amorphous carbon were trans ferred to the steel after only a short period of wear. A similar experiment in air produced thinner carbon transfer films. The apparatus provides a co nvenient way of measuring chemical changes that may occur on surfaces under going wear and provides a new capability for EPSRC Users of the Scienta ESC A300 XPS at Daresbury Laboratory. (C) 1999 Elsevier Science S.A. All rights reserved.