A sample holder for the X-ray photoelectron spectroscopy analysis of multiple mini-traction machine ball samples

A simple sample holder has been developed for the K-alpha and Nexsa range of X-ray photoelectron spectroscopy instruments that allows for the rapid throughput analysis of the ball sections of mini-traction machine, the analysis of which can be useful in understanding of, for example, surface scuffing and other phenomena in mini-traction machine instruments

Given the inherent surface sensitivity of X-ray photoelectron spectroscopy (XPS), it is ideal to study a surface after a tribochemical reaction, resulting from the thermal and mechanical stresses involved in the wear process.An understanding of the chemical changes in the surfaces of these materials is paramount in the elucidation of whether a tribochemical reaction has, for example, a beneficial 9 or deleterious effect 10 or an idea of the elemental distribution from depth profiling experiments. 8 is common for XPS analysis to be performed solely on the MTM disc themselves, and despite their size can fit in to most modern XPS spectrometers if mounted correctly (Figure 2).However, there is an interest also in the corresponding wear tracks of the ball and how the surface chemistry of that component may be different from that of the disc or to investigate 'scuffing' of the lubricant-metal system. 11th the advent of coincident techniques, such as Raman (allowing phase identification) and ion-scattering spectroscopy (ISS) 12,13 and depth profiling using monoatomic ions or argon clusters, 12,14 significantly more chemical information can be extracted from a single sample.

| DESIGN OF THE NEW HOLDER
Wear tracks on both disc and ball are typically narrow and consequently small-area analysis is required within the central area of the wear track to avoid potential artefacts from uneven edges.As seen in Figure 2, the mounting of the discs is straight forward, and hence alignment for analysis can be relatively facile depending on camera geometry or using XPS imaging to determine the area for analysis. 15alysis of the balls is more difficult.No longer do we have a flat surface for mounting, but instead a sphere, typically coated in a layer of lubricant, which can make adhesion difficult.To circumvent this, we developed a holder that would not only remove the need for the balls to be (precariously) held with tape but give a method of mounting, which would also allow a fixed alignment of the wear track on the ball with the geometry of the X-ray spot, or other coincident techniques. 12e rationale for the current design of the holder is twofold; the first is to enable a higher throughput of samples for analysis and secondly for the ease of positioning and lack of adhesive tape as already stated.The holder has been designed for ThermoFisher Scientific Kalpha and Nexsa systems, which utilise the same sample holder design, although simple modification can be made for inclusion into similarly designed sample platters.
The evolution of the design can be seen in Figure 3.A proof-ofconcept and successful prototype was formed using strips of nickel foil, together with polythene stoppers taken from common laboratory 25 mm diameter, flat bottomed glass sample tubes, the tops of which were cut off.The polythene rings were secured using smaller strips of nickel spot welded to the larger strip, which also had the advantage of acting like springs to allow for a tighter fit of the balls in the polythene rings.This prototype holder could be held down with double-sided carbon tape or clamped using clips, which can be inserted into the base of the ThermoFisher Scientific holder.
The final version of the sample holder was printed on an Ender 3 Pro 3D printer, using a polylactic acid (PLA+) filament source.As shown in Figure 3B, the holder is printed with recesses for grub screws, which have their thread tapped later, and for the insertion of copper pads to allow conduction to the base of the holder.[18] As PLA for 3D-printing can contain additives, especially if coloured, the holder was thoroughly rinsed in deionised water and placed in a dedicated vacuum chamber to outgas for 24 hours.After rough vacuum and turbomolecular pumping for 30 minutes, the outgassing vacuum chamber was ca.5x10 À5 mbar, whilst after 24 hours, the chamber pressure was in the low 10 À8 mbar regime.F I G U R E 4 (A) Holder pictured using the system load-lock camera, (B) in-analysis chamber camera view of the sample at the correct analysis height, and (C) survey spectrum taken from the wear track.

1
Schematic of the (A) ball and disc setup and (B) interaction of ball and disc with a lubricating layer within a mini-traction machine (MTM).F I G U R E 2 (A) Typical mounting of an MTM disc for analysis of a wear track and internal image of the wear track with examples of (B) XPS imaging, (C) snapshot XPS data from a depth profile, and (D) in-situ Raman analysis of such a wear track.

F
I G U R E 3 (A) Prototype of the holder (bottom insert: example of glass vial used to source the polymer top), (B) CAD-style projections of the holder for printing, (C) final 3D-printed version of the sample holder and (D) close-up of a 3 /4 00 ball in the holder where the wear track is visible and aligned to the most vertical point of the holder.