Micromechanisms of transfer film formation and sliding wear of PEEK

Debashis Puhan, Janet S.S. Wong

Tribology Group, Department of Mechanical Engineering, Imperial College London, United Kingdom SW7 2AZ

INTRODUCTION:

Polyetheretherketone (PEEK) have been replacing many metallic components in industrial equipment such as seal rings, transmission gears, bushings, sliders, bearings and valves. Its good mechanical properties, light weight, chemical and heat resistance, low friction, and wear behaviour make it an ideal material for such applications¹. It is well known that PEEK, PBI (polybenzoimidazole) polymer blends and PEEK+CF (carbon fibre) composites show significantly less wear than individual polymers. The wear mechanisms of these blend are however, unclear^2,3. The structure and function of the transfer film formed on metal counterface from the blend, which is the key process for reducing friction and wear, have not yet been understood well. This study is aimed to understand the micromechanisms of transfer film formation of PEEK on steel conterface. Further studies are planned to elucidate transfer film formation of its blends.

METHODS:

A 6 mm diameter steel ball were slid against a polymer disc on a unidirectional pin-on-disc tribometer. Load, speed and temperature were varied to study their dependence on transfer evolution in a period of 30 minutes. Measurements of ball and disc temperature, charge and plasma luminescence were recorded using the setup shown in Fig 1. The ball material AISI52100 steel.

Figure 1: Schematic of Experimental set-up

RESULTS AND DISCUSSION:

When discs made of PEEK, PBI and their blends are rubbed against steel balls, time varying luminescence was observed at the contact interface. In samples containing CF, luminescence was completely absent which is attributed to the increase in conductivity of disc samples. The occurrence of luminescence suggests the generation of plasma in-situ termed as triboplasma⁴. Triboplasma is generated at the inlet as well as the outlet of the contact and is more intense at the contact outlet (see Figure 2).

Figure 2 Distribution of average triboplasma emission around PEEK/Steel contact during sliding for 30 minutes at 6 N and 2 m/s, as observed from the side.

The charge generated due to rubbing creates a high potential at the interface, leading to corona discharge. The current from this high potential region flows into the surrounding air and ionizes it. As a result, the triboplasma spectrum resembles the nitrogen discharge spectrum. Peaks of OH and CH radicals are observed along with the peaks corresponding to N₂ molecule excitation and ionization. The OH radicals could be generated from adsorbed water on the surface and the CH radicals could originate from the breaking of the hydrocarbon chains.

Figure 3: FTIR Spectra (center), wear track on PEEK disc (left) and transfer film on the steel ball (right)

The wear track on the PEEK disc surface shows grooves and fish scale like features (Figure 3a). SEM images clearly shows the presence of fine cracks that extend in the direction perpendicular to the sliding direction. These cracks are related to the mechanical action of the sliding friction. On the steel surface (see Figure 3c), three distinct features are observed (i) wear scar covered with thin (~200nm) film, (ii) thick PEEK films (2 μm) at exit and inlet of the wear scar, and (iii) loosely adhered shredded polymer debris which are present everywhere. FTIR analysis of strongly adhered thick and thin polymer films show strikingly different features. The thin film shows ketone C=O and OH peaks suggesting formation of carboxylic group which were absent on the wear track and the thick films. These IR results suggest that PEEK carboxylate chain ends chelate to the steel surface under the transfer film⁵. The thick films showed CH peaks. The luminescence and IR spectra suggest chain scission and radical driven mechanism which relies on mechanical energy input.

CONCLUSION:

IR spectra indicate the presence of chelated carboxylate polymer chain ends in the transfer films. Luminescence spectra showed nitrogen discharge, emission of OH and CH radicals. The formation of transfer film is thus suggested to be initiated by radical-driven mechanism which itself relies on the mechanical input of energy to cause chain scission which formation. Further studies are planned to elucidate micro mechanism of the transfer film formed from the blend.

REFERENCES:

1. J.P. Lu, Wear, (1995) 2. B.H Stuart, Tribol. Int. (1998), 3. P.Liu, J Phys. Chem. B (2017) 4. Nakayama, Tribol. Lett. (2010), 5. K.L. Harris, Macromolecules (2015)