Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


The ‘Mid-Range’ polymers, often termed ’engineering polymers’, such as typically PA (polyamides, ‘Nylon’) and PET/PBT (polyesters) have higher strengths of the order of 75MPa with continuous operating temperatures up to 110/120°C and a cost range up to £10/kg.


melting point around 343°C and a usable temperature range to 250o


C, produced by specialist companies with prices beginning in the region of £100/kg and upwards.


Polymers and the Laws of Friction: Polymer tribology differs from metal/ceramic tribology because polymer/metal or polymer/ polymer contact is predominantly elastic. The Plasticity Index of Greenwood and Williamson predicts the onset of plastic flow as:


ψ = (E/H) (σ*/r)½


where E is Youngs Modulus (the major contributor), H-hardness, σ*- the standard deviation of asperity heights and r is the assumed spherical surface radii. E/H for metals is of the order of 100, for polymers it is of the order of 10. Friction coefficients for un-addivitised polymer against polymers, metals or ceramic pairs are 0.1–0.6, with a lower bound of 0.10 for PTFE.


But more importantly, polymer tribology does not follow the Laws of Friction, because friction coefficients, µ’s, vary according to:


- normal load - should be independent but µ’s often decrease with increasing load, up to a limit determined by deformation and contact temperature limits,


- sliding speed, should be independent but µ’s often decrease with increased relative velocity, limited by heat generation and increased contact temperatures,


- temperature, as polymers have much lower melting points than metal, related to the normal load and sliding speed used,


- the effect of Tg , the 2nd order phase transition, on friction are usually different, reflecting the changed nature of the


coefficients due to the semi-crystalline to amorphous phase transition of a polymer is often not considered for friction/ wear/temperature performance, particularly for surface layers. It is a fundamental property of each polymer, e.g., +63°C for nylon-6, +143°C for PEEK, and -1°C for PP. Friction coefficients above and below the Tg


value for a polymer


polymer mass at the surface. The contributions of the Tg transition to friction and wear performance of a polymer, particularly for surface layers, is not often considered in the literature.


LUBE MAGAZINE NO.135 OCTOBER 2016 33 Figure 2. PV diagram plots for two different polymers


- Dynamic Friction Coefficients of Polymers: For objective comparison between the friction coefficients of polymers, the test conditions and apparatus must be specified. Different test methods can give different friction coefficients for the same polymer, the dynamic friction coefficients in Table 1 used the ASTM D3702 method and apparatus at a load of 2.77kg/cm2 and 253mm/s.


The ‘High Performance’ polymers tend to be produced in developed countries and have a higher level of physical properties, higher melting points and very good resistance to degradation and cost up to £100/kg. These merge into the ‘Ultra Polymers’, with very high melting points, higher strength such as the polyaramids (Kevlar/Nomex) and the PAEK family which includes PolyEtherEtherKetone, or ‘PEEK’, with Ts


up to 100MPa,


No.106 page 2


The PV Envelope: The PV envelope, is the locus of the product of normal load (P) x sliding speed (V). It is an essential tool which defines a reliable working region for a polymer application. The test method, humidity and sample % crystallinity are further 2nd order effects on friction coefficient values. Figure 2 shows the PV envelope for two different polymers, emphasizing that each individual envelope can be different. It is essential that the PV envelope of a polymer specified for a proposed component design is known to avoid its premature failure in service.


, which applies to the established motion of a body on that surface. The static friction coefficient of two material surfaces in contact is always higher than the dynamic friction coefficient. Contrary reports have been few, far-between and not sustained.


experienced friction coefficient is the dynamic friction coefficient, µd


Static and Dynamic Friction Coefficients: The static friction coefficient applies to the initial effort required to start moving a body on a surface, µ’s


described later. The more commonly


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