It is not considered meaningful to tabulate results for design properties in one large table, and so each property is discussed separately and relevant values included in their correct context. All values quoted are actual results, and if applied to design calculation a relevant factor of safety should be applied.

• Friction :

Most studies on the friction of PTFE have been carried out with unfilled PTFE. Whilst the exact mechanisms involved are still not fully understood a picture emerges in which the 'dry' coefficient of friction is dependent upon the pressure, the speed, the temperature, the mating surface, the orientation of the PTFE, the environment and the time of running. Coefficients from 0.016 to 0.36 have been quoted and while this work is discussed in detail on page 25 it may be summarized (with some additional information) as follows:

The classical laws of dry friction state that the friction force is independent of the apparent area of contact, making the friction force proportional to load rather than pressure. Many investigators quote the coefficient of friction of PTFE as a function of load and show it to rise steeply at very light loads (below 5 lb) and decrease with increasing load. R B Lewis (ref. 104) does not support this, but suggests the coefficient of friction µ is proportional to the applied pressure P (lbf/in2) according to the formulae: µ= CP-0.2.

where C = 0.12 ± 0.03 at velocities below 10 mm/s (2ft / min).
        C = 0.35 ± 0.10 at 50 mm/s (10 ft / min).
        C = 0.45 ± 0.15 at 500 mm/s (100ft / min) and above.

The coefficient of friction falls markedly at low speeds (below 50 mm/s; 10 ft/min) and increases with increasing speed.

The coefficient of friction appears to be stable over the range -45 to 100°C (-49 to 212°F) but to rise at lower temperatures and fall at higher temperatures.

Study showed that sliding of PTFE against steel gave lower coefficients of friction than sliding bulk PTFE against bulk PTFE. When mating areas are large, friction is primarily due to adhesion.

It has been shown that the coefficient of friction can be affected by up to 30% depending upon the the orientation of PTFE molecules.

It is found that prolonged and continuous running under dry nitrogen (5-10 parts per million of water) gave rise to intermittently high coefficients of friction but this was alleviated as soon as normally moist air (50% r.h.) was admitted. The short term tests at temperatures from-1 to + 60°C(30 to 140°F) in helium, oxygen, nitrogen and air showed no such effect and neither did tests in air room temperature with relative humidities in the range 12 to 54%. in the friction of PTFE in vacuum (10-9 mm Hg) coefficients of friction is obtained as 0.25 with a load of 1 kg. It is also found that the coefficient to be constant over the speed range < 50mm-5 m/s (<10-1000 ft/min).

Several investigators have shown that the coefficient of friction is decreased dramatically by-the addition of lubricants. This is not surprising since, if a full film of oil is present, the friction is virtually independent an of the mating surfaces.

The study shows that the coefficient of friction for PTFE on PTFE is influenced by the number of traverses, the time lapse between runs, the nature (especially velocity) of the preceding sliding, and the thermal history of the sliding components. It is experimentally found that a similar increase in friction with time for PTFE on steel, up to a steady level (from 0.05 to 0.20 in 4 hours), and showed this to be due to a in) change in the surface of the PTFE rather than a change in the surface of the steel (i.e., the transfer of PTFE to the steel).

Research suggest that, using molybdenum disulphide (MoS₂) , asbestos, carbon, graphite, and copper as fillers, as the volume of filler increases the coefficient of friction increases from 0.016 (no filler) to ~ 0.030 (30% of filler), but that there is little difference in this effect between the various fillers. For a similar range of fillers it is found that the coefficient to be independent both of the type of filler and its volume addition. In particular, MoS₂ and graphite showed no advantage over glass fibre, asbestos and copper. Their results for all types ranged from 0.09 to 0.22 depending on velocity, load, etc. They concluded that the filler was effectively encapsulated and the friction was that of PTFE only. The friction is dependent more upon the volume than the type of filler, although cadmium oxide is claimed to be an exception. At the very low temperatures of liquid oxygen and nitrogen and under conditions of high vacuum the there is considerable variation in the coefficient, but this does not appeared to be correlated with either filler type or volume. In practical tests with the Wankel engine using various grades of PTFE as a seal, the coefficient was again found independent of the filler, whilst in a laboratory test, it is found that the type of filler is important; however, they appear to quote the filler content as % by weight so that filler type and filler volume are not separable. It is again found that MoS₂ and graphite fillers to give high coefficients of 0.26 to 0.34.

In some other studies it is found that filler type has a greater effect than filler volume, with MoS₂ giving a lower coefficient than unfilled PTFE. Study suggests that volume of filler is not directly related to friction coefficient but fillers in general raise the coefficient under these particular test conditions by a factor of about two. It has also been suggested that the addition of MoS₂ and carbon to glass fibre compounds reduces the coefficient of friction, although figures quoted show only a marginal decrease. some tests found no improvement when working under vacuum.

Similarly, practical tests showed no advantages for adding MoS₂ to glass although this combination was suggested as a possible means of reducing the scoring of shafts, and for use in very dry gases. It is conceivable that after prolonged continuous running under dry conditions, the MoS₂ is not subject to the rise in friction reported for PTFE.

There is therefore conflicting evidence as to the effect of filler type and volume upon the coefficient of friction of PTFE.

It is difficult to separate the effects of particle size and shape from those of filler type, since specific forms of particle, tend to be used with specific types of filler (eg, glass fibre, irregular particles of graphite and MoS₂ spherical bronze, etc.). Moreover, in much of the published work no details of filler particle are given.

Study with the very hard particles of alumina (Al2O3), it is found spherical particles gave coefficients of friction similar to that of unfilled PTFE (0.05—0.08) whereas irregular particles gave significantly higher results (0.14-0.15). They attributed this increase to cleavage of the irregular Al203 which saturates the surface until the coefficient is that of Al2O3 on steel. The abrasive nature of the filler also gives a 'rough' surface finish to the steel, thereby giving a coefficient approximately double that of a 'smooth' steel surface. This effect is less likely to occur with softer fillers, and this has been found true with bronze, where no difference in friction has been found between spherical and irregular particles, although it is suggest that particle size can have an effect in extreme cases. Other parameters (load, speed etc.)

The statements made in the first part of this section for the effects of load, speed, temperature, etc, upon the coefficient of friction of unfilled PTFE in general hold good for filled PTFE, although some research shows that the coefficient of glass-filled PTFE does not rise at low loads, whilst other investigations suggest that it does.

Work with filled PTFE at low temperature and in contact with liquid oxygen and nitrogen shows the coefficient to rise with the passage of time (eg, 0.18 to 0.43 in 23 hours). Although actual coefficients are lower (0.07 to 0.20 in 20 hours). There is some evidence therefore that the coefficient of friction increases in the presence of liquid oxygen or nitrogen. High coefficients (0.2 to 0.4) were also found by Buckley et al. for filled PTFE under high vacuum, but some of the fillers, notably copper, silver and powdered coke gave coefficients lower than for unfilled PTFE under the same conditions. The reasons for these effects are not known: the effects may be due to temperature or environment, or the mechanisms may be similar to that experienced with graphite where the low coefficient of friction is attributed to the presence of adsorbed gases at the Crystallite interfaces where cleavage occurs.

• WEAR :

The mechanisms responsible for the wear of PTFE are not fully understood, but it is generally thought that adhesion and the freeing of transferred wear fragments, either in terms of surface energy or by virtue of fatigue, are of major importance. It is known that when PTFE is rubbed against other materials a transfer takes place and it is believed that the wear process involves the laying down and subsequent removal of such transferred layers. An ideal situation is given as having a highly oriented mono-molecular layer of PTFE bonded to the metal surface which then rubs against as smooth a mating surface of PTFE as possible.

What is not clear is exactly how and why fillers and conditions affect both the initial laying down and subsequent removal of the PTFE particles. It is suggested that a minimum temperature at the interface is required to promote adequate bonding and that certain fillers function by causing frictional heat. It is also clear that surface finish will affect this transfer, and whilst there is wide agreement that too rough a mating surface will cause rapid wear, one school of thought suggests that too smooth a surface finish leads to high wear rates, while others suggest that is not so. The answer may be that although too fine a finish may well inhibit good transfer, many filled compounds are sufficiently abrasive to roughen the mating surface adequately. However, if the filler or environmental conditions are too abrasive, rapid wear will occur through ploughing. The entrapment of wear debris can have a similar affect.

It has been suggested that chemical reactions at the interface may be important. Buckley and Johnson consider that wear is related to the decomposition mechanism and hence to the temperature at the interface, while Hargreaves and Tantam I suggest lead oxide can be an oxygen carrier to other metals, giving selective oxidation of roughness on the mating surface. Mitchell and Pratt have noted the formation of copper fluoride at the interface of bronze- filled PTFE, presumably caused by local degradation of the PTFE and bronze. They do not, however, attribute the reduction in wear accompanying the formation of copper fluoride to the chemical action, but rather to the fact that the area of contact at the interface increases with time, which reduces the interface temperature. Vinogradov did however attribute a reduction in friction between copper and PTFE to the formation of the solid lubricant copper fluoride.

The most widely quoted formula for the wear of filled PTFE is that of Archard and Hirst which states that the volume wear (W) is proportional to the relative speed at the interface (V), the load supported (M) and time run (T), ie, W a MVT or using the 'wear factor' K W=KMVT (Equation 1).

(i) By quoting the maximum PV value the material can withstand (termed limiting PV value). This is found experimentally either by determining the maximum load that can be applied at constant velocity while still maintaining temperature and/or frictional torque equilibrium, or by determining the PV value at which the wear rate suddenly increases.

(ii) By quoting the PV value which gives a specified wear rate. This is generally quoted as the PV value to give 0.127 mm(0,005 inch) wear in 1000 hours, and is determined from specific wear tests.

(iii) By determining the constant K in Equation 2. This is known variously as the 'wear factor' or 'K-value'. Work has been carried out to determine these factors for various filled PTFE materials. However, during these and subsequent investigations it has been shown that there are two major errors in accepting results derived from Equation 2.

(a) K is not necessarily a constant for a given material, but will vary with the load applied, the velocity, the length of time run, the temperature, and other factors such a; clearance and conditions at the interface.

(b) The method of determining the factors is invariably to test specimens. The conditions of test have a considerable effect upon the results and universal values cannot be obtained from one series of tests.

R B Lewis suggests that each material has two wear rates (metric), mild wear (K- 7) and severe wear (K > 35) which is attributed to a rise in temperature at the interface. The actual temperature at which the transition occurs is reported to depend upon the load. He concludes that the PV value at which transition begins depends upon the application geometry, ambient temperature, and manner and amount of cooling, whilst the slope of the transition depends upon the application parameters and properties of the compositions. The mild wear is reported to be characterized by wear of the surface layers whilst severe wear is characterized by bulk removal of material. Similar conclusions were drawn by Summers-mith who considered the composite to be hard granules in a 'cement' of softer materials. He suggests the mild wear region corresponds to a gradual attrition of the hard granules, the change to severe wear occurring when the 'cement' becomes softened by heat and the granules are plucked bodily out of the matrix. From work carried out by Mitchell and Pratt, it is concluded that the entrapment of wear debris as well as surface temperature is a very important factor in determining whether severe wear occurs or not. For example, although the difference in running conditions between a thrust washer and a piston ring is mainly considered to be one of interface temperature, it is also true that wear debris is far less likely to become entrapped in the piston ring. It is also true that differences in wear rate can be attributed to differences in behaviour (abrasive or otherwise) when trapped wear debris is present. Nevertheless, whatever the mechanisms, it is generally accepted that an increase in interface temperature increases the wear rate.

At room temperatures and above it is generally agreed that a hard, approximately 900 VPN (Vickers Pyramid Number), mating surface is beneficial. Softer materials can be used providing the filler will not abrade them. The materials with good dry bearing properties of their own (eg, bronze) are preferred to the softer more easily damaged materials (eg, aluminium). There is some divergence of opinion as to the suitability of chromium plating. Pratt shows chromium plating to be advantageous whereas O'Rourke et al. show it to give poor results. The answer might well be that the fillers used by Pratt were less abrasive than those used by O'Rourke.

The surface finish of a material is generally quoted as a mean of the 'peaks and valleys' of its surface as detected by traversing a diamond stylus across it. This does not fully specify a surface however, since a turned and a ground surface of the same Ra value will be different. It is now generally accepted that a ground surface is superior to a turned surface and that above 0.75Mm the wear rate of the filled PTFE will increase. The existence of a lower limit is still in dispute and so the best compromise is to use a ground surface finish of 0.2—0.4 µm.

'Lubricant' is a very general term and it used to be stated that any liquid will act as a lubricant and be beneficial to PTFE, To some extent this is true in that, if hydrodynamic conditions are established, no wear will take place, but filled PTFE may run under conditions of boundary lubrication. Hydrocarbon oils are generally advantageous, with a significant reduction in wear rates. This is not so with water. The wear factor increased for unfilled and various filled PTFE compounds when running against steel with water boundary lubrication, It is found that boundary lubrication with water gave a reduction of wear life of 50% when filled PTFE ran against steel.