PTFE wear resistance

Hydrodynamic principles for gas bearings are similar to those involved with Hquid lubricants except that gas compressibility usually is a significant factor (8,69). With gas employed as a lubricant at high speeds, start—stop wear is minimized by selection of wear-resistant materials for the journal and bearing. This may involve hard coatings such as tungsten carbide or chromium oxide flame plate, or soHd lubricants, eg, PTFE and M0S2. 

For wear resistance and low friction, coatings of PTFE or M0S2 generally have been satisfactory. Use of low thermal expansion filler in PTFE helps minimise cracking and loss of adhesion from metal substrates with their lower coefficients of expansion. 

Figure 12-8A. Piston rings. The piston rod is manufactured from heat-treated stainless steel and is coated with wear-resistant overlays, such as ceramic, chromium oxide, and tungsten carbide applied by plasma techniques. Piston rod cross-head attachment has mechanical preloading system for the threads. Rider rings and seal rings are manufactured from PTFE filled resins fillers are matched to the gas, piston speed, and liner specifications. Typical fillers are glass, carbon, coke, or ceramic. (Used by permission Bui. BCNA-3P100. Howden Process Compressors Incorporated. All rights reserved.)...

After the micro wear tests, the dependence of worn depth of PTFE and PTFE/Si3N4 film on load is shown in Fig. 13. The worn depth of both PTFE and PTFE/Si3N4 film is in the nanometer scale. It can be seen that the worn depth increases linearly with load. However, the worn depth of PTFE/Si3N4 multilayers is about one-tenth of PTFE film at the same load. All these results demonstrate that the wear resistance of PTFE/Si3N4 multilayers is greatly improved after micro-assembling of soft and hard layers. 

PTFE/Si3N4 multilayers not only have the property of PTFE s low friction coefficient but also have the property of Si3N4 s high wear resistance. 

To improve the properties of the raw polymer (wear resistance, creep resistance, thermal and electrical conductivity), various fillers, such as glass fibers, powdered metals, and graphite, are combined with all three types of PTFE polymers mostly by intimate mixing. Filled fine powders are produced mostly by adding fillers into a dispersion and then coagulating the mixture. 

These materials have good wear resistance and a low coefficient of friction, both of which are factors that can be further improved by including additives like PTFE. Self-lubricating parts containing graphite powders have flexural strengths above 69 MPa (10,000 psi.) Their... 

PVDF and PTFE have wide application in industry due to their strength, chemical and wear resistance, and dielectric properties. However, VF2 and TFE may be copolymerized with comonomers such as hexafluoropropylene (HFP) and... 

Compared with metals or ceramics, polymers are lighter, softer, weaker, less thermally stable and less wear-resistant. They are also poor conductors of heat and electricity. However, their properties can be enormously modified by the incorporation of fillers, reinforcements, and other components such as plasticizers. For most purposes it is useful to consider polymers in three separate groups, namely thermosetting, thermoplastic and PTFE. 

Like other composite properties, the wear resistance is influenced by the orientation of reinforcing fibres. Sung and Suh found that with biaxially-oriented glass fibre and molybdenum disulphide in a PTFE matrix (Duroid 5813), the wear resistance was greatest when the highest proportion of fibre was normal to the sliding surface. The same effect was found with a graphite fibre/epoxy composite and a Kevlar fibre/epoxy composite. 

The main advantage of polyimides is that they retain their structural strength to temperatures approaching 350 C, but they also have good outgassing characteristics in vacuum. For such applications the thermal stability of a PTFE filler is a limitation, and 5% of molybdenum disulphide may be added to a polyimide for friction reduction. The wear resistance of this material was inferior to some of the filled nylons, acetals and polycarbonates, and it would probably be chosen mainly for its high temperature stability. 

Thermosetting resins are usually used lubricated, often with water. When they run in dry situations they benefit considerably from the addition of PTFE, graphite or molybdenum disulphide. Their wear resistance is still not as good as that of the better PTFE composites , but they can be used with higher loads because they are stronger and are not so catastrophically affected by heat. The effect of heat on the polymer is to cause surface charring, which does not seriously degrade performance. 

Typical fillers used for reduction of wear include PTFE, silicone, graphite powder, molybdenum disulfide, and aramid fibers. Good results were also reported with mica and zirconia combination. Figure 8.36 shows the effect of mica and mica in combination with zirconia on the wear resistance of an epoxy resin. ... 

Typical fillers carbon fiber, glass fiber, graphite lubricant for wear resistant applications molybdenum sulfide, PTFE. antimony trioxide, barium titanate, clay, silica, aluminum nitride, smectite... 

These new PTFE-PA materials can be processed without any problems on commercial processing equipment like twin-screw extruders and injectionmoulding machines. The specific adjustment of the melt viscosity could be realised through additives (lowering of MFI additive [10,18] increase of MFI acid-terminated oligoamides [9,10]). The new materials combine the good material properties of PTFE and polyamide with the processability of polyamide and are particularly well-suited for wear-resistant maintenance-free bearings. 

The tribological investigation of the chemically bonded PTFE-PA materials shows that besides the low friction coefficients the high wear resistance is the most important property of such products. Examinations with several different PTFE polyamide materials certify this predicate. This phenomenon could be explained by the chemical coupling of the PTFE distributed in PA. The compati-bilisation of PTFE by block copolymer formation causes a better connection to the PA matrix as compared to physical blends. 

Lehmann D et al (2002) New PTFE polyamide compovmds for wear resistant maintenance-free slide bearings. Part 1 Chemistry of polyamide, 13th international colloquium tribology, Technische Akademie Esslingen, pp 2309-2314, 15-17 Jan 2002, ISBN 3-924813-48-5... 

Carbon reduces creep, increases hardness and elevates thermal conductivity of polytetrafluoroethylene. Wear resistance of carbon filled compounds improves, particularly when combined with graphite. Carbon-graphite compounds perform well in non-lubricated applications such as piston rings in compressor cylinders. Carbon-filled PTFE has some electrical conductivity. Close tolerances can be achieved... 

RT/Duroid M. [Rogers] PTFE, glass-reinforced wear-resistant matei for seats, seals, bearings for chemical and food processing equip., backup rings in hydraulic service, rub strips in B-IB bomber, microwave dielec, thermal insulation, clutch face gaskets. 

In blends, fluoropolymers are used in small quantities to enhance throughput, reduce the frictional properties and increase the wear resistance. Blends comprising 0.3-50 wt% of a low molecular weight PTFE (T 350°C) with engineering resin showed improved anti-friction properties [Asai et al, 1991]. 

PPS and PEEK blended with a fluoro(co)-polymers and reinforced with either CF or GF were wear resistant with a short break-in period for forming a self lubricating film [Davies and Hatton, 1994]. Many commercial blends contain fluoropolymers (primarily PTFE) for the improved weatherability, wear and solvent resistance SUPEC — self-lubricating blend of crystalline PPS with PTFE and 30 wt% GF, Lubricomp blends from LNP and similar/JTP blends from RTP Co. (e.g., 15 wt% PTFE, 30 wt% GF and any of the following resins ABS, PA, PEST, PC, PE, PEI, POM, PP, PPE, PPS, PS, PSF, PVDF, SAN, TPU, PEEK, PES, etc.), Sumiploy from Sumitomo Chem. Co., etc. [Utracki, 1994]. 

PPS/PTFE (polyphenylene sulfide/polytetra-fluoroethylene) high durability and wear resistance, tensile strength, hydraulic stability, heat resistance. Applications include seals valves and bushings. 

The polymer can also be blended up to 25% with PTFE. Such blends have good temperature and wear resistance and are self-lubricating. Applications include seals, bearings, and rotors. 

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