Parametric Analysis of Reciprocating Friction and Wear Behaviour of PTFE Composite

When filler materials like carbon, graphite, glass fibres are added in the PTFE, a composite is formed which improves mechanical & thermal properties of that composite. It increases hardness and wear resistance, while the coefficient of friction is slightly affected and remains low. In this survey, we study various papers in which various methods & experiments were carried out for calculating friction & wear resistance of various compositions of polymers. So it is easy to understand properties & behaviour of polymers under various test conditions.


Introduction
Polytetrafluoroethylene (PTFE) is an important polymer based engineering material. When rubbed against a hard surface, PTFE exhibits a low coefficient of friction but a high rate of wear. It is white or gray in color. It is an ideal bearing material for heavy and light load pressures with medium and low surface speeds. The development of the fluoropolymer industry began with the discovery of the polytetrafluoroethylene (PTFE) by Dr. Roy J. Plunkett  at DuPont in 1938 and introduction as a commercial product in 1946.Polytetrafluoroethylene (PTFE) is a high performance engineering plastics which is widely used in industry due to its properties of self-lubrication, low friction coefficient, high temperature stability and chemically resistant. While PTFE exhibits poor wear and abrasion resistance, leading to early failure and leakage problem in the machine parts.
To minimize this problem, various suitable fillers added to PTFE. Generally, reinforcements such as glass fibres, carbon fibres and solid lubricants are added internally or incorporated into the PTFE. Its relative softness and poor heat conductivity limit its suitability as a bearing material to applications involving low speeds and low unit pressure, the tribological behaviour of polymers is affected by environmental and operating conditions and by the type, size, amount, shape and orientation of the fibres.
A relationship between the wear of the polymers and operating parameters is desirable to obtain the better understanding on the wear behaviour. In 1953, a new fluoropolymer, polychlorotrifluoroethylene (PCTFE) was commercialized by M. W. Kellog Company under the trade mark Kel-F. PCTFE is produced by the free radical polymerization of chlorotrifluoroethylene (CTFE) with a linear polymer chain structure.
Polytetrafluoroethylene (PTFE) polymer is a widely used solid lubricant. It is commonly used to provide a dry sliding friction coefficient μ < 0.2 on a variety of counterface types including stainless steel. PTFE is a popular polymer solid lubricant because of its resistance to chemical attack in a wide variety of solvents and solutions, high melting point, low coefficient of friction, and biocompatibility.
It is commonly used in bearing and seals applications. Unfortunately, PTFE suffers from poor wear resistance. Gong et al. and Blanchet and Kennedy report unfilled wearrates of 7.36×10−4 and 7.41×10−4 mm3/Nm, respectively. A hypothesis for the low friction of PTFE is that the long chains of PTFE, -(CF2-CF2)n-, orient on the transfer surface during sliding, forming a low shear-strength interface with the bulk PTFE bearing material. PTFE has a 13/1 or 15/1 helicalmolecular structure with the fluorine atoms close enough to form a smooth cylindrical surface against which other molecules can easily slide .Hard particulate fillers made of ceramic or metal particles, and fiber fillers made of glass have been used with a PTFE matrix to dramatically improve the wear resistance, even up to three orders of magnitude. The use of such hard particles increases the dry sliding friction coefficient and abrades the counterface. Counterface abrasion is not desirable in a solid lubricant bearing system. Novel non-abrasive PTFE systems with wear resistance similar to filled PTFE composites have been made using gamma and electron irradiation by Briscoe and Ni and Blanchet and Peng respectively. With the recent availability of nanoparticles it is hypothesized that composites made with such small fillers will be significantly less abrasive than their micrometer sized counterparts. Perhaps such composites will provide a new family of nonabrasive and wear-resistant solid lubricant bearing materials. The Piston on Cam n follwer wear testing machine represents a substantial advance in terms of simplicity and convenience of operation, ease of specimen clamping and accuracy of measurements, both of Wear & Frictional force.

Coefficient of Friction
The coefficient of friction is generally depends on the Load, sliding speed. Material should possess low coefficient of friction.

1.3.2
Wear rate Wear is the removal of material from either or both of the contacting surfaces. Material should have improved wear resistance under load and permanent deformation. , glass fibre reinforced (GFR) and bronze and carbon (C) filled PTFE polymers under ambient conditions in a pin-on-disc arrangement concluded that The friction coefficient of pure PTFE and its composites decreases when applied load increases also pure PTFE is characterised by high wear because of its small mechanical properties. Therefore, the reinforcement PTFE with glass fibres improves the load carrying capability that lowers the wear rate of the PTFE.   investigated the wear mechanisms of the hybrid SFRPs filled with nano particles on pin on disc arrangement. It was found that the load carrying capacity of the SFRPs is mainly determined by the properties of the fibers. However, the tribological performance of SFRPs can be significantly improved by using nanoparticles due to their friction reducing abilities, especially under extreme loading conditions.

Conclusion
From the overview of above study concludes that, 1) Addition of filler materials such as bronze and carbon to PTFE causes an increase in wear resistance, while the coefficient of friction is slightly affected. 2) The friction coefficient of pure PTFE and its composites decreases by increasing applied load, which can be improved by reinforcement PTFE with glass fibres. 3) For the specific range of load and speed, the load has stronger effect on the wear behaviour of PTFE and its composites than the reciprocating velocity. 4) Addition of filler materials such as carbon, graphite, glass fibers and PPDT to PTFE causes an increase in hardness and wear resistance, while the coefficient of friction is slightly affected and remains low. Filler materials in general are effective in impeding large-scale fragmentation of PTFE, thereby reducing the wear rate.