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MANE 6960 - Friction & Wear of Materials
Wear Behavior of PTFE and PTFE-Based Composites in Sliding Contact with Common Titanium Alloys
15 December 2009
1.0 Polymeric Materials
Polymers play a very important role in modern mechanical engineering and machine design. The utility of engineering plastics and elastomers is realized where applications require structural materials that possess a level of chemical resistance or reduced component weight that can not be achieved by metals or ceramics. It is the tribological properties of these materials, however that makes them invaluable in applications where dynamic contact exists between machine components.
Significant advances have been made in recent years in the field of polymer tribology, enabling the use of these materials in systems with very demanding operating conditions. Of particular interest is the usefulness of polymer materials as solid lubricants in sliding bearing applications where boundary lubrication is not feasible or cost-effective. In modern tribology, only a few select materials have been identified as possessing the unique molecular properties necessary to make for an effective bearing polymer.
Polytetrafluoroethylene (PTFE) is one such material that is frequently employed in bearing applications due to its inherently low coefficient of friction. PTFE is a linear fluorocarbon that is prone to slippage in the crystalline formation of its molecular bond structure when subjected to shear loading. This ‘slipping’ phenomenon afforded by its unique molecular structure is the most likely reason that PTFE maintains such a low coefficient of friction (as low as .02 under specific operating conditions) , making it an excellent solid lubricant material. When coupled with a suitable counter surface in dynamic contact, a thin layer of PTFE is often transferred from the bulk material to the surface of the mating component. This condition is commonly referred to as the transfer film layer in the field of tribology. It is the subsequent PTFE-PTFE contact that results in a low coefficient of friction and earns this material its ‘self-lubricating’ designation. This low COF material property is typically attributed to the synergy of the PTFE chains (shearing interface) of the deposited surface and the bulk PTFE surface. Figure 1 presents a series of SEM micrographs showing the transfer film layer and wear debris from pin-on-disk testing using a 1045 steel disk and PTFE / PTFE composites (3N normal load, .15 m/min relative surface velocity) .
Figure 1. Transfer Film on AISI 1045 Steel Plate (Left Column) and Wear Debris (Right Column) of PTFE (A) Filled PTFE/15 vol.% MoS2 (B) Filled PTFE/15 vol.% graphite (C) 
Despite its self-lubricating properties and relatively high temperature stability, PTFE has been shown to exhibit relatively poor wear and abrasion resistance when compared to similar polymer materials such as polyethylene and polyamide . The high rate of wear exhibited by this material tends to limit its effectiveness as a bearing material in its virgin form. Several test studies have demonstrated that a direct correlation exists between the bulk wear rate (volume material loss) experienced by PTFE and both bearing pressure and sliding velocity. Increasing the dynamic bearing pressure seen by a PTFE tribosystem, (typically coupled with a metallic material of higher material hardness) results in an exponential increase in the volume loss of PTFE as a function of distance traveled. A similar relationship exists between wear and relative surface velocity. Increasing velocity at the bearing interface also yields an exponential increase in volume loss of the softer PTFE material due to the resultant increase in localized surface heating, a phenomenon commonly referred to as flash heating. Elevated surface temperatures in conjunction with a change in the crystalline shear mechanism of PTFE at increased speed ultimately results in larger wear debris particles and higher wear rates.
Figure 2. Wear Rate of a PTFE-Based Composite as a Function of Pressure and Velocity 
To make for a useful tribological material, significant efforts have been made to increase the wear resistance of PTFE, leading to the development of a number of PTFE composite materials. These composites can typically be classified according to two basic categories which describe the general method of modification ; Bulk-modified composites and Interface-modified composites. Bulk-modified composites employ ‘hard’ filler materials such as ceramics, metals or synthetic fibers directly in the bulk self-lubricating material (i.e. PTFE). The function of these ‘hard’ filler materials is essentially to strengthen the polymer matrix, increasing its load-carrying capacity and wear-resistance. Some of the more common filler materials used to increase bulk material strength are glass, aramid and carbon (all usually in short fiber form).
On the other end of the composites spectrum, interface-modified composites employ a softer self-lubricating material (i.e. PTFE, graphite, molybdenum disulfide) as a ‘filler’ in a harder polymer matrix. The PTFE fillers (also commonly in short fiber form) provide the composite with the desired low friction properties, while the strength and toughness of the bulk matrix (some common polymers are epoxy, phenolic and PEEK), is augmented by additional fillers. Alumina (aluminum oxide) and titanium oxide are frequently employed as strengthening fillers in interface-modified polymer composites and their particle size is central to the level of wear resistance that can be achieved using these fillers. Recent studies  have shown that using fillers with particle size on the nano-scale (termed nanocomposites) can effectively reduce wear rates by one and in some cases, two orders of magnitude when compared to micro-sized filler particles. This is also true of bulk-modified composites.
There are two primary subcategories that exist for interface-modified composites, thermoplastic materials, and those that employ a thermosetting matrix (such as epoxy compound). The performance of thermoset composites is of particular interest to the author of this paper and will be the focus of its content. The wear performance of neat PTFE will also be examined as it is the primary constituent in most polymer bearing composites and its relative performance is of great interest.
2.0 Titanium and Titanium Alloys
The corrosion resistance, comparatively low weight and high strength of titanium and its alloys makes it very desirable as a structural material in mechanical design. Titanium alloys, however, have a tendency to gall and seize due to low work hardening behavior in applications where bearing pressure and sliding velocities are high . As such, much work has been accomplished in research and development of coatings and surface treatments aimed at increasing the surface hardness (and wear resistance) of this material. Despite the progress made through surface treatment of titanium with carbides, oxides, nitrides (etc), the wear resistance of titanium is still considerably higher than most alloy steels when evaluated in similar operating conditions .
The exceptional strength-to-weight ratio of titanium makes this an interesting material for bearing applications in the aerospace industry, where weight savings is often a critical factor to system performance. Aerospace components are typically sized and designed to the upper strength limits of the respective material substrate. Subsequently, bearing contact stresses are often significantly higher than would be encountered in standard machine design practices. The poor tribological behavior of titanium necessitates the use of surface coatings to mitigate friction and wear resulting from dynamic contact between components manufactured from these alloys. Given the strength and general material properties of common titanium aerospace alloys, PTFE composites and engineered plastics with their relatively lower material hardness and shear properties would seem a plausible couple for titanium in bearing tribosystems.
3.0 Titanium-Polymer Tribosystems
Little work has been published in recent years concerning the wear performance of titanium in sliding contact with PTFE and its composites, however, the work that has been accomplished using varying operating conditions tend to show significantly increased wear rates when compared to high hardness stainless steel / PTFE couples. This general relationship was witnessed in experimental work performed by Qu, Blau et. al.  concerning the friction and wear behavior of two different titanium aerospace alloys against PTFE. Qu subjected Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo titanium alloys to standard pin-on-disk sliding wear experiments at different speeds (0.3 and 1.0 m/s) using a 10N normal load. Additional slider materials examined during this testing were AISI 440C stainless steel, silicon nitride (NBD200 Grade 5) and alumina (AFBMA Grade 25). The results obtained by Qu are illustrated in Table 3 below.
Table 1. Friction and Wear Results Obtained by Qu  for Ti64 and Ti6242 in Sliding Contact (Pin-On-Disk) with PTFE and Other Slider Materials
As is denoted by the N/Ma specific wear value reported for the titanium substrate in Table 1, the neat PTFE slider demonstrated its characteristic self-lubricating qualities during each of these sliding tests. In each instance, a thin film transfer layer was deposited on the surface of each titanium sample at test’s end, leading to lower COF values and higher wear rates than the other materials tested. When comparing the results of this test to very similar pin-on-disk test results using neat PTFE and a hardened 440C stainless steel counterface , PTFE wear rates were found to be up to three orders of magnitude lower when using 440C over titanium alloys. Historically, surface finish and preparation has shown to have a considerable effect on specific wear rates and could also have contributed to the significant difference in wear rates; however most likely not to the degree shown.
Probably the most interesting work encountered during the course of this research in the field of polymer-titanium interface engineering actually involved the wear performance of ultra-high molecular weight (UHMW) polyethylene as a tribomaterial rather than PTFE. Researchers Li, Dong and Shi  took an interest in the sliding wear behavior of Ti6Al4V when coupled with UHMW polyethylene as this material combination is frequently employed in biomedical applications. Their experimental examination originally initiated as a study into potential titanium surface treatment methods aimed at increasing the wear resistance of Ti64 . Using diamond-like coatings, thermal-oxidation (TO) and oxygen diffusion (OD) surface engineering techniques, Li was able to demonstrate significant gains in the tribological performance of titanium when coupled with UHMW in dynamic contact.
During the course of their testing (using standard 5Mpa pin-on-disk test methods), a wear phenomenon was witnessed whereby the un-treated baseline 6-4 titanium sample consistently exhibited significant wear damage when operated against the relatively softer UHMW slider. Deep, wide grooving was witnessed in the wear scar on the untreated titanium, with a depth and pattern indicating a severe abrasive wear mechanism, with relatively low wear displayed by the UHMW slider. As this author can testify (as a result of numerous bearing endurance tests), this is a fairly common condition when coupling untreated 6-4 titanium against epoxy-based PTFE composites. Where one would expect the softer polymer material to display the bulk of the material wear in this couple, the titanium substrate exhibits deeper and more pronounced wear scarring when operated at high bearing pressures.
In their original work , Li, et. al. identified this abnormal wear phenomenon and attempted to explain the wear as a function of poor adhesive attraction between the UHMWPE and the un-treated titanium countersurface due to an ionic character of the surface interface layers. It wasn’t until two years later that this team developed an interesting hypothesis for this wear condition; hydrogen-induced wear. This theory was originally introduced by a team of Russian researchers in the late 1970’s  when examining the tribological performance of polymers when operated against tungsten carbide-cobalt coatings, however the theory was not pursued and developed after the journal article was published in 1993. Li et. al. recognized the plausibility of this theory as an explanation for the wear mechanisms displayed during their experimentation and they attempted to prove this concept through additional experimentation using UHMW and titanium alloys.
During testing, Li (and Zaitsev) noticed a trend wherein hard alloys experienced higher than expected wear rates when coupled in dynamic contact with polymers such as polycaproamide (PCA), phenolformaldehyde polymer (PF) and expoxy compound (EDC), which all contain reactive groups such as amide, ether and hydroxyl groups. The theory of hydrogen-induced wear assumes that the excessive wear induced in the harder alloys was primarily attributed to a chemical action of these reactive groups. The destruction of hydrogen-containing groups from the high localized heat developed by dynamic friction leads to the inward diffusion of the excess hydrogen into the alloy materials during operation, accelerating subsurface cracking and resulting in increased specific wear rates. Titanium and its alloys are well known to be subject to hydrogen embrittlement, which has shown to reduce the toughness and ductility of this material.
Li conducted pin-on-disk testing using UHMW pins and disk samples manufactured from both Ti64 and 316L in an effort to analyze this condition. As-polished 6-4 titanium was evaluated as well as thermal-oxide treated 6-4 titanium (a common method used to develop a hard titanium oxide layer on the surface of the titanium disk). After testing was completed (5 Mpa effective pressure, 0.3 / 1.0 fpm relative surface velocity), a composition depth profile was generated using glow-discharge spectrometry, and each wear surface was also characterized using SEM and TEM microscopy. After sectioning, additional spectro-analyses were performed using Fourier-transform infrared (FT-IR) and electron spectroscopy for chemical analysis (ECSA) in an exhaustive review of the wear composition.
As a brief summary of Li’s findings , the presence of TiHx, titanium hydride was found to be present in significant concentrations both in the Titanium oxide layer, and more importantly in the α-phase bulk below the TiO layer. The presence of TiHx below the direct interface layer indicates that the diffusion of hydrogen into the titanium bulk could well have contributed to the deep abrasive wear experienced during this test. This condition was witnessed in both the as-polished titanium sample as well as the TO-treated sample. The percent composition of TiHx found in the α-Ti of the TO-treated sample was significantly lower than was witnessed in the as-polished titanium sample. This stands to reason as testing has shown the Titanium oxide serves as an effective barrier against hydrogen diffusion . Figure 3 provides a graphical illustration of the hydrogen-assisted wear mechanism as proposed by Li et. al.
Figure 3. Illustration of Proposed Hydrogen-Assisted Wear Mechanism for the Tribosystem UHMW/Ti6Al4V 
This finding shows promise in explaining the poor wear performance exhibited by titanium alloys in PTFE composite tribosystems. Future development efforts with PTFE composite bearing design should include consideration for the hydroxide content of any epoxy-compound matrices employed in the liner system. Coupled with the application of thermal-oxide surface treatments, significant gains could potentially be achieved in the performance of self-lubricating composites coupled with titanium alloys.
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