S. Bahadur

The development of transfer layers and their role in polymer tribology

Abstract When polymers slide against metal counterfaces, transfer films are formed. This is also the case when sliding occurs between a polymer and another polymer. In the latter case, the transfer of material has been documented by infrared studies which show that material transfer occurs from a polymer of low cohesive energy density to one of higher cohesive energy density. The transfer film formed on a non-polymer counterface is governed by the counterface material and roughness, and of course the sliding conditions. The growth of transfer film with the number of passes is presented and the effect of counterface roughness is examined. The mechanism of wear is discussed with respect to the transfer film. It is shown that when polymers are modified, such as by the addition of fillers, the transfer film affects the tribological behavior. Some fillers affect the development of transfer film and enhance its adhesion to the counterface. Such fillers reduce the wear rate of polymer, often drastically. On the other hand, there are many fillers which have no such effect on the transfer film and wear in these cases is increased. The results of friction and wear behavior for both types of fillers are discussed in view of the transfer film characteristics. The analyses of transfer films as performed by XPS for a few cases are also presented.

Studies on the tribological behavior and transfer film-counterface bond strength for polyphenylene sulfide filled with nanoscale alumina particles

Abstract The tribological behavior of polyphenylene sulfide (PPS) composites made with nanoscale particles (also called “nanoparticles”) of Al2O3 was investigated using the pin-on-disk configuration. The volumetric proportions of the filler used were 0%, 1%, 2%, 3%, 5%, and 10%. The counterfaces were made from hardened tool steel and were prepared with three different surface roughnesses: 0.027, 0.060, and 0.100 μm Ra. Flexure tests on the composite specimens were run to investigate the effect of weakening from the presence of heterogeneous particles and strengthening because of the addition of a harder phase. The lowest wear rate was obtained for the composite with 2 vol.% filler and the counterface with 0.060 and 0.100 μm Ra roughness. The composites with 3 vol.% or greater filler proportions had higher wear rates in all cases. The coefficient of friction was also higher than that of PPS for all composites except with 10 vol.% alumina sliding on a 0.060 μm counterface. The bond strength between the transfer film and the counterface was measured and related to wear rate. It was found that the transfer films with stronger bond strengths led to lower wear for all the composites. The results have been interpreted in terms of the ability of nanoscale particles to anchor the transfer film to the counterface thereby increasing bond strength and lowering wear.

Tribological studies of glass fabric-reinforced polyamide composites filled with CuO and PTFE

Abstract The effect of glass fabric reinforcement on the friction and wear behavior of nylon was investigated for varying fabric proportions. The effect of the addition of particulate CuO filler, and the filler and PTFE to the glass fabric-reinforced composites on the tribological properties of the composites was also studied. The composites with different proportions of fiber, filler and PTFE were compression molded and tested for friction and wear in a pin-on-disk configuration. It was found that the proportion of glass fabric for optimum wear resistance was about 20% by volume. The addition of CuO filler did not contribute to improvement in the wear resistance of the fabric-reinforced composites because a good transfer film did not develop in the presence of fibers and the composites became fragile. PTFE was very effective in reducing the wear of filled and fabric-reinforced composites. The friction and wear behavior of these composites has been interpreted in terms of their ability to form transfer film on the steel counterface and the changes occuring on the pin surface. The composition for maximum wear resistance was 25 vol.% CuO-11.3 vol.% glass fabric-10 vol.% PTFE-nylon.

The role of copper compounds as fillers in the transfer and wear behavior of polyetheretherketone

CuO, CuS and CuF2 have been used as fillers in polyetheretherketone (PEEK) and the influence of these fillers on the friction, wear and transfer behavior of the PEEK composites has been studied. Sliding tests were conducted with a pin-on-disk machine at 1.0 m s−1 speed and 0.65 MPa nominal contact pressure under ambient conditions. The pins were made of polymer composites with a filler content of 35 vol.%. The composites were prepared by compression molding. The disks were machined from tool steel blocks and hardened. Experimental results showed that all these fillers reduced the wear of PEEK. The coefficients of friction of the CuS-PEEK and CuF2-PEEK composites during steady state wear were slightly higher than that of unfilled PEEK, but the coefficient was much higher in the case of the CuO-PEEK composite. The transfer films were studied by optical and scanning electron microscopy and their bonding to the counterface by X-ray photoelectron spectroscopy. It has been argued that the ability of these fillers to reduce wear depends upon their ability to form transfer films of the composites on the counterface which are thin and uniform and are strongly bonded to the substrate.

The role of copper compounds as fillers in transfer film formation and wear of nylon

Abstract The friction and wear behavior of filled nylon 11 composites was studied. The fillers used were CuS, CuO, CuF 2 and Cu(C 2 H 3 O 2 ) 2 ·H 2 O (copper acetate). A fixed filler proportion of 35% by volume was used. Sliding tests were performed in a pin-on-disk configuration at 1.0 m s −1 speed and 0.65 MPa nominal contact pressure under ambient conditions. The pin was made of composite specimens and the disk of hardened tool steel. It was found that CuS, CuF 2 and CuO as fillers reduced the wear rate of nylon while Cu(C 2 H 3 O 2 ) 2 ·H 2 O increased it. The cause of this behavior was investigated in terms of the ability of these materials to develop a transfer film and its bonding to the counterface. The wear rates of nylon and copper-acetate-filled nylon were high because the ability of nylon to form a transfer film was poor and the composite transfer film had poor adhesion to the counterface. On the other hand, the wear-resistant nylon composites which were filled with CuS, CuO and CuF 2 transferred well to the counterface and their transfer films were thin, uniform and strongly adherent to the counterface. The adhesion of the transfer film to the counterface for CuO-nylon composite was investigated by X-ray photoelectron spectroscopy analysis.

The action of fillers in the modification of the tribological behavior of polymers

Abstract This paper discusses the role of fillers such as solid lubricants, pure metals and inorganic compounds in modifying the tribological behavior of polymers. It examines critically the mechanisms and hypotheses proposed by a number of workers for wear reduction. This is done from the overall perspective of the data published in the literature as well as the recent work done by the authors on the subject. Some of the hypotheses have been negated while some are supported with discretion. The critical filler volume model is discussed and the filler volume for minimum wear is shown to agree with that calculated by the model. Some guidelines for the selection of fillers and their proportion for minimum wear rate are provided and the areas of research needing further attention are indicated.

Erodent particle characterization and the effect of particle size and shape on erosion

SiC, Al2O3 and SiO2 particles in various grit sizes are characterized by image analysis in terms of their area, area diameter, width, length, width-tolength ratio W/L and perimeter squared-to-area ratio P2/A. Along with the mean, median and r.m.s. deviation of these parameters, the cumulative frequency distributions of the area and W/L are also obtained. W/L and P2/A are used as the indicators of particle shape. The particles become more elongated and less circular as the size of SiC and A12O3 particles increases and that of SiO2 particles decreases. The characterized particles are used for erosion studies on 18Ni(250) maraging steel in aged condition in a sand-blast type rest rig. The variation of erosion is studied with changes in size and shape of these particles. Erosion rate increases with increasing particle size for SiC and Al2O3 particles up to a certain value and decreases for SiO2 particles. However, it increases with increasing P2/A and decreasing W/L for all three types of particles. Erosion behavior is analyzed considering the effect of rake angle in ploughing and microcutting as observed in erosion. It is the changing contribution from ploughing and cutting with changes in shape and size of particles that accounts for erosion variation.

The growth and bonding of transfer film and the role of CuS and PTFE in the tribological behavior of PEEK

Abstract Sliding experiments on the PEEK composites filled with CuS and PTFE powders while rubbing against tool steel surfaces were performed under ambient conditions in a pin-on-disk configuration. A constant sliding velocity of 1 m s−1 and a contact pressure of 0.654 MPa were used in all experiments. The steel counterfaces were finished by abrasion to a surface roughness of 0.11 μm Ra. The optimum CuS proportion for minimum wear rate as determined experimentally was 35 vol.%. With the addition of CuS, the coefficient of friction increased. The addition of PTFE to these composites reduced both the wear rate and the coefficient of friction. The wear rate of the different compositions depended upon their ability to form transfer films on the steel counterfaces. The study of the transfer film growth by scanning electron microscopy revealed that it was a mechanical process in which the fragments of the material removed were locked into the crevices of counterface asperities. The compositional changes in the transfer film were studied by XPS analysis of the top surface of the transfer film and also closer to the steel substrate. It was found that Cu ions and the compounds of FeF2, FeSO4 and FeS were formed near the interface of the transfer film with its steel substrate. These compounds contributed to enhanced bonding between the transfer film and the counterface. Of the various mechanisms of wear, the cracking and delamination of the transfer film will also be affected by the bonding of transfer film to the counterface.

The effect of laser surface treatments on the tribological behavior of Ti-6Al-4V☆

Abstract Laser surface treatments were tried with the objective of improving the tribological performance of Ti-6Al-4V. The surface treatments used were laser surface melting, laser nitriding and laser nickel alloying, which were performed using a 1.4 kW CO 2 laser. The process parameters for laser surface treatments were optimized. The microstructures produced and the compounds formed were analyzed using standard metallographic and X-ray diffraction techniques. The abrasive, erosive and sliding wear behaviors of the laser-treated surfaces were studied. It was found that, whereas laser nitriding and nickel alloying reduced abrasive wear, laser surface melting did not. The reduction in the abrasive wear rate of laser-nitrided and nickel-alloyed specimens is attributed to the formation of TiN and Ti 2 Ni respectively in the molten and resolidified region. All three laser surface treatments resulted in improving the dry sliding wear resistance. This is attributed to the increased hardness as well as the formation of wear-resistant compounds in nitriding and nickel alloying. The mechanism of wear in the untreated titanium alloy was primarily ploughing but in the laser-treated cases material was removed by the formation and rupture of adhesive bonds. The increased hardness is believed to change the mechanism of sliding wear from ploughing to adhesion. It was found that the laser treatments did not have any effect on the erosion resistance of Ti-6Al-4V when impacted by coarse particles at a high velocity. The above results are supported by electron microscopy of the worn surfaces.

The effect of reinforcement and the synergism between CuS and carbon fiber on the wear of nylon

Abstract The polymer composite specimens were prepared with nylon 11 as the matrix material, CuS as the filler, and short fibers of carbon as the reinforcement. This was done because CuS filler has shown an unusual ability in reducing the wear of a number of polymers including nylon. It was expected that the synergistic action of filler and fiber would produce a highly wear-resistant material. The friction and wear behaviors of the composite materials were investigated in a pin-on-disk system where sliding occurred under ambient conditions between the composite pin and a tool steel counterface. The sliding conditions used were 1 m s −1 speed and 0.65 MPa contact pressure. When nylon was reinforced with carbon fiber, wear and the coefficient of friction were reduced considerably. Wear rate was the lowest for 20 vol.% fiber content and it was higher for both the lower and higher percentages of fiber. Wear reduction was still more significant when carbon fiber was used for reinforcement along with CuS as the filler. These differences in wear behavior have been investigated in terms of the synergism between fiber and CuS. This synergistic action occurred because of the role of fiber in enhancing the decomposition of CuS which increased the adhesion between the transfer film of the composite and the steel substrate.