Peihong Cong

Friction and Wear of Polyphenylene Sulfide (PPS), Polyethersulfone (PES) and Polysulfone (PSU) Under Different Cooling Conditions

The friction and wear properties of polyphenylene sulfide (PPS), polyethersulfone (PES) and polysulfone (PSU), which have similar molecular structure, were investigated using an end-face contact tribometer in three different cooling ways: sliding without air cooling, sliding with air cooling, and sliding in water. The worn surface and wear debris were observed using a scanning electron microscope (SEM). The effect of frictional heat on the tribological properties of the polymers was comparatively studied. When sliding in air, with increasing applied load, the wear rate of PPS decreased slightly initially then increased later while the wear rate of PES and PSU increased through out. The results suggested that the friction coefficient was mainly affected by the temperature of the worn polymer that was controlled by the balance of heat flow of the whole sliding contact system. When sliding in water, the friction coefficients of the three polymers decreased compared to that sliding in air and remained relatively steady through the whole process under different load. The wear rates of the three polymers had a close value and, remarkably, increased compared to that sliding in air. The water cooling and lubrication role decreased the tribological properties difference between the polymers.

Investigation on Tribological Properties of Fluorinated Polyimide

The 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) was used to synthesize polyimide to introduce different amounts of fluoride to the main chain of polyimide. The infrared spectra indicate that the imidizing process was almost complete and fluorinated monomer was formed in the structure. Fluoride-containing polyimide showed better thermal stability, higher tensile strength, and lower surface energy than neat polyimide. The increase of the fluoride monomer ratio contributed to the tribological properties of polyimide. The friction coefficient decreased with the increase of the fluoride monomer ratio. Surface-free energy and friction heat can alter the physical state of polymer sliding surfaces, and had a great effect on the tribological behaviors. Abrasive wear was designed and executed in this work. The wear rate decreased with the increase of the fluoride monomer ratio.

Comparative Study of Tribological Properties of Polyphenylene Sulfide (PPS), Polyethersulfone (PES), and Polysulfone (PSU)

The tribological properties of polyphenylene sulfide (PPS), polyethersulfone (PES) and polysulfone (PSU), which have similar molecular structures, were investigated using an end-face contact tribometer and a reciprocating tribometer. The thermomechanical behavior of the polymers was analyzed using dynamic mechanical analysis (DMA). PPS exhibited a maximum friction coefficient with increasing load and sliding speed, while the friction coefficients of PES and PSU decreased only slightly. The wear rate of PPS was much lower than that of PES and PSU under high loads and speeds. It is suggested that the main factors influencing the friction and wear properties of the neat polymers are their condensed state and heat resistance. Amorphous PES and PSU showed liquid-like behavior and very low friction when the frictional surface was in the molten-flow state. The macromolecular crystals of crystallizable PPS give it some solid-like behavior and load-carrying capacity; hence PPS exhibited lower wear than PES and PSU.

Effects of film characteristics on frictional properties of carboxylic acid monolayers

Monolayers of long-chain carboxylic acids, with various fluorination ratios, were deposited on solid substrates to investigate the effects of surface properties on frictional behavior. It is found that a lower surface free energy correlates to a lower friction coefficient for the monolayers of partially fluorinated carboxylic acids. However, a stearic acid (C17H35COOH) monolayer shows the lowest friction coefficient, although its surface free energy is relative high. The two-dimensional elastic modulus, which might be used as a parameter to quantitatively characterize the film strength, was developed. Friction coefficients of the monolayers show a strong correlation with their elastic modulus, that is, the higher the elastic modulus, the lower the friction coefficient. The research results indicate that frictional properties of ultrathin films are mainly dependent on film strength. Enhancement of intermolecular attractive force might be a more effective way to improve the lubricating properties of ultrathin films.

Tribological Behaviors of Fluorinated Polyimides at Different Temperatures

4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA)-based copolyimides were synthesized and the tribological properties of the copolyimides with different heat histories were studied at different temperatures. Fluoride-containing polyimide (PI) showed better thermal stability, decreased friction coefficients, and postponed the consequence of friction variation, which depended on temperature, than nonfluorinated PI. Thermal treatments seemed to increase the friction coefficients of copolyimides, and reduced the tensile strengths of the materials. The effects of applied load (P) and sliding speed (V) on tribological behaviors of thermally treated copolyimides were also examined and the variations of friction coefficient depending on PV values were investigated for clear understanding of its relationship between PV value and friction coefficient with different thermal treating time. Distortions of net structures of the chains and molecular motion contributed to variations of tribological properties of thermal treated copolyimides.

Application Study of a Modified Phenolic Resin as Binder for Hybrid Fibers Reinforced Brake Pad for Railroad Passenger-coach Braking

A modified phenolic resin was designed and developed in our laboratory to manufacture friction material with high performance. The potential application of the resin as a binder for hybrid-fibers reinforced brake pads for railroad passenger-coach braking was evaluated on a full-scale test bench according to Standard of China Railway Ministry TB/T 3118-2005 “The brake lining made of composite materials for rolling stock”. The test results indicated that the physical and mechanical properties, the braking performance and wear resistance of the brake pad fully met the corresponding requirements; the whole polymer structure survived after the severe braking tests. This result may be due to the metal fibers and/or particles in the friction material that can effectively export frictional heat during the braking process, and the heat stability of the formed tribochemical products. The brake pad with this new binder has achieved practical application for passenger-coach braking in China.

Investigation of the structure and tribological characteristics of monolayers deposited by the Langmuir–Blodgett technique

Abstract A series of partially fluorinated ethyl esters with the same hydrophobic group length were deposited by the Langmuir–Blodgett (LB) technique on an Al plate coated with a NiP film. Film structures were inferred through surface pressure–molecular area isotherms ( π – A isotherms). The results indicate that the film structures are determined by van der Waals forces between hydrophobic chains. Stronger mutual forces lead to stable solid films. Frictional properties and load-carrying capacities of the monolayers were evaluated using a ball-on-plate-type sliding apparatus. The friction coefficient and load-carrying capacity are dependent on the monolayer film structures. The solid film shows a lower friction coefficient and a higher load-carrying capacity. Friction tracks of the monolayers were examined by time-of-flight secondary ion mass spectroscopy (TOF-SIMS). We found that the liquid film may be a fluid monolayer, and that the structure of the solid film changes under frictional force.

Tribological Characteristics and Tribochemical Reactions of Various Ceramics Lubricated with HFC-134a Gas

The role of tribochemical products in the friction and wear reduction of ceramics with different fractional ionic character in CF3CH2F (HFC-134a) gas was investigated using a ball-on-disk type tribometer. Without exposure to air, the wear tracks on the disks were characterized with the aid of a micro-spot X-ray Photoelectron Spectroscope (XPS) whose analytical chamber was connected to the friction chamber of the tribometer. Further, the adsorption and desorption behaviors of HFC-134a molecules on the nascent surfaces of the ceramics were studied using an adsorption test apparatus in high vacuum. It was found that the lubricating effect of HFC-134a gas was closely related to the fractional ionic or covalent characters of the ceramics. HFC-134a gas was more effective in lubricating ionic ceramics than the covalent ceramics. XPS analysis revealed that metal fluorides were mainly formed on the frictional surfaces of the ionic ceramics, whereas the composition of the tribochemical products on the frictional surfaces of the other ceramics was complicated. The adsorption tests proved that HFC-134a was decomposed to an olefin CF2=CHF on the nascent surfaces of the ionic ceramic Al2O3 and the covalent ceramics. However, the formation of organic fluorine-containing compounds was not detected on the frictional surfaces of the ionic ceramics by XPS. This result implies that the mechanism of tribochemical reactions is strongly dependent on the bond type of ceramics. It is concluded that the low friction and wear of the ionic ceramics in HFC-134a gas result from the metal fluorides formed with high surface concentration on the sliding surfaces.

Effect of the Rubber Components on the Mechanical Properties and Braking Performance of Organic Friction Materials

Vegetable oil modified phenolic resin (PF) mixed with four kinds of rubber modifiers, i.e., styrene butadiene rubber, styrene butadiene 2-vinyl pyridine rubber, nitrile butadiene rubber, and carboxyl nitrile butadiene rubber (CNBR), were used as matrices for organic friction materials. The mechanical and thermal degradation properties of all of the blends were investigated. Friction and braking tests of the organic friction materials based on the different matrices and reinforced with hybrid fibers were carried out. The results showed that the resin was most compatible with CNBR; the CNBR/PF blend possessed much higher impact and toughness, and the friction material based on this blend as a matrix exhibited better friction and braking performance. It was concluded that CNBR, the rubber with the most reactive groups, resulted in better mechanical properties of the friction material, and hence optimized the friction, wear and braking performances.

Braking Performance of an Organic Brake Pad Based on a Chemically Modified Phenolic Resin Binder

An organic brake pad for railroad passenger-coach braking was prepared using a chemically modified phenolic resin (PF), that was designed and manufactured in our laboratory. The braking performance of the pad was investigated on a full-scale test bench. For comparison, a brake pad based on straight PF was also prepared and investigated. The results showed that the modified PF pad possessed much higher impact strength, and better braking stability and wear resistance than the straight PF pad. It is thought that the better overall properties of the modified PF pad resulted from the changes in chemical structure of the resin during the frictional process and the strong interreaction of the modified resin binder with the hybrid fibers.