PTFE-based Coatings Research Articles

Characterization and Tribological Behavior of Electroless-Deposited Ni-P-PTFE Films on NBR Substrates for Dynamic Contact Applications

The use of rubber in dynamic contacts often results in severe degradation and wear of the rubber surface, which is why dynamic rubber seal contacts are usually oil lubricated to ensure their functionality. However, the increasing demand for more convenient and environmentally friendly sealing solutions has prompted the development of dry low-friction rubber coatings. In this work, and for the first time, Ni-P and polytetrafluoroethylene (PTFE) particles were co-deposited by electroless plating on Nitrile Butadiene Rubber (NBR), as a low-cost solution to improve the NBR tribological behavior. A cationic surfactant, cetyltrimethylammonium bromide (CTAB), was added to the plating bath to ensure a homogeneous and efficient incorporation of PTFE into the Ni-P. The optimized PTFE incorporation reached 6.8%, and the composite coating adhesion to NBR was 20% higher than that of nickel-phosphorous (Ni-P) films. The tribological properties of the coatings evaluated by pin-on-disk tests showed a marginal decrease in the coefficient of friction (CoF) (10%, 1 N load), compared to that of Ni-P. However, the tested PTFE-based coatings displayed significantly smoother surfaces with less debris and cracks, clearly demonstrating the benefits of the PTFE in terms of wear resistance for loads up to 5 N.

Degradation Mechanisms Occurring in PTFE-Based Coatings Employed in Food-Processing Applications

The application of polytetrafluoroethylene (PTFE) coatings to metal surfaces is a well-known procedure carried out to avoid fouling phenomena on food-processing surfaces. Fluorine-based polymers are generally chemically and thermally stable, thus allowing them to be the preferred choice when designing anti-stick coatings in the food service industry. Their lifespan, however, depends on the environmental conditions. It is well known that thermal ageing can affect the properties of PTFE polymers and reduce their mechanical, thermal, and chemical properties causing failures and contaminating food. The main goal of the study is to identify the different failure mechanisms occurring in PTFE-based coatings, using both SEM/EDXS and ATR FT-IR data to reveal the starting point of degradation phenomena in food processing applications. The results from this research reveal that the preferential points for failures are mainly the polymer/substrate interfaces, the polymer/filler interfaces, or the polymer matrix itself.

Environment-friendly aqueous PTFE based bonded solid lubricating coatings: Mechanical and tribological properties under diversified environments

In the paper, the two kinds of environment-friendly aqueous PTFE based bonded solid lubricating coatings were prepared via facial paint spraying method, significantly reducing the waste of organic solvents. The basic physicochemical properties of two kinds of aqueous PTFE based coatings are comparable to that of organic-solvent based coatings. In addition, the tribocorrosive properties and active mechanism of two kinds of aqueous PTFE based coatings were thoroughly investigated under diversified environments, and the results illustrate that aqueous PTFE-ADPs based coatings exhibit superior mechanical and tribological performances to that of PTFE-PPs based coatings, and the tribocorrosive properties of two PTFE based coatings show major variation under different environments, which provides valuable references for environmental adaptability of the aqueous PTFE-based coatings, and further promotes the development of bonded solid lubricating coatings toward the direction of low carbon and environmental protection.

Novel Superhydrophobic Surface with Solar-Absorptive Material for Improved De-Icing Performance.

Ice accretion is detrimental to numerous industries, including infrastructure, power generation, and aviation applications. Currently, some of the leading de-icing technologies utilize a heating source coupled with a superhydrophobic surface. This superhydrophobic surface reduces the power consumption by the heating element. Further power consumption reduction in these systems can be achieved through an increase in passive heat generation through absorption of solar radiation. In this work, a superhydrophobic surface with increased solar radiation absorption is proposed and characterized. An existing icephobic surface based on a polytetrafluoroethylene (PTFE) microstructure was modified through the addition of graphite microparticles. The proposed surface maintains hydrophobic performance nearly identical to the original superhydrophobic coating as demonstrated by contact and roll-off angles within 2.5% of the original. The proposed graphite coating also has an absorptivity coefficient under exposure to solar radiation 35% greater than typical PTFE-based coatings. The proposed coating was subsequently tested in an icing wind tunnel, and showed an 8.5% and 50% decrease in melting time for rime and glaze ice conditions, respectively.

Properties of POB reinforced PTFE-based friction material for ultrasonic motors

Abstract Polytetrafluoroethylene (PTFE) and its composite coating with various poly-p-oxybenzoyl (POB) proportions was prepared by spray suspensions. The friction and wear behavior were evaluated against a GCr15 steel ball on a ball-on-disc tribometer under dry sliding. The effect of the content of POB on the hydrophobic, mechanical and tribological properties of the PTFE-based coatings and the performances of the corresponding ultrasonic motors (USMs) were studied. Experimental results showed that the optimal content of POB not only increased the hardness, adhesion force and contact angle (CA), but also increased the coefficient of friction and wear resistance of the PTFE coatings. Especially, the wear rate of PTFE coating filled with 15 wt.% POB (3.42×10−4 mm3/N·m) was only a quarter of pure PTFE. The morphologies of the worn surfaces of the PTFE coatings were observed by scanning electron microscopy to discuss the wear mechanism. The mechanical output properties of USMs were the best with filling of 10 wt.% POB into PTFE matrix.

Micromechanical properties of polymeric coatings

In earlier studies, PTFE-based coatings showed superb tribological performance under aggressive conditions, relevant to air-conditioning and refrigeration compressors. In this work, micromechanical properties of different polymeric coatings (PTFE-, PEEK-, resin- and fluorocarbon-based coatings) coated on gray cast iron, were examined using instrumented microindentation. The load–unload responses were used to measure the load-bearing properties of the coatings, as well as to extract their elastic modulus and hardness values. Induced structural differences between the PTFE- and PEEK-based coatings were confirmed using scanning electron microscopy. These measurements were used to explain the difference in the tribological performance between PTFE- and PEEK-based coatings. Such correlation is useful as simpler indentation experiments could be readily performed, compared to more complex and application-specific tribological experiments.

Tribological performance of PTFE- and PEEK-based coatings under oil-less compressor conditions

Due to favorable tribological performance, PTFE- and PEEK-based polymeric coatings have received interest in air-conditioning and refrigeration compressor applications, as a potential solution to supplement and potentially replace conventional oil lubricants. The literature in this area is somewhat scarce, especially on the tribological performance of PTFE- and PEEK-based polymeric coatings under aggressive conditions simulating compressor operation. In this work, several PTFE-, PEEK-, resin- and fluorocarbon-based polymeric coatings, coated on gray cast iron were tribologically evaluated using a specialized tribometer under compressor specific conditions, that included oscillatory motion (simulating piston-type compressors) and unidirectional motion (simulating swash plate compressor operation). The coatings showed good to excellent tribological performance, and in general PTFE-based coatings exhibited better friction and wear behavior than the rest of the coatings, including PEEK-based coatings. Long-term durability experiments also showed the superiority and suitability of PTFE/Pyrrolidone coating for potential use in oil-less compressors (where oil-less conditions refer to operation in the absence of any liquid lubricant).

Tribological Properties of DLC Coatings in Helium

The aim of this research work was to investigate tribological properties of low-friction DLC coatings when operating in helium atmosphere. Two commercial DLC coatings (a-C:H and Me-C:H) were included in the investigation and compared to reference PTFE-based coatings, normally used on components operating in helium. Coatings were deposited on hardened 100Cr6 bearing steel discs and tested against uncoated steel balls in low-load pin-on-disc contact configuration. Investigation was focused on the effect of substrate roughness (R a = 0.05–0.2 μm) and contact conditions, including contact pressure (150–350 MPa) and sliding speed (0.2–0.4 m/s) on the coefficient of friction of DLC coatings operating in helium. Results of this investigation show that for low-load sliding contact DLC coatings provide low friction in helium atmosphere, similar to soft PTFE-based coatings. At the same time DLC coatings investigated were found to substantially reduce wear of the coated surface. However, while the wear of the coated part has been more or less eliminated, application of DLC coating prolongs running-in and increases wear of the steel counter-part. Furthermore, also in helium atmosphere tribolgical behaviour of DLC coatings showed dependence on the coating type and contact conditions.

The significance of tribochemistry on the performance of PTFE-based coatings in CO 2 refrigerant environment

This work reports on the tribological performance of three commercially available PTFE-based coatings (PTFE/pyrrolidone (C1), PTFE/pyrrolidone (C2) and PTFE/MoS 2 (C3)) deposited onto engineered disk samples (substrates) made out of three different commonly used materials (gray cast iron, sintered iron and Al390-T6). Controlled oscillatory experiments were conducted to simulate the operating conditions of piston-type compressors in the presence of environmentally friendly CO 2 refrigerant. It was found that the substrate played a major role on the tribological performance of the coatings and one of the coatings (PTFE/MoS 2) was found to significantly improve the scuffing performance of Al390-T6 substrate. Furthermore, the tribological performance was found to improve with increasing CO 2 pressure since a thicker patchy PTFE transfer layer was formed at the substrate–pin interface. XPS analysis showed that metal fluorides have a beneficial role on the tribological performance of these coatings.

Tribological performance of PTFE-based coatings for air-conditioning compressors

Research on coatings in environments simulating compressor conditions, especially in a carbon dioxide (CO 2) environment, has been limited. In this study, polymeric, PTFE-based coatings were investigated for use in air-conditioning compressors. Tribological experiments were performed to simulate piston-type compressors as well as swash plate compressors. Specifically, the tribological contact at the connecting “rod-wrist” pin interface of a piston-type compressor and the “shoe-disk” interface of a swash plate compressor were simulated. Machined 52100 steel wrist pins from a piston-type compressor and 52100 rounded steel pins, referred to as shoes, from a swash plate compressor were used. In both cases the counterparts were gray cast iron disks coated with three commercially available PTFE-based coatings, namely, DuPont® 958-303, DuPont® 958-414 and Whitford Xylan® 1052. These coatings were tested in CO 2 environment, while testing was also performed in ambient air, and R410a, a common hydrofluorocarbon refrigerant, and the results were compared with a diamond-like-carbon (DLC) coating. The PTFE-based coatings showed low friction characteristics and high load carrying capacity and it was found that unlike the DLC coating they were not greatly affected by the testing environment. There was a significant increase in wear of the coatings under high contact pressures, however, it was shown that the wear debris generated acted as a third-body lubricant with a beneficial role in the overall wear performance. Due to their excellent tribological behavior under a range of experimental conditions, the coatings tested in this work could find use in compressor systems and other engineering applications.

A review and comparative study of release coatings for optimised abhesion in resin transfer moulding applications

In this study, a number of abhesion-promoting coatings were considered in terms of their physicochemical and release properties. The techniques used to further this study include; field emission gun scanning electron microscopy, atomic force microscopy (AFM), profilometry, X-ray photoelectron spectroscopy, Auger electron spectroscopy, static secondary ion mass spectrometry, Fourier transform infra-red analysis and contact angle analysis for coating physical and chemical characterisation along with pulsed force mode atomic force microscopy (PF-AFM) and other adhesion and mechanical tests to determine surface release properties. These coatings were applied to metal substrates and were based upon silicone, fluoropolymer or metal–PTFE composite chemistry, all being potentially useful as release films for resin transfer moulding applications. The semi-permanent Frekote B15/710 NC mould release coating system, which is based on PDMS, proved extremely effective in terms of release against a cured epoxide applied under pressure. Although fluoroalkylsilane coatings offer a number of technological advantages for release applications, they generally produce very thin coatings which conform to any existing surface topography and adhesion through mechanical interlocking. The commercial PTFE-based coatings were found to provide poor release properties due to the presence of surface microcracks which allowed epoxide penetration when cured under elevated pressure and temperature. Electroless Ni/PTFE composite coatings comprise a hard nickel–phosphorus matrix containing a very fine dispersion of PTFE particles. The matrix proved sufficiently robust for industrial applications and the low friction and surface energy provided by the embedded PTFE combined with macroscopic-scale surface roughness provided efficient mould release.