Formation Of Transfer Film Research Articles

Tribological properties of polyimide coating filled with solvent-free covalent carbon quantum dots nanofluids

Novel organic coatings were developed by incorporating solvent-free covalent carbon quantum dots nanofluids (CQDs NFs) into a polyimide (PI) matrix. The tribological properties of CQDs NFs/PI composite coatings were tested using a ball-on-disk tribometer, revealing that even a low loading of CQDs NFs significantly improved tribological performance of PI. Molecular dynamics (MD) simulations and nanoindentation tests showed that CQDs NFs restricted PI molecular chain movement, enhancing mechanical properties. Additionally, CQDs NFs promoted the formation of stable transfer films on steel surfaces, improving wear resistance. This study confirms CQDs NFs' effectiveness in modifying PI composite coatings' friction and wear properties.

A-C/Au Film with Low Humidity Sensitivity of Friction by Forming Au Transfer Film.

Amorphous carbon is recognized as an excellent lubricating material; however, its tribological properties are significantly influenced by humidity. To elucidate the mechanism underlying this humidity dependence and to propose a novel enhancement method, we investigated and compared the tribological properties of hydrogenated amorphous carbon (a-C:H) and amorphous carbon/gold (a-C/Au) composite films. First, the friction coefficient of these carbon films under different humidity conditions was tested using a rotational ball-on-disk tribometer. Subsequently, we analyzed the morphology and structure of the sliding interface employing optical microscopy (OM), Raman spectroscopy, transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Finally, first-principle calculations were carried out to calculate the adsorption energy of water molecules on different surfaces. The results indicate that the friction coefficient of a-C:H film and the area of transfer film increase with the increase of humidity. This phenomenon can be attributed to the fact that water molecules enhance the interaction between the a-C:H film and steel counterfaces. Notably, in contrast, the friction coefficient of a-C/Au film demonstrates low sensitivity to humidity due to the formation of an Au transfer film that exhibits weak interaction with water molecules. These findings provide a promising strategy for developing environment-adaptive amorphous carbon films and play an important role in promoting the development of intelligent lubricating film.

Enhancing the anti-wear performance of graphene sheets embedded carbon films through interface nanotexture fabricated on the substrate

The effect and mechanism of surface texture on friction have been extensively explored, such as storing the abrasive and reducing the real contact area. However, studying the impact of the interface texture remains crucial, especially as the texture size approaches the nanoscale. In the present work, the silicon substrate with different parameters nanotexture was fabricated using a metal-assisted chemical etching process, and the graphene sheets embedded carbon (GSEC) film was deposited on the nanotextured substrate via a low-energy electron irradiation process to produce the interface nanotextured carbon films. After the addition of the interface nanotexture, the tribological performance of carbon films exhibited significant enhancement, with the degree of improvement being influenced by the parameters of the interface nanotexture. A super-low wear rate was achieved, with a value of 6.54 ± 0.70 × 10−8 mm3/N m. The improved tribological performance is due to the enhancement of film-substrate adhesion strength facilitated by interface nanotexture, and its role in promoting the rapid formation of a stable transfer film on the counter-grinding ball. The carbon film with nanotexture displays a small size of the abrasive particles, which contributes to the stabilization of the friction process.

Effects of chemical etching on surface structure and tribological behavior of silicate substrates

Abstract This study investigated the effect of chemical etching on the surface structure and tribological behavior of silicate substrates. Silicate surfaces were etched using a mixture of nitric acid (HNO3) and ammonium bifluoride (NH4HF2) for durations ranging from 1 to 60 min. The etched surfaces were characterized using optical microscopy, scanning electron microscopy, surface profilometry, water contact angle measurements, and UV–vis spectroscopy to evaluate the changes in surface morphology, roughness, wettability, and optical properties. Tribological performance was assessed using reciprocating ball-on-plate friction tests. The results showed that increasing the etching time resulted in the formation of microscale surface features, increased surface roughness, enhanced hydrophilicity, and reduced optical transmittance. The average friction coefficient decreased with an increase in the etching time up to 30 min, beyond which a slight increase was observed. The 1-minute etched specimen exhibited the best wear resistance with the narrowest wear track and the least material removal. The improved tribological performance was attributed to the formation of a stable transfer film, reduced real contact area, and entrapment of wear debris. This study highlights the potential of chemical etching as a technique to tailor the surface structure and tribological properties of silicate materials for various applications.

Filler processing and mixing effects of polyoxymethylene/graphene nanocomposite on tribo-mechanical performances

Binary nanocomposites based on polyoxymethylene (POM) and graphene nanoplatelet (GNP) were fabricated to improve the tribo-mechanical performances of high-precision sliding parts in linear drive system applications. Liquid-phase exfoliation technique and surface modification by 3-aminopropyltriethoxysilane (APTES) were used to process GNP filler for improved endowment of POM/e-GNP matrix-filler interfacial adhesion. Results show that processed e-GNP at optimal 0.5 wt% loading is distributed uniformly in the matrix and enables excellent stress-transfer during the mechanical loading, increasing flexural modulus, impact strength and elongation at break by 51.3 %, 41.9 % and 24.5 %, respectively, in contrast to neat POM. Simultaneously, coefficient of friction (COF) and specific wear rate (Ws) of POM in ambient temperature were also reduced substantially by e-GNP incorporation under both dry (COF by 19.5 %, Ws by 40.6 %) and grease lubricated (COF by 38.2 %, Ws by 76.4 %) sliding conditions. Smoothened wear surface and formation of homogeneous transfer film on the steel counterface were accounted for large specific surface coverage and low-shear strength of e-GNP at the contact interface, especially during the dry sliding. In addition, the accumulated energy dissipation by frictional work was calculated in the sliding interfaces based on friction force-displacement (F-D) hysteresis loop. However, pristine POM/GNP exhibited poor matrix-filler adhesion and deteriorated composite properties due to the aggregated structure which can cause subsurface defects and serve as a failure site. The proposed new material would extend the applications of precision parts by overcoming the tribo-mechanical related issues for quieter and accurate linear motion systems.

Enhancement on the thermal and tribological behaviors of polyurethane/epoxy-based interpenetrating network composites by orientationally aligned CNF/MXene/WPU aerogels

Tribological behaviour of polyurethane/epoxy interpenetrating polymer network (PU/EP IPN) as friction materials was investigated. This was a combination of the good flexibility of PU and high hardness of EP to realize polymer synergistic effect of interpenetrating network structure, aiming to broaden and develop the application of polymer-based composites in friction engineering materials. A novel CNF/MXene/WPU aerogel (CMW) was prepared by employing flexible NCO-terminated waterborne polyurethane prepolymer (WPU) as the supporting skeleton of aerogel, and synchronously combining the reinforcing-phase carbon nanofibers (CNF) and lubricating-phase MXene. Subsequently, the prepared ultralight porous yet mechanically robust CMW aerogels were encapsulated with IPN by vacuum impregnation. As expected, the addition of aerogel imparted IPN matrix with enhanced thermal and tribological properties due to the thermal dispersion and enhancement effects conferred on the matrix by the highly aligned and ordered three-dimensional interconnected structure of aerogel. Meanwhile, CMW reinforced IPN composites were explored for the formation of a high-quality transfer film on the contact surface during friction. The transfer film prevented direct contact between the friction couples, which facilitated the improvement of wear resistance of composites, thus giving this material great potential for tribological applications.

Tribological behavior of poly(ether ether ketone)/synthetic Eucommia rubber composites

AbstractOur previous study (Polymer Composites 2023, 44: 1252–1263) revealed the positive role of rubber in modifying the tribological properties of polymer composites. This concept was applied here by incorporating synthetic Eucommia rubber (TPI) to 3D printed polyether ether ketone (PEEK) skeletons with different infill densities. The formation of TPI/PEEK composite improved the friction and wear of PEEK matrix with some reduction in mechanical performance. The composite with 70% infill density is recommended in terms of its overall performance. Based on the morphological and chemical analysis, the composite's wear mechanism was discussed. The findings of this present study could pave a new route to modify friction‐reduction and anti‐wear performance of PEEK.Highlights Novel composites were successfully prepared from thermodynamically incompatible synthetic Eucommia rubber (TPI) and polyether ether ketone (PEEK). The tribological properties of TPI/PEEK composites were studied in association with the infill density of PEEK. The TPI rubber helped improve the friction and wear properties of PEEK thanks to its role in enhancing the formation of transfer films on the counter steel surface.

Nano-innovations in lubrication: Unveiling the synergistic boundary lubrication potential of zinc titanate with phosphonium ionic liquid

This study investigates the potential of Zinctitanate (ZnTiO3) nanoparticles (APS <100 nm) as nano-additives in lubricant formulations, in conjunction with phosphonium phosphate ionic liquids. Comparative analysis against well-established nano-additives highlights ZnTiO3's exceptional dispersion stability, enduring beyond 225 days. Tribological assessments unveil significant enhancements in antiwear, antifriction, and extreme pressure properties, with 1 wt% ZnTiO3 outperforming the base oil. The ZnTiO3 hybrid nanolubricants exhibited notable synergy, achieving a 49 % reduction in wear compared to hexagonal boron nitride (hBN) and Boehmite NPs. Surface characterization confirms the formation of a protective transfer film (≥100 nm thick), attributed to ZnTiO3's disintegration under tribostress. This results in the development of a mixed metallic metal oxide interfacial film composed of (FeTiO3, FePO4, and ZnO) domains on exposed nascent steel surfaces through tribochemical reactions. These findings suggest ZnTiO3 as a promising nano-additive for advanced lubricants, offering substantial improvements in dispersion stability and tribological performance.

In-situ vibrational spectroscopic observation for thermally activated structural changes of 100% cellulose nanofiber molding with ultralow friction

This paper reports the thermally activated ultralow friction of 100% cellulose nanofiber (CNF) molding. The mechanism of friction reduction was investigated using a laboratory-built in-situ Raman tribometer. Our experimental results showed that a CNF molding exhibited an ultralow friction coefficient of below 0.04 in a CNF ring and steel disk tribopair under high-temperature conditions (T &amp;gt; 100°C). The results of the temperature-rise friction test showed that the friction coefficient of the CNF molding strongly depended on the temperature and decreased linearly with increasing temperature. The in situ tribo-Raman monitoring results, during friction, indicated a change in the structure of the CNF molding. Therefore, the crystallinity indices and lengths of the CNF fibers gradually changed as the temperature increased. Moreover, transfer tribofilms were observed on the counter-steel surface against the CNF rings. When the CNF molding exhibited thermally activated ultralow friction, the tribofilm was mainly composed of cellulose and graphitic carbon. Our results suggest that the thermal and friction-activated structural transformations of CNF molding and CNF-derived transfer film formation are pivotal factors contributing to the ultralow friction phenomenon observed in CNF molding at high temperatures.

Influence of hBN on the abradable performance of AlSi-based coatings: Bridging the gap between tribological assessment and abradability testing

Abradable coatings have been proven to significantly improve the performance of turbomachinery by serving as a protective barrier between rotating blades and stationary casing. However, there is still a pressing need to develop more efficient abradable materials that can withstand the growing energy requirements and prolong the service life of gas turbines, which continues to pose a significant challenge to the aerospace industry. In the present study, two thermally sprayed aluminum‑silicon-based (AlSi) coatings were investigated, the first containing 40 wt% polyester (referred to as AlSi-Poly), and the second containing 6 wt% hexagonal boron nitride (hBN) and 20 wt% polyester (referred to as AlSi-hBN-Poly) as filler material. The microstructure, HR15Y hardness, and surface roughness were systematically examined in the fabricated coatings. Tribological studies were conducted using ball-on-flat and ball-on-disk testing conditions against titanium counterballs. In addition, a cost-effective abradable test rig has been developed to assess the abradability performance of both coatings under closer simulated aircraft turbine conditions. The microstructural examination of the coatings revealed that the filler materials were uniformly distributed throughout the coating, with the hBN particles embedded within the matrix. The average hardness of the AlSi-hBN-Poly coating was higher than that of the AlSi-Poly coating, which is consistent with the area fraction of filler materials present in each coating. The tribological assessment of the coatings revealed similar frictional coefficients, no apparent wear on the counterballs and higher wear depth and width for AlSi-hBN-Poly coating. This was attributed to significant third-body abrasion and higher transfer during testing in the case of the AlSi-hBN-Poly coating. Similarly, the abradability evaluation showed no blade wear against both coatings and slightly different transfer film formation. However, the AlSi-Poly coating presented a smoother rubbing wear track than the AlSi-hBN-Poly, which showed deeper rubbing grooves. These findings provide valuable insights into the tribological behaviour of these coatings and can aid in the development of more effective wear-resistant coatings.

Polymer transfer film formation from cryogenic to elevated temperatures

This study reports on the tribological performance of aromatic thermosetting co-polyester (ATSP) and polyether ether ketone (PEEK)-based polymer composite coatings mixed with PTFE filler. The coatings were tested across a wide temperature range from −180 to 110 °C to simulate the environmental temperatures on Titan, Moon, and Mars, which are of particular interest for NASA’s future exploratory missions. An experimental setup was developed to conduct the pin-on-disk experiments under dry sliding conditions and extreme temperature and contact pressure. Transfer film formation and its characteristics were found to play significant roles in the tribological performance, and the characteristics of the film were temperature-dependent. The XPS and SEM analysis indicated the increase of the PTFE content in the transfer film as the temperature decreased to cryogenic conditions. The coefficient of friction did not follow a linear trend with temperature and was minimum at 110 °C and maximum at −180 °C. ATSP coating showed superior performance with lower friction and unmeasurable wear at all temperatures, whereas PEEK coating exhibited maximum wear at 25 °C followed by −180, and 110 °C.

Improved tribological properties of W-S-C-N film contributed by the friction-induced well crystallized thick transfer layer

Amorphous W-S-C-N lubricating films were prepared by sputtering two WS2 targets and an additional WC target in Ar&N2 atmosphere. The W-S-C-N film demonstrated a notably stable low coefficient of friction (COF) of approximately 0.01, as well as a wear rate of 1.50 × 10-7 mm3/Nm over extended sliding cycles. These characteristics indicated superior tribological properties compared to the deposited WS2, W-S-C, and W-S-N films. The formation of the friction-induced well-crystallized WS2 transfer film with its (002) basal plane parallel to the sliding surface of the counterpart ball contributed to the low friction of the W-S-C-N film. Furthermore, the embedded amorphous and nanocrystalline WC phase within the crystallized WS2-based transfer film further enhanced the stability and durability of the transfer film.

Revealing the low-temperature friction behavior and mechanisms of hydrogenated amorphous carbon films with Al/Cr/Si doping

Hydrogenated amorphous carbon (a-C:H) film has emerged as a significant contender for solid lubrication applications in low-temperature environments. Here, a set of a-C:H films doped with varying contents of elements (Al, Cr, Si) (AlCrSi/a-C:H) were fabricated, and their low-temperature frictional evolution pattern and mechanisms were explored. The results revealed that the films maintained a superlubricity state as the temperature decreased to − 40 °C, and when the temperature further decreased, the coefficient of friction increased followed by a significant fluctuation. It was related to the competition between shear graphitization, abrasive wear, transfer film formation, and film embrittlement in different low temperature ranges. This work provides new evidence for understanding the carbon film low-temperature friction behavior.

Tribological behaviour of a-C:H films with Cu cover layers: Formation and influence of tribo-induced Cu/Cu2O nanograins

This study focuses on the tribological behaviour of Cu-layer covered a-C:H films and the transfer film formation mechanisms, by means of tribological test, surface profiling, Raman spectroscopy and in-depth transmission electron microscopy, etc. Results showed that introducing a Cu cover layer with proper thickness would significantly enhance the tribological performance of a-C:H films at low sliding speed. The CoF of a-C:H was reduced to 0.004 (superlubricating) at the optimal Cu-layer thickness of ∼32 nm. The transferred material on counterpart was composed of a dense Cu layer and an a-C/(nc-Cu/Cu2O) composite layer, which promoted the formation of a similar layer on a-C:H wear track. The strength and durability of a-C/(nc-Cu/Cu2O) composite layer was double-reinforced by “physical pinning” and Cu-O-C “chemical bonding”.

Enhancing high-temperature tribological properties in a detonation-sprayed CoCrNiAl0.3Ti0.3 medium-entropy alloy coating via in-situ formed Al2O3-type oxides and rich-Ag phases

Heterogeneous high-entropy/medium-entropy alloy coatings have the potential of being advanced tribo-components for service in demanding environments. We incorporate dual-heterostructure (Al2O3-type oxides and Ag-rich phases) into the deposited CoCrNiAl0.3Ti0.3 medium-entropy alloy coating employing in-situ flight oxidation and solute addition during detonation spraying. Such heterostructures improve the coating quality and serves twofold, through deposition densification and solid-solution strengthening, contributing to an ultra-low porosity of < 0.6 %, a high bond strength of 61 MPa, and a hardness of 7.6 GPa. The self-organized tribo-layer activated by the heterostructures inhibits abrasive wear at room-temperature, but struggles to resist surface softening and adhesion at 600 ℃. Heterostructures govern the formation of protective glaze-layers and Ag transfer films at 800 ℃ and 1000 ℃, minimizing adhesion and delamination between the tribo-pairs.

Tribological evaluation of thermoplastic polyurethane-based bearing materials under water lubrication: Effect of load, sliding speed, and temperature

Polymers are widely used in bearing applications. In the case of water-lubricated stern tube bearings, thermoplastic polyurethane (TPU)-based composites are used due to their excellent wear resistance, corrosion resistance, and tunable mechanical properties. Their tribological performance, however, depends on operating conditions. In this work, TPU was blended with carbon fiber, graphene platelet, and ultra-high molecular weight polyethylene (UHMWPE). Friction tests of TPU based-composites against copper countersurface were carried out in water to mimic the actual operating conditions of the bearing. Most of the resulting contacts were in the boundary lubrication regime, in which friction was attributed to both contact mechanics of asperities as well as water lubrication. Our results show that the viscoelasticity of TPU has a considerable impact on its tribological performance. Water lubrication at 50 °C promotes the softening of polymer surface material during sliding, resulting in higher fluctuation in the coefficient of friction and wear loss. This is attributed to the reduced thermomechanical properties. In addition, Schallamach waviness is observed on worn surface. The tribological properties of TPU are significantly improved by the inclusion of carbon fiber, graphene platelet, and UHMWPE. The formation of graphene transfer-layers and UHMWPE transfer film reduces friction and wear loss, while the inclusion of carbon fiber enhances wear resistance due to improved mechanical properties and load bearing capacity.

Tribological synergy of copper‐based metal–organic frameworks, carbon fibers and SiO2 in modifying the friction and wear properties of epoxy composites

AbstractCopper‐based metal–organic frameworks is anticipated to find applications in the field of friction materials owing to its plentiful surface‐active groups and distinctive framework structure. In this study, copper‐based metal–organic frameworks was synthesized through ultrasound‐assisted method and incorporated alone into carbon fiber reinforced epoxy composites or together with SiO2 nanoparticles towards the goal of improving the composites' friction and wear properties. It is revealed that copper‐based metal–organic frameworks alone cannot improve the friction and wear of carbon fiber reinforced epoxy composites simultaneously. The combination of copper‐based metal–organic frameworks and SiO2 nanoparticles was more effective in enhancing both the mechanical and tribological properties of studied composites. The composite material exhibits a friction coefficient of 0.104 under the specified test conditions of 10 MPa and 3.75 m/s. Additionally, the specific wear rate is impressively low, measuring only 5.64 × 10−7 mm3 N/m. Through morphological and chemical analysis on the worn surfaces and transfer films, the functioning mechanism of copper‐based metal–organic frameworks in modifying the mechanical and tribological properties was discussed. It was discovered that a tribological synergy exists between short carbon fibers, copper‐based metal–organic frameworks, and SiO2 fillers, leading to a shift in the wear mechanism of the composite materials from abrasive wear to adhesive wear. Concurrently, a shear‐prone transfer film forms on the surface of the counter steel surfaces. Findings of the present study pave a new route of designing high performance epoxy based tribo‐composites by using metal–organic frameworks.Highlights Cu‐BDC nanosheets were synthesized using ultrasound‐assisted techniques and incorporated as tribological fillers in EP composites. The combined use of Cu‐BDC, SiO2, and SCF fillers synergistically enhances the mechanical and tribological properties of EP composites. The composite material exhibits a friction coefficient of 0.104 under the specified test conditions of 10 MPa and 3.75 m/s, the specific wear rate is impressively low, measuring only 5.64 × 10−7 mm3N/m. During the sliding friction process of the composite materials, Cu‐BDC and SiO2 play a crucial role in reducing SCF pull‐out wear through their anchoring effect. The dynamic coordination bond structure of Cu‐BDC contributes to the formation of uniform and continuous transfer films on counter metal surfaces.

Comparative investigation on the tribocorrosion resistance of Ti6Al4V and 60NiTi alloys inimulated seawater environment

The Ti6Al4V alloy is extensively utilized in critical marine equipment components due to its low density, high specific strength, and exceptional corrosion resistance in marine environments. However, its low hardness and inadequate wear resistance pose challenges in meeting the urgent demand for prolonged service life of relative motion friction pairs under complex wear and corrosion conditions. As a newly developed lightweight friction pair material, hardened 60NiTi is considered highly desirable owing to its combination of high hardness, high compressive strength, and excellent corrosion resistance. Nevertheless, the tribocorrosion properties of 60NiTi compared to those of Ti6Al4V are not yet fully comprehended. The present study conducted a series of sliding wear tests using a ball-on-plate configuration in artificial seawater environment to compare the response of 60NiTi with Ti6Al4V. In order to gain a comprehensive understanding of the factors contributing to the divergence in tribocorrosion response between both materials, further analysis was performed on the wear track subsurface and transfer film using focused ion beam-scanning electron microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy respectively. The 60NiTi demonstrates significantly enhanced wear resistance in artificial seawater compared to Ti6Al4V, attributed to its combination of high hardness and excellent corrosion resistance. However, Ti6Al4V exhibits a lower friction coefficient (∼0.30) than 60NiTi (∼0.45) due to the formation of a stable transfer film primarily composed of SiO2, Al(OH)3, and Al2O3.

Mechanical properties and tribological behaviors of Ag/graphene composite coating under sliding friction and current-carrying fretting

Charging connector of electric vehicles faces two working conditions of sliding friction and current-carrying fretting. Traditional Ag coating is difficult to meet the performance requirements. Herein, Ag/graphene composite coatings were prepared by electroplating, and the chemical compositions, crystal phase, mechanical and tribological properties were studied. The results showed that adding graphene increased the hardness, elastic modulus and toughness. Under sliding friction and current-carrying fretting, Ag/graphene coating exhibited low COF and wear, stable and low ECR. The sliding friction mechanism was dominated by abrasive wear, while the current-carrying fretting mechanism was dominated by adhesive and fatigue wear. The worn surface analysis indicated that the superior tribological properties were attributed to the improved mechanical properties and the formation of transfer film.

Effect of Europium and Gadolinium Alloying Elements on the Tribological Response of Low Hydrogen Content Amorphous Carbon.

Dopants and alloying elements are commonly introduced in amorphous carbon (a-C) materials to tailor their mechanical and tribological properties. While most published studies have focused on doping and alloying a-C coatings with metals or metalloids, doping a-C films with rare-earth elements has only recently been explored. Notably, our understanding of the shear-induced structural changes occurring in rare-earth-element-containing a-C films is still elusive, even in the absence of any liquid lubricants. Here, the friction response of Eu- and Gd-containing a-C films with low hydrogen content deposited by HiPIMS on silicon was evaluated in open air and at room temperature. The load-dependent friction measurements indicated that the introduction of Gd ((2.3 ± 0.1) at.%) and Eu ((2.4 ± 0.1) at.%) into the a-C matrix results in a significant reduction of the shear strength of the sliding interfaces ((41 ± 2) MPa for a-C, (16 ± 1) MPa for a-C:Gd2.3at.%, and (11 ± 2) MPa for a-C:Eu2.4at.%). NEXAFS spectromicroscopy experiments provided evidence that no stress-assisted sp3-to-sp2 rehybridization of carbon atoms was induced by the sliding process in the near-surface region of undoped a-C, while the amount of sp2-bonded carbon progressively increased in a-C:Gd2.3at.% and a-C:Eu2.4at.% upon increasing the applied normal load in tribological tests. The formation of an sp2-bonded carbon-rich surface layer in a-C:Gd2.3at.% and a-C:Eu2.4at.% films was not only proposed to be the origin for the reduced duration of the running-in period in tribological test, but was also postulated to induce shear localization within the sp2-carbon-rich layer and transfer film formation on the countersurface, thus decreasing the interfacial shear strength. These findings open the path for the use of Gd- and Eu-containing a-C even under critical conditions for nearly hydrogen-free a-C films (i.e., humid air).