Wear of Teflon coating on non-stick pan in hot oil lubrication

doi:10.21203/rs.3.rs-1445170/v1

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Wear of Teflon coating on non-stick pan in hot oil lubrication

https://doi.org/10.21203/rs.3.rs-1445170/v1

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The wear of Teflon coatings on non-stick pans has drawn wide attention since the pans are daily used in cooking. In most of the national standards for non-stick pans, the wear of Teflon coatings on the pan is tested in dry friction at room temperature, while it actually works at high temperatures with oil lubrication. It is still unknown whether the wear of the coating will be more serious or not in the hot oil lubrication condition. Therefore, in this work, the wear of Teflon coatings of non-stick pans is investigated with oil lubrication at the temperature of 150°C. The testing results illustrate that the hot oil lubrication brings about an increase of the wear of the Teflon coatings, and that is attributed to the diffusion of the hot oil into the cracks of the coating, which accelerates the detachment of the coating from the substrate. This finding will inspire us to review the testing standard of the coatings on non-stick pans.

Teflon coating

wear

abrasion resistance

high temperature

oil lubrication

Teflon is a kind of fluorine containing resin with high non-combustibility, chemical stability and self-lubricating [1–3]. As one of the industrial coatings of DuPont Co., Ltd, Teflon has been widely used as coatings for non-stick pans. The coatings usually work at high temperatures and experience lots of scratches in daily cooking. Since the debris of Teflon has the possibility to be eaten with food, it is important to have the knowledge of the wear of coatings under different cooking conditions.

Currently, there are national standards on the abrasive resistance of the coatings of non-stick pans. For example, the French standard for non-stick coatings AFNOR NF D 21–511, the Chinese standard GB-T 32095.2–2015, et al [4, 5]. However, in most of the national standards, the effects of hot oil used in cooking on the wear of non-stick pans are not considered. The oil lubrication is commonly considered to reduce the wear of materials, thus in most of the standards, the test is performed in dry friction condition. But for Teflon, it has a porous structure, which will absorb the lubricants. The absorbed lubricants will accelerate the growth of the cracks generated during the friction testing [6, 7]. Also, the lubrication film will increase the wear of the Teflon compared with dry friction, because the water or oil will prevent the transfer film of Teflon from adhering to the opposite friction surfaces [8–10]. Up to now, it is still unclear whether the hot oil lubrication will exacerbate the wear of the coatings on the non-stick pans or not, and it inspires us to reconsider the testing standard of the coatings.

In this work, we study the wear of Teflon coatings on non-stick pans in hot oil lubrication. The results are compared with that from the dry friction under room temperature, and the mechanism for different wear rates is studied.

The samples of Teflon coatings are taken from non-stick pans (SUPOR, China), which are widely used in daily cooking. The screening process of sample among the three Teflon–based coatings for non-stick pans is shown in supplementary material. The samples are cut into small squares with the length of 30 mm and thickness of 0.2 mm. The coating layers of the non-stick pan and the composition of each layer are shown in Fig. 1(a). The substrate of the sample is a die-casted aluminum alloy, and the top layer is a Teflon coating. Its hardness is HV_0.1 10.7 tested by microhardness tester (Qness Q60A+, Austria). A transition layer is on the substrate, mixed by the Teflon and aluminum. Before the experiment, the surface of the sample is cleaned in an ultrasound washer filled with acetone and ethanol respectively for 10 minutes. After drying, the samples to be tested in the oil lubrication are put in rapeseed oil (Fulinmen, COFCO, China) for 24 hours. Its viscosity under different temperatures is shown in Table 1.

Table 1

The viscosity-temperature of rapeseed oil used in the study
Temperature(°C)	25	50	75	100	125	150
Viscosity(mPa·s)	75.8	32.9	15.8	9.2	6.0	4.04

The experiments are carried out using a standard tribometer (UMT-5, BRUKER). A GCr15 steel ball with the diameter of 9.5 mm is placed on the sample with the load of 15 N. The steel ball is used in the experiment to simulate the steel shovel scratched on the surface of non-stick pans in daily cooking. The sample rotates with the speed of 150 r/min with the running radius of 4 mm (the velocity is 0.0628 m/s), as shown in Fig. 1(a). The load capacity of 15N and the running velocity of 0.0618m/s are higher than the value commonly used in daily cooking, which is to accelerate the wear of the coating. A high-temperature module is installed in the tribometer to adjust the temperature. Room temperature is set at 25 °C, while the high temperature is set at 150 °C, which is common temperature in cooking. The schematics of the study is shown in Fig. 1(b).

The samples are tested under four kinds of conditions: dry friction at room temperature (DR), dry friction at high temperature (DH), oil lubrication at room temperature (OR), and oil lubrication at high temperature (OH). The surface topography of the samples after the test is observed by an interference microscope (ZYGO NewView 5000). The surface structure and the composition of the worn area are studied using a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (SU8220, Hitachi). The surface morphology on the GCr15 steel balls is examined with an optic microscope (VHX-6000, KEYENCE).

The friction coefficients of the Teflon coating under the four different conditions are measured in 10 minutes. The friction coefficients of the samples in the DR and DH conditions are relatively higher and more stable, as shown in Fig. 2(a). The friction coefficients of the samples with oil lubrication are much lower, and it is attributed to the hydrodynamic lubrication generated in the oil film. Moreover, the friction coefficients change with the testing time. For example, in OH condition, the friction coefficient drops rapidly after 50 seconds, then increases with time, and finally decreases gradually. It indicates that the state of surface of the coating changes during the friction test.

The changes happened on the surfaces of the samples during the test are examined by the interference microscope at the time of 40 s, 4 min and 10 min, as shown in Fig. 2(b-e). The corresponding wear rates are statistically calculated based on the morphology of the worn surfaces, as shown in Fig. 2(f). According to Fig. 2(f), two main results are found:

(1) Under room temperature, the dry friction causes higher wear rate than the oil lubrication does;

(2) While under high temperature, on the contrary, the oil lubrication causes a much higher wear rate.

The first result can be explained since the dry friction displays a higher friction coefficient at the room temperature, while the oil film provides a good lubrication environment to reduce the wear of the coatings. However, the second result is counterintuitive. The friction under oil lubrication has a lower friction coefficient but a higher wear rate. Actually, the wear rate of the coatings under OH condition is the highest, and it is much higher than the wear rate in other 3 conditions after the experiment is running for minutes. It indicates that there is a different wear mechanism of the coating with hot oil lubrication.

To reveal the wear mechanism of the coating under OH condition, the surfaces of the worn coatings under OH and DH conditions after 1-minute friction tests are examined using SEM, and the corresponding micrographs of worn surfaces are shown in Fig. 3. Under DH condition, the coating materials are compacted to the substrate of the sample, and the surface of the grinding crack is more smooth compared with the original porous surface, as shown in Fig. 3(a). The composition of the worn surface is examined using the Energy Dispersive Spectrometer (EDS) of SEM, as shown in Fig. 3(a1-a4). According to the evenly distribution of the elements C and F on the surface, the coating is considered to remain on the substrate and the coating layer is not broken.

While under OH condition, the coating in the wear track is found broken and detached from the aluminum substrate, as shown in Fig. 3(b). Under this condition, large debris of the coating is generated, which explains the notable raise of the wear rate, as shown in Fig. 2(f). According to the distribution of the elements on the surface as shown in Fig. 3(b1-b4), the coating is broken and the substrate is exposed. But unlike the scarce element O in DH, local oxidation under the OH is observed, which implies much more severe tribochemical oxidation reaction, and thus wear rate. Therefore, the hot oil accelerates the wear of the coating, and the wear mechanism of the coating with hot oil lubrication is different from that in DH or OR.

The Teflon coating has a porous structure, and the porosity is usually inevitable since its fibrils nature [11], verified by SEM shown in Fig. 3(a)(b). In hot oil lubrication, oil film will spread out on surface at the beginning, and flow into the holes in the Teflon coatings, as shown in Fig. 3(c1). The oil then permeates through the porous structure, and the high temperature accelerates the diffusion of the oil in the coating. Cracks form, and the oil enters the cracks under the friction, and the hydraulic pressure of the oil helps the expansion of the cracks. When the cracks approach the substrate, the coating material detaches and the wear debris is generated.

The schematics of the wear process with hot oil lubrication is shown in Fig. 3(c2). This explanation is consistent with the result found in the experiment. In Fig. 3(b), the coating is bulged in the grinding track that is contrary to the compacted surface in dry friction, which is attributed to the expansion of the oil in the cracks that peels the coating off the substrate. The wear of coating with the hydraulic pressure has also been found in the Huang’s experiment ^[6], where the hydraulic pressure is used to break the rocks in the quarrying. This effect has also been found in the cavitation erosion on metal surfaces [12, 13], where the hydraulic pressure accelerates the growth of the cracks and causes loss of the materials.

Another question is why this accelerated wear happens at high temperature rather than at room temperature. It is owing to the different diffusion rate and difficulty of the oil into the coating materials under different temperatures. A water washable HG-Z99S2 type dye penetrant flaw detection agent is used to simulate the oil flow in the coating under OH and OR in a visual way. Figure 3(d1) shows the optical images of as-bought pan’s cross-section morphology, and Fig. 3(d2-d3) show the different results of the agent in the coating under the high temperature and room temperature. At high temperature, the agent infuses into the coating materials, while the agent only stays at the surface of the coating at the room temperature. It illustrates that the temperature also plays an important role in the hot oil lubrication, and it also validates the effect of the hydraulic pressure in the enhanced wear of the coating.

Usually, the friction force on the Teflon will be reduced by a thin and uniform transfer film generated on the counterfaces, which can promote the wear resistance of Teflon [14–16]. In the experiment, the effect of transfer film is also investigated. Indeed, a layer of the Teflon transfer film is generated on the counterfaces of GCr15 steel balls, as shown in Fig. 4. Under the dry friction conditions (DH, DR), transfer film of Teflon is found to adhere on the counterfaces of the GCr15 steel balls stably, as shown in Fig. 4(a1-a3) and (c1-c3). But under oil lubrication conditions (OH, OR), transfer film is not found to adhere on the steel ball surface, as shown in Fig. 4(b1-b3) and (d1-d3). According to the friction coefficient shown in Fig. 2(a), the generation of the transfer film on the counterfaces does not play a dominant role in the friction reduction as the oil film does. Moreover, the lower wear rate under the OR condition confirms that the lack of the transfer film on the counterfaces is not the reason to cause the high wear rate under the OH condition.

The temperature-dependent wear resistance of the coatings is also considered [1, 10, 11, 17]. As the temperature rises, the mechanical properties of the Teflon will degrade [18], which will cause the friction coefficient to decrease and the wear rate to increase [19]. However, it cannot explain why the wear rate of the coating under OR condition is much lower than that under OH condition.

According to the experimental results, it can be concluded that the wear resistance of the Teflon coatings is strongly influenced by the lubrication conditions and the temperature. In this work, the synergy effect of high temperature and oil on the wear rate of the Teflon coatings is investigated.

The coating on the non-stick pan with hot oil lubrication has the highest wear rate compared with those in dry friction and in room temperature. According to the examination on the cracks and the transfer film on the counterfaces, the reason is that the diffusion of the oil in the porous coating materials in high temperature, which will generate the hydraulic pressure to accelerate the growth of the cracks in the coatings, leading to the loss of the materials from the coating.

This finding inspires us to review the testing standard of the coatings on non-stick pans, which commonly requires the testing in dry friction at room temperature or in high temperature, while the increased wear rate under hot oil lubrication found in this work is not considered. We wish more attention could be drawn to the phenomenon and more efforts put into the revise of testing standard through this work.

Acknowledgments

The authors are thankful to Wenyan Yang, Yimai Liang and Yuji Chen for technical support on the tribotest and surface characterization. This research was supported by the National Natural Science Foundation of China (No. 52025051).

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contributions

All authors contributed to the study conception and design. Investigation, conceptualization, methodology, formal analysis, writing-original draft, data curation were performed by Yang Zhang. Validation, reviewing and editing were performed by Weiqi Chen. Supervision, visualization, and funding acquisition were performed by Haosheng chen and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.”

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Wear of Teflon coating on non-stick pan in hot oil lubrication

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1. Introduction

2. Experiment Setup

3. Results And Analysis

4. Discussion And Conclusion

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