Influence of heating temperature and time on mechanical-degradation, microstructures and corrosion performances of Teflon/granite coated aluminum alloys used for non-stick cookware

This study explores the functional characteristics (erosion, corrosion, mechanical damage, and microstructural features) of non-stick cookware made from aluminum alloys. Typically coated with polytetrafluoroethylene (PTFE-Teflon) or ceramic for non-stick properties, we conducted a systematic investigation using corrosion, abrasion, and mechanical tests on six types of cookware from different manufacturers (Manuf-1-6). The cookware was heated at various temperatures [Room temperature (RT), 100, 175, 250, & 350 °C] and times (45 & 120 min). Tests included Taber wear, Adhesive Pull-off, hot & RT corrosion, and surface roughness measurements. Characterization involved optical microscopy, scanning electron microscope (SEM) with electron backscattered diffraction (EBSD), and x-ray diffraction (XRD). Ceramic-coated cookware from Manuf-4 demonstrated superior mechanical strength, wear, and corrosion resistance due to refined microstructures. Manuf-1's PTFE-coated cookware also performed well. Optimal results were observed when heating below 250 °C for up to 45 min. Prolonged heating and temperatures beyond 250 °C adversely affected internal structures of all cookware. Thus, it is advisable to use Al-based non-stick cookware below 250 °C for a maximum of 45 min.

When the two abraded wheels rotate, the turn-able sample against sliding rotation of two abraded wheels introduces rub-wear action.Here, the two abraded wheels (one is left, and the other is right) rotate opposite each other on a horizontal axis by the sample.One abraded wheel rubs the sample inward towards the center, whereas the other rubs outward towards the periphery during the test.A vacuum system will be connected to extract the debris produced during the test.
Further, two abraded arms can be lowered or raised for examining the sample, which are independently operating ones.Each arm is a precisely balanced one.The test is usually carried out by fixing the number of cycles like 500, 1000, and 1500 at 60 or 72 rpm speed, which a speed regulator can vary.The evaluation criteria for the cookware is a loss in coating materials over the substrate.Hence, the Taber abrasion test is selected in this research work.After heating the pans, a circular disc was cut with a diameter of 105 mm using a laser cutting machine.F shows the photograph of circular-shaped cut samples prepared for the Taber abrasion test from cookware supplied by different manufacturers at different heating temperatures and times.

Pull-off adhesion test
Portable pull-off adhesion test is the best method for examining the strength of protective coating materials over the substrate.This test is a uniaxial one as per ASTM: D4541 standard.This method can also be used to find-out the strength of coated materials in civil infrastructures.In this test, the sample surface is to be cleaned, a strong glue is to be applied over certain area on the sample, then a dolly (20 mm in diameter) is to be pressed over the substrate, leave the dolly and sample together for curing (at least one day).Next, portable pull-off adhesion tester is to be used for exerting a perpendicular force which remove the dolly along with coated materials from the substrate.A force at which the coating fails per dolly contact area will give adhesive strength.Various factors would affect the performance of pull-off adhesion test, namely, the mixing of epoxy glue, types of glue, sample surface preparation method (either by applying acetone/ethanol or ultrasonic cleaning), the shape and size of dolly, test temperature (room and elevated one), and curing time.Usually, two pack of epoxy (Part-A and Part-B) is to be used during the adhesion test.The Part-A is resin and the part-B is a hardener which are to be mixed in equal manner (approximately 1:1).The weight loss of Manuf-2 pan at 45 min heating exhibited 0.0220, 0.0242, 0.0307, 0.0381, 0.0402, and 0.0631g for RT, 100, 175, 250, 350, and 450°C, respectively.The percentage of variation of weight loss as a function of temperature compared to the RT sample was 9.82%, 39.54%, 73.25%, 82.82%, and 186.6% for 100, 175, 250, 350, and 450°C, respectively.The weight loss of Manuf-3 pan at 120 min heating was 0.0220, 0.0365, 0.0573, 0.0615, 0.0821, and 0.1027g for RT, 100, 175, 250, 350, and 450°C, respectively.In terms of percentage of weight loss, PTFE Manuf-3 pan after 120 min heating compared to RT sample was 65.39%, 160.3%, 179.5%, 273.3%, and 366.7% for 100, 175, 250, 350, and 450°C, respectively.The weight loss of PTFE Manuf-3 pan after 45 min heating was 0.0155, 0.0169, 0.0177, 0.0197, 0.0221, and 0.0309 g for RT, 100, 175, 250, 350, and 450°C, respectively.The weight loss variation percentage compared to the RT sample was 9.03%, 14.06%, 27.1%, 42.78%, and 99.25% for 100, 175, 250, 350, and 450°C, respectively.Similar manner, the weight loss of PTFE Manuf-3 pan after 120 min heating was 0.0155, 0.0183, 0.0199, 0.0237, 0.0281, and 0.0421g for RT, 100, 175, 250, 350, and 450°C, respectively.In terms of percentage of weight loss compared to the RT  450°C, respectively.These results demonstrate low surface roughness variation up to 250°C.However, beyond 250°C, the variation of surface roughness value was increased drastically due to the weak bonding of C-F atoms over the substrate.Similarly, after heating at 120 min, the surface roughness value of PTFE Manuf-1 pan was 0.6033, 0.6365, 0.7853, 0.8699, 1.0234, and 1.1365 µm for RT, 100, 175, 250, 350, and 450°C, respectively.The variation of the percentage of surface roughness value as a function of temperature compared to the RT sample was 5.51%, 30.18%, 44.19%, 69.64%, and 88.60% for 100, 175, 250, 350, and 450°C, respectively.The surface roughness value of PTFE Manuf-2 pan at 45 min heating exhibited 0.7057, 0.7124, 0.8237, 0.9024, 1.1236, and 1.2456 µm for RT, 100, 175, 250, 350, and 450°C, respectively.The percentage of variation of surface roughness value as a function of temperature compared to the RT sample was 0.944%, 16.714%, 27.86%, 59.21%, and 76.51% for 100, 175, 250, 350, and 450°C, respectively.The surface roughness value of PTFE Manuf-2 non-stick cookware at 120 min heating was 0.7057, 0.7728, 0.8636, 0.9864, 1.2366, and 1.3652 µm for RT,100,175,250,350,and 450°C,respectively.In terms of percentage of variation of surface roughness value, PTFE Manuf-3 non-stick cookware after 120 min heating compared to RT sample was 9.50%, 22.37%, 39.77%, 75.22%, and 93.45%, respectively.The surface roughness value of PTFE Manuf-3 non-stick cookware after 45 min heating was 0.9137, 0.9250, 0.9356, 0.9563, 0.9633, and 0.9837 mm for RT, 100, 175, 250, 350, and 450°C, respectively.The percentage of variation of surface roughness value was 1.24%, 2.40%, 4.66%, 5.42%, and 7.66% for 100, 175, 250, 350, and 450°C, respectively.The observed variation of surface roughness value of PTFE Manuf-3 non-stick cookware was very low due to the strong bonding of C-F atoms and effective concentration of PTFE materials over the substrate.Similar manner, the surface roughness value of PTFE Manuf-3 non-stick cookware's after 120 min heating was 0.9137, 0.9954, 1.0237, 1.0456, 1.1237, and 1.2037 µm for RT, 100, 175, 250, 350, and 450°C, respectively.In terms of variation of percentage of surface roughness value compared to the RT sample, PTFE Manuf-3 non-stick cookware exhibited 8.94%, 12.03%, 14.44%, 22.98%, and 31.74% for 100, 175, 250, 350, and 450°C, respectively.These results demonstrate the poor bonding of C-F atoms in PTFE materials after a long heating of 120 min.
For ceramic Granite coated cookware, the surface roughness value of Granite Manuf-4 non-stick cookware after 45 min heating exhibited 0.4237, 0.4376, 0.4437, 0.4563, 0.4654, and 0.4966 µm for RT,100,175,250,350,and 450°C,respectively.The percentage of variation of surface roughness value compared to the RT sample was 3.29%, 4.71%, 7.71%, 9.84%, and 17.21% for 100, 175, 250, 350, and 450°C, respectively.Here, the variation of surface roughness value with the function of temperature was very low.This was expected due to the effective concentration of silane materials over the substrate.

Fig
Fig S1 shows the PTFE coated (Teflon) sauce and fry pans and Fig S2 and Fig S3 show the ceramic coated (silica and granite) cookware's.

Fig.
Fig.S3shows the photograph of heating of as-purchased pans supplied by various manufactures which are all heated at different temperatures and time using infrared electric oven (100, 175, and 250°C) and electric induction furnace (350, and 450°C).

Figure
Figure S4 Photograph showing the heating source of as-purchased pans made from different manufactures using temperature controlled equipment: (a) electric infrared stove/oven; (b) electric induction furnace

Fig
Fig S6.(a) Schematic diagram representing the Pull-Off adhesive test; (b) Pull-Off adhesive tester used during experiment Fig. S6a shows the schematic diagram indicating the pull-off adhesive test and Fig. S6b shows the photograph of pull-off tester used in this work.Fig S7 show the photograph of new surfaces (after heating) and damaged surfaces (after Taber abrasion test) of samples which were prepared for pull-off adhesive tests from cookware at different heating temperature and time.

Fig. S11 -
Fig. S11-Fig S16 shows the optical microstructural changes in the cookware produced by different suppliers after heating under different conditions.These optical microstructural images are also evidenced by the refinement of α-Al grains and increased FeA3 precipitates with increasing temperatures.

Table S1
. Variation of average weight loss and Taber wear index at different operating conditions (RT, 100, & 175 o C for 45 & 120 min) measured from Taber rotary abrasion test as per ASTM D4060 standard Sample Made Operating Temperature, o C Time, min Average weight loss, g