Physiochemical Effects of Sic and Zro2 Particle Fillers on The

coatings

Article Physiochemical Eﬀects of SiC and ZrO2 Particle Fillers on the Properties of Enamel Coatings

Hyunseok Ko 1 , Sung-Jin Lee 2, Jae-Jong Oh 2, Seungho Lee 3,* and Hyung Mi Lim 1,*

1 Convergence Technology Division, Korea Institute of Ceramic Engineering and Technology, Jinju 52851, Korea; [email protected] 2 HaeKwang Enamel Industrial Co. Ltd., Incheon 21697, Korea; [email protected] (S.-J.L.); [email protected] (J.-J.O.) 3 Korean Material Technology Cooperative, Seoul 06977, Korea * Correspondence: [email protected] (S.L.); [email protected] (H.M.L.); Tel.: +82-10-2740-3211 (S.L.); +82-55-792-2450 (H.M.L.) Received: 24 December 2019; Accepted: 28 January 2020; Published: 31 January 2020

Abstract: The ceramic particles of SiC and ZrO2 are embedded as fillers in the enamel coating with varying contents of 0.03, 0.05, and 0.1 wt %, and the surface properties are measured. It is found that the addition of ceramic fillers indeed causes changes in surface properties. The roughness was influenced mainly by the particle size embedded. The contact angle decreased at 0.03 and 0.05 wt % and equivalent to that of no filler at 0.10%. Our analysis suggested that the contact angle is influenced by both surface roughness and surface morphology (with chemical composition). The microstructure and elemental analysis suggest that the chemical composition and shape of Al, Ce, Ca, and P-rich aggregates on the enamel surface are showing significant changes when fillers are added. It is observed that the Al- and Ce-rich aggregates decrease both in number and size as the filler content increases, and Ca-rich aggregates change their shape from needle to spot at 0.1 wt % inclusion. The washability is notably improved at 0.1 wt %, which corresponds to the content where the drastic microstructure change occurred. The examination of the contaminated surface revealed that the phosphate component in the contamination has reacted with the Ca-rich phase of the needle-shape during the process of burning, thus inhibiting an easy removal of the contamination. Therefore, the formation of the Ca-rich phase of the needle-shape on the enamel surface should be suppressed for easy-cleaning enamel coatings for cooking wares.

Keywords: enamel; coatings; ceramic ﬁller; SiC; ZrO2; washability

1. Introduction Vitreous enamel coating is a substantially glassy inorganic coating applied to a metal substrate by thermal fusion. It has good abrasion, wear, and chemical resistance, high hardness, various colors, and incombustibility. While the properties of the vitreous enamel are closely related to the properties of its components, these properties can be controlled by tailoring the composition [1–3], microstructure [4–6], and crystallization of the coating [7–9]. One of the aspects desired for the enamel coating is the non-stick coating or easy-cleaning coating to easily detach the burned contamination in the high-temperature cooking apparatus, such as the gas or electric ovens. While there are a number of patents and articles [10–12] on the durability of enamel coatings, especially on the abrasion [2,3,13,14] and corrosion resistance [2,6,7,15–17], less is understood how the surface microstructural features of enamel scales inﬂuence the washability. For the easy-cleaning enamel coatings, there are an array of self-cleaning coatings on the industry that are typically categorized into two types depending on the cleaning mechanism: catalytic and

Coatings 2020, 10, 121; doi:10.3390/coatings10020121 www.mdpi.com/journal/coatings Coatings 2020, 10, 121 2 of 13 pyrolytic. The catalytic-enamel coating has a porous structure where these pores can absorb the burned contamination, and the catalytic substances like metal oxides in the enamel coating help the decomposition of foods into carbon dioxides and water. During this cleaning cycle, the product is heated to 200 ◦C or higher to burn off and soften excess grease deposits. One can then simply wipe away any residue with some soapy water. The pyrolytic-enamel cleans by elevating the temperature to an extreme degree over 500 ◦C. The heat reduces the baked crust and grease to powders, which are easily removed. Since the process creates smoke and fumes, some pyrolytic-cleaning coatings are provided in a hybrid type containing catalytic components. Nevertheless, there are several issues associated with both types of catalytic- and pyrolytic- enamels. The lifespan and hardness are limited due to the porosity in the structure, and the increase of energy index due to high-temperature cleaning as well as the production of ashes. Therefore, there still are needs for the development of an easy-cleaning enamel that is durable and cleansable at not even high temperatures. Incorporation of additives in the coating is a well-known method to introduce both microstructural change and surface chemistry, which can alter the repellence to undesired matters, such as water and impurities [18]. One could introduce a secondary substance to modify the crystallography and morphology of the enamel scale [19,20]. For instance, the addition of Al2O3 in the enamel improves the hardness of the coating but also induces the formation of a fine-grained enamel coating [21], thus altering the microstructure. Rossi et al. [22] showed that the presence of the micro-Al2O3 particles improved the abrasion resistance of the enamel coating, which resulted in limited corrosion resistance. Likewise, the crystal distribution and surface chemistry can be tailored by controlling contents. In the present study, ceramic particles are incorporated into the enamel coatings to induce physiochemical changes such as surface roughness, chemical reactivity, and morphology. We focused on (i) how the surface properties of enamel coatings incorporated with ceramic particles are modified when SiC and ZrO2 particles are utilized as ceramic fillers, and (ii) how these changes influence the washability.

2. Experimental Details

2.1. Preparation of Enamel Coating Enamels are silicate-based materials that are deposited by a spray gun then fused at high temperatures. A commercially-available alkali borosilicate glass frit from Hae Kwang enamel coating Co. Ltd. was used. The composition of frits is designed for easy-cleaning coatings for cooking wares [23–25]. In Table1, the chemical composition of enamel frit from inductively coupled plasma (ICP) results is shown. Firstly, raw materials of glass frits and clay (kaolin) were ball-milled to achieve homogenized powders. For SiC or ZrO2 filler-added cases, fillers were ball-milled together at this stage with glass frits and clay. The powders were sieved with 180 µm of mesh, and the average size of sieved particles was 30–40 µm. The enamel slurry was prepared by mixing powders in the water to provide a stable suspension. The enamel slurry without filler was used as a reference enamel coating, which will henceforth be referred to as NF. The contents of SiC and ZrO2 particles were set to be 0.03%, 0.05%, 0.1%, and 0.2%, and this is given in Table2 along with the particle information. The field emission scanning electron microscopy (FESEM) images of SiC and ZrO2 particles are shown in Figure1. The SiC and ZrO 2 particles were chosen as reprehensive non-oxide and oxide ceramic fillers, and their high melting point makes them suitable for investigating their physiochemical effects. A particle size between 0.5 and 3 µm was chosen to induce an effect on surface roughness. For SiC, micron-grade was chosen as nano-grade can undergo surface oxidation during the oxidation. The change to SiO2 on its surface may undermine the generic effect of SiC, which is against our initial intention. It should be noted that there is a difference in number density (i.e., spatial distribution) for two cases that may have influenced the changes in observed aggregates. However, the effect by their presence will be confirmed later in this paper, and the difference in the number density does not change our major conclusions. Coatings 2020, 10, 121 3 of 13 Coatings 2020, 10, x FOR PEER REVIEW 3 of 13 changesFor in the observed substrate, aggregates. a cold-rolled However, steel plate the effect of 0.8–1 by mm their thickness presence and will 100 be confirmed150 mm later in size in wasthis × paper,used. Before and the coating, difference all plates in the were number sand-blasted density does and cleanednot change with our a commercially-available major conclusions. cleansing solvent.For the The substrate, cleaned plate a cold was‐rolled sprayed steel withplate aof prepared 0.8–1 mm slurry thickness and firedand 100 at 840 × 150◦C mm for 5in min, size thenwas used.finally Before cooled tocoating, room temperature.all plates were The firingsand‐blasted treatment and at highcleaned temperature with a meltedcommercially the glass‐available frit and cleansingchemically solvent. bonded The the cleaned enamel plate to the was steel. sprayed During with enameling, a prepared the slurry plates and were fired loaded at 840 in °C the for furnace. 5 min, thenThe final finally thickness cooled ofto theroom enamel temperature. scale was The measured firing treatment as 135 15at µhighm. temperature melted the glass frit and chemically bonded the enamel to the steel. During enameling,± the plates were loaded in the furnace. The final thicknessTable 1. Chemical of the enamel composition scale was analysis measured of enamel as used135 ± in 15 this μm. study.

Components Contents (wt %) Table 1. Chemical composition analysis of enamel used in this study. SiO2 55.4

ComponentsNa2O Contents 14.9 (wt %)

SiOB22O 3 14.855.4

NaTiO2O 2 2.8714.9 B2ONiO3 2.6614.8 TiO2 2.87 Al O 1.8 NiO2 3 2.66 Al2SrOO3 1.371.8

SrOLi2 O 1.061.37 Li2O 1.06 K2O 0.98 K2O 0.98

Table 2. Ceramic ﬁllers used in the study and their fundamental properties. Table 2. Ceramic fillers used in the study and their fundamental properties. Filler Manufacturer Particle Diameter (um) BET (m2/g) Filler Manufacturer Particle Diameter (um) BET (m2/g) SiC Sinxing Advanced Material Ltd. (Hongkong, China) 2.61 1.68 3.17 SiC Sinxing Advanced Material Ltd. (Hongkong, China) 2.61± ± 1.68 3.17 ZrOZrO SinoceraSinocera (Shandong, (Shandong, China) China) 0.510.51 0.38± 0.38 9.079.07 2 2 ±

Figure 1. FESEM imagesFigure of (a 1.) SiCFESEM and images(b) ZrO of2 particles. (a) SiC and (b) ZrO2 particles. 2.2. Evaluation of Enamel Coating Properties 2.2. Evaluation of Enamel Coating Properties As the homogeneity can be an issue in evaluating surface properties of sprayed enamel coatings, As the homogeneity can be an issue in evaluating surface properties of sprayed enamel coatings, multiple locations of three specimens of identical conditions were subjected to the measurements. multiple locations of three specimens of identical conditions were subjected to the measurements. The surface roughness of the scale was assessed using a laser microscope, OPTELICS (Lasertec, The surface roughness of the scale was assessed using a laser microscope, OPTELICS (Lasertec, Yokohama, Japan). For the measurement, three specimens were prepared for each condition (by filler Yokohama, Japan). For the measurement, three specimens were prepared for each condition (by filler type and content), and the surface roughness was measured at 25 different positions of each specimen. type and content), and the surface roughness was measured at 25 different positions of each To evaluate the surface roughness of the enamel scale, we introduced the roughness average, R . The specimen. To evaluate the surface roughness of the enamel scale, we introduced the roughnessa roughness average is the arithmetic average of the absolute values of the profile heights over the average, Ra. The roughness average is the arithmetic average of the absolute values of the profile evaluation length, which can be expressed in the equation (Equation (1)) as: heights over the evaluation length, which can be expressed in the equation (Equation (1)) as: Z L 11 Ra = | z(x)| dx (1)(1) L 0

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Coatingswhere 2020L is, the10, x evaluation FOR PEER REVIEW length, x is each of measured points on the evaluation length, and z(x)4is of the 13 profile height function. whereThe is contact the evaluation angle of water length, on the is enamel each of coatings measured was points directly on measured the evaluation on the length, coating and surface of isthe the steel profile substrate height using function. Phoenix 300 ET goniometer (Surface Electro Optics, Suwon, Korea). A syringe withThe a 2 contact mm inner angle diameter of water was on usedthe enamel to supply coatings liquid was into directly the sessile measured drop from on the above coating the surface sample. ofA the sessile steel drop substrate was about using 6–8 Phoenixµm3 in 300 volume ET goniometer and 0.1–0.2 (Surface cm in diameter Electro Optics, at room Suwon, temperature. Korea). The A syringethree-phase with a contact 2 mm inner line of diameter the droplet was used was thento supply drawn liquid automatically into the sessile by computer. drop from Theabove above the sample.procedures A sessile were drop measured was about for fifteen 6–8 μ dropsm3 in volume on the new and surfaces 0.1–0.2 cm of ain single diameter specimen at room and temperature. repeated on Thea total three of‐ threephase specimens. contact line All of readings the droplet were was then then averaged drawn forautomatically each condition. by computer. The above proceduresThe microstructure were measured and for the fifteen elemental drops on analysis the new of surfaces the coatings of a single were performedspecimen and by repeated means of onfield a total emission of three scanning specimens. electron All readings microscopy were (FESEM, then averaged JSM 7610F, for each JEOL, condition. Tokyo, Japan) coupled with energy-dispersiveThe microstructure X-ray and spectroscopy the elemental (EDS). analysis of the coatings were performed by means of field emission scanning electron microscopy (FESEM, JSM 7610F, JEOL, Tokyo, Japan) coupled with energy2.3. Contamination‐dispersive X on‐ray Enamel spectroscopy Coating and (EDS). Cleaning Test The burned contamination is applied on the enamel coating using a mixture of foods, which are 2.3. Contamination on Enamel Coating and Cleaning Test specified in Table3. The 1g mixture of food was pasted on a 50 mm 70 mm area on the enamel scale. × TheThe pasted burned enamel contamination plate was kept is applied in a 250 on◦ Cthe electric enamel oven coating for one using hour, a mixture then cooled of foods, down which to room are specifiedtemperature. in Table The 3. entirely The 1g mixture cooled down of food samples, was pasted Figure on2 aa, 50 were mm then× 70 mm immersed area on in the 60 enamel◦C water scale. for The30 mins. pasted For enamel the cleaning plate was test kept of burnt in a 250 contamination, °C electric oven an abrasion for one rubbinghour, then tester cooled (KP-M4250, down to KIPAEroom temperature.Ent., Suwon, The Korea) entirely was utilized,cooled down where samples, a constant Figure force 2a, was were exerted then immersed on the sample in 60 surface °C water while for the30 mins.sample For moved the cleaning back and test forth of burnt in horizontal contamination, directions. an Aabrasion 60 60 rubbing mm non-scratch tester (KP scouring‐M4250, pad KIPAE was × Ent.,attached Suwon, on Korea) the equivalent was utilized, size of where the steel a constant plate to force aid was homogenize exerted on the the exerted sample forces, surface as while shown the in sampleFigure2 movedb. The sampleback and was forth fixed in on horizontal the abrasion directions. testing A machine 60 × 60 and mm a non 2 kg‐scratch weight scouring was added pad on was top attachedof the axial on the bar. equivalent The cleaning size test of the was steel performed plate to by aid having homogenize the axis the traveling exerted 10 forces, cm back as shown and forth in Figureover the 2b. contaminated The sample was area fixed for 50 on cycles, the abrasion followed testing by a wash-out machine and of debris a 2 kg with weight flowing was water.added Theon topspecimen of the axial was then bar. driedThe cleaning for 10 mins test in was a 100 performed◦C electric by oven. having the axis traveling 10 cm back and forth over the contaminated area for 50 cycles, followed by a wash‐out of debris with flowing water. The specimen was thenTable dried 3. List for of 10 food mins and in weight a 100 °C used electric for burnt oven. contamination test. Contents Quantity Units Table 3. List of food and weight used for burnt contamination test. Cherry (Canister) 200 mL Contents Quantity Units Tomato juice 200 mL Cherry (Canister) 200 mL TomatoEgg (yolk) juice 200 6-7 CountsmL EggMilk (yolk) (2%) 6 112‐7 Counts g TapiocaMilk (powder)(2%) 112 5 g g Tapioca (powder) 5 g Pork fat 112 g Pork fat 112 g CheeseCheese 112 112 g g

FigureFigure 2. 2. OpticalOptical images images of of (a (a) )enamel enamel-coated‐coated steel steel substrate substrate after after burnt burnt contamination contamination treatment treatment and and ((bb)) experimental experimental settings settings for for washability washability test. test.

The washability was then calculated by the following equation (Equation (2)), which essentially is the weight portion of contamination removed divided by the weight of contamination initially applied:

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([W1 W0] [W2 W0]) Washability (%) = − − − 100 (2) W1 W0 × − where W0 is the weight of as-coated specimen and W1 is the weight of specimen after burning the food on top of as-coated (burnt contamination). The W2 is the total weight of the test specimen after the cleaning test.

3. Results and Discussions

3.1. Changes in the Fundamental Surface Properties

3.1.1. Roughness

Figure3a shows the result of measuring the surface roughness value, Ra, by filler contents. The effect of substrate roughness on the measured Ra values are neglected because the highly viscous enamel slurry forms a fairly uniform net-thickness of 135 µm, and this thickness is much greater than the surface roughness of the substrate (Ra = 2.9–3.2 µm). For NF, Ra was measured as 1.7 µm. The Ra value of the SiC-added sample did not change significantly in the 0.03 and 0.05 wt % contents but increased to 2.1 µm at the 0.1% content, which is approximately 25% augmentation compared to the NF. On the other hand, ZrO2-added samples showed a slight increase in Ra value at 0.03 wt %, and a minute decrease at 0.05 and 0.1 wt % compared to NF. The particle size of the frit powder used in this work is within range of 30–40 µm, which was melted and sintered during the spraying process, resulting in approximately a 135 µm-thick enamel scale. The melting points of bulk SiC and ZrO2 are 2830 and 1855 ◦C[26], respectively, which are embedded in the enamel without any melting during the coating process. For SiC fillers, the average particle size is 2.3 µm, and it appears that the relatively large SiC particles are encompassed by the enamel coating, causing an effective increase in roughness when the content is sufficiently high (in this case, 0.10 wt %). In contrast, ZrO2, whose average particle size is 510 nm, has no significant effect on roughness for 0.03–0.10 wt %. The small particles are expected to settle in the free volume between the large enamel particles. Furthermore, the roughness of NF (i.e., no filler) is greater than its size. For these reasons, the effect of such sub-micron particles on the contact angle is considered to be negligible. As a matter of fact, the studs of SiC composition were observed on the surface (Supplementary Materials S4), whereas none observed were for ZrO2 in the later results of element mapping via energy-dispersive spectroscopy (not shown in Supplementary Materials as none detected).

3.1.2. Contact Angle The contact angle is a method of conﬁrming the hydrophilicity of the surface, and when the contact angle is high, the cleaning ability of the surface may be superior to that otherwise. Figure3b shows the contact angle results of the enamel surface with SiC and ZrO2 ﬁllers. In the case of SiC, the lowest contact angle was observed at 0.03 wt % content. At 0.05 and 0.1 wt %, the contact angle increased as the content increased. For ZrO2, the contact angle decreased as the content increased at 0.03% and 0.05%. It should be noted that the contact angle reduction is much greater than that of SiC at these contents. At 0.05%, the lowest contact angle was observed, and at 0.1%, a similar value compared to that of NF was observed. Coatings 2020, 10, 121 6 of 13 Coatings 2020, 10, x FOR PEER REVIEW 6 of 13

Figure 3. (a) The surface roughness R and (b) water contact angle of the surface of enamel coatings Figure 3. (a) The surface roughness a and (b) water contact angle of the surface of enamel coatings withwith SiCSiC/ZrO/ZrO22 ﬁllers.fillers.

The roughness of the surface may have influencedinfluenced the contact angle, as the water droplet size is µ approximately 100100 μm,m, soso the the micron-scale micron‐scale roughness roughness can can play play a a role. role. For For the the contact contact angles angles of of SiC, SiC, a similara similar relationship relationship was was observed observed for for the the roughness. roughness. It It seems seems that that the the roughness roughness ofof thethe surfacesurface have influenced Ra and the contact angle; they both decrease at low contents but increase at high contents. influenced and the contact angle; they both decrease at low contents but increase at high contents. Yet,Yet, itit is is premature premature to to conclude conclude that that the the factor factor that that aff affectsects the the contact contact angle angle is merely is merely roughness. roughness. In the In case of ZrO2, the roughness did not change significantly depending on the content, whereas the contact the case of ZrO2, the roughness did not change significantly depending on the content, whereas the anglecontact showed angle showed a similar a similar tendency tendency to SiC. to In SiC. addition, In addition, considering considering that the that roughness the roughness values values of SiC and ZrO2 at 0.03 and 0.05 wt % are within statistical error, it is hard to explain the large decrease in of SiC and ZrO2 at 0.03 and 0.05 wt % are within statistical error, it is hard to explain the large decrease contact angle observed in ZrO2 alone. In summary, for both SiC and ZrO2, the contact angles tended to in contact angle observed in ZrO2 alone. In summary, for both SiC and ZrO2, the contact angles tended decreaseto decrease at 0.03%at 0.03% and and 0.05% 0.05% and and increase increase at 0.1%, at 0.1%, reaching reaching an equivalent an equivalent value value compared compared to NF. to TheNF. roughnessThe roughness of the of enamelthe enamel surface surface likely likely to playto play a role a role in in contact contact angles angles measured, measured, but but it it is is expectedexpected therethere areare other other factors factors influencing influencing the contactthe contact angles. angles. The surface The phasesurface change phase induced change by induced embedded by ceramicembedded particles, ceramic which particles, will which be later will discussed be later discussed in microstructural in microstructural and elemental and elemental analysis, analysis, possibly influencedpossibly influenced the contact the angle contact as well.angle as well. 3.2. Microstructural and Elemental Analysis 3.2. Microstructural and Elemental Analysis The microstructure of the enamel scale is observed by FESEM and EDS, and the results are The microstructure of the enamel scale is observed by FESEM and EDS, and the results are summarized in Figures4 and5. In the EDS analysis, only the elements showed a correlation to surface summarized in Figures 4 and 5. In the EDS analysis, only the elements showed a correlation to surface morphology are presented, which are Al, Ce, Ca, and P. morphology are presented, which are Al, Ce, Ca, and P. For NF (0.00% of Figures4a and5b), aggregates for three elements are observed; Al, Ce, and Ca For NF (0.00% of Figure 4a and Figure 5b), aggregates for three elements are observed; Al, Ce, (P is presented to make comparison with later results in Section 3.4). Al- and Ce-rich aggregates were and Ca (P is presented to make comparison with later results in Section 3.4). Al‐ and Ce‐rich both in spot shapes with diameters approximately 1–4 and 3–7 µm, respectively. On the contrary, the aggregates were both in spot shapes with diameters approximately 1–4 and 3–7 μm, respectively. On Ca-rich aggregates were in a needle-shape with the dimension being approximately 0.5 4 µm. Since the contrary, the Ca‐rich aggregates were in a needle‐shape with the dimension being approximately× none of the elements with atomic number (Z) > 8 showed clear appearance with Ca-rich aggregates, 0.5 × 4 μm. Since none of the elements with atomic number (Z) > 8 showed clear appearance with Ca‐ it is reasonable to presume that these Ca-rich aggregates are compounds with light elements such as rich aggregates, it is reasonable to presume that these Ca‐rich aggregates are compounds with light H, C, and O. In the further discussion, these aggregates will be notated as ‘X-phase’, where X is the elements such as H, C, and O. In the further discussion, these aggregates will be notated as ‘X‐phase’, element (e.g., Ca-phase). It is noteworthy that the distributions of Al and Ce/P elements were observed where X is the element (e.g., Ca‐phase). It is noteworthy that the distributions of Al and Ce/P elements mutually exclusive (i.e., Al depleted in a Ce- and P-rich region, and vice versa), but the distribution of were observed mutually exclusive (i.e., Al depleted in a Ce‐ and P‐rich region, and vice versa), but Ca element was found independent of Al, Ce, and P. the distribution of Ca element was found independent of Al, Ce, and P. When ceramic fillers are added, the change in surface crystallography is observed, as illustrated When ceramic fillers are added, the change in surface crystallography is observed, as illustrated in Figure4. At SiC contents of 0.03%, the distribution of Al-phases increases in number while being in Figure 4. At SiC contents of 0.03%, the distribution of Al‐phases increases in number while being equivalent in size. For Ce and Ca aggregates, the shape and number were similar to those observed in equivalent in size. For Ce and Ca aggregates, the shape and number were similar to those observed NF. When SiC contents are increased to 0.05%, both Al and Ce aggregates decreased both in size and in NF. When SiC contents are increased to 0.05%, both Al and Ce aggregates decreased both in size number. Furthermore, in the case of Ca, the needle-shape phases were no longer observed. In SiC and number. Furthermore, in the case of Ca, the needle‐shape phases were no longer observed. In SiC 0.1%, it can be seen that the number of Al and Ce aggregates decreased notably. Ca‐phases were no longer in a needle‐shape; they were in the spot shapes.

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Coatings0.1%, it 2020 can, 10 be, x seen FOR PEER that theREVIEW number of Al and Ce aggregates decreased notably. Ca-phases were7 of no 13 longer in a needle-shape; they were in the spot shapes.

Figure 4. FESEM images and corresponding EDS analysis (Al, Ce, Ca, and P) for enamel-coated surface Figure 4. FESEM images and corresponding EDS analysis (Al, Ce, Ca, and P) for enamel‐coated with the addition of SiC at (a) low and (b) high magniﬁcation. surface with the addition of SiC at (a) low and (b) high magnification.

As for ZrO2 (Figure5), a similar tendency was observed; the Al-phase and Ce-phase became As for ZrO2 (Figure 5), a similar tendency was observed; the Al‐phase and Ce‐phase became smaller and less frequent as the filler content increased. On the contrary, the needle-shape Ca-phase smaller and less frequent as the filler content increased. On the contrary, the needle‐shape Ca‐phase appeared up to 0.05 wt % and diminished at 0.1 wt %. At 0.1 wt %, no visible needle-shape configuration appeared up to 0.05 wt % and diminished at 0.1 wt %. At 0.1 wt %, no visible needle‐shape is observed in the FESEM images, and EDS analysis also indicates that there is no clear Ca-rich phase configuration is observed in the FESEM images, and EDS analysis also indicates that there is no clear with a high aspect ratio. Instead, they seemed to be in an intermediate state since the Ca-phases are Ca‐rich phase with a high aspect ratio. Instead, they seemed to be in an intermediate state since the found in either spot or short needle. It may be argued that the shape is changed, but there is also a Ca‐phases are found in either spot or short needle. It may be argued that the shape is changed, but possibility that the growth of the crystal is inhibited due to the presence of the fillers, and the crystal there is also a possibility that the growth of the crystal is inhibited due to the presence of the fillers, may exist in a small size. and the crystal may exist in a small size. With the observed changes in the surface chemistry, the contact angle change with respect to the With the observed changes in the surface chemistry, the contact angle change with respect to the filler contents can be further discussed. As stated in the previous section, changes in contact angles filler contents can be further discussed. As stated in the previous section, changes in contact angles cannot be understood solely by roughness. Although the exact influencing chemical compositions cannot be understood solely by roughness. Although the exact influencing chemical compositions cannot be identified, due to the experimental challenges, some implications can be drawn. When cannot be identified, due to the experimental challenges, some implications can be drawn. When comparing 0.03% and 0.05% to NF, there is a significant increase in Al-phase and a decrease in the comparing 0.03% and 0.05% to NF, there is a significant increase in Al‐phase and a decrease in the number density of Ce-phase. It can be inferred that the increased Al-phase and decreased Ce-phase number density of Ce‐phase. It can be inferred that the increased Al‐phase and decreased Ce‐phase contribute to decreasing the contact angle. At 0.1% SiC and ZrO2, the surface change (non-needle-shape contribute to decreasing the contact angle. At 0.1% SiC and ZrO2, the surface change (non‐needle‐ Ca-phase such as spot-shape or none) may have increased the contact angle. These implications are yet shape Ca‐phase such as spot‐shape or none) may have increased the contact angle. These implications are yet speculative, but more importantly, they provide a case that the surficial changes in crystallography might be determined by other properties such as contact angle. From the microscopic and elemental analysis, it was confirmed that the distribution and shape of the crystal phases on the enamel surface depend on the content of ceramic filler. The addition of

Coatings 2020, 10, 121 8 of 13 speculative, but more importantly, they provide a case that the surﬁcial changes in crystallography might be determined by other properties such as contact angle.

CoatingsFrom 2020 the, 10, x microscopic FOR PEER REVIEW and elemental analysis, it was confirmed that the distribution and shape8 of 13 of the crystal phases on the enamel surface depend on the content of ceramic filler. The addition of fillersfillers notnot onlyonly causecause thethe changeschanges inin roughness oror suppressionsuppression ofof crystalcrystal growth,growth, butbut changeschanges inin locallocal chemistrychemistry thatthat completelycompletely altersalters surfacesurface crystallography.crystallography. Moreover, drasticdrastic changeschanges werewere observedobserved atat 0.1 wt %, %, where where the the Al Al and and Ce Ce phases phases decrease decrease in inboth both number number and and size, size, and and the theneedle needle-shape‐shape Ca‐ Ca-phasesphases are areno longer no longer observed. observed.

Figure 5. FESEM images and corresponding EDS analysis (Al, Ce, Ca, and P) for enamel-coated surface Figure 5. FESEM images and corresponding EDS analysis (Al, Ce, Ca, and P) for enamel‐coated with the addition of ZrO2 at (a) low and (b) high magnification. surface with the addition of ZrO2 at (a) low and (b) high magnification. 3.3. Washability of Enamel Coatings and Its Relation to Surface Properties 3.3. Washability of Enamel Coatings and Its Relation to Surface Properties The effect of SiC and ZrO2 particles on enamel washability is examined and summarized in TableThe4 along effect with of SiC the and previous ZrO2 particles roughness on and enamel contact washability angle results. is examined In Figure and6, summarized the results from in Table the contamination4 along with the cleaning previous test areroughness shown. Forand both contact fillers, angle 0.03 results. and 0.05 In wt Figure % showed 6, the a minuteresults decrease,from the orcontamination negligible change cleaning in washabilitytest are shown. compared For both to fillers, NF; for 0.03 SiC and (ZrO 0.052 )wt contents % showed of 0.03 a minute and 0.05 decrease, wt %, 3.7%or negligible (2.8%), and change 3.3% in (5.4%), washability respectively. compared The di toff erenceNF; for in SiC washability (ZrO2) contents between of NF 0.03 and and low 0.05 contents wt %, (0.033.7% and(2.8%), 0.05 and wt %) 3.3% was (5.4%), within therespectively. statistical error.The difference On the other in hand,washability for 0.1 between wt % of SiC NF and and ZrO low2, thecontents washability (0.03 and increased 0.05 wt by %) 16.3% was within and 21.3% the comparedstatistical error. to NF. On It is the obvious other thathand, the for content 0.1 wt beyond % of SiC a certainand ZrO amount2, the washability indeed increases increased the washability. by 16.3% and 21.3% compared to NF. It is obvious that the content beyond a certain amount indeed increases the washability.

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Coatings 2020Table, 10, 4.x FORA summary PEER REVIEW of evaluated properties for ceramic ﬁller (SiC or ZrO2)-added enamel scales.9 of 13

Filler Type Contents (wt %) Contact Angle ( ◦ ) Roughness, Ra (µm) Washability (%) Table 4. A summary of evaluated properties for ceramic filler (SiC or ZrO2)‐added enamel scales. No Filler (NF) 0.00 62.4 2.7 1.67 0.13 80.9 2.1 ± ± ± Filler Type Contents (wt0.03 %) Contact 52.8Angle 1.4( ˚ ) Roughness, 1.62 0.13 (μm) Washability 77.9 3.5 (%) ± ± ± No Filler (NF)SiC 0.00 62.4 ± 2.7 1.67 ± 0.13 80.9 ± 2.1 0.05 55.6 1.5 1.70 0.20 78.2 1.8 0.03 52.8 ± 1.4± 1.62 ±± 0.13 77.9± ± 3.5 0.10 64.0 1.7 2.10 0.20 94.1 2.4 SiC 0.05 55.6 ± 1.5± 1.70 ±± 0.20 78.2± ± 1.8 0.10 0.0364.0 46.9 ± 1.7 4.4 1.812.10 ± 0.200.15 78.694.14.7 ± 2.4 ± ± ± ZrO 2 0.03 0.0546.9 45.6 ± 4.4 2.1 1.451.81 ± 0.150.17 76.578.68.0 ± 4.7 ± ± ± ZrO2 0.05 45.6 ± 2.1 1.45 ± 0.17 76.5 ± 8.0 0.10 61.2 2.8 1.55 0.14 98.1 1.0 0.10 61.2 ± 2.8± 1.55 ±± 0.14 98.1± ± 1.0

Figure 6. The washability of burnt contamination on enamel coatings with different contents of Figure 6. The washability of burnt contamination on enamel coatings with different contents of SiC/ZrO2 fillers. SiC/ZrO2 fillers. Based on the previous results, the mechanism responsible for the change in the washability is Based on the previous results, the mechanism responsible for the change in the washability is expected to be a combined effect of roughness and surface chemistry. However, the mechanisms expected to be a combined effect of roughness and surface chemistry. However, the mechanisms might be different for two fillers. For SiC additions, it is considered that roughness contributes to might be different for two fillers. For SiC additions, it is considered that roughness contributes to the the increased washability when the filler is sufficiently added (about 0.1). The chemical composition increased washability when the filler is sufficiently added (about 0.1). The chemical composition change of the surface further affects the washability (needle-shape Ca-phase completely disappeared at change of the surface further affects the washability (needle‐shape Ca‐phase completely disappeared 0.1 wt %). The ZrO2 particles, whose particle size is an order smaller than SiC and two orders smaller at 0.1 wt %). The ZrO2 particles, whose particle size is an order smaller than SiC and two orders than enamel powder, is simply sandwiched between enamel powders and show a minute change in smaller than enamel powder, is simply sandwiched between enamel powders and show a minute surface roughness. Thus, in this case, the increased washability at 0.1 wt % is attributed to surface change in surface roughness. Thus, in this case, the increased washability at 0.1 wt % is attributed to compositions produced or changed by the addition of ZrO2. Therefore, in the case of SiC, both the surface compositions produced or changed by the addition of ZrO2. Therefore, in the case of SiC, both morphology and the composition of the crystals on the surface have effects on the washability, and in the morphology and the composition of the crystals on the surface have effects on the washability, the case of ZrO2, the composition is more influential. and in the case of ZrO2, the composition is more influential. Interestingly, the 0.1 wt % content, which showed improved washabilities for both SiC and Interestingly, the 0.1 wt % content, which showed improved washabilities for both SiC and ZrO2, ZrO , is consistent with the filler content at which drastic changes occurred in the microstructure is consistent2 with the filler content at which drastic changes occurred in the microstructure analysis. analysis. As the distribution of surface crystal phases changes with varying filler contents, one can As the distribution of surface crystal phases changes with varying filler contents, one can conclude conclude the following: the shape and composition of the crystals on the surface affect the easiness the following: the shape and composition of the crystals on the surface affect the easiness of washout of washout of burnt contamination, which means that certain crystal phases have a higher chemical of burnt contamination, which means that certain crystal phases have a higher chemical or or mechanical coherency to burnt contamination than other crystal phases. In order to fortify the mechanical coherency to burnt contamination than other crystal phases. In order to fortify the aforementioned statement, we further investigated the surface of enamel coating after fully removing aforementioned statement, we further investigated the surface of enamel coating after fully removing the burnt contamination in the following section in order to support such effects of surface crystals the burnt contamination in the following section in order to support such effects of surface crystals discussed above. discussed above. 3.4. Surface Change in Enamel Coating After Burnt Contamination Cleaning 3.4. Surface Change in Enamel Coating After Burnt Contamination Cleaning To evaluate the coherency of burnt contamination on the enamel coatings, we have compared the surfaceTo evaluate morphology the coherency after wiping of burnt out the contamination burnt contamination. on the enamel An identical coatings, abrasion we have testing compared machine the surface morphology after wiping out the burnt contamination. An identical abrasion testing machine used in the cleaning test is again used, with the increased number of cycles, 200 times. After the wipe‐out, no visible residue of burnt contamination is observed. Then the surface microstructure is analyzed in the same manner as in Figure 7.

Coatings 2020, 10, 121 10 of 13 used in the cleaning test is again used, with the increased number of cycles, 200 times. After the wipe-out, no visible residue of burnt contamination is observed. Then the surface microstructure is analyzedCoatings 2020 in, 10 the, x sameFOR PEER manner REVIEW as in Figure7. 10 of 13

FigureFigure 7.7. TheThe surfacesurface ofof SiCSiC 0.030.03 wtwt %-added %‐added enamel enamel coating coating after after the the washability washability test. test. ((aa)) FESEMFESEM imageimage andand ((bb)) imageimage overlaidoverlaid withwith Al,Al, Ce,Ce, Ca,Ca, andand P. ((cc-‐ff)) elementalelemental mapmap ofof eacheach elementelement (Al,(Al, Ce,Ce, Ca, andand P).P).

SimilarSimilar to to NF NF without without burnt burnt contamination, contamination, the needle the needle-shape‐shape of Ca‐phase of Ca-phase is observed is (circled observed in (circledFigure 7e). in FigureHowever,7e). a However, change in aphosphate change in distribution phosphate is distribution observed at is the observed burnt contamination at the burnt ‐ contamination-removedremoved region; phosphate region; is concentrated phosphate is along concentrated with the alongdistribution with the of distributionCa‐phase, which of Ca-phase, was not whichthe case was for not NF the (Figure case for 4). NF This (Figure infers4). that This the infers needle that‐shape the needle-shape of Ca‐phase of is Ca-phase attracted is to attracted phosphate to phosphateduring the during burning the process burning of processcontamination. of contamination. Phosphate Phosphate is prevalent is prevalentin the daily in diet, the daily and of diet, course, and ofwas course, included was includedin the organ in the meat, organ milk, meat, and milk, eggs, and which eggs, were which used were for used the forfood the contamination food contamination in this inwork. this work.Therefore, Therefore, the Ca the‐phase Ca-phase is believed is believed to tocapture capture phosphate phosphate during during the the carbonization ofof contaminants.contaminants. In Figure8 8,, an an illustrative illustrative comparison comparison is is presented presented to to depict depict how how needle-shape needle‐shape and and spot-shapespot‐shape Ca-phases Ca‐phases react react di differentlyfferently with with the the phosphate phosphate in in the the food food paste. paste. TheThe phosphatephosphate cancan eithereither formform epitaxialepitaxial crystalcrystal structuresstructures onon toptop ofof thethe Ca-phaseCa‐phase oror growthgrowth onon thethe Ca-phase,Ca‐phase, formingforming calciumcalcium phosphates such as hydroxylapatite (Ca (PO ) (OH)). phosphates such as hydroxylapatite (Ca55(PO44)3(OH)). OneOne hypothesis hypothesis is thatis thethat Ca the oxalate Ca phases,oxalate whose phases, decomposition whose decomposition temperature istemperature approximately is 200approximately◦C, became 200 chemically °C, became active chemically during food active burning during at food 250 burning◦C then reactedat 250 °C with then phosphates reacted with to formphosphates hydroxylapatite. to form hydroxylapatite. The presence of carbon The presence in the enamel of carbon scale isin verifiedthe enamel by comparing scale is verified the carbon by contentscomparing in NFthe coating carbon to contents that in thein NF original coating frit powderto that in (Supplementary the original frit Material powder S7). (Supplementary In an attempt toMaterial understand S7). In the an attempt crystal structureto understand for Ca-phase the crystal in structure the as-coated for Ca and‐phase after-contaminated in the as‐coated and regions, after‐ wecontaminated proceeded withregions, the XRDwe proceeded analysis. Itwith is worth the XRD mentioning analysis. that It theis deconvolutingworth mentioning peaks that of thethe multi-elementdeconvoluting polycrystalline peaks of the multi phase‐element and interpreting polycrystalline them phase is a challenge and interpreting to achieve them reliable is a challenge results. Theto achieve XRD results reliable are results. presented The inXRD Supplementary results are presented Materials in S8, Supplementary although identifying Materials the S8, crystalline although phasesidentifying was ditheffi cultcrystalline for such phases cases. was Based difficult on the for EDS such analysis, cases. itBased is suggested on the EDS that theanalysis, Ca-phase it is beforesuggested contamination that the Ca‐ (needle-shape)phase before contamination is calcium oxalate, (needle and‐shape) the is phase calcium becomes oxalate, hydroxylapatite and the phase (Ca (PO ) (OH)) after being exposed to high temperatures with plenty of phosphate sources. Albeit becomes5 4 hydroxylapatite3 (Ca5(PO4)3(OH)) after being exposed to high temperatures with plenty of thephosphate crystal structures sources. Albeit playing the a crystal role need structures to be identified playing in a role further need studies, to be identified it clearly shows in further the ceramicstudies, fillerit clearly altered shows the surfacethe ceramic crystallography, filler altered which the surface in turn crystallography, affected the washability which in of turn enamel affected coatings. the Givenwashability these points,of enamel the Ca-phasecoatings. Given chemically these attracts points, the the phosphate Ca‐phase duringchemically the contaminationattracts the phosphate process andduring makes the the contamination removal of contamination process and dimakesfficult, the thus removal the demoting of contamination growth of a specificdifficult, phase thus (the the needle-shapedemoting growth Ca-phase of a specific in the present phase (the study) needle might‐shape be useful Ca‐phase to be in considered the present as study) a design might criterion be useful for non-stickto be considered or easy-cleaning as a design enamel criterion coatings. for non‐stick or easy‐cleaning enamel coatings.

Coatings 2020, 10, 121 11 of 13 Coatings 2020, 10, x FOR PEER REVIEW 11 of 13

FigureFigure 8. 8. AnAn illustration illustration depicting depicting the difference the difference in the in chemical the chemical reaction reaction with phosphate with phosphate (a) needle (a‐) b shapeneedle-shape and (b) spot and‐ (shape) spot-shape Ca‐phases. Ca-phases. The enamel The scale enamel (blue) scale The (blue) Ca‐phases The Ca-phases are represented are represented with red, with red, and the phosphate distribution is shown with white color. (a) Phosphates are distributed and the phosphate distribution is shown with white color. (a) Phosphates are distributed along the along the Ca-phases; (b) phosphates are distributed independently from the Ca-phases. Ca‐phases; (b) phosphates are distributed independently from the Ca‐phases. 4. Conclusions 4. Conclusion The ceramic particles of SiC and ZrO2 are embedded as fillers in the enamel coating with varying the contentsThe ceramic of 0.03, particles 0.05 and of 0.1SiC wt and % toZrO investigate2 are embedded the effect as offillers ceramic in the fillers enamel on the coating surficial with properties varying theand contents further impact of 0.03, on 0.05 the and washability. 0.1 wt % The to followinginvestigate are the the effect key conclusions of ceramic fromfillers the on present the surficial work. properties and further impact on the washability. The following are the key conclusions from the presentThe work. presence of SiC/ZrO2 filler not only changes surface roughness but also changes the surface • ⦁crystallography, The presence whichof SiC/ZrO2 in turn filler influences not only the contactchanges angle surface and roughness washability. but also changes the Forsurface the enamel crystallography, powers with which sizes ofin 30–40 turn influencesµm, 3 µm SiCthe contact powders angle increase and thewashability. surface roughness • ⦁when For su thefficient enamel amount powers is added. with Insizes contrast, of 30–40 a small μm, powder 3 μm SiC of 500 powders nm ZrO increase2 infiltrates the the surface voids between enamel powders and causes a negligible change in the roughness. roughness when sufficient amount is added. In contrast, a small powder of 500 nm ZrO2 The contact angle is affected by the roughness and the chemical composition of the surface. Both • roughnessinfiltrates and the chemical voids between composition enamel played powders a role forand SiC, causes while a onlynegligible chemical change composition in the playedroughness. a major role for ZrO2. ⦁Both The roughness contact angle and morphology is affected by of the the roughness surface are and responsible the chemical for the composition change in theof the washability. surface. • TheBoth washability roughness is notably and chemical augmented composition at 0.1 wt %,played which a corresponds role for SiC, to thewhile content only wherechemical the drastic microstructure change (and the contact angle) occurred. composition played a major role for ZrO2. The changes in the surface crystallography are observed with the presence of the fillers. Al, Ce, • ⦁ Both roughness and morphology of the surface are responsible for the change in the Ca, and P aggregates showed significant changes (see main text for each change). Thewashability. Ca aggregates The waswashability observed is innotably a needle-shape augmented in at NF 0.1 and wt 0.03-0.05%, which wt corresponds %. This phase to the is • reactivecontent with where phosphates the drastic in the microstructure contamination change during (and the firingthe contact process angle) and makes occurred. the removal of ⦁contamination The changes more in the di surfacefficult. crystallography are observed with the presence of the fillers. Al, Ce, Ca, and P aggregates showed significant changes (see main text for each change). Throughout this study, the results presented in this paper can provide a useful guide in designing an easy-cleaning⦁ The Ca aggregates enamel coating. was observed The washability in a needle is‐shape notably in augmented NF and 0.03 when‐0.05 thewt %. fillers This are phase added is more thanreactive a particular with phosphates amount (in ourin studythe contamination 0.1 wt %), where during a drastic the microstructurefiring process changeand makes occurred. the A drasticremoval change of can contamination be verified by more EDS difficult. analysis methods presented in this study, but also might be guidedThroughout by measuring this roughnessstudy, the and results contact presented angles which in this are paper relatively can less provide time- anda useful cost-consuming guide in designingmethods. an In easy addition,‐cleaning we enamel have identified coating. aThe certain washability phase (needle-shapeis notably augmented Ca-phases when in thisthe fillers work) areimpedes added removal more than of the a particular contamination. amount Suppression (in our study of such 0.1 crystal wt %), formation where a drastic needs to microstructure be considered changefor easy-cleaning occurred. A enamel drastic coatings. change can be verified by EDS analysis methods presented in this study, butSupplementary also might be Materials: guided Theby measuring following are roughness available online and contact at http: //angleswww.mdpi.com which are/2079-6412 relatively/10 /less2/121 time/s1. ‐ and cost‐consuming methods. In addition, we have identified a certain phase (needle‐shape Ca‐ Author Contributions: Conceptualization, H.K., S.L., and H.M.L.; methodology, H.K. and S.L.; validation, H.K., phasesS.-J.L., and in this J.-J.O.; work) formal impedes analysis, removal H.K.; investigation, of the contamination. H.K. and S.L.; Suppression resources, S.-J.L. of such and J.-J.O.; crystal data formation curation, needsH.K.; writing—original to be considered draft for preparation,easy‐cleaning H.K.; enamel writing—review coatings. and editing, H.K.; supervision, S.L. and H.M.L.; projectSupplementary administration, Materials: H.M.L.; funding The following acquisition, are H.M.L. available All authors online have at www.mdpi.com/xxx/s1. read and agreed to the published version of the manuscript. Author Contributions: Conceptualization, H.K., S.L., and H.M.L.; methodology, H.K. and S.L.; validation, H.K., Funding: This research was funded by Technology Development Program (S2428572) funded by the Ministry of S.SMEs‐J.L., and StartupsJ.‐J.O.; formal (MSS, analysis, Korea). H.K.; investigation, H.K. and S.L.; resources, S.‐J.L. and J.‐J.O.; data curation, H.K.; writing—original draft preparation, H.K.; writing—review and editing, H.K.; supervision, S.L. and H.M.L.;

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Conﬂicts of Interest: The authors declare no conﬂict of interest to enclose.

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