7.1 Introduction

Porcelain enamel is an inorganic coating of a vitreous nature that can be applied to both glass and metallic substrates, such as steel and aluminum alloys, using different techniques. Enamel is a term of medieval origin derived from the Germanic smaltjan, which means melt. This coating, in fact, is produced from vitreous flakes which, with the addition of pigments and additives, are fired at relatively high temperatures (between 780 and 850°C, in the case of steel substrate). Then, as a matter of fact, enamel is essentially glass and does not require the application of any kind of toxic substances, such as solvents or other chemical components.¹

During the heat treatment there is fusion of the frit flakes and solubilization of most of the oxides and compounds added during the mill grinding. The vitreous flakes, called frit, are obtained by the melting, rapid cooling, and subsequent grinding with the other materials that constitute the glass. Porcelain enamel is an ancient material. Due to its characteristics of brightness and durability this material has been used for many centuries in artistic fields for manufacturing jewels, art objects, religious items, and other pieces, owing to its attractive appearance with brilliant colors. With the advent of the industrial revolution in the eighteenth century, the use of enamel began to spread, and it was used in several products on an industrial scale, such as household goods, furnishings, and interior decoration items.² In recent decades vitreous enamel has been used both for decorative purposes and for protection against corrosion and for its engineering properties. Today there is a tendency to try to combine in a material, and particularly in the coating, more properties of different natures. These coatings are therefore called intelligent or smart materials or coatings. Even the enamels have followed this trend. In recent years, the research has tried to combine in these layers different functional properties, such as antibacterial properties and optical properties with corrosion and high temperatures.

The research to produce luminescent enamels, the object of this chapter, has the purpose of bringing together the excellent functional and aesthetic properties of this traditional type of coating with the luminescence characteristic, which could lead to new uses of the enamel. Luminescent materials, such as polymers, organic coatings, and inks are already on the market; however, these products are based on pigments that do not show corrosion resistance at high temperatures. In addition these materials do not often present resistance to natural weathering, UV radiation, and chemicals.

The luminescent enamel could have interesting commercial applications (components of outdoor furniture and interior design, architecture, applications in the safety field). Applications in fields in which luminescent paints are not suitable could be very interesting. Luminescent enamel is a not material that is much studied. In the literature, there are very few references to this material. The properties of these innovative materials are not well studied, especially considering their resistance to weathering. The purpose of this work is to study and characterize some types of luminescent porcelain enamel considering both microstructural and the luminescence properties.

To consider several possible applications, it is necessary to investigate the properties of the material using tests that simulate different types of aggressive environments in order to check the maintaining of the luminescent properties during exposure to aggressive environments and whether the addition of luminescent pigments could reduce or modify the excellent properties of traditional porcelain enamel. In addition, it is not known if the heat treatment of enamel curing could modify the behavior of luminescent pigments.

7.2 The Most Important Properties of Vitreous Enamel

Porcelain enamel presents numerous properties that allow many applications in different fields. This type of layer is one of the best methods to guarantee corrosion protection in numerous environments due to its glassy nature, the absence of defects, and its high thickness and excellent adhesion. Figure 7.1 shows the typical cross-section of an enamel layer.

The excellent adhesion is obtained during the process of curing. During the thermal treatment the oxidation of the iron of the substrate happens with the formation of oxides with a dendritic growth. These physical-chemical reactions occurring at the interphase enamel/metal guarantee a strong adherence between the two materials, thus providing the enamel/metal system with good mechanical properties and excellent corrosion resistance.

The firing temperature is between 760 and 870 °C. During this process the different components interact in enamel glass, gas bubbles can be produced, and the layer surface is leveled. Then, a diffusion process of the melted frit is present, obtaining excellent adhesion properties thanks to the presence of oxides including cobalt.^3,4

From Figure 7.2 it possible to observe the typical porosity formed during the curing process due to the development of gas and water vapor. The porosity remains limited to the layers closest to the substrate. In addition, this porosity is not interconnected and then the contact between aggressive environment and metallic substrate is avoided. Due to this aspect and to chemical inertia of the glass matter, this coating shows excellent corrosion protection. Thanks to the glassy nature, the coating is resistant to most of the solutions with the exception of strong basic solutions of fluoridric acid. The enamel shows also resistance to high temperature and it is easily cleanable.^3–5

The glassy nature of the matrix and of the colored pigments produces an excellent resistance to UV radiation with the maintaining of the color.⁴

The hardness also provides good resistance to mechanical damage, the exclusion of impulsive collisions, due to the brittleness of the glassy matter. This coating, in addition to good protection properties, shows very interesting aesthetic qualities connected with its vitreous nature. Its smooth surface provides high gloss and a great color palette with different color shades and textures with special effects available.^2,4

By modifying the frits chemical composition it is possible to control the properties of the layers, improving some aspects. For example, potassium feldspar and zirconium silicate are added in the frit composition to promote the pyrolytic properties at high temperatures and to improve flame resistance, avoiding unaesthetic surface changes.

Usually, in order to guarantee a very good adhesion a groundcoat enamel ensures the adhesion between the coating and the substrate; the followed covercoat enamels provide different surface properties.¹

7.3 Luminescent Properties

Luminescent materials have the characteristic of emitting light when excited by cathode rays, UV radiation, or visible light.

The luminescence is a physical process derived from electronic transitions: only some materials exhibit this phenomenon, such as phosphors.⁶

Usually the luminescent material is formed by a matrix doped with activators, which can be a transition metal or rare earth ions that act as emitting centers. When the material is irradiated with a suitable wavelength, the activators absorb the energy, reaching an electronic excited state. The absorbed energy must equal to the distance between the two energy levels. This condition is unstable, and the system goes back to the ground state by releasing the energy in the form of radiation emission.⁷

To obtain materials that show at the same time luminescent properties and not radioactive or toxic aspects, the addition of rare earths to the oxide matrix is very frequent. The optical excitation of rare earth ions occurs by means of transitions between the 4f orbitals or form 4f to 5d levels. Some tetravalent ions (Ce^4 +, Pb^4 +, Tb^4 +) and some trivalent ions (Dy^3 +, Eu^3 +) have transitions between 4f levels; while other trivalent (Ce^3 +, Pb^3 +) and bivalent (Sm^2 +, Yb^2 +, Eu^2 +) ions have transitions between 4f and 5d levels.⁷

Luminescent materials, which exhibit the phenomenon of the afterglow, are particularly interesting. In fact, in these materials the luminescence can be observed for a long time after the end of the excitatory pulse. This phenomenon is possible by the storage of excitation energy in the lattice for a certain time period.⁷ This category includes the long-afterglow phosphors, which have a luminescence decay time ranging from seconds to minutes.

Considering rare earths, a phosphor can be obtained by exploiting the electronic transitions in the energy levels of europium and dysprosium ions, in particular the electronic transfer from Eu^2 + to Dy³ during arousal and from Dy^3 + a Eu^2 + during the release.⁶ At this type of luminescent pigment often strontium is added, which helps to increase the emission intensity.⁸

7.4 Luminescent Porcelain Enamel Coatings

In this chapter luminescent porcelain enamel is considered, which emits a light radiation after being irradiated with UV or visible light due to the adding of pigments containing europium and dysprosium.

The luminescence properties of these pigments is ensured by the electronic transitions of europium ions, while the presence of dysprosium ions is fundamental to obtain a long afterglow. In particular the latter ions act as hole traps capturing the gaps formed by the excitement of the europium ions.⁹

In europium and dysprosium doped matrices, Dy^3 + is indispensable to obtaining a long luminescence decay time, and, in the same time, its concentration influences the emission spectra. This has been verified by Song et al.¹⁰ in Al₂O₃-SiO₂ glasses doped with Eu and Dy, where an increasing of Dy concentration leads to an increase in green emission effect, which was attributed to the presence of defects in the lattice.

7.5 Materials and Experimental Procedures

Low-carbon steel panels (10 × 10 cm) were coated with different luminescent vitreous enamel systems made of three layers. The first was the ground layer, which functioned to guarantee good corrosion protection and adhesion of the coating;³ the second one guarantees corrosion protection; and the top layer shows the luminescent properties. The coatings are obtained by wet spraying in an industrial laboratory (Wendel Email Italia, Chignolo d’Isola—BG). Drying and firing treatments at a temperature of 850 °C were carried out (three applications and two firing processes).

The first two layers present a typical commercial composition based on SiO₂, B₂O₃, Na₂O, and other oxides; TiO₂ was added in the intermediate layer as an opacifier agent. The total thickness of both layers together is 220 μm.

To produce the luminescent deposit a standard frit was used with an adding of 50% wt. of luminescent pigments, containing europium and dysprosium oxides, and of 1% wt. of visible color pigments. The other components of the frit were typical of enamel with the presence of Al₂O₃, SiO_2, SrO, MgO, CaO, and B₂O₃.

The luminescent pigments are made of a vitreous matrix, 50% wt. Al₂O₃, 25-30% SiO₂, 30% wt. SrO, 10-15% wt. MgO, 2% wt. CaO, 1-2% wt. B₂O₃ wt. with the addition of Eu₂O₃ and Dy₂O₃.

To permit the wet application to the frits are added 5% wt. clay, 0.5% wt. sodium aluminate, and 50% wt. water, producing the necessary slurry.

Three different types of samples are investigated considering the visible color and the luminescent effect produced with different Eu/Dy ratios, as shown in Table 7.1 In addition the dimension of luminescent pigments used for samples M1 and M2 are smaller than those used for sample M3.

The microstructural characterization was carried out on both the cross-section and the surface using an ESEM Philips XL30 microscope. The typical properties of traditional enamel, such as corrosion protection, resistance of color, and gloss and abrasion resistance, were evaluated to observe if the presence of luminescent can affect these properties. The luminescent properties of the different layers and their persistence during accelerated testing were also evaluated. Luminescent characteristics were evaluated measuring the emission spectrum (excitation wavelength of 350 nm) and excitation spectra (emission wavelength 500 nm), using a Jasco spectrofluorimeter FP6300. To evaluate the stability of the properties, different accelerated tests were then carried out, normally performed on coatings and enamels. An exposure to UV-A radiation for 500 h, following ASTM G154 standard, was made. Before and after the exposure the color was measured using a spectrometer CM-2600d Konica Minolta with illuminant D65/10° following the international method CIELab.^11–13 Several parameters were considered. The total color differences ΔE* before and after the test was calculated as follows in Equation (7.1):

$\Delta E*={\left({\left(\Delta L\right)}^2+{\left(\Delta a\right)}^2+{\left(\Delta b\right)}^2\right)}^{1/2}$

where L is the lightness (0 is black, and 100 is white), a is the red-green variation (positive values are red, negative values are green, and 0 is neutral), and b is the yellow-blue variation (positive values are yellow, negative values are blue, and 0 is neutral).

To check the endurance of the luminescent properties in case of cyclic stimulus, 24 cycles of alternate radiation exposure, consisting in 1 h of UV-A exposure followed by an hour without irradiation, were carried out. During this test, the performance of the luminescence as a function of time was continuously monitored using an Ocean Optics Spectrometer USB4000. To collect the emission, a 600 μm silica optical fiber was fixed in front of the sample. As reference sample, the light reflected from a white, not luminescent enamel was measured during the cycles. The spectra and the signal as a function of the time was analyzed by means of SpectraSuite. During the cycling test the intensity of the maximum peak of the spectrum (associated with a particular wavelength) in function of time test was monitored with a measurement every minute.

From the emission spectrum, detected by the optical fiber, was subtracted the reflected spectrum of the UV-A lamps from the white enamel in order to obtain as a result only the emission spectrum of the luminescent enamel. To evaluate the chemical resistance, the samples were subjected to both acid and alkaline attack. For the acid resistance test, the enamel specimens were immersed for 24 h in a 10%wt. citric acid solution at room temperature at around pH 2, following ASTM C282. For the alkaline solution resistance test, the samples were immersed in a 52.64 g/l solution of tetra potassium pyrophosphate at pH 10 for 6 h at 96 °C.

To evaluate the produced damage, color, gloss, and roughness were measured at the beginning time and after 3, 6, 9, and 24 h of immersion. A glossmeter digital Erichsen NL3A was used for the measurement of gloss. The roughness was measured using a MAHR MarSurf PS1 tester at a measurement length of 5.6 mm and a cut-off length of 0.8 mm. R_a was recorded as an average value of five measurements. The device measures at an accuracy of 0.01 μm. Using the spectrofluorimeter, luminescence characteristics were measured before and after the attack.

After immersion the surface and the cross-section were observed using an environment scanning electronic microscope ESEM Philips XL30 to observe the possible damage of the luminescent enameled surface.

To investigate the corrosion protective properties and to check the possible influence of the presence of luminescent layers, samples with an artificial linear defect of 2 mm wide were exposed in a salt spray chamber, according to ASTM B117, for a total of 1000 h. After 500 and 1000 h of exposure, the state of the surface close to the defect was observed using the optical microscope Zeiss Stemi 2000-C to evaluate the adhesion of the luminescent enamel and the possible presence of enamel detachment phenomena.

To evaluate the potential reduction of the luminescence characteristics, the luminescence was measured at the beginning, after 500, and after 1000 h of exposure, using the Jasco spectrofluorimeter FP6300. Finally the resistance to mechanical damage and maintenance of the luminescence properties were evaluated. An abrasion Taber test according to ASTM D-4060 with the use of Taber Abraser 5135 was carried out. H22 grinders with 1 kg applied weight were used.^14,15 The wheels were composed of vitrified clay, silicon carbide, and alumina particles embedded in an organic matrix. After 1000 cycles the change of color, gloss, and surface roughness were measured to highlight the produced damage on the surface. The gloss measured at angle of 60° obtained from the average of more than 12 measures considered.

Roughness measurements R_a, 12 for each sample, were carried out. The decrease in luminescence was evaluated using a FP6300 Jasco spectrofluorimeter. Luminescence spectra were collected on each sample from several points.

To highlight the damage morphology, the samples surfaces were observed using a scanning electron microscope. To study the change of the corrosion protection properties produced by mechanical damage, the abrasion test has been carried out up to the loss of protection. To highlight the protection properties loss, electrochemical impedance spectroscopy (EIS) measurements were the carried out. This technique is widely used to evaluate the protective properties of coatings.^16–18

The measurements were carried out in a classic three-electrode system: the sample was the working electrode, a platinum counter electrode was used, and Ag/AgCl (0.205 V vs. standard hydrogen electrode SHE) was used as the reference electrode. The analyzed area was 30.43 cm², corresponding to the circularly damaged area during an abrasion test. The electrolyte was a 3 wt.% Na₂SO₄ solution.

The electrochemical impedance measurements were carried out every 1000 abrasion cycles. The EIS measurements were carried out using a potentiostat and a Frequency Response Analyzer instrument (Princeton PARSTAT 2273) connected to a PC. The AC perturbation amplitude was 15 mV, the frequency ranged from 10⁵ to 10^− 2 Hz. The EIS data are fitted using a simple equivalent electrical circuit made of a capacity in parallel with a resistance R(RQ),^16,18 using the Z SIMP WIN fitting program. The first resistance is the electrolyte resistance, the R_p is the coating resistance, and the C_p represent the coating capacity.

The R_p threshold of 10⁶ Ωcm² is considered as the limit of the loss of protection properties of the coating.^2,14 Below this threshold, in fact, the coating could be considered not protective. After 8000 cycles of abrasion the samples surface were observed at an optical microscope Zeiss Stemi 2000-C to highlight the damage morphology and to highlight the difference among the luminescent samples.

7.7 Conclusion

The porcelain enamels are glass coatings deposited on steel that guarantee, at the same time, excellent technical properties and aesthetic-perceptive ones. The vitreous matter, which constitutes these coatings, is inert in many environments and aggressive substances. Thanks to the deposition and curing technologies, the adhesion between the enamel layer and the steel substrate is very good, and it remains even in the presence of defects and cracks in the coating and corrosive phenomena on the noncovered substrate.

The luminescent enamel layers obtained with an adding of rare earth compounds such as europium and dysprosium are an interesting example of smart coatings, which show very particular optical properties together with the traditional one, presenting at the same time eco-friendly and nonhazardous aspects.

In literature there are no studies on these innovative coatings. Thus, the aim of this chapter is to present experimental tests to check whether the presence of the luminescent layer would change the traditional enamel properties and to highlight the degradation resistance of these luminescent layers. Three different types of luminescent enamel systems were studied, which differ in the ratio between the rare earths, (europium and dysprosium), that produce the luminescent emission. Different size of the luminescent pigments was considered. The emission spectra were due to the contribution of two peaks (one in blue and one in green), and the increase of dysprosium increased the contribution of the green peak.

M1 and M2 luminescent enamels showed a different behavior to chemical solutions than did the M3 deposit. Only sample M3 was subject to attack by the citric acid solution with the formation of localized attacks, which produced a gloss decrease, increase in roughness, and color change. On the contrary, the particles, which were emerged in the surface, seemed to hinder the aggression on the glassy matrix in alkaline solutions. In this environment, sample M3 results showed much less damaged than that shown in samples M1 and M2, presenting only small localized attacks. These last two types highlight a heavy damage of the glass matrix with a dramatic gloss drop, roughness increasing, and degradation of color pigments. The dimension of luminescent pigments and their position in the enamel layers played a fundamental role in these behaviors.

The luminescent emission intensity remained constant after UV-A radiation exposures (both continuous and cycled tests) and also after salt spray chamber exposure, indicating that the luminescent pigments would not undergo any alteration in aggressive environments. From the experimental data, it is also possible to assume that the thickness of the enamel layer can affect the luminescent emission intensity, but that only great material removals may cause the appreciable decrease in intensity of luminescent emission; the removal of localized few μm does not involve measurable changes in luminescence.

The presence of the external luminescent enamel layer does not affect the excellent resistance properties of the traditional enamel systems. The corrosion protection properties are lost only with great mechanical damage caused by the total removal of the luminescent layer and almost all of the intermediate layer, and, at the same time, the production of cracks.