Protecting silver cultural heritage objects with atomic layer deposited corrosion barriers
© Marquardt et al. 2015
Received: 1 July 2015
Accepted: 10 November 2015
Published: 1 December 2015
Silver, prized throughout history for its luster and shine, develops a black Ag2S tarnish layer that is aesthetically displeasing when exposed to atmospheric pollutants. Tarnishing, and subsequent polishing, leads to irreversible material loss and object damage. Currently, nitrocellulose coatings are often used to prevent silver from tarnishing, however non-uniform coatings and degradation over time limit their effectiveness. Atomic layer deposition (ALD) has been explored as a new method for creating dense, uniform, and conformal coatings on 3-dimensional (3D) objects that are more effective than nitrocellulose in preventing silver from tarnishing.
To create high quality ALD coatings on 3D objects, slowing down the ALD process is critical to ensure proper precursor exposure. Non-ideal deposition of organo-oxy-metallic compounds can occur with fast deposition rates that do not allow sufficient flow around 3D objects. The coatings can be removed by dissolving the Al2O3 ALD films in aqueous NaOH. Thicker ALD films prevent defects from occurring on non-ideal surfaces and effectively prevent silver objects from tarnishing under ambient aging conditions.
Thick ALD films, deposited with sufficiently long precursor pulse and purge times, may be effective in preventing complex, 3D non-mixed media silver cultural heritage objects from tarnishing.
Silver has been a precious and valuable metal dating back nearly 5000 years. Used as both a traded commodity and an artist’s material, prized for its luster and shine, silver is plentiful in cultural heritage and private collections. Because of its value and softness, it was rarely used in its pure form (fine silver, 99.9 % silver), instead, silver is commonly alloyed with copper. For example, the sterling standard stipulates a 7.5 % copper concentration. Unlike other precious metals such as gold and platinum, silver is known to tarnish with time forming Ag2S when exposed to low concentrations (<1 ppb) of H2S in ambient environments [1–3]. While Ag2S is the main component in silver tarnish, sulfate, chloride, oxide, and oxygenated carbon species can also be present . As the Ag2S tarnish layer increases in thickness, the initially white color will change to yellow at 10 nm, violet at 26 nm, blue at 47 nm, and finally dark gray . Because silver sulfide tarnish is a surface phenomenon, it does not tend to threaten an object’s mechanical integrity. It can be, however, aesthetically displeasing and interfere with the readability of surface details.
Tarnish, which often results in undesirable color change, is a damaging process in which copper and silver at the surface are consumed to form sulfide products. Tarnish is commonly removed by polishing silver with an abrasive, although other methods of cleaning silver include chemical, electrolytic, plasma and laser treatments. Polishing, for example, in addition to being a time and labor-intensive process, irreversibly damages the silver object by removing a small amount of metal. Repeated polishing inevitably leads to the loss of surface details in the object . Silver plated pieces stand to lose both surface details and the thin layer of surface silver that provides the artist-intended luster and shine. Ideally, silver objects would be preserved in low humidity environments, free of pollutants and corrosive gases, to reduce the need for subsequent polishing. Practically, silver is often stored in controlled environments ideal for mixed collections at 21 ± 2–3 °C and 50 ± 5 % relative humidity (RH) with limited or no filtration of pollutants . It is often not practical to provide the ideal pollutant free and low humidity environment due to cost, space, and accessibility considerations. Because of this coatings are often used to create a physical barrier between the silver object and the environment. Nitrocellulose lacquer is one of the more common barrier films used by conservators to prevent silver objects from tarnishing . Multiple coats of nitrocellulose are often applied by brushing, spraying or dipping the object to ensure complete coverage . The benefits of nitrocellulose lacquers include easy application, improved appearance over other lacquers and a 5-tenfold reduction in required polishing treatment frequency compared to un-coated silver objects [8, 10–12].
Despite these benefits, this approach is far from ideal: achieving a uniform film with nitrocellulose is extremely difficult, especially on three-dimensional objects with protrusions such as handles, feet, or lips. Pores or cracks in nitrocellulose coatings are found to occur due to incomplete coverage, causing points of failure and corrosion pitting . Additionally, as nitrocellulose films age, they are known to degrade and yellow from UV exposure, causing an undesirable color change on silver objects and difficult removal after aging [8, 14, 15].
The limitations of nitrocellulose coatings may potentially be avoided by instead coating silver objects with atomic layer deposited (ALD) metal oxide thin films. ALD diffusion barriers have been successfully used to protect new silver jewelry and silver coins from tarnishing . Conformal, dense metal oxide thin films deposited with ALD on non-mixed media silver and silver alloy art objects meet the cultural heritage conservation requirements of being more effective than current protection methods, reversible without significant substrate alteration, and optically transparent [17, 18].
ALD films have been demonstrated to be effective diffusion barriers for corrosion protection [25–29]. The density of Al2O3 ALD films (3.0–2.5 g/cm3, depending on deposition temperature) is at least 1.5 times greater than the density of nitrocellulose (1.6 g/cm3, depending on the nitrogen content), creating better barriers that slow the rate of gas diffusion [30, 31]. For applications in cultural heritage, the thickness of a barrier film involves a tradeoff; a thick barrier film results in a long diffusion path for corrosive gases, while thicker films tend to more profoundly affect the optical properties and visual appearance of the film. The visual appearance of the ALD films can be tuned by varying the thickness and refractive index of the ALD film structures used on a substrate . Single or multilayer ALD films consisting of multiple materials or film thicknesses can be deposited with sub-nanometer control over the coating thickness. Optical modeling techniques, adopted from high precision optical coatings for devices like telescopes and spectrometers, can be used to predict, optimize, and minimize surface coloration [33, 34]. This flexibility allows for the creation of nearly invisible ALD films on both coated silver and sterling silver substrates .
Nearly perfect conformality with ALD is achievable even for substrate feature aspect ratios far exceeding those typically found on cultural heritage objects. As a result, ALD can create uniform coatings on intricately detailed surfaces with inscriptions and surface texturing. Coating 3-dimensional (3D) objects with ALD is possible, as it is a ‘non-directional’ technique, where unlike a directional technique, such as spray coating, the location of the source of the ALD precursor chemicals relative to the object does not affect deposition uniformity. Rather, as long as the ALD precursors are allowed sufficient time to surround and bind to the object, reaching all crevices, a conformal coating can be achieved. Indeed, due to the non-directional nature of the ALD process, any surface within the reactor can, in principle, be coated, allowing the process to be scaled up to treat large objects or simultaneously coat multiple objects, the limiting factor being only the size of the ALD coating chamber. Large volume ALD reactors are commercially available though modifications to the technology may be required for cultural heritage applications.
Additional challenges arise due to the complexity of many cultural heritage objects. ALD films are limited to use on silver and silver alloy objects as current alkaline solutions used for ALD coating removal are not recommended for use on mixed media objects or objects with microcracks or pores. ALD films thicknesses may need to be tailored to minimize appearance changes on objects with changing alloy compositions or the presence of solder or gilding. Additionally, the elevated temperatures, 100–150 °C, used to apply the ALD films in this study may cause microstructure changes to silver alloys. The vacuum level used for deposition, 3–5 mbar, may not be suitable for cultural heritage objects. However, ALD systems for atmospheric pressure, low temperature processing have been developed [30, 35, 36], and would likely be more suitable for cultural heritage objects.
Our recent work demonstrates that ALD films are a promising technique to prevent damage from the corrosion process, and subsequent polishing, for silver cultural heritage objects. Our previous results find that 20 nm Al2O3 ALD films protect silver from tarnishing nearly 15 times longer than microns-thick nitrocellulose films: this amounts to a potential ambient effective film lifetime of 150 years. We note, however, that this may represent an upper limit of ALD effective lifetimes—in our earlier work coatings were applied to flat, clean, 2-dimensional ideal silver substrates [16–18]. In the work presented here we investigate the effectiveness of ALD coatings on non-ideal, 3D historic objects. As we show below, additional challenges arise in creating quality ALD coatings on realistic art objects. Extended precursor pulse/purge times are required to adequately coat complex 3D geometries. Adjusting the concentration of aqueous NaOH solutions allows for complete removal of the ALD. In addition, we find that thicker ALD films are needed to reduce pinhole defects in ALD films on non-ideal substrates.
Coating 3-dimensional silver objects
EDS analysis of ALD coating removal
100 nm Al2O3 ALD
140 nm Al2O3 ALD
0.05 wt% NaOH for 2 h
0.5 wt% NaOH for 25 min
Clear area (At %)
Hazy area (At %)
Clear area (At %)
Clear area (At %)
Hazy area (At %)
Clear area (At %)
Hazy area (At %)
34.27 ± 2.4
40.47 ± 2.5
4.64 ± 1.1
53.85 ± 2.6
47.16 ± 2.6
88.12 ± 2.7
88.36 ± 2.8
3.50 ± 0.3
6.12 ± 0.4
8.34 ± 0.5
7.23 ± 0.5
11.88 ± 0.5
11.64 ± 0.5
5.70 ± 0.2
4.77 ± 0.2
17.47 ± 0.8
2.54 ± 0.1
3.67 ± 0.2
56.53 ± 3.4
48.65 ± 3.2
77.89 ± 5.0
35.27 ± 2.3
41.96 ± 2.8
Raman spectroscopy analysis was also performed at clear and hazy regions of the knife, which are circled in Fig. 3b and the results presented in Fig. 4c. In both the hazy and clear regions peaks at 1355, 1594 and 2919 cm−1 were identified. The 1594 and 1355 cm−1 signals are near frequencies attributed to ring stretching in polyaromatic compounds, or disordered graphitic carbon species, at 1580 and 1360 cm−1. These are generally referred to as the G band and D band, respectively [37–39]. The ratio of the G and D band intensities (ID and IG) is a measure of the graphitization level, as the G band alone is present in highly ordered graphite while the D band appears, and increases in intensity with increasing disorder [39, 40]. The ID/IG ratios for the hazy and clear regions of the knife are 0.97 and 0.98, respectively, indicating a relatively low degree of graphitization. Further, the intensity of both the G and D bands is much higher in the hazy region of the knife. Taken together, these observations indicate a higher abundance of a disordered C–H species in the visibly hazy regions. Additionally, the region from 2700 to 3000 cm−1 in Raman spectra is generally attributed to the C–H stretching mode in alkyl hydrocarbons [37, 39]. Both the clear and hazy regions of the knife contain a peak at 2919 cm−1, however, the intensity is significantly higher in the hazy region.
We next consider the significance of the Raman analysis. Trimethylaluminum (TMA) and water were the two precursors used to deposit the Al2O3 ALD films. TMA (Al2(CH3)6 is composed of a central aluminum atom surrounded by three methyl groups (–CH3). The intense Raman signal of alkyl groups in the hazy region suggests an incomplete ALD reaction, in which the methyl ligands in the chemisorbed species from the TMA sub-reaction were not fully replaced during the subsequent H2O precursor sub-reaction. We conclude that a pure Al2O3 film was likely not deposited in the hazy regions, but instead a mixture of Al2O3 and AlxOy(CH3)z organo-oxy-metallic compounds is likely present. It is likely that both precursors, TMA and water, had insufficient precursor flow through this region and prevented the complete replacement of each methyl group with a hydroxyl group (–OH), which is required for dense and uniform, layer-by-layer Al2O3 ALD growth.
Our results show that increasing the precursor pulse and purge times is an effective way to suppress the formation of hazy regions in the ALD coatings. The knife coated with 140 nm Al2O3 ALD film (Fig. 3c), where a 1 s TMA pulse and 4 s nitrogen purge (bottom) was used, showed no visible hazy regions in the coating (note that the colored areas at the tip of the knives are artifacts of edge diffraction, which produces dispersion according to wavelength, along with the chromatic aberration of the lens used in making the image; these are present even in the absence of a coating, as seen in Fig. 3a). EDS analysis, shown in the second column of Table 1, revealed the presence of aluminum across the surface of the knife, indicating complete coverage by the Al2O3 ALD film. Our results indicate that longer pulse and purge times are required for complex 3D objects, particularly near supports, to allow the precursors and by-products adequate time to diffuse to all portions of the object. Understanding the precursor reaction and transport, tortuous diffusion paths and ALD chamber flow dynamics around complex objects is critical and an area of ongoing study [41–43]. While this topic is beyond the scope of this paper, we note that the apex of the prominent hazy plume in Fig. 3b coincides with the position of one of the supports that was used to hold the knife in place.
ALD coating removal
Removal of the ALD coating without significant alterations to the underlying substrate must be demonstrated to meet one of the requirements in cultural heritage conservation. Previously, we have shown that aqueous NaOH solutions as weak as 0.05 % (pH 11) can be used to effectively remove Al2O3 ALD films with no significant change to flat sterling silver substrates. However, the reversibility of hazy coatings, such as those shown in Fig. 4b, was not addressed. To be discussed here is whether the organo-oxy-metallic residues in the films might affect reversibility and if these residues found in the hazy areas are as easily removed as the clear areas. To address this, the knife shown in Fig. 3b was immersed in 0.05 wt% aqueous NaOH for 2 h to remove 100 nm Al2O3 ALD, which is 37 min more than the time required to remove 100 nm of a typical amorphous ALD alumina (clear area). EDS analysis, however, showed the coating remained both in the clear and hazy regions, as shown in Table 1. The concentration of NaOH was increased to 0.5 wt% (pH 11) for 25 min in a second attempt to remove the ALD coating. EDS analysis confirmed the complete removal of the ALD coating with higher concentrations of NaOH in both the hazy and clear areas of the coating, also shown in Table 1. Seemingly the residual organo-oxy-metallic residues have a passivating effect, which is overcome by the higher NaOH concentration.
Overcoming pinholes in ALD films
Contiguous ALD films have been proven to be effective diffusion barriers for preventing corrosive gases in the environment from reacting with the substrate [25–29]. Recent results show that such films can be effective in reducing the rate of silver tarnish formation by preventing atmospheric H2S gas from reaching and reacting with the silver surface . The effectiveness of such an approach for protecting art objects, however, can be limited by holes or point defects, which reduce the effective film lifetime at the defects. Holes in the barrier coating act as fast paths for corrosive gas diffusion to the underlying metal, resulting in more rapid tarnish rates than in the coated areas. Once tarnishing occurs at such defects, the entire coating must be replaced and the tarnish spot removed through mechanical polishing. While ALD films have been reported to be nearly defect free coatings, pinholes can occasionally occur [44, 45].
The samples used for the EIS measurements shown in Fig. 7 were relatively clean, flat silver surfaces with little chemical heterogeneity, providing a nearly ideal silver substrate for ALD; thus these results likely set a lower limit on ALD coating porosity to be expected on art objects, such as the sterling silver knives discussed above. The porosity of the 10 nm Al2O3 ALD film on the knives is likely greater than the same coating on an ideal silver substrates. The silver knife surface is likely more chemically heterogeneous due to the Ag–Cu alloy composition and likely has existing oxides or residues present from handling and previous treatments. This speculation is supported by our previous reported results showing that the porosity of thin 10 nm Al2O3 ALD films depended on the prior treatment of the silver surface, while 100 nm thick Al2O3 ALD films had low porosity that was nearly independent of the surface treatment . For art objects, we expect that thicker ALD coatings will be required to prevent surface heterogeneities, residues or particles from causing defects in the ALD coating.
Cast and surface chased sterling silver butter knives from a personal collection were used to test the application of ALD films on 3D objects. The knives were polished using a slurry of ethanol and 0.04 μm precipitated chalk (CaCO3) followed by a quick polishing with a silver cleaning cloth (Goddard’s Long Shine®) to remove tarnish. Before ALD films were applied, the knives were cleaned by immersion in ultrasonic baths of (all reagent grade) trichloroethylene, acetone, methanol and isopropanol, followed by a deionized water rinse and blow drying with N2 gas.
Silver thin films for electrochemical impedance spectroscopy were deposited on Si wafers using the Metra Thermal Evaporator. A 10 nm Al2O3 ALD adhesion layer was deposited on the Si wafers before Ag deposition to prevent the Ag delaminating from the Si wafer. 99.999 % Ag (Sigma-Aldrich) was deposited at a rate of ~10 Å/s under a base pressure of 2.0 × 106 Torr.
Atomic layer deposited Al2O3
A Beneq TFS 500 atomic layer deposition (ALD) system with a 200 × 170 × 25 mm sub-chamber produced all Al2O3 films in a Class 1000 cleanroom. The ALD reactor was evacuated to a base pressure of 3–5 mbar. Samples were allowed to equilibrate for 1 h before deposition began. The silver knives discussed in this study had ALD films applied at 100 °C, while thermally evaporated silver thin film samples were deposited at 150 °C. Trimethylaluminum and H2O were used as precursors to deposit Al2O3 with a deposition rate of 0.9 Å/cycle, adjusting the number of cycles to produce the desired film thickness. N2 gas was used to purge the ALD sub-chamber of any by-products or remaining precursors between precursor pulses. During purge and pulse cycles, gases continuously flowed through the reactor sub-chamber. As shown in Fig. 2, the gases are introduced through the left side of the reactor and exit on the right. Precursor pulse and N2 gas purge times were adjusted from 0.5 to 2.0 s, depending on the particular experiment, when depositing on 3D objects, to ensure gas delivery in the entire ALD chamber.
The sterling silver knife was characterized after ALD film deposition using a Digital Instruments Dimension™ 3100 atomic force microscope (AFM). Reported surface roughness values were calculated from the root mean square (RMS) of the surface profile. Five AFM scans in each area were obtained to calculate the RMS roughness and representative scans are reported.
Raman measurements of the sterling silver knife after ALD film application were obtained using a HORIBA Jobin–Yvon LabRAM ARAMIS Confocal Raman Microspectrometer. An excitation laser wavelength of 532 nm and a 1200 L/mm grating were used for all measurements reported.
A scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) was used to characterize the surface of the silver knives after film deposition and ALD film removal. A Hitachi S-3400 Variable Pressure SEM was used with an accelerating voltage of 10–20 kV.
Surface composition of ALD film defects in the sterling silver knife was determined through x-ray photoelectron spectroscopy (XPS) measurements using a Kratos Axis 65 X-ray Photoelectron Spectrometer, employing a mono-chromatic Al(Kα) source with a 125 mm Omnicron (EA 125) hemispherical analyzer with a seven-channel detection capability for photoemission analysis. The charge neutralizer was employed to prevent surface charging. All peaks were calibrated to the C s peak centered at 284.6 eV. XPS surface concentrations were calculated using CasaXPS™ from the peak areas of O 1s, Ag 3d, Al 2p, S 2p and C 1s using manufacturer supplied relative sensitivity factors.
Electrochemical analysis was carried out on ALD coated thermally evaporated silver substrates using a three-electrode cell with a Ag/AgCl reference electrode and a platinum counter electrode. Before measurements, the Kapton™ tape was removed from the corners of the silver sample. An area of 1.77 cm2 on the working electrode was created using a Neoprene® o-ring. A 1.0 molar NaCl (Sigma-Aldrich, reagent grade) solution made with ultra-pure water (resistivity >17 MΩ) was de-aerated by bubbling 150 sccm N2 gas for 30 min prior to testing and 50 sccm N2 gas while measurements were taken. Electrochemical impedance spectroscopy (EIS) was performed immediately after the electrolyte was introduced to the electrode cell to determine the electrochemical response due to the pinholes, not the formation of tarnish. Measurements were performed in the frequency range of 10 mHz–100 kHz with a 5 mV exciting signal amplitude, at open circuit potential (OCP).
While ALD has previously been proven highly effective in protecting flat, 2-dimensional silver alloy surfaces from corrosion, coating non-ideal 3D objects can be more challenging. Limited flow of the ALD precursors around 3D objects and supports is an important factor. The precursor pulse and subsequent purge times must be increased to allow sufficient diffusion of precursors and by-products around complex surface topographies. Relatively thin ALD films offer insufficient corrosion protection on art objects, where existing local surface oxides and/or surface pre-treatment residues may cause pinhole defects. Thicker ALD films are found to suppress pinhole defects that may be present during the initial film growth. The Al2O3 ALD coatings can be effectively removed from the silver surface by dissolving the coating in 0.5 wt% (aq.) NaOH. Our results demonstrate that relatively thick ALD films, deposited with sufficiently long precursor pulse and purge times, may be effective in protecting non-mixed media silver and silver alloy cultural heritage objects.
AM carried out the experiments and analysis described here, and contributed to writing and revisions of the article; EB, TW and GG contributed to the experimental design and interpretation and writing and revisions of the article, RP contributed to the experimental design, interpretation, the writing and revisions of the article and the direction of the project. All authors read and approved the final manuscript.
This work was supported by the National Science Foundation under SCIART grants DMR 1041809 and DMR 1041803. The authors would also like to thank Professor Eric Wachsman and Professor Liangbing Hu and their respective group members for access to EIS equipment used for this analysis.
The authors declare they have no competing interests connected with the information contained in this manuscript.
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