SCIART: Collaborative Research: Protection of Silver Objects from Corrosion using Atomic Layer Deposited Barrier Coatings

非技术性描述银制品的腐蚀，特别是抛光的银表面，是全世界艺术收藏品面临的一个重大问题。由于大型博物馆中的物品通常都是独一无二的，因此保护方法和技术需要有大量证据证明处理的有效性、对现有方法的改进以及可逆性。这项研究将开发一种新型的多层，多功能的透明屏障涂层银使用一个非常强大的技术称为“原子层沉积”（ALD），它允许创建纳米厚的金属氧化物层与精致的控制水平，字面上在原子水平。 由此产生的多层膜将被优化以降低银腐蚀的速率，同时符合艺术保护实践的严格标准。这一博物馆和大学的合作将导致一个有效的，低成本的战略，以减少银文物腐蚀，这也保留文物的外观和组成，而不排除未来的保护处理策略。 这些好处将通过出版物和演示与全球博物馆保护界分享。技术装备在这项工作中，多层结构的多功能原子层沉积（ALD）薄膜用于保护银艺术品的制造，表征和优化。银的失泽是一个严重的问题，目前对世界各地博物馆收藏的无价艺术品造成不可弥补的损害。 该方法基于ALD：一种创新的热活化气相工艺，用于通过连续暴露于2种或更多种气体反应物来合成纳米厚的固体膜，以诱导自限化学吸附表面反应，从而降低氧化剂到达底层表面的速率。探索多种组合物和层结构以优化阻隔性能和光学透明度。通过反射光谱法评估变色，并使用X射线光电子光谱法（XPS）测量一系列暴露后剥离氧化物后表面上的硫含量。 使用暴露于具有受控的、升高浓度的H2S的气氛以及增加ALD涂覆的样品的温度来加速氧化剂通过膜的传输以及在银表面处的反应，以建立可能数十年或更长的特征时间尺度。评价ALD金属氧化物涂层的可逆性以确定金属氧化物薄层在银上的沉积或去除是否改变了银表面的物理特性或化学组成。 通过沉积多层铝和钛的交替氧化物来防止氧化剂分子通过阻隔涂层中的针孔的直接撞击。 新的氧化剂吸杂功能引入通过沉积埋层的铂到薄膜。 银基底的图案化用于量化涂层对微观和宏观特征的光学性质的影响，并评估在ALD氧化物沉积、去除期间起始形貌对工艺物体表面的形貌和组成稳定性的作用，以及对局部失泽速率的作用。 研究生和本科生都接受了尖端ALD薄膜制造和表征技术的培训，并在沃尔特斯艺术博物馆和马里兰州大学之间的合作中进行博物馆保护实践。

NONTECHNICAL DESCRIPTION The corrosion of silver artifacts, especially polished silver surfaces, is a monumental problem for art collections throughout the world. As objects in major museums are typically one of a kind, conservation methods and techniques require overwhelming evidence of treatment effectiveness, improvement over existing methods, and reversibility. This research will develop a novel multilayer, multifunctional transparent barrier coating for silver using a very powerful technique known as "atomic layer deposition" (ALD), which allows for the creation of nanometer thick layers of metal oxides with an exquisite level of control, literally at the atomic level. The resulting multilayer films will be optimized to reduce the rate of silver corrosion, while complying with the rigorous standards of art conservation practice. This museum and university partnership will result in an effective, low-cost strategy to reduce silver artifact corrosion, which also preserves artifact appearance and composition without precluding future conservation-treatment strategies. These benefits will be shared with the global museum conservation community through publications and presentations.TECHNICAL DETAILS In this work multilayer-structured, multifunctional atomic layer deposition (ALD) films for conservation of silver art objects are fabricated, characterized and optimized. Tarnishing of silver is a critical problem, presently producing irreparable damage to priceless art objects in museum collections throughout the world. The approach is based upon ALD: an innovative, thermally activated gas phase process for synthesizing nanometer-thick solid films by sequential exposure to 2 or more gas reactants to induce self-limited chemisorbed surface reactions, which reduces the rate of oxidant arrival at the underlying surface by orders of magnitude. Multiple compositions and layer structures are explored to optimize barrier performance and optical clarity. Tarnishing is evaluated via reflectance spectroscopy, and using x-ray photoelectrons spectroscopy (XPS) to measure the amount of sulfur on the surface subsequent to stripping the oxide after a series of exposures. Accelerated transport of oxidants through the film and reaction at the silver surface, using both exposure to atmospheres with controlled, elevated concentrations of H2S, and increasing the temperature of ALD coated samples are employed to establish the characteristic time scales, likely decades or longer. The reversibility of ALD metal oxide coatings is evaluated to determine if either the deposition or the removal of thin layers of metal oxides on silver changes the physical characteristics or chemical composition of the silver surface. The direct impingement of oxidant molecules through pinholes in barrier coatings is prevented by depositing multiple layers of alternating oxides of aluminum and titanium. Novel oxidant gettering functionality is introduced via deposition of buried layers of platinum into the films. Patterning of silver substrates is used to quantify the effect coatings have on the optical properties of micro and macro features and evaluation of the role of the starting topography on the topographical and compositional stability of the surfaces of art objects during ALD oxide deposition, removal, and on the local rate of tarnishing. Students at both the graduate and undergraduate level are trained in cutting-edge ALD film fabrication and characterization techniques, and in museum conservation practices in this collaboration between the Walters Art Museum and the University of Maryland.