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

银制品的腐蚀，特别是抛光银表面的腐蚀，是全世界艺术收藏的一个重大问题。由于大型博物馆里的文物通常都是独一无二的，因此保护方法和技术需要大量的证据来证明处理效果、对现有方法的改进以及可逆性。这项研究将使用一种非常强大的技术，即“原子层沉积”（ALD），为银开发一种新型的多层多功能透明屏障涂层，该技术允许在原子水平上精确控制纳米厚度的金属氧化物层。由此产生的多层膜将被优化，以减少银腐蚀的速度，同时符合严格的艺术保护实践标准。博物馆和大学的合作将产生一种有效的、低成本的策略来减少银器的腐蚀，这也保留了人工制品的外观和成分，而不妨碍未来的保护处理策略。这些好处将通过出版物和演讲与全球博物馆保护界分享。在本工作中，制备、表征和优化了用于银器保护的多层结构、多功能原子层沉积（ALD）薄膜。银的褪色是一个严重的问题，目前对世界各地博物馆收藏的无价艺术品造成了无法弥补的损害。该方法基于ALD：一种创新的热活化气相工艺，通过连续暴露于2种或更多的气体反应物来诱导自限化学吸附表面反应，从而将氧化剂到达下垫表面的速率降低了几个数量级。探索多种成分和层结构来优化屏障性能和光学清晰度。通过反射光谱学来评估光泽，并使用x射线光电子能谱（XPS）来测量一系列曝光后剥离氧化物后表面上的硫含量。通过加速氧化剂通过薄膜的运输和在银表面的反应，使用暴露在控制H2S浓度升高的大气中，以及提高ALD涂层样品的温度来建立特征时间尺度，可能是几十年或更长时间。对ALD金属氧化物涂层的可逆性进行评估，以确定在银上沉积或去除薄层金属氧化物是否会改变银表面的物理特性或化学成分。通过沉积多层铝和钛的交替氧化物，可以防止氧化分子通过屏障涂层中的针孔直接撞击。通过在薄膜中沉积埋藏的铂层，引入了新的氧化剂吸附剂功能。银基板的图案用于量化涂层对微观和宏观特征的光学特性的影响，并评估在ALD氧化物沉积、去除过程中，初始地形对艺术品表面的地形和成分稳定性的影响，以及对局部失光率的影响。研究生和本科生都接受了尖端ALD薄膜制作和表征技术的培训，以及沃尔特斯艺术博物馆和马里兰大学合作的博物馆保护实践。

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.