Metal, heal thyself
Materials science: Researchers have devised an ingenious way for the damaged surfaces of metals to repair themselves when they come to harm
SADLY for engineers, inanimate objects cannot yet repair themselves. But work by Claudia dos Santos at the Fraunhofer Institute for Manufacturing Engineering and Automation, and Christian Mayer at Duisburg-Essen University in Stuttgart, has brought the day when they will be able to do so a little nearer. They and their colleagues have invented a way for damaged metals to heal themselves.
对工程师来说悲哀的一面是，没有生命的物体还无法实现自我修复。但是在德国弗劳恩霍夫制造工程和自动化学会的Claudia dos Santos和斯图加特杜伊斯堡—艾森大学的Christian Mayer的共同研究下，无生命物体实现自我修复那一天的到来被拉近了一点。他们与同事研发出了一种能让受损金属自我愈合的方法。
The surfaces of many metal objects are coated with other metals for protection. Iron, for instance, is frequently galvanised with zinc. The basic idea of the new technology is to infiltrate this coating with tiny, fluid-filled capsules. When the metal coating is punctured or scratched, the capsules in the damaged area burst and ooze restorative liquids, in the form of compounds called trivalent chromates. These react with nearby metal atoms and form tough, protective films a few molecules thick to ameliorate the damage.
The idea of doing this has been around for years, but it has proved difficult in practice because the capsules used were too big. Surface coatings tend to be about 20 microns (millionths of a metre) thick. The capsules were 10-15 microns across—large enough to disrupt the coatings, and thus do more harm than good. The trick worked out by Dr dos Santos and Dr Mayer is how to create capsules a few hundredths of this size.
这种构想已经现世好几年了，但是由于过去所使用的胶囊太大，在实践中应用起来难度很大。金属表面的覆盖层厚度趋近于20微米（微米为一米的百万分之一），而过去用的胶囊跨度为10到15微米，这种跨度足以打乱覆盖层的结构，因此对金属造成的弊大于利。dos Santos 博士和Mayer博士想出来的诀窍就是如何创造跨度仅为过去的百分之几的胶囊。
The capsules the researchers have come up with are made by mixing butylcyanoacrylate, a chemical found in superglue, with an oil carrying the healing compounds. This mixture is then mixed with dilute hydrochloric acid. The result is an emulsion of droplets between 100 and 300 nanometres (billionths of a metre) across. Each droplet has an oil core surrounded by a thin layer of butylcyanoacrylate. To make the droplets stable, phosphate is added to the emulsion. This triggers the polymerisation of the butylcyanoacrylate into a tough plastic, which forms the outside of the capsule.
The greatest challenge for the team, however, was not making the capsules in the first place, but stabilising them during the plating process. Though galvanisation is often done by dipping steel in liquid zinc, it is sometimes done by electrolysis—nickel and copper plating are normally done this way. The capsules, though, tend to stick together in the liquids used as electrolytes during electroplating, and are also destroyed by the extreme acidity or alkalinity that is often involved in the process. To overcome these problems, Dr dos Santos and Dr Mayer used special detergents that stick to the polymerised butylcyanoacrylate shell around each capsule, which stops them sticking together and protects them from the electrolytes.
然而，对于这组研究人员来说，最大的挑战并不是开始时胶囊的制造过程，而是在电解过程中如何稳定这些胶囊。尽管一般是通过液态锌蘸洗钢铁来镀锌，有时候也会用电解的方法，在电镀镍和铜时就通常使用电镀。在电镀过程中，不过，胶囊容易在使用的电解质液体中黏在一起，而且也会由于这一过程中通常出现的极酸性和极碱性而受到破坏。为了克服这些问题， dos Santos博士 和Mayer博士使用了一种特殊的去污剂，这种去污剂可以粘附在每个胶囊周围聚合氰基丙烯酸正丁酯的外壳上，阻止这些胶囊黏在一起，并保护它们免受电解液的破坏。
The researchers have now proved their techniques in electroplated layers of copper, nickel and zinc, and believe that self-repairing metals should commonly be available in the years ahead. Moreover, their nanocapsules may have other applications. Lubricants such as silicone oils can be included in them, to make the damaged surfaces of ball-bearings that have run out of oil more slippery, so that they are not scratched too rapidly. Anti-fouling compounds can be placed in capsules on the surfaces of metals intended for use in marine environments. And, in a nod to butylcyanoacrylate’s origins in superglue, capsules containing chemicals that will react to form adhesives when two surfaces are put together are also on the horizon.