by Prof. Minlin Zhong
One of the hottest wonder-materials in current research, graphene forms mono-atomic layers of carbon atoms tightly packed into a two-dimensional honeycomb structure. To help you visualize that, imagine chicken wire with hexagonal meshes and there is a carbon atom in each corner of the mesh. This “chicken wire” has a lot of interesting properties that make it extremely interesting for electronics applications. But that is not all: its high mechanical and thermal stability and chemical inertness practically cry out for it to be used as extremely thin anti-corrosion coating.
There is more
Dr. Minlin Zhong, Professor, Head of Laser Materials Processing Research Center, School of Materials Science and Engineering, Tsinghua University, Beijing China, focusing on laser micro-nano fabrication, laser surface engineering, laser additive manufacturing and novel material development. Dr. Zhong is a Fellow of Laser Institute of America.
The goal of our experiments was to fabricate graphene layers directly on a workpiece to function as an anti-corrosion coating.
Intensive research into graphene over the past few years has led to the development of a variety of coating methods. When it comes to larger areas, the principal approaches are CVD (chemical vapor deposition) and various laser-assisted processes that combine laser energy with chemical processes. Although all these methods have their individual strong points, it would be good to have a method for practical applications that fulfills all the main requirements: namely, one that grows graphene directly, coats large areas quickly, allows freely programmable patterns, works under normal ambient conditions, does not use or release hazardous substances and employs tried-and-tested industrial tools. We set of to achieve just this.
Straight to the goal
The goal of our experiments was to fabricate graphene layers directly on a workpiece to function as an anti-corrosion coating, as earlier experiments had shown that graphene fabricated directly on the surface of a workpiece is many times more effective than transferred graphene layers.
High-power industrial lasers were used to fabricate the graphene. The experiments were carried out on polycrystalline nickel sheets. A paste made of graphite nanoparticles suspended in ethanol and evenly spread into a 20-micrometer-thick layer served as the carbon source. This solid carbon source is less hazardous than the gaseous sources often used in CVD. Next, the surface of the nickel sheets was irradiated with laser light: a diode laser with a flat-top beam profile and a wavelength of 0.98 micrometers was employed to evenly remelt larger areas. In order to remelt only parts of the surface according to CAD-programmed patterns, the team used a fiber laser with a round beam, a Gaussian beam profile, and a wavelength of 1.06 micrometers.
The diode laser irradiated the workpieces with a 16-millimeter-wide and one-millimeter-long beam profile at a process speed of 18 centimeters a minute. This remelted the surface to a depth of almost 0.3 millimeters, and graphene formed on the fresh surface at the rate of 28.8 square centimeters a minute. During the experiments to produce freely programmed patterns, the solid-state laser irradiated the sheets along two pre-programmed path patterns: the first was a spiral, which the laser melted into the surface with a beam diameter of three millimeters and a scanning rate of 24 centimeters a minute; the second was a maze, which the laser executed with a beam diameter of one millimeter and a scanning rate of 60 centimeters a minute.
Graphene formed on the fresh surface at the rate up to 30 square centimeters a minute.
Of course, any other patterns are possible. Examinations using optical and electron microscopy and various spectroscopic methods revealed that the entire irradiated surface in all experiments was covered by a one- to four-layer graphene film. Three- to four-layer graphene made up just short of a third of the entire film area. However, it was not concentrated in one region, but covered the entire surface in an even, meshed pattern.
Effective anti-corrosion protection
Although just a few atomic layers thick, these graphene films provide extremely effective anti-corrosion protection, as anticipated. This was demonstrated by measuring the corrosion current densities and potentials of various samples in a NaCl-solution. The rate of corrosion and initial resistance to a corrosive environment can be derived from these measurements. The initial resistance of the nickel sheets with a closed graphene film turned out to be significantly higher and the corrosion rate a thousand times slower than for unprotected samples.
A cross-check using sheets whose surface was just remelted without producing graphene showed that this effect was not simply down to the remelting of the surface, with measurements for the cross-check sheets indicating an even faster corrosion process than for completely untreated sheets. Conversely, a graphene film covering only fifty percent of the surface already achieves a considerable reduction in the corrosion current measurements. In short, the graphene produced by this method exhibits the desired properties.
At the same time, the experiments indicate that the number of graphene layers in the film can be influenced to a certain degree. The formation and number of layers are strongly dependent on the cooling rate, a relationship that earlier experiments had already shown and this series of experiments confirmed. Extremely fast cooling of the surface following heat treatment produces the thin film of one to four layers described above. Slower cooling leads to a thicker film with many layers. Very slow cooling processes – in an oven, for instance – produce no graphene at all. The reason for these differences is carbon’s solution behavior in solid nickel.
In short, the graphene produced by this method exhibits the desired properties.
Under the heat of the laser beam, carbon atoms separate from the graphite nanoparticles in the molten nickel. However, because solid nickel permits only a maximum 2.7 percent carbon solution according to the Ni-C binary alloy phase diagram, and the carbon solution rate decreases when the nickel cools down, the excess carbon atoms out-diffuse from the solid solution to the surface, where they meet and form two-dimensional graphene meshes. As the sample is melted only approx. 0.3 millimeter deep at room temperature, it solidifies very quickly and many of the carbon atoms lose their mobility before they reach the surface. Accordingly, examinations of the samples show tiny carbon particles near the surface. This is the carbon that was forced out of the solution but did not get as far as the surface. Slower cooling means that more carbon reaches the surface and forms more graphene layers.
Advantages of graphene films
The series of experiments therefore lays the foundations for a new process to coat workpieces with large-area, thin graphene layers that protect it very effectively against corrosion. The laser method uses common industrial solid-state and high-power diode lasers and works under normal ambient conditions. It can produce not only closed layers but also layers in arbitrary, freely programmable patterns. This is achieved in just a single process step, without using chemicals or chemical processes that are potentially harmful to human health or the environment. We hope that this work will offer industry a productive way to benefit from the fascinating properties of graphene films in the near future.