The story of one discovery
When I was in my second year at the institute, when we were told the “History of Science”, I remember listening to a student who forgot to turn off the crucible and who made the discovery, or one famous scientist who got an apple on his head, and thought it was a fairy tale, and in modern science does not happen. In principle, if you look at publications in top physical journals, you can see that all of them are the result of a long hard digging in one direction. Novoselov and Geim even got the Nobel Prize for the discovery, as such, but for the “systematic study of properties”. But, nevertheless, discoveries in modern science do happen, and I want to tell about one of them, being its co-author.
From the beginning - a small excursion into that field of physics and nanotechnology, I study bark - this is micro- and nano-tribology. Tribology itself is a very respectable age science that deals with friction and wear. It would seem that everything in this area has long been known - poured more grease, and no friction. And to study here, from a scientific point of view, nothing special. But with the development of microminiaturization, tribology received three times the breath. Because the methods of the macro world (pour out a bucket of oil) at the micro level no longer work - and not because everything just drowns, can we add the same oil and drop by drop?
The problem is that as the size of moving parts decreases, the contribution of the surface increases. And any surface effects that are insignificant at the macro level begin to dominate at the micro level. In particular, surface tension. Therefore, at reduction of components, after a certain limit, it is impossible to use greasing. And dry friction appears on the scene. For example, the coefficient of dry friction of silicon on silicon (the most common material for MEMS) reaches 0.7. Those. 70% of the power of such an engine will go simply to rotate the rotor. We need to somehow deal with this. The obvious way is to apply a hard , low-friction coating to the components. Since we are talking about micro-components, and the thickness of the coating should be very small - usually we are talking about tens of nanometers, but there are also ultra-thin coatings with a thickness of 1–2 nm. In principle, there is a fair amount of coatings that can be used to reduce friction and wear — soft metals, organic self-orienting molecules, graphene, and diamond-like films. There is a lot of materials, but they all have certain disadvantages, and have not yet invented any universal one.
From this list, perhaps, diamond-like films (Diamond-like coatings, DLC) are best known. Moreover, they can be equally successfully applied both at the micro and macro levels. So, Hyundai currently uses DLC to cover the surface of valves in engines installed on top-end car models. It is planned to use DLC in HDD for hardening the seating surfaces of hydrodynamic bearings. You can find hundreds of other DLC applications in real life, including the coating on the cutting edge of shaving blades. In most cases, magnetron sputtering is used to apply DLC — a well-known and proven method. But, as always, there are nuances. The most important - all this is very expensive. There are also purely technical problems, such as high levels of internal stress, sensitivity to humidity, and the like. Therefore, attempts to make DLC cheaper and even better, do not stop.
')
Together with colleagues from my alma mater, for several years we have been developing one, one could say alternative technology — deposition of diamond-like films by an ion beam, in which the material is not atomic carbon, but fullerene C 60 molecules. Fullerene is ionized, accelerates to 5 keV and pounds on the substrate. In this case, the molecules break up, and an amorphous structure with interesting properties is formed from debris. Details can be found in this article . This method has its advantages, in particular, our films are not afraid of moisture, and the use of an ion beam allows you to apply a coating on objects of arbitrary shape, which is somewhat difficult in the case of magnetron sputtering. The disadvantage of our films is a rather high level of self-stress. The film tends to expand, to occupy a larger volume than it has. This leads to unpleasant consequences - if you apply such a film on a thin substrate - the substrate can be bent. If the substrate is thicker, and the adhesion between the film and the substrate is not good enough - the film will simply peel off.
We had the idea to dilute the solid mass of the DLC with something soft to compensate for internal stresses. And, since fullerene was used as the base material, it was added. It turned out that if, parallel with the ion beam, a molecular beam is directed onto the substrate, the result is a certain nano-composite in which fullerene molecules are surrounded by solid amorphous carbon. As expected, the level of stresses in such a film turned out to be significantly less. Generally speaking, we found no stresses. Of course, the hardness of the film also decreased - if the value of the film deposited from the ion beam is 50–60 GPa, the nano-composite showed 25–30 GPa. But it is still quite a lot - for example, the hardness of monocrystalline silicon is ~ 10 GPa. Hurray, the problem is solved. Here, in the process of measuring hardness, the discovery, which I spoke at the very beginning, crept up.
But before we get to the point, we need to make another digression. Tell about how to measure the hardness of the films. In principle, the method is the same - we take a calibrated diamond pyramid and press it into the surface with a certain effort. The softer the material, the deeper the pyramid is pressed. We measure the size of the print - we get the hardness. All this is easy when you need to measure the hardness of the rail. And it becomes difficult when it comes to films with a thickness of 100 nm. For these purposes, a nanoindentation method (nanoindentation, depth sensing indentation) was developed. The bottom line is that we gradually increase the load on the pyramid (indenter) and at the same time fix the depth of penetration. The linear law of loading and unloading is usually used. Well, the pyramid needs a special one. In our case, this is a triangular pyramid with a tip diameter of 100 nm.
As a result of “controlled penetration”, for example, of a soft fullerene film, the following curve is obtained:
Here, the X axis is the depth of penetration of the indenter (in nanometers), Y is the force applied to the indenter. The red arrow shows the direction of the load, the green - the discharge. Depth of penetration depends on hardness. The softer the material, the deeper the indenter penetrates under the same load. In this case, the elasticity (Young's modulus) can be calculated from the angle of inclination of the load curve. Load curves and unloading curves do not match as a result of plastic deformation at the point of contact. If we examine the imprint with an atomic-powered microscope, we get the following picture:
At the left - the top view, on the right - sections along the red and green lines. It is clearly seen that the indenter is a trihedral pyramid :). In the case of elastic materials, such as rubber, the load curve will coincide with the unloading curve, because in this case only elastic deformation takes place, up to a certain limit, of course, well, there will be no imprint on the surface. The case when the discharge curve will lie higher than the load curve, in principle, is not possible.
Well, once, this “impossible” curve was experimentally fixed (figure d):
At first I just decided that it was some kind of glitch in the device. Then checked again. Reproduced. Did not believe. Then he began to understand. As it turned out, this phenomenon is characteristic of nano-composites consisting of a mixture of fullerene molecules and solid amorphous carbon. Depending on the speed at which the load and unloading is performed during the test, the curve changes its shape. When we press quickly - we get a typical picture for a solid film (a). We push slowly - we get “what cannot be”. It is obvious that at a low speed of forcing in the film, some additional driving force arises, which pushes the indenter back. But which one?
A detailed analysis showed that in the case of “anomalous” indentation, instead of a print, a hill with a height of several tens of nanometers (a, b) is formed:
Detailed analysis showed that the height of the "hills" depends on the ratio of ionic and molecular beams in the manufacturing process (s).
Obviously, the material swells under load, which leads to the indenter being pushed out and the formation of hills instead of prints. So at the expense of what? With the joint use of ionic and molecular beams, polymerization of fullerene molecules occurs. In the normal state, they are connected by weak van der Waals bonds. However, if they are well “kicked,” a much stronger covalent bond is formed between two neighboring molecules. These two types of bonds, in addition to strength, differ in length. The covalent bond is shorter, and the polymerized molecules are packed more tightly. When "piercing", polymer complexes at the site of contact are deformed, and covalent bonds are destroyed. As a result, tightly packed molecules tend to move away from each other, which leads to an increase in volume, filling in the imprint and forming a hill. Why this effect is observed only with slow indentation? We believe that the yield of depolymerized molecules to the surface is a diffusion process, and with the rapid indentation they simply do not have enough time.
In addition to “self-healing” surfaces, such nano-composite films demonstrate another interesting property - dynamic hardness. The film is very hard in the case of a shock load, while being relatively soft and pliable in the case of a load that is constant or slowly increasing. Why this is needed - we have not yet invented, while thoughts like “nano-bulletproof vests for nano-robots” are floating in the air. Any ideas?
A more detailed description can be found in this article . She's on sci-hub: http://pubs.acs.org.sci-hub.org/doi/abs/10.1021/nl500321g .
______________________
From the beginning - a small excursion into that field of physics and nanotechnology, I study bark - this is micro- and nano-tribology. Tribology itself is a very respectable age science that deals with friction and wear. It would seem that everything in this area has long been known - poured more grease, and no friction. And to study here, from a scientific point of view, nothing special. But with the development of microminiaturization, tribology received three times the breath. Because the methods of the macro world (pour out a bucket of oil) at the micro level no longer work - and not because everything just drowns, can we add the same oil and drop by drop?
The problem is that as the size of moving parts decreases, the contribution of the surface increases. And any surface effects that are insignificant at the macro level begin to dominate at the micro level. In particular, surface tension. Therefore, at reduction of components, after a certain limit, it is impossible to use greasing. And dry friction appears on the scene. For example, the coefficient of dry friction of silicon on silicon (the most common material for MEMS) reaches 0.7. Those. 70% of the power of such an engine will go simply to rotate the rotor. We need to somehow deal with this. The obvious way is to apply a hard , low-friction coating to the components. Since we are talking about micro-components, and the thickness of the coating should be very small - usually we are talking about tens of nanometers, but there are also ultra-thin coatings with a thickness of 1–2 nm. In principle, there is a fair amount of coatings that can be used to reduce friction and wear — soft metals, organic self-orienting molecules, graphene, and diamond-like films. There is a lot of materials, but they all have certain disadvantages, and have not yet invented any universal one.
From this list, perhaps, diamond-like films (Diamond-like coatings, DLC) are best known. Moreover, they can be equally successfully applied both at the micro and macro levels. So, Hyundai currently uses DLC to cover the surface of valves in engines installed on top-end car models. It is planned to use DLC in HDD for hardening the seating surfaces of hydrodynamic bearings. You can find hundreds of other DLC applications in real life, including the coating on the cutting edge of shaving blades. In most cases, magnetron sputtering is used to apply DLC — a well-known and proven method. But, as always, there are nuances. The most important - all this is very expensive. There are also purely technical problems, such as high levels of internal stress, sensitivity to humidity, and the like. Therefore, attempts to make DLC cheaper and even better, do not stop.
')
Together with colleagues from my alma mater, for several years we have been developing one, one could say alternative technology — deposition of diamond-like films by an ion beam, in which the material is not atomic carbon, but fullerene C 60 molecules. Fullerene is ionized, accelerates to 5 keV and pounds on the substrate. In this case, the molecules break up, and an amorphous structure with interesting properties is formed from debris. Details can be found in this article . This method has its advantages, in particular, our films are not afraid of moisture, and the use of an ion beam allows you to apply a coating on objects of arbitrary shape, which is somewhat difficult in the case of magnetron sputtering. The disadvantage of our films is a rather high level of self-stress. The film tends to expand, to occupy a larger volume than it has. This leads to unpleasant consequences - if you apply such a film on a thin substrate - the substrate can be bent. If the substrate is thicker, and the adhesion between the film and the substrate is not good enough - the film will simply peel off.
We had the idea to dilute the solid mass of the DLC with something soft to compensate for internal stresses. And, since fullerene was used as the base material, it was added. It turned out that if, parallel with the ion beam, a molecular beam is directed onto the substrate, the result is a certain nano-composite in which fullerene molecules are surrounded by solid amorphous carbon. As expected, the level of stresses in such a film turned out to be significantly less. Generally speaking, we found no stresses. Of course, the hardness of the film also decreased - if the value of the film deposited from the ion beam is 50–60 GPa, the nano-composite showed 25–30 GPa. But it is still quite a lot - for example, the hardness of monocrystalline silicon is ~ 10 GPa. Hurray, the problem is solved. Here, in the process of measuring hardness, the discovery, which I spoke at the very beginning, crept up.
But before we get to the point, we need to make another digression. Tell about how to measure the hardness of the films. In principle, the method is the same - we take a calibrated diamond pyramid and press it into the surface with a certain effort. The softer the material, the deeper the pyramid is pressed. We measure the size of the print - we get the hardness. All this is easy when you need to measure the hardness of the rail. And it becomes difficult when it comes to films with a thickness of 100 nm. For these purposes, a nanoindentation method (nanoindentation, depth sensing indentation) was developed. The bottom line is that we gradually increase the load on the pyramid (indenter) and at the same time fix the depth of penetration. The linear law of loading and unloading is usually used. Well, the pyramid needs a special one. In our case, this is a triangular pyramid with a tip diameter of 100 nm.
As a result of “controlled penetration”, for example, of a soft fullerene film, the following curve is obtained:
Here, the X axis is the depth of penetration of the indenter (in nanometers), Y is the force applied to the indenter. The red arrow shows the direction of the load, the green - the discharge. Depth of penetration depends on hardness. The softer the material, the deeper the indenter penetrates under the same load. In this case, the elasticity (Young's modulus) can be calculated from the angle of inclination of the load curve. Load curves and unloading curves do not match as a result of plastic deformation at the point of contact. If we examine the imprint with an atomic-powered microscope, we get the following picture:
At the left - the top view, on the right - sections along the red and green lines. It is clearly seen that the indenter is a trihedral pyramid :). In the case of elastic materials, such as rubber, the load curve will coincide with the unloading curve, because in this case only elastic deformation takes place, up to a certain limit, of course, well, there will be no imprint on the surface. The case when the discharge curve will lie higher than the load curve, in principle, is not possible.
Well, once, this “impossible” curve was experimentally fixed (figure d):
At first I just decided that it was some kind of glitch in the device. Then checked again. Reproduced. Did not believe. Then he began to understand. As it turned out, this phenomenon is characteristic of nano-composites consisting of a mixture of fullerene molecules and solid amorphous carbon. Depending on the speed at which the load and unloading is performed during the test, the curve changes its shape. When we press quickly - we get a typical picture for a solid film (a). We push slowly - we get “what cannot be”. It is obvious that at a low speed of forcing in the film, some additional driving force arises, which pushes the indenter back. But which one?
A detailed analysis showed that in the case of “anomalous” indentation, instead of a print, a hill with a height of several tens of nanometers (a, b) is formed:
Detailed analysis showed that the height of the "hills" depends on the ratio of ionic and molecular beams in the manufacturing process (s).
Obviously, the material swells under load, which leads to the indenter being pushed out and the formation of hills instead of prints. So at the expense of what? With the joint use of ionic and molecular beams, polymerization of fullerene molecules occurs. In the normal state, they are connected by weak van der Waals bonds. However, if they are well “kicked,” a much stronger covalent bond is formed between two neighboring molecules. These two types of bonds, in addition to strength, differ in length. The covalent bond is shorter, and the polymerized molecules are packed more tightly. When "piercing", polymer complexes at the site of contact are deformed, and covalent bonds are destroyed. As a result, tightly packed molecules tend to move away from each other, which leads to an increase in volume, filling in the imprint and forming a hill. Why this effect is observed only with slow indentation? We believe that the yield of depolymerized molecules to the surface is a diffusion process, and with the rapid indentation they simply do not have enough time.
In addition to “self-healing” surfaces, such nano-composite films demonstrate another interesting property - dynamic hardness. The film is very hard in the case of a shock load, while being relatively soft and pliable in the case of a load that is constant or slowly increasing. Why this is needed - we have not yet invented, while thoughts like “nano-bulletproof vests for nano-robots” are floating in the air. Any ideas?
A more detailed description can be found in this article . She's on sci-hub: http://pubs.acs.org.sci-hub.org/doi/abs/10.1021/nl500321g .
______________________
The text was prepared in the Blog Editor from © SoftCoder.ru
Source: https://habr.com/ru/post/222055/
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