Diamond-like coating

DLC coating for optical and tribological applications

Naturally occurring diamond is almost always in crystalline form with a purely cubic orientation of sp ³ bonded carbon atoms. Sometimes defects occur in the crystal lattice or the inclusion of atoms of other elements that add color to the stone, but the arrangement of carbon in the lattice remains cubic with an sp ³ bond. The internal energy of cubic polytypes is slightly less than the hexagonal forms , and the crystal growth rate from molten material during natural and mass production of synthetic diamonds is quite slow, so that the lattice structure grows in a low (cubic) shape energy, which is possible for sp ³ bonds of carbon atoms. In contrast, DLC coatings are typically produced by processes in which carbon atoms are rapidly cooled and quenched on relatively cold surfaces at high energy. Such processes can be, for example, in plasma , in filtered cathode-arc deposition , in sputtering and .

In these cases, the cubic and hexagonal lattices can be randomly mixed, the atomic layer behind the atomic layer, because there is no time for one of the crystal geometries to grow due to the other before the atoms are frozen in place in the material. Amorphous DLCs can lead to materials that do not have a long crystalline order. Without long-range order, there are no brittle crack planes. Therefore, such coatings are flexible and conformally coated to the basic form, but also durable as diamond. In fact, this property was used to study atom-by-atom wear at the nanoscale in the DLC. ^[7]

Here are a few DLC production methods that rely on lower sp ² carbon densities. Thus, the use of pressure, external influences, catalysis, or some of their combinations on an atomic scale can affect sp ² bonded carbon atoms closer to each other in sp ³ bonds. This should be done sufficiently strong so that the atoms could not simply spring from each other at distances characteristic of sp ² bonds. Typically, methods combine this compression with pushing a new sp ³ bound carbon cluster deeper into the coating, so that there is no way to return to sp ² . Or a new cluster is “buried” with the advent of new carbon, intended for the next cycle of impacts. It is advisable to provide a process of “hail” of shells, which occurs localized, faster, nanoscale versions of the classic combinations of heat and pressure that produce natural and synthetic diamonds. Because they occur independently of each other in many places across the surface of a growing film or coating. "Recipe" is used depending on the specific option. There are cycles of carbon deposition, exposure to continuous proportions as carbon arrives, and projectile transfer shells necessary to form sp ³ bonds. Classically, “environment”, morphology for ta-C film is shown in the figure.

As the name implies, diamond-like carbon (DLC), the cost of such coatings is determined by their ability to provide some of the properties of diamond on the surface of almost any material. The main desirable qualities are hardness, wear resistance, and smoothness (the coefficient of friction of a DLC film against polished steel ranges varies from 0.05 to 0.20 ^[8] ). DLC properties strongly depend on the plasma treatment ^{[9] [10] of} the deposition parameters, such as the effect of bias voltage ^[11] , the thickness of the DLC coating ^{[12] [13] the} thickness of the intermediate layer, etc. In addition, heat treatment also changes the properties coatings such as hardness, toughness and wear rate. ^[14]

However, there is a decrease in production costs due to the amount and type of diluents, 7 forms, properties, and also the degree to which the DLC is added to the surface. In 2006, the Association of German Engineers, VDI, the largest engineering association in Western Europe, issued the authoritative report VDI2840 to clarify the existing plurality of confusing terms and trade names. It was accompanied by a unique classification and nomenclature for diamond-like (DLC) carbon and diamond films. This is necessary to identify and compare the various DLC films that are offered on the market. Excerpt from this document: - “These (sp ³ ) bonds can occur not only with crystals, in other words, in solids with a long order, but also in amorphous solids, where the atoms are in a random arrangement. In this case, there will be a bond only between several individual atoms, and not as in the long-range order covering a larger number of atoms. Types of bonds have a significant effect on the material properties of amorphous carbon thin films. If sp ² type is present, the film will be softer; if sp ³ type is present, the film will be more rigid. ”

It was found that the secondary determinant of quality is the fractional state of carbon. Some production methods use hydrogen or methane as a catalyst, and a significant percentage of hydrogen may remain in the DLC material. If we recall that soft plastic and polyethylene are made of carbon, which is connected by a pure diamond-like sp ³ bond, but also including the chemical bond of hydrogen, it is not surprising that the fraction of hydrogen remaining in the DLC films worsens their properties almost as well. as the remnants of sp ² carbon bonds do. The VDI2840 report confirmed the usefulness of locating a specific DLC material on a 2-dimensional map, on which the X axis describes the fraction of hydrogen in the material and, along the Y axis, describes the proportion of sp ³ bound carbon atoms. The highest quality of diamond-like properties was confirmed, which correlates with the proximity of the map plot point (X, Y) coordinates of a particular material in the upper left corner at point (0,1), namely 0% hydrogen and 100% sp ³ bonds. This “pure” DLC material, that is, ta-C and others, has approximate values ​​that decompose under the influence of diluents, such as hydrogen, sp ² bonded carbon and metals. The value of the properties of such a material is that it is fully or partially ta-C.

Hardness

In “cobblestones,” nodules, clusters, or “sponges,” the associated angles may be distorted. As a result, there is an internal stress that can increase the hardness of the DLC sample. Hardness is often measured by the nanoindentation method, a stylus with a natural diamond, pressed into the surface of the sample. If the sample is so thin that there is only one layer of nodules, the stylus can enter the DLC layer between the hard cobblestones and push them apart without feeling the hardness of the sp ³ bound volumes. Measurements may be reduced. And vice versa, if the stylus enters the film with a thickness sufficient to have several layers of nodules, so it cannot spread in the transverse direction, or if it enters the top of the “cobblestone” in one layer, then not only the real hardness of the diamond bond will be measured , but the apparent hardness is even greater, because the internal compressive stress in these nodules, provide resistance to penetration of the stylus material. In this case, the hardness is 50% higher than the hardness of natural crystalline diamond. Since the stylus in such cases is dulled and may even break, real numbers for hardness in this case do not make sense. They only show that solid particles of ta-C material will destroy natural diamond. However, from a practical point of view, it does not matter how the resistance of the DLC material develops. One of the methods for testing the hardness of the coating is carried out using the Persozu pendulum.

DLC Coating Linkage

The same with internal stress, which benefits the diamond-like material. Internal stress makes it difficult to bond such coatings on the substrate, which must be protected. This complex lack of hardness is noted in several ways, depending on the particular method of the manufacturing process. The simplest is to use a natural chemical bond, which occurs in those cases in which the incident carbon ions supply the material to be exposed to sp ³ unbound carbon atoms, and by the action of energy that compresses the previously condensed volumes of carbon. In this case, the first carbon ions will act on the surface of the product to be coated. If this element is made of a carbide-forming substance, such as Ti or Fe in steel, a carbide layer will be formed so that it will subsequently be bonded to the DLC grown on top of it. In 2006, many successful methods were implemented for bonding DLC ​​coatings.

Tribological

DLC coatings are often used to prevent wear due to their good tribological properties. DLCs ​​are very resistant to abrasion and adhesion. This makes it suitable for use in parts with strong pressure contact, in cases of both rolling and sliding. DLCs ​​are often used to prevent wear of razor blades, and metal cutting tools, including turning inserts and milling cutters. DLC is used in bearings, cams, pushers and shafts, in the automotive industry. Coatings reduce wear during the break-in period (for trains).

DLCs ​​can also be used as coatings for chameleons, which are designed to prevent wear during launch, orbit, and the return of launched spacecraft to earth. DLC provides lubricity in the atmosphere and vacuum, unlike graphite, which requires moisture. Despite the favorable tribological properties of the DLC, it must be used with caution for ferrous metals. If DLC is used at high temperatures, the substrate may be carbonized, which may lead to loss of function due to changes in hardness. This phenomenon does not allow the use of DLC coatings on steel parts of machines.

Electric

If the DLC material is close enough to ta-C in the regions of the binding coefficients and the hydrogen content, it may be a dielectric with a high resistance value. Perhaps more interesting is if they are prepared in an average cobblestone version such as shown in the figure above, electricity passes through it using the hopping conduction mechanism. In this type of electrical conductivity, electrons move by quantum mechanical tunneling between pockets of conductive material insulated in an insulator. The result is a process that makes the material look like a semiconductor. In future studies on electrical properties, it is necessary to explicate such conductivity in ta-C in order to determine its practical value. Nevertheless, various practical properties of emissivity have been shown to occur at unique levels for ta-C. Such high values ​​allow electrons to be emitted from the ta-C coating electrodes into vacuum, or into other solids using a moderate level of applied voltage. This supports important advances in medical technology.

The use of DLC usually increases the abrasion resistance of the material. Tool components such as end mills, drills, dies, and molds are often used with the DLC. DLCs ​​are also used in engines of ultra-modern sports motorcycles, Formula 1 racing cars, NASCAR cars, and also as a coating on the plates of the hard disk and the head of the hard disk, to protect against head malfunctions. Almost all multi-blade razors used for wet shaving have a face coated with a hydrogen-free DLC to reduce friction, preventing abrasion and wear on sensitive skin. DLC is also used as cover by arms manufacturers and gunsmiths. Some forms are certified in the European Union for catering services, and are widely used in high-speed actions involved in the processing of novelty products, such as chips and in the management of material flows in food packaging with plastic wraps. DLC covers the cutting edges of tools for high-speed dry complex formation of open surfaces of wood and aluminum, for example, on car dashboards. The wear, friction and electrical properties of the DLC make it an attractive material for medical applications. Fortunately, DLC has excellent biocompatibility. This allows you to do many medical procedures, such as percutaneous coronary intervention using brachytherapy, using the unique electrical properties of the DLC. At low voltage and low temperature of the electrodes, the DLC coating can emit enough electrons to be organized into disposable micro-X-ray tubes, as small as the radioactive seeds that are introduced into the artery, or tumors in conventional brachytherapy. The same dose of prescribed radiation can be applied from the inside with the additional possibility of turning radiation on and off in the established “picture” of x-ray radiation. DLC is an excellent coating to extend the life and reduce the complications of replacing hip joints and artificial knees. It has also been successfully used in stents of coronary arteries, which often reduces the occurrence of thrombosis. The implanted pump of the human heart can be considered the pinnacle of biomedical applications, where the DLC coating is used on the blood of contact surfaces, a key component of the device.

Space black stainless steel Apple Watch ^[15] coated with diamond-like carbon.

An increase in the service life of DLC coated products results from increased wear resistance. Abrasion can be described by the formula f = (g) µ, where g is the number characterizing the type of thin film, type of abrasion, substrate material, and, µ is the thickness of the DLC coating in microns ^[16] . For an abrasive with a “low impact” (pistons in cylinders, impellers in pumps for sandy liquids, etc.), g for pure ta-C for 304 stainless steel is 66. This means that the thickness of one micrometer (about 5 percent human hair) can increase the life of the product from a week to a year. The coating thickness of 2 microns increases the service life from 2 to 85 years. Values ​​are measured here; although in the case of measuring a sample with a thickness of 2 mm, the service life was measured until the wear of the device itself. Environmental arguments are also given here, that a sustainable economy should encourage products not designed to increase productivity, or prematurely fail. This, in turn, can reduce the need for large-scale production of parts and their frequent replacement, which were constrained by the economic factor. There are currently about 100 autosync manufacturers of DLC coatings that are based on graphite and hydrogen, and therefore give much lower G-numbers than 66 on the same substrates.

• Vacuum Arc Coating
• Chemical vapor deposition

1. Robertson, J. (2002). "Diamond-like amorphous carbon." Materials Science and Engineering: R: Reports. 37 (4-6): 129-281. doi: 10.1016 / S0927-796X (02) 00005-0.

• Useinov A., Gogolinsky K. Mechanical properties of ultrathin carbon diamond-like coatings // Nanoindustry. - 2010. - No. 5. - S. 54-56. - ISSN. - URL: [1]