Complete Guide To DLC Coating (Diamond Like Coating)

In the contemporary era, the use of DLC coatings has gained a lot of popularity and recognition in the commercial space. As a result of its outstanding hardness, friction resistance, wear, and corrosion resistance properties, DLC coatings have surfaced as the ideal solution for many applications such as tribological applications and many more. This application is evident especially where components are subjected to extreme friction, wear, contacts with other parts, and high loads. Hence, it makes it possible for DLC coating to help prevent components from galling, seizing, pitting, and failing when in action.

For more understanding of the essential information about DLC coatings from points of view of producers and consumers, I write this complete guide and put important contents about DLC coatings in sections for you to understand easily.

Chapter 1: What is DLC Coating?

DLC Coating which is also referred to as Diamond Like Coating is a nanocomposite coating to a host of substrate materials to display the unique properties of natural diamonds such as high hardness, resistance to corrosion, low friction, electrical insulation, and many more. In this regard, the main properties desired among these sets of properties is the hardness of the substance obtained. However, this DLC coating can be applied using multiple deposition techniques. There is a wide range of techniques, but the most common include Physical Vapor Deposition (PVD) and Plasma Assisted Chemical Vapor Deposition (PACVD).

This method of coating centers around carbon which exists in 2 allotropic forms, including carbon in the diamond crystal structure and carbon in the graphite crystal structure. However, these two forms exhibited different and useful properties. The diamond crystal structure is the hardest substance known whereas the graphite crystal structure is soft and lubricous. DLC coatings make it possible for the exhibition of the combination of a high level of hardness and low level of friction in tribological and wear applications. These coating can be done at low substrate temperature (<200C) and can be deposited on metals, alloys of metals, and non-metals including glass, plastics, silicon, and many more.

Chapter 1.1: Categories of DLC coatings

DLC coatings exist in varieties of forms They include:

Form a-C (hydrogen-free amorphous carbon)
Form ta-C (tetrahedrally bound hydrogen-free amorphous carbon)
Form a-C:H (An amorphous carbon with hydrogen
Form a-C:H:Me (Me = W, Ti, metal-doped amorphous carbon with hydrogen)
Form a-C:H:Si (This is a Si-doped amorphous carbon with hydrogen)
Form a-C:H:X (A non-metal-doped amorphous carbon with hydrogen)
Form a-C:Me (Me = Ti, metal-doped hydrogen-free amorphous carbon)
Form ta-C:H (A tetrahedrally bound amorphous carbon with hydrogen

Chapter 2: Properties of DLC Coating

Solid substances that are formed mainly of carbon possess excellent mechanical properties. Such properties exhibited by DLC Coatings (Diamond-Like Carbon) include:

Mechanical Properties
Low coefficient of friction
Corrosion Resistance
Electrical properties
Surface Properties of DLC coatings

Chapter 2.1: Mechanical Properties

The great types of DLC coatings and composition result in a wide range of mechanical properties. In this sense of properties, the hardness of these DLC structures varies from a few GPa for a-C to slightly above 60GPa fir ta-C. Also, for the elastic modulus, it ranges from several tens of GPa for a-C up to several hundreds of GPa for ta-C.

Although a-C and ta-C tend to decrease in elasticity and hardness, then higher sp3 fractions contents can help to improves these mechanical properties. As a result of these properties, DLC films are harder than most metallic materials used in the industry.

In a nutshell, these mechanical properties in form of hardness (wear resistance) and elasticity make DLC coatings more applicable in almost all manufacturing industries.

Chapter 2.2: Low Friction Performances

This property is known as the tribological performance of DLC coatings. In addition to hardness, Diamond possesses one of the lowest coefficients of friction to sliding tribological interfaces. This combination of ultra-low hardness and extreme hardness is rare in tribology and it has made DLC ideal for a wide range of tribological usage.

However, the low friction of diamond is largely attributed to the ultra-high inert or passive nature of its sliding contact surfaces. In essence, the DLC coating’s low-friction property is associated with a lack of adhesive forces between sliding diamond surfaces.

While some tribological applications are in the exploratory stage with the need for further development some tribological application has been established and are offered on a commercial scale. Examples of the fully utilized tribological applications are in the cutting industry and many more.

Chapter 2.3: Corrosion Resistance

DLC (Diamond-Like Cabon) coatings have a wide application in automobiles and electronic equipment, as a result of its superior corrosion resistance in corrosive environments. This corrosion resistance performance of Diamond-Like Carbon is attributed to its physical and chemical properties.

In the industry, there have been several ways to improve the anti-corrosion properties of DLC including chemical doping with foreign atoms. This doping exists to be an effective way to enhance the intrinsic properties of the host materials. Examples of metals used for this doping include Si, F, Cr, Ti, and many other metals to provide good corrosion resistance properties for the substrates.

In addition to this, the corrosion resistance performance of DLC coating can be enhanced by increasing the thickness of the coatings, However, increasing the thickness may increase the internal stress which may eventually lead to cracking of the coatings as a result of high internal stress. In a bid to solve this issue, the multiple-layer coatings are used because they possess small nanopores and obvious interfaces, crevices, or columnar structures and low internal stress.

Chapter 2.4: Electrical Properties

When considering electrical applications, the basic properties that come into play is electrical resistance. The Diamond-Like Coatings in terms of electrical properties can vary from being a semiconductor to being a wide band-gap insulator. These properties are determined by doping elements, impurities, bonding sate (SP²/SP³), growth defects, ad structure.

The resistance of a DLC coating can also be influenced by process parameters such as deposition rate substrate temperature, ion energy, and incident angle. DLC coatings possess a modest band-gap, still, they don’t behave like the typical semiconductors.

At room temperature, the mobility of DLC ranges from 10^-11 to 10^-12 cm2/V. As for the high electrical resistance DCL coatings possess a high range of values ranging from 10² to 1016 Ωcm. Hence, the reason for being a wide-band insulator.

Chapter 2.5: Surface Properties

The surface properties of DLC coatings are characterized by its texture which can be smooth or rough. As a result, DLC coating is known for having a good sliding property. To describe the roughness of the DLC coatings surface, the Ra and Rz are used.

If the surface of the coating is too rough, it will wear out the counterpart, thus not solving the wear issues. It is important to note that the correct specifications of Ra and Rz values must be used because it will greatly affect the adhesion of the DLC coatings and its behavior during operation.

DLC coating helps to increase the life span and to improve the performance of the structure or system. To achieve this, the surface is strictly monitored when coating components are employed to increase the wear resistance of the surface. This is however done by increasing the toughness and hardness of the coating.

Chapter 3: The Benefits of DLC Coating

Recently, a wide range of DLC coatings now features a lot of applications as opposed to when it was first introduced. In its early development, DLC coatings were primarily used for its hardness with a problem of adhesion and man more. Consequently, a wide range of engine components are coated with Diamond-Like Carbon (DLC) coatings and are also used in many varieties of industry.

DLC coatings are beneficial in many ways which include:

High hardness
Low coefficient of Friction
Wear and Abrasion Resistance
Layer structure
Biocompatibility
Chemical Inertness of DLC coatings

Chapter 3.1: High Hardness

The hardness property of DLC coating is the basic benefit to their usage. As a result, the coating of the substrate with DLC coatings makes it possible for a great improvement in performance and life span. DLC coatings exist in different forms including a-C, ta-C, and many more.

However, no matter which form is used for DLC coatings, all varieties of DLC are somewhat hard or even harder than the natural diamond. For example, the ta-C form of DLC measures ranges from 5000HV to 9000HV while other forms of DLC measures between 1000HV to 4000HV.

In addition to this, when optimized, DLC coatings improved the longevity of the coated structures in multiple folds up to a factor of 10.

Chapter 3.2: Wear and Abrasion Resistance

In addition to the hardness of DLC coatings, surface coated with DLC is resistant to abrasion which in turn helps to maintain smooth movement during operation. Due to this, these coated tools possess a longer life span in contrast with uncoated tools.

Also, its application is widely used in the manufacturing industry for engines that possess mechanical parts that slide, rotate, and experience abrasion. Examples of such include camshafts used in cars, boats, motorcycles, and many more like the ones used in Formula 1 racing.

Chapter 3.3: Low Friction Coefficient

Another important benefit of DLC coatings is its tribological application. These coatings confer a very low coefficient of friction to coated surfaces thereby improving performances in all moving parts especially for cutting tools, in engines, plastic injection molds, cams, bearings, a machine of cast & wrought aluminum, and many more.

With help of DLC coatings, efficiency increases during production because industrial machines can now operate at any speed with low friction. Also, the cost of production in these industries using DLC coatings has drastically reduced. This is because there has been low spending on lubricants.

Chapter 3.4: Layer Structure

This layer structure of DLC coating is related to the thickness of the coatings and the incompatibility of DLC coatings with the substrate. In time immemorial, DLC coating has undergone a series of development and has faced the problem of adhesion. This problem may be caused by the thickness of the DLC coating. If the thickness of the DLC coatings is increased, the corrosion resistance of the coating will be improved.

However, internal stress may increase if proper care is taken. This occurs when DLC coatings are combined with incompatible substrate leading to adhesion. This can be solved by using a multi-layer coating stack that has an adhesion layer.

The multi-layer coatings serve as a buffer that helps to reduce stress. This multi-layer coatings also allows for thicker coatings to achieve an impressive combination of properties including ultra-high hardness, ultra-low coefficient of friction, ultra-low rate of wear, and many more.

Commonly used tacks include titanium and other compounds of titanium such as titanium carbide, titanium nitride, titanium carbonitride, and many more.

Chapter 3.5: Chemical Inertness of DLC coatings

The DLC coating is highly chemically inactive and possesses not reactivity potential with acid and alkaline. The Diamond-Like Carbon coatings are an amorphous stable carbon layer in high density that functions to prevent corrosion and oxidation. Also, this inert state of Diamond-Like Carbon coatings is beneficial to reduce the reaction of the surface with other metals and cold-welding reduction.

Other benefits include:

Minimal temperature deposition
Excellent anti-adherence effect

Chapter 4: Applications

The DLC Coating has a wide range of applications in almost every branch of industry, including, manufacturing, electronics, optics, mechanical equipment, microelectromechanical circuits, medicine, and many more.

Chapter 4.1: DLC Coating in Molding Industries

Many moving components of the equipment used in the molding industries are DLC coated. The essence of this coatings is to give the injection molds the proper properties such as a reduction in wear and corrosion protection. When this is achieved, then a better dry-running or emergency operating properties are added.

Others have reported reduced cycle times with the utilization of DLC coatings. In the long run, the components possess a longer lifespan with less maintenance which in turn leads to higher productivity. DLC coatings are hence used to coat.

Cavities & Cores
Ejector Pins
Ejector Sleeves
Core Pins
Plastic Injection Molds
Rubber Molds

Chapter 4.2: DLC Coating in Metal Forming Industries

DLC coatings also have high application in the metal forming industries. Whether you are interested in piercing or stamping hard or soft materials, DLC coatings will help your dies or punches perform effectively. The ideal situation is hard and low friction coatings which helps to solve metal abrasive issues (wear and erosion) and metal adhesion issues (galling and pickup). DLC coatings are used in:

Coining Dies
Dies
Fine Blanking Tools
Punches

Chapter 4.3: For Engines

The use of DLC coatings is popular for engine applications in enhancing properties and reduction of wear of sliding parts. For engines, DLC coatings are widely used as they help to reduce friction, wear, and it confers longevity to this equipment. Thanks to the hardness of DLC coatings and the graphitization which ensures low frictions and longevity.

A typical example is the usage of DLC coating in the Formula 1 cars and motorcycle where higher horsepower is obtained for higher performance during racing. Other examples include:

Pistons
Tappets
Valves
Wrist Pins

Chapter 4.4: Spare Parts and Components

Diamond-Like Coatings (DLC) coating is also useful for camshaft, bearings, and more since they are subject to severe sliding conditions with higher rotating speeds. This helps to increase endurance reliability by strengthening the surface, optimizing the hardness, and reduction in friction for higher performance and productivity.

DLC coatings can be used to coat the following:

Cams or Slides
Bearings
Gears
Shafts

Chapter 4.5: Cutting Industries

DLC coatings also have a wide range of applications in the cutting industries. DLC coatings are used for many cutting tools including razor blades. Based on the evidence, the coating of medical saws used for bone ligation has demonstrated that DLC coating enhances the life span of the tools with very low wear. This is due to the low frictional coefficient of the DLC coating. Also, there was a low amount of necrosis and new tissues were able to form around the cut area. Examples of other coated tools:

Carbide Insert
Bone Saws
Drills
End Mills
Razor Blades

Chapter 4.6: Optics

DLC coatings are uniquely suited for the provision of long-lasting protection to decorative glass products. DLC coatings are chemically inert and they provide additional benefits to glass products in the form of easy-clean properties, corrosion resistance, and protection against bonding with Inorganic contaminants.

Such products that can be coated with DLC include:

Display cabinets
Mirrors
Shelving
Shower enclosures
Tabletops

Chapter 4.7: Decorative

DLC coatings are now being used extensively for decorative purposes. Especially in applications where wear and scratch-resistant are important to allow for a perfect look for a longer lifespan. These DLC coatings are used in different field of decorative including:

Building Hardware; both exterior & interior
Firearms
Luxury goods – watches
Sports and Hobby
Vehicles, Aircraft, and Boats

Chapter 5: Deposition of DLC Coating

There are several methods in the market available for the deposition of DLC coatings including ion beam, electron beam, PAVCD, sputtering, cathodic arc, and laser. In the market today, the use of PVD and PAVCD for DLC coatings is used because it is regarded as the most consistent and the highest quality. Here is a description of the methods and coating process.

Chapter 5.1: PVD Technology

PVD is referred to as a Physical Vapor Deposition. This method is used to produce metal-based coatings. This involves the generation of partially ionized metal vapor that reacts with certain gases which then forms a thin film. This thin film is created with a specified composition on the substrate.

The most commonly used method includes cathodic arc and sputtering. For cathodic arc, repetitive vacuum arc discharges to strike the target (metal) and to evaporate the material. Whereas, sputtering involves the formation of vapor by a metal target being bombarded with energetic gas ions. The PVD process is used to deposit coating made of carbides, nitride, carbonitrides of Cr, Ti, Zr, and alloys.

PVD coatings can be deposited in different layers such as mono-, multi-, and graded layers. Recent generations of PVD coatings are nano-structured and enhance properties are due to superlattice variations. Additionally, the PVD coating process can be tuned to produce a structure with desired properties. Such properties include hardness, friction, adhesion, wear, and many more.

Physical vapor deposition is carried out under a high vacuum condition at a temperature between 250℃ and 450℃. Although some cases require temperature below 70℃ or u to about 600℃. Coating thickness depends on choice and application but it mostly ranges between 2 and 5 µm. It can also be as thin as a few hundred nanometers or thickness and be >=15.

Chapter 5.2: PACVD Technology

PACVD is the short form for Plasma Assisted Chemical Vapor Deposition. This method of deposition is widely used to deposit Diamond-Like Carbon coatings. This process is vacuum-based and all educts of the PACVD process occur in a gaseous state.
In contrast with PVD, the gaseous depositing process makes it suitable for 3D coating with no need for rotation. PACVD coatings contain around 70% sp3 bonding and are amorphous. The sp3 bonding accounts for the ultra-high hardness (10-40GPa) property of the coating. At temperature below 200℃, the PACVD can deposit coatings for a wide range of non-conductive and conductive substrate materials.

Chapter 5.2.1: Advantages of PACVD

PACVD possess quite many advantages, they include:

Post-treatment is not required
Can deposit on a broad range of substrate
No occurrence of distortion in a high precision substrate
Uses a gaseous process that allows for uniform coating

Chapter 5.3: Ion Beam Deposition

Ion Beam Depositing is another method used for DLC coating on substrate materials. This method is an ion beam based with high quality of coats that can be deposited at a very low temperature (room temp). It comes with a disadvantage which is the rate of deposition. This process deposits 1 µm per hour – maximally and to ensure uniform deposition, the substrate of simple geometry requires complex manipulation.

Chapter 5.4: Process of Coating

The products or substrate is loaded into the stainless-steel vacuum chamber via a fixturing carousel and the chamber is then evacuated. The product undergoes a preheating phase at a low temperature that will not exceed 3000F (1500C). The preheating phase is carried out to ensure that all the absorbed moisture by the substrate materials has been gassed out. This process should be done before the deposition is carried out on the substrate.

After the completion of the preheat phase, there is a transition into the ion etching phase. During this phase, the product is bombarded with ions from argon gas. This is used to clean to scrub or sputter clean the product’s surface and to improve the adhesion of the coating to the substrate.

The phase then transitions into the coating phase immediately after the learning phase. At this stage, a lot of consideration must come into play to increase or improve the performance of the product at the end usage.

Amongst the thing that can be done is the use of the sputtering process to deposit a dense adhered smooth underlayer. After depositing the underlayer, the Deposition process transition into the DLC coating phase. This coating phase then deposits a dense and smooth amorphous hydrogenated carbon layer onto the surface of the product.

Note: Not all applications require underlayers. The use of underlayers only helps to improve performances for final use. However, if no underlayer is used, a dense uniform DLC coating can still be directly deposited on the substrate.

Chapter 5.5: What Happens During the Coating Phase?

A carbon carrying gas is introduced into the chamber during the coating phase of the process. The gas introduced into the chamber is the source of the amorphous carbon DLC coating. This carbon carrying gas introduced into the chamber is then ionized by auxiliary anodes.

This then undergoes what is known as cracking or separation of the hydrogen & carbon in the gas. There is an application of electrical charge to the carousel that draws the ionized hydrogen and carbon atoms in the gas.

Chapter 5.6: Final Steps of the Process

The carousel carrying the products undergoing DLC coating operates by rotation in the chamber. This rotation of the carousel carrying the products can be single, double, or triple-axis rotation. The choice of the rotation to be used depend on the coating uniformity or the complexity of the geometry of the products.

The essence of this is that there will be a uniform deposition of coats on the surface of the product compared to the conventional coating process. The application of the electrical charge draws the carbon/hydrogen to the surface. This then results in the formation of amorphous carbon or DLC film.

Chapter 6: Challenges

DLC coatings are used to increase the performance as well as the life-time of a component. There is a need to differentiate between wear and friction as they are interlinked. There are different mechanisms for different coating solutions.

Abrasive wear - the first wear mechanism

This wear type involves the removal of parts from the surface of the worn-out components. Solving this problem requires increasing the superficial hardness of the surface with a coating. This is done by increasing the superficial hardness of the DLC coatings to over 20GPa. When this is done, the component can overcome the abrasive and as well last for a full cycle of the engine.

The table below explains the different wear rates that can be obtained with a harness.

Wear Rate (mm³/Nm)	Type
1.8 X 10^-8	DLC 3
2.5 X 10^-8	DLC 2
2.O X 10^-8	DLC 1

It is also important to note that in this case, then we must consider the substrate materials. The hard DLC coatings must be supported by a sufficient amount of hard choke of the substrate. This way, we can effectively improve the lifespan of the coated component. When high pressures are applied or there is a high concentration of pressure at a point, then the hardness requirements can be increased.

Galling - the second wear mechanism

This involves the transfer of materials in the tribological system between two parts. This can also be solved by using different properties of DLC coatings. The first mechanism is that DLC coatings are an amorphous carbon-based film acting as a barrier and the second is the low friction properties which help to prevent the phenomena known as cold-welding.

For consideration in solving this kind of wear, the hardness of the substrate is less important. However, the hardness must also be considered because the roughness of the surface will greatly affect the overall result. In addition to this, another substrate material can be considered to allow for cheap and easier machine materials.

Chapter 7: F.A.Q

Final Thought

Recently, there has been an advancement in the development of harder, yet smooth Diamond-Like carbon coatings. Therefore, we can say that the driving force is a refinement in the application process. In all ways, the Diamond-Like Carbons (DLC) coatings remain one of the ideal solutions for demanding tribological applications. In this environment, components are subjected to extreme friction, contact with other parts, and wear. Additionally, high hardness properties also make it relevant in the industry.

Thank you for reading this guide, hope it is useful for you. If you like this post, please share it on your social networks, as an encouragement to me. Thanks in advance!

Written by Austin