Acree Technologies Inc.

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DLC | Diamond-Like Carbon Coatings

DLC is an acronym for diamond-like carbon. DLC has some of the valuable properties of diamond, including:  high hardness, low friction, resistance to wear, chemical inertness, biological compatability, electrical insulation, optical transparency, and smoothness. In common terms, DLC is harder than natural diamond and slicker than "Teflon."
DLC coatings are used to impart some of the useful characteristics of diamonds onto other materials. DLC coatings can be deposited on nearly all metals, metal alloys, and also on nonmetals such as silicon, glass, ceramics, plastics, etc. DLC can be deposited at low (<200C) substrate temperature.
DLC coating has many commercial applications, including machine tools, aerospace parts, engine parts, medical implants, and high-end watches. Depending on the application, different formulations of DLC coatings are used.

FORMS OF DLC

Carbon comes in several allotropes, each of which has its own unique structure. Two of these allotropes, diamond (sp3) and graphite (sp2), are found in DLC. Diamond (sp3) has carbon atoms arranged in 3 dimensional cubic lattices while graphite has a layered, planar structure in which the layers are arranged in a honeycomb lattice.  These two allotropes are the main ingredients of DLC, of which there are 7 different forms.  
Despite the name "diamond-like," DLC is not in fact like  natural diamond. DLC coatings do not have the crystalline geometries that are found in nature, but instead are amorphous. DLC coatings are made from random alternations between cubic and hexagonal lattices, which creates no long-range order and therefore no fracture planes along which to break. The result is an exceptionally hard material. DLC looks smooth when seen with visible light, but under a microscope it actually resembles a cobblestone street.
DLC comes in 7 forms, of which "tetrahedral amorphous carbon (ta-C)" is be considered to be the "pure" form, since it consists only of sp3 bonded carbon atoms. However, due to patent restrictions and expensive licensing fees, pure ta-C is typically reserved for high value components. The other 6 forms of DLC, which include mixtures of sp2 and sp3 carbon along with other elements, are therefore more economical and more commonly applied.
Different process parameters control the characteristics of DLC coatings. These include factors such as:  the deposition method, the ratio of sp2 to sp3 carbon, substrate bias voltage, process time, ion energy and density, and substrate temperature. Thus many attributes, such as coating thickness, hardness, resistivity, hydrogen content and others can be controlled as needed for various applications.

DLC LAYER STRUCTURE

DLC, in its early development, had problems with adhesion. DLC tends to have high flim stresses, which when combined with lattice mismatches between DLC and many substrates, led to poor adhesion. However, this problem was solved by the use of multilayer coating "stacks" that include an adhesion layer.  These stacks reduce Hertzian stress concentrations near the coating/substrate interface by virture of graded interfaces, which create a higher modulus of elasticity. This ensures that there are no abrupt changes in composition, and that the stress is introduced into the coating gradually, resulting in excellent adhesion of the DLC. Today, all DLC coatings are stacked, and this influences other important coating properties besides adhesion. The multilayer structures act as buffers, which reduce film stresses. This allows for thicker coatings, which creates excellent properties, such as: extremely high microhardness, low coefficients of friction, slower rates of wear, etc. A commonly used stack is: titanium, titanium nitride, titanium carbonitride, titanium carbide, and finally, DLC.

DLC HARDNESS

DLC coatings produce dramatic improvement in performance and life of tools, components, and machines. The hardness of DLC coatings is the foundation for their benefits. DLC in all forms is extremely hard. Depending on which form is applied, DLC is as hard, or even harder, than natural diamond. In ta-C form, DLC typically measures between 5000-9000HV. Other forms range from 1000-4000HV.
The high hardness of DLC coatings reduces the likelihood of hard particle penetration into tools or parts. Optimized DLC coatings have been shown to improve the lifespan of tools by up to a factor of 10. For example, DLC coatings created major improvements harsh environment of machining stainless steel. Prior to DLC coatings, jobs were done with uncoated tools and hard to work at low speeds and feeds. DLC was a massive game changer, improving process speeds and tool longevity by an order of magnitude. Because of its durability, DLC is used as tribological coating for machine tools such as drill bits, saws, and dies.

DLC RESISTANCE TO WEAR AND ABRASION

DLC’s hardness also makes it durable. DLC coating protects moving parts from abrasion maintaining smooth movement much longer than uncoated parts. Engines with DLC coated parts create more horsepower, and have longer lifetimes from mechanical parts that rotate, slide, and face other types of wear. For example, DLC is now standard practice on camshafts in all types of Formula 1 racing including cars, motorcycles, and boats.

DLC LOW COEFFICIENT OF FRICTION

DLC coatings creater lower coefficients of friction. As friction is the enemy of almost all moving parts, lowering it creates nearly universal improvement, regardless of the industry. Thus, DLC is found in engines, tools, machining of cast and wrought aluminum, plastic injection molds, pumps, machine parts, bearings, cams, and even razor blades. Reduced friction also reduces the need for lubrication, which improves efficiency within the supply chain from raw material through to the end user.

DLC CHEMICAL INERTNESS

DLC coating is an amorphous, stable carbon layer that does not react to acids or alkaline. It is highly resistant against oxidation and corrosion. The high density and amorphous structure of DLC inhibit corrosive by-products from penetrating into tools. The chemically inert characteristics of diamond-like coatings dramatically reduce possibility of cold welding and material pickup on the surface of the tool

DLC BIOCOMPATIBILITY

DLC has been tested extensively for biocompatibility. Studies (both in vitro and in vivo)have focused on the interaction between DLC and macrophage cells (large white blood cells that engulf foreign bodies), fibroblasts (connective tissue forming cells) and osteoblasts (bone-forming cells).

Some key findings include:

  • Cells derived from the tissues that surround a DLC coated joint replacement (macrophages, fibroblasts and osteoblast-like cells) showed no evidence that DLC coatings caused cytotoxicity
  • DLC coating was found to have no adverse effects on cellular metabolism when it was investigated by measuring the production of three osteoblast-specific marker proteins: alkaline phosphatase, osteocalcin, and type I collagen
  • DLC does not induce inflammatory reactions in cells
  • Fibroblast and osteoblast cells attach to DLC coatings- required for both good contact between the implant and the bone, and to allow growth of bone over/around the implant
  • Osteoblast cells spread well on DLC. Spreading of the osteoblast cells indicates potential of bone to grow over a DLC-coated implant
  • DLC deposited on a Ti alloy has a low level of cytotoxicity and acts as a diffusion barrier between the titanium alloy (which caused otherwise the death of many cells) and the fibroblast cells

The bottom line is that DLC has been proved safe and effective for implanted medical devices such as stents, hip and knee joints. DLC coatings allow implants to maintain integrity, avoid formation of debris, prevent uncontrolled cell growth, and to not cause infections.

DLC DECORATIVE

DLC is a popular decorative coating on fine watches. When used on a watch, DLC coatings provide superior durability and wear resistance. The coatings, which are shiny and black, also create aesthetic appeal. Glancing blows against a hard surface, which may dig in and damage a normally coated watch, are far less likely to mar a DLC coated case. These characteristics, along with the beauty of DLC, have helped it to grow in popularity as a hard surface coating for high-end watches.

DLC BENEFITS SUMMARY

DLC offers a wide variety of properties in one coating:
high hardness
lowest available coefficient of friction
no surface roughness changes that could influence galling and release properties
corrosion protection
coating temperature below 200°C
removable
biocompatible

DLC AT ACREE

Acree Technologies offers a complete range of DLC coatings, in all phase compositions. Based on your application, Acree will help you determine which DLC coating structure is best suited to your project.

 

DLC coating on shock shafts


Telephone: 925-798-5770