Advances In Diamond Coating Technology

By Dr Thoguluva Raghavan Vijayaram PhD

Senior Lecturer, Department of Manufacturing Process and System,
Faculty of Manufacturing Engineering,
UTeM Universiti Teknikal Malaysia, Melaka Ayer Keroh, 75450 Melaka Malaysia.
Email: thoguluva@utem.edu.my

Diamond coating technology is used to deposit a uniform layer of diamond on almost any type of material ranging from glass and plastic to metals. It is done by using carbon dioxide from the air, as the carbon source and subjecting it to a combination of lasers. This laser supported process creates pure diamond and bonds it to a surface of a material with the ease of paint on a brush. This article discusses on the advances in diamond coating processing, advantages, and its applications in various engineering fields.

Diamond Coating process can do in seconds than the conventional chemical vapor deposition process which takes hours. It is possible to coat the cutting edges of all types of tools that will last much longer and dull only after prolonged use. Valves, casings and blades of rotating machinery are subjected to wear during operations. Hence, diamond coating is applied to avoid these. Besides, longer-lasting tools, instruments, and wind shields are only a few of the available applications for diamond coating.

Important advances have been made in the diamond coating of metals, glass, ceramics, and plastics, using various techniques, such as CVD, plasma- assisted vapor deposition, and ion-beam-enhanced deposition. Examples of diamond-coated products are: scratchproof windows such as those used in aircraft and missile sensors for protection against sandstorms; sunglasses; cutting tools such as inserts, drills, and end mills; wear faces of micrometers and calipers; surgical knives; razors; electronic and infrared heat seekers and sensors; light emitting diodes; diamond-coated speakers for stereo systems; turbine blades; and fuel-injection nozzles. Techniques have also been developed to produce free-standing diamond films on the order of 1 mm thick and up to 125 mm in diameter; these include smooth, optically clear diamond film, unlike the hazy gray diamond film formerly produced. The film is then laser cut to desired shapes and brazed onto, for example, cutting tools.

The development of these techniques, combined with the important properties of diamond like hardness, wear resistance, high thermal conductivity, and transparency to ultraviolet and microwave frequencies, have enabled the production of various aerospace and electronic parts and components.

Chemical Vapor Deposition: CVD can be directly synthesized to a cutting tool substrate, eliminating many steps in the PCD fabrication process. It also allows diamond to be used on intricate shapes such as cutting-tool inserts with molded-in-chip breaker geometry, twist drills, and taps.

The process and most of its variants consist of the following steps:

The first step is to produce atomic hydrogen from the diatomic hydrogen molecule. This can be done by any method that adds enough energy to the gas for dissociation. The typical methods are thermally assisted and plasma-assisted processes. The hydrogen passes over a hot filament or through plasma and separates into atomic hydrogen.
Then the gaseous hydrocarbon source, such as methane or propane, is mixed with the hydrogen and passed over a substrate.
Diamond condenses and falls onto the substrate as crystals, which fuse into a polycrystalline layer.

Diamond coatings are still relatively expensive, and the CVD process is still a batch processing method. Deposition rates range from less than 1 micron to 5 microns per hour. The substrate to be coated must be thoroughly cleaned and preheated in a vacuum chamber that limits the size of the articles to be coated. It is restricted to limited surface chemistry. A batch of articles in a reactor may take 24 hours or more to be coated. The major difficulty with the cutting-tool applications is the adhesion of the diamond coating to the tungsten carbide substrate. This has caused producers to use costly methods of substrate preparation to achieve good bonding.

A major breakthrough in diamond deposition technology occurred when Pravin Mistry, a Metallurgist, was trying to fabricate hard materials, using lasers to synthesize ceramics and metal-matrix composites on aluminium extrusion dies to improve their performance and longevity. During the laser synthesis of titanium carbide, Mistry switched carbon dioxide for nitrogen and produced a coating speckled with some black particulate inclusions. Analysis of the coating’s surface indicated the presence of polycrystalline diamond and found a radical method for synthesizing polycrystalline diamond films. The QQC diamond coating process uses the carbon dioxide from the atmosphere as the carbon source and subjects it to multiplexed lasers to produce a diamond film that can be deposited on to almost any material.

QQC, Inc. is located at Dearborn, Michigan, USA, founded by Pravin Mistry, an Indian Born British Metallurgist. Since it was developed by that concern, it was named as QQC process. The QQC process creates diamond in an ordinary atmosphere, not the high temperature vacuum used in standard diamond manufacture. Multiple laser beams are directed through a cloud of carbon dioxide at a tungsten carbide surface. The lasers break the carbon dioxide into oxygen and carbon. Diamond is formed from the bonding of this carbon with carbon atoms that the laser energy has put into motion from the rotating object surface.

Steps involved in the QQC process:

Laser energy is directed at a substrate to mobilize, vaporize, and react a primary element, carbon, contained within the substrate.
This changes the composition particularly the crystalline structure of the basic element and diffuses the modified constituent back into the part, as an addition to fabricating a coating diamond or diamond like carbon on the part surface.
This creates a conversion zone immediately beneath the substrate and results in diffusion bonding of the coating to the substrate.
Additional (secondary) similar (e.g, carbon) or different elements may be introduced in a reaction zone on and above the part surface to expand the fabrication and the composition of the coating.
The laser energy is provided by a combination of lasers: excimer, Nd: YAG, and CO2 is shown below in Figure 1.

Figure-1 QQC process is used to produce a diamond coating by a combination of lasers: excimer, Nd: YAG and CO2

The output beams are directed through a nozzle delivering the secondary element to the reaction zone.
The reaction zone is shielded by an inert, non reactive shielding gas, like Nitrogen, delivered through the nozzle.
Flat plasma is created by the lasers, constituent element and secondary element on the surface of the substrate, and the flat plasma optionally extends around the edges of the substrate to fabricate the coating.

Certain advantageous metallurgical changes are created in the substrate due to the pretreatment. The processes (pretreatment and coating fabrication) are suitably performed in ambient, without preheating the substrate and without a vacuum. The object to be coated can be moved around by a robotic arm under the laser to control the deposition of the diamond. Adjustment of the lasers can control crystal size and structure. Most synthetic diamond is made by CVD. In spite of years of efforts, the CVD process can still coat only a few, coin-sized shapes and requires a vacuum chamber that must be heated to 800 degree centigrade.

The thickest layer of diamond made so far by the QQC process has been 1000 microns, compared with the 7 to 22 micron layers usually created by CVD. Most amazing is his how fast the diamond forms, at a rate of about 1 micron per second, while it bonds metallurgically to the surface below. This compared with a few microns per hour for CVD.

The initial claims of field tests stated that tools coated both in diamond and in TNC (tetrahedrally bonded non crystalline carbon) are being used for some production automotive power train and chassis components shown below in Figure-2.

Figure-2 QQC coating process can be applied to a wide variety of products

These include gears, shock-rods and struts, and brake rotors to provide corrosion-proof properties, to improve wear and tear, and sin some cases to replace chromium and cadmium plating. Some tests found QQC coated tools to be the best in terms o performance, wear, and adherence on carbide tool inserts.

The key advantages of the QQC process over existing technology include:

Superior adhesion and reduced interfacial stress result from a graded metallurgical bond between the diamond and the part.
Only carbon dioxide is used as a primary / secondary source for carbon with nitrogen acting as a shield and possible stoichiometric (stock-piling) process ingredient. This replaces the use of dangerous gases such as hydrogen and methane, critical ingredients in the CVD process.
Deposition rates are dramatically increased, with linear growth rates exceeding 1 micron per second, as opposed to 1 to 5 microns per hour by CVD. This is a key economic factor in commercialization of the process.
The process can be applied to almost any substrate such as stainless steel, high-speed steel, iron, plastic glass, copper, aluminium, titanium, and silicon.
Cobalt content limitations for CVD require special substrates for tungsten carbide cutting tools that can compromise the insert’s toughness. The multiplexed laser process can accommodate any percentage of cobalt without affecting the diamond synthesis.
Unlike CVD, the process can be changed automatically to control crystal size, orientation, and morphology. The system can produce tetrahedrally bonded non crystalline carbon, hydrogenated diamond like carbon super lattice hard coatings, and other coatings to achieve desired properties.

Diamond coating processing is used to deposit a uniform layer of diamond on almost any type of material ranging from glass and plastic to metals. The process is carried out without the restrictions of a vacuum chamber. Controlling movements of the lasers or work piece can coat almost any size or shape. Operations such as coating continuous wire, fiber, or coiled stock are possible. Pretreatment and or preheating of the substrate is not required, permitting coating of the substrate of as-manufactured components and elimination of wet chemistry pretreatment. It is concluded that the system is production engineered to permit the economical coating of production components with 24-hour unmanned operation.

About The Author:
Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy, and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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