What Are Hard Coatings?
Hard coatings, in one form or another, have been around for a long time. The need to preserve or protect the surface of materials is not new. From the prehistoric period of the Stone Age to the advent of metal working, man has been trying to keep implements with an edge, a point or a percussion surface more durable and lasting. In more recent times, a great deal of research has been carried out to create hard and wear resistant coatings that can be applied to different surfaces that alter the physical properties of those materials in a positive and meaningful way. Whether those applications are applied to high speed steel (HSS) tool coatings to produce hard and wear resistant surfaces, to the development of aerospace applications, to creating decorative coating products, to enhancing medical devices, or in the establishment of photovoltaic devices; vacuum deposited thin films are often employed. These films can be applied to substrates through several Physical Vapor Deposition (PVD) technologies which are all carried out within a vacuum environment; including magnetron sputtering, thermal evaporation, cathodic arc source, ion beam deposition, etc. Each of which can be used to produce coatings, in one form or another, with hard and wear resistance characteristics.
To take a typical example, the cathodic arc deposition of titanium nitride (TiN) onto cutting tools is now a well-established practice producing a wide variety of industrial products around the world. TiN is a refractory compound with extremely high micro hardness as well as exhibiting chemical and thermal stability. TiN coated products not only include standard metal cutting tools such as the flutes of drill bits, tools for boring mills, lathe bit tooling, slotting saw blades, counterbores, etc. but also all types of die casting equipment, forming tool applications, injection molding parts, die punches, etc.
Of the various forms of PVD used to produce thin film coatings in a vacuum, Cathodic Arc Evaporation (CAE) is most often utilized for producing hard coating surfaces for metal cutting applications. CAE, as explained in a previous Plasmaterials, Inc. BLOG, is a low voltage, high current plasma discharge between two cold electrodes. The localized power density generated on the target surface can be extremely high. The cathodic arc generates a small high energy “spot” on the target surface creating extremely high localized temperatures and thus vaporizing the cathode target material. The power density is switched off and on in short intervals. During the “off cycle” the position of the preceding power cycle is relocated slightly away from the previous “spot”. Through repetitive cycles an arc is formed across the surface of the target. By adjusting the input to the power supply, the power density generated at the target surface can be regulated to match the evaporation temperature for a specific vacuum level (vapor pressure) to create a vapor stream for any given target material. CAE is widely used to produce a highly ionized plasma for the deposition of hard, wear resistant TiN thin films on (HSS) tooling. These coatings typically range from one to two thousand angstroms in thickness depending on the specific application. Measurements from 2400 – 2600 Vickers Hardness can routinely be achieved. From a processing standpoint, the substrate temperature is elevated to between 700-800 degrees Fahrenheit while a partial pressure of nitrogen is fed into the vacuum chamber as the arc strikes an elemental titanium target creating a reactive deposition.
Additional cathode target materials can be used to create alternative hard coatings for specific applications for color variations, corrosion resistance or temperature application requirements. These include:
Titanium Carbon Nitride (TiCN) or titanium carbonate
Titanium Aluminum Nitride (TiAlN)
Titanium Aluminum Carbon Nitride (TiAlCN)
Cathodic arc deposition of TiN for hard coating applications constitutes an approximately three billion US dollar market worldwide and is increasing rapidly as new applications are being developed. Many decorative coating products are showing up in the automotive industry, such as wheel and trim designs as well as paint pigment applications. Sporting goods products are also seeing new applications for golf clubs, handguns and gun barrels, skis, racquets etc.