THE VACUUM ARC is a form of electrical discharge that is sustained primarily on the electrons and ions that originate from the electrodes used to produce the arc. The value of using vacuum arc deposition to produce coatings stems from the copious quantity of ions of electrode material composition that are generated during the discharge. Because the output of the vacuum arc is highly ionized, it is possible to control both the trajectory of the coating material during its transit from source to part and the energy with which ions impinge on that part. This level of control can be contrasted to competing vacuum coating technologies, such as electron-beam evaporation and magnetron sputtering, where the atoms of coating material travel from the source to the part to be coated in an electrically neutral state.

Adjusting the deposition energy can produce coatings that have greater density, purity, and adhesion. Under favorable circumstances, the quenching of ions can produce coatings with structures that have unusual properties, such as the extremely hard and smooth amorphous diamond coatings that will be described in this article. Although the use of ion trajectory control to improve coating properties has not been extensively explored, it has been used to overcome the major drawback of the cold cathodic arc process: the production of micron-scale particles of electrode material, or macroparticles. Macroparticle formation and the approaches used for removal are described in Ref 1, 2, and 3.

Due to the ion charge state, vapor produced by vacuum arc techniques is typically more reactive than that produced using evaporative or sputter techniques. This increased reactivity can lead to compound coatings in which better stoichiometry is produced when deposition occurs in the presence of a reactive gas. For instance, when compared with electron-beam evaporation and magnetron sputtering, the cathodic arc can produce stoichiometric titanium nitride over a much wider range of nitrogen partial pressures (Ref 4). This can be particularly important when depositing compound coatings on complex shapes.

The most widely used type of vacuum arc is a cathodic arc with a cooled cathode. Because the source material remains solid, it can operate in any orientation. This avoids the difficulties associated with the reactivity of liquid metals. Once the arc is initiated, it self-focuses into a small spot at which the heat and the electron flux are sufficient to vaporize and ionize the cathode material and liberate enough electrons to sustain the discharge. The arc currents are typically 100 A, whereas the ion currents fraction is approximately 10% of that.

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