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APPLIED RESEARCH LABORATORY AT THE PENNSYLVANIA STATE UNIVERSITY

Materials & Manufacturing (MM) | ac-cathodic Arc

Cathodic arc falls into the classification of Physical Vapor Deposition (PVD) coating techniques.  The term PVD denotes those vacuum deposition processes where the coating material is evaporated or removed by various mechanisms (resistant heating, ablation, high-energy ionized gas bombardment, or electron gun), and the vapor phase is transported to the substrate forming a coating.  PVD is often classified as a “line-of-sight” process in which evaporated atoms travel from the source material to the substrate in a straight path.  The residual stresses in the PVD coating are generally compressive due to coating bombardment, but can be controlled depending on the deposition parameters.  These compressive stresses are often beneficial as they retard the formation and propagation of cracks in the coating.  PVD coating processes generally take place between temperatures of 200°C to 500°C to minimize stresses associated with thermal expansion mismatch as compared to the high temperatures (1000°C) of CVD. 

In the cathodic arc deposition process, a pulsed or continuous high current-density, low voltage electric current is passed between two separate electrodes (cathode and anode) under low pressure vacuum, vaporizing the cathode material while simultaneously ionizing the vapor, forming a plasma. The high current density (usually 104-106 A/cm2) causes arc erosion by vaporization and melting while ejecting molten solid particles from the cathode surface, with a high percentage of the vaporized species being ionized with elevated energy (50-150eV) and some multiply charged [1].

For example, in the case of TiN, as the vaporized titanium (cathode target) passes through the arc, it becomes ionized forming a plasma.  The plasma is directed towards the substrate’s surface, and in the presence of nitrogen, forms a TiN coating.  Similarly, TiAl, TiSi, TiCr, etc. targets can be used to produce TiAlN, TiSiN, TiCrN, etc., or using acetylene, carbides can be formed  ARL-PSU cathodic arc system will allow up to three target materials (cathodes) of different composition to be used allowing easy deposition of multilayer coatings.  The individual layer thickness (i.e., interfacial size, volume, and structure) as well as the residual stress state can be controlled by altering the deposition parameters such as deposition rate, temperature, pressure, rotation time, etc.  The kinetic energy of the depositing species in cathodic arc, are much greater than those of other PVD processes with energies between 50-150 eV. Therefore, the plasma becomes highly reactive as a greater percentage of the vapor is ionized.  In addition, the cathodic arc process allows tailoring of the interfacial products, especially in multilayer coatings, and does not produce a distinct coating / substrate interface, which may be undesirable. As a result of the high kinetic energy, an intermixed layer of the substrate and coating or between layers of a multilayer coating (10-300 Å thick) can be formed that increases the degree of coating adhesion while minimizing residual stresses [2-3].  Some of the advantages and disadvantages of cathodic arc are briefly described below.

Advantages of cathodic arc processes are:

  1. High level of atom ionization in the plasma
  2. As a result of the high level of energy produced by this process, materials can be co-evaporated at the same rate, thus producing stoichiometric compounds
  3. Better adhesion can be achieved as a result of the intermixed reaction zone
  4. Low-processing temperatures allow for the coating of heat-sensitive substrates/components
  5. Multilayered coatings and functionally graded compositions can easily be produced

Disadvantages of cathodic arc processes are:

  1. Macroparticles of metals and liquid droplets (1-15 µm) during flash evaporation occur
  2. Coating complex geometries can be difficult due to line-of-site process

The main disadvantage of cathodic arc deposition is that it produces macroparticles of metals and liquid droplets that are the result of intense, localized heating from the arc.  This heating produces small amounts of liquid metal that become entrapped within the depositing coating (similar to plasma spraying) and serve as stress concentrations and crack initiation sights.  To decrease the number of molten macroparticles, filtered cathodic arc systems are often used [4].  An electromagnet field is applied, and due to momentum, the larger macroparticles do not deposit on the substrate surface as they get filtered away from the substrate.  In general, filtered arc deposition usually has a lower deposition rate than unfiltered.  In addition, target efficiency can be increased by steering the arc across the cathode surface [5].  A magnetic field is applied that steers the arc across the surface of the targets and thus reduces the dwell time (localized heating) at any given point.