NOVELTY - An apparatus for the application of coatings in a vacuum, comprises cathodic arc source(s) and a bi-polar pulsed DC power supply (801). The cathodic arc source includes cathode(s) (12) contained within a cathode chamber, anode(s) (601) associated with cathode(s) for generating an arc discharge, and coating chamber(s) (42) containing substrate holder(s) (2) for mounting substrate(s). The bi-polar pulsed DC power supply comprises at least one primary power supply, and at least one switching circuit (501, 502) for alternately switching a direction of a current applied to substrate holder.    USE - For the application of coatings in a vacuum (claimed), particularly for depositing multilayer and nano-structured coatings such as corrosion resistant coatings for substrates (e.g. fuel cell plates and fasteners), and thermal bearing coatings to substrates such as engine components; useful in mechanical engineering, instrument and tool making, in the production of electronic equipment, and in other fields of industry.    ADVANTAGE - The bi-polar pulsed DC power supply has switching circuit that alternatively switches a direction of current applied to the substrate holder, to prevent the formation of micro-arcs on the substrate surface. The cathodes on the side of the inactive coils serve as powerful electron emitters, improving ionization of the gaseous component of the plasma flowing past shutter 80a and into the coating chamber, and significantly improving the properties of the resulting coating. The coating apparatus produces interfacial multi-layer coating which is uniform, pin-hole free and stable in an oxidizing atmosphere up to 1200degreesC.    DETAILED DESCRIPTION - An apparatus for the application of coatings in a vacuum, comprises at least one cathodic arc source and a bi-polar pulsed DC power supply (801) having input and output terminals. The cathodic arc source includes at least one cathode (12) contained within a cathode chamber, at least one anode (601) associated with the cathode(s) for generating an arc discharge, and at least one coating chamber (42) containing at least one substrate holder (2) for mounting at least one substrate to be coated. The bi-polar pulsed DC power supply comprises at least one primary power supply having a positive output terminal and a negative output terminal, and at least one switching circuit (501, 502) for switching a direction of a current applied to the substrate holder. The switching circuit is connected to the positive and negative output terminals of the primary power supply and cathode(s), such that the switching circuit alternately applies positive and negative currents to the substrate holder(s). The switching circuit comprises high speed switching components. The apparatus also comprises focusing conductors disposed adjacent to the metal vapor plasma source and a plasma duct on upstream and downstream sides of the metal vapor plasma source, for focusing a plasma flow from the metal vapor plasma source to the plasma duct. An INDEPENDENT CLAIM is included for coating a substrate in a coating apparatus by alternately applying positive and negative currents to the at least one substrate holder comprising the steps of alternately involving connecting to the substrate holder(s) one of a positive or negative output terminal of a power supply connected to at least one cathodes; and disconnecting from the substrate holder(s) one of the positive or negative output terminals of the power supply and connecting to substrate holder(s) the other of the positive or negative output terminals of the power supply.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic plan view of a dual filtered arc coating apparatus.    substrate holder (2)    cathode (12)    coating chamber (42)    switching circuits (501, 502)    anode (601)    bi-polar pulsed DC power supply. (801)