September AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 8 2 1 ARC PLASMA ACCELERATION The APA technique is a modified cathodic arc vapor deposition process patented by Phygen Coatings Inc. [1] The process uses a magnetic field gener- ator to create a field with a distinctive cusp shape, which provides enhanced trapping of plasma particles generat- ed from the cathodic source. The con- toured field creates an electron trap with an aperture through which plas- ma ions are directed at the substrate; the plasma deposition rate is higher per unit of magnetic field strength than can be obtained with conventional designs. The APA process enables control over coating growth, both via the intensity of ion bombardment (through plasma density control) and the energy of ar- riving particles (through the substrate bias potential). It is necessary to ensure that a large number of ions bombard the surface with a velocity in a specific range, and by tuning that range, crystal- line configurations with weaker bond- ing can be minimized while preserving the strongest bonds. This phenomenon results in growth of a dense, highly tex- tured coating with an excellent metal- lurgical bond to the substrate (Fig. 2). Another benefit of the APA ap- proach for producing CAE coatings is that the volume fraction of macropar- ticles within the coating is significant- ly reduced (Fig. 1b), and both average macroparticle size and volume fraction are significantly reduced compared with conventional CAE techniques. De- creasing the volume fraction of mac- roparticle defects within a coating can improve performance and significantly extend coating life. Typical properties of CrN and AlCrN coatings produced using the APA process are listed in Table 1. The APA process is also used to produce CrN-DLC and CrN-SiC bilayer coat- ings (Fig. 3). Properties of CrN-DLC and CrN-SiC bilayer coatings are listed in Table 1 as well. COATING PERFORMANCE PVD coatings are used in many ap- plications where surface performance is crucial. As noted previously, one of the main benefits of thin film coatings is the ability to modify surface behavior independent of bulk performance. This is illustrated in several examples using both single-layer and bilayer coatings produced by the APA process. Corrosion protection. For many ap- plications, PVD coatings cannot provide TABLE 1 — PROPERTIES OF ARC PLASMA ACCELERATION COATINGS Coating compound (product name) Typical coating thickness, µm Nanoindentation hardness, GPa Maximum operating temperature (based on oxidation resistance), °C Adhesion strength (by scratch test) Critical load, N CrN (FortiPhy) 3-6 22-26 800-850 110-120 CrN(a) (FortiPhy Plus) 3-6 23-27 825-875 112-135 AlCrN (CertiPhy) 3-6 31-35 ~950 115-130 AlCrN(a) (CertiPhy Plus) 3-6 31-35 ~950 115-130 CrN-DLC ~2 each layer 23 ~350 80-105 CrN-SiC ~2 each layer 30 >600 89-90 (a) Duplex coating where substrate material is plasma ion nitride prior to coating for improved mechanical support. Fig. 2 — Coatings produced using the arc plasma acceleration method are dense and free of defects as illustrated in this cross section of a 3-µm thick CrN coating deposited on an AISI 4340 alloy steel substrate. Fig. 3 — SEMmicrographs showing cross sections through bilayer PVD coatings deposited onto AISI 4340 alloy steel substrates using the arc plasma-acceleration process. In (a), CrN-DLC coating; (b) CrN-SiC coating (in each image, the left-hand side is viewed using secondary electrons, while the right-hand side is viewed in electron-backscatteredmode). (a) (b)