Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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33 MATERIALS PERFORMANCE: VOL. 58, NO. 6 JUNE 2019 novel PVD application method called the arc plasma acceleration (APA) process that can produce thin-film coatings with very low-defect contents necessary to provide corrosion protection to metal substrates. Before describing the APA process in detail, it is worthwhile to provide a brief summary of PVD coating processes. PVD Coating Processes PVD involves the vaporization of atoms from a solid metallic source, and the trans- portation and deposition of these atoms onto a substrate of interest. Some of the most commonly used PVD coatings for wear protection are metal nitrides, which are produced by bleeding low pressures of nitrogen gas into the PVD deposition cham- ber, allowing the metallic atoms vaporized from the metal target to react with the nitrogen gas during deposition on the sub- strate. PVD coatings are normally ~1 to 6 µm in thickness (Figure 1). A number of commercial PVD processes exist, and CAE is one of the most commonly used, as it produces coatings with extremely high levels of adhesion, cohesion, density, and hardness. One of the drawbacks of the cathodic arc process is the ejection from the target material of relatively large (~2 to 10 µm diameter) macroparticles, 5 which can become incorporated into the coating (Fig- ure 2[a]). These macroparticles form when unwanted droplets of liquid metal splashed from the arc source land on the substrate during coating growth. As these particles are similar in size to the thickness of the PVD coating, and often are poorly adhered to the substrate, they are detrimental to the coating's integrity, and significantly reduce corrosion protection. 5 A modified CAE process called the APA process, patented by Phygen † , 7 can signifi- cantly reduce the number of macroparticles and other defects within the PVD coatings (Figure 2[ b]), and thereby significantly improve corrosion resistance. The APA pro- cess utilizes a magnetic field generator that creates a magnetic field with a distinctive cusp shape, which provides enhanced trap- ping of the plasma particles generated from † Trade name. FIGURE 1 Metallographic cross section though a 3-µm thick CrN coating on a steel substrate. FIGURE 2 CrN coatings produced by cathodic arc evaporation. (a) Conventional CAE process, where coating contains detrimental white metallic Cr macroparticle inclusions and dark pores of various sizes. (b) Coating produced using the APA process, essentially free of macroparticles. the cathodic source. The APA process per- mits control over the growth of the coating, both via the intensity of ion bombardment through the plasma density control, and the energy of arriving particles through the sub- strate bias potential. A key to the process is to ensure that a large number of ions are bombarding the surface with a velocity in a specific range, and by tuning that range, cr ystalline configurations with weaker bonding can be minimized, while preserving the strongest bonds. This results in growth of a dense and highly textured coating, hav- ing a low-defect content, and an excellent metallurgical bond to the substrate. A recent study 1 performed in collabora- tion with the U.S. Army Armament Research, Development, and Engineering C enter, Benét Laboratories compared the corrosion resistance of coatings produced by the APA process with conventional SAE AMS 2460, 8 Class 2D electroplated chromium (12 and 40 µm thick) and SAE AMS 2404F, 9 Class 2, Grade C electroless high phosphorous nickel (40 µm thick). The PVD coatings produced by the APA process were a single layer CrN (3.5 µm thick) and a duplex CrN/SiC coating (total thickness of 3.5 µm). All coatings were deposited onto AISI 4340 steel substrates. Laboratory corrosion testing was performed in accordance with the GM9540P cyclic cor- r o si o n t e st i n g sp e c i f i c a t i o n , 1 0 w h i c h included 30 cycles of 16-h exposure to chlo- ride solutions at 49 °C. The results showed that the CrN PVD and the CrN/SiC duplex PVD coatings produced using the APA pro- cess exhibited significantly better corrosion resistance than the chromium coatings, and equivalent corrosion resistance to the elec- troless high phosphorous nickel plate

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