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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34 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM COATINGS & LININGS (Figure 3). For comparison to other PVD processes, Figures 3(c) and (f ) give addi- tional images following the same corrosion testing protocol for a conventional cathodic arc deposited CrN and a high-power impulse magnetron sputtering (HIPIMS) deposited nitride, respectively. HIPIMS is a high-power density PVD process shown to produce higher density coatings and significantly enhance corrosion resistance over conven- tional PVD coatings. 11-12 Note that the image in Figure 3(c) is following just three cycles as opposed to 30, illustrating the lack of corro- sion protection provided with conventional cathodic arc. Significant improvement in corrosion protection is realized with the HIPIMS process vs. conventional arc but the best performance among the nitride coat- ings were those deposited by the APA process. Mulligan 1 characterized the defect con- tent of the PVD coatings produced using the APA process, and found that the average macroparticle size was four to eight times lower than values typically reported for coatings prepared by conventional cathodic arc, with the maximum particle size in the range of 2 to 16 times smaller. They attrib- uted the excellent corrosion protection of the APA deposited coatings, particularly when compared to conventional cathodic arc (Figure 3[c]), to the high density and this low-defect content. Case Study of Coating Produced Using APA Process There is a case study of the APA process used to produce corrosion protection thin- film coatings for breech spindles for howit- zers. 13 The problem being addressed by the U.S. Army was that wear and corrosion require the replacement of the spindles well before they reach their usable end-of-life. The spindle is the component that seals the breech of the howitzer. Historically, electro- plated chromium coatings were applied to the spindles, but wear and corrosion cause problems, such as a loss of seal, which lead to a shortened spindle life when the howit- zers are subjected to typical usage. Figures 4(a) and (b) illustrate the spindle compo- nent in the as-coated condition for the leg- acy Cr plated spindle and the APA process CrN. Note that the legacy Cr plated spindle also has a heavy manganese phosphate treatment on the top half due to process limitations with the Cr plate. For the case of the APA process, the entire exterior is coated with CrN, further improving perfor- mance and producibility. Reference 13 details the evaluation of 12 separate coating systems and it was found that PVD coatings produced by the APA process vastly outperformed chrome and the other candidates in terms of corrosion and wear. Initially, coated howitzer breech spindles were tested at a live firing on a howitzer range, and to simulate combat conditions, the spindles were weathered for 30 days in a caustic acid + acidic propellant by-product. This was followed by another round of firing plus a second weathering treatment. Following this testing, conventional chromium-coated spindles exhibited severe corrosion, but the PVD-coated spindles' performance was significantly better. Fol- lowing down-select of the APA-processed CrN, more extensive testing involved firing the most aggressive rounds along with intermittent GM9540P-accelerated cyclical corrosion testing in an environmental chamber. Figures 5(a) and (b) are photo- graphs of the APA process CrN-coated spin- dles, and (c) and (d) are photographs of leg- acy Cr-plated spindles at the completion of accelerated corrosion testing, showing the superior performance of the PVD coated spindles. The U.S. Army is now using spin- dles coated with CrN using the APA process, and officials at the Picatinny Arsenal have estimated a $5 million saving over the life of the M777A2 howitzer, with even greater potential savings possible if this PVD coat- ing technology is applied to other artillery systems. 13 Other Factors Affecting Corrosion Protection Reports from technical literature indi- cate that other process parameters may fur- ther improve the degree of corrosion pro- tection offered by coatings produced using PVD processes. These include optimization of surface roughness and the use of a nitrid- ing pre-layer. These are briefly discussed in the following paragraphs. A number of authors 4-6 have noted that substrate surface roughness affects the abil- ity of a PVD coating to provide adequate corrosion protection. For example, Durst, et al., 6 summarized research results that dem- onstrated that, as a substrate surface be- came rougher, the number of visible coating FIGURE 3 Photographs of the various coatings following 30 cycles of the GM9540P accelerated corrosion testing. (a) APA CrN coating, 1 (b) APA CrN/SiC coating, 1 (c) Conventional arc-deposited CrN (three cycles), (d) 40 µm thick electroplated Cr coating, 1 (e) 40 µm thick electroless Ni coating, 1 and (f) HIPIMS-deposited nitride. FIGURE 4 Photographs of the (a) legacy Cr plated spindle and (b) APA process CrN coated spindle.

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