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 2 adequate corrosion protection due to the presence of defects within the coat- ings, such as macroparticles, pits, and flakes. These defects originate during the deposition process along with a co- lumnar growth structure that often is not entirely dense. However, the U.S. Army Armament Research, Develop- ment and Engineering Center at Benet Laboratories [2] recently evaluated the corrosion protection of steel substrates by several coatings produced using the APA process. Tests were performed in accordance with the GM9540P specifi- cation [3] involving 30 cycles of 16-hour exposure to chloride solutions at 50 ° C. Results show that a CrN coating and a CrN-SiC bilayer coating exhibit signifi- cantly better corrosion resistance than an electroplated chromium coating (12 and 40 µm thick), and equivalent cor- rosion resistance to a 40-µm thick elec- troless high-phosphorous nickel plate. Figure 4 shows the surface of the PVD CrN coating following accelerated cor- rosion testing, which exhibited a sig- nificantly smaller amount of corrosion than steel substrates with electroplat- ed chromium coatings. The excellent corrosion protection of APA CrN and CrN-SiC coatings is at- tributed to their lower concentration of defects [2] and dense structure. For ex- ample, APA coatings were found to con- tain a macroparticle density of 3.5 x 10 3 / mm 3 , which is four to eight times lower than values typically found in coatings produced using conventional cathodic arc processes [2] . In addition, the mac- roparticles were significantly smaller; average and maximum particle sizes were 0.67 and 4.2 µ m, respectively, two to 16 times smaller than particles with- in coatings made using conventional cathodic arc processes [2] . Applications requiring solid lubri- cants. Coefficient of friction (CoF) mea- surements were performed at Benet Labs on several APA coatings by slid- ing coatings against 6-mm alumina balls at room temperature using a ball- on-disk tribometer [2] . Results shown in Table 2 reveal that two bilayer coatings (CrN-DLC and CrN-SiC) have lower CoF values than single-layer CrN PVD coat- ings, electroless Ni-P, and electroplated Cr coatings. The bilayer coatings are suitable in applications requiring solid lubricants [2] . Wear resistance. PVD coatings have excellent wear resistance due to their high as-deposited hardness. Table 3 shows wear rates from ball-on-disk tri- bometer tests of the three PVD coat- ings produced using the APA process, conventional Ni-P and Cr coatings, and bare AISI 4340 alloy steel substrate. The PVD coatings have significantly lower wear rates. Metal forming and casting tools. PVD coatings are also commonly ap- plied to metal forming and casting tools, such as stamping tools and core pins used in die casting. A driveshaft housing produced at Mercury Castings, Fond du Lac, Wis., provides an example of the benefits of using PVD coatings [4] . Casting production involves the use of several long cores, and when using un- coated H13 tool steel cores, the alumi- num die casting alloy rapidly solders to the steel core (Fig. 5), creating high loads during casting ejection, which causes the entire housing to bend. Ap- plying a 5- µ m AlCrN coating using the APA process to the core eliminates sol- dering (Fig. 5), avoids bending the cast- ing during core pull, and eliminates the need for 100% inspection of the castings. Recent research has taken the con- cept of coatings on forming and casting tools even further, with the goal of elim- inating the need to lubricate dies used to produce die castings. During die cast- ing, a water based organic lubricant is applied to the hot faces of the steel casting die prior to each shot to prevent the aluminum castings from soldering (sticking) to the die. However, a number of undesirable outcomes can arise from applying these lubricants, including lower casting quality (higher residual porosity), shorter die life, and effluents that need to be disposed of. TABLE 2 — STEADY-STATE COEFFICIENT OF FRICTION FOR SELECT COATINGS [2] Coating Average steady-state coefficient of friction CrN-DLC 0.07 ± 0.01 CrN-SiC 0.07 ± 0.01 CrN 0.28 ± 0.02 Ni-P 0.23 ± 0.02 Cr 0.55 ± 0.09 TABLE 3 — WEAR RATES OF SELECT COATINGS [2] Coating Wear rate, mm 3 /N m CrN 5.30 × 10 − 7 CrN-DLC 7.32 × 10 − 7 CrN-SiC 7.90 × 10 − 7 Ni-P 25.5 × 10 − 7 Cr 263 × 10 − 7 Uncoated AISI 4340 alloy steel 556 × 10 − 7 Fig. 4 — Surface of samples following accelerated corrosion tests consisting of 30 cycles of 16 h exposure to chloride solutions at 50°C in accordance with specification GM9540P [2] : (a) APA CrN coating, (b) 40-µm thick electroplated Cr coating, and (c) 12-µm thick electroplated Cr coating. (a) (b) (c)