Feb_March_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 | F E B R U A R Y / M A R C H 2 0 1 8 3 8 iTSSe TSS iTSSe TSS E lectrolytic hard chrome (EHC) coatings are used to en- hance wear and corrosion resistance in a wide variety of markets and applications, including hydraulics, tooling, and aerospace components. However, the EHC process pres- ents serious health and safety concerns, generating a haz- ardous waste stream of known carcinogens. More stringent regulations and penalties issued by the U.S. Occupational Safety and Health Administration (OSHA) for unsafe handling of the waste products add to the urgency of developing alter- natives to EHC. Many users turned to WC-Co coatings applied using the high-velocity oxygen fuel (HVOF) thermal spray process, which has its own drawbacks and limitations. For example, HVOF operates at combustion flame temperatures over 5000 ° F (2760 ° C) [1] and requires a specially designed sound-deaden- ing booth to protect the hearing of operators [2] and others nearby. These booths often have a large footprint and require additional high-capacity dust collection systems to operate. Further, the high operating temperature of HVOF can cause decarburization and oxidation of WC-Co feedstock, degrading coating quality and performance [1,3] . High jet temperatures also limit the types of parts and materials that can be coated using HVOF. Because HVOF can melt small parts and degrade the temper of larger parts, measures typically must be taken to keep parts sufficiently cool during processing, including re- stricted preheating, in-process cooling, interpass spray paus- es, and controlled post-deposition cooling rates [3,4] . ADVANTAGES OF KINETIC METALLIZATION Kinetic metallization* (KM) offers a number of advan- tages over HVOF including low process temperature, efficient powder use, and inherent process control and documenta- tion. The KM process operates at a nominal gas pressure of 70 psig (480 kPa), using a patented friction-compensated son- ic nozzle to accelerate specially formulated feedstock parti- cles to velocities of over 1000 m/s [5] . As particles impact the substrate, they deform and expose a new oxide-free surface, which enables the formation of strong cohesive and adhesive bonds. A patented powder fluidizing unit (powder feeder) and friction-compensated sonic nozzle from Inovati enable KM equipment to reliably deposit a wide range of materials in- cluding WC-Co coatings (Fig. 1) [6] . Process characteristics for KM and HVOF WC-Co coatings and EHC are shown in Table 1 EXPLORING LOW-TEMPERATURE ALTERNATIVES TO ELECTROLYTIC HARD CHROME COATINGS Kinetic metallization offers certain advantages over hard chrome and thermal spray coatings, especially when elevated process temperatures would degrade the coating or part. Ralph Tapphorn and Howard Gabel, Inovati, Santa Barbara, Calif. 4 FEATURE Fig. 1 — Kinetic metallization production coating system for application of WC-Co and other hard phase wear and corrosion- resistant coatings. and a detailed comparison of KM and HVOF WC-Co coatings is provided in the literature [7] . The advantages of KM versus other techniques include the following considerations: Low process temperature . KM is a low heat input process. KM coatings are deposited at temperatures of only a few hun- dred degrees Fahrenheit, precluding melting of feedstock and substrate. A low KM deposition temperature prevents decar- burization and oxidation of the feedstock, resulting in higher quality WC-based coatings. The process works particularly well for small parts and can be deposited on internal diame- ters as small as 50 mm. Lowheat input enables providing the requiredwear resis- tance in components that cannot withstand the high tempera- tures of HVOF and other thermal spray processes. In addition, independent testing performed by U.S. Navy, Naval Air Sys- tems Command (NAVAIR) verified that coatings using KM pro- vide a good low-temperature alternative to hard chrome plate [8] . For example, NAVAIR’s Fleet Readiness Center South- west conducted special accelerated wear testing to compare material loss fromuncoated, hard chrome plated, and KMWC- Co coated AMS 6265 alloy gearshafts. In the test, the shaft was rotated at a fixed rpmwith the shaft seal installed in a NAVAIR F/A-18E/F Super Hornet jet fighter airframe mounted accesso- ry drive (AMAD) gearbox contaminated with a grit slurry. After roughly 20 hours of testing, the uncoated and plated shafts