Chrome electroplating is a widely used surface treatment process throughout the military, and it represents the most significant contribution to hazardous carcinogenic waste generation and pollution control costs. Alternative technologies, such as the high-velocity oxygen-fuel (HVOF) process, are gradually replacing chrome electroplating for some applications. However, there exists a need for additional alternate technologies because in some cases, existing technologies cannot be applied as a result of geometry constraints or service conditions that exceed the damage resistance of the HVOF coating.
The objective of this project is to develop process control sensors, process parameters, equipment, and techniques using electro-spark deposition (ESD) to coat inside diameters and other difficult geometries with robust-wear and corrosion-resistant coatings that will replace current chromium electroplating applications.
ESD is a pulsed-arc micro-welding process that uses short duration, high current electrical pulses to deposit, with very low heat input, a consumable electrode material on a metallic substrate. The process releases no hazardous wastes, fumes, or effluents; is cost-effective; and requires no special chambers, spray booths, or operator protection. The technical approach consists of developing the process parameters for selected material coatings required for specific military applications and the process control sensors and algorithms necessary to achieve those parameters in non-line-of-sight applications. Power supply modifications and automated control devices will be developed and used to apply coatings to representative components for each military service. The components will be tested as part of the process optimization efforts using specific test conditions defined by the military services.
The selection of candidate coating materials has been completed, and WC-25TaC-13Co and Stellite 6 were chosen as the primary candidate coatings. Research has determined the effect of wave form on coating quality, and the development of force sensors and controls is under way along with the development of controls and algorithms to maintain optimum deposition parameters. The systematic characterization of these parameters has begun, with over 300 specimens coated.
This research will lead to a cost-effective, environmentally-benign process and low-capital expense equipment capable of providing an improved level of performance relative to current electroplated coatings, while reducing or eliminating the need for hard-chromium electroplating. This will complement current replacement technologies, such as HVOF, by allowing coating of nonline- of-sight geometries that HVOF and other thermalspay processes cannot coat. Cost benefits will include low capital expense (~$30K) compared to new chromium plating lines (>$1 million) or HVOF (>$400K); elimination of waste disposal costs ($0 for ESD compared to >$10 million per year reported for chromium plating for the Army alone); reduced or eliminated surface preparation costs relative to either chromium plating or HVOF processes; and savings derived from the portability of the process, allowing use in the field or shipboard to coat or repair components in place, with minimum setup.