HVOF gun in action
Traditionally, chrome plating has been a process often contracted to specialty shops using an electroplating process carried out under strict OSHA controls. Today, many overhaul shops have been able to bring chrome equivalent repair work in house using High Velocity Oxy/Fuel (HVOF) coating technology. The new method replaces a slow process with a coating development of approximately one mil per hour which depends on environmentally unfriendly chrome plating operations using toxic chemicals.
The Benefits of HVOF Coatings versus Hard Chrome Plating
The opportunity to bring chrome plating operations in house is a result of work done twenty years ago by the Hard Chrome Alternatives Team (HCAT) (1). HCAT was an industry and government consortium with members from the United States Department of Defense and United States and Canadian manufacturers. Setting a goal to develop technology to replace hazardous chrome plating operations with HVOF coatings of Tungsten Carbide-Cobalt (WC-Co) and Tungsten-Carbide Cobalt/Chrome (WC-CoCr), the task force adopted a Joint Test Protocol to determine the necessary testing and criteria for success. (Note: chrome in WC-CoCr is not hexavalent chrome which is a carcinogenic associated with chrome plating). Tests included corrosion, wear, fatigue, hydrogen embrittlement and impact resistance. Materials and methods were also prescribed to insure reproducibility. The coalition succeeded in establishing a fully defined and proven procedure that could be adopted by overhaul shops.
Advantages of HVOF coatings over hard chrome plating as documented by HCAT include: (2)
- A hard corrosion resistant surface capable of being applied in-house
- A coating with increased abrasion resistance
- Extended seal life for hydraulic components
- Equal or superior corrosion resistance to chrome plating
- Minimum impact on the fatigue performance of the substrate
- Reduced impact on the environment
- Reduced biological and health and safety concerns
- Simplified masking
Airline repair depots now use HVOF for landing gear overhaul (3). Boeing is using HVOF coatings for 767 and 777 landing gear and has developed repair specifications based on the HVOF process (4). HVOF is also being used for hydraulic rams in heavy construction machinery and drill components for oil and gas exploration. HVOF is being used as a hard chrome replacement in hydraulic and cylinder piston rods (5) diesel engines, turbines, hand or foot pumps, construction equipment, manufacturing machinery and civil engineering. It is also servicing paper mill rolls. The paper and pulp industry relies on HVOF coatings for wear resistance, corrosion resistance, low coefficient of friction, cleanability and traction.
What is needed?
Hipojet 2700 High Velocity Oxygen Fuel Spray System
Any overhaul operation that incorporates gas welding in their operations is three quarters of the way there for adding HVOF capability. Gas and air requirements are very similar as is safety training and awareness. Often the same handling equipment used for HVOF operations is the same as used for welding. As with welding, HVOF operations require control of fumes and radiation using proper exhaust systems and shielding. The main expense is the HVOF gun and controller.
What are the limits?
The HVOF process is a line-of-sight process and is not able to coat around corners. The second issue is speed. Thickness per pass depends on the gun and material but generally ranges from 0.0002” per pass up to 0.015” per pass. The coating speed in surface feet per minute can be high. This is no problem when coating a shaft mounted in a lathe but may require a robot when coating large flat areas.
How to proceed?
The starting point is to work with a supplier to jointly review requirements and develop a plan that considers the cost of present plating operations and estimates the cost of incorporating the HVOF process. At the same time, other thermal spray processes for different repair needs can be evaluated.
(1) Report “Replacement of Chromium Electroplating Using HVOF Thermal Spray Coatings” B. D. Sartwell, P. M. Natishan, I. L. Singer Naval Research Laboratory, Washington, DC, K. O. Legg, Rowan Catalyst, Inc., Libertyville, IL, J. D. Schell, GE Aircraft Engines, Cincinnati, OH, J. P. Sauer, Metcut Research Inc., Cincinnati, OH
(2) “Choosing a Hard Chrome Alternative” Keith Legg Rowan Technology Group
(3) Report “Status of HCAT/JG-PP Program on Replacement of Hard Chrome Plating With HVOF Thermal Spray Coatings on Landing Gear” Bruce D. Sartwell, Naval Research Laboratory, Washington, DC, Keith Legg, Rowan Technology Group, Chicago, IL, Philip E. Bretz, Metcut Research, Inc., Cincinnati, OH. Paper published in Proceedings of AESF Aerospace Plating and Metal Finishing Forum, March 2000, pp. 131-139
(4) HVOF Technology Insertion @ Delta Air Lines, Inc., Presentation by: Jay Randolph, Lead / Principal Engineer- Component Engineering & Nick Cortese Senior Engineer – Repair Process Engineering Presented at HCAT meeting, November 18/19, 2003, Kennedy Space Center
(5) Report “HCAT PROJECT- FUNCTIONAL ROD/SEAL TESTING AND QUALIFICATION OF HVOF COATINGS ON NAVY ACTUATORS”, Jeff Gribble & James Magno, NAS Patuxent River, MD, January 24, 2006