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Surface Treatments for Stainless Steel Actuators


OBJECTIVE: Develop surface treatments to improve the wear and damage tolerance of steel actuators for aircraft subsystems. DESCRIPTION: Stainless steel ballscrew components, such as those found in landing gear and flap actuators, experience wear due to friction and foreign object damage (FOD). The Air Force incurs a high maintenance burden associated with frequent inspections and applications of lubricants in the field in an effort to keep actuators functioning at a high availability rate. Coatings such as Electrolytic Hard Chrome (EHC) have been utilized on actuator components for numerous years. There are many benefits to EHC such as providing an excellent wear surface, a good corrosion barrier and ability for surface restoration to dimensional tolerances during the repair and overhaul process. However, EHC has been systematically phased out due to its environmental and health hazard risks, and replacement coatings have not offered comparable hardness or low friction performance. New surface treatments are needed that improve the surface hardness and lubricity of the steel actuator surface, in order to minimize FOD damage and provide a permanent dry lubricant. Current ballscrew, jackscrew and sliding rail actuator components for flaps and landing gear are made with 4XXX series steels. These steels must be protected with corrosion protection coating systems to improve actuator life. Current specified coatings include magnesium phosphate, zinc phosphate and hard chrome coatings. Phosphate coatings are porous, have high friction, and must be repeatedly sealed with lubricants to improve both friction and corrosion performance. These coating systems are prone to hydrogen embrittlement and use environmentally hazardous materials/processes. Current components are qualified to 16,000 hours but fall well short due to environmental (corrosion, wear, contamination) issues. There are a myriad of issues on the different components. Landing gear ballscrew actuators may wear out prior to a full PDM cycle due to wear/galling - rocks and debris from unimproved runways damage the surface of the actuator, causing accelerated wear. Flap actuators/rails must be completely disassembled every 270 days to reapply corrosion-inhibiting brush coatings, causing a huge amount of maintenance labor. Without these coatings, actuators would likely fail within 1 year or less as the 4XXX steel is not very corrosion resistant on its own. In addition, brush coatings on the sliding rails must be reapplied after every aircraft wash cycle (100-150 days). Current surface hardness requirement for the steel substrate and coatings is 55-59 HRC (~700-900HV). As stated previously, this is a problem as contaminants/FOD that would damage the coatings are often harder (1100-1200HV). Advanced carbon-based coatings (DLC, vanadium carbide, chromium carbide) provide hardness in the range of 1500-3000+ HV (75-90 HRC). These coatings also have the benefit of being extremely dense with coefficients of friction below 0.1, which means no secondary lubricant/corrosion inhibitor topcoat is required. Current phosphate (lubricated) and chrome coatings have COF of 0.12-0.16. The goal of this project is to develop surface treatment processes that both increase the surface hardness as well as the lubricity of the steel actuator substrate while maintaining high fatigue resistance. The process must be both cost-effective and suitable for large-scale manufacturing applications. PHASE I: Investigate the application of surface engineering techniques for steel actuator components in order to achieve lower friction and improved surface hardness while maintaining high fatigue and corrosion performance. Phase I will consist of coating and testing on representative surface geometries. PHASE II: Demonstrate and validate Phase I effort on coupons simulating landing gear components. Conduct coupon-level validation testing for corrosion, surface hardness, fatigue, microstructure, coating bond strength and coating integrity. Demonstrate surface treatments on full-scale actuator components and perform system-level validation test. No government test facility should be needed. PHASE III: Military application: To be used on DOD military aircraft landing gear and flap actuators. Commercial application: There is a possible use on civilian cargo and passenger aircraft landing gear and flap actuator components. Industrial uses also include sealing surfaces. REFERENCES: 1. Menthe, E., Rie, K-T, Schultze, J. W., & Simson, S. (1994). Structure and properties of plasma-nitrided stainless steel. Surface and Coatings Technology (Switzerland), 74(1-3), 412-416. 2. Pfaffenberger, E. E., & Tarantini, P. (1993, June). High temperature corrosion resistant bearing steel development. Paper presented at the AIAA/SAE/ASME/ASEE 29th Joint Propulsion Conference, Monterey, CA. 3. Menthe, E., Bulak, A., Olfe, J., Zimmerman, A., & Rie, K.-T. (2000). Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding, Surface and Coatings Technology, 133/134(2000), 259-263. 4. Larisch, B., Brusky, U., & Spies, H.-J. (1999). Plasma nitriding of stainless steels at low temperatures. Surface and Coatings Technology (Switzerland), 116-119, 205-211.
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