OBJECTIVE: Develop a coating for engine fuel systems that will form a protective tribofilm that enables higher-pressure fuel systems to accommodate the variations possible in F-24 fuels. DESCRIPTION: The increasing use of unmanned ground and aerial vehicles has exposed the need for higher performance engines for these systems. Increasing the performance of these engines requires operation at higher fuel pressures, which creates a challenge related to fuel lubricity. Sufficient fuel lubricity in the engine fuel systems is a necessity for effective operation. At higher pressures that need is more acute, as insufficient lubricity could lead to catastrophic failure due to wear or damage. Most of these vehicles use F-24 jet fuel. There are many different blends of F-24 due to variations in the base fluid and contaminants present. This makes it very difficult to employ in higher-pressure common rail engines, as they may not have sufficient lubricity under all the potential F-24 formulations available. It may not be possible to obtain the ideal formulation of F-24 in the field, and the use of an unsuitable formulation may impair or damage the engine. For higher-pressure fuel systems to be practical in a logistical sense, a solution that enables consistent lubricity for these engines under a variety of conditions and formulations is required. The most practical solution would be a coating that provided protection and sufficient lubricity to the engine parts during operation by formation of renewable protective coatings. A recently proposed solution is the use of coatings that form self-lubricating carbon-based diamond like coatings due to catalytic breakdown of hydrocarbons and contaminants in the fuel [1,2]. This previously was very difficult as these catalysis reactions are facilitated by the use of precious metals such as platinum and gold [3]. In addition, the coating also had to be nanostructured to obtain the desired level of interaction with the organic compounds in the fuel. Recent publications, however, have established it is possible to create nanocrystalline coatings using physical vapor deposition and sputtering techniques [4]. In addition, advances in the field of nanocrystalline alloy design have led to the discovery of a number of potential nanocrystalline alloy compositions that are suitable to fabrication via sputtering or other surface coating techniques [5]. This opens up the possibility of creating a stable nanocrystalline coating using more cost effective alloying elements. In fact, there have been recent publications demonstrating protective tribofilm formation using coatings containing Co, Mo, Cr, Al, and other alloying elements [1, 6]. This makes it plausible that a practical nanocrystalline alloy that forms protective tribofilms during engine operation could be developed for wide scale employment in the field. While some coatings have been demonstrated in the laboratory, there is still a great deal of development required before they can be utilized in an engine fuel system. Any coating used for this application must be stable under a variety of conditions and robust under mechanical stresses. The tribofilm formed must be protective, otherwise the coating would prematurely wear and cause the engine to fail. In addition, the processing challenges related to industrial scale application of the coating and consistent application to complex geometries have to be addressed. PHASE I: The offeror(s) shall develop a coating for AISI 52100 steel that will form protective tribofilms in alkane based fuels. A composition shall be selected that will form a stable nanocrystalline structure, not utilize precious metals such as platinum, palladium, and gold, and be mechanically stabile under load. The mechanism used to create the tribofilm can be based on any type of or combination of mechanisms (e.g. compositional, morphological, mechanical, etc.). However, the tribofilm created should provide protection for the coating and the steel and renew through continuing interactions with the fuel. The coating itself should not require reapplication for long-term function. The final deliverable shall be at least one candidate coating composition, lubricity testing results, and evidence of tribofilm formation. The coating shall be applied to a ball of AISI 52100, and tested for lubricity under following conditions: 1) in comparison to the standard hardened AISI 52100 ball, 2) with at least two fuels selected from diesel, Jet-A, and F-24, 3) using fuel lubricity standard ASTM D6079 (High-Frequency Reciprocating Rig), and 4) using fuel lubricity standard ASTM D5001 (Ball-On-Cylinder Lubricity Evaluator). F-24 can be obtained from the Army sponsor. PHASE II: The offeror(s) shall continue to refine the coating composition, and begin to develop the processing approach for uniform application of the coatings to conformal and complex parts. The compositions identified in Phase I should be adjusted to be suitable for wide scale industrial fabrication. The offeror(s) should select a processing approach, and develop methodologies for uniform application of the coating on conformal AISI 52100 steel parts relevant to high-pressure fuel pumps. The final deliverables shall be four components (pumping piston, piston bore, small crowned cylinder, large cylinder) of a pump rated to reach 1500 bar or greater, with the coating uniformly applied to all contacting surfaces and tested for lubricity in two mechanical interfaces. Interface 1: One piston/bore conformal interface shall be tested with coating on the external surface of a piston and internal surface of a bore (both 5 to 8 mm diameter with clearance of less than 2 micrometer between piston/bore), reciprocating between 1-cm and 2-cm axial interface length, at a reciprocating frequency of at least 30 Hz. Interface 2: One cam-like interface shall be tested between a coated crowned cylinder of 5 to 8 mm sliding on a larger coated cylinder at 4 m/s. Lubricity testing in these two interfaces shall be conducted: 1) in comparison to hardened AISI 52100 steel in the same geometries and conditions, and 2) with three fuels (diesel, Jet-A, and F-24). Alternatively, the coating may be applied to components of a commercial high-pressure fuel pump with an oblong, rotary cam system capable of reaching 1500 bar fuel pressure, to include the pumping piston/cylinder and sliding interfaces of the cam system, and demonstrated with respect to an unmodified pump under conditions similar to those for the two interfaces. PHASE III DUAL USE APPLICATIONS: The technology will support reliable engine operation of internal combustion reciprocating engines for Class II and III Unmanned Aerial Systems, for medium-sized ground vehicles, and for generators. The offeror(s) will adapt the processing technology to apply it to existing engine fuel systems. Coatings for other metallic alloys or composite materials will be developed. The process will be adapted so that engine components with the protective component are commercially available to researchers working on high-pressure fuel systems designs. The coatings could either be used to modify commercially available high-pressure fuel system components in an aftermarket process, or the coating could be integrated into new fuel system designs. REFERENCES: 1. Erdemir, A; Ramirez, G.; Eryilmaz, O; Narayanan, B; Liao, Y.; Kamath, G; and Sankaranarayanan, S; Carbon-based tribofilms from lubricating oils, Nature, Vol 536, Aug 4 2016; 2. Ali, M.; Xianjun, H.; Abdelkareem, M; Gulzar, M; and Elsheikh, A.H., Novel approach of the graphene nanolubricant for energy saving via antifriction wear in automobile engines, Tribology International, Vol 124, 2018, pp. 209-229; 3. Argribay, N; Babuska, T.F.; Curry, J.F.; Dugger, M.T.; Lu, P.; Adams, D.P.; Nation, B.L.; Doyle, B.L.; Pham, M.; Pimentel, A.; Mowry, C.; Hinkle, A.R.; and Chandross, M., In-situ tribochemical formation of self-lubricating diamond-like carbon films, Carbon, Vol. 138, 2018, pp. 61-68; 4. Zhou, XY.; Kaub, T; Martens, RL; and Thompson, GB., Influence of Fe(Cr) miscibility on thin film grain size and stress, Thin Solid Films, Vol. 612, Aug 1, 2016, pp. 29-35; 5. Gibson, M; Schuh, C, Segregation-induced changes in grain boundary cohesion and embrittlement in binary alloys, Acta Materialia, Vol. 95, 2015, pp. 145-155; 6. Bobzin, K.; Br gelmann, T.; and Kalscheuer, C.; Arc PVD (Cr, Al, Mo)N and (Cr,Al,Cu)N coatings for mobility applications, Surface & Coatings Technology, Vol. 384, 2020, pp. 125046 KEYWORDS: Coatings, fuel lubricity, catalysis, tribology, nanostructured coating, active coating
