Iron Tungsten Alloy Plating with Low Friction and Wear Resistance
The modern automotive industry has been striving to reduce energy loss caused by friction. An alloy plating film developed by finely crystallizing iron and tungsten demonstrates both a high level of hardness and excellent ductility. In addition, low friction can be achieved by its application under a layer of lubricant. It also has the merit of greater cost performance as compared with DLC produced with PVD technology.
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Product Development Engineering
Yuken Industry Co. Ltd.
Kariya City, Aichi Prefecture, Japan
Yuken America, Inc.
Novi, Michigan USA
Editor’s Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2016 in Las Vegas, Nevada on June 7, 2016. A printable PDF version is available by clicking HERE.
An alloy plating film developed by finely crystallizing iron and tungsten demonstrates both a high level of hardness and excellent ductility. In addition, low friction can be achieved by its application under a layer of lubricant. The modern automotive industry has been striving to reduce energy loss caused by friction. Low friction and improved durability can be attained by applying this technology to the surface of parts which constantly slide in operation. This in turn contributes to better fuel efficiency. On the other hand, diamond like carbon (DLC), a typical low friction coating, causes peeling at early stages when loaded under high pressure and shows poor synergistic effects with lubricants. Therefore, the application of this coating is not recommended for sliding parts. Alternatively, the wet plating technology we developed does not have these disadvantages. It also has the merit of greater cost performance as compared with DLC produced with PVD technology.
The largest challenge facing the automotive industry these years is CO2 reduction (through lower fuel consumption). They need to meet fuel efficiency regulations that will take effect in various regions in the near future. In mid- to long-term policies, all of the automotive makers have been rigorously working to reduce CO2 in exhaust to cope with global climate changes.
In particular regard to the reduction in friction loss in the internal combustion engine, the industry holds high expectations toward the development of surface treatment technologies that can both decrease friction coefficients for engine components sliding with each other and establish high wear resistance for them. Conventionally, dry plating like DLC (diamond-like carbon) and CrN (chromium nitride), and wet plating including electroless nickel plating and hard chromium plating have been used for these engine components (such as piston rings, bubble lifters and crankshaft bearings) which have metal-on-metal sliding contact.
DLC provides low friction and excellent wear resistance, but peeling is also likely to occur when the ductility is insufficient. In addition, a peeled hard film is damaging to the contacting surface, and the compatibility is extremely limited to only certain types of lubricants. Furthermore, dry coatings like DLC require a batch process which raises the cost, and so it is not widely used.
Electroless nickel plating has excellent covering power, but its friction properties and wear resistance are somewhat insufficient. Further, it requires a high bath renewal frequency and large volumes of waste water subsequently become a burden to the environment. Hard chromium plating is a functional plating process that has been widely used for a long time to achieve wear resistance properties. The greatest concern with this plating is the hexavalent chromium contained in the plating bath that users have to deal with to protect the environment.
We have developed a functional plating method which can be wet processed, thereby promoting process cost control, and which also has high wear resistance and low friction properties without nickel and chromium.
The plating equipment used in this development is shown in Fig. 1. We used insoluble anodes made of platinized titanium. The bath was mixed with a magnetic stirrer during plating. The plating bath parameters used are shown in Table 1. The bath temperature was 75 ± 5°C and the pH was adjusted to 6.5 with diluted sulfuric acid. Deposition could start around 2.0 A/dm2 of the cathode current density. However, in consideration of practical plating speeds, we proceeded with 7.0 A/dm2 as our main focus. The bath temperature was slightly high, but overall it was a typical electroplating method.
The bath was mixed with ferrous sulfate hydrate and sodium tungstate hydrate as shown in Table 2. The total of these metallic salt concentrations was 0.20 mol/L. By using the same amount of the complexing agent, the iron and tungsten became ion-complexed together.