Hitachi Tools and Hitachi Metals Improve Adhesion of DLC Coatings for Automotive Parts
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According to the two companies, the current demand for reducing the weight of the auto parts and increasing the durability is high, including the reduction of the engine valve system (valve lifter, rocker) and the sliding parts of the fuel injection system (fuel nozzle, fuel supply pump, etc.) The friction demand is high. In addition, the sliding parts used in the engine are subject to high temperatures, so it is also necessary to improve the combustion resistance. In the past, the sliding parts were coated with a nitriding coating, which was not applicable in some fields. Therefore, DLC coatings capable of reducing friction are popularized. However, current DLC coatings have problems with adhesion.
Figure 1: Friction coefficient and film hardness of the denser DLC coating "L-Frex" series.
In this regard, Hitachi Tools and Hitachi Metals have studied the adhesion of DLC coatings using their respective coating techniques and material technologies. As a result, it was found that a thin oxide film of several tens of nm located at the interface between the coating film and the base material is a cause of lowering the film adhesion. By completely removing the oxide film, the adhesion is increased by more than 50% compared to the current DLC coating (Figs. 1, 2).
In addition, the new coating also increases the film hardness. The new coating has three materials of "L-Frex S" with a Vickers hardness of 2000 HV, "L-Frex M" of 3000 HV, and "L-Frex H" of 4000 HV. The friction coefficient is 0.10 to 0.15, and the peeling load is 85N.
In addition, the base materials developed by the two companies are based on high-Cr martensite steel. In order to achieve both high fatigue strength and corrosion resistance, the added elements were adjusted. Rockwell hardness can be maintained at about 60 HRC even when the temperature of the coating is increased. When the coating treatment time is shortened to about 450 ° C, high hardness can be maintained and adhesion can be ensured. In addition, the new base material can improve the adhesion even if it is used for a coating other than a DLC coating such as a chromium nitride treatment or a titanium nitride treatment.
Hitachi Tools will provide coating services at the Surface Modification Center in Songjiang City, Shimane Prefecture. Hitachi Metals will manufacture base metals at the Anla Plant of the Special Steel Company (Anlai City, Shimane Prefecture). In addition, the surface modification center has been transferred from Hitachi Metals to Hitachi Tools with the integration of the coating operations of the two companies.
Figure 2: Results of the scratch test (when the scratch load is 40N). The left side is the new coating and the other is the original DLC coating example.