1.  Abstract

In the field of cold forging, the zinc phosphate coating, invented in 1934, has been widely used as a lubricating coating. Although this coating has excellent lubricity, its efficiency of coating formation is poor owing to the use of chemical reactions. Furthermore, it has a significant negative impact on the environment, such as a large amount of waste emitted from the reaction process. Therefore, since 2000, a gradual shift has occurred to dry-in-place lubrication coatings, which do not require chemical reactions for its formation.

To solve these problems, we developed an organic-inorganic hybrid solid lubricant in which an alkylammonium cation, as a lubricating agent, is inserted into the interlayer region of mica via an intercalation reaction. Applying this solid lubricant to the dry-in-place lubrication coating can simultaneously solve the problems of low coating efficiency and high environmental burden of the conventional zinc phosphate coating.

Fig. 1 Examples of cold forged parts

2. Technical Details

2.1 Intercalation Reaction

Mica is a layered clay mineral that consists of silicate layers approximately 1 nm in thickness. The negatively charged silicate layers are loosely bonded to each other via electrostatic attraction with Na⁺ cations, which exist between these layers. These Na⁺ ions can be replaced with cations through ion exchange in water; this method is called the intercalation reaction (see Fig. 2).

In the organic-inorganic hybrid solid lubricant, double chain-type alkylammonium cations with an alkyl chain length of C=18 are intercalated into the mica interlayer region, resulting in smooth cleavage on the friction surface.

Fig. 2 Synthesis of organic-inorganic hybrid solid lubricant

2.2 Performance of Organic−Inorganic Hybrid Solid Lubricant

Fig. 3 shows the lubricity test results for each lubrication coating evaluated by an upsetting and ironing-type tribometer. In this test, cylindrical billet, whose surface was coated in advance by the lubrication coating, is first processed into a barrel-shaped sample for the upsetting process. And then, side of the barrel-shaped sample is squeezed in an ironing process by pushing down three bearing balls simultaneously from the top to bottom. Lubricity is evaluated based on the maximum ironing load and appearance of the ironed surface. It can be seen that the lubricity of mica is improved to the same level as that of the conventional zinc phosphate coating by hybridization with alkylammonium cations. Furthermore, the anti-seizure ability of the organic-inorganic hybrid solid lubricant was better than that of molybdenum disulfide, a conventional solid lubricant.

Table 1 shows the coating performance related to the efficiency of formation and environmental characteristics. The dry-in-place lubrication coating, which consists of an organic-inorganic hybrid solid lubricant, does not involve chemical reactions for coating formation. Therefore, the coating forming time was significantly shortened compared with the zinc phosphate coating. Furthermore, industrial waste, such as wastewater, reaction by products, and CO₂ emissions, can be drastically reduced to less than 10 % compared to zinc phosphate coatings.

Fig. 3 Evaluation result of lubricity by an upsetting and ironing type tribometer

Table 1 Coating performance related to efficiency of formation and environmental characteristics

3. Summary

The development of an organic-inorganic hybrid solid lubricant has improved the efficiency of coating formation and reduced the environmental burden, while maintaining the same level of lubricity as the conventional zinc phosphate coating.

In the intercalation reaction, there are numerous potential cation candidates that can be applied to layered clay minerals. Therefore, by selecting the appropriate cation candidate, it may be possible to easily impart various functions to the layered material based on the target application. The organic-inorganic hybrid solid lubricant developed herein will greatly contribute to the diversity of solid lubricants and conservation of the global environment in the near future.

Kenichiro Oshita,
Member, Central Research Laboratories, Nihon Parkerizing Co., Ltd (2784 Ohkami, Hiratsuka-shi, Kanagawa, 254-0012, Japan)

Shinobu Komiyama,
Corporate Planning Division, Nihon Parkerizing Co., Ltd (1-15-1 Nihonbashi, Chuo-ku, Tokyo, 103-0027, Japan)

Shinya Sasaki,
Fellow, Department of Mechanical Engineering, Tokyo University of Science (6-3-1 Niijyuku, Katsushika-ku, Tokyo, 125-8585, Japan)