Nanotribology Application in the Coining Industry (II) Optimization of Lubricant Film Formation on Blanks

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Amy Reeves
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1 Nanotribology Application in the Coining Industry (II) Optimization of Lubricant Film Formation on Blanks Extended Abstract As discussed in the Nanotribology surface thermodynamic model, the reaction rate between the absorbed molecules and surface atoms is increased by the sliding movement due to the high entropy in the system. The reacted products depend on the properties of the molecules applied into the system. A burnishing process for blank cleaning in coining industry is used again to analyze the reacted products generated during the sliding movement. There are two kinds of reactions. In one case, the applied chemicals react with surface to form an ionic molecule, and surface then be removed layer by layer. For example, to remove the oxide layer on coin blanks, citric acid was used in the burnishing solution. In this reaction, the acid reacts with copper oxide layer to form copper citrates. The copper atom loses an electron to become an ion. Then, the copper atom is dissolved into the water. In the other case, the applied chemicals may react with surface to adhere on it. For example, to apply a lubricant layer on blank surface, oleic acid was used in burnishing solution. The oleic acid reacts with copper to form copper oleate, an organic salt layer. Although these two reactions are completely different, the rubbing movement in the burnishing process increases the reaction rate in both cases. For material removal etching reaction, the weight lost was compared with and without burnishing process for the same chemical solution. With the rubbing movement, the weight lost was increased several times in a short burnishing test period compared to that in an immersed case. For the adhering case, the FT-IR spectrum of applied lubricant before and after rubbing were analyzed. The FT-IR absorbance spectrum of oleic acid applied on a copper surface without rubbing is shown in figure 1. Fig. 1 Oleic acid physically adsorbed on copper surface after immersion in emulsion solution for 30 min.

2 Fig. 2 Oleic acid reacted with copper surface to form copper oleate after burnishing for 1 min. When the blanks burnished in the same emulsified oleic acid solution for a very short time, the carbonyl peak at 1712 cm -1 shifts to 1587 cm -1 due to the oxygen atom is replaced by the copper atom. (Mass increases, the frequency decreases.) The peak shift indicates the oleic acid reacts with copper to form a copper oleate. Burnishing process increases the reaction rate, and lubricant layer can be formed on the coin blanks in a very short time as coining production needs. With this concept, a special chemical Carboshield BTA was developed and applied on the coin blanks with burnishing process. (Carboshield BTA is a reaction product of dideyle dimethyl ammonium bicarbonate (DDBAC) and benzotriazole (BTA).) The reacted surface layer not only provide a strong anti-tarnish function but also works as a good lubricant layer. The detail information of molecular structure and film formation will be discussed in the presentation. As discussed in the adhering case and the thermodynamic modeling, once the sliding movement is not stopped, the chemicals continuously react with the copper surface, a multiple-layer reacted products are generated. The longer the burnishing lasts, the thicker the reacted product layer forms. This thicker product layer caused problem during coining. When a layer of reacted products can adhere on blank surface, this layer can be transferred to the die surface during stamping. When the layers from each blank are transferred to die surface, it will be accumulated there. With a 750 strikes per minutes high speed press, after a few minutes, a thick organic salt layer are formed. These layer causes surface quality issues, the die must be re-polished or replaced. To stop the chemical accumulation on die surface, but also keep a good lubrication function, a monomolecular lubricant layer on blanks must be prepared before striking. The challenge is that the lubricant molecules must be applied on the blank surface, but the extra lubricant layer must be removed from the first reacted chemical layer. These two processed must be completed in a single burnishing process, and after burnishing, only single monomolecular layer is left on the blank surface. As discussed above, the citric acid removes metal atoms, but oleic acid adheres on the surface. There should be some chemicals of which the properties are between them. For multiple reacted product layers, the first layer of organic metal salt are formed on the surface by chemical bonding. The second layer physically absorbs on the first layer. Third layer physically absorbs on the second layer, and so on. If there is a special chemical, it can open the physical absorbing bonding, but cannot open the chemical reacting bonding, the lubricant molecules will react with

3 the metal surface stay there, and extra layer will be removed during the burnishing process. This chemical is called Extra Layer Remover, ELR. With this concept, many chemicals were screened, and finally one ELR met the requirement, and was tested in the lab. When the ELR was added into the lubricant compound during the burnishing process, when the blanks were stamped on the press, no accumulated organic salt was formed. However, it was not proved that the lubricant was a monomolecular film. To prove this monomolecular layer, a special FT-IR calibration process was developed. The model compound used, a multiply alkylated cyclopentane (MAC), was measured at film thicknesses from about 1 to 10 nm using the peak height of the CH2 asymmetric stretch frequency. The key to calibrating the thickness of that compound relied on developing a special freeze evaporation process for uniformly applying a known amount of MAC molecules to a surface in a dilute solution, freezing the solution and evaporating the solvent so that the MAC molecules ended up as a thin film on the surface. Uniformity of the film was confirmed using X-ray photoelectron spectroscopy and found to be better than 5% over a large (28 mm diameter) sample surface spot size. Grazing angle FTIR spectra that included the CH2 asymmetric stretch region were quantified using peak height. Correlating the peak height against the deposited film thickness for a series of different concentrations of MAC in solvent resulted in a calibration curve of about 400 nm per absorbance unit of MAC film. Because the IR spectrum peak near 2932 cm 1 is specific to CH2, one can estimate a CH2 contribution to film thickness from the MAC and use that value to roughly estimate an approximate film thickness for compounds containing CH2 by factoring in the relative percentage of CH2 in that compound to that in MAC. In the case of MAC versus DDBAC this is not necessary because they both contain the same concentration of CH2 bonds per molecule. MAC contains 53 CH2 bonds out of a total of 65 carbons per molecule (82%). DDBAC contains 18 CH2 units out of a total of 22 carbons per molecule (also 82%). This suggests that a DDBAC film analyzed by the same grazing angle FTIR procedure with a CH2 peak height of 0.01 absorbance units should be about 4 nm thick. After the burnishing process with DDBAC BTA and ELR solution, a uniform monomolecular film was formed. Its FT- IR absorbance spectrum is shown in figure 3 in blue color. This film will be used on the coining production line. Fig. 3 FTIR spectra of DDBAC-BTA on blank surfaces generated with different conditions

4 Nanotribology Application in the Coining Industry (II) Optimization of Lubricant Film Tony Ying, United States Mint Richard Gates, National Institute of Standard and Technology 70 th STLE Annual Meeting, Dallas, TX.

5 The Basic Concept K c Aexp E W ( RT f ) The frictional work increases the chemical reaction rate on a solid surface. The reacted product may be dissolved into solution. The reacted product may be stay on the surface.

Reacted Products Are Removed From Sliding Surface 200 ml 20% AC67 (Citric acid compound+surfactants) 10 minutes test 3 one dollar annealed coin blanks One group was immersed in

6 Reacted Products Are Removed From Sliding Surface 200 ml 20% AC67 (Citric acid compound+surfactants) 10 minutes test 3 one dollar annealed coin blanks One group was immersed in solution One group was burnished in solution Immersed Burnished Weight Loss 0.6 mg 4.6 mg Wear tester in Lab, by removing the liner, adding ball medium, Lab burnishing ball mill.

7 From Lab to Production Equipment 200 RPM 50 Kg stainless steel 3 mm balls 20 kg blanks 1.0 liter of AC67 solution

8 Oxide Layer Could not Be Completely Removed Element depth profile before burnishing. Element depth profile after burnishing.

9 Diagnose Burnishing System Blanks and medium have the same speed. The balls do not rub the blank surface. Modify the rotator by adding ribs. Balls slide on blanks. Oxide layer is removed.

10 Reacted Products Stay on Sliding Surface Hu, Z., Hsu, S., and Wang, P. (1992), Tribochemical and Thermochemical Reactions of Stearic Acid on Copper Surfaces Studied by Infrared Microspectroscopy Low, M., Brown, K., and Inoue, H. (1967), The Reaction of Oleic Acid With Copper Surfaces Yoshioka, S. and Yamamoto, H. (1955), On the Structure of Thin Film of Oleic Acid on Metallic Surfaces. Menter, J. and Tabor, D. (1951), Orientation of Fatty Acid and Soap Films on Metal Surfaces.

11 FT-IR Spectrum Comparison Sample immersed in oleic emulsified solution for 30 minutes, rinsed with water. Sample burnished in oleic emulsified solution for 1 minutes, rinsed with water.

12 New Anti-Tarnish Compound Carboshield 1000 (Dimethyl Didecyl Ammonium Bicarbonate) + Benzotriazole + = + H 2 O +CO 2 DDABC BTA

13 FT-IT Sepctrum of Carboshield-BTA

14 Function Groups in the Spectrum

15 Test Conditions Sample material: One dollar top surface layer Sample size: 75 mm x 25 mm x 1.5 mm Sample surface: Polished with 1 µm diamond compound Solution: 200 ml 5% Carboshield BTA aqueous solution One sample was burnished with solution for 10 minutes Once sample was immersed in solution for 10 minutes

16 FT-IR Spectrum Comparison Burnished Immersed

17 FT-IR Spectrum Analysis With burnishing process, the film is thick, due to the higher reaction rate. With burnishing process, at 1620 cm -1 wave number, there is a board peak that indicates the BTA react with copper to form salt. Multi-layer of reacted BTA salt formed due to the continuously burnishing. Thickness cannot be controlled. The film will be transferred to the die surface during stamping.

18 Reacted Film Transferred to Die

19 Two Different Bonds Copper Oleate Layers on a Copper Surface Van Der Waals Bonds Covalence Bonds

20 Extra Layer Remover Citric acid removes copper and copper oxide from a surface into water. Oleic acid adheres on a copper surface to form copper oleate. + Concept of an Extra Layer Remover: A chemical cannot open covalence bonds. A chemical can open Van der Waals bonds. Remove the second adsorbed layer and keep the first reacted layer. ±

21 With/Without ELR Without ELR With ELR

22 freeze evaporation process Multiply alkylated cyclopentane (MAC) MAC molecules is diluted into a solution. Freezing the solution and evaporating the solvent. Uniform file is formed on the surface. IR spectrum peak near 2932 cm 1 is used to estimate a CH2 contribution to film thickness from the MAC.

23 Intensity of Absorbance Peak VS. Film Thickness After burnishing with ELR, the thickness of DDA-BTA film is about 4 nm.

24 Improve Lubrication function Increase the length of hydrocarbon chains.

25 Conclusions The rubbing movement during the burnishing process increases the reaction rate between applied chemicals and surface. Depends on the properties of the chemicals: The reacted product may be removed from the surface. The reacted product may stay on the surface. If the reacted products adhere on the surface, the thickness of the reaction layer cannot be controlled. To generate a monomolecular reaction layer in burnishing process, an extra layer remover (ELR) is used. Compare the MAC FT-IR spectrum, the film thickness is about 4 nm, in the monomolecular level.

How Lubricants Work. If a protective film were present on each of the surfaces, the surfaces could be separated:

How Lubricants Work. If a protective film were present on each of the surfaces, the surfaces could be separated: How Lubricants Work An understanding of how lubricating systems work is crucial to the selection of a lubricant for a particular application. This essay could be summarized in one sentence: lubricants