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Nanotribology Application in the Coining Industry (I)-Turn off Stamping Oil Sprayer during Coining [Tribology & Lubrication Technology](Tribology & Lubrication Technology Via Acquire Media NewsEdge) KEYWORDS Nano tribology; Additive-Deposited Film; Economics; Friction Mechanism; Anti-Tarnish; Forming; Infrared ABSTRACT The most common tribological cases involve two solid surfaces repeatedly contacting each other. During coining, however, a pair of dies strikes each blank that has been stamped only once. The requirements of the surface physical and chemical properties for coining are different compared to the common tribological conditions. For example, a low-viscosity stamping oil is used during coining in the United States Mint, as well as other industries. Under coining tonnage this small amount of liquid on die surfaces may promote microcrack propagation. In this case, lubrication does not prolong die life but shortens it. In this work, the lubrication mechanism during coining was studied. A lubricant layer was applied on blank surfaces before coining. Based on the special stamping condition, blank surfaces must meet several tribological requirements. According to these requirements, a new technology was employed and the use of a new lubricant was implemented. As a result, fatigue die life was increased almost three times on high-volume production lines and coin surface quality was improved. INTRODUCTION There are two main goals in the minting of circulating and collectible coins. One is to make high-surface-quality coins. The other is to reduce manufacturing costs by extending die life. The United States Mint has explored all possible ways to achieve these two goals. One approach was to change the die steel. Traditionally, 52100 steel was used to make coining dies. Although it had good wear resistance, its toughness and fatigue resistance were low. A new tool steel, Crucible Powder Metallurgy steel CPM 3V, which has both high wear resistance and toughness, was tested as a potential alternative to replace the 52100 steel. Coining tests of this steel showed no improvement in coin surface quality or die life. Later testing involved a CrN hard layer coated on the die surface with a magnetron spattering system (Teer, CFUBMSIP system). After coining tests, although the wear marks on the coin surfaces were reduced, the die life for circulating coins was the same. These facts led us to investigate other aspects, such as blank material and the interface between the die and blank surfaces. The coin materials cannot be changed. For quarters, dimes, and nickels the surface composition is 75% copper and 25% nickel. For the $1 coin, the surface composition is 77% copper, 12% zinc, 7% manganese, and 4% nickel. The only aspect left is the interface tribological properties, which were not previously studied. Although a die surface may strike 1 million times, each blank is only stamped once. The lubricant is brought into the contact zone by blanks. Therefore, understanding the tribological characteristics under this special condition is essential for development work. Some test methods and test instruments also need to be modified to simulate this coining condition. Because the lubricant is carried by blanks, the chemical and physical properties of the top blank surfaces directly affect lubrication. These properties are formed during the blank preparation process before coining. Thus, understanding the mechanisms during the blank preparation process is also critical. The research works were focused on two aspects: the lubrication mechanism during coining and the film-forming mechanisms during the blank preparation process. After obtaining this knowledge step by step, we developed a successful process to treat the blank surfaces. After treatment, the tribological properties on blank surfaces perfecdy matched the coining conditions. With the new treated blanks, the fatigue die life on all production lines (dollar, quarter, dime, and nickel) increased almost three times. The die manufacturing cost savings is $1.5 million each year at one mint. The coins also looked shining and beautiful. Because many research and development tests were involved, in this article, the work is divided into three sections. The first step was to understand the principles associated with the blank preparation process. Circular blanks are punched from a rolled sheet and then sent to the annealing furnace to remove the residual stress formed in rolling. Hydrogen and carbon monoxide generated from the partial combustion of natural gas are used as a protective atmosphere. After annealing, the blanks are quenched in cool water and delivered into a rotation barrel. The blanks are exposed to sliding impact with other media (stainless steel balls) or each other when the barrel rotates. Pickling compounds are added into the rotating barrel to remove the metal oxide layer generated during annealing. After rinsing, antitarnish compounds are added to form a protective film on blank surfaces as the barrel continuously rotates. Then the blanks are rinsed and hot air dried. All surface chemical reactions occur under sliding conditions. In general, this process is called burnishing. Applied research was needed to understand what occurs when a liquid chemical is trapped between two moving solid surfaces and the difference compared to the surface being immersed in aqueous chemical solution without shding. A thermodynamic model was generated. This model predicted the effect of mechanical sliding on the chemical reaction rate. If a chemical could not react with surfaces in a simple immersed case, it might form a reaction film on burnished surfaces. This model revealed how we could purposely use the burnishing process to form a special chemical reacted film. The thermodynamic model is discussed in the third part of this article. In the second part of this article, work was focused on simulation of the blank production process in a laboratory. Simple lab equipment was used to generate nanoscale lubricant and antitarnish film on metal surfaces by burnishing. Fourier transform infrared (FTIR) spectra of the tested samples were analyzed for the film structure and thickness. These data were used to determine whether a film met the technical requirements for coining. Generating a nanoscale film in the laboratory is different from implementing the technology on production lines, because many conditions are different. For example, in the laboratory, after being treated in an aqueous solution, each metal sample was dried with an air nozzle. High surface tension after surface treatment was not an issue. On production lines, where a large number of blanks are dried by hot air, the high surface tension causes water stains. During the development work, there were several iterations between the laboratory tests and production line tests. In the first part of the article, the work was focused on press floor environment analysis and the implementation of nanotribology technology on the production line. Specifically, the lubrication between one die and countless blanks is discussed. A Weibull distribution is used to clearly illustrate the die life increment using the nanotribology application. UNDERSTANDING OF THE TRADITIONAL COINING PROCESS In order to understand the mechanism and factors contributing to fatigue die life, the traditional lubrication condition in the coining process and chemicals used in the blank preparation process are reviewed. The presses used for coining are horizontal high-speed mechanical presses. The reverse die (the tail side) is the anvil, and the obverse die (the head side) is the hammer, as shown in Figure 1. Above each coining press there is a circular vibrator. Coin blanks are dropped into the vibrator from the conveyor and aligned in a single row. A pressurized air nozzle sprays stamping oil lubricant on the blank surfaces. These lubricated blanks then fall individually into a track and are fed between an obverse and a reverse die. Liquid stamping lubrication and lubricants have been widely studied in order to achieve low friction between work pieces and dies (Groseclose (1), (2); Kim and Altan (3); Chandrasekharan, et al. (4); Cubberly and Bakerjian (5)). Solid lubricant was also tested in the forming process (Wei, et al. (6)). The coining press manufacturer's operating manual details the lubrication process and requirements of the liquid lubricant (Schuler (7)). The lubricant must meet the following conditions: high adhesive and shearing strength as well as good wetting capability, no involuntary physical or chemical reactions on the surface of the frictional partners, and easy and complete removal of the lubricant from the finished part. According to these requirements, a vanishing oil, Fisk's #7, with very low viscosity (1 cSt @ 100°F) was used as the stamping lubricant on the presses. After stamping, the coins were free of oil when packaged for delivery. This low-viscosity oil was thought to provide the lubrication during coining. This liquid lubrication concept is misleading. A varnishing oil has a short hydrocarbon chain. The average carbon atom number in a volatile oil is about or less than 12 atoms long. According to lubrication theory (Buckley (8)), when the chain length is less than 14 carbons, the paraffin oil does not provide lubrication. Although there is a small percentage of long-chain hydrocarbons in the stamping oil, it does not adequately cover the blank surface. Therefore, this stamping oil does not provide lubrication during stamping. To understand the influence and mechanisms of the oil during stamping, spray oilers on the presses were turned off during the coining tests. Once the oilers were turned off, the die surfaces were covered with a layer of black deposit after less than 4 or 5 min of coining. This deposit is referred to as dirt on die by the press operators. The presence of this deposit makes the stamped coins visually unattractive. The dies must be polished to remove the deposit in order to keep the coin surface quality consistent. On the high-speed coining lines (750 coins/min), the press is not stopped for polishing. Thus, the oilers were turned on again. With the oilers reactivated, no deposition occurred. This test process was repeated several times. The blank deposit disappeared when the oilers were turned on. The more oil that was sprayed, the cleaner the coins looked, but the die fatigue life was reduced. From this test, it was clear that the stamping oil functioned as a solvent rather than as a lubricant. A very small amount of liquid-phase stamping oil might adsorb on coin blank surfaces and be delivered to the die surface during coining. When there was some contamination on a die, the oil on a coin blank dissolved it and carried it away after stamping. However, this hquid-phase lubricant acted as an accelerator in reducing the fatigue life of the dies. Similar to the pitting wear on gears (Blau (9)), this low-viscosity stamping oil might flow into some small defects or microcracks on the die surface. The exposed cracks on the surface of defects might first be closed, when the die just contacts a blank. As the die is cyclically pressed down, the hydrostatic pressure of the trapped oil propagates the crack tip inward. Stamped blank after stamped blank, the oil was brought into the contact zone, which increased the crack propagation rate. Some cracks propagated back to the surface and small pieces of metal were chipped off from the surface. An example of die cracking and chipping is shown in Figure 2. The cracking and chipping are the main failure mode for die retirement. If too much oil is sprayed, the die fatigue life accelerates. A Nicolet Magna-IR750 FTIR spectroscope was used to measure the FTIR absorption spectra on blank surfaces. By analyzing the spectra, we would determine which chemicals were left on the surface and how much. An 85° grazing angle bench was used to measure the surface layer. With this grazing angle, the spectra of a single layer of the chemicals on a surface could be detected. Blanks that were burnished on a production line were measured on the bench. One spectrum is shown in Figure 3. This spectrum illustrates that the re- sidual chemicals from the burnishing process remained on the blanks. To further investigate these residual chemicals, the compound used in the final burnishing process on the production line was analyzed using the same FTIR instrument. A drop of the burnish compound, VC-2 (Chem Tech, Inc.), was spread on a gold-coated glass slide. Then the water component of the compound was evaporated and the stain was analyzed with a direct reflective method. The spectrum of the compound VC-2 is shown in Figure 4. By comparison with the FTIR spectra stored in the computer software, it was determined that the main chemical in the VC-2 compound was coconut diethanolamine (CDEA). At the Denver Mint, the final burnish compound used is Oakite 12, which is made by a different company. When Oakite 12 was analyzed, the FTIR spectrum was identical to that of the VC-2, as shown in Figure 4. In metalworking fluids industry, CDEA is widely used in many burnishing compounds with different trade names. In Figure 4, the sharp peaks at 2950 and 2850 cm-1 indicated CH2 and CH3 vibrations. The peak around 1630 cm-1 is the carbonyl vibration. The peak near 1450 cm-1 is the CH2 vibration. The peak at 1070 cm-1 is the vibration of the C-N bonds. The dull peak at 3300 cm-1 is the OH group vibration. The majority of the peaks in Figure 4 (pure VC-2) can also be seen in Figure 3 (residual chemicals on blank), although the positions of the peaks were shifted slightly. CDEA has two OH groups. The nitrogen atom is an sp3 hybrid. The lone pair of electrons and the two OH groups make the molecule hydrophilic. CDEA is highly soluble in water. When it contacted the blank surface in the burnishing process, the hydrogen atoms in the alcohol groups were displaced and the oxygen atoms bonded with the copper atoms. The nitrogen atom also formed a complex with copper. The whole molecule tightly bonded on the blank surface. Thus, the hydrocarbon chain with 12 to 14 carbon atoms stood up on the surface. All of the CH2 branches in the CDEA molecules were exposed and aligned away from the blank surface. The surface became very hydrophobic, causing the rinse water to bead up on the blank surfaces. These water beads were easily shed from the blank surfaces, resulting in stain-free drying. Additionally, the carbon chains standing up on the surface functioned as a lubricant during coining. The thickness of the CDEA was not controlled during the burnishing process. How many layers of CDEA were adsorbed on the blanks was unknown. In the spectrum shown in Figure 3, the dull peak around 3300 cm-1 is still visible. This indicates that some CDEA absorbed on the blank had not reacted with the surface yet or the molecules were physically adsorbed on the first reacted layer. The OH groups might belong to the molecules in the second layer. These physically absorbed molecules could be transferred to the die surfaces when the blanks were sent to coining without stamping oil. The transferred chemical accumulated on the die surfaces strike after strike. After 4 to 5 min (3,000-4,000 coins), the accumulated CDEA was thick enough on the die surfaces to be observed as the black dirt on die deposit. Following the above discussion, lubrication in the traditional coining process can be summarized as follows: 1. Coining requires a lubricant; otherwise, adhesion wear occurs between the dies and coins. 2. The lubricant was apphed on the blank surfaces during the blank burnishing process. These physically adsorbed molecular layers lubricated the die surfaces during stamping. 3. The layers of lubricant protected contact surfaces from adhesion, but extra layers of lubricant gradually accumulated on die surface during continuous striking and formed a black dirt on die deposit. 4. The varnishing stamping oil sprayed on the presses by the oiler did not provide lubrication but functioned as a solvent to flush away the black deposit. 5. Once liquid-phase oil appeared on the die surface, the crack propagation rate increased and die fatigue life was shortened. The conclusion is that a monomolecular lubricant film on blank should be developed without a liquid-phase lubricant. This monomolecular film must have multiple functions to meet all of the technical requirements. First, the functions of CDEA film are analyzed as follows: 1. The CDEA provided insufficient lubrication because the chain length was around 12 to 14 carbon atoms. 2. The molecular thickness on a blank surface could not be controlled. Multiple layers might form and cause CDEA molecules to accumulate on the die surfaces. 3. The CDEA layer had low surface tension. Residual water was easily shed from the surfaces. 4. The CDEA layers did not provide strong anti-tarnish protection due to its molecular structure. The third and the fourth properties of the film are not related to lubrication but have the same importance for the coin production. One function of the surface layer on blanks is to protect the coins from tarnish or oxidation. All of the burnishing processes are conducted in water. After the final rinse, the water must bead up and be shed from blank surface. The hot air only removes a small quantity of the residual water left on the blank surfaces. If the surface tension on the blanks is high, the water will stick on the surface. The hot air cannot provide sufficient energy to evaporate all of the water, and water stains appear on the blanks. The surface quality is therefore damaged before coining. After coining, although CDEA provides some anti-tarnish function on coins, it is limited. The nitrogen atom in the CDEA is an sp3 hybrid, as shown in Figure 5. When a CDEA molecule adsorbs on a surface, it looks like an unequal-sided three-face pyramid. The molecules are not closely packed and there are small holes among the molecules. The oxygen molecules, which are smaller than the holes, can penetrate the layer and react with the copper surface to form metal oxide. The oxide area gradually grows from this point, causing the surface to tarnish. One example of tarnishing is shown in Figure 6. The blank was burnished with CDEA. After coining, the coin was put in an environmental test chamber for a 2-h accelerated environment test at 100% relative humidity and 100°C. This test is called the 2-h steam test. After the steam test, the coin surface was tarnished. The oxygen atoms or water molecules first penetrated through the small holes in the absorbed CDEA molecular layer and reacted with copper. From these small holes the oxide are gradually grew and merged into other oxide areas until the whole coin surface was covered with oxide stains. This process is similar to the grain growth in crystal materials. To avoid this tarnish, the chemical layer on a surface must be closely packed without holes. From the above analysis, to provide good lubrication during the coining process and long-term tarnish protection after coining, a layer of chemicals must be apphed on blanks before stamping. This layer of chemicals must meet the following six requirements: 1. The molecules in the layer must be so closely packed that oxygen cannot pass through it. 2. The surface must be free of oxide generated in the annealing process. 3. The layer on the surface must be hydrophobic. Thus, after burnishing in water, the blanks can shed rinse water easily to avoid forming water stains on the blank surfaces. 4. The layer must provide a lubrication function during coining to reduce friction at the die-blank interface. 5. This layer must be self-aligned on the surface. The functional group of the molecules must react with the metal surface to form strong bonds. 6. Only one layer of molecules is allowed on the blank surface. The extra layers of chemicals will accumulate on the die surfaces during coining. Development of a New Blank Preparation Process It took several years to develop the new blank preparation process, because all aspects of coin production and coin quality must be considered. Each requirement listed above was determined after many test failures. There were no routine tribological test methods that could be directly used to measure or evaluate the surface physical and chemical properties for the unique blank preparation process. To meet each condition listed above, a special test method or technique must be created and designed. Some test instruments were modified. Because tribology is a multidisciplinary science, the measurements were not limited in friction and wear. Six techniques were used to study the blank surface properties during the process development. The first technique was to adopt hue color measurement to evaluate the surface anti-tarnish layer (Klein (10)). The $1 material was patented by the Olin Corporation (Brauer, et al. (11)). One of the special purposes of the alloy is tomake the color of the coins looks like gold. Unlike cupronickel alloy, the $1 material is very easily tarnished. To keep the golden color, the surface must be protected by a closely packed and strongly reacted anti-tarnish layer. Many chemicals, such as amines, azoles, fatty acids, and silicon oils, as well as all anti-tarnish compounds used by foreign mints, were screened. None of the chemicals on the market met the requirements. The surfaces treated by different chemicals were similar to the oxidized coin shown in Figure 6 after the 2-h steam test. Due to the structure, the molecules could not form a closely packed film. Oxygen and water molecules that penetrated the chemical film continuously reacted with the metal surface and the film lost its anti-tarnish function. To form a closely packed molecular film, a new chemical, Carboshield BTA, was created at the United States Mint's laboratory, and later it was produced by Lonza. Years ago, the molecule in the formula did not exist. The main ingredient in CarboShield BTA is the reaction product of dimethyl didecyl ammonium bicarbonate (DDABC) and benzotriazole (BTA). The chemical molecular structures of DDABC and BTA are shown in Figures 7 and 8, respectively. The trade name of DDABC is CarboShield 1000. When the two molecules react, FICOß + H = > H20 + C02|. The DDABC becomes a cation and the BTA becomes an anion, as shown in Figure 9. In the BTA molecule, the two double-bonded nitrogen atoms are an sp2 hybrid. The two lone electron pairs of the nitrogen react with copper to form a copper complex (Rubim, et al. (12); Nilsson, et al. (13); Mansikkamaki, et al. (14); Kim, et al. (15)). The negative charge of the BTA attracts the positive DDABC ion next to it. When a BTA molecule reacts with metal atoms, one of hydrogen atoms on the benzene ring forms a hydrogen bond with the adjacent nitrogen atom that lost the hydrogen in the reaction. Thus, the BTA can closely link together on the surface to form a line as shown in Figure 10. This molecular mode was reported by Fang (16). Because all of the BTAs are negative ions, they attracted the DDABC ions behind the line. Thus, one line of BTA standing by one line of DDABC forms 2D coverage for the whole surface without any small holes. The surface is protected from tarnishing by blocking oxygen molecules. The Mint does not have the equipment to measure the exact molecular pattern on a flat surface in a production environment. However, with the hue color measurement detected by a spectrophotometer, the tightly arranged film can be indirectly proved on production lines. The hue color coordinate is a 3D system used to digitally evaluate a surface color. The value L* is the reflectivity, a* is the redness, and b* is the yellowness. The target color range for dollar coins is set between 14 K gold and 22 K gold as shown in Figure 11. The initial color of the coin (green diamond) was close to 14 K gold. During a 2-h environmental test, the oxygen and water molecules penetrated the small holes in the film. Because it was covered by a layer of CDEA, the coin was oxidized after test. The thicker the oxide layer was, the higher the values were in the red and yellow coordinates. Its color (dark blue diamond) was beyond the color range of 22K gold after the test. After the CarboShield BTA-treated blank was stamped, it was tested using the same steam test for 2 h. The closely packed film blocked the oxygen and water molecules from the metal surface. The film was not attacked and damaged and the metal was protected. The color change was almost unnoticeable after the same environmental test (light blue square in Figure 11). The surface was not oxidized, as shown in Figure 12. This untarnished surface indicated the presence of closely packed molecules of CarboShield BTA on a blank surface. The a* and b* values of the coins with the CDEA and CarboShield BTA layer after the steam test are listed in Table 1. This color measurement method was adopted on the production line to monitor the coin surface quality. The second technique was to measure the oxygen depth profile to control the material removal thickness. The purpose was to ensure an oxide free surface. Although this re- quirement was not directly linked with the lubrication on the interface, it was extremely important. In the discussion of the first technique, a closely packed CarboShield BTA film was not damaged during the environmental test, but it could not completely prevent the surface from tarnishing. Copper oxidation could occur beneath the protective layer. The $1 coin tarnish issue resulted in public criticism. An element depth profile technique was initiated to further investigate the oxidation mechanism. The depth profile of an annealed blank is shown in Figure 13, immediately after annealing but before pickling. Only three elements, copper (Cu), manganese (Mn), oxygen (O), were calculated and plotted for ease of reading. Because the concentrations of Zn and Ni did not vary much, their depth profiles were omitted. The profile was measured by a glow discharge optical emission spectroscope. The dollar coin is made from a clad sheet whose outer layers are made of brass in which the manganese content is 7% (by weight), copper is 77% (by weight), and the remainder is Zn and Ni. In Figure 13, the top surface had a copper concentration less than 10% and a manganese concentration greater than 50%. The surface was actually covered by a layer of manganese oxide (MnO). X-ray photoelectron spectroscopy was used to measure the depth profile with the same results. The XPS spectrum chart was omitted. The Ellingham diagram shows that the free energy of formation of oxide for manganese is about -600 kj/ mol at around 800°C (Galwey and Brown (17); Buckley (18)). The free energy for copper is about -200 kj/mol. Therefore, the manganese acted as a reducing agent for the copper on the alloy surface due to the lower free energy of MnO formation. In this high temperature range, the atoms in the alloy diffused much faster than at room temperature. The diffusion directions were random for both manganese and copper. However, once a manganese atom reacted with oxygen, it stayed at the surface. Thus, after 40-min annealing at 800°C in an H2 and CO atmosphere, a thick manganese oxide layer formed. (During annealing, blanks are continuously fed into and sent out from the furnace, and ambient air flows inside the furnace.) Manganese gradually reduced to the nominal concentration and copper gradually increased to its nominal value with increasing depth. Oxygen gradually disappeared. These gradual changes indicated that at certain depths the metal and metal oxides were intermixed. The purpose of the pickling process was to remove all metal oxide and to expose fresh metal to the anti-tarnishing compounds. During the pickling process, the oxide layer could not be completely removed due to an incorrect burnishing pattern. (Details on oxide removal will be discussed in the second part of this article.) The pickling depth was limited to around 1.0 pm. One example of a partially pickled blank depth profile is shown in Figure 14. According to Figure 13, only the top 0.75 pm of the material was removed. When a CarboShield BTA film was applied on this surface, Cu, CuO, Mn, and MnO coexisted beneath the layer. Over time, the blank eventually tarnished, although the film was not damaged. Mn took the oxygen from CuO to reduce Cu and became MnO, which showed a pink color under the anti-tarnish film. After a 2-h steam test, the measured a* and b* values for this blank were much higher than after pickling. The hue color data are listed in Table 1. A correct burnishing pattern is critical to removing the entire oxide layer. An oxide-free surface after the correct pickling process is shown in Figure 15. Compared to Figures 13 and 14, the profile in Figure 15 indicates that at least 2 pm of material was pickled away. After these tests, the measured hue color values were interpreted as the thickness of a metal oxidation layer. To monitor whether the oxide layer was completely removed during the process, the hue color specification was established based on the oxygen depth profiles. Thus, a simple optical measure accurately predicted the oxide layer thickness and eliminated the unqualified blanks on the production lines. Note that the coin shown in Figure 12 was pressed by a completely pickled blank with a CarboShield BTA film. The third technique was to measure the surface tension with surface tension fluid. A series of surface tension test fluids was used to rank the surface tension. Each fluid had a different surface tension value varying from 30 to 60 dyn/cm in 5 dyn/cm increments. A drop of given fluid was applied on a surface with a swab. If the fluid beaded up, the surface tension of the surface was less than the fluid value. This method was used to replace a relatively complicated surface contact angle measurement on production lines. With this method, the measured surface tension of the CDEA-coated surface was less than 40 dyn/cm. The third requirement for the new thin layer was to form a hydrophobic surface. With a CarboShield BTA layer, the water did not bead up and was not shed from blank surfaces as seen with a CDEA layer. The surface tension was higher than 55 dyn/cm. This high surface tension stretched water to cover large areas on blanks. During the hot air drying cycle, evaporating a large amount of the residual water absorbed the limited energy, the hot air became cold, and blanks were not completely dried, resulting in water stains on the blank surfaces. To solve this issue, the CarboShield 1000 molecule was modified. Two decyls were replaced by two octadecyls and it was called dimethyl dioctadecyl ammonium bicarbonate (DDOABC). The new name is CarboShield BTX. Once the carbon chain became longer, the surface was hydrophobic. The measured surface tension on the CarboShield BTX-treated blanks was lower than 40 dyn/ cm and was the same as that of the CDEA-coated blanks. Due to the low surface tension, there were no water stains on the cleaned blank surfaces after hot air drying. The hydrophobic property of the CarboShield BTX can be attributed to the exposed long hydrocarbon chains of DDOABC. The fourth technique was to compare the coefficient of friction on the interface by measuring the coin edge thickness. There was no way to measure the coefficient of friction between a die and a blank. A developmental spiral pin-ondisk test was developed to simulate the coining condition. In a standard pin-on-disk test the pin does not move. The test is used to simulate common sliding cases in which the two surfaces repeatedly contact each other. Films form on both sides. The measured coefficient of friction is from the two surface layers. For example, a steel pin slides on a steel disk under a dry condition. After two or three rotations, a layer of metal oxide can be found on the wear track of the disk and on the ball contact point. The measured friction force is actually from the two metal oxide layers. Therefore, the friction curve always shows a running-in period for the film formation. After running in, the friction value becomes relatively stable. To simulate the coining process, the ball must contact an untouched surface on which there is an existing thin film. The data measured on the film generated during the fixed radius tests do not represent the actual coining condition. In a developmental spiral curve test, a 52100 steel ball was used as the pin. (The dies were made of 52100 steel.) A blank prepared via the burnishing process was used as the disk. When the disk was rotated, the ball continuously moved out in the radial direction. In this way, the ball always contacted a fresh surface film. This configuration was used to simulate the coining process in which a die always stamps a fresh coin blank. The measured coefficient of friction of a CarboShield BTX-coated blank is shown in Figure 16. The normal load was 3 N and the linear sliding speed was 12.56 mm/s. The ball diameter was 6.35 mm. The friction curve did not show a running-in period. The measured coefficient of friction immediately jumped to the average value, which was about 0.14. If the blanks were pickled without the lubricant layer, the coefficient of friction was above 0.23. The true friction effect of the CarboShield BTX on coining was unknown at this point. After researching tribology textbooks (Altan, et al. (19)), a flat stamping test was introduced. Two flat dies were used to stamp a flat washer. When the friction at the interfaces was higher, the inside diameter was reduced and the outside diameter increased slightly and vice versa. However, in the coining process, this method introduced too much error. Without a constraining collar, after stamping, the outside diameter was either elliptical or irregularly shaped. In addition, the sample was no longer flat, due to the elastic bounce back of the die. The flat stamping test was discontinued. Next, a special test was designed to evaluate the frictional effect by comparing the edge thickness of coins with different lubricant layers. The die was modified as shown in Figure 17. The shoulder where the blank material flowed in to form the edge of a coin was cut deeper. The dot lines show the deepened shoulder. Once the die shoulder that constrained the coin edge thickness was removed, a stress-free surface was formed during coining by metal flow into that space. In effect, the edge thickness grew freely. With low friction, the metal was easily squeezed out from center area to fill the edge of the coins. The thicker the edge thickness was, the lower the coefficient of friction was between the die and blank. The dime coins were tested. The blanks burnished with CDEA were stamped as the baseline. The oilers on the presses were turned on to provide a thin layer of stamping oil before striking. The load tonnage was 29 tons, and the average edge thickness of four measurements taken around the circle was 1.285 mm. With the blank burnished with CarboShield BTX, the stamping load was reduced to 23 tons. The measured thickness was 1.314 mm, which was still 30 pm thicker than that with CDEA. The tonnage and the edge thickness data are listed in Table 2 for comparison. Although the exact value of the coefficient of friction was unknown, the edge thickness indicated that the friction at the interface was significantly reduced due to the 18-atom hydrocarbon chain in the molecule. The fourth requirement for the film was met. On production lines, the die shoulders were maintained to constrain the edge thickness of coins, and the tonnages were not changed for all denominations. The fifth technique was to evaluate the film bonding strength by burnishing. The film bonding strength is an important parameter in tribology. However, no method was found in tribology textbooks or research papers to obtain an accurate value. To ensure that the applied chemicals can survive the burnishing process, the functional groups of a molecule must react with the clean surface and the whole molecule must line up. If there were no strong reaction bonds between the chemical and metal, the chemical molecules could adsorb on the surface in random orientations. Without a reaction, the chemical layers are very sensitive to the change in burnishing operating conditions, because a burnishing movement can make the chemicals adsorb on the surface or remove them from the surface. Silicone oil (siloxanes) was used as an example to study the bond effect. Siloxane is an inert lubricant (Jones, et al. (20)). Fifty to 200 mL of 1/100% of silicone oil (Emulsified Fluid 8, Dow Corning) was added to a washing barrel with blanks and stainless steel balls. After 30 s of burnishing, a layer of residual silicon oil formed on the surfaces. As more oil was added, the layer became thicker. After this cycle, deionized (DI) water was flushed into the process with the drain open to rinse the extra silicone oil away. Because silicone oil does not have a functional group, it could not selfalign on the surface. Thus, the absorbed layer was either too thick or too thin. If too much silicon oil was added during the film adsorption cycle and flushing cycle time was too short, more residual silicone oil was left on the blank surfaces. As discussed above, the extra silicon oil molecules were redeposited on the die surfaces during coining, causing dirt on die deposits. The tests were performed by two groups: One group conducted the burnishing tests and the other group evaluated the die and blank surfaces. According to the color intensity on the die surfaces, the accumulated film was ranked at three levels, where the third level was the worst case. If a small amount of silicone oil was added during the film adsorption cycle and the flushing time was prolonged, the silicone oil was totally removed by flushing with DI water and the surface became hydrophilic. Water spots were left on the blank surfaces after drying, although there was no molecule accumulation on the dies. The spots were also ranked at three levels, where the third level was the heaviest case. The lubrication parameter was defined as the amount of added silicone oil (milliliters) divided by flushing time (seconds). To view the relationship between the functions of the residual oil and the lubrication parameter, the data are plotted in Figure 18. The residual silicon oil film was either too thick or too thin, resulting in either dirt on die deposits or water stains. On a large production line, it is impossible to maintain accurate conditions to obtain a perfect film thickness. Therefore, the silicon oil was not selected as the lubricant for blanks. The DI water flushing process was used to evaluate the CarboShield BTX film. After the blanks were burnished in the barrel with acid, the surfaces were oxide free. CarboShield BTX was added to the barrel as the final burnishing compound. The dosages varied from 50 to 200 mL. The BTA reacted with copper to form a complex. There was no other atom in this molecule that could react with copper surface. Thus, the molecules were self-aligned on the surface with the nitrogen atoms. To prove that CarboShield BTX strongly bonded to the surface, DI water flushed into the rotating blanks and stainless steel balls and drained away with residual chemicals, when the outlet was open for 1-10 minutes. After rinsing with DI water, the CarboShield BTX layer was intact. The residual CarboShield BTX on the blank surface could be detected by FTIR. The lubrication parameter changed from 0.08 to 3.3. There were no water spots left on the blanks for all of the lubrication parameters, but the deposit was always found on dies after a 4- to 5-min stamping test. The results indicate that after the molecules self aligned on the surface, the layer had a stronger bonding strength compared to silicon oil. However, the molecule layer thickness was not controlled. The last technique was to use an extra layer remover (ELR) to form a monomolecular chemical layer. Currently, the Langmuir-Blodgett technique is often used to generate a monomolecular layer. Dominguez, et al. (21) generated a carboxylic acid monomolecular layer on steel and glass surfaces using this technique. Ratoi, et al. (22) used the same technique to coat a multilayer of stearic acid on steel by multiple pulling. In the blank preparation process, pulling blanks from a water bath is not reahstic. The monolayer can only be formed in the burnishing process for large-scale production. As discussed with regard to CDEA, the CarboShield BTA and CarboShield BTX also formed a multiple molecular layer during the burnishing process. Extra CarboShield BTA or BTX was deposited on the die surfaces as well. Similar to CDEA, once the oiler was turned on during coining, the CarboShield BTX layer was flushed away. To remove the extra layers on the blanks, the surface layer properties were analyzed. In multilayer absorption, the first layer of BTA reacted with the copper to form a metal complex. The second layer was physisorbed on the first layer, and the bonding energy of the physical adsorption was much less than the chemical reaction bonding on the surface. As estimated by Myers (23), the physical bonding energy is around 10 kj/mol, but the chemical reaction bonding energy can be a few hundred kilojoules per mole. If a chemical that could break the physical adsorption bonding of BTAs but did not rupture the chemical reaction bonding between BTA and metal could be found, extra layers would be washed away and a monomolecular layer would be left on the surface. After many tests, one chemical was found to have this property, the ELR. The ELR was mixed with the CarboShield BTX in an aqueous solution. During burnishing, the monolayer CarboShield BTX was gradually formed, after which extra layers were continuously removed. With this ELR added to the solution, process variations in washing time, chemical dosage, DI water flushing time, etc., could be tolerated. The quality of tons of washed blanks was maintained. When these blanks were sent to the presses, the oilers on the presses were turned off. No dirt on die deposit was observed on the die surfaces after 1 million strikes. The ELR concept and its application are new, and no references could be found in tribology or monomolecular layer research work. Detailed film analysis is discussed in the second part of this work. Results from Coining Production Lines After the issues on the production line were solved, the new chemical compound, CarboShield BTX, was implemented in the blank preparation line at the United States Mint in Philadelphia on June 2010. With this new chemical compound, the surface quality of the coins was significantly improved and die fatigue life was almost tripled. The surface quahty improvement was almost immediately noticed by the American public after the coins were released. Different appraisals for the coins made before and after the new process was implemented were published in coin collector magazines. The light yellow-tinged surfaces were replaced by the proof coin quality surfaces (Daley (24); Harper (25)). The appearance of the two coins is compared in Figure 19. The blank on the left was burnished using the traditional process. Contamination on the coin can be seen clearly. The coin on the right was burnished with CarboShield BTX. A scattered light source was used to eliminate direct reflective light. After the blanks burnished with the new process were coined on presses without any liquid-phase lubricant, the die fatigue life was obviously increased. The dime die life charts, for example, are shown in Figure 20. The average die life increased 200-300%. Such an increase in die life in an industrial environment suggests that the lubrication mechanism may be dramatically changed (Such (26); Koc (27)). The main die failure mode is fatigue (i.e., cracking and chipping on the die face) as shown in Figure 2. Once cracks appear on a die surface, they are duplicated on the coins upon stamping. After finding cracks under 7x magnification, operators who monitor the coin surface quality will retire the die and record the die life. Because most dies fail after 100,000 strikes, each die is like a test specimen during fatigue tests. The common characteristic of a fatigue test is the scattering of the data points. The fatigue data are plotted in Weibull distribution charts in which the average fatigue life and the scatter of the data points can be easily visualized. Thus, two average points in Figure 20, before and after the new process, were plotted in a Weibull distribution chart as shown in Figure 21. In the chart, each data point presents one die life. There is no overlap between the two data sets. Improved die life was conclusively proven by comparing the two distributions. In 2011, the blank preparation process was implemented at the United States Mint in Denver and the die life was almost tripled. CONCLUSIONS In coining processes, the stamping oil actually shortens the die life by increasing the crack propagation rate. Therefore, the stamping oil sprayers on the presses were turned off. A new lubricant was developed in the laboratory and implemented on the production lines. This new chemical, CarboShield BTX, was applied on blank surfaces to replace traditional CDEA. This monomolecular layer provided a low friction on the interface, eliminated contamination from the die surface, and prevented liquid-phase lubricant from permeating into the microcracks. As a result, the die fatigue life was more than doubled and coin surface quality was significantly improved. After coining, the layer further protected the coin surface from tarnishing. In the second part of this work the scientific measurement of a monomolecular layer on blank surfaces will be discussed. Infrared absorption spectra will be used to identify the residual chemicals on the surface and estimate the film thickness. This article not subject to US copyright law Manuscript received Aug. 14,2012 Manuscript accepted Dec. 18,2012 Review led by Liming Chang ©STLE Editor's Note: Many of us carry around excellent examples of tribological advancements in our pockets in the form of loose change. The process for making coins requires significant technology to ensure the coins are created without flaws and at the lowest tooling cost possible. This month's Editor's Choice paper investigates the use of a newly developed lubricant to prelubricate the coin blanks before they are stamped instead of constantly spraying lubricant on the die presses themselves. The results show an improvement in die fatigue life and an increase in the coin surface quality. An interesting side effect was a reduction in tarnishing due to the remaining lubricant layer's protection. Evan Zabawski, CLS Editor REFERENCES (1) Groseclose, A. (2009), "Evaluating Lubricant Performance for StampingPart I," Stamping Journal, March, pp 12-13. 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Tony Ying,1 Uvon Tolbert,1 David Zipkin,2 and Etienne Kunderewicz2 1 United States Mint Washington, D.C. 2 United States Mint Philadelphia, Pennsylvania (c) 2014 Society of Tribologists and Lubrication Engineers |