**1. Introduction**

Microforming technology has been receiving much attention as one of the most economical mass production methods for micro components (Geiger et al., 2001). Especially, metal foils have the great advantage to produce high-aspect three-dimensional shapes by miniaturizing the process dimensions of sheet metal forming technologies.

Since the relative ratio of the surface area to the volume of metal foils becomes significantly larger with miniaturization, tribological behaviour is of great significance for the microsheet formability. Over the last decade, basic researches of the size effect of tribology in microforming have been performed worldwide (Vollertsen et al., 2009). One of the representative reports of scale effect in bulk metal forming is the double-cup-extrusion (DCE) test (Engel, 2006). By scaling the diameter of CuZn15 specimen from 4mm to 0.5mm, the scaled DCE test was conducted. Identified with a FE analysis, the significant increase of the friction factor *m* with decreasing the scale was confirmed. Another reports regard to the size effect of coefficient of friction in bulk metal forming was done by Putten et al. (Putten et al., 2007). The scaled plane strain compression tests were conducted with the aim of application on flat rolling. As similar tendency as DCE test, the friction coefficient increased with decreasing scale dimension.

The work focused on the sheet metal forming has been done by Vollertsen and Hu (Vollertsen & Hu, 2006, 2007). The strip drawing method allowing the determination of friction parameters for micro-deep drawing was developed. It was shown that the friction coefficient again increased with decreasing the size. Additionally, the scale dependent friction coefficient was determined by strip drawing test and the calculated value was introduced to the FEM simulation. Especially if the non-uniform pressure distribution was

© 2012 Shimizu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Shimizu et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

taken into account in the deep drawing simulation, relative good results were derived for different dimensions (Vollertsen et al., 2008).

Impact of Surface Topography of Tools and Materials in Micro-Sheet Metal Forming 113

*μ***<sup>d</sup>** : Deformation component

**Deformation of surface asperities in contact**

> **Climbing of contact asperities against each**

**other**

*μ***r**: Ratchet component

**Soft**

**Soft**

Although these applied researches on the field of dry-microforming are well developed for the practical application, the basic research on the dry friction behaviour during microforming was quite few and it was only worked for bulk metal forming (Vollertsen et al., 2009). Krishnan et al. investigated the scale effect on dry friction by scaled forward extrusion test of brass (Cu:Zn 70:30) with the scale range of 0.57-1.33mm in outer diameter of extruded pin (Mori et al., 2007; Krishnan et al., 2007). In addition, the other author conducted the scaled ring compression test of CuZn15 alloy with different grain size. The test was scaled in the range of 0.5 to 4.25mm of ring inner diameter (Vollertsen et al., 2009). Although the purpose of those studies is to determine the size effect under dry friction, the statement for the size effect of dry

According to the tribology theory for dry contact friction (Bhushan, 2003), friction resistance

• So called, ratchet mechanism, which is due to a lateral force required for the contact

**Hard**

**Hard**

friction seems to be very doubtful and there are no general explanations.

**Adhesion of the sliding surface**

> **Plowing by wear particles and hard asperities**

**Figure 1.** Contributions of the four different components of friction coefficient under dry friction

As illustrated in Fig.1, the geometry of surface asperities would contribute to the real area of contact and sliding resistance, and it is a dominant factor over the dry friction behaviour.

However, basic research on the meso-scale tribological behaviour of dry friction, such as the contact interaction of the surface asperities, is not well discussed, especially for micro-sheet metal forming (Vollertsen et al. 2009). In view of the surface functionalization and the structural optimisation, construction of the surface design guide based on the

is contributed by:

**Hard**

**Hard**

• Adhesion of the sliding surface,

• Deformation of surface asperities in contact, • Plowing by wear particles and hard asperities, and

asperities to climb against each other.

*μ***<sup>a</sup>** :Adhesion component

*μ***<sup>p</sup>** :Plowing component

**Soft**

**Soft Wear particles**

These tendencies of the increasing friction coefficient with decreasing dimension in many experiment of forming process have been mainly explained by the "lubricant pocket model" (Engel, 2006). The lubricant pocket model is the only one available model for the description of size effects in lubricated friction (Vollertsen et al., 2009). The basic feature of describing the size effect based on this model is that the increasing relative ratio of OLPs (Open lubricant pockets), which cannot keep the lubricant. With decreasing the scale dimension, the relative ratio of OLPs increases and it results in the increase of friction resistance (Engel, 2006). As overviewed above, the overall investigation suggests the low effect of lubricant in micro-scale region.

From the other point of view of:


the microforming process would be preferred not to use a lubricant (Aizawa et al., 2010).

In response to these findings, the activity of the application of the coating treatment on the die substrate, targeting the microforming, is gradually increasing. Hanada et al. fabricated a micro-die utilizing chemical vapour deposition (CVD) diamond coating (Hanada et al, 2003). The surface roughness of the diamond dies was approximately 10 nm, and the diamond dies showed good lubricating ability in the microcoining of polymethylmethacrylate (PMMA). Fuentes et al. investigated the tribological properties of Al thin foils (0.2mm nominal thickness) in sliding contact with PVD-coated carbide formingtools for microforming (Fuentes et al., 2006). Uncoated, CrN-coated and WC-C-coated tools were tested, using a pin-on-disk configuration. They have shown that the sticking of Al was retarded using low friction magnetron sputtered WC-C-coated carbides. Fujimoto et al. proposed a novel surface treatment process for micro-dies (Fujimoto et al., 2006). They developed a high-energy ion beam irradiation for finishing die-surfaces and a CVD diamond-like carbon (DLC) was coated on the die surface after finished. They have succeeded to reduce the surface roughness by ion beam irradiation process and the high wear resistance of the DLC coating was demonstrated with the 50,000 shots of the microbending tests. In the recent work on the coating treatment for micro die, an impressive coating technology was proposed by Aizawa et al. (Aizawa et al., 2010). The nano-laminated DLC coating was invented and applied to improve the coated tool life, where delamination of coated layers was significantly retarded or saved by optimum interlayer and nano-scopic lamination of DLC sublayers. Micro stamping system, which included the severe wearing condition of ironing and bending step, was employed in the 10,000 shots progressive dry micro-stamping, where SKD-11 punches underwent severe wear in stamping of AISI-304 stainless sheets or pure titanium sheets.

Although these applied researches on the field of dry-microforming are well developed for the practical application, the basic research on the dry friction behaviour during microforming was quite few and it was only worked for bulk metal forming (Vollertsen et al., 2009). Krishnan et al. investigated the scale effect on dry friction by scaled forward extrusion test of brass (Cu:Zn 70:30) with the scale range of 0.57-1.33mm in outer diameter of extruded pin (Mori et al., 2007; Krishnan et al., 2007). In addition, the other author conducted the scaled ring compression test of CuZn15 alloy with different grain size. The test was scaled in the range of 0.5 to 4.25mm of ring inner diameter (Vollertsen et al., 2009). Although the purpose of those studies is to determine the size effect under dry friction, the statement for the size effect of dry friction seems to be very doubtful and there are no general explanations.

According to the tribology theory for dry contact friction (Bhushan, 2003), friction resistance is contributed by:

• Adhesion of the sliding surface,

112 Metal Forming – Process, Tools, Design

micro-scale region.

From the other point of view of:

1. Dirt handling of the tiny work pieces, 2. Contamination of fine products,

stainless sheets or pure titanium sheets.

the effects of meniscus and viscous forces,

3. Lubricant clogging between the micro scale clearance, and

different dimensions (Vollertsen et al., 2008).

taken into account in the deep drawing simulation, relative good results were derived for

These tendencies of the increasing friction coefficient with decreasing dimension in many experiment of forming process have been mainly explained by the "lubricant pocket model" (Engel, 2006). The lubricant pocket model is the only one available model for the description of size effects in lubricated friction (Vollertsen et al., 2009). The basic feature of describing the size effect based on this model is that the increasing relative ratio of OLPs (Open lubricant pockets), which cannot keep the lubricant. With decreasing the scale dimension, the relative ratio of OLPs increases and it results in the increase of friction resistance (Engel, 2006). As overviewed above, the overall investigation suggests the low effect of lubricant in

4. Unstable formability due to the high sensitivity of the variation of lubricant quantity or

In response to these findings, the activity of the application of the coating treatment on the die substrate, targeting the microforming, is gradually increasing. Hanada et al. fabricated a micro-die utilizing chemical vapour deposition (CVD) diamond coating (Hanada et al, 2003). The surface roughness of the diamond dies was approximately 10 nm, and the diamond dies showed good lubricating ability in the microcoining of polymethylmethacrylate (PMMA). Fuentes et al. investigated the tribological properties of Al thin foils (0.2mm nominal thickness) in sliding contact with PVD-coated carbide formingtools for microforming (Fuentes et al., 2006). Uncoated, CrN-coated and WC-C-coated tools were tested, using a pin-on-disk configuration. They have shown that the sticking of Al was retarded using low friction magnetron sputtered WC-C-coated carbides. Fujimoto et al. proposed a novel surface treatment process for micro-dies (Fujimoto et al., 2006). They developed a high-energy ion beam irradiation for finishing die-surfaces and a CVD diamond-like carbon (DLC) was coated on the die surface after finished. They have succeeded to reduce the surface roughness by ion beam irradiation process and the high wear resistance of the DLC coating was demonstrated with the 50,000 shots of the microbending tests. In the recent work on the coating treatment for micro die, an impressive coating technology was proposed by Aizawa et al. (Aizawa et al., 2010). The nano-laminated DLC coating was invented and applied to improve the coated tool life, where delamination of coated layers was significantly retarded or saved by optimum interlayer and nano-scopic lamination of DLC sublayers. Micro stamping system, which included the severe wearing condition of ironing and bending step, was employed in the 10,000 shots progressive dry micro-stamping, where SKD-11 punches underwent severe wear in stamping of AISI-304

the microforming process would be preferred not to use a lubricant (Aizawa et al., 2010).


**Figure 1.** Contributions of the four different components of friction coefficient under dry friction

As illustrated in Fig.1, the geometry of surface asperities would contribute to the real area of contact and sliding resistance, and it is a dominant factor over the dry friction behaviour.

However, basic research on the meso-scale tribological behaviour of dry friction, such as the contact interaction of the surface asperities, is not well discussed, especially for micro-sheet metal forming (Vollertsen et al. 2009). In view of the surface functionalization and the structural optimisation, construction of the surface design guide based on the characterization of tribological behaviour under dry friction is a pressing need for the further high precision forming process design.

Impact of Surface Topography of Tools and Materials in Micro-Sheet Metal Forming 115

Fig.4 shows the appearance of designed micro-deep drawing die assembly. The die assembly is designed to compact palm-size with around 10cm square. Blanking punchdrawing die is mounted on the upper die, while the blanking die and the drawing punch is set up to the lower die. Furthermore, in order to measure the forming load during the process, the micro-compression load cell (TC-SR50N, TEAC Co.), which has the rating

**Blanking punch**

**Blanking die**

**Drawing punch**

**Blank holder**

**Drawing die**

*Blanking process Drawing process*

**30**

μ

**m**

capacity of 50N, is aligned directly below the drawing punch of the lower die.

**Blanking punch-Drawing die**

*Initial State*

**Figure 3.** Schematic illustration of micro sequential blanking-drawing process

**Figure 4.** Appearance of designed micro drawing die assembly (a) micro compression load-cell, (b)

The micro-drawing die assembly is mounted on the desktop size miniature press machine (MS-50M, Seki Co.), which is custom-designed for the micro deep drawing experiment by Seki Corporation. The press machine is driven with a servomotor, which has a high motion resolution of 400nm and a maximum instantaneous velocity of 28mm/s. The motion control is based on the high precision digital displacement sensor (GT2 Keyence Co.) mounted on the press machine. In addition, a variety of press ram motion can be realized by developed

micro drawing die assembly, (c) tool set of micro sequential blanking-drawing process

**Blank**

**Drawing punch**

In view of the significant importance of tribology in micro-sheet metal forming, this chapter creates an overview of the effect of surface topography of tools and workpieces on microsheet formability. Starting with an introduction of the newly developed micro-deep drawing experimental system and that of a finite element simulation model with surface asperities, the impact of surface topography on the tiny micro-scale forming behaviour is discussed.
