Fabrication of stainless steel micro components using softlithography Mohamed Imbabya, Kyle Jiang*a, Isaac Changb a b
School of Mechanical engineering, University of Birmingham, Edgbaston, Birmingham, UK School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, UK
Abstract 316-L stainless steel has good mechanical properties and has been widely employed for making different devices. This paper presents a study for making micro 316-L stainless steel components by soft lithography in combination with powder metallurgical processes. The process involves producing deep and solid micro moulds using SU-8 photo resist, making soft replica of the moulds using silicon rubber (PDMS), forming green patterns by filling stainless steel slurry into the PDMS moulds. The green parts are de-moulded, de-bound, and finally sintered in tube furnace including nitrogen atmosphere to obtain the final micro parts. The resultant micro components show good quality micro parts with complex geometry. The density of the sintered parts reaches 91.5% of the theoretical one and the linear shrinkage of the micro components after sintering is investigated and it is found to be dependent on the percentage of the solid loading in the green patterns. The fabrication process is described in detail and the results of characterization in shrinkage and density have been analysed. Keywords: SU-8 master mould, PDMS, Duramax D-3005, 316L stainless steel, micro components
1. Introduction Micro electro mechanical systems (MEMS) were used in various applications such as pressure sensors, biomedical sensors, drug delivery systems, fluid management processing devices etc. in which the fabrication technology was evolved from silicon based integrated circuits techniques [1]. The demands for multiple material micro components encourages researchers to develop new fabrication techniques that allow metals, ceramics, alloys, and polymers be used. In terms of good mechanical properties and medical applications, stainless steel components are good materials for these purposes. 316L stainless steel grade that contains chromium-nickel and molybdenum provides higher resistance to pitting and crevice corrosion in chloride environments. These properties in addition to low carbon contents make it the best candidate for the implanted applications because of the decreasing in vivo corrosion [2]. Different methods have been used in micro components fabrication, such as LIGA process based on combining synchrotron radiation lithography and galvanoforming [3], focus ion beam (FIB) [4], laser micro machining [5] and micro electro discharge machining (MicroEDM) [6]. These methods are either very expensive, such as LIGA, FIB and MicroEDM, or of low quality and resolution, such as laser. Micro injection moulding (MIM) is another fabrication technique suitable for polymers [7], metals [8], alloys [9], and ceramics [10]. However, micro injection moulding relies on precise metal mould that is fabricated by electroformed process [11] which can increase the overall cost of micro fabrication. This paper presents a study on fabrication of 316-L stainless steel micro parts with high shape retention and complex shapes using softlithography and powder metallurgy processes. Softlithography is a relatively new process that relies on soft mould inserts. The process includes fabrication of SU-8 master moulds
and replication with PDMS negative replicas moulds. Powder metallurgy process includes preparation of stainless steel slurry, filling the PDMS moulds, demoulding, de-binding, and sintering to obtain the final micro part. The research work is based on previous successful experience in fabrication of micro components [12, 13] and the techniques have been developed further. The fabrication process is investigated in details. The linear shrinkage and the density of the sintered micro parts are discussed. 2. Fabrication Process 2.1. Fabrication of SU-8 master mould SU-8 is a negative tone resist. It can be fabricated to a thickness higher than 1mm because of its very low optical absorption in the UV range 360-420 nm. Thus, a relatively constant exposure dose can go through the entire resist thickness without losing much energy and excellent vertical sidewalls are obtained. Moreover, SU8 is highly resistant to solvents and acids when cured [14]. Deep x-ray lithography is one of the fabrication techniques used for fabrication ultra thick layers of SU8 [15]. Due to higher cost of x-ray source, UV lithography was successfully developed to fabricate ultra thick layers of SU-8 micro engine parts [16]. The fabrication process of ultra thick SU-8 master moulds used in this research was based on [12, 13, 16], and modification was made due to the change of the SU-8 type. SU-8 2075 [MicroChem, USA] is the type of photo resist used in this work in which 1mm thick is fabricated. The SU-8 mould fabrication procedure started with casting SU-8 on the 4 inch wafer and soft baking at 65Cº for 2 hours followed by 95Cº for 34 hours. Exposure was done in Canon PLA-501 FA UV-mask aligner. As discussed in [16], the exposure time was 17.5 units, but in this work PL-350 filter was used for reducing the overall exposure time to 13 units. Afterwards, the wafer went through post exposure bake
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and developing in EC solvent. The SU-8 master moulds achieved from the fabrication process are shown in Figure 1 in which uniform and straight sidewalls are obtained. 2.2. Replication with PDMS mould PDMS is viscoelastic, non toxic, non flammable, optically clear. Moreover, very accurate impressions of micro structures can be obtained. The steps needed to replicate PDMS mould inserts are as follows. Firstly, PDMS raw material (DOW Sylgard Silicone) and curing agent were added in 10:1 in weight then the mixture was vacuumed in order to remove all bubbles formed during the mixing. Secondly, the mixture is purred on SU-8 moulds and vacuumed again. Finally the PDMS on top of the moulds was cured in an oven at 90 Cº for 2 hours. The cured PDMS moulds were peeled off from SU-8 master moulds with the help of cutter and tweezer. Figure 2 shows PDMS replication mould. It was observed that very thick PDMS layers damaged the SU-8 master moulds during the peeling off, on the other hand very thin PDMS layer was damaged during the peeling off. 2.3. Preparation of 316-L stainless steel slurry Stainless steel 316L powder supplied by Sandvik Osprey Ltd., UK is used in this research. The particle size distributions and the chemical compositions of the powder delivered by the supplier are presented in Table 1. The powder was examined in SEM (JOEL 6060) and its image is shown in Figure 3. The powder shape is spherical with different size distributions. In softlithography where soft mould (PDMS) insert is used, no injection pressure or heating temperature are needed, thus, the binder must have low viscosity at room temperature during the moulding and high viscosity during the drying processes. In this work, Duramax D-3005 delivered by Chesham Speciality Ingredients Limited, UK was used as a binder. Duramax D-3005 is the ammonium salt of an acrylic homo-polymer and it has been used as a dispersant in the ceramic and nickel fabrications [17, 18]. In this work, stainless steel slurry containing 80% and 85% weight of stainless steel powder were prepared. Duramax D-3005 and de-ionized water were mixed in beaker 30:70 by weight respectively, then stainless steel powder is added. The mixture was homogenized by mechanical stirrer for one hour and put under vacuum to remove the bubbles formed during mixing.
Fig. 2. PDMS moulds insert Table 1 Chemical compositions and size distributions Chemical composition % size distribution Cr
Ni
Mo
Mn C
D10 D50 D90
18.5
11.6
2.3
1.4 .048
P
Si
S
Fe
1.1 um
.027
0.65
.008
Bal.
1.8 um
3.6 um
Fig. 3. 316L Stainless steel powder
2.4. Mould filling and preparation of green parts The slurry was poured in the PDMS mould cavities under gravity. It was observed that the moulds showed incomplete filling because air trapped in small cavities such as gear teeth is difficult to be removed. Therefore, vacuum was applied to let the trapped air to escape from the mould. After that, the filled mould was left to dry at room temperature for 1-2 hours before the green parts were successfully de-moulded with more than 80% defect free samples. Figure 3 shows defect free green part. 2.5. De-binding and sintering
Fig. 1. SU-8 master moulds
De-binding and sintering were carried out in the same heating cycle in a tube furnace (ELITE thermal system limited) in nitrogen atmosphere. In the debinding stage, the temperature was slowly ramped up at 3 ºC/Min until it reached 600 ºC, and it was maintained at that temperature for one hour. Then it came to the sintering stage. The temperature was ramped to 1200 ºC at a rate of 6 ºC/min and maintained at this temperature for 90 minutes.
5. Conclusion Good shape retention with complex shape stainless steel micro parts were successfully fabricated using softlithography in combination with powder metallurgy processes. Also, Duramax D-3005 was successfully used as a binder in the preparation of stainless steel slurry. The linear shrinkage of the sintered parts was investigated and found to be inversely proportional to the solid loading of the powder in the preparations of green pattern. Acknowledgements Fig. 4. Green part
The authors would like to thank Sandvik Osprey Ltd. for the supply of superfine stainless steel 316L powders. The research was supported by the Ministry of Higher Education, Egypt.
3. Component characterization 3.1. Dimensional shrinkage The parts were shirked after sintering and the linear shrinkage was calculated based on the diameters D of the gear as follows:
Shrinkage % =
D SU-8 gear - D sintered gear D SU-8 gear
× 100
4. Results and discussions The fabrication of SU-8 mould is optimized with little difference from the previous work [16] due to the change of SU-8 type. On the other hand, vacuum has the greatest effect of degassing the bubbles formed during the PDMS mould replication and mould filling. Moreover, the sintered gear is examined under SEM and its image was shown in Figure 5. It has been shown that the gear retained all features in which homogenous shrinkage was obtained. The linear shrinkage% of sintered gears containing 80% and 85% solid loading was measured and found to be 17% and 15% respectively. Increasing solid loading during the green part preparation decreases the shrinkage after sintering because of decreasing the amount of the binder that burned out during the heating (de-binding and sintering). The density of the sintered part was measured according to Archimedes principal and found to be 7.23 g/cm3 around 91.5% of the theoretical one.
Fig. 5 Sintered part
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