Abstract
In this research, the Synchrotron X-ray lithography (X-ray LIGA) was implemented to fabricate imitative Stress Corrosion Crack (SCC) for using as a reference specimen of electromagnetic NDT. A pattern of the imitative SCC was created by using X-ray LIGA with controllable shape and size in precise details. The pattern created had a high aspect ratio, which was formed on SU-8 polymer. As a preliminary experiment, the prepared pattern was inserted into a block of pure Nickel formed by an electroplating technique. To develop a reference comparable to stainless steel in terms of electromagnetic response, the technique called “Suspension Electroplating” was implemented to create a block of material from its powder form. A rough magnetic response was then measured by Eddy Current Testing (ECT). The results showed the possibility of creating the desired properties of stainless steel. The SCC pattern will be then inserted to become the reference specimen.
Introduction
Stainless steel is an engineering material that is used for many industries due to its strength, toughness, and high-temperature resistance. However, it could experience the problem of Stress Corrosion Cracking (SCC), which could cause leakage and failure. It is difficult to detect the SCC due to its small size. Non Destructive Testing such as Eddy Current Testing (ECT) has been implemented for inspection effectively. The disadvantage of this technique is related to reference specimens that must be precise in size and shape as well as close to inspect the material in magnetic properties. There several studies to develop SCC reference specimens such as bounding and laser 3D printing to fabricate imitative SCC. However, the specimens were not identical to the actual SCCs as they were quite large, even though they could provide similar properties to stainless steel in terms of ECT signal [1,2]. There was also an attempt to fabricate the imitative SCC by using X-ray LIGA for the crack pattern and was then inserted into a mixture of stainless steel and Sn powders in a soft mold by using less pressure filling method. It could provide a similar response to the ECT signal, but the shape and size were difficult to control [3].
In this research, the Synchrotron X-ray Lithography (X-ray LIGA) was implemented to produce the pattern of the imitative SCC. The pattern would be small with high resolution and high aspect ratio [4]. The pattern was then inserted into a block of material created from the electroplating technique. To archive the electromagnetic properties comparable to the actual stainless steel, another technique called “Suspension Electroplating” has been developed to be prepared for pattern insertion. A rough magnetic response was then measured by Eddy Current Testing (ECT) which then would be created as the reference specimen.
Materials and methods
Metal powders
The small size of stainless steel (SUS 316L) powders with the average particle size of 10 μm as delivered by suppliers were employed in this research. The chemical compositions were shown in Table 1. Additionally, the 99.9% of aluminum (Al) powder with the particle size of 1 μm was also used in the solution mixed with stainless steel powder (SUS 316L) into the total weight of 2 g in nickel solution which was used for the suspension electroplating technique.
Chemical composition of SUS 316L powders
Chemical composition of SUS 316L powders
A picture of SCC was selected from a micrograph of SCC. It was then used to transfer 2D with the Layout Editor Program. In preparation, a graphite plate as a substrate is thoroughly cleaned, and the AZp4620 photoresist was spin-coated on the graphite plate substrate into a thickness of 15 μm. Firstly, the pattern onto thick film photoresist using X-ray radiation, an X-ray mask absorber pattern must be fabricated by using UV lithography and electroplating technique [5]. Figure 1 shows the X-ray mask comprised of 2 parts, a graphite plate as a substrate and an absorber pattern. For the absorber pattern, the material must efficiently absorb X-ray irradiation to provide image contrast on the photoresist. Materials with a high atomic number and high density, such as gold (Au) silver (Ag) and tantalum (Ta), could be employed in this role.
The fabrication of patterns for imitative SCC in this research was based on the beamline 6a: Deep X-ray lithography (DXL) at the Synchrotron Light Research Institute (Public Organization), Thailand. This beamline provides synchrotron light in low energy X-ray range in lithography of ultra-thick photoresists to fabricate high aspect ratio microstructures. In preparation, a graphite plate as a substrate was thoroughly cleaned and placed face up onto a 500 μm thick SU-8 photoresist dry sheet. The lamination is moved through the heated rollers at a speed of 1 ft/min at 60–75 °C. The graphite as a substrate with the SU-8 photoresist dry sheet were baked using a leveled hot plate, at 65 °C for 5 min then at 95 °C for 10 min and left them cool down to room temperature [7] resulting in very smooth surfaces to be used as a substrate as shown in Fig. 2(a). X-ray mask was placed on photoresist and set up in the X-ray scanning system at the BL6a, with the X-ray exposure dose at 68,000 mJ/cm2 under the synchrotron beam current of 125.45 mA as shown in Fig. 2(b). The SU-8 pattern was then developed by the solvent washing, as shown in Fig. 2(c). Figure 3 shows the vertical sidewall of the SU-8 pattern SCC template with a high aspect ratio attached to the graphite substrate ready for suspension electroplating.
X-ray mask for SU-8 photoresist exposure.
Fabrication sequence of SCC by using X-ray lithography.
The pattern SCC was then placed on a graphite plate as a substrate for suspension electroplating (Fig. 3). The suspension electroplating was a mixture of nickel solution and stainless-steel powder with the addition of aluminum powder into the total weight of 2 g.
The SU-8 pattern SCC template for electroplating, (a) top view, (b) side view.
The process was performed using the custom design stirrer with an agitator which was performed at 200 rpm, as shown in Fig. 4. This caused the suspension of powders in the solution. The stainless steel layer was slowly grown with a current density of 6 mA/cm2 with the temperature-controlled at 30 °C until reaching the designate thickness of about 5 mm.
Fabrication sequence of the SCC by using suspension electroplating.
Characteristics of the imitative SCC
After the specimen fabricated by using the X-ray LIGA with suspension electroplating reached a thickness of more than 5 mm, the specimen was ground to get a flat surface on the top surface and graphite on the back was also removed to reveal the crack pattern as imitative SCC. Polishing was also followed to achieve the smooth surface of crack to 0.04 μm abrasive powder. This process was performed to prepare the surface in case that more thickness is required, which is adapted from the preparation of electrochemical testing specimens [6].
Figure 5(a) showed the imitative SCC, which was the cracking pattern of SU-8 polymer. The imitative crack created could have the resolution or size as small as 30 μm, as shown in Fig. 5(b). Details of fine cracks could be imitated on the crack tip of SCC.
The imitative SCC was the cracking pattern of SU-8 polymer, and the imitative SCC in block stainless steel has equal size, as shown in Fig. 6. However, with the X-ray lithography with suspension electroplating technique, there is accuracy in size and shape unchanged.
The imitative SCC in block stainless steel, (a) the imitative SCC pattern of SU-8 polymer, (b) the small size of imitative SCC.
The size and shape of the imitative SCC, (a) the SU-8 pattern of SCC, (b) stainless Steel-Block with imitative crack.
The microstructures of the imitative crack specimen fabricated as shown in Fig. 7. It consisted of a Nickel matrix with SUS316 powders dispersing around the specimen. The sizes of stainless powders were range between 10–20 μm approximately. There were also some porosities formed which could be formed the incompatible deposition rate between SUS316 powders and Nickel solution.
Energy Dispersive X-ray was also performed on the specimen, as shown in Fig. 8. Both SUS316 and Ni matrix were confirmed respectively. The dark islands (spectrum 1) had compositions of Fe, Cr, Ni, and Mo, which were the main elements of SUS316. The nickel matrix had a majority of Ni in spectrum 2.
The Microstructures of the specimens.
The composition chemical analysis by using Energy Dispersive X-ray.
The imitative SCC was then placed to detect by scanning the eddy current probe across the imitative SCCs compared to the reference notch at depth (crack length) about 500 μm and width of 30–1000 μm. The results reveal indicating the existence of SCC could be detected of the measured ECT signals as shown in Fig. 9. Nevertheless, in Fig. 10, another SCC pattern was formed and the imitative crack was fabricated. ECT signal was also detected but form another side of the crack which imitates the sub-surface crack.
Eddy Current Signals, (a) SCC pattern #1, (b) Stainless Steel-Block, (c) Stainless Steel-Block with imitative crack.
Eddy Current Signals, (a) SCC pattern #2, (b) Stainless Steel-Block, (c) Stainless Steel-Block with imitative crack.
Synchrotron deep X-ray lithography with suspension electroplating could be utilized to fabricate complex shapes with high resolution and aspect ratio of the imitative SCC. Microstructure, EDS, and ECT signal of the specimen showed the possibility of creating a block of stainless steel. The imitative SCC fabricated could be detected by ECT, which gives the signal similar to the other standard specimens used for ECT measurement verification.
Footnotes
Acknowledgements
The authors would like to thank the Thailand Science Research and Innovation (TSRI), Research and Researchers for Industries (RRi), and STP Advance Products co. Ltd for funding this study, and grateful to the Beamline 6a: Deep X-ray lithography (DXL) of the Synchrotron Light Research Institute (Public Organization) for the support in the processing of the X-ray LIGA technology, and grateful to the Institute for Scientific and Technological Research and Services, King Mongkut’s University of Technology Thonburi for the support in the eddy current testing.