Published: April 7th, 2021
Presented here is a procedure for measuring fundamental material properties through micromechanical tension testing. Described are the methods for micro-tensile specimen fabrication (allowing rapid micro-specimen fabrication from bulk material volumes by combining photolithography, chemical etching, and focused ion beam milling), indenter tip modification, and micromechanical tension testing (including an example).
This study presents a methodology for the rapid fabrication and micro-tensile testing of additively manufactured (AM) 17-4PH stainless steels by combining photolithography, wet-etching, focused ion beam (FIB) milling, and modified nanoindentation. Detailed procedures for proper sample surface preparation, photo-resist placement, etchant preparation, and FIB sequencing are described herein to allow for high throughput (rapid) specimen fabrication from bulk AM 17-4PH stainless steel volumes. Additionally, procedures for the nano-indenter tip modification to allow tensile testing are presented and a representative micro specimen is fabricated and tested to failure in tension. Tensile-grip-to-specimen alignment and sample engagement were the main challenges of the micro-tensile testing; however, by reducing the indenter tip dimensions, alignment and engagement between the tensile grip and specimen were improved. Results from the representative micro-scale in situ SEM tensile test indicate a single slip plane specimen fracture (typical of a ductile single crystal failure), differing from macro-scale AM 17-4PH post-yield tensile behavior.
Mechanical material testing at the micro- and nano-scales can provide important information on fundamental material behavior through identifying length-scale dependencies caused by void or inclusion effects in bulk material volumes. Additionally, micro- and nano-mechanical testing allows for structural component measurements in small-scale structures (such as those in micro electromechanical systems (MEMS))1,2,3,4,5. Nanoindentation and micro compression are currently the most common micro- and nano-mechanical material testing approaches; however, the resulting compression and modulus measurements are often insufficient to characterize material failure mechanisms present in larger bulk material volumes. To identify differences between bulk and micro-mechanical material behavior, particularly for materials having many inclusions and void defects such as those created during additive manufacturing (AM) processes, efficient methods for micro-tension testing are needed.
Although several micromechanical tension testing studies exist for electronic and single-crystalline materials3,6, specimen fabrication and tension testing procedures for additively manufactured (AM) steel materials are lacking. Material length-scale dependencies documented in2,3,4,5,6 suggest material hardening effects in single-crystalline materials at sub-micron length-scales. As an example, observations from micro-mechanical tension testing of single-crystal copper highlight material hardening due to dislocation starvation and truncation of spiral dislocation sources4,5,7. Reichardt et al.8 identifies irradiation hardening effects at the micro scale, observable through micro-mechanical tension testing.
Micro-tensile material measurements requiring attachment of the indenter probe to the specimen are more complex than corresponding micro-compression tests but provide material fracture behavior applicable for bulk material volume predictions under more complex loading (axial tension, bending, etc.). Fabrication of micro-tensile specimens often relies heavily on Focused Ion Beam (FIB) milling from the bulk material volumes. Because FIB milling processes involve highly localized material removal (at the micro and nano-scales), large area removal through FIB milling often results in lengthy micro-specimen fabrication times. The work presented here explores a methodology to improve efficiency in micro-tensile specimen fabrication for AM 17-4PH stainless steels by combining photolithographic processes, chemical etching, and FIB milling. Additionally, procedures for the micro-mechanical tension testing of fabricated AM steel specimens are presented and testing results are discussed.
1. Sample preparation for photolithography
4. Focused Ion Beam milling of specimen geometry
5. Grip fabrication
6. Micro-tensile test
A material sample from an AM 17-4 PH stainless steel specimen (previously tested in low-cycle fatigue) was prepared and tested using the protocol described, to understand the fundamental material behavior of AM metals (independent of structural defect influence). Typical sample volumes used for material characterization can contain distributed fabrication/structural defects that make discerning between actual material behavior and structural fabrication effects difficult. Following the protocol described in sections 2 through 6 a micro specimen was fabricated and tested to failure in tension, successfully demonstrating the described techniques and producing material test data at scales free from volumetric defect influences. Prior to micro-mechanical testing, X-ray diffraction (XRD) spectra from the prepared steel surface (see Figure 13), show a mostly martensitic grain structure as would be expected from a previously strained material10.
Figure 14 shows the resulting load-displacement behavior of the micro-tensile AM 17-4PH steel sample, having a maximum tensile strength of 3,145 µN at a displacement of 418 nm. From in situ SEM observations during loading, fracture of the micro-specimen occurred along a single slip plane (typical of a ductile single crystal failure) and different from typical post-yield strain hardening behavior observed during macro-scale material tension testing of AM 17-4PH stainless steels. Frames 4-6 of Figure 14 show the single failure slip plane during tension testing of the fabricated micro specimen.
Figure 1: Bulk material where the sample was taken from. The material sample for micro-mechanical testing (~6 mm in thickness) was cut from the gage section of an AM 17-4 PH fatigue specimen. Please click here to view a larger version of this figure.
Figure 2: Material section having an array of squares (70 µm x 70 µm) patterned using photolithography. The 70 µm x 70 µm photoresist array allows for selective etching of the steel surface for bulk surface material removal. Please click here to view a larger version of this figure.
Figure 3: SEM images of the AM 17-4PH steel surface following etching. Surface high-relief locations created by the protective photoresist pattern following etching allow micro-specimen fabrication above the specimen surface elevation. Please click here to view a larger version of this figure.
Figure 4: Sample holder set-up that helps the direct contact of the sample once the micro-tensile specimen is fabricated. The etched AM 17-4 PH sample is placed on the nanoindentation device stub before being mounted to a 45-degree SEM stub (using carbon tape) to reducing handling of the specimen after micro-specimen fabrication. Please click here to view a larger version of this figure.
Figure 5: Illustration of first FIB milling step with area to be removed by FIB (left), and remaining material (right). The surface high-relief material remaining after etching is removed using FIB milling, leaving a rectangular volume of material. Please click here to view a larger version of this figure.
Figure 6: Illustration of second FIB milling step. The rectangular volume of material is further reduced using FIB milling, approaching the desired specimen outer dimension tolerances. Please click here to view a larger version of this figure.
Figure 7: Illustration of third FIB milling step. The remaining material volume is refined using FIB milling to the desired specimen outer dimension tolerances. Please click here to view a larger version of this figure.
Figure 8: SEM image of a micro-tensile sample. Using FIB milling, the profile of the remaining material volume is reduced to create the final micro-tensile specimen geometry. Please click here to view a larger version of this figure.
Figure 9: Micro-tensile specimen dimensions. Between the specimen grip areas, a reduced cross-sectional dimension measuring 1 μm by 1 μm is located within a 4μm gauge length. Please click here to view a larger version of this figure.
Figure 10: Alignment marks performed in the tip for reference. A semi-circular edge hole and circumferential scribe mark provide two sources of indenter tip alignment prior to fabrication of the tensile grip. Please click here to view a larger version of this figure.
Figure 11: Sequential tensile grip fabrication steps. (A) Formation of tensile grip outer profile using FIB milling. (B) Reduction in tensile grip thickness following 90° rotation. (C) Formation of tensile grip inner profile from original orientation. Please click here to view a larger version of this figure.
Figure 12: Grip and sample aligned to perform the tensile test. The fabricated tensile grip is positioned around the micro-tensile specimen such that an upward movement of the tensile grip will engage with the specimen. Please click here to view a larger version of this figure.
Figure 13: XRD spectra of tested sample. Shown is the relationship between X-ray scatter intensity and sample angle. Please click here to view a larger version of this figure.
Figure 14: Tensile load-displacement curve of AM 17-4 PH Steel. (Top) Frame-by-frame progression of applied specimen displacement. (Bottom) Resulting sample behavior comparing measured load (in μN of force) and applied displacement (in nm), indicating a material ultimate strength of 3,145 μN at an applied displacement of 418 nm. Please click here to view a larger version of this figure.
|From 0 to 500 rpm at 100 rpm/s
|From 500 rpm to 3,000 rpm at 500 rpm/s
Table 1: Parameters used for the spin-coating. Process steps are to be performed consecutively.
Table 2: Chemical composition of the etchant used for AM 17-4PH Stainless Steel9. All solution chemical quantities are listed as percentage by weight.
A verified methodology for AM 17-4PH stainless steel micro-specimen fabrication and tension testing were presented, including a detailed protocol for fabrication of a micro-tensile grip. Specimen fabrication protocols described result in improved fabrication efficiency by combining photolithography, wet-etching, and FIB milling procedures. Material etching prior to FIB milling helped to remove bulk material and reduce material re-deposition that often occurs during FIB use. The described photolithography and etching procedures allowed for fabrication of the micro-tensile specimens above the surrounding material surface, providing clear access for the tensile grip prior to testing. While this protocol was described and performed for micro-tensile testing, the same procedures would be helpful for micro-compression testing.
During the development of this process, variation within the photo-resist mask patterning was noticed, as shown in Figure 2. This is likely caused by surface inconsistencies created during dicing or poor adhesion of the photoresist to the sample surface. It was noticed that when wet etching was performed at room temperature, much of the photoresist was removed, due to under etching or poor adhesion; therefore, it is recommended to warm the sample before and during the etching process, as mentioned in the protocol. If significant under-etching (etching below the photoresist) is noticed, increasing the sample temperature may help. The provided protocol uses an SU-8 photoresist due to availability; however, other photoresist and etchant combinations may also be effective.
Tensile-grip-to-specimen alignment and sample engagement were the main challenges of micro-tensile testing. By reducing the indenter tip dimensions as described in the protocol, alignment and engagement between the tensile grip and specimen was improved. Due to SEM view perspective limitations, it was often difficult to tell whether the sample was within the tensile grip. Reducing the grip thickness will likely provide better perspective control.
Micro-specimen preparation and micro-tensile material testing is often a lengthy process, requiring several hours of FIB fabrication time and indenter alignment. The methods and protocols prepared herein serve as a verified guide for efficient micro-tensile fabrication and testing. Note that the micro specimen protocol allows for high throughput (rapid) specimen fabrication from bulk AM 17-4PH stainless steel volumes by combining photolithography, chemical etching, and focused ion beam milling.
The authors declare that they have no competing financial interests.
This material is based upon work supported by the National Science Foundation under Grant No. 1751699. In-kind support of AM material specimens provided by the National Institute of Standards and Technology (NIST) is also acknowledged and appreciated.
|45 ° SEM stub
|Branson 1510 Ultrasonic Cleaner
|Carbon conductive tabs
|PELCO image tabs
|FEI Nova Nanolab 200 Dual-Beam Workstation
|Hydrochloric Acid (12.1M)
|CAS: 7647-01-0, HX0603
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|SimpliMet 4000 Mounting Press
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|Kayaku Advanced Materials (MicroChem)
|Tescan VEGA 3 SEM
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