Micromechanical Tension Testing of Additively Manufactured 17-4 PH Stainless Steel Specimens

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).

Transcript

The presented protocol helps to reduce the required time for fabrication and testing of microscale material specimens and provides clear guidance for the micro mechanical testing of metal materials, which is applicable to many engineering fields. This technique allows high throughput application and testing of microscale material groupings. Additionally, by creating micro current via photolithography, material reposition is reduced and micro tensile grip maneuverability around the specimen is improved.

Although this method was applied to steel similar methodology could be used for other materials, such as silicon to improve the designs of micro electromechanical systems or MEMs. Wet etching and grip sample alignment are challenging steps. It is recommend to perform the wet etching with the sample warm.

For improved alignment, use the microscope working distant or focus to verify that the sample is engaged. Begin by cutting a section of six millimeters from the area of interest with a slow dicing saw or band saw. Using a semiautomatic polisher.

Start polishing the sample using a 400 grit abrasive paper by maintaining a flat surface. Then progressively move to one micrometer diamond particles alternating the polishing direction by 90 degrees, following each grit level. Then use a slow speed saw to align the material and cut it into a thin 0.5 to one millimeter section.

Place the sample on the spin coder with the polished side up and use compressed air to remove any dust or particle on the surface, apply photo resist to the sample and run the spin coder. After coding heat the sample on a hot plate at 65 Celsius for five minutes. And then at 95 degrees Celsius for 10 minutes.

When the sample has cooled to room temperature, use a photo mask with an array of squares measuring 70 micrometers on each side. To expose the sample for 10 to 15 seconds, At a power density of 75 milli Joles centimeter squared. Heat the sample on the hot plate as previously demonstrated and cool it to room temperature.

Then submerge the sample in propylene glyco monomethyl ether, acetate or PG MEA with the pattern facing up and agitate it for 10 minutes. Inside a fume hood, heat the sample in a beaker on a hot plate at 65 to 70 degrees Celsius for five minutes. Add a few drops of the prepared etchent to completely cover the pattern surface.

After five minutes, remove the sample from the beaker and neutralize the etchent with water. Following the wet etching, perform initial milling on the sample using maximum power to remove any undesired bulk material from the platform, then switch to lower power to make a rectangle with slightly larger dimensions than needed for the final specimen geometry. Further reduce the power and make cross section cuts that are closer to the final micro tensile specimen dimensions.

Rotate the sample 180 degrees and perform the final milling using low power to create the desired specimen geometry. Mount the specimen and indenture tip on the nano indenture device. Install the nano indentation machine in the SEM following the manufacturer's recommendations avoiding significant machine tilt.

Perform the desired displacement based tensile loading protocol in air, away from the sample to prevent an unexpected event during the tensile testing. Then slowly move the indenture tip to the sample surface. Move and align the tensile grip with the test sample and perform the tensile test.

In the representative analysis x-ray to fraction spectrum from the prepared steel surface showed a mostly martincic grain structure as would be expected from a previously strained material. The load displacement behavior analysis of the micro tensile AM 17 four pH steel sample, had a maximum tensile strength of 3, 145 micro Newtons at a displacement of 418 nanometers. Further increase in the displacement showed the single failure slip plane during tension testing of the fabricated micro specimen, which corresponds to fracture of the micro specimen from in-situ SEM observations.

In situ micro mechanical material testing allows visual observations of sample deformation during loading and helps an understanding complex material behavior and subsequent measured material performance. The most important factor to remember while attempting this procedure is to prewarm the sample before the wet etching. Perform and aim attention to verify the loading protocol and ensure proper sample engagement with the grip pre to the stencil test.

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Journal (Engineering)

Journal (Engineering)

Micromechanical Tension Testing of Additively Manufactured 17-4 PH Stainless Steel Specimens

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