Testing Tensile Strength of Microscale AM Objects

Nanoscale and microscale objects can show different material characteristics from macroscale objects, but until now, little testing has been conducted.

In academic and industrial environments, material properties like surface roughness, conductivity, and tensile strength are critical data points for researchers—it is not viable to assume the same values as bulk metals, and this might be true specifically in the case of new techniques in additive micromanufacturing (µAM).

Exaddon has collaborated with the Swiss Federal Laboratories for Materials Science and Technology (EMPA) and Alemnis to analyze its printed objects by performing a range of tests on material properties. Among these properties, tensile strength is a characteristic crucial for any part that experiences any mechanical forces.

Testing Tensile Strength of Microscale AM Objects

Tensile strength denotes a material’s strength under tension, and the yield strength is the point at which the elastic behavior of a material ends and its plastic behavior starts. This offers additional understanding about how a material will behave under tension.

Researchers from EMPA used the ASA system from Alemnis to perform tensile strength and yield tests. Alemnis is itself a former spin-off from EMPA and specializes in micro- and nanoscale materials science testing equipment. This equipment is remarkable in its ability to grip and test microscale parts for testing - the jaws of the gripper depicted in the video below are located less than 20 µm from each other.

Tensile Test of Exaddon CERES 3D Printed Microscale Metal Objects - 30 nms

Video Credit: Exaddon AG

Within the ASA system, a microscale copper dogbone with a diameter of 5 µm and printed with Exaddon’s CERES system was used as the test piece. This copper dogbone underwent slow and constant elongation at a standardized speed, in compliance with standard tensile test protocol.

The test piece was strained evenly along its length, and material behavior was plotted as a set of values on a force elongation diagram, in real time. The force elongation diagram maps force (x axis) against the elongation delta (y axis), typically illustrating the force with which the test piece resists the force that the ASA system imposes on it.

Yield Strength and Tensile Strength

The typical behavior expected in a tensile test is as follows: initially, the force increases quickly, and the initial linear curve on the graph denotes the material’s elastic behavior. When the point of maximum force (that the test piece can withstand) is reached, a neck (narrower section) starts to form. All successive plastic deformation is restricted to this neck, until fracture eventually takes place at this area. 

From the video below, it can be observed that this is precisely what happened with the microscale, additively manufactured test piece.

Tensile Test of Exaddon CERES 3D Printed Microscale Metal Objects - 100 nms

Video Credit: Exaddon AG

The tensile strength was calculated to be 342.5 MPa and the yield stress was found to be 320 MPa. These values are in line with cold-drawn copper. Sample failure occurred within the central gauge section at a few percent of inelastic strain.

When the yield stress is analyzed using the Hall-Petch relationship for grain boundary strengthening, the copper grains in the µAM test piece have an estimated diameter of about 160 nm.

 

Tensile Strength—Cyclic Strain Holding

Further tests were performed for investigating the tensile strength with cyclic strain holding, and the microscale 3D-printed test piece again showed the same behavior as would be anticipated from a traditionally manufactured macroscale piece.

Tensile Test with Cyclic Strain Holding of Exaddon CERES 3D Printed Microscale Metal Object

Note that the test piece reacts exactly according to the expected behavior of a macroscale or “standard” test piece. Video Credit: Exaddon AG

Material Density and Conductivity

In other tests conducted by Exaddon, material density was analyzed. In the context of a voxel spacing/merging test, an array of copper micropillars was printed in such close proximity that they merged into an approximate cube, with an overall diameter of about 50 µm. Then, a Focused Ion Beam (FIB) was used to slice it to evaluate the homogeneity of the grain structure.

The external surface shown in the left image of Figure 1 illustrates the external relief of the individual pillars, whereas the cross-section SEM depicted on the right illustrates that the deposited material is highly even and homogeneous from within, and no visible traces of the individual voxels exist.

FIB of a solid cube shows that printed material shows homogeneous, uniform structure.

Figure 1. FIB of a solid cube shows that printed material shows homogeneous, uniform structure. Image Credit: Exaddon AG

Such a grain structure homogeneity is crucial in several cases of use, for example, heat-sensitive applications. Low porosity is also essential in applications where electrical conductivity is crucial, such as defect repairs and wire bonds on microelectronic applications.

These porosity and conductivity tests found the following:

  • Exaddon’s printed microcrystalline copper exhibits a density of >99%
  • Printed structures exhibit a conductivity close to that of bulk copper

This information has been sourced, reviewed and adapted from materials provided by Exaddon AG.

For more information on this source, please visit Exaddon AG.

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