MATERIAL PROPERTIES of µAM
Material Characterization of 3D Printed Microscale Metal Objects
Microscale vs Macroscale
Micro- and nanoscale objects can exhibit different behavior to their macroscale counterparts.
Material characteristics such as conductivity, tensile strength, and surface roughness are crucial data points to researchers in academic and industrial settings – one cannot assume the same values as bulk metals, and this is perhaps especially true of new approaches in µAM.
In partnership with the Swiss Federal Laboratories for Materials Science and Technology (EMPA) and Alemnis, we have been conducting analysis of our printed objects through various tests of material properties. Of these, tensile strength is a key characteristic for any parts being subjected to any mechanical forces.
Testing Tensile Strength of Microscale AM Objects
Tensile strength is the strength of a material under tension, whilst the yield strength denotes the point at which elastic behavior ends and plastic behavior begins; this provides further insight into how a material will act under tension.
Scientists at EMPA conducted tensile strength and yield tests using the ASA system from Alemnis, itself a former spin-off from EMPA specializing in micro- and nanoscale materials science testing equipment. This equipment is unique in its ability to grip and test microscale parts for testing; the jaws of the gripper in the video are less than 20 µm apart.
A microscale copper dogbone of 5 µm diameter printed with our CERES system was used as the test piece within the ASA system. This copper dogbone was subjected to slow and constant elongation with a standardized speed, as per typical tensile test protocol. As the piece was strained uniformly along its length, material behavior was plotted in real-time as set of values on a force elongation diagram. This diagram maps force (x axis) vs the elongation delta (y axis), and essentially shows the force with which the test piece opposes the force imposed upon it by the ASA system.
Yield Strength and Tensile Strength
In a standard tensile test,
the expected behavior is as follows; the force rises rapidly at first, and the initial linear curve shows the elastic behavior of the material. At the point of maximum force, a neck (narrower section) begins to form; all subsequent plastic deformation is confined to this neck, until fracture finally occurs there. As can be seen in the video, this is exactly what happened with our microscale test piece.
The yield stress was calculated to be 320 MPa and the tensile strength 342.5 MPa. These values are in line with cold drawn copper. Failure of the sample occurred within the central gauge section at a few percent of inelastic strain. When analyzing the yield stress using the Hall-Petch relationship for grain boundary strengthening, the copper grains in our µAM test piece are likely of a diameter of around 160nm.
Tensile Strength - Cyclic Strain Holding
Further tests analyzing the tensile strength with cyclic strain holding were carried out, and again our microscale 3D printed test piece exhibited the same behavior as would be expected from a conventionally manufactured macroscale piece.
Material Density and Conductivity
As part of a voxel spacing/merging test, we printed an array of copper micropillars in close proximity, such that they merged into an approximate cube (roughly 50 µm diameter). We then sliced it with a Focused Ion Beam (FIB) in order to assess the homogeneity of the grain structure.
Whilst the external surface in the left image shows the external relief of the individual pillars, the cross-section SEM on the right shows that the deposited material is nicely uniform and homogeneous inside, and there are no visible traces of the individual voxels.
This homogeneity of grain structure is essential in many use cases, such as heat-sensitive applications. Low porosity is also crucial, especially in uses such as wire bonds and defect repairs on microelectronic applications, where electrical conductivity is paramount.
Our printed microcrystalline copper has > 99% density
Resistivity of printed structures: ρcopper = 1.9E-8 Ω.m
This is around 87% of the conductivity of bulk copper
Substrate Adhesion and Mechanical Testing
We printed a microscale copper spring to test the mechanical properties of our printed objects, specifically substrate adhesion and mechanical hysteresis:
The copper is mechanically stable
The copper behaves elastically
There is a good adhesion to the substrate
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