Quality over Quantity: How Different Dispersion Qualities of Minute Amounts of Nano-Additives Affect Material Properties in Powder Bed Fusion of Polyamide 12

Abstract

The great interest, within the fields of research and industry, in enhancing the range and functionality of polymer powders for laser powder bed fusion (LB-PBF-P) increases the need for material modifications. To exploit the full potential of the additivation method of feedstock powders with nanoparticles, the influence of nanoparticles on the LB-PBF process and the material behavior must be understood. In this study, the impact of the quantity and dispersion quality of carbon nanoparticles deposited on polyamide 12 particles is investigated using tensile and cubic specimens manufactured under the same process conditions. The nano-additives are added through dry coating and colloidal deposition. The specimens are analyzed by tensile testing, differential scanning calorimetry, polarized light and electron microscopy, X-ray diffraction, infrared spectroscopy, and micro-computed tomography. The results show that minute amounts (0.005 vol%) of highly dispersed carbon nanoparticles shift the mechanical properties to higher ductility at the expense of tensile strength. Despite changes in crystallinity due to nano-additives, the crystalline phases of polyamide 12 are retained. Layer bonding and part densities strongly depend on the quantity and dispersion quality of the nanoparticles. Nanoparticle loadings for CO₂ laser-operated PBF show only minor changes in material properties, while the potential is greater at lower laser wavelengths.

Keywords: LB-PBF; additively manufactured parts; dispersion; laser powder bed fusion; mechanical properties; nanocomposites; nanoparticles; polyamide 12.

Figure 1

Flowability characteristics of PA12 powder and its composites measured by a ring shear cell at a pre-consolidation stress of 1 kPa. Due to the additional washing step before the colloidal additivation, colloidal composite powders are compared to washed PA12 powder while dry-coated composites are compared to PA12 powder as received. An increase in significance is depicted with an increase in the number of asterisks, while no significant differences are declared as “ns” ($$p>0.05$$). Results are based on three measurements.

Figure 2

Three heating curves of the LB-PBF-P specimens made of (a) pure PA12, (b) PA12 and 0.005 vol% carbon nanoparticles, (c) PA12 and 0.05 vol% silver nanoparticles, and (d) PA12 and 0.05 vol% carbon nanoparticles. The addition of the nanoparticles was performed via colloidal additivation. The heating rate was 10 K/min.

Figure 3

Averaged thermal values of different material compositions showing their results of (a) peak melting temperature values, (b) heat of fusion, and (c) crystallinity. The level of significance increases with the number of asterisks, while “ns” stands for an insignificant difference.

Figure 4

Microscopic polarized images of 10 µm sliced LB-PBF-P specimens made of (a) pure PA12, (b,c) PA12 and 0.005 vol% carbon nanoparticles, (d) PA12 and 0.05 vol% silver nanoparticles, and (e,f) PA12 and 0.05 vol% carbon nanoparticles. PA12 powders were additivated (b,d,e) with the colloidal deposition and (c,f) with the dry coating method. The images provide an overview of the processed layers in a horizontal position, where higher amounts of carbon nanoparticles lead to poorer layer bonding.

Figure 5

Magnified microscopic polarized images of 10 µm sliced LB-PBF-P specimens made of (a) pure PA12, (b,c) PA12 and 0.005 vol% carbon nanoparticles, (d) PA12 and 0.05 vol% silver nanoparticles, and (e,f) PA12 and 0.05 vol% carbon nanoparticles. PA12 powders were additivated (b,d,e) with the colloidal deposition and (c,f) with the dry coating method. The images provide a more detailed view of the developed crystalline structures and the positions of carbon and silver nanoparticles in the cooled polymer melt.

Figure 6

The overview of the tensile test results shows the LB-PBF-P specimens of different material compositions with regard to their (a) measured dimensions, (b) exemplary stress–strain curves, and (c) mechanical properties of the ultimate tensile strength ($${\sigma }_{ult}$$), ultimate elongation ($${\epsilon }_{ult}$$ ) and Young’s modulus ($$E$$ ). The level of significance increases with the number of asterisks, while “ns” stands for an insignificant difference.

Figure 7

Collection of scanning electron microscopy images of the fractured surfaces of the tensile bars and of the top surface of the specimens. The left column (a,d,g,j,m,p) depicts the edges, and the middle column (b,e,h,k,n,q) shows the center of the fractured surface. The condition of the specimens’ top surface can be seen in the right column (c,f,i,l,o,r). The quantity of nanoparticles increases from top to bottom. Exemplarily, the red arrows mark unmelted polymer particles, while the red circles highlight voids.

Figure 8

(a) Scanning electron microscopy images of the ductile area of fractured surfaces of the PA12 specimens. A more detailed view of the spherical fibrillated structures responsible for the ductility can be seen in (b).

Figure 9

Diffraction patterns of PA12 powder and specimens of different compositions additivated by colloidal deposition and dry coating, depicted (a) as overviews separated from each other and (b) on top of each other with a zoomed-in picture of the shifted main reflex positions.

Figure 10

Results of µ-CT scans for specimens of pure PA12 in (a) top and (c) side view and of PA12 dry-coated with 0.05 vol% CNP in (b) top and (d) side view.

Figure 11

(a) Relative density and (b) pore size distribution of pure PA12 and PA12 specimens with CNP additivation.

Figure 12

Sphericity measurements of the pores in specimens of (a) pure PA12, (b) PA12 dry-coated with 0.005 vol% CNP and (c) PA12 dry-coated with 0.05 vol% CNP.