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Ollekkatt Sivadas, Binsha (2025) Laser powder bed fusion of boron nitride nanotubes (BNNTs) and multi-wall carbon nanotubes (MWCNTs) functionalised Polyamide-12 nanocomposites with improved powder reusability. PhD thesis, University of Nottingham.

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Abstract

Polymer Laser Powder Bed Fusion (P-LPBF) is a widely used Additive Manufacturing (AM) technique that is known to be one of the cheapest AM methods for batch fabrication. The limited material palette is one of the drawbacks of P-LPBF. Most commercially available PA12 powders for P-LPBF have poor reusability (~50%) of unfused powder. The poor reusability makes it prohibitively expensive to develop Polymer Nanocomposite (PNC) powders, especially when using costly nanomaterials. Hence, no study has explored functionalisation with exotic nanomaterials such as hexagonal Boron Nitride Nanotubes (BNNTs) that cost ~$600 per gram. Thus, choosing a highly reusable polymer matrix is key to reducing waste, cost and carbon footprint.

BNNTs are relatively new and complement Carbon Nanotubes (CNTs) in the materials pallet. BNNTs are currently scarce and expensive as the manufacturing methods are not yet mature. However, BNNT PNCs are still viable for high-value applications such as in aerospace, where multifunctional BNNTs can deliver a cost reduction through weight reduction. Most current research on BNNTs focuses on maturing their synthesis and developing novel solutions. However, this thesis is focused on the AM of BNNT PNCs for the first time via P-LPBF.

Despite commercially available PA12 powder for P-LPBF, such as Orgasol-Invent Smooth (PA12OIS) and its variant with flow additive (PA12OISA) being ~90% reusable due to polymer chain end passivation, they have not been preferred for conversion to nanocomposites until now as they were perceived as less tolerant to variations in process parameters and materials properties. This work demonstrated functionalising PA12OISA with MWCNTs (MWCNT-PA12OISA) and BNNTs (BNNT-PA12OISA) with 0.1 wt% loading via wet mixing PNC powder preparation method and part fabrication using the nanocomposite powder via P-LPBF for the first time and achieved it without sacrificing processability.

Processing PA12OISA via P-LPBF yielded parts with average Ultimate Tensile Strength (UTS), Elastic modulus (E) and Elongation at Break (EaB) of 43.63 MPa, 1736 MPa and 19.46% respectively. Compared to PA12OISA, MWCNT-PA12OISA parts had ~5.3% lower elastic modulus, 3.3-7.0% higher UTS, with a negligible 0.69% reduction in EaB while BNNT-PA12OISA parts had ~4.37% higher elastic modulus, ~13.69% higher UTS and ~2.31% reduction in EaB.

Unfused PA12OISA, MWCNT-PA12OISA and BNNT-PA12OISA powders from builds were recycled, and parts were fabricated via P-LPBF across multiple cycles during parameter optimisation. The recycled powders still yielded curling- and warping-free functional parts, demonstrating their reusability. To systematically quantify powder reusability, recycled PA12OISA and MWCNT-PA12OISA powders from the builds were further oven-aged to simulate the thermal cycling across a few builds and then successfully processed via P-LPBF without refreshing. The Aged-PA12OISA parts (from recycled powder) had ~18% higher elastic modulus and ~8% higher UTS than PA12OISA, with only a ~5% reduction in EaB. The Aged-MWCNT-PA12OISA parts (from recycled composite powder) had ~10% higher elastic modulus and ~11% higher UTS than PA12OISA, with only a ~6% reduction in EaB. This increase in strength was unprecedented for parts fabricated via P-LPBF from any extensively recycled PA12-based powder. The practically 100% reusability of unfused powder effectively eliminates the main drawbacks to P-LPBF regarding materials wastage, cost, and carbon footprint. It drastically reduces the cost of material development, thus opening up possibilities for developing similar reusable PNC powders for P-LPBF and other AM methods.

To achieve success, this study had to challenge some of the well-established practices in P-LPBF and push some of the processing boundaries, such as setting the powder bed temperature (TPB) more than 4 oC above the melting onset temperature (TMO) without suffering from stickiness and caking, compared to the current practice of setting TPB below TMO to reduce stickiness and caking. A new metric, Effective Mass-Energy Density (EMED), was proposed in this study and quantified (with some assumptions) to compare the energy density available to raise the temperature of the melt pool beyond the melting endset and relate it to part properties. Scanning Electron Microscopy (SEM) imaging of the fracture surface revealed the influence of internal features, such as gas pores, on part properties, such as elongation at break, enhancing understanding of the subject. An unexpected simultaneous increase in porosity and tensile strength was observed in the parts, which EMED explained. A new term, vertical growth, was introduced in this work to differentiate the influence of gravity from lateral growth and was used to explain part growth observed in this study.

Though accidental, laser-induced carbon-rich features, including platelets, were synthesised in PA12 and PA12-based nanocomposites for the first time by P-LPBF. PNC parts were synthesised in-situ from unreinforced PA12 powder by synthesising laser-induced carbon-rich features, thus adding one more route for creating PNC parts via P-LPBF. The ability to modulate process parameters of standard P-LPBF machines, such as the EOS P100 used for this study, to synthesise laser-induced carbon-rich features opens up the possibility of tailoring the concentration of the nanomaterials by location, which could be used to fabricate three-dimensional electrical or thermally conductive pathways in insulating polymer parts.

The study of microtome part cross-sections enabled the identification and distinction of numerous features such as bubbles, deformed bubbles, and Lack-of-Fusion (LoF) pores via SEM. The study also demonstrated the influence of process parameters in creating gas pores or bubbles at specific locations in the part, which could theoretically be controlled to yield parts with engineered porosity analogous to bones and other cellular structures. Such lightweight and strong parts have applications in medicine and space exploration, among other fields. Thus, numerous capabilities were demonstrated for the first time via this work.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Khlobystov, Andrei Ashcroft, Ian Goodridge, Ruth
Keywords:	BNNT; MWCNT; Space flight; Polymer; Nanocomposite; SLS, Laser Sintering; Additive Manufacturing; Nanomaterials; Laser Powder Bed Fusion; LPBF; PA12; Polyamide 12; Nylon; Laser induced graphene; Platelets
Subjects:	T Technology > TS Manufactures
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID:	80085
Depositing User:	Ollekkatt Sivadas, Binsha
Date Deposited:	31 Jul 2025 04:40
Last Modified:	31 Jul 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/80085

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