The current research aims to improve the mechanical properties of the lunar basalt fibre in an automated manufacturing process on the Moon, compared to the fibres developed so far in earlier studies [1,2,3,4,5,6,7,8,9], by identifying the best available simulant for fibre extrusion, optimizing the fibre manufacturing parameters, decreasing the final fibre diameter and post-processing the fibres. In addition, the fibres will be mechanically, structurally and thermally characterized for more comprehensive overview of the final fibre properties. The manufacturing of fibres will also be investigated in vacuum and compared to the ones extruded in ambient conditions, in order to understand the effects of space environment on fibre extrusion. In the later stages of the study the application of the developed fibres will be investigated in manufacturing of lunar basalt fibre composites. For that purpose, a suitable resin needs to be identified based on its compatibility with the fibre, ease of in-situ manufacturing and required final properties of the composite.
Basalt fibre, very similar to fibreglass, is made of volcanic rock, mainly found in the lunar maria. It is composed of the minerals plagioclase, pyroxene, and olivine. The main components of basalt are the metal oxides SiO₂, Al₂O₃, CaO, MgO, Fe₂O₃, and FeO. Possible other components in smaller amounts are K₂O, Na₂O, and TiO₂. Basalt is categorized, based on its main component SiO₂, into alkaline (up to 42% SiO₂), mildly acidic (43 to 46 % SiO₂) and acidic basalts (over 46% SiO₂), whereas only acidic basalts are suitable for continuous fibre production. The main difference compared to other metal oxide fibres, such as glass fibres or ceramic fibres, is the content of iron oxides in the basalt fibres. This gives the basalt fibres the dark coloration in contrast to the white and transparent glass and ceramic fibres.
Fibre Production Process
The production of fibre based materials begins with the manufacturing of fibres by using different mechanical and chemical processes to draw fibres from viscous melt by melt spinning or from solution by gel, wet or dry spinning techniques. While the fibres are extruded through the spinneret, the collector on the opposite side helps to draw the fibres into continuous textile filament yarns. The filaments exiting the spinneret do not yet have the properties of a textile yarn due to disorderly placed molecular chains. The drawing process, however, is necessary to reduce the diameter of the filament, orientate the molecular chains along the axis of the filament and increase the crystallization phase by elongating the fibre, which enhance the mechanical properties of the yarn. The continuous filament yarns can be used either as a single filament or as a bundle of multiple filaments, called roving, in a textile or composite assembly process. The textile fibre based materials have a number of advantages over materials produced by extrusion and sintering methods:
- The fibres can be individually oriented and placed in a structure for creating local variations in material properties
- The use of continuous filament ensures a higher-fidelity manufacturing process due to better control over the placement of the material when compared to powders and liquids
- Fibre based materials are suitable for both compression and tension structures, which extends the number of possible application areas
- Fibrous materials are highly formable which allows production of complex shapes in response to unique performance criteria or site conditions
- Fibres enable the production of light-weight and highly optimized structures
- Fibrous materials might offer a better performance in response to thermal stresses
Fibre Production on the Moon
On Earth the basalt fibre is produced using only one component, the grinded and melted basalt rock  For the basalt to be spinnable it needs to contain at least 46% or more silica. Only then the stone can be melted completely without residues, a suitable viscosity can be reached for fiber formation, and homogenous amorphous phase without crystalline areas can be gained after cooling down. On Earth the basalt fibre manufacturing can be divided into five main steps: preparation of raw materials, melting of the feedstock stone, homogenization of the melt, the spinning of the fibre, and finally drawing the fibre to a specific diameter . On the Moon the fibre production would be similar to the production on Earth. However, the lunar fibre extrusion process would require full control and automation to eliminate the need for the involvement of astronauts. The goal would be to install a continuous basalt fibre production line which would allow spinning of monofilaments and assembling these into rovings, possible addition of sizing to improve the mechanical properties of the filaments, and winding the filaments into spools. The spools can then be used in robotic manufacturing of habitats and other fibre based structures and components. The sequence of the manufacturing process could look like as follows:
- Collection of regolith
- Formation of regolith melt in furnace
- Homogenization of the melt
- Melt drawing through a bushing with multiple nozzles
- Filament cooling
- Possible application of sizing
- Filament winding into bobbins
Basalt fibre is a good candidate for use in lunar applications due to following reasons :
- Basalt-based materials are non-hazardous
- Simpler manufacturing process than that of a glass fibre due to less complex composition, which can be produced in a single feed line because there is no need for secondary materials
- High strength and high modulus with excellent shock resistance
- Similar mechanical properties than glass fibres
- High chemical durability against the impact of water, salts, alkalis and acids
- High service temperature and fire resistance
- Can be post-processed to change thermal and mechanical properties, e.g. via doping, plasma treatment or using a sol-gel technology for the application of metal oxide coatings
The fibres produced in lunar environment, in fact, may have better mechanical properties than the fibres produced on Earth. A number of previous studies have suggested that the fibres may reach higher tensile strength properties when produced in lunar environmental conditions, such as high vacuum and low-gravity. The first reason for this is the anhydrous nature of the materials on the Moon and because the hydrolytic weakening process is inhibited in vacuum . The other reason mentioned is the smaller amount of ferric oxide (Fe₂O₃) in lunar soil which is considered as contaminant to high-strength glass products . It is therefore even suggested that lunar glass products could be competitive with, or even superior to, metals extracted from regolith, with considerably less processing effort.
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