WOVEN CARBON FIBRES
When we talk about woven carbon fibres, we generally refer to a composite material reinforced with carbon fibres. The carbon fibre strengthens a matrix of epoxy, polyester, or phenolic resin to form a composite.
History
The production of carbon fibres from polymer monofilaments (the PAN method) as used in watchmaking was patented in 1963. The industrialisation of carbon parts began in 1970. Due to high production costs, their use was initially restricted to the aerospace and aeronautics industries. As production costs fell, carbon fibres became more widespread in the 1990s. By the end of that decade, carbon fibres were present in all decorative components of a watch. Then, ten years later, the first movement elements (mainplates and bridges) made from this material were introduced.
Synthesis of Fibres
The synthesis of carbon fibres used to reinforce composite materials can be achieved using polymer monofilaments (PAN), hydrocarbons (petroleum, coal), or cellulose. In watchmaking, the PAN method is generally used. Various chemical and thermal steps (pyrolysis) eliminate all atoms except carbon (carbonisation at around 1500°C), and carbon atoms may be added (graphitisation) to obtain filaments with a carbon content exceeding 90%.
Weaving the Fabric
Filaments with diameters ranging from 5 to 10 micrometres are then woven to form a fabric. By orienting the carbon fibres within the woven fabric, can optimised the strength of the piece along specific axes (longitudinal, lateral, or torsional).
Application of the Matrix and Curing in an Autoclave Oven
Most objects made from carbon fibres are directly formed on a pre-fabricated mould. The carbon fibre fabric is cut to the mould’s dimensions according to a chosen orientation of the weave to optimise the piece’s resistance along precise axes. The fabric is carefully applied to the surface of the mould and coated with resin. Several layers can be superimposed. The composite is cured in an autoclave oven at temperatures of up to 250°C and pressures up to 1.5 MPa (15 bar). For watch components, direct moulding does not achieve the precision required by the industry. Instead, blocks are moulded in the autoclave (using the same process described earlier), which are then machined to achieve the necessary precision and tolerances in watchmaking. Other reinforcing elements, such as aramid fibres, are often used in carbon fibre composites to further enhance their properties.
Machining
Machining carbon fibre blocks involves the usual operations: turning, milling, and drilling. The tool materials for such operations include ceramic, cemented carbide, polycrystalline diamond, and cubic boron nitride. Due to its low thermal conductivity, carbon retains heat and does not dissipate it easily to the tool. However, excessive heat can alter the matrix resin and compromise the material’s properties. Therefore, carbon fibre machining requires extended machining times, especially since it typically does not tolerate coolant. Once the component is created, various finishing operations are possible, including trimming, polishing, and applying a matt or glossy varnish.
Properties of Carbon Fibres
Carbon fibres in composite form are lightweight, have excellent tensile and compressive strength, and are flexible. They have high resistance to crack propagation (toughness) and excellent resilience to abrasion and corrosion. Additionally, carbon fibres have a very low coefficient of thermal expansion. The elasticity modulus of carbon fibres varies depending on the fibre weave and the nature of the matrix, but it remains consistently high. Consequently, carbon fibre components resist deformation well but can break beyond a certain strain.
Watchmaking Applications
Carbon fibres are prized in the manufacture of all decorative components (cases, straps, dials). In some rare instances, they may be used to produce mainplates and bridges, although forged carbon is often preferred in such cases.