Understanding Carbon Fiber Preforms
Carbon fiber preforms are the foundational building blocks of all carbon-carbon (C/C) and carbon-ceramic (C/SiC) composite products, including brake discs, thermal protection systems, and structural components. A preform is essentially a shaped arrangement of carbon fibers that defines the final part’s geometry, fiber architecture, and mechanical properties.
The quality of the preform directly determines the quality of the finished composite product, making preform manufacturing one of the most critical steps in the production chain.
Types of Carbon Fiber Preforms
1. Needled Preforms (2.5D/3D)
Needled preforms are the most common type used for brake disc applications. The process involves:
- Stacking layers of carbon fiber fabric or nonwoven mat
- Using barbed needles to punch fibers from one layer through adjacent layers
- Creating z-direction fiber connections that prevent delamination
The “2.5D” designation indicates that while the primary fiber reinforcement is in the X-Y plane, the needling process introduces some Z-direction fibers, creating a quasi-three-dimensional structure. True “3D” preforms have more extensive through-thickness reinforcement.
2. Woven Preforms
Woven preforms use interlaced carbon fiber tows arranged in specific patterns (plain weave, twill weave, satin weave). These offer:
- More predictable and uniform mechanical properties
- Better control over fiber orientation
- Higher fiber volume fractions possible
3. Braided Preforms
Braided preforms are created by interlacing carbon fiber tows around a mandrel. They’re particularly suited for tubular or complex-shaped components.
The Needled Preform Manufacturing Process
For aircraft brake disc production, the needled preform process is preferred. Here’s a detailed look at the steps:
Step 1: Fiber Selection
The process begins with selecting the appropriate carbon fiber precursor. For brake applications:
- PAN-based fibers: Most common, offering excellent mechanical properties and consistent quality
- Pitch-based fibers: Used when higher thermal conductivity is required
- Oxidized PAN (OPF): Sometimes used as a cost-effective alternative, converted to carbon during subsequent heat treatment
Step 2: Web/Mat Formation
Carbon fibers are processed into a flat web or nonwoven mat through carding and cross-lapping. The fiber length, orientation distribution, and areal weight of the web are carefully controlled to achieve the desired preform properties.
Step 3: Layup
Multiple layers of carbon fiber web are stacked to achieve the target preform thickness. The number of layers depends on the final disc thickness required — typically 20-40 layers for an aircraft brake disc.
Step 4: Needling
This is the critical step that differentiates needled preforms. A needling loom with thousands of barbed needles repeatedly penetrates the stacked layers:
- Needle density: typically 20-50 punches per cm²
- Penetration depth: controlled to ensure through-thickness connection
- The barbs catch fibers and push them through adjacent layers
- This creates mechanical interlocking between layers
Step 5: Shaping
The needled preform is cut or shaped to the disc geometry — typically an annular (donut) shape with appropriate inner and outer diameters.
Step 6: Carbonization (if needed)
If OPF fibers were used, the preform undergoes carbonization at 1,000-1,500°C in an inert atmosphere to convert the oxidized fibers to carbon.
Quality Control in Preform Manufacturing
Critical parameters that are monitored include:
- Fiber volume fraction: Typically 20-30% for needled preforms
- Bulk density: Target ranges depend on the application (typically 0.4-0.6 g/cm³)
- Z-fiber content: The percentage of fibers oriented in the through-thickness direction
- Uniformity: Consistent thickness and density across the preform
- Dimensional accuracy: Meeting tight tolerances for diameter, thickness, and roundness
From Preform to Finished Product
After the preform is manufactured, it undergoes densification to become a functional C/C or C/SiC component. The main densification methods are:
- Chemical Vapor Infiltration (CVI): Hydrocarbon gas is decomposed at high temperature to deposit carbon within the preform pores. This is the primary method for aircraft brake discs.
- Liquid Silicon Infiltration (LSI): For carbon-ceramic composites, molten silicon is infiltrated into a pre-densified C/C preform, reacting with carbon to form SiC.
CMCMAT’s Preform Capabilities
CMCMAT specializes in manufacturing high-quality carbon fiber preforms with both 2.5D and 3D needled architectures. Our preforms are used in aircraft brake discs, carbon-ceramic automotive brake rotors, and various industrial applications.
We offer preforms tailored to customer specifications, with control over fiber type, architecture, density, and geometry. Contact our engineering team to discuss your preform requirements.