Fraunhofer Institute for Surface Engineering and Thin Films IST
As part of the ECO program, waveguides were developed – some with copper plating and some without.
Chromium(VI)-free metallization of CFRP – from material expertise to industrial implementation
Carbon-fiber-reinforced plastics (CFRP) play a central role, particularly in the aerospace and defense industries. Their high specific strength and stiffness, combined with low weight, make them an important material for demanding lightweight construction applications. By metallizing the CFRP surface, the mechanical advantages of CFRP can be combined with the functional properties of metals, such as electrical conductivity.
In addition to performing electrical functions, metallic coatings on CFRP also serve purposes in thermal management, corrosion and diffusion protection, and tribological and optical functionalization. They also enable the integration of additional functions directly into the structure, such as temperature and strain sensors. However, to ensure that these functions are reliably fulfilled, the metallization of CFRP must meet high standards. Key requirements include high layer homogeneity, excellent adhesion, a defined surface roughness, and reliable and reproducible coverage even on complex geometries. This applies in particular to applications in satellite and communications technology, such as waveguides, where low-loss signal transmission, low surface resistance, and minimal scattering and reflection losses are additionally required.
The challenge of chromium(VI) substitution
For decades, the required surface properties were achieved through pretreatments based on compounds containing chromium(VI). The strong oxidizing effect of chromic acid enables the targeted etching of the polymer matrix, the formation of a micro-rough surface for mechanical anchoring of the metal layer, the introduction of polar functional groups, and improved wettability. The interplay of these effects creates a reactive and easily metallizable surface with high adhesion strength. At the same time, chromic acid-based processes are characterized by high process robustness and fault tolerance.
However, due to the regulatory requirements of the REACH Regulation, the previously established pretreatment system based on chromic acid is increasingly being phased out. The substitution of chromium(VI) represents far more than just the replacement of a single process step. Rather, the entire process chain must be reevaluated—from surface activation through metal deposition to ensuring the required component and coating properties.
Surface activation as the key to reliable metallization
A key challenge lies in the reproducible activation of the CFRP surface. Due to the combination of electrically conductive carbon fibers and an insulating polymer matrix, as well as varying resin systems, fiber orientations, and curing conditions, CFRP components exhibit an intrinsically inhomogeneous surface. Chromic acid was able to compensate for these heterogeneities with relatively high reliability. This is significantly more difficult with alternative pretreatment methods. If the polymer matrix is not sufficiently structured, the mechanical anchorage required for lasting metal adhesion is lacking. Conversely, if too much material is removed, there is a risk of exposing carbon fibers, thereby adversely affecting the surface structure and subsequent coating behavior. A pretreatment that delivers optimal results for a specific material system cannot therefore be readily transferred to other CFRP materials.
Copper-plated waveguide from the ECO line—installed in the fully assembled unit.
It is therefore crucial to create a surface topography that, on the one hand, enables high bond strength and, on the other hand, supports uniform and controlled metal deposition. Inhomogeneous or non-reproducible surfaces can have a direct impact on the functional, mechanical, and electrical properties of the coating. Consequently, the development of REACH-compliant metallization solutions requires a coordinated interplay of materials science, surface engineering, coating technology, analytics, and process engineering.
An overview of the services offered by Fraunhofer IST in the field of chromium(VI)-free CFRP metallization.
From individual procedures to a coordinated process chain
This is precisely where Fraunhofer IST’s development approach comes into play: The focus is not on a single pretreatment or coating process, but rather on the optimal alignment of the entire process chain with the material, component geometry, and function.
The broad technology portfolio includes various coating processes such as electroplating, PVD coatings, atmospheric pressure plasma activation, and metal blasting. Depending on the material, component geometry, and functional requirements, these processes can be applied selectively or combined. This portfolio is complemented by many years of experience working with various CFRP systems—ranging from different resin systems and fiber materials to various fiber architectures and laminate structures. This material expertise enables application-specific process development and forms the basis for high adhesion strength, functional reliability, and long-term durability of metallized components.
From existing CFRP components to functional metallization
A key benefit for customers is that existing materials and components can be specifically adapted for functional metallization. Since the CFRP system used is typically already determined by the intended application, surface pretreatment, metallization, and process parameters are tailored to the specific properties of the respective material. The accompanying material and coating analysis provides a thorough understanding of the interactions between the CFRP surface, the coating, and component behavior. This allows the relevant influencing factors to be specifically identified and processes to be optimized in terms of adhesion, functionality, and long-term durability. At the same time, attention is paid to robust and scalable process control as early as the development phase, ensuring that successful processes can be efficiently transferred to pilot and series production. This reduces development efforts, increases process reliability, and shortens the time to industrial implementation.
The goal, therefore, is not merely to replace established chromium(VI)-based pretreatments, but to develop high-performance and cost-effective metallization solutions that are specifically tailored to the respective material and application. This results in robust and scalable processes that ensure reproducible surfaces, high and durable coating adhesion, and reliable functionality throughout the component’s entire life cycle.