Fraunhofer IST expands process chain for high-performance boride coatings

For industrial project partners and clients, all relevant process steps can be developed and tested in an integrated, reproducible, and application-oriented manner - from thermochemical diffusion treatments and modern coating technologies to high-temperature tests under realistic conditions. 

According to the institute, the new gas boriding plant at Fraunhofer IST uses diffusion treatments at temperatures of up to 1,050 °C to create boride boundary zones with the reported technological properties described below.

According to the institute, the centerpiece is a completely refurbished PECVD system, which is scheduled to be put back into operation by mid-year with modernized equipment. In addition to a modern control system, both visualization and data acquisition systems have also been updated. 

According to the institute, the integrated and now expanded precursor system enables the development of new multi-component coatings, such as nanocomposite layers based on titanium, silicon, boron, carbon, and nitrogen (Ti-Si-B-C-N). The institute reports that these coatings exhibit high hardness, thermal stability, and oxidation resistance up to approximately 900 °C. The coating temperatures of around 520 to 540 °C are reported to allow the uniform coating of complex-shaped components for demanding industrial geometries. 

To further improve mechanical stability, coating processes can be combined with upstream treatment steps, e.g., in so-called duplex processes. In these processes, a diffusion zone created beforehand in the edge area is reported to ensure a stable transition between the substrate and the coating.

In addition, a completely newly developed gas boriding system is available at the site. According to the institute, it enables the development and optimization of boron diffusion processes from the gas phase using boron trichloride (BCl₃) at temperatures up to 1050 °C. Compared to solid- or paste-based methods, the institute reports that gas-phase boriding offers the following advantages: boron activity and layer growth can be controlled via gas composition through defined addition of hydrogen and nitrogen, which is reported to allow accurate process control and increase reproducibility. The institute states that the process does not require plasma excitation, which simplifies the equipment design and reduces investment costs. The resulting diffusion layers are reported to exhibit good material adhesion and tribological properties at high temperatures, which the institute indicates can reduce wear and extend maintenance intervals or tool replacement cycles in many technical applications. 

According to the institute, following the boriding process, the mechanical properties of steel materials are adjusted through hardening and tempering, and further increases in hardness are achieved through phase transformations. For nickel-based materials, heat treatment steps for precipitation hardening can also be integrated directly into the treatment process; post-hardening is not required. The institute states that other material classes, such as CoCrMo or Mo alloys, can also be gas borided.

For a high-gloss polish or to adjust the surface topography, the process chain can also be supplemented with dry electropolishing. According to the institute, the method is suitable both for polishing complex geometries prior to subsequent surface treatment and for finishing the surface of applied coatings. This is reported to enable the reproducible achievement of surface quality levels up to Rz ≤ 0.5 µm depending on the material and initial condition. A dry electrolyte granulate with a low acid content is used, which the institute distinguishes from conventional wet chemical methods through its automatability, surface quality, low material removal, and more environmentally friendly process control.

An additional component of the expanded process chain is a new high-temperature vacuum furnace. It enables testing at temperatures up to 1600 °C and pressures below 10⁻⁵ mbar, as well as in defined atmospheres, such as hydrogen or inert gas mixtures. In addition, a high-temperature tribometer is available, which can be used to investigate the tribological behavior of materials, coatings, and diffusion layers to determine friction coefficients and wear patterns at temperatures up to 1000 °C. According to the institute, this allows customers and project partners to evaluate the performance of materials, coatings, and diffusion layers under realistic operating conditions before components enter production.

According to the institute, the recently available high-temperature furnace enables investigations in the temperature range of up to 1,600 °C.

According to the institute, the improved surface properties can contribute to efficiency gains in numerous industrial applications. For example, forging tools in hot forming can be used significantly longer, or in copper extrusion processes, material adhesion and scrap rates can be reduced. The institute notes further applications, for instance, in the manufacture of high-quality aluminum components, in the bending of stainless steel, or in the production of precision fittings and valves.

The institute states that it is opening up new fields of research and development with its new infrastructure. New gaseous precursor materials are reported to enable the development of further nanocomposite coatings in carbon-free quaternary systems such as Ti-Si-B-N. In addition, the institute is expanding its expertise in electrochemical characterization to enable the early evaluation and targeted optimization of the corrosion behavior of surfaces. 

Image Credits: Fraunhofer IST