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Cold atmospheric plasma (CAP) treatment of metals for adhesive bonding

This project, part of TWI’s Core Research Programme, aims to reduce the dependence on wet chemical processes as a pre-treatment to adhesive bonding of metals by replacing the etching chemicals with cold atmospheric plasma (Figure 1). This process successfully increased the lap shear strength of 316 stainless steel from 33 MPa (untreated) and 44 MPa (grit blasted)  to 51 MPa (grit blasted and CAP treated), while a grit blasted and acid etched joint failed at 46 MPa (Figure 2). This was achieved while maintaining the joint’s mechanical properties and limiting the use of wet chemicals to solvent degreasing only

Overcoming inefficiencies

When metals are bonded using current methods, they are pre-treated using wet chemicals and energy-intensive processes. There are significant disadvantages to these processes, such as high energy demands, hazardous waste generation, and associated health and safety concerns.

A further issue is that each metal generally requires a different etch solution. This can limit design choices or increase the cost of manufacture considerably, requiring several chemical baths. CAP has the ability to treat different materials, with only a modest change in plasma chemistry required between each. The plasma has also the potential to be monitored to ensure that it meets specification during the treatment.

Objectives

The project’s first objective is to use CAP to produce a joint which maintains parity with the equivalent wet chemical treatment over the following tests:

  • lap shear
  • Boeing wedge test
  • aged lap shear
  • failure surface analysis.

It also seeks to understand the treatment in terms of chemistry and mechanisms using techniques such as the following:

  • X-ray photoelectron spectroscopy
  • scanning electron microscopy
  • optical emission spectroscopy.

Finally, the project aims to investigate energy usage across the lifecycle of the process to enable an accurate comparison to equivalent processes.

Figure 1 Helium:nitrogen plasma plume
Figure 1 Helium:nitrogen plasma plume

Early results show promise

Initial testing performed using 316 stainless steel as a substrate increased the bond strength while maintaining durability. Initial analysis of titanium-6Al-4V is underway, with early results suggesting CAP has produced a bond strength 93% as strong as chemical etching; this data will be refined during the remainder of the project, which is due to end in September 2017. Chemical analysis of the plasma plume and how it affects the surface is also ongoing.

Benefits to industry

Ultimately, this project will improve the efficiency of the CAP treatment and optimise its parameters, establishing the plasma composition which produces an adhesion interface equal to or better than current wet processes.

For more information, please email contactus@twi.co.uk.

Figure 2 Lap shear results for 316 stainless steel
Figure 2 Lap shear results for 316 stainless steel
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