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Surface mapping and control during atmospheric plasma treatments

Final Report Summary - PLASMACLEAN (Surface mapping and control during atmospheric plasma treatments)

Executive Summary:
This report provides an overview of the PlasmaClean project which investigated the use of air atmospheric plasma treatments for the removal of organic contaminants on both titanium and composite substrates. Process monitoring was evaluated using reflectance infra-red spectroscopy combined with optical emission spectroscopy (OES) and acoustic techniques. Based on this study the air plasma treatment combined with OES has been shown to exhibit considerable potential for controlled contaminant removal during the manufacture of aerospace composites. After the Introduction section this report summarises the results of the main project tasks as follows:.

- Evaluation results on the use of nebulizers for the controlled contamination of composite surfaces
- Evaluation reflectance IR and / or mass spectrometry for the
chemical examination of contaminants
- Evaluation of the effectiveness of the atmospheric plasma jet system for
contaminant removal based on chemical analysis and paint adhesion study
- Surface mapping and control during atmospheric plasma treatments

Project Context and Objectives:
The objective of this project is to develop a novel non contact diagnostic technique for the determination of the chemical species present on the surface of metallic materials and composites. The use of atmospheric plasma techniques for the removal of contaminants will be investigated. The performance of the novel non contact diagnostic technique will be benchmarked based on conventional surface chemical analysis techniques (i.e. XPS), in addition to paint / sealant adhesion testing. It is planned to use LabVIEW software to facilitate real-time process control based on the output data provided by the non contact surface analysis technique.


Specific Objectives
• Evaluate the use of pneumatic nebulization technique for the controlled reproducible delivery of contaminants onto the surface of composites.
• Evaluate the use of reflectance FTIR spectroscopy and / or mass spectrometry for the detection of organic contaminants on contaminated composite surfaces. Selection of the most appropriate technique for purchase / detailed study on the correlation between surface contaminant level and its removal.
• Demonstrate the removal of contaminant on composite after atmospheric plasma treatments. FTIR and XPS will be used to demonstrate the removal of contaminants. Paint spraying of composites followed by adhesion testing will demonstrate if the resultant surfaces meet the requirements for adhesive bonding.
• Evaluate if the electro-acoustic technique can be used for routine process control once FTIR reflectance / mass spectrometry have determined the conditions for contaminant removal.


Project Results:
The research focus is on the removal of organic contaminants from composite surfaces. As part of their review paper on bond strength assessment Crane & Dillingham (2008) outlined how a significant cause of failure in adhesive bonding is due to surface contamination. In their review which investigated automated contamination detection systems it was concluded that no suitable detection technique was available. Smith & Lindeburg also assessed techniques such as ellipsometry, surface potential difference, photoelectron emission and water contact wettability. These tools were also found to be insensitive for this type of analysis. Park et al. review the non-destructive inspection techniques available for aluminium substrates for the period 1995 to 2008 and concluded that no significant advances had been made in the intervening years. The Center of Excellence for Advanced Materials in Transport Aircraft Structures (AMTAS) undertook a project to Identification and Validation of Analytical Chemistry Methods for Detecting Composite Surface Contamination and Moisture. This work is focused on the development of a method using Surface Potential Difference along with Atomic Force Microscopy, both of which are contact methods of assessment.

A wide range of non contact techniques are potentially available to determine surface chemistry particularly X-ray Photoelectron Spectroscopy (XPS), Time of Flight and Static Secondary Ion Mass spectrometry (SIMS), Infrared spectroscopy (IR spectroscopy), energy and wavelength dispersive x-ray spectroscopy (EDS or WDS), ellipsometry and Raman spectroscopy based LIDAR. The XPS, SIMS, EDS and WDS techniques however rely on the use of high vacuum systems. From this study of non contact surface chemical analysis techniques, two potential systems were identified. The first is based on a newly developed high intensity IR reflection system. The second utilises the fact that the treatment of composite / metal surface using a plasma liberates chemical species, which can then be detected using mass spectrometry.

In this project the use of a high resolution infra-red surface reflection system from QCL IR Spectrometers (http://www.blockeng.com/products/laserscope.pdf) is evaluated as a non contact analytical technique for determining contaminants on composites / steel surfaces.
A difficulty with the wider exploitation reflectance IR as chemical detection / control systems for use with composites, is the cost of these detection systems. This could limit broader commercial impact of the developed technology. In order to address this, one objective of this proposal will be to determine if the low cost acoustic detection technique can be used as a sensitive measurement of surface treatment. The plan was to develop a set of plasma processing conditions which the reflectance IR / mass spectrometry detection techniques demonstrate will remove all contaminants for a given composite / processing application. The acoustic technique will then be used to demonstrate that these processing are monitored and controlled.

Potential Impact:
Over an 18 month period the PlasmaClean project has investigated plasma removal on contamination with thickness in the range 5-10 nm. The main contamination type investigated being FreKote 710-NC, either sprayed or dabbed on to the surface of silicon wafer and aircraft grade composite substrates. Atmospheric plasma jet conditions were investigated in the removal of FreKote from composite surfaces. The degree of contamination removal and damage where investigated using reflection FTIR spectroscopy, electro-acoustic emission and OES. The FTIR measurement was found not to true real-time metrology technique as an A-B measurement had to be used, whereas electro-acoustic and particularly OES metrology can be used in real-time, once synchronised to the plasma scan over the composite surface was achieved.

With respect to composite surface mapping during plasma processing:

1. Stand-off FTIR can detect down to approx. 2 nm of FreKote on a composite surface. The removal of this contaminant can also be detected. This FTIR technique cannot however be performed in real-time due to the heterogeneous nature (resin and weave), of the composite. The onset a composite thermal decomposition however can be readily detected using these measurements. Once this deterioration occurs there is a rapid deterioration in the composite-to-composite or composite to paint bond adhesion strength.
2. Conditions have been developed to successfully remove a FreKote layer (thickness 5-8 nm) from composite surfaces, using the PlasmaTreat air plasma system. After the contaminant removal the composite surface exhibited the same adhesion enhancement in pull adhesion test studies, that is achieved for composites which had been plasma activated but whose surface had not been contaminated with FreKote. In particular composite-to-composite bond tests were obtained both for the freshly deposited FreKote and for the surface that had been left to air dry for 1 hour after the application of the FreKote. The effectiveness of the plasma treatment was not found to be influenced by FreKote drying time.
3. The air plasma system was also found to be effective in the removal of contaminants prior to painting. Cross hatch tests demonstrated that the plasma was effective in the removal of contaminants both on both metallic (titanium) and composite surfaces.
4. Off-axis real-time electro-acoustic measurements provide information on the nozzle-to-surface distance within the visible plume distal point. This measurement can be correlated directly with the composite surface temperature under a given set of plasma treatment conditions. It can therefore be used as a low cost process control technique.
5. Off-axis real-time OES of the plasma-composite reaction volume provides information on emission species (i.e. OH band) emanating from the composite surface. This can be used for in-process control to avoid thermal damage by the plasma. The technique is also sensitive to changes in the composition of the surface (metal to composite) and also height differences (to 1 mm) on a given surface.
6. For industrial applications electro-acoustic and OES both appear to be cost effective process control techniques for mapping the treated surface temperature as well as its surface chemistry. Both techniques have potential for process control for the atmospheric pressure plasma processing of composites, once they are calibrated using more expensive analytical methods such as reflectance FTIR.

In conclusion therefore this project has successfully demonstrated that air plasma treatments can be successfully used to remove organic contaminants on the surface of both aerospace composites and metals. The optical emission spectroscopy technique can be used as non contact diagnostic method for the determination of the chemical species present on the surface of metallic materials and composites during atmospheric plasma jet treatments. Any thermal damage caused by the air jet can be identified by changes in the OES spectra obtained of the plasma plume during treatment.

List of Websites:
Dr. Denis Dowling, Room 223, Engineering and Materials Science Centre, UCD, Belfield, Dublin 4, Ireland
Ph. 0035317161747
EMail: denis.dowling@ucd.ie