Surface Characterization Techniques for Chemical and Physical Analysis of Composite Materials by Lucideon

Topics Covered

Introduction
Chemical and Physical Analysis of Composite Materials
X-ray Photoelectron Spectroscopy (XPS) in Surface Chemical Analysis
Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS) in Surface Characterization
Adhesion Failure Analysis of Composite Materials
Dynamic Secondary Ion Mass Spectrometry (DSIMS)
3D Profilometry Using White Light Interferometry (3DP)


Introduction

Lucideon is a global expert in materials analysis, research and quality testing, providing customised solutions and consultancy that help clients to measurably improve performance and profitability through safer, regulatory-compliant and better-engineered products. Setting new standards in materials testing, Lucideon works as an extension of customers' teams, applying its expertise and capabilities to a wide range of sectors

Lucideon has been working in partnership with R&D and production personnel in companies around the world for over 25 years. Using our expertise, experience and a comprehensive suite of advanced instrumentation we have been able to solve problems and create competitive advantage for our clients in the composites and related industries.

Chemical and Physical Analysis of Composite Materials

With composites the interface between one material and another (eg glass fibre and resin or thermal barrier coating and substrate) is critical to the performance of the product in operation. The analytical techniques operated by Lucideon are used to understand the nature and functionality of material interfacial interactions in terms of their composition and shape. This is particularly useful in both the initial development of composite materials and in failure investigations (eg disbondment between fibre and matrix). Chemical mapping of materials can tell what is present, where it is and how much is there - for all the elements and molecular fragments as large as 10000 mass units, with high sensitivity. In the sections below we look at three chemical analysis techniques and one physical surface profiling method and their application to the characterisation of various composite materials.

XPS 'heat scale' chemical map images of PEO coating on PP mesh

X-ray Photoelectron Spectroscopy (XPS) in Surface Chemical Analysis

X-ray Photoelectron Spectroscopy (XPS) uses an X-ray beam to analyse the surface for its elemental chemical composition. It samples the top 10nm of the surface under investigation and is quantitative to an accuracy of 0.1 atomic percent. It can also be applied in high resolution mode to unravel the nature of elemental bonding (eg C-O, C-C) or the oxidation state of metallic elements. The normal analysis area is 700µm x 300µm with small spot options down to 55µm x 55µm. The technique can also be used to construct 2D chemical maps for particular elements of interest (as in the images above) and for quantitative elemental depth profiling of either the top 10nm (by precise adjustment of the X-ray beam angle of incidence) or more deeply by using argon ion beam sputtering.

The surface specificity of the technique can be seen on a polyethylene oxide (PEO) coated polypropylene (PP) mesh where the quantitative spatial distribution of the coating is tagged using the C-O signal from the spectrum and the substrate is tagged using the C-C signal.

Time-of-Flight Secondary Ion Mass Spectrometry (ToFSIMS) in Surface Characterization

Static secondary ion mass spectrometry uses a primary ion beam to sputter material from the sample surface. The sputter cascade contains both neutral and ionic species and the secondary ions are focused using ion optics towards a charge detector. Mass detection is achieved by the use of an extended flight path such that the lighter ions arrive at the detector in advance of the heavier ions giving 'time-of-flight' spectral separation. In contrast to the XPS output this generates a mass spectrum extending to molecular fragments as large as 10000 mass units. ToFSIMS samples the top 3nm of the surface and is sensitive to ppm levels. Although highly sensitive, ToFSIMS is a qualitative method. ToFSIMS is routinely used to investigate organic material at surfaces and interfaces.

Adhesion Failure Analysis of Composite Materials

Adhesion failure analysis is a common application where the presence of a very wide range of contaminants can be confirmed. Species specific images are obtained by scanning the sample surface with the primary ion beam and generating spectra for every pixel. This data can then be used to select a characteristic mass fragment and plot its distribution within the area sampled with micron scale resolution. When this is done on a cross section bulk distribution in complex systems can be informed.

ToFSIMS 'false colour' chemical map of boron segregation in stainless steel

ToFSIMS 'false colour' chemical map image of a cross-section through a glass fibre composite

 

Dynamic Secondary Ion Mass Spectrometry (DSIMS)

For many applications the region of interest is not only the surface or interfacial condition of a material but also the immediate sub-surface. This is particularly the case where multilayer coatings are concerned or where embedded material distribution with depth is of interest. For these applications Dynamic Secondary Ion Mass Spectrometry (DSIMS) is the technique of choice. In this case the primary ion beam is used to continuously sputter the area of interest with sufficient energy to generate a crater in the material under investigation. The secondary ions so generated are continuously detected and plotted against the sputter rate which is subsequently calibrated against the crater depth.

Because of the relatively high energy required to generate the sputter crater molecular information is not available from this technique. However, atomic masses can be summed to differentiate and track higher mass fragments characteristic of specific material types of interest. Where deep profiles are of interest (several hundred microns) sputtering times can be extended with an adverse cost impact. In such cases it can be beneficial to work on a cross section and use the imaging capability. DSIMS can operate in imaging mode and offers both particularly good spatial resolution at micron scale together with excellent sensitivity at ppb levels.

DSIMS chemical map images of an optical fibre bundle at three resolutions

 

3D Profilometry Using White Light Interferometry (3DP)

The three previous techniques all generate chemical information at surfaces, sub-surfaces and interfaces. 3D profilometry is a technique for measuring surface topography which generates quantitative information on the physical nature of surfaces and sub-surfaces by using white light interferometry. The 3D image is half micron pixellated in the x-y plane but nanometre resolved in the vertical (z) axis. The technique can also generate line scans from any part of the area sampled, 2D colour height maps and 3D video output. Most importantly, the technique gives statistical averaging of the surface roughness. These parameters can be used to specify surface condition where adhesion between different materials is critical. The field of view for a single image is 3mm x 5mm although much larger fields can be measured by 'stitching' areas together.

Moreover, it is not necessary to present the actual sample to the instrument - areas of interest can be replicated using a liquid silicone and the 'negative' so generated analysed for surface topography. For transparent coatings such as lacquers coating thickness can be determined by profiling both the lacquer and substrate surfaces and subtracting for the difference. In composite manufacture, surface roughness measurement is important from both the raw material and finished product standpoint. Furthermore, surface deterioration measurement (eg on a turbine blade leading edge) can be accurately quantified by the 3DP technique as an in-service monitoring method by using the replication procedure.

3D topographic profile of turbine blade with line scan and statistical data

Source: Lucideon

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