Testing techniques to analyse and control surface properties of materials

There are numerous techniques for manipulating the surface properties of a material. Heat treatment, anodising, shot blasting, peening, plating, painting; the list goes on.

However, perhaps less well known and utilised are the techniques to assess the chemical composition and physical topology of a material's surface.

Obtaining an accurate profile of a surface is vital for many applications and its composition can tell a lot about what can be expected in terms of performance. The surface of an engineered component is its physical interface and connection to the surrounding environment, and getting them right is often crucial to the overall performance of product or system.

Making a surface harder, for example, requires accurate measurement both before and after the appropriate process. Ideally this is an activity that will happen during product development to ensure that the surface properties are as expected. However, surface profiling commonly happens later, when products fail.

"Material failure is often about understanding the interface and interaction of materials," says Dr Chris Pickles, a consultant at materials testing company, Ceram. "It might be corrosion on the surface, a stress fracture due to wear or the delamination of a coating. These all began at the surface and you need to understand what caused them. In order to prevent it you need to first understand the surface chemistry and topography."

From corrosion to contamination, keeping surfaces clean and free of impurities is as vital when applying paint to a vehicle as it is when sterilising a medical implant. Stents, for example, are treated with a slow release anti-rejection drug held in an ethylcellulose coating. The drug can only be released from the top 2-3µm of the coating but due to the surface roughness of the metal 5µm of coating is placed on the stents to guarantee coverage.

"We have done a lot of work on stents, coating thickness, what the drug concentration is through the coating, and the thickness at different points on the substrate," says Dr Pickles. "The process put the drug on all 5µm, but when we knew the drug didn't come out deeper than 2.5µm we suggested the first 2.5µm coating had no drug in it, and only the top 2.5µm contained the drug, halving the drug cost. There are lots of things in the coatings area where surface characterisation can help in the development process.".

Surface profiling and analysis is not something engineering firms tend to do themselves. There are exceptions, of course, such as the semi-conductor industry that needs to precisely measure the dopants used, and in the production of sheet glass which adds a multi-layer thermal barrier coating to its products. For the most part however, it is more cost effective to take a sample and send it to the necessary lab for analysis.

"It depends what is happening as to how you take your sample," says Dr Pickles. "If it was a corrosion effect you can put down some adhesive tape and peel off what is on the surface, you could take a scalpel and scrape off the surface, or you can take some abrasive paper to remove some of the surface."

However, sending a sample to a lab is not always necessary and sometimes analysis can be carried out without taking any material from the surface of a part at all.
For surface roughness measurements using White Light Interferometry, an imprint of a surface can be taken using a liquid silicone that is applied, left to dry and then removed.

"We don't always need the part," says Dr Pickles. "We did work with a tyre moulder that produced large sheets of rubber to place over a mould. To make the large flat sheet of rubber necessary, rubber is fed through successive nip rollers. We were asked to look at the degradation and measure any wear on the roller surfaces as some of the ingredients in the rubber mixture were quite aggressive. So we applied the silicone replica technique to take samples and measured it."

Surface Analysis Techniques

X-Ray Photoelectron Spectroscopy (XPS): XPS uses an X-ray beam targeted at a sample. This disturbs the electron structure of atoms close to the surface and 'photoelectrons' are emitted. These can be used to determine the elemental content, the oxidation state of metallic elements and for depth profiling.
Application: XPS is commonly used to measure component cleanliness and for validating a cleaning process. This is particularly relevant to the medical device sector and other high performance engineering sectors such as scientific instrumentation and aerospace. It can quantify surface contaminants that can also limit performance in areas such as coating adhesion. XPS can also discern metal oxidation states and is frequently used to investigate corrosion phenomena and stress failures.

Dynamic Secondary Ion Mass Spectrometry (DSIMS): DSIMS uses a primary ion source to continuously sputter a sample. This generates a 'crater' that releases secondary ions. These are continuously detected and plotted against the sputter rate. Fragment masses can be identified and characterised to determine elemental abundance at a given depth.
Application: For many applications the region of interest is not only the surface but also the immediate sub-surface. This is particularly the case for multi-layer coatings of depths up to several hundred microns.

Time-Of-Flight Secondary Ion Mass Spectrometry (ToFSIMS): ToFSIMS uses a pulsed ion beam to sputter material from a sample surface. This sputter releases secondary ions that are focused toward a detector. As the lighter ions arrive at the detector before the heavier ones, the 'time-of-flight' can be calculated and used for mass spectrum analysis.
Application: ToFSIMS samples the top 3nm of a surface and is often used to investigate organic materials at a surface or interface. Adhesion failure is a common application where a wide range of contaminants can be detected. It is also useful in tribological investigations of lubricant performance by assessing the distribution of lubricant additives on and off wear scars.

3D Profilometry using White Light Interferometry (WLI): This measures surface topography of surfaces and sub-surfaces. The microscope illuminates a sample surface and also an optically flat reference plane. The interference pattern generated by the recombination of reflected light from both surfaces allows the construction of a 3D image with nm resolution in the z-axis.
Application: This can be used to specify surface conditions where adhesion between different materials is critical, for example when adhesives or coatings are used. Furthermore, surface deterioration can be accurately measured using the silicone replication procedure.

Justin Cunningham

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