Surface and Interface Analysis by Glow Discharge Spectrometry

Introduction

The possibilty of performing surface and interface analysis using a glow discharge source is linked to the flat sputter craters which can be created with this technique. For good depth resolution in depth profiling, it is important to choose the source parameters correctly to obtain sputter craters with flat bottoms

Flat bottomed craters correspond to the ‘abnormal’ discharge region, the region for which all of the cathode surface participates in the discharge. Concave craters generally arise when the potential is too low for the sample at the chosen pressure; convex craters are the reverse, the potential is too high for a given sample and pressure.

When operating with constant power, the pressure is adjusted before analysis to give a flat bottomed crater.

In bulk analysis, the crater shape is generally not important. High power and pressure (high voltage and current) are usually selected to increase sputtering to gain the maximum signal. These hard conditions may have to be reduced for temperature sensitive materials. For many non-conductive materials, being particularly sensitive to heat, an efficient sample cooling is required. In some cases, such as organic paint coating, cooling with liquid nitrogen helps stabilising the discharge. Also pulsed glow discharge as been found to reduce the thermal stress on the sample surface.

References:

M Bouchacourt and F Schwoehrer, in R Payling, D G Jones and A Bengtson (Eds), Glow Discharge Optical Emission Spectrometry, John Wiley, Chichester (1997), pp 62-67;
M S Marychurch and R Payling, in R Payling, D G Jones and A Bengtson (Eds), Glow Discharge Optical Emission Spectrometry, John Wiley, Chichester (1997), pp 67-69.
Th. Nelis, J. Pallosi; Glow Discharge as a tool for surface and Interface analysis; Applied Spectroscopy Review, 41; 2006; pp 227-258; DOI 10.1080/05704920600620345

Author: Richard Payling & Thomas Nelis; First published on the web: 1 June 2000.

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Compositional Depth Profiling by GD OES

The algorithms for quantitative depth profiling (QDP) were described in Depth Profiling. The means of controlling the GD source were described in Modes of Operation. The method of calibration was described and illustrated in Calibration. So now we put it all together and present some examples of QDP using RF-GD-OES.

Metal Coatings

Two examples are: hot-dipped Galvanized steel and Zinc-Nickel coated steel. These were used to introduce Glow Discharge. A description of these and other coated products can be found in the next section .

Another important commercial product is Galvalume. The coating is 55% Al, 43.5% Zn and 1.5% Si, and typically 20 µm thick:⁽¹⁾

Galvalume - Al-Zn-Si coated steel
Key: Si x5, Vdc /10, QM a.u., Dns x10

From the Al and Zn profiles we can determine that the coating is 20 mm thick. The Al and Zn profiles are not constant through the coating indicating the coating is not homogeneous with depth. Si is concentrated more towards the steel substrate. Vdc is higher in the steel than in the Al-Zn-Si coating. The sputtering rate QM varies in the coating up and down with the Zn content but overall is about the same in the coating and steel. The density is about 4 g/m² in the coating and increases to 7.8 g/m² in the steel.

Hard Coatings

Some of the most important hard coatings are based on TiN. They are used to increase wear and corrosion resistance. They are commonly produced by chemical vapour deposition (CVD) or physical vapour deposition (PVD). An example of such a CVD coating is:⁽¹⁾

The analytical result displayed in the graph has been obtained with a TiN coated tool steel, showing an intermediate TiCN layer

Polymer Coatings

One of the important steps in producing car bodies is the cataphoresis (e-coat) coating applied before painting. It is used to improve paint adhesion and corrosion resistance. One such coating, produced in a laboratory test, is shown here, as Atom% vs Depth (mm), to emphasise the light elements, C, O, N, etc.:⁽²⁾

Cataphoresis on steel: the steel substrate is to the right, the first layer on the steel is a zinc phosphate, and then the thick outer polymer coating to the left.

References:

R Payling, M Aeberhard and D Delfosse, in Proc. 12^th Intl. Federation for Heat Treatment and Surf. Eng. Cong., Melbourne, Australia, 29 Oct.-2 Nov. 2000, Vol. 2, pp 97-100 (2000).
R Passetemps, R Payling and P Chapon, Pittcon99, Florida (1999).

First published on the web: 8 October 2000.

Author: Richard Payling

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Continuous Metal Coating Lines

The following table summarises some metal coating typically used in industrial applications, car manufacturing and the building industry.

No.	Product Name	Major Elements in Coating (approx. mass%)	Typical Coating Mass/Thickness (g m^-2/um)	Coating Properties
	Electroplated Zinc	Zn 99.9	75/10
	Galvanneal	Zn 89 Fe 11 Al 0.2	40-60/6-8	Zn rich surface, relatively uniform centre section in good coatings and increasing Fe near steel
	Hot-dipped galvanized steel	Zn 99 Al 0.3 Pb/Sb 0.2	70-140/10-20	Al enriched at surface and coating/steel interface. Sometimes C and H seen at the interface depending on the level of steel cleanliness
	Galfan	Zn 95 Al 5% La+Ce trace	60-140/9-20
	Aluzinc	Zn 70 Al 30	60-140/10-20
	Galvalume	Zn 44 Al 55 Si 1.5	80/21	Regions rich in Zn and others rich in Al (because of limited solubility), Si increasing towards coating/steel interface, a Al/Zn/Si/Fe quaternary
	Aluminium coated steel	Al 99	40/5
	Electroplated Zinc-Nickel coated steel	Zn 89 Ni 11	45/6	Normally very uniform coating
	Tinplate	Sn 99.9	90/5

First published on the web: 15 January 2000.

Author: Richard Payling