Test Methods

2.5.1 Glow Discharge Spectroscopy

Glow Discharge Optical Emission Spectroscopy (GDOES) relies on excitation of atoms within the sample being analysed. Electrons pass over to a lower energy level during relaxation and emit radiation quanta with characteristic frequency and wavelength [68]. Hence excitation of different elemental atoms will give rise to the emission of characteristic spectral lines by which those elements may be identified. The intensities of the lines are proportional to the number of emitted quanta, and hence to the element concentration. This is the essence of atomic emission spectroscopy [54].

In the Glow Discharge Optical Emission Spectroscopy (GDOES) the sample to be analysed forms the cathode at one end. The hollow cylindrical anode is in a small vacuum chamber, filled with argon as a working gas. The other end of the vacuum chamber contains a window through which the photon emission is detected via a polychromator.

An applied voltage between the anode and the cathode causes ionisation. Those argon ions are then accelerated towards the cathode. On impact, kinetic energy is transferred which causes the sputtering of neutral cathode atoms (at a sputtering rate in the range of 10 to 100 nm per second) and secondary electrons (sustaining the discharge). The sputtered cathode atoms diffuse across the dark space into the negative glow region and are excited or ionised by collision with secondary electrons or argon ions. These excited atoms will emit characteristic line spectra.

The quantification of the sputtered depth from the sputtering time can be subsequently checked by surface profiling the sputter erosion craters after analysis and comparing with the glow discharge plots and can be calibrated using a surface profilometer or directly from available sputter yield data.

The determination o f the chemical content from the emission line intensities is depending on the voltage, current, sputtering rate and chemical composition. Intensity calibration is performed using certified reference materials of known chemical composition.

The analysed composition is normalised to 100%, so it is important to know all the major elements present in the sample.

- depth profiles of chemical composition

- depth range from tens of nanometers to tens of microns - depth resolution can be as low as 5 to 10 nm

- can cover all elements of the periodic table - simultaneous analysis of 40 elements

- analysis is quantitative and accurate, provided suitable calibration samples available - ppm accuracy is possible

- easy sample preparation, only flat surface necessary - area of analysis is 4 mm diameter

2.5.2 Adhesion and Scratch Test

Adhesion is defined as "the state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking forces or both" (ASTM D907-70). The nature o f these bonding forces are van der Waals, electrostatic and / or chemical bonding forces which are effective across the coating-substrate interface. The practical adhesion is a macroscopic property which depends on chemical and mechanical bonding at the interface, residual stress and the presence o f any stresses imposed [66].

Depending on the brittle / ductile properties of the coating and substrate a number of possible failure modes can occur in any application. If a break occurs at the interface it is termed adhesive failure, and if it occurs within the substrate or the coating it is named cohesive failure. A number of techniques have been developed in order to improve the adhesion, for example:-

(i) pretreatments - cleaning and degreasing of components prior to loading coating systems

(ii) in situ treatments - such as heating, plasma treatment, sputter cleaning. Higher process temperatures lead in general to better adhesion, caused by increased surface mobility, inter-diffusion and the formation of chemical bonds.

(iii) bonding layers - to form strong interfacial phases, minimise interfacial stresses and getter contaminations [66].

At the present time there are no tests for the measurement of practical adhesion which fulfil requirements like easily adaptable to routine testing, relatively simple interpretation, amenable to standardisation, reproducibility, quantitative and directly related to coating reliability. All of the commonly used tests are destructive in nature [69, 70].

Methods of adhesion evaluation are eg. Pull-Off methods (ie tape test) and the scratch test. The methods used in this thesis are the scratch test and the indentation test, which are described in more detail below.

Scratch Adhesion Test

The scratch test was introduced by Heavens and was later developed by Benjamin and Weaver. In the scratch test, an indentor is drawn across the coated surface while a continuos or stepwise increasing normal load is induced (Figure 2.31). When the critical normal force Fc is reached, a failure event occurs which is detected by optical or other means. Provided that the observed event represents the loss of adhesion at the

coating-substrate interface, the critical load can be used as a qualitative measure of the coating-substrate adhesion [71]. The critical force value cannot be directly related to the strength o f the coating- substrate interface. Coating thickness, substrate hardness, surface roughness, stylus radius, scratching speed, loading rate and gradual wear of the diamond stylus have a significant influence on Fc.

Figure 2.31: Scratch adhesion test

Identified mechanisms o f coating damage and detachment are given in figure 2.32 [72] and figure 2.33 [73].

a

n

a

£ 1

a

La

Pronounced plastic deformation PPD Stick-slip deformation SSD

External parallel cracking EPC

Internal transverse cracking ITC External transverse cracking ETC

Coating debris removal CDR

Discontinuous chip removal DCR

Continuous chip removal CCR

Splinter-like parallel flaking SPF

Sideward lateral flaking ELF

Forward lateral flaking FLF

Figure 2.32: Coating damages

Figure 2.34: Schematic damage mechanism

V*f*»Qen.

vom Diaman<«n «ingebett«(