Analytical Services

Acree Technologies has a testing lab for determining material and coating properties. Acree offers testing services on a fee basis. Please call for a quote. In-house testing capabilities include:

Wear Properties

Many of the coating applications at ATI are based in the improvement of the mechanical properties of surfaces. These include components used in aircraft engines, body armor, optical components, and internal pipes. These represent a very diverse range of applications with very different needs and therefore very different testing protocols. Therefore, a variety of testing systems exist at ATI to quantify the testing of these films. In house testing capabilities include:

1. Air borne particle erosion test stand (testing standard XXXXX) – an example of testing is shown in Figure 1 for an uncoated and coated sample. Partial breakthrough occurs at the change in the rate of erosion for a coated part.

2. Pin on disk wear test (ASTM G99) - used for evaluating contact wear between surfaces. The wear track is measured using our inhouse surface profilometer and analysis software.

3. Abrasion resistance of optical coatings (Mil Spec 48497 and 43511, ASTM F735)- testing evaluates resistance of optical coatings to abrasion by contact with an rough surface (Taber eraser) or contact with moving sand. Analysis is performed through a combination of visual inspection, profilometry of wear track, and scatter/haze measurements.

Ellipsometry

Introduction
For transparent and semitransparent coatings the thickness, index, and absorption contain vital information and are a very sensitive indicator of changes in film stoichiometry or contamination. The measurement of these characteristics is therefore of great interest. Ellipsometry is a powerful tool to measure changes in properties of transparent or semitransparent thin films. Acree Technologies uses a Gaertner multiangle ellipsometer for measurement of film properties. Combined with inhouse software, the system allows the extraction of the full film index (real and imaginery components) at 632 nm as well as the film thickness.

Background
A reflection ellipsometer measures the change in polarization between the incident and reflected beam from a sample surface. The geometry of such a measurement is shown in Figure 1. The incident beam can be separated into two distinct polarizations: Ep (perpendicular to direction of travel, in plane of incidence) and Es (perpendicular to direction of travel, perpendicular to plane of incidence). For an incident angle of f1 the Fresnel reflection coefficient, r which is the amplitude of the reflected to incident wave, are given by:

where rP and rS are the Fresnel coefficients for the two incident polarizations, f2 is the angle of the transmitted beam and N1 and N2 are the complex refractive indices of the incident medium (usually air) and the sample. The reflected intensity is defined as the square of the amplitude:

the above equations assume a single interface. For a thin film the contributions of the multiple reflections from each interface must be taken into account and summed. Using the above relations, the fundamental equations of ellipsometry can be derived. The first parameter is the change in phase upon reflection delta, ?:

where d1 is the phase difference between the parallel and perpendicular polarizations of the incident wave and d2 is the phase difference between the parallel and perpendicular polarizations of the reflected wave. This value can vary between 0 and 360°. In addition to a change in phase, the amplitudes of the perpendicular and parallel components may also change upon reflection. The quantity psi, ?, is defined such that:

Often, the amplitude coefficients are shown as RP and RS rather than rp and rs when thin film systems are being considered since the equations for the amplitudes will be different. The ratio of the amplitudes can therefore be expressed as;

which is the fundamental equation for ellipsometry. The ellipsometry equipment measures the psi and delta values. The information about the sample is contained in the amplitude ratio. This raises the cautionary note regarding ellipsometry. The system, when operating correctly, will always measure the correct delta and psi values. However, the calculation for the material parameters must assume a certain model. For example, if measuring films deposited on silicon wafers whether or not a native surface oxide is present between the film and Si wafer will affect the interpretation of the delta, psi values. It is therefore of importance to know the history of what is being measured. Examples of trajectories for two absorption free coatings are shown in Figure 2. The refractive indices of the materials are 1.613 and 1.70. Each is assumed to be on glass. As can be seen the delta-psi trajectories fold back on themselves on a thickness given by:

where ? is the wavelength used in the measurement, n2 is the index of the film (non-absorbing) and f1 is the incident angle. On multiples of d the delta, psi values return to that of the substrate without coating. Therefore, when measuring samples an absolute thickness cannot be determined from a measurement at only one angle. If two angles are measured only one unique thickness solution will exist for the combination of angles allowing an absolute thickness to be determined. Figure 3 shows the trajectory for an absorbing layer. The delta-psi trajectory does not close back upon itself but instead follows a different trajectory on each multiple order. As the thickness is increased the trajectory continues to spiral eventually reach a fixed point which corresponds to the delta-psi value for an infinitely thick film of the absorbing layer (i.e. a thickness which becomes opaque at the test wavelength). This can be used to examine the evolution of thin metal coatings.