Dieter Schneider

Non-destructive evaluation of diamond and diamond-like carbon films by laser induced surface acoustic waves

Abstract Youngs modulus of diamond-like carbon films varies in a wide range which suggests the use of this material parameter to characterize the film quality. It can be obtained non-destructively by means of surface acoustic waves. A reliable and quick photoacoustic technique has been developed which enables the film modulus to be determined for rather small samples such as cutting tools. The technique is based on the measurement of the surface wave velocity vs. frequency. This spectrum is derived from laser induced surface wave impulses by Fourier transformation. Comparison with the theoretical results may provide Youngs modulus, density, and film thickness simultaneously. Less information is obtained for very thin films so that less than three parameters can be derived. By taking into account an empirical relation between Youngs modulus and density which has been found for diamond-like carbon films, two parameters can be obtained even for films about 100 nm thick. The informational content of the measurement depends on the bandwidth of the equipment and on the substrate material. Diamond and diamond-like carbon films were studied. They were deposited using different technologies onto silicon single crystals, steel, and WC-Co cemented carbide. The film thickness was in the range 60 nm d

A photoacoustic method for characterising thin films

Abstract A photoacoustic method is presented for characterising thin films. The method uses acoustic surface waves with frequencies up to 200 MHz induced by short laser pulses and detected with a piezoelectric transducer. The surface wave signals are processed by cross-correlation and Fourier transformation to determine a dispersion spectrum (phase velocity depending on frequency) with optimal signal-to-noise ratio. The theoretical curve is fitted to the measured dispersion spectrum to derive film parameters as Youngs modulus, density and/or film thickness. The conditions are discussed which enable one, two, or three of these parameters to be obtained. The dispersion spectrum may show normal and anomalous dispersion, which depends on the combination of film and substrate material. Diamond-like carbon and polyamide films on (100) silicon are used to demonstrate this phenomenon. The effects of firm thickness, film and substrate material, and the error of the input parameters on the results are discussed. The peculiarity of surface wave propagation in cubic single crystals is described. The photoacoustic method is particularly suitable to determine the Youngs modulus of the film. This material parameter is sensitively related to important microstructural properties and bonding conditions. The large modulus variation of some important covalent and ionic film materials recommends to use Youngs modulus for quality control.

Determination of elastic modulus and thickness of surface layers by ultrasonic surface waves

A method was developed to determine simultaneously the thickness and the elastic modulus of surface layers from surface wave dispersion. Surface wave pulses with a broad bandwidth are generated by an impulse laser. The pulses are received with interdigital transducers in the frequency range 12–31 MHz. A Fourier Transform technique is used to determine the dispersion curve representing the phase velocity depending on frequency. The problem of the n2π ambiguity has been overcome by a measuring procedure based on the determination of the phase shift for several increasing measuring distances between the sound source and the receiving transducer. To calculate thickness and elastic modulus of the layer, the inverse solution of the surface wave dispersion in a homogeneous, isotropic material coated with a homogeneous, isotropic layer is carried out by non-linear regression. Two materials, (W, Ti, Ta)C-Co cemented carbide coated with either nickel or TiC, represent the two cases of normal and anomalous surface wave dispersion and were investigated with the surface wave method. The layers had thicknesses of 3–80 μm. In the case of normal dispersion in nickel-coated cemented carbide, measurement of the dispersion curve in the range up to a ratio of layer thickness to wavelength dλ = 0.05 is sufficient to determine thickness and elastic modulus simultaneously. In the case of anomalous dispersion in TiC-coated cemented carbide, measurement in the range up to dλ = 0.21 is necessary. Good agreement has been found between the surface wave results for the layer thickness and those obtained with the stylus instrument and the weighing method. The measured values of the elastic modulus of the layer indicate structural changes.

Non-destructive characterization of mechanical and structural properties of amorphous diamond-like carbon films

Abstract Amorphous diamond-like carbon (DLC) films show promising properties for wear protection applications. Pulsed laser deposition (PLD), laser-induced pulsed vacuum arc deposition (laser-arc) and mass-selected ion beam deposition (MSIBD) are techniques which enable deposition of DLC films with the desired properties by providing sufficiently high carbon energy. Two non-destructive methods were used to determine the properties of films deposited by these three deposition techniques: (1) ellipsometry, which gives information about the micro-structure through the optical properties; and (2) laser-acoustic analysis enabling the measurement of the Youngs modulus of thin films down to less than 100 nm film thickness. These methods were used to investigate the effect of the carbon ion energy and the substrate temperature during deposition on the film quality. A clear correlation between the Youngs modulus and the optical parameters was found. All three deposition techniques are characterized by a critical substrate temperature above which sp 2 rich films are deposited. The PLD and MSIBD systems have a higher ion energy and a lower deposition rate than the laser-arc system. This is the reason for the higher critical substrate temperature below which sp 3 rich films are formed for PLD and MSIBD compared with laser-arc.

Testing ultra-thin films by laser-acoustics☆

Abstract Evaluating ultra-thin films of a few nanometers thickness is still a challenge for mechanical testing. The capability of the laser-acoustic technique was investigated to measure the Young’s modulus of diamond-like carbon films with thickness down to 5 nm. The elastic modulus has proved to characterize an important mechanical property of hard films which are used for wear protection. It can non-destructively be determined by the laser-acoustic technique that is based on measuring the dispersion of surface acoustic waves. This acoustic wave mode is very sensitive to thin films which was previously shown for diamond-like carbon films with thickness down to 60 nm. Diamond-like carbon films with thickness from 5 to 30 nm were deposited by the pulsed arc technique on pure silicon and on silicon coated with a metal layer of 80 nm thickness. The demand for the accuracy of the measuring instruments and for the measuring conditions, such bandwidth and sample dimension, is discussed for testing such ultra-thin films.

A laser-acoustic method for testing and classifying hard surface layers

Abstract The laser-acoustic method is accepted to be an interesting method of testing thin films. It is based on measuring the dispersion of surface acoustic waves which are generated by short laser pulses. A reliable test equipment was developed that allows a user-friendly operation. The method is non-destructive, the test takes little time and special sample preparation is not required. It is mainly applied to measure the Young’s modulus of thin films with thickness down to less than 50 nm. Recent studies showed these results to correlate with important microstructural and mechanical properties of hard and superhard films. The laser-acoustic technique was improved to test multilayer films consisting of two components. The approach of an effective medium of transversal symmetry is used to describe the elastic behavior of multilayer films. It enables the elastic anisotropy of the multilayer film to be evaluated. Applications are presented, performed at multilayers of diamond-like carbon and aluminum deposited by laser-arc on steel and silicon. The films consisted of four and twenty single layers, respectively. The Young’s modulus of the diamond-like carbon in the multilayer was determined with the laser-acoustic technique. The results reveal the reproducibility of the deposition technique and demonstrate the potential of the laser-acoustic technique to test multilayer films. The laser-acoustic method is shown to be sensitive to machining layers. The effect of grinding and polishing steel surfaces was studied. Studies were performed to compare the results of the laser-acoustic technique with those of membrane deflection and micro-indentation. TiN, CrN and TiCN films (thickness: 0.8–2.3 μm) were tested with laser-acoustics and micro-indentation, polysilicon films (thickness: 0.46 μm) with laser-acoustics and the membrane deflection technique.

Non-destructive characterization of plasma-sprayed ZrO2 coatings by ultrasonic surface waves

A surface wave method has been used to determine the elastic modulus of ZrO2 coatings plasma-sprayed onto a nickel-base superalloy and aluminium. The phase velocity depending upon the frequency of laser induced surface waves has been measured. The inverse problem of the surface wave dispersion relation has been solved to calculate the elastic modulus of the coatings from the measuring data. Modulus values in the range 11.2-46 GPa were obtained for the coatings applied on a nickel base superalloy by atmospheric plasma spraying. These moduli are lower by up to one order of magnitude than those measured on compact ZrO2 samples (240 GPa) owing to a network of long stretched-out pores which intersperse the coating material. An elastic modulus of 90 GPa has been determined for the ZrO2 coating vacuum-plasma-sprayed on aluminium. The micrograph of this coating showed a structure with more isolated pores having almost equiaxial form. The nature of the pore structure can be predicted from the measured elastic modulus using the theory of elastic modulus of heterogeneous solids.

Quality control of ultra-thin and super-hard coatings by laser-acoustics

Non-destructive testing of hard-coatings is highly desirable. The available test methods are continuously faced with many new demands for engineered surfaces; this arises from reducing film thicknesses, more complicated film composition, and extreme mechanical requirements such as high hardness, stiffness and adhesion. The laser-acoustic technique based on surface acoustic waves is a relatively new surface test method, but its capability for testing thin and hard coatings has already been demonstrated. The laser-acoustic method yields the Youngs modulus, revealing the effect of varying bonding structure of the material, porosity and other micro-defects, including insufficient adhesion. Efforts have been made to adapt the method to the requirements for testing ultra-thin films. The special methodical aspects of testing these films are discussed, such as the effect of measuring accuracy, bandwidth and sample dimension. For nanometer films, the theoretical assumption of a homogenous film hardly applies. The gradient interlayer and the incomplete micro-structural development in the process region at the surface evidently influence the calculated film modulus for the thickness of the carbon films lower than 20 nm. Although the results of elastic modulus only represent an effective value for these films, it can be used as an indirect indicator for the composition of the films varying with reducing film thickness. The results presented illustrate the way the laser-acoustic results can help to optimize the deposition processes. Ultra-thin amorphous carbon films deposited by high current pulsed vacuum arc discharges were tested. The effects of the substrate temperature during the deposition, the nitrogen content, and the film thickness were studied. The minimum film thickness was less than 3 nm. The coherent trend of elastic and plastic deformability, frequently reported for thicker carbon films as the correlation of hardness with Youngs modulus, was also found for the ultra-thin carbon films. The plastic behavior was studied by means of measuring the nano-scratch resistance.

Elastic properties of Ti,Al,Si N nanocomposite films

Ž. Ti,Al,Si N films have been prepared by d.c. and rf reactive magnetron sputtering, with Si contents in the range 211 at.% and Ž. Al contents between 4 and 19 at.%. Samples prepared in rotation mode three magnetrons presented densities between 4.0 and 3 Ž. 4.6 gcm , while samples prepared in static mode magnetron with Ti target with small pieces of Si and Al displayed densities mainly in the range 3.03.9 gcm 3 . For comparison purposes, the evaluation of Youngs modulus was performed by both Ž. depth-sensing indentation and surface acoustic wave SAW techniques. Indentation results revealed systematically higher values than those obtained by SAW. These discrepancies might be related with the relatively low density of the films. Hardness values of approximately 60 GPa were obtained with samples with a composition of approximately 28.5 at.% titanium, 12 at.% aluminium, 9.5 at.% silicon and 50 at.% nitrogen. XRD patterns showed the presence of two different crystalline phases, as in the case of Ž.

Characterization of thin diamond-like carbon films by ultrasonic surface waves☆

Abstract The method of ultrasonic surface waves is suitable to determine the elastic modulus of thin diamond-like carbon films deposited onto silicon (111) wafers by laser-induced vacuum arc deposition (Laser-Arc) if frequencies in the range up to 100 MHz are used. Surface wave pulses were generated in the thermoelastic regime by an excimer-laser (XeCl, 308 nm). Ultrasonic signals influenced by the film structure were detected by a piezoelectric transducer. The surface wave phase velocity, which is dependent on frequency, was measured using a Fourier transformation technique. To calculate the elastic modulus of the films the inverse problem of surface wave dispersion in coated materials was solved. Carbon films with a thickness of 300 nm deposited at different substrate temperatures in the range between 60 and 500 °C were studied. Youngs modulus values up to 230 GPa were obtained for substrate temperatures below 150 °C. The very small modulus for T > 150 °C suggests a drastic decrease in the sp 3 -to-sp 2 bonding ratio in the amorphous diamond-like carbon structure. These results correlate with optical studies. High frequency surface waves were demonstrated to be a sensitive indicator for evaluating the structure of the carbon film as dependent on the deposition conditions.