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

Abstract

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 Young's 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 Young's modulus, was also found for the ultra-thin carbon films. The plastic behavior was studied by means of measuring the nano-scratch resistance.