Measurement of Thin film Interface Strength by a Laser Spallation Technique

Dartmouth researchers have developed a laser spallation technique for measuring the tensile strength of any planar interface between a thin coating (thickness greater than 0.1 µm can be tested) and a substrate. In this technique, a laser-produced compressive stress pulse in the substrate, reflecting from the coating's free-surface pulls the interface in tension and leads to its failure if the tensile amplitude is high enough. The interface stress is determined directly by recording the coating or substrate free-surface velocities using a laser displacement interferometer. The recorded surface velocity is related to the interface stress via an elastic wave mechanics simulation. Several interface stress charts are given for wider applicability of the modified technique.

The potential of the modified laser spallation technique has been demonstrated by applying the technique successfully to several interface systems, including fiber/coating interfaces in composite materials, interfaces between functional (magnetic, conducting, optical, electrical), protective (thermal barrier, corrosion, wear resistant), or decorative coatings and their underlying substrates, and several metal/ceramic and metal/polymer interfaces for applications in multilayer devices, and magnetic disc and head technology. In all of these applications the tensile strength of the interface is an important property that directly controls the interface decohesion process, and often controls the reliability of the coated components. Both control and measurement of the interface tensile strength are of importance in the above applications.

To quote some results, the tensile strength of interfaces between 3-µm thick coatings of Sn, Sb, Cu, Nb, Al, and Cr on carbon substrate were measured in units of MPa to be 18.53, 5.99, 15.46, 41.16, 16.53, and 15.47, respectively. Interfaces between the diamond, SnO2 (Tin Oxide) and Nb coatings on polycrystalline alumina substrates were measured to be 140 MPa, 286 MPa, and 280 MPa respectively. Recently 0.1 µm thick Cu coatings on oxidized surfaces of Si were successfully measured. Also metal/polymer coatings were tested. These systems have applications in the microelectronic industry. In addition, the modified technique allows us to quantitatively study the effect of different interlayers to optimize the interface strength. The Nb/Al2O3 (Aluminum Oxide) interface strength can be controlled from 0.16 GPa to 0.35 GPa by depositing Cr and Sb interlayers up to a thickness of 70 Å.

By using a novel displacement interferometer and a setup to produce hitherto unreported short stress pulses, the spallation technique has become more versatile as coatings with rough surfaces and thickness as thin as 0.1 mm can be tested. In addition the technique can also measure the tensile strength and Young's modulus of the coating. Future potential application of the short stress pulses include, non-destructive testing for small flaws for quality control in the manufacturing industry, laser cutting applications in microelectronic and other industries, materials processing, and shock explosion applications in the Defense industries.

This invention may drastically improve the reliability of the coatings against mechanical failures under service conditions. In addition, using this technique the interfaces between the coating and the substrates can be modified using different interfacial chemicals or changing coating deposition parameters and the effect assessed by the laser spallation experiment. We are seeking an industrial partner to further refine and market this technology.

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