Ultrasonic characterization of mechanical properties of Cr- and W-doped diamond-like carbon hard coatings
Introduction
Ultra-hard coatings such as diamond, diamond-like carbon (DLC), cubic BN, etc., are of great interest in technological applications due to their unique mechanical, thermal, and electrical properties. In particular, the high hardness and stiffness of diamond-like thin films make them an excellent coating material for tribological applications. DLC can be vapor deposited at low temperatures, has good substrate compatibility, and results in very good surface quality. DLC is a meta-stable form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H) containing a fraction of sp3 bonding, which leads to diamond-like properties [1]. Therefore, DLC coatings also exhibit high mechanical hardness, low wear rate, and chemical inertness. In recent years, DLC films have been applied as protective coatings on cutting tools, magnetic hard drive disks, as wear-resistant layers in micro-electro-mechanical systems devices [1], [2], [3], [4]. In all these applications, it is critical to ensure that the mechanical properties of DLC coatings are carefully controlled during fabrication so that the desired properties are obtained. The intrinsic high hardness of DLC coatings can, however, be significantly affected by the microstructure of the coating, including the size and number of graphite clusters, as well as the presence of any impurities at cluster grain boundaries [3], and in turn can lead to a wide variation in the mechanical properties of the coatings.
Current testing techniques that are used to investigate the mechanical properties of thin films mainly include nano-indentation, micro-beam bending technique, tensile testing, bulge testing, surface Brillouin scattering (SBS), and broadband surface acoustic wave spectroscopy (SAWS) [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The first four methods require direct loading on the material surface, and furthermore the influence of the substrates or supporting structures sometimes can overwhelm any effort to obtain the film properties [7]. Also, these methods are either destructive or cannot be used for in situ material evaluation. SBS is a nondestructive method based on the inelastic scattering of photons by thermal phonons in the film. The surface wave velocities can be measured by SBS at very high frequencies (GHz or above) [11], but this is a very time consuming measurement due to the low photon count in the backscattered direction, which is the preferred direction to achieve the highest signal-to-noise ratio.
The SAWS method is perhaps the most suitable nondestructive method to measure the acoustic wave related mechanical properties of thin films. Here high frequency ultrasonic waves are intentionally launched into multilayered thin film–substrate systems and their guided-wave behavior is monitored. Dispersion of SAW is dependent upon the mechanical properties and physical parameters of every layer in the layered structure, and the experimentally measured dispersion relations can be analyzed to obtain the desired mechanical properties of thin films. In this work, a guided-wave photoacoustic method and the line-focus acoustic microscopy (LFAM) technique are used to measure the SAW and leaky SAW (LSAW) dispersion curves in multi-layered specimens. LFAM is a well-established technique that has been utilized to evaluate various types of materials, including semiconductors, thin films, piezoelectric materials, etc. [13], [14], [15], [16]. In the guided-wave photoacoustic method (also known as laser ultrasonics), a well-focused ultra-short pulsed laser beam illuminates the sample causing rapid local thermal expansion and resulting in the launching of broadband SAWs. The SAW wavepacket is also typically monitored optically. This photoacoustic technique has been applied to investigate the elastic properties of various types of film structures, including hard coatings, anisotropic semiconductor films, and also to study nonlinear shock wave effects in thin films [13], [17], [18], [19]. Due to the all-optical configuration, photoacoustic techniques are potentially suitable for in situ measurements during the fabrication process of the coatings and they can be used as quality control tools in a production environment.
This paper reports an effort to develop a guided-wave photoacoustic method to determine the mechanical properties of ultra-hard metal-doped DLC films that can be used in situ and on complex geometries. To benchmark the results from the photoacoustic measurements, LFAM and nano-indentation measurements were also performed. In the first part, the metal containing DLC coatings under investigation are described. These metal-doped DLC coatings with various compositions were deposited with different recipes during the physical vapor deposition (PVD) fabrication process. Secondly, the theoretical background for the propagation of surface acoustic waves in multi-layer thin film materials is briefly explained. The transfer matrix method for numerical calculation of the SAW dispersion curves is utilized in our work. In the third part, the ultrasonic experiments using the photoacoustic method and line-focus acoustic microscopy are introduced, including their principles and setups, and the measured dispersion curves of SAW and LSAW are presented. For the inverse problem of determining the mechanical properties from the measurements, a nonlinear optimization method is applied. After the fitting procedure, the mechanical and physical constants of the metal containing DLC coatings, i.e. Young's moduli, Poisson's ratios and coating densities, etc., are derived simultaneously. This inverse procedure is performed on photoacoustic and LFAM signals independently. The independently obtained elastic constants are cross-correlated with the nano-indentation tests on the metal-doped DLC specimens and good agreement is observed with these three approaches. The comparison of various techniques shows that the surface acoustic wave methods are reliable and robust in characterization of the mechanical properties of thin film materials. Finally, photoacoustic measurements on a curved gear component are described as a representative real-world application of the method.
Section snippets
Metal-doped diamond-like carbon coatings
The chromium-doped diamond-like carbon (Cr-DLC) films studied in this work were provided by Caterpillar Inc. The coatings were deposited on heat-treated, polished, flat AISI 52100 steel substrates in Caterpillar's closed-field unbalanced magnetron sputtering chamber using a reactive PVD process. Caterpillar has an in situ spectroscopic ellipsometer installed on the PVD chamber so that the optical constants, i.e., index of refraction and dielectric constant, of the coating can be analyzed
Acoustic guided waves in multi-layer thin film materials
The acoustic wave motions along the surface of the materials are very sensitive to coatings and films, even for films that are thinner than the acoustic wavelength. For multi-layer materials, the acoustic properties for each layer will affect the surface wave motion. The penetration depth of SAWs is proportional to the wavelength, with the higher frequency components propagating shallower than low frequency components. Surface waves with higher frequencies are thus more influenced by the films,
Ultrasonic characterizations of Cr- and W-DLC coatings
In this section, we describe both the photoacoustic measurements and the line-focus acoustic microscopy measurements on Cr-DLC and W-DLC coatings.
Results and discussions
The fitted dispersion curves for Cr-DLC and W-DLC coatings are shown in Fig. 6, Fig. 9, respectively, as solid lines. The calculated parameters by photoacoustics and LFAM are listed in Table 1. Nano-indentation tests were also performed to get the elastic constants for the same set of Cr-DLC and W-DLC coatings that we measured using the ultrasonic approaches. The nano-indentation tests for Cr-DLC coatings were measured using a Hysitron Nanoscope nano-indenter with a Berkovich tip. The tests
Acknowledgments
This work was supported through a NIST Advanced Technology Program, cooperative agreement number 70NANBOH3048. The authors acknowledge Professor Peter Barna at Hungarian Academy of Sciences for the TEM image, and Dr. Mahmoud A. Taher of Caterpillar Inc. for helpful discussions about coating structures. The authors thank Bo He in Professor Qian Wang's group in the Department of Mechanical Engineering and Yanfeng Chen in Professor Y.W. Chung's group in the Department of Materials Science and
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