Characterization of mechanical properties of tungsten carbide/carbon multilayers: Cross-sectional electron microscopy and nanoindentation observations

Multilayers of tungsten carbide/carbon (WC/C) deposited by physical vapor deposition onto steel substrates were subjected to depth-sensing indentation testing. The investigation aimed at probing the influence of dissimilarities between the microstructure of the multilayers and substrate on the system mechanical properties. The resultant load-displacement data were analyzed both by conventional load-displacement (P-δ) and load-displacement squared (P-δ^2) plots. Furthermore, it was demonstrated that the occurrence of annular through-thickness cracks around the indentation sites can be identified from the load-displacement curve. Also, analysis of the lower part of the unloading curve permitted us to identify whether the coating had popped up by localized fracture. The cracking mechanism was characterized using a new technique for cross-sectional electron microscopy of the nanoindentations. The information retrieved with this technique eliminates the problems, inherent in assessing at this small contact scales, whether the fracture is by coating decohesion or by interfacial failure. In our case, it was demonstrated that the failure mechanism was decohesion of the carbon lamellae within the multilayers. The mechanical properties (hardness and effective Young’s modulus) were also assessed by nanoindentation. The hysteresis loops were analyzed and discussed in terms of a method developed previously.

Microstructure of WC/C coatings deposited on steel substrates

Electron microscopy, including scanning (SEM), transmission (TEM) and high-resolution (HRTEM) were employed to characterise slightly different tungsten carbide/carbon coatings deposited onto steel substrates. Complementary techniques, such as X-ray diffraction (XRD), Auger electron spectroscopy (AES) and energy filtered TEM (GIF) were also used. The coatings were deposited by magnetron sputtering of pure WC and Cr targets in a plasma decomposition of C2H2 in mixed Ar-C2H2 discharges. The coatings are made up of a chromium interlayer, a coarse WC/C intermultilayer, a WC layer, and the WC/C multilayer. The chromium interlayer has a body centred cubic phase and a dense columnar structure, while the remaining coating is truly amorphous, with the exception of polycrystalline particles and clusters that are present within some layers. Crystalline particles and clusters were identified as having the cubic beta -WC1-x phase. Defects in the coatings were also found, due to substrate surface irregularities and to the growth structure of the chromium columns.</p

Microstructure investigation of magnetron sputtered WC/C coatings deposited on steel substrates

Electron microscopy, including scanning (SEM), transmission (TEM) and high-resolution (HRTEM) were employed to characterise slightly different tungsten carbide/carbon coatings deposited onto steel substrates. Complementary techniques, such as X-ray diffraction (XRD), Auger electron spectroscopy (AES) and energy filtered TEM (GIF) were also used. The coatings were deposited by magnetron sputtering of pure WC and Cr targets in a plasma decomposition of C2H2 in mixed Ar-C2H2 discharges. The coatings are made up of a chromium interlayer, a coarse WC/C intermultilayer, a WC layer, and the WC/C multilayer. The chromium interlayer has a body centred cubic phase and a dense columnar structure, while the remaining coating is truly amorphous, with the exception of polycrystalline particles and clusters that are present within some layers. Crystalline particles and clusters were identified as having the cubic β-WC1-x phase. Defects in the coatings were also found, due to substrate surface irregularities and to the growth structure of the chromium columns.