ICMCTF2015 Session B4-3: Properties and Characterization of Hard Coatings and Surfaces

The unique properties of titanium carbide (TiC), such as high hardness and elastic recovery, high corrosion resistance at elevated temperatures and high electrical conductivity make TiC very attractive as a protective coating for nuclear, aerospace and tool steel applications. In this study, coatings were prepared by Radio-Frequency Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) onto silicon and Ti-6Al-4V alloy substrates using titanium tetrachloride (TiCl₄) and acetylene (C₂H₂) in argon-hydrogen mixtures. The deposition was performed at a substrate temperature of 400° C and an RF self-bias voltage of -400 V while studying the effect of the H₂ concentration and the C₂H₂:TiCl₄ ratio on the coating structure and mechanical properties. The controlled parameters were found to affect the phase formation, grain size and orientation, as well as hardness H, Young's modulus E, internal stress σ, wear and adhesion. In order to improve the adhesion to the Ti-6Al-4V substrates, nitriding or carburizing was performed prior to the deposition. XRD revealed that the grain size varied between 6 and 12 nm for different H₂ and C₂H₂ concentrations. The high hardness of 32-35 Gpa as well as the highest H/E, and H³/E² ratios were correlated either with the smallest grain size and (111/200) peak ratio, or to the highest H₂ concentration (67 vol.%) in the gas feed. Alternatively, the coating with the maximum H (36 Gpa), H/E, and H³/E² was obtained with lower H₂ (~20 vol.%) and low C₂H₂ concentrations (~1 vol.%). The role of hydrogen and acetylene in the gas feed as dominant parameters affecting the structure and properties of the produced coatings is presented and discussed.

The scratch test on boriding layers was analyzed. Experiments tests and numerical simulations by finite element method of the scratch test were development on a FeB/Fe2B coating. The boride layers were formed at the surface of AISI 304 steels by developing the powder-pack boriding process at temperature of 1223 K with 2, 6 and 10 h of exposure times. From the set of experimental conditions of boriding process, scratch test were performed with linearly-increasing load mode of 1 to 90 N on 7 mm in length to determinate the most effective and informative testing conditions and to determine the critical load (Lc) for coating failure. The damage in the coating was examined by high resolution SEM. Experiments tests indicated that at a critical load the coating fails through brittle fracture. Numerical calculations considering the residual stress field generated by the scratch load showed that at this load the tensile stresses inside the coating become large enough to cause brittle failure. The residual stress field generated by the scratch load was analyzed and related with the failure mechanics observed by the experimental test.

We have proposed an atmospheric controlled induction-heating fine particle peening (AIH -FPP) system which creates thick and stable modified layers with element of shot particles at the surface of steel. The aim of this study is to create a Ti-Al intermetallic compound at the carbon steel surface by AIH-FPP treatment. AIH-FPP treatments were performed on the carbon steel surface with Ti and Al mixed particles at 900 ℃ for 10 seconds. The particles were prepared by means of a planetary mill using isopropanol as a process control agent. The treated surfaces were analyzed using a scanning electron microscope, an energy dispersive X-ray spectrometer, and an X-ray diffractometer (XRD). The result of XRD analysis showed the generation of Ti-Al intermetallic compound at the treated surface. The Vickers hardness test results also showed the hardened modified layer comparable to Ti-Al intermetallic compound. Consequently, the AIH-FPP treatment can create the Ti-Al intermetallic compound at the treated surface within a short amount of time.

In recent years, post-deposition treatments of hard coatings for cutting tools have gained increasing interest owing to their high potential to further improve coating properties and thus, the tool performance. The aim of the present work is to give an overview on post-treatment methods and their effect on the coating properties. In general, mechanical and thermal methods can be distinguished, where mechanical treatments enable to enhance surface topography and to tailor residual stresses. Mechanical post-treatments can be e.g. polishing or brushing with abrasive media or blasting processes, where the coating is bombarded by granulate material. This can be done in dry or wet environment. Several blasting treatments and their effect on different coating materials grown by chemical and physical vapor deposition are discussed. Thermal post-treatments can be used to improve the microstructure and to modify the surface. Exemplarily, the spinodal decomposition and thus, age-hardening of metastable Ti_1-xAl_xN at elevated temperatures and annealing of TiAlTaN coated samples in methane to reduce friction between work-piece and coating is discussed. In order to determine the influence of the respective treatment on the coating properties, sophisticated characterization methods like synchrotron X-ray nanodiffraction are of crucial importance.

We have fabricated Ta, Nb, Mo and W thin films on Si (100) p-type substrates with film thicknesses ranging from 20 to 500 nm using DC magnetron sputtering technique. Distance between substrates and the target during the depositions was 18 cm. Ar⁺ gas was used for generating plasma and its working pressure was set to 4÷5×10^-3 mbar. RF power was set to 150W, 200W, 150W, and 200W during W, Nb, Mo, and Ta film depositions, respectively.

Quartz crystal microbalance attachment, placed next to the substrates, was used at every deposition to determine the film densities. The change in mass on the surface of quartz crystal as it gets loaded by the deposited material was calculated from the difference in its natural oscillation frequency before and after film depositions. Film thicknesses were determined by cross-sectional SEM.

Young’s moduli of these “slow” refractory films on “fast” Si substrate were obtained by measuring ultrasonic phase velocity of nanosecond laser pulse-induced surface acoustic waves as a function of wave frequency (phonon dispersion), and by taking into account measured film thicknesses and mass densities.

Results show that for W, Ta and Mo films Young’s moduli increase and approach their bulk values with the growth of films thicknesses. The thinnest and the thickest Nb films have Young’s modulus values higher than the bulk values. For the thinnest Nb films this is probably due to the substrate effect on the measurements since Nb bulk Young’s modulus is lower than that of Si substrate (165.3 GPa). With the increase of Nb film thickness effect reduces. But for the thickest Nb film, its effect probably appears again due to buckling/delamination or cracking within the film, which can be caused by high intrinsic stress generation as the film thickness grows.

Crystal microstructure and orientation of the films were studied from θ-2θ scan X-ray diffraction analysis. XRD profiles show that for almost all samples peaks appear at slightly smaller angles than the nearest expected peaks should appear for each of the materials used. This is especially significant for the thickest films and originated from higher compressive stress.

For different refractory metals obtained results on measured densities is in agreement with their elemental masses, but differs for film and bulk Young moduli of Ta and Mo. There is an additional XRD peak for the thickest Ta film at lower angle, which is not associated with any of the Ta related peaks in JSPDS database, and might have originated from an unstable transition phase of Ta. Optimum thickness of refractory sputtered films can be reached based on the highest values of Young’s modulus.