D. Galvan

Breakdown of the Coulomb friction law in TiC/a-C:H nanocomposite coatings

Advanced TiC∕a‐C:H nanocomposite coatings have been produced via reactive deposition in a closed-field unbalanced magnetron sputtering system (Hauzer HTC-1000 or HTC 1200). In this paper, we report on the tribological behavior of TiC∕a‐C:H nanocomposite coatings in which ultralow friction is tailored with superior wear resistance, two properties often difficult to achieve simultaneously. Tribotests have been performed at room temperature with a ball-on-disk configuration. In situ monitoring of the wear depth of the coated disk together with the wear height of the ball counterpart at nanometer scale reveals that the self-lubricating effects are induced by the formation of transfer films on the surface of the ball counterpart. A remarkable finding is a breakdown of the Coulomb friction law in the TiC∕a‐C:H nanocomposite coatings. In addition, the coefficient of friction of TiC∕a‐C:H nanocomposite coatings decreases with decreasing relative humidity. A superior wear resistance of the coated disk at a level o...

Microstructure and Properties of TiB/Ti-6Al-4V Coatings Produced With Laser Treatments

TiB/Ti-6Al-4V metal-matrix composite (MMC) layers were produced on Ti-6Al-4V substrates by laser cladding. A TiB2/Ti powder mixture was used as a precursor to obtain a dispersion of TiB needles in the Ti alloy matrix, with the aid of an exothermic reaction between TiB2 and Ti. A eutectic microstructure was obtained that consisted of an extremely homogeneous dispersion of TiB eutectic needles in the Ti alloy matrix, having a volume fraction as high as 0.33. Also, an equilibrium-like microstructure was found, consisting of a dispersion of both primary and eutectic TiB needles inside the Ti alloy matrix. An analysis of the geometry of the layers was performed and proved successful in determining the percentage of B. Further, it correctly predicted the variation of atomic B content as a function of laser power. The relative wear resistance coefficient, defined as the wear coefficient of the uncoated matrix divided by that of coating, shows an improvement by a factor as high as 1500 for the eutectic microstructure.

Tribological behavior and thermal stability of TiC∕a-C:H nanocomposite coatings

Advanced TiC∕a-C:H nanocomposite coatings have been produced via reactive deposition in a closed-field unbalanced magnetron sputtering system (Hauzer HTC-1200). These wear-resistant coatings are targeted for automotive applications where high load-bearing capacity and thermal stability, low friction, and wear resistance are the primary requirements. In this article the tribological behavior of the nanocomposite coatings is scrutinized by means of ball-on-disk tribotests at elevated temperature or after annealing in the temperature range of 150–400°C. The thermal stability of the coatings in terms of critical temperatures, at which the degradation of wear resistance and friction of the coatings starts, is monitored with depth profiling of oxygen content using Auger spectroscopy in conjunction with detailed examinations of the mechanical properties of the annealed coatings. A striking result is that a coating thermally stable up to 350°C may fail at much lower temperatures during elevated-temperature tribot...

Reactive magnetron sputtering deposition and columnar growth of nc‐TiC∕a‐C:H nanocomposite coatings

TiC∕a‐C:H coatings were deposited using closed-field unbalanced reactive magnetron sputtering deposition from Ti targets. Different acetylene gas flow and substrate bias values were employed to vary the coatings’ compositions and microstructures. The application of an external negative substrate bias increased the deposition rate of the plasma-enhanced chemical vapor deposition process from the reactive atmosphere. The sputtering of carbonaceous species from the poisoned targets surface was a minor source of C flux to the substrates. The application of an external substrate bias during deposition yielded films with a featureless cross-sectional structure, but the dominant parameter controlling the columnar growth was the chemical composition of the films. An explanation of this effect was formulated based on the microstructures observed and the deposition technique employed.

TiC/a-C Nanocomposite Coatings for Low Friction and Wear Resistance

TiC/a-C:H nanocomposite coatings have been deposited by magnetron sputtering and are composed of 2-5nm TiC nanocrystallites well separated by amorphous hydrocarbon (a-C:H) of about 2nm separation width. A transition from columnar to glassy microstructure has been observed with increasing substrate bias or carbon content. Micro-cracks induced by nanoindentation or wear tests readily propagate through the column boundaries whereas the coatings without a columnar microstructure show supertough behavior. The nanocomposite coatings exhibit hardness of 5~20 GPa, superior wear resistance and strong self-lubrication effects with a friction coefficient of 0.05 in air and 0.01 in nitrogen under dry sliding against uncoated bearing steel balls. Especially, the transitions from low to ultralow friction or the reverse are repeatedly switchable if the atmosphere is cycled between ambient air and nitrogen. The lowest wear rate is obtained at high humidity.

TEM Characterization of W-O-N Coatings

Recently, a new class of coatings based on oxynitrides has drawn much attention in the research field as well as in industrial applications, as shown by either the large numbers of recent publications on TM O N systems (TM—transition metal) such as Ti-O-N, Zr-O-N and Ta-O-N, or the development of Si O-N for opto-electronic devices. The properties of these coatings are related to the chemical composition and the structural arrangement. However, the knowledge about the structure of TM-O-N systems is very limited, especially how the structural arrangement of the non-metallic elements is in the lattices. To the best of our knowledge, only a few studies exist on the development of structural models for oxynitrides, based on XRD and/or XPS analysis, as e.g. Si-O-N and Ti-O-N, or on Mossbauer spectrometry for Fe-O-N. TEM was used scarcely for the characterization of TM-O-N coatings possibly due to the damage of the structure by the electron-irradiation as it is reported for Cr-O-N. This work is aimed at the crystallographic understanding of W-O-N sputtered films by using TEM and HR TEM techniques for complementing the information provided by XRD characterization.

Self-Lubrication and Wear Behavior of TiC/a-C:H Nanocomposite Coatings

Advanced TiC/a-C:H nanocomposite coatings have been produced via reactive deposition in a closed-field unbalanced magnetron sputtering system. In this paper, we report on the tribological behavior of TiC/a-C:H nanocomposite coatings in which ultra-low friction is tailored with superior wear resistance, being two properties often difficult to achieve simultaneously. In-situ monitoring the wear depth at nanometer scale reveals that the self-lubricating effects are induced by the formation of transfer films on the surface of ball counterpart. In addition, the CoF of TiC/a-C:H nanocomposite coatings decreases with decreasing relative humidity. This phenomenon can be interpreted in terms of water molecule interactions with the wear track. The influence of the volume fraction and grain size of TiC nanocrystallites on the coating properties has been examined. A superior wear resistance at a level of 10-17 m³/(N m lap) has been achieved under the condition of super-low friction and high toughness, both of which require fine TiC nano-particles (e.g. 2 nm) and a wide matrix separation that must be comparable to the dimensions of the nano-particles.

Nanocomposite TiC/a-C coatings : Structure and properties

This contribution deals with fundamental and applied concepts in nano-structured coatings, in particular focusing on the characterization with high-resolution (transmission) electron microscopy. Both unbalanced and balanced magnetron sputtering systems were used to deposit nc-TiC/a-C nanocomposite coatings with hydrogenated or hydrogen-free amorphous carbon (a-C) matrix, respectively. The contents of Ti and C in the coatings have been varied over a wide range (7 similar to 45 at.% Ti) by changing the flow rate of acetylene gas or the locations of substrates relative to the center of C/TiC targets. Different levels of bias voltage and deposition pressure were used to control the nanostructure. The nanocomposite coatings exhibit hardness of 5 similar to 35 GPa, hardness/E-modulus ratio up to 0.15, wear rate of 4.8 x 10(-17) m(3)/Nm, coefficient of friction of 0.04 under dry sliding and strong self-lubrication effects. The nanostructure and elemental distribution in the coatings have been characterized with cross-sectional and planar high-resolution transmission electron microscopy (HRTEM) and energy filtered TEM. The influences of the volume fraction and size distribution of nanocrystallites TiC (nc-TiC) on the coating properties were examined.

Electron microscopy characterization of W-O multilayers

In the decorative field, besides the optical characteristics needed for achieving desired colours, the material should have suitable properties able to assure the daily use of the component without significant degradation. Oxides are well known materials with a high wear and corrosion resistance which make them a preferential choice of coatings for environmental protection. However, the intrinsic colours possible to be achieved in a single oxide system are very limited. For example in W-O system, besides the different tonalities of grey, only the dark blue can be obtained for slightly substochiometric WO3 compound. The pallet of colours exhibited by WO3 coatings is due to interferometric effects. Recently, it was shown that playing with the gradient of the chemical composition of W-O layers it was possible to tune the global colour of the coatings, which made this system an attractive candidate for decorative applications.

The Influence of Chemical Alloying on the High Temperature Wear Resistance of H-Free DLC Coatings

A commercial RF-sputtering deposition rig was employed to deposit H-free diamond-like carbon (DLC) coatings. The influence of alloying elements such as Ti and Si on the structure, mechanical and tribological properties of the coatings was investigated. The coating was observed in cross section and in plan view with SEM, TEM and AFM. Because of the highly-ionized plasma generated by the RF-powered glow discharge, ion bombardment suppresses the formation of a columnar structure regardless of the composition of the coatings. The method produces featureless microstructures and smooth surfaces. TEM investigations confirm that no crystalline phases form in the coatings regardless of the presence of considerable concentrations of Ti and Si. Tribological tests were performed with a high-temperature tribometer in a ball-on-disk configuration, using coated disks and different materials for the ball countepart. At ambient T the sliding friction coefficient decreases as the concentration of alloying elements increases. Nevertheless, high-T tribotests with a constant thermal load showed that the presence of alloying elements decreases the thermal stability of the coatings. For each coating a temperature exists above which a sudden increase of friction coefficient is observed, with subsequent detachment and failure of the coatings. The mechanism of disruption of the self-lubrication effects is identified and the influence of the alloying elements on the thermal degradation of tribological performance of the coatings is discussed.