J.-P. Celis

Oxidational wear of TiN coatings on tool steel and nitrided tool steel in unlubricated fretting

The fretting wear behaviour of PVD TiN coatings against corundum has been investigated in unlubricated contacts. Analysis of the debris indicated that titanium oxides of different composition are being generated. The volumetric wear on the TiN coatings is shown to be directly proportional to the dissipated friction energy measured on-line during the fretting tests. Furthermore, the effect of oxide debris present in the vibrating contact on the velocity accommodation and, consequently, on the coefficient of friction is discussed. The hardness of some of the ASP 23 tool steel substrates was increased with a plasma nitriding treatment prior to the coating deposition. It was found that the coefficient of friction and the fretting wear rate of the TiN coatings remain unaltered by the plasma nitriding treatment. After perforation of the TiN coating, however, wear proceeds faster on nitrided tool steel substrates.

Sol-gel preparation of high-Tc Bi-Ca-Sr-Cu-O and Y-Ba-Ca-O superconductors

High‐Tc superconducting oxides have been prepared by a liquid‐mix technique using etylene‐diamine‐tetra‐acetic acid (EDTA) as a complexing agent. Bi‐Sr‐Ca‐Cu oxides and Y‐Ba‐Cu oxides were made using this technique giving a better compositional homogeneity, a more precise control of the cation stoichiometry and a decreased firing temperature as compared to conventional material produced by a solid‐state reaction. This technique extends the amorphous citrate process to those systems where no citrate complex exists. EDTA binds with most metallic elements of the periodic table, making this technique a versatile tool in the production and study of these new ceramic materials. Therefore, the method is easily adapted to the preparation of new superconducting oxides.

Sliding wear behavior of thermally sprayed 75/25 Cr3C2/NiCr wear resistant coatings

Abstract One of the foremost coating methods for combating wear is thermal spraying, however, despite its widespread industrial use, little is known about the basic friction behavior and the mechanisms by which such coatings wear. The manner of processing in thermal spraying inevitably leads to inhomogeneities, such as unmelted particles, oxide inclusions and porosity, in the sprayed deposits resulting in a structure markedly different from that of cast, wrought or even powder metallurgy materials. It is expected that thermal spray coating behavior would be even more complicated than that of homogeneous, uncoated bulk materials. Results of an investigation to determine the effects of some wear test variables on high velocity oxy-fuel (HVOF) sprayed Cr 3 C 2 NiCr coatings using a pin-on-disk tribometer are presented. Room temperature sliding friction and wear behavior of coatings are discussed with respect to load, relative velocity and counterbody material. The present investigation shows that the tribological behavior of HVOF sprayed Cr 3 C 2 NiCr coatings is significantly affected by its microstructural constituents such as splats, porosity and form and dispersion of second phases. It is also shown that changes in imposed sliding wear test conditions varied the friction and wear behavior of thermally sprayed coatings considerably. The break-in sliding coefficient of friction is found to be more significantly affected by load than other test parameters. Results also indicate that friction decreased with increasing velocity but wear decreased then increased with increasing velocity. By proper control of test conditions and by selected changes of those conditions, the physical wear mechanisms involved in thermally sprayed coatings could be understood. This study showed that the pin-on-disk is a well controlled test and can be used to understand certain basic relationships between the sliding friction and wear behavior of thermally sprayed coatings.

Do oral biofilms influence the wear and corrosion behavior of titanium

The main aim of this work was to study the simultaneous wear-corrosion of titanium (Ti) in the presence of biofilms composed of Streptococcus mutans and Candida albicans. Both organisms were separately grown in specific growth media, and then mixed in a medium supplemented with a high sucrose concentration. Corrosion and tribocorrosion tests were performed after 48 h and 216 h of biofilm growth. Electrochemical corrosion tests indicated a decrease in the corrosion resistance of Ti in the presence of the biofilms although the TiO2 film presented the characteristics of a compact oxide film. While the open circuit potential of Ti indicated a tendency to corrosion in the presence of the biofilms, tribocorrosion tests revealed a low friction on biofilm covered Ti. The properties of the biofilms were similar to those of the lubricant agents used to decrease the wear rate of materials. However, the pH-lowering promoted by microbial species, can lead to corrosion of Ti-based oral rehabilitation systems.

Effect of crystallographic orientation on fretting wear behaviour of MoSx coatings in dry and humid air

The tribological behaviour of MoS2 coatings in air of high humidity is critical for their application in air. An improved friction and wear resistance of sulfur-deficient MoSx coatings in humid air is reported based on fretting wear tests. For random-oriented MoSx coatings, the coefficient of friction and the wear volume are still significantly dependent on the relative humidity. On the contrary, basal-oriented MoSx coatings show an interesting low sensitivity to humidity. MoO3 and SO42− have been detected in the wear track and the debris. The amount of MoO3 and SO42− is related to the crystallographic orientation of the coatings and the relative humidity in the fretting wear tests. However, the structure and grain size of the MoO3 debris are not dependent on the relative humidity and the crystal orientation of the coatings. The effect of humidity on the friction and wear of these sulfur-deficient MoSx coatings with different crystallographic orientations is discussed.

Electrodeposition of magnetic multilayers

Abstract In this contribution, results obtained in a BRITE–EURAM project on the electrodeposition of composition-modulated alloys (CMM) are presented. Electrodeposited Co-rich Co–Pt–P, natural, very thin multilayers are studied as possible substitutes of Co-based sputtered magnetic recording media, for their easy production, high magnetisation and coercivity. The effects of hypophosphite addition to the electrolyte on its electrochemical behaviour and on the magnetic characteristics of the films are reported. Magnetic coercivity is strongly increased when hypophosphite is added, but the improvements are not simply related to the P content of the alloy. Co–Pt–P films maintain a high coercivity (>2xa0kOe) even at high thicknesses, showing the possibility of their application in microelectromechanical structures. The interaction occurring between Co and Pt is shown by ESCA measurements of the density of states in the valence-band region. The stability of the CMM structure is studied for multilayers of Cu and Ni as a function of time and temperature from the saturation magnetic moment change, determining the diffusion coefficient variation with temperature for the formation of non-magnetic interfaces.

Role of overpotential on texture, morphology and ductility of electrodeposited copper foils for printed-circuit board applications

A study was made to correlate different electrodeposition parameters, like, e.g., cathodic overpotential, bath composition, and bath ageing, with characteristics as crystallographic texture and roughness, and the ductility of electrolytic copper foils. Copper foils with a low (220) preferred crystallographic orientation, and a smooth surface are obtained when depositing at 87 to 113 mV cathodic overpotential from copper sulfate solutions with a low chloride content. Under these plating conditions copper foils with the highest ductility were achieved

The tribochemical behaviour of TiN-coatings during fretting wear

Abstract To reveal the tribochemical reactions during fretting wear of TiN-coatings sliding against corundum, the microstructure and the chemical composition of debris have been investigated by analytical TEM. Novel observations on fretting wear debris are presented. These observations were possible thanks to a TEM-sample preparation procedure by ultramicrotomy. Based on friction force measurements, the tribological life of the TiN-corundum tribo-pair, was divided in five periods, namely: a running-period, a first transition, an intermediate steady state, a second transition, and a final steady state. Fretting experiments were stopped at different periods after which the debris were collected and characterised. The structure of fretting wear debris appeared to be dependent upon the moment of formation. The collected debris consisted of either amorphous or nanocrystalline rutile-like material. For comparison, fretting tests were also performed on pure titanium sliding against corundum. The debris originating from titanium consisted of nanocrystalline rutile. The observed difference in microstructure between the debris from TiN and pure titanium is mainly due to differences in the tribochemical reactions occurring during fretting wear. The evolution from amorphous to nanocrystalline debris on TiN, is proposed to be due to the combined effect of relative humidity, energy input, and continuous oxidation.

Fretting wear of selected ceramics and cermets

Abstract Fretting experiments were performed on sialon ceramic, zirconia-toughened alumina ceramic, titanium carbonitride cermet and tungsten carbide-cobalt cemented carbide materials to determine their relative wear performance when tested against Cr-steel counterbodies. All of the materials, with the exception of the titanium carbonitride cermet, display fretting steady state coefficient of friction values similar to that of steel on steel. The wear behavior of the sialon/steel couple appears to involve transfer layers, indicating that tribochemical interactions play an important role in this case. Both the zirconia-toughened alumina/steel and tungsten carbide-cobalt cemented carbide/steel couples undergo abrasive wear during fretting testing, though the tungsten carbide-cobalt cemented carbide is worn to a lesser degree than the zirconia-toughened alumina. The titanium carbonitride cermet/steel couple, on the other hand, undergoes a special abrasive wear mechanism in which hard particles from the cermet embed themselves in the soft steel, where they preferentially abrade the cermet while protecting the steel.

Development of low friction wear-resistant coatings

Abstract Research has been performed to design a multifunctional coating combining TiN with MoS x resulting in a wear-resistant coating with a low coefficient of friction (COF). The coatings are deposited by a combined physical vapour deposition technique of E-gun evaporation and magnetron sputtering in a BAI640 equipment. Pin-on-disk (POD) measurements at approximately 50% RH against a corundum (Al 2 O 3 ) ball show a significant decrease in COF compared with the results for a standard TiN coating. The incorporation of a soft phase in the TiN matrix did not lead to a decrease in hardness. The combination of a low COF and high hardness results in a decreased wear rate approximately 20 times lower in POD and 500 times lower in fretting compared to the standard TiN coating. The performance of the composite coating is tested in a drilling application and shows an increased lifetime.