ICMCTF2011 Session EP: Symposium E Poster Session

Silver oxovanadate (Ag₃VO₄) powder was synthesized by a simple wet chemical route. The synthesized Ag₃VO₄ precipitate resulted from the reduction of Ag₂VO₅ in solution phase. Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) were used to evaluate the thermal and structural properties of the precipitant, respectively. The silver oxovanadate was then subsequently investigated as a potential solid lubricant by burnishing it onto textured VN. The textured vanadium nitride (VN) coatings were created by depositing VN on an Inconel substrate by unbalanced magnetron sputtering and then etched; using reactive ion etching; through a mask to create a periodic array of micro-sized dimples on the surface. The effectiveness of this new design was tribologically tested against Si₃N₄ balls at different temperatures. A significant decrease in the wear rate and coefficient of friction was achieved and was maintained due to the micro-sized depressions on the surface acting as reservoirs to replenish Ag₃VO₄ to the sliding contact throughout the tribological tests. After wear testing, Raman spectroscopy and XRD was performed to identify the phase composition that was tribochemically formed at the surface.

Using the plasma-detonation technology, coatings of 80 to 150 µm thickness on a steel 3 substrate were fabricated. To improve properties of these coatings and decrease their porosity, we repeated a plasma-jet melting, which melted the layer up to 45 to 50 µm. A plasma-detonation gun “Impulse- 6” with 0.3 msec pulse duration and 10⁶Wt/cm²power flux was employed. We used a powder fraction of 28 to 43 µm size, the Russia standard (Ni was the base, Cr was 10 to 14 wt.%, B was 2.9 to 3.2; Si = 2,5, Fe was to 6wt.%). Samples were 10 x 20 mm and reached 4 mm thickness. After deposition they were cut for analysis and corrosion wear and nano- and microhardness tests. After deposition, we found a nanocrystalline γ-phase based on Ni and a micro-crystalline phase based on CrN3 (of 50 to 150nm size) in the coating. Regions with nanograins having different crystallographic orientations were found in the nanocrystalline phase.

Using HRTEM and XRD analysis, we investigated the phase composition and grains sizes of these phases. RBS and EDS were employed to perform an element analysis both over the coating depth and over transversal cross-section.

It was demonstrated that doping of the coating, which was realized in the process of deposition, reached the whole coating depth (though we observed also the regions without Mo – TEM analysis). After melting, the coating nano- and microhardness was 8.1GPa. Before irradiation, it was 6.8 GPa. The wear resistance increased almost by a factor of three, in comparison with the substrate and by a factor of 1.7 in comparison with the non-melted Ni-Cr-B-Si-Fe coating. The mass-transfer processes and redistribution of impurities in the coating due to the repeated melting were investigated.

DLC films are meta-stable amorphous films that exhibit unique combinations of properties such as high hardness, low friction coefficient, and good wear resistance, etc. However, DLC films have several known limitations, such as high internal stress, low thermal stability. In order to solve these problems, enhance the film properties, Si has been incorporated into the amorphous hydrocarbon films. Our study aimed to study the effects of silicon and oxygen incorporation on the tribological properties and thermal stability of C-Si-O composite thin films deposited by PBII method.

The films were deposited by PBII method with gaseous mixtures of C₂H₂:TMS:O₂ on Si (100) wafers. The flow rate ratio range between C₂H₂ and TMS were from 10:1 to 100:1, while oxygen was kept constant at 1 sccm. The deposition pressure range was from 2-6 Pa. The bias voltage was set 0 kV, at RF power of 300 W. The total deposited thickness of the films was approximately 500 nm. An annealing temperature range of 200-600°C was investigated under high vacuum, air atmosphere and argon atmosphere for 1 hour. The film structure was analyzed using Raman spectroscopy. The hardness and elastic modulus were measured by a nano-indentation hardness tester. The tribological properties were measured using a ball-on-disk friction tester under air condition. From the results deposited at 2 Pa pressure, hardness and elastic modulus of films decreases with silicon incorporation, believed to be partly due to the changes in the microstructure, indicates that increasing graphite dominance, as could be concluded from Raman analysis. For annealed films under high vacuum, results also decreases with silicon incorporation. The films at flow rate ratio of 10:1 are stable and low friction coefficient of 0.04, due to high hardness and elastic modulus.

Amorphous hydrogenated carbon thin films were deposited by reactive magnetron sputtering, using a CemeCon industrial scale unit. The films were deposited from two carbon targets in a mixed argon and acetylene atmosphere. In order to investigate the influence of hydrogen on the mechanical properties and structure of the films the acetylene flow rate was varied. Furthermore the effect of substrate bias voltage was investigated. The hardness and elastic modulus of the coatings were measured by nanoindentation, and were in the range from 2 to 21 GPa and 30 to 215 GPa respectively. Raman analysis indicated that a optimum in hardness occurred when the highest structural disorder in the sp² matrix was present. This disorder probably originates from C-C sp³ sites distorting the matrix. A combination of nanoindentation and Raman spectroscopy was used to calculate the compressive stress in the films, and showed that the hardest films were the most stressed, with compressive stress values peaking at 5.5 GPa. Post-deposition annealing at 430°C of the films showed a significant reduction in the stress from 5.5 GPa to 1 GPa, while almost no loss in hardness was observed. The stress reduction was attributed to a relaxation of the film, where bond angles and bond lengths change to a more stable configuration. The stress reduction was thus not caused by a graphitization, where C-C sp³ sites are converted into C-C sp². Also, the relaxation was not attributed to hydrogen emission from the films, since NRA measurements revealed an almost constant hydrogen content of the annealed films.

Nano-impact test on PVD coatings is an efficient method for investigating film failure mechanisms. During this test, the coating is subjected to repetitive impacts by a diamond indenter, inducing high local deformations and stresses into the film material, which may lead to coating failure.

In the paper, coated specimens with a TiAlN PVD film were investigated by nano-impact tests. The nano-impacts were conducted at several loads and for various test durations. For explaining the attained results, the nano-impact test was simulated by a developed three dimensional FEM model, considering a piecewise linear plasticity material law. The film elasto-plastic properties, used in the FEM-calculations, were determined by nanoindentations and analytical evaluation of the related results. During the nano-impact indenter penetration, it was assumed that the coating material at the FEM model node regions can withstand the applied load up to a maximum value, which corresponds to the coating rupture stress. Over this load limit, the related nodes are disconnected from the neighboring finite elements. If all nodes of an element are disconnected, the element is released for simulating a crack formation and it becomes an inactive separate entity. In this way, the stress fields developed in the film material and its coating fracture progress in terms of imprint depth versus the repetitive indenter penetrations are analytically described. The attained results converge sufficiently with the experimental ones. The developed nano-impact FEM-simulation predicts the film failure initiation and evolution, which depend on the impact load.

The research efforts on Diamond-Like Carbon (DLC) films have strongly increased over the last decade. Those coatings exhibit a wide range of attractive properties such as a low friction coefficent, a high wear resistance as well as good thermal and chemical stabilities. However, the main issues in tribological applications are related to smoothness, heat transfer and abrasion resistance especially under severe conditions like in an automotive engine.

For that reason, we have investigated amorphous hydrogenated silicon-doped carbon (a-C:H:Si) films deposited on various substrate materials. The DLC films were grown by using a combined PVD and Plasma Enhanced Chemical Vapor Deposition (PECVD) technique with the help of the LARC^® technology.

The adhesion strategy, the transition layer as well as the film structure have been tuned to match the application requirements on various components like lifters and valves. The tribological properties were investigated by a pin-on-disc test at room temperature and up to 400°C. Hardness and elastic modulus were measured and considered as an additional reference in terms of coating quality. Finally, results of the spintron and dyno testing will be detailed for the mentioned applications.

Fluorinated and chlorinated amorphous hydrogenated carbon films also containing silicon were produced by plasma enhanced chemical vapor deposition from hexamethyldisiloxane-argon mixtures together with either sulfur hexafluoride or chloroform. Chemical structure and composition were examined using infrared reflection-absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). Infrared spectra show the presence of silicon-containing functionalities and XPS allows quantification of the degree of halogenation of the films. Thus correlations may be made between surface contact angles, measured using a goniometer, film composition, and surface roughness obtained via atomic force microscopy. Nano-indentation was used to investigate the nano-hardness, elastic modulus and stiffness of the films as a function of the degree of fluorination or chlorination.