Violeta Merie

Experimental investigation by atomic force microscopy on mechanical and tribological properties of thin films

Abstract For this paper, a two part approach was taken to develop a fundamental understanding of the surface properties of four different hard thin films. On one front, atomic force microscopy was used to quantitatively measure both the adhesion and friction forces between the tip and sample surfaces. On the other front, the indentation technique was used to determine the mechanical properties of these materials (Youngs modulus and hardness). The main purpose of this study was to investigate different thin films deposited on silicon wafer substrate for improving the wear life and reducing the coefficient of friction. Nanomechanical and nanotribological characterization of thin films of chromium, nickel, platinum and titanium deposited on silicon were performed.

The influence of titanium dioxide on the tribological characteristics of a Fe-based friction composite material:

New Fe-Cu-graphite-Ni-TiO2 (2 to 8 wt% TiO2) composite materials are studied for friction applications. The tribological characteristics of these materials were monitored by a pin-on-disc method against a cast iron plate. An increase of the friction coefficient when the titanium dioxide content was increased up to 6 wt% was noticed. Instead, adding 8 wt% of titanium dioxide determined the decrease of friction coefficient. A gradual decrease of the wear rate was marked out when titanium dioxide content was increased. The best behavior determined by the optimal ratio friction coefficient/wear rate was the Fe-Cu-graphite-Ni-TiO2 composite containing 6 wt% titanium dioxide.

Analysis on temperature effect on the mechanical and tribological properties of titanium nitride thin films

The main goal of this paper is to study the influence of the temperature on the mechanical and tribological characteristics of titanium nitride thin films. The titanium nitride thin films were deposited by reactive magnetron sputtering on silicon substrates using a titanium high purity target. The films were deposited in different conditions. Several films were deposited on silicon substrate at room temperature while the others were obtained after the substrate was preheated. The majority of the films were deposited on non-biased substrates while the rest were deposited on substrates to which a negative bias was applied. Once the films were deposited, the characterization was realized by atomic force microscopy investigations determining the topographical parameters as well as the mechanical properties such as the modulus of elasticity and the hardness. The mechanical properties mentioned before were determined at 20 °C, 40 °C, 60 °C, 80 °C and 100 °C in order to establish the effect of testing temperature on the mechanical characteristics. The results highlighted a significant influence of temperature on the mechanical and tribological properties of the investigated titanium nitride thin films.

Re-sintering Influence on Tribological and Structural Characteristics of Iron-Based Composites for Automotive Applications

Iron-based sintered composites can be used for manufacturing different friction applications for the automotive industry (i.e. brake pads). The present research is a study concerning the influence of the re-sintering on the tribological and structural characteristics of three iron-based friction materials with application in vehicle brake systems. These composites were elaborated by powder metallurgy methods. The chemical composition of the materials contained iron, copper, graphite, nickel, titanium dioxide and/or barium carbonate. First, the three mixtures were homogenized. Green compacts were formed at a compaction pressure of 600 MPa. The sintering was done at 1050 °C, in vacuum for 30 min. The obtained samples were further re-sintered in vacuum, at 1050 °C for 20 min to improve their tribological characteristics. The elaborated materials were characterized by the tribological and structural point of view. The re-sintered samples presented, in general, a higher porosity than the sintered samples. An increase in the friction coefficient was marked out for all three re-sintered materials. Instead, a continuous increase or decrease in the wear rate was not observed. The material that contains titanium dioxide presented the best tribological properties in both sintered and re-sintered states.

Estimation of Tribological Properties for Fe-Cu-Graphite-Ni-TiO2 Composites for Automotive Industry

Automotive industry is constantly developing due to the use of new technologies and materials that are allowing the finding of novel solutions in order to solve existing problems. This study is focused on using a mathematical predictor called Kriging for the tribological properties of iron-based composites so as to estimate their values. First, experimental data regarding the tribological properties was obtained for four Fe-Cu-graphite-Ni-TiO2 composite materials. Based on this data the appropriate Kriging estimator is established and the values for the friction coefficient and wear rate are predicted at intermediary TiO2 contents. Therefore, without using expensive equipment and time-consuming experimental tests such as trial and error method, the tribological properties of iron-based composite materials can be estimated. Based on the predicted values and using an optimization algorithm, the material with the optimal tribological behavior can be determined in order to be used in applications for the automotive industry, such as the manufacturing of brake pads.

Analysis of the bending stiffness and adhesion effect in RF-MEMS structures

Microelectromechanical system (MEMS) is a special branch with a wide range of applications in sensing, switching and actuating devices. Designing the reliable MEMS for thin free-standing structures like as bridges and cantilevers requires understanding of the tribomechanical properties of the materials and structures. The effect of geometrical dimensions (cross-section dimensions and length) on mechanical and tribological behavior of free-standing MEMS structures made of electroplated gold was analyzed in this paper. Special attention was given to the dependences between stiffness and cantilever length and the dependences between bending stress and variable travel range of actuated load. The force position was moved from the beams free-end toward to the anchor. The tests were performed at room temperature (22°C) and relative humidity RH of 40% with a noise- and vibration-isolated and environment-controlled XE-70 AFM from Park Systems using the contact mode. Each measurement was repeated many times in order to improve the accuracy of the experimental results. The stiffness of a microcantilever varies if the position of the acting force is changed. The experimental results obtained were in good correlation with those obtained analytically.

EVALUATION OF THE MECHANICAL PROPERTIES AND SURFACE TOPOGRAPHY OF AS-RECEIVED, IMERSSED AND AS-RETRIEVED ORTHODONTIC ARCHWIRES

Background and aims This experimental study mainly aims at comparing the most important mechanical properties of the new orthodontic archwires, those immersed in fluorinated solution, the as-retrieved ones and the intra-oral used ones. Methods A total of 270 arch wires were tested, using tensile testing and three-point bending tests. The tested archwires were made of Stainless Steel, Nickel Titanium, Beta-Titanium and physiognomic covered Nickel Titanium. The tested archwires were subjected to three types of treatments: immersion into fluorinated solution, immersion into carbonated drinks and intra-oral use. Results The immersion caused variations of the activation and deactivation forces of all arch wires. The most affected arch wires, in terms of bending characteristics, were the intra-oral used ones. Conclusions The alteration of mechanical properties of the orthodontic arch wires by their immersion into fluorinated solutions and soft drinks could not be statistically demonstrated.

Substrate Influence on the Mechanical and Tribological Characteristics of Gold Thin Films for MEMS Applications

The development of micro-and nanoelectromechanical systems (MEMS/NEMS) makes use of different thin films such as aluminum, gold, silicon, silver, titanium nitride, silicon carbide etc. This study is a research concerning the influence of substrate nature on the tribological and mechanical characteristics of gold thin films elaborated by thermal evaporation method, for space applications. Three different substrates were employed, namely: C45 steel, plastic (polycarbonate) and glass. Atomic force microscopy investigations were performed in order to characterize the obtained thin films at nanoscale. The nanohardness, Young’s modulus, roughness and the friction force are some characteristics that were determined. A significant influence of substrate nature on both mechanical and tribological properties of researched gold thin films was marked out. Regarding the topography, the smallest roughness was determined on the gold thin films deposited on glass substrate.

The Influence of Barium Carbonate on the Tribological Characteristics of Some Iron-based Friction Composites

New iron-based composite materials with addition of barium carbonate (2 to 8 wt% barium carbonate) for friction applications are investigated. The tribological behavior of the studied materials was determined by a pin-on-disk method when a cast iron disk was employed. The addition of 2 wt% barium carbonate determined a significant increase of the average friction coefficient. Instead a further increase of barium carbonate content determined a gradual decrease of this parameter. The improvement of wear resistance was marked out for a barium carbonate content of up to 6 wt%. The optimal ratio between the average friction coefficient and the wear rate for the researched Fe-Cu-graphite-Ni-BaCO3 composites was determined for the iron-based material containing 2 wt% barium carbonate. SEM, EDX and XRD analysis marked out a complex structure containing alloyed ferrite, pearlite and carbides, traces of nickel and barium carbonate and free graphite.

Tribological and micro/nano-structural characterization of some Fe-based sintered composites

Abstract New iron-based composites were investigated for applications within the friction materials domain. This study presents the elaboration and the structural and tribological characterization of iron-based friction composites containing iron, copper, graphite and/or nickel, introduced in a powder state. The technology employed for obtaining these materials is via a classical powder metallurgy route. The porosity of the studied materials, the tribological characterization, the roughness of researched composites and their structure are marked out. Results showed that the samples elaborated from the material with 10 wt.% Cu, 7 wt.% graphite, 12 wt.% Ni and the rest iron, compacted at 600 MPa, presented the best tribological behavior. The presence of pearlite, ferrite, nickel-based and copper-based solid solutions and free graphite in composite structures is highlighted.