Marko Sedlaček

Use of the Roughness Parameters Ssk and Sku to Control Friction—A Method for Designing Surface Texturing

ABSTRACT The aim of this work was to show that with the use of the surface roughness parameters Ssk and Sku we can predict tribological behavior of contact surfaces and use these parameters to plan surface texturing. This article presents a continuation of our research on virtual texturing and experimental work on surface textures in the form of channels. For this investigation, steel samples were laser surface textured in the shape of dimples with different spacings between the dimples and different dimple depths. The experimental results confirmed that the parameters Ssk and Sku can be used to design the surface texturing, where a higher value of Sku and more negative Ssk lead to lower friction.

Validation of linear damage rules using random loading

Fatigue damage is a parameter which plays an important role in lifetime and reliability predictions of randomly loaded structures. In this paper, four different forms of the Miner linear accumulation damage rule are examined. They are used to compare observed experimental damage and the results of numerical simulations for both uniaxial and multiaxial fatigue. First, critical fatigue damage values for all forms of the Miner accumulation damage rule are calculated for uniaxial cases. Based on the results presented in this paper, it has been established that for high strength materials the critical damage value is around 0.3. These results are then applied to the multiaxial cases. The multiaxiality is taken into account using either the signed von Mises or critical plane method. Finally, the fatigue fracture surface has been calculated and compared to the experimental pattern of fatigue fracture. The results show that the fatigue fracture surface determined by the numerical simulation is comparable to the fatigue fracture observed experimentally.

Analysis of Factors Influencing Measurement Accuracy of Al Alloy Tensile Test Results

Abstract In order to properly use materials in design, a complete understanding of and information on their mechanical properties, such as yield and ultimate tensile strength must be obtained. Furthermore, as the design of automotive parts is constantly pushed toward higher limits, excessive measuring uncertainty can lead to unexpected premature failure of the component, thus requiring reliable determination of material properties with low uncertainty. The aim of the present work was to evaluate the effect of different metrology factors, including the number of tested samples, specimens machining and surface quality, specimens input diameter, type of testing and human error on the tensile test results and measurement uncertainty when performed on 2xxx series Al alloy. Results show that the most significant contribution to measurement uncertainty comes from the number of samples tested, which can even exceed 1 %. Furthermore, moving from experimental laboratory conditions to very intense industrial environment further amplifies measurement uncertainty, where even if using automated systems human error cannot be neglected.

Influence of Surface Texturing Sequence on Fatigue Life and Tribological Properties of Coated Tool Steel

The aim of this study was to investigate the influence of laser surface texturing on fatigue life and tribological properties. Textures were introduced before and after hard TiAlN coating deposition in order to investigate the sequence of surface texturing process. It was found that the sequence of surface texturing has an effect on tribological and fatigue life behaviour. If laser texturing is done after coating deposition, friction is lower because of the better wettability of steel in comparison with coating, but suffers in fatigue life properties.

Microstructure and Heat Treatment of Hot Work Tool Steel: Influence on Mechanical Properties and Wear Behaviour

Good mechanical and wear properties of hot-work tool steels are needed for tools to withstand severe service conditions during their operational lifetime. Thus, the aim of this investigation was to correlate mechanical and wear properties with changes in microstructure of commercially available hot work tool steel Sitherm S361R. Hardness, impact toughness, tensile strength and wear tests were performed. Hot-work tool steel was heat treated at austenitizing temperature 1030 °C for 15 min in a horizontal vacuum furnace and gas quenched using nitrogen. One set of samples was investigated in as quenched state. Double tempering of samples was performed after quenching for 2 h at each of chosen temperatures, with first tempering temperature of 500 °C for the whole set of tempered samples. The second tempering was conducted at temperatures from 520 °C to 640 °C with increment of 30 °C for each set of samples. Microstructure of differently heat treated samples showed martensitic matrix, but different fraction and distribution of carbides, consequently influencing hardness, impact toughness, tensile strength, yield strength and wear resistance. Reciprocating sliding wear tests were carried out at room temperature in order to correlate microstructure of differently heat treated hot-work tool steel with wear. In order to achieve adhesive and abrasive wear mechanisms, 100Cr6 and Al2O3 balls were used as counter-body, respectively. Combination of adhesive and abrasive wear was observed for all specimens with different hardness when using 100Cr6 material as a counter body. However, in the case of Al2O3 abrasive wear was found as the prevailing wear mechanism.

Characterization and Design of Laser Textured Surfaces

Increased requirements for improved performance and reliability of mechanical systems dictate the use of new advanced materials and surface engineering techniques, which would increase efficiency and reduce energy losses. Already with a small change in surface roughness or topography lubrication mechanism can be change, thus leading to substantial improvement in tribological behaviour of contact surfaces. One of the ways of altering surface topography, known as surface texturing is by introducing small dimples or channels at the contact surface. Surface texturing is already successfully used in different applications, however, they are mainly based on trial and error approach. The aim of this investigation was to identify surface roughness parameters which could be used as a design parameter when preparing textured surfaces and to determine the effect of texturing parameters on roughness and tribological properties of boundary lubricated surfaces. Results of this investigation show that textured surfaces can be treated as an ordered roughness, with skewness and kurtosis parameters showing good correlation with the change in tribological behaviour of contact surfaces. Negative skewness and high kurtosis values, which can be achieved through wedge-shaped dimples, reduced dimple size and increased dimple spacing, were found to result in lower friction.© 2009 ASME

Modeling of Surface Topography for Reduced Friction

The aim of the present research was to investigate surface topography in terms of different surface roughness parameters and to correlate surface topography change to friction of contact surfaces. For this purpose, different 100Cr6 plate samples with different surface topography were prepared. Using different grades and combinations of grinding and polishing samples with similar Ra values, but different Rku and Rsk values were obtained. To evaluate influence of roughness parameters on friction and wear, dry and lubricated pin-on-disc tests were carried out under different contact conditions. Test results indicate that high Rku and negative Rsk values lead to decrease in friction. To investigate the effect of surface texturing on surface roughness parameters, real roughness profiles were virtually altered to achieve virtually textured surfaces. Using NIST SMATS softgauge for calculation of surface roughness parameters, virtually altered roughness profiles were investigated in terms of texture size, shape and spacing, and their influence on surface roughness parameters, especially on skewness and kurtosis. Lower diameter, higher spacing and wedge-shaped dimples reflect in higher Rku and more negative Rsk parameters.Copyright