Miroslav Babic

Effect of vacuum-treatment on deformation properties of PMMA bone cement.

Deformation behavior of polymethyl methacrylate (PMMA) bone cement is explored using microindentation. Two types of PMMA bone cement were prepared. Vacuum treated samples were subjected to the degassing of the material under vacuum of 270 mbar for 35 s, followed by the second degassing under vacuum of 255 mbar for 35 s. Air-cured samples were left in ambient air to cool down and harden. All samples were left to age for 6 months before the test. The samples were then subjected to the indentation fatigue test mode, using sharp Vickers indenter. First, loading segment rise time was varied in order to establish time-dependent behavior of the samples. Experimental data showed that viscous part of the deformation can be neglected under the observed test conditions. The second series of microindentation tests were realized with variation of number of cycles and indentation hardness and modulus were obtained. Approximate hardness was also calculated using analysis of residual impression area. Porosity characteristics were analyzed using CellC software. Scanning electron microscopy (SEM) analysis showed that air-cured bone cement exhibited significant number of large voids made of aggregated PMMA beads accompanied by particles of the radiopaque agent, while vacuum treated samples had homogeneous structure. Air-cured samples exhibited variable hardness and elasticity modulus throughout the material. They also had lower hardness values (approximately 65-100 MPa) than the vacuum treated cement (approximately 170 MPa). Porosity of 5.1% was obtained for vacuum treated cement and 16.8% for air-cured cement. Extensive plastic deformation, microcracks and craze whitening were produced during indentation of air-cured bone cement, whereas vacuum treated cement exhibited no cracks and no plastic deformation.

Tribological Behavior of Aluminum Hybrid Composites Studied by Application of Factorial Techniques

ABSTRACT This article analyzes the influence of graphite reinforcement, load, sliding speed, and sliding distance on tribological behavior of A356 aluminum matrix composites reinforced with silicon carbide and graphite using the full-factorial design. The wear rates of A356/10SiC composite material and A356/10SiC/1Gr and A356/10SiC/3Gr hybrid composites have been analyzed. The composites were obtained by a modified compocasting procedure. Tribological tests were performed on a block-on-disc tribometer without lubrication. The testing included sliding speeds of 0.25 and 1.0 m/s, normal loads of 10 and 20 N, and sliding distances of 300 and 900 m. The analysis of the obtained results was performed using the full-factorial method based on the signal-to-noise (S/N) ratio. The effects of load, sliding speed, weight percentage of graphite reinforcement, and sliding distance on the wear rate are 38.99, 17.87, 13.95, and 11.25%, respectively. The best tribological characteristics were exhibited by the A356/10SiC/1Gr hybrid aluminum composite.

Optimization of tribological properties of aluminum hybrid composites using Taguchi design

This paper analyses the influence of graphite reinforcement, load and sliding speed with constant sliding distance on tribological behavior of A356 aluminum matrix composites reinforced with 10 wt.% silicon carbide and graphite using the Taguchi design. Hybrid composites were produced in the compo-casting process. Tribological tests were performed on a block-on-disc tribometer where the weight percentage of graphite has three variations (0, 3, and 5), as well as load (10 N, 20 N, and 30 N) and sliding speed (0.25 m/s, 0.5 m/s, and 1 m/s), with sliding distance of 300 m. The wear of the composite is investigated under dry sliding condition. The specific wear rate was analyzed using Taguchi method with the aim of finding the optimal parameters. By applying analysis of variance, it was determined that the best tribological properties has A356/10SiC/3Gr hybrid composite. It was also found that the greatest impact on specific wear rate has load with the percentage effect of 69.163%, then sliding speed with 14.426% and the interaction between wt.% graphite and load. The dominant wear mechanism is adhesive wear as confirmed by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS).

Microstructure and mechanical properties of Zn25Al3Cu based composites with large Al2O3 particles at room and elevated temperatures

Abstract Microstructures and compressive properties of Zn25Al3Cu alloy and Zn25Al3Cu/Al2O3 particulate composites with large reinforcing particles (250 μm) were examined. The composites were obtained by the compocasting technique through infiltration of 3, 8 and 16 wt.% Al2O3 particles into the semi-solid metal matrix. The influence of temperature in the range from 70 to 170°C on compressive yield strength of the matrix alloy (as-cast and thixocast Zn25Al3Cu alloy) and the composites was investigated. Above 70°C compressive yield strength of all materials decreases, but the rate of decrease is different for the matrix alloy (as-cast and thixocast) and composites. It was found that the abrupt decrease in compressive yield strength of the matrix alloy occurred at temperatures higher than 70°C, whereas composites retained relatively high values of compressive yield strength till the end of the testing temperature range regardless of the amount of reinforcing particles.

Optimization of hybrid aluminum composites wear using Taguchi method and artificial neural network

Purpose This research aims to describe the influence of weight per cent of graphite (Gr), applied load and sliding speed on the wear behavior of aluminum (Al) alloy A356 reinforced with silicon carbide (SiC) (10 Wt.%) and Gr (1 Wt.% and 5 Wt.%) particles. The objective is to analyze the effect of the aforementioned parameters on a specific wear rate. Design/methodology/approach These hybrid composites are obtained by means of the compo-casting process. Tribological analyses were conducted on block-on-disc tribometer at three different loads (10, 20 and 30 N) and three different sliding speeds (0.25, 0.5 and 1 m/s), at the sliding distance of 900 m, in dry sliding wear conditions. Optimization of the tribological behavior was conducted via the Taguchi method, and ANOVA was used for the analysis of the specific wear rate. Confirmation tests are used to foresee and check the experimental results. Examined samples were analyzed via a scanning electron microscope (SEM). Regression models for predicting specific wear rate were developed with Taguchi and ANN (artificial neural network) methods. Findings The biggest impact on value of specific wear rate has the load (43.006%), while the impact of Wt.% Gr (31.514%) was less. After comparison of the results, i.e. regression models, for predicting the specific wear rate, it was observed that ANN was more efficient than the Taguchi method. The specific wear rate of Al alloy A356 with SiC (10 Wt.%) and Gr (1 Wt.% and 5 Wt.%) decreases with a decrease in the load and weight per cent of Gr-reinforcing material, as well as with a decrease in sliding speed. Originality/value The results obtained in this paper using the Taguchi method and the ANN method are useful for improving and further investigating the wear behavior of the SiC- and Gr-reinforced Al alloy A356.

Artificial aging of thixocast ZA27 alloy and particulate ZA27/SiCp composites

Abstract Thixocast ZA27 alloy and particulate ZA27/SiCp composites were subjected to artificial aging at 80, 120 and 160°C (T5 regime). Composites with 5 and 10 vol.% SiC particles were produced via the compocasting method. The influence of the aging was investigated using different techniques. Differential scanning calorimetry was performed to reveal the presence of phases created in the thixocast ZA27 alloy and the composites. Microstructures and fracture behavior were examined by means of optical microscopy, scanning electron microscopy and X-ray diffraction. Aging processes in the matrix alloy and composites were followed using hardness measurements. A decrease in the hardness values was noticed with the increase in the aging temperature. At higher temperatures diffusion of zinc atoms from supersaturated phases in the matrix alloy was enhanced, which resulted in faster changes in the hardness. Aging processes in the composites were significantly accelerated compared to the matrix alloy, due to the presence of particulate reinforcements. The maximum in hardness was achieved for shorter time in the ZA27/SiCp composite with higher volume fraction of SiC particles.

Tribological Properties of Ground Surfaces

This paper presents the results of tribometric investigations of ground surfaces machined with different combinations of grinding regime. Though they belong to the same class of surface roughness, they show significant differences in tribological properties. This is a consequence of differences in the state of material in the surface layers, which arise from the machining regime parameters variation. These results emphasize the essential importance of correct definition of tribological criteria for contact surface states in the design phase.Copyright