Papers by Author: Csaba Balázsi

Abstract: Si₃N₄/hBN composites were fabricated by hot isostatic pressing at 1700°C/3h with 1, 3 and 5 wt. % micro-sized or nano-sized hexagonal boron nitride particles added to silicon nitride matrix. An indentation quench test method was used for estimation of thermal shock resistance of monolithic Si₃N₄ and Si₃N₄/hBN composites. Thermal shock resistance of the composites increased with the increase of size and volume of hBN particles. The critical temperature difference for the composites with micro-sized hBN was significantly higher (over 900°C) compared to the monolithic silicon nitride (580°C).

Abstract: This study presents the production of silver doped bioactive calcium-phosphate (CaP) coatings on commonly used orthopaedic implant materials (Ti6Al4V). The deposition process was performed by pulse current technique from electrolyte containing the appropriate amount of Ca(NO₃)₂ and NH₄H₂PO₄ components at 70 °C. In order to modify the CaP layer, Ag⁺ ions were added to the base electrolyte. The electrochemical behaviour of the coatings was investigated by potentiodynamic polarization method in conventional Ringer’s solution in a three-electrode open cell. The corrosion properties of samples prepared with different parameters were compared. During immersion, the coating comes into contact with the electrolyte and corrosion occurs. Due to the potential difference between layer and the metallic substrate, discrete anodic and cathodic areas can be formed, which result in the release of silver and calcium ions. For antimicrobial applications of the modified CaP coated implant alloys, it is important to maintain a continuous release of silver ions, while the bioactive CaP layer enhance the biocompatibility properties of the layer by fostering the bone cell growth. The morphology and grain size of coatings as deposited have been investigated and confirmed by different methods such as Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) analysis.

Abstract: Nanostructured silver layer was deposited by pulse current technique onto three different implant materials-TiAl6V4, CoCrMo alloy and stainless steel–that are commonly used in orthopedic surgery. The electrochemical behavior of the coatings in isotonic salt solution was investigated by potentiodynamic polarization measurements over a period of several weeks. The corrosion properties of silver coated different implant materials were compared. Degradation of silver coatings have been traced and confirmed by different methods such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) measurements.

Abstract: In this paper, the correlation between structural and mechanical properties of nanostructured austenitic oxide dispersion strengthened (ODS) steel prepared by powder metallurgy were studied. High efficient milling process was applied to achive an optimal dispersion of ceramic oxide particles in the steel matrix. The austenitic powder with or without Y₂O₃ addition was milled by two different milling process (wet and dry). Spark plasma sintering as a fast and effective compaction method was used as sintering equipment. The morphology and structural properties of ODS were studied by X-ray diffraction and scanning electron microscopy. Mechanical properties, such as microhardness, bending strength and modulus of nanostructured ODS were also determined.

Abstract: As graphene nanomaterials present exceptional mechanical, thermal and electric properties therefore it can be an excellent reinforcement material for ceramic composites. The research of ceramic composites incorporated with carbon-based fillers has focused on carbon nanotubes until now. In this work silicon nitride-based nanocomposites have been prepared with different ammount (1 and 3 wt%) of multilayer graphene (MLG) made by mechanical milling-based method as well as exfoliated graphite nanoplatelets (xGnP) and nanographene platelets (Angstron) in comparison. The morphology and microstructure of the sintered samples were studied by scanning electron microscope and transmission electron microscope. Phase compositions were determined by X-ray diffractometer. The bending strength and elastic modulus of MLG-silicon nitride composites showed enhanced values compared to the other graphene reinforced silicon nitride ceramic composites.

Abstract: Multiwall carbon nanotubes were dispersed with a concentration of 3wt% in silicon nitride ceramic host. A high efficiency attritor mill has been used for an effective dispersion of the filler phase in the matrix. In this work we have developed a spark plasma sintering process (SPS) suitable to consolidate and tailor the microstructure of CNT-reinforced silicon nitride-based ceramic composites. Mechanical measurements, micro-indentation investigations of the hardness and fracture toughness have been performed. Scanning electron microscopy has been involved in order to reveal the microstructure of the resulting composites.

Abstract: Hydroxyapatite, (Ca₁₀(PO₄)₆(OH)₂ is chemically similar to the mineral component of bones and teeth. HAp is among of the few materials that are classified as bioactive, meaning that it will support bone ingrowth and osseointegration when used in orthopaedic, dental and maxillofacial applications. Hydroxyapatite may be employed in forms such as powders, porous blocks and hybrid composites to fill bone defects or voids. These may arise when large sections of bone have had to be removed or when bone augmentations are required (e.g. dental applications). In this work, nanohydroxyapatite (nanoHAp) was successfully produced by using recycled eggshell and phosphoric acid by mechanochemical activation method (e.g. attrition milling). nanoHAp bioactivity was evaluated in animal (rabbit) models. Sixteen 4-month-old New Zealand white rabbits with an average weight of 2.8kg were used in experiments. After bilateral parietal bony defects formation (diameter: 8.0mm), nanoHAp was grafted. The control was unfilled defect. The bone regeneration was evaluated by micro-computerized tomograms (μCT) and histomorphometric analysis at 4 and 8 weeks. In conclusion, nanoHAp from eggshell showed much more bone formation compared to unfilled control group in both μCT analysis and histomorphometric analysis. Considering that the eggshell is easily available and cheap, nanoHAp from the eggshell can be good calcium source in tissue engineering.

Abstract: Tribological behavior of ZrO2 and Si3N4 based nanocomposites with addition of carbon nanofibres and nanotubes has been studied by the pin-on-disc technique. Friction coefficients were measured and recorded, wear rates were calculated in terms of material volume loss per load and sliding distance. The wear damage was studied using optical and electron microscopy and its mechanisms were identified. In monolithic materials the dominant wear mechanism was abrasion, in composites with CNF and with higher volume fraction of CNTs (5 and 10%) fiber pull-out and lubricating by the carbon phases occurred.

Abstract: Hydroxyapatite (HAP) is the main inorganic component of mammal bone and teeth. It is one of the few bioactive materials, which helps cell growing. The objective of this study was to fabricate hydroxyapatite-biopolimer nanofiber mats. To achieve this goal, two different suspensions were used. Approximately 1 μm diameter size fibers with large HAP agglomerates were fabricated using acetone and acetic acid. Fibers with a larger diameter size and homogenous distributed HAP particles were produced applying acetone and isopropanol. During the experiment the effect of the processing parameters: applied voltage, diameter of needle, the distance of the needle tip and the collector, flow rate was analyzed.

Abstract: Tribology tests were conducted on silicon nitride-based nanocomposites with various carbon additions to explore the effects of microstructure, the type and quantity of carbon additives and the preparation routes on the behavior. The nanocomposites consisted of Si3N4 and C in the proportions of 1 – 10 wt % carbon nanotube (CNT), or carbon black (CB), or graphite, or graphene. Specimens were produced by hot isostatic pressing. X-ray diffraction and scanning electron microscopy were used to reveal phase composition and microstructure. Unlubricated ball-on-disk tribology tests with silicon nitride counter face were carried out at room temperature in ambient atmosphere. Contact profilometer was used to profile the wear tracks. The friction coefficients of the pure Si3N4 and Si3N4 samples with 3% CNT varied between 0,77-0,81. Addition of 10% graphite and 3% CB to Si3N4 resulted in friction coefficients of 0,83 and 0,72 respectively. Si3N4 samples with 3% graphene showed distinctly lower friction levels of 0,52 and smaller scatter of the measured values. The wear track study revealed that high graphene content in the Si3N4 matrix caused relatively big wear particles and an uneven wear track.