Papers by Author: Gabriela Sima

Abstract: Recent developments in the field of tissue engineering recommend the 3D printing to obtain the complex shape of the final alloplastic grafts (for soft or hard tissues). The medium pressure injection moulding (MEDPIM) technology could be a cost-effective alternative to 3D printing. This paper presents the first studies for the MEDPIM applicability in the case of some alloplastic bone grafts manufacturing. Two biocomposite feedstocks have been researched in order to obtain the injected parts (herein after named green parts). The Ti6Al4V respectively TiH₂, both as powder particles, were mixed with hydroxyapatite (HAP) powders in the W 50 EHT measuring mixer. Both powder mixtures contain NaCl as foaming agent as well as some wax-based binders. The MEDPIM process is developed in the laboratory-scale device, designed and manufactured in the frame of the research project BONY. This device replicates, at lab-scale, the MEDPIM process developed by the Goceram MEDPIMOULD equipment, able to work at mass-production scale. The injection moulding tests were performed at the temperature range of 50…110°C, respectively at 30-45 MPa as injection pressure. The green parts quality was evaluated by means of the physical characteristics (dimensional and density measurements) as well as macroscopic evaluation using the stereomicroscope NIKON SMZ 745T. The influence of the biocomposite feedstock type as well as the injection temperature on the green pats quality is studied in this research.

Abstract: Designing and processing of the alloplastic bone grafts represent one of the newest trends in bone tissue engineering, solving a lot of trauma problems of the patients simultaneously with technological and economical achievements. Recent developments in the field provide advantageous aspects concerning the internal architecture, mechanical properties and biocompatibility of the alloplastic bone grafts processed by the powder metallurgy (PM) technology. In this respect, the PM biocomposite materials based on hydroxyapatite powder particles reinforced by metallic or ceramic powders afford great benefits combining classic PM processes with different foaming techniques. The obtained biocomposites present special morphological and structural features matching the genuine bone tissue to be grafted, cortical respectively trabecular. This study focuses on the mechanical testing of the hydroxyapatite-based biocomposites reinforced by different foaming agents, specifically TiH2, CaCO3 and NH4HCO3 up to 25% mass. The overlapping of the obtained experimental results with those reported by the literature leads to the conclusion that the mechanical response of the PM biocomposites studied in this paper may play as a selection criteria to depict their application in hard tissue engineering.

Abstract: The raw material to be compacted by moulding is represented by aluminium alloy (ALUMIX 321) powder particles as metallic matrix and carbamide as foaming agent. The raw material to be injected is represented by the mixture (feedstock) between the wax-based binder system (40-60% mass) and the aluminium alloy (ALUMIX 321) powder particles (balance). The binder system is made of paraffin wax and stearic acid. The foaming effect is generated by addition of carbamide as foaming agent. Both categories of raw samples were washed in the ultrasonic machine and the aim of research was to study the physical properties and the macroscopic analysis of this materials.

Abstract: The paper focuses on the mechanical characterization of porous biocomposites based on hydroxyapatite submicronic powders (< 200 nm), respectively micronic powders particle (30-50 μm) as matrix, reinforced by titanium hybrid powders (15 and 20% mass; 100-150 μm) as foaming agent. Another foaming agent used is calcium bicarbonate powder (5 and 10% mass). The mixture homogenization was made in a Frisch-Pulverisette 6 type planetary mill (n=200 rpm), for 30 minutes. The green compacts were processed by unilateral cold compaction at 150 MPa. The two-step sintering technology (TSS) has been applied to the green parts, on the Nabertherm conventional furnace: at 900°C for few minutes (first step) and at 800°C for 450 minutes, respectively 600 minutes (the second step). The mechanical characteristics (compression modulus G [MPa] and ultimate compression strength σ_UTS [MPa]), were studied using the universal mechanical testing machine INSTRON 3382 and compared with the mechanical characterization of the human bone.

Abstract: This paper presents a comparative analysis of the foaming process developed in hydroxyapatite (HAp)-based bicomposites as a function of the foaming agent. The matrix of the biocomposite consists of either submicronic or micronic powder particles of HAp. The titanium hydride powder was added as reinforcement’s precursor as well as blowing agent, and in order to increase the biocomposites’ porosity calcium carbonate was added as space holder agent. The powders mixture was homogenized in a planetary ball mill with a single grinding bowl for 1 minute in air. Uniaxial cold compaction at 120-170 MPa was performed in order to obtain cylindrical green parts, which next were heated in argon atmosphere using the two step sintering technique at temperatures of 900 °C for 1 minute and 800°C for 450 - 600 minutes. The porosity of the biocomposite is analysed through calculations and SEM and EDS analysis highlighting the influence of the above mentioned foaming techniques (blowing and space holder).

Abstract: Using powder metallurgy techniques new porous materials for self-lubricating bearings were developed. These materials are characterized by total porosity, which represents their major advantage for tribological applications, acting like their own oil reservoir. Sometimes the presence of pores can be also detrimental to the part performance. Among the causes of the bearings failure is their increased porosity for improving the lubricant retention capacity. Consequently, this can lead to a significant loss in strength. In the present work tensile test specimens based on Fe-Cu/brass-Sn-Pb powders were prepared in order to investigate the morphology of the fracture surfaces and to analyze the effect of pores on the failure process of these materials subjected to tensile loads. Distinct morphologies of the pores area were revealed by SEM images of the fracture surfaces.

Abstract: This research focuses on Ag-Cu powder particles processing by mechanical alloying (MA) route. The powder mixture is representative for the eutectic composition, respectively 72%wt. Ag + 28% wt. Cu. The milling process is developed in high energy ball mill Pulverisette 6, using different size for the milling balls, in wet conditions for 80 hours. One of the most important parameter studied in this research is the particle size distribution of the processed powder mixture. Thus, it changes along the milling time, from 10…75 µm at the beginning of MA process up to (60 – 80) nm at 80 h. The milling parameters will be optimized in future research depending on the particle size distribution related with thermophysical and thermodynamic properties focused on electrical and optical properties improvement.

Abstract: The objective of this research is the development of a detailed structural analysis of biocomposites with ceramic matrix of hydroxyapatite (Hap) reinforced by titanium (Ti), elaborated by powder metallurgy technology. Nanometric Hap powders (<200nm) 75% wt and micrometric Ti powders (<150μm) are homogenized in a high energy ball mill Pulverisette 6. Spark plasma sintering (SPS) is the sintering route able to lead to nanostructured sintered samples when nanopowders are used as raw material. The SPS parameters are: the sintering temperature, T=(1000-1100)°C and the maintaining time, t=(10-20) minutes in vacuum. The influence of the sintering parameters on the composites structures is monitored using the optical microscopy (OM), electronic microscopy (SEM) and the X-Ray diffraction (XRD).

Abstract: The paper presents the experimental results regarding the influence of the reinforcing elements on the wear behavior of Al-matrix composites discontinuously reinforced by SiC and Graphite. This antifriction composite material is processed by Reactive Mechanically Alloyed and then by Spark Plasma Sintering technology. In order to optimize the processing technology, especially the sintering parameters, the Spark Plasma Sintering process was applied because of its advantageous aspects: lower sintering temperatures, shorter sintering time and higher properties values of the sintered material vs. the corresponding ones obtained by the classical sintering route. The authors realized a comparative analysis on the wear behavior of the researched composite materials.