Carlos Alberto Alves Cairo

Functionally gradient ceramic coating for carbon–carbon antioxidation protection

Functionally gradient ceramic coating provides an eAective antioxidant protection to carbon/carbon composites at intermediate temperatures (below 1000C) by the formation of a B2O3 glass from the oxidation of boron phases. The glass flows and seals the cracks to prevent further oxygen penetration. A protection layer based on SiC, B4C and Al8B4C7 phases was obtained by carbon conversion of the composite using a pack cementation process with a powder mixture of the ZrB2‐SiC‐Al2O3 system. # 2001 Elsevier Science Ltd. All rights reserved.

Kinetic study by TGA of the effect of oxidation inhibitors for carbon–carbon composite

The corrosion by air oxidation of the carbon fiber reinforced carbon composites can be inhibited by adding glass-forming particles to the matrix during the impregnation process. The effectiveness of the inhibition is related to the formation, wetting and spreading of the glass on the carbon matrix. The glass acts as a blockage onto the carbon active sites forming a thin protection barrier against oxygen diffusion. This work intends to evaluate the influence of the reactivity of the composite components on the inhibition effectiveness obtained by the addition of boron carbide and amorphous boron to the carbon matrix. Bi-directional composites were obtained with carbon matrix derived from phenolic resin using both carbonized and graphitized carbon fibers. The oxidation resistance of the composites was carried out by a weight loss in a thermogravimetric analyzer and activation energy calculation through Arrhenius plot for oxidation in air. The inhibition effectiveness depended on the reactivity of the individual components of the composite.

Porous Structure Characterization in Titanium Coating for Surgical Implants

Powder metallurgy techniques have been used to produce controlled porous structures, such as the porous coatings applied for dental and orthopedic surgical implants, which allow bony tissue ingrowth within the implant surface improving fixation. This work presents the processing and characterization of titanium porous coatings of different porosity levels, processed through powder metallurgy techniques. Pure titanium sponge powders were used for coating and Ti-6Al7Nb powder metallurgy rods were used as substrates. Characterization was made through quantitative metallographic image analysis using optical light microscope for coating porosity data and SEM analysis for evaluation of the coating/substrate interface integrity. The results allowed optimization of the processing parameters in order to obtain porous coatings that meet the requirements for use as implants.

Production of titanium alloys for advanced aerospace systems by powder metallurgy

Titanium alloys parts are ideally suited for advanced aerospace systems because of their unique combination of high specific strength at both room temperature and moderately elevated temperature, in addition to excellent corrosion resistance. Despite these features, use of titanium alloys in engines and airframes is limited by cost. The alloys processing by powder metallurgy eases the obtainment of parts with complex geometry. In this work, results of the Ti-6Al-4V and Ti-13Nb-13Zr alloys production are presented. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering between 900 up to 1500 °C, in vacuum. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. It was shown that the samples were sintered to high densities and presented homogeneous microstructure from the elements dissolution with low interstitial pick-up.

Porous Titanium Scaffolds Produced by Powder Metallurgy for Biomedical Applications

Porous titanium scaffolds are promising materials for biomedical applications such as prosthetic anchors, fillers and bone reconstruction. This study evaluated the bone/titanium interface of scaffolds with interconnected pores prepared by powder metallurgy, using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Porous scaffolds and dense samples were implanted in the tibia of rabbits, which were subsequently killed 1, 4, and 8 weeks after surgery. Initial bone neoformation was observed one week after implantation. Bone ingrowth in pores and the Ca/P ratio at the interface were remarkably enhanced at 4 and 8 weeks. The results showed that the interconnected pores of the titanium scaffolds promoted bone ingrowth, which increased over time. The powder metallurgy technique thus proved effective in producing porous scaffolds and dense titanium for biomedical applications, allowing for adequate control of pore size and porosity and promoting bone ingrowth.

Sintering behaviour of alumina–tungsten carbide composites

Abstract Alumina reinforced with tungsten carbide has been investigated as an alternative material for metalworking, combining resistance to high service temperatures and improved toughness. Pressureless sintered and hot-pressed Al 2 O 3 –WC composites were manufactured and characterised. The use of Y 2 O 3 as a sintering additive has also been evaluated. Additions of up to 30 wt.% WC resulted in limited grain boundary pinning and corresponding high densification. Although the addition of Y 2 O 3 improved sintering, the presence of a residual grain boundary phase (YAG) was harmful to the fracture toughness of the composites, as it affected the effectiveness of the crack deflection mechanism that takes place at the interfaces between Al 2 O 3 and WC grains. Hot-pressing resulted in hardness ∼17.5 GPa and fracture toughness ∼7 MPa m −1/2 , which is an improvement compared to alumina reinforced by other refractory carbides.

Evaluation of bone ingrowth into porous titanium implant: histomorphometric analysis in rabbits

A porous material for bone ingrowth with adequate pore structure and appropriate mechanical properties has long been sought as the ideal bone-implant interface. This study aimed to assess in vivo the influence of three types of porous titanium implant on the new bone ingrowth. The implants were produced by means of a powder metallurgy technique with different porosities and pore sizes: Group 1 = 30 % and 180 µm; Group 2 = 30% and 300 µm; and Group 3 = 40% and 180 µm;. Six rabbits received one implant of each type in the right and left tibiae and were sacrificed 8 weeks after surgery for histological and histomorphometric analyses. Histological analysis confirmed new bone in contact with the implant, formed in direction of pores. Histomorphometric evaluation demonstrated that the new bone formation was statistically significantly lower in the group G1 than in group G3, (P = 0.023). Based on these results, increased porosity and pore size were concluded to have a positive effect on the amount of bone ingrowth.

Reinforcing Al2O3 with W-Ti mixed carbide

Abstract Recent advances in high-speed cutting materials have focused on reinforcing alumina with different carbides and nitrides in order to improve hardness and fracture toughness. However, data on mixed carbides is still scarce. The potential use of this type of material for cutting tool applications has yet to be determined. The present study reports some preliminary results obtained reinforcing Al 2 O 3 with WTiC in the range of 5–30 wt.%. The material was hot-pressed at 1650°C for 30 min and then characterized. Vickers microhardness ( H V ) and fracture toughness ( K IC ) were evaluated by the indentation method. The addition of WTiC did not result in any significant change in the fracture toughness of alumina, however hardness values in excess of 22 GPa were obtained.

Titanium–35niobium alloy as a potential material for biomedical implants: In vitro study

Research on new titanium alloys and different surface topographies aims to improve osseointegration. The objective of this study is to analyze the behavior of osteogenic cells cultivated on porous and dense samples of titanium-niobium alloys, and to compare them with the behavior of such type of cells on commercial pure titanium. Samples prepared using powder metallurgy were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and metallographic and profilometer analyses. Osteogenic cells from newborn rat calvaria were plated over different groups: dense or porous samples composed of Ti or Ti-35niobium (Nb). Cell adhesion, cell proliferation, MTT assay, cell morphology, protein total content, alkaline phosphatase activity, and mineralization nodules were assessed. Results from XRD and EDS analysis confirmed the presence of Ti and Nb in the test alloy. Metallographic analysis revealed interconnected pores, with pore size ranging from 138 to 150μm. The profilometer analysis detected the greatest rugosity within the dense alloy samples. In vitro tests revealed similar biocompatibility between Ti-35Nb and Ti; furthermore, it was possible to verify that the association of porous surface topography and the Ti-35Nb alloy positively influenced mineralized matrix formation. We propose that the Ti-35Nb alloy with porous topography constitutes a biocompatible material with great potential for use in biomedical implants.

Osteoblast response to porous titanium and biomimetic surface: In vitro analysis

OBJECTIVE This study analyzed the behavior of human osteoblasts cultured on porous titanium specimens, with and without biomimetic treatment, compared to dense titanium. DESIGN The experiment had seven groups: Group 1: cells cultured on polystyrene of culture plate wells; Group 2: cells cultured on dense titanium specimen; Group 3: specimen with 33.79% of pores; Group 4: 41.79% of pores; Groups 5, 6 and 7: specimens similar to groups 2, 3 and 4, yet with biomimetic treatment. Real time-polymerase chain reaction with reverse transcription of the following genes was performed: prostaglandin E2 synthase, integrin β1, osterix, Runx2, Interleukin 6, macrophage colony stimulating factor, apolipoprotein E and others. The study achieved data on cell adhesion, growth and viability, total protein content, alkaline phosphatase activity and quantity of mineralized nodule formations. Data were statistically evaluated. RESULTS Adherent cells and alkaline phosphatase activity were similar in titanium specimens, regardless of the groups. Biomimetic treatment reduced the total protein activity and the viability of tested cells. Most tested genes had statistically similar expression in all groups. CONCLUSION The tested porosities did not cause alterations in osteoblast behavior and the biomimetic treatment impaired the biocompatibility of titanium causing cytotoxicity.