F.J. Oliveira

Three‐dimensional printed PCL‐hydroxyapatite scaffolds filled with CNTs for bone cell growth stimulation

A three-phase [nanocrystalline hydroxyapatite (HA), carbon nanotubes (CNT), mixed in a polymeric matrix of polycaprolactone (PCL)] composite scaffold produced by 3D printing is presented. The CNT content varied between 0 and 10 wt % in a 50 wt % PCL matrix, with HA being the balance. With the combination of three well-known materials, these scaffolds aimed at bringing together the properties of all into a unique material to be used in tissue engineering as support for cell growth. The 3D printing technique allows producing composite scaffolds having an interconnected network of square pores in the range of 450-700 μm. The 2 wt % CNT scaffold offers the best combination of mechanical behaviour and electrical conductivity. Its compressive strength of ∼4 MPa is compatible with the trabecular bone. The composites show typical hydroxyapatite bioactivity, good cell adhesion and spreading at the scaffolds surface, this combination of properties indicating that the produced 3D, three-phase, scaffolds are promising materials in the field of bone regenerative medicine.

Effect of α-/β Si3N4-phase ratio and microstructure on the tribological behaviour up to 700°C

Abstract The present work analyses the effect of mechanical and microstructural characteristics in the tribological behaviour of Si3N4 materials. Special attention was given to the α/β phase ratio and the coarseness of the microstructure. Wear experiments were performed in a pin-on-disc tribometer using self-mated Si3N4 pairs varying the sliding speed and the test temperature. In the case of fine and medium microstructures, fracture and delamination was the main wear mechanism while for heterogeneous coarse material deformation related to intergranular microfracture was prevalent. The latter present higher wear rates at room temperature and low sliding speed but showed the best wear resistance when either sliding speed or temperature were increased.

Mesoporous bioactive glasses: Mechanical reinforcement by means of a biomimetic process

Mesoporous bioactive glasses (MBGs) constitute a new family of bioceramics with the fastest in vitro bioactivity studied so far. In this work, pieces with the composition 85SiO(2)-10CaO-5P(2)O(5) (mol.%) were prepared as MBGs and also by the conventional sol-gel method. After in vitro tests in simulated body fluid, the MBG pieces exhibited compression resistance twice as great than before, whereas conventional sol-gel glasses showed similar values. Scanning and transmission electron microscopy demonstrate the development of an apatite-like phase not only on the external surface, but also on the grains located within the MBGs pieces. In contrast, conventional sol-gel glasses only developed an apatite-like phase on the external surface. This work presents for the first time a new property of MBGs: the mechanical reinforcement of a bioactive glass through a biomimetic process. This ability is explained in terms of the outstanding bioactive behavior and the three-dimensional mesoporous structure that is exclusive for this bioceramics family.

Tribological characteristics of self-mated couples of Si3N4–SiC composites in the range 22–700°C

Abstract The tribological behaviour of self-mated pairs of Si 3 N 4 , Si 3 N 4 /SiC P (platelets) and Si 3 N 4 /SiC N (nanosized) hot pressed materials was studied in a pin-on-disc configuration. Tests were performed without lubrication, at a fixed load of 5 N, using different temperatures (room temperature −700°C) and sliding speeds (0.5–2 m s −1 ). Friction coefficients were usually above 0.5, being almost independent of sliding speed and test temperature. For all test conditions, the Si 3 N 4 /SiC P composite had the lowest friction coefficient, due to the presence of SiC platelets with smooth cleavage planes oriented parallel to the sliding surface. Wear coefficients were above 10 −6 mm 3 N −1 m −1 denoting a generalised severe wear mode, controlled by surface microcracking. No net differences were found between the wear behaviour of monolithic and composite materials under this wear regime. Wear decreased with sliding speed due to the protective action of a coherent tribofilm resulting from debris aggregation. Conversely, wear coefficients showed a steep increase above room temperature, to values around 10 −4 mm 3 N −1 m −1 at 700°C.

Microstructure, toughness and flexural strength of self-reinforced silicon nitride ceramics doped with yttrium oxide and ytterbium oxide.

Self‐reinforced silicon nitride ceramics with additions of either yttrium oxide or ytterbium oxide have been investigated at room temperature after various processing heat treatments. Devitrification of the intergranular phase in these materials is very sensitive to the heat treatment used during processing and does not necessarily improve their strength and toughness. Hot‐pressed ceramics without a subsequent devitrification heat treatment were the strongest. The ytterbium oxide‐doped silicon nitride ceramics were consistently tougher, but less strong, than the yttrium oxide‐doped silicon nitride ceramics. In all the ceramics examined, the fracture toughness showed evidence for R‐curve behaviour. This was most significant in pressureless sintered ytterbium oxide‐doped silicon nitride ceramics. A number of toughening mechanisms, including crack deflection, bridging, and fibre‐like grain pull‐out, were observed during microstructural analysis of the ceramics. In common with other silicon nitride‐based ceramics, thin amorphous films were found at the grain boundaries in each of the ceramics examined. Arrays of dislocations left in the elongated silicon nitride grains after processing were found to belong to the {101¯0}<0001> primary slip system.

Relationship between flexural strength and surface roughness for hot-pressed Si3N4 self-reinforced ceramics

Abstract The hot-pressed Si3N4 self-reinforced ceramics with Y2O3 and Al2O3 addition were first ground using the diamond wheel with the diamond grit of 64 μm size, and then polished using the diamond abrasives of 15, 6 and 1 μm size. The surface morphologies were examined by scanning electron microscopy (SEM) and atomic force microscope (AFM). The flexural strength at room temperature showed a net relationship with surface roughnesses (Ra, Rz, R3z, Rt, Rq) the increase of which followed the surface finished and defect disappearance. The flexural strength and the negative square root of surface defect size, maximum peak-to-valley height of surface, fit the following equation: σ f = A R t − B R t +C , where surface compressive residual stress and defect size interact competitively on strength. Sharp peaks and deep grooves were undoubtedly the fracture origins after the first grinding by the diamond wheel. The obvious surface defects had been eliminated by the following polishing, where fracture may be initiated from subsurface defects; however, there was still some local damage observed by AFM due to diamond abrasion. The mirror finished surfaces should be stress free.

Tailored Si3N4 Ceramic Substrates for CVD Diamond Coating

Abstract A review is presented of chemical vapour deposition (CVD) diamond coating of silicon nitride (Si3N4) materials. Microcrystalline and nanocrystalline diamond films were grown using microwave plasma (MPCVD) and hot filament (HFCVD) reactors, respectively. Scanning electron and atomic force microscopy, μ-Raman spectroscopy, low incident angle and classical X-ray diffraction, acoustic emission assisted Brale indentation and thermal conductivity measurements were employed for the full characterisation of the diamond/ Si3N4 system. Using these techniques, the nucleation and growth stages as a function of substrate composition and surface pretreatment were characterised, as well as the diamond quality, the existence of residual stresses and the adhesion between the diamond film and the substrate. Based on this study, a tailored material was developed and tested in the machining of hardmetal workpieces with encouraging results.

Residual stress minimum in nanocrystalline diamond films

Nanocrystalline diamond films have been deposited on silicon nitride substrates by hot filament chemical vapor deposition. Gas mixtures of CH4–H2–Ar were used with variation of the Ar∕H2 ratio in order to study the influence of the Ar content on the formation of nondiamond phases at the grain boundaries and thus in the film residual stress assessed by x-ray diffraction techniques. By varying this ratio it is possible to optimize conditions, decreasing the film’s residual stress to a minimum of 0.09GPa.

Infiltration of SiC preforms with iron silicide melts: microstructures and properties

Spontaneous infiltration of SiC preforms with FeXSiY (=Fe3Si, Fe5Si3 and FeSi) melts produced SiC/FeXSiY composites with nearly full density (≥96.5% theoretical density). The phases, microstructures and mechanical properties have been studied with conventional materials characterization techniques including X-ray diffraction, optical microscopy, scanning electron microscopy etc. A key issue behind these studies is dissolution of SiC in FeXSiY melts during infiltration. The dissolution of SiC in Fe3Si melt led to carbon precipitation and phase changes (Fe3Si disappeared and Fe5Si3 and FeSi formed). However, Fe5Si3 and FeSi infiltration gave no carbon precipitation, but SiC sintering, particle coalesce and grain growth instead. Extra-large SiC grains and SiC single crystals were found in fine SiC (0.5 μm) infiltrated by Fe5Si3. Phase equilibrium calculations for Fe–Si–C system were performed at 1873 K using chemsage in order to study SiC solubility and the condition for the precipitation of SiC rather than C in Fe–Si melt. Mechanical properties including micro-hardness, bending strength and Weibull modulus of all infiltrated samples were tested and analyzed on the basis of phase and microstructure observations.

MODELING OF CHEMICAL WEAR IN FERROUS ALLOYS/ SILICON NITRIDE CONTACTS DURING HIGH SPEED CUTTING

The wear resistance of SisN4 in machining of iron alloys can be surprisingly low due to chemical affinity for dissolution in the metal. This limits the use of SisN4 inserts in high speed machining of steels, while Si3N4 retains the best performance of all cutting materials in turning of grey cast iron, a different fer- rous alloy. The chemical wear of several ceramics has been investigated on the basis of dissolution in pure iron by Kramer and Suh. Nevertheless, the influence of alloy elements was not studied. In the present work, solid solution thermodynamics is applied to predict chemical wear of S&N4 by setting the influence of interaction coefficients of the alloy elements in the Henrian activity of Si and N in austenite. The model predicts the relative order of magnitude of the crater wear of SisN4 inserts in machining of tool steels, car- bon steels and grey cast iron. 0 1998 Acta Metallurgica Inc.