Manuel Belmonte

Carbon nanofillers for machining insulating ceramics

The implementation of ceramics in emerging applications is principally limited by the final machining process necessary for producing microcomponents with complex geometries. The addition of carbon nanotubes greatly enhances the electrical properties of insulating ceramics allowing electrical discharge machining to be used to manufacture intricate parts. Meanwhile other properties of the ceramic may be either preserved or even improved. For the first time, a silicon nitride/carbon nanotubes microgear is electrically discharge machined with a remarkably high material removal rate, low surface roughness, and low tool wear. This offers unprecedented opportunities for the manufacture of complicated ceramic parts by adding carbon nanotubes for new engineering and biomedical applications.

Thermal conductivity of Al2O3/SiC platelet composites

The thermal conductivity of hot-pressed Al2O3/SiC platelet composites is determined as a function of the platelet content, from 0 to 30 vol.% of SiC. Existing heat conduction models are employed to discuss the experimental data. Data agree with the presence of an interfacial thermal resistance at the Al2O3/SiC grain boundaries, which precludes the effect of percolation on the thermal conductivity for the higher percentage of SiC platelets. The observed orientation effect on the thermal conductivity due to an alignment of the platelets is also modelled using the Hasselmans approach. The thermal conductivity of the SiC platelets is calculated from the effective thermal conductivity of the composites.

Wear resistant CVD diamond tools for turning of sintered hardmetals

Abstract Sintered hardmetals are very hard materials that are usually machined using diamond grinding wheels and electro-discharge machining. Dry cutting with super-hard cutting tools like cubic boron nitride (c-BN), polycrystalline diamond (PCD) and chemical vapour deposition (CVD) diamond is an ecological alternative to reduce operation times and, therefore, to improve the productivity. In the present work, cylindrical forging dies of WC–27 wt.% Co hardmetal grade were turned at fixed operating parameters (cutting speed=15 m/min; depth-of-cut=0.2 mm; feed rate=0.03 mm/rev.) using CVD diamond tipped hardmetal inserts. Commercial PCD and c-BN inserts were tested for comparison. The cutting tool behaviour was studied in terms of both the tool wear and the finishing quality of the workpiece. The tool damage was evaluated using a special probe for edge roughness evaluation, together with scanning electron microscopy observations. The CVD diamond tools survived the task showing slight cratering, whereas flank wear was the main wear mode for the other superhard tools. Amongst all the tested tools, PCD presented the worst performance in terms of tool wear and workpiece surface quality. Furthermore, the operation time was reduced to one tenth with respect to conventional diamond wheel grinding.

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.

Obtention of highly dispersed platelet-reinforced Al2O3 composites

The rheological behaviour of alumina/platelets (Al2O3 or SiC) suspensions has been established by measuring zeta potential versus pH and the relative viscosity as a function of pH, solids loading and platelets content. Highly homogeneous mixtures of alumina/platelets, with different platelets content (5, 8, 12 and 20 vol %), have been obtained by controlled flocculation at a pH value in the range 6–7. From these powders, very close to theoretical density (⩾ 99% dth) and slightly oriented platelet-reinforced alumina compacts have been obtained by hot pressing at temperatures ranging from 1500–1550 °C.

Carbon nanotubes functionalization process for developing ceramic matrix nanocomposites

The carbon nanotubes (CNTs) functionalization for developing ceramic matrix composites with an optimum interface between the matrix and nanotubes is presented. The functionalization processes were successfully performed by means of oxidation and boron nitride and silicon oxide coatings, which were characterized, among others, by O2/N2 adsorption-desorption method, micro-Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. In all cases, the functionalized CNTs exhibited an enhanced thermal stability compared to the original nanotubes. Silicon nitride (Si3N4) nanocomposites having 5.3 vol% of multi-walled CNTs, both pristine and functionalized, were developed. Dense nanocomposites having well dispersed nanotubes were attained under all circumstances, although the nanocomposites containing the coated CNTs required the use of a dispersant agent to prevent nanotubes agglomeration. Besides, slightly higher sintering temperatures were needed to densify the composites containing the coated CNTs. A stronger mechanical interlocking between the matrix and the nanotubes was achieved by the functionalization processes, which led to some improvement of the mechanical properties of these nanocomposites, compared to those containing pristine CNTs; and actually values that were close or even higher than those measured for the blank Si3N4 were achieved.

Low incident angle and classical x-ray diffraction analysis of residual stresses in diamond coated Si3N4

The overall residual stresses and the stress distribution on chemical vapor deposition diamond coated silicon nitride (Si3N4) are investigated by x-ray diffraction using the classical sin2 ψ method and the low incident beam angle diffraction technique, respectively. The deposition time and the substrate surface roughness evidence a clear effect on the film texture and residual stress generation on the diamond coatings. For the roughest substrate, 15 μm polished diamond, as the deposition time increases from 2 to 6 h, a remarkable improvement of the (220) texture and compressive stress relaxation occur from −0.75 to −0.14 GPa. A change to less compressive and even to slight tensile stress takes place for the smoothest substrate, colloidal finished silica, giving stress values of −0.40 and 0.21 GPa for 2 and 6 h of deposition time, respectively. The stress profile along the film thickness for the 6 h grown diamond film on the 15 μm polished substrate exhibits a linear decrease, in absolute values, from a co...

Role of triboelectrification mechanism in the wear behaviour of Al2O3SiC platelet composites

Abstract Wear behaviour of Al 2 O 3 SiC platelet composites has been analysed as a function of the content and the orientation of the platelets. The results have shown that wear volume increased with platelet content. For long sliding times (≥ 60 min) and high SiC platelet additions (≥ 12 vol.%), a mild-severe wear transition took place. A mechanism based on the triboelectrification concept has been developed to explain the wear behaviour of Al 2 O 3 SiC platelet composites. A strongly anisotropic wear behaviour, that has been directly associated to the anisotropy in the electrical and mechanical responses, was observed. This behaviour was a consequence of the platelet orientation during hot pressing.

Sensitivity of the resonant ultrasound spectroscopy to weak gradients of elastic properties

The applicability of resonant ultrasound spectroscopy on materials with weak spatial gradients in elastic coefficients and density is analyzed. It is shown that such gradients do not affect measurably the resonant spectrum but have a significant impact on the modal shapes. A numerical inverse procedure is proposed to explore the possibility of reconstructing the gradients from experimentally obtained modal shapes. This procedure is tested on synthetic data and applied to determine the gradient of the shear modulus in a continuously graded silicon nitride ceramic material. The results are in a good agreement with the gradient calculated for the examined material theoretically as well as with the results of other experimental methods.

Carbon nanotube-based bioceramic grafts for electrotherapy of bone

Bone complexity demands the engineering of new scaffolding solutions for its reconstructive surgery. Emerging bone grafts should offer not only mechanical support but also functional properties to explore innovative bone therapies. Following this, ceramic bone grafts of Glass/hydroxyapatite (HA) reinforced with conductive carbon nanotubes (CNTs) - CNT/Glass/HA - were prepared for bone electrotherapy purposes. Computer-aided 3D microstructural reconstructions and TEM analysis of CNT/Glass/HA composites provided details on the CNT 3D network and further correlation to their functional properties. CNTs are arranged as sub-micrometric sized ropes bridging homogenously distributed ellipsoid-shaped agglomerates. This arrangement yielded composites with a percolation threshold of pc=1.5vol.%. At 4.4vol.% of CNTs, thermal and electrical conductivities of 1.5W·m(-1)·K(-1) and 55S·m(-1), respectively, were obtained, matching relevant requisites in electrical stimulation protocols. While the former avoids bone damaging from Joules heat generation, the latter might allow the confinement of external electrical fields through the conductive material if used for in vivo electrical stimulation. Moreover, the electrically conductive bone grafts have better mechanical properties than those of the natural cortical bone. Overall, these highly conductive materials with controlled size CNT agglomerates might accelerate bone bonding and maximize the delivery of electrical stimulation during electrotherapy practices.