Characterization of diamond films deposited on titanium and its alloys

Titanium and its alloys have important applications for example in aerospace or as bioimplants. Some of these applications would be improved by diamond coatings. However the large thermal expansion mismatch between diamond and titanium or its alloys creates high residual stresses, up to about 7 GPa at 800 °C, which represent an important drawback. In this study, polycrystalline diamond films were deposited on pure titanium and Ti-6Al-4V in a classical tubular microwave plasma reactor from C-H(-O)-containing gas mixtures, at a temperature in the range 600–900 °C. Raman spectroscopy provided information about the diamond grain stress, which is obviously related to the deposition temperature. X-ray diffraction indicates the presence of titanium carbide or oxycarbide. Some other characterizations by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) are reported. It is shown that XPS coupled to argon ionic etching allows us to study the first steps of the deposition process. The structure and the chemical composition at the interface of a thicker deposit are obtained by TEM and EELS.     

Direct calculation by analogue computer of sensitivity coefficients of linear and nonlinear systems

This letter gives a method which enables a direct analysis of sensitivity to parameter variations of linear or nonlinear systems to be simulated on an analogue computer. Based on the automatic changes ± ?q of a parameter q around a value q0, this method enables the dynamic sensitivity coefficient u(t, q0) to be computed with respect to this parameter by comparison of the disturbed solutions obtained in such a way.     

Pseudotumor forms of neurosarcoidosis in children. Diagnostic difficulties and therapeutic management

In systemic sarcoidosis, neurologic manifestations are possible and occurred in 5 p. 100 of cases. The diagnosis is easier when sarcoiddosis is known, but in half case neurologic symptoms are inaugural. In these cases, involvement of other sites (liver, lung, eyes or skin) by the disease is needed to confirm sarcoidosis. Main manifestations in neurosarcoidosis are cranial nerve palsy, diabetes insipidus or chronic aseptic meningitis, but sometimes sarcoidosis can present as an intracranial mass. The diagnosis of neurosarcoidosis may be difficult and could be confused with infection (such tuberculosis or mycosis), with inflammatory diseases (such multiple sclerosis) or with neoplasm, particularly in case of oedema. Brain biopsy may be necessary in isolated neurosarcoidosis if no peripheral histology evidence of noncaseating granulomas has been obtained. Treatment with steroids alone is the best choice for initial therapy but must be prolonged. It is difficult to predict accurately the response to steroids and relapse may be possible after a long time. Other immunosuppressive therapeutics or radiotherapy have been used in patient with refractory neurosarcoidosis. Sequential magnetic resonance imaging is the useful mean for the follow-up of neurosarcoidosis.     

z transfer functions of sampled-data control systems with a discontinuous error signal

Sampled-data control systems, of which the open-loop transfer function is such that the corresponding impulse response is discontinuous at the origin, cannot be studied with the usual z formula. The z transfer function of such systems is established when it can be assumed that there is no instantaneous transmission, or when the sampler is not a usual Dirac sampler.     

Interface characterization by TEM,AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase

The fibre-matrix interfacial zone formed during the isothermal/isobaric chemical vapour infiltration processing of SiC fibres (ex-polycarbosilane)/boron nitride/SiC matrix composites has been analysed by TEM/electron energy loss spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy. In the composites, the boron nitride interphase (deposited from BF₃-NH₃) is made of turbostratic boron nitride, almost stoichiometric but containing some oxygen (less than 5 at %). The boron nitride layer stacks are randomly orientated except in the very vicinity of the fibre surface where they lie almost parallel to the substrate. The long chemical vapour infiltration treatment at 1000 °C used to infiltrate the SiC matrix acts as an annealing treatment for the metastable ex-polycarbosilane fibres which gives rise to the growth of an SiO₂/carbon amorphous double layer at the boron nitride/fibre interface. Deflection of microcracks arising from the failure of the brittle SiC-matrix occurs at the boron nitride/SiO₂ interface considered to be the weaker link in the matrix/boron nitride interphase/SiO₂/carbon/fibre sequence. It is suggested that the combination of the boron nitride layered interphase and SiO₂/carbon fibre decomposition products might play an important role in determining the propagation path of microcracks in the fibre/matrix interfacial zones and could be responsible, at least to some extent, for the non-brittle behaviour of such composites.     

Boron Nitride Interphase in Ceramic-Matrix Composites

A BN interphase has been deposited, by isothermal/isobaric chemical vapor infiltration (ICVI) from BF₃─NH₃, within a preform made from ex-polycarbosilane (ex-PCS) fibers, at about 1000°C. In a second step, the BN-treated preform was densified with SiC deposited from CH₃SiCl₃─H₂ at about the same temperature. From a thermodynamic standpoint, ex-PCS fibers could be regarded as unreactive vs the BF₃─NH₃ gas phase assuming they are coated with a thin layer of carbon or/and silica. The as-deposited interphase consists of turbostratic BN (N/B < 1) containing oxygen. The SiC infiltration acts as an annealing treatment: (i) the BN interphase becomes almost stoichiometric and free of oxygen; (ii) the fibers undergo a decomposition process yielding a SiO₂/C layer at the BN/fiber interface. The weaker link in the interfacial sequence seems to be the BN/SiO₂ interface. Deflection of microcracks arising from the failure of the matrix takes place at (or nearby) that particular interface.     

Correlations between gas phase supersaturation,nucleation process and physico-chemical characteristics of silicon carbide deposited from Si-C-H-Cl system on silica substrates

In the CH₃SiCl₃-H₂ CVD system, from which SiC-based films are prepared, the supersaturation of the gas phase increases when temperature and total pressure decreases and when a diffusion-controlled kinetic process is changed in a reaction-controlled one. These conditions variations seem to induce a transition from a growth regime to a nucleation regime, as evidenced by a study of the initial stages of the deposition. A transition from a crystallized film with columnar crystals to a nanocrystalline deposit is also reported on the basis of accurate experiments using TEM and related techniques.