New method to measure thermal shock resistance in ceramics using a piezo-spectroscopic technique

A new method for assessing the critical temperature in thermally shocked ceramics is proposed. It is based on the measurement of stress relaxation of residual stresses as a consequence of thermal shock. The change in the stress-field is determined by piezo-spectroscopic technique. The technique is described and the results analyzed. The values obtained are compared with those obtained by the conventional method based on strength degradation measured on test pieces quenched at different temperatures. The agreement among the data is very good

Phase composition and morphology as a function of depth for boride coatings grown on Fe-Ni alloys

Considerable R&D efforts are being carried out in order to produce materials with satisfactory resistance to wear, corrosion or both. In many applications, in fact, the in-service life of components is determined by the surface properties. In the important field of thermochemical treatments of steels, based on diffusion of species such as carbon, nitrogen or boron, boriding is in a peculiar position. Indeed, coatings constituted by iron borides generally display very high hardness, even in excess of 20 GPa, as well as good wear resistance under both sliding and abrasive conditions. Borided steel components display excellent performance in several tribological applications in the mechanical engineering and automotive industries. In spite of several researches performed since many years, some problems remain to be solved as, in particular, the role of alloying elements in the steel on growth and properties of the boride coating. In the present work the attention has been addressed to the role of Ni, investigated by boronizing binary alloys in order to avoid any interference by other alloying elements. Five Fe-Ni alloys with a Ni contents in the range from 11 to 80 wt.% were borided by a pack-cementation process, using a high boron potential powder mixture constituted by amorphous boron activated with 10 wt.% of potassium fluoborate. The thermochemical treatments were carried out at 850°C for exposure times up to 8 h. Phase composition and morphology of the boride coatings were characterized as a function of depth by the layer-by-layer removal technique as well as by observations carried out on metallographic cross-sections, using different and complementary techniques: X-ray diffraction (XRD), Mossbauer spectroscopy (MS), micro-Raman spectroscopy, optical (OM) and scanning electron microscopy (SEM), Vickers microhardness measurements (MHV)

Effect of Superficial Residual Stresses on Abrasive Wear Resistance of Al2O3/Al2O3 + 3Y-TZP multilayers

Multilayered composite specimens consisting of Al2O3 / Al2O3+ 3Y-TZP (A/AZ) layers with different compositions and thicknesses were prepared starting from ceramic sheets obtained by tape casting. Residual stresses arisen from mismatch in thermal expansion coefficient during sintering were evaluated using luminescence piezo-spectroscopy. The stress in the superficial A layer was found to be compressive, and its value depended on the ratio between thickness of A and AZ layer. The influence of the superficial compressive stress on the abrasive wear resistance was investigated using microscale ball cratering test; results were correlated with the superficial compressive stress and compared with a specimen of pure unstressed Al2O3 prepared both by lamination and by cold isostatic pressing. Experiments show an improvement of performances in the samples containing compressive residual stress in the surfac

Nota editorial, diciembre 1966

Esta sección recoge una diversidad de textos que dan cuenta de la contribución de expertos analistas que desde distintos enfoques se han referido sobre los acontecimientos financieros y económicos más relevantes ocurridos en Colombia y otros países. Se halla, en especial, noticias relativas a la producción y comercio del café dentro del panorama nacional y mundial; cifras de interés; también se rememora el contenido de revistas anteriores. En esta sección se compilan, además, reseñas, reflexiones, cartas y notas sobre la pluralidad de eventos académicos, sociales y culturales que han configurado el escenario económico nacional.En esta edición se tratan los siguientes temas:-       La situación general; el costo vida; giros por importaciones; la banca y el mercado monetario; el mercado bursátil; el petróleo; el café; personal directivo-       El mercado mundial del café-       VIII Reunión de técnicos de bancos centrales del continente americano:Discurso pronunciado por el señor vicepresidente, doctor Antonio Micele, en la sesión inauguralRelatoría de la comisión IRelatoría de la comisión IIRelatoría de la comisión IIIInforme del relator general -       Veinticinco años atrás (número 170, diciembre 1941

Microscale computational simulation and experimental measurement of thermal residual stresses in glass-alumina functionally graded materials

Glass-alumina functionally graded materials are new attractive composite materials, that can achieve peculiar mechanical properties due to their gradual compositional variation. Nevertheless, the difference between the coefficients of thermal expansion of the constituent phases may result in significant thermal residual stresses in service or during fabrication. A proper (glass formulation can minimize the mismatch in thermo-mechanical properties, thus relevantly reducing the mean value of the resultant thermal stresses. However, it is a crucial requirement to evaluate the effect of microstructural discreteness and randomness oil the actual stress distribution in functionally graded materials. With this aim,a computational model which applies the finite element method at the microscale is used. The careful modelling of the real microstructural details enables to accurately predict the local stress values and distribution. In order to verify the reliability of the computational simulations, the residual thermal stresses were also experimentally measured by means of a piezo-spectroscopic technique. The comparison between the numerical and the experimental results validate the microstructure-based model