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Silicate glasses are known to be brittle at the macroscopic scale but they can also undergo plastic deformation at the microscopic scale. More specifically, plastic densification occurs under contact loadings. In this paper, a constitutive model, which allows to take into account this phenomenon, is presented. An identification of the material parameters using instrumented indentation results is proposed. This identification is based on the comparison of local residual indentation-induced densification field obtained using Raman spectroscopy and computational results. This constitutive model is then used to analyse the distribution of densification after three-dimensional Vickers indentation and Cube Corner indentation. Finally, a finite-element analysis of the scratch test is done to show the plastic densification of such materials under scratching conditions. These different results show that the size of the densified zone under indentation-scratch loading is strongly related to the contact size. [ABSTRACT FROM AUTHOR]

Aspherical surfaces are more and more used in high precision optical systems of Thales Angénieux. They must satisfy the same quality standards of form and roughness as the other dioptre surfaces. However, aspherical dioptres can not be manufactured with conventional methods used for spherical or plane surfaces, because form and roughness required can not be obtained simultaneously. Among the various available techniques, Thales Angénieux has chosen to develop an industrial manufacturing process, with two successive methods: micro-grinding and magneto-rheological finishing. Furthermore, micro-grinding phase is divided into two parts: firstly, a brittle one with a wheel with large size abrasive grains, secondly a ductile one with a wheel with small size abrasive grains. However, a transition phase, flexible polishing, was inserted between micro-grinding and magneto-rheological finishing phases to reach the objectives. Finally, this manufacturing process made it possible to produce aspherical glass surfaces with a defect of form (PV) around 0.150/µm and a roughness (Ra) less than 3 nm. Moreover, this process can be integrated in an industrial manufacturing process. [ABSTRACT FROM AUTHOR]

Oxydo-reduction phenomenon concerns glass manufacturing as well as its properties. Various oxidation scales have been used by glassmakers, the most widely used being the ratio between Fe2+ and the total iron content. Comparisons are made with others, such as the partial pressure of oxygen or the redox potential. The influence of the chemical composition of the glass is also studied as well as that of the temperature, through the results obtained in Saint-Gobain Recherche and in other laboratories. In industrial glass manufacturing, the oxidation equilibrium is usually achieved during the fining process, an intense ‘bubbling’ being necessary to obtain a homogeneous glass. Thus, the oxidoreduction state of the glass does not depend on the atmosphere under which it was made, but depends on the mixture of gases generated during the fining process, which is described through the reactions generated by the two main fining systems, arsenic oxide or antimony oxide + sodium nitrate and sodium sulphate + a reducing agent. The former provides oxygen and the latter, a mixture of oxygen and sulphur dioxide. Consequently, a glass melted with arsenic oxide is more oxidised than a glass melted with sodium sulphate. The equilibrium constant used to interpret the latter system, mostly used for soda lime silica glasses fining, is given as a function of the temperature and some of the consequences are examined. Obviously, the coloration by iron oxide impurities is influenced by the manufacturing process. Some other colouring oxides are submitted to a change of oxidation state and thus to a modification of their colouring action. A quick review is done of the transition metal coloration, of the electron-charge-transfer absorption due to sulphides or selenides, and of aggregate formation inducing the well-known red ruby glass, for instance. [Copyright &y& Elsevier]

This article describes a new process for mounting an ivory miniature in a frame. The restoration comprises the following stages: removal of the backing from the painting and conservation treatment, reconstitution of part of the frame, and making a flexible support using silicone rubber. This support, which protects the miniature and holds it in place, is made in tow parts: a backing which is moulded to the reverse of the ivory sheet, and a surround obtained by embossing. Finally, the protective glass and the flexible support are mounted in the original frame. [ABSTRACT FROM AUTHOR]