1999

Graded structures obtained by coupling different WC grades, also containing cubic carbides type (Ta, Nb)C with Co as binder, were produced by vacuum sintering. Different batches of cylindrical samples were compacted by coupling two different grades and were sintered at 1450°C under vacuum. The hardness properties as well as the shrinkage due to sintering process are function of the employed grades and show gradual change between the single values of each of the coupled grades.

Sintered samples containing Mo, Ni and Mn, were plasma nitrided or nitrocarburized or sulphonitrocarburized with seven different cycles. After characterization the dry wear resistance of the treated samples was evaluated by an Amsler tribometer. The samples with the best wear resistance have a surface layer formed by martensite and retained austenite. A good wear resistance is also observed for the samples with a surface layer formed by a high amount of ε solid solution, or ε solid solution with a great carbon content.

Sintered components are gaining more and more applications, their success towards the most traditional technologies is due to the possibility of low cost manufacturing of parts having complicated shape coupled with restricted tolerances, to the very high yield of the utilisation (>95 %) of the raw material, as well as to the availability of a large choice of powders with a wide range of chemical composition.

The preparation of NASICON powders has been achieved by sol-gel methods. The precursor solution was either rapidly hydrolyzed, to get a precipitate, or slowly gelated under exposure to atmospheric humidity. The slow hydrolysis process was studied using nuclear magnetic resonance (NMR) spectroscopy in liquid phase. Two-dimensional experiments indicate the occurrence of ligand exchange reactions in solution. Monodimensional 1H, 13C, 31P, and 23Na NMR spectra were also measured. NMR allowed us to follow the advancement of the reaction and determine the degree of polymerization.

NASICON powders having the Na3Zr2Si2PO12 composition were prepared using a sol-gel method. All the reagents used were alkoxides. The precursor solution was rapidly hydrolized in order to get a precipitate, or slowly gelated under exposure to low atmospheric humidity. The slow hydrolysis process was studied using nuclear magnetic resonance (NMR) spectroscopy in liquid phase. 1H, 13C, and 31P NMR spectra were performed. NMR allowed us to identify the advancement of the reaction and the degree of polymerization.

Low-load indentation experiments have been performed on electrophoretic-deposited films of SiC particles on a graphite substrate. Films with thicknesses between 60 and 300 μm prepared at two current intensities and subsequently dried under different humidities were indented with spherical indenters with nominal radii of 10, 50, and 150 μm. Force-displacement data were analyzed to determine contact pressure and elastic modulus versus depth results.