E. Lucchini

Influence of residual stresses on the mechanical properties of a layered ceramic composite

Abstract A layered Al2O3-ZrO2 ceramic composite has been fabricated using a colloidal processing method. The technique uses sequential centrifuging of slurries containing suspended ceramic powders to form a three-layered structure. The outer layers have a high alumina content while the central layer contains mainly stabilized zirconia. These laminae are subjected to residual stresses due to their different thermal expansion coefficients. These stresses depend on the configuration of the system as well as on the amount of the zirconia in the two layers. If the residual stresses exceed the strength of the inner layer, periodic parallel cracks are produced. Such cracks of course adversely influence the structural performance of the composite and should be avoided. A model for this problem is presented. Vickers indentations were also placed into the tensile layer with the intent to explore the crack propagation in such system. Cracks were split after they reached the compressive layer. The effects of the layer thickness on the depth of the cracks beneath the interface were systematically explored.

Wear mechanism of ceramic tools

Abstract Cutting tests were performed using ceramic cutting tools under continuous cutting conditions. The tests were carried out on AISI 1040 steel, with cutting speeds ranging from 5 to 11 m s −1 . The wear mechanism was investigated for both crater and flank. Alumina-toughened zirconia of submicron grain size showed the best wear resistance. Alumina with TiC, TiN and ZrO 2 inclusions exhibited a wear resistance a little lower than the above-mentioned materials. Low chemical stability seems to be the reason for the poor performances of the silicon carbide whiskers-reinforced alumina, silicon nitride and the tungsten carbide inserts.

The effect of wear on the tetragonal-to-monoclinic transformation and the residual stress distribution in zirconia-toughened alumina cutting tools

Abstract Zirconia-toughened alumina cutting tools have been examined after machining an AISI 1040 steel. The Raman spectrum of zirconia and the fluorescence spectrum from alumina have been used to determine the extent of the tetragonal-to-monoclinic (t-m) transformation and the residual stresses in and around the worn-out crater, respectively. Although about 15% vol. m-zirconia is present even in a brand new cutting tool, it was shown that the crater region undergoes the t-m transformation up to 40% vol. In both cases, the transformed zone is concentrated near the surface. The residual stresses in alumina are slightly compressive in the new cutting tool and away from the crater region, but they become tensile inside the crater, where the t-m transformation of zirconia has occurred to a larger extent. A stochastic model has been used to predict the stresses and it has been found to reproduce very well the experimental data. Some evidence of the formation of a spinel reaction product in the crater region has been found, likely (Mg,Fe)AlO 4 .

Sintering of glass bonded ceramic barium hexaferrite magnetic powders

Mixtures of low-melting glasses (glazes) and barium hexaferrite magnetic powders sinter at relatively low temperatures (800° C) and produce dense items in which excessive grain growth has been inhibited.

Ceramic materials wear mechanisms when cutting nickel-based alloys

Abstract In this paper the performances of some commercial ceramic inserts when cutting AISI 310 steel are investigated and compared to those of a traditional carbide based tool. The most important wear mechanism in the ceramic inserts is related to the segmented edges of the chips that abrade a notch at the end of the cut zone. Alumina–zirconia inserts are very sensitive to this kind of wear, whereas tools made of Sialon and alumina with SiC whiskers exhibit slightly better performances despite the concomitant chemical wear mechanisms.

The performance of molybdenum toughened alumina cutting tools in turning a particulate metal matrix composite

In this paper, a study of the tool wear mechanism in turning aluminium alloy reinforced with alumina using molybdenum-toughened alumina tools is presented. Alumina tools with three different amounts (15, 20 and 25 vol.%) of molybdenum were prepared and tested. The wear type was identified and its evolution with cutting time was measured. The results show that the main mechanism of tool wear is abrasion and not heat. The best overall performance was achieved, as far as flank wear is concerned, using the tool with 20 vol.% of molybdenum added. This has been explained using some of the composites intrinsic properties. SEM examination revealed that molybdenum particles are easily torn from the matrix by flowing chips. Under these conditions the molybdenum particles are not able to carry out their toughening action and they are responsible for the flank wear. Some ideas have been proposed as to engineer the alumina/molybdenum interface in order to increase their adhesion and the composite toughness.

Alumina/zirconia multilayer composites obtained by centrifugal consolidation

Abstract An attractive method for the fabrication of Al 2 O 3 -ZrO 2 laminar composites is described in this paper. The process consists in the centrifugation of colloidal suspensions containing alumina and zirconia particles. With this procedure, it is possible to grow thin layers with different composition and smooth interfaces. The effects of the slurry compositions, as well as aging times and centrifuging conditions on the morphology of the laminae are discussed.

Cutting Temperatures Evaluation in Ceramic Tools: Experimental Tests, Numerical Analysis and SEM Observations

The authors propose a multiple approach for the evaluation of cutting temperatures in ceramic tools. The first approach was the experimental evaluation of equitemperature lines, obtained in three-dimensional cutting by employing constant melting point powders scattered on planes parallel to rake face. In the second approach, a numerical finite element analysis was performed. At this step the determination of the percentage of total heat produced in the operation that flows into the tool was considered and, consequently, the temperature distribution within the whole volume of the insert. Finally, the examination of the crater zone with SEM microscopy confirmed the temperature levels as previous estimated.

Wear performance of ceramic cutting tool materials when cutting steel

Abstract Some test cycles have been carried out in continuous cutting conditions, employing cutting parameters (feed, depth of cut and cutting speed) chosen following experimental planes and suitable test fields. The wear tests have been carried out on AISI 1040 steel with cutting speeds from 5m/sec to 11m/sec. The silicon nitride, sintered carbide, cubic boron nitride and alumina reinforced with SiC whiskers inserts, have shown, at each assigned cutting parameter, poor wear resistance when cutting steel. Alumina and alumina in submicron grain, which has been toughened by ZrO2 phase transformation, and the oxide-based alumina, have been the better wear resistance. The mixed based alumina has shown a wear resistance, a little lower than the previous materials. The low chemical stability seems to be the reason for the poor performances of the silicon nitride and the silicon carbide whiskers reinforced alumina inserts. The high wear rate of the sintered carbide and the cubic boron nitride is essentially due to the pull out of the ceramic particles. From the other hand, the chemical stability, together with their good mechanical properties can explain the appreciable results obtained with the other alumina based materials.

A new method for low temperature preparation of barium hexaferrite powders

The heating behaviour of lyophilized gels composed of ammonium pectate and Ba2+ and Fe3+ ions was investigated by differential thermal analysis, thermogravimetric analysis and X-ray diffractometry. The formation of barium hexaferrite was found to be complete after 22 h in an oxygen atmosphere at 600° C, whereas in air at 700° C it has been attained after 30 min only. Examination by scanning electron microscopy of the powders fired at temperatures below 800° C showed almost all of the particles to be less than 1 micron in diameter, a condition necessary for the attainment of high coercivity.