Developments in Processing of Functionally Gradient Metals and Metal–Ceramic Composites:A Review

Functionally gradient/graded materials(FGMs), an emerging new class of materials, are the outcome of the recent innovative concepts in materials technology. FGMs are in their early stages of evolution and expected to have a strong impact on the design and development of new components and structures with better performance. FGMs exhibit gradual transitions in the microstructure and/or the composition in a specific direction, the presence of which leads to variation in the functional performance within a part. The presence of gradual transitions in material composition in FGMs can reduce or eliminate the deleterious stress concentrations and result in a wide gradation of physical and/or chemical properties within the material. Functionally graded metal–ceramic composites are also getting the attention of the researchers. Among the fabrication routes for FGMs such as chemical vapour deposition, physical vapour deposition, the sol–gel technique, plasma spraying, molten metal infiltration, self propagating high temperature synthesis, spray forming,centrifugal casting, etc., the ones based on solidification route are preferred for FGMs because of their economics and capability to make large size products. The present paper discusses and compares various solidification processing techniques available for the fabrication of functionally gradient metals and metal–ceramic composites and lists their properties and possible applications. The other processing methods are briefly described.     

NEMA Standards for Induction Motors and Their Application to the Cement Industry

Familiarity with NEMA standards and the influence of the power supply on the motor-load system permits the user to make the necessary decision as to whether a standard or special motor is needed for his particular application. This paper highlights the more important aspects of NEMA standards for ac induction motors from the application engineer's standpoint and the effect of the power supply and the load on motor characteristics.     

Thermal degradation kinetics of polyesters containing mesogenic aromatic diols

A series of polyesters were synthesized by reacting structurally differing aromatic diols with either saturated (flexible) or unsaturated (rigid) dicarboxylic acid halide by a stirred interfacial polycondensation technique. Thermal degradation kinetics of these polyesters were investigated by applying Coats–Redfern and Horowitz–Metzger nonisothermal procedures. The dynamic thermogravimetry experiments were conducted in nitrogen to obtain differential thermogravimetric plots. Thermal stability of these polyesters was discussed on the basis of semiquantitative methods such as differential procedural decomposition temperature, integral procedural decomposition temperature, and fraction decomposition temperature (e.g., 10% DT). Degradation proceeded in multiple stages. The thermal degradation patterns and activation energies in these stages were discussed in relation to central bridging moieties of aromatic diol. The activation energies of these polyesters were found to be in the range of 100 to 200 kJ/mol. The effect of spacer type on activation energy was also reported. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 784–792, 2005     

Sub-Critical Crack Growth Behavior of a Low-Temperature Co-Fired Ceramic

The strength of sintered low-temperature co-fired ceramic (LTCC) disks was measured under dry (<2% relative humidity [RH]) and wet (98% RH) conditions, using ring-on-ring, biaxial-flexure testing in the stressing rate range of 0.002–2 MPa/s. The empirical sub-critical crack growth parameters, n and A , were calculated. The values of the parameters in the wet environment were higher than in the dry environment. It is argued that the high RH n value is more representative of material behavior. Results of fractographic examination are reported and expected lifetime and safe design stresses for LTCC are calculated.     

The oxidation of titanium silicide

This paper investigates the morphology changes that occur with the oxidation of a ti-tanium silicide—polysilicon system. These changes were studied as a function of poly-silicon doping and silicide formation parameters. Emphasis was placed on transmission electron microscopy studies of the samples by planar and cross sectional techniques. Various surface analysis methods have also been used to characterize the films. This study helps to define the possible use and shortcomings of a self aligned titanium silicide insulator. The results show that varying quality insulators result, dependent largely on the initial conditions of the titanium silicide. After oxidation the Auger and TEM anal-ysis show that in all cases some form of silicon dioxide was created, but typically a considerable amount of titanium oxide was also present. For instance, it was apparent that more titanium oxide formed on the samples RTA’ed for 1 min at 700° C than the 5 min at 800° C and considerably more on the arsenic doped sample than the boron doped. The silicide also had morphology changes as the result of the oxidation. There was a phase change from the C49 to C54 phase for the 1 min at 700° C samples as would be expected at the time and temperature of the oxidation. There also was a sig-nificant amount of agglomeration and epitaxial growth observed. Further work is re-quired to completely characterize these phenomena.     

Studying orientation relationships in heteroepitaxial systems

To date, the crystallography of deposits on single-crystal substrates has been studied by the use of linear transformation theories, and the results of the two-dimensional invariant line criterion are quite successful in rationalizing the epitaxial orientation of most metal/metal systems. In this article, however, we describe the use of a lattice potential-energy model (LPEM) to duplicate the results of the invariant line criterion by examining the case of silicides on (001) Si. Unlike geometric models that ignore the chemical nature of atoms and look only for strain energy, the LPEM has the potential of simulating real chemical bonding by altering the nature of the Fourier series used to generate the potential surface.     

Interfacial reactions in aluminosilicate short fiber-reinforced aluminum matrix composites

Aluminosilicate short fibers are one of the less expensive reinforcements used for the fabrication of metal matrix composites (MMCs). The present investigation evaluates the interfacial characteristics of Al-7Si-0.4Mg (356) alloy reinforced with 10 wt pct aluminosilicate short fibers using optical microscopy, electron microscopy, and X-ray analysis. The fibers used are standard- and zirconiagrade aluminosilicate short fibers. The interfacial analysis has shown the formation of MgAl₂O₄ and Si in both grades of fibers. In addition, ZrAl₃ formation is observed in the zirconia-grade fiber because of the interaction between the matrix and the dispersoid. The zirconia-grade fiber is more susceptible to interfacial reaction than the standard-grade fiber because of the presence of the highly reactive ZrO₂ phase and a lower amount of the Al₂O₃ phase, which provides resistance to the reaction.