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Clad Core Clad

Clad

Core Clad

Core Clad

Clad Core Clad

Clad Core Clad

FIGURE 10.7 Schematic illustrating differences between (a) discrete architecture (multimode step index), (b) continuous architecture (multimode graded index), and (c) non-graded index (single-mode step index). (Taken from [3].)

in semiconductor lasers. Further details of these applications may be found elsewhere [3].

Materials in which magnetic properties are graded may be desired in applications where a magnetic sensor may be used to measure position [33]. A unique approach to processing graded magnetic steels is by rolling deformation of wedge-shaped samples [34]. In this technique, different sections of the wedge experience different strains (as measured by the rolling ratio) and thus exhibit different degrees of transformation from austenite (low saturation magnetization) to martensite (high saturation magnetization).

10.4 FABRICATION 10.4.1 General

It is useful to view fabrication methods as falling into one of the two categories shown in Fig. 10.1: constructive processes and transport-based processes. Constructive processes are those in which material is placed in the appropriate locations by some technique such as vapor deposition or powder metallurgy. Transport-based processes utilize the presence of a steep gradient to promote mass transport (e.g., atomic diffusion) from one location to another. A functionally graded material is in some sense inherently unstable since at a high enough temperature and long enough time, atomic or molecular diffusion occurs until there is no gradient. This natural tendency for the material to homogenize limits its use conditions. As with many material fabrication processes, the temperature of fabrication of FGM components should be higher than the service temperature or else structural and compositional evolution of the FGM may occur during service. However, the greater challenge with respect to the compositional or microstructural instability may be in processing the material because the processing conditions are typically more severe than the service conditions, as is true with many engineering materials. Furthermore, methods for producing many conventional materials seek to produce uniform composition and microstructure and therefore may not apply to graded material fabrication. Some methods not discussed here but used to produce FGMs include electrophoresis, electrodeposition, reaction synthesis (or self-propagating high-temperature synthesis), cladding, and sedimentation.

10.4.2 Thermal Spraying

Because of the ability to control composition with relative ease and the potential for batch manufacturing, thermal spraying is one of the easiest methods to fabricated graded coatings. In addition, it may be used to form bulk materials [35]. Generally, feedstock consists of powders, though rods or wire may also be used. The feedstock is mixed in the appropriate composition while it is fed into an intense heat source, such as a combustion plasma, an arc, or a laser beam. A torch is frequently used to generate the combustion plasma, and this is usually referred to as plasma spraying. The particles of material melt while they feed through the heat source, and they impinge on the substrate, typically undergoing rapid solidification. For more details on thermal spraying, the reader is referred to references [35-38]. One of the reasons composition may be easily controlled is that plasma spraying may involve multiple feeders that either feed into a single torch or multiple torches, thereby resulting in blended powders. Additionally, very dissimilar materials, such as refractories and metals may be simultaneously melted in the desired composition. One of the disadvantages is that the microstructure, including the texture, porosity, and presence of nonequi-librium phases is somewhat difficult to control. Some of these latter aspects may be affected by adjusting the cooling rate during deposition. Certain applications in which porosity gradients are desired (e.g., where increased compliance toward the coating surface is required) may utilize the ability to control porosity through the thickness of the coating by adjusting parameters such as the particle size, the plasma gas pressure, and the torch-to-substrate distance.

10.4.3 Vapor Deposition

10.4.3.1 Physical Vapor Deposition

Physical vapor deposition (PVD) provides a great degree of compositional control because multiple sources and targets may be employed. The most common method for producing graded coatings, electron beam physical vapor deposition (EB-PVD) utilizes an electron beam to heat the target material. Evaporation from the target to the substrate then occurs, the rate depending on a number of factors including temperature, constituent vapor pressures, and the geometrical conditions. The most successful method to produce graded compositions is to employ multiple targets using multiple electron guns. Single-gun EB-PVD units may be programmed so that the gun jumps back and forth between targets, at frequencies as high as kilohertz [3], varying whatever parameters (e.g., energy) are necessary to achieve the desired evaporation rates. Sputtering may also be employed to form films and coatings, though the rates are generally lower than EB-PVD. In sputtering, an inert gas is ionized using a high voltage. The resulting high-energy ions accelerate toward the cathodic target, and this results in the release of target material atoms that then deposit on the substrate. Sputtering is typically used to form wear-resistant coatings, such as in compositionally graded TiN-coated titanium alloys for surgical implants [3]. In this latter case, pure titanium is sputtered in an Ar-N2 atmosphere, forming an amount of TiN that depends on the relative Ar-N2 concentration. The Ar-N2 concentration is varied during the deposition to produce a graded composition.

10.4.3.2 Chemical Vapor Deposition

Chemical vapor deposition (CVD) utilizes a source gas fed into a reaction chamber that excites the gas, typically by using heat, light, or a plasma. The gas reacts to form products that subsequently deposit onto a substrate. Control over composition of a CVD coating is achieved by modifying the gas mixture, the gas flow rate, the gas pressure, or the deposition temperature. Some common graded coatings fabricated by CVD include C-SiC, ZrC-C, and diamond-metal.

10.4.4 Powder Metallurgy

The techniques used to form graded materials by the sintering of powders are similar to those used for ceramics and metals, and the reader is encouraged to refer to other chapter of this handbook. A layered geometry is made by stacking powders of varying composition prior to consolidation. It may be difficult to achieve layer thickness uniformity when stacking powders of different compositions, particularly when the layer of each composition is thin (e.g., less than about 1 mm in the final dense part). Automated systems have been designed to maximize uniformity of layer thickness, allowing very large diameter (up to 300 mm) parts to be fabricated [39].

One of the most significant issues in processing graded materials by powder metallurgy concerns the residual stresses that may develop from differential shrinkage characteristics. This residual stress is distinctly different from that developed due to differential thermal expansion coefficients. Because different materials exhibit different (1) initial packing densities, (2) sintering start temperatures, and (3) sintering rates, mismatch strains develop during densification. Two methods used to minimize this mismatch include [40]: (1) adding different amounts of an organic binder phase to different regions of the FGM so that the initial packing densities are closely matched, and (2) altering the constituent particle sizes in different regions to produce different sintering start temperatures, sintering rates, and potentially initial packing densities. An example formulation is given for a Ni-Al2O3 system in Fig. 10.8 [40]. The same principle could be applied to other materials. The procedure is to conduct a set of experiments to determine the differential sintering characteristics (sintering rate and sintering start temperature) and the initial packing densities for each compositional layer. Once these are determined, matching the amount of shrinkage in each layer may be achieved by adding a binder phase that burns out and altering particle sizes, provided the latter does not change the resulting microstructure in an undesirable way. The same procedure may be used to intentionally produce gradients in porosity in a material, for example, for fabricating a graded porosity preform that is subsequently infiltrated; an example is described in [41]. Other techniques for producing compositional gradations during powder preparation include powder

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