Whiskers intermetallics

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

Metal-Ceramic Composites. Metals such as aluminum, titanium, copper and the intermetallic titanium aluminide, which are reinforced with silicon-carbide fibers or whiskers show an appreciable increase in mechanical properties particularly at elevated temperatures. These composites are being considered for advanced aerospace structures.1 1... 

The use of lead in tin plating and solder has been used to inhibit tin whisker growth. Rare reports of tin whiskers have been reported from tin-lead coatings. Whiskers evolve from tin surfaces as a stress-relief mechanism between tin crystals. Most stress in tin deposits arises from the growth of copper-tin intermetallics at the interface with underlying copper. With a much slower rate of nickel-tin intermetallic formation, tin whiskers form at a much lower rate than tin coatings on nickel underplate. 

The generalized BE approach also finds application in the P/M processing of composite materials for high-temperature service— i.e., the term could be applied to the introduction of oxide particles or unconstitutional precipitates (e.g., refractory intermetallic compounds) as dispersion strengtheners, or of suitably coated SiC or refractory metallic whiskers for fiber reinforcement. 

Fibers and whiskers of intermetallic materials also are being considered for strength-enhancing fillers in metal- and ceramic-matrix composites. 

Compressive stresses in the film are built up over time due to the diffusion of copper atoms from the substrate into the tin film, and the subsequent formation of the intermetallic CueSns. The as-deposited stress state for the tin film was net tensile (+11 MPa) and changed to a compressive stress (—8 MPa) after a few days. Subsequent to whisker formation and about 50 days later, the stress level decreased (—5 MPa). 

FIG. 20 The first published FIB cross-sectional micrograph of a tin whisker grown on a copper substrate showing an intermetallic SnCu particle at the root of the whisker. (From Ref. 41.)... 

FIG. 22 (a) SEM micrograph of a tin whisker before FIB cross-sectioning, (b) FIB cross-sectional micrograph of a whisker grown by thermally cycling a 10-pm-thick matte-tin film over a 1-pm-thick nickel underlay on a copper substrate. The whisker was determined to be pure tin and appeared to form from the surfaee grain. Intermetallics were observed to form at the grain boundaries. (From Ref. 49.)... 

FIG. 30 An FIB cross-sectional micrograph of the bright-tin nodule-whisker structure shown in Fig. 29 taken closer to the center of the nodule-whisker structure. In this micrograph, there is a large SnCu intermetallic particle at the base of the nodule. (Courtesy of G. Galyon and L. Palmer, IBM Corporation.)... 

There is no industry standard acceleration test to characterize the whisker-formation capabihty of a given film. Accelerated tests are typically performed at elevated temperatures to increase the reaction kinetics. For Sn films, it is necessary to keep the temperature below about 80°C to avoid the creation of intermetallics compounds (CusSn) not seen at lower temperatures. At these lower temperatures whisker growth rates are still quite slow. At temperatures above 80°C, diffusion is much faster but the microstructure is not equivalent to the room temperature microstructure as a result of the formation of the CusSn intermetallic compound. In addition, at higher temperatures the stresses are relieved quicker due to creep mechanisms, which decreases the driving force necessary for whisker formation. Hence, there exists a tradeoff between driving force and kinetics in standard temperature-based acceleration testing. 

Metal matrix composites (MMCs) are a group of materials (such as metals, alloys or intermetallic compounds) incorporated with various reinforcing phases, such as particulates, whiskers or continuous fibres. Based on the mechanical properties of the reinforcing phases, the composite materials could be simply divided into two categories [1]. In the first category, the matrix is reinforced with a ductile component, typically a refractory, such as... 

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