Multiphase hard materials

Multiphase Hard Materials Based on Carbide-Nitride-Boride-Silicide Composites... 

Thermoplastic elastomers are often multiphase compositions in which the phases are intimately dispersed. In many cases, the phases are chemically bonded by block or graft copolymerization. In others, a fine dispersion is apparentiy sufficient. In these multiphase systems, at least one phase consists of a material that is hard at room temperature but becomes fluid upon heating. Another phase consists of a softer material that is mbberlike at RT. A simple stmcture is an A—B—A block copolymer, where A is a hard phase and B an elastomer, eg, poly(styrene- -elastomer- -styrene). 

Thermoplastic elastomers are multiphase composites, in which the phases are intimately depressed. In many cases, the phases are chemically bonded by block or graft copolymerization. At least one of the phases consists of a material that is hard at room temperature. ... 

The fact that crystalline polymers are multiphase materials has prompted a new approach in characterizing their internal structure (lamellar thickness, perfection, etc.) and relating it to the hardness concept (volume of material locally deformed under a point indenter). In lamellar PE microhardness is grossly a given increasing function of lamellar thickness. In using the composite concept care must be exercised to emphasize and properly account for the non-crystalline phase and its various... 

The Knoop test is a microhardness test. In microhardness testing the indentation dimensions are comparable to microstructural ones. Thus, this testing method becomes useful for assessing the relative hardnesses of various phases or microconstituents in two phase or multiphase alloys. It can also be used to monitor hardness gradients that may exist in a solid, e.g., in a surface hardened part. The Knoop test employs a skewed diamond indentor shaped so that the long and short diagonals of the indentation are approximately in the ratio 7 1. The Knoop hardness number (KHN) is calculated as the force divided by the projected indentation area. The test uses low loads to provide small indentations required for microhardness studies. Since the indentations are very small their dimensions have to be measured under an optical microscope. This implies that the surface of the material is prepared approximately. For those reasons, microhardness assessments are not as often used industrially as are other hardness tests. However, the use of microhardness testing is undisputed in research and development situations. 

The measurement of local mechanical properties is an important step in understanding of the macroscopic behavior of multiphase materials. The indentation hardness test is probably the simplest method of measuring the mechanical properties of materials. Figure 12.2b shows the evolution of the microhardness as a function of the thermal treatment temperature of a Nasicon sample. The use of load-controlled depth-sensing hardness testers which operate in the (sub)micron range enables the study of each component of the composite more precisely. 

All materials subject to size reduction through grinding will exhibit a distribution of particle sizes, often skewed in shape, with the result that a single estimate of size will not represent the sample as a whole. In addition, the grinding of multiphase samples most often results in differential size reduction of hard and soft phases. In practice, the difficulty in obtaining an accurate estimate of individual particle sizes usually means that analysts make an informed guess at the value of D. Thus, the value used may be empirically based to achieve a desired phase abundance rather than a value based on sound measurement. Widespread misuse of microabsorption correction was clearly demonstrated in the lUCr quantitative phase analysis round robin. ... 

Figure 20.3. Comparison of the predicted Young s moduli of binary multiphase materials with morphologies best described by the aligned lamellar fiber-reinforced matrix model (Equation 20.1), the blend percolation model (Equation 20.2), and Davies model for materials with fully interpenetrating co-continuous phases (Equation 20.3). The filler Young s modulus in Equation 20.1 was assumed to be 100 times that of the matrix, and calculations were performed at Af=10, At-=100 and Af=l()00 to compare the effects of discrete filler particles with differing levels of anisotropy. It was assumed that E(hard phase)=100, pc=0.156 and (3=1.8 in Equation 20.2. For...

In order to determine the local mechanical properties such as the hardness and modulus of non-flat samples or heterogeneous multiphase materials, a DSI system was integrated in a scanning device. This system combines SFM-like topography imaging with the ability of DSI tests to be performed at selected areas of interest by using a well-defined diamond tip. 

Second, there is no reproducibility of the resulting compositions in duplicate experiments. Third, die final products of the same gross composition reactions, for instance, R+2S or RS. 50+0-5S, are not always identical, and depend on the origin of the starting products and the sequence of temperature and pressure variation. With these principles, one can suggest that the powdered polysulfides prepared by different authors (see sect. 3.3) can hardly be related to equilibrium materials. A multiphase surface state of powders has a serious effect on many properties and, thus, merits detailed consideration here. It is apparent that special means must be used to provide data on the gross composition and the spatial variation of the sulfur content over the grain. 

Amalgam butter—a commonly used term in mercury cell operations—is a multiphase material, which has not been characterized thoroughly. The butter formed with Fe, Ni, and C consists of finely divided metal particles dispersed in mercury, whereas graphite butter is shiny and foamy. Ca butter is hard and compact, while Fe butter is blackish. 

For ceramic materials that are not extremely hard (e.g., stabilized zirconium oxide or multiphase materials), surface relief can be created within a few minutes by means of final polishing with colloidal silica on a chemicaUy resistant, short-napped fiber cloth. Because removal is dependent on the grain orientation and the type of phase, it is possible to distinguish between grains of a single phase and between different phases during microscopic examination. Application of the DIC method makes it possible to use even poorly defined reUef to display microstructure under the optical microscope. A brief final polishing step with very fine alumina (0.05 pm) will help reveal the spinel phase in aluminum oxide materials, for example. 

Despite the above-mentioned problems, hardness testing is a recognized method for evaluating components made of multiphase ceramics (Fig. 154) and making rough assessments of a material s mechanical properties. 

Multiphase polymers are commonly toughened plastics which contain a soft, elastomeric or rubbery component in a hard glassy matrix or in a thermoplastic matrix. An example of the typical brittle fracture morphology of an unmodified thermoplastic is shown by SEM of nylon (Fig. 5.45A). Addition of an elastomeric phase modifies the brittle fracture behavior of the matrix, as shown in a fracture surface of a modified nylon (Fig. 5.45B). The modification depends on the composition and deformation mechanism of the material [204, 215], but normally it increases the fracture toughness and strength from that of the unmodified matrix resin. Impact strength, as measured for instance by an Izod impact testing apparatus, is affected by the dispersed phase... 

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