Hard-material composite alloys

Hard alloys and hard composite materials are defined as metallic materials on an iron, nickel, or cobalt basis with a volume content up to ≈ 50 per cent of hard particles, such as carbides, borides, and nitrides for protecting against wear. Hard alloys are obtained by solidification of a melt with the precipitation of hard phases. The alloy and composite material can constitute mixed forms, as is the case with thermal spraying with a non-fused hard material component.

Metal carbides, borides, and nitrides are especially well suited for use as hard phases for the following reasons:

(a) The high solubility of the components in the melt is in contrast to the low solubility of these components in the solid state; consequently, solidification results in a high yield of hard phases.

(b) An increase in the share of covalent bonding results in a several-fold increase in the hardness of the hard phases, in comparison with that of the metallic matrix, and thus provides effective protection against wear by abrasive particles.

(c) Nevertheless, the metallic bonding component provides higher ductility for these brittle hard phases, in comparison with that of abrasive oxide mineral particles. Consequently, fracture of the mineral is more probable upon contact than that of the hard phase.

(d) Strong bonding is present between carbides, borides, and the surrounding metal matrix and thus enhances the mutual adhesion of these structural components. When subjected to stress, oxides of comparable hardness become more readily detached from the metal matrix at the interfaces because of the weaker bonding.

If hard phases with ceramic properties are embedded in a matrix with metallic properties, the resulting materials provide a favourable combination of wear resistance and fracture resistance. By appropriately adjusting the quantity, type, size, shape, and distribution of the structural components, the properties of components can be varied from metallic-ductile to ceramic-hard within wide limits and adapted to match the particular application. Thus, this material group is well suited for protecting against wear in a wide range of applications, /Bürgel98/.

In contrast to the hard alloys, particles of hard materials are added in the solid state in the case of hard composite materials, rather than being formed in-situ from a melt. To an increasing extent, alloyed metal powder is mixed with powdered hard material, and the resulting mixture is subsequently employed by powder-metallurgical methods. In this manner, structural building blocks can be arbitrarily combined and arranged. Excellent wear-resistant properties are obtained at both room temperature and elevated temperature with the use of tungsten carbides (eutectic WC/W₂C) in a high-temperature-resistant steel matrix as well as in a precipitation-hardenable Ni matrix.

Physical and mechanical properties of a few hard materials suitable for use as weld fillers are compiled in table 2.

Table 2: Physical and mechanical properties of hard materials