Erosion has been defined as the progressive loss of original material from a solid surface due to the mechanical interaction between that surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles (Hutchings, 1983). In solid particle erosion, the particle sizes are typically between 5 and 500 µm, and the relative velocities between 5 and 500 m/s. In most cases, the particles are traveling at high speed, when they strike the surface. However, the reverse is possible, as in the case of helicopter blades in dust-laden air.
In solid particle erosion, the impact angle (defined as the angle between the plane of the surface and the particle trajectory) is very important. Ductile materials appear to suffer most at impact angles between 20 and 30°, whereas brittle materials exhibit maximum rates of degradation when the impact angle is 90° (Hutchings, 1983). The former has been described as ductile erosion behavior, involving plastic flow as the primary mode of degradation. The latter has been described as brittle erosion behavior, with brittle fracture as the predominant degradation mechanism.
Rates of degradation during solid particle erosion are strongly dependent upon the velocity with which the particles strike the surface (or vice versa). In fact, for a constant impact angle, the rate has been found to be proportional to velocity to the power n, where n usually falls within the range 2.3 to 2.5 for ductile materials, and within the range 2 to 4 for brittle materials (Hutchings, 1983).
Except in the case of very fine particles, the relationship between the erosion rate of brittle materials and particle radius also appears to obey a power law, with exponents in the range 0 to 1. Fine particles, surprisingly, induce a response in brittle materials that is pseudo-ductile, i.e. the angular dependence is similar to that for ductile materials. The erosion of ductile materials is generally independent of particle size, with diameters above 100 to 200 µm. At smaller diameters, the relationship is approximately linear, though little erosion is experienced with particles of diameter 5 µm and less (Hutchings, 1983).
With regard to the nature of the eroding particles, sharp particles obviously cause more damage than rounded particles. Surprisingly, however, there is not a strong relationship between erosion rate and particle hardness, provided the particles are harder than the surface being eroded. For particles softer than the surface, the erosion rate falls sharply with reduced particle hardness (Hutchings, 1983).
With regard to metallic materials, attempts have been made to establish relationships between microstructural characteristics and solid particle erosion resistance. Several studies indicate, for example, an inverse relationship between the hardness of martensitic steels and their erosion resistance (Gulden, 1979 and Green et al, 1981).
Likewise, carbides can be deleterious to the solid particle erosion resistance of white irons, if they are softer than the eroding particles (Aptekar and Kosel, 1985). If they are harder, the opposite is true. From these studies, it is evident that hardness, especially if due to the presence of martensite or carbides in metallic microstructures, is, if anything, a measure of lack of resistance to this form of wear.
Cobalt alloys have been included in several room temperature, solid particle erosion studies through the years, notably those described in Ninham, 1987 and Levy and Crook, 1991. It is valuable to review some of these results, in light of the principles presented in Hutchings, 1983. In the Ninham, 1987 study, Alloys 6, 6B, and HAYNES® 188 (a low carbon Co-Cr- Ni-W alloy designed for use in the hot sections of flying gas turbine engines) were tested, along with several chromium- bearing nickel alloys and stainless steels. Three variables were studied, namely the type of eroding particle (silicon carbide or quartz), the impact angle (30, 60, or 90°), and the condition of the material. One of the test alloys (718) was age-hardenable, so was tested in both the age-hardened condition and the annealed condition. Two of the alloys (188 and C-276) can be cold-reduced, to enhance their room temperature strength, so these were tested in both the annealed and cold-reduced conditions.
The apparatus used to assess the effects of these variables is described in detail in Levy, 1981. Essentially, it comprises a vibrating hopper, to feed the erosive particles into a high velocity air stream, and a test chamber, in which impingement occurs. A particle velocity of 60 m/s was used, and sample weight measurements taken every 20 or 40 g of erosive particles used. The silicon carbide particles were angular and had diameters between 250 and 300 µm. The quartz particles were between 75 and 200 µm diameter, and of an unspecified shape. The hardnesses of these two materials (taken from Hutchings, 1983) are 2100 to 2480 kgf/mm2 for silicon carbide and 820 kgf/mm2 for quartz (SiO2).
One of the main conclusions of this work was that aging and cold-working have little effect upon the solid particle erosion resistance of alloys of this type. Also, the effect of impact angle was small. In the case of silicon carbide, the angle effects were in line with those defined for ductile erosion behavior in Hutchings, 1983 (a 30° impact angle causing the highest rate, and 90° causing the lowest). In the case of quartz, however, the angle effects were both mixed and minimal. As might be expected, the angular silicon carbide particles did more damage than the quartz particles.
Perhaps the most important fact to emerge from the work described in Ninham, 1987 was that there is not much difference between the various alloys (under these test conditions), irrespective of the alloy base (cobalt vs. nickel vs. iron), and irrespective of the microstructural condition (annealed vs. aged vs. cold-reduced). Although the carbides present in Alloys 6 and 6B did not appear to be of benefit, at least they were not detrimental, as they were for the white irons in Aptekar and Kosel, 1985.
The Levy and Crook, 1991 study, involving many of the same wrought alloys tested under abrasive wear conditions, was limited in scope, but again provided evidence that alloy base and carbides are of little importance in solid particle erosion, at room temperature. One of the materials, ULTIMET® alloy, was tested at different impact angles, with two different types of eroding particle (400 µm angular silicon carbide and 80 µm aluminum oxide, of an unspecified shape). The erosion rates measured are shown below, as a function of impact angle. As in the Ninham, 1987 study, silicon carbide induced ductile erosion behavior, whereas aluminum oxide produced a slightly different response, the erosion rate at an impact angle of 60° being equal to that at 30° impact. According to Hutchings, 1983, the hardness of aluminum oxide is similar to that of silicon carbide; however, the shape of the aluminum oxide particles was not specified.
An important part of the Levy and Crook, 1991 study was some solid particle erosion testing at high temperature (850°C), using the same aluminum oxide particles and an impact angle of 30°. The alloy with the lowest strength, 316L stainless steel, exhibited the lowest erosion rates by far in this test, suggesting that the particles might have become embedded in the surface, rather than causing the surface to deform and fracture. Of course, oxides films are very important at such high temperatures.