Of the metallic materials, cobalt-based alloys are generally regarded as the most resistant to sliding wear at high loads, in the absence of lubrication. Under such conditions, a form of damage known as “galling” is common; this is thought to occur by gross plastic deformation, atomic bonding (or cold welding), then fracture on one or both surfaces, leading to material transfer. As will be shown, test results confirm that cobalt-based alloys are resistant to galling. Moreover, they indicate that the stainless steels are very prone to this form of damage, independent of their atomic structure.
Galling, however, is not the only form of damage encountered in unlubricated sliding systems. At lower loads, and particularly at high relative velocities (which induce considerable heating of the metallic surfaces), oxide growth and stripping is a possibility. Under such circumstances (sometimes termed “oxidative” wear), the characteristics of the base metal are somewhat masked. This is certainly true at temperatures in excess of about 500°C, where oxides tend to control the sliding wear process, as long as the underlying material is sufficiently supportive. At very high temperatures, very smooth oxide surfaces can be generated on sliding metallic materials; it has been surmised that these so-called “glazes” might involve some form of thermal transformation, or that fine oxide particulates are easily sheared and even out the contact surfaces.
The history of metal-to-metal wear testing at Haynes International (and former companies) spans 40 years. In the early years (1978 to 1986), galling and pin-on-block tests were used to evaluate and compare the characteristics of the cast and weld overlay materials (mostly cobalt- and nickel-based) in the company portfolio at that time (the hard-facing segment of the business was sold in 1986). A pin-on-disc (POD) unit, capable of elevated temperature testing, was built, but gave dubious results, and quickly fell out of favor. Some room and elevated temperature, metal-to-metal contract testing (at General Atomic, in California) was carried out in 1981 (Crook and Li, 1983), and gave clear indications of the benefit of cobalt, even though it was concerned with weld overlay (hard-facing) materials and not wrought products.
The second period of metal-to-metal testing at Haynes International coincided with the development and promotion of ULTIMET® alloy, the first wrought, cobalt-based, corrosion and wear resistant material to emerge from the laboratories of Haynes, following the sale of the hard-facing business. This period was from 1989 to 1992 and involved mostly galling tests of wrought material, although other forms of ULTIMET® alloy were tested, in particular weld overlays made from Haynes-produced wrought wire consumables.
Haynes International has only recently entered its third period of metal-to-metal testing, by virtue of the purchase of an ASTM G99 compliant pin-on-disc tester, plus the re-furbishing of Haynes’ galling test fixtures. More importantly, Haynes International has acquired a wide-area, laser-based, 3D measurement system, to enable extremely accurate surface analyses.
The ASTM G99 compliant, pin-on-disc test and the Haynes galling test are at opposite ends of the metal-to-metal sliding wear spectrum. The pin-on-disc test simulates high speed/low load conditions (found in many bearings and rotating seals, for example), whereas the galling test simulates low speed/high load conditions (found during the high torque attachment and detachment of bolts, for example). In the absence of liquids, high surface temperatures can be generated during high speed/low load, metal-to-metal sliding; these can result in debris containing oxide particles.
Galling, on the other hand, can result in seizure (due to bonding of the materials) and can generate relatively massive amounts of damage over small sliding distances (large chunks of material being transferred from one surface to the other, or released at the interface).
To illustrate the influence of alloy base and of strengthening mechanism on high speed/low load sliding wear, early work at Haynes International on the ASTM G99 compliant, pin-on-disc tester has been focused on (self-coupled) HAYNES® 25, 230®, and 282® alloys. HAYNES® 25 alloy is a cobalt-based alloy containing 10 wt.% nickel to adjust its transformation tendency. HAYNES® 230® alloy is a nickel-based alloy with remarkably similar alloying additions, at similar levels. HAYNES® 282® alloy is a gamma-prime strengthened, nickel-based material.
The extent of pin wear on the three alloys is shown in the following bar chart, which indicates that the cobalt-based, HAYNES® 25 alloy is the most resistant of the three alloys to wear at a load of 30 N, and a linear speed of 2 m/s.