Case Study

Abstract

The sealing elements are the most critical part of the mechanical sealing assembly and the main aspects of these are the tribological ones. Industrial applications require the sealing of high-quality materials that resist friction against each other for different operating conditions (e.g. sealed fluid pressure, rotational speed and viscosity). Most dynamic seals, such as mechanical seals, fall into the category where friction surfaces are separated and lubricated with a thin film of lubricant [1]. For devices executed from thin films is evident that stress-strain relationship is affected by the relatively high area to volume ratio. As a result, typical properties used for the description of bulk material strength and deformation do not apply to thin films and micro devices. In particular, this size effect relationship is not yet well understood when structural dimensions decrease from millimeters to micro- and nanometers [1]. Furthermore, sealing rings are subjected to chemical attack by a wide range of fluids present in the mechanical system. The structural behavior of the sealing components also plays a crucial role in determining the sealing performance. In general, a mechanical seal works with an extremely thin film lubricant in the sealing space, the thickness thereof being typically in the range of 0.1–1.0 μm to reduce leakage and to limit friction and wear [1–3]. When the sealing rings engage each other, the tribological characteristics of the material combination of this seal determine either the durability or the failure of the sealing. It is recommended to choose a suitable couple of sealing materials, so that the relative sliding movement does not generate excessive heat in the seal. The best sealing materials have low friction, high hardness and difference in value, are not soluble in one another, good corrosion resistance, good bearing properties, good machining capacity and high thermal conductivity [4, 5]. Wide varieties of materials are used as seal face materials in practice. In this chapter we focus on a couple of sealing materials CoCrWMo alloy/four different types of materials (copper-zinc alloy – brass, tin-copper alloy - bronze, graphite bronze and Teflon – PTFE). Dry and lubricant friction and block on ring configuration are considered using a tribometer. Rings made from a CoCrWMo alloy were manufactured using the selective laser melting (SLM) process. Tribological tests were run at room temperature (RT) on different couples. These materials present different tribological behaviors in terms of sliding wear evaluation. In this chapter, based on the experimental data performed, we appreciate that in contact with the proposed CoCrWMo alloy material as the sealing friction semi coupling the material with the best tribological behaviour under both dry and lubricated conditions is graphite bronze. At lower loads, the CoCrWMo/PTFE alloy can be considered as a couple of material for a mechanical sealing system.