Tribological characterization of dynamic braking systems

For a recently developed high precision module, typical materials would not provide an adequate level of friction and wear to the dynamic braking systems for use in both vacuum and ambient. Therefore, a tribological characterization was carried out to provide insight in the material selection. The material selection included different stainless steel grades, hardening processes, roughness level, and Physical Vapor Deposition (PVD) coating to cope with a required minimum level of friction (even with possible presence of grease) to ensure braking with low particle generation.

On request of a semiconductor OEM, Philips Engineering Solutions has developed a high precision positioning module. This module uses brake systems during standstill to increase stiffness, carry the load and maintain position. The main requirements, among others, include a required minimum level of friction (e.g., CoF ≥ 0.3 to ensure braking and load carrying capacity), prescribed contact pressure (e.g. ranging from 100 to 200 MPa), operating in both vacuum and ambient, as well as the possible presence of high vacuum grease.

Main questions:

• Is it possible to design the braking system with conventional hardened stainless steel?
• What is the risk of cold welding/ fretting / galling? Can risks be mitigated by any special hardening (thermal and/or chemical treatment) and/or coating (PVD coating)? Which treatment and/or coating?
• What is the influence of roughness on friction and wear? Can the required low wear (particle generation) be achieved?
• Can the required coefficient of friction be ensured with low wear (particle generation) even in the worst-case scenario with the presence of grease?

Approach

Philips Engineering Solutions managed to translate the customer needs into a pin-on-plate testing campaign by mimicking the operation conditions of the dynamic braking system (load, speed, atmosphere, etc.). See Figure 1 for the test setup.

Figure 1: Vacuum tribometer

The systematic test campaign included variations in:

• Materials (different stainless steel grades)
• Hardening processes (thermal and thermo-chemical treatments)
• Roughness levels
• PVD coatings
• Loads and speeds
• Lubricated conditions: grease as well as dry. The selected greases are normally used to lubricate mechanical parts operating at high vacuum

Additionally, outgassing measurements of the proposed coatings and greases (including untreated and baked) were carried out. Next to that, physical (viscosity) and chemical (composition) analysis of the grease were performed.

Result

This tribological characterization provided design rules with respect to material selection including substrate material (stainless steel), hardening process, PVD coating and roughness level, thus ensuring that the requirements critical to the function of the dynamic braking system were met. Figure 2 shows some typical results of the tested surfaces: PVD coating 2 (Figure 2c) provided approximately 2.5 times higher static coefficient of friction and hardly visible wear as compared to uncoated (Figure 2a) and PVD coating 1 (Figure 2b).

Figure 2: Tested surfaces: a) uncoated, b) PVD coating 1 and c) PVD coating 2