Wear Mechanism of Surfacing Materials on Oil Drilling Rig Disc Brake Surface

Literature Overview

The paper by Wang Xinhua, Zhang Siwei, and Wang Deguo, published in China Surface Engineering in 2008 (Volume 21, Issue 3, pages 44-49), presents a comprehensive study on the wear mechanism of surfacing materials developed for oil drilling rig disc brake surfaces. Funded by the China National Petroleum Corporation's "Ninth Five-Year" Science and Technology Development Project, this research addresses a critical component in drilling operations where disc brakes must withstand extreme thermal and mechanical loads during drilling and tripping operations.

Core Technical Content

The study develops a specialized surfacing material for disc brake surfaces and investigates its tribological behavior under variable-temperature friction and wear conditions. The research employs scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), and X-ray diffraction (XRD) to characterize the worn surface morphology, composition, and phase structure at thermal equilibrium temperatures of 150 °C and 300 °C.

The surfacing material is designed to improve both wear resistance and thermal fatigue resistance of the disc brake. The material contains iron-based solid solutions of alpha-Fe and Cr, along with carbides (Fe3C, Fe5C2, SiC) and intermetallic compounds (NbNi, NbNi5) dispersed throughout the matrix.

Phase Structure and Wear Mechanism Analysis

The XRD analysis reveals several important metallurgical phenomena:

• The alpha-Fe and Cr iron-based solid solutions provide excellent crack-arresting capability, which is crucial for resisting thermal fatigue cracking in the brake disc.
• Under friction heating, an alpha-Fe to gamma-Fe transformation occurs, generating a friction-induced austenite phase gamma-(Fe,Ni) + alpha-Fe + carbides.
• This friction austenite phase enhances the plastic deformation capacity of the worn surface material, thereby improving thermal fatigue resistance.
• The dispersed carbides Fe3C, Fe5C2, SiC and intermetallic compounds NbNi, NbNi5 have high hardness and significantly enhance the wear resistance of the brake disc.

The SEM and EDAX analysis reveals that the wear process follows a dynamic cycle: surface film formation, growth, rupture (tearing), detachment, and regeneration. The overall wear mechanism is a combined result of abrasive wear, oxidative wear, and adhesive wear during the friction process.

Engineering Significance

The development of this surfacing material addresses a specific industrial need in the oil drilling industry. Disc brakes in drilling rigs experience:

• High friction velocities during emergency braking, generating significant friction heat.
• Repeated thermal cycling between ambient temperature and high operating temperatures.
• Contact with abrasive drilling fluids and debris.
• Cyclic loading that can initiate fatigue cracks.

The surfacing material's ability to form a protective surface film during operation is particularly significant. This film acts as a barrier against direct metal-to-metal contact, reducing adhesive wear and moderating the friction coefficient. The regeneration capability of this film ensures sustained protection over extended service periods.

Key Questions and Reflections

Several aspects of this research warrant further consideration. The study characterizes the wear mechanism at 150 °C and 300 °C, but actual disc brake operating temperatures during emergency braking can exceed 500 °C locally. The behavior of the surfacing material at higher temperatures, particularly the stability of the intermetallic compounds and the extent of the alpha-to-gamma transformation, remains to be investigated. Additionally, the long-term durability of the surfacing layer under millions of friction cycles is critical for field application, and accelerated life testing would provide valuable data for service life prediction.

The finding that friction-induced austenite formation enhances thermal fatigue resistance is particularly insightful. This suggests that the material has a self-reinforcing mechanism where the wear process itself generates phases that improve performance. This principle could be extended to other tribological applications where controlled phase transformation during service is beneficial.

Study Insights and Implications

This research demonstrates the power of multi-scale characterization in understanding tribological phenomena. By combining XRD for phase identification, SEM for morphology, and EDAX for elemental mapping, the study provides a comprehensive picture of the wear mechanism. For engineers designing surfacing materials for high-temperature tribological applications, the key lesson is that the material response during operation is dynamic and self-evolving. The design of surfacing alloys should account for in-service phase transformations and the formation of protective surface films, not merely the as-deposited microstructure. The incorporation of elements such as Ni to promote friction austenite formation and Nb to form stable intermetallic compounds represents a rational alloy design approach for improving both wear and thermal fatigue resistance simultaneously.