High-Temperature Friction-Wear Characteristics of Plasma Overlay Stellite Alloys

Literature Overview

This paper, authored by Qu Shengguan and colleagues from South China University of Technology and Huaiji Dengyun Auto Parts Co., Ltd., published in Tribology International in 2016 (Vol. 36, Issue 3, pp. 362-370), investigates the high-temperature friction-wear behavior of Stellite 1 and Stellite F alloys deposited by plasma arc overlay welding on valve steel substrates. The study employs a MMU-10G high-temperature end-face friction-wear tester to evaluate the performance of both Stellite alloys at elevated temperatures, providing valuable data for applications in high-temperature wear environments such as valve seats, turbine components, and exhaust system parts.

Core Technical Findings

The study begins with a comprehensive characterization of the overlay microstructure, including metallographic examination, XRD phase analysis, and hardness measurement. Both Stellite 1 and Stellite F coatings exhibit a matrix of austenite or austenite-ferrite, with a high volume fraction of hard carbides including Cr7C3, Cr23C6, and Mo2C. The specific carbide composition and distribution differ between the two alloys due to their different compositions: Stellite 1 contains approximately 5% Cr and 1% Mo, while Stellite F contains approximately 27% Cr and 4% Mo, which significantly affects the carbide type and volume fraction.

The high-temperature friction-wear testing reveals a complex relationship between temperature and tribological behavior. As temperature increases, the friction coefficient of both Stellite alloys increases, while the wear volume decreases. This counterintuitive finding—where wear volume decreases with increasing temperature—is attributed to the rapid formation of protective oxide films on the wear surface at elevated temperatures. The oxide films act as a lubricant and protect the underlying metal from direct contact, reducing material removal despite the increased friction.

Wear Behavior at Different Temperatures

The most critical temperature is 400°C, where both Stellite alloys exhibit the most severe wear behavior. At this temperature, the oxide film formation is not yet sufficient to protect the surface, but the substrate hardness has begun to decrease, creating an unfavorable combination. The wear surfaces at 400°C show extensive fatigue cracking and wide ploughing grooves, indicating that the coating is being subjected to both cyclic stress and direct mechanical removal. This temperature represents a critical service limit for Stellite overlay coatings in high-temperature applications.

At 500°C and 600°C, the wear behavior improves as the oxide film becomes more effective. The wear traces narrow at the center and widen at the edges, where wear debris accumulates and is compacted, leading to severe adhesive wear at the trace edges. This edge adhesive wear is responsible for the increase in friction coefficient at higher temperatures, as the compacted debris creates a third-body layer that increases shear resistance.

Engineering Practice Implications

For engineers designing components that operate at elevated temperatures—such as valve seats in internal combustion engines, exhaust system components, and turbine blade coatings—this study provides critical temperature-dependent performance data. The identification of 400°C as the most severe wear temperature is particularly important for service life prediction and maintenance scheduling. Components operating near this temperature should be designed with additional safety margins or protected with additional thermal barrier coatings.

The difference in performance between Stellite 1 and Stellite F at various temperatures provides guidance for alloy selection. Stellite F, with its higher Cr and Mo content, generally exhibits better high-temperature oxidation resistance due to the formation of more protective chromium oxide films. However, Stellite 1 may be preferred in applications where lower cost is a priority and the operating temperature is below 400°C or above 500°C. The study's findings suggest that the choice between these two alloys should be based not only on room-temperature properties but also on the specific operating temperature range of the application.

Study Insights and Reflections

This study makes a valuable contribution to the understanding of high-temperature tribology of overlay coatings, an area that is often neglected in favor of room-temperature testing. The identification of the 400°C critical temperature is a practically important finding that can directly inform design decisions for high-temperature components. The detailed analysis of wear mechanisms at different temperatures—fatigue cracking, ploughing, fatigue spalling, abrasion, and adhesive wear—provides a comprehensive picture of the complex interactions that govern high-temperature wear behavior.

One area that deserves further investigation is the effect of sliding speed and load on the high-temperature wear behavior. The study uses a single set of sliding conditions, but in actual applications, sliding speed and load can vary significantly. For example, valve seat applications involve intermittent contact at high speeds, while turbine blade applications involve continuous sliding at moderate speeds. Understanding how the temperature-dependent wear mechanisms evolve with sliding speed and load would provide more comprehensive guidance for engineering design. Additionally, the long-term stability of the oxide films under cyclic temperature conditions—relevant for components that experience thermal cycling—would be valuable to characterize. The oxide films that protect the surface at steady-state high temperatures may behave differently under thermal cycling, where repeated expansion and contraction can cause film cracking and delamination. Overall, this work provides essential data for the design of high-temperature wear-resistant components and highlights the importance of temperature-dependent tribological characterization in overlay coating development.