Friction and Wear Properties of Nanocrystalline Surface Layer on Cladding Deposits

Literature Overview

This research by Bademar, Meng Fanjun, Sun Xiaofeng, and Qiu Ji from the Academy of Armored Force Engineering was published in the Journal of Tribology in 2014. Funded by the National Natural Science Foundation of China (grants 51105376 and 51005245), this study investigates the tribological behavior of a nanocrystalline layer formed on the surface of weld overlay cladding deposits. The research bridges fundamental materials science and practical tribology by examining how surface nanocrystallization affects friction and wear performance under various sliding conditions.

Core Technical Content

The study begins with the observation that weld overlay cladding deposits often develop a nanocrystalline layer on their surface during the welding thermal cycle, particularly in the region immediately below the surface where rapid cooling rates produce very fine grain structures. This nanocrystalline layer, typically 10-50 μm thick with grain sizes of 10-50 nm, has properties that differ significantly from the bulk microstructure of the deposit. The researchers systematically characterized the tribological properties of this nanocrystalline layer using pin-on-disk wear testing under dry sliding conditions.

The experimental matrix included cladding deposits made with different welding processes (GMAW, SAW, and laser cladding) to produce varying degrees of surface nanocrystallization, tested against steel counterfaces (45# steel) under loads of 5, 10, 20, and 50 N. The friction coefficient and wear rate were measured as functions of sliding distance, and the worn surfaces were analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to identify the dominant wear mechanisms.

Wear Mechanism Analysis

The SEM analysis of worn surfaces revealed that the dominant wear mechanism for the nanocrystalline surface layer was adhesive wear, characterized by the formation of transfer films and micro-ploughing. At low loads (5-10 N), the wear rate was extremely low (less than 5×10⁻⁷ mm³/N·m) and the worn surface showed minimal material removal, indicating that the nanocrystalline layer provided excellent resistance to adhesive wear. At higher loads (20-50 N), the wear rate increased significantly, and the worn surface showed evidence of delamination wear where the nanocrystalline layer was removed in thin flakes.

The researchers proposed that the superior wear resistance of the nanocrystalline layer is attributed to two mechanisms: first, the Hall-Petch strengthening effect of the nanocrystalline grains increases the hardness and yield strength of the surface layer; second, the high density of grain boundaries in the nanocrystalline structure facilitates dislocation absorption and plastic deformation accommodation, which reduces the tendency for crack initiation and propagation. The nanocrystalline layer effectively acts as a protective barrier that prevents the bulk material from coming into direct contact with the counterface, thereby reducing adhesive wear.

The friction coefficient was found to be strongly dependent on the sliding distance. In the running-in period (0-500 m), the friction coefficient decreased rapidly as the nanocrystalline surface was partially removed and a stable tribolayer formed. In the steady-state period (500-5000 m), the friction coefficient remained relatively constant at 0.35-0.45, indicating a stable wear regime. The EDS analysis of the tribolayer revealed that it consisted primarily of iron oxide and iron carbide phases, which provided a low-friction surface.

Engineering Practice Considerations

From an engineering perspective, the key finding of this research is that the welding process used for cladding has a profound effect on the surface nanocrystallization and consequently on the tribological performance of the cladding deposit. Laser cladding produces the finest surface grains and the best tribological properties, but it is also the most expensive and least productive process. GMAW and SAW are more practical for industrial applications but produce coarser surface grains and somewhat inferior tribological properties.

For industrial hardfacing applications where tribological performance is critical, the researchers recommend using laser cladding or hot-wire TIG cladding to produce fine-grained surface layers. If conventional welding processes must be used, post-weld surface treatment such as shot peening or surface mechanical attrition treatment (SMAT) can be employed to introduce nanocrystallization into the surface layer. The economic trade-off between process cost and tribological performance should be carefully evaluated for each specific application.

Study Insights and Reflections

This research demonstrates that the surface nanocrystalline layer formed during welding is not merely a microstructural feature but a functional tribological layer that significantly affects the wear performance of cladding deposits. The practical implication is that welding process selection should consider not only deposition rate and cost but also the resulting surface microstructure and its effect on tribological performance. For applications where wear resistance is the primary design requirement, investing in advanced welding processes that produce nanocrystalline surface layers can provide substantial benefits in terms of component life and maintenance cost reduction.