Surface Engineering: Enhancing Hardness and Reducing Friction

Even with advanced materials and optimized designs, surface engineering techniques can further enhance wear resistance by modifying the top few microns of a component’s surface. These treatments improve hardness, reduce friction, and create barriers against corrosion—all of which extend service life in abrasive environments.

Thermal diffusion coatings (such as boronizing or chromizing) penetrate the surface of steel or cast iron components, forming hard intermetallic layers. Boronizing, in particular, diffuses boron atoms into the material at high temperatures (800–1,000°C), creating a layer of iron borides (FeB and Fe₂B) with hardness up to 1,800 HV. This layer is 50–100 μm thick and strongly bonded to the substrate, providing excellent resistance to sliding and impact abrasion. Boronized blades have shown a 2–4 times increase in lifespan compared to untreated blades in tests with silica sand abrasives. However, the process can increase brittleness, so it is often applied only to the wear surfaces (e.g., blade tips) rather than the entire component.

Physical vapor deposition (PVD) and chemical vapor deposition (CVD) coatings deposit thin films (1–10 μm) of hard materials like titanium nitride (TiN), chromium nitride (CrN), or diamondlike carbon (DLC) onto component surfaces. PVD coatings, applied via sputtering or arc evaporation, offer high adhesion and uniform coverage, making them suitable for complex geometries like blade edges. CrN coatings, with hardness around 2,000 HV, provide good corrosion resistance in addition to wear resistance, making them ideal for blast wheels in humid environments or processing corrosive media (e.g., saltladen marine components). DLC coatings, with properties similar to diamond, reduce friction coefficients by up to 50%, minimizing abrasive wear from media sliding across blade surfaces. While PVD/CVD coatings are thin, they are particularly effective in lowimpact applications (e.g., shot peening with small steel shot) where surface scratching is the primary wear mechanism.

Laser surface alloying uses a highpower laser to melt the surface of a component, along with a powdered alloy (e.g., chromium, tungsten, or silicon carbide), creating a melted pool that solidifies into a wearresistant surface layer. This process allows for precise control over layer thickness (50–500 μm) and composition, enabling customization for specific abrasive types. For example, laser alloying blade tips with a tungsten carbideiron mixture creates a surface hardness of 1,200–1,500 HV, while maintaining a tough substrate. The rapid cooling during laser processing produces a finegrained microstructure, enhancing both hardness and toughness. This technique is particularly useful for repairing worn components, as it can rebuild worn areas to their original dimensions with material properties superior to the base metal.

Shot peening is a surface treatment that induces compressive stress in the component, improving resistance to fatigue wear. By bombarding the surface with small, highvelocity shot, shot peening creates a layer of compressed material (typically 0.1–0.5 mm deep) that resists crack formation and propagation. This is especially beneficial for blast wheel blades, which experience cyclic stress from centrifugal forces and media impact. Shot peening can extend fatigue life by 50–100% in highspeed blast wheels, reducing the risk of sudden blade failure due to stress cracks. The process is often applied after other surface treatments, as it does not compromise hardness but enhances the component’s ability to withstand repeated impact.