Scuffing is a rapid and severe form of failure commonly found in high-speed and heavy-duty gear transmission systems. Improving the scuffing load capacity has become a key technical challenge to ensure the service reliability of high-power-density gear systems. Currently, enhancing gear surface resistance to both fatigue and scuffing through surface modification technology represents a major approach for manufacturing high-performance and highly reliable gears. Laser cladding is an emerging technology for surface repair and additive manufacturing. Due to its many advantages, it has been widely used to improve the performance of critical components in industries such as automotive, mold-making, and railways. While most studies on laser cladding repair for gears focus on tempered gears, this work targets carburized and hardened gears which are the most commonly used type in heavy-duty industrial gear drives. To explore the feasibility of using laser cladding to improve scuffing resistance and repair damaged tooth surfaces, 18CrNiMo7-6 gear steel was selected as the base material. Three types of substrates were prepared: tempered, carburized, and carburized samples with pre-induced scuffing damage. A NiCr20-3%ZrO2-1%MoS2 laser-cladded coating was fabricated on the outer circumferential surface of disk samples by a MobiMRO-2 laser cladding system with synchronous powder feeding. The cladded samples were then machined to meet the requirements for subsequent scuffing tests. After machining, the cladded coating exhibited a thickness of approximately 1 mm and a surface roughness of Ra = 0.3 μm. Subsequent scuffing tests were conducted on an MJP-30A rolling contact test rig with a step-wise loading method, with the load increasing by 300 N at each stage. The test conditions included rotational speeds of 500 r/min and 150 r/min for the driving and following samples, respectively, resulting in a slip ratio of 70% and a sliding speed of 1.04 m/s (based on a diameter of ϕ57 mm). The tests were performed under sufficient lubrication with VG40 industrial oil, supplied at a flow rate of 0.5 L/min and a temperature of 40 ℃. A sudden increase in the friction coefficient was used as the criterion for scuffing failure. The scuffing load capacity and failure behavior of the different cladded samples were compared with those of uncladded carburized samples. The microstructure of the laser-cladded coating and the effect of the cladding process on different substrate materials were analyzed through scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), a Vickers hardness tester, and a metallographic microscope before and after the scuffing tests. The results showed that the laser-cladded coating mainly consisted of dendritic structures, cellular crystals, and irregular particles. It had a thickness of approximately 1.2 mm and a hardness between 690HV0.1 and 730HV0.1, which was comparable to that of a carburized tooth surface. During the cladding process, the tempered steel near the cladded coating underwent a secondary quenching process, forming lath martensite and lower bainite, while the lower part of the heat-affected zone experienced tempering, resulting in spheroidized structures. For the carburized steel, its heat-affected zone showed coarse, needle-like martensite at the top, refined structures in the middle, and troostite at the bottom, with an overall tempering and softening trend. Compared to the uncladded samples, the resistance to scuffing increased by 94.4% for the cladded tempered samples, 61.3% for the cladded carburized samples, and 50.7% for the repaired samples. This significant improvement was primarily attributed to the self-lubricating effect of the laser-cladded layer. Analysis of the failure mechanism indicated that the cladded coating significantly increased the critical failure load by altering the path of crack initiation and propagation. However, for the repaired samples, the entire cladded coating was prone to spalling off due to weaker interfacial bonding. This study provides both experimental evidence and theoretical support for gear surface strengthening and repair.
Key words
laser cladding /
self-lubricating coating /
scuffing /
gear steel /
microstructure /
surface repair
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Funding
National Key Research and Development Program of China (2024YFB3410302)
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