Gordon Wang, Yifei Zhang, Xinhui Zhang, Hua Liu – Shanghai Pioneering Surface Material Co. Ltd; Shanghai, CHINA

In this article, ion-sulfurization and lubricating gradient material (LGM) are introduced. The unique tribological advantages with respect to actual engineering applications are reported for the first time.

Ion-sulfurization and lubricating gradient material (LGM) technology signals a unique contribution in the fight against friction and wear. It is characterized as a solid lubricating effect and lubrication-reserving capacity. The low friction coefficient can be retained at elevated temperature, which satisfies requirements for high-temperature dies and molds. A benefit is that the LGM-forming process is conducted at low temperature, which minimizes substrate microstructure and hardness variation and preserves geometric precision. It also identifies surface cracks. The technology is environmentally friendly because low hazardous emissions are generated.

Introduction

Tribological property is one key requirement for sliding and rolling parts used in vehicles and machines. Industrial equipment fails through diverse and interacting forms of wear, seizure, scuffing, contact fatigue and flake. Friction alone generates heat, induces wear and other damage, and reduces power efficiency.

    There are numerous technical approaches to impart friction reduction and wear resistance on metallic components, including chemical/phase/composite design, substrate microstructure modification, surface microstructure modification, chemical heat treatment, surface coatings or lubrication. Each approach suits specific service conditions. However, ever-increasing and more aggressive engineering, economical and environmental challenges and regulations inspire the development of technology improvements to substitute for or use in conjunction with existing processes in order to optimize machine/vehicle performance.

    This paper introduces a unique anti-wear technology – ion-sulfurization and LGM. It is wear-preventive, power-efficient, market-profitable and environment-friendly. To this point, it has proved to be a milestone in anti-wear efforts.

What is ion-sulfurization and LGM?

Back in the 1980s, physicist Yifei Zhang developed a low-temperature ion-sulfurization technology. During the transition from development to industrialization, the substantial functional and commercial benefits drew more and more attention.

    Ion-sulfurization can be described as a process in which ionized sulfur reacts with metallic articles so that a layer, composed primarily of sulfides, is formed. An active-element gradient exists subsurface. Therefore, we technically name the sulfur-proliferated zone as LGM (Figs. 1 & 2).

Why does industry need ion-sulfurization and LGM?

Let’s look at five reasons why industry needs ion-sulfurization and LGM.

1. Low friction coefficient, extraordinary wear/seizure resistance, fast run-in, self-replenishing property

In a vacuum environment, ionized sulfur diffuses into the metal, and a layer with a gradient content of sulfur is formed via the reaction M + S → M[S]. Sulfides FeS, CuS, CrS, NiS, CoS and MoS₂ possess hexagonal lattice structure and extreme lattice anisotropy, which constitutes a uniform solid lubricant.

    LGM reduces friction power consumption, dissipates friction heat, inhibits thermal softening of the substrate and reduces occurrences of asperity welding and adhesive wear under insufficient or non-uniform lubricant. The propensity of adhesive seizure and scuffing is retarded, especially under high-speed rotating and sliding, due to lattice incompatibility with the substrate and low friction coefficient.

    Owing to the lattice characteristic of sulfides, ion-sulfurization reduces run-in time because it facilitates a surface smoothing process that enables hydrodynamic lubrication. Poor lubrication aggravates seizure and galling and leads to premature failure. Because of its microporous feature (Fig. 3), the ion-sulfurized LGM layer holds a more sustainable lubricant agent.

    Ion-sulfurized LGM imparts a unique active lubricating property in which the sulfur continuously diffuses deeper into the substrate and provides endurance in the solid-lubricating effect.

2. Retention of excellent tribological property up to 2190°F (1200°C)

Under elevated-temperature and/or radiating conditions, the sulfurized article still maintains a solid lubricating and wear-resistant surface. Conventional lubricating grease can work up to 390-480°F, and MoS₂ survives 570°F (higher under vacuum condition). Ion-sulfurized LGM is able to maintain its lubricating property at a temperature as high as 2190°F (1200°C). The comparison is shown in Fig. 4.

    Ion-sulfurization is being utilized in molds/dies for aluminum due to the chemically inert characteristics of the sulfide layer to impede reaction of iron with aluminum (Fe + Al → FeAl). The ion-sulfurized surface of the mold is much smoother (Fig. 5).

    The chilling caused by mold-releasing agents introduces tensile stress and hot cracks in the mold, but the LGM layer allows less mold-release agent to be used, which inhibits the occurrence of hot fatigue cracks (Fig. 6).

    Ion-sulfurization is also being applied in the steel industry. The service life of ion-sulfurized rolls, made of either alloy or cast iron, can increase by 50-100%.

3. Benefit of nondestructive evaluation

Ion-sulfurization reveals the sharp geometric non-continuities visible via color contrast. This happens when sulfur plasma interacts with geometric edges, which can be specified to delineate surface cracks induced by heat treatment, forging, welding, hydrogen embrittlement, and grinding or hard turning. Therefore, open defects can be identified visually, which precludes the extra cost for conventional dye-penetrant or magnetic-powder inspection. This approach helps avoid assembling flawed parts into machines or automobiles and reduces latent maintenance cost. One example of defect detection is shown in Fig. 7.

4. Dimension stability

Ion-sulfurization is carried out at temperatures as low as 190-360°F, which is lower than the tempering temperature for most carbon and alloy steels. Thus, the microstructure and properties of the substrate are not impaired or compromised. It inflicts no geometric distortion, and the maximum size variation is no larger than 0.0005 mm. There is no need for post-sulfurization grinding in most circumstances. As a finish process, it suits most steels, cast iron, copper and its alloys, carburized/nitrided parts, and nickel coatings, etc. Dimension stability and its comparison with carburization and nitriding processes per our experience are listed in Table 1.

5. A green technology

Ion-sulfurization is an environmentally friendly technology. Little hazardous emission and waste is released during the sulfurizing process. Also, the service life of the ion-sulfurized part is significantly enhanced so that the raw-material and energy costs drop.

    Additionally, ion-sulfurization gives low-cost metals excellent tribological properties in certain circumstances, which reduces alloy costs. As the friction coefficient plummets, power and fuel consumption is reduced by a noticeable level. Therefore, this technology is strategically advantageous since energy drives the economy. With wear damage reduced, ion-sulfurization helps garner extra profit by reducing maintenance time and cost.

Engineering Applications

We have included some examples of LGM technology in the machinery and automotive industries. The relative benefits are listed for reference and are only as good as demonstrated under the test conditions. The customers remain anonymous in order to observe commercial codes. The quantification of performance may be different under particular applications.

    In one typical plunger pump (Fig. 8), LGM resulted in a 20% friction-temperature reduction and a 3.5% power efficiency increase.

    In one model of auto air-conditioner compressor (Fig. 9), LGM contributed an average friction coefficient reduction of 39.5%, and the average power consumed by friction decreased 51.3%. Service life of LGM parts was 1.3-2.4 times longer (Table 2).

    LGM significantly increases the service life of bearings. In LGM-processed sliding bearings (Fig. 10), the wear loss dropped to 0.5 mm/year from 2 mm/year.

    LGM technology has been widely used in molds and cold-/hot-work dies. For an LGM-processed bearing-retainer die, the service life increased by 100%. It has also been employed for hot rolls used in the steelmaking process with success at temperatures as high as 1800°F (982°C).

    Multiple components in one type of gas engine were LGM processed, including piston ring, cylinder, crankshaft, camshaft, tappet, valve and the rocker-arm shaft. Only 23 hours of run-in stabilized the friction power loss, while the non-LGM engine required 74 hours. Additionally, the average frictional power-loss dropped by 4.7% for the engine with LGM parts. Under high speed, it reached 7%.

    In a diesel engine used in one brand of 10,000-ton cargo ship, seizure and galling took place in full load and super-speed run-in stage. In order to avoid such failure, 18 parts were processed with LGM technology, including bearing roller (Fig. 11),
roller-pin and bushing cylinders (Fig. 11). Run-in passing ratio was boosted from 20% to 100%.

    In one world-class automobile enterprise group, each heavy truck was equipped with 10 LGM and sulfurized gears. Those gears were used in the new-generation 10- to 12-speed transmission gearbox. An ion-sulfurized gear is shown in Fig. 12 (intro). Other LGM and sulfurized parts include limit washer and securing collar. The friction coefficient was 30% lower, and the area with wear marks was only 20% of that seen in original non-sulfurized counterparts.

    In ion-sulfurized parts, the wear resistance, thermal stability, fatigue and assembly reliability improves, even under heavy load, high rotating rate and harsh impact from gear switching. The transmission system with ion-sulfurized parts passed the three-year-long 200,000-kilometer tests under all driving conditions.

    Due to its characteristics of solid lubrication, LGM suits rotating or sliding units requiring limited or no addition of oil/grease lubricants.

Conclusions

In general, ion-sulfurization and LGM is an unprecedented surface-processing technology entailing the following tribological advantages:

•   Solid lubrication effect

•   Micro-reservoirs to hold lubricant

•   Prevention of adhesive contact

•   Reduction in run-in time

•   Low friction coefficient at elevated temperatures

•   Nondestructive evaluation side benefit

•   Low-temperature processing with dimension/geometric stability

•   Environmentally friendly nature

    As ion-sulfurization and LGM applications grow, its benefits will be further exploited via utilization in specific machinery/mechanical units. Our goals are to offer a substantially effective engineering solution against friction and wear and to gain wide recognition in industry through unique performance. IH

For more information: Contact Gordon Wang, Shanghai Pioneering Surface Material Co. Ltd.; tel: 708-415-5993; e-mail: services@sam-metallurgical.com or gordonwang15@aol.com; web: www.sam-metallurgical.com

References

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3. Bharat Bhushan, B. K. Gupta, Handbook of Tribology, McGraw-Hill, INC., New York, 1991

4. Ernest Rabinowicz, Friction and Wear of Materials, John Wiley & Sons, Inc., New York, 1965