More efficient surface treatment

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2013

A new boriding process produces an efficient coating that uses less energy and doesn’t generate emissions.

KEY CONCEPTS
• Conventional boriding is a surface treatment used to improve the surface hardness of ferrous metals that is inefficient, consumes high levels of energy and is not very environmentally friendly.
• A new boriding process has been developed on a commercial scale that can produce a very effective coating much more efficiently.
• Ultra-fast and large-scale boriding produced coated 9310 gear steel that exhibits a lower coefficient of friction and wear.

SURFACE TREATMENTS ARE USED TO IMPROVE the physical and mechanical properties of a metal, which facilitates better performance in many applications. There are a wide range of techniques used, including carburizing and nitriding.

A previous TLT article discussed the development of a more cost-effective procedure for conducting a surface treatment, called cladding, that is used to produce corrosion-resistant metal alloys (1). Problems encountered in prior techniques included slow processing and cost. A high-energy-density process was developed using a high-density, infrared arc lamp that enables cladding to be done more efficiently and with a wide range of metal alloys.

In a similar fashion to cladding, carburizing and nitriding are also time-consuming, expensive and energy- intensive. Another option that has also been looked at is boriding. STLE Fellow Dr. Ali Erdemir, Argonne Distinguished Fellow in the Energy Systems Division of Argonne National Laboratory in Argonne, Ill., says, “Boriding involves the diffusion of boron atoms into the metal substrate and the formation of a boron compound on the metal surface. During the process, known as pack-boriding, boron powders such as boron carbide, (B₄C), ferroboron and amorphous boron, as well as numerous catalyzers or boron activators such as sodium fl uoroborate (NaBF₄), potassium fl uoroborate (KBF₄), barium fl uoride (BaF₂) etc. (which are considered environmentally hazardous) have to be added to the pack containing metal parts or substrates at a temperature of 1,000 C. But the process is lengthy in duration because it takes about eight hours to generate a coating that is about 50 microns in thickness.”

The key components formed when a ferrous alloy is treated are iron borides prepared from the reaction of iron and boron. Erdemir says, “A combination of iron (I) boride and iron (II) boride are typically formed during boriding. The amount of each species is dependent upon the concentration of boron available to react with iron and on process parameters.”

Another problem is that boriding is simply not very environmentally friendly. Erdemir says, “Besides consuming high levels of energy, conventional boriding also generates a large amount of carbon dioxide.”

But boriding is more effective than other surface treatment techniques in improving the surface hardness of ferrous metals. Typically, boriding produces a layer with a hardness ranging from 1,500 to 2,800 HV. Other surface treatments display hardness values ranging from 750 to 2,000 HV.

If a more efficient boriding process can be developed and commercialized, then it will lead to a new approach for strengthening the physical and potentially improving the tribological properties of metals. Such an approach has now been established.

ULTRA-FAST & LARGE-SCALE BORIDING
Erdemir, in collaboration with researchers at Istanbul Technical University and Bodycote Thermal Processing, has optimized and scaled up a new boriding process that is much more effective, energy efficient, environmentally friendly and economical. The process is known as ultra-fast and large-scale boriding.

The process is conducted at a temperature between 770 C and 950 C and produces a very dense, iron (II) boride coating over 90 microns in thickness after only 30 minutes of processing. In contrast, pack-boriding produces a fairly open and defective coating that is 50 microns in thickness after eight hours of heating at elevated temperatures (see Figure 1). The structure of the coating also contains a mixture of iron (I) and iron (II) borides.


Figure 1. A new process known as ultra-fast boriding has been commercialized that produces a much more effective coating on metal alloys than conventional pack boriding in a more efficient manner. (Courtesy of Argonne National Laboratory)

Two cylindrical specimens prepared from 9310 gear steel were treated by ultra-fast boriding and carburizing and then evaluated in a reciprocating cylinder on flat-bench top-test coated with a synthetic lubricant. Under a 1 gigapascal loading, the coefficient of friction and wear for the borided steel specimen were both lower than for the carburized steel specimen (2).

One of the additional benefits of ultra-fast boriding is that it can be used to coat all ferrous alloys, including stainless. Erdemir adds, “Our industrial- scale process also can be used to coat nonferrous metals and alloys, including nickel, molybdenum, titanium, superalloys, etc.

The initial work done to demonstrate the effectiveness of ultra-fast boriding involved the use of a 21-inch diameter graphite furnace. The researchers realized the potential value of this process by scaling it up to a commercially viable system.

Erdemir says, “We developed the concept after evaluating materials and parts and built a full-scale production system in less than a year. The system has a melt capacity of approximately 3,500 kg and can handle thousands of small parts.” No emissions are produced from this environmentally friendly process.

The technology has been licensed to Bodycote Thermal Processing, and the researchers have been recognized as a winner of the 2012 R&D 100 Award. An economic analysis shows the cost of ultra-fast and large-scale boriding is substantially lower as measured in dollars per 100 microns per pound than pack-boriding, carburizing and nitriding.

For the future, Erdemir says, “We are thinking about applying the boriding technique to coating cutting tools. The hope is that this will produce tooling that can retain its hardness and performance over an extended period.”

Additional information can be found in a recent presentation made at the ASME/STLE 2012 International Joint Tribology Conference (3) or by contacting Erdemir at erdemir@anl.gov.

REFERENCES
1. Canter, N. (2011), “A Cost-Effective Metal-Cladding Process,” TLT, 67 (12), pp. 10-11.
2. Greco, A., Mistry, K., Sista, V., Eryilmaz, O. and Erdemir, A. (2011), “Friction and Wear Behavior of Boron-Based Surface Treatment and Nano-Particle Lubricant Additives for Wind Turbine Gearbox Applications,” Wear, 271 (9-10), pp. 1754-1760.
3. Erdemir, A., Eryilmaz, O., Sista, V., Timur, S., Kahevcioglu, O. and Kartal, G., (2012), “Ultra-Fast and Large-Scale Boriding of Metals and Alloys for Demanding Tribological Applications,” Presented at the ASME/STLE 2012 International Joint Tribology Conference, Oct. 7-10, Denver, Colo.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.