Pulsed Power Engineering and Exploratory Technology Applications: Research at the Materials Modification Laboratory

The Materials Modification Laboratory produces repetitively pulsed ion beams for

• Surface Modification
• Thin-Film Synthesis
• Chemical-Free Surface Treatment

500 - 750 kV, < 250 kA/cm2 ion range of 2 - 10 micrometers, 2 - 8 J/cm2 for melt rapid cooling (109 K/s) by thermal diffusion into substrate

IBEST efficiently modifies the surface of materials by controlled ion deposition using short-pulse (typically <100 ns) technology.

Materials modification is accomplished with intense ion beams
The materials modification is possible because of Sandia's repetitive pulsed power capability combined with a robust intense ion beam design developed at Cornell University. Such a beam from the Repetitive High-Energy Pulsed Power (RHEPP) I facility can be used to deposit high-energy ions in the top 1 - 10 micrometers of material surfaces. The depth of treatment is controllable by varying the ion energy and species. Efficient deposition of the ion energy in a thin surface layer allows melting with relatively small energies (1 - 10 J/cm²). Rapid cooling and resolidification of the melted layer occurs by thermal conduction into the underlying substrate. Typical cooling rates of greater than 10⁹ K/sec are sufficient to cause amorphous and nanocrystalline grain layer formation and the production of non-equilibrium near-surface microstructures. This largely non-ablative mode of beam operation is referred to as Ion Beam Surface Treatment, or IBEST.

With increased energy deposition (10 - 20 J/cm²) ablation and redeposition of target material makes possible intense pulsed ion beam deposition (PIBD), leading to the creation of thin films for a number of applications (e.g., diamond-like carbon).

Ions are generated by the magnetically-confined anode plasma (MAP) source.
In this source (shown in ablation mode), gas is injected from a central plenum. The gas propagates to the beam generation location (annulus), where a fast-rising magnetic field breaks down the gas, forming a plasma. Any number of gases, including high-Z gases such as krypton, can be injected to form a beam. Since each pulse delivers ~10¹³ ions/cm², implantation effects are negligible.

The efficiency of pulsed power (15% wall-plug to beam) makes IBEST or PIBD a scalable technology for industrial applications.
Compared to ion implantation, costs can be significantly lower at doses sufficient to modify metallurgical properties. And while cooling rates comparable to those seen here occur with pulsed laser processing, the in-depth energy delivery with ions means that a deeper melt layer can be created before ablation occurs. Reflection of laser light also limits laser use with metals. In addition, IBEST represents a processing technology free of added solvents or heavy metals.

a	b

IBEST has been shown to increase surface hardness and corrosion resistance.
Shown are scanning electron microscope (SEM) images, magnified 2000 times, of the surface of 440C stainless steel before and after IBEST treatment. The dark areas in a) are predominantly chromium carbide particles. These are largely missing in the treated surface, which shows a lath martensitic (needle-like) structure. Tests show significant increases in hardness and wear durability of the treated surface. Similar improvements in corrosion resistance and wear durability have been seen in other steel and titanium alloys subjected to IBEST treatment.

c

d	e

Surface alloying results in 25 - 50 atomic weight % Pt concentration near the surface of a titanium, compared to 1% typical for standard alloying. Figures d) and e) show SEM images of wear tracks in the TI surface.

Surface alloying can extend the range of property improvement.
Additional elements can be added in thin-film form to a metal substrate. During the pulse, this coating mixes into the substrate, leading to surface property improvements beyond that possible by IBEST without the surface layer. An example of Pt added to Ti alloy can be seen in c), d), and e). In c), a treated layer of 1800 ? Pt was mixed into Ti Grade 2, and was analyzed by Rutherford Back-scatter Spectrometry (RBS). The curve shows the inferred Pt concentration with depth after beam treatment, compared to the original layer thickness (cross-hatched). The Pt has become incorporated to a depth far beyond the original coating thickness, and the coating itself has ceased to exist as a separate entity. Figures d) and e) show wear tracks on Ti Grade 5 sample surfaces subjected to testing with a linear reciprocating tribometer (ball-on-flat). The untreated sample d) shows wear tracks indicative of the relatively poor wear resistance of Ti.

Partnering with industry is possible.
These partnerships rely on cooperative research and development agreements (CRADAs) as well as through User Facility arrangements. Experiments at the Materials Modification Laboratory are supported by a team of materials scientists located at Sandia as well as at Cornell University and the Naval Research Laboratory.

For further information about the Materials Modification Laboratory and RHEPP technology, contact

• Bobby Turman, Program Manager, Beam Applications and Initiatives, 1-505-845-7119, bnturma@sandia.gov
• Timothy Renk, Project Leader, Materials Applications of Ion Beams, 1-505-845-7491, tjrenk@sandia.gov

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