Method and apparatus for altering material using ion beams

Abstract

A method and apparatus for treating material surfaces using a repetitively pulsed ion beam. In particular, a method of treating magnetic material surfaces in order to reduce surface defects, and product amorphous fine grained magnetic material with properties that can be tailored by adjusting treatment parameters of a pulsed ion beam. In addition, to a method of surface treating materials for wear and corrosion resistance using pulsed particle ion beams.

Description

The United States Government has rights in this invention pursuant to Contract No. DE-AC04-76DP00789.

This application is a Continuation-In-Part of U.S. patent application Ser. No. 08/153,248 filed Nov. 16, 1993 μs) gas puff which is delivered through a supersonic nozzle 406 to produce a highly localized volume of gas directly in front of the surface of a fast driving coil 408 located in an insulating structure 420. After pre-ionizing the gas with a 1 ms μs induced electric field, the fast driving coil 408 is fully energized, inducing a loop voltage of 20 kV on the gas volume, driving a breakdown to full ionization, and moving the resulting plasma toward the flux filled shaping anode electrodes 410 in about 1.5 ms μs to form a thin magnetically-confined plasma layer. The pre-ionization step is a departure from the earlier MAP reference which showed a separate conductor located on the face of a surface corresponding to the insulating structure 420 herein. Since this conductor was exposed to the plasma, it broke down frequently. One of the inventors herein discovered that the separate pre-ionizing structure was unnecessary. The gas can be effectively pre-ionized by placing a small ringing capacitor in parallel with the fast coil. The field oscillators produced by this ringing circuit pre-ionize the gas in front of the anode fast coil.

We have also discovered that, prior to provision of the main pulse to the fast coil, it is beneficial to have the ability to adjust the configuration of the magnetic field in the gap between the fast coil and the anode to adjust the initial position of plasma formation in the puffed gas pulse prior to the pre-ionization step. This is accomplished by the provision of a slow bias capacitor and a protection circuit both being installed in parallel with the fast coil and isolated therefrom by a controllable switch. A slow bias field is thus created prior to pre-ionization of the gas by the fast coil.

After pre-ionization the fast coil is then fully energized as described above to completely breakdown the gas into the plasma. After this pulse the field collapses back into the fast coil which is connected to a resistive load which is in turn connected to a heat sink, not shown. In this manner heat build up in the fast coil is avoided.

The fast coils 408 have been redesigned from the reference fast coils in several ways as well as the heat sinking mentioned above. The gap between the fast coil and the anode electrodes 410 has been reduced with the result that the amount of necessary magnetic energy has been decreased by over 50%. The lower energy requirement permits repetitive use at higher frequencies and reduces the complexity of the feed system voltages for the fast coils. The design of the new flux-shaping anode electrode assembly has also contributed to these beneficial results.

The pulsed power signal from the power system is then applied to the anode assembly 35, accelerating ions from the plasma to form an ion beam K. The slow (S) and fast (F) magnetic flux structures, at the time of ion beam extraction, are shown in FIG. 4A. The definite separation between the flux from the fast coil from the flux from the slow coil is shown therein. This is accomplished by the flux-shaping effects of the anodes 410 and also by the absence of a slow coil located in the insulating structure 420 as was taught in the earlier MAP reference paper. The slow coils in the present system are located only in the cathode area of the MAP. This anode flux shaping in conjunction with the location of slow coils in the cathode assembly is different from that shown in the MAP reference paper and permits the high repetition rate, sustained operation of the MAP diode disclosed herein. This design allows the B=0 point (the separatrix) to be positioned near the anode surface, resulting in an extracted ion beam with minimal rotation. This minimal rotation is necessary for effective delivery of the beam to the material to be treated.

FIG. 4B is a detailed view of the gas valve assembly 404 and the passage 425 which conducts the gas from the valve 404 to the area in front of the fast coil 408. The passage 425 has been carefully designed to deposit the gas in the localized area of the fast coil with a minimum of blow-by past this region. The gas valve flapper 426 is operated by a small magnetic coil 428 which opens and closes the flapper 426 upon actuation from the MAP control system. The flapper valve is pivoted on the bottom end 427 of the flapper. The coil 428 is mounted in a high thermal conductivity ceramic support structure 429 which is in turn heat sinked to other structure, not shown. This heat sinking is necessary for the sustained operating capability of the MAP. The gas is delivered to the valve from a plenum 431 behind the base of the flapper. The vacuum in the nozzle 406 rapidly draws the gas into the MAP once the flapper 426 is opened. The function of the nozzle is to produce a directed flow of gas only in the direction of flow and not transverse to it. Such transverse flow would direct gas into the gap between the anode and the cathode which would produce detrimental arcing and other effects. The reduction of the fast coil-anode gap discussed above makes the design of the nozzle very important to the successful operation of the MAP. Fortunately, gas flow design tools are available and were used to develop a nozzle with improved gas flow (higher mach number) and minimal boundary effects. This improved nozzle has an enlarged opening into the gas between the fast coil and the near edge of the anode which tapers from 9 to 15 mm instead of the straight walled 6 mm conduit in the reference MAP. The operating pressure of the gas in the puff valve has been increased from the range of 5-25 psig to the range of 35-40 psig. Experiments have confirmed much improved MAP operation as a result of this new design.

The ion diode of this invention is distinguished from prior art ion diodes in several ways. Due to its low gas load per pulse, the vacuum recovery within the MAP allows sustained operation up to and above 100 Hz. As discussed above, the magnetic geometric is fundamentally different from previous ion diodes. Prior diodes produced rotating beams that were intended for applications in which the ion beam propagates in a strong axial magnetic field after being generated in the diode. The present system requires that the ion beam be extracted from the diode to propagate in field-free space a minimum distance of 20-30 cm to a material surface. The magnetic configurations of previous ion diodes are incapable of this type of operation because those ion beams were forced by the geometries of those diodes to cross net magnetic flux and thus rotate. Such beams would rapidly disperse and be useless for the present purposes. By moving the slow coils (the diode insulating magnetic field coils) to the cathode side of the diode gap eliminated the magnetic field crossing for the beam but required a total redesign of the magnetic system for the anode plasma source. The modifications to the fast coil discussed above result in an energy requirement that is 5-10 times less than previous configurations. The modifications include: the elimination of a slow coil on the anode side of the diode and its associated feeds, better control over the magnetic field shaping and contact of the anode plasma to the anode electrode structure through the use of the partially field-penetrable electrodes, the elimination of the separate pre-ionizer coil from the prior ion diodes, the circuit associated with the fast coil to provide "bias" current to adjust the magnetic field to place the anode plasma surface on the correct flux surface to eliminate beam rotation and allow optimal propagation and focusing of the beam, and the redesign of the gas nozzle to better localize the gas puff which enable the fast coil to be located close to the diode gap which in turn reduces the energy requirements and complexity of the fast coil driver.

The plasma can be formed using a variety of gas phase molecules. The system can use any gas (including hydrogen, helium, oxygen, nitrogen fluorine, neon, chlorine and argon) or vaporizable liquid or metal (including lithium, beryllium, boron, carbon, sodium, magnesium, aluminum, silicon, phosphorous, sulfur, and potassium) to produce a pure source of ions without consuming or damaging any component other than the gas supplied to the source. The ion beam K propagates 20-30 cm in vacuum (∼10-3) to a broad focal area (up to 1000 cm²) at the target plane, not shown, where material samples are placed for treatment and can thermally alter areas from 5 cm² to over 1000 cm².

The ion beam or MAP source 25 is capable of operating at repetitive pulse rates of 100 Hz continuously with long component lifetimes >10⁶. The ion beam or MAP source 25, according to the principles of the present invention, draws ions from a plasma anode rather than a solid dielectric surface flashover anode used in present single pulse ion beam sources. Use of a flashover anode typically introduces a variety of contaminants to the surface of the material, often with detrimental results. One of the significant advantages of the using the improved MAP source disclosed herein is that one has precise control over the components in the ion beam by controlling the composition of the gas source.

The present invention combines the pulsed power supply P and the MAP ion source 25 to obtain a system for repetitively generating pulsed high voltage ion beams in a manner that allows the use of this technology for the efficient treatment of surfaces in commercial applications. In particular, the ion voltage is in the range 0.1-2.5 MeV per ion, the energy per pulse is as large as 2.5 kJ, and the ion source impedance is significantly less than 100Ω, allowing the pulse width to be as small as 30 ns. These numbers are characteristic of the present embodiment, and may be superseded by design changes obvious to worker in the art.

The detailed description of the new class of ion beam generators having been completed, attention now turns to the many applications made possible and practical in an industrial sense by said generators.

There are three broad classes of surface effects upon which the aforementioned applications depend. These are: a) Surface Smoothing; b) Evaporation and Ablation from a Surface, and; c) Generation and Quenching of Non-Equilibrium Surface Structure. Other types of effects exist, and are not intended to be removed from the scope of the claims, but the effects listed above illustrate the enormous breadth of the present invention.

Surface Smoothing has a sphere of influence far wider than the innocuous name would suggest. Every surface has an energy (or surface tension) raising the energy of the atoms which make up to the surface above the energy they would have if located in the bulk of the material. Accordingly, given the opportunity any surface structure which increases the surface area (thereby increasing the number of surface atoms) will adjust by moving material around to reduce the total surface area. As described in the Background section, Surface Smoothing is driven by the surface tension of the molten surface following surface heating by the ion beam, but before sufficient heat has conducted into the body of the material to allow the near-surface regions to resolidify. During this time, the surface morphology will become less jagged and smoother, the improvement limited primarily by the duration of surface melting.

Another effect which can add to the smoothing of the surfaces of fine-grain sintered materials, such as ceramics or materials resulting from powder metallurgy, via ion beam surface melting. In these cases, when proper process parameters are used, a glass or alloy surface may be formed, thereby eliminating the grain structure from the surface in favor of a smooth glassy surface. Note further that the glass or alloy need not be equilibrium forms of the material, as the rapid quenching will preserve many forms of molecular solid solutions which do not exist in the relevant equilibrium phase diagram.

The process conditions for Surface Smoothing are not onerous, so long as the near-surface region of the material does melt to some depth. In contrast to some of the techniques to be described later. Surface Smoothing can often be carried out in a number of smaller ion pulses, each one melting the surface, thereby allowing said surface to become a little smoother.

Having described how to smooth a surface using ion beam surface heating, the range of applications of Surface Smoothing must be described. Again, these examples are simply for illustration, and there is no intent to limit the present invention to a scope inferior to that of the attached claims.

The simple process of smoothing a surface, e.g., to remove surface defects resulting from etching or machining, is straightforward. Example 1 describes the removal of etching defects on a copper surface using the Surface Smoothing process. The surface initially consisted of canyons and mountains some 3-5 μm in height having sharp edges and points. Following Surface Smoothing, the surface exhibited surface roughness only on a size scale of less than 0.5 μm.

Example 2 described the polishing of machining marks from a machinable titanium alloy. The marks were originally some 5 μm, the remnants of a precision machining operation. The process of Surface Smoothing reduced the surface roughness to less than 0.1 μm, again removing the sharp, abrupt initial features and leaving only a gently rolling surface. This polishing of machining marks will also be useful in polishing of diamond-turned optics, allowing such polishing to be executed without danger of changing the carefully controlled surface generated by the machining process, thus greatly reducing the cost of such optical elements. Another application will be in the treatment of machine tool surfaces, so that a minimum of machine marks may be made in the first place.

Example 3 describes the smoothing of an Al₂ O₃ ceramic surface by conversion of the surface to a glassy layer. Such a process should be useful on a wide range of ceramics and other materials having a pronounced grainy structure. There are certain materials, such as most stainless steels, which do not form glassy layers. They can, however, be melted to form a solid layer of metal in these circumstances, said layer of metal having a very-fine-grained structure.

Surface Smoothing makes two primary alterations in surface morphology, it reduced the average surface roughness and it reduces the surface area of the body treated. Both of these effects have clear applications. The phenomenon of adhesion between two materials is now well-understood. However, it is clear that the more surface area upon which two materials meet, the more adhesive force will exist between them. In fact, the function of many adhesives is not only to stick to the surfaces of both bodies being glued together, but also to maximize the area of contact by flowing into small grooves and crevasses before hardening. The increase in surface area which occurs in this process is enormous, and also increases with time, explaining why fast-curing epoxies are generally not as strong as their slower-curing cousins.

If one produces a surface which is (approximately) maximally smooth using Surface Smoothing, the result will be a surface which will experience minimal adhesive forces to another body in contact. In other words, Surface Smoothing is another approach to non-stick surfaces. Note that a non-stick surface need not be a low-friction surface, as the one refers to the force required to start the body into motion and the other to the force needed to keep it in motion some moving.

Another general result of the Surface Smoothing process, resulting directly from reduction in surface roughness, is reduction of wear between two elements in contact and in relative motion. As discusses in the Background section, the amount of material lost in a given time to adhesive wear should be a linear function of the surface roughness of the two elements. Although that estimate is oversimplified, it is clear that less wear will result from the mechanical interaction of two surfaces after Surface Smoothing has been performed, beyond any surface hardening which might also have taken place.

Related to the above is the fact that a smooth surface can increase the working toughness of a material, although the actual micro-properties of that material are not altered. The materials used for mechanical applications are rarely, if ever, completely homogeneous. Among other defects, incipient surface cracks provide sites for failure of the element under stress. If the surface of such a body is essentially smoothed, all incipient cracks are located below the surface, and thus have two closed ends rather than one. Such cracks are nearly twice as resistant to growth as is a crack which intersects the surface. Thus, a smooth surface gives a tougher part.

Corrosion resistance can also be increased through the use of Surface Smoothing. Increased surface area, cracks, and other defects associated with rough surfaces increase the rate of corrosive processes, including in particular pitting, stress corrosion, and attack by microbiological organisms. A number of processes exist which directly attack the chemistry of corrosion, such as formation of a layer of corrosive-resistant surface alloy, but all such techniques work better if the surface is also smooth and relatively free of cracks. This is the role of Surface Smoothing in preventing corrosion. Several examples have been investigated, which will be discussed in the section on Non-Equilibrium Surface Structures.

An application of Surface Smoothing closely related to the above is that of passivation or protection of welds against corrosion. Exposed welds, particularly between dissimilar materials, offer fertile ground for corrosive processes. The reason is at least two-fold. Generally, the region of the weld is rather heterogeneous in composition and structure. Any corrosive process is thus likely to act with different rates in different regions, resulting in a surface of increasing micro-roughness as corrosion continues. Also, the initial surface of a weld is usually very rough, having many flaws and cracks on a small size scale. The effect of Surface Smoothing following the welding process thus acts to ameliorate both effects, resulting in a more corrosion-resistant weld.

A final illustration of the use of Surface Smoothing is in application to amorphous magnetic materials. When a thin layer of a magnetic material is considered, the surface roughness can have a significant effect of magnetic properties, including coercive field and dc hysteresis losses. An example of great industrial significance is METGLAS™, a class of magnetic alloys produced by shooting a jet of the molten alloy at a spinning metal wheel which cools the alloy into a ribbon quickly enough that the resulting structure is amorphous. One negative aspect of this means of production is that the side of the ribbon opposite the wheel has a very rough surface. This roughness also limits the thickness of material that can be commercially produced, limiting the high frequency range of METGLAS™ applications. Surface cracking of the METGLAS™ ribbon also limits the thickness of material that can be produced commercially, increasing the cost of METGLAS™ cores for power distribution and related applications. As a result, although the potential of METGLAS™ in power handling devices is enormous, it has not yet realized that potential. Surface Smoothing is a technique capable of smoothing and even forming METGLAS™, with the hoped-for improvement in magnetic properties, as described in Example 4. The technique of Surface Smoothing can, of course, be applied to any amorphous or fine-grained material, with beam kinetic energy and ion species tailored to obtain the proper cooling rate. Due to the extremely rapid quench rate, Surface Smoothing can also be used to produce or modify new magnetic materials not accessible using existing techniques.

A related technique can be applied to thin layers of amorphous or nanocrystailine material, given only that these layers are deposited on a substrate having high thermal conductivity (roughly speaking, metals and ceramics rather than polymers and insulators). The physics behind the design of a smoothing treatment is the same as above, except that the heat from the ion pulse is conducted into the substrate instead of into the bulk of a thick sample. Examples of such processes include smoothing e.g., plasma spray deposited films, filling in pinhole defects in the amorphous film, and precisely controlling the grain size of fine-grain films by melting and recrystallization.

Having described a number of applications for the process of Surface Smoothing as made possible by the present invention, attention is now focused on Evaporation and Ablation from a Surface (EAS for short). One of the most important applications of EAS is the simple task of cleaning surfaces. Simple, that is, except that one wants to consistently clean a surface to an environmentally-limited amount of contamination, without the use of EPA- of OSHA-regulated solvents, preferably immediately before using the clean surface (e.g., in welding, flux-free soldering, vacuum deposition, and the like). If cleaning is also extended to the removal of, for example, oxide layers from a metal surface, it becomes clear that cleaning can be an essential and difficult part of the manufacturing process. The process of EAS has many uses in this domain.

A conventional form of cleaning is degreasing parts prior to some assembly step, such as welding, soldering, gluing, etc. As will be shown in Example 5 below, a 100 nm thick layer of conventional lubricating oil is easily removed from a stainless steel surface using a single pulse of about 1-2 J/cm², a very small dosage for the present class of ion beam generators. Note that no attempt is made to restrict the beam to the contaminant layer alone, as an extremely low beam energy would be required, owing to the low density and small thickness of the contaminant. Rather, the ion species and the energy of the beam is adjusted to superheat a thin layer of the metal surface, which then vaporizes the hydrocarbon contaminant before the bulk of the steel can cool the surface.

A further extension of cleaning a surface is the rapid and thorough sterilization of surfaces subjected to appropriate EAS treatment. Such techniques are likely to have impact in the manufacture of the pre-sterilized medical implements.

The technique described above is quite general, and may be used on any form of contamination that has a significantly lower boiling point than the substrate material. In fact, in cases where a natural passivating layer, e.g., a surface oxide, must be removed before soldering, for example, can take place, and the relative characteristics of the bulk material and the surface passivating layer are as outlined above, the passivating layer can be removed by superheating the underlying material.

In most cases, however, the materials encountered in both natural and artificial surface layers have higher vaporization points than do the materials they protect. In such cases, the EAS technique can still be used to remove the surface layer provided only that loss of a few microns of the underlying material is acceptable. This is accomplished by ablating the surface layers of the underlying material, taking along the unwanted overlayer. The total energy required for ablation is generally quite high (>10 J/cm²), and should be restricted to as thin a layer of material as is reasonable (perhaps 0.5-1.0 μm).

These numbers, like all specific numbers appearing in the specification, depend to some extent on the ion species used and the type of bulk material being processed. Note particularly the difference caused by attempting to treat a polymer substrate, whose thermal conductivity is perhaps 1000-10000 times smaller than that of a metal alloy. The ablation temperature will be about the same, and the energy contained in a given layer is perhaps 10-20% that of an equivalent metal layer, owing to the lower density of the polymer. As a result, the characteristic time to remove energy from a heated surface layer will be on the order of 10 times that for a typical metal. In addition, the range of ions in the polymer is much greater for a given beam kinetic energy than in normal structural metals. The net effect is that a much greater thickness (say, × times the distance in the metal, for example) will be heated by a beam of given kinetic energy. As the characteristic time depends quadratically on this thickness and inversely on the thermal conductivity, the characteristic time in polymer heating will be ∼(10² -10³)ײ longer than that in a metal. Extremely rapid quenching thus cannot be produced on a polymer surface by the techniques of the present invention. The time required for heating, however, is limited only by the maximum peak power of the ion beam generator. The EAS techniques therefore apply to polymers, whereas most of the Surface Smoothing and Non-Equilibrium processes do not.

If an patterned ion-absorbing mask or compound is used to prevent the ion pulse from affecting certain areas of the element being treated, a surface having a pattern of varying surface properties can be generated. Such a pattern can range from removing an oxide layer in certain areas to obtain patterned etching of a surface by chemical action to direct etching of ablated patterns in large scale solar cells to manufacture of patterned printed circuit boards. The EAS process offers the advantage of limiting the use of solvents and powerful acids in such procedures.

When a higher level of pulse power (>>10J/cm²), is deposited in a thin surface layer (∼μm in thickness), violent ablation occurs. The expanding gases accelerate the evaporated layer outward from the body of the material at extreme velocity, generating as a result of momentum conservation a strong pressure wave in the material. As most materials exhibit a nonlinear stress-strain relationship, the pressure wave rapidly sharpens into a shock wave. As this shock wave propagates inward through the material, it generates dislocations, twinning planes, and complex systems of these structure defects, thereby dissipating its power and eventually (within perhaps 100 μm or more) ceases to exist as a cohesive entity. This damaged region, however, has undergone a phenomenon known as shock-hardening, an extreme form of work-hardening. Even though the direct heating action of the ion beam may be limited to the first few μm, the shock hardening effect penetrates much deeper, offering a surface treatment which cannot be directly obtained using the present invention.

EAS uses the pulsed ion beam generators of the present invention to rapidly vaporize material from the surface of a body. This vaporized material can be used as a source material for vapor deposition processes, having the advantage that chemical compositions will not be charged by segmentation effects due to the phase diagram of the alloy system or chemical reactions with a resistive heating element, as is often used in vapor phase deposition. In addition, the vapor deposition will take place in a very short period of time (<1 μs). As a result the heat of adsorption will rapidly conduct away into the bulk of the substrate, and one will again obtain a rapidly quenched material, given only that the substrate has large thermal conductivity. The large surface area that the ion beam generators of the present invention can vaporize makes this approach available to large-scale manufacturing efforts.

Another effect associated with EAS used in this mode has been observed. A layer of material a few μm thick is vaporized within the period of a few tens of nanoseconds. This converts a metal layer having a given density into a plasma which initially has very nearly the same density, as it has not yet had time to expand away from the bulk of the material. The energy distribution of this layer follows a Boltzmann distribution, meaning that a significant percentage of the vaporized material has kinetic temperatures significantly less than the average temperature of the plasma. Because of this, and because the plasma is so close to a relatively cool conducting surface, a small amount of the vaporized material redeposits on the surface from which it came. In doing so, that surface acquires a structure which is extremely rough on a nanoscale, particularly having numerous protuberances much smaller than a μm in size, possessing unique properties.

EAS processes can be used for many other manufacturing purposes, and presentation of these examples is not intended to limit the scope of the invention beyond the limitations outlined in the attached claims.

The final major class of processes made practical for large-scale manufacturing by the new category of pulsed ion beam generators made possible by the current invention is the production of non-equilibrium surface structures (NESS for short). The name is a bit misleading, as some near-equilibrium applications also come under this title, but the general concept is that one heats a surface having an initial structure rapidly to some depth with a pulsed ion beam, the heat is rapidly lost to conduction into the material, and the result is a product surface having a structure with different properties than those of the initial structure. As the structure of many of the product surfaces is non-equilibrium, that term is used herein to describe the whole family of processes.

A good example of the production and retention of high-temperature structures is offered by Example 6, in which an NESS-type process is applied to the surface of a tool steel component. (Such a process is not limited to the hardening of steel.) The hardness of the surface roughly tripled, but the important point is how this increase in hardness came about. X-ray and electron microscope analysis of the untreated surface shows the simple co-ferrite phase with a significant density of cementite precipitates. However, the treated surface showed the presence of small crystallites of austenite, the possible presence of martensite, and no carbide precipitates. This is significant in that austenite is stable only at high temperatures, and that the equilibrium structure at room temperature is a mixture of ferrite and cementite (Fe₃ C precipitates). At high temperature, the carbon dissolves into the matrix, producing austenite in the process. The NESS process has thus quenched a high-temperature phase structure so that it exists at room temperature. Conventionally hardened tool steels are composed either of a very fine grain pearlite or of tempered martensite. The structure obtained from the NESS treatment differs from these, thus providing another surface microstructure useful for hardening steel alloys. Other precipitates than carbon, of course, can be dissolved and retained in a non-equilibrium solid solution using the NESS technique, and other materials than steel can be successfully treated.

Another approach toward hardening the surface of steel (or other alloys) is to add elements, usually carbon and/or nitrogen, which encourages the formation of high-hardness carbides and nitrides in the near-surface region. The NESS process offers an alternate approach to the usual process of addition, which involves long periods of diffusion in hot environments. For carburization, it is possible to start by depositing a glassy layer of carbon on the surface to be treated (this deposition may use an EAS process, but need not). The layer of carbon and a suitable thickness of the underlying metal would then be melted by the pulse of an ion beam, whereupon the carbon would dissolve into the steel. Further heat treatment may be necessary to obtain optimal surface conditions, depending on the starting alloys. A similar technique which may work for nitriding would require deposition of a layer of a high-temperature nitride, such as titanium or vanadium nitride. (The titanium or vanadium also improve the properties of the resulting steel. However, this hardening process is not limited to these two elements, but may use any nitride which can withstand the high process temperatures without volatilizing.) The remainder of the process is carried out as for carbon above, save that further thermal treatment are generally not useful in nitridization. Other elements can be introduced into the surface layers of a compatible body using this type of NESS technique.

The beneficial effects of Surface Smoothing on corrosion resistance was discussed earlier. Additional phenomena more closely related to the NESS processes are also of value in holding back corrosion. This is illustrated in Example 7, in which a stainless steel surface is treated with a mixed carbon-hydrogen ion beam pulse from an early device utilizing a flashover ion source. Although this technology is primitive compared to that offered by the current invention, in particular not allowing industrial scale-up, it did prove adequate to demonstrate the increase of corrosion resistance.

When 304 stainless steel is annealed at high temperatures as described in the Example, chromium-depleted regions form near the grain boundaries of the metal. The chromium precipitates out in large chromium carbide particles in the interiors of the grains. The chromium-depleted regions are intrinsically more susceptible to corrosion, and the chromium carbide particles present intergranular surfaces which are also particularly susceptible to corrosion. As a result, 304 stainless steel, when subjected to the described heat treatment, becomes extremely susceptible to corrosion, primarily preferential grain boundary corrosion. When the heat-treated surface is subjected to a 0.3 MeV. ∼300 ns pulse of mixed ions with a total energy of 2-3 J/cm², the rapid melting and recrystallization removed the chromium-depleted grain boundaries and caused the chromium carbide particles to redissolve in the metal. This treatment was observed to increase corrosion resistance essentially back to the pre-heat treatment level. Similar work aimed at studies of pitting susceptibility of 316L and 316F stainless steels has also been undertaken with similar results.

Aluminum alloys have also been subjected to NESS processes to increase their corrosion resistance. Again, the pulsed ion beam used was a mix of carbon and hydrogen ions accelerated to 0.7 MeV. The pulses were ∼100 ns wide, and the total energy of each pulse was ∼2-3 J/cm². Exposure testing for the alloys used was conducted in a saturated salt fog environment. The alloys treated have included 2024-T3, 6061-T6, and 7075-T6. In all cases the NESS treatment increased the corrosion resistance of the samples. This should be true for all structural aluminum alloys.

Another approach to increasing corrosion resistance through NESS treatment can be illustrated best by considering a carbon steel (i.e., low chromium content). Such steels are extremely susceptible to corrosion, rusting in moist air, disintegrating over time in saline environments, and failing even more quickly in more hostile conditions. The addition of chromium to such steels produces stainless steels, which do not share this extreme sensitivity to environment. However, stainless steel is expensive, especially considering that the mechanical properties of stainless steels are suboptimal, and that the property of being "stainless" need only exist at the surface of the element. NESS treatment can help to solve this problem by mixing a surface-deposited layer of chromium with the near-surface regions of the steel element. The result will be an element having the superior mechanical properties of carbon steel combined with an outer layer of stainless steel perhaps 5-20 μm thick (depending on conditions) which is both smooth and uniform, thus providing excellent corrosion resistance. This sort of technique is extendible to many metal alloy systems, including welds, the scope of which are well-known to practitioners in the metallurgical arts.

The Examples referred to above will now be described in detail. These Examples are not intended to limit the scope of the claims appended in any manner, but rather to illustrate their application in specific instances.

EXAMPLE 1

A sample of nominally pure Cu was etched in 1 molar nitric acid for one minute. Scanning electron microscopy (SEM) analysis of the resulting surface showed a roughened surface with hillocks and "sharp" features approximately 3-5 μm in height. These samples were treated using a single pulse of an ion beam generated using a RHEPP prototype power source and a flashover ion source. (In a flashover ion source an electrical discharge volatilizes the surface of a polymer, resulting in the generation of mixed carbon and hydrogen ions.) The beam kinetic energy was 1.0 MeV, the pulse width was approximately 60 ms, and the total pulse energy density at the treated surface was ∼3J/cm³.

Post-treatment SEM analysis revealed a smoother surface with more gradual changes in surface configuration and an average surface roughness of ∼0.5 μm. In this example the Cu surface was molten for ∼500 ns. The driving force of surface tension during this period was clearly sufficient to produce nearly complete removal of the original surface morphology.

EXAMPLE 2

A piece of Ti-6Al-4V alloy (a common machinable titanium alloy) was machined using conventional precision machining techniques, leaving a nominally flat surface with machining marks producing a surface roughness of ∼5 μm. This surface was treated by exposure to four pulses, each pulse having a beam kinetic energy of ∼3.0-0.4 MeV, a duration of ∼400 ns, and a total pulse energy density of ∼7 J/cm². SEM analysis of the treated surface revealed surface roughness had been reduced to ∼0.1 μm. The time the metal surface was liquid was again some 250-500 ns, suggesting that the effect of multiple pulses in the smoothing process is additive, i.e., that more pulses give a smoother surface.

EXAMPLE 3

One side of an alumina (Al₂ O₃ ceramic) sample was polished using submicron abrasive grit suspensions. Following characterization of the surface with an SEM, the polished surface was subjected to a single ion pulse having a beam kinetic energy of ∼1.0 MeV, a beam duration of ∼60 ns, and a total pulse energy density of ∼10 J/cm². Post-treatment analysis showed evidence for melting and resolidification resulting in reduction of surface porosity. There remained, however, some microcracking on a 0.1 μm size scale. It is considered likely that further treatment would yield a uniformly smooth surface.

EXAMPLE 4

Because of its unique magnetic properties, various amorphous magnetic alloys known by the registered trademark (Allied-Signal, Inc.) METGLAS™ are desirable in high frequency applications, including pulsed power supplies and control. These materials are made by spraying the molten alloy on a cooled rotating wheel, thereby quenching the material at ∼10⁶ °K/sec and forming an amorphous ribbon having thicknesses in the range of 15-50 μm. Due to hydrodynamic instabilities during the cooling process, one side of such ribbons has significant ripples in thickness having a period similar to the thickness of the ribbon. This non-uniformity is important for two reasons. First, the magnetic properties at high frequencies are a function of the thickness of the ribbon; hence the variation in thickness limits the performance of devices constructed of non-uniform ribbon. Second, the size scale of the surface roughness is sufficient that when the ribbon is formed into a coil, or similar structure, the layer of insulation between alternate layers of ribbon must be very thick to prevent formation of short-circuits. The thick insulation reduced the density of magnetic material in a given construct, lowering performance and increasing the physical dimensions of the ultimate device.

An experiment was performed to discover if Surface Smoothing with ion beam pulses could even out the non-uniformities of a METGLAS™ surface while retaining the unique magnetic properties which result from the amorphous structure. METGLAS™ 2605CO material was chosen for the test, as it is perhaps most widely used in commercial applications at this time. The nominal composition of METGLAS™ 2605CO is Fe₅6 Co₁8 B₁5 Si₁, and it is produced using the wheel-quenching technique described above. A sample was selected, and subjected to a single 2 J/cm² pulse of mixed carbon and hydrogen ions from a flashover source. The beam kinetic energy was ∼0.6 MeV, and the pulse width was ∼60 ns. The resulting surface was virtually flat.

A second concern, of course, was that the nanostructure which helps to give METGLAS™ 2605CO its unique properties might be damaged by remelting and quenching at a rate different than encountered in the original manufacture. Tests have shown that the amorphous structure of the original METGLAS™ is unchanged by the ion pulse treatment.

EXAMPLE 5

A 0.1 μm layer of machining fluid (a hydrocarbon mixture) was applied to the surface of a sample of 304 stainless steel. The surface was examined using x-ray photo-emission spectroscopy (XPS) to verify the thickness of the hydrocarbon layer. The sample was then exposed to three ion pulses, each having a total energy density of 2-3 J/cm², a beam kinetic energy of 0.5-0.75 MeV, and a pulse duration of ∼50 ns. Following treatment, XPS was again performed, and showed only that amount of hydrocarbon expected from atmospheric contamination (about a monolayer). The surface cleaning was thus totally successful.

EXAMPLE 6

A sample of 0-1 tool steel was subjected to ion pulses to determine if the surface could be hardened thereby. The sample was subjected to a single pulse having a beam kinetic energy of ∼1 MeV, a duration of ∼40 ns, and a surface energy density of ∼5 J/cm². On recovery, the top few microns of the sample showed only fine grains on the order of 20 nm in size, compared to the initial material which had grain size on the order of 1 μm in size. The initial material had a significant density of iron carbide precipitates, whereas the surface layers did not, having apparently redissolved the carbon into the iron matrix.

Hardness testing on the samples was done using micro-indentation techniques. A Knoop indentor tip was pressed into the samples with a 25 gram load, producing indentations about 5 μm in thickness. A direct reduction of this data showed that the untreated surface had a Knoop hardness of 330, while the treated surface has a Knoop hardness of 900, roughly three times higher. Further, indentation hardness tests are influenced by the hardness of the material out to a distance of perhaps 10 time the size of the indentations. Since the treated layer is only ∼7 μm thick, this means that it is actually much harder than the indentation testing revealed.

θ-2θ x-ray diffraction measurements were taken of the treated and untreated surfaces. The untreated surface shows only a sharp peak corresponding to large ferrite grains (the Fe₃ C precipitates would not diffract at the angles examined). The treated surface, however, showed three interesting differences from the untreated surface. First, austenite peaks appeared, showing that high-temperature species had been successfully recovered in the rapid quench. Second, the diffraction peaks were all quite broad, in agreement with the observation that the grain size in the treated material was very small. Finally, the ferrite peak in the treated sample is asymmetric, suggesting the existence of lattice strains consistent with the presence of martensite. It is likely that all of these effects combine to increase the hardness of the surface of the treated sample.

EXAMPLE 7

Four flat samples of 304 stainless steel were prepared to determine if ion beam pulses could eliminate preferential grain boundary corrosion due to heat treatment. All samples were held at 1100°C for 24 hours, and then quenched in cold water. Two of the samples were sensitized to corrosive action by heating them at 600°C for 100 hours, followed by cooling in air. This second anneal produces precipitation of chromium carbide particles, formed through depletion of the grain boundaries of the metal of their chromium, a well-known problem in the application of stainless steels having too much carbon.

All samples were polished to a mirror finish. Two of the samples, one from each group of annealing conditions, were subjected to four pulses each having a surface energy density of ∼3 J/cm², a beam kinetic energy of ∼0.3 MeV, and a duration of ∼300 ns. Each pulse was a combination of carbon and hydrogen ions, the ions source using flashover technology.

The degree of sensitization was determined using potentiokinetic reactivation in a 0.5M H₂ SO₄ plus 0.01M KSCN solution held at 30°C The charge per unit area Q/A required for reactivation is a measure of the susceptibility of the surface of the corrosive effects of this solution. The sample exposed only to the 1100°C anneal had a Q/A value of 0.018 Coulombs/cm². The sample having the same heat treatment but also exposed to the ion beam pulse had a Q/A value of 0.057 and 0.084 Coulombs/cm² (on separate measurements), suggesting that the beam treated surface was somewhat more susceptible to corrosion.

The more important results, however, are on the samples which had undergone both annealing cycles. The sample which only received both annealing cycles had a Q/A value of 0.825 and 0.817 Coulombs/cm² (again two measurements were made), an enormous increase from the value of 0.018 for the sample which only received the high-temperature anneal. This huge difference in corrosive sensitivity explains why temperatures in the 600°C range are avoided in application of most stainless steels. However, when such a sample is treated with the above described ion beam pulse schedule, the Q/A value dropped to 0.027 and 0.028 Coulombs/cm², a value nearly as low as the original material.

One example of why this result is important lies in the problem of welding stainless steel for applications in which corrosive environments are to be encountered. In welding there will clearly be a zone of material which will slowly cool from a temperature in the sensitization range (roughly 400°-800°C). This zone will be somewhat sensitized to corrosion, although not to the extreme of the experimental sample described above. Unless the entire assembly can be subjected to high-temperature annealing when complete, most stainless steels will not be practical choices for corrosive environments. When stainless steels must be welded now, a steel is chosen having so little carbon that the grain boundary sensitization process cannot occur, thus solving the corrosion problem. However, low-carbon steels are generally soft and weak by comparison to other possibilities, so this choice is a compromise. The ion beam pulse surface modification technology described herein will reduce the number of design compromises required, in this problem and in many others.

The capacity of the present invention for producing high energy, high average power pulsed ion beams results in a new, low cost, compact surface treatment technology capable of high volume commercial applications and new treatment techniques not possible with existing systems. Having thus described the present invention with the aid of specific examples, those skilled in the art will appreciate that other similar combinations of the capabilities of this technology are also possible without departing from the scope of the claims attached herewith.

Claims

1. A process for uniformly altering a characteristic of a surface of a material to a depth of<several hundred microns comprising the step of irradiating a surface of the material with a repetitively pulsed ion beam from an ion beam source having an anode electrode and a cathode electrode, defining therebetween an acceleration gap, and producing ions created by ionizing an injected gas, wherein each pulse of the pulsed ion beam has a duration of ≦1000 ns at (an said accelerating gap) between an anode electrode means and a cathode electrode means n the ion beam source, a total beam energy delivered to the material of >1 Joule/pulse, an impedance of <1000Ω, a repetition rate of >1 Hz, an ion kinetic energy of >50 keV, and an ion penetration depth of <50 microns.

2. The process of claim 1 wherein the depth of ion penetration is controlled by controlling the kinetic energy of the ion beam.

3. The process of claim 1 wherein the depth of ion penetration is controlled by controlling the atomic mass of the ions in the ion beam.

4. The process of claim 1 wherein the depth of ion penetration is controlled by controlling the atomic number of the ions in the ion beam.

5. The process of claim 1 wherein the characteristic is surface smoothness which is modified to a surface roughness of <0.5 microns.

6. The process of claim 5 wherein the material is a fine grain, sintered material.

7. The process of claim 5 wherein the surface is a food preparation surface.

8. The process of claim 7 wherein the food preparation surface is a food cooking surface.

9. The process of claim 5 wherein the material is an amorphous magnetic alloy.

10. The process of claim 9 wherein the alloy has the approximate composition of Fe₆6 Co₁8 B₁5 S₁.

11. The process of claim 1 wherein the surface characteristic is the presence of an unwanted contaminant.

12. The process of claim 11 wherein the unwanted contaminant is a machining lubricant.

13. The process of claim 11 wherein the unwanted contaminant is solder flux.

14. The process of claim 11 wherein the unwanted contaminants is biological contamination.

15. The process of claim 11 wherein the unwanted contaminants is a surface coating.

16. The process of claim 1 wherein the total beam energy delivered to the material per pulse is <10 Joules/pulse and the surface characteristic to be altered is the presence of the top 1-2 microns of the material which is removed by ablation.

17. The process of claim 1 wherein the total beam energy delivered to the material per pulse is <20 Joules/pulse and the surface characteristic is shock hardening.

18. The process of claim 16 wherein the ablation produces vaporization of the surface of the material which redeposits upon the surface of the material.

19. The process of claim 16 wherein the ablation produces vaporization of the surface of the material which redeposits upon a surface of a second material.

20. The process of claim 16 further including protection of certain areas of the surface of the material by mask means which protect the surface from the ablation.

21. The process of claim 1 wherein the surface characteristic to be altered is hardness.

22. The process of claim 1 wherein the surface characteristic to be altered is corrosion resistance.

23. The process of claim 22 wherein the material is steel.

24. The process of claim 22 wherein the material comprises aluminum.

25. The process of claim 23 wherein the material is stainless steel that has been heat treated to above 600°C

26. The process of claim 1 wherein the surface characteristic to be altered is resistance of welds to stress cracking.

27. The process of claim 1 wherein the surface characteristic to be altered is resistance of welds to corrosion.

28. The process of claim 1 wherein the surface characteristic to be altered is the formation of non-equilibrium structures within the surface.

29. The process of claim 28 wherein the non-equilibrium structures are selected from the group consisting of amorphous structures, disordered crystalline structures, and nanocrystalline structures not present in the original material.

30. The process of claim 1 wherein the area of continuous and uniform alteration of the characteristics is >5 cm².

31. The process of claim 1 wherein the ion species are selected from the group consisting of hydrogen, helium, oxygen, nitrogen fluorine, neon, chlorine, argon, lithium, beryllium, boron, carbon, sodium, magnesium, aluminum, silicon, phosphorous, sulfur, potassium and the isotopes thereof.

32. The process of claim 1 wherein the material is selected from the group consisting of intermetallic materials, amorphous materials, crystalline materials, nano-crystalline materials, dielectrics, polymers, semiconductors, ceramics and glasses.

33. A method of smoothing a surface of a bulk material comprising:

(a) generating an ion beam using a magnetically confined anode plasma ion source comprising a vacuum chamber having an anode assembly including an anode electrode and a fast driving coil and a cathode assembly including a cathode electrode and a slow driving coil, the anode electrode and cathode electrode defining there between an acceleration gap said step of generating further comprising:

(i) introducing a puff of gas into said vacuum chamber to produce a localized volume of gas adjacent said fast driving coil but insulated from said fast driving coil;

(ii) pre-ionizing said gas using said fast driving coil;

(iii) further ionizing said gas while moving said gas away from said fast driving coil to create a thin, magnetically confined plasma layer;

(iv) applying a pulsed power signal to said anode electrode to form an ion beam from said plasma;

(v) extracting said ion beam with little or no rotation thereof; and

(b) directing said beam at said surface to reduce the total surface area. 34. The method according to claim 33, wherein said step of directing comprises melting a near surface layer of said bulk material, and resolidifying said near surface layer without melting said bulk material. 35. The method according to claim 34 further comprising controlling the duration of exposure of said surface to said

ion beam to improve surface morphology. 36. The method according to claim 34, wherein said bulk material is a fine grain sintered material. 37. The method according to claim 36, wherein said material includes ceramic materials and powder metallurgy materials. 38. The method according to claim 36, wherein said steps of melting and resolidifying produce a glassy surface. 39. The method according to claim 36, wherein said steps of melting and resolidifying produce a alloy surface. 40. The method according to claim 38, wherein said surface comprises a non-equilibrium form of the bulk material. 41. The method according to claim 39, wherein said surface comprises a non-equilibrium form of the bulk material. 42. The method according to claim 34, wherein said method is repeated whereby said surface is repeatedly melted, each time resulting in a smoother surface. 43. The method according to claim 34, further comprising the step of removing surface

defects. 44. The method according to claim 43, wherein said surface defects comprise machining marks from a machining process. 45. The method according to claim 44, wherein said post smoothing process surface is free of sharp or abrupt features. 46. The method according to claim 44, wherein the surface contour generated by the machining process is unchanged by the smoothing process. 47. The method according to claim 34, wherein the surface to be smoothed is a wear surface. 48. The method according to claim 43, wherein said surface defect comprises a surface crack. 49. The method according to claim 43, wherein said surface defects comprises potential corrosion sites and said method results in a corrosion resistant surface. 50. The method according to claim 34, wherein said bulk material comprises an amorphous magnetic material. 51. The method according to claim 34, wherein said bulk material comprises a nanocrystalline material.

52. The method according to claim 51, wherein said bulk material is formed in thin layers deposited on substrate having a relatively high thermal conductivity and wherein said method further comprises directing said ion beam onto said substrate. 53. The method according to claim 51, wherein defects in the bulk material are removed by melting and recrystallization. 54. The method according to claim 33 further comprising the step of melting a near surface layer of said bulk material followed by the step of rapid thermal quenching. 55. The method according to claim 54, wherein said step of rapid thermal quenching comprises removing heat at a rate greater than about 10⁸ k/sec. 56. The method according to claim 36, wherein the bulk material is a ceramic and a glassy surface is formed. 57. The method according to claim 37, wherein the bulk material is a powder metallurgy material and an alloy surface is formed. 58. The method according to claim 36, wherein a surface having reduced surface porosity is formed. 59. A method of evaporation and ablation from a surface comprising the steps of:

(a) generating ion beam using a magnetically confined anode plasma ion source comprising a vacuum chamber having an anode assembly including an anode electrode and a fast driving coil and a cathode assembly including a cathode electrode and a slow driving coil, the anode electrode and cathode electrode defining therebetween an acceleration gap; said step of generating further comprising:

(i) introducing a puff of gas into said vacuum chamber to produce a localized volume of gas adjacent said fast driving coil but insulated from said fast driving coil;

(ii) pre-ionizing said gas using said fast driving coil;

(iii) further ionizing said gas while moving said gas away from said fast driving coil to create a thin, magnetically confined plasma layer;

(iv) applying a pulsed power signal to said anode electrode to form an ion beam from said plasma;

(v) extracting said ion beam with little or no rotation thereof; and

(b) directing said ion beam at said surface to evaporate a layer of said

surface. 60. The method according to claim 59, wherein said surface comprises a layer and an underlying surface of bulk material and further comprising the steps of removing at least said layer. 61. The method according to claim 60, wherein said bulk

material is metal and said layer is an oxide layer. 62. The method according to claim 60, wherein said bulk material is metal and said layer is a hydrocarbon. 63. The method according to claim 60, wherein said layer is a contamination layer, said contamination having a boiling point lower than that of said bulk material. 64. The method according to claim 60, wherein said layer is a passivation layer and said bulk material is part of a substrate, said passivation layer having a lower boiling point than said substrate; and further comprising the step of removing said passivation layer by super heating the surface of said substrate by said ion beam. 65. The method according to claim 60, wherein said layer is an overlayer of an unwanted material having a higher vaporization point than the bulk material, said method further comprising ablating a surface layer of the bulk material beneath said overlayer, thereby removing said unwanted material. 66. The method according to claim 60, wherein said bulk material is a polymer substrate. 67. The method according to claim 59, wherein the step of directing further comprises interposing a mask or compound to prevent said ion beam from effecting selected portions of said surface. 68. The method according to claim 65, further comprising the step of shock-hardening of the bulk

material to a depth greater than the melt depth. 69. The method according to claim 59, further comprising the step of redepositing vaporized material onto said surface. 70. A method of producing non-equilibrium or near equilibrium structures on the surface of a bulk material comprising the steps of:

(a) generating an ion beam using a magnetically confined anode plasma ion source comprising a vacuum chamber having an anode assembly including an anode electrode and a fast driving coil and a cathode assembly including a cathode electrode and a slow driving coil, the anode electrode and cathode electrode defining therebetween an acceleration gap, said method comprising the steps of:

(i) introducing a puff of gas into said vacuum chamber to produce a localized volume of gas adjacent said fast driving coil but insulated from said fast driving coil;

(ii) pre-ionizing said gas using said fast driving coil;

(iii) further ionizing said gas while moving said gas away from said fast driving coil to create a thin, magnetically confined plasma layer;

(iv) applying a pulsed power signal to said anode electrode to form an ion beam from said plasma;

(v) extracting said ion beam with little or no rotation thereof;

(b) heating a bulk material having an initial structure to a predetermined depth with said ion beam; and

(c) rapidly conducting the heat into said bulk material whereby a surface having a structure different from said initial structure is formed.

71. The method according to claim 70, wherein said bulk material is a metal alloy. 72. The method according to claim 70, further comprising the step of depositing a layer of material on the bulk material and whereby the step of heating further comprises melting said layer of material and a surface layer of said bulk material whereby said material is dissolved into said melted bulk material. 73. The method according to claim 72, wherein said layer material comprises carbon and said bulk material comprises steel. 74. The method according to claim 72, wherein said layer material comprises a nitride and said bulk material comprises steel. 75. The method according to claim 72, wherein said bulk material is selected from the group consistent of 304 stainless steel, 316 L stainless steel and 316 F stainless steel. 76. The method according to claim 72, wherein said bulk material comprises an aluminum alloy. 77. The method according to claim 72, wherein said layer material is chromium and said bulk material is carbon steel and wherein said step of melting further comprises melting said chromium and

carbon steel. 78. The method according to claim 72, wherein the layer material and bulk material are immiscible when solid and wherein the step of melting further comprises forming a single phase liquid comprised of the layer material and bulk material. 79. The method according to claim 78, wherein said step of rapidly conducting heat further comprises quenching the molten material produced an amorphous alloy consisting of said layer material and said bulk material. 80. The method according to claim 78 wherein said step of rapidly conducting heat comprises quenching at a rate whereby nanoscale precipitates are formed after quenching. 81. The method according to claim 72 further comprising the step of rapid quenching whereby an alloy of said bulk material and layer material is formed. 82. The method according to claim 81, wherein said alloy comprises a metastable alloy on the surface of said bulk material. 83. The method according to claim 70, wherein said bulk material is a fine-grain sintered material. 84. The method according to claim 83, wherein said bulk material is a ceramic and a glossy surface is formed. 85. The method according to claim 83, wherein said bulk material is a powdered metallurgy material and an alloy surface is formed.