Coating the surface - with nitrogen?
Wear is a problem with virtually all manufacturing operations where one material contacts another. In our business, we usually think of wear with respect to the parts we are trying to make. This includes tires, seals, rolls and other machinery components.
However, wear is also a factor affecting the tools we use to produce rubber parts. It is particularly evident in precision molds and extrusion dies. In internal mixers, the wear that occurs on the metal surfaces is often tracked from year to year as metal is worn off by the mixing of rubber.
One of the more common ways used to reduce the effect of wear on metal caused by rubber is to apply a very hard surface coating of a material over the base steel. Rubber molds are often chrome plated to reduce wear (and improve release) while mixer rotors are generally stellite coated. In both cases, one of the primary problems associated with the coating is poor adhesion, causing flaking and peeling of the coating and loss of the wear resistant surface.
Recently, new methods of surface coating and hardening have been discovered that offer some new and unique advantages. One of the more interesting of these is now being offered commercially by Ion Surface Technology, called "ion implantation."
What is ion implantation?
For some time now, scientists and engineers have been looking at ways to alter the surface characteristics of metals without changing the base structure. Coatings of various types along with chemical treatments have become popular for a variety of applications, including coatings as simple as paint or as hard as various carbides and nitrides. Practically all these methods involve application of a layer of some different material that is either laid across the metal surface or chemically adhered to the surface. Most are prone to chipping, cracking and peeling.
Ion implantation - and its cousin, ion beam mixing - are significantly different. Using these techniques, it is possible to alter the metal surface at the atomic level. By changing the atomic structure at the atomic level, it is possible to design and make a material with a microalloy at the surface. The result is not a coating that can peel, chip and delaminate. It is an integral part of the material although it may in fact have vastly different properties.
While some surface treatments do not involve coatings, they do involve high process temperatures. These high temperatures unfortunately limit the usefulness of the treatment since distortions are often possible. Ion implantation avoids use of high temperatures, thus avoiding this problem.
In fact, ion beam technology involves very low process temperatures. In spite of this, it has unlimited potential for changing the surface characteristics of a material. In theory, any combination of elements and materials can be used with some result. In practice, practical limitations narrow this field. However, the possibilities are still numerous. Because the technology is not limited by conventional constraints of alloying, new types of alloys are possible.
How does it work?
Ion implantation involves the bombardment of materials with ions of a given atomic element traveling at very high speed. It is a very violent process at the atomic level.
In practice, the material to be coated becomes the target for a small linear accelerator in which the material to be implanted is the material that is passed through the accelerator. While any material can be used, nitrogen gas is popular and offers a number of benefits. For the rest of our discussion we will assume nitrogen is the implantation media that will be used and the substrate will be steel.
The accelerator works by heating a tungsten element inside a graphite box that has been evacuated to approximately 10-6 torr. The element is kept quite hot, using a current of 100 amps or more. This causes an electron field to be generated inside the box in the neigborhood of the filament. This field is "swirled" and contained magnetically while the nitrogen is introduced into the box.
The electron cloud in the box physically knocks electrons off the nitrogen, creating a positively charged nitrogen gas, a type of plasma. This plasma is then accelerated and drawn to the target, which is maintained at a voltage difference of approximately 100,000 volts. It is estimated that, at impact, the nitrogen is traveling at a speed of 2 million miles per hour!
As mentioned earlier, this results in a very violent impact. The impact of the nitrogen at the steel surface results in a churning of the surface. If that surface has been previously coated with another metal or material, such as chrome, the chrome will be churned into the outer surface of the steel substrate. In this fashion, an alloy is created on the surface that is still an integral part of the substrate.
However, as violent as the impact is, it generally only enters the surface a small distance - perhaps 0.1 micron (4 millionths of an inch).
While the actual penetration of the surface is small, the effects can be significant. First, of course, if the steel is chrome plated and assuming the thickness of the chrome plate is approximately 0.1 micron, a surface alloy consisting of steel, chrome and chrome nitride will be formed. This surface will exhibit much greater hardness and lubricity than the steel alone.
Next, nitrogen that does not react with the chrome to form chrome nitride will fill micro-fissures in the steel, effectively locking them in place. Wear in metal surfaces relates to some extent to the movement of microfissures around in the metal. If the microfissures are not free to move, wear will be reduced. Thus, nitrogen packing of the microfissures tends to reduce wear. Finally, the surface of the steel will be work hardened. Work hardening is believed to occur by introducing defects into the lattice structure of the steel. In this case, nitrogen is forced into the crystal structure of the steel. These defects reduce the ability of the steel to deform, thus increasing its hardness and wear resistance. Work hardening resulting from the impact of the nitrogen on the surface appears to affect a depth of 50 times the penetration level of the nitrogen. Therefore, the surface will have been work hardened to a depth of approximately 5 microns.
If the steel surface has not been plated with a material to chemically react with and lock the nitrogen in place, nitrogen implanted tends to be fugitive. While it will not be removed in normal use, heating will remove unreacted nitrogen from the crystal structure. The degree of loss will vary with the temperature. At 700 [degrees] F, all the nitrogen will be gone.
Ion beam mixing is very similar to ion implantation with one exception. Again, using nitrogen, chrome and steel, the nitrogen is passed through the linear accelerator and focused on the steel target. However, instead of having the chrome already plated on the surface, chrome is introduced into the gas stream prior to impacting the steel. The chrome and nitrogen are effectively mixed in the beam and hit the steel surface together at the same speed.
There is no apparent benefit to ion beam mixing versus simple ion implantation.
What about PSII?
There is one other technique, however, that does offer some additional benefits. It is called plasma source ion implantation - PSII. Figure 1 shows the primary difference between the two techniques.
Ion implantation works by accelerating an ion beam and focusing that beam on a target. It is a line of sight technique and requires that the target be moved and re-oriented in the beam in order to "coat" the target surface. The linear accelerator used can be relatively small with the plasma stream exiting the end towards the target much like water out of a hose. This technique also keeps the target electrically neutral while the plasma is accelerated in a separate system.
PSII requires a much larger plasma generator. With this technique, the nitrogen is again passed through an electron cloud creating a nitrogen plasma. However, in this case, the target is placed in the middle of the plasma generator and kept at a negative potential of approximately 100,000 volts. In this way it provides the acceleration for the nitrogen plasma. Since the target is in the middle of the plasma, it is not necessary to move the target in the gas stream to get complete coverage. Coverage of the surface is more complete and uniform. It is also a much faster process.
What are characteristics of ion implanted material?
As mentioned earlier, ion implantation results in the formation of a microalloy on the metal surface that is a part of the substrate material. It increases the hardness and wear resistance of that surface. Table 1 shows some of the relative effects of the technique.
In virtually all cases, wear resistance was improved and surface lubricity was increased. In the case of the rubber molds, there was less need of mold releases and less adhesion of material on the mold.
The increase in surface hardness cannot be measured using conventional techniques. Conventional instruments break through the thin surface coating and wind up measuring the hardness of the substrate material. Only by using micro-hardness instruments is it possible to actually measure the effect of implantation.
As mentioned earlier, nitrogen is a preferred material for implantation. First, it is gaseous, readily available and easy to use. Second, nitrogen by itself has a beneficial effect on the steel. Third, it will react with surface materials to form nitrides. Nitrides are very hard, wear resistant materials and, for that reason, are often used as surface coatings. If the surface is precoated with chrome, chrome nitrides are formed. If titanium is used as a precoating, titanium nitride is formed.
It is important to remember that any pre-coatings used must be very thin - 0.1 micron or less. Otherwise many of the advantages of ion implantation will be lost since there will be little, if any, mixing with the substrate.
Plating over an ion implanted surface is of no value. There are no advantageous reactions with either the nitrogen or the substrate metal.
Chrome can be used instead of nitrogen for implanting. When this is done with steel, the corrosion resistance of the steel is significantly improved by forming a chrome alloy steel on the surface. Also, the implanted chrome is harder than "regular" chrome.
Other implanting materials that have been used include the following:
* Boron - reduced surface friction compared to nitrogen.
* Carbon - works much like nitrogen, only forming carbides on the surface.
* Oxygen - difficult to implant. Reacts with all materials in area.
* Titanium and carbon - in combination form titanium carbide; very hard.
What are the problems?
As with all processes, ion implantation has its problems. One of the most pronounced occurs with undercuts. Undercut areas can be very difficult or impossible to implant using current equipment. Ion implantation works on a line of sight basis. If the angle of incidence on the target is over 45 [degrees] (angle between the surface and the beam is less than 45 [degrees]), the result is that blind areas cannot be implanted well. Also, inside surfaces can only be effectively coated as deep as the diameter of the part.
Also, there are size limitations. Ion implantation allows coating of larger parts than PSII. Since PSII requires the target surface to fit inside the linear accelerator, it requires a much larger (physically) accelerator. Current limitations are 10 inch diameter or a flat piece, 10" x 16" or narrower. Work is being done to develop machines that will allow larger pieces
PSII is the lower cost of the two techniques. The process is much simpler and requires no manipulation of the target. Cost of implantation varies depending on the requirements of the part. Costs are calculated on a batch basis and vary from $200 to $400 per batch.
Both ion implantation and PSII appear to offer significant advantages for molds and dies that have very critical dimensional requirements or high wear. Manufacturing operations with critical dimensions and close tolerance requirements would probably benefit form this technique. This would include precision extrusion and molding operations. Reduction in wear and rework of dies and molds would be expected to more than pay for the cost of the process.
This could certainly help advance rubber processing and operations. [Tabular Data Omitted] [Figure Omitted]
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|Title Annotation:||Tech Service|
|Date:||Nov 1, 1989|
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