Planting ions: seeds for precision-tool life extension.
For example, ion nitriding, chemical vapor deposition (CVD) and, more recently, physical vapor deposition (PVD) are all means for significantly extending tool life (see Tooling & Production, Oct '84, pg 34). An alternative is ion implanting, which doesn't plate, bond, or coat. Instead, ions are embedded in a tool's surface--there's neither dimensional change nor a coating that can peel or delaminate. Most tools and dies subject to adhesive or mild abrasive wear are candidates for treatment.
Evolving from World War II isotope separators, ion-implanting equipment was first used in the late '63s for manufacturing semiconductors. Over 2000 implanters now are used throughout this industry, often on a two- or three-shift basis.
Zymet Inc, Danvers, MA, an affiliate of Eaton Corp, recently introduced an ion-implanting system (called Z-100), Figure 1, for treating tools and dies. The firm received the IR-100 Award, sponsored by Research & Development magazine, for their effort. The award recognizes the 100 most significant technological achievements of the year.
The Zymet process begin by feeding a small stream of nitrogen gas into an ion source. Electrons are emitted at high velocities from a hot tungsten filament and collide with the nitrogen atoms, stripping off an electron to form ions.
The nitrogen ions then are electrostatically extracted from the ion source, focused into a beam, and accelerated to 100 KeV. When the beam impinges on a metal surface, the ions become part of the near surfce region. All this happens in a vacuum.
To understand how any surface treatment inhibits wear, it's necessary to look at what causes it. While there are many types of wear, tool engineers should be most concerned with adhesive and mild abrasive wear, which combine to cause billions of dollars in tool replacement costs every year.
There's a complex relationship between near surface structures and the interaction between two faces in sliding contact. A magnified cross section of a typical tool surface reveals myriad peaks (asperities) and valleys. Asperities will adhere to one another when faces are in sliding contact. The result? Microscopic pieces of the less durable surface are plucked out (adhesive wear).
This subsequently leads to more damaging abrasive wear. Here, dislodged particles work harden and combine with other hard particles to gouge the contacting surfaces.
Other factors are at work as well. Examining a subsurface structure, for example, reveals small grains that form a lattice of different defects. One that's especially troublesome is called a dislocation. As dislocations move under applied loads, surface microcracks are initiated that eventually propagate and aggravate particle loss.
Both coatings and heat treatments can easily improve wear resistance. Titanium-nitride coatings and hard-chromium platings, for example, resist wear better than most tool-material substrates; however, the applied thickness changes dimensions and even can delaminatte in severe service.
Heat treatments provide material hardness at an atomic level, so delaminating isn't a problem. The process temperatures, though, can distort precision tooling. Ion implanting, which takes place at room temperature, is a viable alternative when dimensionality and distortion are concerns.
Embedded nitrogen ions act as atomic anchors by strongly coupling with structural defects such as dislocations. Injecting ions into a near-surface region induces compressive stresses (similar to the benefit of shot peening) that reduce the tendency of surface cracks to open.
With tool steels, some nitrogen ions even bond with alloying elements (e.g., chromium or vanadium), forming extremely hard nitrides. These also protect the surface from fracturing. (Note: Because nitrides are formed in such minute quantities, they can't be detected readily with conventional hardness-testing techniques.)
Further, certain oxide films eliminate severe adhesion and reduce the coefficient of friction. Steels in sliding contact in a vacuum, for instance, lose oxides and experience rapid adhesive wear. The same materials sliding in a good lubricant have oxides replenished and wear more slowly.
By modifying the compositon of the uppermost layers of a tool material, ion implanting reduces the chemical affinity of surfaces in contact, promotes normal oxide growth, and strengthens the metal-oxide/metal interface, allowing a naturally formed surface oxide to serve as a solid lubricant.
Although nitrogen ions penetrate less than 1-micron deep, during wear the implants debond and retreat from the wear front to form new quasi-nitrides. This provides a persistence of wear resistence that's 10t to 50 times deeper than the initially treated zone--ions that fight and run away live to fight another day.
Cases in point
Examples of successfully treated tools and dies follow, Figure 2.
Scoring die. An aluminum can manufacturer found that implanting nitrogen ions into precision dies used for scoring the pop-out tab on the top of beverage cans increased tool life sevenfold. Made from D2 tool steel, the dies cost several hundred dollars to produce because of the precision machining required.
To reduce tooling costs, the company's manufacturing engineers investigated several other surface treatments; however, none could provide wear resistance without also changing the diehs dimensions, distorting critical areas, or losing temper.
Forming die. Another can maker discovered that implanting nitrogen ions into a D2 die (used for forming the walls and bottom of a two-piece aluminum beverage can) not only increased tool life, but resulted in less material pickup by reducing surface friction. During forming, a hard, abrasive Al.sub.2.O.3 layer built up on the die. Production was stopped repeatedly to polish this away.
A treated die experienced less surface friction as it formed can walls, resulting in markedly reduced pickup. Now the tooling runs for at least 1.5 million hits without a need for polishing.
Printed-circuit-board drills. Several companies report that ion-implanted drill bits used for making holes in glass-filled, epoxy-resin printed-circuit boards last longer, are self honing, and have lower operating temperatures. While individual tool bits aren't expensive ($2 $3 each), it's commong for a high-volume manufacturer to use from 100,000 to 200,000 annually.
When implanted with nitrogen ions, these tungsten-carbide drills last two to three times longer for an annual savings in excess of $100,000. Other advantages--such as lower drilling temperture--are more important to many electronics manufacturers, however.
Untreated drills typically operate at 375 F, which causes the epoxy resin to smear into a hole as the tool cuts through. This requires postprocess cleaning (desmearing), which takes time, adds cost, and involves using hot sulfuric acid or a plasma etch system--messy, dangerous processes that require disposing of hazardous chemicals.
An ion-implanted drill obviates desmearing. By reducing friction between the drill and work material, the tool operates at temperatures as low as 160 F.
Wire-drawing die. A wire manufacturer reports ion implanting increases tungsten-carbide drawing-die life. Wire production typically consumes several hundred dies annually at a cost of about $200 each. Once a die insert is treated, the tool often lasts 10 times longer.
Another benefit is the reduced friction between the tool's forming edge and the work material. This decreases the drawing force required to pull wire through the die by 10 to 15 percent. Because less electricity is consumed driving the motors that draw the wire, the user is saving energy.
For more information about ion implanting, circle E2.
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|Author:||Coleman, John R.|
|Publication:||Tooling & Production|
|Date:||Dec 1, 1984|
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