Abstract: Thermal-spray coatings for computer components
Authors: McClocklin Robert S. , Teal Brad A. 
Published: July 2002

The technology of plasma flame spraying is being used extensively by IBM to combat wear and prolong the life of computer components. The wear environments which cause the most severe problems are abrasive in nature—card stock, paper documents, and magnetic tape. Currently, IBM is using plasma-deposited tungsten carbide, chromium oxide, aluminum oxide, and alumina/titania coatings on over 50 component parts which must function in these severe wear areas. Traditionally, wear resistance has been the prime application area for flame-spray-deposited coatings; however, during the past few years many other applications have been developed which should be cited.

A plasms may be formed from a diatomic gas by first dissociating the molecules into atoms and then ionizing the atoms. To accomplish the dissociation, energy must be put into the system. The energy is produced by a constant high-potential discharge across the electrodes in a confined space under high pressure. The process is accomplished by a large temperature increase. This temperature is dependent upon the energy which must be introduced into the system to finally dissociate the gas.

When the plasma enters the air, it tends to recombine to form the more stable gases originally introduced. During recombination, considerable energy is released as heat.

The powder is introduced into the flame and the particles reach the molten or plastic state in a fraction of a second. The high velocity of the stream leaving the gun propels the particles to the part, and upon impact with the base material these particles resolidify to form the coating. The following is a list of general characteristics common to all flame-spray-deposited coatings: (1) Porosity: A flame-sprayed coating contains a certain degree of porosity. Depending on the coating type, the porosity can range from 0.5% to 10%. (2) Sprayable materials: Any material which does not decompose by sublimation and is available in fine powder form can be plasma flame sprayed. (3) Substrate materials: All metallic, ceramic, and selected polymer substrates can be coated by the plasma-flame-spray process. (4) Bond to the substrate: Most plasma-sprayed coatings are mechanically bonded to the substrate. Certain metallic, exothermic coatings acheive a true metallurgical bond, as confirmed by SEM and microprobe studies. The general range of bond strengths is 4000–10 000 psi. (5) Heat effect: Depending on the coating type, substrate temperatures are maintained in the range of 200 °–500 °F. Carbon-dioxide and liquid-nitrogen cooling techniques can keep the substrate temperature as low as ambient. (6) Nonoxidizing flame: The plasma flame is basically nonoxidizing as compared to the oxyacetylene flame. (7) Thickness limitation: Plasma coatings are thickness limited. Because of internal stresses and expansion coefficient differences, the susceptibility to chipping and spalling increases with coating thickness.

The unique characteristics of the plasma-flame-spraying process give rise to many different types of applications other than for wear resistance. The following is a list of some of the more important application developments: (1) Magnetic head coatings: Dense alumina/titania coatings have been developed for use on magnetic tape read/write heads. These new coatings are providing increased wear resistance and superior anticontamination properties. Finishes as low as 4 μin. (10.16 μm) arithmetic average are being obtained. (2) Tape ceaner blocks: Tungsten-carbide coatings with a controlled topography are providing increased tape-cleaning properties. Precise peak-to-valley and surface-roughness criteria are maintained so as not to damage the magnetic tape, yet at the same time to provide the cleaning action. (3) Armatures/bails: Dense, highly bonded tungsten-carbide coatings for use on print armatures and punch bails are providing increased impact/wear resistance and at a lower cost than conventional case hardening and plating. (4) Friction feed wheels: Many types of card-, document-, ribbon-, and label-moving devices are using feed wheels coated with precisely meshed tungsten-carbide materials. Surface-roughness and friction coefficients can be tailored to the application by proper powder selection and spray parameters. Diametrical tolerances as close as ± 0.0004 in. (± 0.010 mm) can be maintained. (5) Optical reflecting coatings: Various optical-character-reading machines are using dense blue-black alumina/titania coatings on read-area bed plates, wherewear resistance and low optical reflectance are required. Also, pure white alumina coatings are being used where high reflectance is required. (6) Electrically conductive coatings: The increased use of plastics and thermoplastic foams in data processing machines has caused the development of metallizing techniques to solve complex electrical shielding, static discharge, and electromagnetic interference problems. Wire metallized zinc, aluminum, and copper are providing solutions to these problems, and at cost savings over the other tecniques of vacuum metallizing and conductive paint systems. (7) Die and strippers: Plasma-flame-sprayed tungsten-carbide coatings are being used on die plates for edge retention and wesr resistance. Because of the extremely fine mesh of the powder, the coating is used in the as-sprayed condition, and consequently the high finishing costs are eliminated. (8) Dielectric coatings: Ceramic coatings providing dielectric strengths as high as 450 V/mil are being used for insulators and corona discharge electrodes. (9) Tooling applications: High-wear areas in molds, dies, and grinding fixtures are being coated with various tungsten-carbide and ceramic coatings. An extensive program to salvage worn tooling and redesign wear surfaces throughout ceramic chip processing lines will realize savings of over $ 200 000. Savings in excess of $ 75 000 per year are generated by the salvage of mismachined or worn tooling.