I. THERMAL SPRAY TECHNOLOGY
During the past 50 years, thermal spray technology has gained widespread attention as a means of providing machinery parts with wear and corrosion-resistant coatings. These coating are used not only for enhancing the mechanical properties of the base material, but are very useful in salvaging worn machine elements, the latter being the most dominant in thermal spray service repair shops.
To date we have a number of thermal spray processes which we can use to improve a part's surface characteristics, namely:
Wire flame spraying Electric arc flame spraying Powder flame spraying Plasma flame spraying Hypersonic flame spraying
These processes use either wire or powder feedstock materials.
Wire, electric arc, and powder flame spraying are considered to be low thermal and low kinetic energy processes. The most popular process used in this category is the wire arc flame spray system. This equipment was primarily developed for the rapid application of corrosion-resistant coatings (i.e., aluminum and zinc) to large surface areas in the steel construction and fabricating industries. The process has since been successfully adapted to the application of higher melting point metals and enables the use of two separate coils of wire, to create "pseudo" alloys of dissimilar materials.
In comparison with other wire and powder-formed coatings, arc sprayed coatings are generally more porous. This porosity is unacceptable for components requiring critical surface finishes, for which we use powder spraying techniques. However, the relatively coarse, tough coating matrix, created by the arc process, exhibits excellent, hard, wear-resistant, resilient qualities. Therefore, the electric arc spraying process has become a necessity when refurbishing large components requiring heavy metal-sprayed deposits.
Plasma spraying process technology was primarily developed for the aerospace industry. This process falls into the category of high thermal, low-kinetic energy processes. The production of coating qualities suitable to the exacting needs of aerospace has involved major commitment from the flame spray equipment manufacturers and several large aircraft manufacturing organizations.
The plasma spray process is generated by a gas, or mixture of gases, passing through an electric arc created between a tungsten cathode and a water-cooled anode (nozzle). This causes an extreme energy transfer, resulting in excessive heating, disassociation, and partial ionization, thereby forming an electrically-neutral plasma. Due to its resultant rapid expansion, the plasma is forced to exit through the nozzle bore, liberating additional heat through recombination.
Into this plasma of ionized gas is introduced an accurately metered flow of powder material suspended in a carrier gas. As the powder particles are fed into the high velocity/high temperature plasma, they are rapidly and thoroughly heated and accelerated to the surface to be coated.
Coatings created by this process are relatively low in oxide content, exhibit minimal porosity, and have high bond strengths.
The physical and metallurgical properties of plasma sprayed coatings are generally superior to all other flame spraying methods, with the exceptions of hypersonic combustion and detonation gun systems.
Unlike plasma spraying (electrical energy), the hypersonic method of thermal spraying falls into the category of a high-velocity combustion process (chemical energy) in that coatings are formed by particles becoming molten by convective heat transfer from homogeneous beam of hot exhaust gases, and accelerated to high velocities by the rapidly moving flame.
The primary component of the hypersonic flame spraying process is an internal combustion device, which produces an "exhaust" similar to that found in a rocket. This exhaust is produced by the internal combustion of oxygen and fuel gases. A combustion flame temperature of approximately 5500°F. is created, with exhaust velocities of 4,500 feet per second.
Powder materials, suspended in a carrier gas, are axially fed into the center of the exhaust within the gun nozzle, where they are heated and accelerated to approximately 2,500 feet per second.
When the powder particles impact a solid workpiece, they possess such exceptional thermal and kinetic energy that an extremely dense, well-bonded and smooth coating of the highest quality is created.
Hypersonic coatings can be routinely reproduced by controlling all the operation's variable parameters, which are less critical than for other flame spray methods. In particular, coating integrity is not significantly affected by changes in "gun-to-workpiece" distance. This relative insensitivity to spray distance, and the portability of this system, make it ideal for on-site spraying and for applications where the gun must be hand held.
II. MARKET IMPACT — APPLICATIONS OF THERMAL SPRAY AS A SUBSTITUTE FOR PLATINO
A. Tungsten carbide has replaced chrome plating on oil field piston rods over the last five years.
Chrome plating had a flaking problem; the chrome flakes would work into the cylinder, causing additional wear on the piston and cylinder.
The tungsten carbide has excellent wear characteristics and does not flake. Piston rods coated with tungsten carbide do not need recoating like chrome did except for external damage. The benefit to the user is longer operational life, thus, lengthening the period between overhauls.
B. Ceramic coatings have replaced chrome plated water rolls in the printing industry. These ceramic rolls are used because of their excellent wetting action. Chrome plated rolls require acid etching, which is in itself an environmental concern. Additionally, with chrome rolls isopropyl alcohol is used to increase wetting action. The volatility of this alcohol is an environmental concern. Alcohol substitutes are more expensive and are increasing in price. The use of ceramic rolls has reduced the need for wetting agents and, in most cases, completely eliminated their use. Ceramic rolls do not need acid etching. In fact, they are inert. Since ceramic does not flake, there is no contamination of rubber rolls in the water or ink train similar to chrome flakes.
c. Hasteloy C has replaced nickel and chrome plating on print cylinders and on composite rolls in the plastics industry. Hasteloy C provides a smooth finish comparable to chrome or nickel. These rolls vary in diameter from 6" to 16" and in length from 2' to 7'. There has been a sharp decline in available suppliers of chrome. Additionally, there has been a reduction in nickel platers because of increasing environmental pressure. The particular advantage to the customer in the use of Hasteloy C is a faster turnaround time.
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