Coating processes – Spray coating utilizing flame or plasma heat – Organic containing coating
Reexamination Certificate
2000-05-22
2003-01-07
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Organic containing coating
C427S449000, C427S453000, C427S454000, C427S455000, C427S456000, C427S446000
Reexamination Certificate
active
06503575
ABSTRACT:
TECHNICAL FIELD
The invention relates to a novel thermal spray process for the deposition of coatings with a graded or layered composition on a substrate and the coated articles produced thereby. More particularly, the invention relates to feeding at least two coating materials to a thermal spray device and continuously or intermittently changing the composition of the deposited coatings by changing the thermal spray operating parameters. The change in the composition of the coating during deposition creates a graded or layered coating structure.
BACKGROUND ART
The family of thermal spray processes includes detonation gun deposition, high velocity oxy-fuel deposition and its variants such as high velocity air-fuel, plasma spray, flame spray, and electric wire arc spray. In most thermal coating processes metallic, ceramic, cermet, or some polymeric materials in powder, wire or rod form is heated to near or somewhat above its melting point and droplets of the material accelerated in a gas stream. The droplets are directed against the surface of a substrate to be coated where they adhere and flow into thin lamellar particles called splats. The coating is built up of multiple splats overlapping and interlocking. These processes and the coatings they produce have been described in detail in the following: “Advanced Thermal Spray Deposition Techniques”, R. C. Tucker, Jr., in Handbook of Deposition Technologies for Films and Coatings, R. F. Bunshah, ed., Second Edition, Noyes Publications, Park Ridge, N.J., 1994, pp. 591 to 642; “Thermal Spray Coatings”, R. C. Tucker, Jr. in Handbook of Thin Films Process Technology, Institute of Physics Publishing, Ltd., London, 1995; and “Thermal Spray Coatings”, R. C. Tucker, Jr., in Surface Engineering ASM Handbook, Vol. 5, ASM International, Materials Park, Ohio, 1994, pp 497-509.
In virtually all of the thermal spray processes two of the most important parameters controlling the structure and properties of the coatings are the temperature and velocity of the individual particles as they impact on the surface to be coated. Of these, the temperature of the particles is of greatest importance relative to the present invention. The temperature the particles achieve during the deposition process is a function of a number of parameters including the temperature and enthalpy (heat content) of the process gases, the specific mechanisms of heat transfer to the particles, the composition and thermal properties of the particles, the size and shape distributions of the particles, the mass flow rate of the particles relative to the gas flow rate, and the time of transit of the particles. The velocity the particles achieve is a function of a number of parameters as well, and some of these are the same as those that affect the particle temperature including the composition, velocity and flow rate of the gases, the size and shape distributions of the particles, the mass injection rate and density of the particles. Thus the thermal gas dynamics characteristics of the thermal spray process determine the quality of the resulting coating.
In a typical detonation gun deposition process, a mixture of oxygen and a fuel such as acetylene along with a pulse of powder of the coating material is injected into a barrel, such as a barrel of about 25 mm in diameter and over a meter long. The gas mixture is detonated, and the detonation wave moving down the barrel heats the powder to near or somewhat above its melting point and accelerates it to a velocity of about 750 m/sec. The molten, or nearly molten droplets of material strike the surface of the substrate to be coated and flow into strongly bonded splats. After each detonation, the barrel is generally purged with an inert gas such as nitrogen, and the process repeated many times a second. Detonation gun coatings typically have a porosity of less than two volume percent with very high cohesive strength as well as very high bond strength to the substrate. In the Super D-Gun™ coating process, the gas mixture includes other fuel gases in addition to acetylene. As a result there is an increase in the volume of the detonation gas products which increases the pressure and hence greatly increases the gas velocity. This, in turn, increases the coating material particle velocity which may exceed 1000 m/sec. The increased particle velocity can result in an increase in coating bond strength, density, and an increase in coating compressive residual stress. In both the detonation gun and Super D-Gun coating processes, nitrogen or another inert gas can be added to the detonation gas mixture to control the temperature of the detonated gas mixture and hence the powder temperature. A number of parameters can be used to control both the particle temperature and velocity including the composition and flow rates of the gases into the gun.
In high velocity oxy-fuel and related coating processes, an oxygen, air or another source of oxygen is used to burn a fuel such as hydrogen, propane, propylene, acetylene or kerosene in a combustion chamber and the gaseous combustion products allowed to expand through a nozzle. The gas velocity may be supersonic. Powdered coating material is injected into the nozzle and heated to near or above its melting point and accelerated to a relatively high velocity, such as up to about 600 m/sec. for some coating systems. The temperature and velocity of the gas stream through the nozzle, and ultimately the powder particles, can be controlled by varying the composition and flow rate of the gases or liquids into the gun. The molten particles impinge on the surface to be coated and flow into fairly densely packed splats that are well bonded to the substrate and each other.
In the plasma spray coating process a gas is partially ionized by an electric arc as it flows around a tungsten cathode and through a relatively short converging and diverging nozzle. The partially ionized gas, or gas plasma, is usually based on argon, but may contain, for example, hydrogen, nitrogen, or helium. The temperature of the plasma at its core may exceed 30,000 K and the velocity of the gas may be supersonic. Coating material, usually in the form of powder, is injected into the gas plasma and is heated to near or above its melting point and accelerated to a velocity that may reach about 600 m/sec. The rate of heat transfer to the coating material and the ultimate temperature of the coating material are a function of the flow rate and composition of the gas plasma as well as the torch design and powder injection technique. The molten particles are projected against the surface to be coated forming adherent splats.
In the flame spray coating process, oxygen and a fuel such as acetylene are combusted in a torch. Powder, wire or rod is injected into the flame where it is melted and accelerated. Particle velocities may reach about 300 m/sec. The maximum temperature of the gas and ultimately the coating material is a function of the flow rate and composition of the gases used and the torch design. Again, the molten particles are projected against the surface to be coated forming adherent splats.
Thermal spray coating processes have been used for many years to deposit layered coatings. These coatings consist of discrete layers of different composition and properties. For example, the coating may be a simple duplex coating consisting of a layer of a metal alloy such as nickel-20 chromium (compositions herein are in weight percent unless otherwise noted) adjacent to the substrate with a layer of zirconia over it. In this case the undercoat of nickel-chromium may be used to enhance the mechanical or thermal shock resistance of the coating system or to protect the substrate from corrosion. An increase in mechanical or thermal shock resistance may be achieved by adding a third layer of coating consisting of a mixture of nickel-chromium and zirconia between the pure nickel-chromium and zirconia layers. Alternatively, perhaps even better thermal or mechanical shock resistance could be achieved by using two or more intermediate layers, each with an incr
Payne William A.
Stavros Anthony J.
Vickrey Michael W.
Bareford Katherine A.
Biederman Blake T.
Praxair S.T. Technology, INC.
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