molten metal flame spraying, a thermal spraying process variation in which the metallic material to be sprayed is in the molten condition. ⁽¹⁾

It has long been recognized that fluids may be broken up into very fine particles by a stream of high velocity gas emanating from a nozzle. Early experiments using this atomizing approach appear to have been directed at producing metallic powders rather than coatings. It was left to Schoop to appreciate the possibility that a stream of metallic particles, formed from a molten source, could produce a coating. Myth has it that Schoop developed the concept when playing "soldiers" with his son and observing the deformation of lead pellets being fired from a toy cannon against a brick wall. Whatever the rationale, it can be stated that the pioneer work of Schoop resulted in the discovery and development of metal spraying and subsequently the "Thermal Spray Process".

The first spray technique developed by Schoop was the outcome of experiments in which molten metal was poured into a stream of high velocity gases. Schoop's apparatus consisted of a compressor supplying air to a heated helical tube. The heated air was used to pressurize a crucible filled with molten metal and eject it out as a fine spray that would adhere to a suitable surface. This system was bulky, primitive and inefficient; however, the concept did lead to the development of portable and user friendly equipment.

There are no further accounts of molten metal spraying by Schoop, it appears that his efforts were directed at developing and improving powder and wire flame spraying. However, work by others continued as a 1924 Dutch patent, describing equipment for spraying low melting point metals, was granted to Jung and Versteeg ⁽²⁾. Mellowes Ltd commercialized the process in the UK. Their system consisted of a gun, a furnace, an air compressor and a fuel supply. The gun had many air and gas valves, a heating chamber (burner), nozzle, handle and a melting pot. The pot was bulky having the ability to store 1.8 kg (4 lb) of molten lead. The pot sat atop the heating chamber, which was similar in construction to a Bunsen burner. Compressed air, fed to the burner, intensified the flame. The handle jutted out and downward from the pot; it was insulated using wood and asbestos. Metal exited the pot through a front orifice where it was directed into a nozzle. Compressed air surrounded the nozzle, atomizing the molten metal and propelling it to the surface to be coated.

The molten metal process has advantages and disadvantages. Advantages include: cheap raw materials; use of inexpensive gases; and, gun design is very basic. Noteworthy disadvantages are: gun is cumbersome to use in the manual mode, can only be held in a horizontal plane; high maintenance due to high temperature oxidation and molten metal corrosion; and, useful only with low melting temperature metals.

Uses for the molten metal thermal spray process include the fabrication of molds, masks and forms for the plastics industry, using low melting point bismuth based alloys (the Cerro family of alloys); the deposition of solder alloys to joints that would be coalesced using torches or ovens; and, the production of metal powders.

ceramic rod flame spraying, a spraying process in which material to be sprayed is in ceramic rod form.⁽¹⁾
The spraying of ceramic rods dates back to the early 1950's when a demand arose for heat resistant refractory coatings. Plasma had not come into its own and flame sprayed powder coatings, due to their porous nature, lacked the integrity and protection required. The solution was rather simple - Coors Ceramic and Norton developed ceramic rods, referred to as Rokide, while the Metallizing Engineering Company (Mogul), modified a wire gun to spray rods. What differentiated the guns were the drive rolls. A wire gun had serrated steel rolls to grip and feed the wire while the Rokide gun employed "V" slotted fiber rolls that pinched the rods and fed them forward. The principle of operation in either gun is similar - the nozzle's flame is concentric to the wire or rod in order to maximize uniform heating. In the graphic below, a coaxial sheath of compressed gas around the flame atomizes the molten material and accelerates it to the workpiece. Particle velocities in both the wire and rod process are approximately the same - 185 m/sec (600 ft/sec) while coating densities have been measured at approximately 95 v/o.

Ceramic rods are 61 cm (24 in.) in length and offered in three diameters 1/8, 3/16 and 1/4 in. Compositions include several stabilized zirconias, white and gray alumina, and a spinel.

cold spray, a kinetic spray process utilizing supersonic jets of compressed gas to accelerate near-room temperature powder particles at ultra high velocities. The unmelted particles, traveling at speeds between 500 to 1,500 m/sec plastically deform and consolidate on impact with their substrate to create a coating. ⁽¹⁾

A mention is made of "Cold Spray" although the name seems to contradict the concept of "Thermal (heat) Spraying"; however, the method has garnered significant research interest over the last five years. Developed in the former Soviet Union in the mid 1980s by Papyrin ⁽⁴⁾, the process is, however, now being commercialized in both Europe and the United States. A typical system is shown above.

The basis of the cold spray process is the gas-dynamic acceleration of particulates to supersonic velocities (300-1200 m/sec-1), and hence high kinetic energies, so that solid-state plastic deformation and fusion occur on impact to produce dense coatings without the feedstock material being significantly heated. This is achieved using convergent-divergent, de Laval nozzles, high pressures (up to 500 psi [3.5 MPa]) and flow rates (up to 90 m3/hr) of gases such as helium or nitrogen. The gases are pre-heated to about 800°C (1472°F), or below the melting point of many metals, to increase the velocity. Pre-heating also aids in particle deformation. The spray pattern is roughly 20 to 60 mm2 (0.031 to 0.093 sq in.); spray rates - 3-5 kg/hr (6.5 to 11 lb/hr), with build ups of about 250 µm (10 mils) per pass and DEs of 70 wt %. Feedstock particle sizes are typically of the order of 1-50 µm.

The advantage of cold spray versus the "hot" spray processes, which melt or soften the feedstock, is a significantly reduced level of coating oxidation. Electrical conductivity of cold sprayed copper has been reported at about 90% of wrought material - a significant increase over the <50% typical for other sprayed copper deposits.Cold spray coatings also exhibit improved adhesion, reduced material loss by vaporization, low gas entrapment, insignificant grain growth and recrystallization, low residual stress, phase and compositional stability, reduced masking requirements and improved surface finishes ^{(5) (6)}.

Cold spray, owing to its principle of impact-fusion coating build-up, is limited to the deposition of ductile metals and alloys (Zn, Sn, Ag, Cu, Al, Ti, Nb, Mo, NiCr, Cu-Al, nickel alloys and MCrAlYs) and polymers, or blends of >50 vol % ductile materials with brittle metals or ceramics. The absence of a heated jet also yields a low heat input to the substrate.

Obvious disadvantages to the cold spray process include the use of high gas flows, increased gas costs, especially in the case of helium, recycling would be needed. Consequently, lower cost gases as nitrogen are being investigated as alternatives. Also, high gas pressures have required the development and modification of powder feeders. Solid materials traveling at high velocities are abrasive, so the lifetime and dimensional stability of key components are emphasized. Nozzle lifetimes in excess of 100 hours have been reported ⁽⁷⁾.

Applications for cold spray coatings include corrosion protection, where the absence of process-induced oxidation may offer improved performance; deposition of electrical conductors and solders; and, the application of metallic coatings to ceramic and glass substrates.

ceramic rod flame spraying, a spraying process in which material to be sprayed is in ceramic rod form.⁽¹⁾
The spraying of ceramic rods dates back to the early 1950's when a demand arose for heat resistant refractory coatings. Plasma had not come into its own and flame sprayed powder coatings, due to their porous nature, lacked the integrity and protection required. The solution was rather simple - Coors Ceramic and Norton developed ceramic rods, referred to as Rokide, while the Metallizing Engineering Company (Mogul), modified a wire gun to spray rods. What differentiated the guns were the drive rolls. A wire gun had serrated steel rolls to grip and feed the wire while the Rokide gun employed "V" slotted fiber rolls that pinched the rods and fed them forward. The principle of operation in either gun is similar - the nozzle's flame is concentric to the wire or rod in order to maximize uniform heating. In the graphic below, a coaxial sheath of compressed gas around the flame atomizes the molten material and accelerates it to the workpiece. Particle velocities in both the wire and rod process are approximately the same - 185 m/sec (600 ft/sec) while coating densities have been measured at approximately 95 v/o.

Ceramic rods are 61 cm (24 in.) in length and offered in three diameters 1/8, 3/16 and 1/4 in. Compositions include several stabilized zirconias, white and gray alumina, and a spinel.

high velocity oxy/fuel spraying (HVOF), a high velocity flame spray process ⁽¹⁾
In the early 1980's Browning and Witfield, using rocket engine technologies, introduced a unique method of spraying metal powders. The technique was referred to as High Velocity Oxy-Fuel (HVOF). The process utilizes a combination of oxygen with various fuel gases including hydrogen, propane, propylene, hydrogen and even kerosene. In the combustion chamber, burning by-products are expanded and expelled outward through an orifice where at very high velocities. Often times they produce "shock diamonds" exiting the spray gun as in the graphic below.

Powders to be sprayed via HVOF are injected axially into the expanding hot gases where they are propelled forward, heated and accelerated onto a surface to form a coating. Gas velocities exceeding Mach 1 have been reported with temperatures approaching 2,300°C (4,172°F). The coupling of inertially driven/highly plasticized particles can achieve coatings approaching that of theoretical density. Disadvantages include low deposition rates and in-flight the oxidation of particles. Future efforts will focus on applying thick coatings and improvements in processes control including in-flight transit time and exposure to atmospheric oxygen.

Powder flame spraying, a thermal spray process in which the material to be sprayed is in powder form. ⁽¹⁾
Powder flame spraying is probably the simplest of all the spray processes to describe - feed a powder through the center bore of a nozzle where it melts and is carried by the escaping oxy-fuel gases to the work piece. Unfortunately, this approach yields coatings high in oxides and with void contents approaching 20 volume percent (v/o). However, coating quality can be improved by feeding air to the nozzle through a small jet, which reduces the pressure in a chamber behind the nozzle. This chamber is connected to the powder feed hopper. In this way a gentle stream of gas is sucked into the gun and carries powder with it. A typical gun is illustrated below.

wire flame spraying, spray process in which the feed stock is in wire or rod form.⁽¹⁾
In about 1912 Schoop developed the first device for spraying metal wires. The apparatus consisted of a nozzle in which a fuel, probably acetylene or hydrogen, was mixed with oxygen and burned at the nozzle's face⁽³⁾. A stream of compressed air surrounding the flame atomized and propelled the liquefied metal. Process continuation depended on feeding the wire at a controllable rate so it melted and was propelled in a continuous stream.

Schoop approached this problem by using a turbine to actuated gears and drive rolls that pulled the wire into the nozzle. This apparatus appeared to him to be similar to a pistol or gun, and because of this, he and we, refer to thermal spray devices as "guns" or "pistols" and never "torches". A typical wire spray gun is shown in the graphic above. Schoop' concepts of spraying solid metals has given rise to the thermal spray industry and for this reason it is sometimes referred to as the "Schoop Process". Regardless, the wire flame spray gun has not radically changed since the days of Schoop. While there have been changes in nozzle and air cap design, replacement of the air turbine with an electrical motor and even the use of barrel valves the basic principal, however, remains the same "push or pull a wire into a flame, melt and atomize it and deposit the molten droplets to form an adherent coating".