Metal Finishing Industry
Alternative Methods of Metal Deposition
Methods for depositing metal coatings such as chromium, nickel, cadmium, and copper in traditional electroplating processes have inherent pollution problems. Several alternative technologies exist to coat a substrate with metal without using electrolytic solutions or plating baths. These technologies do not eliminate the use of metal coatings, but they do eliminate the use of non-metal toxic components such as cyanide from the plating process. They also can reduce the amount of metal-contaminated wastewater and sludge that is generated from plating. These alternative technologies include thermal spray coating, vapor deposition, and chemical vapor deposition (EPA 1995).
In the future, these technologies might play a greater role in metal finishing operations. However, many of these alternative processes have high unit-plating costs and, therefore, are used only for special applications where the cost of coating is not a major consideration. Another drawback to alternative metal deposition methods is that metal overspray or tailings from remachining thick coatings from the alternative processes can actually increase waste generation (Davis 1994).
Alternative technologies for metal finishing have several features in common that distinguish them from conventional technologies. A general overview of each feature is presented below:
Table 24 presents a summary of the various alternative coating technologies and their applications and limitations. Table 25 compares these alternatives to their conventional counterparts and presents information on the status of the technology; the surface preparation required; the relative capital and operating costs; and the relative environmental, health, and safety (EHS) risks (EPA 1995).
C=Commercial, P=Pilot plant, R=Research; EHS=Environmental, health, and safety
The basic steps involved in any thermal coating process are:
The basic parameters that affect the deposition of metals in thermal spray applications include the particle's temperature, velocity, angle of impact, and amount of reaction with gases during the deposition process. As with traditional electroplating, part geometry also influences how the surface coating is deposited. Several industries use thermal spray coatings as a substitute for plating. They include:
u Tungsten carbide replacement of chrome plating on oil field piston rods: Prior to the adoption of thermal spraying, there were considerable problems with chrome flaking. The flakes would work themselves into the cylinders, causing additional wear on the piston and cylinder. The tungsten carbide substitute has shown excellent wear characteristics and does not require recoating as the chrome finish did (except when damaged externally, resulting in longer operational life).
u Replacement of chrome-plated water rolls in the printing industry: Ceramic coatings applied by thermal spraying have replaced chrome-plated water rolls. Ceramic rolls are used because of their excellent wetting action. Chrome-plated rolls required acid etching and the use of volatile isopropyl alcohol to increase wetting action. The use of the ceramic rolls has reduced or, in some cases, eliminated the need for wetting agents. Since the rolls do not flake, they do not contaminate the water or ink (Gansert 1989).
Three basic categories of thermal spray technologies are combustion torch (e.g., flamespray, high-velocity oxy fuel, and detonation gun), electric (wire) arc, and plasma arc.
Flame spraying involves feeding gas and oxygen through a combustion flame spray torch. A coating material in powder or wire form is fed into the flame. The coating is heated to near or above its melting point and accelerated by the combustion of the coating material. The molten droplets flow together on the surface of the workpiece to form the coating. Platers can use this technique to deposit ferrous-, nickel-, as well as cobalt-based alloys and ceramics. Companies use combustion torches to repair machine-bearing or seal areas and to provide corrosion and wear resistance for boilers and structures (EPA 1995).
Combustion torch deposits are noted for their relatively high porosity, low resistance to impact or point loading, and limited thickness (0.5 to 3.5 millimeters). Advantages include low capital costs, simplicity of use, and relative ease of operator training. In addition, the technique uses materials efficiently and has low maintenance requirements (EPA 1995).
With high-velocity oxy fuel (HVOF) systems, the coating is heated to near or above its melting point and is deposited by a high-velocity combustion gas stream. Continuous combustion of fuels typically occurs in a combustion chamber, enabling higher gas velocities. Typical fuels include propane, propylene, or hydrogen. This technique might be an effective substitute for hard chromium plating for certain jet engine components. Typical applications include worn parts reclamation and machine buildup, abradable seals, and ceramic hard facings (EPA 1995).
This technique has high-velocity impact. Coatings applied with HVOF exhibit little or no porosity. Deposition rates are relatively high, and the coatings have acceptable bond strength. Coating thicknesses range from 0.000013 millimeters to 3 millimeters. Some oxidation of metallics or reduction of oxides can occur, altering coating properties (EPA 1995).
Combustion torches and detonation guns mix oxygen and acetylene with a pulse of powder containing carbides, metal binders, and oxides. This mix is introduced into a water-cooled barrel about 1 meter in length and 25 millimeters in diameter. A spark initiates detonation, resulting in expanding gas that heats and accelerates the powder materials so that they are converted into a plastic-like state at temperatures ranging from 1,100 degrees Celsius to 19,000 degrees Celsius. A complete coating is built up through repeated, controlled detonations (EPA 1995).
This technique produces some of the densest thermal coatings. Platers can use almost any metallic, ceramic, or cement materials that melt without decomposing to coat parts. Typical coating thicknesses range from 0.05 millimeters to 0.5 millimeters, but both thinner and thicker coatings can be achieved. Because of the high velocities in this application, the properties of the coatings are much less sensitive to the angle of deposition than most other spray coatings (EPA 1995).
This technology is used with a narrow range of coating materials and substrates. Oxides and carbides commonly are deposited. Because of the high-velocity impact of depositing materials such as tungsten carbide and chromium carbide, combustion torches and detonation guns can be used only on metal substrates (EPA 1995).
During electric arc spraying, an electric arc forms between the ends of two wires that are made of coating material. The arc continuously melts the ends of the wire while a jet of gas (e.g., air or nitrogen) blows the molten droplets toward the substrate. Platers can use electric arc spraying for simple metallic coatings such as copper and zinc and for some ferrous alloys. Coating deposits can be applied thinly or thickly depending on the end use. Electric arc spray coatings have high porosity and low bond strength. Industrial applications include coating paper, plastics, and other heat-sensitive materials. It is also used in the production of electromagnetic shielding devices and molds (EPA 1995).
Plasma spraying involves the introduction of a flow of gas (usually argon-based) between a water-cooled copper anode and a tungsten cathode. A direct current arc passes through and is ionized to form a plasma. The plasma heats the powder coating to a molten state. Compressed gas propels the material to the workpiece at high speeds. Materials suitable for plasma spraying include zinc, aluminum, copper alloys, tin, molybdenum, some steels, and numerous ceramic materials. Platers can use plasma spraying to achieve thicknesses from 0.3 to 6 millimeters depending on the coating and substrate materials.
With proper process controls, this technique can produce coatings with a wide range of selected physical properties including coatings with a wide range of porosities (EPA 1995).
Companies can use plasma spraying to deposit molybdenum and chromium on piston rings, cobalt alloy on jet engine combustion chambers, tungsten carbide on the blades of electric knives, and wear coatings on computer parts (Kirk-Othmer 1987).
Vapor deposition technologies include processes that put materials into a vapor state via condensation, chemical reaction, or conversion. Manufacturers use these processes to alter the mechanical, electrical, thermal, optical, corrosion resistance, and wear properties of substrates. They also use vapor deposition technologies to form freestanding bodies, films, and fibers and to infiltrate fabric-forming composite materials (EPA 1995).
This section describes physical vapor deposition (PVD) and chemical vapor deposition (CVD). In PVD, the workpiece is subjected to plasma bombardment. In CVD, thermal energy heats gases in a coating chamber, driving the deposition reaction. Vapor deposition processes usually take place within a vacuum chamber (EPA 1995).
Physical vapor deposition involves dry vacuum deposition methods in which a coating is deposited over the entire object rather than in certain areas. All reactive PVD hard coating processes combine:
The primary PVD methods are ion plating, ion implantation, sputtering, and laser surface alloying. The production of metals and plasma differs in each of these methods (EPA 1995).
Plasma-based plating is the most common form of ion plating. In plasma-based plating, the substrate is placed in close proximity to a plasma. Ions then are accelerated from the plasma by a negative bias onto the substrate. The accelerated ions and high-energy neutrons from the charge-exchange processes in the plasma deposit the coating on the surface substrate with a spectrum of energies (EPA 1995).
This technique produces coatings that typically range from 0.008 millimeters to 0.025 millimeters. Advantages of ion plating include the excellent surface covering ability, good adhesion, flexibility in tailoring film properties (e.g., morphology, density, and residual film stress), and in-situ cleaning of the substrate prior to film deposition. Disadvantages include tightly controlled processing parameters, potential contamination activated in the plasma, and potential contamination of bombarded gas species into the substrate and coating (EPA 1995).
Ion plating can deposit a wide variety of metals including alloys of titanium, aluminum, copper, gold, and palladium. Manufacturers use plasma-based ion plating in the production of X-ray tubes, piping threads used in chemical environments, aircraft engine turbine blades, steel drill bits, gear teeth, high-tolerance injection molds, aluminum vacuum-sealing flanges, and decorative coatings and for corrosion protection in nuclear reactors. In addition, ion plating is widely used as an alternative to cadmium for applying corrosion-resistant aluminum coatings. Compared to other deposition processes, ion plating is relatively inexpensive (EPA 1995). Capital costs are high for ion plating, creating a significant barrier to its use. Ion plating is used mainly in value-added equipment such as expensive injection molds rather than inexpensive drill bits (EPA 1995).
Ion implantation does not produce a discrete coating; rather, the process alters the elemental chemical composition of the surface of the substrate by forming an alloy with energetic ions. A beam of charged ions of the desired element is formed by feeding a gas into the ion source where electrons, emitted from a hot filament, ionize the gas and form a plasma. An electrically biased extraction electrode focuses the ions into a beam. If the energy is high enough, ions alloy with the substrate instead of onto the surface, changing the surface composition. Three variations of ion implantation have been developed: beam implementation, direct ion implantation, and plasma source implementation (EPA 1995).
Cleaning is critical to the success of this technology. Platers must pretreat (e.g., degrease, rinse, and ultrasonically clean) the substrate to remove any surface contaminants prior to implantation. The process is performed at room temperature. Deposition time depends on the temperature resistance of the workpiece and the required dose (EPA 1995).
Platers can use ion implantation for any element that can be vaporized and ionized in a vacuum chamber. The benefits of this process include high reliability and reproducibility, elimination of post-treatment, an easily controlled process, and minimal waste generation. Ion implantation does not produce a stable finish if the coating is exposed to high temperatures. When this happens, implanted ions diffuse from the surface because of limited depth of penetration. Commercial availability of this technology is limited by a lack of familiarity, scarcity of equipment, and the need for strict quality control. Manufacturers commonly use nitrogen to increase the wear resistance of metals because nitrogen easily produces ion beams (EPA 1995).
Implantation is used primarily as an anti-wear treatment for components of high value such as biomedical devices (e.g., prostheses), tools (e.g., molds, dies, punches, cutting tools, and inserts), and gears and balls used in the aerospace industry. Other industrial applications include depositing gold, ceramics, and other materials into plastic, ceramic, and silicon and gallium arsenide substrates for the semiconductor industry (EPA 1995).
The initial capital cost of ion implantation is relatively high although large-scale systems have proven cost effective. An analysis of six systems manufactured by three companies found that coating costs range from $0.04 to $0.28 per square centimeter. Depending on throughput, capital costs range from $400,000 to $1.4 million and operating costs range from $125,000 to $250,000 (EPA 1995).
Sputtering and Sputter Deposition
Sputtering is an etching process that alters the physical properties of a surface. In this process, a gas plasma discharge is set up between two electrodes: a cathode plating material and an anode substrate. Positively charged gas ions are attracted to and accelerated into the cathode. The impact knocks atoms off the cathode, which impact the anode and plate the substrate (Davis 1994). A film forms as atoms adhere to the substrate. The deposits are thin, ranging from 0.00005 millimeters to 0.01 millimeters. The most commonly applied materials are chromium, titanium, aluminum, copper, molybdenum, tungsten, gold, and silver. Three techniques for sputtering are available: diode plasmas, RF diodes, and magnetron-enhanced sputtering (EPA 1995).
Sputter deposition provides a versatile process for depositing metals, alloys, compounds, and dielectrics on surfaces. Manufacturers have used this technology to apply both hard and protective industrial coatings. Areas requiring future research and development include better methods for in-situ process control, stripping, and understanding of process controls that affect coating properties (EPA 1995).
Sputter-deposited films are used routinely in decorative applications such as watchbands, eyeglasses, and jewelry. The electronics industry relies on heavily sputtered coatings and films (e.g., thin film wiring on chips and recording heads as well as magnetic and magneto-optic recording media). Companies also use sputter deposition to produce reflective films for large pieces of architectural glass and decorative films for plastic used in the automotive industry. The food packaging industry uses sputtering to produce thin plastic films for packaging. Compared to other deposition processes, sputter deposition is relatively inexpensive (EPA 1995).
Laser Surface Alloying/Laser Cladding
Increasingly, lasers are being used for surface modification. Surface alloying is one of the many kinds of alteration processes that use lasers. This technology is similar to surface melting, but it promotes alloying by injecting another material into the melt pool that alloys into the melt layer. Surface characteristics of this technology include high-temperature performance, wear resistance, improved corrosion resistance, better mechanical properties, and enhanced appearance (EPA 1995).
One of many methods of laser surface alloying is laser cladding. The overall goal of laser cladding is to selectively coat a defined area. In laser cladding, a thin layer of metal (or powder metal) is bonded with a base metal via a combination of heat and pressure. Specifically, ceramic or metal powder is fed into a carbon dioxide laser beam above the surface of a substrate, melted in the beam, and transferred to the substrate. The beam welds the material directly into the surface region, providing a strong metallurgical bond. Powder feeding is performed using a carrier gas in a manner similar to that used for thermal spray systems. Large areas are covered by moving the substrate under the beam and overlapping deposition tracks. Pretreatment is not vital to the successful application of laser cladding coatings although the surface might require roughening prior to deposition. Grinding and polishing generally are required after the coating is applied (EPA 1995).
This technique can apply most of the same materials as thermal spraying technologies. Materials that are easily oxidized are difficult to deposit without using inert gas streams and envelopes. Deposition rates depend on laser power, power feed rates, and traverse speed. Coating thicknesses can range from several hundred microns to several millimeters. If the density is too high, however, cracking and delamination can occur as is the case with aluminum and some steels. This technology also is unable to coat areas that are out of the line of sight. Although laser processing technologies have been in existence for many years, industrial applications are limited, partly because of high capital costs (EPA 1995).
Chemical vapor deposition (CVD) is a subset of PVD as described above. Over time, the distinction between PVD and CVD has blurred as new technologies have been developed. Chemical vapor deposition includes sputtering, ion plating, plasma-enhanced CVD, low-pressure CVD, laser-enhanced CVD, active reactive evaporation, ion beam, laser evaporation, and other variations. The variations are distinguished by the way that the precursor gases are converted into the reactive gas mixtures (EPA 1995).
In CVD processes, a reactant gas mixture comes in contact with the substrate. Gas precursors are heated to form the reactive gas mixture. The coating is delivered by another precursor material, known as a reactive vapor, that can be dispensed in either a gas, liquid, or solid form. Gases are fed to the chamber under normal pressures and temperatures while solids and liquids require high temperatures and/or low pressures. Once in the chamber, energy is applied to the substrate to facilitate the coatings reaction with the carrier gas. The basic steps in the CVD processes are:
Substrate pretreatment is important in vapor deposition, particularly in CVD. Pretreatment involves using mechanical and chemical means to minimize pretreatment before placing the substrate in the deposition reactor. Substrates must be cleaned prior to deposition and the deposition reactor chamber must be clean, leak-tight, and free of dust and moisture. Cleaning is usually performed using ultrasonic cleaning and/or vapor degreasing. To improve adhesion, vapor honing might follow. During the coating process, operators must maintain surface cleanliness to prevent particulates from accumulating in the deposit. Manufacturers use mild acids or gases to remove oxide layers formed during heat-up. Post-treatment can include heat treatment to facilitate diffusion of the coating material (EPA 1995).
Companies use CVD mainly for corrosion and wear resistance. CVD usually is applied to obtain specific properties that are difficult to obtain with other processes. CVD's ability to control the microstructure and/or chemistry of the deposited material makes it important for some applications. The microstructure of CVD deposits depend on the chemical makeup and energy of atoms, ions, or molecular fragments; chemical composition and surface properties of the substrate; substrate temperature; and presence or absence of a substrate bias voltage. The most commonly used metals in CVD coatings are nickel, tungsten, chromium, and titanium carbide (EPA 1995).
Companies also use CVD to deposit coatings and to form foils, powders, composite materials, free-standing bodies, spherical particles, filaments, and whiskers. The majority of applications are in electronics production including structural applications, optical, opto-electrical, photovoltaic, and chemical industries. Startup costs are high (EPA 1995).
Davis, Gary A. et al. 1994. The Product Side of Pollution Prevention: Evaluating the Potential Safe Substitute. Cincinnati, OH: Risk Reduction Laboratory, Office of Research and Development.
EPA. 1995. Waste Minimization for Metal Finishing Facilities. Washington, DC: Office of Solid Waste.
Freeman, Harry J. 1995. Industrial Pollution Prevention Handbook. New York, NY: McGraw Hill, Inc.
Gansert, Daren, and George Grenier. 1989. Substituting Thermal Spraying for Electroplating.
Metal Waste Management Alternatives. Pasadena, CA: Alternative Technology Section, Toxic Substances Control Division, California Department of Health Services.