Wear need not be catastrophic to cause substantial reductions in the operational efficiency of production equipment due to increased power loss, equipment downtime and lubricating-oil consumption subsequently leading to the replacement of equipment components. Thus, component wear necessitates an ongoing need to develop components with improved wear performance.

For example, in the chemical, petroleum and petrochemical industries, with the demand for increased productivity in the presence of an aggressive-use environment, equipment components are subjected to thermal cycling, abrasion, erosion and corrosion. Component wear reduction requires the use of protective coatings produced by such processes as the thermal spray, PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposition) and electrolytic hard-chromium processes, among others.

Among the more common surface coating processes, thermal-spray processes are often preferred because of the following:[1,2]
  • Ease of coating for large and complex parts
  • Low cost
  • High rates of material deposition[3]
  • Possibility of coating components and parts with little or no equipment disassembly of the components
  • Recovery of the used part
  • Possibility of the deposition of various materials including: ceramics, metals, “cermets” or polymers with relatively low process-heating requirements to produce the coated part
These coating processes permit the use of high-performance material deposition on a low-cost metal base, resulting in reduced wear and/or corrosion leading to increased equipment lifetime while simultaneously reducing overall material and production costs.

In this paper, a brief overview of various common surface-treatment methodologies will be provided. This will be followed by a discussion of the use of HVOF (High Velocity Oxygen Fuel) deposition of two alloys – Cr3C2-NiCr and WC-CoCr – to replace an electrolytic chromium-deposition process used to form a hard-chromium coating for wear reduction.

Fig. 1. Schematic illustration of a typical thermal-spray process[7]

Thermal-Spray Processes

Thermal spray is a generic term used for a coating process in which the coating material is warmed or melted quickly by means of combustion, electric arc or plasma, and simultaneously projected by gases under high pressure and high speed onto a prepared substrate.[1] The particles striking against the surface are flattened and adhere to the material base or onto previously deposited particles to form a surface layer. These layers are composed of small particles flattened in a parallel direction relative to the substrate and typically exhibit a lamellar structure with oxides, inclusions and porosity. Figure 1 provides a schematic illustration of a typical thermal-spray process.

Many materials, including the majority of metals and their alloys, ceramic materials, carbides, borides and hydrides, as a powder or a wire shape can be deposited by thermal spray. These thermal-spray processes are classified as a function of the method of energy generation. The most common power sources are electric energy or gaseous combustion.[1,2] Thermal-spray processes include the following six different processes.

Flame Spray
Flame-spray methods utilize the heat generated by the combustion of a mixture of gases (oxygen-acetylene or oxygen-propane) to melt the material to be deposited. Acetylene is the combustible gas most commonly used due to the high temperature – 3100°C (5600°F) – of the oxygen-acetylene flame. The materials to be deposited are supplied in powder or wire shapes and may be metals, metallic alloys, ceramic material and some plastics. Particle velocities can reach 90m/s when using powder and between 150-270m/s when wire is used.[5]

Electric-Arc Spray
In the electric-arc thermal-spray process, two consumable wire electrodes that are initially isolated from each other advance until a potential difference of 18-40 volts is applied, which initiates an electric arc that melts the wire tips. The casting material is atomized and accelerated by means of compressed air toward the exterior of the pistol under the shape of a jet against a previously prepared substrate. For special applications, inert gases such as argon and helium can be used instead of compressed air to reduce oxidation of the coating materials.

Plasma of No-Transferred Arc
In the plasma-spray process, a “non-transferable” electric arc is generated between a tungsten electrode coaxially aligned in a constriction nipple. The central zone of the plasma consists of an ionized inert gas (usually argon or nitrogen) involving one or more protective inert gases (argon, helium, hydrogen, nitrogen or gaseous mixtures).[10]

During the heating, the gas is partially ionized to create a plasma. When the plasma leaves the pistol, dissociated molecules of a diatomic gas recombine and liberate heat. The powder is introduced in the plasma, melted and accelerated against the substrate through a high-speed flow.

The temperature of the gases in the plasma can reach 17000°C.[14] Usually, the working gas is Ar or a mixture Ar+H2, Ar+He and Ar+N2. Sometimes N2 and a mixture of N2+H2 can also be used.

Plasma of Transferred Arc
The plasma of transferred-arc thermal-spray process – commonly known as PTA – adds to the plasma-spray process the capacity of heating and melting the surface of the substrate. Practically, it is a combination of welding and thermal-spray processes. In this technique, a secondary current is established between the electrode of the pistol and the conducting substrate. Material addition can be in the powder or wire shape.

The advantages of this process are the metallurgical interface, high coating density, high deposition rates and high bypass thicknesses. In this technique, relatively little energy is necessary when compared with the plasma of no-transferred-arc process.

The HVOF (High Velocity Oxy Fuel) Thermal-Spray Process
The HVOF process is based on a high-pressure internal-combustion system similar to a miniature rocket propellant. Fuel and oxygen are mixed to produce a supersonic gas jet that reaches 2000m/s and 2800°C (5070°F). Fuels include kerosene, hydrogen, propylene, propane, acetylene and natural gas. The powder materials to be deposited are introduced into the hot flame inside the exit barrel and can produce coatings on almost any substrate. The semi-molten powder particles that are accelerated in the gas stream and in the nozzle exit reach velocities of approximately 400m/s. These conditions normally result in porosities of less than 1% and an oxide content less than 1% with a bond strength greater than 60 MPa.

These coatings exhibit higher hardness and durability, stronger bonding to the substrate and greater deposition-thickness capability than those produced by other thermal-spray processes. Carbides of all types are especially well suited for the HVOF process because they do not require significant melting to deposit and form an effective wear surface. Typical compositions contain 8-30% matrix alloy content, which serves primarily as a binder for the carbide particles.[2,4]

The Hard-Chromium Process
The hard-chromium coating process is used in aircraft manufacturing, for maintenance of civilian or military vehicles and ships, and for a wide range of industrial applications. These include connecting rods of hydraulic actuators, bearings and retainers, among others. This coating provides an effective surface treatment for wear and corrosion reduction while providing decreased friction and dimensional restoration of worn and undersized parts.

Hard chromium has been used for more than 70 years, and it has provided an effective solution at relatively low cost. Environmentally, the problem of such coating is not the chromium but the coating process that uses a chromic-acid solution, which is released to the environment during the coating process in the form of a fine fog. This fog contains chromium ions in the hexavalent state, which is carcinogenic and causes several other health problems such as perforation of the respiratory ways and skin eruptions. Therefore, the Environmental Protection Agency (EPA) generated new and more restrictive regulations to limit the release of hexavalent chromium to the environment. Additionally, the Occupational Safety and Health Administration (OSHA) established exposure limits for hexavalent chromium in the work environment of 0.1mg/m3. Recent studies indicate that the risk of cancer is still significant at this level, and therefore it is anticipated that OSHA will define a substantially lower level in relation to the current value.[6,7] This will increase the cost of hard-chromium coating operations, making it prohibitively expensive to use in certain applications. This is the primary motivation for the development of a substitute coating for hard chromium.

The objective of the work to be discussed subsequently was to evaluate the performance of cermet layers produced by the HVOF process and hard-chromium layers produced by the hard-chromium process to determine if these cermets can replace the chromium-deposition process.

Fig. 2. Micrograph of the transverse section of the WC-CoCr coating

Cermet HVOF Deposition

The HVOF thermal-spray process is the most promising candidate to substitute the electrolytic hard-chromium deposition process and can be used for a wide range of applications where the electrolytic-chromium coating process is still used.[8-12]

The tungsten-carbide-cobalt-base and tungsten-carbide-cobalt-chromium-base alloys deposited by HVOF have shown promising results for substitution of the electrolytic-chromium coating process in aircraft components.[9,11] HVOF chromium-carbide coatings on diesel-engine piston rings provide performance improvements of up to three times greater than that exhibited by electrolytic hard chromium.[10] The automotive industry is actively engaged in the development of thermal-spray coatings for cylinders, pistons, piston rings, valves and other components.

The HVOF process is particularly suited for application of carbides (cermets) due to their low flame temperature and low residence time in the flame relative to the plasma process.[14] These coatings provide greater hardness, decreased porosity and oxide content combined with decreased residual stresses of the depositions.[8]

During the last 20 years, many carbide compositions have been used successfully in the coating process by HVOF thermal spray. Materials such as WC-10Ni and WC-10Co-4Cr were developed to deliver an excellent combination of wear and corrosion resistance. These materials are now being specified for critical applications for aircraft components such as landing gears.[12]

To evaluate the effectiveness of the cermet coatings produced by HVOF for potential substitution for electrolytic hard chromium, two alloys were selected to be deposited by HVOF – Cr3C2-NiCr and WC-CoCr.

The abrasive low-tension wear test (ASTM G65-94 Standard) is widely used to measure abrasion resistance.[15] In this test, a specific type of sand flows between the sample and the rotary disk during a defined testing time, and the abraded material is determined by volume or gravimetric weight loss.[16] This test was used for performance comparison of the abrasive wear resistance of coatings to be discussed here.

Fig. 3. Micrograph of the transverse section of the Cr3C2-NiCr coating

Micrograph Analysis

Figures 2, 3 and 4 show the micrographs of the transverse sections of the three surface coatings of interest. The Cr3C2-NiCr and WC-CoCr materials resulted in coatings with homogeneous carbide distribution in the respective metallic matrices as shown in Figures 2 and 3. Average thickness of the layer was 0.7mm for Cr3C2-NiCr and WC-CoCr after grinding.

The tungsten-carbide particles from the WC-CoCr coating were smaller than the chromium carbide of the Cr3C2-NiCr coating. The Cr3C2-NiCr coating exhibited the largest amount of oxides and porosity produced during the thermal-spray process.

Fig. 4. Micrograph of the transverse section of the hard-chromium coating

The hard-chromium coating exhibited a zone of pores at approximately 0.2mm of the substratum surface that was consistent with the observation of the presence of cracking inherent to the process as shown in Fig. 4. The average thickness of the hard-chromium layer after grinding was 0.6mm.

The results of the microhardness tests of the coatings are shown in Table 1, and Table 2 shows the results of the Pycnometry density analysis. The relatively large difference between the deposited materials is due to the density difference of each carbide composing the coating – Cr3C2 has a density of 6.66 g/cm3, whereas the WC has a density of 15.63 g/cm3.[2] The results of the abrasive-wear tests according the ASTM G65-94 Standard for the analyzed coatings are shown in Figure 5.

The wear volume-loss data shows that the WC-CoCr coating provided the best performance. The high hardness of the tungsten-carbide particles – harder than the abrasive sand – and its high compactness so that the distance between the carbide particles is less than the size of the abrasive particles are the primary reasons for the performance. Because of its more compact carbide structure (compared to the Cr3C2-NiCr carbide particles), the removal, or wear, rate of the binder material is lower. Therefore, the more compact the carbide structure, the more protected is the binder from wear, resulting in a lower wear rate.

Analysis of the worn surfaces showed that the dominant wear mechanism for the coating in this test was the pulling out of the carbides from the matrix. This process starts with the removal of small portions of the CoCr binder around the carbide particles in the surface. Eventually, the carbide becomes loose. This is more evident in regions with pores.

A factor that can also influence the adhesion-force performance of sprayed coatings is the cohesive resistance of the layer (between lamellas),[3,6] which is influenced mainly by the level of oxidation of the particles during the aspersion process. As observable in Figure 3, the Cr3C2-NiCr coating possesses a greater number of pores and oxides between the lamellas resulting in the greatest volume loss among the coatings tested. Its lower cohesive force and lower hardness relative to the WC-CoCr coating contribute to the increased wear.

Fig. 5. Graph of volume loss for the three analyzed coatings

Conclusion

A brief overview of different thermal-spray coating processes was provided. The HVOF coating process offers one of the best alternative coating methodologies to replace electrolytic hard-chromium processes, which produce various toxic workplace hazards and are being increasingly regulated. To illustrate the potential of the use of cermets to replace hard chromium for wear reduction, two HVOF coatings – Cr3C2-NiCr and WC-CoCr – were evaluated and compared to chromium. Among the three coatings tested, the WC-CoCr coating provided the best performance in the wear tests, indicating the possibility of substitution of the hard-chromium coating in some applications. These examples illustrate the potentially enormous impact of HVOF-coating methodologies for surface modification.

For more information: Contact George E. Totten Ph.D., FASM, Portland State University - Department of Mechanical and Materials Engineering, P.O. Box 751, Portland, OR 97207-0751; tel: 206-788-0188; fax: 815-461-7344; e-mail: totten@cecs.pdx.edu; web: www.getottenassociates.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: physical vapor deposition, chemical vapor deposition, electrolytic hard chromium, high velocity oxygen fuel, plasma

See January's issue for list of References