Abstract
Molybdenum thermal spray coatings are used in aerospace and other industries for wear resistance applications. Metallographic sample preparation of molybdenum coatings presents unique challenges. The purpose of the investigation described in this article is to determine Accepted Practices for sample preparation to better understand the process related microstructures of thermal spray molybdenum powders. The committee followed a round robin approach to assess metallographic sample preparation by a variety of laboratories. The article summarizes the results of the committee’s work.
Introduction
The prime user of thermal spray molybdenum coatings has traditionally been the aerospace industry (see Fig. 1); however, other industries have begun to utilize this coating as well. Metallographic sample preparation of molybdenum coatings presents unique challenges. The purpose of the investigation described in this document was to determine Accepted Practices for sample preparation to better understand the process related microstructures of thermal spray molybdenum powders. The committee followed a round robin approach to assess metallographic sample preparation by a variety of laboratories. This document summarizes the results of the committee’s work.
Cutaway view of an AE 3007 engine. Molybdenum coatings are used for a range of applications in the “cold” sections of a gas turbine. Photo courtesy of Rolls-Royce Corporation
Cutaway view of an AE 3007 engine. Molybdenum coatings are used for a range of applications in the “cold” sections of a gas turbine. Photo courtesy of Rolls-Royce Corporation
Background
The goal of surface engineering is to extend the life of parts and thus save time, energy, and money. Perhaps the biggest challenge is solving wear problems. Modifying the surface properties of a material or by application of a “surface engineering” process can minimize wear. Thermal sprayed molybdenum bonds well to metals, especially steel, and has desirable material properties.
General characteristics of molybdenum include high thermal conductivity, low thermal expansion, and excellent wear resistance. When molybdenum is sprayed, oxides are produced inside the coating. These oxides are beneficial for service performance because they have a low friction coefficient and contribute to overall coating hardness
Wear is the removal of material from a solid surface by the action of another material. There are five principal wear mechanisms: abrasion, adhesion, surface fatigue, fretting, and erosion. Wear by abrasion is due to hard particles that are forced against and move along a solid surface and is the costliest and most damaging of the wear mechanisms. Adhesive wear occurs when a surface and its complementary component come into contact and move relative to one another. Under the proper conditions the two surfaces may fuse together locally. In extreme cases the surfaces will seize or weld to each other causing considerable surface damage; this phenomenon is referred to as galling. If the load has a cyclic component the parts may fail by contact fatigue. If the load contains a vibratory component, then fretting wear may occur. Erosion wear is the loss of material that results from repeated impact of small particles. These mechanisms do not include dimensional losses of parts due to plastic deformation or corrosion, which are usually associated with wear damage. The severity of wear damage is dependent on load, temperature, and oxidation tendency of the material.
In an effort to improve wear, industry has often employed the use of specialty steels by alloy additions. Typically, the component is placed in an application where the crack resistance of the alloy is poor, which in turn limits the effectiveness. This limitation has led to molybdenum deposition on metal substrates, where higher hardness and low friction coefficient are needed to offset the wear mechanisms. On steel substrates, no traditional bond coat material such as NiAl is required. Molybdenum has been successfully applied to titanium and aluminum substrates as well. As a further advance, pure molybdenum has been alloyed with specific elements. For example, carbon additions form carbides within the molybdenum matrix can improve erosion resistance. Depending on the industry and part configuration, molybdenum has been deposited by various combustion and plasma processes.
Operating limits for pure molybdenum in oxidizing conditions is about 300 °C (570 °F). In reducing environments, temperature exposure can be increased. Plasma sprayed molybdenum coatings are used in aerospace, automotive, marine, and heavy industry applications. The coating is sufficiently porous to operate under lubrication and can be impregnated with oil after spraying. Molybdenum based coatings are used on journal and bearing shafts, piston rings, valves, cylinder rods and gears.
Metallographic Preparation Considerations
Due to molybdenum’s wide variety of applications, preparation of this coating for metallographic analysis can present challenges. Certain applications require a thick and dense build-up while other applications may need controlled porosity with an associated hardness range. Molybdenum is susceptible to smearing. Furthermore, damage to hard/brittle phases may also occur during improper sample preparation.
Round Robin Testing
Round robin testing was performed on this coating by the member laboratories of the TSS Accepted Practices Committee on Metallography. Each laboratory was provided with an air- plasma spray coupon, from which a minimum of one cold mount and one hot mount sample was prepared. Preparation recipes were collected from each laboratory; example recipes are given in Tables 1–3.
| Step | Cloth | Abrasive | Lubricant | Speed, RPM | Force per sample, N | Time, minutes |
|---|---|---|---|---|---|---|
| Planar grinding | MD Piano 120 | Bonded diamond | Water | 300 | 30 | 3 |
| Fine grinding I | MD Piano 220 | Bonded diamond | Water | 300 | 20 | 3 |
| Polishing I | MD Largo | 9 μm diamond suspension | Struers Green | 150 | 25 | 6 |
| Polishing II | MD-Dac | 3 μm diamond suspension | Struers Green | 150 | 30 | 4 |
| Final polishing | MD-Chem | OP-S colloidal silica | Water | 150 | Low | 1 |
| Step | Cloth | Abrasive | Lubricant | Speed, RPM | Force per sample, N | Time, minutes |
|---|---|---|---|---|---|---|
| Planar grinding | MD Piano 120 | Bonded diamond | Water | 300 | 30 | 3 |
| Fine grinding I | MD Piano 220 | Bonded diamond | Water | 300 | 20 | 3 |
| Polishing I | MD Largo | 9 μm diamond suspension | Struers Green | 150 | 25 | 6 |
| Polishing II | MD-Dac | 3 μm diamond suspension | Struers Green | 150 | 30 | 4 |
| Final polishing | MD-Chem | OP-S colloidal silica | Water | 150 | Low | 1 |
| Step | Surface | Abrasive | Lubricant | Speed, RPM | Force per sample, N | Time, minutes | Relative rotation |
|---|---|---|---|---|---|---|---|
| Planar grinding | Diamond grinding disc | 70 to 45 μm | Water | 250 | 22 | Until planar | Comp |
| Coarse polishing | Ultra-Pol | 9 μm MetaDi Supreme | … | 150 | 22 | 3 | Contra |
| Trident | 3 μm MetaDi Supreme | … | 150 | 22 | 3 | Contra | |
| Final polishing | MicroCloth or ChemoMet | MasterMet2 + 30% H2O2 | … | 150 | 22N 36N | 4 | Contra |
| Step | Surface | Abrasive | Lubricant | Speed, RPM | Force per sample, N | Time, minutes | Relative rotation |
|---|---|---|---|---|---|---|---|
| Planar grinding | Diamond grinding disc | 70 to 45 μm | Water | 250 | 22 | Until planar | Comp |
| Coarse polishing | Ultra-Pol | 9 μm MetaDi Supreme | … | 150 | 22 | 3 | Contra |
| Trident | 3 μm MetaDi Supreme | … | 150 | 22 | 3 | Contra | |
| Final polishing | MicroCloth or ChemoMet | MasterMet2 + 30% H2O2 | … | 150 | 22N 36N | 4 | Contra |
| Step | Surface | Abrasive | Lubricant | Speed, RPM | Force per sample, N | Time, minutes |
|---|---|---|---|---|---|---|
| Planar grinding | 120-grit SiC paper | SiC (2 papers used) | Water | 300 | 30 | 1.5 |
| Fine grinding I | 240-grit SiC paper | SiC (2 papers used) | Water | 300 | 20 | 1 |
| Polishing I | 320-grit SiC paper | SiC (2 papers used) | Water | 300 | 25 | 1 |
| 600-grit SiC paper | SiC (2 papers used) | Water | 300 | 30 | 1 | |
| Polishing I | MD-Dac | 3 μm polycrystalline diamond suspension | Struers Green | 150 | 30 | 5 |
| Final polishing | MD-Chem | Struers OP-S | Water (to wet cloth only) | 150 | Low | 1 |
| Step | Surface | Abrasive | Lubricant | Speed, RPM | Force per sample, N | Time, minutes |
|---|---|---|---|---|---|---|
| Planar grinding | 120-grit SiC paper | SiC (2 papers used) | Water | 300 | 30 | 1.5 |
| Fine grinding I | 240-grit SiC paper | SiC (2 papers used) | Water | 300 | 20 | 1 |
| Polishing I | 320-grit SiC paper | SiC (2 papers used) | Water | 300 | 25 | 1 |
| 600-grit SiC paper | SiC (2 papers used) | Water | 300 | 30 | 1 | |
| Polishing I | MD-Dac | 3 μm polycrystalline diamond suspension | Struers Green | 150 | 30 | 5 |
| Final polishing | MD-Chem | Struers OP-S | Water (to wet cloth only) | 150 | Low | 1 |
Observations
The majority of the laboratories’ identified vacuum impregnation of a low viscosity castable epoxy as the preferred step to ensure coating integrity during sample preparation. When necessary, the epoxy should be dyed with a suitable material.
Two laboratories noticed that hot mounted samples showed less porosity than the cold mounted samples, possibly due to smearing (see Fig. 2).
One laboratory noticed that the linear splat lines in the cold mounted sample were wider than the hot mounted sample (see Fig. 3 and 4). The linear splat lines can be defined as linear porosity. The dyed epoxy validates the observation.
One laboratory noticed that when using a heat cured epoxy (fast cure time) the coating separated from the substrate. The sample was remounted in a slow cure epoxy and no separation was noted.
Optical micrographs of cold mounted (top) and hot mounted (bottom) molybdenum coating samples. The above samples were prepared using the same grinding and polishing method. Note the dense structure of the hot mounted sample. With the exception of the very top of the hot mounted sample, no details such as linear porosity or cracks within the splat lines are present. The hot mounted sample appears to be smeared.
Optical micrographs of cold mounted (top) and hot mounted (bottom) molybdenum coating samples. The above samples were prepared using the same grinding and polishing method. Note the dense structure of the hot mounted sample. With the exception of the very top of the hot mounted sample, no details such as linear porosity or cracks within the splat lines are present. The hot mounted sample appears to be smeared.
Micrographs of a cold mounted sample with a florescent dye in the epoxy taken at an original magnification of 200×. The top image was taken in brightfield. The bottom image was taken using a filtered UV light source. The green fluorescent light areas are showing the dyed epoxy that has completely impregnated the sample down to the substrate.
Micrographs of a cold mounted sample with a florescent dye in the epoxy taken at an original magnification of 200×. The top image was taken in brightfield. The bottom image was taken using a filtered UV light source. The green fluorescent light areas are showing the dyed epoxy that has completely impregnated the sample down to the substrate.
Micrographs taken at an original magnification of 500×. The red arrow points to an area of loosely bonded globules. Note the effectiveness of vacuum impregnating the sample: the dyed epoxy helps support the coating microstructure and prevents coating damage during preparation.
Micrographs taken at an original magnification of 500×. The red arrow points to an area of loosely bonded globules. Note the effectiveness of vacuum impregnating the sample: the dyed epoxy helps support the coating microstructure and prevents coating damage during preparation.
Accepted Practice
Due to thickness and porosity considerations, vacuum impregnation with a low-viscosity cold mount epoxy is the recommended mounting method.
As with all thermal spray coatings, sufficient material must be removed during the planar grinding stage to ensure that all sectioning damage has been removed. While all laboratories indicated a planar grinding step, only one laboratory specified material removal of 0.060 in. The amount of material which must be removed is typically considered to be a function of the sectioning methods employed.
Preparation recipes supplied by the member laboratories contained a range of recipes from predominately SiC papers (120-4000 grit), to strictly resin bonded diamond discs and polishing cloths.
All accepted practice preparation procedures utilized combined diamond steps totaling a minimum of five minutes
Note
This Accepted Practice is intended to be used as a baseline, but it does not replace local test or laboratory instructions. Additional requirements may apply based on the available equipment, testing materials, customer requirements, and other criteria.
Acknowledgments
This document was prepared in 2009 by the ASM Thermal Spray Society Accepted Practices Committee on Metallography. The Accepted Practice outlined in this document is based on tests by the ASM Thermal Spray Society Round Robin Laboratories:
Buehler Ltd.
Chromalloy Gas Turbines
Deloro Stellite
Goodrich Power Systems
HEICO Aerospace
IMR Test Labs
Praxair TAFA
Pratt & Whitney Aircraft
Standard Aero
Struers
