Final Report: Meta-Structured Cements for Chrome ReplacementEPA Contract Number: EPD06053
Title: Meta-Structured Cements for Chrome Replacement
Investigators: Sherman, Andrew J. , Garrett, William R.
Small Business: Powdermet, Inc.
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2006 through August 31, 2006
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2006) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , SBIR - Pollution Prevention , Small Business Innovation Research (SBIR)
Electrolytic hard chrome plating is extensively used to impart wear resistance and to rebuild worn components in the aircraft, oil and gas, and other general industry applications. Approximately $3 billion of chrome plating is conducted each year, generating more than 1.4 million pounds of hexavalent chrome sludge and 130 million gallons of hexavalent chrome contaminated water, annually. With hex-chrome being environmentally and occupationally hazardous, the process of plating with this material is under scrutiny by both the U.S. Environmental Protection Agency and the Occupational Safety and Health Administration (OSHA), and is being phased out in Europe. Previous market studies and completed hard chrome alternatives work had projected that carbide thermal spray technology would replace 70 percent of chrome plating in aerospace and 25 percent of all other chrome plating operations by 2011, based on an $8 per pound cost for tungsten carbide (WC) from China and on OSHA adopting a low permissible exposure limit (PEL). Unfortunately, WC prices have increased—not decreased—in the face of increasing demand, and the final OSHA ruling was 5 μg/cm3. This level can be met with much lower investment than the feared 1 μg/cm3 limit. These two facts reduce the economic viability and hence market penetration of hard chrome alternatives. The purpose of this Phase I research project was to demonstrate a much lower cost hard chrome alternative utilizing nanocomposite technology.
In this research project, Powdermet, Inc., developed a nanocomposite thermal spray coating trade named PComP™ that provides a cost-effective alternative to electrolytic hard chrome plating. Current hard chrome replacement alternatives such as trivalent chrome plating or tungsten-carbide thermal spray coatings are much more expensive than hard chrome and are both heavy and brittle—greatly limiting their rate of adoption. The nanocomposite technology developed by Powdermet enables the application of coatings that are ductile, while still retaining their hard protective features. Moreover, because of a much lower density compared to chrome and WC-Co alternatives, this new nanocomposite can be used more efficiently, allowing the same quantity of material to coat four times the surface area compared to existing hard chrome alternatives. This fact, when combined with using new high throughput thermal spray gun designs and low-cost finishing operations enabled by the nanocomposite coatings, can result in an 80 percent cost reduction for hard chrome alternatives.
The main advantages of this technology, in terms of total value added are described below.
Powdermet’s PComP™ nanocomposite thermal spray coatings have one-quarter the density of WC-Co and one-half the density of hard chrome. Because coatings are applied to a given thickness/volume, this results in 1 pound of PComP™ covering four times the area than 1 pound of WC-Co covers. Because thermal spray gun output is based on mass and not volume, this means that four times the coverage is achieved per hour of operations, reducing application cost by 75 percent. This application cost can be further reduced using new high throughput gun technology that enables 3 times the throughput of conventional thermal spray guns, with a combined productivity 10 – 12 times that of current chrome alternatives.
Ease of Machining/Finishing
Current chrome plate and WC-Co thermal spray coatings are diamond ground, a long and expensive process that can take 12 hours for a typical landing gear application. Because of the unique structure of the coating, including the lack of large abrasive grains, PComP™ coatings can be ground much faster and with SiC wheels, reducing finishing costs by up to 80 percent compared to WC-Co.
Coating Ductility/Toughness with High Hardness
The meta-structured lamillar microstructure of PComP™ nanocomposite coatings incorporates micron-scale ductile phase toughening into a high hardness micro/nanocomposite material leading to 3–5 percent true ductility in the coatings, combined with hardness greater than chrome and in some cases exceeding 1100VHN.
In this project, Powdermet developed synthesis and production processes to produce nanocomposite thermal spray powders using low-cost starting materials, such as reclaimed scrap cutting tools, as raw material sources. Pilot plant production equipment was purchased with company matching funds. Processing conditions were developed for preparing nanocomposite feedstocks, spray drying these feedstocks to form spherical nanocomposite powders, densifying these powders to provide suitable thermal spray feedstock, and then encapsulating and infiltrating these powders with a ductile phase to provide the desired meta-structured coating microstructure. These powders then were thermally sprayed in a diamond-jet High Velocity Oxy-Fuel (HVOF) system and significantly higher wear performance was demonstrated compared to hard chrome. Nanocomposite cermets with hardnesses exceeding 1150VHN and a ductile WC-Co nanocomposite with 1415VHN hardness were produced.
To prepare the nanocomposite feedstocks, low-cost milling techniques were developed to reduce ceramic particle sizes to the low-submicron to nanometer size range, and then to intimately mix these materials with a metal binder. After particle size reduction and intimate mixing, binders were added and the feedstocks were prepared for spray drying. Spray drying was performed in nitrogen using a Niro production minor spray dryer with a centrifugal atomizer. After spray drying, fast-sintering profiles were developed to remove the nontoxic organic binder and to sinter the powders into a dense particle, without significant coarsening of the powders and without breaking the spherical agglomerates. After sintering, the powders were further infiltrated and encapsulated with metal binders using a fluidized bed vacuum metallizing process pioneered by Powdermet.
To verify performance of the nanocomposite materials, developmental quantities of 10 different powders (5–20 pounds of each variant) were generated and provided to a thermal spray service provider to develop appropriate HVOF coating parameters. Thermal spray parameters were successfully developed and deposited coatings were evaluated. Microstructural analysis, hardness measurements, and G65 sand erosion tests were completed showing that the nanocomposite coatings performed substantially better than hard chrome in all cases.
This program demonstrated feasibility and economics for producing nanocomposite thermal spray powders that can potentially lead to an order of magnitude reduction in the cost of hard chrome alternatives. The key demonstration of utilizing low-cost ceramic and reclaimed waste carbide feedstocks to produce nanocomposite thermal spray powders was demonstrated. Economic feasibility for producing nanocomposite feedstocks at equivalent or lower cost than current carbide feedstocks, and producing coatings with much lower density but competitive with carbides in terms of hardness was demonstrated. Market analysis indicated that reducing the cost of hard chrome alternatives 3 – 5 times would lead to significant adoption of thermal spray technology, which has had poor market penetration outside of aerospace, because of the negative cost impact. Phase II efforts will focus on refining the optimum nanocomposite coating formulation(s), and on validating performance and economic assumptions at various user sites.