Life Cycle Engineering of Green Gun Barrel Technology
Kenneth R. Stone
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
513/569-7474, Fax: 513/569-7111
[email protected]
John D. Vasilakis, Ph.D
Benet Laboratory
Watervliet Arsenal
Watervliet, New York 12189-4050
518/266-5615, Fax: 518/266-4661
[email protected]
ABSTRACT
The Green Gun Barrel project is designed to develop a technology to relieve the DoD of the
hazardous wastes associated with electroplating and toxic chromium by both changing the
process and the material. The physical vapor deposition (PVD) technique, known as sputtering,
involves the evaporation of solid sources in a vacuum environment and deposition of the vapors
on the interior surface of the gun barrel, controlled by a cylindrical magnitron. The material
substitute for chromium is tantalum. This paper presents the application of LCA to the Green
Gun Barrel project to determine the environmental impacts of the PVD process in comparison to
chromium electroplating. The barrel selected for the program is the M242 Bushmaster and the
GAU-12.
Co-sponsored by the Strategic Environmental Research & Development
Program (SERDP) and EPA, the Life Cycle Engineering evaluation of
PVD plating was applied to an emerging technology (PVD) and to an
established process (electroplating) to draw comparative information.
INTRODUCTION
The purpose in plating the interior surface of a gun barrel is to extend the useable life of the
barrel. Since the advent of hot and fast-burning smokeless powder, cannon barrels and rapid-fire
gun barrels have required the addition of either a sleeve or lining made of a hard metal to protect
the barrel from the effects of heat, friction and corrosion. For small cannon, such as the M242, a
chrome coating is applied to protect the steel barrel.
Each of the military services uses chrome-plated gun barrels in their weapons systems. Presently,
chromium is deposited on the barrels electrochemically. The chromium used in the plating bath
is present as hexavalent chromium (chromate), which is carcinogenic. In addition to the exposure
problem this method poses, a considerable amount of hazardous waste is generated.
Physical Vapor Deposition:
This project entails the development of an alternative technology for plating gun barrel steel to
replace the process electroplating of chrome (Cr-electroplate) with physical vapor deposition of
tantalum (Ta-PVD). Developed by Benet Laboratory at Watervliet Arsenal, this project’s
objective is to develop a superior coating application at a lower cost and lower environmental
impact. To date, Benet has built Ta-PVD technology at the bench and pilot scale. First efforts
were made on 45mm test blanks and on French 45mm barrels measuring 55 inches in length.
Currently, Benet has completed a Ta-PVD rig that can accommodate the 25mm M242 . Benet’s
partners in this project are: Naval Air Warfare Center - China Lake, Eglin Air Force Base, the
Army's Construction Engineering Research Laboratory at Champaign, Illinois, EPA’s National
Risk Management Research Lab in Cincinnati, Ohio, and Battelle Pacific Northwest Labs.
Ta-PVD is being developed for
its potential as an innovative,
non-aqueous, non-polluting
method for applying
environmentally safe coatings
to protect gun barrels and
replace chromium. In the TaPVD process, a tantalum rod is
inserted and centered inside a
gun barrel, and the atmospheric
pressure within the barrel is
reduced using vacuum pumps.
An argon plasma is then
generated between the target
and the substrate which causes
tantalum atoms to be sputtered
off the rod and onto the barrel.
(See Figure1)
Figure 1 PVD via Cylindrical Magnitron Sputtering
In selecting PVD, several technologies were examined and dismissed including: molten salt,
plasma spray, chemical vapor deposition, ion implantation, ion plating, explosion bonding,
metalliding. PVD was selected over these other technologies because in addition to being a dry
(non-aqueous) process with good adhesion characteristics and a deposition rate comparable to
that of electroplated chromium, it eliminates hazardous materials (hexavalent chromium),
exposure problems, and the generation of wastewater associated with the process.
Technical Status:
This year the process has been scaled up to coat a full size 25 mm barrel. This barrel will be
tantalum-sputtered and test fired with several rounds to verify adhesion of the coating. Additional
barrels will then be supplied by each of the services, coated with tantalum, and extensively test
fired alongside a chrome-plated barrel. The barrels will be characterized and evaluated to
determine how well their performance compares to conventional chrome-plated barrels.
Ultrasound, heat effects, crack density, thermal shock, porosity, chemical composition, and
metallurgical tests will be performed to analyze the coatings after firing. Theoretical molecular
modeling, metallic surface characterization, and thermo-elastic and -inelastic stress analysis of
the coating and gun tube are also being conducted. The project will culminate in an advanced
technology demonstration addressing specific Army, Navy, and Air Force requirements.
LIFE CYCLE ENGINEERING OF Ta-PVD v. Cr-ELECTROPLATING
LCE Methodology:
This LCI was developed using the methodology
introduced in "Life Cycle Assessment: Inventory
Guidelines and Principles" (EPA, 1993). As discussed in
Chapter 1, an LCI is a comprehensive inventory of all
process inputs and outputs for all life cycle stages - raw
materials acquisition through final disposal. To help
organize this effort, each lifecycle stage is reduced to a
series of individual processes, or "black boxes." This
black box is illustrated here in Figure 2, the life cycle
Figure 2 Life Cycle Inventory Template
inventory template. The LCI template is the basic
building block of the LCI process. For each process stage, the raw material inputs, water and
energy requirements, process outputs, and wastes are identified.
Functional Unit:
The functional unit - a standard unit of output - provides the basis for comparing different gun
barrel processes. Following identification of the process inputs and outputs using life cycle
inventory templates, these requirements are then quantified and scaled to the functional unit. The
functional unit for this report is 10 ft2 inner bore surface area of a gun barrel. Ten ft2 is
approximately the average of the surface area of the M256 120 mm smoothbore cannon gun
barrel (21.03 ft2) currently produced via Cr-electroplate at Watervliet Arsenal and that of the 45
mm test barrel (2.13 ft) used to test the Ta-PVD process. This was done because a common
barrel size was not available, and no data were available for the 25mm M242. By averaging the
45mm and the 120mm, we were able to get a common denominator for our calculations without
pushing the scaling process too far from either process.
Inventory:
A materials requirements analysis was conducted to identify and quantify the material inputs and
outputs for each process stage. The material inputs and outputs provide the basis to scale energy
requirements and environmental emissions associated with each process stage on a per gun barrel
basis. Following a detailed material balance for each process stage, a comprehensive material
balance for the entire life cycle is built. In this LCI, the first step identifies and quantifies the
principal ingredients for each process. The next step reduces each principal ingredient down to
the raw material extraction stage (i.e. coming from the earth), via intermediate production stages,
if required. Finally, each material input and output is scaled to the functional unit.
An energy requirements analysis was conducted of the energy use associated with the Ta-PVD
process on a single gun barrel, then scaled to the functional unit of 10 ft2. This analysis identifies
and quantifies the energy requirements for each process within the LCI scope in terms of fuel and
electricity units. Included in this analysis are the fuel requirements for materials transport.
The LCI also analyzes the atmospheric emissions, waterborne emissions, and solid wastes
discharged to the environment at each stage. Emissions data are first collected on a per gun barrel
basis prior to scaling to the functional unit. Airborne, waterborne, and solid waste emissions are
not combined since their environmental effects can differ substantially. For example, ammonia
emissions to the atmosphere have different environmental effects compared to ammonia
emissions into a lake.
Primary data gathered at Watervliet Arsenal were used when available. Data that could not be
gathered at Watervliet Arsenal was extrapolated from industry averages, commercially available
LCI datasets, and government statistics. Further, data was collected on the life cycle stages from
raw material extraction to manufacturing. The use, demil and disposal stages will be added after
data becomes available from the live fire tests to be conducted this year.
Life Cycle Cost:
The approach used in this study is consistent with DoD Instruction 5000.1. DoD life cycle costs
are defined as the costs associated with total system ownership, from procurement and
installation, through use and maintenance, to disposal and demilitarization. Included in life cycle
costs are training and labor requirements, security costs, compatibility, interoperability, potential
environmental impacts and associated compliance costs. This LCC does not evaluate external
costs - that is, costs incurred external to the facility- such as environmental impacts, although
some of the data collected could potentially affect human health and the environment. Since
external costs are difficult to monetize, it was determined that this study shall focus on the
internal direct costs incurred by the Army as a result of the Ta-PVD operation. The following list
of cost categories illustrates what types of costs were considered by the LCE team:
Direct Costs
Indirect Costs
Sunk Costs
Capital Costs
Operating Costs
Net Present Value
- Included – e.g., equipment acquisition and maintenance.
- Excluded – e.g., overhead and regulatory compliance.
- Excluded – e.g., Ta-PVD R&D, Cr-electroplate investment costs
- Included – e.g., Ta-PVD equipment sufficient to plate 150 barrels/year
- Included – e.g., labor, utilities and materials.
- Included - NPV takes future forecasted costs and revenues and translates
them into today's dollar values using a stated discount rate.
Indirect costs are excluded because they would likely be similar regardless of the gun barrel
coating technology. Although the Ta-PVD process would likely require fewer compliance
requirements, the savings will be minimal because other Arsenal activities still require
compliance staff. Although research and development is usually considered an investment cost,
in this study it is considered a sunk cost -a cost that has already incurred - and therefore
excluded. Likewise, Cr-electroplate investment costs are also considered sunk costs.
Data Reliability
The sources of data for this study include actual operations data, vendors, and estimates provided
by Watervliet Arsenal and Benet Laboratories staff. Due to the developmental nature of the PVD
tantalum technology, the data cannot yet be compared to an industry average.
Several data points in the study, such as the number of gun barrels produced per year, affect
many other data points in the system. A change in these assumptions can cause cascading effects,
changing many other values in the study.
Key Assumptions:
In a study of this magnitude, some assumptions are necessary to limit the scope of the data
collection effort. For example, this study does not scale the costs of the Cr-electroplate and TaPVD to a common gun barrel size and production level because of data uncertainty and technical
differences. The baseline Cr-electroplate operation was projected to be capable of plating 300,
120 mm gun barrels per year. The Ta-PVD tantalum process is currently in the development
stage, but is being designed to operate at 150, 25mm barrels annually.
Watervliet Arsenal personnel indicated that the Cr-electroplating line required a major overhaul
approximately every ten years. Benet Laboratories staff believed that ten years was a reasonable
assumption for the useful life of the Ta-PVD tantalum equipment.
EPA believes the data used in this study are accurate for the tantalum process. It should be noted
that the data shown represents 95% of the total contribution on a mass basis. Any material
representing less than 5% of the total mass was omitted from the analysis.
LCI Findings:
Although there are no significant waste emissions from the PVD tantalum process itself, there are
emissions resulting from production of energy and raw materials inputs used in the PVD
tantalum process. This LCI is a preliminary dataset. Several important aspects of the Crelectroplate LCI dataset were not available.
Energy consumption, as shown in Figures 3 and 4, is subdivided into two categories. The first
category, "cradle-to-gate" energy, represents the energy used during raw materials acquisition and
manufacture. The second category, “gate-to-gate" energy, represents the energy used on-site in
the Cr-electroplate and Ta-PVD. Ta-PVD uses more energy. Approximately 12 million Btus
(8.86 million Btu for "cradle-to-gate and 3.31 million Btu for "gate-to-gate") are used throughout
the Ta-PVD life cycle per functional unit, compared to about 9 million Btu for Cr-electroplate
(5.79 million Btu for "cradle-to-gate" and 3.65 million Btu for "gate-to-gate"). Cr-electroplate's
use of steam heat is reflected in the "gate-to-gate" natural gas use shown in Figure 3. Figure 4
shows that Ta-PVD relies exclusively on electricity for “gate-to-gate" energy requirements.
Figure 3 Resource Use Cr-Electroplate per FU
Figure 4 Resource Use Ta-PVD per
FU
There are also significant differences in emissions between the processes. While there are no
significant environmental emissions directly associated with Ta-PVD, the Cr-electroplate has
"gate-to-gate" air emissions containing chrome, waterborne emissions containing chromium and
phosphoric acid, and hazardous waste emissions containing chromium compounds and
phosphoric acid. The most striking differences between the two processes are in waterborne
emissions and hazardous waste emissions. Cr-electroplate generates wastes in the tens and
hundreds of pounds per functional unit versus the half pound to four pound range in the PVD
tantalum process. Further, Cr-electroplate generates over 500 pounds of chromium compounds
per functional unit compared to zero hazardous waste emissions directly associated with TaPVD.
Although Ta-PVD has no significant "gate-to-gate" environmental emissions, there are
environmental emissions resulting from previous life cycle stages. Ta-PVD generates more
carbon dioxide emissions per FU (3,3 70 pounds for Ta-PVD tantalum versus 1,972 pounds for
Cr-electroplate), which is a reflection of the PVD tantalum process' greater electricity use.
LCC Findings:
The NPV for Cr-electroplate was projected to be $9,804,000 and the NPV for Ta-PVD was
projected at $3,599,207. As discussed earlier, the Ta-PVD analysis is based on production of
150 barrels per year while the Cr-electroplate analysis is based on production of 300 barrels per
year.
The largest costs in Cr-electroplate are wastewater treatment, labor and natural gas. The most
significant operating costs for the Ta-PVD are labor, tantalum, and electricity. Therefore, there
appears to be more emissions and waste-related costs associated with chrome.
Preliminary performance testing indicates that the Ta-PVD process could provide a superior gun
barrel coating surface. For the purposes of setting a baseline, this study treated performance as
being equivalent (e.g., barrels from either process will support the same number of firings). A
longer lasting barrel would reduce material, energy, and labor requirements and the
corresponding costs.
CONCLUSIONS
This analysis showed that the chrome process generates more waste emissions than the Ta-PVD
tantalum process per functional unit, particularly hazardous waste. The chrome LCI is dominated
by inputs directly associated with the plating processes such as chromic acid, phosphoric acid,
sodium hydroxide, water, and sulfuric acid. Water, tantalum, limestone, argon, and copper are
the primary constituents of Ta-PVD. Although there are no significant waste emissions directly
associated with Ta-PVD, the life cycle does have emissions, notably global warming potentials,
associated with tantalum extraction and electricity generation.
The highest operating costs for Cr-electroplate are wastewater treatment, labor, and natural gas.
The highest operating costs for Ta-PVD are labor, tantalum sleeves, and electricity. Therefore,
Cr-electroplating costs are driven by emissions and waste-related costs, and costs for the PVD
tantalum process are driven by raw materials (tantalum), labor, and electricity. This indicates
that, if performance expectations are validated, Ta-PVD is the more sustainable coating system.