Advances, Applications, and Opportunities
for Coated Iron Powder for Electromagnetic Applications
Michael L. Marucci & K. S. Narasimhan
Hoeganaes Corporation, Cinnaminson, NJ
Presented at PM2TEC2003, Las Vegas, NV
ABSTRACT
This paper reviews how the advances made in iron coating technology over the last fifteen years can be
applied to a variety of common electromagnetic applications. The properties of several traditional alloys
are reviewed. In addition, properties of newly developed insulated materials are also presented. Specific
application examples include use of coated iron powder in brushless DC motors. Opportunities for
application of insulated material in components for the 42V automotive electrical system are also
outlined.
IP Overview
The development of insulated particle (IP) comprising of pure iron powder and a uniform dispersed
insulting coating have opened several new opportunities for the use of particulate material in the place of
traditional lamination steels. The permeability of IP's is substantially lower than the permeability of
lamination steel due to the distributed air gap induced by the insulating coating.[1] In addition, the
magnetic saturation is lower due to the lower metal loading in the IP material.
However, these materials possess several advantages that when incorporated into the designing of an
electromagnetic component make IP the best material alternative.[2] The advantages of IP materials
include the ability to mold complex net shapes with minimal waste, the ability to engineer special
material systems to specific electromagnetic applications, excellent high frequency response, and the
ability to carry flux in all three dimensions.
IP materials fall into three general categories. The three types include fluid bed coated materials,
conventionally compacted materials, and annealable composite materials.
MATERIAL PRODUCTION AND PROCESSING
Fluid Bed Coated Materials
Fluid bed coated materials are produced by the process shown in Figure 1. The process includes several
optional steps that can be incorporated only if needed for the specific application. This flexibility in the
process allows the material/processing system to be tailored to yield the properties needed via the most
economical process.
The iron powder for these materials is coated in the virgin state or after an optional oxide coating with a
high performance engineering polymer. The polymer loading falls in the range of 0.5-1.0 w/o. The
coating imparts a non-conductive layer surrounding the iron particles isolating them, confining the core
losses to individual particles. The polymer has been designed specifically for the warm compaction
process and results in green compacts that have high strength. An example of the strength and density
achievable in SC120, a fluidized bed coated material, is shown in Figure 2
Pure Iron Powder
Optional Oxide
Coating
Fluidized Bed Coating
with a Polymer
Warm Compaction
Optional Curing
Finished Part
Figure 1: Material and part production process steps using fluid bed coated IP materials.
7.45
30,000
Strength
Density
7.40
25,000
7.30
20,000
7.25
15,000
7.20
Density (g/cm3)
Cured Strength (psi)
7.35
7.15
10,000
7.10
5,000
7.05
0
7.00
25
30
35
40
45
50
55
Compaction Pressure (tsi)
Figure 2: Strength and density of SC120 at various compaction pressures.
A summary of typical magnetic properties of fluidized bed coated IP materials is given in Table I. The
magnetic properties of the fluid bed coated IP materials are dependent on the coating process. For
example, the permeability will increase as the amount of polymer coating decreases, this is charted in
Figure 3. This increase in permeability is due to the increased metal loading and reduced distributed air
gap. In addition, the high frequency performance also changes. For instance, the data plotted in Figure 4
show that as you increase the amount of coating the stability of the permeability improves at higher
frequencies..
In summary, fluidized bed coated IP material provided high strength for this type of composite materials,
can be machined, possess good corrosion resistance arising from the polymer coating, are suitable for
operating temperatures of up to 400°F (200°C), and have good high frequency electromagnetic
properties. Some of the inherent limitations of this type of material include the use of warm compaction
(powder and die heating), the requirement of die wall lubrication for certain part geometries, and less
than optimal low frequency properties.
Table I: Typical magnetic properties for several fluid bed coated materials
Material
SC100
SC120
SC600
TC80
Polymer
Coating
Oxide
Coating
w/o
0.75
0.60
0.25
0.75
No
No
No
Yes
Density at
Initial
50 tsi
Permeability
(690 MPa)
g/cm3
7.20
100
7.30
120
7.40
140
7.15
80
Maximum
Permeability
400
425
600
210
Coercive
Induction
Force at
at 40 Oe
40 Oe
Oe
KGauss
4.8
10.9
4.7
11.2
4.7
12.7
4.7
7.7
700
600
Initial
Maximum
Permeability
500
400
300
200
100
0
SC120 0.6 w/o
Polymer
SC100 0.75 w/o
Polymer
SC600 0.25 w/o
Polymer
TC80 0.75 w/o
Ploymer + Oxide Coat
Figure 3: Permeability of several fluid bed coated materials.
Figure 4: Permeability of several fluid bed coated IP materials as a function of frequency.
Conventionally Compacted IP Materials
To overcome some of the processing challenges inherent to the fluidized bed materials several
conventionally pressed and cured IP grades have been developed. These newly developed materials
employ an oxide coating applied using a proprietary process in combination with several special
lubricant/insulating polymer materials that can be chosen to attain the desired properties.
These materials possess properties similar to the fluidized bed materials but have the added advantage of
utilizing conventional compaction techniques. The main drawback is a reduction in part strength. The
part production process for these materials is shown in Figure 5. The amount of polymeric insulating
material is usually 0.5 w/o, but can be adjusted for specific applications. The processing conditions can
also be tailored for specific desired properties. For instance, a cold die can be used and the part can be
subsequently cured at 300°F (150°C) to achieve lower density electromagnetic parts or a warm die set to
280°F (138°C) can be used to compact higher density parts and the curing can be skipped.
Pure Iron Powder
Proprietary Oxide
Coating Process
Compact with
Optional Warm Die
Optional Curing
Admixing with Specialized
Insulating Material
Finished Part
Figure 5: Material and part production process steps using conventionally compacted IP materials.
Table II shows a comparison of how changing the material and the processing parameters effects the
density and strength at a given compaction pressure. In addition to the strength, the magnetic properties
also are impacted. One other important item to note is that the materials listed in Table II pressed at
280°F (138°C) employ a warm die only. The powder was not heated prior to being filled into the die.
Figure 6 shows the typical compressibility and strength for this type of material. The plot shows the
strength and density advantage in using a warm die.
Table II: Comparison of the effect of material/processing differences on the final mechanical properties
of conventionally compacted IP materials.
Material
Die
Temperature
o
AP500 LS
AP500 HS Cured
AP500 HS Warm Press
AP500 High Perm
F (oC)
Room Temp
Room Temp
280 (138)
280 (138)
Curing
Temperature
o
F (oC)
900 (480)
355 (180)
None
900 (480)
Density at
50 tsi
(690 MPa)
g/cm3
7.22
7.15
7.45
7.20
Strength at
50 tsi
(690 MPa)
psi (MPa)
5,257 (36)
13,822 (96)
12,969 (89)
4,850 (33)
A summary of the typical magnetic properties of conventionally compacted IP materials is summarized in
Table III. As stated above, these materials are all produced from pure water atomized iron powder and
subsequently oxide coated. The combination of the oxide coating and the polymeric insulating material
create a distributed air gap. This reduces the density and the permeability of the material. However, the
permeability is not solely a function of density. It is also dependent on the amount of deformation due to
cold working of the iron powder, which creates dislocations pinning the magnetic domains. Therefore
the materials that can be stress relieved following molding result in higher permeabilities. This is clearly
seen in Figure 7, which shows the increased maximum permeability of AncorPerm 500LS and 500HP
that were cured at 900°F (480°C).
14,000
7.50
12,000
7.40
AP 500HS
Warm Die
7.30
7.10
AP 500LS
6,000
7.00
3
Strength (psi)
7.20
8,000
Density (g/cm )
10,000
4,000
6.90
2,000
6.80
0
6.70
25
30
35
40
45
50
55
Compaciton Pressure
Figure 6: Typical compressibility and strength of conventionally mixed IP materials.
Table III: Typical magnetic properties for several conventionally compacted IP materials.
Material
AP500 LS
AP500 HS Cured
AP500 HS Warm Press
AP500 High Perm
Polymer
Coating
Oxide
Coating
(w/o)
0.50
0.50
0.50
0.50
Yes
Yes
Yes
Yes
Density at
50 tsi
(690 MPa)
Initial
Permeability
Maximum
Permeability
85
80
90
80
425
230
300
520
3
(g/cm )
7.22
7.15
7.45
7.20
Coercive
Core Loss
Induction
Force at
at 60 Hz,
at 40 Oe
40 Oe
15 Oe
(Oe)
3.0
4.7
4.8
3.8
(KGauss)
12.7
8.6
9.9
12.9
Wb/lb.
0.79
0.56
1.38
2.77
600
500
Initial
Maximum
Permeability
400
300
200
100
0
AP500LS Cured @
900F
AP500HS Warm Die @
300F
AP500HS Cured @
355F
AP500HP Cured @
900F
Figure 7: Permeability of conventionally compacted IP materials.
The oxide-coated materials also have the advantage of good performance at higher frequencies. The total
core loss is a function of hysteresis and eddy current losses. At lower frequencies the hysteresis losses
dominate, however, at higher frequencies the eddy current losses, which are confined within the insulated
particles. However, at higher frequencies the eddy current losses begin to overlap from particle to
particle leading to a reduction in permeability. The oxide coating increases the distributed air gap of the
material that in turn decreases the total core loss at higher frequencies.
120
100
Permeability
80
60
AP500LS Cured @ 900F
40
AP500HS Warm Die @ 300F
AP500HS Cured @ 355F
20
AP500HP Cured @ 900F
0
100
1,000
10,000
100,000
1,000,000
Frequency (Hz)
Figure 8: Permeability of conventionally compacted IP materials as a function of frequency with a
constant 10 Gauss applied field.
The high frequency stability of the permeability of the conventionally compacted IP materials is shown in
Figure 8. This plot shows that at low frequency the materials that have high densities or were cured at
900°F (480°C) possess the best permeability. The stability of these materials are good up to about 10
kHz except for the AncorPerm 500HP, which uses a coarser base iron material. The coarser iron lowers
the distributed air gap and also causes eddy current losses to increase at higher frequencies. For this
reason, the AncorPerm 500HP is best suited for low frequency applications that require higher
permeablities and induction levels. The core loss over the same frequency range for these materials is
shown in Figure 9.
1000000
100000
AP500LS Cured @ 900F
AP500HS Warm Die @ 300F
AP500HS Cured @ 355F
Core Loss (Wb/lb.)
10000
AP500HP Cured @ 900F
1000
100
10
1
100
1,000
10,000
100,000
1,000,000
0.1
0.01
Frequency (Hz)
Figure 9: Core loss of conventionally compacted IP materials as a function of frequency with a constant
10 Gauss applied field.
Some of the limitations of the conventionally compacted IP materials include achievable strength levels
that are approximately half that of the fluid bed coated materials, the continued need for die warming for
most applications. However, the combination of magnetic performance and processing ease makes this
type of insulated particle suitable for a variety of static electromagnetic applications.
Annealabel IP Materials
Annealable IP materials were developed to combat the intrinsic problem of the increase in hysteresis
losses due to cold working of the iron powder.[3] The deformation due to cold working can be relieved
by annealing the iron at temperatures in excess of 1000°F (538°C). Conventional polymer coating
systems cannot withstand these temperatures and therefore Hoeganaes Corporation has developed a
proprietary coating system that can be annealed. The process to produce parts from the resulting material
system, called Ancorsteel LCM, is shown in Figure 10.
Pure Iron Powder
Proprietary
Organometallic Coating
Anneal at 1200°F
(650°C)
Finished Part
Warm Compact with Die
Wall Lubrication
Figure 10: Material and part production process steps using annealable IP materials.
A summary of the typical magnetic properties for an annealable IP material is shown in Table IV. This
material has a higher than usual amount of insulating coating which leads to lower strength following
annealing. The coating creates a continuous insulating matrix that completely isolates the iron particles.
Table IV: Typical magnetic properties for an annealable IP material.
Material
LCM
Insulating
Coating
Density at
50 tsi
(690 MPa)
w/o
g/cm
2.0
7.25
3
Strength at
50 tsi
(690 MPa)
psi (MPa)
3,600 (25)
Initial
Permeability
Maximum
Permeability
125
245
Coercive
Induction
Force at
at 40 Oe
40 Oe
Oe
KGauss
4.4
8.0
Figure 11 shows the AC hysteresis loops of LCM. The annealed material exhibits a much smaller loop
area and low coercively as compared to other materials. This indicates that the losses will be less. This
is confirmed by the data presented in Figure 12. The plot shows that the core loss is lower that the
SC120.
The main limitations of this type of material are the low strength due to the limited strength of the
coating material and the lowered magnetic performance imparted by the large air gap. The use of these
materials should be limited to applications that require low core losses to reduce heat emission.
10
8
6
B (KGauss)
4
LCM
2
0
-50
-40
-30
-20
-10
0
10
20
30
40
50
-2
-4
-6
-8
-10
H (Oe)
Figure 11: AC hysteresis curves of LCM at 60 Hz.
16.0
14.0
LCM
12.0
Core Loss (Wb/lb.)
200 Hz
SC120
10.0
8.0
6.0
60 Hz
4.0
2.0
0.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
Induction (kG)
Figure 12: Core Loss of LCM and SC120.
APPLICATIONS FOR IP MATERIALS
IP materials are suitable for a number of applications such as iron powder cores for inductors, switch
mode power supplies, R F transformers, EMI filters, saturable reactors, oscillators, and tank circuits. The
following gives more descriptive examples for the use of IP materials in electric motors.
Permanent Magnet DC Motors
Permanent magnet DC motors (PMDC), such as brushed DC motors, have permanent magnets attached to
a stator backing material. The rotor is wound (armature) and is energized using a brushed commutator.
A schematic of this arrangement is shown in Figure 13. This motor architecture has some drawbacks that
include RF interference due to the sparks caused by the brushes sliding on the commutator, reduced
motor performance due to brush friction, heat build up in the armature that can only be transmitted via
the shaft, and the accumulation of brush dust.
Stator
Rotor
(Lam Steel)
Permanent
Magnet
Figure 13: Schematic of PMDC motor architecture.
These drawbacks in combination with the limited strength of IP materials limit the applicability of using
insulated iron material in new PMDC motor applications. However, there may be an opportunity in the
stator backing material. The more modern brushless DC motor that has become popular in recent years
offers significant opportunities for IP materials is described below.
Brushless DC Motors
IP materials are applicable for the stator in brushless DC (BLDC) motors. The motor architecture, shown
in Figure 14, employs high power permanent magnets on the rotor with a copper wound stator. The
stator in these devices has traditionally been produced from lamination steel that requires a good deal of
processing including rolling and stamping. Since the stator is fixed the strength of the material is less
important than the ease of fabrication. Therefore, the net shape capability of IP has the potential to
reduce manufacturing costs.
The stator material must possess several characteristics to ensure adequate performance. For instance,
the material must have the saturation flux density and the capacity to carry magnetic field
circumferentially through the body of the core and stator teeth. The material also should have core losses
low as possible to reduce heat emissions during operation and also must have sufficient thermal
conductivity to dissipate heat created by the windings.
Stator
(Lam Steel)
Permanent
Magnet
Rotor
Figure 14: Schematic of BLDC motor architecture.
Since IP materials exhibit lower permeability and induction than lamination steels, IP material cannot be
directly substituted for the lamination steel without a substantial drop in motor performance. The motor
must be redesigned to accommodate the differences in the materials. Since the induction is less in IP
materials, the magneto magnetic force (MMF) must be increased to drive the same amount of flux
through the core as lamination steel. This can be accomplished by increasing the tooth area. However,
the tooth area cannot be increased without changing the slot width therefore, the motor size must increase
to accommodate this change. This is not an acceptable solution because it reduces the power density of
the motor and therefore its competitiveness for a variety of applications. A better solution is to increase
the MMF by using higher coercivity permanent magnets. This will allow for the same power output in a
similar package size that includes the ease of fabrication inherent in using the P/M IP materials for the
stator.
OPPERTUNITIES FOR IP MATERIALS
The 42V System
At the current rate of increasing power demand, the traditional 14V automotive electrical will cease to be
a viable source of on board electrical power in passenger cars. The current trend is to increase the
electric and electronic features in automobiles, all of which draw more power. A recent report [4] shows
that by 2010 large and medium cars will have reached the 4500-Watt limit of the 14V system. One major
factor driving the increase in electrical equipment is the trend to change accessories from belt drive to
electrical drive to improve fuel economy. Figure 15 shows the potential improvements in fuel economy
that can be realized in moving toward electrical motor drive for these accessories.
16
14
Alternator Efficiency Improvement
Electromagnetic Valve Operation
12
Electric Power Steering
Percent Improvement
Stop/Star Operation
10
Electric Water Pump
8
6
4
2
0
Figure 15: Potential fuel economy benefits in passenger cars resulting from conversion to electrical
drive.
In order to meet the future demands of these and any additional electric systems the automotive industry
is transitioning from a 14V to a 42V automotive electrical system. In addition to being able to provided
more power output (Wattage) for the devices described in Figure 15, the 42V system has the advantage
of reduced wire size due to the Ohm's Law relation that increasing the voltage will lower the I2R heating
while supplying the same current.
A potential system layout of the 42V system is shown in Figure 16. The system shown is a two-voltage
system that uses the standard 12V battery to power traditional electronics such as the engine management
system, lamps, audio system, etc… The 36V power pack is the source of energy for the integrated
starter/generator (ISG), and some of the more energy reliant devices such as electric power steering or an
electric water pump.
Figure 16: Potential 42V system layout utilizing a two voltage approach (Source: Delphi).
Shifting to this system offers a variety of opportunities for IP materials. The following sections outline
some of these opportunities.
Power Generation and Storage
The automotive power generation and storage system has several opportunities for IP. One opportunity
is use in DC/DC converters to reduced voltage in for automotive components that operate at 12V. This
inverter can then be used in the place of a separate battery for these devices. Another opportunity lies in
using regenerative brake assist. This would reduce the brake pad wear and return some energy to the
battery packs. The opportunity here is in the cores needed to produce the electric current at each wheel.
The final opportunity lies in the integrated starter motor, which also could be used as a supplement for
the IC engine during acceleration. This hybrid concept could incorporate IP in this electrical motor to
save manufacturing costs.
Powertrain Systems
The main powertrain system that provides an opportunity for IP materials is the aforementioned ISA
being integrated with the IC engine and the transmission. The ISA makes the idle/engine stop function
possible to increase fuel economy and reduce emissions. As mentioned above the ISA could benefit from
the reduced manufacturing costs that IP has to offer.
There are three types of ISA motors. They include induction, permanent magnet, and switched
reluctance. The AC induction motors provide smooth torque output and high efficiency but require a DC
to AC converter when taking power from the battery packs. Switched reluctance motors offer a scalable
power output but produce a non-smooth power output that may degrade ride and smoothness of
acceleration. The permanent magnet offers a very efficient design due their not being a need for an
external field excitation and also offers a smaller package.
In addition to ISG's, the powertrain offers other opportunities for IP. One example is the use of insulated
iron for electromagnetic valvetrain components. The solenoid like devices employed could use the net
shape capability of IP for cost savings. EGR and turbocharger control and actuation systems also provide
opportunities for IP.
Chassis Systems
The chassis systems employ several motors that would have to be redesign for the 42V system. The
redesign could incorporate IP technology. Electric motors would be used for power steering pumps and
power braking pumps. In addition the increased use of magnetic damping systems for active suspension
also provides an opportunity for this type of electromagnetic material.
Thermal Systems
IP could also be used in the motors for HVAC system in the automobile. These include blower motors
and perhaps electrically driven air conditioner compressors.
CONCLUSIONS
•
•
•
•
•
IP materials produced by the fluid bed coating process offer the best combination of magnetic
performance and strength, however, require advanced processing technology such as warm
compaction and die wall lubrication to produce optimal parts.
Newly developed conventionally compacted IP materials that use an oxide coating in
combination with specialized admixed insulating lubricants offer magnetic performance
comparable to the fluid bed materials with the main drawback being a loss in strength.
Annealable IP materials have been developed for use in applications that warrant low core losses.
Pressed stators for brushless DC motors (BLDC) are an excellent application for IP materials,
which simplify the production of these machines by using the net shape capability of the powder
process.
The 42V system offers a variety of opportunities for IP materials over the next few years with the
required redesign of electrical components in the automobile. The use of IP is best suited for
BLDC motors for automotive use.
REFERENCES
1. Hanejko, F.G., Marucci, M.L., “Soft Magnetic Materials Utilizing Conventional and Warm
Compaction Technology,” SAE World Congress 2003 paper No. 2003-01-0448.
2. Narasimhan, K.S., Miller, T.J., "42 Volt Architecture on Powder Metallurgy-Opportunities," SAE
World Congress 2003 paper No. 2003-01-0443.
3. Hanejko, F.G., Ellis, G.W., Hale, T.J., "Application of High Performance Material Processing Electromagnetic Products," Advances in Powder Metallurgy and Particulate Materials, Vol. 2, pp.813-8-26, Metal Powder Industries Federation, Princeton, NJ, 1998.
4. Narasimhan, K.S., "More Power all Sparks Soft Compact Research," Metal Powder Report, May
2003, Vol. 58, No. 5, pp. 12-21.