optimizing soft magnetic composites for power frequency applications and power-trains

Optimizing Soft Magnetic Composites for Power FrequencyApplications and Power-Trains

PATRICK LEMIEUX,1,2,3 RODERICK GUTHRIE,2 and MIHAIELA ISAC2

1.—IMFINE Corp., Ste-Julie, QC J3E 1W2, Canada. 2.—Department of Mining and MaterialsEngineering, McGill Metals Processing Centre, McGill University, Montreal, QC H3A 2B2,Canada. 3.—e-mail: [email protected]

A new approach, together with a new family of soft magnetic composites(SMCs), has been developed and optimized for power alternating-currentapplications. The different technological and economic restrictions needed tomaximize a composite’s performance-to-cost ratio are presented. The experi-mental procedures to produce sintered lamellar SMCs are reported, togetherwith magnetic results and the effects of different processing parameters ontheir performance. The present results are compared with corresponding dataavailable for soft magnetic materials available on the market (laminations andcomposites). Data on the mechanical strength of these new SMC structuresare also given. The new process results in magnetic and mechanical propertiesof different alloy systems that are better than those of any of the SMCsavailable. The present materials’ energetic losses can be under 2 W/kg at60 Hz, at 1 T, whilst their permeability exceeds 2000, while maintainingmaximum induction above 1.7 T. These properties are very close to the bestresults for standard laminations on the market. The present process has thepotential to be very inexpensive, owing to its simplicity. Even though not fullyisotropic, recent three-dimensional machine designs and process advantagesconferred by powder metallurgy techniques can be applied to this new familyof lamellar particle composites. Through theoretical calculations and modelingexercises, it is briefly shown that this new kind of material can result in animprovement to the transportation sector where weight and efficiency ofnewly emerging electrical and hybrid power-trains are of prime importance.

INTRODUCTION

Since the end of the nineteenth century, electricmotors have been constructed by stacking stamped,insulated, ferrous-type laminates so as to form acore, around which copper wires are wound. Thisdevelopment followed Thomas Edison’s key discov-ery of how to limit eddy currents generated withinthese solid, magnetizable cores. Before the end ofthe 1980s, insulated consolidated powders weredeveloped, but only for high-frequency applications,starting with telecommunication technologies, andextending to filters, chokes, inductances, and high-frequency transformers. The application of suchpowders to form magnetic cores for electric motorsoperating at low frequencies met with limitedsuccess.

In the last 15 years, however, powder metal pro-ducers have carried out new trials aimed atimproving the performance of soft magnetic com-posites (SMCs) for lower frequencies, using theirregular water-atomized powders.1–8 They discov-ered, through a joint effort with motor designers,that even with their poorer magnetic properties, itwas possible to achieve performance comparable tomotors constructed with standard laminationstacking for specially designed permanent magnetmotors, particularly for high-frequency (400 Hz)machines.9–12 The lower magnetic performance atlower frequency could be compensated by the use ofnew three-dimensional (3D) designs that allowed foran important decrease in resistive losses within thecopper windings.9,13–15 However, for induction ma-chines with very low air gaps, operating at regular

JOM, Vol. 64, No. 3, 2012

DOI: 10.1007/s11837-012-0262-z� 2012 TMS

374 (Published online March 8, 2012)

power frequencies (50–60 Hz), and for high-power-density machines such as those used in hybridvehicle power-trains, the poor properties of pow-dered materials with high hysteresis losses arebarely acceptable. In addition, their mechanicalstrength is very low (maximum �100 MPa). This iscaused by the importance of limiting interparticlemetallic contact, in order to keep a high electricalresistivity. This issue severely limits their fields ofapplication, particularly for the transportation sec-tor.5,16,17 With these considerations in mind, effortswere conducted to develop new SMCs with proper-ties comparable to the best laminations available onthe market.18 The results demonstrate the bestdirections to pursue to optimize SMCs for powerfrequencies. Computer modeling and theoreticalcalculations are conducted, using the properties ofthe new material family, and the properties avail-able in literature for lamination stacking and water-atomized powder-based SMCs. The analysis is car-ried out for different potential designs that couldbe used in power-trains, in order to estimate thepotential of this newly proposed technology.

THEORY BEHIND THIS NEW APPROACH

Sintered soft magnetic parts, produced from wa-ter-atomized iron or stainless-steel powders, arereadily available.19,20 They are also sometimesmisnamed as ‘‘composites.’’ They are commonlyused for direct-current (DC) applications, or for verylow-frequency devices, such as actuators (electro-magnets). When an actuator is used at frequencyhigher than a few cycles per second, overheatingrapidly occurs in the magnetic parts unless there isproper insulation at distances shorter than themean magnetic depth of penetration into the mate-rial. This value, commonly termed the skin depth ofthe magnetic material in an alternating-current(AC) magnetic field, is proportional to the squareroot of its electrical resistivity (qe), and inverselyproportional to the square root of its permeability(l) and the frequency (f) of the applied field, i.e.,d = C(qe/lf)�0.5, where C is a constant depending onthe units used.

This limitation is due to the formation of eddycurrents in the part. In addition to causing over-heating and energy losses, eddy currents oppose themagnetic field applied to the part, thereby limitingthe magnetic field within the part. As the electricalresistivity increases, the penetration depth, d, of themagnetic field within the part increases, leading tomore efficient use of the material. A decrease in thepermeability of the material will give the sameresult, but will also limit the strength of theinduction and torque that can be generated by thepart for a given applied magnetic field, H, inaccordance with Maxwell’s equation, B = lH. Theonly way to increase the penetration of the magneticfield, and thereby the efficiency of the device at agiven frequency, without affecting its magnetic

induction, is to increase its electrical resistivity.Obviously, if the material is isotropic, comprising anarray of insulated, irregularly shaped particles, aswith SMCs, and is used in an application where themagnetic field has a preferential direction, thenmany demagnetizing gaps will be present in thedirection of the main field, decreasing the material’smagnetic permeability, increasing the coercive field,and thus increasing hysteresis losses within thematerial. These facts lead to the notion that thematerial should not be fully isotropic, so as to min-imize the presence of nonmagnetic material in themain direction of the field. This matter, which actsas a demagnetizing field, is similar to an air gap in amagnetic circuit.21 Efforts should therefore considerusing insulated lamellar particles, oriented mainlyin the main direction of the magnetic field, andideally sintered along their edges. However, if theiredges are sintered, then the electrical insulation inthe direction normal to the applied field will bedecreased by each contact zone between adjacentparticles. This potentially represents a zone ofdouble thickness without insulation. In fact, to limiteddy currents as well as those in the stacking ofstandard laminations, particles must have a thick-ness a few times lower than comparable laminationsused for the same application. An interestingdirection of research would be to determine the levelof mechanical strength that can be achieved in sucha composite without losing too much electricalinsulation. Another question is how much thecoercive field and permeability would benefit fromthis new approach, which allows for distributedmetal contacts between lamellar insulated particles.

OPTIMIZED EXPERIMENTAL PROCEDURES

A new process has been developed to insulatelamellar particles without covering their edges, so asto leave the uncoated edges available for developingmetal–metal contact during subsequent sintering ofthe part in a reducing atmosphere. This processshould lead to a minimum of nonmagnetic materialwithin the final composite, keeping the saturationinduction of the material as high as possible. Theinsulation layers between the lamellar particlesshould also be stable at temperatures commonlyused to sinter metallic particles, i.e., at 80% of theirabsolute melting temperature (up to 1300�C forcommon ferromagnetic materials). This limits thechoice of nonmagnetic materials to a few very stablemetal oxides, or their mixtures. Very few possibili-ties exist to uniformly coat such refractive dielectricmaterials as a very thin layer on a metal substrate.Physical/chemical vapor deposition (PVD/CVD)processes and sol–gel techniques are two of them.Both techniques were applied to metallic ribbons ofvarious common compositions (pure iron, and alloysof iron–3% silicon, iron–49% nickel, and iron–48%cobalt–2% vanadium). These compositions wereselected because they give high maximum inductions

Optimizing Soft Magnetic Composites for Power Frequency Applications and Power-Trains 375

and can be sintered at high temperature withoutlosing their good magnetic properties (thereforeexcluding use of amorphous or nanomaterials).

A technology was then developed to cut 2-inch-wide, thin, alumina-coated steel strips into thin,small, rectangular- or square-shaped particles. Asexplained above, the particles must be as thin aspossible to limit eddy currents. However, any mass-production cutting technology requires a properclearance between the blades of the knives, in orderto avoid exaggerated burrs on the material and ra-pid wear of the knife. This limits the thickness ofthe ribbon that can be used. For high-performancemotors used nowadays in industry, and particularlyfor hybrid automobiles coming onto the market,lamination thicknesses are �350 lm. Economicrestrictions preclude use of thinner laminations,limiting applications to �400 Hz according to theskin depth equation. In our case, this means thatlamellar particles must be less than �100 lm thick,in order to limit eddy currents, taking into accountlayered sintered joints, as explained earlier. As aresult, the base ribbons used were selected withthicknesses varying between 50 lm and 125 lm,depending on their availability on the market.Meanwhile, research is ongoing at the McGillMetals Processing Center, aiming to produce therequired thin strips, or ribbons, using an economical,direct casting, operation. This latter avenue is veryinteresting since, contrary to the use of standardlaminations, the final stacking factor, or the finalelectromagnetic material density of the part, does notdepend on the surface finish of the base material or onits uniformity of thickness (planarity). The pressingoperation, and any subsequent standard densifica-tion process, typically used in powder metallurgytechnology (i.e., double pressing–double sintering,hot pressing, sizing, and hot forging), basicallyeliminates interparticle voids and defects.

Regarding the desired thickness of insulation,common high-efficiency, high-performance motorshave stacking factors above about 95%. This tells usthat the insulating layer must not exceed a maxi-mum of 5% by volume of the composite, in order tobe competitive. Particles having a maximum thick-ness of 150 lm, with coatings of insulating materialon both sides (meaning that two intermediate layersare present when stacking), require that the thick-ness of one layer must not exceed a maximum of2.5% of the particle thickness, or 3.75 lm. However,we know that the compacted parts will not be fullydense, except when using the powder forging pro-cess, and in the best cases will have a few percent-age points of residual porosity remaining. It wastherefore an objective of the present investigation tokeep the insulator thickness under 1% of the par-ticle thickness or 2% of the volume of the compositeif possible (�1.5 lm), so as to exceed 95% of thetheoretical ferromagnetic density.

The optimum dimensions (length and width)of the particles for the cutting operation were

determined by taking into account the feedability ofthe particles in a standard powder metallurgypressing process and associated tooling, and themagnetic performance of the composite, with longerparticles giving better performance. As such, thedimensions of the particles were set to approxi-mately 1.5 mm 9 1.5 mm, representing the bestcompromise for parts having a minimum sectionwidth of �5 mm.

The simplicity of the process avoids the need forany screening operation to eliminate under- oroversized particles, as is the case for the regularpowder metallurgy process, nor are any sophisti-cated coating operations involving fluidized beds orvacuum drying mixers with injectors to encapsulatethe particles required. Mixing operations are alsoavoided, provided that one has developed a suitabledie wall lubrication technique. Figure 1 presentsthe flowchart of the new process.

The sol–gel operation, which is preferred for massproduction, comprises spray nozzles with dryingfurnaces, in line. The viscosity of the sol–gel solu-tion can be adjusted by adjusting its concentrationand the level of plasticizer additives. Viscosity var-iation allows one to obtain different thicknesses ofcoatings. These were analyzed and adjusted to avoidcoating cracks and peeling during subsequent cut-ting operation of the ribbons.22 The sol–gel solutionis water based, and uses aluminum isopropoxide asa precursor. The plasticizers used are polyvinyl-pyrrolidone and polyvinyl alcohol. The ribbons arecoated at a speed of 20 cm/s.

The plasma vapor deposition (PVD) techniqueused at the start of the project, in order to estimatethe potential of the sintered lamellar soft magneticcomposite (SL-SMC) technology, was a magnetronsputtering process using two magnetrons to coatboth sides of the ribbons. We know that this processis hardly a competitive mass-production method forthe automotive industry. As soon as the concept wasproven, our efforts were next directed towardsdeveloping an economical sol–gel method, using thecheapest precursors. Aluminum isopropoxide is nowan abundant product, easily produced by recuper-ating the dross from the aluminum can recyclingprocess.23 For the original magnetron sputteringexperiments, two approaches were tried. For thefirst one, the alumina layers were produced bysimple deposition of an aluminum layer of 0.7 lm.After cutting the ribbons into particles, the lamellarpowders were boiled in water containing a low levelof an amine, to help in the transformation of alu-minum to its oxide/hydroxide. The second series ofexperiments was conducted in a reactive sputteringprocess, in which a small amount of oxygen wasadded to the vacuum chamber while sputteringaluminum with a pulsed DC voltage. After an initialionic etching of the ribbons, a first sublayer of purealuminum was deposited to assure good adhesion ofthe coating. Then, progressively, the partial pres-sure of oxygen was increased in the system to form

Lemieux, Guthrie, and Isac376

the desired oxide layer on the ribbons while keepinga high evaporation rate of the target (nonoxidized orpoisoned mode).24 The deposition was as follows:0.3 lm of pure aluminum, followed by 0.4 lm ofreactive sputtering. As can be seen in the processflow sheet in Fig. 1, the resulting layers arereheated during the subsequent processes and havethe chance to form a stable stoichiometric oxide ofaluminum. Some experiments were also conductedwith magnesium targets, but it proved close toimpossible to reach stable, high deposition rateswith pulsed DC reactive sputtering, particularlywith the use of two magnetrons on both sides of theribbons. Trials with direct deposition of a layer ofmagnesium did not give good results after an oxi-dation process in water, or at high temperature inair. It is well known that the Pilling–Bedworth ratiofor magnesia on magnesium is less than 1, giving aporous layer that is unsuitable as an electricalinsulation and diffusion barrier. As seen in Fig. 1,after the coating operation, an optional grain-coarsening thermal treatment can be applied beforecutting the particles. This was not used in thepresent study. Figure 2 shows a batch of lamellarpowders, together with a few parts produced usingthis new SL-SMC process.

We verified the extent to which high sinteringtemperatures during the final thermal treatment ofthe parts might affect the resistivity of the thinlayers of alumina coating the steel. The followingsimple calculations were made. Given that the dif-fusion coefficient of iron in the alumina layer at

around 1000�C is on the order of 10�16 cm2/s,25 thenthe ‘‘diffusion distance’’, x, which represents thedistance at which the concentration reaches half ofthe original one (C = C0/2), is given by

x �ffiffiffiffiffiffiffiffiffi

D � tp

; (1)

where D is the diffusion coefficient of an atom in thelattice and t is time. The Arrhenius equation is usedto calculate the diffusion coefficient as a function oftemperature as

D ¼ D0 exp�Q

RT

� �

; (2)

where D0 is the maximum diffusion coefficient (atinfinite temperature), Q is the activation energy fordiffusion in dimensions of energy per unit mass ofmaterial, R is the gas constant (energy and tem-perature per unit mass), and T is absolute temper-ature. This equation tells us that, for every increaseof 100�C, the diffusion coefficient is increased byapproximately three times. At 1175�C, it is close toten times higher than at 1000�C, giving an affectedzone of 0.1 lm with pure iron, for 1 h of treatment.For 3 h at 1175�C, the known optimal treatment forthe Fe-Ni alloy, roughly 0.2 lm is affected, and thisstarts to be important. For Fe-3%Si alloy, sometreatments were done at 1350�C for 10 min. Fol-lowing the same rule, we can say that the diffusioncoefficient at that temperature is roughly five timeshigher than at 1200�C, giving 0.5 lm affected in 1 h,or 2.5 times less in 10 min. If we take into account

Fig. 1. Flowchart of the new process to produce sintered lamellar soft magnetic composites (SL-SMCs).


the heating and cooling time above 1000�C to reach1350�C at 5�C/min, we can evaluate the affectedzone as approximately 0.35 lm, or half of the typicalsol–gel insulating layer formed, which is far fromnegligible. However, for sintering conditions up to1250�C, and times less than 30 min at temperature,the insulator should be resistant to metal diffusion.

A special grain-coarsening thermal treatmentwas sometimes tried with pure iron at the end of thesintering cycle. Parts were simply oscillated slowlya few times above and below the alpha–gammaphase transformation temperature of pure iron at912�C. This heat treatment leads to a great increasein grain size.26 The treatment contained a minimumof three ferritic–austenitic phase changes and isnamed ‘‘Osc’’ (Figs. 3 and 4).

For mechanical and magnetic characterization,standard rings and bars were produced. The barsmeasured 31.8 mm 9 12.7 mm 9 6.4 mm, being so-called transverse rupture strength (TRS) speci-mens. They were used to determine the mechanicalproperties of the parts, as pressed (green strength)and fully processed (forged/sintered). The three-point bending test was used, as described by MPIFstandard 15 and 41.27

Magnetic characterization was done on rings withouter diameter of 53.6 mm, inner diameter of40.6 mm, and height of 6.25 mm. DC and AC mag-netic characterization was done on a KJS AssociatesHysteresis Graph (model ACT-500, SMT-500,7385K Fluxmeter), according to ASTM standard A773.28 For AC characterization, 250 turns of 24AWG wire and 250 turns of 30 AWG wire, respec-tively, were used for the primary and secondarywindings, while 450 turns and 150 turns, respec-tively, were used for DC characterization. For theshaping of the parts in the present study, an

atomized polyethylene wax was used as lubricant indifferent amounts, and die wall lubrication with aZnSt aerosol was also used.

Fig. 2. Powder produced with the new process and example of part pressed for characterization or motor teeth prefrom ready for forging.

Fig. 3. Losses, permeability, and coercive field (910) as a functionof the sintering temperature and conditions for sample 3 (four thinlayers, DPDS, 7.32 g/cm3).

Fig. 4. Losses, permeability, and coercive field (910) as a functionof the sintering temperature and conditions for sample 4 (four thinlayers, 0.5% BN, DPDS, 7.34 g/cm3).


To evaluate the influence of different processingparameters on the magnetic properties of the com-posites, the following parameters were varied:

(a) Composition/ribbon thickness: pure iron/75 lm, Fe-3%Si/125 lm, Fe-48%Ni/50 lm, andFe-48%Co-2%V/50 lm

(b) Shaping process:

(i) Cold pressing at 830 MPa(ii) Double pressing–double sintering (DPDS)

with a first step of cold compaction at690 MPa, followed by a delub/stress reliefthermal treatment at 900�C for 30 minunder hydrogen, and a second cold compac-tion step at 830 MPa

(iii) Hot forging (HF) with a first cold pressingoperation at 830 MPa, followed by anoptional delubing/curing treatment in airat 525�C for 15 min, followed by preheatingat 1000�C in argon for 4 min, then hotcompaction with MoS2 or BN aerosol lubri-cants, at speed of 25 mm/s and compactingpressure of 550 MPa.

(c) Final sintering treatment: under pure hydrogen(less than 4 ppm oxygen), a first delubingplateau at 700�C for 15 min, followed by themean plateau at elevated temperature, withheating and cooling rates of 5�C/min and tem-perature varied between 850�C and 1210�C

(d) Additives:

(i) BN: fine hexagonal structure boron nitridepowders, commercial grade PHPP325B, fromSaint Gobain Ceramics, D50: 3 lm

(ii) Al: air-atomized pure aluminum powders,grade AM650-PM, supplied courtesy ofAMPAL Inc., a subsidiary of United StatesMetal Powders Inc., D50 = 100 lm

RESULTS

Magnetic Results with Different Alloysand Additives

Table I reports typical results for the differentprocessing conditions. The first column gives thealloy system used, and the second gives the numberof coating layers applied and an indication of eachlayer thickness depending on the viscosity of the soldeveloped (thin, medium, or thick). The fourth col-umn indicates the shaping process used by givingthe pressure for a simple cold pressing operation, orDPDS for a double cold pressing operation sepa-rated by a stress relief treatment, or forged for theless compressible alloys.

It can be seen that, by adjustments to the coatingthicknesses and sintering conditions, one can createany composite with properties ranging from fullydense and sintered DC magnetic material to thebest composite for AC applications. Indeed, for ironin particular, sample 1 had a very thin coating and T

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was sintered at 1150�C, giving it a relative perme-ability above 5000, but with losses approachingvalues typical of any powders not insulated at all.On the other side, sample 3 with four layers ofcoating and sintered at a very high temperature stillregistered losses, at 60 Hz, of under 3.5 W/kg, whilehaving permeability above 1000 and coercive fieldaround 90 A/m. This is exceptionally good for aniron part. Part 5, which includes 0.2% aluminum(that should have completely diffused into the ironparticles), exhibited a coercive field of only 75 A/m.The aluminum could have helped to improve thecoercive field of pure iron. The coercive field alsobenefits from a transient liquid phase that helps tocreate better bridges, or magnetic couplings,between particles.19 It is interesting to see that thepresence of a good conductor additive such as alu-minum does not significantly harm the insulation ofthe composite with iron particles. With the otheralloys, in addition to not lowering the insulation atall, it allows one to reach the best coercive fieldvalues, even at a very low temperature for sinteringtreatments such as 920�C (see sample 9 or 12). Itwould be interesting to try other low-melting-tem-perature additives to see whether the same resultscan be obtained, or whether it is only aluminumthat results in such phenomena. In past experi-ments, we found that an undercoating of aluminumunder the alumina layer was beneficial. However,this coating needed to be done by a physical vapordeposition process, which increased the cost of theprocess. If the same result can be obtained with asimple mixture of standard-grade aluminum pow-ders for powder metallurgy (D50 50–100 lm), itopens the door to an economical and easy process toobtain interesting improvements. It is thought thataluminum helps to correct any defects formed in thealumina coating, and could even increase the duc-tility of the coating during the deformation of thepart, by creating an extremely low partial pressureof oxygen in the alumina coating in contact with it.This creates voids in the alumina coating structurethat result in an important increase in oxygen atomdiffusion in the alumina and its ductility. Theremaining free aluminum can diffuse into the ironmatrix after the pressing operations, where itshould improve magnetic properties slightly. Higherconcentrations of aluminum are under investiga-tion.

Finally, regarding additives, addition of fine bor-on nitride powders to the mix (sample 4) did notseem to improve particle insulation for pure iron,but it did help with Fe-3%Si (sample 8) and Fe-Co(sample 14). It also did not harm the DC properties,since the coercive field value still remains at 100 A/mand the permeability is above 1000 for pure iron, thebest permeability being obtained with BN forFe-3%Si. BN, being expensive, is not a very inter-esting additive to try at higher concentrations. Italso acts to decrease the concentration of ferro-magnetic material within the composite.

As a general observation, we see that pure ironand Fe-Ni are so compressible that the final densityof the parts is not improved using a double pressingoperation, or by pressing at higher pressure. Thedensity is related more to the coating thickness thatlimits the final density than to the pressing opera-tion used. The precision of the density measurementby water displacement on those rings was limited to±0.03 g/cm3. The value of the maximum inductionreached is sometimes a more revealing indication ofthe density reached than the measurement itself(see samples 2 and 3). Sample 3, with its thickercoating, appears less dense than sample 2, as shownby its maximum induction, and contrary to thedensity measurement with its uncertainty.

Overall, it is worth noting the tremendous effectof the coating on the particles, even if it is not totalencapsulation. Indeed, it changes the losses at 400Hz from 465 W/kg for sample 1, to only 28.5 W/kgfor sample 3. This dissipates any concerns that theencapsulation of particles should be total. For com-parison purposes, the best grades of SMCs on themarket have losses at 1 T of induction of 10 W/kg at100 Hz and 46 W/kg at 400 Hz. Hoganas claimedrecently to have developed a grade giving losses of 6W/kg at 60 Hz and 32 W/kg at 400 Hz.29 Pure Fe SL-SMC has still lower losses than those values, whileretaining permeability values at least twice as high.Moreover, SL-SMC parts such as sample 3 followprocessing conditions similar to any regular powdermetallurgy structural part, being single or doublecold pressed, and sintered at 1200�C, under areducing atmosphere. As such, this new SL-SMC isvery convenient for the powder metallurgy industry.As can be seen in Fig. 3, under certain conditions,properties of pure iron parts continue to improve,and losses even decrease, as the sintering temper-ature increases. To date, no treatments have beenrun above 1210�C.

In general, as expected, the lowest losses, thelowest coercive fields, and the highest maximumrelative permeability were obtained with the Fe-Nialloy, followed by Fe-3%Si and then pure iron. Thehighest maximum induction is reached with the Fe-Co alloy, followed by pure iron and Fe-3%Si. Sur-prisingly, Fe-3%Si had maximum induction valuesup to above 1.7 T, very close to those for the pureiron SL-SMC. This shows that silicon addition of upto 3%, even if it is not a ferromagnetic, is not verydetrimental to the induction. The best compromise,in terms of maximum induction and minimum los-ses for applications such as power-trains in thetransportation sector, seems to be the Fe-3%Si alloy.Maximum relative permeabilities close to 5000 canbe reached with this alloy, compared with well be-low 1000 for current SMCs on the market. We mustalso underline that typical properties for wroughtFe-3%Si at full density, like the best laminations forelectric motors, are permeabilities around 5000,coercive fields around 40 A/m, and a maximuminduction at an applied field of 12,000 A/m in the


range of 1.7 T. It is noteworthy that the SL-SMCprocess does not involve any compromise for thoseDC magnetic properties, when processed in the bestconditions. The improvement of the DC magneticproperties (hysteresis curve) for Fe-3%Si comparedwith pure iron explains overall why the total lossesare lower with this alloy, potentially well under 2W/kg, i.e., close to 50% lower than figures for pureiron. Fe-3%Si is also roughly five times more elec-trically resistive,30 giving it a better chance toobtain lower eddy current losses. This statement ishowever mitigated to some extent by the fact thatsome parts can exhibit losses exceeding 15 W/kg at60 Hz and 400 W/kg at 400 Hz, when improperlyinsulated (results not presented here). Similarly,poor insulation with pure iron SL-SMCs can gen-erate losses above 30 W/kg at 60 Hz and above900 W/kg at 400 Hz. Such parts were processedusing steel strips or ribbons that were either poorlycleaned of their rolling oils before applying the sol–gel coating or had received too thick a sol–gel coat-ing that peeled off when the ribbon was cut intoparticles.

Effect of Processing Parameters

The following graphs show the trends for themaximum induction (at applied fields of 12,000 A/mand 10,000 A/m), permeability, losses, and coercivefields of the parts as a function of apparent densityor sintering temperature, for the various coatingsand pressing conditions. In Figs. 5 and 6, we seethat for pure iron, as for all alloys studied, all partsfollow a general trend for improvement withincreasing part density. There is a very high corre-lation, apart from those samples with additives orthose which cracked during the second densificationprocess. Cracks were sometimes formed because therings had to enter the same die as for the firstpressing operation, even though their inner diame-ter was slightly smaller than that of the core rod.

Regarding additives, as for any DC magneticparts, the presence of nonmagnetic additivesdecreases the maximum induction. As can be seen

in Table I, this trend is not present for the energeticlosses. Losses are more related to the sinteringtreatment strength, or the amount of diffusion be-tween particles. Concerning the effects of thermaltreatment, there is an improvement in the proper-ties owing to the relief of all remaining stresses inparts from room temperature to the common stressrelief temperature of about 850�C. This is not shownhere, as the graphs start above 900�C, but it is wellillustrated by the recent progress in totally encap-sulated water-atomized powder composites on themarket.29 In fact, as the resistance of the coatingmaterial applied to the particles was improved, theindustry succeeded in retaining good insulationvalues up to the stress relief temperature of steel.This allowed for an important decrease in the hys-teresis losses by improving the coercive field and thepermeability. The coercive fields of water-atomizedSMCs was recently decreased from a few hundredA/m to 120 A/m, and the permeability was increasedfrom �300–400 to �800. In the case of SL-SMCs,with particles partially insulated with purealumina, this trend continues as the temperatureincreases, as can be seen in Figs. 3 and 7.

In Fig. 7, the coercive field is around 140 A/m(right vertical axis 910) at sintering temperature of

Fig. 5. Maximum induction at applied field of 12,000 A/m (uppercurve) or 10,000 A/m (lower curve) of parts made with pure iron rib-bons processed in different conditions as a function of their density.

Fig. 6. Maximum permeability of a selection of parts made with pureiron ribbons processed in different conditions as a function of theirdensity.

Fig. 7. Losses, permeability, and coercive field (910) as a functionof the sintering temperature for a sample similar to the sample 2 inTable I (four thin layers, pressed at 60 tsi, 7.17 g/cm3)


920�C. It decays close to linearly until 90 A/m at1200�C and becomes much lower at 1210�C. Per-meability values follow the same trend, increasingfrom �1020 to 1150 and 1275. These two parame-ters reveal a constant decrease in hysteresis lossesas a certain degree of sintering occurs betweenparticles. This is not related to the relief of internalstresses because the temperatures are too high forany residual stresses to remain. However, this sin-tering between particles will also decrease theoverall insulation or electrical resistivity of thecomposite, and the improvement in DC magneticproperties, or hysteresis losses, should eventuallybe outweighed by an increase in eddy current losses.This is the other important part of the total losses ina magnetic part submitted to an alternating elec-trical field. In fact, we see that this crossing pointwhere the effect of the improvement in hysteresislosses is exceeded by an increase in eddy currentlosses occurs around 1175�C to 1200�C, according toFig. 7. This crossover occurs at different tempera-tures depending on the efficiency of the coating andthe density of the part. This can be seen with theFig. 3 reporting the same properties for sample 3 inTable I. In fact, the part in Fig. 7 has a low density,only 7.17 g/cm3, but part 3 in Fig. 3 has 7.32 g/cm3,a typical density obtained for pure iron SL-SMCdouble pressed (DPDS). This part has globally lowerlosses, and its crossover point has not yet beenreached, even at 1210�C, while losses are stilldecreasing. This can be explained by the greaterpotential for improvement in hysteresis losses forhigher-density parts. This assumption is confirmedby the more important improvement in permeabilityand coercive fields, both starting from worse valuesat 920�C than for the part in Fig. 7 (�140 A/m), butreaching the same value at 1200�C (�90 A/m). Interms of hysteresis losses, increasing sinteringtemperature brings the performance of the twoparts together.

Regarding the effect of the oscillating thermaltreatment around the phase change temperature ofpure iron applied during the cooling of the partsfollowing the sintering treatment to encouragegrain growth, parts 3 and 4 show important de-creases in hysteresis losses, and therefore in totallosses (see the circled points in Figs. 3 and 4). Thistreatment was applied for the repeated sinteringtreatment at 1120�C. Indeed, a first treatment wasapplied at 1120�C without the oscillating treatment.After testing the part, it was unwound, andretreated, this time with the oscillation treatment atthe end. We see that all the DC magnetic properties(or hysteresis properties, Hc, and lmax) wereimportantly improved with the grain growth treat-ment. Finally, a negative effect on lmax and even Hc

is seen as the sintering temperature was increasedfor sample 4. This is probably due to the presence of0.5% BN, and can be explained by the productreacting with oxygen or humidity released from theiron particles during the reduction of iron oxides.

Indeed BN above 900�C, in the presence of oxygen,transforms into boric acid, and the released boroncan then diffuse into the iron matrix, harming thepermeability and the coercive field.

Regarding Fe-3%Si alloy, which has very lowcompressibility and must be forged to reach inter-esting densities for magnetic applications, the opti-mum sintering temperature also seems to be around1200�C, even with the use of BN (sample 8, Table I).In fact, in Fig. 8, inflection points are visible at1200�C for the parts containing BN, or a smallamount of aluminum. It is remarkable that thealumina coating resists such aggressive sinteringtreatments, particularly following the high levels ofdeformation produced during the forging step. Alu-minum additions definitely have good potential forimproving mechanical properties without harmingother properties, or even while improving the insu-lation of particles. Further tests are underway tovalidate the beneficial effects of aluminum addition.BN additions are also not detrimental to anymagnetic properties, while improving losses withFe-3%Si. The best results come from a part with0.75% BN in Table I (part 8). Its permeability ap-proaches those of the best laminations (4722). It isthought that, contrary to pure iron, BN cannot betransformed with Fe-3%Si, even at high tempera-tures, since any oxygen released through iron oxidereduction is trapped by the silicon content of thealloy and transforms into silica. Thermodynami-cally, SiO2 formation is favored over BN decompo-sition. However, conditions to reduce SiO2 can bepresent above 1175�C, depending on the dew pointof the gaseous atmosphere. This could start to de-crease the insulation of particles, if SiO2 has con-tributed towards insulating them during thedifferent thermal treatments applied to the parts.This would explain the inflection points at 1200�C.In fact, Fig. 9 (left), giving the oxide-reductioncurves of SiO2 for different sintering conditions,shows that, given a good dew point of �60�F(�51�C) that can be obtained in typical industrialsintering furnaces for magnetic materials, SiO2 insolution in a powder metallurgy compact wouldstart to be reduced at around 1200�C (2200�F). The

Fig. 8. Losses as a function of sintering temperature for varioussamples.


tubular furnace used during the present experi-ments certainly had this dew point or an even lowerone. The hydrogen bottle used had less than 4 ppmoxygen, and only pure alumina tubes and platenswere used when sintering Fe-3%Si.

Another important point for the forged Fe-3%Siparts is the effect of any residual carbon in theparts. Since a forging operation is used to increasethe apparent density of the parts, it was importantto know whether a delubing treatment prior to theforging operation was needed to decrease the carboncontent, before closing the porosities with furtherdensification. It was clear that parts at low densi-ties, such as the Fe-3%Si green parts, need to bemanipulated as little as possible owing to their verylow green strengths.31 In fact, green strength whenpressed at 830 MPa was only around 3 MPa(450 psi) under the best conditions at 6.6 g/cm3,compared with 70 MPa (10,500 psi) in the bestconditions for pure iron at 7.3 g/cm3. As such, itcould be interesting to avoid a delubing process.

Figure 9 (right) was used as a base graph fromthe literature. Carbon level measurements wereplotted against corresponding coercive fields. Onecan see, first of all, that the present results aregenerally in good agreement with literature data,32

except for the slightly better values of coercive fieldsin this work. This is probably due to the advantagesconferred by the lamellar-shaped particles. Itshould be underscored that a comparison of thepresent results is made against noninsulated,water-atomized standard particles, fully sinteredunder the best industrial conditions, and destinedfor DC magnetic applications. It is therefore note-worthy to be able to achieve such results with acomposite designed for AC applications. The dots,

from the least contaminated condition reported tothe most contaminated condition, correspond to:

1. Curing of cold-pressed parts in air at 525�C for 15min, as is commonly done for Fe-3%Si SL-SMCparts reported in this paper. After the forgingoperation, a sintering treatment follows at1120�C under pure hydrogen, for 10 min. Thiscondition corresponds, for example, to sample 6reported in Table I. All other parts reported inthis paper have thus better, or equal, carboncontents, since they were cured in air, in additionto receiving a higher-temperature sinteringtreatment after forging.

2. No curing in air before forging, sintering at1350�C for 10 min. No parts reported in thispaper were processed with this condition. It wastested only to see what was needed to decarbu-rize the part as completely as when curing in airwas applied. We see that, even above 1350�Cunder pure hydrogen, decarburization is not asgood as applying a regular sintering treatmenton a part that has previously been burnt in air at525�C.

3. No curing in air. After forging, a sinteringtreatment at 850�C for 30 min in pure hydrogenatmosphere.

4. No curing in air. After forging, a sinteringtreatment at 850�C for 30 min in a 10% H2/90%Ar atmosphere.

Mechanical Properties

Table II reports the densities and TRSs of thecomposite parts formed, using the different alloysstudied, and depending on their processing condi-

Fig. 9. Left: dew point required to reduce SiO2 pure ceramic, or in solution in iron, as a function of sintering temperature and for different reducingatmosphere compositions. Right: coercive field (Hc) as a function of the carbon and nitrogen content in a Fe-3%Si regular powder metallurgysintered part. Dots added to the original graph are from this work with SL-SMC using Fe-3%Si base alloy.32


tions. The results show that all these compositeshave sufficient strength to withstand any magneticapplications. We can see that, for the forgedFe-47.5Ni alloy, when the temperature is increasedfrom 850�C to 1175�C, the TRS value nearly triples,indicating that a strong sintering bond existsbetween the metal components of the SL-SMC. Thiscombination of treatment and deformation istherefore probably too aggressive for the insulatinglayer of Fe-Ni parts. Optimum magnetic propertiesfor AC application are thought to be obtained withonly slight metallic bonding between particles inorder to keep the resistivity as high as possible.

Present SMCs on the market, formed from irreg-ular ‘‘water-atomized’’ powders completely encap-sulated and joined by an organic or an oxide matrix,have TRS values on the order of 40 MPa to100 MPa. Forged or DPDS SL-SMCs sintered atoptimum temperature for their magnetic properties(�1175�C) exhibit superior TRS values. Aluminumaddition seems to give slightly improved mechanicalproperties at low sintering temperature, but testsare still ongoing regarding additive additions. Tin orcopper infiltration or additions will also be investi-gated in the future. Impregnation with resins willalso be tested to increase strength in large pucksand facilitate prototyping by machining. In general,regarding density values presented in this, or in theother table, if one takes into account the presence ofthe less dense insulating coatings (theoretical den-sity of the mix), close to no porosity remains in theforged, or DPDS parts. Those values are signifi-cantly higher than common SMCs in the market-place, enabling higher maximum induction values.Future efforts will be oriented towards retainingthose values, while limiting the deformation ofparts, rather than increasing it.

MODELING AND PERFORMANCEESTIMATION FOR POWER-TRAINS

As has been well demonstrated by Professor A.G.Jack’s research group over the past 15 years, goodredesigns of rotating electric machines are possible,

making better use of space by replacing the two-dimensional (2D) lamination stackings. By usingmore isotropic SMCs, and permanent magnets, it ispossible to decrease the volume and the weight usedin the machine by 30% to 40%, for the same poweroutput.9,10,12,13 Space and weight savings aremainly related to an important decrease and betterorganization of the copper windings around theferromagnetic parts. Performance of electric motorscan also be maintained even using materials havingten times lower maximum relative permeability,and also lower maximum induction, largely inrelation to the savings in Joule energy, or electricalresistance heating losses, caused by the copperwindings. Copper losses, which often account forhalf or more of the total losses in a motor, are cut byan amount that is closely proportional to its volumedecrease (up to 40% in some designs). Such new 3Ddesigns with permanent magnets using SMCs havealso been developed by the Cros and Viarougeteam,11,14,15 and by others. However, none of themtried to develop new 3D designs without the use ofpermanent magnets, i.e., with ferromagnetic partsfor the stator and the rotor. The following resultsand calculations using regular two-dimensionaltopologies, with and without permanent magnets,explain why efforts with water-atomized SMC haveonly been directed towards motors using permanentmagnets.

The total performance of an electromechanicaldevice such as a motor, or an alternator, is depen-dent upon its total magnetic permeability, accordingto the following equations33:

f ¼ N � I ¼ H � L ¼ B � Lltot

¼ / � <tot; (3)

and

<tot ¼ <1 þ <2 ¼1

A

L1

l1

þ L2

l2

� �

¼ Ltot

Alequ

; (4)

where f is the magnetomotive force (ampere-turns,A), N is the number of copper winding turns, I is the

Table II. Mechanical properties and density of the composite parts produced

AlloyAfter ForgingT (�C)–t (min) Density (g/cm3)

TRS, MPa (psi)

Mean SD

Fe 1175–15 7.62 392 (57,700) 35 (5166)Fe-3Si 1175–15 7.39 126 (18,600) 5 (725)Fe-47.5Ni 850–30 7.90 266 (39,100) 41 (5940)Fe-47.5Ni 1175–15 7.88 674 (99,150) 50 (8660)Fe-49Co-2V 920–30 7.46 223 (32,800) 5 (725)After DPDS

Fe 1175–15 7.62 120 (17,660) 5 (725)Fe-47.5Ni 1175–15 7.90 136 (20,000) 5 (725)

SD, standard deviation


electric current (A), H is the magnetic field strength(A/m), L1, L2, or L is a partial or the total magneticcircuit path length (m), B is the magnetic flux den-sity (T or Wb/m2, or kg/A s2), l1, l2, or lequ is themagnetic permeability of a part of the circuit or itstotal equivalent permeability (=l0lr, H/m or m kg/s2 A2), / is the magnetic flux (Wb or kg m2 A�1 s�2),<1, <2, and <tot are the partial or total reluctance ofthe circuit (H�1 or A2 s2/kg m2), and A is the sectionof a magnetic flux path (m2).

These equations state that, for a fixed magneto-motive force, a higher permeability will give ahigher magnetic flux density, and thus a highertorque, or power. We see also that reluctance, andnot permeability, is summed in a magnetic circuit.From this perspective, the way to estimate theequivalent permeability, lequ, for a multicomponentcircuit, follows from Eq. 4 as

lequ ¼l1l2 L1 þ L2ð Þl1L2 þ l2L1ð Þ : (5)

For an air gap, the magnetic relative permeabilitylr is 1. A permanent magnet behaves as an air gapregarding its magnetic permeability, since its polar-ization is fixed. For the soft magnetic materials of astator or rotor, lr depends on the material (Table III).If one computes values for the different magneticmaterials in a typical 1-HP permanent magnet mo-tor, with a large equivalent gap, including the mag-nets (6.25 mm), or an alternative current motor witha very narrow gap (100 lm), both motors having atotal magnetic path length of 15 cm, one obtains theequivalent permeabilities presented in Table III forthe different motors (Eq. 5).

From these results, one sees that, for permanentmagnet motors, a decrease in the soft magneticmaterial’s permeability is not too critical for overallmotor performance. The water-atomized SMCs leadto only a 4% decrease in the equivalent permeabilityof the whole magnetic path, compared with lami-nations which have a ten times superior maximumrelative permeability. However, for a smaller gapmachine such as an AC motor, the handicap is tre-mendous, decreasing the permeability by a factor 3,as compared with lamellar composites. The lamellarcomposite suffers only an 11% decrease in equiva-lent permeability compared with the standard M19lamination steel. These results illustrate why sin-tered-lamellar SMCs could potentially be used in

any motor configuration versus water-atomizedSMC, which are limited to permanent magnetmotors. With this comparison in mind, one maywonder what could be the potential improvementachieved by using SL-SMCs in terms of volume andweight in a motor without permanent magnets, ifwater-atomized SMCs gave access to 40% smallerand lighter motors (with permanent magnets),while having such inferior magnetic properties ascompared with lamination stacking. A seriousredesign exercise, similar to that achieved by Jackand Viarouge, must be done for motors withoutpermanent magnets, using the properties ofSL-SMCs. This work becomes important with theincreasing shortage and prices in the permanentmagnet industry these days.

Modeling work was also done in a recent 3Dtopology machine using permanent magnets.34

Figure 10 shows a cut of the stator and rotor of themachine modeled. This optimized machine for iso-tropic water-atomized SMC, a claw-pole transverseflux design using flux concentrators, was modeled tosee if there is an advantage of using SL-SMCs ra-ther than fully isotropic SMCs. This kind of largemachine can be used in wind turbine generators, orfor large power-trains in the transportation sector.They will necessarily result in SL-SMCs being usedin their worst direction during a portion of the cycle.In fact, SL-SMCs have particles oriented in a pref-erential direction, perpendicular to the pressingaxis of the parts. In addition to also being the casefor regular lamination stack parts, these kinds oftopologies are practically impossible to produce withlaminations due to economic restrictions. Everylayer of steel sheet will, in fact, have to be cut dif-ferently from its neighboring layers.

The properties of the SL-SMC were measured intheir best and worst directions to be entered into themathematical model following the Bertotti or clas-sical loss equations.35 Comparison with other iso-tropic SMCs, where electrical resistivity can beevaluated with a simple four-point technique(�15,000 lX-cm), allowed us to determine that theSL-SMC made with Fe-3%Si had electrical resis-tivity 150 times higher in its best direction (7500lX-cm) and three times in its worst direction, com-pared with the base material used. Analysis of themodeling results allowed determination that, evenin those designs optimized for isotropic SMCs, using

Table III. Equivalent magnetic permeabilities of regular 2D topologies permanent magnet motors (PM) andAC motors (AC) with a 15 cm flux path

Soft MagneticMaterial

RelativePermeability (lr)

PM EquivalentRelative Permeability (lr)

AC EquivalentRelative Permeability (lr)

SMC water-atomized 500 23.5 375SL-SMC forged (Fe-si) 3000 24.5 1155Lamination M-19 5000 24.6 1305


SL-SMCs in the flux concentrators (stack of mag-nets/SMC in the rotor) and retaining regular SMCsin the stator feet to complete the C cores made withlaminations, a machine with the SL-SMC will have‘‘no-load’’ core losses in SL-SMC concentrators 12%lower than if using SMC for the concentrators. Theapparent power developed by the machine will alsobe increased by 13.6%. This is mainly due to anincrease of magnetic flux in the air gap between thestator and rotor. Even better results would havebeen obtained by replacing the stator feet withSL-SMCs as well, since losses increased in thoseparts due to an increase in the magnetic field.

One then sees that, in addition to potential sav-ings of up to 30% to 40% in volume or weight byusing SMCs, mainly due to the amount of coppersaved in the machine, use of SL-SMCs will allow forat least a 10% more powerful machine for the samevolume. All these potential improvements are veryimportant for the transportation sector, whereevery weight decrease is crucial.

CONCLUSIONS

The SL-SMC process to date typically gives thefollowing properties, respectively, for pure Fe,Fe-3%Si, Fe-47.5%Ni, and Fe-Co:

� Losses at 1 T 60 Hz of 3 W/kg to 5 W/kg, 1.5 W/kgto 2.5 /kg, 0.9 W/kg to 1.5 W/kg, and 4.5 W/kg,

� Bmax at 12,000 A/m of 1.55–1.65 T, 1.60–1.70 T,1.3–1.5 T, and 1.85–2 T

� lmax of 1000–1250, 1000–5000, 2000–16,000, and1350.

The optimum sintering temperature for the gradesis as follow:

� Pure iron without additives: around or above1200�C; with aluminum powders: not determinedyet; with BN: around 1120�C, but BN is mostlyuseless

� Fe-3%Si with or without additives: around1175�C to 1200�C

� Fe-Ni, pure or with any of the additives tested:1150�C to 1175�C

� Fe-Co, pure: above 1200�C (not known yet); withBN: 1175�C

The grain growth oscillation thermal treatment,around 910�C for pure iron SL-SMC, has a tremen-dously beneficial effect on properties. This effect isobvious at 1120�C, as it can cut losses by up to 30%.

� Maximum induction is a linear function of den-sity. Higher density values are obtained withDPDS and by forging.

� SL-SMC gives the same level of maximum induc-tion as standard lamination of the same alloy, atcomparable density or stacking factors. They havehigher values than other SMCs on the market.

� SL-SMC gives permeabilities doubled comparedwith other SMCs, close to values given bywrought alloys in their best condition. Thisproperty is very process sensitive, improving withgrain size, density, purity, and particle jointstrength, and thus with longer thermal treatmentat higher temperature.

� Hc values typical of the best laminations on themarket, less than half of other SMC.

� Losses less than half of other SMCs at 60 Hz,twice those of the best lamination on the markettested with Epstein frame. Epstein frame test forlamination does not reveal consolidation of stack-ing effects like edge welding or residual stress.

� Losses are similar at 400 Hz for SMC, SL-SMC,and lamination stacking (M15).

� SL-SMC exhibits ductile behavior, and mechani-cal properties ranging from values reached withthe best SMC (100 MPa) to close to those reachedwith the same wrought alloys (700 MPa), depend-ing on the consolidation process and the sinteringtreatment selected.

� For all the alloys studied, improvements are stillpossible through development of better perform-ing coatings, use of additives, or infiltrationduring sintering that could form a liquid phase(transient or not), such as the aluminum powdersused in the present study. Aluminum contentswill be pushed up to 2% to further evaluate itspotential and will be added also as infiltrate afterpressing rather than as an additive. Copper-, tin-,and nickel-based commercial brazing alloys willalso be tested for the infiltration process. The goalis to enhance diffusion between particles at lowersintering temperatures without harming the effi-ciency of the alumina coating. Aluminum powderadditives clearly showed no harm in terms ofelectrical resistivity and could potentiallyimprove the DC magnetic and mechanical prop-erties. More tests are needed to validate thispotential. It is not clear whether the effect of thepowders could have the same impact as anevaporated underlayer of aluminum under thealumina coating, as was done with PVD technol-ogy in the past.

� SL-SMC did not show any significant deteriora-tion of properties as the sintering temperaturewas increased up to 1210�C. Properties need to bestudied at higher temperatures.

Fig. 10. Clawpole Transverse Flux Machine with a hybrid stator(Laminated C-core and composite feet).34


� With its high permeability and induction values,the SL-SMC process is capable of supplyingcomposites with only a minor decrease in perfor-mance compared with conventional 2D designmotors without permanent magnets with thin airgaps such as induction machines. These designswere developed for lamination stacking technol-ogy more than 100 years ago. If designs arereviewed, as was done for permanent magnetmotors using SMC recently launched to themarket, to take full advantage of the 3D designenabled by the composites, SL-SMC could producemotors without permanent magnets with greatlyimproved level of performance and power densi-ties. Similarly to the results obtained with SMCin new 3D optimized designs with permanentmagnets, important weight and volume decreasescould be obtained. These new developments areparticularly important given the shortage insupply of rare-earth elements.

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