ma e ials
A icle
S ibeck Cu e o Magne o heological Fluid
wi hin Pin-on-Disc Con igu a ion:
An Expe imen al In es iga ion
Jakub Roupec, Filip Jeniš, Zbynˇek S ecke , Michal Kubík * and Ondˇ ej Macháˇcek
Facul y o Mechanical Enginee ing, B no Uni e si y o Technology, Technická2, 616 69 B no, Czech Republic;
[email p o ec ed].cz (J.R.); ilip.Jenis@ u b .cz (F.J.); [email p o ec ed].cz (Z.S.);
ond ej.Machacek@ u b .cz (O.M.)
*Co espondence: michal.kubik@ u b .cz
Recei ed: 1 Sep embe 2020; Accep ed: 16 Oc obe 2020; Published: 20 Oc obe 2020
Abs ac :
The pape ocuses on he coe icien o ic ion (COF) o a magne o heological luid
(MRF) in he wide ange o wo king condi ions ac oss all he lub ica ion egimes—bounda y, mixed,
elas ohyd odynamic (EHD), and hyd odynamic (HD) lub ica ion, speci ically ocused on he common
wo king a ea o MR dampe . The coe icien o ic ion was measu ed o MR luids om Lo d
company wi h concen a ions o 22, 32, and 40 ol. % o i on pa icles a empe a u es 40 and 80 ◦C.
The esul s we e compa ed wi h a e e ence luid, a syn he ic liquid hyd oca bon PAO4 used as a
ca ie luid o MRF. The esul s show ha a bounda y egime and empe a u e 40
◦
C all he luids
exhibi simila COF o 0.11–0.13. Di e ences can be ound in he EHD egime, whe e he MR luid
COF is signi ican ly highe (0.08) in compa ison wi h PAO4 (0.04). The COF o MR luid in he HD
egime ose e y s eeply in compa ison wi h PAO4. The e ec o pa icle concen a ion is signi ican
in he HD egime.
Keywo ds:
ic ion; S ibeck cu e; magne o heological luid; pin on disc; MR dampe ;
ibological pe o mance
1. In oduc ion
Magne o heological (MR) luid belongs in o he g oup o sma ma e ials enabling a change in an
appa en iscosi y in o e a g ea ange, and his swi ching be ween wo s a es can be eached wi hin
1 ms [
1
]. The luid is he suspension o mic on-sized pa icles, which a e usually made o pu e i on
because o hei a o able magne ic p ope ies. These pa icles ha e a globula shape, and hey a e
suspended in he ca ie luid, o ins ance, wa e , silicone oil, o , in mos cases, a syn he ic hyd oca bon,
say, polyalphaole in (PAO). When he magne ic ield is applied, he e omagne ic pa icles a e
conca ena ed in o chain o ma ion along he magne ic lux lines. The cohesion o chain o ma ions
causes an inc ease in he appa en iscosi y o MR luid [
2
]. This p ope y is mos o en exploi ed by
MR dampe s [
3
–
5
], b akes/clu ches [
6
–
8
], seals [
9
,
10
], o lexible s uc u es [
11
,
12
]. The applicabili y o
hese MR de ices in sma mechanical sys ems has se e al limi a ions. The main issues a e as ollows:
(i) he sedimen a ion s abili y [
13
,
14
]; (ii) an inc ease o MR luid consis ency du ing he MR luid
loading called an in-use- hickening [
15
,
16
]; (iii) and he ab asi eness and poo lub ica ing p ope ies
o he MR luid [
17
]. Cu en ly, he i s wo issues a e being su icien ly sol ed by using sui able
addi i es o p e en he sedimen a ion and oxida ion o i on pa icles.
The MR de ice li e ime is limi ed by wo majo sou ces o wea : (i) luid low wea and (ii) wea
inside he ibological con ac pai s by i on pa icles o addi i es. Wea due o luid low in he dampe
has been desc ibed in, e.g., [
18
]. Wea causes a dec ease in he equi ed o ce o o que due o an
Ma e ials 2020,13, 4670; doi:10.3390/ma13204670 www.mdpi.com/jou nal/ma e ials
Ma e ials 2020,13, 4670 2 o 11
inc ease in he gap size. Howe e , his p oblem can be sol ed by a sui able dampe con ol s a egy.
The wea o highe ic ion losses o ibological con ac pai s is a signi ican p oblem o MR de ice
li e ime and pe o mance. In he case o he MR dampe , se e al ibological con ac pai s can be
ound: sealing and guiding o (i) he loa ing pis on, (ii) he pis on, and (iii) he pis on od. Wea o
hese pa s can cause wo se unc ionali y exhibi ed as a highe ic ion o ce and MR luid leakage
om he de ice o in o he gas chambe in he dampe . The e o e, he basic knowledge o wea and
lub ica ing beha io o MR luid in di e en ibological con ac pai s can signi ican ly help in p ac ice.
Jolly [
19
] measu ed he coe icien s o ic ion o MR luid lub ica ed i on-on-i on con o mal in e aces.
He compa ed he ou ypes o MR luid om Lo d wi h d y ic ion (0.18), and he measu ed coe icien
o ic ion (COF) was an in e al om 0.04 o 0.07 o all samples. Song [
20
] es ed ma e ial pai s o
s eel-on-s eel and aluminum-on-aluminum. The no mal o ce was applied a h ee le els and a h ee
le els o speed o a ion. The COF was lowe o he highes speed and he highes load. The lowes
COF was measu ed o he s eel-on-s eel con igu a ion and he highes COF o he aluminum ma e ial
pai . Shah i a [
21
] measu ed S ibeck cu es o e o luids and MR luid MRF-132DG om LORD.
He used con igu a ion ball-on- h ee pla es om s ainless s eel AIS 316. The COF was inc eased om
0.13 o 0.19. Rosa [
22
] measu ed he S ibeck cu e and es ed he in luence o pa icle size. The olume
concen a ion o he pa icle was e y low, and i was se p ope ly o 1 ol. %. The esul s indica e
ha he COF is much lowe o MR luid wi h 0.4
µ
m pa icles han a MR luid wi h 1.3 and 2
µ
m,
which ha e almos iden ical COF. Zhang [
23
] measu ed COF o di e en pa icle olume con en
in comme cial MR luid om Lo d company. The COF we e almos iden ical (ca. 0.35) o 22, 32,
and 40 ol.% o CI pa icles. Leung [
24
] used, o wea and COF measu emen , block-on- ing geome y
equipped wi h a s i ing mechanism. He used a comme cial MR luid MRF-132DG om LORD as a
sou ce o CI pa icles and subs i u ed he o iginal ca ie luid wi h simila iscosi y as an o iginal
ca ie luid and wi h ou imes highe iscosi y. The measu ed COF o suspensions wi h highe
iscosi y o ca ie luid was ca. 0.065 and i was almos iden ical o high concen a ion o CI pa icles.
Fo lowe concen a ion i was signi ican ly lowe . The suspension wi h low iscosi y o ca ie luid
had iden ical COF o all concen a ions (ca. 0.08). I seems ha he ca ie luid wi h highe iscosi y
can o m su icien lub ica ing ilm, which can ensu e he sepa a ion o bo h su aces in con ac in o
such a le el ha he pa icles can mo e in he con ac mo e eely.
The in en ion o de ailed s a e-o -a syn hesis was he compa ison o he esul s om a ious
s udies. Only es s wi h MRF-132DG om Lo d company o luids wi h simila composi ion, pe o med
in modes pin-on-disc, block-on- ing o block-on-ball on h ee pla es we e e alua ed. F om hese da a,
he iscosi y o ca ie luids, he con ac p essu e, and he sliding speed o su aces in con ac om
se e al pape s can be calcula ed o es ima ed. In some cases, he sliding speed was jus es ima ed om
RPM. The iscosi y o a ce ain empe a u e du ing measu emen was also es ima ed. The con ac
p essu e was calcula ed as he a io o he no mal o ce o he con ac a ea calcula ed om he used
geome y acco ding o He z. The es ima ed ( ed) and calcula ed (blue) alues a e p esen ed in Table 1.
In an MR dampe , wo impo an s eel–s eel ibological con ac s a e ope a ing in he MR luid, namely,
he pis on od guide and pis on guide (no e ec o a magne ic ield). Conside ing a pis on eloci y in
he ange o 0.05 o 2 m/s, a la e al o ce in he ange o 50 o 1000 N, a pis on diame e o 32 mm, and a
pis on od diame e o 12 mm, he ange o a He sey numbe be ween 1.E-11 and 1.E-09 o he pis on
od guide and pis on guide was de e mined. I can be seen ha he e a e ela i ely ew s udies in he
usual wo king condi ion o an MR dampe .
The main goal o he pape is o de e mine he COF o MR luids in a la ge ange o a He sey
numbe s and o ocus mainly on he wo king condi ions in he MR dampe , which is essen ial o
p ac ical applica ions. The seconda y aims a e o desc ibe he e ec s o pa icles on COF in he la ges
possible ange o He sey numbe , he e ec o pa icle concen a ion, and he e ec o empe a u e.
Ma e ials 2020,13, 4670 3 o 11
Table 1.
Compa ison o esul s om published s udies ( ed: es ima ion alue, blue: calcula ion)
[20,21,23,25–29].
Au ho Geome y Rela i e
Speed [mm/s]
No mal
Fo ce [N] Con ac Pai Viscosi y
[mPa.s]
COF
[-]
Con ac
P essu e [GPa]
Sohn Block on ing 950 40 S45C/S45C 14.2 0.8 1.433
Song pin on disc 780 50 b ass/b ass 14.2 0.13 0.012
Song pin on disc
780 20 s eel/s eel 13.6 0.12 1.367
780 50 s eel/s eel 13.6 0.11 1.855
780 100 s eel/s eel 13.6 0.1 2.337
780 20 b ass/b ass 13.6 0.16 0.862
780 100 b ass/b ass 13.6 0.1 1.474
780 20 alu/alu 13.6 0.12 0.673
780 50 alu/alu 13.6 0.15 0.913
780 100 alu/alu 13.6 0.11 1.150
330 50 s eel/s eel 13.6 0.12 1.855
1120 50 s eel/s eel 13.6 0.11 1.855
330 50 b ass/b ass 13.6 0.12 1.170
1120 50 b ass/b ass 13.6 0.11 1.170
330 50 alu/alu 13.6 0.14 0.913
1120 50 alu/alu 13.6 0.13 0.913
Wong block on ing 1623 292 s eel/s eel 15.1 0.06 2.658
1623 292 s eel/s eel 15.1 0.08 26.585
Zhang pin on disk
31.25 1 alu/alu 13.6 0.5 0.156
31.25 5 alu/alu 13.6 0.25 0.267
62.5 1 alu/alu 13.6 0.5 0.156
62.5 5 alu/alu 13.6 0.25 0.267
Zhang pin on disk 31.25 10 s eel/s eel 15.1 0.3 0.683
31.25 10 alu/alu 15.1 0.3 0.336
31.25 10 b ass/b ass 15.1 0.25 0.336
Lee pin on disc 390 50 s eel/s eel 15.1 0.16 1.855
390 100 s eel/s eel 15.1 0.2 2.337
560 50 s eel/s eel 15.1 0.13 1.855
Shah i a ball-on- h ee pla es
0.06 10 s eel/s eel 13.6 0.15 0.224
0.1 10 s eel/s eel 13.6 0.14 0.224
1 10 s eel/s eel 13.6 0.145 0.224
10 10 s eel/s eel 13.6 0.15 0.224
100 11 s eel/s eel 13.6 0.17 0.224
1000 12 s eel/s eel 13.6 0.19 0.269
2. Ma e ials and Me hods
2.1. Measu emen Me hod o COF
In his s udy, a o a ing ic ion es e was used o ca y ou all expe imen s. Tes s we e pe o med
on he ibome e B uke ZP-UMT T iboLab (ZP-44957) (Figu e 1a) wi h a pin-on-disc con igu a ion
(Figu e 1b). This es e uses a o a y module o d i e he disc sample ixed a he lowe pa . The pin
was used du ing he measu emen s a iona y wi h he possibili y o se he adial and e ical posi ion
o he disc. The pin was ixed by an uppe holde o he no mal and adial o ce senso . The measu ed
MR luid o oil was applied o he pla e su ace wi h a olume o 12 mL, which ensu ed he ully
looded con ac . Expe imen al da a, such as he ic ion o ce (F), empe a u e, and no mal o ce (N)
a e measu ed using senso s. The COF is calcula ed using he ollowing equa ion:
COF =F
P(1)
whe e COF is he kine ic coe icien o ic ion, F is he nominal measu ed ic ion o ce du ing sliding,
and P is he applied load (no mal o ce). The no mal o ce is ensu ed by an uppe linea d i e and he
pe manen p essu e o he pin o he disc is ensu ed by he sp ingy me al s ips Figu e 1b. A ha dened
Ma e ials 2020,13, 4670 4 o 11
ball bea ing wi h a diame e o 6.35 mm was used as a pin and a ha dened ool s eel as a disc. The disc
had o be gen ly g ound and polished up o less han Ra 0.025
µ
m o a oid he seizu e a highe speed
(Figu e 2a). Gene ally, highe disc oughness esul ed in seizu e, especially a highe sliding speeds.
Howe e e en polishing he disc did no ully p e en he con ac om seizu e (Figu e 2b) because
o he lexible moun o he pin, which s a ed o ib a e a high u ning speeds. Fo his eason,
he maximal o a y speed had o be se on ca. 2400 e /min. The co esponding sliding speed was
insu icien o achie e he equi ed alues o he He sey numbe . The e o e, o measu e he S ibeck
cu e wi h his egime o lub ica ion, a no mal o ce canno s ay cons an o all measu ing s eps, and
i had o be g adually educed when he maximum pe mi ed o a y speed was eached.
Ma e ials 2020, 13, x FOR PEER REVIEW 4 o 11
COF =F
P
(1)
whe e COF is he kine ic coe icien o ic ion, F is he nominal measu ed ic ion o ce du ing
sliding, and P is he applied load (no mal o ce). The no mal o ce is ensu ed by an uppe linea d i e
and he pe manen p essu e o he pin o he disc is ensu ed by he sp ingy me al s ips Figu e 1b. A
ha dened ball bea ing wi h a diame e o 6.35 mm was used as a pin and a ha dened ool s eel as a
disc. The disc had o be gen ly g ound and polished up o less han Ra 0.025 µm o a oid he seizu e
a highe speed (Figu e 2a). Gene ally, highe disc oughness esul ed in seizu e, especially a highe
sliding speeds. Howe e e en polishing he disc did no ully p e en he con ac om seizu e
(Figu e 2b) because o he lexible moun o he pin, which s a ed o ib a e a high u ning speeds.
Fo his eason, he maximal o a y speed had o be se on ca. 2400 e /min. The co esponding sliding
speed was insu icien o achie e he equi ed alues o he He sey numbe . The e o e, o measu e
he S ibeck cu e wi h his egime o lub ica ion, a no mal o ce canno s ay cons an o all
measu ing s eps, and i had o be g adually educed when he maximum pe mi ed o a y speed was
eached.
(a)
(b)
Figu e 1. (a) T ibome e UMT B uke ; (b) pin-on-disc con igu a ion wi h he hea ing module.
Thus, he no mal o ce was g adually dec eased om 20 o 1 N. The e olu ion o he o a y es
module was se acco ding o he equi ed sliding speed and ac ual adius o he pin on he disc
sample. The sliding speed was changed om 5 o 4000 mm/s in 20 s eps, which app oxima ely
co esponds o 2400 pm o maximal sliding speed; see Table 2. The ini ializa ion p ocedu e was
pe o med o 120 s a sliding speed 125 mm/s and 60 s a sliding speed 1500 mm/s o neglec he
e ec o wo n su aces on measu ing COF.
(a)
(b)
Figu e 2. (a) Polished disc—Ra 0.014 µm; (b) ack oughness a e es —Ra 0.110 µm.
Du ing he p ocedu e o he measu emen o one S ibeck cu e, he ha dened ball as a pin was
wice changed o a oid con ac seizu e because o he escala ed pin wea a a high o a ional speed.
The epea ing o measu emen s was ca ied ou o ob aining h ee S ibeck cu es o one sample
Figu e 1. (a) T ibome e UMT B uke ; (b) pin-on-disc con igu a ion wi h he hea ing module.
Ma e ials 2020, 13, x FOR PEER REVIEW 4 o 11
COF =F
P
(1)
whe e COF is he kine ic coe icien o ic ion, F is he nominal measu ed ic ion o ce du ing
sliding, and P is he applied load (no mal o ce). The no mal o ce is ensu ed by an uppe linea d i e
and he pe manen p essu e o he pin o he disc is ensu ed by he sp ingy me al s ips Figu e 1b. A
ha dened ball bea ing wi h a diame e o 6.35 mm was used as a pin and a ha dened ool s eel as a
disc. The disc had o be gen ly g ound and polished up o less han Ra 0.025 µm o a oid he seizu e
a highe speed (Figu e 2a). Gene ally, highe disc oughness esul ed in seizu e, especially a highe
sliding speeds. Howe e e en polishing he disc did no ully p e en he con ac om seizu e
(Figu e 2b) because o he lexible moun o he pin, which s a ed o ib a e a high u ning speeds.
Fo his eason, he maximal o a y speed had o be se on ca. 2400 e /min. The co esponding sliding
speed was insu icien o achie e he equi ed alues o he He sey numbe . The e o e, o measu e
he S ibeck cu e wi h his egime o lub ica ion, a no mal o ce canno s ay cons an o all
measu ing s eps, and i had o be g adually educed when he maximum pe mi ed o a y speed was
eached.
(a)
(b)
Figu e 1. (a) T ibome e UMT B uke ; (b) pin-on-disc con igu a ion wi h he hea ing module.
Thus, he no mal o ce was g adually dec eased om 20 o 1 N. The e olu ion o he o a y es
module was se acco ding o he equi ed sliding speed and ac ual adius o he pin on he disc
sample. The sliding speed was changed om 5 o 4000 mm/s in 20 s eps, which app oxima ely
co esponds o 2400 pm o maximal sliding speed; see Table 2. The ini ializa ion p ocedu e was
pe o med o 120 s a sliding speed 125 mm/s and 60 s a sliding speed 1500 mm/s o neglec he
e ec o wo n su aces on measu ing COF.
(a)
(b)
Figu e 2. (a) Polished disc—Ra 0.014 µm; (b) ack oughness a e es —Ra 0.110 µm.
Du ing he p ocedu e o he measu emen o one S ibeck cu e, he ha dened ball as a pin was
wice changed o a oid con ac seizu e because o he escala ed pin wea a a high o a ional speed.
The epea ing o measu emen s was ca ied ou o ob aining h ee S ibeck cu es o one sample
Figu e 2. (a) Polished disc—Ra 0.014 µm; (b) ack oughness a e es —Ra 0.110 µm.
Thus, he no mal o ce was g adually dec eased om 20 o 1 N. The e olu ion o he o a y es
module was se acco ding o he equi ed sliding speed and ac ual adius o he pin on he disc sample.
The sliding speed was changed om 5 o 4000 mm/s in 20 s eps, which app oxima ely co esponds o
2400 pm o maximal sliding speed; see Table 2. The ini ializa ion p ocedu e was pe o med o 120 s
a sliding speed 125 mm/s and 60 s a sliding speed 1500 mm/s o neglec he e ec o wo n su aces on
measu ing COF.
Du ing he p ocedu e o he measu emen o one S ibeck cu e, he ha dened ball as a pin was
wice changed o a oid con ac seizu e because o he escala ed pin wea a a high o a ional speed.
The epea ing o measu emen s was ca ied ou o ob aining h ee S ibeck cu es o one sample
because o he highe s a is ical signi icance o da a. An e o ba was c ea ed om hese da a. The new
adius, new pin, and new es ed luid we e used o each epea ing.
Ma e ials 2020,13, 4670 5 o 11
Table 2. Condi ions o S ibeck cu e measu emen .
S ep Fz(N) (mm/s) S ep Fz(N) (mm/s)
1 20 5 11 20 2500
2 20 10 12 20 3000
3 20 25 13 20 3300
4 20 50 14 15 3300
5 20 125 15 10 3300
6 20 250 16 5 3300
7 20 500 17 2.5 3300
8 20 1000 18 2 4000
9 20 1500 19 1.6 4000
10 20 2000 20 1 4000
The measu emen was ca ied ou o empe a u es o 40 and 80
◦
C using a empe a u e chambe
by B uke . The deli e ed so wa e wi h a ibome e canno compile a S ibeck cu e including he
sliding speed, con ac p essu e, and dynamic iscosi y on he x-axis. This so wa e wo ks only wi h
e olu ions o o a y d i e, which is misleading when using a di e en adius o he pin. The e o e,
he new sc ip in Ma lab o da a e alua ion was c ea ed. The one measu emen (one epea ing) o he
S ibeck cu e consis s o 20 s eps, as men ioned abo e when each s ep co esponds o one poin on he
S ibeck cu e. Fo each s ep, he 16 changes in sense o he o a ion we e se o be e looding o he
con ac . Some imes, he measu ed da a included se e al ypes o ins abili ies ha we e sol ed by his
sc ip . Viscosi y is an impo an inpu o calcula ion o He sey numbe . The suspension iscosi y (MR
luid) is dependen on he olume ac ion o pa icles and ca ie luid iscosi y. Howe e , he amoun
o pa icles in con ac is no known because some pa icles a e p obably excluded om he con ac
zone [
30
]. The e o e, he au ho s decided o use o calcula ion o He sey numbe ca ie luid iscosi y
o MRF. The sc ip calcula es a ela ion among ela i e sliding speed o su aces in con ac , con ac
p essu e p
z
,and dynamic iscosi y o ca ie luid
η
and d aws i on he x-axis (He sey numbe ).
This ela ion is gi en by (2):
axis alue =η·
pz(2)
2.2. Tes ing Samples
Comme cial MR luids supplied by he LORD companyo a ion (MRF-122EG, MRF-132DG,
and MRF-140CG) we e chosen as he MR luid samples. The p ope ies om LORD Technical
Da ashee s a e s a ed in Table 3.
Table 3. P ope ies o MR luid samples and PAO.
Pa ame e MRF-122EG MRF-132DG MRF-140CG PAO4
Viscosi y a 40 ◦C (Pa.s) 0.042 ±0.02 0.112 ±0.02 0.28 ±0.07 0.014
Densi y (g/cm3)2.28–2.48 2.95–3.15 3.54–3.74 0.82
Solids con en o weigh (%) 72 80.98 85.44 0
Solids con en o olume (%)
22 32 40 0
The Lo d MR luids we e compa ed wi h base oil om he g oup o syn he ic liquid hyd oca bons
(poly-alpha-ole in). The es ed MR luids di e ed in i on pa icle concen a ion om 22 o 40 ol. %.
The commonly accep ed heological model o MR luid is he Bingham model:
τ=η0.
γ+τ0(H)sign.
γa |τ|≥
τ0(H)
,.
γ=0 a |τ|≤
τ0(H)
, (3)
Ma e ials 2020,13, 4670 6 o 11
whe e
τ
is shea s ess,
η0
is plas ic iscosi y,
τ0
is yield s ess, H is magne ic lux in ensi y in MR
luid, and
.
γ
is shea s ain a e. In he case o expe imen s, magne ic lux in ensi y is ze o (H =0 A/m).
MR luid in he non-ac i a ed s a e (H =0 A/m) exhibi s yield s ess in ens o Pa. Howe e , he MR
luid supplie usually s a es he New onian beha io o MR luid and p o ides he alue o iscosi y a
high shea a es. The pa icle size and dis ibu ion we e measu ed by a scanning elec on mic oscope
FEG SEM ZEISS Ul a Plus and analyzed by sc ip using ools o pic u e analysis in Ma lab (R2018b).
The pa icles we e sphe ical and he a e age size (diame e app oxima ely 2.1
µ
m) and dis ibu ion
acco ding o Q3 we e iden ical o all MR luids (Figu e 3).
Ma e ials 2020, 13, x FOR PEER REVIEW 6 o 11
whe e 𝜏 is shea s ess, 𝜂0 is plas ic iscosi y, 𝜏0 is yield s ess, H is magne ic lux in ensi y in MR
luid, and 𝛾 is shea s ain a e. In he case o expe imen s, magne ic lux in ensi y is ze o (H = 0
A/m). MR luid in he non-ac i a ed s a e (H = 0 A/m) exhibi s yield s ess in ens o Pa. Howe e , he
MR luid supplie usually s a es he New onian beha io o MR luid and p o ides he alue o
iscosi y a high shea a es. The pa icle size and dis ibu ion we e measu ed by a scanning elec on
mic oscope FEG SEM ZEISS Ul a Plus and analyzed by sc ip using ools o pic u e analysis in
Ma lab (R2018b). The pa icles we e sphe ical and he a e age size (diame e app oxima ely 2.1 µm)
and dis ibu ion acco ding o Q3 we e iden ical o all MR luids (Figu e 3).
(a)
(b)
Figu e 3. MRF-122EG (a) SEM pic u e o cen i uging pa icles; (b) his og am o 1399 de ec ed
pa icles.
3. Resul s and Discussion
3.1. COF Compa ison o Oils and MR Fluid
Figu e 4 compa es he S ibeck cu es o measu ed PAO and MRF-132DG o a empe a u e o
40 °C. The esul s in Figu e 4 indica e e y high epea abili y o he measu emen ; see e o ba s. Only
a he bounda y lub ica ion egime was he a iance o alues sligh ly highe . Howe e , ha can be
unde s andable because in his a ea he solid su aces come in o con ac and his e en has a
s ochas ic cha ac e . I can be seen ha he bounda y, mixed, elas ohyd odynamic (EHD), and
hyd odynamic egimes o lub ica ion we e measu ed; see Figu e 4. Howe e , he EHD egime ( he
a ea wi h he lowes COF) a MR luid came on a signi ican ly highe COF han PAO, and hen i
apidly ose. The EHD is de ined as an a ea whe e he lub ica ion ilm has he same o de o
magni ude as he su ace oughness. In his a ea, he i on pa icles o MR luid p obably wo k as a
media o a he in e ac ion o bo h solid su aces. The apid inc ease o COF a he hyd odynamic
egime a MR luids can be explained by he p esence o i on pa icles, which can o m a highe
lub ica ing wedge unde he pin. The non-New onian heological beha io o MR luid may also
a ec he apid inc ease o COF. The esul s indica e ha he a ea wi h high ela i e sliding speed
canno be ecommended as an ope a ing poin o MR luid, which is he main con as o common
oils. Gene ally, o common oils, he hyd odynamic lub ica ion egime is desi able as an ope a ing
a ea. The hyd odynamic egime is de ined as a egime ully sepa a ed om con ac ing su aces by
lub ica ion ilm and he e o e he wea and COF a e he lowes in his a ea. The measu emen s
showed ha he HD egime is also ecommended o MR de ices, bu only in he na ow band up o
sliding speed co esponding o COF o ca. 0.14. The measu ed alue o COF in a mixed egime
co esponds o he da a published by Sohn [25] (COF 0.1–0.12) which was es ed in he same
con igu a ion.
Figu e 3.
MRF-122EG (
a
) SEM pic u e o cen i uging pa icles; (
b
) his og am o 1399 de ec ed pa icles.
3. Resul s and Discussion
3.1. COF Compa ison o Oils and MR Fluid
Figu e 4compa es he S ibeck cu es o measu ed PAO and MRF-132DG o a empe a u e o
40
◦
C. The esul s in Figu e 4indica e e y high epea abili y o he measu emen ; see e o ba s. Only a
he bounda y lub ica ion egime was he a iance o alues sligh ly highe . Howe e , ha can be
unde s andable because in his a ea he solid su aces come in o con ac and his e en has a s ochas ic
cha ac e . I can be seen ha he bounda y, mixed, elas ohyd odynamic (EHD), and hyd odynamic
egimes o lub ica ion we e measu ed; see Figu e 4. Howe e , he EHD egime ( he a ea wi h he
lowes COF) a MR luid came on a signi ican ly highe COF han PAO, and hen i apidly ose.
The EHD is de ined as an a ea whe e he lub ica ion ilm has he same o de o magni ude as he
su ace oughness. In his a ea, he i on pa icles o MR luid p obably wo k as a media o a he
in e ac ion o bo h solid su aces. The apid inc ease o COF a he hyd odynamic egime a MR luids
can be explained by he p esence o i on pa icles, which can o m a highe lub ica ing wedge unde
he pin. The non-New onian heological beha io o MR luid may also a ec he apid inc ease o
COF. The esul s indica e ha he a ea wi h high ela i e sliding speed canno be ecommended as an
ope a ing poin o MR luid, which is he main con as o common oils. Gene ally, o common oils,
he hyd odynamic lub ica ion egime is desi able as an ope a ing a ea. The hyd odynamic egime
is de ined as a egime ully sepa a ed om con ac ing su aces by lub ica ion ilm and he e o e he
wea and COF a e he lowes in his a ea. The measu emen s showed ha he HD egime is also
ecommended o MR de ices, bu only in he na ow band up o sliding speed co esponding o
COF o ca. 0.14. The measu ed alue o COF in a mixed egime co esponds o he da a published by
Sohn [25] (COF 0.1–0.12) which was es ed in he same con igu a ion.
Ma e ials 2020,13, 4670 7 o 11
Ma e ials 2020, 13, x FOR PEER REVIEW 7 o 11
Figu e 4. S ibeck cu es compa ison o PAO 4 and magne o heological luid MRF-132DG a 40 C.
3.2. Pa icle Concen a ion E ec
I is su p ising ha he COF in he bounda y lub ica ion egime is almos iden ical o all luids,
despi e he p esence o i on pa icles in MR luid. P obably, only he iscosi y o he ca ie luid and
he ic ion su aces a ec he COF. I can be also deduced ha he concen a ion o i on pa icles was
al eady high enough o he luid MRF-122EG (22 ol.%) o ully ill he lub ica ion gap be ween
su aces. Many s udies sugges ha he concen a ion abo e 10 ol. % o i on pa icles does no
in luence COF and wea [20]. In he hyd odynamic egime, he si ua ion is di e en . The e ec o
pa icle concen a ion on COF is obse able; see Figu e 5. The highe he pa icle concen a ion, he
highe he COF. The iscosi y o he suspension (MR luid) p obably has a signi ican e ec in he
HD egime. Simila conclusions a e p esen ed in he pape [27].
(a)
Figu e 4. S ibeck cu es compa ison o PAO 4 and magne o heological luid MRF-132DG a 40 ◦C.
3.2. Pa icle Concen a ion E ec
I is su p ising ha he COF in he bounda y lub ica ion egime is almos iden ical o all luids,
despi e he p esence o i on pa icles in MR luid. P obably, only he iscosi y o he ca ie luid and
he ic ion su aces a ec he COF. I can be also deduced ha he concen a ion o i on pa icles was
al eady high enough o he luid MRF-122EG (22 ol. %) o ully ill he lub ica ion gap be ween
su aces. Many s udies sugges ha he concen a ion abo e 10 ol. % o i on pa icles does no
in luence COF and wea [
20
]. In he hyd odynamic egime, he si ua ion is di e en . The e ec
o pa icle concen a ion on COF is obse able; see Figu e 5. The highe he pa icle concen a ion,
he highe he COF. The iscosi y o he suspension (MR luid) p obably has a signi ican e ec in he
HD egime. Simila conclusions a e p esen ed in he pape [27].
Ma e ials 2020, 13, x FOR PEER REVIEW 7 o 11
Figu e 4. S ibeck cu es compa ison o PAO 4 and magne o heological luid MRF-132DG a 40 C.
3.2. Pa icle Concen a ion E ec
I is su p ising ha he COF in he bounda y lub ica ion egime is almos iden ical o all luids,
despi e he p esence o i on pa icles in MR luid. P obably, only he iscosi y o he ca ie luid and
he ic ion su aces a ec he COF. I can be also deduced ha he concen a ion o i on pa icles was
al eady high enough o he luid MRF-122EG (22 ol.%) o ully ill he lub ica ion gap be ween
su aces. Many s udies sugges ha he concen a ion abo e 10 ol. % o i on pa icles does no
in luence COF and wea [20]. In he hyd odynamic egime, he si ua ion is di e en . The e ec o
pa icle concen a ion on COF is obse able; see Figu e 5. The highe he pa icle concen a ion, he
highe he COF. The iscosi y o he suspension (MR luid) p obably has a signi ican e ec in he
HD egime. Simila conclusions a e p esen ed in he pape [27].
(a)
Figu e 5. Con .
Ma e ials 2020,13, 4670 8 o 11
Ma e ials 2020, 13, x FOR PEER REVIEW 8 o 11
(b)
Figu e 5. S ibeck cu es o (a) MR luids MRF-122EG, MRF-132DG, and MRF-140CG a 40 °C; (b)
de ail on HD egime.
3.3. Tempe a u e E ec
Figu e 6 shows he compa ison be ween 40 and 80 °C, which can be conside ed as an ope a ing
poin o linea ac ua o s such as dampe s. The iscosi y was kep he same o bo h empe a u es
(0.01 Pa.s). No signi ican empe a u e e ec was obse ed o MR luids 122 and 140. Fo MR luid
132, he e is a sligh di e ence in he minimum COF alue. Howe e , he PAO was signi ican ly
a ec ed by empe a u e. A signi ican inc ease in COF o e he en i e He sey numbe ange was
obse ed a 80 °C; see Figu e 6d.
(a)
(b)
(c)
(d)
Figu e 6. Compa ison o indi idual measu ed luid a empe a u es 40 and 80 °C: (a) MRF-122; (b)
MRF-132; (c) MRF-140; (d) PAO.
4. Conclusions
In his wo k, he COF measu emen s o h ee comme cial MR luids wi h 22–40 ol. % o CI
pa icles om LORD company o a ion we e done. These luids we e compa ed wi h syn he ic liquid
hyd oca bon PAO4, which co esponds o he base oil o MR luids. The p ope ies o all he luids
Figu e 5.
S ibeck cu es o (
a
) MR luids MRF-122EG, MRF-132DG, and MRF-140CG a 40
◦
C; (
b
) de ail
on HD egime.
3.3. Tempe a u e E ec
Figu e 6shows he compa ison be ween 40 and 80
◦
C, which can be conside ed as an ope a ing
poin o linea ac ua o s such as dampe s. The iscosi y was kep he same o bo h empe a u es
(0.01 Pa.s). No signi ican empe a u e e ec was obse ed o MR luids 122 and 140. Fo MR luid 132,
he e is a sligh di e ence in he minimum COF alue. Howe e , he PAO was signi ican ly a ec ed
by empe a u e. A signi ican inc ease in COF o e he en i e He sey numbe ange was obse ed a
80 ◦C; see Figu e 6d.
Ma e ials 2020, 13, x FOR PEER REVIEW 8 o 11
(b)
Figu e 5. S ibeck cu es o (a) MR luids MRF-122EG, MRF-132DG, and MRF-140CG a 40 °C; (b)
de ail on HD egime.
3.3. Tempe a u e E ec
Figu e 6 shows he compa ison be ween 40 and 80 °C, which can be conside ed as an ope a ing
poin o linea ac ua o s such as dampe s. The iscosi y was kep he same o bo h empe a u es
(0.01 Pa.s). No signi ican empe a u e e ec was obse ed o MR luids 122 and 140. Fo MR luid
132, he e is a sligh di e ence in he minimum COF alue. Howe e , he PAO was signi ican ly
a ec ed by empe a u e. A signi ican inc ease in COF o e he en i e He sey numbe ange was
obse ed a 80 °C; see Figu e 6d.
(a)
(b)
(c)
(d)
Figu e 6. Compa ison o indi idual measu ed luid a empe a u es 40 and 80 °C: (a) MRF-122; (b)
MRF-132; (c) MRF-140; (d) PAO.
4. Conclusions
In his wo k, he COF measu emen s o h ee comme cial MR luids wi h 22–40 ol. % o CI
pa icles om LORD company o a ion we e done. These luids we e compa ed wi h syn he ic liquid
hyd oca bon PAO4, which co esponds o he base oil o MR luids. The p ope ies o all he luids
Figu e 6.
Compa ison o indi idual measu ed luid a empe a u es 40 and 80
◦
C: (
a
) MRF-122;
(b) MRF-132; (c) MRF-140; (d) PAO.
Ma e ials 2020,13, 4670 9 o 11
4. Conclusions
In his wo k, he COF measu emen s o h ee comme cial MR luids wi h 22–40 ol. % o CI
pa icles om LORD company o a ion we e done. These luids we e compa ed wi h syn he ic liquid
hyd oca bon PAO4, which co esponds o he base oil o MR luids. The p ope ies o all he luids
we e measu ed a empe a u es 40 and 80
◦
C. The esul s indica e ha in he ange o bounda y
lub ica ion, all he luids exhibi simila COF, despi e i on pa icles in he MR luids. In he EHD a ea
( ansi ion be ween mix and hyd odynamic lub ica ion), whe e he COF is gene ally he lowes , he MR
luids exhibi sligh ly highe alues o COF han PAO. The bigges di e ence can be obse ed in he
hyd odynamic egime. The COF o PAO ises wi h a much gen le slope han MR luids. The e ec o
pa icle concen a ion on COF is obse able. The empe a u e e ec on he COF o MRF is insigni ican .
The esul s can be also used as a guide o de elope s o MR de ices. The impo an esul is he
speci ica ion o a sui able ope a ing ange. Conside ing low ic ion and o al sepa a ion o con ac
su aces by lub ica ion ilm, he hyd odynamic lub ica ion a ea seems o be sui able. On he con a y,
he con ac looded by MR luid should no be ope a ed a highe He sey numbe s, because he COF
ises e y s eeply o speci ic numbe s, highe han he EHD lub ica ion egime.
A compa ison o COF measu ed by au ho s men ioned in he in oduc ion (see Figu e 7) shows
qui e a la ge a iance, al hough only measu emen s wi h MRF132DG o luids wi h simila pa icle
concen a ions a e included in his igu e. The majo i y o he alues we e measu ed in bounda y
and mixed egimes, whe e he COF is e y dependen on con ac su ace oughness. Only a ew
alues we e measu ed in he hyd odynamic egime. In he HD egime, he su ace oughness and
geome y should ha e a much smalle impac on measu ed COF; he e o e, a smalle a iance can be
expec ed. This wo k p o ided in o ma ion abou he COF in he usual wo king a ea o he MR dampe
(HD egime).
Ma e ials 2020, 13, x FOR PEER REVIEW 9 o 11
we e measu ed a empe a u es 40 and 80 °C. The esul s indica e ha in he ange o bounda y
lub ica ion, all he luids exhibi simila COF, despi e i on pa icles in he MR luids. In he EHD a ea
( ansi ion be ween mix and hyd odynamic lub ica ion), whe e he COF is gene ally he lowes , he
MR luids exhibi sligh ly highe alues o COF han PAO. The bigges di e ence can be obse ed in
he hyd odynamic egime. The COF o PAO ises wi h a much gen le slope han MR luids. The
e ec o pa icle concen a ion on COF is obse able. The empe a u e e ec on he COF o MRF is
insigni ican .
The esul s can be also used as a guide o de elope s o MR de ices. The impo an esul is he
speci ica ion o a sui able ope a ing ange. Conside ing low ic ion and o al sepa a ion o con ac
su aces by lub ica ion ilm, he hyd odynamic lub ica ion a ea seems o be sui able. On he con a y,
he con ac looded by MR luid should no be ope a ed a highe He sey numbe s, because he COF
ises e y s eeply o speci ic numbe s, highe han he EHD lub ica ion egime.
A compa ison o COF measu ed by au ho s men ioned in he in oduc ion (see Figu e 7) shows
qui e a la ge a iance, al hough only measu emen s wi h MRF132DG o luids wi h simila pa icle
concen a ions a e included in his igu e. The majo i y o he alues we e measu ed in bounda y
and mixed egimes, whe e he COF is e y dependen on con ac su ace oughness. Only a ew
alues we e measu ed in he hyd odynamic egime. In he HD egime, he su ace oughness and
geome y should ha e a much smalle impac on measu ed COF; he e o e, a smalle a iance can be
expec ed. This wo k p o ided in o ma ion abou he COF in he usual wo king a ea o he MR
dampe (HD egime).
Figu e 7. Compa ison o esul s o COF om ou s udy ( ed line) and esul s om p e ious wo ks o
o he au ho s o MR luids MRF-132DG o simila MR luid.
Au ho Con ibu ions: Concep ualiza ion, J.R. and Z.S.; me hodology, J.R. and F.J.; so wa e, J.R.; alida ion,
M.K, O.M. and Z.S.; w i ing—o iginal d a p epa a ion, M.K.; w i ing— e iew and edi ing, F.J. and Z.S.; p ojec
adminis a ion, J.R.; supe ision, Z.S. All au ho s ha e ead and ag eed o he published e sion o he
manusc ip .
Funding: This pape and expe imen s ha e been made possible hanks o he kind sponso ship o a ious g an s
and nume ous agencies. We would like o hank speci ically he g an Agency o he Czech Republic (GAČR),
numbe s 17-10660J and 20-23261Y.
Con lic s o In e es : The au ho s decla e no con lic o in e es . The unde s had no ole in he design o he
s udy; in he collec ion, analyses, o in e p e a ion o da a; in he w i ing o he manusc ip , o in he decision o
publish he esul s.
Re e ences
1. Goncal es, F.D.; Ahmadian, M.; Ca lson, J.D. In es iga ing he magne o heological e ec a high low
eloci ies. Sma Ma e . S uc . 2005, 15, 75–85.
Figu e 7.
Compa ison o esul s o COF om ou s udy ( ed line) and esul s om p e ious wo ks o
o he au ho s o MR luids MRF-132DG o simila MR luid.
Au ho Con ibu ions:
Concep ualiza ion, J.R. and Z.S.; me hodology, J.R. and F.J.; so wa e, J.R.; alida ion,
M.K, O.M. and Z.S.; w i ing—o iginal d a p epa a ion, M.K.; w i ing— e iew and edi ing, F.J. and Z.S.; p ojec
adminis a ion, J.R.; supe ision, Z.S. All au ho s ha e ead and ag eed o he published e sion o he manusc ip .
Funding:
This pape and expe imen s ha e been made possible hanks o he kind sponso ship o a ious g an s
and nume ous agencies. We would like o hank speci ically he g an Agency o he Czech Republic (GA ˇ
CR),
numbe s 17-10660J and 20-23261Y.
Con lic s o In e es :
The au ho s decla e no con lic o in e es . The unde s had no ole in he design o he
s udy; in he collec ion, analyses, o in e p e a ion o da a; in he w i ing o he manusc ip , o in he decision o
publish he esul s.
