(2000), 2, 26+27a. Silicon Nitride Boundary Lubrication: Effect of Sulfonate, Phenatq and. Salicylate Compoundse. RICHARD S. CATES and STEPHEN M. HSU.
vol.
/$
(2000), 2, 26+27a
TRIBOLOGY TFANSACTIONS
Silicon Nitride Boundary Lubrication: Effect of Sulfonate, Phenatq and Salicylate Compoundse RICHARD S. CATES and STEPHEN M. HSU National Institute of Standards and Technology Ceramics Division Gaithersburg, Maryland 2089
tive and diesel engines on the friction and wear chatacteristics of silicon nitride (SirN.) ceramics. There are very few previous studies on lubricating propenies of deteBents on ceratnics, Some (1.), (2) have used firlly formu-
phen' The boundary lubticalion chorocteristics of sulfoMtes,
nitride contoct oles, ond salicilates in a silicon nitride on silicon were invesrigared- The additfues were blended a'l onc percent by weighr solution in o puified paraffnic hse oil ond tested on a
lated lubricants lhat contain deteBents for lheit studi6. The authors eadie, study (3) evaluated several individual compone s normally found in engine oil formulations. Both phosphorus-containing compounds (ZDP included) and deleBcnr-q?c compounds werc effective in lubricating Si3Na at l%(w) coocenrrations in a base fluid. A more ind€prh study of phosphotls{on-
geometry in the Four-Boll Testcr' These compounds all exhibited efective lubricating propcnies for silicon nitride, inctuding both low base ond high base types Lner fric' porufin rion and wear v'ere obsened as compared to the base
batl-on-three
flot
base-oil case.Iood capocity tests indicated that these compounds hod enended lood ranges for silicon nitride Boundory films were
taining compounds (4) confmed the ability of this class of compounds in lubricaring SilNa and suggested a mechanism of addi-
obserued in the weor scar suggesting lhe occurrcrce of tribo'
tive adsorption and decomposition to form phosphorous-rich fiIms in the contac't. This paper examines the effects of individual detergent compounds in detail and Provides exPerimental detail to aid
chemical reoctions. Surface onolysis ol the worn surlace retealed the presence
the
of
etol sah portion of the comPound bur without the calcium
in lhe mechanistic interpretation of lhe lubrication process. Sincc boundary lubrication deals with chemical reaclions at the surfac., th€ chemical nature of ihe Si3N! surface is imPonanl.
corbonate y,hich *'as usedlor overbosing.This suggests lhot lhesc compoun^ probablJ oidize ond then odsorb o'tlo the surfoce to
lubricoting filnts containing the metal sohs h is htpothesized thot upon decomposition, the melol Porlion of lhe deteryenl molecule rcacts h'ith thc silicon oride tuioce loyer of the Sitll!
fom
XPS and Auger analyzes p€rformed on the PrePared Si3N. surfacc (5] confinn€d the prcsence of a silicon oxide laycr on the surface
apFoximately 6 nm thick. Thes€ observations are consisteot with SirN. surface aDalysis pe ormed by other researcheE such as
producing o complex silicale omorPhou.$ 'gloss" strucnre which prctects the Si3l'lalron fracture and daaoge.
Crdnmer (6,).
KEY WORDS
EXPERIMENTAL MATERIALS, APPARATUS AND PBOCEDURES
Detergents; Friclion; Lubrication; Silicon Nitri&: Tribochem-
istry; weat
The monoli$ic silicon nitride cerarnics used in lHs study \Pere
Adva[ced cerarnics are being used h beatings, tools, ard engine components. Thc durability of the ceramic parts' esPecial-
commelcially available fully dense materials known as Nonoo NCl32 and NBDI0O. These two materials stafled from de samc initial powder mi.xlure, but NCl32 was densified by hol prcssing while MDl00 was hot isostatically pressed (HIP). For th€ Pur-
ly under lubricated conditions, is of concem. This Fper examines
pose of this study, there was no significant chemical or wear
INTRODUCTION
$e
effects of deter3ent-tyPe additives commonly used in automo-
Pre3ent d
.t th.
5alh Annud
a.a
Fldal lranu3ctlpt
ProP
erty di-ffereDce b€tween these rwo materials. Wear tests were conducted using a Four-Ball Wear Tester wifi a ball-on-tlrce-flar (BTD geometry. This configuratior (Fi8. l), utilizrs a 12.7 mm (0.5 in) diameter ball and thre€ 6.35 mrn (0.25 in) diarleler flats, L59 mm (0.0625 or l/16 in) rhick. Flas werc
llccdllg
V.4t, llgtldl
prepar€d from 6.35 rnm diameter rcd slock of SirN. using a sequencs of diamond cuning, grinding, and polishing stcF. This
tbl *21,1g,,re .Po.o!td Jul, 13' 1909 269
R. Grres
2't0
I
n!.rd!g UFctBdt
C
ta7
o
rxo
S. Hsu
(0J h)DirnclcrBdl
ArplidIr.& 60fS
NaulIr..I2{ON Spcc{
r)
03 dr(500tP)
V Losq SblionaY FLlr __l_
FlO.'l-8611-on.throo-rlat (BTF) wear tcst oonllgurdlon and test condl' Uonr.
ffiEDETERGElrr-IYpE O*crmc Covrouxo Cmrncet.
G2o
srRusruREs
lli rlr
Useo nr Trns SruoY
..r|
C
o
SrRugruPc
-o\ c-o o
l
L c
Salicylate
t-
a.rs
b)
C
a o
ar0
aa (,o ara
xSD .
a.aar.r
I
a.o
Phenarc
Sulfonate
[with overbasing]
flma, mlnut
'-i-"['-*l'-"-i-'
resulted in a mirror surface frnish with a roughness of less than 0.01 pm (RMS). Specimens werc cleaned just prior to testing using a sequence of solvents of hexane, acetone' detergent in 18 MO deionized @[) water and pure DI water. Wear tests were conducted at conditions of 60 kg applied load (2CI N normal load), 0.23 m/s (600 rpm), 30 min. duration, 2l'C, using l'5 mL lubricant. After a test, the lubricant was removed and the specimens were rinsed with hexane and dried with nitrogen gas' Samples uralyzed by SEM were coated with conductive carbon films' Wear of the flats was measured at the end of.the test using a
t0ox optical microscope with a calibrated graduated reticle. Each of thc wear scars was measurcd both parallel and perpendicular to
t a
Flg.2-PPO hffcsted BTF tesl 8t 60 k9. (!) opthal Pholornlcrograph O) ftlction tracs
in this study was a =2.7 x t0'5 m% (27 cSO viscosity (measured at 37 .7'C ( lm'D parafFrnic oil purified by percolating it through 200 mesh activated alumina. This resulted in a purified paraffrn oil (PPO) free of polar impurities that might influence the tes6. General chemical structures for the detergent-type additive compounds are provided in Table l. The'R" groups in these structures represent long chain organic groups that make the compounds soluble in base oils. All of lhe detergents contain metals (Ca, Mg or
Zn) incorporated tfuough oxygen linkages to either carbon or sulfur. In the case of the overbased detergents, calcium carbonates have been incorporated into micellar structures associated with the molecules (7) to neutralize acidic reaction products of lubri-
the direction of sliding. The resulting six measurements werc averaged to give the wear scar diameter for the test. For very effective lubricants, it is possible for specimens to have little
cant degradation.
appreciable wear, but have the Hertzian contact surface roughened slightly so that oPtically, a Hertzian-sized wear scar can be seen'
The base oil iself, PPO exhibits limited boundary lubricating ability for Sirl,{n below 30 kg applied load in the BTF test (J).
The Hertzian wear scar diatneter thercfore rcPresents a theoreticd minimum wear scar diameter for optical measurements. A bener way to gauge the effectiveness of differcnt lubricant chemisries therefore is to compare the wear scar diameters after subtracting
Wear scar diameters are close to the Hertzian contact diameter and
thc Herrzian wear scar diameter. Both wear scar diameter and
at the end of the test is 0.65 mm which is significantly larger than
diameter increase above Hertzian contact diameter values are Providcd in this study. Friction was continuously monitored throughout each tesr by a force transducer. Values provided for coefficient of friction are the stcady-state values obtained at the end of the
the Hertzian contact diameter (0.38 mm) at this load. The optical micrograph in Fig. 2 reveals that the wear sciu is relatively smooth and symmetric. Small striated discolorations within the wear sc:u
test.
The compounds used in thesc tcsts werc obtained in additive component form, i.c. compounds in diluent oil. The basc oil used
{
ta
PPO BASELINE CASE
friction coeffrcient is low (p=0.09) and steady. Under a higher load of60 kg (Fig. 2), PPO exhibits an increase to higher friction coefficient
04.13),
and the wear is high. The wear scar diameter
suggest a micro-abrasion process by opposing asperities. Thesc discolorations are brownish and arc indicative of high molecular wcight compounds polymerized from oxidized paraffrnic structures (8).
27r
Silicon Nitridc Boundary Lubrication: Effect of Sulfonatc, Ptr-narc urd Sdicylate Compounds
AND
ON
Drusrtn
Diamctcr
lncreasc' (mm)
0.645
0.265
0.tt7
I
0.391
o.olt
0.rffi
TV
o.112
0.032
0.(m
w
0.412
0.032
0.103
Iv
l%
0.422
0.042
0.096
l
l5
t%
0.423
0.u3
0.089
I
