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M e c h a n i c a l s e a l s : s o m ed e v e l o p m e n t s in f a c em a t e r i a l sK. W akely*Carbon has bee n used as a face material in mec hanical seals o r well overf i f ty years. The f irst grades of carbon used were developed from carbonelectrical brush m aterials, bu t the co nt inu ous increase in the severity ofmechanical seal operat ing con dit ions has necessitated ever improving gradesof carbon. T his paper detai ls some of the m eans available to th e carbontechnologist to achieve these improvements and gives examples of somespecial applications.Keyword$: carb on, face mater ia ls, me chan ical seals,

Ca rbonEl em en t a l c a r bon ex i s t s na t u r a l l y i n m an y pa r t s o f t he w or l di n t h e f o r m s o f g r a p h i te a n d d i a m o n d . W h e n m i n e d ,g r aph i t e i s no t pa r t i cu l a r l y u s e f u l a s an eng i nee ri ng m a t e r i a l ,becaus e o f i t s s o f t ne s s , h igh i m pur i t y l eve l and l i m i t ed s i z e .T h e m a n u f a c t u r e o f c a r b o n a n d g r a p h i t e h a s d e v e lo p e d f o rove r t h r ee - qua r t e r s o f t he cen t u r y w i t h t he p r ope r t i e s o ft he r e s u l t ing p r oduc t s be i ng cons i de r ab l y i m pr oved ove rt hos e o f t he na t u r a l m a t e r i a l . C a r bon has been l ong e s t ab -l i shed a s a p r i m e m a t e r i a l f o r one o f t he r ubb i ng pa i r s i n am echan i ca l s ea l , and t he con t i nuous m anuf ac t u r i ng deve l op -m e n t s have k ep t p ace w i t h t he i nc r ea s i ng ly s eve r e r equ i r e -m en t s o f t he app l i ca t i on .B y j ud i c i ous s e l ec ti on o f r aw m a t e r i a ls , and by us i ng a va r i e tyo f m anuf a c t u r i ng p r oces s e s , c a r bon an d g r aph i t e c an bem od i f i ed t o g ive a r ange o f p r ope r t i e s s u it ab l e f o r d i f f e r en tsea l appl ica t ions . The raw mater ia l s a re se lec ted f romna t u r a l l y occu r r i ng g r aph i t e s , f r om w hi ch s om e o r a l l o f t hei no r gan i c i m pur i t ie s have been r em oved . The o r i g i na lm a t e r i a l s i nc lude c oke de r i ved f r om coa l s ou r ce s , r e s iduest aken f r om o i l d i s t i l l a t i on ( pe t r o l eum cokes ) , and ca r bonb l acks w h i ch a re ob t a i ned f r om t he bu r n i ng o f o il s andgases in l imi ted suppl ies of a i r .T h e r a w m a t e r ia l s ar e p o w d e r e d a n d b o n d e d t o g e t h e r w i tha p i t ch o r t a r , and t hen m ou l ded unde r p r e s s u r e t o f o r m aconven i en t s iz e and s hape . The r e s u lt i ng p r oduc t i s f i r ed ina p r o t ec t i ve a t m os phe r e , t o app r ox i m a t e l y 1000 C . D ur i ngt h i s p r oces s, t he t a r and p i t ch a r e conve r t ed t o a coke ,t he r eb y cem en t i ng t he pa r t i c le s o f the o t he r r aw m a t e r i a l st oge t he r t o f o r m a cohes ive m as s . Such a p r oduc t i s kno w nas ca r bon- g r aph i t e .Som e g r ades o f c a r bon a r e f u r t he r hea t - t r e a t ed t o t em p e r a -t u r e s o f up t o 25 00 C , a t w h i ch leve l t he am or ph ous ca r boni s tr ans f o r m ed i n t o a c r ys t a l li ne e l ec t r o - g r aph i te . T h i sm a t e r i a l ha s g r ea t e r pu r i t y , i m pr oved t he r m a l conduc t i v i t y ,be t t e r o x i da t i on r e s i s t ance , bu t l ow er s t r eng t h . C a r bon an dgr aph i t e m a t e r i a l s t ha t a r e p r odu ced by t h i s m ean s a r epor ous . The p r ope r t i e s , how eve r , c an be m od i f i ed byi m pr eg na t i on w i t h s yn t he t i c r e s i n s o r m e t a l s . The i m pr eg -nan t s con f e r i nc rea s ed s t r eng t h , l ow er pe r m eab i l i t y and

Morganite Special Carbons Ltd, Northfields, Wandsworth, LondonSW18 1NG, UK

i m pr o ved w ea r r e s i st ance cha r ac t e r is t i c s t o t he ba s i c c a rbons .I m pr egna t i on w i t h ox i da t i on i nh i b i t o r s w i l l i m pr ove t heox i da t i on r e s i s t ance and w i t h s om e bas e m a t e r i a l s bo t hreduce and s tabi l i ze f r i c t ion.C a r b o n d e v e l o p m e n t i s a c o n t in u o u s p r o c e ss t o m e e t t h eneeds o f t he s eal and other i ndus t r i e s . Tab l e s 1 - 4 i nd i ca t et he e f f ec t s o f va ri ous changes in t he m anuf ac t u r i ng p r oces s .The va r i ous pos s i b l e p r oces s changes ou t l i ned a r e u s ed i nt he deve l op m en t o f s pec i a l g r ades t o m ee t s pec i f ic app l ica -t i ons . Som e exam pl e s o f t h i s a r e g i ven l a t e r.Si l icon carb ideI n t he cons t an t ques t f o r i m pr ov em e n t s in m echan i ca l s ea lpe r f o r m ance , new f ace m a t e r i a l s a r e r egu l a r ly t e s t ed . O neof t he s e m a t e r i a ls w h i ch ha s s ho w n s upe r i o r qua l i ti e s a s aface mater ia l i s s i l i con carbide . Si l i con carbide 's uni formha r dnes s and dens i t y m ake i t an ex t r em e l y ab r a s i on r e si s t an tm a t e r i a l . S i l icon ca r b i de ha s exce l l en t t he r m a l s ho ck r e s i st -ance becaus e o f i t s h i gh t he r m a l conduc t i v i t y com bi nedw i t h h i gh t ens il e s t r eng t h and l ow t h e r m a l exp ans i on .Thes e p r ope r t i e s , t oge t he r w i t h i t s exce l l en t co r r os i onr e s i st ance and l ow coe f f i c i en t o f f r i c t ion , m ake s i li conca r b i de an exce l l en t s ea l f a ce m a t e r i a l . Four t ypes o f s i l iconcarbide are in genera l use . These are : r e ac t i on - bonded p r e s s u re l e s s s i n t e r e d s u r f ace conve r t ed ca r bon and g r aph i t e s i li con ca r b i de / g r aph i t e com pos i t e sThese var ious s i l i con carbide mater ia l s a re di scussed la ter int he pape r .Carbon grade developmentsThe alkylation processI n t he pe t r o l eum i ndus t r y , a p r oces s kno w n a s a l ky l a t i on i sca r r i ed ou t . Th i s i nvo l ves t he chem i ca l com bi n a t i on o fi s opa r a f f in w i t h any on e o r a com bi n a t i on o f o le f i ne s t of o r m a m i x t u r e o f h i gh l y - b r anched pa r a f f i n s t ha t have ah i gh an t i - knock r a t ing and good s t ab i li t y . Th i s p r oces s isexpand i ng a s l e ad - fr ee pe t r o l i s dem anded f o r env i r onm en t a lreasons .

1 9 8 0 3 0 1 - 6 7 9 X / 8 6 / 0 3 0 1 9 8 - 0 6 0 3 . 0 0 B u t t e r w o r t h & C o ( P u b li s he r s) L t d A u g u s t 8 6 V o l 1 9 N o 4

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Table 1 Pr ime factors affect ing physical propert iesWakely developments in face materials

Transverse Bu lk H a rd n e s s Elast icbend density modulusstrengthThermal Thermalconduct ivi ty expansion

Part icle size ~ I ~ ?Pore size ~ ?Total poros i ty ~. ? ? ? ? ?Anisot ropy ? ? ?(parallel grains)Coke ~ ~ ~Graphite I ~ I ~Resin impregnation 7 ~Meta l impregnat ion ~ 7 ~ 7 7Table 2 Pr ime factors a f fect ing per formance

Oxidat ion Wear St rength Fr ict ion H ea tresistan ce resista nce generatedPart icle size ~Co ke I ~ ~ I ~Gr aphite t ~ ? ~ ~Resin ? ~ ? 7 ' ~ IMetal I~ ~ ? 7 ? ~A n i s o t r o p y 1 ~ ~ ? ~ ~(parallel grains)

Table 3 E f fect o f impregnantsSeal Frict ion Strength Ox idat ionperformance

Oi l , wax ~Resin ~ ~ ~ 7Metal I 1 ? I ~Salts ; ~ ~

Alk y la t i o n r eac t i o n s i n v o lv e t h e u se o f e i t h e r su lp h u r i c ac ido r , m o r e c o m m o n l y , a n h y d r o u s h y d r o f l u o r i c a c id s a sca t a ly s t s. Th e r eac t i o n s a r e ca r r i ed o u t a t t em p era tu r es an dp ressu res su f f ic i en t l y h ig h t o k eep t h e f ee d an d r eac t i o nm i x t u r e s i n a l i qu i d p h a s e. T h e c o n c e n t r a t i o n o f h y d r o g e nf lu o r id e i s u su a l l y 3 o r 6% , b u t u n d e r b reak d o w n co n d i t i o n scan r i se t o 9 0 % .Th e p l an t i n wh ich t h e r eac t i o n s t ak e p l ace i n co rp o ra t esp u m p s f i t t e d w i th m e ch an i ca l sea ls h av in g ca rb o n seal in gr in g s. Co n s id e rab l e p ro b l em s were en co u n t e red w i th t h esep u m p s b e c a u s e o f a n a l m o s t c a t a s t r o ph i c f a i l ur e o f t h ecarb o n sea li ng r i n g s wh e n b ro u g h t i n to co n t a c t wi th t h eh y d ro f lu o r i c ac id an d i so p ara f f in . Th e f a i l ed ca rb o n s u su a l l ysh o we d a d e l am in a t i o n s t ru c tu re as i l l u s tr a t ed i n F ig 1 .An a ly s i s o f t h e d ep o s i t s fo u n d o n f a i l ed r i n gs su g g es ted t h a tt h e d i s r u p t i o n o f t h e c a r b o n r i n g w as c a u s e d b y t h e a t t a c ko f h y d ro f lu o r i c ac id o n t h e i n o rg an i c co n s t i t u en t s , su ch ass i l ico n o r a l u m in iu m , o f th e r aw m ate r i a l s u sed i n t h e m an u -f a c t u r e o f t h e c a r b o n . A n u m b e r o f g r a d e s f r o m d i f f e r e n tso u rces b eh av ed i n t h e sam e m an n e r . Tes t s were ca r r i ed o u tan d t h ese sh o w ed t h a t a p h en o l i c r es in - im p reg n a t ed h a rdcarb o n g rad e , wi th m in im u m in o rg an i c im p u r i t i e s , wi th s to o dth e ch em ica l co n d i t i o n s o f t h e a lk y l a t i o n p ro cess . Un d erd ry co n d i t i o n s t h i s h a rd ca rb o n cau ses h ig h wear o f t h e

co u n t e r f a ce . In t h ese c i r cu m s tan ces , a so f t e r , r e s i n im p reg-n a t ed e l ec t ro g rap h i t e ca rb o n g rad e can b e u sed , t h o u g h a tt h e ex p en se o f a r ed u c ed wear l i fe .Developm ent o f an ant i -b l is ter ing carbonC a r b o n b l i st e ri n g h a s b e e n a r e c o g n iz e d p h e n o m e n o n f o ro v er f i f t een y ear s. S ea l i n g f a i l u res d u e t o t h i s p h en o m e n o nap p eared t o i n c rease a t a p ro d ig io u s r a te s i n ce t h is m o d e o fseal face degradat ion was f i rs t repor ted . Whi ls t th is fai lu rem o d e i s r eco g n i zed as b e in g o n e o f t h e m ajo r cau ses o f sealf a i l u re en co u n t e re d i n th e p e t ro -ch em ica l i n d u s t ry , i t m u s tb e e m p h a s i z e d t h a t m a n y o t h e r f a i lu r e m o d e s h a v e b e e nwro n g ly a t t r i b u t ed t o b l i s t er i n g .I t i s s ign i f ican t that b l i s ter ing i s exp er ien ced o nly in associa-t i o n wi th t h e sea li ng o f li g h t h y d ro carb o n s an d o rg an i cre f r i g e ran t s . Man y t h eo r i es h av e b een ad v an ced as t o t h ecau se o f b li s t e ri n g . Th e co m m o n th em e to a l l t h e t h eo r i es i s

Fig 1 Delarnin ation structure o faile d carbon sealing rings

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Wake/y - deve lopments in face m ater ia lsTable 4 Physical properties of typical gradesG ra d e Description Transversereference bendstrength(kgf/cm 2 )

Bulkdensity(kg/m 3 x 103)Hardnessscleroscope Dynam ic Thermal Coefficiente l as t i c conduct iv i ty o f t he rma lmodulus (Wm K) exp ans ion(kgf/cm 2 x 103) C-1 X 10 -6)

1 Carbon

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Wa ke ly de ve lo pm e n ts i n fa c e m a te r ia l s( 7 ) Th e co u n te r f ace r o u gh n ess h o ld s a k ey to b l i ste rin g . V er yh ar d su r faces w h ich h av e a su r f ace n e tw o r k o f lu b r ica t io nchanne ls will p revent b l is ter ing provided th at the se channelsa r e r e ta in ed an d n o t w o r n .( 8 ) M at t l ap p in g tu n g s ten ca r b id e r e ta r d s th e o n se t o fb l is ter ing dras t ical ly for s tand ard ca rbon grades .A n t i - b l i s t e r g r a d eA car b o n -g r ap h ite w i th a ca r b o n ized im p r eg n an t to c o n t r o lp o r o s i ty , f o l lo w ed b y an an t imo n y an d th en a p h en o l icres in impregn ation , has been fo un d to g ive the best b l ister-res is tan t mater ial . Ph ysical p roper t ies of th is grade are:Transverse bend s treng th 840 kgf /c m2Bu lk d en s i ty 2 .4 0 k g /m a x 1 0Hardness ( scleroscope) 80D y n amic e las ti c mo d u lu s 3 4 0 k g f /cm2 x 10aTherm al cond uctiv i ty 13 .0 Wm 1 K -1Co ef f ic ien t o f th e r mal 4 .5 C - t x 1 0 - 6ex p an s io n

A ircraf t turb ine sealsTh e mo d er n g as tu r b in e em p lo y ed f o r p r o p u ls io n o f co m-mer c ia l an d m i l i t a r y a i rc r a f t i s a co mp lex a n d h ig h tech -n o lo g y p iece o f mach in er y . I t h as th r ee b as ic co m p o n en tsec t io n s , n am ely co mp r esso r, co m b u s t io n an d tu r b in e . Th eturb ine p resents a num ber o f d if f icu l t sealing problems, andspecial g rades of carbon have been developed for use inthese applicat ions .Th e tu r b in e sec t io n is d iv id ed in to a h ig h an d lo w p r es sur etu r b in e w h ich a r e m ech an ica l ly l in k ed to r esp ec t iv e h ig hand low pressure compressors. The turb ine b lades ex tra ctk in e t ic en er g y f r o m th e ex p an d in g a i r an d b u r n in g g asesb e in g rece iv ed f ro m th e co mb u s t io n sec t io n , co n v er t in g i tto sh a f t h o r sep o w er w h ich i s u sed to d rive th e co mp r esso rand al l o f i ts accessor ies . The rem ain ing energy , representedb y th e r a t io o f th e r emain in g p res su re r e la tiv e to am b ien tp r essu r e, an d th e r emain in g temp er a tu r e to am b ien t ,g en er a tes th r u s t . Th e m ain sh af t in a tu r b in e en g in e r o ta tesa t sp eed s ap p r o ach in g 1 0 0 0 0 r / r a in an d o p er a tes in anen v i r o n men t o f ch an g in g h ig h temp er a tu r e co n d i t io n s .Th e sh a f t i s su p p o r ted b y a se ries o f b ea r in g co mp o n en tsas so c ia ted w i th an ex ten s iv e lu b r ica t io n sy s tem. M ain sh af tb ea r in g co mp ar tm en t sea ls a re u sed to p r o tec t th e r o to rsu p p o r t b ea r in gs f r o m th e h o t g ases flo w in g th r o u g h th een g in e an d to p r ev en t th e lo s s o f th e lu b r ican t b e in g f ed tothe bearings.Ma in shaf t seals are designed as labyr in th , r ing and facetypes . T he lab yr in t h seals are g iv ing way to r ing and facedesigns due to decreases in weight and space a nd an increasein en g in e e f f ic ien cy a f f o r d ed b y th e la t t e r ty p es . Th er e i sa lso a t r en d to w ar d f ace seal s h avin g an a lu min iu m o x id eco a t in g o n th e in sid e d iamete r o f th e seal to p r o v id ead d i t io n a l w ear res i s tan ce a t th e c o n tac t p o in t w i th me ta lp is ton r ing- type secon dary seals .Th e ca r b o n m ate r ia ls u sed to d a y f o r th ese sea ls a r e th er esu lt o f ma n y y ear s o f d ev e lo p men t . Th e ca r b o n mu s t h av eh ig h s t r en g th , g o o d o x id a t io n r es i s tan ce , lo w f r ic t io n an dlong wear l i fe . These proper t ies have been achieved by these lec t io n o f o p t im u m r aw m ate r ia l s an d man u f ac tu r in gp ro cesses an d th e a d d i t io n o f an t io x id a t io n imp r eg n an ts .Tw o special carbon grades have been d eveloped for a ircraf ttu rb ine m ain sha f t sealing applicat ions . The f ir s t o f these is

a h igh s trength , f ine-grain carbon-graphite con tain ing aspecial h igh temperature res is tan t impregnant that issu i table u p to 4 5 0 C ( 8 5 0 F) in o x id iz in g en v i r o n men tsan d in ex cess o f 6 5 0 C ( 1 2 0 0 F) in n eu t r a l o r r ed u c in gatmo spheres . T his grade is used for bo th r ings and seals;i t h as o v er 3 0 y ear s an d mi l l io n s o f f ly in g h o u r s to i t scred it .The second grade has been developed to meet the h ighertem peratu re and ro tat io nal speeds o f newer engine designs .I t has a special non -exuding impre gnant th at g ives greateroxid at ion res is tance and lower running f r ic t ion . I ts g reatertherm al condu ctiv i ty derives f rom th e use of a f ine grainedelectrographite base s tock .

Pressed-to-shape comp onents fo r m echanical sealsA t th e o th e r en d o f th e sp ec t r u m to th e ap p l ica t io n o fcarbo n in aircraf t tu rb ine seals is i ts use in low du tymech an ica l s eal s as f o u n d in th e au to mo t iv e an d d o m es t icappliance industr ies : However , per form ance a nd l i fe is o fequal impor tance whatever the applicat ion . These industr iesalso require h igh volume, low cost p rod ucts a nd specialcarbo n grades and ma nufa ctur ing processes hav e beend ev e lo ped to m ee t th i s d u a l r eq u i r emen t o f p e r f o r man cean d co s t .The pressed- to -shape PTS) processPTS is a me thod used to produce carbon com ponents inhigh volum e at low cost. The process involves th e simul-taneous shaping of a com pon ent a nd ma ter ial p rocess ing .Th e e lem en ts o f sh ap in g ar e in te r ac t iv e w i th th o se o fmate r ia l p r ocess in g an d sa t i s f ac to r y co mp o n en ts can o n lyb e p r o d u ced b y ca r e f u l p lan n in g an d co n t r o l . Th e m aino b jec tiv e is to p r o d u ce a co m p o n en t w h ich r eq u i res amin im u m o f su b seq u en t mach in in g f o r co mp le t io n .P r o c e s s c a p a b i l i t ySize , sh ap e an d to le r an ce l imi t s a re th e main f ea tu r es w h ichn eed to b e co n s id e r ed w h en d esc rib in g th e cap ab i l i ty o f th ePTS process. The fo l low ing guidel ines relate to typ ica l p lan tand processes, the specific detai ls ma y vary f rom man ufac-tu r e r to man u f ac tu r e r .SizeSize is l imite d by press capacity and s ize of the special k i ln :( 1 ) Th e co m p o n en t sh ap e sh o u ld b e co n ta in ed w i th in acircle of 79 m m (3 .11 in) d iam eter (p lan v iew) .( 2 ) Th e d imen s io n o f th e co m p o n en t in th e d i r ec t io n o fp r ess in g sh o u ld n o t ex ceed 3 6 mm ( 1 .4 in ) .To le rancesDime nsional con tro l in the PT S process is ach ieved bycarefu l select ion and b lending of powde rs to achieve aco n s i s ten t co n t r ac t io n f ig ur e . Th is an d c a r e f u l co n t r o l o fk i ln t emp er a tu r e an d sp eed in f lu en ce th e s ize o f th e p o s tk i ln com pon ent . D etai led below are the to lerance capabili -t ies of the process . When consider ing mult i- featured com-p o n en ts , i t i s imp o r ta n t th a t th e t ig h tes t to le r an ces a re o n lyapplied to cr i t ical d imensions .To ta l t o le ra n c e e xp re ss e d a s a pe rc en ta ge o f t h e d im e n s io n s(1) Dim ensions at r igh t angles to the d irect ion o f pressing:

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Wakely developments in face materialsGen era l t o l e r an ce T ig h t es t t o l e r an ce

Carb o n -g rap h i tes 1 .2 5% 0 .8 0 %Meta l im p reg n a t ed 2 .5 0 % 1 .6 0%carb o n -g rap h i t e(2) Dimensions in the d i rect ion of p ressing :As (1 ) , b u t wi th a m in im u m g en era l t o l e r an ce o f 0 .3 m m( 0 . 0 1 2 i n ) a n d a m i n i m u m t i gh t e s t to l e r a n c e o f 0. 2 m m(0.00 8 in) for al l g rades.(3 ) Co n cen t r i c i t y an d o v a l i t y :0 . 1 5 m m ( 0 . 0 0 6 i n ) T I R u p t o 3 0 m m ( 1 . 18 i n ) .0 . 2 0 m m ( 0 . 0 0 8 i n) T I R o v e r 3 0 m m ( 1 . 1 8 in ) .(4) Parallelism:0 . 1 0 m m ( 0 . 0 0 4 i n ) T I R u p t o 3 0 m m ( 1 . 1 8 i n ) ac r os s .0 .1 5 m m (0 .0 0 6 i n ) T IR o v er 3 0 m m (1 .1 8 i n ) ac ro ss .ShapeTh i s i s t h e m o s t d i f f i cu l t f ea tu re o f t h e P TS p ro cess t oprescr ibe in detai l, bu t the re are basic ru les to be co nsidere d .(1 ) P T S is a v e r t i ca l fo rm in g p ro cess , p r essu re n eed s t o b eap p l i ed ac ro ss t h e w h o le p i ece an d i t m u s t b e p o ss ib l e t oe j ec t t h e p i ece f ro m th e d i e .(2 ) T o o l s wi th a r ad ia l wa ll t h i ck n ess o f le s s t h an 1 m m(0 .0 4 i n ) a r e t o b e av o id ed .(3 ) S t ep s o r r ecesses m o re t h a n 3 0 % o f o v era l l l en g th a r e t ob e av o id ed i f p ro d u ced i n f i x ed t o o l f ea tu res .(4 ) T h e m a x im u m ra t i o o f ax i a l t h i ck n ess t o r ad i al t h i ck n essis 4 :1 .(5 ) Gro o v es an d u n d erc u t s can n o t b e p ro d u c ed i n d iam ete r sdur ing the pressing process .P TS gradesTh e p h y s i ca l p ro p er t i e s o f two g rad es t h a t h av e b eensp ec i a ll y d ev e lo p ed fo r P TS co m p o n e n t s a r e g iv en h e re .Th ese co n t a in e i t h e r a p h en o l i c r es in o r an t im o n y im p reg -n an t i n t h e sam e b ase ca rb o n -g rap h i t e , th e l a t t e r a l so m an u -fac tu red w i th a f in a l re s in im p reg n a t i o n wh en en h an cedp ressu re t i g h tn ess i s r eq u i r ed . T h e p h y s i ca l p ro p er t i e s o ft h ese two g rad es a re :

I m p r e g n a n tTran sv erse b en d s t r en g th (k g f / cm 2 )Bu lk d en s i t y (k g /m a x 1 0 3 )Hard n ess ( sc l e ro sco p e)Dy n am ic e l ast ic m o d u lu s(k g f / cm 2 1 0 a )Th erm al co n d u c t i v i t y (Wm -z K -1 )Co ef f i c i en t o f th e rm al ex p an s io n( C - I 1 0 -6 )P o ro s i t y ( ap p aren t ) (% )

G r a d e A G r a d e BP h e n o l i c A n t i m o n yresin6 0 0 6 3 01 .8 0 2 .3 07 5 7 52 2 5 2 8 013 174 .2 4 .92 2

ApplicationsT h e m a i n m a r k e t s f o r P T S c a r b o n c o m p o n e n t s l ie i n t h eau to m o t iv e an d d o m es t i c ap p l ian ce i n d u s tr i e s , a l t h o u g hth ere a r e i n c reas ing r eq u i r em e n t s f ro m in d u s t ri a l s ea l an dsu b m ers ib l e p u m p m an u fac tu re r s .(1 ) Au to m o t iv e ap p l ica t i o n s :Th e m ajo r cu r r e n t d em an d i n Eu ro p e i s fo r ca rb o n f aces inau to m o t iv e co o l an t wa t e r p u m p sea l s , an d a n u m b er o f

l a rg e co n s t ru c to r s a r e ch an g in g t o ca rb o n f r o m res in -b o n d edmater ials . T here are also appl icat ions f or seal faces in ai rco n d i t i o n in g u n i t s .(2 ) D o m es t i c ap p l i an ce ap p l i ca t i o n s :Sealing r ing appl icat ions are fo un d in do m est ic d ishwashers ,was t e d i sp o sal u n i t s an d d o m es t i c wa t e r p u m p s .(3 ) In d u s t r i a l ap p l i ca t io n sTo j u s t i fy t h e co s t o f t o o l i n g t h e P TS p ro cess is n o rm al lyo n ly u sed fo r q u an t i t i e s o f 2 0 0 0 0 an d ab o v e fo r s im p ler in g s an d 1 0 0 0 0 an d ab o v e fo r co m p l i ca t ed co m p o n en t s .Mo s t d em an d s f ro m th e i n d u s t r ia l s ec to r d o n o t r each t h eselevels, bu t , in som e areas , dem and fo r me chanic al seals forg en era l ap p l i ca t i o n h as i n c reased t o a l ev e l wh ere t h e P T Sp ro cess can b e u sed .

S i l i c o n c a r b i d e sA range o f s i l icon carb ide m ater ials i s availab le to them ech an i ca l sea l d es ign en g in eer , an d t h e i r u se , b o t h as t h eco u n t e r f ace an d sea l face , is g ro win g . Each o f t h ese m ate r i a lso f f e r s ce r t a in co m m o n ad v an t ag es as a seal m a t e r i a l an dthese include: low speci f ic g rav i ty ex ce l l en t ab ras io n r es i s tan ce g o o d t h e rm a l sh o ck r es i st an ce ex t r em ely h ig h h a rd n ess h igh elast ic mo dulus h ig h t h e rm al co n d u c t i v i t y l o w th e rm a l ex p an s io n l o w f r i c t i o n g o o d ch em ica l r es i s tan ce ab i l i ty t o wi th s t an d h ig h t em p era tu resAl l av ai l ab l e s i li co n ca rb id e m ate r i a l s co m b in e t h ese p ro p er -t i e s t o a l e s se r o r g rea t e r ex t en t an d a n u m b er o f f ac to r sb e c o m e o f p r i m e i m p o r t a n c e w h e n s e le c ti n g t h e c o r r e c tg rad e o f s i l i co n ca rb id e f o r a sp ec i f ic ap p l i ca t i o n . Th esein c lu d e : ost avai lab i l i ty in the s ize and shape requi red co r ro s io n r es i st an ce t ech n i ca l p e r fo rm an ceDetail s o f the va r ious types of s i l icon carb ide are g iven inth e fo l l o win g su b - sect i o n s an d g en era l p h y s i ca l p ro p er t i e sfo r t h ese m ate r i a l s a r e li s t ed in Tab l e 5 , t o g e th er wi th so m eo th er h a rd f ace m ate r i a ls i n c lu d ed fo r co m p a r i so n .Reaction-bonded silicon carbideTh i s t y p e o f m at e r i a l was t h e f i r st s i l i co n ca rb id e i n t ro d u cedto t h e m ech an i ca l sea l m ark e t . I t is m ad e b y su b j ec t i n g aco m p ac t o f a lp h a s i l ico n ca rb id e an d ca rb o n t o l i q u id s i li co na t h ig h t em p era tu re . Th e l i q u id s i l ico n p en e t r a t es t h e co m -p ac t an d r eac t s wi th t h e ca rb o n t o fo rm b e t a s i li co n ca rb id ewh ich bond s the or ig inal a lpha s i l icon carb ide grainsto g e th er . S o m e r es id u a l s i li co n r em ain s , wh ich wi l l v a ryf ro m 1 0 -2 0 % d ep en d in g o n t h e m an u fac tu re r , an d t h i s f i l l su p t h e r em ain in g p o ro s i t y . Th e r esu l t an t m a t e r i a l i s im p er -' m eab l e .A n a d v a n ta g e o f t h i s m e t h o d o f m a n u f a c t u r e i s t h a t c o m -p o n en t s can b e sh ap ed i n t h e ' g r een ' s t a te b e fo re co n v er s io n .Negl ig ib le d imensional changes take p lace dur ing conversionan d i f n o n -c r i t ica l d im en s io n s can b e u sed as co n v er t ed t h e

2 0 2 A u g u st 8 6 V o l 1 9 N o 4

8/12/2019 Wakely 1986

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Tab le 5 Ty pical physical properties o f si l icon carbidesWakely developments in face materials

Materia l Therm al Dens ity Therm alshock (kg /m 3 x 103) cond uct iv i typarameter at r o o m(Cal /sm s t ) tempera ture(W m -1 k -1)

Hardnessk noop(kg /mm 2 )Compressive Flexura l Modu lusstrength strength elas ticity(MPa) (MPa) (GPa)

Reactio n -bonded 59 2. 9- 3. 10 120silicon carbideSin te red s i li con ca rb ide 60 2 .9 5-3 .12 11 5-1 25Surface converted > 50 2.2carbonS i l icon ca rb ide /g raph i te 2 .6 0-2 .80 6 0 - 7 5compositesHot-presse d silicon 29 3.2 1 7 . 5 ( 5 0 0 C )n i t r ideTungsten carbide 48 14.8 86Carbon< jraph ite > 50 1 .8 -1 .9 10 -1 8(resin-impregnated)

2 5 0 0 - 3 5 0 02 4 0 0 - 2 8 0 0

8 0 0 - 1 0 0 02 5 0 0 - 3 5 0 01 8 0 0

3 4 0 0 4 7 5 4 0 03 9 0 0 5 3 0 4 0 0

9 0 1 0 54 0 0 - 5 0 0 1 6 0 - 2 3 0 1 40

6 9 0 3 0 04 5 0 0 1 6 5 0 6 0 0

2 0 0 - 2 5 0 6 5 2 3

m al m ac h in in g co s t i s r ed u ced . W h ere r eq u i r ed , c l o sed im en s io n a l t o l e r an ces an d a f 'm e f 'm i sh can b e o b t a in e d b yf inal g r ind ing .Th e m ain d i sad v an t ag e o f r eac t i o n -b o n d e d s i li co n ca rb id e ist h e p resen ce o f t h e r es id u a l s il i co n, wh ich m ak es i t u n su i t -ab l e fo r ce r t a in ch em ica l co n d i t i o n s . Th e m ain ch em ica l s t ob e av o id ed a re cau s t i c a l k a l is an d n i t r i c / h y d ro f lu o r i c ac idm ix tu res .Carb o n -g rap h i t e ag a in s t r eac t i o n -b o n d e d s i l i co n ca rb id e i san ex ce l l en t co m b in a t i o n o f m at e r i a l s fo r m ech an i ca l sea ls .Th e r e ac t i o n -b o n d ed s i l i con ca rb id e h as b een ru n ag a ins tca rb o n -g rap h i t e fo r P V l im i t s o f u p t o 4 2 0 0 b ar m / s . Th i s isa p p r o x i m a t e l y 5 0 % h i gh e r t h a n t h e P V l i m i t u s e d f o rt u n g s t en ca rb id e ag a in st ca rb o n -g rap h i t e .Pressureless sintered silicon carbideTh i s m ate r i a l, m ad e b y s i n t e ri n g , h as th e ad v an t ag e t h a t i td o es n o t co n t a in an y f r ee s i li co n . Th i s p ro v id es i t w i th o u t -s t an d in g co r ro s io n r es i s tan ce . Th e r e su l t an t m a t e r i a l d o esh av e so m e p o ro s i t y , b u t t h e p o res a r e i so l a t ed in d iv id u a lp o res so t h e m ate r i a l is im p erm eab l e . P ressu rel es s s i n t e r edm ate r i a l h as g o o d ' a s p ressed ' su r f ace f i n i sh es b u t i s m o re -b r i t t l e t h an r eac t i o n -b o n d ed g rad es .Surface converted carbo n and graphiteIn t h i s p rocess , t h e su r f ace o f a ca rb o n o r g rap h i t e co m -p o n en t i s co n v er t ed t o b e t a s i li co n ca rb id e b y a v ap o u r p h asereac t i o n . Un l ik e co n v en t i o n a l ch em ica l v ap o u r d ep o s i t i o n ,t h e s u r f a ce c a r b o n o f t h e c o m p o n e n t r e a c t s w i t h t h e v a p o u rto fo rm s i l ico n ca rb id e in situ. A high d eg ree o f i n t eg r i t y i nth e s i l i co n ca rb id e / ca rb o n i n t e r f ace d e r iv es f ro m th e p ro cessan d , h e n ce , t h e re i s n o ch an ce o f t h e s i l i co n ca rb id e l ay er

p ee l i n g o f f i n se rv i ce. An ad v an t ag e o f t h i s p ro cess i s t h a tc o m p o n e n t s c a n b e p r e - m a c h i n e d i n c a r b o n o r g r a p h it er a t h e r t h a n e x t r e m e l y h a rd s i li c on c a r b id e . T h i s m e t h o d o fm an u fac tu re l en d s i t se l f t o l a rg e q u an t i t y p ro d u c t i o n .Mate r i a l fo r su r f ace co n v er s io n t r ea tm en t m u s t b e se l ec t edf r o m t h e w i d e r a n ge o f u n i m p r e g n a t e d c a r b o n s a n d g r a p h it e s ,p ressed g rad es b e in g t h e m o s t su i t ab l e . To ach i ev e a co n -v er t ed l ay er o f s i l i co n ca rb id e wi th m in im u m s tr es ses , t h ei m p o r t a n t p a r a m e t e r s a r e t h e p o r o s i t y o f t h e b a s e m a t e r i a la n d t h e e x t e n t o f m i s m a t c h b e t w e e n t h e c o e f f i c i e n t s o fe x p a n s i o n o f t h e b a s e m a t e r ia l a n d c o n v e r t e d l a y e r o f b e t as i l ico n ca rb id e . In p rac t i ce , t h e p o ro s i t y o f t h e b ase m a te r i a li s e f f ec t i v e ly r ep ro d u ced i n t h e co n v e r t ed s i l ico n l ay er .Ty p i ca l p ro p er t i e s o f t h i s m at e r i a l v a ry acco rd in g t o t h ed ep th o f s i li co n ca rb id e co n v er s io n an d t h e su b s t r a t e u sed .T h e d e p t h o f c o n v e rs i o n c a n b e m a d e t o v a r y b e t w e e n 0 . 5an d 2 .0 m m . S in ce t h e co n v er t ed l ay er r e t a in s t h e p o ro s i t yo f t h e b ase m ate r i a l , i t is n ecessa ry t o r es in im p reg n a t e t oo b t a in im p erv io u sn ess . Th e t y p e o f r es in u sed wi ll d i c t a t eth e ch em ica l r es i s t an ce o f t h e m ate r i a l .Silicon carbide/graphite componentsTh e l a t es t , an d p o ss ib ly m o s t ex c i t i n g , d ev e lo p m en t i n t h eran g e o f s i li co n ca rb id e m ate r i a l s is a co m p o s i t e o f s i l i co ncarb id e an d g rap h i t e . Th i s m at e r i a l i s m a d e b y i n f i l t r a t i n g asp ec i a ll y s t ru c tu red g rap h i t e b ase w i th l i q u id s i l ico n . Th i sr esu l t s i n s i l ico n ca rb id e co n t a in in g a su b s t an t ia l am o u n t o fg rap h i t e (3 0 - 5 0 % ) an d a v e ry lo w l ev e l o f f r ee s il i co n .Th i s m at e r i a l co m b in es t h e h ig h s t r en g th o f s i li co n ca rb id ewi th t h e l u b r i ca t i n g p ro p er t i e s o f g rap h i t e . I t h as b e t t e r d ryru n n in g p ro p er t i e s t h a n so l id s i l ico n ca rb id e b u t i s n o t sores i s t an t t o ab ras io n . I t w i ll b e ch em ica l l y a t t ack e d b yn i t r i c / h y d ro f lu o r i c ac id m ix tu res .

TR I B O L O G Y i nt e rn a ti o na l 2 0 3