M u

Chng i: Tng quan v ma st- mn1. Cc l thuyt v ma st.Cng nh cc ngnh khoa hc khc, khoa hc v ma st pht trin khng ngng v gn lin vi cc ngnh cng nghip c lin quan nh cng nghip t, giao thng ng st v gia cng kim loi. Vic nghin cu, tm hiu v bn cht ca ma st l rt cn thit trc ht c th m hnh ho, tip theo l lng ho v iu khin c cc qu trnh xy ra khi ma st. Cc kin thc v ma st gip cho cc nh nghin cu nh hng c cng vic v gii quyt c cc vn do thc tin t ra.

Nhng nh khoa hc u tin nghin cu trong lnh vc ma st l Amoton, Culong, Leona d Vinxi v sau ny l M.L.Lomonoxop,.

Nga, khoa hc v ma st, bi trn v mi mn c hnh thnh t khi thnh lp Vin hn lm khoa hc Nga. Nh khoa hc v i M.L.Lomonoxop thit k thit b u tin nghin cu s lin kt gia cc phn t ca cc vt th. l tin thn cho cc thit b hin i sau ny nghin cu v mi mn ca vt liu.

Ma st c nghin cu t lu nhng cc l thuyt v ma st mi mn vn khng ngng c b xung v pht trin cng vi s pht trin ca cc ngnh khoa hc khc. Bt u t l thuyt c in, l thuyt phn t, l thuyt in cho n cc l thuyt hin i ngy nay.

1.1 L thuyt c inNh bc hc u tin nghin cu v xut l thuyt c in v ma st l Amoton. Trong ti liu vo nm 1969, tc gi a ra nhng hiu bit v ma st v m t dng c dng trong th nghim nghin cu. l mt dng c n gin bao gm mt l so gn vo vt th nghim v tt c c t trn mt mt phng. u kia ca l so c gn vi mt ti trng nh m vt nghin cu c th trt trn mt phng.

Nhiu th nghim c thc nghim v i n kt lun sau:

- Lc ma st cn bng vi ti trng m vt bn trn tc ng xung vt th bn di.

- Vi cc vt th khc nhau lc cn do ma st xp x 1/3 ti trng.

- Lc ma st c lp vi din tch tip xc v ph thuc vp bn cht ca hai vt th tip xc vi nhau.

Leona d Vinxi khi nghin cu v ma st cng ch ra rng: lc ma st khng ph thuc vo hnh dng m ch ph thuc vo trng lng ca vt th v rp b mt.

C th ni giai on pht trin u tin ca khoa hc kinh in v ma st ngoi l nhng cng trnh nghin cu ma st tnh. Bn cht ng ca ma st ngoi ln u tin c t ra trong cng trnh nghin cu ni ting ca S.Culong vo nm 1785. Quan im ca S.Culong gn ging vi Amoton, cho rng lc ma st cn bng vi ti trng, khng ph thuc vo din tch tip xc nhng ph thuc vo bn cht ca vt liu. Culong cng ch ra l cn phn bit ma st tnh vi ma st ng v lc ma st ng c lp vi tc chuyn ng. Culong tip tc nghin cu nh hng ca lc phn t v coi l phn ca lc ma st. Thnh cng ln ca tc gi l tng hp ca hai sc cn, sc cn th nht l ca lin kt khng ph thuc vo din tch tip xc m t l vi ti trng, sc cn th hai l ca lin kt ph thuc vo din tch tip xc. Bn cht hai mt c th hin qua phng tnh sau:

(1)

Trong : F l lc ma st.

A l hng s c trng cho kh nng lin kt tng h ca vt th.

l h s ma st

N l phn lc php tuyn.

V mt gia cng th, tr s A l rt nh c th b qua c. Khi cng thc (1) chuyn thnh:

F= N (2)

Cng thc (2) thng c gi l nh lut Culong- Amoton.

Tm li, l thuyt c in v ma st cho rng: lc ma st ph thuc vo hai yu t chnh l ti trng t ln vt th ( bao gm c trng lng ca vt th v tng ngoi lc tc dng ln vt th) v bn cht ca hai vt th tip xc vi nhau

1.2 L thuyt phn tCc tc gi thuc trng phi ny m i din l F.P.Bowden, D.Tabor lm nhiu th nghim nghin cu nh hng ca rp b mt, din tch tip xc, tc trt v s chuyn di ca vt liu t b mt tip xc ny sang b mt tip xc khc n lc ma st. V nh hng ca rp b mt n h s ma st, Athur Morin (1934) cho rng: lc ma st khi hai vt th chuyn ng tng i vi nhau l khng i khi rp b mt thay i v h s ma st gim i dn n mi mn thp.

Khi nghin cu nh hng ca din tch tip xc, cc tc gi cho rng khi b mt hai vt th rt gn nhau lc s im tip xc thc t s cn bng vi din tch tip xc gia hai b mt v khi lc ma st l ln nht. Nh vy lc ma st ph thuc vo din tch tip xc thc t gia hai b mt.

Mt nh bc hc khc nh: H.Shaw (1986), F.P.Bowden (1938- 1942) khi nghin cu v ma st nhn thy b mt ca cc vt th thng b co xc v ny sinh vic kho st qu trnh vn chuyn vt liu t b mt ny n b mt khc. Cc tc gi trn cho rng bn cht ca qu trnh l do lc tng tc gia cc phn t trn b mt ca hai vt th v rt ra nhn xt sau: khi lng chuyn di ph thuc vo qung ng ch khng ph thuc vo ti trng. Khi mt vt th cng hn chuyn ng trn b mt ca vt th mm hn th lng vt liu chuyn sang vt th cng hn ph thuc vo rp b mt ca hai vt th.

L thuyt phn t cng cp ti nh hng ca tc trt ti lc ma st. Galton (1978) khi nghin cu ma st xy ra khi tu chuyn ng trn ng ray a ra nhn xt sau:

Khi tc trt thp, lc ma st tng ln cng vi tc .

Ti tc trt trung bnh khong 1 m/s ti vi ft/s, lc ma st gn nh khng ph thuc vo tc .

Vi tc trt cao, lc ma st gim i cng vi tc .

Ti cn di ca di tc P.P.Ewald, T.Poschl, L.Prandtl (1930) a ra cng thc lin h gia h s ma st v tc trt nh sau:

Trong : : h s ma st tnh.

: tc trt (m/s).

Tuy nhin, cc tc gi cha ch ra l cng thc trn c p dng c hay khng khi hai vt th chuyn ng vi tc ln, b mt chuyn ng sang trng thi mm cao thm ch b chy nht lm gim ng k h s ma st.

Tm li, l thuyt phn t v ma st cho rng bn cht ca lc ma st l lc ht gia cc phn t khi hai b mt vt th tip xc vi nhau. Do lc ma st khng ph thuc hay ni cch khc l c lp vi ti trng. Lc ma st ch ph thuc vo din tch tip xc thc t v tc chuyn ng tng i ca hai vt th.

1. 3 L thuyt in.L thuyt ny c xut do trong thc t khi hai vt th tip xc vi nhau gy ra s pht sng v c kh nng ht cc vt th nh khc. Hin tng ny nhn thy nhiu nht trong cng nghip giy khi giy c cn qua hai trc. Cc tc gi J.A.Jon (1925), P.Schnurmann (1940), E.Warlow- Davises (1942) cho rng in ma st (triboeletricity) v ma st l hai mt ca cng mt s kin: ph hu lin kt gia cc nguyn t ca cc b mt tip xc vi nhau.

Theo l thuyt in th ma st l tng hp ton b cc lin kt in gia cc nguyn t ca hai b mt khi chng tip xc vi nhau. Tng tc nguyn t- phn t c coi l ch yu cn cc tng tc c hc ch l kt qu ca cng lin kt nguyn t- phn t.

1.4 L thuyt hin i.Cc l thuyt trnh by trn tuy c tha hng, pht trin nhng kt qu t c ca cng trnh l thuyt trc , nhng do iu kin, mc nghin cu v theo cc xu hng khc nhau nn xut hin nhng quan im khng ging nhau v ma st. Mt khc, do cha c nhng phng tin my mc hin i nghin cu mt cch chnh xc cc hin tng xy ra trong qu trnh ma st nn cc kt lun rt ra thng t trc quan v dn ti s bt ng v quan im.

Nhc im ln nht ca cc l thuyt v ma st trc y l chng u da trn cc hin tng c bit nh c hc, vt l hoc in m khng xt ti nh hng tng h gia chng, do khng phn bit c lc ma st vi sc cn ca cc khuyt tt trn b mt vt liu.

S pht trin ca khoa hc v ma st c gn lin vi s pht trin chung ca khoa hc. Nh nhng nh lut v ph hu bin dng do- n hi, cc cng trnh nghin cu v b mt ca vt th khi chuyn ng, kt cu mng ca kim loi cc b mt khi lm vic m cc nh nghin cu v ma st a ra l thuyt v ma st ngoi tng qut bao gm c ch vt l, c hc, ho hc, nng lngTheo cc l thuyt hin i th lc ma st khng ch l hm ca lc php tuyn m cn ph thuc vo t hp cc yu t: tc trt, vt liu, iu kin mi trng,S ph thuc ny c th biu din bng cng thc tng qut nh sau:

T(n) = f(n,v,c) (4)

Trong : T(n)- L lc ma st ng vi ti php tuyn n.

v- Tc trt.

c- Cc thng s nh mi trng, vt liu,Cc khi nim mi v ma st sau ny do Suh v Sin xng (1981). Theo cc tc gi ny th cc tnh cht v c hc c nh hng ln hn so vi cc tnh cht ho hc i vi lc ma st, nu trong qu trnh chuyn ng khng c hin tng tng nhit . Theo quan im ny c th phn chia lc ma st ra lm 3 thnh phn:

Bin dng ca cc nhp nh trn b mt.

S bm dnh ca cc din tch tip xc.

S trc ca b mt.

Cc nh nghin cu khc cng a ra nhng l thuyt v ma st, bi trn, hao mn cho tng trng hp c th. Khi nghin cu v ma st xy ra mt vt th c kh nng phc hi cao tng t nh cao su, G.M.Bartenev, V.V.Lavrentjev v N.A.Konstantinova (1971) a ra cng thc sau:

EMBED Equation.3 Trong : F- Lc ma st.

c- H s ph thuc vo gc trt.

-Din tch tip xc thc t.

SO- Din tch tip xc khi khng c p lc (PO).

- Yu t hnh hc ca b mt vt th.

N- Ti trng.

Vt liu ma st trn c s nha phenol- formandehyt rt ph bin, do vy thu ht c d quan tm ca cc nh nghin cu S.K.Rhee v cng s (1971). Cc tc gi nghin cu v nh hng ca cc yu t nh p lc, vn tc v thi gian ti lng mi mn ca vt liu ma st trn c s nha phenol- formandehyt n si amiang v a ra cng thc sau( p dng cho nhit b mt nh hn 220oC ):

W= K.Pa.Vb.c.

Trong : W- Lng vt liu b mt i.

P- Ti trng.

V- Tc .

- Thi gian.

a,b,c- Hng s ph thuc vo tng cp ma st.

ng dng cc l thuyt v ma st em li li ch to ln gip cho nhng nh nghin cu, ngi sn xut nhn thc v khc phc c nhng hn ch cn tn ti ca vt liu. Hn ch ti mc ti a nh hng c hi do ma st gy ra.

2. Cc loi vt liu c kh nng lm gim ma st

Gim ma st v mi mn c ngha quan trng i vi nn kinh t quc dn. Cc nh nghin cu a ra 3 loi vt liu chnh c kh nng lm gim ma st v mi mn bao gm: gm, hp kim v polyme.

So vi kim loi v polyme, gm c u im l h s ma st thp, cng b mt tng i cao, t b mi mn v khng b xy ho trong qu trnh lm vic. Tuy nhin, gm li b nh hng ca nhit, dao ng v c bit d b v khi va p.

Kim loi v hp kim c s dng rng ri nht trong lnh vc vt liu chng ma st do nhng u im ni bt nh: c cng b mt cao, c kh nng lm vic nhit cao.....Nhc im ca chng l kh gia cng, c gi thnh cao v khng bn ho cht.

Vic nghin cu s dng vt liu Polyme cho cc kt cu ma st cho kt qu bt ng. So vi kim loi, polyme c h s ma st nh hn, t mn hn, t b nh hng ca dao ng v va p, c gi thnh r, tnh cng ngh cao hn trong vic ch to chi tit, c kh nng lm vic trong mi trng ho cht v bi trn bng nc.

Tuy nhin s thay th kim loi bng Polyme khng phi lc no cng c li. i vi kt cu chng ma st, hng nghin cu c nhiu trin vng nht l kt hp gia polyme v cc vt liu khc.

3. Cc phng php nghin cu ma st v mi mn

Trong qu trnh c xt s xy ra tng tc cc b ca cc lp b mt vt liu trn cc din tch rt nh. S tng tc ny lm thay i cu trc v tnh cht vt liu trn b mt c xt. i vi cc cht do, cc thay i ny rt mnh v chng xy ra di tc dng ca nhit, tc ng c hc, cc cht hot ng b mt, in tch xut hin sn phm t cht dro vi mt t hp tnh cht cho trc, iu quan trng l s dng cc s th nghim ma st m phng c cc iu kin d s dng nht, in hnh nht.

Khi nghin cu su v ma st v mi mn trong iu kin bi trn ngi ta s dng n rbt.

Trong a s cc trng hp, cc c trng cn nh gi hn c l lc ma st, bm dnh, cc thay i c l b mt, chuyn v ngang v vung gc.

Lc ma st thng c o bng cc phng php cn lc k hoc tenx, hoc theo tt dn ca con lc. V tnh cht b mt c xt thng xt theo kt qu o tuyn hnh b mt c nh gi theo kt qu khi ph, ph in t v phn tch cu trc Rghen. S tng tc , nhit v ti trng k thut hin i dn n s gia tng khc nghit ca cc th nghim v mi mn vt liu. Bn cnh , i khi phi tnh n cc yu t nh hng nh chn khng su, bc x v mi trng xm thcH s ma st ph thuc rt ln vo ti trng vung gc, tc trt, nhit v cc yu t khc.

S ph thuc ca cc h s ma st vo ti trng thay i theo nhit . nhit th nghim c nh th h s ma st gim khi tng ti trng cn nu ti trng c nh h s ma st tng khi ti trng tng.

Khi thay i nhit th c vn tc trt ca cht do cng c th c cc c tnh khc nhau nh mt vt thu tinh, vt mm cao hoc vt do.

Khi nh gi chu mi mn ca cht do nn chn mt c trng khng thay i theo cng c xt. V d: chn t s gia cng mi mn chung v cng c xt. S nh gi chu mi mn, theo t s trn c tnh cht gn ng do s ph thuc ca s mi mn khng ch vo tnh cht vt liu m c vo iu kin th nghim.

Cng mi mn c th nh gi nh lng theo mt i lng thng s o:

In = h/L = V/(A. L)

Trong : h- b dy lp b mi mn i.

V- th tch lp b mi mn i.

L- qung ng c xt.

A- din tch chun ca b mt.

mi mn cng c th nh gi bng ch tiu nng lng

In = V/W

Trong : W- nng lng c xt.

Chng II: Tng quan vt liu polyme Compozit- Vt liu ma st, cc yu t nh hng1. Tng quan v vt liu Polyme Compozit. Khi nim c bn - c im - Phn loi.

1.1 Khi nim.Vt liu Compozit c lch s pht trin t rt sm. Ngay t khi hnh thnh vn minh nhn loi nhng vic nghin cu ch to vt liu Compozit mi c thc s ch trong nhng nm 50 tr li y. Mc ch ch to vt liu Compozit l lm sao phi hp c cc tnh cht m mi vt liu ban u khng th c c. Nh vy c th ch to vt liu Compozit t nhng cu t m bn thn chng khng th p ng c cc yu cu i vi vt liu.

Vt liu Compozit ni chung l loi vt liu ng nht trong th tch ln nhn c bng cch hp nht cc th tch nh ca cc vt liu khc. V bn cht vt liu Compozit l h thng hai hay nhiu pha khc nhau v bn cht ho hc gn nh khng tan ln trong nhau, phn cch nhau bng ranh gii pha trong pha lin tc hay nn l polyme, pha phn tn l ph gia tng cng. Ngoi ra cn c mt s hp cht khc nh: cht to mu, cht tng cng c bit.

Vt liu Compozit c th ch to bng cc phng php c nhit t cc thnh phn hay bng phn ng ho hc m dn n s phn chia pha trong hn hp ng nht ban u. Vt liu Compozit c th c ch to bng nhiu phng php khc nhau nh c p, qun ng...

1.2 c im.

Vic a cht gia cng vo nn polyme a li cho vt liu Compozit nhiu u im ni bt so vi vt liu truyn thng: bn c nng cao, m un n hi cao, t trng thp, n nh tnh cht trong nhiu mi trng ho cht, chng mi mn tt...

Nhng c im ni bt ca vt liu Compozit l: L vt liu nhiu pha, trong thc t ph bin nht l loi vt liu Compozit hai pha, pha gin on l ct c bao bc bi nhiu pha lin tc l nn polyme. Cc pha tng tc vi nhau qua b mt phn chia pha nn tnh cht ca vt liu khng ch ph thuc vo bn cht ca cc thnh phn m c ph thuc rt nhiu vo lin kt gia cc thnh phn. Vt liu c tnh cht cng cao khi lin kt gia cc thnh phn cng cht ch. C tnh ca vt liu cn ph thuc vo hnh thi hc nh hnh dng, kch thc....mt phn b v phng, th t sp xp ca ct. Mt ca ct c xc nh qua t l khi lng hoc th tch. y l mt thng s quan trng trong vic quyt nh tnh cht c hc ca vt liu. Khi ct phn b khng ng u vt liu s b ph hu trc ti nhng v tr mt ct thp do lm gim bn kt cu ca vt liu. y l mt c trng tri nht ca vt liu Compozit. V vy vi nhng phng php ch to khc c th a li nhng vt liu Compozit c tnh cht c l khc mc d c cng thnh phn ging nhau nh phng php qun ng v c ko lin tc.

- Trong vt liu Compozit t l hnh dng - kch thc cng nh s phn b ca nn, ct tun theo cc qui nh thit k trc, ni cch khc vi s la chn thch hp cht tng cng v nha nn tnh cht ca vt liu Compozit c th tnh ton trc.

- Tnh cht ca cc pha thnh phn c kt hp to nn tnh cht chung ca Compozit. Tuy vy, tnh cht Compozit khng bao hm tt c cc tnh cht ca cc pha thnh phn khi chng ng ring r m ch la chn v pht huy thm nhng tnh cht tt.

- C nhiu phng php gia cng cho php ch to nhiu dng sn phm c c tnh khc.

Tuy nhin vt liu Compozit cn c mt s nhc im:

- L vt liu a thnh phn nn vic ng thi tho mn tt c cc yu cu ca vt liu rt kh khn. Ch c th la chn cc thnh phn v phng php ch to thch hp c th p ng nhng yu cu quan trng nht.

- M un n hi khng cao.

- Nhit lm vic tng i thp.

1.3 Phn loi vt liu Compozit.

phn loi vt liu compozit ngi ta da vo cc c im chung ca chng:

Theo bn cht vt liu nn: compozit nn polyme, gm, graphit, kim loi.

Theo hnh hc ct: compozit ct ht (th, mn), compozit ct si (di, ngn).

Theo cu trc vt liu: compozit tm, lp, tm 3 lp, khi, t ong..

Theo phng php ch to: c, p, c phun, ln l Theo phm vi ng dng: compozit cao cp, compozit k thut.

a. Phn loi theo bn cht vt liu nn:* Nn kim loi: Kim loi c bn, cng, do kh cao, c th chu c nhit cao hn polyme trong mi trng khng c oxy. Vt liu compozit nn kim loi c modun rt cao (110GPa) nn i hi si tng cng phi c modun cao. Al, Mg, Ti, Ni, Cu c nghin cu ng dng lm nn compozit nhiu hn c, c bit cc kim loi nh rt c a chung trong ngnh hng khng.

Nn kim loi c mt s u im so vi nn polyme:

Kim loi ni chung do, kh bn nn ci thin ch tiu c l ca polyme nh modun n hi.

C dn in, nhit cao hn.

Khng bc chy, t b nh hng ca mi trng m.

C tnh n hi tt nn trong mt s trng hp d gia cng lp rp.

Nhc im chnh ca nn kim loi l c khi lng ring ln. Cng ngh ch to compozit nn kim loi phc tp hn rt nhiu so vi nn polyme v do vy gi thnh sn phm s cao hn. Chnh iu ny hn ch vic la chn v trin khai ng dng chng trong cng nghip.

Compozit nn nhm ct si bo c c tnh cao l mt trong nhng compozit nn kim loi c ng dng kh nhiu trong cc ngnh cng nghip nh hng khng, v tr, ch to t.

*Nn polyme: Compozit nn polyme chim 90% trong tng s cc loi compozit. Nn polyme rt c a chung do c t trng thp, cch in, cch nhit tt, d gia cng. Polyme dng lm nn c 2 loi: nha nhit rn v nha nhit do.

1. Nha nhit do c cu trc mch thng hoc mch nhnh, chy mm nhit cao v c th ly li trng thi rn khi lm lnh. Nha c th dng tinh th hoc v nh hnh. Kh nng chu dung mi, chu nhit km hn nha nhit rn. Ngi ta thng tng bn, chu ti v chng ro cho vt liu bng cc loi si ngn. Nha nhit do gia cng ch yu bng phng php c. nhit thng, compozit nha nhit do c tnh dai, bn, cng nhng u b mt tnh bn nhit cao. C th khc phc phn no nhc im ny bng cch tng cng cht n. Cc loi nha nhit do thng dng: polyetylen (PE), polypropylene (PP), polystyrene2. Nha nhit rn c to thnh t cc polyme cng chnh l compozit nhit rn, c kh nng to c cc lin kt ngang (lin kt ho hc). Qa trnh to cc lin kt ngang gi l qu trnh ng rn. Cc phn ng to lin kt ngang ny lin kt cc phn t polyme l vi nhau to thnh mng li khng gian ba chiu c khi lng phn t ln. Chnh iu ny khin cho nha nhit rn sau khi ng rn khng c kh nng nng chy hoc ho tan m ch c th b phn hu.

T hp ca nha phenol fomandehyt vi si aming bn vi axit, kim nng nh hn 40%, bn vi axeton 500C v bn vi axetic bt k nhit no.

Vt liu polyme compozit trn c s nha phenol fomandehyt vi cht n thch hp nh: si thu tinh, si amiang, c kh nng lm vic nhit cao ti 2500C hoc trong iu kin nhit lm vic thay i trong mt phm vi rng m khng c nguy c b thay i kch thc.

chu mi mn ca tm phng t nha phenol - fomandehyt c th so snh vi nhm, ng. Tuy nhin kh nng ny b suy gim khi m ca mi trng tng. H s ma st ca nha phenol- fomandehyt tng t nh cc loi nha nhit rn khc, nm trong khong 0,2- 0,3, tng i thp so vi cao su v kim loi nhng vi cc cht n thch hp c th tng h s ma st ca vt liu ln ti 0,7- 0,8.

Vt liu polyme compozit i t cc loi nha nhit rn c tng cng cc loi si di s dng nhiu trong cng nghip hng khng, v tr, qun s, t, Nha nhit rn ph bin nht l epoxy, polyeste khng no, phenolic, polyuretanTnh cht c l ca phenol- fomandehyt so vi cc loi nha nhit rn khc c trnh by qua bng sau:

Tnh chtn vPolyeste

Khng noEpoxyPhenol- fomandehytUre- fomandehyt

bn koMpa25803010025652545

bn nnMpa6016060904512540120

bn unMpa701406018045954595

ht ncMpa103072015301530

T trngg/cm31,11,151,151,251,151,31,11,4

* Nn gm v thu tinh: T chc in hnh ca gm l a pha v a tinh th. Hai pha chnh to nn t chc ca gm: pha v nh hnh phn b xen ln gia cc vng pha tinh th v gn kt chng li vi nhau. Gm l vt liu c bn cao, duy tr c bn nhit cao (1650oC), kh nng chu oxy ho cao. Tuy nhin c bn ko thp, chu va p km.

Thu tinh v c l vt liu nhn c bng cch lm ngui mt hp cht v c t trng thi nng chy hon ton nhit cao n trng thi rn khng kt tinh. Qa trnh ny ging nh i vi nha nhit do nn c th p dng cc phng php gia cng ca cht do. Thu tinh c bn v modun n hi cao (69GPa), gin n nhit thp. Nh tnh chy tt nn khi tng nhit v p lc p th hm lng si tng cng c th s tng nhng khng lm gim bn vt liu.

* Nn cacbon/ graphit : L vt liu chu nhit rt tt, cng, bn nhit (25000K) in hnh l compozit cacbon- cacbon. Vt liu ny c s dng nhiu trong ngnh hng khng, ho dc, y t...

b. Phn loi theo hnh hc ct:* Compozit ct ht: c cu to gm cc phn t ct dng ht ng trc phn b u trong nn. Cc phn t ct rt a dng: cc loi khong t nhin, oxit, cacbit, nitritCompozit ct ht gm hai loi: ct ht th v ct ht mn.

- Compozit ct ht mn thng c nn l kim loi hoc hp kim, ct c kch thc nh (