G rind ing of Tra nsf o rmation-Toug hened Y -TZ P Ceramics

R. Krishnamurthy, L. M . Arunachalam, C. V. Gokularathnam, Indian Institute of Technology, Madras 600 036/lndia Submitted by V. C. Venkatesh (1). Tennessee Technological University, Cookeville, Tennessee/USA

Received on January 15,1991

ABSTRACT

Deveiopments in ceramics have introduced a new c!ass of ZQ cenmics called i s cenmic steel. By controlling the ml. percentage and panicie size. a Zirconia-Y:ma cenmic was aeveioped which exhibits roughening by terngonai :o mnoclinc !rsnsiormation. The inserts made oi (Transformation Toughened Zirconia! we= ,mund for apoiication 3s a cutting m i . During surface grinding, roughening of suriace material was observtd; he amount of ransformarion was more with CBN than diamond k~nding, indicanng the smss induced narure of mansfomation. This rnnsfomtion has intluenced significmt!y specific grinding ?ressure. nomal ginding force component and surface quality.

Key Words: Transformation toughening, Grinding, Zirconia.

1. INTRODUCTION

The demand for higher productivity at reduced manufacturing cost is i:n?osing i need for cutting tools which can withstand severe themial influences associated wirh higher cutring speeds and enhanced material removal rares. Development in tool materials has i n d u c e d SULON Ceramics. which exhibit improved oxidation resistance and resistance to creep and other chemical phenomena. Howeve:. this material has suffered from traditional kacrure toughness problems. To over come this. reinforced ceramics and ceramic comesites have k e n introduced. Tne basic concept is in reinforcing a ceramic matrix wirh extremely strong. stiff. single crystals of S i c or Si&. Of iate. developments based on micro suuctural engineering concepts have introduced a new ciass of c:rmics called Zirconia toughened Ahnind (ZTA). This material utilizes the phase ransfomation of Zirconia (1.1) for enhanced toughcess qualities. Yct another new cbss of cenmic tools utilizing the tetragonal monoclinic transformation in Zirconia - YtIT;a Systems - has been deveiopea: these tools (Transformation roughened Zirconia (TTZ)). developed :it IIT, bladns, have perfonned satisfac:orily in machining both free machining s t d :in4 spheroidal graphite i d s t iron. Figure 1 illustrates :he potential ;ipplic;ition range of dirTerent cenmics. For an effective commercial utilization. it is essenmtl to generate dau in grinding of TTZ ceramics. In the following sections of the pnper. data on grinding of T I 2 is presented.

0 so0 1000 Applied stress IMP01

Figure I Potrntiul Application Range of the Engineering Ceralnics SIC, Si,N,, Z?A, PS.4. TZP, and ZrO, - l'oughetled Ceramics

t. (;KINDING OF Trz CERmiics

Grinding of ceramics assunies grater significaiice with its widespread use as structural engineering materials. Grindability of cenmics largely depends on localized defects (in ceramics). their size. density and dismbution. Depending upon :he stress field (grinding stresses) and strength tield (defects in ceramics) intensities. grinding may be associated with plastic deformation and shearing or perkct sheuinghpturing. The material removal which is the sum of microscopic deformations and fractures at the point ofcontact of abrasive grains with the ceramics 131 also depends on frac:ure :oughness. hardness. and thermal conductivity. When a sharp abrasive grain comes into contact with a hard brittle material like ceramics, the ceramic-work surface experiences laterdl and Cone type cracking due to cohesion failure. It is thus possible that the size of

the chip becomes grcater than the depth of grinding, posing additional problems in dimensional control. An expression for minimum threshold load for occurrence of such cracking has been derived as (41

where

A . dimensioniess constant. K, - fracture roughness. E - young's modulus. and H - hardness.

The ibove expression suggesrs that for conmlled grinding of ceramics, the normal grinding !odd has to be h i t & based on fracture toughness and the rat10 EH. The newly developed material exhibits a K, value much higher than any of the traditional ceramics.

I t is known that mechanisms responsible for toughening of Zirconia cermiics are stress induced transformation, nucleation of micnxracks. and crack-branching. The micro-srructural ptuameters, such as volume fracti.on. chemical composition. size and dismbution of Zro, panicles. can control the dismburion of :ecagonal Zirconia in a given matrix which enhances the :neckanicsl properties of TTZ during tetragonal to monoclinic transformation. as shown in the phase diagram in Figure 2. It is seen that for the m undersrudy. containing 3 M0l.Q of Yttria. it is possible to achieve tetragocal monoclinic rransformatioiis enhancing its toughness. It is reported tha! tetragonai is a metastable phase and transforms to monoclinic at lower temperatiires undergoing :in increase in volunie, thereby promoting matrix roughening. The tetragorid to monoclinic transformation is analogous to ni:irtensitic transformation; it star~s around 350" C. When the tetngonal ZrO, panicles are coiistreined, this temperature can k lowered. This has k e n observed in the "surface toughening of TTZ ceramics" ground by surface grinding.

3000 . .

L - Llquid

F - F I u o r i t c lcubic)

T - Tetrogonal

T T - Tronsforrnabk Tetragoml

PI- Monoclinic

T '- Non transformable Te trogonol

M.1

I ; s 10 1s 20 I 1 1

no1 Y. YO1.s

Figure 2. Low Yttria Region of rhe ZrO,-YO,,, Phase Diagram

? . I . Translormation Touthenine of IT2 Ceramics During Grinding

TTZ ceramic inserts were surface ground using both diamond and CBN wheels as per derails presented in Tdble 1. During grinding, a thin layer of

Annals of the CIRP, Vol. 40/1/1991 33 1

marcrial is cxpihed to higher order remperJriire iind strsses. The :ipp;!ren! e.xpansion nf t!iis hented zoiir: of rna:erid wou!d be resijred b< :he bulk of the m:iterial. resulting in loducriorl of hydrostatic compressive sires$ over the surf:ice matend In the TTZ cc~31:iics t h i s coinpresi\e stress con prm!lice r-m rianr.iormatiori l3y sclzc!inp the fine grained 21-0, and :iIm restricting the Yrtria to 3% (inoi;. i t I*;IS possit.le to reduce the transformation temper:irure below the grindin3 temperatures. X-:ay diffrac:io:i icrensity proli!es for monoclinic and reaagor.al were recorded for as sintered and sinrered.grr)r!nd TI2 inserts. The phase rrai:siormation oi retragona! Lo monoc!inic on grinding could be evaluated from :he intecsi:y ratio. <,, g iven as

r

ThBLE 1 Surface Grinding of Ceramics

350. 700. 900. 1500, 1Scx) mimin

I

h1,03 , 3.98 ; 18.5 1 344 'I

ion - - 9 0

-

- 8 0

- - 70

- - 60 - -50

- - LO

- - 30 -

-20

I ?06 li zro, (3 Mol. % j 5.9 ' , 12.5 il Yt:riai

Grinding Wheel

1 /I

Wheel Speed

/I /I Table Feed II I

I 2 1 5 mmisec (stepper motor dnven! , 1

Figure 3 illustrates rypical X-ray intensity profiles. I t is seen that with CRX gnndmg. more r-rn urnsfomation has occurred This shows that the TTL ceramics experience. IS more stressing during CBN - ~ n d i n g than i t 1s d u m g diamond gnnding.

I

TI101

Grindrng speed - 1800m/mtn F e e d - 21 Smm/sec Deprh o f cut - 2 O p m Wheel -Oiomord b C O N W o r k - 2 r 3 2 - U n p r o v i d - - - - Diamond gf- iding - _- C B H gr inding

(1011

'I 1 ' I I1

I !

I t ' l I t ' L 9

I ; I ! I I I I

TI1011

! !/ Ji

4 . i i 1 t = 3 . 6

! I

Figure 5 X-ray Diffracromenr Traces of lvl(l11) T(101) and M(1IT'I of Unground 2nd Ground Surfaces of Transformation roughened Zirconia !TTZI using Diamond Grinding and CBN Grinding

2 . 1 . Influmet. of c ; z ~ ~ ~ n . S ~ ~ ~ d _ ~ n ~ p . ~ c i - G ~ n ~ n . Pressure

For assessins rhe grinciahilit) of TI'%. g!nnding force components Fn (rorinal) and Fr (tangziitisi) %ere measured usiily an octagonal r i ng dyiiamometer. The specific ?rindin: p.eisur: w;is cslcul3ted.

Fisiie -1 iliutrraiss a typicai influence of grinding speed o!: specific gr:iiding pressure fur CB4' and diamond g h d i n g it i s F C S ~ ti:nt CBN grinds TTZ :vi;h hipher specific grinding pressure than diamond ,qndiny. CBN xs:sts ox:ca:ion kip 10 13(Joc C. while diamond :s stoble only :o a much :ewer remper;irure of around 3W' C. I r is known !lint CBS 1s mucn softer tnm finrnond: :his .isuJiIy resalts ;n grain :Iattening. unlike rhe case of Ciiimocd H hich : l a better se!f-shqening action. Further. the surface hexing ivould 5e mucn recuccd N.ZES ceing a better conductor! with CBX ,mnding. This Iras resulred ;r: highe: order specific grinding pressure. I t is worch noting here k i t higher specitic ynnding prersurc ioiild Jiso be due to Stress induced :r.in~tormntion af i -m phase. resulting i n roughening iarface i n x c i s l . Grindin$ o i roughened marenal would be ssswiared .with de%rmat:on w d shexinu,. resulang :n higher orde: gnnding smsses. This i-m :ranrii~mdtion vas observed by X-ray inrenairy profiles 1 s stated c;trlicr.

F e e d - 2 1 . 5 m m / r e c . Cepfh of cut - 20pm 2 5 1 W o r k - W h e e l - Zr32 e y

n 0 O i o m o n d - z 8 y1

Figure -I. Infiutnce of Gnnding Speed on Specific Grinding Pretsure

Refenins to Figurc 4 , i r is :iIso seen tli:it there IS d steep increase in griiidin: pressure around 100 m/mm oi ginding. This could be attributed 10

possible thermal decrmposition of diamond. For comparx'ive ev.iiuadons, yrmding rrudies on commercial grade. soid iornpacted white AI& Inserts have also been carried out. Figure 5 illusrrares the cornpanrive evaluxion of yr!nding oi the two seramlcs I t is seen that wirh Al@,, :here i s only a s m d l varxition in [he specific grinding pressure with pnding speed. This indicates th:it h!20, i d rather insensitive to ;hem31 intluences, while 210: undergoes transforni:ition touphening due to grinding trmperatures and pressure.

Depth at cut - 20pm Wheel - Oiomond h C B N W o r k - ZrOZ and A1203

1110'2

A1203

C B N

1'1010 L-L1or+- 500 Gr inding speed, m/min

Figire 5. Gr;nd:ibi!ity of Ceramics

2.3. Influence of Grinding

.As \rated earixr. ,piding force components in directions normal fFn) m d :mgenriai ! F i ) lo !he work surface were metisured using an octagonal nng !ype w d i n gnus: dyinmvmeter. Figure 6 IS 1 :ypic;il vilniltlon 3i Fn w i t h ;mnding soeed for both AI#, and ZOO, ceramics. It is seen that with grinding speed.

332

gindinp force decreases. whiie there is increase :n specific ciirring pressore (F:gure 5). >laterial removal in grinding of ceramics i s usoally associated predominately with fracture mechanisms and. to a smaller extent. with plastic flow and she.mng. The normal corrponen! Fn causes ploughing of tlic abrasive, elasric deformation ot wheel surface and cracking of work msterial by cohenion failure. With small grinding velacit:es. the wheel work contact s u r f m s evperience ploughing and. consequently, higher normal force. AS the grinding veiocity :ncreases. the grinding becomes steady. resulring in a reduc:ion normal force compocent. Fvrrher. with CBS gnndinp, thes is an increase in f n . Aker dressing, a; !he stan of gnnding !especially with smaller ve:ocitles). :he abnsive p i n s become dull and rlatrened. resulnng in an increase i n grinding force. This happened most with CBN pndin:.

2.4. Influence of Grindint! SDeed on Surface Finish

blatcnal removal dunng gmding depends upon the size and density of defecu :n :he work rnatefial. such as :laws. cncks lrnd size of !he stress field. When the applied stress field i s smaller than the size of the defect. ma:er:d swouid be mos;ly removed by plastic deformation l i larger. :ocaiired brirt!e . ture w0u.d be ihe mechiinism of :he material removsl. S:nca cernmics ue kiown for :he isolared pockets of defects. the mater.al rmoval would not be smoother. Consequently, it would be difficult to achievc sontro1:ed surface finish. Observation on ground surface texture indicated folding of aspenries. crpec:a!!y with 2x0,. It is known that in surface grinding. the tex:ure presents a uniformly serrated lay. with a number of parallel grooves over the surface If _rr:nding is associated with plastic deformation. then folding of :ispenties over the grooved positions would occur, rewlting in a reduc:ion in Rr:Ro value. (R: - p& 10 valley heigh:. Ra - CL.4 value.)

Fisure 7 :s a typical illusuadon of Rr!Ra dunng grinding of ceramics. I t is seen :hat ZrO, exhibits the leas: Rr!Ra when Found with CBX. It IS seen in

:he e;u!ier section rhat ZrO: experiences roughening dunng CBN grinding. due to stress indued i-m innsfomanon. Thus, toughening of :he surface mxenal of ZrO: facilitates more plastic deformation and consequent folding of ssper:ties over :he surface.

.Also during certimic grinding, it 1s known rhat the grinding stiffness ~ 5 ! pnm:irily i i i the i ices the depth of ginding. With higher order foorcc experienced !n CBN grinding, rhe ginding stiffness increases. This would iesuit :n :educed d e p h or' grinding and consequrnriy detonmng the asperities.

I 1 Feed - 21 Smn/Erc D e p t h a t cut- Z O p m Wheel -0iamond 6 CBN Work- ZrO2and A1203

I I I I I i z o o 1500 1 B O O

'0 600 900

Cnndin g speed, m/min.

Wheel- Ojamond 6 C B N W o r h - Zr02 ond All03

1 I I 1 1 600 90 0 1200 1500 1800

Grinding speed m/min .

Figure 7. Influence of Grinding Speed on RoRa Value

3. CONCLUSIONS

The data reported in :his paper i s from grinding studies on 3 new class of Z.Q ~Transr'orrnat~on Toughened Zirconia TE)). developed in >[&as, India. The :ou:h:-ing of :his cemmics. without sacnficing the hardness. could be used to advanrase 3s n cutting tool and internal combustion engine parts. The fol!owing major mnclusions are drawn from this study:

ZrO: undergoes more s:ress induced rrdnsformarion during CBN grinding ris conrimed by X-ray iniensiry profiles.

Be:iond around 900 m h i n of grinding velocity, diamond undergoes thermal degadation. resulting in higher <pecific grinding pressure.

Touzheniiig of Z d , surface material results In asperity folding and rzducrion iii RNRa vducs.

REFEKESCES

bl. Kriven Waluaud, "Possible Alternative Transformation Toughener to Zirconia." Jwrna! ~f American Ceramic Society. :](I?), 1989, p. 1021.

A.H. Hener. "Transformation Toughening in ZrO, Containing Ceramics." Journul of .Arnerinrn Ceraniic Socieiy. 70(10). 1987, p. 689.

S . Mnlkin, J.E. Rirter. "Grinding Mechanism and Suength Degradation for Ceramics," A W E Journal of Enelneering fur Industry. ?Say. Vol 111, 1989. pp. 167-171.

A G. Evans. D.B. Marshall, "Wsar Mechanism in Ceramics Fundamentals of Fracture and \ V e x of Mat." D A. Risney. ASME. 1981, p. 439

1. In:isaki. "Hish Efficiency Grinding of Advanced Ceramics. Awals of CIRP, '401. 35, I . 1986. F 211.

Figtire 6. Intluence of Grinding Sperci on Grinding Force

333