Study on Mechanism of Ceramics Grinding

Study on Mechanism of Ceramics Grinding K. Kitajima, Kansai University; G. 0. Cai, Northeast University; N. Kurnagai, Y. Tanaka, Kansai University/Japan; H. W. Zheng (l), Northeast University/China Received on January 14,1992 Summary: The grindabilities of ceramic materials are evaluated through the measurement 01 the gnnding forces, energy, temperature and wheel wear, and the SEM examination of !he ground surface and !he grinding swarf. Comparing with Sic and Si3N4, the grinding force. energy, temperature and the wheel wear in A1203 grinding with the same removal rate are lowest. The ground surface roughness of Sic is better Secause no any bulge formed on the side edges of streaks. The grindability of ceramics is related to the mechanical behavious of materials at grinding temperature.

Keywords: Ceramics and Grindability Address: 3-3-35, Yamate-cho, Suita. Osaka, 564 Japan 1. INTRODUCTION

Ceramic materials have attrac?ive properties: high hardness, high thermal resistance, chemical inencess. and low thermal or elec!rcal conductivity, to name a lew. Tne applicaricn of advanced ceramics as a substitute for classical materials is increasing. To meet the increasing demand for ceramic components, ef!icient machining technologies are needed. Because of their great hardness and brittleness, ceramic materials are extremely difficult to machine. Grinding with a diamond wheel is the only method which can meet the above-mentioned demand

[I1 [21. However, the material removal rates must be increased in current processes to improve productivity rates for economic justification and the surface finish must be improved because ceramic materials react sensitively to cracks and damage produced during grinding. which may lead to premature lailure in the event of mechanical stressing of the component. For these, the knowleges about the grinding mechanism of ceramics materials are needed. However, the processes taking place in the contact zone during grinding are not yet fully understood. One model of chip formation is based on the supposition that the ceramic material is softened by the high temperature at Ihe cutting point. thus becomes plastically deformable and can thereiore be machined just as other materials. Another model is based on the Hertzian surface pressure Of two bodies in contact with each other, where stresses and deformations produce microcracks. which cause break down of ceramic grains, so that brittle material erosion takes place (31 [4]. A combination of the two extremes according to the type of ceramic and the machining condilions is probably involved in reality. Grinding involves a complex interaction between a number of variables: workpiece material properties. wheel specifications. machine tool selection, and wheel preparation. Llnasaki has outlined the characteristic features of hard and brittle materials and discussed the found mental principle of ceramics grinding [2]. N.Kayaba lound that the specific grinding energy of ceramics is strongly correlated with fracture toughness and hardness IS]. T.Ueda measured the grinding temperature for S ~ ~ N using J infrared radiation pyrometer with optical fiber and found that the maximum temperature of cutting point was up to 1200 - 1300%

was determind by the relationship below:

There, the Vs is wheel speed, the VWis work speed, the ap is the depth of the cut and the B is the grinding width. The grinding temperature was measured using a thermocouple which is a couple of pre-welded nickel-alloy wires to be mounted in the workpiece consisting of two parts. Because of the thermal inertia of the pre-welded thermocouple the actual temperature has to be corrected by its time constant and the contact time between wheel and thermocouple. In order to examinate the matenal removal process under a scanning electron microscope (SEM). the workepiece surface was polished carefully. Then it is ground using a eccentric wheel specially mounted with high table speed. So that a group of intermittent grinding slreaks formed by grains can leave on the polished surface as shown in Flg.1. which are just like the streaks obtained by cluster wheel grinding. Table 1. Characlerislicfeaturesof lest material Mark 01 workpiece 1 A-l 1 A-2 'SN-1 ISN-2 SN-3ISN-41SC.1 ,SC-2 Matenal I A1203 I SnNi i, Sic Specificgravity gf/cm' 3 7 3 9 i 2 9 3 3 1 3 2 13 25 I 3 1 ' 3 1 Hardness HRA 951 187 8 7 1 91 i - j 9 2 1 . vickers.kgl/mm* - j i8ooi . i - j19Ool - ' 2 2 0 0 Bendingstrength' 20% 35 j 50 40 35 j 50 82 5055 kgfirnrn' 1200% 20 .' - 50 --'Fracture toughness KIC , I kgf/mm" 13 4 . - 19 7 1 4 5 1 3 5 Young's modulus E I x I O'kgt/mrn' 4.3 4.0 1 2.4 1.8 3.0 2.9 4.2 4.2 -Specific heat.Cal/gC . 0.19 1 . - 0.17 01 Heat conductivity . Calicm s C 0.07 0.07 - 0 04 0.07 0.16 0.2 Size Forsurfacegrinding 120XSOX20 (rnm) For cylindrical grinding d 1OOX3OXlOO (mm)

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able 2. Surface grinding conditions M7120A surface grinder Machinetool

[el. In this paper, the grindabilities 01 A1203. SIC and Si3N4 are evaluated through the measurement of the grinding forces, energy, temperature and wheel wear, and the SEM examination of the ground surface and the grinding swarf. Based on this the grinding mechanism lor ceramic materials is discussed also.

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Machine 1001 Wheel

2. T E S T METHOD A N D CONDITIONS Wheelspeed. VO

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GUP32X50 cylindrical grinder

1 SOC14ONlOOB(SOC).1A,300X1 OX127mm

1 SDC14OB75B.lA.300Xl OX127mm ~

33 mkec

The 99 5% Al203. SIC and S3iN.i oescribed in Table 1 served as test materials Surface grinding and cylindrical grinding have been done in this study The test conditions are delaited in Table 2 and 3 The normal and tangential grinding forces, Fn and Ft. were measured using a octagonal ring dynamometer and the specific grinding energy E'

Annals of the CIRP Vol. 41/1/1992

367

eccentric wheel

wheel'scenter-

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.--/ grinding streaks

Fig2. shows the normal and tangenttal specific grinding forces, Fn' and

F!', obtained under different conditions for the surface grinding of A1203 Si3N.i and SIC rnarerials. And the gnnding force ratios A ( A =FrvFt) are shown in Fig.3. From Figs.2 and 3 we can see that for surface grinding and when the feed rate Vw is over 3 mimin, the normal specific force Fn' in grinding Si3N4 is highest. then followed by Sic. and that in grinding A1203 is lowest. The tangential specific force Ft' 01 Sic grinding is highest. however, then followed by Si3N4. and that of A1203 grinding is lowest also. Both Fn' and Ft' increase with increse with increasing depth of cut ap or work speed VW. The ratios between normal and tangential forces for the surface grinding of Si3N4. SIC and A1203 are much different. The force ratio of A1203 grinding is highest, which changes from 7 to 11 and averages 8.6. The value of A in grinding Si3N4 changes in a narrow range of 5-6 and its average is 5.36. The lowest ratio A appears in grinding Sic. which changes from 3.1 to 4.4 under test conditions and its average is about 3.5. Fig.4 shows the specific grinding forces Fn'. Ft' and force ratio FvFn in the cylindncai grinding of Si3N4. SIC and A1203 with different material removal rate ZW.These results are quite similar with above results But the force ratio Fn/Ft are some lower than that in Fig.2 an 3. it may be caused by the difference between two wheels.

3.2. Specific Grinding Energy E' Fig.5 shows the relationship between specific grinding energy E' and the average cross-sectional area of the effective cutting edges am in the surface grinding test. The am is given by the equation bellow:

Where, ds is the diameter of wheel and the W is the average distance between the active cutting edges which can be determind by the measurement of grinding thermal signal using thermocouple. Rg.6 shows the specific energy E in the cylindrical grinding with different material removal rate Zw. We can find from Figs.5 and 6 that the larger the average cross-sectional 01 the effective cutting edges am or the material removal rate Zw. the less the grinding energy needed for any ceramic gnnding. For fine grinding with very small am the grinding energy needed can be 2-3 times larger than that of rough grinding with large am. When the am or Zw is constant. the specific grinding energy in gnnding A f f i wiil be much lower than that of Si3N4 and S i c grinding. Fig.7 shows the influence of work's bending strength 6 B, fracture toughness KIC and the modulus of elastic energy M.O.R. on specific grinding force on unit cross-sectional area of removed material Ks. The Ks. which is the same with E' in meaning, increases with increasing u 8, Klcand M.O.R.

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v s 3 7 . 6 8 misec Vw=3-12 mlmin ap=0.005-0.02 mm

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Fig.2 Specific grinding forces

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Tangential force Ft.N Fig.3 Grinding force ratios

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2 4 6 8 1 0 1 2 1 4 1 6 Material removal rate Zw,mm'/mm.s Fig.6 Specific grinding energy in cylindrical grinding

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3.4. Ground Surface roughness Fig.10 shows the influence of the kind of ceramic material and material removal rate on ground surface roughness. The surface roughness obtained in A1203 grinding and Si3N.i grinding will be much rougher :han that of SIC grinding under the same grinding conditions.

Material removal rate Zw,mm3imm.s Fig.10 Influence of the kind of ceramic material and material removal rete 3n surface roughness

3.5.Grinding Temperature

It is obvious that the grinding ratio will decrease with increasing

material removal rate and the grinding ratio of A1203 grinding is highest. then followed by that of S i c grinding, and that of Si3N4 grinding IS lowest. The relationship between grinding ratio and the mechanical characteristics of workpiece material is shown in Fig.9. it is found from Figs.8 and 9 that the material characteristic has a great influence O n grinding ratio.

There is a large number of researches on grinding temperature [q.But there is still the scarcity of date on the diamond grinding, specially on ceramic grinding. Fig.1 1 summarizes the grinding temperature of A1203. Si3N4 and Sic under different grinding conditions shown in Tab.2. It is evident that the grinding temperature increases with increasing both depth of cut ap and feed rate VW.According to the equation of the maximum grinding temperature, the temperature should be decrease slightly with the increasing movement speed of heat source. When the feed speed increased, however, the material removal rate and therefore the source heat intensity increases proportionally also. So that results shown i n Fig.11 can be understood. We can also find from Fig.11 that the grinding temperature for Sic and Si3N4 are close to each other. In the case of higher removal rate, the former is some higher than the latter. The temperatures in SIC and Si3N6 grinding can change from 290% to 1335°C and from 400% to 1120Y: Vw=Bm/min

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Vw=l 2mlmin

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!f 600t

- 1 0.01 0.02 Depth of CUI ap.mrn Fig.11 Grinding temperature of ceramics 0.01

4 6 8 10 12 14 16 Material removal rate Zw,mm3/mm.s Fig.8 Relationship between grinding ratio and material removal rate 0

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Grinding ratio.mm3/mm2 Fig.9 Relationship between grinding ratio and u ,and M.O.R.

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60 0 20 40 Tanqential force Ft.N Fig. 12 Relationship between grinding temperature and tangential grinding force 20

40

60

under different conditions respectively. While the grinding ternperatwe in grinding alumina only changes from 120% to 575% under the same conditions, which is much lower than that of Sic and Si3N4 grinding Comparing Fig.11 and Fig.2 it can be found that the situation of grinding temperature for different ceramic grinding is very similar to that of tangential grinding forces. The relationship between tangential force Ft and grinding temperature is shown in Fig.12. When the depth of cut am is constant, the grinding temperature seems directly proportional to the tangential grinding force. The proportionaiity will decrease with increasing the depth of cut am.

plastically removed material and very fine particulate swarfs. The swarfs of Si3N4 grinding are mixture of short strip-type chips and fine particles (Fig.lG(b)). This strip-type chip is partly formed through the plastic deformation. The Sic chip fragments are shown in Fig.lG(c). It is mainly formed through the micro-brittle flacture in grinding. Therelore. the chips have sharp edges and corners. It is evident there must be a remarkaole difference in the chip removal process between Sic grinding and A1203 sr Si3N4 grinding.

Examination of the grinding streaks on ground surface under a SEM shows that the chip formation process of AIz03. SisNa and Sic grinding are much different. Figs. 13.14 and 15 are the SEM photographs of grinding streaks lor A k a . Si3N4 and Sic respectively. For A1203 grinding, plastic bulge appears on the sides of streak grooves formed by diamond grains. And the larger the depth of groove, the more obvious the plastic deformation of workpiece material. The walls and bottom of the streak grooves are smooth, just like that of plastic metal grinding. For Si3N4 grinding, the pattern of streaks is some similar to that of A1203 grinding: the bulge exists on the sides of streak grooves. This bulge material is composed by the material flowed out from groove plastically and the material dug out from groove by fracture. This is to say that the streak bulge in Si3N4

(bl LaDed arindina streaks(X850) Plastuc bulge on edges

grinding is formed by both plastic deformation and brittle fracture. So that the walls and bottom of the streak groove are uneven on which there are many cracks. The pattern of streaks in Sic grinding, however, is very different from that of above two materials. No any bulge but chipping appears on the side edges of ground grooves. The chipping and the material removing IS completely caused by brittle fracture. When many streak grooves laoed

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(a) End of grinding streak(X425) Fig.14 SEM photographs of grinding streaks for Si3N4(SN-4)

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(c) Laped rrrindino streaks with larqerdepth of cut(X850)

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nrindinn streaksfX850) ~~, Edges

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(bl LaDed arindina streaks with small depth of cutlX8501

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Fig.13 SEM photographs of grinding streaks for A1203(A-2) each other, due to the successive cutting ac!ion by many graics on wheel surface, a lot of chipping with big piece will happen on the hill edges between grooves. In this case the walls of groove become intermittent even beyond recognition. 4.2, The Grinding Swarf The grinding swarfs of Al203, Si3N4 and Sic are different also. Tne swarfs obtained in A1203 grinding cmsis: of spongy nodular cnips am2 some fine parltcles (Fig.lG(a)). The ncdular chips may be made up O f

I b l Sinole arindina streak(X850)

(a) End of arindina streak(X42.51 Fig.15 SEM photographs of grinding streaks fo SiC(SC-2)

5 . DISCUSSION The comparison of grinding features and material characteristics for ditferent ceramics is shown in Table 4. It is obvious from Tab.4 that the grinding features on the grindability of different ceramic materials is direct related to the workpiece material characteristics, mainly the hardness, the bending strength, !he fracture toughness and the Young's modulus of ceramics. All of A1203. Si3N4 and

Sic are hard and bri:tle materials. But the ratios between covalent and ionic bonding of these materials are different. As well known, strength and fracture toughness of materials which have a large ratio of ionic bonding are considerably affected by temperature materials with covalent bonding, on the other hand, are not as affected by eleva!ed temperalure. Therefore, at high point temperature the materials with more ionic bonding such as A1203 and SnN4 can be remove through plastic deiorrnation more or less during grinding. This is to say, the grinding temperature plays an important part in ceramic grinding process. So the mechanical characteristic at grinoing temperature must be taken into consideration in the study on ceramic grinding mechanism.

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6. CONCLUSION 1 ) Comparing with Sic and Si3N4, !he grinding force, grinding energy,

grinding temperature and the wheel wear in A1203 grinding with the same removal rate are lowest. The grinding of Sic and Si3N4 IS more dilficult than A1203 grinding. However, the ground surface roughness of Sic is better because no any bulge formed on the side edges of streaks. Force ratio FNFt 2) The grinding temperatures of tested ceramics are much different. For A1203 grinding it is lowest and for Sic grinding it is highest. The maximum surface temperatures on contact area in A1203. Si3N4 and SIC grinding under test conditions are 575% ,112O'C and 1335% respectively. 3) The grindability of ceramics is related to Ihe mechanical behavious

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Plastic deformation

Surface roughness Ratio between covalent bonding and ionic bondig Affecte of temperature on strength

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of materials at grinding temperature.

REFERENCES [I] Saije.E.. Damlos, H.H. and Mohlen. H..1985, Internal Grinding of High Strength Ceramic Workpiece Materials with Diamond Grinding Wheel, Annals of the CIRP, 34/1, p.263-266 (21 Inasaki, I.,1987, Grinding of Hard and Brittle Materials, Annals of the CIRP. 3612, p.463-471 [3] Pluta, 2. and Kacalak. W., 1983. Microscopic Investigations of Cutting Marks on Aluminium Oxide Ceramics, lndustric Diamanen Rendschau. 17. N0.3. p.124 [4] Sheppard.L.M., 1987, Machining of Advanced Ceramics, Advanced Materials & Processes inc. Metal Progress, 12/87, p.40-48 151 Kayaba. N. and Furisawa.M.. 1989, A Study on Efficient Grinding of Ceramics by Fracture Machining. JSPE. 55, No.7, p.1289-1294 [6]Ueda,T., 1989, The Measurement 01 Grinding Temperature for Si3N4 Using lnlrared Radiation Pyrometer with Optical Fiber, JSPE, 55. N0.12. p.2273-2276 Snoeys.R.. Leuven,K.U.. Maris,M. and W0.N.F.. 1978, Thermally Induced Damages in Grinding, Annals 01 the CIRP. 2712, p.571-581 181 Zhang,Q.C.. 1987, "Mechanical Behavious of Ceramic Materials". Publishing House of Science, China

Hardness

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