Ceramic Tools — Material Characteristics and Load Types Determine Wear Mechanisms

Ceramic Tools - Material Characteristics and Load Types Determine Wear Mechanisms

E. Brinksmeier, S. Bartsch - Submitted by H. K. Tonshoff (1); University of Hanover / West Germany Received on January 1,1988

Abstract Material characteristics of ceramic cutting materials have been measured for description and verification of the ceramic specific material properties. Young's modulus, bending strength, specific heat capacity and heat conductivity of various ceramic tool materials have been determined in a temperature range up to 120OOC. These properties are significant for wear behaviour in cutting processes. For simulation of thermal load components comparable to cutting loads a laser beam method has been developed, because only this method enables temperature - time - functions, which occur in cutting processes. The material response on thermal loading is described. Keywords : Ceramics, material characteristics, thermo shock, thermally induced wear

1. Introduction

Ceramic cutting materials are of high interest in production engineering due to high applicable cutting speeds. This enables high material removal rates. Ceramic materials used as cutting tools are based on aluminum oxide and silicon nitride nowadays, so that two material groups can be distinguished, which in their part are subdivided too. The materials in the subgroups differ by composition, manufacturing process and properties. As a consequence of the specific properties of these ceramic materials, distinct wear types and application ranges exist /1,2/. In compa- rison with traditional tool materials like high speed steels and carbides, the most important advantages of ceramics are their higher wear resistance, higher heat strength and higher chemical stability. Critical drawbacks are the ceramic-specific hard and brittle material characteristic, which causes low tensile strength, high compression strength and little toughness. The predominant bond-type of ceramic crystal lattices is the ion-binding type. This bond- type involves high lattice energies, which hinder the few existing creep-systems, so that healing up processes like flowing or creeping do not occur /3/. Consequently structural defects like microcracks and pores, which cannot be avoided totally in technical materials, lead to fracture more likely than in plastic materials like metals. Stress maxima at crack tips are not reduced by plastic flowing so that the speed of crack propagation increases. This disadvantage occurs in ceramic cutting tool in- serts too, but the reliability can be enhanced, if critical application conditions and critical load types for different cutting materials are disclosed, so that these can be avoided. To this end it is necessary to carry out extended investigations on the wear behaviour and wear mechanisms of ceramics dependent on the type of loading. In this connection the combination of tool material characteristics and load types determine predomi- nantly wear behaviour and wear mechanisms in cutting processes.

2. Load- and wear types

Cutting tools have to resist a complex load spectrum, which consists out of mechanical, thermal and chemical loads. Besides load type temporal and local dependence is importance. In this connection stationary loads with no or only little changes in dependence on time are distinguished from instationary loads with time-dependent gradients. Cyclic loads make greater demands upon the tool material toughness, which is just a weakpoint of ceramics. These different kinds of loading cause tool wear. There are distinct wear types like abrasion,adhesion, fracture, cracking, exfoliation, diffusion and oxi- dation, which never occur seperately. Rather a superposition of several wear types happens, which shapes the tool wear, whereby certain wear typs can be predominant. The specific wear type and wear behaviour of a tool material is determind by the tool material properties and the workpiece material above all. This is valid for ceramic cutting materials obviously too, so that oxide cutting tools and nitride cutting tools show characteristic wear behaviour and caused by that specific fields of applica- t ion. A typically worn oxide tool shows regular abrasion on the flank and rake face, only little crater wear, cracks and sometimes a notch at the end of the width of cut. Crater wear increases in discontinuous cuttinq. Cracks in continuous cuttinq take a predominant

course parallel to the cutting edge, whilst in discontinuous cutting comb cracks are observable, which are situated perpendicular to the cutting edge / 4 , 5 / . In / 6 / and / 7 / is reported about own investi- gatiovs on notch wear and crack-formation in alumina ceramics, so that this shall not be discussed fur- thermore here. As a consequence of crack formation tool fracture occurs, which leads to unpredictable failure, which diminishes the reliability of alumina tools. The wear behaviour of silicon nitride tools is cha- racterised by regular wear growth as well on the flank as on the rake face. Cracks cannot be detected usually. But the wear behaviour is highly dependent on the workpiece material. Steel materials generate rapidly increasing crater and flank wear, so that tool life of about 1 min is attainable only. On the contrary in cutting gray cast iron these tools achieve high tool lifes with high material removal rates. In this connection it is unimportant, whether the cutting process is of continuous or discontinous kind. For an explanation of this tool material-specific wear behaviour, the tool material properties shall be considered more detailed in the next sec- t ion. 3. Material Characteristics

For a comparsion of tool material properties, measurements were executed, because producer specifications are not existant or the present specifications are not comparable due to different measuring methods used. A further aim was to detect the temperature dependence of important material characteristics, so that eventually existing regions of thermal softening can be detected. In this connection the following parameters were investigated: Young's modulus, specific heat capacity, heat conductivity and bending strength. These measurements were executed particulary for aluminum oxide ceramics and the obtained results are compared with random tests for silicon nitride ceramics. The materials investigated were three A1 0 /zrO -ceramics, in the following named by RK1, &2, RK3, and one A1203/TiC-ceramic, in the following named by MK1.

3.1 Young's moduls

The Young's modulus was measured by a dynamic testing method, because results obtainable by such methods are more accurate. The basis of this method is the measurement of the propagation-velocity c of a lon- gitudinal compression valve in a long, slim bar. This velocity can be calculated from the bar length 1 . and the vibration frequency v :

c = 21v For the Young's modulus E is valid

L E = c . Q whereby 9 describes the density. So a frequency measurement is executed finally. A control device produces a voltage U , which is transformed into displacements by a diecoelectric exciter system. These displacements are transferred into the specimen by a fine quartz thread, which is fixed elastically at the front side of the sample-bar. In this way the bar is caused to vibration. From the other front side the vibration of the sample-bar ist received by an other quartz thread. A piecoelectric receiver re- transforms the displacements into charges p, which are amplified and converted into voltages U . The vibration frequency v is derivable from this 3oltage U2'

Annals of the CIRP Vol. 37/1/1988 97

The ceramic bars used had following dimensions: 3,5 mm x 4,5 mm x 4 5 mm. The results achieved with this method are assorted in Figure 1. It is evident that all four examined A1 0 -ceramics show a linear decrease in this tempe?a&re range. The three A1 0 /ZrO -ceramics show no virtual differences. The A1203/TiC2ceramic has a uniform higher Young's modulus 3as a consequence of the non-oxide additivgs. These results show a linear decrease up to 1200 c. This characteristic results from an increasing interatomic distance at increasing temperatures, so that lower forces are to be applied for further distance extensions / 3 / .

GPO

vI 390 W

3

3 v

-

in m 5 330 0 >

temperature 5

Figure 1: Youngs's modulus versus temperature

3.2 Specific heat capacity

The specific heat capacity was measured by the technique of differential calorimetry. Two identical calorimeters are situated thermally isolated in a furnace. The heat loss of both chambers is identical, because both are arranged symmetrically. The specimen is placed into the specimen calorimeter. The second calorimeter, the reference chamber, remains empty or is filled with a reference material with a well known heat capacity. During heating a temperature difference occurs between both calorimeters due to the different heat capacities. This difference is measured by a differential thermocouple. The calorimeter with the lower temperature is heated by an additional heating in such a way, that the temperatures of both calorimeters are balanced. The additional heating energy, which must be provided for the temperature equilibrium, is a quantity for the unknown heat capacity of the specimen.

Figure 2 shows the measured heat capacities of the four A1 0 -ceramics in dependence on temperature. In additioA aalues of Si N are marked in. All ceramic materials have degressi$ely rising slope. This is a result of energy transformation in the crystal lattice. The absorbed heat is transformed in kinetic energy in the lattice by increasing frequency of atomic vibration. This type of energy transformation is possible up to a certain frequency f only. At further temperature increase the specifi!!a#eat capacity remains constant then / 3 / .

At low temperatures all ceramics have nearly identical heat capacities, whereby Si N has lowest and the A1 0 /ZrO highest values. DuriAg'heating Si N shows ths iighegt increase, the oxide ceramics shdw4compa- rable developments with lowest values for the Al2O3/TiC-cerarnic in the whole temperature range.

1.2S1 I I I I

1.15

' I 1 .

!emperalure J 0.74 1w 200 300 A 5b.l & 7M ' C do

Figure 2: Specific heat capacity versus temperature

3 . 3 Heat conductivity

The basics of heat conductivity determination are derived from Fourier's differential equation:

Experimental arrangements for measurements are usually set up in such a way that particular terns in this equation become neglectable so that simple s0- lutions of the differential equation follow. For this reason stationary conditions and one-dimensional heat conduction are desired. If this two restrictions are fulfilled the above mentioned equation reduces to ..

d xL This principle is pursued in utilization of compari- son method f o r heat conductivity measurement. speci- mens are plane parallel plates, which are situated between two reference samples of identical geometry and with known heat conductivity. This three samples are arranged between two heating plates, which can be controlled. separately. In this way a temperature gradient 1s generated, which causes a heat flow through the three samples. The temperature is measured at both sides of each sample. Measuring faults due to heat loss to ambiance are avoided by instal- lation in a heat isolated box. An external filament winding enables measurement at higher temperatures. The plane parallel, symmetrical arrangement provides constant surface temperatures, so that a one-dimensional heat flow results parallel to the surface normal.

The heat flow through a surface element dA in the interval dt is calculable by

Integration of reduced Fourier's equation yields

d x 6 so that for the heat flow follows

Q " A = - A ( T i - Ti+, ) 6

As a result of the geometric identity of all Samples the heat flow through each sample is equal.

From this the demanded quantity A m can be determined easily /a/.

T T

In Figure 3 the measuring results are depicted. Be- sides the curves for the four A1 0 -ceramics, values for sintered Si N (SNS) and hotZp3essed Si3N4 (SNH) are marked in 202 comparsion. The three A1 0 /ZrO - ceramics show no dicisive differences. The A$ 8 /Ti& ceramic has equel values like the other oxide3cera- mics up to 500 C. At higher temperatures the heat conductivity is better. Si N has higher values over the whole range irrespectfvd of manufacturing process.

The typical degressivly decreasing behaviour is a result of enhancing lattice vibrations, which cause diminution of free path length in the lattice. Due to the direct proportionality between heat conductivity and this quantity, the detected behaviour follows /3/.

R K 2

I loo0 K 1250

I M3 7%

temperature T

Figure 3 : Heat conductivity versus temperature

3.4 Bending strenght

The determination of strength characteristics for ceramic materials suffers from the problem, that failure occurs in a wide stress ranqe. Thus indica-

98

tion of critical limiting characteristics values is problematical. The reason for this fact is the hard- ness and brittleness of ceramics.

As a consequence strength properties of ceramics are described usually by statistical methods. The draw- back of statistical examination is the great number of tests to be executed for a verification.

For bending strength measurements the samples ( 3 , 5 mm x 4 , 5 mm x 4 5 mm) are loaded by the four point method. The result is the fracture probability for different stress values. The experimental fracture probability p is calculated by

p = - I

n + l whereby i is the number of broken samples at a distinct stress value and n is the total number of samples. According to Weibull's theory the following relation exists

In this equation a is the ultimate stress, the characteristic streagth and m the Weibull moklus. Figure 4 shows measured values (spots) and calculated values (curve) for ceramic R K 3 . In linear illustra- tion a typical s-shaped course follows, whereby a indicates the position of abrupt drop and m is % quantity for the slope.

In Figure 5 values measured for the four oxide ceramics are plotted in Weibull paper. This scaling li- nearizes the s-shaped curves, so that valuation of measurement data achieved becomes easier.

Figure 5 reveals evident differences between the ceramics. Ceramic MK1 has the best values, because this material has a high reference stress value and a high Weibull modulus, whereby a high Weibull modulus is more important. This is also the reason why ceramic R K 3 has to be rated hisher than ceramic R K 1 . Ceramic RK2 has the lowest Weibull modulus and the lowest reference strength. The reason for this behaviour is probably the fact, that RK2 has the lowest content of ZrOZ, which enhances the toughness behaviour. Temperature dependencies were not measured, because this was beyond the scope.

a1

2 0,8

2 0,s

k 0,4

2 0 P 2

rn 11,83 .- -

0

7

0 Lc

bending strength ab

Figure 4 : Bending strength of ceramic R K 3

'50 200 300 LOO 5w 600 7co B a 3 b ' P O r n bending s r r e i g ' h a b

Figure 5: Bending strength at ambient temperature

4 . Thermal load simulation

The brittle, hard material characteristic of ceramic materials leads to a high sensitivity to instationary loading, because these materials have low toughness characteristics, like fracture toughness or bending strength. In addition especially A 1 0 -materials have low heat conductivity values, so Ihht instationarv

thermal loads seem to be particularly interesting for wear investigations. The cutting tool is subjected to instationary thermal loads in each cutting process. In continuous turning processes such load types are active in the cut-in-phase and cut-out-phase only. But in discontinuous turning and milling the thermal load changes at each cutting interruption. During the contact period between tool and workpiece or tool and chip heat is flowing into the tool permanently, so that an instationary temperature field results with high gradients. Temperature values Xary between ambient temperature and more than 1000 C in these temperature fields. From cutting edge exit on, the tool is cooled bv ambient air or coolant fluid. The heat flow changes it's sign and the tool is cooling down. A s well local as temporal temperature variations cause thermally induced stresses in the tool tip. These stresses can produce cracks in the tool material, if the material strength is exceeded, so that this mechanism is the origin for tool failure. The capability of materials to resist temporal and local temperature variations is called thermoshock- resistance. The thermoshock-behaviour of a material is dependent on - type of external loading - geometric shape of samples - material characteristics of samples. Hereby external load type and sample geometry determine conditions and intensity of the thermoshock, whilst material characteristics are responsible for produced stresses and possibly originated cracks. For an investigation of thermoshock-behaviour the following methods have been developed in the past: - quenching of heated samples by immersion into cold fluids, fo r high heat transfer values salt bathes are well suited

- blasting with gas - temperature variation between two different heated - contact with a cold plate - heating and quenching in a whirly chamber, filled furnaces

up with air and solid particles

The evaluation of the executed test occurs generally

- optical investigations, which leads to crack de-

- strength measurement - resonance frequency measurement - mass loss - crack detection by interruption of conducting sur-

by

tection

face layers

The thermoshock-resistance is quantified by counting of temperature cycles, which are resisted. It is evident that results obtained with different methods are hardly comparable / 9 , 1 0 / , All these above mentioned methods are not suited for a thermal load simulation of thermal load cycles, which occur in cutting processes, because they do not fulfil the following requirements: - local limitation to the cross sectional area of - high temporal cycle-frequency - high energy density By application of a laser aggregate these requirements can be performed. The laser beam is focused in such a way, that an area of the cutting edge is co- vered, which corresponds to the cross-sectional area of cut. The laser works intermittent in this application. In this thermal time-load-functions can be adjusted, which are comparable to those of the cutting process. The laser control used allows pulse widthes and pulse repetition periods less than 1 ms, so that extreme short pulses can be produced. Consi- dering the temporal process of cutting performance P (t) in milling, there exist distinct time intervals ag shown in figure 6 . The tool revolution time T consists of cut-in-time T and cut-out-time T . The whole time for a distince manufacturing procsss is named T, which is equal to the total load duration of the tool. The cycle number N can be determined by division of T by T These time values are derivable form real cutting processes, which depend on tool diameter, workpiece dimension, cutting speed and feed. the tool input heat energy can be estimated from the cutting performance and the loaded area on the tool is given by the cross sectional area of cut. So all required quantities are present to execute the thermal load simulation tests, whereby following quantities can be varied: - laser performance PL - loading time T - cycle number N - period T - pulse pepiod Te, - pulse pause period Ta

cut

P'

99

C l

a U C 0

5 0

01 Q

c L

t i m e I Figure 6:

In this paper only the principal of this method and a few first results shall be presented. Thus here no dependencies on test variables are discussed, but the material specific response on thermal loading. Figure 7 and 8 show thermal loaded A1 0 /ZrO (RK?) and A1 0 /Tic (MK1) ceramic structures2a?ter Identi- cal lo~d3cycles. The wear type of RK3 is characteri- sed by a thin layer of molten material, which covers partially the fine grained ceramic structure. This structure in the worn region reveals a greater number of fine pores, which can result from melting processes or microfracture. But closer consideration lets tend to the first alternative, because material breakage seems to happen by shell-like cracking. So two mechanisms of material remove occur. Firstly in an extreme thin surface layer ceramic material is molten. Secondly caused by the low heat conductivity the heat energy is concentrated in the surface near region, which leads to high stresses /7/, so that material cracking results. Considering the A1 0 /Tic-ceramic in Figure 8 it becomes obvious thaz 'melting processes have greater signifance here, because more molten material is detectable and though a greater magnification was pointed no single grainfi can be identified. In Figure 9 a thermal loaded Si N ceramic is shown. At this material other test-para%e&rs were adjusted, because especially the low energy causes no detectable wear here. This can be explained by the hiqher

Temporal process of cutting performance

Figure 8: Thermally worn A1203/TiC-ceramic

heat conductivity of this material, so that the heat energy is conducted from the surface, so that generated temperatures are lower. The wear behaviour of this ceramic is quite different. Molten material emerges out of the ceramic matrix, so that course pores are remaining. This behaviour can be attributed to the fact, that Si N -ceramics possess secondary glassy phases, which a?e4necessary for densification. This glassy phase, which covers grain boundries pre- dominantly, melts during heat loading and shapes the round material deposits observable. If this mechanism runs in the cutting process too, the smooth surfaces of worn Si N -tools become declarable, because the molten glas%y4 phase is smeared over the tool tip by

Figure 9: Thermally worn Si3N4-ceramic

the chip or the workpiece.

5. Conclusions

Measurements of material characteristics in dependence on temperature reveal that commercial alumina cutting materials have no decisive differences. Fur- thermore no thermally induced apupt drop on strength can be detected up to 1200 C. The only pa- rameter determined, which reveals obvious differences between the ceramics investigated was bending strength. But these differences cannot be tranpferred to the wear behaviour of these materials. In comparsion to Si N -ceramics the lower heat conductivity and lower t%u&hness is remarkable, which must be re- garded in investigations on wear behaviour. These material parameters are of importance for the thermo shock resistance too. For experimental simulation of thermal loads camparable to cutting processes, application of laser beam technique is favourable. By these technique termal load-time-functions can be realized, which are indentical to the cutting process. Further advantages are precise power adjustment and adjustability of loaded area by different objec- tives. In this way the facility is offered to inve- stigate the influence of the thermal load component separately, which is impossible in cutting processes, because mechanical, thermal and chemical influences cannot be separated there. First results obtained by this simulation technique show that the thermally affected zone is limited in alumina ceramics by the low heat conductivity. There occur melting processes irrespective of the ceramic material. In A1 0 /ZrO only thin molten layers are detectable. In %130 / h C greater amounts of molten material are gene3aged. But in both material groups homogeneous melting of the structure is observed. on the contrary Si3N4-ceramics show heterogeneous melting. In these structures molten material emerges out of the matrix like water out of a sponge. This molten material is probably the glassy phase used for densification in this material. The remaining Si N - matrix has course pores, which were filled w h e h glassy phase before. By this mechanism the smooth exterior of worn tool surfaces becomes explainable.

References

/1/ Wertheim, R., Agranov, D., 1986, Wear Behaviour of Silicon Nitride Tools as a Function of their Specific Properties, CIRP Annals, 35: 63-66

/2/ Vigneau, J., et. al., 1987, Influence of the Microstructure of the Composite Ceramic Tools on their Performance when Machining Nickel Alloys, CIRP Annals, 36: 13-16

/3/ Richerson, D.W., 1982, Modern Ceramic Enginee- ring, Marcel Dekker, Inc.

/4/ Pekelharing, A.J., et. al., 1978, The Exit Fai- lure in Interrupted Cutting, CIRP Annals, 27: 5- 10

/5/ Loladze, T.N., 1975, Nature of Brittle Failure of Cuttinq Tool, CIRP Annals, 24: 13-16

/6/ Tonshoff, H.K., Brinksmeier, E., Bartsch, S., 1987, Notch Wear and Chemically Induced Wear in Cutting with A1203-Too1s, CIRP Annals, 36: 537- 54 1

/7/ Tonshoff, H.K., Bartsch, S., 1987, Wear Mechanism of Ceramic Cutting Tools, Proceedings of the 1987 Intersociety Symposium "Machining of Ceramic Ma- terials and Components", 276-292

/8/ Eder, F., 1956, Moderne MeRtechnik der Physik 2, VEB Verlag Berlin

/9/ Hennicke, H.W., 1970, uber die Temperaturwechsel- bestandigkeit keramischer Werkstoffe. In: Hand- buch der Keramik, Schmidt Verlag, Freiburg

/lo/ Buessen, W.R., 1955, Thermal Shock Testing, Jour. Amer. Cer. SOC., 38: 15-17

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Ceramic Tools — Material Characteristics and Load Types Determine Wear Mechanisms