Trent E.M., Wright P.K. Metal Cutting

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PRACTICAL TECHNIQUES OF USING CEMENTED CARBIDES FOR CUTTING 223

FIGURE 7.42 Variation of tool-life with (a) feed (depth of cut = 2.54 mm), and (b) depth of cut (feed =

0.25 mm/rev.) for four different grooved tool inserts (cutting speed = 274 m/min., work material = 1037M

steel)

224 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

7.11 CONCLUSION ON CARBIDE TOOLS

Comparison of the performance of the cemented tungsten carbides with and without the addi-

tion of the cubic carbides demonstrates very well that wear rate is very dependent on the type of

wear process or mechanism involved. Thus wear resistance is not a unique property of a tool

material which can be determined by one simple laboratory test, or correlated with one simple

property such as hardness. Correct diagnosis of the controlling wear mechanism for a particular

operation can often be the starting point in selecting the optimum tool material.

The concept of indexable inserts and the ready availability of different types of tools with

clamps for the inserts, in a range of shapes and sizes for different operations, offers the potential

for many new lines of development in tooling materials and geometries. In particular, the mate-

rial on the surface of the insert can be selected for maximum resistance to wear for specific con-

ditions, while material for the body of the insert can be selected for resistance to bulk stresses

causing deformation and fracture.

There is much scope for further development of coated tools. The existing commercial range

of coatings has demonstrated that very hard, brittle materials such as TiC, TiN and A1

when

in the form of very thin layers - less than 10 μm thick can resist the stresses of cutting operations,

when supported on a rigid but tough substrate. Solid tools made from the same materials would

fracture if used as tools in the same operations. Diamond coatings are the subject of intense

research at the time of writing.

So far coatings have been developed almost entirely by tests on high speed cutting of cast iron

and steel. Coatings may be found for cutting materials other than iron and steel and, in parallel

with modified tool design, the range of operations for coated tools is likely to be extended.

Further developments in the process of coating may result in improvements. A number of

physical vapor deposition (PVD) methods are being explored.

29,30

In these, the surfaces to be

coated are first cleaned by bombardment with inert gas ions in a low pressure chamber. Next, the

pressure in the chamber is reduced, and metal atoms for the coating are evaporated into the

chamber. The atoms are ionized and attracted by high voltage to the tool surface in the presence

of a low pressure gas, which contains nitrogen or carbon atoms. Adherent coatings can be

formed on the tool at temperatures of 500°C or lower.

The lower deposition temperature of PVD, results in lower residual stress in the coatings

compared with CVD. This may be advantageous, although the residual stress and stress-cracking

in CVD coatings does not appear to affect significantly their performance in cutting applications.

PVD coatings produced so far have been less strongly adherent to the substrate than CVD coat-

ings. This disadvantage may be eliminated and the potential of PVD for development of new

coatings will continue.

The greatest potential of the PVD approach may well be the multiple and nanolayer coatings.

These create a variety of properties on the surface of the tough carbide core. The nanolayers in

particular provide alternate layers of tough and hard materials. The evidence indicates that such

“sandwiches” arrest microcracks at the interfaces. There is also the possibility of adding extra

layers for oxidation resistance, and solid lubrication for example with nanolayers of molybde-

num disulphide.

32,33,34

These composite nanolayer materials will provide exciting new possibilities in the future.

Referring back to the beginning of Chapter 6, perhaps some of the “early magic” of cutting tool

REFERENCES 225

development will return in the 21st century, if these nanolayer materials can create an all pur-

pose cutting tool that can be manufactured to “give the best of all worlds” - highly wear resistant

at high cutting speeds; but extremely tough in milling and interrupted cutting. Viewed from

another perspective, such an “all purpose” tool would be greeted with great enthusiasm by the

purchasing department in any organization. Today, the great proliferation of grades and styles

causes much overstocking and, in the end, probably much unused, wasted material in the aver-

age “tool room”.

7.12 REFERENCES

1. Dawihl, W., Handbook of Hard Metals. (English translation), H.M.S.O., London (1955)

2. Schwarzkopf, P. and Kieffer, R., Cemented Carbides, Macmillan, New York (1960)

3. Goldschmidt, H.J., Interstitial Alloys, Butterworths, London (1967)

4. Schwarzkopf, P. and Kieffer, R., Refractory Hard Metals, Macmillan, New York (1960)

5. S. Atkins, A.G and Tabor, D., Proc. Roy. Soc. (A), 292, 491 (1966)

6. Miyoshi, A. and Hara, A., J. Japan Soc. Powder and Powder Met., 12, 78 (1965)

7. Exner, H.D, and Gurland, J., Powder Metal. 13, (5), 13 (1970)

8. Featherby, M., Ph.D. Thesis, University of Birmingham (1968)

9. Burbach, J., Sonderdruk aus Technishe Mitteilungen. Krupp, 26, S 71, 80 (1968)

10. Classification of Carbides According to Use, ISO 513 (1975)

11. Trent, E.M., Proc. Int. Conf. M.T.D.R., Manchester, 629 (1968)

12. Aschan, I.J., et al., Proc. 4th Nordic High Temp. Symp., 1, 227 (1975)

13. Trent, E.M., I.S.I. Report No. 94, pp. 11, 77, 179 (1967)

14. Trent, E.M., I.P.E.J., 38, 105 (1959)

15. Dearnley, P.A. and Trent, E.M., Metals Technol. 9, (2), 60 (1982)

16. Trent, E.M., J. Birmingham Met. Soc., 40, 1 (1960)

17. Trent, E.M., Proc. Inst. Mech. Eng., 166, 64 (1952)

18. Naerheim, Y. and Trent, E.M., Metals Technol. 4, (12) 548 (1977)

19. Kramer, B.M. and Hartung, P.D., Cutting Tool Materials, p.57, Am. Soc. for Metals. (1981)

20. Loladze, T.N., Wear of Cutting Tools, Moscow (1958)

21. Freeman, R.M., Ph.D. Thesis, University of Birmingham (1975)

22. Mayer, J.E., Moskowitz, D. and Humenik, M., I.S.I. Publication 126, P. 143 (1970)

23. Brookes, K.J.A., World Directory and Handbook of Hard Metals, Engineers’ Digest Publi-

cation (1987)

24. Sandvik, Coromant AB, Sweden, Private communication

25. Schintlmeister, W., and Pacher, O., J. Vac. Sci. Technol., 12(4), 743 (1975)

26. Reiter, R.T., Engineers’ Digest Int. Conf New Tool Materials, paper 6 London (March 1981)

27. Stjernberg, K.G., Met. Sci

, 14, (5), 189 (1980)

28. Hale T.E., and Graham, D.E., Cutting Tool Materials, p 175, Am. Soc. for Metals (1981)

29. Bunghah, K.F., Proc. 9th Plansee Seminar, Paper 22 (1974)

30. Yoshihiko, D., et al., Cutting Tool Materials, p. 193, Am. Soc. for Metals (1981)

31. Dearnley, P.A., Surface Engineering l, (l), 43 (1985)

226 CUTTING TOOL MATERIALS II: CEMENTED CARBIDES

32. Komanduri, R., Tool Materials, Encyclopedia of Chemical Technology, Fourth Edition, Vol-

ume 24, 390 (1997) John Wiley and Sons Inc.

33. Shaw, M.C., Marshall, D.B., Dadkhah, M.S., and Evans, A.G., Acta Metall. Mater. 41, (11),

3311 (1993)

34. Keem J.E., and Kramer, B.M., U.S. Patent 5,268,216 to Ovonic Synthetic Materials, (Dec. 7,

1993)

35. Fang, X.D., and Jawahir, I.S., Wear, 160, 243 (1993)

36. Jawahir, I.S., Annals of the CIRP, 37, (1), 121 (1988)

37. Jawahir, I.S., Annals of the CIRP, 39, (1), 47 (1990)

38. Balaji, A.K., Sreeram, G., Jawahir, I.S.,and Lenz, E., Annals of the CIRP, 48, (1), (1999)

39. Jawahir, I.S., Li, P. X., Ghosh R., and Exner, E. L., Annals of the CIRP, 44, (1), 49 (1995)

40. Li, P. X., Jawahir, I.S., Fang X. D., and Exner, E. L., Trans. NAMRI/SME, 24, 33 (1996)

41. Jawahir, I.S., Fang, X.D., Li P.X., and Ghosh, R., Method of Assessing Tool-Life in Grooved

Tools, United States Patent, # 5,689,062, November 18, 1997

CHAPTER 8 CUTTING TOOL

MATERIALS III:

CERAMICS, CBN

DIAMOND

8.1 INTRODUCTION

In this chapter, the properties and performance of ceramic tools, aluminum-based composites,

sialons, cubic boron nitride and diamond will be reviewed. A final survey of the three classes of

tool materials from Chapters 6, 7, and 8 is also provided, and many of the cited references cross-

compare the abilities of these new materials.

1-25

8.2 ALUMINA (CERAMIC) TOOLS

Refractory oxides have been among the many substances of high hardness and melting point

investigated as potential cutting tool materials. Throw-away tool tips consisting of nearly 100%

(alumina) have been available commercially for more than 30 years, and have been used

in many countries for machining steel and cast iron.

The successful tool materials consist of fine-grained (less than 5 μm) A1

of high relative

density, i.e. containing less than 2% porosity. Several different methods have been used to make

tool tips which combine these two essential structural features, including:

(1) Pressing and sintering of individual tips by a process similar to that used for cemented car-

bides. Sintering is carried out in air and the tool tips are white.

(2) Hot pressing of large cylinders of alumina in graphite molds, the tool tips being cut from the

cylinders with diamond slitting wheels. The tool tips are dark gray.

The possibility of ‘cementing’ alumina particles together with a metal bond, using a process

similar to the bonding of carbide by cobalt, has been explored, but no satisfactory metal bond

has been found. (Recall in Section 7.1, the liquid phase solution of WC in cobalt wets and pulls

together the WC particles.) However, many additions, e.g. MgO and TiO, have been made to

promote densification and retain fine grain size. The basic raw material, alumina, is cheap and

228 CUTTING TOOL MATERIALS III: CERAMICS, CBN DIAMOND

plentiful, but the processing is expensive and the tool tips are therefore not cheap compared with

cemented carbides. The room temperature hardness of alumina tools is in the same range as that

of the cemented carbides, 1550-1700 HV.

The room temperature compressive strength is reported to be approximately 2750 MPa (180

tonf/in

). Their major advantages are:

(1) Retention of hardness and compressive strength to higher temperatures than with carbides.

(2) Much lower solubility in steel than any carbide - they are practically inert to steel up to its

melting point.

To offset these advantages, their toughness and strength in tension are much lower. The trans-

verse rupture strength is an inadequate guide to toughness but values reported range from 390-

780 MPa (25-50 tonf/in

), about one third that of cemented carbides. The reported values of the

fracture toughness parameter K

for alumina tool materials are 1.75 to 4.3 MPa m

-1/2

. This is

also much lower than those for cemented carbide (Table 7.2).

Alumina is non-metallic in character, with an ionic rather than a metallic bond. Consequently,

it is an electrical insulator with low thermal conductivity - thus favored for use in the integrated

circuit (IC) packaging industry. Alumina is a true ceramic, the pure form being white, translu-

cent and looking like porcelain to which it is similar. To the new student in the field, it is quite a

surprise to find that a material similar to pottery or porcelain can withstand the rapid fluctuations

of temperature and stress involved in metal cutting operations. Nevertheless, alumina tools can

be used to cut steel at speeds much higher than can be used with conventional cemented carbides

or TiC-based alloys.

Negative rake throw-away tool tips are nearly always used, and it is not difficult to demon-

strate that cutting speeds of 600-750 m min

-1

(2000-2500 ft/min) can be sustained, at a feed of

0.25 mm (0.010 in) per rev, for long periods without excessive wear when cutting cast iron and

many steels. Tools used at high speed show flank wear and a type of cratering starting close to

the cutting edge.

There is evidence to demonstrate that wear, under conditions where the interface temperature

is high, is caused by very superficial plastic deformation and flow of a very thin layer on the

tool, rather than by diffusion into the work material.

Transmission electron micrographs (TEM)

show a high concentration of dislocations just below the worn surface, while the body of the tool

is practically free from dislocations.

Gradual failure involving flaking of thin fragments from

the rake or clearance face is sometimes observed. It seems to be an advantage of alumina tools

that such fractures do not always lead to sudden and massive failure of the whole tool edge with

major damage to the workpiece, such as would occur after fracture of the edge of carbide tools.

The potential speeds using alumina tools, which are three to four times higher than those nor-

mally used with carbide tools, represent an increase in metal removal rate as great as that

achieved by high speed steel and by cemented carbides at their inception. However, in spite of

the efforts of dedicated enthusiasts to introduce these tools on a large scale, the numbers in use

are only a very small percentage of the carbide tool applications. Their main continuous usage is

in cutting gray cast iron where a very good surface finish is required. On clutch facings and

brakes for cars they are being used at speeds up to 600 m min

-1

(2000 ft/min) to give a surface

finish good enough to eliminate a subsequent grinding operation.

ALUMINA-BASED COMPOSITES (Al

+ TiC) 229

8.2.1 Attitudes and Economics on the Factory Floor

It is first difficult to understand why alumina tools have not yet come into more wide spread

use. There are several cases where concentrated efforts in the ‘Research and Development’ labo-

ratories of a large firm have resulted in ceramic tools being introduced into individual factories

for cutting both steel and cast iron.

In the long run, however, their continued use has been abandoned except for specific opera-

tions, mainly on cast iron. Cast iron, (see Chapter 9), machines with a “shower” of discontinuous

chips at low cutting forces. It is easy to machine in this regard and once the operation is “up-and-

running” not much supervision is needed.

However, for other materials with longer chips and higher cutting forces, the inadequate

toughness of ceramics leads to unreliable performance under machine shop conditions. To obtain

consistent day-to-day performance (on materials other than cast iron) continuous attention to

every precise detail of tool setting and monitoring is required to prevent premature tool failure of

ceramics. Such minor, yet frustrating, interruptions to production are bad enough. Worse still,

damage to the component being machined, caused by occasional catastrophic failure of the

ceramic tool edge, will more than off-set the economies gained by peak performance. And, of

course, once a ceramic tool gets a “bad reputation” on a particular factory floor, it takes a long

time to recover and to be considered for use again.

The enormous economies which resulted from the introduction of high speed steel and

cemented carbides naturally whet the appetite for a further round of speed increases on the same

scale.

However, in summary, there seems to be less potential advantage to be gained by increasing

cutting speeds beyond those achieved by cemented carbides, for two reasons:

• when cutting speeds are high, e.g. 200 m min

-1

(600 ft/min), the time required to load and

unload the work in the machine may be quite a large proportion of the cycle time, and further

increases in cutting speed do not achieve a proportional reduction in total machining time.

• at very high speeds, certain difficulties become accentuated - for example the chips coming

off at very high speed are difficult to clear and may be a hazard to the operator. The chips are

less of a problem when cutting cast iron. The machining of cast iron does not form a continu-

ous ribbon, and this is the main reason for the successful application of ceramic tools to cast

iron.

8.3 ALUMINA-BASED COMPOSITES (Al

+ TiC)

The problems discussed above, with materials that are nearly 100% pure alumina, naturally

led to investigations on a mixture of (ceramic + carbide). Ceramic composite tool materials con-

sisting of alumina with 30% or more of a refractory carbide - usually TiC or (Ta,Ti)C - have been

commercially available since the 1960s. The mixture is hot pressed to full density and is dark

gray in color. The structure consists of fine-grained Al

with dispersed carbide grains a few

microns in diameter. These tools are used mainly in high speed machining of cast iron and, as

might be expected, experience shows that they can be applied in a wider range of applications

than pure A1

because of the increased toughness that results from the addition of the TiC.

230 CUTTING TOOL MATERIALS III: CERAMICS, CBN DIAMOND

The 1990s was a period in which the producers of tool materials improved the performance of

alumina tools by exploring the possibility of ‘alloying’ with other substances. The main objec-

tive was to increase toughness, and development was much influenced by the concepts of frac-

ture mechanics and their application in the area of composite materials - particularly the factors

which influence crack propagation and the fracture-toughness value K

Alumina with the addition of up to 10% ZrO

is reported to increase the K

value by about

25% and to improve the capability for machining both cast iron and nickel-based alloys. The

crack-retarding action is attributed to compressive stresses brought about by phase changes asso-

ciated with the ZrO

particles (Table 8.1).

FIGURE 8.1 Structure of Al

/SiC (30 Vol%) whisker ceramic tool (Courtesy of Sandvik)

8.3.1 SiC whisker reinforced alumina

A more recent innovation is incorporation in alumina of up to 25% of SiC “whiskers”, the

mixed powders being consolidated by hot pressing. These whiskers are about 1-2 μm in diame-

ter and about 20 μm long. They are very hard and strong, free from structural defects and are

randomly distributed in the alumina matrix (Figure 8.1).

This material has a higher K

value and reports indicate considerably improved performance

when machining certain nickel-based alloys, hard cast iron and hardened steel.

This is attributed mainly to higher resistance to crack propagation due to deflection of the

cracks at interfaces, or to relief of stress at the crack tip when SiC whiskers are pulled from their

sockets in the alumina.

In future years, such SiC whisker-reinforced aluminas or carbides may be the basis upon

which many new tool materials eventually evolve. Since titanium aerospace work materials are

SIALON 231

chemically reactive with most tool materials, it will be interesting to continue to search for whis-

ker-like additions to alumina that are inert to titanium.

8.3.2 Tool design of alumina based tools

In parallel with these composition changes, a steady development has taken place in adapta-

tion of the tool edge shape to the machining of different work materials and different cutting

conditions. A commonly used edge shape is shown in Figure 8.2. To a large extent, the data

below and the diagram on the next page are related to all brittle type materials. In production, as

much benefit can result from these edge preparations as from the actual choice of material.

A chamfer, C, has a width usually about 0.75 times the feed at an angle β to the rake face (of

between 10° and 30° depending on the work material and the severity of the operation).

The edge itself may also have a honed radius, while the tool tip is presented to the work with a

negative 5° rake angle. Tools with this type of edge configuration are less susceptible to prema-

ture failure, and are now being used in a wider range of operations and for cutting other work

materials. As well as gray and chill cast iron, steels with hardness up to 500 HV, martensitic

stainless steels, plastics and carbon are being machined.

Recall in Chapter 4 that the normal stress is highest on the cutting edge. Also, in some cutting

conditions, there is a tendency for tensile stresses to arise at the rear of the contact length (Fig-

ures 4.21 and 4.22). Based on intuition, it is clear that the chamfer, C, redistributes the stress at

the tool point. Perhaps the normal stress is reduced and the largest stress at the edge now acts

inwards towards the tool (on a vector normal to the land measured by “C” in Figure 8.2) where

the material can bear the stress more easily. Or, perhaps there is now less tendency for the nor-

mal stress to “bend the rake face like a cantilever beam” and create the tensile loads in Figures

4.21 and 4.22. These intuitive thoughts of the authors deserve further research - perhaps with the

birefringent sapphire, ground and polished with different values of chamfer land and chamfer

angle.

8.4 SIALON

A group of ceramics known as ‘sialons’ has been intensively investigated because of their out-

standing properties as high strength refractory materials. Since 1976 their use as cutting tools

has been explored and they have been successful in several applications.

Sialons (Si-Al-O-N) are silicon nitride-based materials with aluminium and oxygen additions.

Silicon nitride (Si

) itself has useful properties, including high hardness (ca. 2000 HV), bend

strength better than that of alumina (ca. 900 MP) and a low coefficient of thermal expansion (3.2

x 10

-6

), giving good resistance to thermal shock. It has been used as a cutting tool material but

has not been accepted widely, partly because it can be produced in high-density form only by hot

pressing, so that the cost of accurately shaped tools is high.

232 CUTTING TOOL MATERIALS III: CERAMICS, CBN DIAMOND

FIGURE 8.2 Edge chamfer commonly used on ceramic tools: c = 0.08-0.5 mm; β = 10 to 30°. Recall - at

the end of Chapter 4 - it was shown that the largest normal stress on the rake face is at the cutting edge.

Values of 1.5 to 2 times the average stress are likely. Also recall that under some cutting conditions -

perhaps those where the feed rate is heavy- there is a tendency for tensile stresses to occur at the rear of

the chip-tool contact length (Figures 4.21 and 4.22). The evidence suggests that the chamfer on ceramic

tools redistributes the stresses and ameliorates both these conditions. The authors suggest that the

chamfer causes the peak stress at the edge to act inwards towards the bulk of the tool, thereby causing

less pressure on the delicate cutting edge itself. At the same time, there is less of a tendency for the rake

face to bend like a cantilever beam reducing the tensile stresses on the tool face near the rear of the

contact length. Both these effects allow the inherently brittle ceramics to perform better without premature

fracture.To investigate these issues further, a fruitful research area would be to use the birefringent

sapphire tools polished to exhibit a variety of chamfer length and chamfer angle styles. The effect of feed

rate (undeformed chip thickness) is probably the most important machining parameter to vary in such tests.