Cutting tool materials coated by chemical vapour deposition

Cutting tool materials coated by chemical vapour deposition

Wear, 100 (1984) 153 - 169 CUTTING TOOL DEPOSITION W. SCHINTLMEISTER, Metallwerk MATERIALS 153 COATED W. WALLGRAM, Plansee G.m. b.H., A-6600 ...

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Wear, 100 (1984)

153 - 169

CUTTING TOOL DEPOSITION W. SCHINTLMEISTER,

Metallwerk

MATERIALS

153

COATED

W. WALLGRAM,

Plansee G.m. b.H., A-6600

J. KANZ

Reutte

BY CHEMICAL

VAPOUR

and K. GIGL

(Austria)

Summa~

Tools coated with hard materials are extremely important in the field of machining. Initially the reasons why these thin layers of hard material give considerable increases in tool life are explained. Subsequently the currently outstanding multilayer coatings are described and practical examples are given which demonstrate their effect when machining both steel and cast iron, The type of machining and the workpiece material play a decisive role in designing the optimum sequence and composition of the layers. Cemented carbides with multilayer coatings based on Tic, Ti(C, N) and TiN with a total thickness of about 10 pm are best for turning normal steels. For turning cast iron, hardened steels and Inconel cemented carbides with multilayer coatings containing special ceramic layers show a significant increase in tool life. These coatings also give a considerable improvement when milling cast iron. In contrast, for milling steel, thinner coatings based on Tic, Ti(C, N) and TiN are more advantageous. In addition to cemented carbide, high speed steel tools are also used in large quantities for machining steel. The coating of high speed steel with hard material layers also increases tool life and this is explained. Practical examples are given which support the possibilities for high speed steel tools coated by chemical vapour deposition (CVD). Finally an explanation is given of the CVD process. This is the process in most common use, especially for coating cemented carbide tools. A brief description of CVD production units is given.

1. Introduction

Within the last ten years very important changes have taken place in the field of machining. The need for economic manufacture has given rise to the requirement for larger chip removal rates. These have been met by longer tool life, with higher cutting speeds and with a more universal use of the tools. In achieving this, cemented carbides, coated with hard material layers by chemical vapour deposition (CVD), play a decisive role. At first, cemented carbide inserts were coated with a TiC monolayer only. On the 0043-1648/84/$3,00

0 EIsevier Sequoia/Printed

in The Netherlands

154

basis of studies of the effect of coatings, multilayer coatings based on TiC, titanium carbonitride (Ti(C, N)) and TiN and also A1203 coatings were developed and introduced to the market. Initially, their application was restricted to turning operations. However, coatings for milling applications were then developed. Figure 1 shows coated cemented carbide inserts which are clamped into tool holders and are used for turning and milling operations. A further expansion of coating applications took place when tools and wear parts made from steel were coated.

Fig. 1, Coated

cemented

carbide

inserts

for machining.

2. Wear characteristics The main field of application of coated cemented carbides is for metal machining (turning and milling). The lifetime of tools is increased by these coatings by three to ten times. To gain an understanding of the wear reducing effect of coatings, we must first look at the most important causes of wear. These are presented in Fig. 2. Figure 3 shows schematically the different kinds of wear related to the cutting speed (temperature at the cutting edge). Increasing speed and feed rate increase the temperature at the cutting edge. Temperatures up to 1200 “C and more can be reached in the zone where the chip flows over the tool surface. In machining steel at lower temperatures (lower cutting speed) the wear is mostly caused by galling, with increasing speed abrasive wear increases and at higher speeds wear by diffusion is dominant. With regard to the basic effect of coatings, measurement of the cutting forces and investigation of the wear patterns are important [ 21.

155

Cuttmg

speed

I- temptvoturel

Fig. 2. Causes of wear. Fig. 3. Schematic representation of wear as a function of cutting speed (according to Vierregge [ 11): curve A, initial edge wear; curve B, abrasion; curve C, tear-off due to galling; curve D, diffusion; curve E, scale; curve F, general wear (sum of curves A - E).

The three cutting force components present during turning are shown schematically in Fig. 4. Figure 5 demonstrates the effect of coatings when machining plain carbon steel with high speed steel inserts (grade M44) at a is shown between the cutting forces speed of 40 m min-‘. A comparison when using a Tic-coated tool and an uncoated tool. The cutting forces are reduced by between 10% and 3076, depending on the component, and the vibration is also greatly reduced. The large amount of vibration on the uncoated tools results from galling. In machining with coated cemented carbides a reduction in the forces of about 20% is also observed. In parallel with this there is a reduction in the friction and the temperature at the cutting edge. Not only the coating but also the products of a reaction between the coating and the oxygen in the air, or with impurities in the workpiece material, can combine to resist wear. At higher cutting edge temperatures, wear is mainly caused by diffusion or by chemical reaction between the tool material and the chip. If diffusion or chemical reaction is prevented by a coating then cratering is almost eliminated. The effect of coatings can be summarized in the following statements [3]: (1) reduction in friction, in generation of heat and in cutting forces; (2) reduction in the diffusion between the chip and the surface of the tool, especially at higher cutting speeds (the coating acts as a diffusion barrier); (3) prevention of galling, especially at lower cutting speeds. As well as wear resistance and resistance to the different causes of wear, some other requirements must be fulfilled by cutting materials for optimum tool life to be attained. The most important are as follows: chemical stability in contact with the workpiece material; resistance against oxidation; resistance against abrasion; prevention of galling; impact strength; resistance against thermal cycling; sufficient adhesion of coatings to the base material. On the basis of investigations of the effect of TiC coatings and on the fact that the large number of requirements for optimum tool life are not

156

Fig. 4. Cutting

force components

during turning.

Fig. 5. Diagram of cutting forces for uncoated high speed steel grade M44 (upper curves) and for the same material coated with TiC (lower three curves).

three

fulfilled by one homogeneous coating, we developed multilayer coatings. Figure 6 demonstrates the increase in wear resistance given by the use of a double layer [4]. Turning tests were carried out on constructional steel comparing a TiN coating 11 pm thick with a two-layer coating consisting of a TiC layer 5 pm thick adjoining the base material with a 6 pm layer of TiN above it. Indexable inserts SPGN 120308 EN of MlO-type hard metal (6% Co; 5% Tic; 5.5% Ta(Nb)C; balance WC) were used. The tool life of the TiN-coated tip was 17 min and that of the TiNcoated tip with the intermediate TiC layer 42 min. It is only as a result of the improved flank wear resistance conferred by the intermediate TiC layer that advantage can be taken of the high crater resistance of TiN. Figure 7 shows the effect of a multilayer coating on the influence on thermal stresses [4]. In a milling test tips were coated with a TiN layer 10 pm thick (Fig. 7(a)) and others were coated with 2 pm of TIC adjoining the base material, then a transitional zone of titanium carbonitrides and finally approximately 5 pm of TiN (Fig. 7(b)). Cracks are visible on the rake face of the TiN-coated tip (Fig. 7(a)) which do not occur on the tip with the multilayer coating (Fig. 7(b)). When used for milling, the tips are subjected to continuous temperature variations due to the in~~pted cutting. The difference in coefficients of thermal expansion between the hard metal (5.5 X 1O”“6 K-l) and TIN (9 X 10d6 K-i) causes the cracks to form. Since TiC has a lower coefficient of thermal expansion (7 X 10e6 K-l)

157

2 L 6 8 10

15 Cuttmg

20

25

Ifme

Imml

30

35

LO

Fig. 6. Flank wear in relation to cutting time (cutting speed u = 200 m min-’ ; feed rate s = 0.41 mm rev-’ ; depth of cut a = 2 mm; workpiece material, C 60 steel; tensile strength, 1000 N mmP2).

(a)

(b) Fig. 7. Rake face of coated inserts after a milling time of 9 min (u = 223 m min-’ ; s = 0.25 mm rev-‘; a = 2 mm; workpiece material, structural steel; tensile strength, 700 N mmP2): (a) TiN coated; (b) with multilayer coating.

158

the multilayer-coated tip showed no cracks under the machining conditions used. These would form only at higher cutting speeds or when milling materials of higher tensile strength.

3. Important

grades and fields of application

3.1, Coated cemented carbides for turning operations In 1968 coated cemented carbide tips were first brought on the market. These consisted of a TiC layer 2 - 3 pm thick but also in the adjacent carbide they had a decarburization zone (the so-called q phase) 3 - 5 pm thick. In spite of this restriction these coated cemented carbides increased the tool life by 100% or more in cutting operations without interruptions. An increase in cutting speed and feed was also possible. Later on, in addition to Tic, coatings of TiN and of HfN were used. In 1973 a multilayer coating which gave a remarkable increase in tool life was brought on the market [ 51. This consists of a TiC layer adjacent to the base material followed by a number of titanium carbonitride (Ti(C, N)) layers and with a TiN layer on the top. Figure 8 shows such a coated cemented carbide. The increase in lifetime is demonstrated in Fig. 9, which shows the lifetime when turning a plain carbon steel with 0.6% C related to the cutting ’ -’ the lifetime of an uncoated speed [6]. For example, at 200 m mm cemented carbide of grade PlO is about 3 min, the life of a cemented carbide coated with 2 - 3 pm of TiC and a 5 pm 7) layer about 7 min and that of a cemented carbide with the coating shown in Fig. 8 about 30 min. Subsequently A1203 layers were used on a TiC or Tic-Ti(C, N) interlayer and also with TiN on the top. Cemented carbide inserts with a homogeneous and relatively thick Al,O, layer (up to 10 pm) have been on the market since 1978. In addition to the homogeneous Al,Os layer this grade is characterized by an enrichment of mixed carbides (W-Ti-Ta mixed

Fig. 8. Multilayer coating based on Tic, Ti(C, N) and TIN.

159

Fig. 9. Lifetime related to cutting speed (workpiece material, C 60 steel; tensile strength, 950 N mm-* ; s = 0.5 mm rev-’ ; a = 2 mm).

carbide) and a reduction in the amount of cobalt in the cemented carbide under the coating [ 71. Finally, a coating consisting of A1203 containing TiN layers has been brought on the market [8]. On the basis of the results of systematic investigations of the interactions between A&O3 layers with the other compositions it was possible to develop a multilayer coating with a sequence of ceramic layers which improved the lifetime considerably and also extended the fields of application. Figure 10(a) shows a pho~micro~aph of this coating which is now on the market as the grade Sr17 [9]. The coating consists of ten layers. Adjacent to the base material (cemented carbide) is a TiC layer, which is followed by a titanium carbonitride layer and then a sequence of four intermediate layers (bright lines) and four ceramic layers (dark lines) based on Al,Os. Figure 10(b) shows a scanning electron micrograph of a fracture (intermediate layers are dark, ceramic layers bright). With special reference to the wear resistance of this coating, the incorporation of special elements into the intermediate layers and into the ceramic layers is important. The grade Sr17 consists of this coating on a cemented carbide with 6% Co, 2.5% Tic, 5.5% Ta(Nb)C and balance WC. The following example demonstrates the increase in tool life in the turning of grey cast iron. Figure 11 shows the results. Ml0 cemented carbide (6% Co, 5% Tic, 5.5% Ta(Nb)C and balance WC) was used: this was coated with TiCTi(C, N)-TiN, with Tic-Al,Os-TiN and the Sr17 coating (multilayer ceramic coating). The lifetimes were as follows: Ml 0, Tic-Ti( C, N )--TIN coated MlO, Tic-A1203-TiN coated MlO, with multilayer ceramic coating (Sr17)

8.5 min 12.5 min 19 min

In the machining of grey cast iron increases in lifetime are achieved in a large number of applications, e.g. brake drums and brake discs, machine parts and housings. Also, in the machining of steel improvement in lifetime are achieved, especially if clearance face wear limits the lifetime [lo].

160

(a)

Fig. 10. (a) Photomicrograph and (b) scanning electron micrograph of the fracture of a grade Sr17 multilayer ceramic coating.

The extremely high abrasion resistance of the multilayer ceramic coating Sr17 enables increases in lifetime to be reached on steels with higher strength, e.g. hot-working steel with a hardness of 54 HRC. In machining the component shown in Fig. 12, two components were machined with inserts coated with Tic-Ti(C, N)-TiN and ten components were machined with Sr17. Also, on some other workpiece materials, where until now coated cemented carbides have produced insufficient improvements, the multilayer ceramic coating shown in Fig. 10 enables increases in lifetime to be achieved. Examples include chilled cast iron with hardnesses up to 55 HRC (a Shore hardness of 75) or superalloys. Figure 13 shows the results of a

161

5

0

Cuttmg

10 15 t/me Imfnutesl

20

Fig. 11. Clearance face wear related to cutting time of Ml0 cemented carbide with various coatings (workpiece, grey cast iron bar with a hardness of 170 HB; v = 200 m mm ’ -i ; s = 0.41 mm rev-’ ; a = 2 mm). Fig. 12. Component face).

TL-Al2Oj-TIN coated

Sr 17

made from hot-working

K 10

steel (the shaded region is the machined

K20

uncoated

Fig. 13. Lifetime of different tool materials in the machining of Inconel 718 (strength, 1200 N mm-*; v = 30 m min-‘;s = 0.25 mm rev-‘;0 = 2.5 mm).

comparison test in the machining of Inconel 718 with a strength of 1200 N mm-*. The lifetime was more than twice that of normally used uncoated cemented carbide grades and a coating consisting of Tic-Al,Os--TiN. The lifetimes achieved on the above-mentioned materials with Sr17 are similar to those with dense ceramic materials or with cutting materials based on Si3N4 or Si-Al-O-N. The advantage of Sr17 is the noticeably higher toughness compared with that of dense ceramic materials [lo]. 3.2. Coated cemented carbides for milling At the present time most coated cemented carbides are used for turning operations. In metal cutting, high crater resistance and high flank wear

162

resistance are needed. For milling applications, impact strength and resistance against thermal cycling are of greatest importance. Therefore additional requirement for the coatings are wanted. The coatings usually used have a tendency to microsnip in milling. There is a need for a coating with excellent adhesion to the base and with a very even grain structure. Additionally, it is important that the coating is thin. Figure 14(a) shows a photomicrograph and Fig. 14(b) a scanning electron micrograph of a coating especially suitable for milling. It consists of a TiC layer 1.5 pm thick adjacent to the base material followed by a 1.5 ,um titanium ~~bonitride layer and a TiN layer 2 pm thick on the top. In the milling of constructional steel with medium to higher cutting speeds this coating increases the lifetime by 50% in comparison with a coating consisting of TiC only [ 11 J. This improvement results from the fact that

Fig. 14. (a) Photomicrograph and (b) scanning electron micrograph of the fracture of the three-layer coating for milling applications.

163

the crater resistance, the impact strength and the resistance to the thermal stresses are all increased by the multilayer coating. The example in Table 1 is typical for the performance of this coating [ 121. The coating enables an increase in lifetime and also in the cutting speed by a factor of more than 2 to be achieved. TABLE 1 Performance of a coating used for milling Operation Workpiece

material

Tensile strength Insert Coating Cutting speed Feed per tooth Depth of cut Number of machined per edge

components

Milling of the component shown in Fig. 15 Heat-resistant steel casting GX 40CrSi22 (material 1.4745) 900 N mm-* SPKN 1504 AE-T Uncoated cemented Coated cemented carbide (grade K20) carbide (grade P30) 120 m min-’ 280 m min-’ 0.14 mm rev-’ 0.14 mm rev-’ 1.5 mm 1.5 mm 1-2 3

In the milling of cast iron further improvements are achieved with the multilayer coating having the ceramic layers shown in Fig. 10. For example, in the milling of the cylinder heads shown in Fig. 16 twice the lifetime of the normally used uncoated grade is achieved with grade Sr17 (Table 2). The high abrasive resistance of the ceramic layers and the increase in impact strength and resistance against thermal cycling given by the sequence of thin layers and the fine-grained structure that results from the addition of special elements are responsible for the improvement.

Fig. 15. Example for milling (machined face shaded). Fig. 16. Example for milling (cylinder head; machined faces shaded).

164

TABLE 2 Milling with Sr17 Cuffing

conditions

alloyed grey cast iron (GG 25) 250 HB SNUN 120412 180 m min-’ 0.17 mm rev-” 2-3mm

Workpiece material Hardness Insert Cutting speed Feed per tooth Depth of cut Machined

components

per edge

Uncoated hard metal (5.5% Co; 6.5% Tic; 3.2% Ta(Nb)C;remainder WC) Hard metal coated with Sr17

100 200

3.3. Coated high speed steels Tools made from steel are used in a variety of ways: for metal cutting and also for chipless forming. With regard to cutting conditions, the cutting speed and feed are lower for high speed steel tools than for cemented carbide tools. Therefore wear is often caused by galling. Depending on the type of workpiece material, abrasive wear can also play an important role. The wear reducing effect of hard material layers in metal cutting under typical high speed steel conditions is demonstrated in Fig. 5 (Section 2). Figure 17 shows the chips of a coated high speed steel cutting tool and of an uncoated tool. The chip from the coated tool is much smoother than that from the uncoated tool. Consequently the surface of the workpiece machined with a coated tool is much smoother. The tool of Fig. 17 was used for machining the bearing of a crankshaft [ 131 (Table 3). Most of the high speed steel cutting tools used so far are reground after the cutting edge is worn. In regrinding, the coating is removed from at least one of the surfaces of the tool. Therefore it is advantageous to use high speed steel inserts. Figure 18 shows such inserts which are used

chip formed when coated toot used

chip formed when uncoated tool used

Fig. 17. Example of the effect of using coated high speed steel in cutting.

165 TABLE

3

Results

from the machining

of a crankshaft

Workpiece material Tensile strength Tool material Cutting speed Feed rate Lifetime Uncoated tool Tool coated with 5 pm TiN

Fig. 18. Coated

high speed steel inserts

bearing Steel 42 CrMo 4 800 N mmP2 High speed steel grade S 10-4-3-10 6 - 20 m min-’ 0.15 - 0.30 mm rev-’

(1.3207)

40 components 120 components

for facing.

for the facing of motor housings. The following conditions were used: workpiece material, plain carbon steel; tensile strength, 400 N mm-*; tool material, high speed steel grade M35 (1.3243); cutting speed, 50 - 60 m min -’ ; feed rate, 0.2 mm rev-‘; depth of cut, 4 mm. The lifetime of the insert coated with a TIN layer 5 I.trnthick was about three times greater than the lifetime of the uncoated insert. Figure 19 shows another example of the replacement of a high speed steel tool by a high speed steel insert. This insert is used for the machining of rings for ball-bearings. The results are given in Table 4. For the CVD coating process temperatures from 700 to 1000 “C are needed. This operation reduces the hardness of steel tools. Therefore, after coating, a hardening and tempering process must be carried out. The coating is not affected by the quenching during hardening [ 131. The heat treatment causes dimensional and shape changes. Therefore tools with very close tolerances (tolerances smaller than 0.01 mm and with a length-to-diameter relationship larger than 15:l) cannot always be successfully coated. Examples are twist drills, taps, hobs etc. and these can be coated successfully with a physical vapour deposition (PVD) process. Possibilities for the application of CVD-coated tools are [13] as follows: turning tools; tools for parting and grooving and for form turning; insert tools for gear manufacture.

166 TABLE 4 Results from the machining of ball-bearing rings Workpiece material Tensile strength Tool material Cutting speed Feed rate Lifetime Uncoated tool Tool coated with 5 m

10006 600 N mrnm2 High speed steel grade M35 36 m min-” 0.06 mm rev‘ ’

TiN

Up to 300 components 1500 components

Fig. 19. Form turning tool consisting of a clamped coated high speed steel insert.

4. Modern chemical vapour dep~ition

technology

For the manufacture of hard material layers both CVD and PVD can be used. In the coating of cemented carbides the temperature required does not influence the properties of the base material. Therefore for coating cemented carbides the CVD process is nowadays well established. However, the PVD process has been worked on for the coating of tools for several years, in particular for high speed steel tools with very close tolerances [ 141. In CVD the coating is built up by a chemical reaction from a gas mixture on the surface of the parts to be coated. Figure 20 shows the principle of the process. If, for example, TiN is deposited the gas mixture consists of TX& (titanium tetrachloride), Hz and Nz :

processes

167

2TiC14 + 4H, + Nz __f

2TiN + 8HCl

If A1203 is deposited, the gas mixture consists of AlCls (aluminium chloride), Hz and COz: 2A1C13 + 3C02 + 3H, -

A1,03 + 3C0 + 6HCl

Important parameters influencing the deposition rate, composition and structure of the coatings are the temperature, composition of the gas atmosphere, flow rate of the gas in the coating chamber and the coating time [151. The CVD process has, in comparison with the PVD process, advantages with respect to the uniformity of the coating thickness and the adhesion of the coating. Even components with very complicated shapes and with holes and also tubes can be coated. The CVD process is used for the coating of very large quantities of cemented carbide tools. With respect to equipment for production purposes, some additional requirements must be fulfilled, such as a large number of

Hz

CO2

H2 N2.CHd

u components

/ hea& lo be coated

Fig. 20. Principle

/

of the CVD process.

components coated in one run with a uniform coating thickness, a minimum rejection rate as a result of the high degree of reproducibility from the process, a high reliability of the equipment and low production and maintenance costs. Figure 21 shows equipment for which the above-mentioned criteria are the basis for the design and manufacture. This equipment consists of four coating chambers and two heaters. Figure 22 shows a unit for smaller coating capacities and consists of one or two chambers and one heater. The coating chambers have a working diameter of 360 mm and a working height of up to 900 mm. These units have a microprocessor-based automated control system. This enables full automatic operation of the complete coating cycle even for coatings with a complex composition and a sequence of layers. For example, the coating described in Section 3.1 which consists of a sequence of ten layers can be produced fully automatically in one cycle.

168

Fig. 21. CVD unit for the production of large quantities.

Fig. 22. CVD unit for the production of smaller quantities.

169

References 1 G. Vierregge, Zerspanung der Eisenwerkstoffe, Stahleisen, Diisseldorf, 2nd edn., 1970, p. 83. 2 W. Schintlmeister, 0. Pacher and T. Raine, Weor, 48 (1978) 251 - 266. 3 W. Schintlmeister and 0. Pacher, J. VW. Sci. TechnoZ., 12 (1975) 743 - 748. 4 W. Schintlmeister, 0. Pacher, T. Krall, W. Wallgram and T. Raine, Powder Metall. Int., 13 (1) (1981) 26 - 28. 5 Metallwerk Plansee, Austrian Pat. 312,952, 1973. 6 W. Schintlmeister and 0. Pacher, Metall, 7 (1974) 688 - 693. 7 T. E. Hale, Ger. Patent DE 2265 603 C2, 1983. 8 U. KSnig, K. Dreyer, N. Reiter, J. Kolaska and H. Grewe, in H. M. Ortner (ed.), Proc. 10th Plansee Seminar, Reutte, June 1 - 5, 1981, Vol. I, Metallwerk Plansee, Reutte, 1981, pp. 411 - 441. 9 Metallwerk Plansee, Eur. Patent Appl. 0083043, 1981. 10 H. Meier, W. Schintlmeister, R. Storf and W. Wallgram, Fachber. Metallbearb., 60 (3 - 4) (1983) 87 - 91. 11 W. Schintlmeister, 0. Pacher, W. Wallgram and J. Kanz, Metal2 (Berlin), 34 (1980 905 - 909. 12 R. Storf, Maschinenmarkt, 102 (1980) 2015 - 2016. 13 W. Schintlmeister, J. Kanz and W. Wallgram, Znt. J. Refract. Hard Met., 2 (1) (1983 ) 41- 43. 14 W. Schintlmeister, W. Wallgram and J. Kanz, Thin Solid Films, 107 (1983) 117 - 127. 15 H. E. Hintermann and H. Gass, Schweiz. Arch., 35 (1967) 157.