Ceramic Cutting Tool Materials

Transcription

1 Ceramic Cutting Tool Materials D H,Jack Sandvik Hard Materials Ltd, P 0 Box 109, Torrington Avenue Coventry, UK Abstract Ceramics, as a class of materials, have always had potential as cutting tools. They are hard, retain their hardness at high temperatures and have a relatively low reactivity with steel. Hence they can be used at high cutting speeds without deformation or dissolution wear processes determining tool life. The drawback to ceramics as tool materials is that they lack toughness and resistance to both mechanical and thermal shock; this has limited the use of ceramic tool materials in the pasl In recent years there have been significant developments in ceramic tool materials; there are three categories available, namely pure oxide ceramic, mixed oxide plus carbide & nitride and silicon nitride based material. Each category has its own characteristicproperties, which must be understood if the materials are to be most effectively exploited in metal cutting. Introduction In the majority of machining operations today the tool material is in the form of what are referred to as indexable inserts, about 1-2 cm across and some 0.5 cm thick, which are mechanically clamped to a tool holder, so that when one edge of the insert is worn it can be indexed to bring another edge into operation. During machining the workpiece material is removed as chips. The terminology of metal cutting, (see Fig. 1), refers to the machining speed, v, at which workpiece material moves over the cutting edge; this can be simply related to the length of chip which is produced in a given time; to the feed, s, which is the distance that the tool moves along the workpiece with each revolution and this is related to the thickness of the chip; and to the depth of cut, a, which is the width of chip which is removed during machining. With today's machine tools and cutting tool materials, speeds of up to a 1000 m min -1 are possible with feeds of up to 1 m min -1. Depths of cut vary from 0.1 mm for fine finishing operations up to as much as several mm for heavy roughing cuts. Fig 1. / / V y /, /' /' / / Definition of speed, v, feed, s, and depth of cut, a, in metal cutting Tool Materials To be of use a tool must be able to withstand the particular cutting conditions for an economic cutting time, which for modern tool materials can be extremely short, of the order of a few minutes, but during which the material is subjected to extremes of stress and temperature. What is the most desirable in the tool material is predictable wear, leading to a predictable life under a given set of cutting conditions. Unpredictable failure by fracture or gross plastic deformation can lead to damage to the part being machined or to the machine tool, both of which can be several orders of magnitude more valuable than the tool itself. Fracture can arise due to mechanical shock or simply to the fact that the tool in use is a cantilever and there is a maximum tensile stress some way away from the region of contact; hence toughness is a requirement. Fracture can also occur due to rapid changes in temperature leading to thermal shock, which is resisted by materials with a MATERIALS & DESIGN Vol. 7 No. 5 SEPTEMBER/OCTOBER

2 low coefficient of thermal expansion and a high thermal conductivity. All intemaittent curing operations, eg milling, can lead to mechanical and thermal shock, but mechanical shock occurs every time the tool first engages the workpiece. Thermal shock can arise if the depth of cut varies during an operation or if the workpiece is small so that the tool is only in cut for a matter of seconds, during which time the tool temperature may have built up to over a 1000 C. To maintain the shape of the cutting edge the tool must resist gross plastic deformation. Once the edge loses its shape the stresses and temperatures rise progressively until collapse or fracture occurs. It is therefore necessary to have tool materials which have a yield stress greater than that of the work-hardened workpiece at the temperature of cutting. Given that the tool does not fail by fracture or gross plastic deformation then its life time is determined by its resistance to wear, which can take place by several mechanisms. Abrasive wear occurs by the mechanical action of hard particles in the workpiece and to resist it, the tool material should itself be hard. Attrition or adhesive wear occurs where there is intermittent sticking between workpiece and tool, for example at the extremities of contact between the chip and the tool. When the solid state weld between the workpiece and tool is broken, small fragments of tool can be plucked out and carried away on the underside of the chip or by the workpiece. Diffusion or solution wear occurs by a process of chemical dissolution of the tool material in the chip, particularly at the hottest point of contact on the rake-face some way back from the cutting edge; it is prevented by having tool material which will not chemically interact or alloy with the workpiece material at high temperatures. The characteristic wear patterns arising from such processes are illustrated schematically in Fig 2.11) Resistance to what is described as a diffusion or dissolution wear is critically important when the workpiece material is ductile. If the workpiece is brittle like cast iron, then little strain can be put into the chip before it breaks and so there is relatively little contact between the chip and the top of the surface of the tool. But with a ductile workpiece material such as steel, the chip is forced over the tool surface and the stresses and temperatures involved create a solid state weld between the chip and the tool. The relative move- Fig 2. rake face honed cutting edge I \ / I / / / / clcaranc~ end clearanc~ fac_ ~ L.~nos~ radius Regions of tool wear: crater wear at A: flank wear at B; depth of cut notch wear at C; trailing wear edge notch at D. Fig 3. Chip formation, indicating regions of highest temperature in the tool. ment of the chip with respect to the tool In the absence of such an ideal, creates a zone of intense shear in the cemented carbide has proved to be a chip close to its interface with the tool; very good compromise material and this shear generates heat which is partly some 70% of machined metal is cut conducted away in the chip, but which using it. raises the temperature to greater than a 1000 C in a localised region of the tool Cemented Carbide as illustrated in Fig 3. As far as cemented carbide tool materials In view of the stringent and complex are concerned, what are referred to as demands put upon tool materials, it is the straight grades containing only often said that the ideal material should tungsten carbide and cobalt can be used therefore have the hardness of diamond, to machine cast iron, the chips from the toughness of high speed steel and which fracture easily and so do not the chemical inertness of alumina. High remain in contact with the tool for long speed steel, however, can only be used enough to cause dissolution wear. For at slow cutting speeds, otherwise it just steel machining however, it is necessary collapses from "the high temperatures to add quantities of alternative carbides, which are generated. Diamond can be such as titanium carbide and tantalum used for high speed machining of non- carbide which are less soluble in hot ferrous materials, but, as well as being steel, (see Fig 4). But even with these so expensive, it has an unacceptably high called mixed crystal grades, wear by reactivity with iron-based metals. Alum- virtue of dissolution of the tool material ina is susceptible to both mechanical in the chip is the process which limits and thermal shock. machining speed. 268 MATERIALS & DESIGN VoL 7 No. 5 SEPTEMBER/OCTOBER 1986

3 log 10 =, Predicted Relative Solution Wear i 10, 10-=, i Rates of Steel Cutting Materials at 1300K 10 ~, Si:lN4 WC TiC TiN TiO Al=O:=ZrO= Fig 4. Predicted solution wear rates of tool materials in iron at 1027 C. One approach to solving this problem is to develop grades based on titanium carbide and nitride with essentially a nickel-cobalt alloy binder. These materials are used for moderate to high speed finish machining of steel, both in turning and milling; in Japan they now make up more than 15% of all inserts, although elsewhere the figure is currently less than 4%. Their reduced toughness makes them unsuitable for rough turning operations. The other approach to combating dissolution wear is to coat a tough cemented tungsten carbide substrate. A coating of titanium carbide some few micrometers thick can be built up on cemented carbide in a few hours by reacting together titanium tertra-chloride, methane and hydrogen gases at 1000 C. Using the appropriate reactants such as chemical vapour disposition (CVD) process can be used to deposit most carbides, nitrites and oxides, either singly or in combination. The principal property required of the coating is resistance to solution wear but it must also both adhere well to the substrate and resist abrasive wear & plastic deformation. The very low chemical wear rate for zirconia for example is not achieved in practice because it is not abrasion resistant enough and most commercial coatings today are combinations of TiC, TiN and A1203, (see Fig 5). Each successive generation of coated carbide has led to an increase in cutting speed for a given tool life, but there is a limit to the speed at which even coated cemented carbide can be used. As speed increases the temperature rises to the point where the metallic cobalt binder softens and the tool distorts by gross plastic deformation. The same restriction applies to cemented titanium carbide bonded with nickel. Ceramics, as a class of materials, have always had potential as cutting tools. They are hard, retain their hardness at high temperatures and have a relatively low reactivity with steel. Hence they can be used at high cutting speeds without deformation or dissolution wear processes determining the tool life. The drawback to ceramics as tool materials is that they lack toughness and resistance to both mechanical and thermal shock. This means that, in general, ceramics can only be used at high speeds and require stable machining conditions free from vibration. Such restrictions, and deficiencies in the tool materials themselves, have limited the use of ceramic tool materials in the past. In recent years, however, the increased use of powerful, rigid, CNC machine tools has allowed for greater exploitation of the benefits of ceramic tool materials and there have been significant developments in those tool materials themselves. Ceramic Tool Materials There are three categories of ceramic tool material available today. The traditional ceramic tool material has been aluminium oxide- alumina which is white and can be manufactured by cold pressing powder in a die with subsequent sintering. The modem alumina ceramic contains a proportion of zirconium oxide, zirconia, which significantly toughens the material. The terms "pure oxide" or"white" or"coldpressed ceramic" are used to describe this category. A major disadvantage wih pure oxide ceramics is that they have a low thermal Fig 5. conductivity which makes them very susceptible to thermal shock. Thermal effects become more pronounced at higher machining speeds and are also caused by short machining cycle times or variable depths of cut. The thermal conductivity, and hence thermal shock resistance, of alumina can be improved by the addition of titanium carbide. The resultant material, which is black, does not sinter as easily as alumimina itself and it has usually been considered necessary to hot-press the powderwith consequent limitations on the shapes which can be produced. A recent development has been the introduction of a material with titanium nitride as the major addition. (2) This improves the thermal shock resistance still further and is also a composition which can be cold pressed & sintered. The material is dark brown or chocolate coloured, hence the use of the terms "black ceramic" and "hot pressed ceramic" are no longer appropriate for this category of material, which is better described as "mixed ceramic". Not only do the mixed ceramics have better thermal shock resistance than pure oxide ceramics, but they are harder and retain their hardness better at high temperatures, making them more suitable for finishing operations and for harder steel and cast iron machining where the combination of high cutting forces and high temperatures can cause surface deformation of pure oxide ceramic. Whilst the newer alumina-titanium nitride mixed ceramic is tougher than the established aluminatitanium carbide, black, hot pressed ceramic, its toughness is not as great, however, as that of the zirconia tough- AI203 Diagram of a coated cemented carbide metal cutting insert. MATERIALS & DESIGN Vol. 7 No. 5 SEPTEMBER/OCTOBER

4 ened pure oxide grades. The third class of ceramic tool material is that based on silicon nitride. Silicon nitride (Si3N4) has a very low coefficient of thermal expansion, which reduces the stresses set up between hotter and cooler parts of an insert, so its thermal shock resistance is excellent. Silicon nitride is not, however, easily sinterable to full density, but by substituting some of the silicon and nitrogen in silicon nitride by aluminium and oxygen, the sialon ceramics are formed, (Fig 6) which have equally good thermal shock resistance, and an ability to be cold pressed and sintered. A metal SiO2 ~ ~ Ciqu,:~;~ x-phase Si2N=O///l~-sialon A1203 Silicon Nitride Sialon Alumirlium; Oxygen Sis N8,~ Sic-, AI= O, Ne-= AI;O Si3N 4 ~,ll ii',i1' rl~,flf' e~-sialon (Projection) AIN Fig 7. The Si-AI-0-N behaviour diagram. Metal cutting compositions lie near the silicon nitride corner of the diagram. Fig 6. Silicon nitride and sialon. oxide, typically yttria, Y203, is commonly added to aid further sintering. During sintering, silica (Si02) on the surface of the silicon nitride particles reacts with alumina (A1203) and yttria to form a low melting point liquid. The silicon nitride reacts with this liquid to form sialon, plus, on cooling, a glass. Depending on the relative proportions of the reactants, the sialon so formed can have the atomic arrangement of beta, or alpha silicon nitride. It is possible to have both beta sialons, (of composition Sit.zAlzOzNs.z, where "z" represents the degree of substitution of silicon and nitrogen by aluminium and oxygen), and alpha sialons, (represented by Mx(Si,AI)12(O,N)~ 6, where M is a metal atom, for example yttrium). Fig 7 shows the position of the beta and alpha sialon compositions in the phase systen~ The microstructure of sialon consists of grains of the crystalline nitride phase held in a glassy, or partly crystallised, matrix. The crystalline grains can either be all beta sialon (Fig 8), or a mixture of alpha and beta (Fig 9). In general the hardness of sialon increases as the amount of the alpha phase rises, and this increase is retained at high temperature (Fig 10). (3) The fact that sialon retains its hardness at temperature better than alumina is one of its outstanding properties. Another is its toughness as a ceramic, but it should be noted that it does not have the same Fig 8. Fig 9. Polished and unetehed section of a B-glass sialon. Light phase is glass. Section of an wb - glass sialon. Lighter phase is glass: mid grey phase is c : dary grey phase is B 270 MATERIALS& DESIGN Vol. 7 No. 5 SEPTEMBER/OCTOBER 1986

5 toughness as cemented carbide of equal hardness. The technical properties of typical ceramic tools are indicated in Table I Applications of Ceramic Cutting Tool Materials Cast Iron Machining Machining of cast iron brake drums and discs for the automotive industry remains the largest application area for ceramics and all three categories of ceramic can be used depending upon the application. Where castings are of high quality and pre-machined, (ie there are relatively few slag inclusions and no as-cast skin) the even flank wear resistance of the pure oxide ceramic is superior to both mixed ceramic and sialon. For the types of components for which ceramics are likely to be used, however, the casting quality and surface condition will vary and there can be degrees of run out leading to variable depths of cut. Variable depths of cut or short cycle times can lead to thermal cracking and the mixed ceramic and nitride based ceramics perform better than pure oxide ceramic under these circumstances. Edge strength can be tested by an intermittent operation such as the facing of a slotted bar, the results of which are shown in Fig 11. At the lower speeds the fracture resistance of pure oxide ceramic and sialon is comparable, but whereas the toughness of sialon in retained throughout the speed range, that of the pure oxide ceramic falls off at high speeds as thermal stresses are superimposed on mechanical ones. The poorer mechanical strength of the mixed ceramic also makes it unsuitable for use at heavy feeds. In finishing operations the tool life criterion is maintenance of the required surface finish on the component and wear at the trailing edge contact point is therefore critical, see Fig 2. The greater attrition wear resistance of CC650 mixed ceramic makes it clearly superior to pure oxide ceramic in this respect as shown by Fig 12. The lower general wear resistance of nitride ceramics precludes their being used effectively in finishing operations. Hence, nitride ceramics find their most suitable application in roughturning and intermittent operations, whereas pure oxide ceramic is likely to give longest tool life in semi-finishing good quality castings under stable continuous cutting conditions. For finishing operations the mixed ceramic CC650 is the first choice. Steel machining The present day use of ceramics for steel machining is largely confined to Hardness 5001 ~ Alumina / 0 i I C Temperature Fig 10. Hot hardness of sialon compared with alumina. Table 1 Characteristics of Ceramic Cutting Tool Materials C~//~ - sialon Ceramic Type Pure oxide Mixed Sialon Sandvik Grade CC620 CC650 CC680 Composition AI203 +Zr02 Al203 +TiN Si3N Colour White Chocolate Grey Hardness-Room Temp (10kg load) Hardness C (10 kg load) Flank Wear Resistance High Very High Medium Solution Wear Resistance Very High High Low (Steel) Bulk Toughness High Medium Very High Edge chipping resistance Medium Medium High Thermal shock resistance Low Medium High Tool life (Number of cuts) Intermittent facing of Cast Iron Tool life criterion: Edge fracture Feed = 0.2mm/rev : Depth of cut = 2.0mm - ~ CC I I I I Fig 11. Tool life in intermittent cutting of grey cast iron. Speed (m/min) 4 JI-AI203 MATERIALS & DESIGN Vol. 7 No. 5 SEPTEMBER/OCTOBER

6 Surface Finish Ra (/~m) 3 -- m 2 -- Facing Cast Iron Cutting speed m/min. Feed - 0,15 mm/rev: Depth of cut = 0.75mm CC 620 variants, (see Fig 13(4)). Where high speed machining of heat resistant steels is required, the mixed ceramic is the preferred material. With titanium alloy machining the strong chemical affinity between titanium and both alumina and silicon nitride generates extensive crater wear after a very short cutting time, making oxide, mixed and nitride ceramics inappropriate choices for such an application f CC 650 I I I I I I I Time (min) Fig 12. Surface finish as a function of machining time with pure oxide and mixed ceramic. turning of hardened or low alloy steels and both pure oxide & mixed ceramic are used. In general harder workpiece materials and higher cutting speeds favour the use of mixed ceramic which can resist both thermally induced stresses and the combined effects of high temperature & cutting forces, better than pure oxide ceramic. The better bulk strength of the pure oxide ceramic favours its use under conditions of higher feed. Sialon, in general, cannot be used for steel machining at high speeds due to its rapid rate of solution wear but in, for example, machining of hardened die steels at speeds of up to 100 m/min, sialon can compete very effectively with cubic boron nitride. Heat resistant steels, nickel and titanium alloys Heat resistant steels, nickel, and titanium alloys pose particular problems in machining as a rule the most critical wear processes are those occurring at the depth of cut points, (see Fig 2). A major factor in this is that these workpiece materials generate highly segmented chips, which means the edge of the chip is in fluctuating contact with the tool and generates fluctuating stresses. The chip can be regarded as alternately seizing and being released, several thousand times a second; as it does so small fragments of tool material are pulled out. This type of wear is best resisted by a tool material with weak adhesion between itself and the workpiece, plus good microtoughness. Sialon has been outstandingly successful in this respect in machining nickel base alloys, although its resistance to even flank wear is not as high as that of the pure oxide and mixed ceramic. The superior performance of alphabeta sialons, as opposed to beta only grades, is particularly noticeable in machining nickel-based alloys, not just, it is considered, due to their superior high temperature deformation resistance, but to the somewhat increased resistance to the solution wear of the alpha-beta Continuous turning of Inconel 718 Tool life (min 10 q% Feed: 0.2 mm/rev ~ Depth of cut: 2ram \\ SANDV,K% S,A'ON AhO3 - based ~ 0 Cemented Carbide 1;0 2;0 3;0 Speed (m/rain) Fig 13. Tool life ofo~/bsialon and the other tool materials when machining Inconel 718. Future Usage of Ceramic Tools The advances in understanding of the relationships between ceramic microstructure and properties, together with improved processing techniques, have led to the existing range of ceramic tool materials, with significantly superior combinations of wear resistance and toughness than the materials of even five years ago. Development continues; there is now on the market a new type of mixed ceramic which is based on Ale0 3 containing whiskers of silicon carbide, (Fig 14) and that appears to be extremely good for high strength nickel alloy machining. The addition of the silicon carbide whiskers leads to improved toughness and increased Weibull modulus, see Fig 15. It is certain also that silicon nitride based ceramics will continue to develop, not so much stimulated by the prospect of a large cutting tool market perhaps, but by the potential in automotive and aerospace Fig 14. Alumina containing 25 vol% of silicon carbide whiskers. 272 MATERIALS& DESIGN Vol. 7 N(~ 5 SEPTEMBER/OCTOBER 1986

7 SiC Whisker-Reinforced Alumina Composites 700 Flexural Strength Bulk fracture resistance (Cast Iron) ~ lb 1'5 2'0 2'5 3'0 % Percent SiC Whiskers Fracture Toughness (MPa m v2 ) Accumulated fracture incidence 100% 50% Cutting speed = 500m/rnin. Feed = 0.5mm/rev : Depth of cut = 3.0mm, Mixed / / Coated Ceramic / /[ Carbide /Pure Pure Alumina 0 ~ 1'0 "t~ Weibull Modulus f 2'0 2'5 3'0 % Percent SiC Whiskers Number of cuts 1 I I 1'0 1'5 20 2'5 3'0 % Percenl SiC Whiskers Fig 15. Properties of SiC-reinforced alumina. Fig 16. Bulk fracture resistance of tool materials in a severely interrupted high speed cut, ting test. applications from which there will be further spin off. Such developments can also be expected to reduce the price of ceramics as the volume increases. Additionally, a higher proportion of the machine tool population will be better suited to the requirements of ceramic tool materials. The materials are still intrinsically brittle, however, and in the context of modem automated manufacture it is essential that the performance of any tool is consistent and that any tool failure does not interrupt production. The point can be illustrated by reference to Fig 16 which shows the accumulated fraction of inserts which fail with number of passes in a particularly demanding interrupted cutting test. A coated carbide grade exhibits a lifetime threshold below which no tool failure occurs, and above which the inserts fracture in a predictable way. Mixed ceramic also behaves consistently, but with an unacceptably short life in this particular test. Pure oxide ceramic has a longer average but more variable life than the mixed variety. Sialon has the best average bulk toughness, better even than cemented carbide, but the probability of insert breakage is still finite, even at very short times. Conclusion Ceramics, then, can offer increased metal removal rates, extended tool life and an ability to machine hard workpiece materials. At present their share of the total indexable insert market is less than 4% That figure will doubtless increase as both the number of machine tools capable of exploiting the benefits of ceramic grows and the development of the materials themselves continues. The intrinsic properties of ceramic materials does mean however that they can never replace cemented carbide as a cutting tool material, but are ideal complements to it. Acknowledgements The work on which this paper is based has been carried out in the laboratories of Sandvik Hard Materials Ltd. in Coventry, England, and AB Sandvik Hard Materials in Stockholm, Sweden; acknowledgement is due to many colleagues in both laboratories. References (1) Dearnley, P.A. and Trent E.M., "Wear mechanisms of coated carbide tools", Metals Technology, Vol. 9, pp , 1982 (2) Brandt, G. and Olsson, B. "AI203_ TiN, a new tougher composition for ceramic cutting tools", in "Advances in Hard Metal Production", Metal Powder Report Conference, Luzern, (3) Ekstrom, T. and Ingelstrom, N. "Characteristics and properties of sialon materials", to be published in the Proceedings of the Conference on Non-Oxide Technical and Engineering Ceramics, Limerick, (4) Aucote, J. and Foster, S.R. "The performance of sialon cutting tools when machining nickel based aerospace alloys", to be published in Materials Science and Engineefin~ MATERIALS & DESIGN Vol. 7 Na 5 SEPTEMBER/OCTOBER