Mechanisms Of Wear In Hss Cutting Tools

Seminar Report on

Mechanisms of Wear in HSS cutting tools

Prepared by JYOTI RANJAN NAYAK [email protected] om

Contents Introduction The cutting process in brief Tool material properties 

High temperature strength



Fracture strength vs. hardness

Common work materials for HSS cutting tools Tool wear Wear mechanisms of uncoated tools 

Abrasive wear



Adhesive wear



Large scale plastic deformation



Fatigue and fracture



Diffusion wear

Wear mechanisms of coated tools 

Coating removal due to poor substrate preparation



Coating removal due to thermal softening of the substrate

Distribution and evolution of edge wear Towards better performance of HSS cutting tools 

Improving the HSS material



Improving the surface integrity

Conclusions References

Introduction Metal cutting puts extreme demands on the tool and tool material through conditions of high forces, high contact pressures, high temperatures, and intense chemical attack by difficult to cut work materials. In addition, the tool geometry and cutting conditions in terms of sharp edges, cyclic engagement and presence of cutting fluid will add to the severity. Most often cutting tools are used close to their ultimate resistance against these loads, especially to the limiting thermal and mechanical stresses.

In spite of the increasing use of high performance tool materials, such as CVD and PVD coated cemented carbides, cermets, ceramics, cubic boron nitride and diamond, high speed steels (HSS) are still frequently used in tools for metal cutting applications. The relatively high toughness and the possibility of economic manufacturing of tools with complicated geometries still justify the use of HSS in many cutting operations. The introduction of powder metallurgical grades in combination with Electro Slag Heating (ESH) and Physical Vapour Deposition (PVD) coating technologies has further improved the performance of HSS cutting tools.

Since the successful introduction of the PVD-Tin coating in the late 70’s, the academic research on HSS metal cutting tools has been concentrated to developing even better coating materials and techniques for their deposition. This paper is a brief overview of the mechanisms of wear of HSS cutting tools and includes illustrations from both uncoated and coated tools. More details on the metal cutting process, the mechanisms of tool degradation, and the properties of HSS materials and their coatings are found in Reference 1to 10.

The cutting process in brief To understand the wear mechanisms in metal cutting it is necessary to have a brief understanding of the severe contact conditions prevailing at the cutting tool/work material interface, see Fig. 1. The common model illustrates orthogonal cutting, but it applies to any cutting operation including turning, milling, sawing, drilling, tapping, broaching, etc. Through plastic shear of the work material and sliding of work material against the tool flank and rake face a characteristic temperature profile is established. The principle heat sources are located at the primary shear zone in the forming chip and in the frictional contact between chip and tool (secondary shear zone), and the highest temperature is consequently reached on the rake face at some distance from the edge.

.

Fig. 1. Principle action and temperature distribution of a HSS metal cutting edge exposed to its practical limit of thermal loading

To illustrate the forces and mechanical stresses acting on the tool edge in one picture is less strait forward since they change considerably with cutting operation and cutting parameters. In intermittent cutting they also may change completely from entrance to exit during the Wear mechanisms of HSS cutting tools individual edge engagements. Generally, the over all cutting force F is related to cutting speed and feed as indicated in Fig. 2. It is indicated that a low friction coating can lower the cutting force and thereby giving a lower edge temperature, which can be utilized to increase the productivity.

We know from the type of failure mechanisms that HSS cutting tools are used close to their limits of yield and fracture stresses, see § 6 and on. Since the cutting edge is forcing its way through the interior of the work piece like a propagating wedge, both surfaces of the opened “crack” represent highly chemically reactive metal. The fact that there is no access to external oxygen or cutting fluids to this region means that there is no formation of oxide films or any other protecting interlayer. Consequently, the tool edge is also exposed to extremely severe conditions.

Fig. 2(a). Schematics of cutting force F vs. cutting speed

Fig. 2(b). Schematics of cutting force F vs. feed

Tool material properties • High temperature strength A metal cutting tool must be able to combine high hardness (or high yield strength) with high fracture strength at elevated temperature, see Fig. 3a. The latter is especially important in Primary shear zone Wear mechanisms of HSS cutting tools interrupted cutting. A high thermal conductivity is also a desired tool property since it will reduce the tendency to local thermal softening. The high thermal resistance of carbides, nitrides and oxides indicates their potential as protective thin PVD or CVD coatings, but also their strengthening ability when present in the form of small particles in the tool material. However, they are also common as strengthening elements in most work materials where they contribute to abrasive wear, see § 6.1.

• Fracture strength vs. hardness High hardness is associated with brittleness, and strengthening metallic materials such as HSS by martensitic hardening, dispersion of hard particles, etc. of a metallic materials most often results in a material with a lower fracture strength as indicated by Fig. 3b.

a)

b)

Figure. 3. a) Hot hardness (HV) of HSS compared to that of carbon steel and austenitic stainless steel. The superior hot hardness of carbides, nitrides and oxides in the whole temperature interval is also indicated. b) Room temperature fracture strength (Rmb) vs. hardness (HV) of some common tool materials.

Common work materials for HSS cutting tools Generally, the work materials in metal cutting with HSS tools are macroscopically much softer than the tools, see Table 1. However, many work materials contain constituents (carbides, nitrides or oxides) that are harder (HV 1500 – 3000) and more temperature resistant than the HSS matrix, as indicated in Fig. 3a, and contribute to the tool degradation by abrasion. High toughness, large fracture elongation (ductility) and ability to work harden all add to generate a high temperature during chip formation. High temperatures reduce the strength of the HSS tool, but will also facilitate chemical reactions and possibility to form inter-metallic phases between tool and work material. This will increase the friction between these materials and thus further aggravate the situation.

Another fact that has to be considered when comparing the mechanical properties of tool materials with those of work materials is that chip formation generally occurs by extremely high shear rates. Taking high strain rate into

account, the work material curves of Fig. 3a are Wear mechanisms of HSS cutting tools lifted up such that the corresponding RT hardness of a carbon steel may well match the hardness of the cutting edge at its working temperature, as indicated by the two ovals in this figure [11]. The illustrated situation is accentuated in intermittent cutting when a hot tool edge suddenly meets cold work material.

Table 1. Work materials and their nominal properties related to tool wear in metal cutting Work material

Hardness [HV]

Hard particles

Ductility

Work harden

C-steels

200 - 250

Cementite

Yes

Yes

Cast irons

200 - 250

Cementite

-

-

γ−steels

180 - 250

-

Yes

Yes

Al-alloys

100 - 150

Oxides, AlFeSi

Yes

-

Ti-alloys

200 - 350

-

Yes

Yes

Ni-based alloys

200 - 350

Yes

Yes

Yes

Tool wear Taking orthogonal cutting as a model the general characteristics of a worn HSS cutting tool are schematically illustrated in Fig. 4. Primarily, depending on cutting operation, cutting parameters, cutting parameters, work material and tool material the performance of the tool is limited by nose wear, flank wear, crater wear, edge chippings, or combinations of these. Depending on the same parameters, the wear either occurs gradually by abrasive or adhesive wear, through plastic deformation, by more discrete losses of material through discrete fracture mechanisms, or by combinations of these. Below, illustrative micrographs from scanning and optical microscopy (SEM and OM, respectively) of used HSS tools will be used to demonstrate the wear mechanisms.

Cutting tools can be used only when their edges produce parts with specified surface finish and dimensional tolerance. When the quality of the cutting edge is lost due to wear, the tool reaches its limit and must be replaced by a new one. In this paper, we will discuss about the mechanisms that lead to the tool wear. Presently, no viable theories exist for predicting tool wear on the basis of properties of tool and work material. Optical and electron microscopic and auto radiographic observations suggest that the tool wear phenomena occur at microscopic and atomic levels. Each tool wear location such as wearland, crater, nose, notch and trailing (see Fig.4 b)involves a different wear mechanism. This is because the temperature, sliding velocity and stresses are different at each location. Also, the applicable mechanism for each location may be different for different tool materials, work materials, and machining speeds, feed, depths and cutting fluid. This multiple nature of tool wear phenomena has led to a number of possible mechanisms but, however, no predictive theories have evolved as yet. Lim and Ashby consider that two major forms of wear are commonly observed on a cutting tool: flank wear and crater wear [3]. This is illustrated in Fig.1. Flank wear (occurring on the major and minor flank of a tool) generally causes an increase in the cutting force and the interfacial temperature, leading normally to dimensional inaccuracy in the workpieces machined and to vibration which makes the cutting operation less efficient. Crater wear takes place on the rake face of the tool where the chip moves over the tool surface. A crater is ussualy formed at some distance from the cutting edge (see Fig.4b) and it is most frequently observed when cutting steels and other highmelting-point metals at relatively high cutting speeds [4]. This crater gradually becomes deeper with time and may lead to the breakage of the cutting edge, rendering the tool useless.

a)

b) Figure. 4. a) Schematic of tool wear distribution. Wear mechanisms of HSS cutting tools b) Typical wear patterns of turning tool (carbide insert)

Wear mechanisms of uncoated tools • Abrasive wear Abrasive wear dominates the flank and crater wear of the HSS tool edge seen in Fig. 5. The grooved pattern is a combination of the scratching action of hard particles in the work material, and the protection against scratching offered by the hard phases in the tool material. Behind large tool carbides, seen in the chip flow direction, there are typical ridges of protected tool material. The individual abrasive scratches are too small to be resolved in the picture. Abrasive wear is counteracted by a high yield strength (high hardness) and large carbide volume of the HSS.

Abrasive wear on the tool surface is caused by hard particles in the work material. Their effect on the tool wear can be explained theoretically by three abrasion models, these are micro ploughing, micro chipping and micro cracking (Knotek, 1993).

Many work materials (cast iron, steel), contain particles of phases which have a hardness that is much higher than of the workpiece. The particles can typically be carbides or oxides, particularly Al2O3, but also silica and some silicates (Trent, 1979). The particles may also be highly strain-hardened fragments from an unstable built-up edge on the tool (Moore, 1975). In particular the wear at the flank face may be attributed to abrasive wear. In abrasive wear the influence of tool surface hardness is considerable (Budinski, 1980; Kramer, 1986; Konig 1992)

Abrasion [1] occurs when a harder material (the tool) shears away small particles from the softer work material. However, softer work material also removes small particles from the tool material although at a smaller rate. The hard tool particles are caught between the hard tool and soft workpiece, and this causes additional abrasion wear. Tool and workpiece contain carbides, oxides, and nitrides with hard microstructures; these cause abrasion wear during machining.

a)a)

b)

c)

d)

Fig. 5. Typical appearance of abrasive wear. a) Wear dominates the crater and flank wear of a milling tool. The arrows point at ridges of HSS material relatively resistant to abrasion. There is also evidence of edge fracture. Work material: C-steel. b) Paper knife. An extremely fine-scaled abrasion, only resisted by the hard carbides, dominates the tool wear. c) and d) Abrasive wear of the cutting tool

• Adhesive wear When viewed in low magnification the dominating wear mechanism of the milling tooth of Fig. 6 appears to be abrasive, i.e. a ploughing action of hard constituents in the work material (carbon steel). However, higher magnification (Fig. 6b) reveals that it is rather a combination of abrasive and adhesive wear. This adhesive component, often referred to as mild adhesive wear, is a tearing of superficial HSS material by high shear forces resulting in a slow drag of the surface layer and removal of small fragments in the direction of chip flow. If the tool is used to its upper limit of heat resistance, severe adhesive wear may result as a large scale plastic flow of surface material in the direction of the chip flow, see Fig. 7. Wear mechanisms of HSS cutting tools

Adhesive wear dominates the flank and crater wear of HSS tools if the edges reach high temperatures, i.e. at high cutting speed. Adhesive wear is further promoted when cutting chemically aggressive materials. Both mild and severe adhesive wear are primarily resisted by the HSS material through its high yield strength at elevated temperature (high hot hardness).

a)

b)

Fig. 6. Crater in a milling tool that has been cutting in low carbon steel. In low magnification (a) the dominating wear mechanism appears to be abrasive.

However, a close up (b) reveals that it is dominated by a mild adhesive component with shear fragmentation of the HSS material in the direction of chip flow (arrow).

a)

b)

c) d) Figure. 7. a) Optical micrograph of cross-sectioned hob tooth after cutting austenitic stainless steel. b) Detail of a). The arrows indicate the chip flow direction and flow pattern of superficial HSS material, respectively. The latter is indicative of severe adhesive wear. c): The wear of cutting tool determined by adhesion and d) built-up edge Adhesive wear is caused by the formation of welded asperity junctions between the chip and the tool faces and the fracture of the junctions by the shearing force so that tiny fragments of the tool material are torn out and adhere to the chip or the workpiece. This kind of wear may occur at the flank face in low speed cutting when the contact temperatures are not so high. It may involve oxidation of the tool surface, or other chemical interaction with the surrounding atmosphere, followed by mechanical removal of the products of the reaction. Another point of view regarding the adhesion wear is presents by Yusuf Altintas and represents the analyze making by Oxley [5]: When there is a relative motion between the two bodies that are under the normal load, fragments of softer workpiece adhere to the harder tool. The adhered material is unstable, and it separates from the cutting tool and tears small fragments of the tool material. The 1819 typical example in metal cutting is a built-up edge, which usually occurs at low cutting speeds when part of the chip material welds to the cutting edge. Depending on the size and stability of the built-up edge, either the forces decrease because the effective rake angle becomes positive or the lumped built-up edge dulls the tool and increases the forces. An unstable, large built-up edge occurs close to the cutting edge at low speeds where the tool-chip interface temperature is low. The material is still strong at this point and difficult to move over the rake face. As the chip moves over the rake face, the chip-tool interface temperature increases, leading to a

softer chip, which is easier to move. As the cutting speed is increased, the magnitude and length of the built-up edge becomes smaller and localizes close to the cutting edge. Predicting the tool chip interface temperature is therefore important in identifying cutting speeds where the built-up edge is minimum.

• Large scale plastic deformation Sometimes, the HSS tool edge is loaded beyond its yield strength and deforms by large-scale plastic deformation, see Fig. 8, resulting in edge blunting. This has been observed when high speed tools soften due to annealing during machining

a)

b) Fig. 8. Plastic deformation of HSS tool edge. a) The edge line in the central part of the picture is being plastically moved downwards and will soon leave the edge as the adjacent parts of the edge already have. b) Cross section of the plastically deformed edge showing signs of adiabatic shear.

• Fatigue and fracture Macroscopic fracture of the whole tool can occur but is a rather scarce event. More common is localised chippings of the tool edge, see Fig. 9. Note that the chippings in (a) seem to be initiated by grinding marks running parallel to the edge.

a)

b) Fig 9. Small (a) and somewhat larger (b) edge chippings due to local overloading and fatigue of hob teeth. Wear mechanisms of HSS cutting tools

• Diffusion wear Diffusion wear characterizes the material loss due to diffusion of atoms of the tool material into the work piece moving over it. Requirements for diffusion wear are metallurgical bonding of the two surfaces so that atoms can move freely across the interface, a temperature high enough to make rapid diffusion possible, and some solubility of the tool material phases in the work material In a theoretical model of diffusion wear proposed by Kramer and Suh (1980) and Kramer (1986) the wear rate is considered to be controlled by the mass diffusion rate. However, tool wear prediction according to the model is not in agreement with experimental results reported by Sproul (1987), Ono and Takeyama (1992) have shown that in diffusion wear at the minor flank (relief face) of the tool the chemical reaction taking place at the interface has a major effect on wear. They showed that oxygen gas accelerates the formation of oxide layers that are continuously torn off resulting in increased wear, whilst wear is decreased by an environment of argon gas. From the analyze proposed by Altintas (2000), the diffusion wear appear when the temperature of the tool and work piece increase at the contact zones, the atoms in the two materials become restive and migrate to the opposite material where the concentration of the same atom is less.

Typically, in a tool material such as tungsten carbide (WC), where carbide (C) provides the hardness, while cobalt (Co) binds the WC grains, carbon diffuses to the moving steel chips, which have less concentration of the same atoms. Progressive diffusion of the tool materials into the chip gradually leads to a weakened cutting edge and eventual chipping or breakage of the tool.

Figure: Diffusion wear of the cutting tool

Wear mechanisms of coated tools Since the late 70’s when the TiN-coating was introduced on HSS metal cutting tools, PVD coating has become standard in tool wear protection, and today coating centres offer a considerable number of thin ceramic coatings for HSS tools [9]. A thin (1 – 10 µm) PVD coating will primarily protect the cutting edge in two ways: • Acting as a shield against abrasive and mild adhesive wear. • Reducing the tool temperature by reducing the friction between tool and work material, especially between chip and rake face. The coatings combine a superior hardness (abrasive wear resistance) with relatively low chemical reactivity with metallic materials (low solubility), the latter giving protection against the welding mechanism that is the prerequisite for adhesive wear. Consequently, most of the common PVD coatings of today rather fail by fatigue and discrete delamination /detachment than removal by slow gradual wear [9]. Once the coating is removed, the wear mechanisms of coated tools are the same as those of uncoated, although more severe because more severe cutting parameters are normally used for coated tools.

• Coating removal due to poor substrate preparation There are primarily two ways by which failure in HSS substrate preparation can occur. • The surface temperature during grinding/polishing reaches above the austenitisation temperature resulting in a brittle interlayer of untempered martensite, see Fig. 10. • The resulting substrate surface is too rough, see Figs. 11 and 12.

a)

b)

Fig. 10. Metallographic cross-sections through surface finished HSS materials. a) Superficial layer of untempered martensite due to excessive heat generation during finishing. b) Properly surface finished HSS. Used as substrate for PVD coating, the untempered martensite in Fig. 11a would constitute a brittle interlayer inferior to coating adhesion. 10 µm 10 µm PVD coatings on HSS tools possess internal compressive stresses of the order of 1-5 GPa. Typically, TiN deposited on HSS has a lateral compressive residual stress of around 4 GPa. This stress acts positively for the coating cohesion, but negatively on its adhesion to the substrate. In combination with a rough substrate, excessively high compressive stresses may cause spontaneous detachment without any external loads [10, 12]. The reason is that lateral compressive stresses in the coating combined with a rough substrate will generate tensile stresses across the coating/substrate interface as illustrated in Fig. 11a [12]. If such a system is externally loaded, coating detachment is facilitated along regions of maximum tensile stress, i.e. along the coarse ridges on the tool of Fig. 11b. These ridges are the result of a too rough grinding process / incorrect grinding parameters.

Another example of topographically induced coating failure is shown in Fig. 12 where it also is indicated that cracks nucleated in the coating may spread to the underlying HSS material. Through fatigue, they may later cause edge chippings and large-scale edge fracture.

a) b) Fig 11. a) The lateral compressive stresses state σ present in most PVD coatings will generate interfacial stresses S. At the top of e.g. grinding ridges this stress is a tensile “lift off” stress that may reach the same order of

magnitude as the residual stress σ [12]. Such ridges can result from rough grinding. b) Tin coating detachment along grinding ridges of a HSS cutting tool.

a)

b)

Fig. 12. Microscopic fatigue cracks observed on the rake face close to the edge of a hob tooth that has been cutting in carbon steel. b) Close up of a). The direction of the cracks coincides with the direction of surface finishing. 25 µm Wear mechanisms of HSS cutting tools

• Coating removal due to thermal softening of the substrate Once the HSS substrate material reaches a temperature level of excessive softening, it fails to resist the contact pressure, and the brittle coating fractures, see Fig. 13a. Note the dark etching contrast underneath the coating, which reveals thermal softening due to over tempering. The coating fractures and individual fragments are then detached in the form of small fragments, see Fig. 13b.

a)

b)

Fig. 13. Coating detachment of hob tooth used for making gears of carbon steel. a) Coating fracture due to thermal softening of the substrate. b) Removal of small coating fragments and initial wear of the underlying HSS material. The thickness of the fragments is the same as the original coating thickness.

Distribution and evolution of edge wear The macroscopic wear pattern of a cutting tool edge was illustrated in Fig. 4. The mechanisms described above will eventually cause wear that exceed the worn out criteria, either as a certain width of the flank, the rake face or as a certain edge blunting. Figure 14 shows the development of a large crater in a TiN-coated hob tooth. The work material was carbon steel and the wear that eventually controlled tool life occurred on the rake face. Initially, the hob teeth suffered from limited edge chipping (Fig. 14b and c). At the same time, thermal softening of tool material in the rake face (over tempering revealed by the dark contrast adjacent to the coating) reduced the load bearing capacity of the coating, which failed by cracking and brittle fracture (Fig. 14b), cp. Fig. 13. Once the coating was removed, a large crater was rapidly developed in the unprotected HSS by severe adhesive wear, Fig 14d. Irrespective of the location of the critical wear, its evolution can be illustrated as in Fig. 15. An initial wear, often involving tip blunting through minor fractures (chipping) is followed by a linear, steady-state wear regime dominated by abrasive and adhesive wear. A gradual tip blunting is one of the

reasons behind a successively increasing edge temperature, and eventually, a situation of accelerated wear through edge fracture or severe plastic deformation is reached. It is also indicated in Fig. 15 that a smoothening of the tool surface and sharpening of the edge will prolong tool life. This is further accentuated by coating. However, the improved wear resistance obtained by coating is often used to increase the productivity rather than to obtain a longer tool life.

Fig. 14a)

Fig. 14c)

Fig. 14b)

Fig. 14 d)

Fig. 14. Successive wear of TiN-coated hob teeth revealed by SEM and etched metallographic cross-sections. a) Overview of one tooth from a used tool. b) Initial stage of edge chipping and thermal softening of the coating substrate on the rake face.

c) Close up of edge chipping in cross-section. Cracks running from both rake and clearance face d) Final stage of severe crater wear. The etching in b) and c) reveals superficial thermal effects.

Fig. 15. Tool wear evolution. The three ovals represent initial, steady state and catastrophic tool wear, respectively. Influence of tool surface preparation and coating is also indicated. Table 2 summarises the wear mechanisms of HSS cutting tools. Normally, the tool suffers from a combination of two or more of these mechanisms, and it can be difficult to judge which is dominating. It is also indicated in the table how the different wear mechanisms result from a combined effect of properties of work and tool materials as well as cutting parameters. Table 2. Common wear mechanisms of HSS cutting tools, their cause and how to fight them. Wear mechanism

Cause of wear

Counteractive tool properties

Abrasive wear

Hard particles or other hard phases in the work material remove material by a ploughing action.

High matrix hardness, large volume of hard phases, hard coating

Mild and severe adhesive wear

High cutting speed generates high tool surface temperatures that facilitate strong adhesion between work and tool materials. The worst situation prevails for tough, ductile and chemically reactive work materials with low thermal conductivity. High cutting speed generates excessive edge temperatures in combination with high loads.

Smooth surface, sharp edge, high hot hardness, high thermal conductivity, chemically inert (anti sticking) coating

Plastic deformation

Fracture and fatigue

High hot hardness, high thermal Conductivity.

Interrupted cutting, Smooth tool surface, especially in high fracture combination with high toughness promoted cutting speed and use of by a defect free HSS cutting fluid, a tough and with a fine grained ductile work material. Use of structure of both insufficiently sharp tool matrix and hard edges. phases

Towards better performance of HSS cutting tools Improving the HSS material From the demonstrated mechanisms of wear of HSS cutting tools we can draw the conclusion that hardness, heat resistance (hot hardness) and fracture toughness both macroscopically and microscopically are the prerequisites of high tool performance. Recent HSS development has focused on the homogeneity and cleanliness of the HSS steel. Through the PM technology it is

possible to obtain a fine grained, homogeneous structure. However, the authors think that it is possible to further improve the hardness/toughness ration demonstrated in Fig. 2b. By further reducing the size of the matrix grains and hard phase particles down to the nanometer range (applying nanotechnology) a further step is possible as illustrated by the general hardness/toughness relation given in Fig. 16.

Fig. 16. Reducing the size of the microstructure constituents and improving the cleanliness improves the hardness/toughness ratio.

Improving the surface integrity Another means to further improve HSS tool performance is to improve the surface finish, i.e. avoiding deterioration of the superficial HSS material by excessive heat generation. The macroscopic strength and the resistance to edge chipping of HSS materials (Fig. 14.c) is very sensitive to surface defects generated by the surface preparation, see Fig. 17, and consequently the surface of the cutting tool edge must be accurately prepared in order to avoid premature failure. A smooth tool surface contributes to the resistance against

micro cracking and to demonstrated in Fig. 11.

avoid

premature

detachment

of

coatings

as

Figure. 17 Four point bend strength of AP 2023 illustrating the influence of surface condition on strength.

Conclusions The most typical models of tool wear in metal cutting are the flank wear at the front of edge of the tool and crater wear at the tool face. Flank wear is caused by friction between the flank face (primary clearance face) of the tool and the machined workpiece surface. At the tool flankworkpiece surface contact area, tool particles adhere to the workpiece

surface and are periodically sheared off. Adhesion of the tool and workpiece materials increases at higher temperatures. Abrasive wear occurs when hard inclusions of work material or escaped tool particles scratch the flank and workpiece surface as they move across the contact area. Although adhesive and abrasive wear mechanisms are predominant in flank wear, some diffusion wear also exists [1]. It is the dominating wear mode at low cutting speeds (Thangaraj and Weinmann, 1992). Crater wear occurs at the tool-chip contact area where the tool is subject to a friction force of the moving chip under heavy loads and high temperatures. At higher speeds, the temperature on the rake face of a carbide tool may reach over 1,000°C. At these high temperatures, the atoms in the tool continuously diffuse to the moving chip. The temperature is greatest near the midpoint of the tool-chip contact length, where the greatest amount of crater wear occurs due to intensive diffusion. As the crater wear approaches the cutting edge, it weakens the wedge and causes chipping of the tool [1]. Crater wear is caused primarily by the dissolution of tool material by diffusion or solution wear since it occurs in the region of maximum temperature rise (Subramanian, 1993). Crater wear is the formation of a groove or a crater on the tool face, typically some 0.2 to 0.5 mm from the cutting edge, at the place where the chip moves over the tool surface. Crater wear can be minimized by selecting a tool material that has the least affinity to the workpiece material in terms of diffusion. Mapping of wear data for flank or crater wear of uncoated high-speed-steel (HSS) cutting tools, during dry turning operations is presents by Lim (1993), (Fig.18). The map for flank and crater wear for HSS tools can be useful because shows that by suitable choice of cutting parameters a safety zone with low wears can be reached.

Figure 18: The wear map of uncoated HSS cutting tools during dry turning operations

References 1. Söderberg, S., Wear mechanisms and tool life of hhigh speed steels related

to microstructure, PhD Thesis of Uppsala University, Sweden, Acta Universitatis Upsaliensis 1982

2. Söderberg, S., Jacobson S., Olsson, M., Wear Atlas of HSS Cutting Tools, Proceedings of the 5th International Congress on Tribology (Eurotrib 89), Helsinki, Finland, Finnish Society for Tribology, 1989

3. Trent, E.M., Metal Cutting. 3rd ed. Oxford: Butterworth-Heineman, 1991.

4. Alden Kendall, A., Friction and wear of cutting tools and cutting tool

materials, ASM Handbook, Vol. 18, 1992, pp 609-620

5. Holmberg, K., Matthews, A., Coatings tribology, Properties, techniques and applications in Surface Engineering, Ed: D. Dowson, Elsevier 1994.

6. Alvelid, B., Wisell, H., Wear of HSS in orthogonal milling, Scand. J. of Metallurgy 9 (1980) 59-67

7. Lim, S.C., Lee, S.H., Liu, Y.B., Seah, K.H.W., Wear maps for uncoated highspeed steel cutting tools, Wear 170 (1993) 137-144.

8. Lim, C.Y.H., Lim, S.C., Lee, K.S., The performance of TiN-coated high speed steel tool inserts in turning, Tribology International 32 (1999) 393-398

9. Hogmark, S. Jacobson, S., Larsson, M., Wiklund, U., Mechanical and tribological requirements and evaluation of coating composites, In Modern Tribology 2000. Ed. B. Bushan, Vol II, 931-959 CRC Press 2001

10.Le May, I., Principles of mechanical metallurgy, Elsevier 1981

11.Larsson, M., Olsson M., Hedenqvist, P., Hogmark, S., Mechanisms of coating failure as demonstrated by scratch and indentation testing of TiN coated HSS - On the influence of coating thickness, substrate hardness and surface topography, Surface Engineering 16, 5 (2000) 436-444

12.Wiklund, U., Gunnars, J., Hogmark, S., Influence of residual stresses on fracture and delamination of thin hard coatings, Wear 232 (1999) 262-269