Wear, 139 (1990) 439-451
WEAR BEHAVIOUR AND WEAR MECHANISM OF CERAMIC TOOLS IN MACHINING HARDENED ALLOY STEEL HONG XIAO Department
of Mechanical Engineering,
University of Technology (China)
(Received July 6, 1989; revised October 17, 1989; accepted February 13, 1990)
Machining tests were carried out on hardened AISI H13 hot-work steel (HRC 43-48) using a series of different ceramic tools, i.e. SiC whiskerreinforced alumina, Si-Al-O-N, ahunina, mixed alumina and cubic boron nitride (CBN). The wear behaviour and wear mechanisms of the tools are discussed. Significant di.Eerences in tool life were noticed between the different ceramic tools under the same cutting conditions. It was found that the tool life was limited mainly by crater wear when using Si-Al-O-N, whiskerreinforced alumina and CBN. Alumina-based ceramic tools showed superior crater wear resistance. Whisker-reinforced ahnnina tools showed microfracture during cratering. CBN tools revealed evidence of diffusion wear.
1. Introduction The technology of ceramic cutting tool materials has made great strides in recent years with substantial improvements in their strength, toughness, and wear resistance. Machining of steels in their hardened condition is now both technologically and economically feasible with advanced ceramic tools. The wear mechanism of ceramic tools is basically different from that of classical cemented carbides. Brandt [ 1 ] observed that cratering in alumina and mixed ahunina tools involved plastic deformation of a thin layer. Such deformation led to ductile or brittle fracture of small pieces from the asperities on the tool rake face. Crater wear of mixed ahunina was more sensitive to cutting speed than that of alumina as the content of Ti(C,N) was worn by diffusion into the workpiece material within the secondary shear zone. A similar phenomenon was reported by Chattopadhyay et al. . Matta et al.  also concluded that when machining steels with alumina-based ceramics, the flank wear was controlled by a thermally activated brittle fracture process whereas crater wear was controlled by a relatively thermal plastic flow mechanism.
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Although Si-Al-O-N tools are used to cut steels, the rate of wear, cratering in particular, is generally too high for these tools to be recommended for application over alumina-based tools. The fundamental reason is the diffusion of the silicon and nitrogen of sialon into the hot steel chip flowing across the rake face. Brun et al.  reported that Si-Al-O-N tools crater much more severely than other ceramic tool materials when machining steels. The very rough crater surface revealed that the tool material was pushed aside by the grams to be pulled out. It seems, therefore, that the wear of Si-Al-O-N was a combination of chemical degradation of the grains and the mechanical pull out of grains from the glass matrix. The strength and fracture toughness of alumina have been improved by the addition of silicon carbide whiskers and the improved mechanical characteristics offer encouragement for the use of this composite as a wearresistant material, e.g. for a cutting tool. The wear behaviour of SiC-whiskerreinforced alumina has been investigated by Yust et al. . The whiskerreinforced composite was found to be significantly more wear resistant than ah.unina. On the severely worn surface, the cracks were initiated at the whisker-matrix interface. Analysis of the mechanical behaviour of the whiskerreinforced ceramic has shown that crack deflection by the whiskers is the major toughening mechanism. An observation of the wear of a whiskerreinforced ceramic tool suggested  that removal of particles occurred by a microfracture mechanism on the rake face; notching wear and attrition on the flank face seemed to be the main mechanisms responsible for wear rather than any chemical reaction. It may be concluded that the Sic whiskers do not modify the main wear mechanism of the cutting tools during machining. CBN tools have attracted special attention since they can be used to machine hardened steels efficiently. It was found that when machining hardened steels, the wear behaviour of the CBN tools was also influenced by the microstructure of the workpiece material [ 71. The microstructures of workpiece materials can strongly influence chip formation and chip flow, although the workpiece materials are of the same hardness. It was reported that when Amborite (CBN, produced by DeBeers Co.) tools were used for machining M2 steel in particular [ 81, a notch often developed on the leading edge of the insert. This was found to be the result of a burr of metal formed on the outer leading edge of the workpiece. Hitchiner et al.  revealed that when machining hardened steels using CBN tools, the wear behaviour appeared to be unaffected by the variation in workpiece hardness. A similar observation was made by Gane and Stephens [lo] who also noted that the wear rate of CBN and Si-Al-O-N tools was higher than that of alumina tools. The hardness of the steel does not seem to be the only factor in determining the wear. The chemical constitution of the workpiece may sometimes be a more important factor than the alumina, zirconia, Si-AI-O-N and CBN when machining hardened steels. Extensive studies have been conducted in recent years to find the limits of application of ceramic tools in the machining of hardened steels with a hardness of 55-65 HRC, whereas little work has been carried out to determine
the potential of machining steels with a hardness in the range of 40-50 HRC using ceramic tools. Moreover, as a newly developed tool material, whisker-reinforced alumina shows different wear behaviour, which has not been much studied. The aim of the present investigation was to compare the wear behaviour of a series of ceramic tools in order to optimize cutting conditions for the different tools used for turning hardened AISI H13 steel, a widely used hotwork steel in the forging industry. The msjor properties of this steel are high hot tensile strength, hot wear resistance and toughness. It is possible therefore, that the machining of this steel in its hardened condition might be different and that severe tool wear could occur. 2. Experimental
2. I. Tool materials Eight grades of different ceramic tool materials were used for this investigation. Table 1 lists the tool materials used, their trade names, suppliers and compositions. 2.2. Workpiece material Cutting tests were performed on a quenched AISI H13 steel bar (43-48 HRC). The initial diameter was 150 mm and the length 800 mm. The chemical composition of the steel is shown in Table 2. Figure 1 shows the microstructure of the AISI H13 hardened steel. Phases of carbides, bainite and tempered martensite can be clearly seen.
2.3. Cutting tests The facilities used in the experiments are shown in Table 3, and in Table 4 the cutting conditions are shown. To overcome systematic wear variations caused by possible inhomogeneity of the workpiece, all of the inserts were consecutively tested for a period TABLE! 1 Tool materials and compositions Tool material
A1203 95.8 wt.%, ZrOz 4.2 wt.%
A1203 70 wt.%, TiN 22.5 wt.%, Tic 7.5 wt.%,
SalIdVik SI,N, 77 wt.%, ~$0, Kennametal Yz03 10 wt.%
G.E. De Beers
Cubic boron nitride
Al,03, Sic(w) 20 vol.%
BN 90 vol.%, Co 10 vol.% BN 90 vol.%, ceramic 10 vol.%
442 TABLE 2 Chemical composition of the workpiece material (wt.%) C
Fig. 1. Microstructure of hardened AK1 H13 steel. TABLE 3 Facilities used in the experiments
Lathe ProJilo?nt?ter Scanning electron microsccpe
Colchester Hydro NC 540 Talysurf module 3 Cambride stereo-scan S4
TABLE 4 Experimental conditions Cutting speed (m min-‘) Feed rate (mm rev-‘) Depth of cut (mm) Environment
50, 100, 250 0.25
of l-3 min. Therefore, the wear figures obtained represent the average from the whole workpiece as all the inserts were tested under similar closely controlled conditions. 2.4. Metallography To determine the wear mechanisms of the ceramic tools used in these tests, the wear areas were observed in a scanning electron microscope. The
tool wear was normally partly covered by traces of adherent workpiece material which were removed by leaching in dilute HCl before observation.
3. Results 3.1. Flank wear Figure 2 shows flank wear vs. cutting time for eight different ceramic tools at a cutting speed of 50 m min-‘. As can be seen, KY3000 and CC680 Si-Al-O-N tools suffered from the most severe wear, followed by: Amborite and BZN CBN tools; CC670 and KY2500 whisker-reinforced alumina tools and CC620 and CC650 alumina-based tools in that order. It is of special interest to note that the wear of CC620 and CC650 was very low compared with that of the other tools and even less than that of CBN tools. There was no distinct difference between pure and mixed alumina tools with respect to flank wear. The flank wear of the whisker-reinforced ahunina group was greater than that of the ahuninas but less than that of the CBN tools, Of the whisker-reinforced aluminas, CC670 showed better flank wear resistance. The Si-Al-O-N group of tools had a relatively high flank wear rate. Experiments at a cutting speed of 100 m min-’ revealed similar trends (Fig. 3). The flank wear behaviour of the tools at 100 m nun-’ cutting speed was not significantly different from that at lower cutting speed. However, the tool failme of BZN was caused by heavy cratering which led to a very sharp tool edge, and hence resulted in fracture. Chipping on the edge caused failure of Amborite. KY2500 failed catastrophically within 2 min. The alurninabased tools showed no damage at this cutting speed. 3.2. Crater wear Wear also took place on the rake face during machining. The crater depth profiles are shown in F’ig. 4. It can be seen that at a cutting speed of 100 m min-’ the alumina-based ceramic tools underwent insignificant 0.4 f=0.25mm/rev 0.3 z !z 2
P As 5 lz
Q * * +
EZN Arnt.xlk CC650
Cutting time (min) Fig. 2. Flank wear vs. cutting time for different tool materials.
Fig. 3. Flank wear vs. cutting time for different tool materials.
KY2500 __ ~
Fig. 4. Crater depth profIles of different tools. VI= 100 m mh-‘;f==0.25 cutting time, 3 min; dry environment.
mm rev’; t= 1.5 min;
crate&g. CC670 and KY2500 of the waker-re~orced ahrmina evidently gave different crater wear resistance. When KY2500 is compared with CC670, a wider and deeper crater developed on KY3500 although flank wear rates of both tools were similar. BZN and Amborite exhibited almost the same crater wear rate under these cutting conditions. Since it was recommended that the BZN be used with a large chamfer on the edge, the crater developed on the chamfer and caused the cutting edge to sharpen. The crater depth profiles obtained at 250 m min-’ cutting speed after machining for 3 min are illustrated in Fig. 5. The crater wear rate of CBN
Fig.5. Crater depth profiles of different tools. v = 250 m min-‘; f= 0.25 mm rev-‘; t = 1.5 mm; cutting time, 3 min; dry environment.
tools and whisker-reinforced ahunina tools increased abruptly, whereas alumina-based tools still gave good wear resistance, but showed evidence of premature fracture recorded within a cutting time of 1 min. Since Si-Al-O-N tools failed within a very short time because of severe cratering, the wear curves and crater depth profiles could not be recorded at cutting speeds higher than 50 m min-‘. 3.3. Wear wuxhanimns The wear area on the CC620 tool flank face is shown in Fig. 6. The flank face was grooved by the workpiece. Very large ridges were formed which had a smooth appearance indicating that in the smoothly grooved worn area the grains were uniformly abraded. This suggests that the aluminabased tools can resist wear satisfactorily. However, some chipping can be observed on the cutting edge because of its low transverse rupture strength. The wear area on the CC650 tool flank (Fig. 7) had a similar appearance to that of CC620. Because of its improved hardness, strength and chemical stability as well as thermal conductivity with the addition of Ti(N,C), the grooves were shallower, but there was also chipping on the tool edge. In both cases mentioned above, the worn marks on the chamfer are seen to be very close to the tool edge. Unlike the alumina-based ceramics, CC670 performed with good fracture resistance. It can be seen in Fig. 8 that the zone close to the edge chamfer
Fig. 8. Flank wear of CC670. 2,= 100 m mid; Fig. 9. Flank wear of KY2500. v = 100 m mid;
f= 0.25 mm rev-‘; t = 1.5 mm. f= 0.25 mm rev-‘; t = 1.5 mm.
showed no wear whereas the wear zone reached the chamfer in the case of the CC620 and CC650 tools. In addition, no chipping was observed on the CC670 tool flank face. However, another grade of whisker-reinforced ceramic tool, KY2500, had significantly Merent wear behaviour from CC670. Chipping on the edge of KY2500 took place to a considerable extent as can be seen in Fig. 9. The flank wear band showed irreguhu scars instead of smooth grooves. The appearance of the Amborite flank wear area is shown in Fig. 10. Voids were observed on the edge and flank face of the tool as a result of particles being plucked out. The sign&ant amount of pull-out exhibited on the flank face of Amborite was probably due to brittleness of its ceramic boundary material. The wear area on the rake face of the CC620 tool is shown in Fig. 11, It can be seen that, after leaching in HCI, a deformed layer is apparent on the crater surface. Deformation was limited to the upper part of the superficial layer of grains. Wear topography of CC650, shown in Fig. 12, is similar to
Fig. 12. Crater wear of CC650. v=250 Fig. 13. Crater wear of Si-AI-O-N.
_f=0.25 mm rev-‘; t= 1.5 mm.
v = 250 m mid;
mm rev-‘; t = 1.5 mm.
that of CC620. However, the scratching marks on CC650 were finer and no cracks were observed in the worn area. The crater wear of Si-Al-O-N tools is shown in Pig. 13. It can be seen that the CC680 and KY3000 tools have undergone significant cratering. Wide, deep, smoothly worn craters indicate a diEusion wear mechanism. However, Si-Al-O-N tools were almost free from fracturing throughout the tests. The topography of the rake face of the whisker-reinforced ceramic tools is shown in Pig. 14. It can be seen that whisker-reinforced ceramic tools wear mainly by microfracture. The bonding at the boundaries between Sic whiskers and the ahunina matrix is weakened by repetitive stresses. The workpiece material adheres to the rake face of the tool. As the chips flow along the rake face, small pieces of the deposited material separate from the tool (Pig. 15). This separation results in tensile or shear stresses which further lead to distortion at the interface. Therefore, in a short span of time, the tool may be damaged by this cyclic process. Particles and lumps of tool material were found to be missing from the rake face.
Fig. 14. Crater wear of KY2500. u =250 m min-‘; f=O.25 Fig. 15. Crater wear of IW2500. v=250
mm rev-‘; t= 1.5 mm. mm rev-‘; t=1.5
Fig. 16. Fractured surface of KY2500. Fig. 17. Crater wear of CBN. v=250
mm rev-‘; t=1.5
Whisker pull-out protrusion and whisker-shaped cavities can be seen in Fig. 16, indicating evidence of microfracture in the crater area. By contrast, CBN tools were strongly resistant to grooving and spahing as can be observed in Fig. 17. The crater surfaces of BZN and Amborite were smooth and polished by chip flow leaving ffne scratches in the direction of flow of the chip. However, the possibility of diffusion wear of CBN tools should be considered as far as the quite high crater wear rate of these tools is concerned.
4. Discussion The ceramic tools, in particular alumina-based tools, are chemically wear resistant materials. Based on the results of the present investigation, it can be concluded that alumina-based tools exhibit better cratering behaviour than both whisker-reinforced ceramic and CBN tools. Quite smooth grooved
worn areas without cracks and voids formed on the flank faces of the CC620 and CC650 tools (Pigs. 7 and 8). This suggests that flank wear of aluminabased tools is mainly caused by abrasion at the tool-work interface, and to some extent, by chipping on the chamfer edge. Plastic deformation is one wear mechanism in which the crater is formed owing to high tensile stresses acting on tool edges. This agrees with Cadoz et al. [ 111 who reported that alumina can be deformed at low temperatures with high hydrostatic pressures. The phenomenon that crater wear was primarily due to superficial plastic deformation of a thin layer of alumina has been reported by other investigators [ 1, 2 1. Evidence of plastic deformation on the CC620 and CC650 tools was also observed at a cutting speed of 250 m min-’ and feed rate of 0.25 mm rev’ as can be seen in Pigs. 11 and 12. Both alumina tools exhibited more or less similar wear patterns and wear rates. In general, the difference in tool life between the two tools is not significant. Among all the tool materials tested, Si-Al-O-N tools had the highest wear rate. Hence in the case of machining hardened AISI H13 steel, Si-Al-O-N tools are not particularly suitable. The characteristics of wear on whisker-reinforced alumina tools (Pigs. 14 and 15) were quite unusual. It appears that removal of particles or lumps by a microfracture mechanism on the rake face seems to be more responsible for wear than any chemical reaction. Vigneau et al.  reported that the whisker toughening effect prevents catastrophic failure. Particle removal and microcracking by debonding of the SIC whiskers and subsequent energy dissipation occurs when cracks propagate and bridge the matrix grains. However, a large broken area can be observed on the rake face of both CC670 and KY2500 which does not appear to be in agreement with the toughening effect of the whiskers. Nevertheless, hot pressing causes the length of the whiskers to be randomly oriented in a plane perpendicular to the hot-pressing axis (Pig. 18) owing to the high average aspect ratios (1 O-80 pm in length and 0.6 pm diameter ) of the Sic whiskers. Therefore, cohesion and further microfracture due to local stress concentration occurs more readily along the interface of the whiskers and the matrix on the rake
Fig. 18. Micrographs
face of the tool which is parallel to the plane in which whiskers lie. This is clearly shown in Fig. 16. It has been reported [ 131 that the tool wear of CBN becomes more severe in the machining of soft steel than in the machining of hardened steel, and large crater wear is generated under certain cutting conditions. The same is true when machining hardened AISI H13 steel with a hardness of 43-48 HRC. However, it is quite obvious that there are some significant differences in the wear behaviour of CBN tools in comparison to those alumina-basedtools. For CBN tools, there is (a) more distinctive and deeper crater wear, (b) some evidence of chemical reactions at the chip-tool interface, and (c) a higher flank wear rate. Moreover, of the two CBN tools, the grain boundary strength of BZN seems to be higher than that of Amborite. This is because Amborite contains ceramic boundary material which has lower fracture resistance. The wear mechanisms should be further verified by the analysis of microstructures and mechanical properties of the different cutting tool materials. Presently such quantitative data about ceramic cutting tool materials properties are insufficient.
The following conclusions are drawn with respect to turning tests carried out on hardened AISI H13 using a range of ceramic tools. (1) The alumina-based ceramic tools, CC620 and CC650, are more suitable for machining hardened AISI H13 steel than other ceramic tools because of their superior flank wear resistance. (2) Removal of particles and lumps by a microfracture mechanism causes a high crater wear rate of whisker-reinforced ceramic tools CC670 and KY2500 because the whiskers are oriented in the plane perpendicular to the hotpressing axis. (3) Among all the tools tested, the highest wear rate was obtained with Si-Al-O-N tools at both low and high cutting speeds. (4) Diffusion wear of CBN tools appears to be responsible for the large crater wear and smooth crater surface.
The author would like to thank Dr. M. L. H. Wise and Mr. D. K. Aspinwall, University of Birmingham, U.K. for their helpful discussions and suggestions. The author is also indebted to the Chinese Government and the British Council for support during the research.
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