1 Introduction

Powder metallurgy (PM) steels have been widely applied in several industrial products due to its high mechanical strength, wear resistance, and capability of near net shape manufacturing [1]. On account of these superior performances, PM steels are used in making automotive sector such as engine valve seats [2]. Machinability of PM steels is affected by the combination of carbides embedded in a hard matrix [3]. PM steels are regarded as difficult processing materials due to the porous structure and contain martensite, pearlite, and carbide. Polycrystalline cubic boron nitride (PCBN) tools are frequently used in the machining PM steels and received extensive attention in the manufacturing field for their excellent mechanical properties, high temperature stability, and high wear resistance [4]. The performance of PCBN tool is depended on many factors, such as CBN content, CBN grain size, varieties of binder phase ,and cutting edge geometry of PCBN tools [5]. The macroscopic and microscopic geometry and parameters of the cutting tool are significantly affected by tool wear and tool life [6]. Wear performance and mechanisms of PCBN tools mainly include crater wear, flank wear, abrasion, adhesion, diffusion, and a combination of these due to complex cutting situation [7, 8].

PCBN tools have different performance properties depending on the CBN content. With the increase of CBN content, hardness, toughness, and thermal conductivity of the tool are improved [9]. Huang et al. [10] investigated the finish turning of AISI52100 steel with low CBN content PCBN tools; it revealed that with the increasing of cutting temperature, adhesion was replaced by diffusion as a dominant wear mechanism. Arsecularatne et al. [11] investigated the machining of AISI D2 steel with high CBN (85% vol.) content PCBN tools. Grooves appeared perpendicularly to the cutting edge along the flank face. It is due to the dissolution or diffusion wear of BN and binder. Gordon et al. [12] found that flank wear and crater wear of low CBN content tools are sensitive to cutting speed. With the increasing of cutting speed, flank wear and crater wear were increased; at the same time, element diffusion and adhesion lead to chemical wear. Chemical wear acted the dominate wear mechanism; diffusion, adhesion, and abrasion occurred at the same time. As for high CBN content tools, flank wear is more serious than low CBN content PCBN tools, but it is not sensitive to cutting speed, and as the cutting speed increases, abrasion and adhesion were the primary tool wear.

PCBN cutting tools with various edge preparations (chamfer, honed cutting edge, or waterfall edges) are preferred to strength and decrease the risk of edge chipping and fracture [13]. PCBN tools with chamfered cutting edge are always used in rough and interrupted turning; chamfered cutting edge can improve strength [14]. Nonetheless, higher temperature and plastic strain will occur on cutting edge due to large negative rake angle [5]. Kurt et al. [15] studied the effect of chamfer angle of PCBN tool in finishing hard turning of AISI 52100 steel; it showed that chamfer angle has a great impact on cutting force and tool stresses; with the increasing of chamfer angle, all cutting force components were improved, wear mechanisms of PCBN tools had been changed. Ventura et al. [16] investigated the customized cutting edge geometries of CBN tools on tool wear performance of hard turning; the result showed that a single chamfered cutting edge is the most appropriate, since it reinforces the cutting edge without excessively increasing mechanical and thermal loads. The main wear mechanism observed for all micro geometries corresponds to attrition.

At present, most of the studies focused on the wear performance and mechanisms of low CBN content and high CBN content PCBN tools in hard turning of hardened steels; only a limited number of publications have been focusing on wear performance and mechanisms of high CBN content PCBN tools which CBN content are changed in certain range in boring of PM steels. The objective of this paper was to evaluate the influences of different CBN content (80 vol% and 95 vol%) and different cutting edge geometry (honed edge radius, 0.05 mm chamfered edge width, and 0.10 mm chamfered edge width) to characterize wear performance and mechanisms of PCBN tools in boring PM steels. The advanced three-dimensional wear parameters include the volume of removed material (WRM), maximum depth of defect (WMD), affected area (WAA), volume of adhered material (WAM), maximum height of defect (WMH), trend of removed material (η1 = WMD/WRM), and trend of adhered material (η2 = WMH/WAM) were used to characterize the PCBN tool wear. Tool life was calculated as the quantity of machined workpieces. Three-dimensional wear parameters based on focus variation microscopy (FVM) was used to quantitatively evaluate tool wear. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) were used to further observe wear mechanisms of PCBN tools.

2 Experimental

This experiment was carried out with high CBN content PCBN tools; CBN content with 80% vol. and 95% vol. (grain size 1–3 μm) were applied; binder phase was ceramic binder which is composed of Ti (C, N) and Al. Three different cutting edge geometries (honed edge radius, 0.05 mm chamfered edge width, and 0.10 mm chamfered edge width) are shown in Fig. 1. The width of chamfered edge was measured by microscope VHX-6000 before every tests. Six groups of PCBN tools were divided as A, B, C, D, E, and F as shown in Table 1.

Fig. 1
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Tool edge microgeometry a honed edge radius and b 0.05 mm edge width chamfered edge, c 0.10 mm edge width chamfered edge

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Table 1 Six groups of PCBN tools
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The workpiece material was PM steels and the shape of PM steel is shown in Fig. 2. The microstructure and alloy element of PM steels mainly include Fe, Cu, and C and are shown in Fig. 3. The mechanical performance with hardness of PM steel was above 50 HRC and the density was 7.6 g/cm3.

Fig. 2
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Workpiece geometry and machine process

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Fig. 3
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Microstructure and element analysis of PM steels

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Boring experiments were performed using Heller MCH350D four-axis horizontal machining center with a maximum spindle rotational speed of 8000 rpm and rated power of 46 kW under dry cutting condition. PCBN inserts were fixed on the cutter bar using special jag and rotating with main axis. PM steels were assembled on engine cylinder head which were fixed on the machining center as shown in Fig. 4. PM steels were machined by six groups of PCBN tools, respectively, and each experiment was carried out two times using a fresh cutting edge. Cutting parameters were used: cutting speed (VC) = 200 m/min, feed rate (f) = 0.04 mm/rev, and depth of cut (ap) = 0.15 mm. The standard for defining the end of tool life was based on the surface finish parameters of workpieces, surface roughness Ra = 1 μm [17].

Fig. 4
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Experimental setup

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After each boring test, tool micro-wear was observed using SEM on a Zeiss EVO 18 with a tungsten filament equipped with an EDS microprobe for elemental analysis. FVM (Alicona, model Infinite Focus G5, Fig. 5a) was used to quantitatively evaluate three-dimensional tool wear parameters, an objective lens with 20× magnification (25 nm in the vertical resolution and 1.1 μm in the lateral resolution) is used in Fig. 5a. Focus variation technology combined the small depth of focus of an optical system with vertical scanning to provide topographical and color information from the vertical of focus, and this technology was used to measure cutting tool microgeometry, precision engineering, and so on [18]. Three-dimensional wear parameters can be outputted using FVM. The calculation method is shown in Fig. 5b. A fresh cutting edge geometry was created using FVM as a reference surface at first; after each test, the cutting edge was repeated measurement 3 times; according to overlapping the models of fresh cutting edge and worn cutting edge, a report can be outputted by software of Alicona; three-dimensional wear parameters were determined by overlapping datasets of fresh cutting edge and wearing cutting edge [19].

Fig. 5
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Methodology of output three-dimensional wear parameters with tool F

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According to reports generated by Alicona and physical meanings of every data, three-dimensional wear parameters WRM, WMD, WAA, WAM, WMH, WMD/WRM, and WMH/WAM are illuminated in Fig. 5. WRM and WMD refer to the defects below the reference surface and indicate the tool wear definition according to ISO 3685:1993 [20]. Interaction between cutting edge and workpieces during machining can result adhere material on the rake face or clearance surface, parameters WAM and WMH refer to the defects above the reference surfaces. Parameter WAA involves all alterations on the cutting edge wear surface compared with the fresh surface. Based on the definition of parameters WRM and WMD, the ratio of parameters (WMD/WRM) can be defined as a tendency to measure removed material that is deep or not convergence. The larger the ratio is, tool wear is trend to be deeper. By contrast, the smaller the ratio is, tool wear is trend to be not convergence. The ratio of parameters (WMH/WAM) can be defined as a tendency to measure the size of adhesion. The larger the ratio is, the adhesion is to get together. By contrast, the smaller the ratio is, the adhesion is trend to be not convergence.

3 Results and discussion

3.1 3D characterization of tool wear

The PCBN tool 3D wear characterizations of six groups are shown in Fig. 6. This experiment used multiple repeated measurements, and there was no significant difference in the three-dimensional parameter values by repeated measurements. These 3D parameters are obtained by Gaussian fitting normal distribution.

Fig. 6
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Three-dimensional wear parameters based on datasets overlapped

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As can be seen in Fig. 7, parameters WRM, WMD and parameters η1 defined WMD/WRM were influenced by the CBN content and tool edge geometry. For the increasing of CBN content, the parameters WRM of tool B, D, and F increased a little compared to tool A, C, and E, respectively. However, there was a contrary tendency that can be seen according to the parameters WMD except of tool E and F. The tendency of parameters η1 was similar to parameters WMD. For the changing of tool edge geometry, there was no significant difference of parameters WRM of tool B and D, but there was a rapid increasing between tool D and F. Parameter WMD of tool D was the lowest among tool B, D, and F; meanwhile, parameter WMD of tool F is the highest value. Parameter η1 showed the decreasing tendency of tool A, C, and E; however, there was an increasing tendency that happened between tools D and F.

Fig. 7
figure 7

Three-dimensional wear parameters WRM, WMD, and η1 with six-group tools

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Basically, the increasing of CBN content can promote hardness and thermal conductive of PCBN tools [9], which can be a reasonable explanation of the decreasing of parameters WMD and can be controlled with the increasing of CBN content. For the interesting increasing of parameters WMD and η1 of tool F compared to tool E, which are supposed to be decreased due to higher hardness of tool F, however, the result showed a contrary tendency. With the increasing of chamfered edge width, the contact area of the tool and workpiece and the chips increases, which increases the temperature from the cutting area, leading to premature tool failure [5]. In the range of 0.05 to 0.10 mm of chamfer edge width, increasing of chamfered edge width also can promote cutting force [21]. These factors can be considered as the reason for the increasing in parameters WMD and η1 between tool E and tool F. As can be seen in Fig. 6, crater wear of tool E and F occurred on chamfered edge; nevertheless, crater wear of tool C and D occurred on chamfered edge and rake face. This phenomenon can be explained as the increased width of the chamfered edge will change the chip flow, thereby changing the wear performance. In this condition, increasing width of chamfered edge is more powerful than increasing CBN content in controlling tool wear. Therefore, it is no significant differences of parameters WRM of tool A and B compared to tool C and D, which means 0.05 mm chamfered edge did not have much advantages than honed edge radius in controlling tool wear.

As can be seen in Fig. 8, parameters WAA decreased slightly with the increasing of CBN content by comparing parameters of tool A and B, tool C and D; however, parameter WAA of tool E and F showed an opposite transformation. For the changing of tool edge geometry, parameter WAA of tool A and B with honed edge radius was higher than the parameters of tool C and D with 0.05 mm edge width chamfered edge; in the meanwhile, parameters WAA decreasing with the increasing of chamfered edge width.

Fig. 8
figure 8

Three-dimensional wear parameter WAA with six-group tools

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As mentioned before, hardness and thermal conductive of PCBN tools were increased with the increasing of CBN content [9], so that affected tool area had been controlled by increasing of CBN content and this phenomenon occurred in certain range of cutting edge geometry (honed edge radius and 0.05 mm chamfered edge width). For the increasing of parameters WAA of tool F compared with tool E which should be decreased, tools have greater surface contact with workpieces and chip with the increasing of chamfered edge width [5], so that WAA showed an increasing tendency and increasing CBN content did not appear significant advantage in controlling tool wear area when edge width increased from 0.05 to 0.10 mm. Parameter WAA of tool B and D did not change too much; however, parameter WAA of tool E is the lowest in these parameters, which means chamfered edge has better performance than honed edge radius in controlling affected tool area; moreover, 0.10 mm edge width is better than 0.05 mm edge width.

Wear parameters WAM, WMH, and the tendency to measure adhered material are gathered or not convergence (η2 = WMH/WAM) that are shown in Fig. 9. With the increasing of CBN content, the parameters WAM and WMH both showed an increasing tendency; nevertheless, parameter η2 was decreased with the increasing of CBN content except for PCBN tools with 0.05 mm chamfered width. For the changing of tool edge geometry, parameters WAM and WMH were increased, parameter η2 showed a decreasing tendency.

Fig. 9
figure 9

Three-dimensional wear parameters WAM, WMH, and η2 with six-group tools

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The parameters WAM and WMH were related to adhesion of PCBN tools, which always occurred on crater wear on rake face and this is a typical wear morphology of PCBN tools [22]. Crater wear on the rake face just locates in the cutting zone very close to the cutting edge suffering higher temperature, greater mechanical load, and consequently induces higher compressive stress and shear stress resulted from the friction effect at the tool-chip interfaces [23]. Thermal conductive of PCBN tools is promoted by increasing CBN content [9]; therefore, when machining same workpieces with different CBN content PCBN tools, higher CBN content PCBN tools suffer higher cutting temperature, which accelerate adhesive wear of PCBN tools. That is the reason of the parameters WAM and WMH both showed an increasing tendency with the increasing of CBN content. Increasing CBN content of PCBN tools can control the size of adhesion which can be proved by the decreasing of parameters η2. As mentioned before, changing tool edge geometry from honed edge radius to chamfered edge and increasing chamfered edge width from 0.05 to 0.10 mm changed chip flowing and increased cutting force [21], which can generate higher cutting temperature and mechanical load on cutting edge, so that WAM and WMH are increased. Moreover, adhesion was not trend to be convergence due to these factors, which can be proved by the decreasing of parameter η2.

3.2 The mechanism of tool wear

In order to analyze the wear mechanism of PCBN tools in detail, SEM equipped with an EDS microprobe has been used. According to the analysis in 3.1 section, with the increasing of CBN content, parameters WRM, WMD, η1, and WAA of 0.10 mm chamfered edge width PCBN tools showed a contrary tendency compared to others, so tool F was used to analyze wear mechanism. Tool D was observed in order to compare the influence of the increasing of chamfered edge width on wear mechanisms.

The wear mechanism of tool F is shown in Fig. 10. As can be seen in Fig. 10 a and e, many deep grooves are observed on the clearance face; these grooves may be resulted either from the loose CBN grains [24] or the find hard carbides [22] rolling and sliding at the tool-workpiece interfaces. Binder phase is prone to chemical, dissolution, and diffusion wear due to high temperature, mechanical load, and stress, resulting in exfoliation of loose CBN grains, which is in good agreement with the literature [25]. Notch wear happened due to the fact that chip flowing is nonlinear on the cutting edge, so that a position far from the cutting edge occur notch wear, which is in good agreement with the literature [4]. As mentioned above, crater wear on the rake face is a typical morphology observed for PCBN tool due to high temperature, great mechanical load, and compressive stress [26, 27], with chip flowing from rake face, adhesion and diffusion wear happened on the rake face. SEM images and EDS analysis of area i are shown in Fig. 10 c and d, which is the magnification of crater wear. As shown in Fig. 10c, area i is covered by an oxide film, which was generated by the high temperature in the cutting zone and the surrounding atmosphere [4]. The EDS analysis of area i verified that oxygen element appeared 24.64 wt.%. Furthermore, diffusion happened on crater wear; Mn, S, and Cr element from the workpiece material occurred as can be seen in the evidence of diffusion. Due to adhesion, cutting temperature, and chip flowing from rake face, TPL–tool protection layer was formed on the cutting edge which can improve cutting performance. However, boring of loop PM steel is a kind of discontinuous cutting, which generates impact load on the rake face, so that the TPL is easy to peel. Through a long time of formation and peeling of TPL, microchipping occurred on the cutting edge as shown in Fig. 10b which is in good agreement with the literature [28]. EDS analysis of point j is shown in Fig. 10f; the obtained element is the composition of the PCBN inserts, which means CBN grain has been pulled out by TPL. Furthermore, this phenomenon will aggravate the wear of PCBN tools. This result is differed from the work of Zhou et al. [29], who affirmed that the chamfered edge helped preventing the occurrence of chipping. The main reason for this difference caused by the impact load generated by intermittent or discontinuous cutting.

Fig. 10
figure 10

Wear morphologies of the tool F: a wear morphologies, b the magnified area of the red dashed line in a, c the magnified area of the light blue dashed line in a, d EDS of area i, e FVM image, f EDS of point j

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As can be seen in Fig. 11, which is the SEM and EDS observations of tool D, crater wear appeared on rake face and abrasive wear appeared on clearance which is similar to the tool F. As can be seen in Fig. 11a, there was a large area of adhesion material on the rake face and this phenomenon did not occur on tool F. In order to deeply study the adhesion material, area 1 has partially enlarged. As can be seen in Fig. 11d, thermal-creak appeared alone adhesion material. The reason of the formation of adhesion material could be the changing of chip flowing generated by the decreasing of width of chamfered edge [5]. As mentioned before, chip flowing was changed due to the width of chamfered edge of tool D is smaller than tool F. Chip will be flowing out from rake face of tool D which can generate high temperature and stress on rake face; adhesion material was formed under this cutting condition. EDS has been used to further analyze the component of adhesion material, as can be seen in Fig. 11f; content of element Fe and Cu was 23.73 and 10.08 wt.%, other elements like S and Si were measured in adhesion material, which belong to workpiece material. As shown in Fig. 11 b and c, many grooves occurred; however, microchipping did not occur comparing to tool F; the reason of this phenomenon could be the decreasing of cutting force and changing of chip flowing which were generated by the decreasing of chamfered edge width [21].

Fig. 11
figure 11

Wear morphologies of the tool D: a wear morphologies, b the magnified area of the red line in a, c the magnified area of the yellow dashed line in b, d the magnified area of area l in e, e FVM image, f EDS of area l

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3.3 Tool life

Figure 12 shows that the workpiece surface roughness Ra vs. the processed the number of parts in boring PM steel with a cutting speed (VC) = 200 m/min, feed rate (f) = 0.04 mm/rev, and depth of cut (ap) = 0.15 mm. It is evident that the PCBN tool provides the standard for defining the end of tool life was based on the surface roughness Ra = 1 μm [17], tool life was calculated as the quantity of machined workpieces. Tool life of tool A processed 300 parts which was the lowest compared with the others in Fig. 12 with black line. Nevertheless, the number of 420 workpieces were machined by tool F, which was obtained the highest tool life in this experiment. Tool life value of tool B was 16% greater than tool A, which was the maximum increasing in three groups. Comparing tool life of tool B, D, and F, tool F with 0.10 mm chamfered edge width achieved a higher tool life at 95% vol. CBN content, life of tool F was about 20% greater than tool B and 16% greater than tool D.

Fig. 12
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Machined surface roughness vs. processing number at (VC) = 200 m/min, feed rate (f) = 0.04 mm/rev and depth of cut (ap) = 0.15 mm

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Hardness and thermal conductive of PCBN tools were increased with the increasing of CBN content [9]. Higher tool life could be achieved by using higher CBN content at the same tool edge geometry. Chamfered cutting edge could protect cutting edge from chipping and improved tool life [13], so that chamfered cutting edge shown a better surface roughness than honed edge radius. A total of 0.10 mm chamfered edge PCBN tools achieved higher tool life than PCBN tools with 0.05 mm chamfered edge width, which means increasing chamfered edge width in certain range could increase tool life.

4 Conclusion

Based on the results obtained in these experiments, it can be concluded that in the boring of PM steels using PCBN tools with different CBN content (80% vol. and 95% vol) and different cutting edge geometry (honed edge radius, 0.05 mm edge width chamfered edge, 0.10 mm edge width chamfered edge):

  • A novel characterization method of removed material (η1 = WMD/WRM) and adhered material (η2 = WMH/WAM) based on three-dimensional wear parameters WRM, WMD, WAA, WAM, WMH, are functional to measure the trend of tool wear and as a complementary method to discuss machining phenomena like tool wear mechanisms.

  • With the increasing of CBN content, volume of removed material (WRM), the maximum depth of defect (WMD), and affected area (WAA) could be restrained. Amount of adhered material (WAM) and the maximum height of defect (WMH) were increased. Removed material (η1) and adhered material (η2) were not trend to be convergence.

  • For the changing of cutting edge geometry, chamfered edge shown better wear performances than honed edge radius. For the increasing of chamfered edge width from 0.05 to 0.10 mm, volume of removed material (WRM) and maximum depth of defect (WMD) were increased. Affected area (WAA) was controlled. Amount of adhered material (WAM) and the maximum height of defect (WMH) were increased. Removed material (η1) was not trend to be convergence and adhered material (η2) was trend to convergence.

  • Diffusion, adhesion, abrasive, notch wear, and microchipping were the dominating wear mechanisms of PCBN tools with 95% vol. CBN content and 0.10 mm chamfered edge width. Adhesion happened on rake face of 0.05 mm chamfered edge width PCBN tools; however, for 0.10 mm chamfered edge width PCBN tools, adhesion occurred on crater wear.

  • PCBN tools with 95% vol. CBN content and 0.10 mm chamfered edge width have achieved a better tool life than other kinds of PCBN tools. Increasing CBN content and increasing chamfered edge width in certain range can improve PCBN tool life.