Wear behaviors of single-layer and multi-layer coated ... - Springer Link

Journal of Mechanical Science and Technology 27 (5) (2013) 1451~1459 www.springerlink.com/content/1738-494x

DOI 10.1007/s12206-013-0325-2

Wear behaviors of single-layer and multi-layer coated carbide inserts in high speed machining of hardened AISI 4340 steel† Satish Chinchanikar* and S. K. Choudhury Mechanical Engineering Department, Indian Institute of Technology Kanpur, Kanpur-208016, India (Manuscript Received August 19, 2012; Revised November 22, 2012; Accepted December 20, 2012) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract Flank wear progression and wear mechanisms of uncoated, coated with PVD applied single-layer TiAlN, and CVD applied multi-layer MT-TiCN/Al2O3/TiN cemented carbide inserts were analyzed during dry turning of hardened AISI 4340 steel (35 HRC). Experimental observations indicate that by applying a coating to the uncoated insert the limiting cutting speed increase from 62 to 200 m/min, which further extends up-to 300-350 m/min when using multi-layer coating scheme. Relatively lower wear rate seen when using single-layer TiAlN coated inserts. However, after removal of the thin layer of coating the wear rate increase rapidly, subsequently dominates the wear rate of multi-layer coated inserts. Cutting forces; especially axial and radial components have also shown the similar behavior and increase rapidly when the tool failure occurs. Flank wear, crater wear and catastrophic failure are the dominant forms of tool wear. Digital microscope and SEM images coupled with elemental analysis (EDAX) have been taken at various stages of tool life for understanding the wear mechanisms. Keywords: AISI 4340 steel; Coated carbide tools; Cutting force; Elemental analysis; High speed machining; Turning; Tool failure modes; Wear mechanisms ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction Dry machining with more wear resistant tool material triggered the fast commercial growth of coatings and stands out to be one of the best solutions to address the environmental problems occurring due to wet machining [1]. The most frequently used coatings for machining of metals are PVD applied TiAlN, CVD applied Al2O3, TiC and combined CVD and PVD applied TiN, TiCN [2-4]. Kyung et al. [5] observed the progress of flank wear and wear mechanisms of multilayer TiCN/Al2O3/TiCN coated carbide inserts and found abrasion as a dominant wear mechanism during machining of AISI 1045 steel. Chakraborty et al. [6] performed the end-milling of AISI 4340 steel with multilayer PVD TiAlN-TiN coated carbide inserts and found diffusion as a major wear mechanism under both semi-dry and dry machining conditions. Avila et al. [7] performed turning of quenched and tempered AISI 4340 steel with TiN, TiCN and TiAlN coated carbide inserts, respectively. They observed higher wear rate with TiAlN followed by TiN and TiCN coated inserts. However, Khrais et al. [8] claimed that the TiAlN coated carbide inserts give best performance under dry *

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condition up-to 260 m/min cutting speed. Oh et al. [9] observed lower wear rate with TiCN coating than TiC and TiN coating. Their study of analysis concluded that harder coating results in low wear rate. Chubb et al. [10] found abrasion and diffusion as dominant wear mechanisms on the flank and rake face, respectively, during machining of EN24 steel with TiC coated carbide tools. Kim et al. [11] investigated the effect of deposition time and temperature on surface characteristics of the coating layer. Deposition time was observed as a more influencing factor than deposition temperature. Noordin et al. [12] observed better performance by multi-layer TiCN/Al2O3/TiN carbide inserts against TiCN coated cermet during dry turning of stainless steel. Jindal et al. [13] evaluated the metal cutting performance of PVD TiN, TiCN, and TiAlN coated cemented carbide tools while turning different work combinations and cutting conditions. They observed better performance by TiAlN coated tools followed by the TiCN and TiN coated tools. Khidhir et al. [14] observed tool wear and insert geometry as crucial factors and had a significant effect on surface roughness. Sahoo [15] investigated machining performance of uncoated and multilayer TiN and ZrCN coated carbide inserts in hard turning of AISI 4340 steel. Experimental results showed better performance when using multilayer TiN/TiCN/Al2O3/TiN coated carbide inserts. Suresh et al. [16] investigated cutting force and surface roughness while machining hardened steel

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using multi-layer TiN/TiCN/Al2O3 coated carbide inserts. They observed minimum cutting force and surface roughness at low feed, low depth of cut and high cutting speed. In another study [17], they observed minimum tool wear at low cutting speed and low feed rate. Sahoo and Sahoo [18] developed a mathematical model, which can be used for predicting surface roughness during turning of high chromium cold worked tool steel with coated carbide inserts. Asilturk et al. [19] optimized cutting parameters to minimize surface roughness during hardened steel turning with Al2O3 and Ti-C coated inserts. Federico et al. [20] performed hardened steel turning using TiCN/Al2O3/TiN coated carbide and PCBN inserts. The results concluded that machining of medium hardened steels was productive with TiCN/Al2O3/TiN coated carbide tools. A group of researchers [21-23] investigated the effect of varying coating thickness (1.8 - 6.8 µm) on the properties of TiN coatings deposited on tungsten carbide inserts for machining of carbon steels. The TiN coating with thickness of 3.5 µm was found to exhibit the best turning performance on the WC inserts. They reported that higher levels of compressive stresses in the thicker coating resulted in coating failure due to chipping of the coating at the cutting edge. PVD applied TiAlN and CVD applied multi-layer TiCN/Al2O3/TiN are well-known coatings, commercially available and recommended for high speed machining of steel and cast iron. TiAlN coating is characterized by high hot hardness and is the first choice for thermally influenced processes [24]. This coating applied by PVD technique employs lower deposition temperature; toughness of the substrate does not decrease considerably as the case with conventional CVD technique. Another advantage is that, under high speed conditions, this coating layer forms protective Al2O3 layer, which protects the tool from further oxidation and excessive tool wear by reducing heat conduction to the substrate. TiCN coating is also characterized by high hot hardness [24], preferred for mechanically influenced processes. Ti-N can work as a good compromise between TiAlN and TiCN coatings [25]. Ceramic coating Al2O3 exhibits good resistance to abrasive wear and has high thermal stability [26]. Cemal et al. [27] investigated the effects of cutting parameters on surface roughness using these well-known coating materials; CVD applied multi-layer TiCN/Al2O3/TiN and PVD applied single-layer TiAlN. CVD coated inserts demonstrated higher values of surface roughness, which further increased at higher cutting speed. However, PVD coated inserts showed almost no variation in surface roughness at higher cutting speeds. Authors claimed that due to dissimilar behavior of these inserts against cutting speed further study will be required concerning the wear behaviors of these inserts. Chinchanikar and Choudhury [28] investigated the performance of these coated carbide inserts during machining of hardened AISI 4340 steel. The results of analysis concluded that the PVD coated TiAlN inserts produced lower surface roughness and incurred lower cutting forces than the CVD coated TiCN/Al2O3/TiN inserts. However, the studies reported in

Refs. [27, 28] neglected the effect of tool wear while investigating the machining performance. Some group of researchers [29-31] observed better surface finish with PVD coated single-layer TiAlN inserts and better tool life with multi-layer CVD coatings during machining of titanium and nickel alloys. However, other groups of researchers [32, 33] observed better or almost the same performance of thin TiAlN/TiN or TiN/(Ti,Al,Si)N/TiN coated inserts (coating thickness < 4 µm) in comparison to thick TiCN/Al2O3/TiN coated cemented carbide inserts (total coating thickness > 10 µm). Sufficient studies were made on comparative performance evaluation of various single-layer coatings [6-8, 13, 24, 25] or multi-layer coatings [5, 12, 15-20] or single-layer and multilayer coatings [27-33]. However, in the machining of hardened steel alloys with well-known PVD coated single-layer TiAlN and CVD coated multi-layer TiCN/Al2O3/TiN carbide inserts studies were mainly concentrated on cutting force and surface roughness measurement [27, 28]. Tool wear which adversely affects the product quality and dimensional accuracy and tool wear mechanisms of the coating materials, which can also affect the tool performance, was neglected while investigating the machining performance. Although, some studies were made towards comparative evaluation of these inserts in terms of tool life/tool wear progression, but workpiece material used were nickel and titanium alloys [2931] or, on the characterization of the coating instead of performance evaluation with varying cutting conditions [32]. Understanding of the tool wear progression and wear mechanisms of these single-layer and multi-layer coatings will be helpful in deciding the limiting cutting conditions to avoid damages to the machine tool, downtime and scrapped components due to catastrophic failure of the tools. The aim of this paper is to investigate and demonstrate the dry cutting performance and hence to obtain the limiting cutting conditions of well-known PVD applied single-layer TiAlN and CVD applied multi-layer TiCN/Al2O3/TiN coated carbide inserts during high-speed machining of hardened AISI 4340 steel. The work mainly concentrates on understanding the wear mechanisms and wear progression together with the cutting force variation of coated inserts under wide range of cutting conditions, which will help in the selection of right kind of a tool: without coating, with single-layer coating or with multilayer coating for achieving an optimum performance in machining in terms of cost, productivity or surface finish.

2. Experimental details 2.1 Cutting conditions Dry cutting tests were carried out on an HMT center lathe using two different sets of cutting conditions. In the first set of experiments the coated inserts were tested at cutting speeds of 142, 200 and 265 m/min and at higher feed higher depth of cut (HFHD), using feed and depth of cut values of 0.2 mm/rev and 1.5mm, respectively, and at cutting speeds of 142, 265, 345 and 487 m/min and at lower feed lower depth of cut

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(LFLD), using feed and depth of cut values of 0.125 mm/rev and 0.8 mm, respectively. The uncoated inserts were tested at cutting speeds of 42, 62 and 84 m/min and at feed and depth of cut values of 0.125 mm/rev and of 0.8 mm. 2.2 Workpiece materials and cutting inserts Experiments were performed on AISI 4340 steel having hardness of 33-35 HRC, which was maintained uniform throughout the cross section by hardening and tempering process. The workpiece material used has a dimension of 400 mm in length and 90 mm in diameter. Cutting tests were carried using uncoated low binder content, fine-grained cemented carbide insert (Kennametal K313), PVD coated single-layer TiAlN carbide insert (Kennametal KC5010) and CVD coated multi-layer MT-TiCN/Al2O3/TiN carbide insert (Kennametal KC9110), designating hereafter as tool numbers ‘T1’, ‘T2’ and ‘T3’, respectively. All the insets have identical geometry designated by ISO as CNMG 120408 (800 diamond shape with 0.8 mm nose radius). A right hand style tool holder designated by ISO as PCBNR 2020K12 was used for mounting the inserts. 2.3 Experimental procedure During experiments, tool height, its overhang and tool geometry was kept constant. Flank wear and its growth was monitored at regular intervals of length of cut. Digital microscope with maximum magnification of 230X was used to evaluate the wear on flank surfaces. A FEI QUANTA 200 ESEM and EDAX system (for elemental analysis) were used for the flank and crater wear analysis and understanding the wear mechanisms at different stages of tool life. The average values of the cutting force components were measured a threecomponent piezo-electric dynamometer (KISTLER Type 9257BA).

3. Results and discussion In this section, experimental observations are summarized and, wear mechanisms are discussed with the images taken by SEM, digital microscope and elemental analysis (EDAX). Referring to ISO 3685-1977 (E), the tool life was considered to be over when the maximum flank wear or maximum end clearance wear or nose wear reached 0.2 mm or the occasion of the catastrophic failure. 3.1 Uncoated inserts Wear mechanisms and failure mode of uncoated inserts ‘T1’ were assessed in the cutting speed range of 42-84 m/min and at feed of 0.125 mm/rev and depth of cut of 0.8 mm. Accumulation and entanglement of chips around the insert were observed, which indicated the loss of chip breaker geometry due to intense heating. This increases rubbing of chips with

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(c) Fig. 1. Flank wear when cutting using ‘T1’: (a) V = 42 m/min; (b) V = 84 m/min; (c) EDAX at location A.

tool surfaces. With continuous rubbing adhered metal from the flank and crater surfaces along with some tool elements plucked away and, attrition mechanism of wear is generated. Similar observation was reported during turning of AISI 4140 steel by uncoated carbides [34]. Uncoated ‘T1’ inserts suffered from excessive crater wear and flank wear even at very low cutting speeds. Nose failure, abrasive marks, metal adhesion and edge deformation can be clearly seen from the backscattered SEM images shown in Figs. 1(a) and (b). Elemental analysis at position A of Fig. 1(a) is shown in Fig. 1(c), which shows workpiece elements like Fe, Mn and Cr at the flank face, confirms the metal adhesion. With continual working with the worn tools a catastrophic failure of the cutting edge was observed due to mechanical fatigue. Experimental observations showed that a cutting speed limit of 62 m/min to have a tool life of more than 5 minute when using uncoated carbide inserts. Haron et al. [35] observed a cutting speed limit of 75 m/min when using uncoated carbide inserts for machining tool steel. 3.2 Coated inserts Coatings are considerably harder, provides more abrasion resistance to the cutting edge and its high temperature resistance enables transfer of maximum part of heat to the chips. In this section, the dry cutting performance of PVD applied single-layer TiAlN (‘T2’) and CVD applied multi-layer TiCN/Al2O3/TiN (‘T3’) coated carbide inserts are presented to get the limiting cutting conditions during turning of hardened AISI 4340 steel. Different tool wear forms and wear mechanisms observed for these inserts is explained in this section with the images taken by SEM, a digital microscope and with

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Fig. 2. Flank wear with time of cutting: (a)‘T2’ at HFHD; (b)‘T2’ at LFLD; (c)‘T3’ at HFHD and (d)‘T3’ at LFLD.

the help of elemental analysis (EDAX). Findings of this research provide the necessary information to the industrial community in deciding the tool replacement policy and setting limiting cutting conditions while machining of such alloys. 3.2.1 Tool life Flank wear progressions for PVD 'T2' and CVD 'T3' at cutting speeds of 142, 200 and 265 m/min and at high feed high depth of cut (HFHD) of 0.2 mm/rev and 1.5 mm, respectively, are shown in Figs. 2(a) and (c). Similarly, flank wear progressions for PVD 'T2' and CVD 'T3' at cutting speeds of 142, 265, 345 and 487 m/min and at low feed low depth of cut (LFLD) of 0.125 mm/rev and 0.8 mm, respectively, are shown in Figs. 2(b) and (d). Plots reveals that the wear land on flank face increase with cutting time and generally confined to three distinct regions, namely, initial breakdown, uniform wear rate and rapid breakdown of the cutting edge. The insert images at the end of cutting (for HFHD condition) are shown in Fig. 3. Flank wear as a dominant wear form with absolutely no crater wear can be seen for CVD ‘T3’. However, flank wear, crater wear and catastrophic failure can be seen as a dominant tool failure mode for PVD ‘T2’ inserts. The comparative performance of these single-layer and multi-layer coated inserts in terms of tool life is shown in the histogram, in Fig. 4(a). It can be seen that CVD ‘T3’ performed better at all the tested conditions. Low tool life for PVD ‘T2’; especially at higher cutting speeds can be seen. This insert was failed by sudden fracture due to the weakening of the cutting edge by an accelerated crater wear rate. It can be clearly seen that the tool life increases with decreasing cutting speeds, feed and depth of cut. Fig. 4(a) reveals that the de-

crease in the cutting speed from 265 m/min to 200 m/min and from 200 m/min to 142 m/min, increased the tool life of about 47% and 18%, respectively, when using CVD ‘T3’ and 70% and 64.28%, respectively, when using PVD ‘T2’ (at HFHD cutting condition). However, by changing the cutting conditions from high feed high depth of cut (HFHD) to low feed low depth of cut (LFLD), tool life increased of about 60% when using CVD ‘T3’ and of about 72.7% when using PVD ‘T2’ while machining at cutting speed of 265 m/min. Relationship between tool life and cutting speed is obtained for both the coated inserts at high feed and high depth of cut (HFHD) and low feed low depth of cut (LFLD) cutting conditions as shown in Fig. 4(b). The Taylor tool life exponent ‘n’ is calculated based on experimental results by obtaining linear trend line. Exponent values obtained are 0.391 and 0.278 when using PVD 'T2' and 0.61 and 0.669 when using CVD ‘T3’ for LFLD and HFHD cutting conditions, respectively. Smaller values of ‘n’ for PVD ‘T2’ for both LFLD and HFHD cutting conditions shows that the tool life of this tool is more affected by cutting speed. The Taylor constant values obtained are 649.36 and 365.768 m/min when using PVD 'T2' and 2298.47 and 1956.67 m/min when using CVD ‘T3’ for LFLD and HFHD cutting conditions, respectively. A higher value of constants obtained for CVD ‘T3’ indicates a longer tool life. From the above discussion, it is clear that when using single-layer PVD coated TiAlN inserts to obtain better surface finish and minimum cutting forces [27, 28], the cutting speed must be limited to 200 m/min to prevent the catastrophic failure and hence the scrapped components and damages to the machine tool. Khrais et al. [8] observed better performance of the TiAlN coated carbide inserts by limiting the cutting speed

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Fig. 3. Images of flank and rake faces at HFHD cutting condition when using ‘T2’ and ‘T3’.

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Fig. 4. (a) Histogram of tool life for ‘T2’ and ‘T3’ and; (b) Plot of Taylor tool life equation for ‘T2’ and ‘T3’.

to 260 m/min while machining AISI 4140 steel. However, when using multi-layer coated carbide tool better tool life can be obtained by limiting the cutting speed to 300-350 m/min. Haron et al. [35] observed a cutting speed limit of 350 m/min for machining tool steel using multi-layer coated inserts. 3.2.2 Wear mechanisms and failure mode In the initial stage of machining, lower flank wear values were recorded with single-layer TiAlN coated inserts (PVD ‘T2’) as seen in Fig. 5(a). However, after some time of cutting, a rapid increase, subsequently dominating the flank wear rate of CVD coated multi-layer TiCN/Al2O3/TiN (CVD 'T3') can be seen. The cutting force variation together with the progress in the flank wear also showed a sudden increase in the three directional components of the cutting force; especially feed and radial forces, for PVD ‘T2’ as seen from Fig. 5(b). It can be clearly seen that beyond a flank wear length of 0.15 mm, the cutting forces P1 (tangential), P2 (axial) and P3 (radial) increased sharply by 26%, 85% and 113% when using PVD ‘T2’. However, the cutting forces P1, P2 and P3 increased by 28%, 55% and 52% only even beyond a flank wear length of 0.2 mm when using CVD ‘T3’. This shows that although lower cutting forces incurred when using single-layer TiAlN coated inserts [27-31], the cutting forces increase sharply beyond the flank wear length of 0.15 mm. This may result into more vibrations and hence chatter marks on the machined surface. However, more or less uniform variation of

cutting forces with the progress in the flank wear in case of CVD ‘T3’ shows the better performance and uniform growth in tool wear as seen in Fig. 5(a). Lower initial wear rate of single-layer coated tools can be explained in terms of relative merits of coating materials used. CVD ‘T3’ insert contains three main layers of coating with a total thickness of 18 microns. The inner layer TiCN deposited using MT-CVD (medium temperature-CVD) process provides a tougher cutting edge and provides protection against flank wear. The intermediate Al2O3 layer provides protection against the elevated temperatures and crater wear. The outer layer TiN/TiCN of 2 µm thickness provides additional wear resistance [36]. ‘T2’ is a PVD coated insert with thin layer of titanium aluminum nitride (TiAlN) coating with an average thickness of 2 microns. Lower initial flank wear values recorded when using PVD ‘T2’ resulted due to higher hot hardness of TiAlN coating material as against the hot hardness of outermost TiN coating layer of multi-layer coated insert [2426, 29]. Further, when aluminum in the TiAlN coating reacts with the oxygen in the air; especially at higher cutting temperatures, a thin Al2O3 layer is created at the cutting edge. This layer provided heat protection for the cutting edge resulted in lower flank wear rate in comparison to multi-layer coated inserts. However, just after the removal of this thin layer during the cutting process; most probably by adhesion and abrasion wear mechanisms, the oxidation-dominated and diffusion wear mechanisms became active, resulted in rapid wearing of

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Fig. 5. (a) Flank wear for ‘T2’ and ‘T3’ and; (b) Plot of % increase in cutting force due to tool wear.

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Fig. 6. (a) ‘T2’ tool image at 265 m/min; (b) EDAX at location A and; (c) at location B.

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Fig. 7. (a) Rake face of ‘T3’ when cutting at 345 m/min; (b) EDAX at location A; (c) at location B.

the substrate, and subsequently dominated the wear rate of multi-layer coated inserts. Similar observations reported while machining of titanium alloys with these tools [29]. Chakraborty et al. [6] also found diffusion as a major wear mechanism with multi-layer PVD TiAlN-TiN coated carbide inserts. Fig. 6(a) depicts the condition of PVD ‘T2’ insert after cutting for 4.5 minute at 265 m/min and at high feed high depth of cut (HFHD) cutting condition. The complete spalling of coating from the flank and rake faces and plastic deformation of cutting edge can be seen. Fracturing of the adhered metal from the flank and rake surfaces resulted in the removal of coating. The EDAX analysis at location A and B of the flank face is shown in Figs. 6(b) and (c) confirms the exposed substrate (elements W and Co at location A) and adhered workpiece material (elements Fe, Mn, Ni and Cr at location B). In case of multi-layer coated tools (CVD ‘T3’), Al2O3 layer is included as one of the layers, which is much thicker than the layer which was formed on the TiAlN coated insert during

machining. This layer provided heat protection with the first cut and benefited the tool from crater wear and severe abrasion at elevated temperatures. However, with continual working under high sliding-speed, flaking or peeling off of the coating layers from tool surfaces (Fig. 7(a)) and chipping of the coating from the flank and nose region was normally observed. Tuffy et al. [21] also observed similar type of tool failure when machining with thick TiN coated carbide insert while machining carbon steel. They reported that higher levels of compressive stress in the thicker coating resulted in coating failure due to chipping of the coating at the cutting edge. Tool image shown in Fig. 7(a) depicts the condition of CVD ‘T3’ after cutting for 17.5 minute at 345 m/min. Elements W, Co (substrate elements) along with the coating elements Ti, C and O are detected from elemental analysis at region A (Fig. 7(b)). The peak of Al and O detected from region B shows the presence of Al2O3 oxide layer (Fig. 7(c)). When these tools were worked at higher cutting speeds

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Fig. 8. (a) ‘T3’ Tool image when cutting at 487 m/min and at LFLD; (b) Magnified view; (c) EDAX at location A.

Fig. 9. Rake face of ‘T2’ at 265 m/min and at LFLD condition.

(> 300 m/min) chipping of the coating layers and severe notch wear was normally observed as seen in Fig. 8(a). Tool image shown depicts the condition of CVD ‘T3’ after cutting for 11 minutes at 487 m/min using low feed low depth (LFLD) cutting condition. Magnified view showing part of flank and rake faces is shown in Fig. 8(b). Fine and uniform grooves on the tool flank resulted from the abrasion of the hard particles of the workpiece and some metal adhesion can be seen (Fig. 8(c)). This shows that abrasion and adhesion as dominant wear mechanisms when working with multi-layer coated tools. However, a rough surface in the crater wear region observed for uncoated ‘T1’ and PVD ‘T2’ (Fig. 9) indicates a severe wear mode. It occurs due to inter-diffusion of tool and workpiece elements (Fe and Co) when fast flowing chips adheres on the rake face resulting in the weakening of the binder. This causes fracturing of small elements of the tool resulting in the formation of rough surface in the crater. It can be seen that abrasion, adhesion and diffusion become dominant wear mechanisms when working with uncoated 'T1' and singlelayer PVD ‘T2’ coated inserts. Low tool life of single-layer TiAlN inserts resulted due to the weakening of the cutting edge by an accelerated crater wear rate; especially at higher cutting speeds. Avila et al. [7] also observed crater wear as a dominant wear form when machining hardened AISI 4340 steel with TiAlN coated insert. They reported that high amount of aluminum in the coating resulted in a high hardness and low toughness film, resulted in severe wear. Another reason of low tool life of TiAlN coating is due to its lower adhesion to the substrate in comparison to TiCN layer in multilayer coating scheme. However, Jindal et al. [13] observed lowest crater wear and flank wear rate with TiAlN coated

inserts in comparison to other single-layer coated inserts. They observed satisfactory performance of these inserts during machining of SAE 1045 steel up-to cutting speed of 300-350 m/min. The contradictory in the results may be due to softer workpiece material (210 BHN) used, resulted in reduced abrasive wear rate in comparison to hardened steel used in the present study. Another reason may be due to flood cooling used, resulted in lowering the temperature in the cutting-zone, and hence, diffusion wear rate as against dry cutting in the present study. It can be seen that a satisfactory performance of single-layer coated insert can be obtained using lower cutting conditions. However, a sizeable tool life (> 10 minute) can be obtained by limiting the cutting speed to 200 m/min (obtained using exponent ’n’ and constant ‘C’ values for HFHD cutting condition). However, this limit extends up-to 300-350 m/min when using multi-layer coated insert. It is suggested that CVD coated multi-layer TiCN/Al2O3/TiN carbide insert is the best option for rough and medium machining of hardened AISI 4340 steel (35 HRC). However, by limiting the cutting speed up-to 200 m/min, PVD coated single-layer TiAlN carbide insert can be used for finishing operation to avail the advantage of better surface finish and minimum cutting forces [27, 28].

4. Conclusions The following conclusions could be drawn on the basis of experimental investigation carried out: • Lower wear rate was observed for PVD coated singlelayer TiAlN carbide inserts. However, rapid deterioration of the cutting edge and catastrophic failure due to accelerated crater wear was normally observed after the removal of the thin layer of coating. As against, CVD coated multi-layer TiCN/Al2O3/TiN carbide insert was observed to be more effective in producing better tool life. • Tool life gets affected mostly by cutting speed. However, this effect was more prominent for uncoated and singlelayer TiAlN coated inserts. A higher value of Taylor tool life exponent and constant for multi-layer inserts indicates a longer tool life. • It has been observed that, beyond a flank wear length of

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0.15 mm, the cutting forces increased sharply; especially axial and radial components in case of single-layer coated inserts. However, forces increased more or less uniformly with the progress of tool wear when using multi-layer coated inserts. • It has been observed that the abrasion and adhesion as dominant wear mechanisms for multi-layer coated inserts. However, abrasion, adhesion, fatigue and diffusion wear mechanisms were observed for uncoated and single-layer coated inserts. • It has been observed that by providing single-layer TiAlN coating to an uncoated insert, the limiting cutting speed increased from 62 to 200 m/min, which further increased to 300-350 m/min by providing the multi-layer TiCN/Al2O3/TiN coating.

Nomenclature-----------------------------------------------------------------------AISI : American iron and steel institute Al2O3 : Aluminum oxide CVD : Chemical vapor deposition EDAX : Energy-dispersive X-ray spectroscopy ESEM : Environmental scanning electron microscope HFHD : Higher feed higher depth of cut ISO : International organization for standardization LFLD : Lower feed lower depth of cut PCBN : Polycrystalline cubic boron nitride PVD : Physical vapor deposition TiC : Titanium carbide TiAlN : Titanium aluminum nitride TiCN : Titanium carbo-nitride TiN : Titanium nitride T1 : Uncoated carbide insert T2 : Single-layer TiAlN carbide insert T3 : Multi-layer TiCN/Al2O3/TiN carbide insert WC : Tungsten carbide ZrCN : Zirconium carbon nitride

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Satish Chinchanikar Graduated in Production Engineering and received Masters Degree with the specialization in Manufacturing from Pune University, India in 1998. He is pursuing research at the Mechanical Engineering Department, Indian Institute of Technology Kanpur, India, for his Ph. D. He has four years of Industrial experience and 11 years of teaching experience as an Assistant Professor of Department of Mechanical Engineering in various Engineering colleges under Pune University, India. His main research interest is in tribological studies in metal cutting and advanced manufacturing processes. Currently, he is working in the area of hardened steel machining using coated carbide tools, as an economical alternative to costly cubic boron nitride and ceramic tools. S. K. Choudhury received his M.Sc. (Mech. Engg.), Ph.D. (Manufacturing Science) and Post Doctor Fellowship from Lumumba University, Moscow, Russia in 1979, 1984 and 1986. He is currently a professor at Mechanical Engineering Department, Indian Institute of Technology Kanpur, India. His research activities focus on condition monitoring of machine tools, on-line monitoring and control of tool wear, adaptive control systems and manufacturing automation.