MACHINING PERFORMANCE AND WEAR MECHANISM OF TiAlN-COATED INSERT

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1 International Journal of Mechanical and Materials Engineering (IJMME), Vol.6 (2011), No.3, MACHINING PERFORMANCE AND WEAR MECHANISM OF -COATED INSERT R.J. Talib, H.M. Ariff and M.F. Fazira AMREC, SIRIM Bhd,.Lot 34, Jalan Hi-Tech 2/3 Kulim Hi-Tech Park, KULIM, Malaysia address: Received 2 December 2011, Accepted 26 December 2011 ABSTRACT Titanium Aluminum Nitride ()-coated cutting tool inserts were subjected to turning of carbon steel at two cutting speeds (75 mm/min and 120 mm/min), whereas depth of cut and feed rate is kept constant at 0.5 mm and 0.06 mm/rev, respectively. The objective of this work is to investigate the microstructural changes on the flank of the insert and effect of cutting speed on the machining performance of the coated insert. Microstructural examinations revealed the following phenomenon during turning process; (i) two-way transfer of material on worn surfaces of the insert and workpiece, (ii) formation of mechanically alloying transferred material, (iii) plastic flow of coated material, and (iv) abrasion wear mechanism. Test results also show that the flank wear reduced as the cutting speed increased due to good oxidation resistance properties of coating layer at high temperatures generated during turning process. Keywords: 3-5 -coated insert, Turning, Wear, Performance, SEM 1. INTRODUCTION Hard coating is used to increase microhardness, wear resistant, corrosion resistant, tool life properties of engineering components. Hard coating can be applied to cutting tool, mold and dies, machine elements, automotive parts, and electrical components. The commercial coatings of TiN, TiCN,, Al 2 O 3 are frequently used in tools industry for machining and drilling of metals (Che-Haron et al., 2001; Sproul, 1996; Talib et al., 2007). Previous study by earlier researchers showed that an increase in tool life may be due to increase in hardness (Zeng et al., 1998), greater bonding energy of the coating elements (Lin et al., 1997), and lower friction coefficient (Sedlacek, 1982). In case of coating, the improvement in the cutting performance is due to the oxidation resistance of properties at higher temperature (Munz, 1986); Leyendecker et al., 1991). High wear resistance even at high temperatures is the outstanding property of (Munz, 1986), a characteristic that makes this coating appropriate to cut abrasive work piece material such as cast iron, aluminium silicon alloys and composite materials at high speeds. The generations of thermal fatigue crack on the substrate are due to the phenomena of thermal cycling coupled with 414 thermal shock during machining (Ghani et al., 2002). During machining process, wear mechanism also take place, depending on the machining parameters setting, coating materials employed, and type of work piece used. In the study on failure mechanisms of -coated insert, Khrais and Lin (2007) identified that the microwear mechanisms in operation during machining of AISI 4140 steel were edge chipping, microabrasion, microfatigue, micro-thermal, and micro-attrition. This work will discuss the wear mechanism operated on the tool insert during turning process and the effect of cutting speed on the performance of the tool insert. 2. METHODOLOGY The investigations were carried out on the tool inserts made from the coated on the tungsten carbide substrate. Cutting tests were carried out on a CNC milling machine model EXCEL SL 360/600I with cutting fluid of metal cut emulsion (SELSO). Cutting ability of the coated insert was conducted at two cutting speed (75 mm/min and 120 mm/min), while feed rate of 0.06 mm/rev, and depth of cut 0.5 mm were kept constant (Table 1). The tests were conducted on 100 mm diameter and 140 mm long medium carbon steel rod (1.25 % C, 0.23% Si, 0.12% Mn, 0.24 %, 5.39% Cr, 0.47 % Mo, 0.93 % V, 0.011% W, 0.075% P and balance Fe) with hardness of 38 HRC. Figure 1 and Figure 2 show the microstructure of the workpiece and coating film deposited on substrate, respectively. After subjected to turning test, the cutting edges of the inserts were examined using a field emission scanning electron microscope (FESEM) model LEO 1525 equipped with energy dispersive X-ray (EDX). FESEM operated at 15 kv, using secondary electrons mode. Sample for microstructural investigation were ultrasonically cleaned for 30 minutes. Depth of cut (mm) Table 1: Machining parameters Feed Rate (mm/rev) Cutting Speed (mm/min)

Build-up edge Figure 1 Microstructure of work piece Figure 3 Build-up edge Figure 2 coating 3.

High temperature generated on the cutting edge formed build-up edge (Figure 3), composed of elements shown in Table 2.

As the turning process progressed, the adhesion of coating with the tungsten carbide substrate will weaken and finally the coating layer was disposed from the substrate material exposing the

60 Totals 100 100 415 Figure 4 SEM image of cutting insert after disposal of coating layer Figure 5a shows the worn surface of insert flank, where the left side is the tungsten carbide substrate

2 Build-up edge Figure 1 Microstructure of work piece Figure 3 Build-up edge Figure 2 coating 3. RESULTS AND DISSCUSSION Heat accumulated during turning process causes high surface temperature on both the cutting tool insert and work piece due friction between the two mating surfaces. High temperature generated on the cutting edge formed build-up edge (Figure 3), composed of elements shown in Table 2. EDX result revealed that the build-up edge mail composed of Fe which is a material transferred from the work piece. As the turning process progressed, the adhesion of coating with the tungsten carbide substrate will weaken and finally the coating layer was disposed from the substrate material exposing the substrate material as shown in Figure 4. Table 2 Composition of build-up edge Element Weight% Atomic% C K Fe K Totals Figure 4 SEM image of cutting insert after disposal of coating layer Figure 5a shows the worn surface of insert flank, where the left side is the tungsten carbide substrate which has been abraded during machining, the middle part is the area in the process of exposing tungsten carbide surface, and the right side is the coating. EDX analysis on the middle part shows that this area composed of magnesium and aluminum. Magnesium probably came from the work piece material, whereas the aluminum came from the coating film (Figure 5b). This is the intermediate layer where the coating film is going to be disposed and finally will expose the tungsten carbide as substrate material as the machining process progresses. Figure 6 shows the worn surface of exposed. It was observed that the worn surface has been abraded by the harder peak asperities, most probably metal carbide which resulted in the formation of grooves on the worn surface of the work piece material. This process is a manifestation of abrasion wear mechanism. The mechanism of edge chipping was also observed of the -coated insert as shown in Figure 7. The cutting tool insert was finally unable to machine the workpiece due to catastrophic failure as shown in Figure 8. It appears from

this figure that there is a good bonding between the film with the substrate.

edge Figure 9 and Table 3 point out that the transfer layers generated on the worn surface contain both materials from tungsten carbide substrate material, coating layer and work piece.

Carbon most probably came from the workpiece material, substrate as well as from the absorbed carbon in the SEM chamber and effect of being exposed to the atmosphere.

Figure 9a shows that this spot composed of only W, Fe and Ti, elements that of high melting point.

3 this figure that there is a good bonding between the film with the substrate. a coating Fe, Mn, C b Figure 5 SEM image of worn surface and EDX spectrum Figure 6 Abrasion wear Edge chipping Figure 7 Edge chipping Figure 8 SEM image of the catastrophic failure on the cutting edge Figure 9 and Table 3 point out that the transfer layers generated on the worn surface contain both materials from tungsten carbide substrate material, coating layer and work piece. EDX results illustrate that all the four spot of analyses composed oxygen and carbon. Carbon most probably came from the workpiece material, substrate as well as from the absorbed carbon in the SEM chamber and effect of being exposed to the atmosphere. The present of oxygen was probably due to residual oxygen in the chamber and formation of oxide layer. Figure 9a shows that this spot composed of only W, Fe and Ti, elements that of high melting point. This indicates that the temperature is quite high which resulting in evaporating of the lower melting temperature elements such phosphorous, aluminium and manganese. W is the substrate material, Fe is the one being transferred from work piece and Ti is the coating film. This implies that there are materials being transfer from the work piece as observed elsewhere (Talib et al., 2007; Chen and Rigney, 1985). Table 3 Composition of spectrum spot measured by EDX Element Weight % Spot 28 Spot 30 Spot 31 Spot 33 CK OK NK AlK TiK FeL WM Total Figure 9b shows that the coating film still protecting the workpiece. However, it was noticed that Al is not observed at this point which could be due to the high temperature at this contact point during turning process. During turning process, the surface temperature increased due to friction between the mating surfaces. 416

High surface temperature can soften the contact region and the transfer layer may behave like a fluid and result in the plastic flow of transfer layer as shown in Figure 9c.

4 High surface temperature can soften the contact region and the transfer layer may behave like a fluid and result in the plastic flow of transfer layer as shown in Figure 9c. At this spot it was also noticed the presence of Fe. Figure 9d illustrates this spot as coating layer which composed high of weight percentage of Ti. Worn surface of the coated insert shows that the iron and carbon from work piece material have been mixed with titanium from the coated insert. This is due to process of mechanical alloying on transfer layers, a phenomenon observed in previous works by Talib et al. (2007) and Chen and Rigney (1985). This is a symptom of adhesion mechanism. d a Figure 9. SEM image of insert worm surface showing location of EDX spot; (a) edge chipping, (b) worm surface, (c) plastic flow of material, (d) coating layer b Turning test Failure test results showed that the flank wear of machining at lower speed (75 mm/min) is almost two times higher than the machining at higher speed (120 mm/min) (Table 4). Thus, it is concluded that coated cutting tool insert is generally good for high speed machining. During turning process, the surface temperature of the cutting insert increased due to the friction between the two contact surfaces. The temperature increases much higher as the cutting speed increased. The flank wear reduced as the cutting speed increased due to good oxidation resistance properties of coating layer at high temperatures. This can be seen at spot 31, where the oxygen weight percentage is the highest due to the formation of aluminum oxide layer. Cutting Speed (mm/min) Table 4 Test results Cutting Time (min) Flank Wear, VB (mm) c 4. CONCLUSION In this work, the effect of cutting speeds on flank wear and wear mechanism of -coated when turning of low carbon steel under lubrication have been studied. It is found that the flank wear reduced as the cutting speed increased due to good oxidation resistance properties of coating layer at high temperature. Microstructural examination revealed that there are the two-way transfers of material on worn surfaces of the insert and workpiece, followed with formation of mechanically alloying of transferred materials. As the temperature increased during turning, the phenomenon known as plastic flow of melted material was observed on the flank of the cutting insert. Micro-structural changes on the worn flank surface of cutting insert revealed that the wear 417

5 mechanisms operated during turning process include edge chipping, adhesion and abrasion. The wear mechanisms operated during turning are rather complex with no single mechanism was found to be operating fully. REFERENCES Che-Haron, C.H., Ginting, A. and Goh, J.H. (2001). Wear of coated and uncoated carbides in turning tool steel, J. Mater. Process. Technol. 116: Chen, L.H. and Rigney, D.A. (1985). Transfer during unlubricated sliding of selected metal systems. Wear 59: Ghani, J.A., Choudhury, I.A., and Hassan, C.H. (2002). Study of Wear Mechanism of P10 TiN Coated Carbide Tools Using SEM Technique. Proc. 12 th Scientific Conference Electron Microscopy Society Malaysia. Johor Bharu: Khrais, S. K. and Lin.Y.J. (2007). Wear mechanisms and tool performance of PVD coated inserts during machining of AISI 4140 steel, Wear 262, Leyendecker, T., Lemmer, O., Esser, S. and Ebberink, J. (1991). The development of the PVD coating TiAIN as a commercial coating for cutting tools, Surface and Coatings Technology, 48: Lin, K.L, Chao, W.H. and Wu, C.D. (1997). The Performance and Degradation Behaviours of the /interlayer Coatings on Drills. Surface & Coating Technology 89: Sedlacek, V. (1982). Metallic Surfaces, Films and Coatings, Czechoslovakia, Elsevier. Sproul, W. D. (1996). Physical vapor deposition tool coatings, Surface and Coatings Technology 81: l-7 Talib, R.J., Toff, M.R.M. and Ariff, H.M.. (2007). Wear Mechanism of TiN, and TiCN Coated Drills During Drilling of Medium Carbon Steel. Physical Science, Vol 18 (1): W.D.Munz. (1986). Titanium aluminium nitride films: A new alternative to TiN coatings, J. Vac. Sci. A 4 : Zeng, X., Zhang, S. and Hsieh, J. (1998). Development of Graded Cr-Ti-N Coatings. Surface and Coating Technology 102:

Jurnal Teknologi WEAR MECHANISM OF TICN AND TIALN COATED DRILL IN DRILLING OF CARBON STEEL. Full Paper

Jurnal Teknologi WEAR MECHANISM OF TICN AND TIALN COATED DRILL IN DRILLING OF CARBON STEEL R. J. Talib a*, S. M. Firdaus a, N. I. Ismail a, M. Hisyam Basri a, H. M. Ariff b a Faculty of Mechanical Engineering,