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Effect of pulsed DC CFUBM sputtered TiN coating on performance of nickel electroplated monolayer cBN wheel in grinding steel
Surface & Coatings Technology 204 (2010) 3818–3832
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of pulsed DC CFUBM sputtered TiN coating on performance of nickel electroplated monolayer cBN wheel in grinding steel D. Bhaduri ⁎,1, A.K. Chattopadhyay 1 Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India
a r t i c l e
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Article history: Received 16 February 2010 Accepted in revised form 27 April 2010 Available online 5 May 2010 Keywords: Pulsed DC CFUBMS Target frequency Bias voltage Electroplated cBN wheel Cross-diffusion Grit fracture and pull-out
a b s t r a c t The present research involves the deposition of pulsed DC CFUBM sputtered TiN on nickel plated steel discs and electroplated monolayer cBN wheels at seven different target frequencies and ten different bias voltages separately. The coating microstructures and the interaction between TiN and nickel were studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analysis (EPMA). Phase detection was carried out using grazing incidence X-ray diffraction (GIXRD) technique. The cohesive and adhesive strengths of nickel layer were assessed by scratch test. After grinding of low carbon steel (AISI 1020) and hardened bearing steel (AISI 52100), the conditions of the uncoated and coated cBN wheels were observed under Stereo Zoom Microscope and SEM. Average column size of TiN was found to decrease with increase in both target frequency and negative bias voltage. The structure of the coating gradually transformed from porous and open columnar (at 0 V bias) to very compact, dense and featureless (at − 80 V bias). EDX line scan and EPMA confirmed the cross-diffusion between TiN and nickel and GIXRD indicated the formation of nickel–titanium intermetallic phases at their interface. The cohesive strength of nickel layer was not effectively enhanced with increase in target frequency, whereas the same was significantly improved with increase in negative bias voltage. Seemingly, TiN coated wheel could not perform better than the uncoated wheel in grinding AISI 1020 steel due to high wheel loading. However, the uncoated wheel was found to undergo fracture wear, which was remarkably absent in the coated wheels. On the other hand, many fractured grits and some grit pull-out were observed in the uncoated wheel when grinding AISI 52100 steel, whereas almost no pull-out along with much less fractured grits were observed in the wheels coated at bias voltages like − 60 V and − 90 V. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Monolayer superabrasive wheels are very much useful in high precision and high material removal grinding and thus prove their potential application in automobile and aerospace industries [1–3]. Typically nickel is used as the bond material during the manufacturing of monolayer wheels using electroplating technique. In this method, no residual stress exists at the bond level because of low plating temperature [4]. However, such wheels are not free from limitations as weak mechanical anchorage between the grits and the nickel bond often results in the pull-out of the grits from the encapsulation of nickel bond [4–7]. The stripping of nickel bond from the wheel core is also a common phenomenon. Apart from grit dislodgement, grit failure is also influenced by the breakage of grits at or above bond level [4].
⁎ Corresponding author. Currently Doctoral Researcher in the Department of Mechanical Engineering, The University of Birmingham, Edgbaston, Birmingham, UK. Tel.: +44 7943852031. E-mail addresses: [email protected] (D. Bhaduri), [email protected] (A.K. Chattopadhyay). 1 Fax: + 91 3222 282278. 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.063
In some patented research works, the advantage of physical vapour deposited hard tribological coatings, such as TiN [8,9], HfN [10] and TiN–ZrN [11], in arresting of grit failures in monolayer wheels has been reported. The coated wheels have been claimed to show a working life much greater than that of their uncoated counterparts. Ghosh and Chattopadhyay [12] studied the effect of TiN coating on grinding performance of nickel bonded underplated (grit protrusion is 60–70% of the grit height) and brazed grinding wheels. The results indicated that TiN coating played a significant role in arresting grit pull-out in electroplated wheel and reducing the number of bond level grit-breakage in brazed wheel. Grinding of low carbon steel is sometimes problematic as it exhibits high adhesion to the grits leading to wheel loading. Monolayer wheels can be useful in this regard as they possess relatively high grit protrusion (with respect to the conventional composite wheels), which indicates higher chip accommodation volume and less chance of wheel loading. However, in most of the previous literatures, grinding of low carbon steel was carried out with conventional wheels [13–17]. Hence, it is imperative to study the performance of monolayer wheels in grinding low carbon steel. Moreover, to the best of the authors’ knowledge, no study has ever been carried out to investigate the
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Table 1 Deposition parameters during TiN coating. Phase of experiment
Target (cathode) frequency (kHz) 80% duty cycle
Substrate bias frequency (kHz) 80% duty cycle
Negative substrate bias voltage On Ni plated C-20 steel disc
On cBN wheels for grinding AISI 1020 steel
On cBN wheels for grinding AISI 52100 steel
1 2
0, 50, 100, 150, 200, 250, 300 200
100 100
60 0, 30, 40, 50, 60, 70, 80, 90, 120, 150
60 0, 30, 60
– 0, 60, 90, 120, 150
possibility of grinding low carbon steel with TiN coated monolayer cBN wheel. Therefore, one of the objectives of the present investigation was to explore the effect of pulsed DC closed-field unbalanced magnetron (CFUBM) sputtered TiN coating on performance of electroplated cBN wheels in grinding C-20 or AISI 1020 steel. TiN was expected to provide two folds benefit: (a) it would render some anti-frictional and antiwear characteristics in grinding a sticky material like C-20 steel and (b) it would prevent the grit pull-out from the nickel bond. The effect of target frequency and bias voltage on physical and mechanical properties of the PVD coating has been reported in many past literatures. However, in most of the cases, the coating was deposited on M2 grade tool steel [18–21], glass [18,19], silicon wafer [18,19,22,23] and stainless steel [23]. However, the effect of target frequency and bias voltage when depositing TiN on electroplated nickel has never been investigated before. Thus, in connection to the application of TiN on electroplated grinding wheel, the second objective of the present research aimed at exploring the effect of the above mentioned deposition parameters on microstructure of TiN and adhesion strength of both TiN and nickel layer on C-20 steel substrate. In the present study, TiN coating was deposited on underplated galvanic cBN wheels (growth of nickel layer was of nearly 30–40% of the average grit height). The underplated wheels have been intentionally used to obtain the benefit of large chip accommodation space as well as the usefulness of TiN coating even for a low level of plating thickness. During the 1st phase of experiment, TiN was deposited on nickel electroplated C-20 (AISI 1020) steel discs and monolayer galvanically bonded cBN wheels at seven different target frequencies keeping the bias voltage and bias frequency constant. The coating microstructure was studied using scanning electron microscopy (SEM). The adhesive strength of nickel with the steel substrate was assessed by scratch test to examine whether there was any increase in adhesion due to TiN coating. Grinding of AISI 1020 steel was carried out with all the coated cBN wheels and result was compared with that obtained with an uncoated one. During the 2nd phase of experiment, TiN was deposited on the nickel plated steel discs at ten different bias voltages, keeping target frequency and bias frequency unchanged. The above mentioned characterisation techniques have been used in the 2nd phase also. The wheels coated at 0, −30 and −60 V bias voltages were used in grinding AISI 1020 steel and those coated at 0, −60, −90, −120 and − 150 V bias voltages were used in grinding AISI 52100 steel. The part of the experiment relating to grinding of AISI 52100 steel has been communicated elsewhere [24]. However, the result has been presented here to make a comparative study on effect of target frequency and bias voltage in grinding of low carbon steel and hardened bearing steel with TiN coated wheels.
comparable with the thickness of the nickel bond in an underplated wheel. Some underplated galvanically bonded grinding wheels (plating area: Φ 15 mm× 10 mm) were also procured from Eastern Diamond Products Pvt. Ltd., Kolkata, India. Grit distribution density was counted on 10 underplated wheel samples and average density was found to be varied in the range of 55–65 numbers per 5 mm2. Grits were monocrystalline cBN of B251 size (mesh width 250/180 μm) and nickel layer thickness was around 70–80 μm. 2.2. Deposition of TiN coating on steel disc and grinding wheel TiN was deposited by pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS) technique on both nickel electroplated C-20 steel discs and electroplated cBN wheels. The coating was deposited in a dual cathode CFUBMS system (TOOL COATER, VTC-01A) manufactured by Milman Thin Film Systems Pvt. Ltd, Pune, India. The detailed description of the system and conditions during target-cleaning and substrate-ionetching have been reported elsewhere [25]. Other deposition parameters are summarized in Table 1. A representative photograph of a TiN coated wheel is shown in Fig. 1. 2.3. Coating microstructure analysis, EPMA and GIXRD The surface morphology and cross-section of the deposited coatings were observed using SEM. Images were taken in Carl Zeiss Supra 40 field emission SEM (FESEM), Zeiss EVO 60 and Jeol JSM 6490 SEM along with INCA FET 3X EDX analysis. The interface of TiN and nickel was examined by EDX line scan and EPMA area mapping. EPMA was carried out in a CAMECA SX100 instrument with LaB6 filament and beam diameter of 1 µm. Grazing incidence X-ray diffraction (GIXRD) with an angle of incidence 1.5° was carried out on the coated samples to find out the formation of any intermetallic phase between TiN and nickel layer. All measurements were carried out in a high resolution Philips, PANalytical PW 3040/60 X'Pert PRO instrument with molybdenum (Mo) target (wavelength = 0.70930 Å). The samples were continuously scanned with a step size of 0.05° and time per step of 1 s. The generator setting was 40 kV and 30 mA. The data were later analysed with X'Pert HighScore software (Philips Analytical B.V., Netherlands) and peaks were identified by comparing with Joint Committee on Powder Diffraction Standards (JCPDS) data files. 2.4. Scratch adhesion test On nickel plated steel discs, scratch test was carried out before and after TiN coating to find out whether there was any increase of strength of nickel layer due to high energy ion impingement during
2. Experimental 2.1. Preparation of steel disc and procurement of electroplated monolayer cBN wheels In order to observe the coating microstructure and assess the adhesive strength of coating and nickel layer, some C-20 (AISI 1020) steel discs were prepared in CNC turning centre and were electroplated with nickel (plating was obtained from Eastern Diamond Products Pvt. Ltd., Kolkata, India). The plating thickness was around 70–80 μm to make it
Fig. 1. Photograph of a TiN coated electroplated cBN wheel.
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deposition. A DUCOM scratch tester TR-101-M5 was used for scratch testing. The scratch tests were performed with a Rockwell C diamond stylus (0.2 mm radius) drawn across the surface of the coating at a constant linear speed of 0.2 mm/s. The load was varied in the range of 10 N to 90 N with a loading rate of 5 N/mm.
2.5. Grinding The grinding experiments have been carried out under dry condition in a surface grinding machine (Make: Praga Tools Ltd., India) retrofitted with a high speed spindle (Make: Prazisionspindilen GmbH, Germany, Model: SC 60-O). The detailed descriptions of the workpiece holding device and dynamometer have been discussed elsewhere [25]. The workpieces were low carbon steel (AISI 1020) and hardened bearing steel (AISI 52100) with a size of 80 mm × 25 mm× 6 mm. Table 2 depicts the parameters used during grinding. A grinding condition 40-4-10 indicates a wheel speed of 40 m/s, table speed of 4 m/min and a downfeed of 10 µm. During grinding, normal and tangential forces were measured following a sequence of grinding conditions based on Taguchi L16 orthogonal array [26]. The sequence was chosen in a way such that chip load increased from the 1st condition to the 16th condition. Chip load on a grit is directly proportional to the maximum grit depth of cut (hm). hm was calculated as:
hm =
3 V · w c tan α Vc
rffiffiffiffi!1 = 2 d D
ð1Þ
where, hm
Maximum grit depth of cut, µm
c
No. of grits/mm2 area
α
Half apex angle of a grit, degree
Vc
Grinding velocity, m/s
Vw
Table speed, m/min
d
Wheel depth of cut or downfeed, μm
D
Wheel diameter, mm
However, for all wheels, c, α and D were assumed to be same as 55–65 numbers per 5 mm2, 60° and 15 mm respectively. So, for calculating chip load, Vc, Vw and d were taken into consideration. Hence, the sequence of operation was chosen with an increasing value part ′ was of hm′, whichis apffiffiffi of maximum grit depth of cut, hm. hm calculated as VVwc d . Five passes were given under each condition and average force was taken into account. Post-grinding conditions of the wheels and the grits were observed under Stereo Zoom Microscope (Olympus SZ 1145TR PT) and SEM (Jeol JSM 6490).
Table 2 Grinding parameters during grinding of AISI 1020 and AISI 52100 steel. Wheel diameter × width Grinding velocity, Vc Table speed, Vw Down feed, d Environment Wheel-workpiece contact width Length of cut per pass Grinding conditions
Ф15 mm × 10 mm 22, 28, 34, 40 m/s 3, 4 m/min 20, 30, 40 μm Dry 6 mm 80 mm 40-4-10, 28-3-10, 34-4-10, 22-3-10, 40-4-20, 40-3-40, 34-3-30, 34-4-20, 34-3-40, 28-4-30, 28-4-40, 22-4-30,
3. Results 3.1. Coating microstructure 3.1.1. With varying target frequencies Fig. 2 represents SEM micrographs of surface morphology and cross-section of TiN deposited on nickel plated steel discs at various target frequencies. Average column size of TiN was found to decrease with increase in frequency up to 250 kHz. The fractured cross-section reveals that the structure of the coating at 0 kHz frequency gradually transformed into very dense and compact at 250 kHz. However, the column size slightly increased when TiN was deposited at 300 kHz. In pure DC sputtering, target poisoning takes place [27], which can be reduced to a great extent with the application of pulsed DC in the target [28]. Moreover, the plasma became more energetic during pulsed DC sputtering resulting in higher degree of ionization [29]. This might have attributed in reduction of column size and formation of dense and compact coating structure. With the help of time and energy resolved mass spectrometry, Arnell et al. [19] previously observed that argon content of the coating increased with increasing target frequency. In their study, the coating deposited at 350 kHz possessed higher surface roughness, poor adhesion and wear resistance. Probably the same reason was prevalent in the present case, where slightly larger average column size was obtained at the frequency of 300 kHz. 3.1.2. With varying bias voltages The effect of bias voltage on coating microstructure was more predominant than that of target frequency as can be observed from Fig. 3. The average column width of TiN was found to noticeably decrease from a bias voltage of 0 V to − 80 V. The porous and open columnar structure (at 0 V bias) gradually transformed into dense columnar (at −50 V bias) and further into compact and featureless (at −80 V bias). The reduction in column size of the coating could be due to increased high energy ion bombardment leading to more surface defects [22] and nucleation sites. The coating topography was lumpy in nature up to − 70 V, however severe micro-cratering marks on the coating surface were observed at and beyond −90 V due to extremely high energy ion impingement, which led to the induction of large compressive stress within the coating [30]. Such brittleness caused vertical cracks, which are visible from the cross sectional view of the coating deposited at −120 V. 3.2. EDX line scan, EPMA and GIXRD Fig. 4 depicts EDX line scan carried out across the polished crosssection at the interface of TiN and nickel layer (TiN was deposited at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz). The polishing was done to keep the two different layers in a single plane. Some amount of cross-diffusion between TiN and nickel at their interface can be seen in Fig. 4. The depth of interdiffusion was found to be in the range 1–1.5 μm. The EPMA area mapping (shown in Fig. 5) across the cross-section also confirmed this observation where an overlapping region among Ti, N and Ni can be seen. The inter-diffusion phenomenon is clearly substantiated from Fig. 6, which displays a representative GIXRD spectra carried out on TiN coated (deposited at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz) nickel plated steel disc. It reveals the presence of Ni–Ti intermetallic phases in addition to the standard TiN phases. 3.3. Scratch adhesion test
40-3-30, 28-3-20, 22-3-20, 22-4-40
Scratches were taken before and after TiN coating on the same nickel plated disc. In all cases, the left one represents graphs obtained before TiN coating, while the right one shows graphs obtained after
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Fig. 2. SEM micrographs of surface morphology and cross-section of TiN deposited at various target frequencies.
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Fig. 2 (continued).
TiN coating. Fig. 7(a) to 7(d) demonstrates the graphs of normal load, traction force and apparent coefficient of friction when TiN was deposited at 0 kHz, 250 kHz, 0 V and − 90 V respectively. Fig. 7(e) shows a representative SEM image of a scratch track taken after coating. All the graphs clearly indicate that there was an increase in traction force after deposition of TiN. However, the difference in increase of traction force after coating at 0 kHz and 250 kHz of target frequency was not significant (Fig. 7(a) and 7(b)). On the other hand, remarkable enhancement in traction force was obtained when bias voltage was increased from 0 V to − 90 V (Fig. 7(c) and 7(d)). Similar enhancement was also observed with coating at − 120 and −150 V (graphs not shown). Hence it can be inferred that bias voltage has much more significant influence in increasing cohesive strength of nickel than
target frequency. This might be attributed to the densification and overall strengthening of TiN coating with the application of negative substrate bias voltage discussed in Section 3.1.2. 3.4. Grinding 3.4.1. Grinding of AISI 1020 steel with cBN wheels coated at different target frequencies and bias voltages Fig. 8 describes the comparison of average normal grinding force (FN) during grinding of AISI 1020 steel with uncoated and TiN coated (at different target frequencies) electroplated cBN wheels. Although the grinding sequence was chosen according to the ascending order of the value of hm′, which is a part of maximum grit depth of cut (hm), the main influencing grinding parameter on normal and tangential
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Fig. 3. SEM micrographs of surface morphology and cross-section of TiN deposited at various substrate bias voltages.
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Fig. 3 (continued).
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Fig. 3 (continued).
grinding forces was found to be downfeed (d) as evident from Fig. 8. The graph of tangential force is not shown here as the trend was more-or-less similar to that of normal force. With the uncoated wheel, grinding could not be carried out after the 13th condition (28-4-30) because of high wheel loading. The grinding had to be stopped after the 4th (40-3-30) and 6th (40-4-20) conditions with wheels coated at 0 and 100 kHz respectively due to severe wheel loading. The same phenomenon observed for the wheels coated at 50 and 150 kHz and grinding was stopped after the 7th
condition (40-3-40). The wheel coated at 250 kHz could grind up to the 8th condition (28-3-20). Only with the wheels coated at 200 and 300 kHz, grinding could be carried out till the 13th condition and then had to be stopped due to high wheel loading. A similar trend of large wheel loading was observed when grinding of AISI 1020 steel with the wheels coated at 0, −30 and −60 V bias voltages. The graphs of normal and tangential forces are not reported here. Apparently it seems that coated wheels performed worse than the uncoated one. However the effect of TiN coating on performance of
Fig. 4. EDX line scan across the cross-section at the interface of TiN and nickel layer at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz.
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Fig. 5. EPMA area mapping across the cross-section at the interface of TiN and nickel layer.
electroplated wheel cannot be judged in the light of wheel loading only. During grinding with uncoated wheel, there was material built-up in the inter-grit space as low carbon steel produced long sticky chips. The built-up of material increased and after some time large scale adhesion [15] occurred between the grits and the workpiece material. When the forces of adhesion acting over the area of metal/metal contact exceeded the sum of the grit retention forces acting on the grits contained within the area of the adherent material, the adherent material was detached from the area along with some part or the whole grits within that area.
Fig. 6. GIXRD spectra of TiN on nickel plated steel disc at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz.
As the number of active grits participating in grinding reduced, the total normal force also decreased as can be observed from Fig. 8, where the uncoated wheel exhibited much lower total normal force than all the coated wheels. As the detachment of workpiece material caused removal of some part of the grits, many fractured grits could be seen after grinding when viewed under SEM. Such adherent material and fractured grit in an uncoated wheel are shown in Fig. 9(a) and 9(b) respectively. On the other hand, high energy ion impingement during TiN coating enhanced the bonding strength of nickel as well as filled up the pores, voids, cracks, surface defects within the crystals of cBN grits. Therefore, neither the grits underwent fracture nor pulled out when wheel loading occurred in the coated wheels. Very little fractured grits can be observed from Fig. 9(c). As the loaded material could not be removed, the grinding had to be stopped much before compared to the uncoated wheel. The level of normal force was also found to be higher because of this reason (Fig. 8). 3.4.2. Grinding of AISI 52100 steel with cBN wheels coated at different bias voltages The situation was entirely different when hardened bearing steel was ground with uncoated and coated cBN wheels and the effectiveness of TiN coating was immediately understood as the wheels were free from the problem of wheel loading. The graphs of average normal (FN) and tangential (FT) forces during grinding of AISI 52100 steel with uncoated and TiN coated (at different bias voltages) cBN wheels are depicted in Figs. 10 and 11 respectively. From both the figures, it is observed that under the 1st, 4th, 7th 13th, 14th and 16th conditions, significantly lower grinding forces were obtained with the uncoated wheel. It indicates that some amount of grits was pulled-out from the encapsulation of nickel bond in the uncoated wheel from the very beginning of grinding. This resulted in the reduction of total grinding force due to a lesser number of active grits participating in the actual grinding operation. However,
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it caused an escalation of specific grit force which in turn resulted in further pull-out of grits under very high chip load conditions like 284-40 and 22-4-40. Fig. 12(a) and 12(b) reveal such grit pull-out from the nickel bond of the uncoated wheel after grinding. The uncoated wheel has also undergone some grit fracture at the 15th condition (22-4-30), which is indicated by the increase in normal force (Fig. 10), while for the other wheels normal force decreased because of reduced downfeed from the 14th condition (28-4-40). Fig. 10 also reveals that the wheel coated at 0 V bias voltage has suffered from significant amount of grit fracture at and above bond
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level during grinding at the 7th, 9th, 11th and 13th conditions. This can be understood as the percentage increase in normal force in this particular wheel was more than that of the other wheels. Some grits were pulled out at the 15th condition (22-4-30), which can be inferred from the higher percentage reduction in normal force than the other wheels. When TiN was deposited at bias voltages like −60 V and −90 V, number of grit pull-out and damage of grits were evidently reduced when observed under Stereo Zoom Microscope after grinding. The BSE image (Fig. 12(c)) of a TiN coated wheel (deposited at −90 V bias)
Fig. 7. Graphs of normal load, traction force and apparent coefficient of friction on nickel plated steel disc before and after TiN coating at (a) 0 kHz, (b) 250 kHz, (c) 0 V, (d) −90 V, (e) SEM image of a representative scratch track after coating.
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Fig. 7 (continued).
revealed that macro-fracturing of grits above bond level and microfracturing at the tip occurred for a lesser number of grits. TiN coating was retained on most of the grits and bond surface even after grinding under very high specific chip load condition. Fig. 12(d) shows the retention of TiN on the very sharp edge of a cBN grit even after grinding of 80 passes. This clearly indicated a very good adhesion of TiN coating with the cBN grits. The calculation of specific grinding energy (Ug) with increase of maximum grit depth of cut (hm) from the 1st to the 16th grinding
conditions during grinding of AISI 52100 steel has also been carried out and represented in Fig. 13. The curves were fitted using a second order polynomial regression equation. The lower specific energy of the uncoated wheel could be ascribed to the lower grinding force obtained in this wheel, which was originally resulted from the pull-out of the grits. However, the curve of Ug in uncoated wheel shows an abnormal behaviour as it at first increased and then decreased with increase in maximum grit depth of cut (hm). It is expected that with an increase of downfeed (d) or maximum grit depth of cut (hm), Ug should reduce,
Fig. 8. Comparison of average normal grinding forces (FN) during grinding of AISI 1020 steel with uncoated and TiN coated (at different target frequencies) electroplated cBN wheels.
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coated wheels with increase in hm. This is in agreement with a previous observation by Paul and Chattopadhyay [31], where Ug was found to decrease with increase in downfeed. The very high value of Ug in the TiN coated (0 V bias voltage) wheel might be attributed to a greater amount of primary and secondary ploughing of large number of fractured grits with the workpiece. This became evident when the wheel was viewed under the microscope and SEM, which revealed many fractured grits at the bond level. The gradual fall of Ug in the wheels coated at −60 V and −90 V bias ensures preclusion of primary and secondary ploughing by retaining the sharpness of the grits and participation of the same in micro-cutting action. 4. Discussion
Fig. 9. (a) Wheel condition of an uncoated wheel after grinding of AISI 1020 steel, (b) Grit fracture in the uncoated wheel, (c) Wheel condition of a TiN coated wheel (target frequency of 200 kHz) after grinding AISI 1020 steel.
because the rubbing and ploughing parts of Ug get diminished and grits take part in shearing or micro-cutting action [31]. The rise of Ug in uncoated wheel up to 12th grinding condition indicates large rubbing and ploughing of the fractured grits with the workpiece surface. Ug again decreased from the 13th condition because of some amount of grit pull-out under very high chip load and consequent fall in total grinding force. On the other hand, Ug gradually decreased in all the
While comparing the effect of target frequency and bias voltage on coating characteristics, it can be inferred that bias voltage has shown more prominent effect than target frequency in reducing the average column size of the coating. The cohesive and adhesive strengths of the nickel was also remarkably increased with the application of bias voltage. Because of high energy ion impingement of TiN within the electroplated nickel layer coupled with the strong affinity of nickel towards titanium, a cross-diffusion occurred between TiN and nickel with the formation of nickel–titanium intermetallic phases up to a certain depth (evident from Fig. 6). As the Ni–Ti intermetallic phase is hard, it helped in augmenting the scratch resistance of the nickel bond. Although TiN deposited at very high bias voltages like −120 V and −150 V could enhance the cohesive strength of nickel, large spallation at the edge of scratch track was also observed. This can be ascribed to the brittle nature of coating due to the induction of large compressive stress within it [30]. Along with the cohesive strength of nickel, the adhesion between nickel and steel substrate was also enhanced. During scratch test before TiN coating, it has been observed that at normal load as high as 85–90 N, nickel was stripped off from the steel substrate. When TiN was deposited with substrate bias below −50 V, the stripping of nickel layer from the substrate could not be prevented even after TiN coating. However, no observable evidence of stripping of nickel was obtained when TiN was deposited at a substrate bias of − 60 V and above that. The effect of increased strength of nickel immediately reflected in actual grinding operation, where wheels coated at −60 V and −90 V bias voltages have undergone much less fracture and pull-out compared to their uncoated counterpart. The retention of TiN on the edge and surface of cBN grits could be attributed to the strong affinity of cBN towards titanium with the formation of TiB2, TiB, (TiB + TiN), TiN, Ti reaction products at their interface [32,33]. It can be further noted that, the present investigation ascertained less number of grit pull-out in the uncoated wheel compared to what was reported by Ghosh and Chattopadhyay [12]. In their study, TiN was deposited at a particular bias voltage of −80 V and the normal grinding force obtained with the TiN coated wheel was 0.9 N higher than that obtained with the uncoated one at a grinding condition of 30-4-40. In the present research, at a similar condition like 28-4-40, the difference in normal forces measured with uncoated and a coated wheel (−90 V bias) was found to be around 2.7 N. This might be due to the difference in active grit density in the uncoated and coated wheels. The coated wheels exhibited higher grit density participating in micro-cutting action and thus higher total normal force. As the monolayer wheels are not required to be trued, it is important to maintain their form stability by preventing the grit fracture at the bond level and restricting the same only at the tip of the grits. While evaluating the effect of bias voltage on performance of the coated grinding wheels, neither a too low nor a very high bias voltage was found to meet this requirement. A too low bias like 0 V was not able to provide sufficient strength to the nickel bond (as also evident from the scratch test) and could not fill up the defects along the
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Fig. 10. Comparison of average normal grinding forces (FN) during grinding of AISI 52100 steel with uncoated and TiN coated (at different bias voltages) electroplated cBN wheels.
cleavage planes in the monocrystalline cBN grits. Therefore grit fracture at the bond level could not be arrested in the wheels coated at 0 V bias voltage. A very high substrate bias voltage (i.e. −120 V and −150 V) also seemed to be detrimental to the grinding performance of the coated wheels. The post-grinding observation of the wheels (coated at − 120 V and −150 V bias) revealed many fractured grits at the bond level. A concentration of very high compressive residual stress at the interface of coated cBN and nickel bond might be imputed to this detrimental effect. Hence it can be inferred that, TiN deposited within a certain range of bias voltage (− 60 V to − 90 V in the present
study) was ideal to arrest the grit fracture and pull-out. It has been further witnessed that the anti-wear property of TiN coating was revealed in the present study rather than its anti-frictional characteristics (although TiN is not an automatic choice as a low friction coating material [34]). In order to preclude any reaction between grinding fluid and TiN coating, all the grinding experiments were carried out under dry condition. This could be a step towards environment friendly grinding technology. The successful implementation of TiN (within a certain range of bias voltage) in grinding necessitates further detailed investigation
Fig. 11. Comparison of average tangential grinding forces (FT) during grinding of AISI 52100 steel with uncoated and TiN coated (at different bias voltages) electroplated cBN wheels.
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Fig. 12. (a) Wheel condition of an uncoated wheel after grinding AISI 52100 steel, (b) Grit pull-out from the uncoated wheel, (c) Wheel condition of a TiN coated wheel (bias voltage of − 90 V) after grinding of AISI 52100 steel, (d) Retention of TiN coating on the cBN grit.
on the application of other types of PVD coatings (like CrN, ZrN, HfN or a co-deposition of TiN with these coatings) in the field of abrasive machining.
5. Conclusions TiN was deposited using pulsed DC CFUBMS technique on nickel plated steel discs and electroplated monolayer cBN wheels at different target frequencies and bias voltages. Grinding of low carbon steel and hardened bearing steel was also carried out with the uncoated and coated cBN wheels. Based on the experimental observations, the following conclusions could be made: (A) With increase in target frequency, average column size of TiN decreased up to 250 kHz and then slightly increased at 300 kHz. The coating structure obtained at 0 kHz gradually transformed into compact and dense at 250 kHz, however at 300 kHz, the larger column width of the coating was again observed. The column size of TiN was prominently reduced when bias voltage was increased from 0 V to −80 V. The structure of the coating gradually transformed from porous and open columnar to very compact, dense and featureless at −80 V. However, at and beyond −90 V, micro-cratering marks and vertical cracks were observed on the surface and cross-section of the coating respectively. (B) EDX line scan and EPMA confirmed the cross-diffusion between TiN and nickel and GIXRD indicated the formation of nickel– titanium intermetallic phases at their interface.
Fig. 13. Variation of specific grinding energy of the uncoated and coated wheels with increase of maximum grit depth of cut during grinding of AISI 52100 steel.
(C) Target frequency could not effectively increase the cohesive strength of nickel layer, which was significantly improved with the increase in negative bias voltage. The stripping of nickel
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from the steel substrate was also arrested at a bias voltage of −50 V and beyond that. (D) Seemingly, TiN coated wheel could not perform better than the uncoated wheel during grinding of low carbon steel due to high wheel loading. However, the uncoated wheel was found to undergo fracture wear, which was remarkably absent in the coated wheel. The situation was totally different during grinding of hardened bearing steel, where benefit of TiN coating within a particular range of substrate bias was clearly revealed. The uncoated wheel was found to undergo many fracture wear with some pull-out of grits. On the other hand, almost no pull-out and much less fractured grits were observed in the wheels coated at bias voltages like −60 V and −90 V. However, neither a very low (like 0 V) nor a too high bias (like −120 V and −150 V) was found to arrest the grit fracture at bond level. Acknowledgments The authors gratefully acknowledge the funding support they received from DST, FIST (Sanction No. SR/FST/ET-II-003/2000 dated 20.5.2002), The authors are also thankful to Prof. N. C. Pant of the Department of Geology and Geophysics, IIT Kharagpur, India, for providing SEM and EPMA facility. The authors also express their sincere gratitude to Dr. Soumya Gangopadhyay of the Department of Mechanical Engineering, IIT Kharagpur for his help and co-operation during the experiments. References [1] S. Malkin, CIRP Ann. 34 (2) (1985) 557. [2] W. König, F. Ferlemann, Ind. Diamond Rev. 51 (5) (1991) 237. [3] A. Ghosh, A.K. Chattopadhyay, Int. J. Mach. Tools Manuf. 47 (2007) 1206.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
A. Ghosh, A.K. Chattopadhyay, Ind. Diamond Rev. 67 (1) (2007) 59. Z. Shi, S. Malkin, CIRP Ann. Manuf. Technol. 52 (1) (2003) 267. R.P. Upadhyaya, S. Malkin, Trans. ASME J. Manuf. Sci. Eng. 126 (2004) 107. Z. Shi, S. Malkin, Trans. ASME, J. Manuf. Sci. Eng. 128 (2006) 110. D. Lynn Julien, US patent no. US5139537 (1992). W.P. Grotepass, P.W. DeLange, US patent no. US5376444 (1994). T.P. Slavin, J.D. Campbell, US patent no. US5398455 (1995). M.S. Hammond, T.W. McClain, US patent no. US5750207 (1998). A. Ghosh, A.K. Chattopadhyay, Int. J. Mach. Tools Manuf. 47 (2007) 1799. S. Malkin, N.H. Cook, Trans. ASME, J. Eng. Ind. (1971) 1120. S. Malkin, N.H. Cook, Trans. ASME, J. Eng. Ind. (1971) 1129. S. Yossifon, C. Rubenstein, Trans. ASME, J. Eng. Ind. 103 (1981) 144. S. Yossifon, C. Rubenstein, Trans. ASME, J. Eng. Ind. 103 (1981) 156. S. Paul, P.P. Bandyopadhyay, A.B. Chattopadhyay, J. Mater. Proc. Technol. 37 (1993) 791. P.J. Kelly, C.F. Beevers, P.S. Henderson, R.D. Arnell, J.W. Bradley, H. Bäcker, Surf. Coat. Technol. 174–175 (2003) 795. R.D. Arnell, P.J. Kelly, J.W. Bradley, Surf. Coat. Technol. 188–189 (2004) 158. S.L. Rohde, W.D. Sproul, J. Vac. Sci. Technol. A 9 (3) (1991) 1178. S.L. Rohde, L. Hultman, M.S. Wong, W.D. Sproul, Surf. Coat. Technol. 50 (1992) 255. Jyh-Wei Lee, Shih-Kang Tien, Yu-Chu Kuo, Thin Solid Films 494 (1-2) (2006) 161. M. Beilawski, D. Seo, Surf. Coat. Technol. 200 (2005) 1476. D. Bhaduri, A.K. Chattopadhyay, Under communication in Surf. Coat. Technol., Manuscript Number: SURFCOAT-D-09–01132R1. D. Bhaduri, R. Kumar, A.K. Jain, A.K. Chattopadhyay, Wear 268 (2010) 1053. T.P. Bagchi, Taguchi Methods Explained: Practical Steps to Robust Design, Prentice-Hall of India Pvt Ltd, New Delhi, 1993. J. Sellers, Surf. Coat. Technol. 98 (1–3) (1998) 1245. P.J. Kelly, R.D. Arnell, Vacuum 56 (3) (2000) 159. K.E. Cooke, J. Hamsphire, W. Southall, D.G. Teer, Surf. Coat. Technol. 177–178 (2004) 789. N.J.M. Carvalho, E. Zoestbergen, B.J. Kooi, J.Th.M. De Hosson, Thin Solid Films 429 (2003) 179. S. Paul, A.B. Chattopadhyay, Int. J. Mach. Tools Manuf. 36 (1) (1996) 63. W.F. Ding, J.H. Xu, M. Shen, H.H. Su, Y.C. Fu, B. Xiao, Mater. Sci. Eng. A 430 (2006) 301. W.F. Ding, J.H. Xu, M. Shen, Y.C. Fu, H.H. Su, B. Xiao, Vacuum 81 (2006) 434. P.J. Kelly, T. vom Braucke, Z. Liu, R.D. Arnell, E.D. Doyle, Surf. Coat. Technol. 202 (2007) 774.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of pulsed DC CFUBM sputtered TiN coating on performance of nickel electroplated monolayer cBN wheel in grinding steel D. Bhaduri ⁎,1, A.K. Chattopadhyay 1 Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India
a r t i c l e
i n f o
Article history: Received 16 February 2010 Accepted in revised form 27 April 2010 Available online 5 May 2010 Keywords: Pulsed DC CFUBMS Target frequency Bias voltage Electroplated cBN wheel Cross-diffusion Grit fracture and pull-out
a b s t r a c t The present research involves the deposition of pulsed DC CFUBM sputtered TiN on nickel plated steel discs and electroplated monolayer cBN wheels at seven different target frequencies and ten different bias voltages separately. The coating microstructures and the interaction between TiN and nickel were studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analysis (EPMA). Phase detection was carried out using grazing incidence X-ray diffraction (GIXRD) technique. The cohesive and adhesive strengths of nickel layer were assessed by scratch test. After grinding of low carbon steel (AISI 1020) and hardened bearing steel (AISI 52100), the conditions of the uncoated and coated cBN wheels were observed under Stereo Zoom Microscope and SEM. Average column size of TiN was found to decrease with increase in both target frequency and negative bias voltage. The structure of the coating gradually transformed from porous and open columnar (at 0 V bias) to very compact, dense and featureless (at − 80 V bias). EDX line scan and EPMA confirmed the cross-diffusion between TiN and nickel and GIXRD indicated the formation of nickel–titanium intermetallic phases at their interface. The cohesive strength of nickel layer was not effectively enhanced with increase in target frequency, whereas the same was significantly improved with increase in negative bias voltage. Seemingly, TiN coated wheel could not perform better than the uncoated wheel in grinding AISI 1020 steel due to high wheel loading. However, the uncoated wheel was found to undergo fracture wear, which was remarkably absent in the coated wheels. On the other hand, many fractured grits and some grit pull-out were observed in the uncoated wheel when grinding AISI 52100 steel, whereas almost no pull-out along with much less fractured grits were observed in the wheels coated at bias voltages like − 60 V and − 90 V. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Monolayer superabrasive wheels are very much useful in high precision and high material removal grinding and thus prove their potential application in automobile and aerospace industries [1–3]. Typically nickel is used as the bond material during the manufacturing of monolayer wheels using electroplating technique. In this method, no residual stress exists at the bond level because of low plating temperature [4]. However, such wheels are not free from limitations as weak mechanical anchorage between the grits and the nickel bond often results in the pull-out of the grits from the encapsulation of nickel bond [4–7]. The stripping of nickel bond from the wheel core is also a common phenomenon. Apart from grit dislodgement, grit failure is also influenced by the breakage of grits at or above bond level [4].
⁎ Corresponding author. Currently Doctoral Researcher in the Department of Mechanical Engineering, The University of Birmingham, Edgbaston, Birmingham, UK. Tel.: +44 7943852031. E-mail addresses: [email protected] (D. Bhaduri), [email protected] (A.K. Chattopadhyay). 1 Fax: + 91 3222 282278. 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.063
In some patented research works, the advantage of physical vapour deposited hard tribological coatings, such as TiN [8,9], HfN [10] and TiN–ZrN [11], in arresting of grit failures in monolayer wheels has been reported. The coated wheels have been claimed to show a working life much greater than that of their uncoated counterparts. Ghosh and Chattopadhyay [12] studied the effect of TiN coating on grinding performance of nickel bonded underplated (grit protrusion is 60–70% of the grit height) and brazed grinding wheels. The results indicated that TiN coating played a significant role in arresting grit pull-out in electroplated wheel and reducing the number of bond level grit-breakage in brazed wheel. Grinding of low carbon steel is sometimes problematic as it exhibits high adhesion to the grits leading to wheel loading. Monolayer wheels can be useful in this regard as they possess relatively high grit protrusion (with respect to the conventional composite wheels), which indicates higher chip accommodation volume and less chance of wheel loading. However, in most of the previous literatures, grinding of low carbon steel was carried out with conventional wheels [13–17]. Hence, it is imperative to study the performance of monolayer wheels in grinding low carbon steel. Moreover, to the best of the authors’ knowledge, no study has ever been carried out to investigate the
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Table 1 Deposition parameters during TiN coating. Phase of experiment
Target (cathode) frequency (kHz) 80% duty cycle
Substrate bias frequency (kHz) 80% duty cycle
Negative substrate bias voltage On Ni plated C-20 steel disc
On cBN wheels for grinding AISI 1020 steel
On cBN wheels for grinding AISI 52100 steel
1 2
0, 50, 100, 150, 200, 250, 300 200
100 100
60 0, 30, 40, 50, 60, 70, 80, 90, 120, 150
60 0, 30, 60
– 0, 60, 90, 120, 150
possibility of grinding low carbon steel with TiN coated monolayer cBN wheel. Therefore, one of the objectives of the present investigation was to explore the effect of pulsed DC closed-field unbalanced magnetron (CFUBM) sputtered TiN coating on performance of electroplated cBN wheels in grinding C-20 or AISI 1020 steel. TiN was expected to provide two folds benefit: (a) it would render some anti-frictional and antiwear characteristics in grinding a sticky material like C-20 steel and (b) it would prevent the grit pull-out from the nickel bond. The effect of target frequency and bias voltage on physical and mechanical properties of the PVD coating has been reported in many past literatures. However, in most of the cases, the coating was deposited on M2 grade tool steel [18–21], glass [18,19], silicon wafer [18,19,22,23] and stainless steel [23]. However, the effect of target frequency and bias voltage when depositing TiN on electroplated nickel has never been investigated before. Thus, in connection to the application of TiN on electroplated grinding wheel, the second objective of the present research aimed at exploring the effect of the above mentioned deposition parameters on microstructure of TiN and adhesion strength of both TiN and nickel layer on C-20 steel substrate. In the present study, TiN coating was deposited on underplated galvanic cBN wheels (growth of nickel layer was of nearly 30–40% of the average grit height). The underplated wheels have been intentionally used to obtain the benefit of large chip accommodation space as well as the usefulness of TiN coating even for a low level of plating thickness. During the 1st phase of experiment, TiN was deposited on nickel electroplated C-20 (AISI 1020) steel discs and monolayer galvanically bonded cBN wheels at seven different target frequencies keeping the bias voltage and bias frequency constant. The coating microstructure was studied using scanning electron microscopy (SEM). The adhesive strength of nickel with the steel substrate was assessed by scratch test to examine whether there was any increase in adhesion due to TiN coating. Grinding of AISI 1020 steel was carried out with all the coated cBN wheels and result was compared with that obtained with an uncoated one. During the 2nd phase of experiment, TiN was deposited on the nickel plated steel discs at ten different bias voltages, keeping target frequency and bias frequency unchanged. The above mentioned characterisation techniques have been used in the 2nd phase also. The wheels coated at 0, −30 and −60 V bias voltages were used in grinding AISI 1020 steel and those coated at 0, −60, −90, −120 and − 150 V bias voltages were used in grinding AISI 52100 steel. The part of the experiment relating to grinding of AISI 52100 steel has been communicated elsewhere [24]. However, the result has been presented here to make a comparative study on effect of target frequency and bias voltage in grinding of low carbon steel and hardened bearing steel with TiN coated wheels.
comparable with the thickness of the nickel bond in an underplated wheel. Some underplated galvanically bonded grinding wheels (plating area: Φ 15 mm× 10 mm) were also procured from Eastern Diamond Products Pvt. Ltd., Kolkata, India. Grit distribution density was counted on 10 underplated wheel samples and average density was found to be varied in the range of 55–65 numbers per 5 mm2. Grits were monocrystalline cBN of B251 size (mesh width 250/180 μm) and nickel layer thickness was around 70–80 μm. 2.2. Deposition of TiN coating on steel disc and grinding wheel TiN was deposited by pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS) technique on both nickel electroplated C-20 steel discs and electroplated cBN wheels. The coating was deposited in a dual cathode CFUBMS system (TOOL COATER, VTC-01A) manufactured by Milman Thin Film Systems Pvt. Ltd, Pune, India. The detailed description of the system and conditions during target-cleaning and substrate-ionetching have been reported elsewhere [25]. Other deposition parameters are summarized in Table 1. A representative photograph of a TiN coated wheel is shown in Fig. 1. 2.3. Coating microstructure analysis, EPMA and GIXRD The surface morphology and cross-section of the deposited coatings were observed using SEM. Images were taken in Carl Zeiss Supra 40 field emission SEM (FESEM), Zeiss EVO 60 and Jeol JSM 6490 SEM along with INCA FET 3X EDX analysis. The interface of TiN and nickel was examined by EDX line scan and EPMA area mapping. EPMA was carried out in a CAMECA SX100 instrument with LaB6 filament and beam diameter of 1 µm. Grazing incidence X-ray diffraction (GIXRD) with an angle of incidence 1.5° was carried out on the coated samples to find out the formation of any intermetallic phase between TiN and nickel layer. All measurements were carried out in a high resolution Philips, PANalytical PW 3040/60 X'Pert PRO instrument with molybdenum (Mo) target (wavelength = 0.70930 Å). The samples were continuously scanned with a step size of 0.05° and time per step of 1 s. The generator setting was 40 kV and 30 mA. The data were later analysed with X'Pert HighScore software (Philips Analytical B.V., Netherlands) and peaks were identified by comparing with Joint Committee on Powder Diffraction Standards (JCPDS) data files. 2.4. Scratch adhesion test On nickel plated steel discs, scratch test was carried out before and after TiN coating to find out whether there was any increase of strength of nickel layer due to high energy ion impingement during
2. Experimental 2.1. Preparation of steel disc and procurement of electroplated monolayer cBN wheels In order to observe the coating microstructure and assess the adhesive strength of coating and nickel layer, some C-20 (AISI 1020) steel discs were prepared in CNC turning centre and were electroplated with nickel (plating was obtained from Eastern Diamond Products Pvt. Ltd., Kolkata, India). The plating thickness was around 70–80 μm to make it
Fig. 1. Photograph of a TiN coated electroplated cBN wheel.
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deposition. A DUCOM scratch tester TR-101-M5 was used for scratch testing. The scratch tests were performed with a Rockwell C diamond stylus (0.2 mm radius) drawn across the surface of the coating at a constant linear speed of 0.2 mm/s. The load was varied in the range of 10 N to 90 N with a loading rate of 5 N/mm.
2.5. Grinding The grinding experiments have been carried out under dry condition in a surface grinding machine (Make: Praga Tools Ltd., India) retrofitted with a high speed spindle (Make: Prazisionspindilen GmbH, Germany, Model: SC 60-O). The detailed descriptions of the workpiece holding device and dynamometer have been discussed elsewhere [25]. The workpieces were low carbon steel (AISI 1020) and hardened bearing steel (AISI 52100) with a size of 80 mm × 25 mm× 6 mm. Table 2 depicts the parameters used during grinding. A grinding condition 40-4-10 indicates a wheel speed of 40 m/s, table speed of 4 m/min and a downfeed of 10 µm. During grinding, normal and tangential forces were measured following a sequence of grinding conditions based on Taguchi L16 orthogonal array [26]. The sequence was chosen in a way such that chip load increased from the 1st condition to the 16th condition. Chip load on a grit is directly proportional to the maximum grit depth of cut (hm). hm was calculated as:
hm =
3 V · w c tan α Vc
rffiffiffiffi!1 = 2 d D
ð1Þ
where, hm
Maximum grit depth of cut, µm
c
No. of grits/mm2 area
α
Half apex angle of a grit, degree
Vc
Grinding velocity, m/s
Vw
Table speed, m/min
d
Wheel depth of cut or downfeed, μm
D
Wheel diameter, mm
However, for all wheels, c, α and D were assumed to be same as 55–65 numbers per 5 mm2, 60° and 15 mm respectively. So, for calculating chip load, Vc, Vw and d were taken into consideration. Hence, the sequence of operation was chosen with an increasing value part ′ was of hm′, whichis apffiffiffi of maximum grit depth of cut, hm. hm calculated as VVwc d . Five passes were given under each condition and average force was taken into account. Post-grinding conditions of the wheels and the grits were observed under Stereo Zoom Microscope (Olympus SZ 1145TR PT) and SEM (Jeol JSM 6490).
Table 2 Grinding parameters during grinding of AISI 1020 and AISI 52100 steel. Wheel diameter × width Grinding velocity, Vc Table speed, Vw Down feed, d Environment Wheel-workpiece contact width Length of cut per pass Grinding conditions
Ф15 mm × 10 mm 22, 28, 34, 40 m/s 3, 4 m/min 20, 30, 40 μm Dry 6 mm 80 mm 40-4-10, 28-3-10, 34-4-10, 22-3-10, 40-4-20, 40-3-40, 34-3-30, 34-4-20, 34-3-40, 28-4-30, 28-4-40, 22-4-30,
3. Results 3.1. Coating microstructure 3.1.1. With varying target frequencies Fig. 2 represents SEM micrographs of surface morphology and cross-section of TiN deposited on nickel plated steel discs at various target frequencies. Average column size of TiN was found to decrease with increase in frequency up to 250 kHz. The fractured cross-section reveals that the structure of the coating at 0 kHz frequency gradually transformed into very dense and compact at 250 kHz. However, the column size slightly increased when TiN was deposited at 300 kHz. In pure DC sputtering, target poisoning takes place [27], which can be reduced to a great extent with the application of pulsed DC in the target [28]. Moreover, the plasma became more energetic during pulsed DC sputtering resulting in higher degree of ionization [29]. This might have attributed in reduction of column size and formation of dense and compact coating structure. With the help of time and energy resolved mass spectrometry, Arnell et al. [19] previously observed that argon content of the coating increased with increasing target frequency. In their study, the coating deposited at 350 kHz possessed higher surface roughness, poor adhesion and wear resistance. Probably the same reason was prevalent in the present case, where slightly larger average column size was obtained at the frequency of 300 kHz. 3.1.2. With varying bias voltages The effect of bias voltage on coating microstructure was more predominant than that of target frequency as can be observed from Fig. 3. The average column width of TiN was found to noticeably decrease from a bias voltage of 0 V to − 80 V. The porous and open columnar structure (at 0 V bias) gradually transformed into dense columnar (at −50 V bias) and further into compact and featureless (at −80 V bias). The reduction in column size of the coating could be due to increased high energy ion bombardment leading to more surface defects [22] and nucleation sites. The coating topography was lumpy in nature up to − 70 V, however severe micro-cratering marks on the coating surface were observed at and beyond −90 V due to extremely high energy ion impingement, which led to the induction of large compressive stress within the coating [30]. Such brittleness caused vertical cracks, which are visible from the cross sectional view of the coating deposited at −120 V. 3.2. EDX line scan, EPMA and GIXRD Fig. 4 depicts EDX line scan carried out across the polished crosssection at the interface of TiN and nickel layer (TiN was deposited at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz). The polishing was done to keep the two different layers in a single plane. Some amount of cross-diffusion between TiN and nickel at their interface can be seen in Fig. 4. The depth of interdiffusion was found to be in the range 1–1.5 μm. The EPMA area mapping (shown in Fig. 5) across the cross-section also confirmed this observation where an overlapping region among Ti, N and Ni can be seen. The inter-diffusion phenomenon is clearly substantiated from Fig. 6, which displays a representative GIXRD spectra carried out on TiN coated (deposited at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz) nickel plated steel disc. It reveals the presence of Ni–Ti intermetallic phases in addition to the standard TiN phases. 3.3. Scratch adhesion test
40-3-30, 28-3-20, 22-3-20, 22-4-40
Scratches were taken before and after TiN coating on the same nickel plated disc. In all cases, the left one represents graphs obtained before TiN coating, while the right one shows graphs obtained after
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Fig. 2. SEM micrographs of surface morphology and cross-section of TiN deposited at various target frequencies.
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Fig. 2 (continued).
TiN coating. Fig. 7(a) to 7(d) demonstrates the graphs of normal load, traction force and apparent coefficient of friction when TiN was deposited at 0 kHz, 250 kHz, 0 V and − 90 V respectively. Fig. 7(e) shows a representative SEM image of a scratch track taken after coating. All the graphs clearly indicate that there was an increase in traction force after deposition of TiN. However, the difference in increase of traction force after coating at 0 kHz and 250 kHz of target frequency was not significant (Fig. 7(a) and 7(b)). On the other hand, remarkable enhancement in traction force was obtained when bias voltage was increased from 0 V to − 90 V (Fig. 7(c) and 7(d)). Similar enhancement was also observed with coating at − 120 and −150 V (graphs not shown). Hence it can be inferred that bias voltage has much more significant influence in increasing cohesive strength of nickel than
target frequency. This might be attributed to the densification and overall strengthening of TiN coating with the application of negative substrate bias voltage discussed in Section 3.1.2. 3.4. Grinding 3.4.1. Grinding of AISI 1020 steel with cBN wheels coated at different target frequencies and bias voltages Fig. 8 describes the comparison of average normal grinding force (FN) during grinding of AISI 1020 steel with uncoated and TiN coated (at different target frequencies) electroplated cBN wheels. Although the grinding sequence was chosen according to the ascending order of the value of hm′, which is a part of maximum grit depth of cut (hm), the main influencing grinding parameter on normal and tangential
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Fig. 3. SEM micrographs of surface morphology and cross-section of TiN deposited at various substrate bias voltages.
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Fig. 3 (continued).
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Fig. 3 (continued).
grinding forces was found to be downfeed (d) as evident from Fig. 8. The graph of tangential force is not shown here as the trend was more-or-less similar to that of normal force. With the uncoated wheel, grinding could not be carried out after the 13th condition (28-4-30) because of high wheel loading. The grinding had to be stopped after the 4th (40-3-30) and 6th (40-4-20) conditions with wheels coated at 0 and 100 kHz respectively due to severe wheel loading. The same phenomenon observed for the wheels coated at 50 and 150 kHz and grinding was stopped after the 7th
condition (40-3-40). The wheel coated at 250 kHz could grind up to the 8th condition (28-3-20). Only with the wheels coated at 200 and 300 kHz, grinding could be carried out till the 13th condition and then had to be stopped due to high wheel loading. A similar trend of large wheel loading was observed when grinding of AISI 1020 steel with the wheels coated at 0, −30 and −60 V bias voltages. The graphs of normal and tangential forces are not reported here. Apparently it seems that coated wheels performed worse than the uncoated one. However the effect of TiN coating on performance of
Fig. 4. EDX line scan across the cross-section at the interface of TiN and nickel layer at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz.
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Fig. 5. EPMA area mapping across the cross-section at the interface of TiN and nickel layer.
electroplated wheel cannot be judged in the light of wheel loading only. During grinding with uncoated wheel, there was material built-up in the inter-grit space as low carbon steel produced long sticky chips. The built-up of material increased and after some time large scale adhesion [15] occurred between the grits and the workpiece material. When the forces of adhesion acting over the area of metal/metal contact exceeded the sum of the grit retention forces acting on the grits contained within the area of the adherent material, the adherent material was detached from the area along with some part or the whole grits within that area.
Fig. 6. GIXRD spectra of TiN on nickel plated steel disc at a target frequency of 250 kHz, bias voltage of −60 V and bias frequency of 100 kHz.
As the number of active grits participating in grinding reduced, the total normal force also decreased as can be observed from Fig. 8, where the uncoated wheel exhibited much lower total normal force than all the coated wheels. As the detachment of workpiece material caused removal of some part of the grits, many fractured grits could be seen after grinding when viewed under SEM. Such adherent material and fractured grit in an uncoated wheel are shown in Fig. 9(a) and 9(b) respectively. On the other hand, high energy ion impingement during TiN coating enhanced the bonding strength of nickel as well as filled up the pores, voids, cracks, surface defects within the crystals of cBN grits. Therefore, neither the grits underwent fracture nor pulled out when wheel loading occurred in the coated wheels. Very little fractured grits can be observed from Fig. 9(c). As the loaded material could not be removed, the grinding had to be stopped much before compared to the uncoated wheel. The level of normal force was also found to be higher because of this reason (Fig. 8). 3.4.2. Grinding of AISI 52100 steel with cBN wheels coated at different bias voltages The situation was entirely different when hardened bearing steel was ground with uncoated and coated cBN wheels and the effectiveness of TiN coating was immediately understood as the wheels were free from the problem of wheel loading. The graphs of average normal (FN) and tangential (FT) forces during grinding of AISI 52100 steel with uncoated and TiN coated (at different bias voltages) cBN wheels are depicted in Figs. 10 and 11 respectively. From both the figures, it is observed that under the 1st, 4th, 7th 13th, 14th and 16th conditions, significantly lower grinding forces were obtained with the uncoated wheel. It indicates that some amount of grits was pulled-out from the encapsulation of nickel bond in the uncoated wheel from the very beginning of grinding. This resulted in the reduction of total grinding force due to a lesser number of active grits participating in the actual grinding operation. However,
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it caused an escalation of specific grit force which in turn resulted in further pull-out of grits under very high chip load conditions like 284-40 and 22-4-40. Fig. 12(a) and 12(b) reveal such grit pull-out from the nickel bond of the uncoated wheel after grinding. The uncoated wheel has also undergone some grit fracture at the 15th condition (22-4-30), which is indicated by the increase in normal force (Fig. 10), while for the other wheels normal force decreased because of reduced downfeed from the 14th condition (28-4-40). Fig. 10 also reveals that the wheel coated at 0 V bias voltage has suffered from significant amount of grit fracture at and above bond
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level during grinding at the 7th, 9th, 11th and 13th conditions. This can be understood as the percentage increase in normal force in this particular wheel was more than that of the other wheels. Some grits were pulled out at the 15th condition (22-4-30), which can be inferred from the higher percentage reduction in normal force than the other wheels. When TiN was deposited at bias voltages like −60 V and −90 V, number of grit pull-out and damage of grits were evidently reduced when observed under Stereo Zoom Microscope after grinding. The BSE image (Fig. 12(c)) of a TiN coated wheel (deposited at −90 V bias)
Fig. 7. Graphs of normal load, traction force and apparent coefficient of friction on nickel plated steel disc before and after TiN coating at (a) 0 kHz, (b) 250 kHz, (c) 0 V, (d) −90 V, (e) SEM image of a representative scratch track after coating.
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Fig. 7 (continued).
revealed that macro-fracturing of grits above bond level and microfracturing at the tip occurred for a lesser number of grits. TiN coating was retained on most of the grits and bond surface even after grinding under very high specific chip load condition. Fig. 12(d) shows the retention of TiN on the very sharp edge of a cBN grit even after grinding of 80 passes. This clearly indicated a very good adhesion of TiN coating with the cBN grits. The calculation of specific grinding energy (Ug) with increase of maximum grit depth of cut (hm) from the 1st to the 16th grinding
conditions during grinding of AISI 52100 steel has also been carried out and represented in Fig. 13. The curves were fitted using a second order polynomial regression equation. The lower specific energy of the uncoated wheel could be ascribed to the lower grinding force obtained in this wheel, which was originally resulted from the pull-out of the grits. However, the curve of Ug in uncoated wheel shows an abnormal behaviour as it at first increased and then decreased with increase in maximum grit depth of cut (hm). It is expected that with an increase of downfeed (d) or maximum grit depth of cut (hm), Ug should reduce,
Fig. 8. Comparison of average normal grinding forces (FN) during grinding of AISI 1020 steel with uncoated and TiN coated (at different target frequencies) electroplated cBN wheels.
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coated wheels with increase in hm. This is in agreement with a previous observation by Paul and Chattopadhyay [31], where Ug was found to decrease with increase in downfeed. The very high value of Ug in the TiN coated (0 V bias voltage) wheel might be attributed to a greater amount of primary and secondary ploughing of large number of fractured grits with the workpiece. This became evident when the wheel was viewed under the microscope and SEM, which revealed many fractured grits at the bond level. The gradual fall of Ug in the wheels coated at −60 V and −90 V bias ensures preclusion of primary and secondary ploughing by retaining the sharpness of the grits and participation of the same in micro-cutting action. 4. Discussion
Fig. 9. (a) Wheel condition of an uncoated wheel after grinding of AISI 1020 steel, (b) Grit fracture in the uncoated wheel, (c) Wheel condition of a TiN coated wheel (target frequency of 200 kHz) after grinding AISI 1020 steel.
because the rubbing and ploughing parts of Ug get diminished and grits take part in shearing or micro-cutting action [31]. The rise of Ug in uncoated wheel up to 12th grinding condition indicates large rubbing and ploughing of the fractured grits with the workpiece surface. Ug again decreased from the 13th condition because of some amount of grit pull-out under very high chip load and consequent fall in total grinding force. On the other hand, Ug gradually decreased in all the
While comparing the effect of target frequency and bias voltage on coating characteristics, it can be inferred that bias voltage has shown more prominent effect than target frequency in reducing the average column size of the coating. The cohesive and adhesive strengths of the nickel was also remarkably increased with the application of bias voltage. Because of high energy ion impingement of TiN within the electroplated nickel layer coupled with the strong affinity of nickel towards titanium, a cross-diffusion occurred between TiN and nickel with the formation of nickel–titanium intermetallic phases up to a certain depth (evident from Fig. 6). As the Ni–Ti intermetallic phase is hard, it helped in augmenting the scratch resistance of the nickel bond. Although TiN deposited at very high bias voltages like −120 V and −150 V could enhance the cohesive strength of nickel, large spallation at the edge of scratch track was also observed. This can be ascribed to the brittle nature of coating due to the induction of large compressive stress within it [30]. Along with the cohesive strength of nickel, the adhesion between nickel and steel substrate was also enhanced. During scratch test before TiN coating, it has been observed that at normal load as high as 85–90 N, nickel was stripped off from the steel substrate. When TiN was deposited with substrate bias below −50 V, the stripping of nickel layer from the substrate could not be prevented even after TiN coating. However, no observable evidence of stripping of nickel was obtained when TiN was deposited at a substrate bias of − 60 V and above that. The effect of increased strength of nickel immediately reflected in actual grinding operation, where wheels coated at −60 V and −90 V bias voltages have undergone much less fracture and pull-out compared to their uncoated counterpart. The retention of TiN on the edge and surface of cBN grits could be attributed to the strong affinity of cBN towards titanium with the formation of TiB2, TiB, (TiB + TiN), TiN, Ti reaction products at their interface [32,33]. It can be further noted that, the present investigation ascertained less number of grit pull-out in the uncoated wheel compared to what was reported by Ghosh and Chattopadhyay [12]. In their study, TiN was deposited at a particular bias voltage of −80 V and the normal grinding force obtained with the TiN coated wheel was 0.9 N higher than that obtained with the uncoated one at a grinding condition of 30-4-40. In the present research, at a similar condition like 28-4-40, the difference in normal forces measured with uncoated and a coated wheel (−90 V bias) was found to be around 2.7 N. This might be due to the difference in active grit density in the uncoated and coated wheels. The coated wheels exhibited higher grit density participating in micro-cutting action and thus higher total normal force. As the monolayer wheels are not required to be trued, it is important to maintain their form stability by preventing the grit fracture at the bond level and restricting the same only at the tip of the grits. While evaluating the effect of bias voltage on performance of the coated grinding wheels, neither a too low nor a very high bias voltage was found to meet this requirement. A too low bias like 0 V was not able to provide sufficient strength to the nickel bond (as also evident from the scratch test) and could not fill up the defects along the
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Fig. 10. Comparison of average normal grinding forces (FN) during grinding of AISI 52100 steel with uncoated and TiN coated (at different bias voltages) electroplated cBN wheels.
cleavage planes in the monocrystalline cBN grits. Therefore grit fracture at the bond level could not be arrested in the wheels coated at 0 V bias voltage. A very high substrate bias voltage (i.e. −120 V and −150 V) also seemed to be detrimental to the grinding performance of the coated wheels. The post-grinding observation of the wheels (coated at − 120 V and −150 V bias) revealed many fractured grits at the bond level. A concentration of very high compressive residual stress at the interface of coated cBN and nickel bond might be imputed to this detrimental effect. Hence it can be inferred that, TiN deposited within a certain range of bias voltage (− 60 V to − 90 V in the present
study) was ideal to arrest the grit fracture and pull-out. It has been further witnessed that the anti-wear property of TiN coating was revealed in the present study rather than its anti-frictional characteristics (although TiN is not an automatic choice as a low friction coating material [34]). In order to preclude any reaction between grinding fluid and TiN coating, all the grinding experiments were carried out under dry condition. This could be a step towards environment friendly grinding technology. The successful implementation of TiN (within a certain range of bias voltage) in grinding necessitates further detailed investigation
Fig. 11. Comparison of average tangential grinding forces (FT) during grinding of AISI 52100 steel with uncoated and TiN coated (at different bias voltages) electroplated cBN wheels.
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Fig. 12. (a) Wheel condition of an uncoated wheel after grinding AISI 52100 steel, (b) Grit pull-out from the uncoated wheel, (c) Wheel condition of a TiN coated wheel (bias voltage of − 90 V) after grinding of AISI 52100 steel, (d) Retention of TiN coating on the cBN grit.
on the application of other types of PVD coatings (like CrN, ZrN, HfN or a co-deposition of TiN with these coatings) in the field of abrasive machining.
5. Conclusions TiN was deposited using pulsed DC CFUBMS technique on nickel plated steel discs and electroplated monolayer cBN wheels at different target frequencies and bias voltages. Grinding of low carbon steel and hardened bearing steel was also carried out with the uncoated and coated cBN wheels. Based on the experimental observations, the following conclusions could be made: (A) With increase in target frequency, average column size of TiN decreased up to 250 kHz and then slightly increased at 300 kHz. The coating structure obtained at 0 kHz gradually transformed into compact and dense at 250 kHz, however at 300 kHz, the larger column width of the coating was again observed. The column size of TiN was prominently reduced when bias voltage was increased from 0 V to −80 V. The structure of the coating gradually transformed from porous and open columnar to very compact, dense and featureless at −80 V. However, at and beyond −90 V, micro-cratering marks and vertical cracks were observed on the surface and cross-section of the coating respectively. (B) EDX line scan and EPMA confirmed the cross-diffusion between TiN and nickel and GIXRD indicated the formation of nickel– titanium intermetallic phases at their interface.
Fig. 13. Variation of specific grinding energy of the uncoated and coated wheels with increase of maximum grit depth of cut during grinding of AISI 52100 steel.
(C) Target frequency could not effectively increase the cohesive strength of nickel layer, which was significantly improved with the increase in negative bias voltage. The stripping of nickel
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from the steel substrate was also arrested at a bias voltage of −50 V and beyond that. (D) Seemingly, TiN coated wheel could not perform better than the uncoated wheel during grinding of low carbon steel due to high wheel loading. However, the uncoated wheel was found to undergo fracture wear, which was remarkably absent in the coated wheel. The situation was totally different during grinding of hardened bearing steel, where benefit of TiN coating within a particular range of substrate bias was clearly revealed. The uncoated wheel was found to undergo many fracture wear with some pull-out of grits. On the other hand, almost no pull-out and much less fractured grits were observed in the wheels coated at bias voltages like −60 V and −90 V. However, neither a very low (like 0 V) nor a too high bias (like −120 V and −150 V) was found to arrest the grit fracture at bond level. Acknowledgments The authors gratefully acknowledge the funding support they received from DST, FIST (Sanction No. SR/FST/ET-II-003/2000 dated 20.5.2002), The authors are also thankful to Prof. N. C. Pant of the Department of Geology and Geophysics, IIT Kharagpur, India, for providing SEM and EPMA facility. The authors also express their sincere gratitude to Dr. Soumya Gangopadhyay of the Department of Mechanical Engineering, IIT Kharagpur for his help and co-operation during the experiments. References [1] S. Malkin, CIRP Ann. 34 (2) (1985) 557. [2] W. König, F. Ferlemann, Ind. Diamond Rev. 51 (5) (1991) 237. [3] A. Ghosh, A.K. Chattopadhyay, Int. J. Mach. Tools Manuf. 47 (2007) 1206.
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