Tribological behavior and cutting performance of monolayer, bilayer and multilayer diamond coated milling tools in machining of zirconia ceramics

Accepted Manuscript Tribological behavior and cutting performance of monolayer, bilayer and multilayer diamond coated milling tools in machining of zirconia ceramics

Chengchuan Wang, Xinchang Wang, Fanghong Sun PII: DOI: Reference:

S0257-8972(18)30901-0 doi:10.1016/j.surfcoat.2018.08.074 SCT 23743

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

21 March 2018 21 June 2018 25 August 2018

Please cite this article as: Chengchuan Wang, Xinchang Wang, Fanghong Sun , Tribological behavior and cutting performance of monolayer, bilayer and multilayer diamond coated milling tools in machining of zirconia ceramics. Sct (2018), doi:10.1016/ j.surfcoat.2018.08.074

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ACCEPTED MANUSCRIPT

Tribological behavior and cutting performance of monolayer, bilayer and multilayer diamond coated milling tools in machining of zirconia

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ceramics

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Chengchuan Wang, Xinchang Wang, Fanghong Sun*

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School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai

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200240, China

*Corresponding Author:

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Tel: +86 21 3420 6557

Fax: +86 21 6293 2610 E-mail address: [email protected]

ACCEPTED MANUSCRIPT Abstract In this work, a comparative study of diamond films consisting of alternate microcrystalline (MCD) and nanocrystalline diamond (NCD) layers is conducted. Diamond films including monolayer, bilayer and multilayer diamond films are coated

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on cemented tungsten carbide (WC-Co) substrates by adopting a bias-enhanced hot

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filament chemical vapor deposition (HFCVD) technique. Tribological properties of the

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diamond films are evaluated by using a reciprocal tribometer without lubrication. Further milling tests are carried out to examine the cutting performances with sintered

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zirconia ceramics as workpiece comparatively. In friction test against zirconia ceramics,

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the monolayer NCD film shows the lowest friction coefficient (0.128) because of its smooth surface. Also, the bilayer diamond film with surface coating of NCD layer

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(MNCD) and both of the multilayer diamond films exhibit good friction property while

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the monolayer MCD film has the largest friction coefficient (0.292). The milling test demonstrates that the monolayer diamond (MCD and NCD) coated milling tools show

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poor tool life due to abrasive action of hard workpiece. Working life of all the bilayer

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and multilayer diamond coated tools is enhanced, but large area shedding of coatings appears after some milling passes except the multilayer diamond film with surface coating of NCD layer (MNMN-CD). The multilayer film (MNMN-CD) presents superior machining performance and its working life increases by 3~7.5 times compared with the monolayer diamond coated ones. Keywords: MCD; NCD; Multilayer; Tribological; Cutting performances.

ACCEPTED MANUSCRIPT 1. Introduction Chemical vapor deposition (CVD) diamond film is widely used in many fields, such as nozzles [1], drawing dies [2-5], sealing rings [6, 7], electrodes [8], and especially cutting tools [9-11]，due to its excellent properties in hardness, thermal

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conductivity, elastic modulus, friction coefficient, wear resistance and chemical

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inertness [12]. CVD diamond coated tools are mostly used in machining carbon fiber

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reinforced plastic (CFRP), printed circuit board (PCB), ceramic, graphite and metal matrix composite (MMC) [13-15]. Compared with uncoated cutting tools, diamond

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coated ones show higher wear resistance and longer working lifetime [16, 17].

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However, performances of diamond films such as tribological and mechanical properties could have distinct differences in view of some factors including diamond

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surface morphology, structure and crystalline quality [18].

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Nanocrystalline diamond (NCD) films, possessing outstanding properties of high hardness and reduced surface roughness, are very suitable for mechanical and

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tribological applications [19, 20]. Lei et al. [21] studied tribological and cutting

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performances of microcrystalline diamond (MCD) and NCD films. In the study, NCD films showed lower friction coefficient and better aluminum alloy machinability. Qin et al. found nanostructured diamond (nano-diamond) coated tools showed greater coating delamination wear resistance than conventional MCD coated tools in composite machining [22]. However, NCD films show weaker chemical bonding to substrates than MCD films [23, 24]. This is caused by formation of non-diamond species at the NCD grain boundaries which reduces their adhesion strength. MCD films exhibit high

ACCEPTED MANUSCRIPT hardness and strong chemical bond to substrates, but their large columnar grains could cause high friction during tribological applications [25]. Therefore, to make full use of their different advantages, composite and multilayer diamond films (bilayer [26, 27], trilayer [28-30], four-layer [31-34] or even tens of layers [35-37]) have been studied by

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depositing MCD and NCD layers alternately. By consisting of MCD films with

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different grain sizes, Chen et al. [38, 39] adopted a novel deposition method for

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multilayer diamond fabrication. A novel composite diamond film comprising of a relatively thick layer of UNCD (ultrananocrystalline diamond) and an underlying

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relatively thin MCD layer was shown by Zeng et al. [40] for film delamination

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resistance under extreme shear stress. Takeuchi et al. [37] synthesized multilayer diamond films and reported that the interfaces formed by the multilayer structure are

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expected to prevent crack propagation. In addition, the bending strength of the

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multilayer diamond film is 30% higher than that of conventional diamond film. However, most works mentioned above are mainly focused on characterization,

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mechanical performances, frictional characteristics and wear resistance of multilayer

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diamond films. There are barely researches about multilayer diamond coated cutting tools with complex shapes, such as mill or drill tools. Salgueiredo et al. [27] presented a novel silicon nitride (Si3N4) ceramic drill bit and carried out drilling experiments to study cutting performances of the novel tools, but the drill tool was just coated with a bilayer coating of micro/nanocrystalline diamond (MCD/NCD). Therefore, further study on the mechanical properties and cutting performances of multilayer diamond coated cutting tools needs to be conducted.

ACCEPTED MANUSCRIPT Hot chemical vapor deposition (HFCVD) is a well-known technique to fabricate diamond films coated on various types of substrates due to its low cost, operational convenience and practicality [41, 42]. In this study, monolayer, bilayer and multilayer (four-layer) diamond films are coated on cemented tungsten carbide (WC-6% Co)

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substrates using a HFCVD apparatus. The objective of present work is to compare the

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characteristics and tribological behaviors of different types of diamond films. Besides,

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Cutting performances of all the diamond coated tools are also assessed by machining

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ZrO2 ceramics.

2.1. Fabrication of diamond films

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2. Experimental Details

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Flat substrates with square geometry (12 × 12 × 4 mm) and ball end mills (2 mm

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diameter) with two-flute, both made up of WC-6% Co, are used as substrates for diamond deposition. Before diamond deposition, a two-step pretreatment must be

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applied in order to decrease the content of surface metallic cobalt [11]. Firstly, the

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Murakami’s reagent (10 g K3[Fe(CN)6] + 10 g KOH + 100 mL H2O) contained in ultrasonic agitation vessel is used to roughen the substrate surface for 20～30 min. Then the WC-Co substrates are immersed in the Caro’s acid (20 mL HCl : 80 mL H2O2) for 30～60 s to further etch the cobalt element still existed on the surface. Thereafter, the substrates are grinded with diamond grits (25 μm) by hard cloth for enhancing diamond nucleation during deposition. Diamond films are deposited on flat substrates and ball end mills respectively by

ACCEPTED MANUSCRIPT adopting a homemade hot filament chemical vapor deposition (HFCVD) apparatus. It is heated by tantalum wires twisted by two single-wires (ϕ 0.3 + ϕ 0.4 mm, 200 mm length). As carbon source, liquid acetone is introduced into the reaction chamber by bubbling part of H2 through the liquid. To control flow rate of acetone accurately, its

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container is immersed in ice-water bath to keep constant temperature (0 ℃) considering

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the temperature effect on acetone vapor pressure. In deposition process, temperature of

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hot filaments is detected by double color integrated thermometer (Raytek MR 1SCSF) and kept at 2100～2200 ℃. Meanwhile, temperature of substrates, set at 750-900 ℃,

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is measured by K-type thermocouple. For flat substrates with large surface areas,

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thermocouples could be bended easily and adjusted to touch the substrate surface directly for temperature measurement. With regard to milling tools, little holes are

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drilled on the copper holders to locate thermocouples [21]. Surface morphology

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transition from microcrystalline diamond (MCD) to nanocrystalline diamond (NCD) could be achieved typically by changing chamber pressure, carbon source content and

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reactive temperature. The detailed deposition parameters of MCD and NCD films are

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listed in Table 1. Then six different types of diamond films are fabricated. Fig. 1 exhibits the schematic diagram of their cross-sectional structures, including (a) monolayer:

MCD;

(b)

bilayer:

NCD/MCD

(NMCD);

(c)

multilayer:

NCD/MCD/NCD/MCD (NMNM-CD); (d) monolayer: NCD; (e) bilayer: MCD/NCD (MNCD); (f) multilayer: MCD/NCD/MCD/NCD (MNMN-CD). Considering different growth rates of MCD and NCD films, MCD layer and NCD layer of different diamond films are coated with different growth time which is shown in Table 2. In this way, the

ACCEPTED MANUSCRIPT thicknesses of different types of diamond films and different layers in the same diamond film could be kept same respectively. This could exclude thickness effects on properties of diamond films. In the case of multilayer diamond films, each MCD layer growth lasts for 2.5 h, while each NCD layer only grows 1.5 h.

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Field emission scanning electron microscopy (FESEM, Zeiss Ultra_55) is

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performed to study surface morphology and cross-sectional topology of the diamond

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films. Raman spectroscopy (SPEC 14-03) is adopted to exam identification of carbon phase using a He–Ne laser at an excitation wavelength of 532 nm. Furthermore, surface

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roughness is measured by surface profilometer (Mitutoyo Surftest SJ-410).

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2.2. Friction tests

Tribological properties of the as-fabricated diamond films are studied by carrying

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out friction tests on a reciprocal tribometer (MFT-R4000) under ambient atmosphere at

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room temperature. Contact type here is a ball-on-disc type. Spherical ZrO2 ceramics (Φ 6 mm) are selected as counterparts to slide on the diamond coated WC-Co substrates. A

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normal load of 7.0 N, corresponding to the Hertzian contact pressure 1.04 GPa, is

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applied on samples. In the test, the reciprocating frequency is 6.0 Hz, providing an average velocity of 60 mm/s for a 5 mm stroke. Each test lasts for 2 h, 86400 strokes totally. The coefficients of friction (COF) is recorded automatically by friction tester. 2.3. Milling tests In order to evaluate cutting performances of diamond coated mills in ZrO2 ceramic machining, dry milling tests are performed on a vertical machining center (VMC 850E). Commercially WC-Co cemented carbide mills coated with mono-, bi-

ACCEPTED MANUSCRIPT and multilayer diamond films are used in zirconia ceramic machining. The details of milling parameters are as follows: spindle speed of 6000 rpm, feed rate of 100 mm/min, cutting depth of 0.1 mm. Flank wear of the diamond coated mills are observed by

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optical microscope (SX-5) periodically.

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3.1. Surface morphology and structural characterization

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3. Results and discussion

Surface morphology of the as-fabricated diamond films shown in Fig. 2 is

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observed by FESEM. Fig. 2 (a) shows surface micrograph of the monolayer MCD

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film. It exhibits a rough surface with faceted crystallites. The NCD film has distinctive nano-features with cauliflower appearance (Fig. 2 d). Apparently, diamond

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films with nano-topography (Fig. 2 d-f) present smaller grain size than that with

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micro-topography (Fig. 2 a-c). The bilayer MNCD and multilayer MNMN-CD have similar micromorphology with that of the NCD film. However, the multilayer

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MNMN-CD film seems rougher than the other two ones because of its protuberant

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clusters which may be caused by the rough MCD layer below its surface coating. Seemingly, Fig. 2 (c) reveals that MCD grain size on the multilayer NMNM-CD surface gets smaller under the smoothening effects of its third NCD layer. The average surface roughness Ra measured by surface profiler with 4 mm scanning length is shown in Fig. 3. The MCD film has the roughest surface (Ra 351 nm) while the NCD film shows smoother surface (Ra 169 nm) than anyone else. The tendency of two curves shown in Fig. 3 matches well with surface features in Fig. 2. Surface

ACCEPTED MANUSCRIPT roughness of the bilayer NMCD film show a sharp decrease from the MCD film in the plot (Fig. 3), which means the bottom NCD layer has a great refinement effect on MCD. It also could be seen that both of the multilayer films MNMN-CD and NMNM-CD have similar roughness values.

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MCD film is typical of columnar structure, different from grainy structure of NCD

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film in the case of their fracture surface [43]. In Fig. 4, the cross-sectional morphology

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of these diamond films is displayed. They all have the same thickness, about 16 μm. For bilayer and multilayer diamond films, there are distinct boundaries between MCD and

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NCD layers, and each layer has the same thickness in each film. The multilayer

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MNMN-CD film, actually comprising four layers, starts with MCD layer (first layer) and ends with NCD layer (fourth layer). Besides, the cross-sectional images of the

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diamond coated milling tools are shown in Fig. 5, which show similar characteristics to

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those of films on flat substrates. The main difference between them is film thickness, and 10 μm-thick coating is very suitable for mills while being too thick will cause film

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delamination.

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Fig. 6 displays Raman spectra of all the diamond films. The spectrum of the monolayer MCD film shows a sharp peak with FWHM of 16 cm-1 at 1336 cm-1, indicating its good crystallinity quality. Some other small peaks with very low intensity are also visible: sp2 bonded carbon of G band (around 1580 cm-1), and peaks at around 1150 cm-1 and 1480 cm-1 are assigned to trans-polyacetylene (TPA) [44] . As for the monolayer NCD film, a broad diamond peak appears at about 1329 cm-1. The broadening and weakening of its diamond peak may be influenced by the disordered

ACCEPTED MANUSCRIPT carbon in the grain boundaries which is located around in 1350 cm-1, the D-band peak [45]. Besides, the small peak located around 1200 cm-1 arises from nanocrystalline diamond [46]. Another band at 1456 cm-1, together with 1132 cm-1, has been discussed as the TPA segments at the boundaries of NCD surface [47]. They are also treated as the

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evidence of NCD.

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In Fig. 6 (a), for diamond films with surface coating of MCD layer, it could be

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observed that non-diamond carbon peaks become more obvious with diamond layers increasing. Whereas, for diamond films with surface coating of NCD layer, intensity

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of characteristic peaks representing NCD features becomes weaker, and some of them

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even disappear in the MNMN-CD spectrum. All the detailed Raman analysis is based on Raman spectra deconvoluted by Gauss-Lorentz peak fitting, as exemplified in Fig. 6

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shown in Fig. 6 (c).

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(b). Besides, its corresponding deconvoluted peaks are identified respectively and

3.2. Tribological behaviors

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Friction curves of the diamond coated specimens are illustrated in Fig. 7. It could

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be seen that all the tribo-tests have similar friction coefficient evolutions, an initial sharp peak following with a rapid value decline, and then gradually reaching a steady state. Therefore, all the coefficient of friction (COF) curves can be divided into three stages, including an initial sharp-peak stage, a run-in stage with rapid decline and a steady-state stage with dynamic equilibrium. A sharp peak of COF curves appearing at the very beginning is attributed to interlocking effect among sharp-shaped asperities distributed on the sliding interfaces [44]. Then, zirconia ceramic counterpart

ACCEPTED MANUSCRIPT balls get worn apparently following a run-in period of ploughing and their worn surfaces are polished. After a while, the friction curves drop and reach a stable state. Besides, unreacted ZrO2 debris fills valleys between diamond crystals, as shown in Fig. 8, which is helpful in COF decreasing. Apparently, diamond films coated with surface

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coating of MCD layer adhere less debris than those coated with surface coating of

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NCD layer because their wider crystal valleys are not easy for chipping insertion.

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Only scattered ZrO2 debris is found on the monolayer MCD surface in the magnified inset-Fig. 8 (b).

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To explain their tribological properties more clearly, the average and maximum

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friction coefficients are depicted in Fig. 10. Diamond films with surface coating of MCD layer have larger maximum coefficient of friction (max-COF) than those with

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surface coating of NCD layer. The average coefficient of friction (ave-COF) is

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obtained by calculating the mean values recorded in stable-state stage. The ave-COF of the monolayer MCD is 0.292, much larger than the other five films. The ave-COF

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values of different diamond films present comparisons, specifically, ave-COFMCD > ave-COFMNMN-CD > ave-COFMNCD >

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ave-COFNMCD > ave-COFNMNM-CD ≈

ave-COFNCD. This comparison result matches well with that of their correspondent surface roughness. Furthermore, the wear volume of counterpart balls is also calculated by observing and measuring the worn scars of balls [48], and the detailed volumetric values are exhibited in Fig. 10. The counterpart balls sliding against the monolayer MCD films present the maximum volume loss, while the one against the NMNM-CD films shows the highest wearing resistance.

ACCEPTED MANUSCRIPT It’s worth noting that two multilayer diamond films MNMN-CD and NMNM-CD films reach the same stable state after 25-min testing although there are some differences between them before reaching dynamic equilibrium, as shown in Fig. 9. At region I, a run-in stage with rapid decline caused by rapid wear of counterpart

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balls (Fig. 9 a), MNMN-CD film has lower COF than NMNM-CD film. One reason is

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attributed to the effects of surface roughness which has been mentioned before. The

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other reason is that higher portion of graphite in MNMN-CD (26.2%) could better serve as lubrication parts than NMNM-CD (22.4 %). At region II, MNMN-CD

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exhibits a slight-upward trend while the NMNM-CD not. As is shown in Fig. 2 (f),

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distinct nano-clusters with different heights are distributed on MNMN-CD surface. During friction, these protruding clusters are ground firstly. With time going on, these

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clusters would be worn away (Fig. 9 b) and then more mating surfaces even including

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some area of the third MCD layer (Fig. 9 b) will join in the friction [49]. Therefore, a 5-min upward trend appears in MNMN-CD film. Region III, a steady state, is in

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oscillation where both MNMN-CD (0.155) and NMNM-CD (0.154) presents the

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almost same ave-COF values. For the steady state reaching of MNMN-CD film, two explanations are put forward. On the one hand, most of the protruding clusters on contact surface of diamond have been flatten, thus only a few more areas of the film would join in the friction test afterwards, which has little effect on COF changing. On the other hand, some pits (Fig. 9 - b) where the third-MCD layer has been exposed are filled by ZrO2 debris and it would counteract the effects of new exposed areas on the COF value increasing.

ACCEPTED MANUSCRIPT 3.3. Cutting performance evaluation In the machining process, the maximum flank wear (VBmax) is measured intermittently every 60 sec. Flank wear near mill nose is the most easily worn parts in the test and occurrence time of film delamination is selected as tool failure criterion.

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After 120 s of milling, film delamination is observed on the NCD coated mill nose. The

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life of the MCD coated mill is less than 240 s, a little longer than that of the NCD one.

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The abrasive wear behavior of zirconia ceramics with high hardness is regarded as the main reason for causing rapid wear and failure of the two monolayer diamond films.

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Similarly, film peeling-off also occurs in bilayer (NMCD, MNCD) and multilayer

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(NMNM-CD) diamond films after milling of 540 s, 780 s and 960 s respectively. Differently, the MNMN-CD films show high wear resistance and its measured VBmax is

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about 55.98 μm after machining of 1020 s, without tool failure occurred.

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The flank wear shown in Fig. 11 is plotted as a function of machining time for all of these diamond coated mills. All the plots except that of MNMN-CD have a sharp

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increase at some time. This means film delamination occurrence. For MNMN-CD film,

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its evolution curve exhibit a steady increase and no sudden aggravation of flank wear appears.

By

comparing

mills

coated

with

surface

coating

of

MCD

(MCD/NMCD/NMNM-CD) and NCD (NCD/MNCD/MNMN-CD) layers respectively, bilayer and multilayer diamond films present less wear and longer working life than the monolayer ones. This well demonstrates composite structure consisting of MCD and NCD layers could improve cutting performance of mills effectively. The flank wear of tools pictured by optical microscope are presented in Fig. 12.

ACCEPTED MANUSCRIPT Film peeling-off appears on flank areas of the NCD coated mill only after two milling passes. It can be observed that wear rates of MNCD, MNMN-CD and NMNM-CD coated mills are very close before film delamination occurrence, while the other ones have more severe wear at the initial passes. One interesting phenomenon is that film

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delamination occurs suddenly after a period of milling. This may be caused by severe

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impact of the ceramic workpiece with high hardness. Furthermore, weak adhesion

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between NCD layer and substrate also aggravates film delamination. Whereas, for MNMN-CD films, the bottom MCD layer ensures coating adhesion and the alternate

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layer structure plays a positive influence on improving film wearing resistance.

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Compared with the monolayer ones, the tool life MNMN-CD coated mills increase by 3～7.5 times.

Diamond

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Conclusion films

consisting

of

alternate

microcrystalline

(MCD)

and

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nanocrystalline diamond (NCD) layers are fabricated on WC-6% Co substrates by HFCVD method, including monolayer MCD and NCD films, bilayer MNCD and

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NMCD films, and multilayer MNMN-CD and NMNM-CD films. All these films are coated with the thickness in the sake of excluding thickness effects on film performances. Friction and milling test are conducted and the following conclusions could be drawn: Diamond films with surface coating of NCD layer exhibit lower friction coefficients than those with surface coating of MCD layer. Debris adhesion and chipping insertion on the NCD ones smoothen frictional interfaces and help decrease

ACCEPTED MANUSCRIPT friction coefficients effectively. The MNMN-CD film exhibits a local slight-upward trend in the friction coefficient evolution because of its distinct nano-clusters worn-away phenomenon. Milling test results show distinct differences between working life of these

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diamond coated milling tools. Film delamination appears on the monolayer MCD and

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NCD coated mills just after a few cutting passes. The abrasive wear behaviors of

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zirconia ceramic with high hardness are regarded as the main reasons for causing rapid wear of the two monolayer diamond films. Similarly, large-area film peeling-off also

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occurs in bilayer (NMCD, MNCD) and multilayer (NMNM-CD) diamond coated tools.

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Differently, the MNMN-CD film shows high wear resistance and good adhesion to substrates, exhibiting working life of 3～7.5 times as long as the monolayer diamond

Acknowledgement

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coated tools.

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This research is supported by the National Natural Science Foundation of China

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(No. 51275302, No. 51370511).

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against Si3N4, Appl. Surf. Sci., 369 (2016) 448-459.

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SiC-based MCD films synthesized using different carbon sources when sliding

ACCEPTED MANUSCRIPT Table 1: Deposition parameters for MCD and NCD layers. Diamond type

MCD

nucleation

NCD

growth

nucleation

growth

2.8

2.5

2.8

2.8

Hydrogen flow rate (sccm)

240

240

240

240

Total pressure (Torr)

12

12

2000±10

2000±10

Substrate temperature (℃)

800±20

800±20

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2000±10

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Filament temperature (℃)

10

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30

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Carbon source content (%)

870±20

2000±10

870±20

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Table 2: Deposition time of each diamond layer during deposition. Deposition time (h)

MCD

—

6

MNCD

5

3

NMCD

5

3

MNMN-CD

2.5

1.5

NMNM-CD

2.5

1.5

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NCD

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—

10

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MCD

NCD

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Diamond film

ACCEPTED MANUSCRIPT List of figure captions: Fig. 1. Schematic diagram of the as-fabricated diamond films. Fig. 2. Surface morphology of the diamond coated WC-Co substrates. Fig. 3. Average surface roughness of diamond films.

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Fig. 4. Cross-sectional micrographs of diamond coated WC-Co substrates.

Raman spectra of (a) all diamond films, (b) the monolayer NCD film and (c)

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Fig. 6.

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Fig. 5. Cross-sectional micrographs of diamond coated mills.

Raman peak identification of the NCD film.

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Fig. 7. Friction coefficient curves of diamond films sliding against ZrO2 ceramic

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balls under ambient air.

Fig. 8. Debris distribution on surface of the (a, b) MCD, (c) NMCD, (d) NMNM-CD,

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(e) NCD, (c) MNCD, (c) MNMN-CD films during friction test.

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Fig. 9. Friction coefficient curves of multilayer MNMN-CD and NMNM-CD films, and worn surface of (a) counterpart ball, (b) MNMN-CD film.

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Fig. 10. Average-maximum friction coefficient of different diamond films and wear

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volume of counterpart balls. Fig. 11. Flank wear evolution curves of diamond coated mills during cutting process. Fig. 12. Images (magnification ×80) of flank wear on monolayer (a～c) MCD; (d～e) NCD; bilayer (f～h) MNCD; (i～k) NMCD; multilayer (l～n) MNMN-CD; (o～q) NMNM-CD after different milling pass (The machining time is marked on the picture).

ACCEPTED MANUSCRIPT Highlights: Diamond films with distinct multilayer structures are fabricated. Multilayer diamond films are consisted of alternate MCD and NCD layers. Multilayer diamond films exhibit good friction properties and low COF values.

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Multilayer diamond films could improve the life of mills greatly.

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Multilayer diamond coated mills show the best potential for zirconia machining.

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