process investigation on meso-scale hard milling of zro2 by diamond coated tools

Accepted Manuscript

Title: Process investigation on meso-scale hard milling ofZrO2 by diamond coated tools

Author: Rong. Bian Eleonora. Ferraris Ning.He Dominiek.Reynaerts

PII: S0141-6359(13)00133-5DOI: http://dx.doi.org/doi:10.1016/j.precisioneng.2013.07.007Reference: PRE 6032

To appear in: Precision Engineering

Received date: 28-3-2012Revised date: 19-7-2013Accepted date: 26-7-2013

Please cite this article as: Bian Rg, Ferraris Ea, Reynaerts Dk, Process investigationon meso-scale hard milling of ZrO2 by diamond coated tools, Precision Engineering(2013), http://dx.doi.org/10.1016/j.precisioneng.2013.07.007

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http://dx.doi.org/doi:10.1016/j.precisioneng.2013.07.007

http://dx.doi.org/10.1016/j.precisioneng.2013.07.007

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Process investigation on meso-scale hard milling of ZrO2 by diamond coated tools

Rong. Biana,b, Eleonora. Ferrarisb, Ning.Hea, Dominiek. Reynaertsb

a College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics & Astronautics, 210016 Nanjing, China

Email: [email protected]

Phone: +86 25 8489 6040

Fax: +86 25 8489 1501 b Department of Mechanical Engineering, KU Leuven, 3001 Heverlee, Belgium

Abstract:

This paper investigates the feasible machining of zirconium oxide (ZrO2) ceramics, in the hard

state, via milling by diamond coated miniature tools (from here on briefly indicated as meso-scale

hard milling). The workpiece material is a fully sintered yttria stabilized tetragonal zirconia

polycrystalline ceramic (Y-TZP). Diamond coated WC mills, 2 mm in diameter, four flutes and

large corner radius (0.5 mm) are chosen as cutting tools, and experiments are conducted on a state-

of-the-art micro milling machine centre. The influence of cutting parameters, including axial depth

of cut (ap) and feed per tooth (fz), on the achievable surface quality is studied by means of a one-

factor variation experimental design. Further tests are also conducted to monitor the process

performance, including surface roughness, tool wear and machining accuracy, over the machining

time. Mirror quality surfaces, with average surface roughness Ra below 80 nm, are obtained on the

machined samples; the SEM observations of the surface topography reveal a prevailing ductile

cutting appearance. Tool wear initiates with delamination of the diamond coating and progresses

with the wear of the WC substrate, with significant effect on the cutting process and its

performance. Main applications of this research include three dimensional surface micro

structuring and superior surface finishing.

Keywords: milling, ZrO2, ceramic, diamond coated tools, surface quality, tool wear

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1. Introduction

Zirconium oxide (ZrO2) is a ceramic material currently used in a broad range of industrial

applications. Unique properties, such as high fracture toughness, chemical and wear resistance,

low heat conductivity, biocompatibility, and high ionic conductivity make it very suitable for

advanced applications and some miniature and precision components, such as pump impellers for

turbo-machinery, diesel injection micro nozzles, micro-fluidic devices, micro-moulds, oxygen

sensors for foundry industry, dental and orthopaedic implants, etc..

At present, manufacturing these components from ZrO2 (as ceramics in general) is still a

difficult task. The entire process chain from powder preparation to sintering is a labour intensive

procedure. Besides, the application of traditional abrasive-based processes, such as grinding

lapping and polishing, is needed to achieve tight tolerances and a high surface quality on the final

component. These processes are well known for being very time consuming, limited in flexibility,

costly, and traditionally reported to may induce surface/subsurface damages into the machined

parts [1]. Damage-free machining of brittle materials has been studied by several authors in recent

years. Shimada et al. [2] claimed that any material, regardless of its hardness, can be machined in

a ductile mode by adjusting the depth of cut to a sufficiently small value. In particular, based on

molecular dynamic analyses, they found that the mode of material removal can be switched either

into ductile or brittle mode, regardless of the material, because of the dependence of the critical

tensile strength to the size effect. Bifano et al. [3] experimentally formulated the critical depth of

cut for plunging grinding as a function of material properties for several ceramics. Beltrão et al.

[4] successfully achieved ductile mode machining of different kinds of commercial PZT ceramic

samples. Zhong [5] reported ductile or partial ductile mode grinding of some brittle materials

including ZrO2, and found ductile streaks on the machined surface. Furthermore, Gao [6]

discussed ductile grinding of nano-engineered ZrO2 ceramics and found that with the assistant of

ultrasonic vibration, the actual critical grinding depth can be enlarged about 30%. Yan [7] also

found the ground surface quality and the life of diamond wheel can be improved when grinding

nano- ZrO2.with the assistant of ultrasonic vibration. The literatures show the effort done on

ductile grinding of ZrO2 ceramics.

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On the other hand, the practical implementation, within an industrial environment, of

extremely small cutting parameters could be difficult because of the given accuracy of

conventional machines. In that respect, micromachining and high precision cutting processes, as

primary users of extremely advanced and ultra-precision equipment, could function as a novel

method for fabricating unique features in brittle materials, which are not achievable by traditional

abrasive based technologies [8].

Alternative processes to machine ceramics have been also studied in the last decades. For

example, Ferraris et al. [9, 10] deeply investigated the machining behaviour of many electrically

conductive ceramics, including Al2O3-, ZrO2- and Si3Ni4-based ceramic composites via Electrical

Discharge Machining (EDM),and also demonstrated the feasible manufacturing of complex

shaped and micro features. Nevertheless, the approach suitability is mostly restricted to

electrically conductive ceramics, such as carbides and composites. Furthermore, the machined

surface presents thermal induced micro cracks, which may cause adverse effect on the operational

performance of some precision components.

The recent entry into the market of new advanced cutting tools, e.g. cubic boron nitride (CBN),

mono crystalline diamond (MCD), poly crystalline diamond (PCD) and chemical vapour

deposition (CVD) diamond coated tools, also opened new possibilities in traditional cutting. Yan

et al.[11], for instance, analysed the effect of rake angle and feed in hard turning silicon carbide

ceramics by MCD cutting plates with large nose radius. Wang et al. [12] employed polycrystalline

CBN tools for turning Si3N4 hard ceramics when cooling with liquid nitrogen. Ferraris et al.

investigated the feasibility of turning zirconia ceramics in the hard state by means of PCD inserts

[13]. However, these investigations are still on an initial phase, besides no exhaustive data have

been delivered so far on ZrO2 ceramics according to the author’s knowledge.

This work specifically aims at investigating the feasible machining of zirconium oxide, in the

hard state, via milling with diamond coated miniature tools, as an alternative technology to

standard abrasive based processes for flexible and high precision machining of ceramics. The

technological approach will later on shortly be indicated as meso-scale hard milling. As directly

scaled down from the macroscopic process, micro/meso-scale milling features relatively high

material removal rates and high machining flexibility; it is compatible with various engineering

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materials, such as polymers, metals, metal alloys, and pre-sintered ceramics [14, 15]; it thus

proves to be a competitive fabrication technology to manufacture miniature parts and micro

features[16]. To study this original research topic, diamond coated WC miniature mills, of stiff

geometry, are employed as cutting tools to withstand the abrasive effect of the workpiece material,

the eventually high cutting forces and to prevent tool breakage. Diamond coating was selected

because of the superior properties such as extreme hardness (80-100GPa) of diamond and the

proven performance in many leading edge cutting fields, including machining of graphite, carbon

fibre composites and aluminium alloys in dry conditions [17-19]. The investigation involves small

scale machining to obtain ductile material removal; accordingly, experiments are conducted on a

high precision machine centre. The influences of cutting parameters and milling time on

machining performance, including surface quality, tool wear and machining accuracy are studied.

Finally, two appealing examples are manufactured, and conclusions are drawn.

2. Experimental set-up

2.1 Workpiece Material

The workpiece material employed in this work is an yttria-stabilized tetragonal zirconia (Y-

TZP) with a 2 wt. % of Al2O3, manufactured by the Department of Metallurgy and Materials

Engineering of KU Leuven. The starting powder mixture (TM2 grade) contains yttria-free

monoclinic ZrO2 powder (Tosoh grade TZ-O) and yttria co-precipitated ZrO2 particles (Tosoh

grade TZ-3Y), mixed in such a ratio that the overall content of Y2O3 stabiliser is 2 mol%; the

material is prepared by hot pressing at a pressure of 28 MPa and at a temperature of 1450°C for 1

hour. The material provides high fracture toughness and modest hardness, the mean grain size is

about 0.4 μm. Table 1 summarizes some relevant properties of the ZrO2 ceramic workpiece used

during the investigations.

Table 1 Characteristics of the ZrO2 ceramic workpiece used during investigation

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2.2 Manufacturing equipment and cutting tools

The experiments were conducted on a high-precision micro milling machine centre (KERN

MMP 2522, KERN Micround Feinwerktechnik GmbH & Co. KG; Germany). This machine tool is

specifically designed for micro milling applications; it is constructed with a C-frame and a

polymer concrete monobloc for higher accuracy and stability; the machine is capable of achieving

positional and repetition accuracy within ±1 μm, and it is equipped with an infrared touching

probe for part alignment and a BLUM laser scanning device for tool measurement and geometrical

compensation. The tool spindle can rotate up to 40000 rpm.

The tools used are commercial available WC miniature mills, specifically designed for

machining hard materials (see Fig. 1a), and successively diamond coated on requested via CVD

by an industrial partner. The diamond coating average surface roughness was measured to be

about 237nm Sa by a white-light-interferometer profiling system, Wyko NT3300. The tools have a

tool diameter of 2 mm, a corner radius (rε) of 0.5 mm and four flutes; the helix angle is 52°. The

measured rake angle (α) and relief angle (γ) are about 0° and 5°, respectively; hence, providing a

robust cutting edge geometry to eventually withstand high forces. A large corner radius will also

allow higher feed rate (increased efficiency), while maintaining a low surface roughness of the

machined component. SEM inspection of the cross sections of the cutting edge reveal an uneven

diamond coating with a thickness about 4.5±0.5 μm, and a cutting edge radius, rβ, of 7.5±0.5 μm,

as shown in Fig. 1b. Table 2 summarizes the main geometrical characteristics of the diamond

coated WC mills used during investigation.

Table 2 Geometry of the diamond coated WC mills used during investigation

Fig. 1(a) SEM picture of a diamond coated WC miniature mill used during investigation; (b) cross section of the cutting edge in the orthogonal plane

2.3 Investigation methodology

Two experimental campaigns were carried out to investigate the machining performance of

hard milling ceramics. Firstly, a set of grooves was milled, as sketched in Fig. 2a, in order to

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investigate the achievable surface quality using different cutting parameters, including the feed per

tooth, fz, and axial depth of cut, ap. Experimental conditions were determined based on a one-

factor variation methodology, and the cutting parameters were varied as listed in Table 3. The

maximum uncut chip thickness, hmax, was also estimated for each experiment; given the condition

22z p pf r a aε< − , it can be calculated as a function of the tool corner radius and cutting

parameters, according to formula (1) [20] (see Fig. 2b):

2 2 2max 2 2z z p ph r r f f r a aε ε ε= − + − − (1)

Secondly, new grooves were milled consecutively with a single tool and for a given combination

of parameters (specifically, test No. 2: fz = 2.6 μm and ap = 5 μm, in middle level of parameters), to

evaluate the overall process performance, including tool wear, surface roughness and accuracy, as

a function of the machining time. For lack of experiential data on milling ZrO2 with diamond

coated tools for reference, a total cutting length of about 2m was used as qualitative tool life

criteria, and eleven grooves machined at different cutting time were kept for investigation as listed

in Table 4. Each groove is about 10 mm long and several cutting layers deep, for an expected

width of about 1 mm.

Table 3 Experimental design matrix selected for process investigation

Table 4 Characteristics of the consecutively machined grooves for process performance monitoring (cutting parameters as: n=30000rpm, fz = 2.6 μm and ap = 5 μm)

Fig. 2 (a) Sketch of the experiments executed for process investigation; (b) sketch of the cutting zone in the tool reference plane (cross section A-A)

As shown in Table 3, a small machining scale (hmax smaller than 1 μm) was chosen in the

present study in order to enhance ductile material removal and surface/sub-surface integrity. The

resulting maximum uncut chip thicknesses is much smaller than the measured tool cutting edge

radius, rβ (7.5±0.5 μm); as a consequence, the effective rake angle would have been negative

during machining; it was estimated about -60° to -80° . Those process conditions are

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traditionally reported to be beneficial for machining hard materials. They promote the formation

of large compressive stress into the chip formation zone and reduce the trailing tensile stresses,

thus obstructing crack propagation of pre-existing of induced flaws and defects, while enhancing

the plastic deformation of the undergoing material and therefore, a ductile chip formation [21-23].

The spindle rotation speed (n) was maintained constant at 30000 rpm throughout all the

experiments, resulting into an average cutting speed around 105 m/min for an effective working

diameter within 1.06 and 1.18 mm when increasing from the smallest to the largest depth of cut.

Before the experiments, the workpiece plate was ground once clamped in the fixture to ensure

optimal surface alignment on the machine, and the ground surface roughness is about 260nm Ra.

All the milling tests were conducted in wet condition using a water based emulsion (7% Blazocut

BC35, with the addition of 0.1~0.15 % antisept Swisscare to prevent formation of bacteria and

algae) to remove chips and debris.

The surface roughness (Ra) of the machined surfaces was measured along the feed direction by

using a contact type Talysurf 120L surface profiler, of stylus radius about 2~3 μm and nanometre

resolution. The same tool was employed to acquire the cross-section profile of the milled grooves,

orthogonally to the tool feed direction. The surface topography and tool wear characteristics were

examined by means of a ZEISS SteREO Discovery.V20 microscope and a scanning electron

microscope FEI XL40 SEM FEG. All the samples were ultrasonically cleaned and dried with

pressurized air before analysis.

3. Experimental results

3.1 Surface quality

3.1.1 Surface roughness

Fig. 3 shows the average surface roughness Ra as a function of the feed per tooth, fz, and the

axial depth of cut, ap, respectively. As shown an increasing of the Ra value is observed with the

rising of both cutting parameters. The best obtained surface roughness is down to 27nm Ra. Even

under the harshest cutting condition: fz = 5.2 μm, ap = 9 μm, the surface roughness obtained is

about 68nm Ra, which still satisfies the requirements of many micro components, including

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micro-optics. These results indicate that mirror surface quality is achievable on ZrO2 ceramics via

hard milling by diamond coated tools when using certain small feed per tooth and axial depth of

cut.

Fig. 3 Surface roughness response of ZrO2 fully hard samples machined by diamond coated mills as a function of (a) the feed per tooth, fz, and (b) axial depth of cut, ap

Furthermore, Fig. 4 plots the evolution of surface roughness (Ra) over the machining time, t

(min); the experimental points are obtained by machining a series of consecutive grooves for a

given combination of cutting parameters (fz = 2.6 μm, ap = 5 μm and vc = 105 m/min) and tools, as

mentioned in Sect. 2. As shown, Ra tends to increase with the milling time while remaining within

the same variation range of previous experiments (< 80 nm). Looking at the single data, the

average Ra response is around 30 nm during the first 3 minutes of machining; an eventual peak (~

40 nm of Ra) is registered within the first minute. Afterwards, the surface roughness increases to

about 65 nm; then, it fluctuates around 55 nm. Experiments were interrupted when achieved a

cumulative cutting length of about 2 m.

Fig. 4 Plot of the surface roughness response over the machining time of ZrO2 fully hard samples machined by diamond coated mills

3.1.2 Surface topography

For each experimental cutting condition, Fig. 5 shows an optical image of the surface

topography and the corresponding profile, acquired via Talysurf along the tool feed direction. The

images reveal, at first view, a prevailing ductile cut surface appearance with clearly visible feed

marks, evenly distributed over the surface; the profile reveals a regular and periodic saw-tooth

shape, of increasing peak-to-valley distance with the rising of the maximum uncut chip thickness

(hmax) and period corresponding to the tool feed per rotation, i.e. four times the feed per tooth, fz.

As an example, the distance measured in Fig. 5b between two main saw-tooth peaks is about 21

μm, i.e. 4 times fz=5.2μm. Secondary signal components (harmonics), corresponding to lighter

marks between two main saw-tooth peaks on the surface, are also visible along the profile.

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Besides, feed marks become more remarkable with the increasing of the depth of cut. The result is

most likely due to inaccuracies, such as tool geometric errors or axial misalignment, resulting in

one flute entering deeper into the material than the others and generating a clearer mark every

rotation, especially with the increasing of the depth of cut. Hence, the stylus tip radius and z-axis

resolution of the touching instruments also play a significant role in this context, since it can filter

relevant information when the features become too small to be detected by the instrument.

The same considerations could be made from the inspection of the machined surface via SEM.

As an example, Fig. 6a shows a SEM picture of the surface topography of test No.5 (fz = 5.2 μm,

ap= 9 μm and vc=~ 100 m/min) which highlights smooth areas (1) spaced by regular cuts (2), and

indicating a ductile material removal mode. No surface damages could be observed even at higher

magnification (see Fig. 6b). The appearance of the feed marks is uneven; their distance

corresponds approximately to the feed per tooth fz. From Fig. 6b, it also can be seen that the

material around the cutting edge is subjected to sufficiently high pressure to cause the material to

flow to the side. The phenomenon also proved that the material was removed in ductile mode. As

for comparison, Fig. 6c shows the SEM image of the original surface by fine grinding, from which

some uneven ductile streaks and micro particles can be seen.

Fig. 5 Optical images of the surface topographies and profiles of fully sintered ZrO2 samples milled by diamond coated tools at different cutting conditions. (Z axis profile enlarged the scale for visual purpose).

Fig. 6 SEM picture of a typical surface topography of the machined surfaces: 1- smooth areas, 2- feed marks; (b) enlarged view of the 3- plastic side flow (cutting parameters: fz = 5.2 μm, ap= 9 μm and vc=~ 100 m/min); (c) original sample surface after grinding

3.2 Machining accuracy

Fig. 7 plots the width of the bottom of the grooves (dw) for a set of grooves consecutively

machined by means of a single tool and using the cutting conditions: fz= 2.6 μm, ap= 5 μm and vc

= 100m/min; the expected value is about 1 mm, since only the bottom of the tool is used for

machining. The figure also shows the profiles of the cross section and the surface topographies of

some typical samples, specifically, taken after about 1, 3 and 7 minutes of machining.

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As shown, the measured width of the machined grooves gradually reduces with increasing

machining time, from 906 μm after 1min milling to 823 μm after 3mins and finally to 672 μm

after 7mins. The cross section measurements reveal a step profile which largely deviates (with 5

μm higher) from the ideal shape after about 3 minutes of machining, and this becomes more

outspoken as the machining time increases. The graphs give an indication regarding the achievable

machining accuracy in hard milling of ZrO2 ceramic via diamond coated tools for micro scale

applications. The results also indicate the traces of tool wear.

Fig. 7 (a) Plot of the measured groove widths over the milling time; (b) surface topographies and the cross section profiles of characteristics samples at different milling time:1, 3 and 7mins; cutting parameters: fz = 2.6 μm, ap = 5 μm and vc= 105m/min; Z axis profile enlarged the scale for visual purpose

3.3 Tool wear characteristics

Due to the uncut chip thickness is much smaller than the cutting edge radius, the main contact

area between the cutting edge and workpiece is on the bottom of the edge arc where near the flank

face. So, main analysis was focused on flank wear. On the other hand, the delamination of

diamond coating always happened suddenly, and the dimension of fracture area was affected by

the crack propagation/parth or inner flaws of the brittle coating. Fig. 8 shows images of the tool

flank face acquired after about 1, 3 and 7 minutes of machining of consecutive grooves, under the

cutting conditions of: fz= 2.6 μm, ap= 5 μm and vc =105m/min. Early damage of the cutting edge is

observed after about 1 minute of machining (Fig. 8b). A closer inspection reveals a slight

delamination of the diamond coating, exposing a portion of the WC substrate (see Fig. 9a). The

coating on the cutting edge arc is normally a bit weaker due to stress concentration there[24].

Furthermore, since the milling process is interrupted, the cutting edges were undertaken high

frequency interrupted cutting force. Under the period crashes, the coating is easier to chip off. The

exposed cutting edge appears intact without any traces of a tool-workpiece contact; on the

contrary, abrasion marks are clearly visible along the fracture edge of the surrounding diamond

coating (Fig. 9b). With increasing machining time, the coating delamination increases, leaving a

larger portion of the tool blank; here, slight wear of the exposed cutting edge is also visible (Fig.

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8c). The phenomenon becomes severe, and part of the exposed WC tool is completely worn out

after 7 min (Fig. 8d). From a closer inspection of the tool flank (Fig. 10), three main areas are

distinguishable on the worn WC substrate: 1) an area where the substrate WC has gone away due

to the serious attrition wear; 2) a very smooth and burnished area as result of prolonged contact

with the workpiece and high friction; 3) a normal topography without any contact with the

workpiece after the diamond coating delamination.

Fig. 8 (a) Optical picture of the bottom of a new tool; status of tool wear flank documented via optical microscope after about 1 (b), 3 (c) and 7 (d) minutes of machining indicating progressive exposing of the underlying tool blank due to fracture of the diamond coating; ①-average flank ware of substrate WC, ②- average delamination of diamond coating

Fig. 9 (a) SEM picture of the status of the cutting edge after about 1 minute of machining revealing a small portion of the underlying WC substrate due to fracture of the diamond coating; (b) view of the fracture edge of the diamond coating

Fig. 10 (a) SEM picture of a diamond coated WC mill after use in ceramic milling; (b) enlarged view of the flank face of a worn cutting edge after coating delamination, including: 1- attrition wear area, 2- smooth and burnished area, 3-untouched area

4. Discussion

It was shown that mirror surface quality is achievable on ZrO2 samples in meso-scale

machining. The material removal mode, in the process window investigated, is prevailing ductile

and the machined surface shows smooth areas delimited by clear-cuts into the material. The

results are in agreement with the theory claiming that any material, regardless its brittleness, can

be machined in ductile mode when properly adjusting the cutting parameters. Theoretical and

experimental studies have been conducted in the recent years to define critical cutting conditions

for certain materials [8]. Among them, Bifano et al. formulated and experimentally verified for

many ceramics, the critical depth of cut in plunging grinding, dc, as a function of the material

properties, according to the formula (2) [3]:

2

0.15 ICc

KEdH H

⎛ ⎞⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠ (2)

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where KIc is the fracture toughness, H is the hardness, and E is the Young’s modulus of the

material. For the given material, the calculated critical value is estimated to about 2.6 μm (much

larger than the maximum uncut chip thickness (hmax) used during experiments). However, that

estimated value does not take into account the specific material characteristics such as

transformation toughening which is reported may enhance the strength and fracture toughness of

ZrO2 [25, 26]. Besides the correction factor of the formula is specifically calculated for plunging

grinding operation, and tailored experiments should be conducted for the given process.

A change in the surface roughness, due to rising of the depth of cut, could also be seen. The

observation is in contrast with the traditional assumption of the only effect of, fz, on surface

quality in ductile removal. Noise factors, such as set-up inaccuracies, tool misalignment, and

coating unevenness etc., of remarkable importance here due to the small machining scale, can be

called in question as explanation for the gap between theory and practise.

Rapid tool wear was registered during the experiments. Based on the data collected, the tool

wear progresses along three main stages: i) early coating delamination; ii) extended coating

delamination with slight wear of the exposed WC cutting edge; and iii) severe wear of the tool

blank. In the experiments described in this paper, the entire process evolves in the order of

minutes. The concept is schematically illustrated in Fig. 11. First, a small coating fracture takes

place at the cutting edge, i.e. where the coating is weaker [27] at the beginning of the process. As

machining continues, the coating damage expands until reaching such a level that the underlying

substrate gets into contact with the workpiece; then, due to the inferior abrasive resistance of WC

as compared to ZrO2, the exposed cutter rapidly wears out.

Fig. 11 Schematic illustration of progressive tool wear, in the orthogonal plane to the cutting edge, of diamond coated mills after machining ZrO2 ceramic: (a) new cutter; (b) early coating delamination, (c) progressive coating delamination and slight wear of the uncoated WC cutting edge; (d) severe wear of the tool blank, including indicative elapsing time

Due to the small uncut chip thickness, the tool wear significantly influences the cutting process

evolution. Considering that the uncut chip thickness (hmax< 1μm) is small as compared to the tool

coating thickness (4~5μm), once a first portion of coating detaches from the substrate, the cutting

process is most likely undertaken by the fractured coating, as schematically illustrated in Fig. 12.

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This is illustrated by the fact that abrasion marks were observed along the coating fracture edge

after about 1 minute of machining, while the exposed WC remained intact (Fig. 9b). The “new

diamond cutter”, on the other hand, is of unpredictable geometry, thus introducing, in practise, an

uncontrollable factor to the cutting process. Similarly, the cutting edge geometry and local shape

of the tool rapidly change with the progressive wear of the underlying substrate; as shown in Fig.

11 of previous section, part of the tool blank has completely worn out after about 7 minutes.

Obviously, the tool wear process also influences the machining performance. As shown in Fig.

4, the surface roughness tends to increase with the milling time; local peaks are eventually

registered in correspondence of a stage transition, e.g. early coating damage (within the first

minute), and initial WC wear (at about minute 3). During that period, the achieved surface

roughness (Ra) is stable at a low value. It is reasonable to say that this may be the result of the

cutting process using the sharper fractured coating edge. Groove bottoms also become thinner as a

result of the decreasing of the effective milling diameter.

Fig. 12 Schematic illustration of the cutting process evolution under the action of (a) fractured coating; (b) uncoated cutting edge

5. Applications

A three-dimensional micro feature was fabricated as an example of the application capabilities

of the process in specific fields, such as surface micro structuring. The feature has a triangular

cross-section, with a side of 200 μm on the top surface, and 800 μm high. Machining conditions

were as previously tested: fz=2.6 μm, ap=5 μm and vc=100 m/min, for a material removal rate of

about 2.7 mm3 per minute. Fig. 13a and b show SEM pictures of the feature obtained; the realized

feature reveals a very good surface quality, but also little damages along the edges. Additionally, a

50 mm2 surface (see Fig. 13c) was milled with the same machining conditions to further validate

the potential of the process in surface finishing applications. The result is a mirror quality surface,

in which the stylus of the profiler distinctly reflects itself; the average surface roughness (Ra)

measured is around 20 nm along the feed direction.

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Fig. 13 Potential applications of the process and demonstration examples: (a) three-dimensional micro feature and (b) closed up view; (c) image of a realized mirror-like surface, 1-surface machined by milling, 2- original surface after grinding

6. Conclusion

This work investigates achievable performance in hard milling of ZrO2 ceramics via diamond

coated miniature tools, as alternative technology, to current standard abrasive ceramic processing,

for flexible and high precision machining of ceramics. According to an industrial/application

oriented approach, commercial available WC end-mills, with cutting geometry specifically

designed for hard machining applications, are here selected for investigation and coated with

diamond on request. Micro scale machining is deliberately chosen to enhance ductile material

removal because of the importance covered by surface integrity in ceramic performance. Based on

the results obtained, within the process window investigated (hmax < 1 μm), the following

performance can be stated:

A small mirror-like surface (5mm×10mm), of Ra down to 20nm in the tool feed

direction, is achievable on ZrO2 samples via hard milling by diamond coated tools.

SEM inspection of the machined surfaces reveal a prevailing ductile material removal

mechanism, characterized by smooth areas, spaced by clearly visible tool feed marks.

Tool wear evolves rapidly, in the order of minutes, and it is mostly related to the integrity

of the diamond coating. Three main stages of wear mechanism could be recognized: i)

early coating delamination; ii) extended coating delamination with slight wear of the

exposed cutting edge; and iii) severe wear of the tool blank; due to the small uncut chip

thickness, the tool wear evolution significantly influences the cutting process

performance.

The work offers new potential perspectives for diamond milling applications, such as three

dimensional micro structuring and ceramic surface finishing. It also points out capabilities and

limitations of current diamond tooling technological solutions, delivering first experimental

evidence on performance of diamond coated tools in ceramic machining and related tool wear

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mechanisms. Coating delamination is here regarded as the most limiting factor at the moment.

Industrial contacts with tool and coating manufacturers have been established to further

investigate the tool wear process, and new experiments are currently in progress. Besides, further

research works, such as 1 single point diamond experiments and/or investigation on ductile-brittle

transition material removal mechanism, should be considered as future necessary steps for a

fundamental understanding of the process.

Acknowledgment

This research was performed at the division of Production engineering, Machine design and

Automation (PMA) of the Department of Mechanical Engineering of KU Leuven. The authors

gratefully acknowledge the financial support of China Scholarship Council and National Nature

Science Foundation of China (Project NO.:50935003). Part of the work was also supported by the

project MiCMa3, Efficient Micromachining of Ceramic Materials in free formed, accurate and

reliable shapes, (IEF- 221709), funded by the FP7-Marie Curie Actions. The authors are also

thankful to Heleen Romanus for the measurement work and technical assistance.

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Table 5 Characteristics of the ZrO2 ceramic work piece used during investigation

Composition Y2O3 content (mol%) 2 Al2O3 content (wt%) 2 Physical and mechanical properties Density ρ (g/cm3) 6.02 Mean grain size s (μm) 0.4 Young’s modulus E (GPa) 223 Fracture Toughness KIC (Pam1/2) 11.1 ± 0.7 Hardness HV10 (kg/mm2) 1180 ± 13

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Table 6 Geometry of the diamond coated WC end mills used during investigation Diameter D (mm) 2 corner radius rε (mm) 0.5 flute number, z 4 helix angle, θ (°) 52 rake angle, α (°) 0 relief angle, γ (°) 5 cutting edge radius, rβ (μm) 7.5±0.5 coating thickness, tc (μm) 4.5±0.5

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Table 3 Experimental design matrix selected for process investigation Cutting parameters

No. n (rpm) fz (μm) ap (μm) hmax (μm) 1 30000 1.3 5 0.18 2 30000 2.6 5 0.36 3 30000 5.2 1 0.30 4 30000 5.2 5 0.71 5 30000 5.2 9 0.96

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Table 4 Characteristics of the consecutively machined grooves for process performance monitoring (cutting parameters: n=30000rpm, fz = 2.6 μm, ap = 5 μm)

Groove No. 1 2 3 4 5 6 7 8 9 10 11 No. of cutting

layers 7 13 15 15 15 17 15 31 28 34 28

Total depth (μm) 35 65 75 75 75 85 75 155 140 170 140 Cumulative cutting

length (mm) 70 200 350 500 650 820 970 1280 1560 1900 2180

Cumulative cutting time (min) 0.22 0.64 1.12 1.60 2.08 2.63 3.11 4.10 5.00 6.09 6.99

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(a)

(b)

Fig. 14(a) SEM picture of a diamond coated WC miniature mill used during investigation; (b) cross section of the cutting edge in the orthogonal plane

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Fig. 15 (a) Sketch of the experiments executed for process investigation; (b) sketch of the cutting zone in the tool reference plane (cross section A-A)

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(a) (b)

Fig. 16 Surface roughness of ZrO2 fully hard samples machined by diamond coated mills as a function of (a) the feed per tooth, fz, and (b) axial depth of cut, ap

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Fig. 17 Plot of the surface roughness response over the machining time of ZrO2 fully hard samples machined by diamond coated mills

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Fig. 18 Optical images of the surface topographies and profiles of fully sintered ZrO2 samples milled by diamond coated tools at different cutting conditions. (Z axis profile enlarged of a 20:1 scale for visual purpose).

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(a)

(b)

(c)

Fig. 19 (a) SEM picture of a typical machined surface after milling: 1- smooth areas, 2- feed marks; (b) enlarged view of the 3- plastic side flow (cutting parameters: fz = 5.2 μm, ap= 9 μm and vc=~ 100 m/min); (c) original sample surface after grinding

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(a)

(b)

Fig. 20 (a) Plot of the measured groove widths over the milling time; (b) surface topographies and the cross section profiles of characteristics samples at different milling time:1, 3 and 7mins; cutting parameters: fz = 2.6 μm, ap = 5 μm and vc= 105m/min; Z axis profile enlarged the scale for visual purpose

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Fig. 21 (a) Optical picture of the bottom of a new tool; status of tool wear flank documented via optical microscope after about 1 (b), 3 (c) and 7 (d) minutes of machining indicating progressive exposing of the underlying tool blank due to fracture of the diamond coating; ①-average flank ware of substrate WC, ②- average delamination of diamond coating

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Fig. 22 (a) SEM picture of the status of the cutting edge after about 1 minute of machining revealing a small portion of the underlying WC substrate due to fracture of the diamond coating; (b) view of the fracture edge of the diamond coating

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Fig. 23 (a) SEM picture of a diamond coated WC mill after use in ceramic milling; (b) enlarged view of the flank face of a worn cutting edge after coating delamination, including: 1- attrition worn area, 2- smooth and burnished area, 3-untouched area

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WC

diamond coating

rake

face

flank face

a b c d

Fig. 24 Schematic illustration of progressive tool wear, in the orthogonal plane to the cutting edge, of diamond coated mills after machining ZrO2 ceramic: (a) new cutter; (b) early coating delamination, (c) progressive coating delamination and slight wear of the uncoated WC cutting edge; (d) severe wear of the tool blank, including indicative elapsing time

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WC

diamond coating

rake

face

flank face

workpiece

h

vcWC

workpiece

h

vc

a b

Fig. 25 Schematic illustration of the cutting process evolution under the action of (a) fractured coating; (b) uncoated cutting edge

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Fig. 26 Potential applications of the process and demonstration examples: (a) three-dimensional micro feature and (b) closed up view; (c) image of a realized mirror-like surface, 1-surface machined by milling, 2- original surface after grinding

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Highlight

We report a first attempting on meso-scale milling of fully sintered ZrO2 ceramics by

diamond coated tools.

Mirror quality surface, with Ra down to 20 nm, on the sample is achieved.

Ductile material removal is the prevailing cutting mechanism.

Tool performance is related to the integrity of the diamond coating.

Due to the rapid wear, industrial application of the process is mainly restricted to

finishing operations and micro applications.

process investigation on meso-scale hard milling of zro2 by diamond coated tools

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