Fine abrasive water jet machining of piezoelectric

ceramics: cutting parameters optimization

M. Annoni, F. Arleo, A. Trolli Politecnico di Milano A. Suarez, A. Alberdi Tecnalia R&I

Piezoelectric ceramics are increasingly applied in optical, electronic, mechanical and

biomedical applications thanks to their inherent physical properties such as electrical

behavior, electromagnetic response, high temperature strength, hardness and corrosion

resistance. Nevertheless, this kind of materials is usually very difficult-to-machine by

conventional technologies, while its applications become more and more demanding in

terms of quality and precision. In such a scenario, the Fine Abrasive Water Jet (jet

diameter less than 0.5 mm) represents a very appealing and promising technology

compared to micro-milling, laser or EDM, offering many advantages such as the absence

of thermal distortions, high flexibility and versatility, small cutting forces and the

increasing capability to cut smaller and smaller features. In the present work, carried out

at Tecnalia R&I in collaboration with Politecnico di Milano, piezoelectric material lead

titanate zirconate (PZT) thin sheets machining is investigated with a DOE approach, in

order to optimize the FAWJ cutting parameters and to test its capability as a concrete

high-precision technology to machine ceramic materials. Finally, an application of PZT

as base material for micro positioning actuators is presented as a case study.

Keywords: fine abrasive water jet, piezoelectric ceramic, cutting optimization, design of

experiments

AWJ Abrasive Water Jet

DOE Design Of Experiments

Ex Modulus of elasticity (longitudinal)

Ez Modulus of elasticity (transversal)

FAWJ Fine Abrasive Water Jet (jet diameter less than 500 µm)

Gyz Modulus of elasticity in shear

HAZ Heat Affected Zone

MAWJ Micro Abrasive Water Jet (jet diameter less than 200 µm)

PZT Lead Zirconate Titanate

Ra Average roughness

ABSTRACT

1 NOMENCLATURE AND ACRONYMS

SEM Scanning Electron Microscope

Tc Curie temperature

WJ Water Jet

ρ Density

νxy Poisson’s ratio of piezoelectric material

νxz Poisson’s ratio of piezoelectric material along the direction of polarization

Advanced piezoelectric ceramics have been increasingly applied in the optical,

electronic, mechanical and biological industries due to their peculiar electromagnetical

response. Unfortunately, these materials are usually very difficult-to-cut, and this is a

great limitation to their further employment in more and more complex innovative

applications.

Cold-cutting and non-intrusiveness are peculiar features of AWJ that make it especially

well-suited for cutting unusual shapes out of exotic and heat-sensitive materials, such as

the piezoelectric ceramics. Particularly, this technology is very appealing because it is a

cold-cutting process, it does not create a heat-affected zone (HAZ), which in turn induce

thermal damage or localized loss of the piezoelectric behavior of the parent material.

Nevertheless, recent technological progresses often bring along a down-sizing of

products together with small tolerances and high precision: this pushes machining

technologies to continuously improve in order to reach higher and higher standards of

quality and precision on the final workpiece, along with the capability to machine smaller

and smaller complex features (1). There are actually great opportunities for low-cost

micro-manufacturing due to the recent advent of micro and nano components, MEMS

devices, green energy products and biomedical technologies: MAWJ could be a

profitable technology, taking advantage of the inherent merits of WJ cutting, namely

versatility (material independence), flexibility, non-intrusiveness and cost effectiveness.

With the aim to start studying these topics and go towards micro machining with AWJ,

the cutting head used during the experimentation has the peculiarity of having an internal

geometry equal to one half that of a standard system: the jet produced is then about

0.5 mm wide and the size of the used abrasive is #200 (particle dimension around

74 μm).

The dimension of the out flowing jet is actually slightly large for defining it strictly as a

MAWJ (jet diameter of around 0.2 mm or lower), but this is a first attempt to gradually

face the arising problems associated with reducing the size of the cutting head towards

MAWJ current standards.

The piezoelectric ceramics are particular ceramic materials which are characterized by a

close correlation between their electrical and mechanical behavior.

If a piezoelectric material is subjected to a mechanical strain, an electrical polarization

will appear within it (direct piezoelectric effect); similarly, if the material is exposed to

an external electric field, it reacts with a mechanical deformation (inverse piezoelectric

effect).

Therefore, these materials are used to convert electric energy into mechanical or vice-

versa and thanks to this unique property they are suitable to be employed both as

actuators or sensors.

2 INTRODUCTION

3 THE CASE STUDY: PIEZOELECTRIC ACTUATORS

Most of the piezo-ceramic materials used in mechanical applications have not a single-

crystal structure, but they have a polycrystalline structure formed by various grains.

In general, the grain polarization in a polycrystalline body is statistically distributed in all

directions: therefore, the global polarization is zero. However, by adopting ad hoc

techniques during the production process, it is possible to align the induced polarization

direction within each individual crystal, obtaining pieces with a uniform polarization (2).

Normally, these materials are divided into two main categories, namely soft and hard,

depending on their piezo-electric behavior and mechanical properties.

For the present work purposes, the only useful information is that significant changes in

their piezo-electric properties starts around 100°C while their Curie temperature is

relatively low, around 150°C.

Because of this reason, thermal processes such as laser or EDM machining are often not

suitable, while water jet represents a perfect alternative technology able to machine hard

piezoelectric materials without thermally or chemically altering the materials often used

in the field of micro applications.

There is a growing demand for actuators with a micro-scale design. This need has been

reported across the micro-robotics industry and the most recent medical applications.

Despite such different fields of use, the core characteristics required by millimeter and

sub-millimeter-scale actuators are basically the same: actuators at these scales are widely

used in smart structure applications due to their high bandwidth, high output force,

compact size, low response times, simple design and simple operating mechanism (3).

The present case study was initially promoted by a specific application regarding some

bending actuators and it was proposed by Università di Pisa.

These actuators exploit the resonant eigenmodes electrically induced in a piezo-electric

material to deform the main body in order to perform controlled movements (4). The

base material is a 0.5 mm thin sheet of PZT ceramic, while the final workpiece

dimensions and characteristics are reported respectively in Figure 1 and Table 1.

Due to the high required precision of the final assembly and the small dimensions of the

actuator, it is very important to obtain a good surface quality on the kerf walls and a

small or even absent HAZ: in fact, when a piezoelectric material approaches the Tc it

progressively loses its electro-mechanical properties until these are completely lost when

that critical temperature is overcome.

Figure 1: piezoelectric actuator geometry and dimensions (4)(Bertini).

3.1 Properties of the polycrystalline piezo-ceramics

3.2 The bending actuators

Table 1: main PZT ceramic properties (www.piezo.com)

ρ [kg/m3]

Ex [MPa]

Ez [MPa]

Gyz [MPa]

νxy [-]

νxz [-]

Tc

[°C]

7800 66∙103 52∙103 21∙103 0.35 0.38 350

Initially, the reference machining process considered for the presented application was

laser machining, since it is already used for industrial cases on similar materials. The

PZT belongs to the category of soft piezoelectric materials and it was initially chosen as a

target material due to its higher Tc (compared to hard piezoelectric material), with the aim

of reducing as much as possible the HAZ during laser machining process.

Later on, water jet machining is considered as a competitor technology in order to

minimize the HAZ, to machine higher thicknesses and to allow the machining of hard

piezoelectric materials which show changes in their electric-mechanical behavior even at

lower temperatures.

In order to investigate the cutting capability of the FAWJ on piezoelectric ceramics, a

feasibility study was carried out at Politecnico di Milano, as presented in Figure 2. Only

pressure and feed rate were varied in these experiments, both on 2 levels (respectively

200-300 MPa and 30-60 mm/min). The experiments were randomized on 3 replicates in

order to take into account variability and to minimize the effects of experimental error on

the analysis of the results.

Analyzing the roughness as output, the feed rate proved to be significant, while the

pressure did not. The best roughness value (Ra = 3.50 µm) was obtained with a feed rate

level of 60 mm/min.

These experiments showed that the FAWJ is an effective alternative for the machining of

such a part and so a deeper analysis was developed.

Figure 2: preliminary experiments setup in Politecnico di Milano.

With the aim to move towards fine machining with AWJ, a cutting head with the internal

geometry equal to the half of a standard system was mounted on a standard WJ plant:

0.15-0.18 mm orifice are employed together with a Ø0.5x76 mm focuser and

Garnet #200 abrasive.

Moreover, a customized abrasive feeding system and a specific fixturing system are

developed.

4 PRELIMINARY EXPERIMENTS

4.1 Fine AWJ preliminary experiments

5 EXPERIMENTAL EQUIPMENT

The implementation of a FAWJ system in a traditional water jet plant involves a new

hopper to improve the versatility of the machine by allowing the operator to switch from

the FAWJ system to the standard one more rapidly and avoiding losing time emptying

and cleaning the hopper. This self-made feeding system is characterized by a small

cylinder that contains the abrasive, a selector to manually switch on and off the abrasive

flow and a selector to regulate the abrasive flow rate, as shown in Figure 3 .

Figure 3: a scheme of the employed abrasive feeding system.

Since the mini-hopper is self-made, the abrasive feeding system requires a specific

calibration in order to determine the abrasive flow rate corresponding to the specific

calibrated hole in the selector base working at different process conditions. The abrasive

flow is mainly influenced by the gravity, the calibrated hole diameter through which the

abrasive flows and the suction caused by the Venturi effect of the jet inside the mixing

chamber depending on the upstream water pressure. To prevent a strong dependence

between the abrasive flow rate and the vacuum pressure inside the mixing chamber, a

connection with the outside ambient is made downstream the calibrated hole selector: in

this way, the system works at atmospheric pressure and the abrasive falls only by gravity,

uncoupling the average flow rate from upstream water pressure.

Anyway, fluctuations of the Venturi effect still can cause fluctuations of the abrasive

suction inside the mixing chamber. Because of this, the calibration is performed by

measuring the difference in the hopper weight after 30 s in real working conditions with

different calibrated holes and upstream pressures; the results are reported in Table 2.

It is necessary to demonstrate that the experimental conditions with the self-made mini

hopper keep constant: this essential condition is verified when working with a new

orifice every run. In fact, a signal of the depression in the abrasive feeding tube is

acquired in order to demonstrate that there is no relationship between the water pressure

level and the suction caused by the Venturi effect: as shown in Figure 4, the suction

capability with a new orifice is relatively low and pressure independent, while with a

worn one the suction power is higher and pressure dependent.

This behavior suggests that working with new orifices prevents water pressure to play a

significant role on the Venturi effect in the selected range; this fact allows abrasive flow

rate not to depend on water pressure.

5.1 The abrasive feeding system

5.2 Process monitoring and abrasive flow rate calibration

Figure 4: suction pressure signals at different upstream pressures and for different

orifice conditions (good and worn).

Finally, the abrasive flow rate calibration is performed by measuring the difference in the

hopper weight after 30 s in real working conditions with two different calibrated holes;

the results are reported in Table 2: the behavior shown in Figure 4 is confirmed by the

low variation of the results with respect to the water pressure variation with new orifices.

Table 2: abrasive flow rate calibration.

Calibrated

hole diameter

[mm]

Water jet

pressure

[MPa]

Measured

abrasive flow rate

[g/min]

Mean

[g/min]

Std dev

[g/min]

2 250 83.6

84.2 5.6 2 300 90.1

2 350 79.0

2.5 250 132.8

134.1 3.5 2.5 300 138.0

2.5 350 131.4

The fixturing system is quite simple but specifically customized for the target material.

In fact, the piezoelectric thin specimen is very brittle, so it needs to be properly fixed on

a rigid substrate in order to avoid shocks, vibrations or inflections which could be

dangerous for its integrity: as a final solution, it is fixed on a piece of hard polystyrene

with an adhesive tape, as shown in Figure 5; finally, some weights are put on the

polystyrene in order to avoid vibrations of the base. Due to the small applied forces on

the workpiece, this was found to be the best and easiest solution.

Figure 5: the fixturing system used during this experimentation.

5.3 The fixturing system

DOE is a very effective way to investigate the influence of a variable factor, as

demonstrated in (5)(6) and other scientific works. This approach is used as a statistical

method of analysis during the present work: the choice of the variable parameters for this

experimentation is made starting from the literature review (7) which points out that

orifice diameter, focus diameter, upstream pressure, feed rate, abrasive dimensions and

mass flow rate are the most influential parameters. In particular, feed rate seems to be the

most important influencing parameter for ceramic machining, while the influence of

focuser length has less importance, thus this parameter is not involved in the present

investigation.

The aim of the optimization experiments is to find the best parameters in terms of kerf

quality, particularly in terms of roughness, in order to verify if water jet is an appealing

technology in the machining of piezoelectric ceramics.

The DOE variable factors are feed rate, pressure and abrasive flow rate, while orifice

diameter is considered as a blocking factor1.

As always, the choice of the levels of each parameter is very important and must be done

carefully as it is presented below.

Some screening tests show that the flow rate is quite regular up to 150 g/min: this is due

to the physical limits of the FAWJ cutting head. Up to this level of abrasive flow rate,

there is no abrasive clogging in the mixing chamber. The lower level is set at the value of

80 g/min.

The upper level of feed rate is fixed at 110 mm/min after being determined by some

screening tests, showing that above this value, up to 190 mm/min, the kerf quality gets

worse and worse in terms of brittle chipping and irregularities on the bottom edge, as

shown in Figure 6. The lower value of feed rate is set to 30 mm/min to keep it in

accordance with the preliminary tests presented in Section 4.1. Additionally, the typical

orifices in FAWJ have a diameter of 0.15-0.18 mm and the Garnet#200 is characterized

by finer abrasive particles. The standoff distance is set at 1 mm.

Figure 6: kerf bottom quality at feed rates of (a) 190 mm/min and (b) 110 mm/min.

Table 3 shows a summary of both the variable factors and the constant parameters used

within this experimentation.

1 A blocking factor is a known and controllable nuisance source of variability whose effects can be

hidden by particular DOE techniques.

6 OPTIMIZATION EXPERIMENTS

Table 3: DOE parameters setup summary.

Variable factors Levels

Orifice [mm] 0.15 0.18

Abrasive flow rate [g/min] 80 140

Feed rate [mm/min] 30 70 110

Pressure [MPa] 250 300 350

Constant parameters

Abrasive type GMA garnet

Garnet mesh number #200

Focuser diameter [mm] 0.5

Standoff distance [mm] 1

The introduction of a block on the factor “orifice” is needed since the operation of

changing the orifice is usually tricky and can easily cause its damage: consequently, in

order to assure the homogeneity of the experimental conditions within all the

experimental runs, it is not possible to randomize this factor (6).

Setting to 4 the number of replicates, the resulting total number of experiments is 144

which are considered a good compromise between experimental effort and results

reliability, as it will be presented in the next Section 6.1 .

The Pearson–Hartley charts can be used in order to perform a statistical power analysis

and help in defining the experimental effort; briefly, using the sample size and the

desired effect size, it is possible to determine which is the statistical power of the carried

out analysis assuming that the effect size in the sample is equal to the effect size in the

population (6).

Generally, when the analysis aims to detect small effects a high power is required which

needs in turn several replicates and so a higher number of experimental runs. In the

present study, 4 replicates were performed, resulting in a statistical power of 41% if the

minimum detectable difference on roughness is set to 0.5 μm or equivalently 90% to

detect a difference of 1 μm. In conclusion, taking into account that the work is a

preliminary study to understand which process parameters are the most influencing and

whether they are affecting the kerf quality in such an application, the actual power seems

a good compromise to achieve enough precise and reliable results with a reasonable

experimental effort.

The quality of the machined surface is the result of four different results analysis,

developed starting from literature (8). Provided that the hypothesis of symmetric kerf is

verified as shown in Figure 7, the analyzed results are the top kerf width, the bottom kerf

width, the kerf taper and the roughness. An optical microscope is used for the analysis of

the geometrical kerf dimensions, while a profilometer is used for the roughness analysis.

6.1 Statistical power analysis

7 DOE RESULTS

Figure 7: the kerf symmetry.

The DOE results are summarized in Table 4 and Figure 9.

Table 4: significant factors summary table.

Orifice

diameter Pressure Feed rate

Abrasive

flow rate

Top kerf width ● ○ ● ●

Bottom kerf width ● ○ ● ●

Kerf taper ● ● ○ ●

Kerf roughness ● ○ ● ○

● : significant factor ○ : not significant factor

Since the thickness of the machined material is very low, the cut is performed mainly in

the “cutting wear region” where the jet still has high energy rather than the “deformation

wear region” (see Figure 8)(9): this could explain the fact that some parameters are non-

significant, even if they are usually influencing in a traditional AWJ cut such as the

pressure and the abrasive flow rate on the kerf roughness and the feed rate on the kerf

taper.

Figure 8: traditional kerf regions forming in an AWJ cut (9).

Moreover, the interactions among any of the parameters have no statistical relevance.

7.1 Statistical results

Figure 9: main effects plots.

Finally, Table 5 reports the values to be assigned to the studied factors in order to

optimize the different considered parameters listed on the left column; in case of non-

significant parameter, its value is chosen as the most economical or productive.

Table 5: optimized parameters summary table.

Orifice

diameter

[mm]

Pressure

[MPa]

Feed rate

[mm/min]

Abrasive

flow rate

[g/min]

Top kerf width 0.15 250 110 80

Bottom kerf width 0.15 250 110 80

Kerf taper 0.15 350 70 80

Kerf roughness 0.15 250 110 80

In order to more deeply understand the machining mechanism and the influence of the

process parameters on it, a further analysis of the kerf surface is carried out by means of a

SEM. The specimens have to be treated, because a SEM analysis is possible only with a

conductive material: the process of gold-plating allows analyzing this non-conductive

ceramic material. The Figure 10 shows that the structure is inhomogeneous, with the

typical appearance of the material produced by powder manufacturing: it is possible to

clearly distinguish the powder grains whose dimensions are around 2 μm.

8 CONSIDERATIONS ON THE MACHINING MECHANISM

Figure 10: kerf surface at an enlargement of 5000X.

The Figure 11 shows how the material removal mechanism is characterized by brittle

fractures among powder grains, but there are also traces of ductile fractures (10) (11). A

possible explanation for the presence of ductile behavior is that the interaction between

the material and the abrasive particles produces friction heat leading to very high local

temperatures, which in turn can cause thermal stresses and reactions which may promote

thermal wear processes and the transition from brittle to ductile behavior of the ceramic

(12) and (13).

Figure 11: traces of ductile fracture on the kerf surface.

Moreover, a common characteristic to all the specimens is the presence of embedded

abrasive particles in the ceramic matrix, as shown in Figure 12. The shape of the particle

is sharp and irregular and the dimension is smaller than the nominal dimension. It is not

possible to exactly determine the specific degree of fragmentation occurring at any

different stage of the process (i.e. during particle mixing and acceleration), even if the

main contribution is probably given by the crumbling during the cutting process since the

hardness of the piezoelectric material is comparable to the abrasive one.

Figure 12: embedded abrasive particle on the kerf surface.

The three main reasons that make FAWJ an appealing technology in the field of

piezoelectric ceramics are the extreme flexibility with short manufacturing time, the

relatively low costs and, above all, the reduction or even the absence of macro thermal

alterations even on relatively thick specimens.

The kerf quality is evaluated in terms of kerf width and roughness: on one hand smaller

kerf width (less than 1 mm) are important to minimize the scraps (piezoelectric is a

valuable and expensive material) and to allow small features machining; on the other

hand, the roughness is also very important, being strictly linked to the material fatigue

performance.

The applied DOE approach pointed out that the only significant factors are the orifice

diameter and the feed rate with their optimized levels respectively of 0.15 mm and

70 mm/min. Moreover, since a 0.5 mm-thick piezoelectric material does not require a

high cutting energy to be machined; the factors water pressure and abrasive flow rate turn

out not to be significant, so in order to reduce the mechanical stresses on the workpiece

and reduce costs, their best levels are respectively 250 MPa for pressure and 80 g/min for

abrasive flow rate (the lower values considered inside the experimentation).

This study has been carried out in the frame of collaboration between Dipartimento di

Meccanica – Politecnico di Milano and Tecnalia R&I. The authors are grateful to both

the institutions.

(1) T. Aklint et al., “Abrasive waterjet cutting for micro manufacturing”, 7th

International Conference on Multi-Material Micro Manufacture, Bourg en Bress

and Oyonnax (France), November 2010.

9 CONCLUSIONS

10 ACKNOWLEDGEMENTS

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