Ceramics are a class of materials widely used during last fifteen years for orthopaedic applications. It is well known that they are characterized by low wear rate, and friction coefficient. However, these materials are very difficult to machine into complex shapes because of their brittleness and high hardness. The most effective method to increase the crack resistance is the formation of a composite structure. This class of materials, composed by two or more different ceramics, can present higher characteristic respect to the single component, like fracture toughness and flexural strength. This paper presents a study of the influence of cutting parameters (cutting speed, feed rate and step number) onto the hole surface roughness and deformation due to the drill operation. The ceramic composite materials AZT (alumina toughened zirconia) and ZTA (zirconia toughened alumina) were first characterized in terms of hardness and roughness. After the drilling test, the holes were analyzed using scanning electron microscope (SEM) and an advanced 3-dimensional non-contact optical profilomete

Gautier, G.

Previtali, B.

Rebaioli, L.

Settineri, Luca

Tebaldo, V.

English

PORTO Publications Open Repository TOrino

____________________ 
* Corresponding author:delle Cacce street 73, Turin, +390113977616, +39011346288, g.gautier@to.istec.cnr.it, 
 
 MICRO-DRILLING OF ZTA AND ATZ CERAMIC COMPOSITE: EFFECT 
OF CUTTING PARAMETERS ON SURFACE ROUGHNESS 
 
 
G. Gautier1,*, V.Tebaldo1, L. Settineri2, B. Previtali3, L. Rebaioli3 
 
1National Research Council- Institute of Science and Technology for Ceramics, Strada delle Cacce 
73, 10135 Torino, Italy 
2 Department of Production Systems and Economics, Politecnico di Torino, C.so Duca degli 
Abruzzi, 24, 10129 Torino, Italy 
3Department of Mechanics, Politecnico di Milano, via La Masa 1, 20156, Milano, Italy 
  
ABSTRACT: Ceramics are a class of materials widely used during last fifteen years for orthopaedic applications. It is 
well known that they are characterized by low wear rate, and friction coefficient. However, these materials are very 
difficult to machine into complex shapes because of their brittleness and high hardness. The most effective method to 
increase the crack resistance is the formation of a composite structure. This class of materials, composed by two or 
more different ceramics, can present higher characteristic respect to the single component, like fracture toughness and 
flexural strength. This paper presents a study of the influence of cutting parameters (cutting speed, feed rate and step 
number) onto the hole surface roughness and deformation due to the drill operation. The ceramic composite materials 
AZT (alumina toughened zirconia) and ZTA (zirconia toughened alumina) were first characterized in terms of hardness 
and roughness. After the drilling test, the holes were analyzed using scanning electron microscope (SEM) and an 
advanced 3-dimensional non-contact optical profilometer.  
KEYWORDS: Ceramic composite, Micromachining, Roughness 
 
 
1 INTRODUCTION 
In the last years there was an increasing use for oxidic 
materials within biomedical fields, especially in 
dentistry and prosthetic. However the results have not 
always met the expectations because of their fragility, 
causing sometimes the prosthesis failure. For example, 
Linkevicius et al. [1] have shown how particular 
ceramic materials exhibit a tendency to fracture when 
subjected to high compression conditions. For this 
reason, the need for ceramics showing a good balance 
between hardness and toughness has increased. 
This balance can be achieved by using zirconia-based 
ceramic, as the tetragonal form of zirconia which has 
excellent mechanical properties. A good compromise is 
to use the alumina-zirconia composites as an 
alternative to pure zirconia.  
Two types of ceramic composites can be prepared: the 
first consisting of a matrix of ZrO2 reinforced with 
alumina particles, called AZT (developed for its 
excellent value hardness-toughness), and the second 
consisting of a matrix of Al2O3 reinforced with 
particles of zirconium oxide, known as ZTA 
(developed to substitute alumina ceramics in 
applications where a higher fracture resistance was 
required). [2-3]. Despite there is a trend today to 
develop alumina–zirconia composites as an alternative 
to monolithic alumina and zirconia, the literature 
contains few formal investigations of the machining of 
alumina–zirconia composites ceramic materials as well 
as no systematic studies on their machining are present. 
The paper experimentally investigates the 
micromachining of green and pre-sintered alumina–
zirconia composites. In particular micro-drilling of 
AZT and ZTA ceramics is investigated making use of a 
high precision machining centre and micro-drills in 
hard metal. 
In the last years there has been an increasing tendency 
to the down-scaling of components and systems in 
different industrial fields (mechanics, biotechnology, 
electronics, energetics, fluidics, optics, etc…); the 
miniaturization is needed, first of all, for functional 
purposes, but it also driven from the necessity of  
saving materials and energy and, this way, making 
costs decrease. 
As shown in [4] and [5], micro components or features 
can be produced by means of several process, such as 
mechanical machining, laser machining, EDM, ECM, 
ultrasonic machining, injection moulding; among these 
different process, mechanical micromachining is one of 
the most widespread since it is very versatile in terms 
of workpiece material and geometry, it allows to 
produce objects with a good quality and, at the same 
time, it has a high productivity.  
As confirmed by Masuzawa, microholes are the 
simplest products of micromachining. Holes with 
diameter of 200-300 μm can be commonly found in 
several products (printed boards, fuel injection nozzles, 
moulds, filters) and recently the challenge has shifted 
to performing holes with even smaller diameters. 
 
 2 EXPERIMENTAL 
2.1 MATERIALS  
Two kinds of ceramic composites were considered: 
• an yttria stabilized zirconia matrix reinforced with 
alumina particles, alumina toughened zirconia 
(AZT); 
• an alumina matrix reinforced with zirconia 
particles, zirconia toughened alumina (ZTA). 
The following raw powders “ready to press” were 
used: 
• for ZTA the granulated powder was an Alumina 
composite (pure at 99.99%), (84% in weight of 
Alumina, 16% of Zirconia); 
• for AZT the powder was a composite of Zirconia 
with 20% in weight of Alumina (Tosoh 
Corporation: ZrO2-20%Al2O3, TZ-3Y20AB).  
In both cases, the zirconia powders contain 3 mol% of 
Yttria (~ 4 wt %), required to stabilize the tetragonal 
zirconia during the sintering, and traces of other 
compounds.  
Green samples were produced utilizing an Uniaxial 
Pressing at low pressure, around 49 MPa; afterwards, 
the samples were subjected to Cold Isostatic Pressing 
(CIP) at a pressure of 245 MPa for 1-2 minutes in order 
to obtain the presintered one.  
The forming phase was carried out in moulds of proper 
dimensions to obtain final green compacts of 33x33 
mm, thickness 11 mm with parallel surfaces. 
Before the drilling operation, the surface were levelled 
with a 6 mm diameter mill. Cutting conditions: n = 
3978 rev/min; Vf  = 300 mm/min; Vc = 75 m/min. 
The composite materials were analyzed by means of 
SEM ZEISS EVO 50xvp equipped with source LaB6 in 
order to evaluate the microstructure surface. The 
specimens were mirror polished with diamond 
suspensions and proper disks, in order to progressively 
reduce the roughness up to 0.025 μm, so that the 
roughness sample surface is suitable for the 
microhardness tests.   
Vickers microhardness, for the presintered composite 
materials, was determined by a FISCHERSCOPE HM 
2000 XYm (Fischer, NIS-University of Turin, Turin, 
Italy) with WIN-HCU software. It’s a computer-
controlled measuring system for microhardness testing 
and determination of material parameters according to 
the standard ISO 14577 [6].  
Ten indentations were made to account for the scatter 
caused by residual porosity or foreign particles. Each 
test was performed to give a cycle with the same rates 
on loading and unloading, using the following 
conditions: Peak load of 1000 mN, loading rate of 50 
mN·s-1. 
 
2.2 MICRODRILLING  
The drilling experiments were carried out on the 
KERN Evo 5 axis high precision machining centre 
available at the “MI_crolab” of the Dipartimento di 
Meccanica of Politecnico di Milano, whose main 
characteristics are listed below. 
Micro drills made of hard metal, with a diameter of 
D = 0.5 mm, were used. 
Surface roughness was measured with a non contact 
profilometer  Talysurf CCI 3000Å using a metered cut-
off length of 0.8 mm. Surface roughness readings were 
taken in three different positions inside the hole and the 
average was taken for the analysis. After data 
collection the image data was levelled to remove slope 
caused by tilting of the tablet surface using a data-
analysis programme Talymap. From the three-
dimensional analysis, the roughness parameters Sa, Ssk 
and Sku were extrapolated.  
The skewness parameter Ssk is a measure of the 
asymmetry of surface deviations about the mean plane. 
It can be a good indicator of the presence of peaks or 
pits of a surface, while Sku, (Kurtosis) is a measure of 
the “spikiness” of the surface, or the distribution of 
spikes above and below the mean line. For spiky 
surfaces, Sku > 3; for bumpy surfaces, Sku < 3; 
perfectly random surfaces have kurtosis of 3. Kurtosis 
is also a measure of the randomness of surface heights. 
[7]. Two parallel series of holes have been produced. 
One was necessary in order to evaluate the hole 
circularity, while the other one has been milled to half 
in order to estimate the internal roughness.  
The holes deformation has been evaluated by means of 
the “noncircularity” parameter, which has been 
calculated measuring by the SEM the holes diameter 4 
times every 45° and then subtracting the minimum 
value from the maximum one. 
 
2.3 DESIGN OF EXPERIMENT  
The factors whose effect is intended to be studied are 
the cutting speed Vc , the feed rate f and the number of 
steps (named “step”) which are necessary to complete 
the hole depth with the “peck drilling” strategy; in 
order to reduce the number of tests to be done and, at 
the same time, to be able to estimate the data variance, 
a 23-1 factorial plan with 4 central points has been 
chosen. The following table shows the experimental 
design, used both for the tests on ZTA and for those on 
AZT. 
Table 1: Experimental design. 
Run order Vc [m/min] 
f 
[mm/rev] step 
1 50 0,02 2 
2 50 0,02 2 
3 25 0,03 1 
4 75 0,01 1 
5 25 0,01 3 
6 75 0,03 3 
7 50 0,02 2 
8 50 0,02 2 
 
 
3 RESULT AND DISCUSSION 
For sake of brevity only the morphologies of the AZT 
composite are shown in Fig. 1 (a) and (b), referred to 
green and presintered respectively. The SEM images 
 show that the alumina and zirconia particles were 
uniformly distributed and closely packed in the green 
and presintered body without surface defects, such as 
crack, void and particles pull-out. 
 
      
                    a)           b) 
Figure 1: SEM image of a) AZT green and b) 
presintered 
Tables 2 shows the micro hardness and young modulus 
of AZT and ZTA presintered samples. 
Analysing the hardness of the composite materials can 
be seen that ZTA presintered sample has a high 
hardness respect to AZT. 
Table 2 - Microhardness and Young's modulus for ZTA 
and AZT 
 AZT ZTA 
HVL-D [N/mm2] 1744± 27 2241± 15 
EIT [GPa] 266± 2,5 348± 2,5 
 
Fig. 2 shows the ZTA green and presintered sample 
holes performed with Vc = 75 m/min, f = 0,01 mm/rev, 
step = 1 (test n°4). The others hole images are not 
reported for sake of brevity. As can be seen from the 
SEM images, the holes shape is regular, in both case, 
with some debris material left inside the hole. Debris 
mainly consists of small drops of AZT and ZTA 
composite material. Considering the area around the 
hole periphery, a little chip was found for the 
presintered composite material, which was not found 
around the green one. This phenomena depends on the 
type of chip. In fact for the green samples only powder 
was found, while for the presintered one, the 
microdrilling operation causes the production of flakes.  
 
    
        a)                      b) 
Figure 2: SEM image of a) ZTA presintered  and  b) 
green area 4 
Figure 3 shows the cross-sections of a) green and b) 
presintered ZTA and c) green and d) presintered AZT 
holes for the area 4. As can be noticed from the picture 
below, a barrelling phenomenon, which defines how 
parallel-sided is the hole, was not found for the cross 
section of all the ceramic composite.  
As can be notice from the picture below, the ZTA 
presintered and AZT green and presintered, show a 
split along all the cross sections. This phenomena was 
due to milling operation necessary to investigate the 
internal roughness.  
 
   
       a)      b) 
  
c)      d) 
Figure 3: Cross-sections of the ZTA a) green and b) 
presintered, and c) green and b) presintered of AZT 
holes of the condition n°4.  
 
Figure 3 shows a three-dimensional analysis of the 
ZTA composite  hole.  
 
 
 
Figure 4: 3D surface roughness profiles of AZT green 
hole 
 
In order to study the influence of cutting parameters on 
the hole roughness and circularity, the ANalysis Of 
VAriance (ANOVA) has been performed on Sa, Sku, 
Ssk and noncircularity data for both ZTA and AZT, 
green and presintered. For sake of brevity the whole 
analysis and its verifications are not shown and only 
results are presented. 
As concerning both green and presintered ZTA, there is 
no statistic evidence that any cutting parameter 
influences all roughness parameters Sa, Ssk, Sku; as 
regarding noncircularity, only in presintered ZTA it is 
influenced by f. 
As regarding green AZT, Sa is influenced by “step” 
factor while Ssk, Sku and noncircularity are not 
influenced by any cutting parameters. On presintered 
AZT there is no statistical evidence of an influence of 
cutting parameters on Sa and noncircularity while Ssk 
and Sku are both influenced by “step” factor. 
 
  
Figure 4: Sa measurement results. 
 
Figure 5: Ssk measurement results. 
 
Figure 6: Sku measurement results. 
 
Figure 7: Noncircularity measurement results. 
An ANOVA carried out on Sa, Sku, Ssk and 
noncircularity data respect to material type (AZT or 
ZTA) and status (green or presintered by means of 
CIP) points out that Sa is statistically influenced by the 
material status and its interaction with the material 
type, Sku is only influenced by the interaction between 
material type and status, no factor has an influence on 
Ssk while material status has an influence on the 
noncircularity. 
 
4 CONCLUSIONS 
An investigative analysis of parametric influence on 
surface roughness in micro drilling operation of AZT 
and ZTA green and presintered ceramic composite has 
been presented in this paper. The influence of drilling 
parameters was validated through analysis of variance 
(ANOVA). 
As a result of drilling test the following points have 
been noted: 
• Damage around the hole caused by microdrilling 
operation are less for the green ceramic composite 
respect to the presintered one.  
• Comparing the Sa parameter, a lower value for 
green AZT composite was found. 
• The Kurtosis (Sku) parameter is for all the materials 
higher than 3, means that is representative of a 
surface with valleys and less peaks.  
• Also for skewness value (Ssk), the surface is 
characterized by high valleys. In fact the values are 
higher than zero. Only ZTA green presents negative 
value.  
• Comparing the micro hole roundness of all the 
ceramic samples, a lower value for the green one 
was found and in particular for the AZT one.  
• As for the ANOVA analysis, only the AZT green 
sample presents an evident influence of machining 
operation onto the roughness parameters. In fact 
drilling with one step, the surface roughness 
presents lower value.  
 
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Metallic materials – Instrumented indentation test for hardness and materials parameters, Switzerland: International Organization for Standardization,

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Micro-Drilling of ZTA and ATZ Ceramic Composit: Effect of Cutting Parameters on Surface Roughness

Micro-Drilling of ZTA and ATZ Ceramic Composit: Effect of Cutting Parameters on Surface Roughness

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