Performance Evaluation of Macor Dental Ceramic:
An Investigation with Rotary Ultrasonic Machining
Ravi Pratap Singh1*, Ravinder Kataria2, Sandeep Singhal3
1Department of Industrial & Production Engineering, Dr. B R Ambedkar National Institute of Technology, India
2Department of Mechanical Engineering, Lovely Professional University, India
3Department of Mechanical Engineering, National Institute of Technology, India
Submission: November 06, 2017 ; Published: April 04, 2018
*Corresponding author: Ravi Pratap Singh, Department of Industrial & Production Engineering, Dr. B R Ambedkar National Institute of Technology, Jalandhar, Punjab, India, Email: firstname.lastname@example.org
How to cite this article: Ravi P S, Ravinder K, Sandeep S. Performance Evaluation of Macor Dental Ceramic: An Investigation with Rotary Ultrasonic Machining.
Adv Dent & Oral Health. 2018; 8(2): 555733. DOI: 10.19080/ADOH.2018.08.555733
Macor dental ceramic has been glowingly observed as a classic ceramic material which holds enlarged applications in the field of numerous manufacturing and medical industries especially in dentistry field, because of its excellent and versatile properties. However, its fruitful and productive processing is quite difficult through conventional methods. This article attempts to experimentally investigate the performance (in terms of surface roughness) of macor dental ceramic with rotary ultrasonic machining. The surface roughness has been investigated under the influence of different input factors, namely spindle speed, feed rate, coolant pressure, and ultrasonic power. The experiments have been conducted through response surface methodology. Microstructure analysis of the machined samples has revealed and confirmed the presence of dominating brittle fracture that caused removal of material along with the slighter plastic deformation of the work surface. The best surface quality has been attained as 0.5130 μm (at experimental run 17) which is corresponding to the lower feed rate and moderate level of other parameters.
Keywords: Ceramic; Dentistry; Machining; Macor; Microstructure; Roughness; RUM
Macor dental ceramic is well known as one of the highly demanding material in the family of technically advanced medical ceramics, which acquires an unrivalled combination of superior thermal, mechanical, electrical and chemical properties i.e.; better hardness, chemically durability, excellent wear resistance, high temperature stability, etc. Because of the above stated versatile properties of macor dental ceramic, it covers a broad range of application in several industries such as; medical and laboratory equipments, aerospace, electronics, automobile, etc [1-3]. More distinctively, macor ceramic is used in welding nozzles, dentistry, space shuttle parts, etc. Regardless of exceptional capabilities, it’s processing with various processing methods does not provide productive solutions and casually results with several drawbacks such as; high processing cost, surface defects, lesser material removal rate, micro cracks, and lesser accuracy, which further entangled its expansion to market [4,5]. Hence, there is a favorable requirement to develop a cost-effective and highly accurate machining solution which can process this highly demanding dental ceramic in an effective way.
Among the available contemporary machining methods introduced for processing typical and advanced materials (i.e. ceramics, composites, etc.), Rotary Ultrasonic Machining (RUM) method has been observed as one of the best suitable candidates which fits for precise processing of macor dental ceramic material
as this process produces thermal damage free profiles along with better surface quality [6-13].
RUM is a hybrid non-traditional machining solution that merges the mechanisms of conventional grinding and static USM, reporting with enhancive Material Removal Rate (MRR) and better surface quality than that attained by either static USM or diamond grinding, utilized potentially to machine a wide range of latest and difficult-to-machine materials including ductile, hard and brittle, ceramics, and composites, etc [14-22]
It is revealed from the literature review that, there has been only a few research studies reported on RUM of macor dental ceramic material. In the light of the above discussion, current article has been focused to experimentally investigate the surface roughness under the influence of several process factors in RUM of macor dental ceramic by using response surface methodology. The microstructure of the machined work samples have been analyzed to understand the different mechanisms of material removal from the work surface using Scanning Electron Microscope (SEM).
The macor dental ceramic’s peerless microstructure makes it distinct from most variant ceramics and glasses [2,3]. The fabrication process of macor dental ceramic starts with the mixing of raw materials in a ball-mill having alumina balls followed by melting at 1550°C for 2 hours in a crucible. The molten metal is
further casted on a steel mould of desired shape (rectangular).
The adequate nucleation was attained by heat treating the
prepared samples at 680°C for 2 hours. During this process, the
re-crystallization of chondrolite phase to smaller platy crystals of
norbergite is also taken place. This further creates an extremely
interconnected array of two-dimensional mica crystals diffused in
a brittle glassy matrix.
The microstructure of the machined surface was observed
using SEM analysis (SEM EVO40, Carl Zeiss AG, Oberkochen,
Germany). The present investigation involves the rotary ultrasonic
drilling of macor dental ceramic under the influence of a distinctive
set of experimental conditions. Table 1 & 2 are demonstrating the
elemental composition and work material properties respectively.
To investigate surface roughness in RUM of macor dental
ceramic material, feed rate, spindle speed, ultrasonic power,
and coolant pressure were selected as four process variables,
as represented in Table 3. In the present experimental study,
macor dental ceramic has been selected as a work material with
the dimensions of 50×50×4mm. The experimentation work was
performed on “Series10 Knee-mill” rotary ultrasonic machine setup
(Sonic Mill, Albuquerque, NM, USA) having power capacity of
1kW. Figure 1 illustrates the major constituents of RUM set-up and
diamond abrasive tool.
In the present study, the main experiments were planned and
designed by using a design of experiments technique called as-
“Response Surface Methodology (RSM)” through CCRD. For this
purpose, statistical software known as- “Design Expert 9.0” (State-
Ease, Inc., USA) was utilized. As per the designed experimental
matrix, holes were drilled in macor dental ceramic workpiece
under the different operating conditions. Table 4 is representing
the complete experimental design plan along with the average
values of SR. The SR of the drilled holes was measured with a
surface tester (Carl Zeiss, MG, USA).
The experimental results for surface roughness have been
further plotted over the line graphs to observe the variation of
surface roughness values under different experimental runs.
Figure 2 is depicting the surface roughness values against the
several conducted experimental runs. It has been clearly observed
from this plot that, the lowest value of surface roughness for
macor dental ceramic has been attained against the experimental
run number 17. This variation can be associated with the lower
feed rate level under this experimental run (Figure 3).
In RUM, the characteristics of the surface under process
are mainly getting influenced by the effect of feed rate, spindle
speed, and ultrasonic power. Moreover, under few experimental
conditions, apart from the brittle and plastic deformation of work
material, the material removal can also take place as a combination
of both the failure modes. Figure 4 represents the SEM set-up used
for microstructure analysis.
Figure 5 shows the microstructure of machined surface
corresponding to experiment no. 1. The parametric setting for this
experimental run was having a combination of moderate level of
feed rate and spindle speed. In RUM process, the diamond abrasive
tool vibrating at ultrasonic frequency hammered the macor dental
ceramic surface because of this the initiation and propagation
of micro cracks takes place. Figure 6 depicts the SEM image
consequent to experiment no. 11. The mixed mode of material
removal has been found as the presence of brittle mode fracture
confirmed with removal of bigger chunks from the surface.
Figure 7 illustrates the microstructure of machined surface
corresponding to experiment no. 17. SEM image reveals the
presence of mixed mode of material removal along with the
leading plastic mode failure of work material. It is also revealed
that, at the lowest level of feed rate (0.012 mm/s), the indentation
depth of abrasives decrements which further promotes the
material to be removed in ductile mode. Figure 8 demonstrates
the SEM microstructure corresponding to experiment no. 18. This
surface microstructure reveals the presence of highly dominant
brittle mode deformation of the work material. At a higher feed
level (0.060 mm/s), the depth of penetration of abrasives into
the work surface incremented considerably and hence resulted
in the presence of profound abrasion marks which further causes
material to be removed. Sharper and pointed edges have been
clearly observed on the machined surface.
The following conclusions can be made from the present
a) The feed rate factor has been revealed as the most
influential for SR, in RUM of macor dental ceramic. Lower
feed rate gives the best solution with respect to SR. This can
be concerned to the decrement in the indentation depth
of abrasives occurs at the diminished feed rate level. An
incremented spindle speed level also promotes the chances of
material removal in a ductile mode, which further produces
the finer surfaces.
b) In RUM of macor dental ceramic, crack propagation often
observed since the work surface under processing is getting
stressed cyclically. As the penetration depth of abrasives
increases (at higher feed rate), the proportion of the brittle
mode deformation has been revealed to be increased.
c) Experimental setting at run number 17 provides the
better results in terms of surface roughness as the value of
roughness has been observed as lowest i.e. 0.5130μm at this
experiment run. A parametric combination possessing feed
rate at a lower level and spindle speed at a higher level, offers
more favorable conditions for the plastic mode deformation to
occur in RUM of macor dental ceramic as for this setting the
indentation depth of diamond abrasives reduces considerably.