Performance Evaluation of Macor Dental Ceramic: An Investigation with Rotary Ultrasonic Machining

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.


Introduction
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][2][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][7][8][9][10][11][12][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][15][16][17][18][19][20][21][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).

Materials and Methods
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

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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.

Experimentation and Data Collection
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).

Results and Discussion
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.

Conclusion
The following conclusions can be made from the present investigation: 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.