Brittle material characters and property

Brittle materials characters and properties

Brittle materials can be divided into three major categories: amorphous glasses, hard crystals, and advanced ceramics. Among them, advanced ceramics are the some of the hardest and most brittle materials [1]. These advanced ceramics are different from traditional ceramics because of their specialized mechanical properties and corresponding sophisticated manufacturing processes [2].

The entire family of advanced ceramics includes silicon carbide (SiC), silicon nitride (Si3N4), aluminum oxide (Al2O3), zirconia (ZrO2), zirconia toughened alumina (ZTA), boron carbide (B4C), and polycrystalline diamond [2].  Ceramic atoms are bonded with high-energy bonds (covalent bonding, ionic bonding, and the combination of these two types), while metal atoms are typically connected by low-energy metallic bonding. In the case of metal oxide ceramic (Al2O3), the ratio of covalent bonding and ionic bonding is about 4:6. However, this ratio is 9:1 for SiC, which is a typical non-oxide ceramic [3]. Strength of materials which have large ratios of ionic bonding, are considerably affected by temperature. Materials with covalent bonding, on the other hand, are not affected by elevated temperatures. Furthermore, low thermal coefficients of expansion and relatively high thermal conductivity have also been proven to be special features of ceramics with covalent bonding [3].

From an engineering perspective, a brittle material is defined as one that does not exhibit plastic deformation preceding the initiation of a crack. When a brittle material needs to be brought to final dimension by hard grinding, it will not exhibit plastic deformation as the diamond grit plows through it to remove material, nor will it exhibit the residual stress profile of a ductile material [4]. Instead, the material will crack, leaving microcracks that remain as subsurface damage. The key issue of grinding brittle materials is to minimize this subsurface damage by following prescribed grinding parameters that will achieve the desired material removal rates and final dimensional accuracy.

 

Ductile-regime grinding principle

When machined, a brittle material can deform via a variety of mechanisms. If the critical resolved shear stress at any point within the material exceeds the elastic yield stress, the mechanism of deformation will change from one of reversible energy storage via elastic stretching to one of irreversible energy dissipation. Examples of irreversible deformation include macroscopic fracture propagation, microcrack formation, phase transformation, dislocation motion (in crystals), and intermolecular sliding (in amorphous materials) [1] [2]. Irreversible material-removal mechanisms can be divided into two types: brittle and ductile. In brittle mechanisms, material removal is accomplished through the propagation and intersection of cracks, while ductile mechanisms produce plastic flow of material in the form of severely sheared machining chips.

Since advanced precision engineering can allow controlled grinding infeed rates as low as several nm per grinding wheel revolution, it is possible to grind brittle materials with this low rate. Thus, the predominant material-removal mechanism is plastic-flow rather than fracture. This process is known as ductile-regime grinding. When brittle materials are ground through a process of plastic deformation, a working surface similar to those achieved in polishing or lapping are produced [1] [5]. However, unlike polishing or lapping, grinding is a deterministic process which permits finely controlled contour accuracy and complex shapes. Ductile regime machining is an alternative method for polishing brittle materials to obtain a high-quality working surface by a ductile or plastic material removal process.

 

Design of experiment

As expected, ductile-regime grinding will apply to Silicon Carbide (SiC). This study attempts to find out the critical deformation point according to relevant surface morphology.

A total of 6 SiC workpieces were processed in this experiment. The workpieces were divided into two groups with two different table speeds (feed speed). Within each group, the workpieces were assigned with 3 different wheel speeds. The minimum step size of depth for each grinding was 0.0001 inch (2.54 μm). After the grinding wheel first touched the peak of the workpieces, the grinding process was repeated several times to equalize the height of the workpieces. Then, all workpieces were cut to three equal lengths: 18mm*, 4mm*, and 5mm. After polishing all the cutting surfaces, these 3 short workpieces were tightly piled together as shown in Fig.1 and were inserted into a specially designed tilt holder: The workpiece slot length is 60 mm. On the longitudinal direction of this slot, a 10 μm height elevation was designed from the left end (zero point) to the right end. Additionally, there are three points designated A, B, and C. These three points are in the middle of each divided piece. Points A, B, and C are convenient for further calculation and data analysis.

A schematic drawing of this design is displayed in Fig.2 and an actual picture of this system is shown in Fig.3. This design guarantees a good observation of the cutting sections. During the next grinding process with the corresponding parameters shown in table 1, the workpieces on this holder would experience a small grinding edge angle (equation 1). Such small grinding edge angles can lead to various depths of cut on the workpiece with normal force. The left end is set as 0, the first cut section point to left end is 18 mm, from equation 1.  The depth of cut at this point is 18mm*sinα=3 μm.  The second cut section point to left end length is 36 mm, and the depth of cut is 6 μm.

Grinding edge angle α = sin^-1(10μm/60mm)=0.001°

Equation 1

Fig.1

Fig.2

Fig 3

Work material Silicon carbide (SiC)
Grinding wheel Winter D1500 730B
Wheel speed 47.87 m/s, 39.89 m/s, 31.92 m/s
Feed rate 0.0105m/s, 0.0211m/s
Depth of cut 3 μm, 6 μm

 

Table 1: The specifics of the grinding conditions

Experimental Set-up

The SiC workpieces in this experiment were from the Ford development department. Their original application was for grinding tools for the automobile industry.

SiC is the only chemical compound of carbon and silicon. It was originally produced by a high-temperature electro-chemical reaction of sand and carbon. Silicon carbide is an excellent abrasive. Thus, it had been widely used in producing grinding chips and other abrasive products. SiC has been developed into a high quality technical grade ceramic with outstanding mechanical properties. SiC ceramics have not only excellent mechanical properties at room temperature (high flexural strength, excellent oxidation resistance, good corrosion resistance, high abrasion resistance and low coefficient of friction and high temperature mechanical properties)[6], but they also have adequate high-temperature strength which can be maintained up to 1600℃.

Grinding machine

Fig 2 Thompson Surface Grinding Machine FSH CNC 30/90

Table 3: Grit size of diamond particles

The grinding machine adopted in this study is a CNC grinder from Thompson Industries, Inc. equipped with a 1500 grit diamond wheel, which can make a minimal infeed of 0.0001inch. To provide real-time dynamic balancing, a dynamic balance system was equipped with the wheel spindle. This system eliminates the imbalance of the grinding wheel and minimizes grinding wheel vibration  while maintaining an optimum grinding process.

The computer located on the right side is connected to the dynamometer set on the grinder table [3]. Between computer and grinder is the control area and grinding wheel auto balance monitoring system.

Fig 3 Kistler dynamometer Type 9257B

 

Results and discussions

Grinding force

 

 

 

Fig 12 grinding Normal force DADisp 6.7

 

Fig.13

 

Fig.14

 

Fig.15

 

Fig.16

 

Fig.17

From Fig.12 we can determine that the normal force kept ascending with longer grinding times. This trend is in consistent with our workpiece holder design: as the barrier (workpiece) height increases across the longitudinal direction from left to right, the grinding wheel experienced an increasing counterforce, proportional to the normal force.

A comprehensive analysis of grinding factors on surface roughness was displayed from Fig.13 to Fig.15. Several issues could be inferred from those results: First, the surface roughness became greater with faster feeding speed. Since higher feeding speed leads to insufficient contact between the grinding wheel and the workpiece. Similarly, the increased wheel speed could enlarge the contact efficiency between the grinding wheel and the workpiece. Therefore, as shown in Fig.15, the surface roughness decreased with increasing wheel speed. Moreover, as the depth of cut increases, the surface roughness also increases. These effects are independent of each other.

From Fig.16, it is clear that the increased depth of cut would enlarge the normal force. Since the depth of cut is proportional to material removal rate (MRR), according to the force model established by Fan et al. [11], the normal force can be divided into vertical and horizontal components:

,

Here, kz, kx, mz and mx are constants while Fz0 and Fx0 are the corresponding rubbing component, respectively. Based on this model, it is clear that higher material removal rate would lead to a higher normal force.

Fig.17 showed that the normal force is reduced with higher wheel speed. This phenomenon can be explained according to our previous discussion; a higher wheel speed reduces  surface roughness. Therefore, the horizontal counter force, which is one component of the total normal force, was reduced due to reduced  friction.

SEM analysis

The aim of the SEM analysis was to estimate the grinding ductility by observing the microstructure of a ground workpiece. Fig.18 and Fig.19 display the surface morphology between point A and B. From Fig 18, the localized micro-plastic deformation has been observed and Fig.19 gave a better estimation that such deformation only happened within several micrometers from the surface of workpiece. Under this region, few structural defects could be observed, indicating that lower depth of cut has less impact of the internal tissues. Moreover, this change on the shoulder edge of a grinding surface could be identified as  plastic flow on lower depths of the cut workpiece. Therefore, a shallow ductile-regime has been demonstrated on the shoulder edge of the grinding surface under lower depths of cut, i,e. lower material removal rates.

 

 

 

 

Fig. 20 displays the surface morphology between point B and C. Here we find fracture cracks due to a higher depth of cutting. The occurrence of cracking pits on the cutting section surface proves the transition of the material removal mechanism from ductile deformation to brittle fracture in the grinding process. This stage can be defined as the critical parameter. Relative to the grinding depth, brittle material removal can be classified into three types: ductile regime grinding, ductile-brittle regime grinding, and brittle regime grinding. The critical depth of cut exists between our experiment range. As point B stood for a depth of cut of 4.5 μm, it can be inferred that the 4.5 μm depth of cut might be the critical point.

Conclusion

This project aimed at a deeper understanding of the grinding process for brittle materials such as SiC. In this project, we focused on the effects of several vital factors (feed speed, depth of cut and wheel speed) on the normal force and processed surface roughness. During our investigation, it was observed that the surface roughness after grinding was enhanced by higher feed speed, higher depth of cut and lower wheel speed. Additionally, normal force increased with an increasing depth of cut or by  reduced wheel speeds. Moreover, surface morphology was assessed to estimate a range for proper grinding. Through the SEM analysis, it is concluded that within a shallow depth of cut (less than 4.5 μm), the workpiece would display a ductile-regime with a plastic flow only until several micrometers from the top surface, leaving the internal tissue unchanged. However, as the depth of cut continues increasing, fracture cracking is triggered at both shallow surface and bulk areas of the workpiece, which indicates  the occurrence of an unwanted brittle-regime. It can be further inferred that as depth of cut increases, the brittle-regime is enhanced and might replace the ductile-regime. Based on our experimental design, it is suggested that a 4.5 μm depth of cut might be the critical point for grinding on an SiC workpiece.

References:

 

  1. Bifano, T. G., 1988, “Ductile-Regime Grinding of Brittle Materials,” Ph.D. Thesis, NC State University, Raleigh, NC.
  2. Chen J, Shen J, Huang H, Xu X. Grinding characteristics in high speed grinding of engineering ceramics with brazed diamond wheels. J Mater Process Technol 2010;210:899–906.
  3. Shearer, Thomas R., “Diamond Wheel Grinding 101,” Ceramic Industry magazine, June 2006, pp. 17-20.
  4. S., andSathyanarayanan.G., 1987, “An Investigation into the Mechanics of Diamond Grinding of Brittle Materials,” 15th North American Manufacturing Research Conference Proceedings, Vol. 2, Manufacturing Technology Review, pp. 499-505
  5. Chen M, Zhao Q, Dong S, Li D. The critical conditions of brittle–ductile transition and the factors influencing the surface quality of brittle materials in ultra-precision grinding. J Mater Process Technol 2005;168:75–82.
  6. Huerta, M., and Malkin, S., 1976, “Grinding of Glass: The Mechanics of the Process,” ASME JOURNAL OF ENGINEERING FOR INDUSTRY, May, pp. 459-467
  7. Krauskopf, B. Diamond Turning: Reflecting Demands for Precision, Manuf Engng, 92 (5), 1984, 90-100
  8. Blake, P. N. Ductile-Regime Diamond Turning of Germanium and Silicon, PhD Thesis, North Carolina State University, 1988
  9. Blake, P. N. and Scattergood, R. O. Ductile Regime Machining of Germanium and Silicon, J Amer Ceram Soc 73 (4) 1990
  10. Cheng, J., Gong, Y.D., 2013. Experimental study on ductile-regime micro-grinding character of soda-lime glass with diamond tool. Int. J. Adv. Manuf. Technol. 69,147–160.
  11. Xiaorui, F,. and Michele H. M., 2007, “Force Analysis for Grinding with Segmental Wheels,” Machine Science and Technology, Vol. 10, pp. 435-455 

g

Introduction

Brittle material characters and property

Brittle materials can be divided into three major categories: amorphous glasses, hard crystals, and advanced ceramics. Among them, advanced ceramics are the some of the hardest and most brittle materials [1]. These advanced ceramics are different from traditional ceramics because of their specialized mechanical properties and corresponding sophisticated manufacturing processes [2].

The entire family of advanced ceramics includes silicon carbide (SiC), silicon nitride (Si3N4), aluminum oxide (Al2O3), zirconia (ZrO2), zirconia toughened alumina (ZTA), boron carbide (B4C), and polycrystalline diamond [2].  Ceramic atoms are bonded with high-energy bonds (covalent bonding, ionic bonding, and the combination of these two types), while metal atoms are typically connected by low-energy metallic bonding. In the case of metal oxide ceramic (Al2O3), the ratio of covalent bonding and ionic bonding is about 4:6. However, this ratio is 9:1 for SiC, which is a typical non-oxide ceramic [3]. Strength of materials which have large ratios of ionic bonding, are considerably affected by temperature. Materials with covalent bonding, on the other hand, are not affected by elevated temperatures. Furthermore, low thermal coefficients of expansion and relatively high thermal conductivity have also been proven to be special features of ceramics with covalent bonding [3].

From an engineering perspective, a brittle material is defined as one that does not exhibit plastic deformation preceding the initiation of a crack. When a brittle material needs to be brought to final dimension by hard grinding, it will not exhibit plastic deformation as the diamond grit plows through it to remove material, nor will it exhibit the residual stress profile of a ductile material [4]. Instead, the material will crack, leaving microcracks that remain as subsurface damage. The key issue of grinding brittle materials is to minimize this subsurface damage by following prescribed grinding parameters that will achieve the desired material removal rates and final dimensional accuracy.

 

Ductile-regime grinding principle

When machined, a brittle material can deform via a variety of mechanisms. If the critical resolved shear stress at any point within the material exceeds the elastic yield stress, the mechanism of deformation will change from one of reversible energy storage via elastic stretching to one of irreversible energy dissipation. Examples of irreversible deformation include macroscopic fracture propagation, microcrack formation, phase transformation, dislocation motion (in crystals), and intermolecular sliding (in amorphous materials) [1] [2]. Irreversible material-removal mechanisms can be divided into two types: brittle and ductile. In brittle mechanisms, material removal is accomplished through the propagation and intersection of cracks, while ductile mechanisms produce plastic flow of material in the form of severely sheared machining chips.

Since advanced precision engineering can allow controlled grinding infeed rates as low as several nm per grinding wheel revolution, it is possible to grind brittle materials with this low rate. Thus, the predominant material-removal mechanism is plastic-flow rather than fracture. This process is known as ductile-regime grinding. When brittle materials are ground through a process of plastic deformation, a working surface similar to those achieved in polishing or lapping are produced [1] [5]. However, unlike polishing or lapping, grinding is a deterministic process which permits finely controlled contour accuracy and complex shapes. Ductile regime machining is an alternative method for polishing brittle materials to obtain a high-quality working surface by a ductile or plastic material removal process.

 

Design of experiment

As expected, ductile-regime grinding will apply to Silicon Carbide (SiC). This study attempts to find out the critical deformation point according to relevant surface morphology.

A total of 6 SiC workpieces were processed in this experiment. The workpieces were divided into two groups with two different table speeds (feed speed). Within each group, the workpieces were assigned with 3 different wheel speeds. The minimum step size of depth for each grinding was 0.0001 inch (2.54 μm). After the grinding wheel first touched the peak of the workpieces, the grinding process was repeated several times to equalize the height of the workpieces. Then, all workpieces were cut to three equal lengths: 18mm*, 4mm*, and 5mm. After polishing all the cutting surfaces, these 3 short workpieces were tightly piled together as shown in Fig.1 and were inserted into a specially designed tilt holder: The workpiece slot length is 60 mm. On the longitudinal direction of this slot, a 10 μm height elevation was designed from the left end (zero point) to the right end. Additionally, there are three points designated A, B, and C. These three points are in the middle of each divided piece. Points A, B, and C are convenient for further calculation and data analysis.

A schematic drawing of this design is displayed in Fig.2 and an actual picture of this system is shown in Fig.3. This design guarantees a good observation of the cutting sections. During the next grinding process with the corresponding parameters shown in table 1, the workpieces on this holder would experience a small grinding edge angle (equation 1). Such small grinding edge angles can lead to various depths of cut on the workpiece with normal force. The left end is set as 0, the first cut section point to left end is 18 mm, from equation 1.  The depth of cut at this point is 18mm*sinα=3 μm.  The second cut section point to left end length is 36 mm, and the depth of cut is 6 μm.

Grinding edge angle α = sin^-1(10μm/60mm)=0.001°

Equation 1

Fig.1

Fig.2

Fig 3

Work material Silicon carbide (SiC)
Grinding wheel Winter D1500 730B
Wheel speed 47.87 m/s, 39.89 m/s, 31.92 m/s
Feed rate 0.0105m/s, 0.0211m/s
Depth of cut 3 μm, 6 μm

 

Table 1: The specifics of the grinding conditions

Experimental Set-up

The SiC workpieces in this experiment were from the Ford development department. Their original application was for grinding tools for the automobile industry.

SiC is the only chemical compound of carbon and silicon. It was originally produced by a high-temperature electro-chemical reaction of sand and carbon. Silicon carbide is an excellent abrasive. Thus, it had been widely used in producing grinding chips and other abrasive products. SiC has been developed into a high quality technical grade ceramic with outstanding mechanical properties. SiC ceramics have not only excellent mechanical properties at room temperature (high flexural strength, excellent oxidation resistance, good corrosion resistance, high abrasion resistance and low coefficient of friction and high temperature mechanical properties)[6], but they also have adequate high-temperature strength which can be maintained up to 1600℃.

Grinding machine

Fig 2 Thompson Surface Grinding Machine FSH CNC 30/90

Table 3: Grit size of diamond particles

The grinding machine adopted in this study is a CNC grinder from Thompson Industries, Inc. equipped with a 1500 grit diamond wheel, which can make a minimal infeed of 0.0001inch. To provide real-time dynamic balancing, a dynamic balance system was equipped with the wheel spindle. This system eliminates the imbalance of the grinding wheel and minimizes grinding wheel vibration  while maintaining an optimum grinding process.

The computer located on the right side is connected to the dynamometer set on the grinder table [3]. Between computer and grinder is the control area and grinding wheel auto balance monitoring system.

Fig 3 Kistler dynamometer Type 9257B

 

Results and discussions

Grinding force

 

 

 

Fig 12 grinding Normal force DADisp 6.7

 

Fig.13

 

Fig.14

 

Fig.15

 

Fig.16

 

Fig.17

From Fig.12 we can determine that the normal force kept ascending with longer grinding times. This trend is in consistent with our workpiece holder design: as the barrier (workpiece) height increases across the longitudinal direction from left to right, the grinding wheel experienced an increasing counterforce, proportional to the normal force.

A comprehensive analysis of grinding factors on surface roughness was displayed from Fig.13 to Fig.15. Several issues could be inferred from those results: First, the surface roughness became greater with faster feeding speed. Since higher feeding speed leads to insufficient contact between the grinding wheel and the workpiece. Similarly, the increased wheel speed could enlarge the contact efficiency between the grinding wheel and the workpiece. Therefore, as shown in Fig.15, the surface roughness decreased with increasing wheel speed. Moreover, as the depth of cut increases, the surface roughness also increases. These effects are independent of each other.

From Fig.16, it is clear that the increased depth of cut would enlarge the normal force. Since the depth of cut is proportional to material removal rate (MRR), according to the force model established by Fan et al. [11], the normal force can be divided into vertical and horizontal components:

,

Here, kz, kx, mz and mx are constants while Fz0 and Fx0 are the corresponding rubbing component, respectively. Based on this model, it is clear that higher material removal rate would lead to a higher normal force.

Fig.17 showed that the normal force is reduced with higher wheel speed. This phenomenon can be explained according to our previous discussion; a higher wheel speed reduces  surface roughness. Therefore, the horizontal counter force, which is one component of the total normal force, was reduced due to reduced  friction.

SEM analysis

The aim of the SEM analysis was to estimate the grinding ductility by observing the microstructure of a ground workpiece. Fig.18 and Fig.19 display the surface morphology between point A and B. From Fig 18, the localized micro-plastic deformation has been observed and Fig.19 gave a better estimation that such deformation only happened within several micrometers from the surface of workpiece. Under this region, few structural defects could be observed, indicating that lower depth of cut has less impact of the internal tissues. Moreover, this change on the shoulder edge of a grinding surface could be identified as  plastic flow on lower depths of the cut workpiece. Therefore, a shallow ductile-regime has been demonstrated on the shoulder edge of the grinding surface under lower depths of cut, i,e. lower material removal rates.

 

 

 

 

Fig. 20 displays the surface morphology between point B and C. Here we find fracture cracks due to a higher depth of cutting. The occurrence of cracking pits on the cutting section surface proves the transition of the material removal mechanism from ductile deformation to brittle fracture in the grinding process. This stage can be defined as the critical parameter. Relative to the grinding depth, brittle material removal can be classified into three types: ductile regime grinding, ductile-brittle regime grinding, and brittle regime grinding. The critical depth of cut exists between our experiment range. As point B stood for a depth of cut of 4.5 μm, it can be inferred that the 4.5 μm depth of cut might be the critical point.

Conclusion

This project aimed at a deeper understanding of the grinding process for brittle materials such as SiC. In this project, we focused on the effects of several vital factors (feed speed, depth of cut and wheel speed) on the normal force and processed surface roughness. During our investigation, it was observed that the surface roughness after grinding was enhanced by higher feed speed, higher depth of cut and lower wheel speed. Additionally, normal force increased with an increasing depth of cut or by  reduced wheel speeds. Moreover, surface morphology was assessed to estimate a range for proper grinding. Through the SEM analysis, it is concluded that within a shallow depth of cut (less than 4.5 μm), the workpiece would display a ductile-regime with a plastic flow only until several micrometers from the top surface, leaving the internal tissue unchanged. However, as the depth of cut continues increasing, fracture cracking is triggered at both shallow surface and bulk areas of the workpiece, which indicates  the occurrence of an unwanted brittle-regime. It can be further inferred that as depth of cut increases, the brittle-regime is enhanced and might replace the ductile-regime. Based on our experimental design, it is suggested that a 4.5 μm depth of cut might be the critical point for grinding on an SiC workpiece.

References:

  1. Bifano, T. G., 1988, “Ductile-Regime Grinding of Brittle Materials,” Ph.D. Thesis, NC State University, Raleigh, NC.
  2. Chen J, Shen J, Huang H, Xu X. Grinding characteristics in high speed grinding of engineering ceramics with brazed diamond wheels. J Mater Process Technol 2010;210:899–906.
  3. Shearer, Thomas R., “Diamond Wheel Grinding 101,” Ceramic Industry magazine, June 2006, pp. 17-20.
  4. S., andSathyanarayanan.G., 1987, “An Investigation into the Mechanics of Diamond Grinding of Brittle Materials,” 15th North American Manufacturing Research Conference Proceedings, Vol. 2, Manufacturing Technology Review, pp. 499-505
  5. Chen M, Zhao Q, Dong S, Li D. The critical conditions of brittle–ductile transition and the factors influencing the surface quality of brittle materials in ultra-precision grinding. J Mater Process Technol 2005;168:75–82.
  6. Huerta, M., and Malkin, S., 1976, “Grinding of Glass: The Mechanics of the Process,” ASME JOURNAL OF ENGINEERING FOR INDUSTRY, May, pp. 459-467
  7. Krauskopf, B. Diamond Turning: Reflecting Demands for Precision, Manuf Engng, 92 (5), 1984, 90-100
  8. Blake, P. N. Ductile-Regime Diamond Turning of Germanium and Silicon, PhD Thesis, North Carolina State University, 1988
  9. Blake, P. N. and Scattergood, R. O. Ductile Regime Machining of Germanium and Silicon, J Amer Ceram Soc 73 (4) 1990
  10. Cheng, J., Gong, Y.D., 2013. Experimental study on ductile-regime micro-grinding character of soda-lime glass with diamond tool. Int. J. Adv. Manuf. Technol. 69,147–160.
  11. Xiaorui, F,. and Michele H. M., 2007, “Force Analysis for Grinding with Segmental Wheels,” Machine Science and Technology, Vol. 10, pp. 435-455

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