Advanced ceramics: evaluation of the mechanism of stock removal and ground surface quality

Advanced Ceramics: Evaluation of the Mechanism of Stock Removal and Ground Surface Quality

Eduardo Carlos Bianchi

Eraldo Jannone da Silva

Carlos Elias da Silva Júnior

Paulo Roberto de Aguiar

Ivan De Domenico Valarelli

Rodrigo Daun Monici

Mechanical Engineering UNESP. Bauru. SP. Brazil

The aim of this work is to evaluate the mechanism of stock removal and the ground surface quality of advanced ceramics machined by a surface grinding process using diamond grinding wheels. The analysis of the grinding performance was done regarding the cutting surface wear behavior of the grinding wheel for ceramic workpieces. The ground surface was evaluated using Scanning Electron Microscopy (SEM). As a result it can be said that the mechanism of material removal in the grinding of ceramic is largely one of brittle fracture. The increase of the h_max can reduce the tangential force required by the process. Although, it results in an increase in the surface damage, reducing the mechanical properties of the ground component.

Keywords: Advanced ceramics, SEM, diamond wheel, grinding

Introduction

Today's advanced ceramics are being used in the manufacturing of workpieces which are subject to many severe and complex demands, with high-performance materials being required due to the unique combination of particular properties to such materials. Typically they may be used in special situations, since their use on a large scale may be limited unless there is economic and technical justifications. Applications include precision bearings for the use in the nuclear industry; automotive components (sensors, insulators, catalysers, pistons, jackets, inserts, valves, linings); biocampatible implants (coxofemoral and dental prostheses, bone replacements, cardiac valves); wear parts (valve seats, mechanical seals, guides); refractories (insulators, rocket lining plates, military linings, furnace components); substrates, bases and insulators in electrical components.

According to Inoue, Kihara and Arakida (1989), the manufacturing process for these components consists essentially of casting, followed by green machining, sintering and finishing machining by an abrasive machining process. This stage involves grinding, lapping and polishing with diamond abrasives.

The cost of machining is the main obstacle to the manufacturing of ceramic on a large industrial scale (Xu and Jahanmir, 1994). An objective is therefore to increase machining performance and efficiency without damaging the mechanical properties of the end product. However, in order to do so it is essential to understand the mechanism of material removal on a microstructural scale and the relationship between the microstructure of the material and the formation of a damaged layer caused by the machining process itself.

The grinding process, the most important stage in the machining of advanced ceramics, is highly complex and involves the contact between a great number of abrasive particles and the surface of the workpiece. Moreover, the characteristics of the grinding of advanced ceramics are very different from those in the grinding of metals, and further studies are still required to obtain a more comprehensive understanding of the process and to obtain improved control of the machining parameters. Studies have been carried out in this area (Malkin and Hwang, 1996).

An understanding of the material removal mechanisms and their effects on the mechanical properties of the workpiece, cost factors and surface quality is extremely important in the manufacturing of advanced ceramics, as faults such as surface and internal cracking, non-uniform voids, shapes and grain sizes serve to increase the stresses in the component, in some cases causing it to fracture at stresses below an acceptable level.

This paper presents a study demonstrating the influence of machining conditions on the grinding of advanced ceramics, according to variations in the equivalent cutting thickness h_eq, resulting in different values of maximum uncut chip thickness h_max, when using diamond resin bond grinding wheel.

Some Concepts on Ceramics Grinding

In the machining of brittle materials, the action of each abrasive grain causes deformation on the material. In the zone next to that one of plastic flow, the cutting depth of the grain increases until it becomes large enough to cause cracks in the structure of the material that is being machined. These fracture mechanisms have been studied by Malkin and Ritter (1989) and are discussed further.

Influence of Process Parameters on Ceramic Machining

In the case of surface grinding, the specific grinding energy (u), which is an important parameter in the grinding of brittle materials since its value results from the mechanisms associated with the interactions between the abrasive grain and the workpiece, is expressed by the Eq.(1) (Malkin, 1989):

where Ft_c is the cutting force, v_s is the grinding cutting speed, v_w is the workpiece speed, a is the cutting depth and w is the grinding width.

The h_eq can be expressed by the Eq.(2) (Malkin, 1989):

In the grinding of ceramic, the material removal mechanism will include chip formation characterized by plastic flow or brittle fracture (shaving) as the cutting depth increases. If the cutting depth is large enough to cause cracks, a chip will be removed due to fracture of the material. When fracture occurs, the specific grinding energy (u) is lower than in chip formation, but the finished surface is damaged and its strength after grinding is reduced. Quantitatively, the parameter h_max(maximum uncut chip thickness) characterizes the depth of penetration of the abrasive grain into the workpiece when it is engaged in cutting. The value of h_maxdepends on both machine and wheel parameters and it is defined by the Eq.(3) (Malkin, 1989):

where: C is the number of active cutting points per unit of area of the wheel periphery (grit surface density); r is the ratio of chip width to avarege undeformed chip thickness; d_sis the wheel diameter.

The Eq.(3) shows that not only the machining parameters (a, v_w, v_s) modify the h_max but also the wheel parameters, such as: C and r. The formers depend on the wheel topography, which includes the distance between the consecutive grains. This distance is related to the grain grit size, the diamond type and the concentration.

According to Guo and Chand (1998), in general, a smaller h_maxis desirable to generate smoother surfaces, which is beneficial to reduce machining induced damage. For achieving high material removal rate, however, a higher value of the product (a.v_w) is desirable. This in turn would make h_max larger. In other words, high material removal rate often contradicts with the smaller h_max, or low damage. Grinding with high wheel speed, called high speed grinding (HSG), can achieve both high material removal rate and low uncut chip thickness (low damage) (Guo and Chand, 1998). Creep-feed grinding (high depth of cut and low workpiece speed) can also lead to both high material removal rate and low uncut chip thickness.

The Fig.(1) illustrates h_max :

Fracture Mechanisms Through Indentation

According to Malkin and Ritter (1989), the indentation fracture mechanisms approach likens abrasive/workpiece interactions in the grinding of ceramics to localized small-scale indentation events. The deformation and fracture pattern observed under normal contact on glass and ceramics using a Vickers indenter is shown schematically in Fig.(2).

Here it can be observed that there is a zone of plastic (irreversible) deformation below the indentation and two main crack systems derived from that zone (the median/radial and the lateral cracks) can be identified. The behavior of these cracks is affected by residual stresses derived from non-uniform deformation of the material into elastic/plastic deformation. The radial cracks are initiated by a wedge-like action when the load is applied and they may continue to propagate as the load is removed due to the residual tensile stresses acting on the tip of the crack. Lateral cracks are observed to initiate and propagate by residual stresses only when the indentation load is removed. The residual stress field drives both types of cracks. While the median/radial cracks are associated with the degradation in the strength of the workpiece material, the lateral cracks are assumed to be responsible mainly for material removal by erosion and abrasion.

After some considerations on this mechanism, Malkin and Ritter (1989) reported that this approach could be extended to abrasive wear and grinding, which was done by Evans and Marshall (1981). They created a physical model modifying that, described previously, taking into account the tangential movement of an abrasive grain along the surface, as shown in Fig.(3), where a region can be observed, around the plastic zone, in which shaving of the material is most intense.

The material removal mechanisms at work in the grinding of ceramics have often been studied through microscopic observations of the ground surface. The removal of an alumina polycrystalline structure when it is being ground with a diamond wheel occurs primarily by fracture mechanisms, although there is also evidence of plastic flow of the alumina (Malkin and Ritter, 1989). Therefore, when the grain initially passes across the surface of the ceramic, internal stresses are created in the subsurface of the material causing cracks to initiate and leading to a degradation in the strength of the material and shaving. Thus, in the subsequent passes of the wheel across the surface, most of the specific grinding energy will not be spent in chip formation because the material is already brittle, that is, some of its bonds have already been broken (there are cracks already); the grain will only remove this material. According to Malkin and Hwang (1996), SEM observations of grinding debris indicate material removal mostly by brittle fracture, except at extremely fine grinding conditions in nanometrical depth of cut.

Adapting the Machining Process to the Ceramic Material

Many different technical ceramics and composites are now commercially available. They are developed to have distinct thermal, mechanical, and electrical properties for various commercial applications. Years of ceramic machining experience has taught that ceramics also have different grinding characteristics (Guo and Chand, 1998). Some ceramics can be ground much faster than the others. However, the material’s data sheet does not provide information regarding the material’s machining conditions. There is no correlation between the machining characteristics of a ceramic and its published properties such as hardness, fracture toughness, flexural strength, elastic modulus and thermal conductivity (Guo and Chand, 1995a). With the development of the ceramic grindability test system (Guo and Chand, 1995b; Chand and Guo 1996), quantitative measurement of machining characteristic of a ceramic material, called "grindability" has become, for the first time, a reality. The grindability results in Fig.(4) indicate that ceramics have distinct grindability. It should be noted that within a family of material such as silicon nitride, there is a wide variation in grindability. The grindability results, however, correlate well with surface grinding parameters such as normal force, and specific energy, etc. as shown in Tab.(1). It is clear that ceramics of higher grindability require lower specific energy and grinding force.

Grindability is defined as a specific characteristic of ceramic material and measured in terms of normalized material removal rate (Guo and Chand, 1998). This information on grindability therefore becomes the basis for choosing diamond wheel and processing parameters. For example, a ceramic with high grindability (Al₂O₃ – grindability 180x10^-4mm³/mN) can be machined with higher material removal rate than ceramics with low grindability (Si₃N₄ – grindability of 20x10^-4mm³/mN). The maximum material removal rate, however, can sometimes be limited by factors such as part geometry and materials susceptibility to machining induced damage. Generally, ceramics with large internal flaws are less sensitive to machining induced damage than ceramics with small internal flaws. This is illustrated in Table 2 where the strength of the bonded silicon nitride (a low strength ceramic) is much less sensitive to machining conditions than hot pressed silicon nitride (a high strength ceramic) (Guo and Chand, 1998).

Description of the Test Bed

In order to carry out tests to evaluate the influence of some process parameters on ceramic machining, a test bed was constructed consisting of two parts, each one with specific functions. The electronic part of the apparatus forms the interface between the mechanical part and the computer, and the computational part processes the data produced in the course of the test and displays the results, which will be studied below. Figure 5 shows a general outline of the rig developed to carry out the tests.

The power in the induction motor is transmitted to the drive pulley, which is attached to the main spindle of the grinding machine. A torquemeter, placed between the motor and the drive pulley, measures the instantaneous torque necessary to carry out the grinding operation. Coupled to the main spindle, which holds the grinding wheel, was attached an encoder that measures the instantaneous angular position of the wheel. This rotation is altered by a frequency inverter according to the need of each test condition. The voltage values, proportional to the torque and the rotation, are simultaneously received by a computer through an A/D data acquisition system. The total instantaneous power required by the operation is obtained by the product: total torque x rotational speed. The instantaneous cutting power (P_ci) on each part of the workpiece is obtained by subtracting the residual average power (no-load, not grinding) from the total instantaneous power. The former is obtained by determining the rotational speed values and the residual torque for a certain period of time and by calculating the average value of both products. The average cutting power (P_cm) in each stroke is obtained from the average of the instantaneous cutting power values at each point. The software developed to control the test bed is responsible for determining the values of the instantaneous and average cutting force (Ft_c) by using a cinematic equation that relates cutting power (P_c) and cutting force (Ft_c).

In the grinding tests, only half of the width of 10mm-wide grinding wheel was used (5 mm). Consequently, after each test, the grinding wheel presented two distinct regions (a consumed and a non-consumed one). In order to evaluate the radial wheel wear, the grinding wheel profile after grinding was marked in a mild steel plate and the differences in the two regions evaluated using a TESA displacement gauger, model TT10. The G ratio for each test was calculated by the ratio of the volumetrically wheel wear and the volume of material removed. A broad band acoustic emission transducer with a frequency range of 50-100 kHz carried out the acoustic emission measurement. The piezoelectric sensor was attached close to the part to be ground, and its module held circuits necessary for the signal processing, providing calibration and sensitivity adjustments through the front panel potentiometers, delivering the RMS signal at the output.

Test Methodology

Preliminary tests were carried out in order to establish the machining conditions that would be applied for the tests. According to data on diamond grinding reported in the literature, a cutting speed of v_s = 35 m/s was adopted which would be a constant for all the trials. It was noted that with low values about 10 mm for the penetration of the wheel into the workpiece, the cutting force was so low that it could not be measured. Consequently a number of tests were carried out increasing values of penetration into the workpiece until it was established values from 80 mm the cutting force became such that it could be read and recorded. This value became the constant for all the tests.

Three values for workpiece speed v_w were determined, 65 mm/s, 87 mm/s and 109 mm/s, used respectively for the three values of equivalent cutting thickness h_eq: 0.15 mm, 0.20 mm and 0.25 mm.

The test piece was rectangular in shape, in standard dimensions for this study - 120 mm long x 5 mm thick x 50 mm high - made of 96% alumina. This is 96% alumina (all with the remaining 4% made up of fluxes such as: SiO₂, K₂O, Na₂O, MgO and CaO. These fluxes are additives which reduce the sintering temperature, increase densification and limit grain size growth.

The diamond grinding wheel used for these tests is specified as D107N115C50, where D indicates the type of abrasive (diamond), 107 refers to diamond grain size (107 mm), N denotes the hardness of the wheel (average hardness), 115 indicates the type of diamond, and C50 refers to the diamond concentration.

The apparent average material density (D_ap) was 3,73 g/cm³ or 95,1% of the theoretical density (D_th), which is equal to 3,92 g/cm³. Its Vickers’ hardness was 3,8Hv (98N) (GPa), or 380Hv in kg/mm². The indentation obtained by Vickers’ macro-hardening is shown at Fig.(6). At the Fig.(7), it’s possible to observe the alumina’s surface before grinding.

Results and Discussion

Results of Cutting Force

The results for cutting force obtained in the tests are shown in Fig.8.

It can be observed in Fig.(8) that the highest values were achieved in the test in which the lowest h_eq value was used (0.15 mm). As the values for the penetration (depth of cut) of the grinding wheel (a) and cutting speed (v_s) were kept constant throughout the tests, the lowest h_eq value means that the workpiece speed (v_w) used was the lowest, according to the Eq.(2). Since workpiece speed is a factor directly related to the impacts of the abrasive grain on the workpiece, a lower value for v_w indicates smaller impact or less intense impacts in the grain/workpiece contact area. Moreover, according to the Eq.(4), keeping constant the others variables, there is a decrease in the parameter h_max, which can lead to reduce the machining induced damage.

Since in the grinding of ceramic both types of material removal mechanism (brittle fracture and/or ductile flow) may occur, at a lower h_eq there is a reduction in the degree of brittleness of the material removal mechanism due to the smaller impacts between the abrasive grain and the workpiece, and mainly by the reduction of the h_max(the lowest value among the tests). There is a decrease in the formation of either radial cracks (relating to a reduction in the strength of the material) or lateral cracks (which are responsible for material removal by abrasion) in the grinding area. Therefore greater grinding energy is required to cut the workpiece material, which, together with a proportion of the dissipative forces (heat produced by friction, acoustic emission, etc.), results in higher values for the cutting force. Although, the material removal mechanism is brittle fracture yet. This statement will be further verified when analyzing the SEM microscopy of the ground surface for this test.

The lowest tangential force values were obtained in the test performed with the highest h_eq parameter value (0.25 mm). This means that the workpiece speed (v_w) was also the highest, resulting in the greatest impacts between the abrasive grain and the workpiece and, besides, it leads to increase the h_max. Therefore, the material removal occurs mainly by brittle fracture, in a highest degree among the tests performed, because the surface shaving of the material is more intense when the workpiece is highly fragile due to the increasing of the depth of penetration of the abrasive grain into the workpiece when it is engaged in cutting. In such a case, comparatively there is a greater formation of lateral cracks (responsible for the material removal) and radial cracks (reduction in strength). Therefore, as the material is already fragile (broken bonds), the grinding wheel only removes this material, which results in lower cutting forces. The dissipative force components are lower as well as a lower cutting effort means a smaller amount of heat generated through friction. The surface finish produced in this case is therefore not of such high quality, because the surface shows a great amount of shaving.

In the test in which the h_eq value 0.20 mm was used, is represented an intermediate machining condition, when compared to those mentioned above (intermediate value of h_max). There were both radial and lateral cracking, but, probably in a proportion between the two conditions referred above.

Results of Acoustic Emission

Acoustic Emissions (AE) are acoustic waves generated inside a material under external pressure. Its frequency range goes from 50 kHz to 1000 kHz (Beattie, 1983; Blum and Dornfeld, 1993), which is above the range of many noises coming from sources outside of the own grinding process. One of the areas of technology that has expanded considerably in the last ten years is the use of acoustic emission for monitoring the machining process and the condition of the tool in the machining of metals (Kannatey, Asibu and Dornfeld, 1987).

The acoustic waves are subjected to many effects such as damping, speed dependent frequency, reflection and other phenomena due to the diffusion of the waves as they pass through the material (Inasaki, 1990).

The acoustic emission is seen as the power resulting from the interaction between the abrasive grain and the workpiece, which diffuses through the structure of the material. In such a case it can be related to the specific grinding energy (u), which is also a power value associated with the grinding process. As referred to above, the parameters cutting force (F_tc), grinding width (w) and equivalent cutting thickness (h_eq) all contribute to the calculation of specific grinding energy. As the same grinding wheel and the same grinding width was used for all the tests, only the F_tc, and h_eq values will influence on the calculation of specific grinding energy.

Based on the results obtained for cutting force, it can be seen that a higher tangential force occurred in the test carried out with the lowest h_eq value of 0.15 mm (the first test), due to the reduction in the degree of brittleness of the material which is directly related to the decreasing of the h_max. Therefore, comparing to the other tests, the energy spent to remove the material is higher because the lateral and radial cracks are less induced. It reduces the surface damage and the proportion of material crushed beneath and ahead the tool. In this case the values obtained through the acoustic emission were greater, as may be seen in Fig.(9), and also observed in the results for the cutting force.

Conversely, in the test carried out with the highest h_eq value (0.25 mm) the force F_tc was lower. Therefore, the material removal occurs mainly by brittle fracture, in a highest degree among the tests performed. There is also a considerable amount of material shaving. As a result, the proportions of dissipative forces (friction, heat, acoustic emission, etc.) are lower because the material is already fragile. Total forces are therefore lower and so the AE values (Fig.9).

In the intermediate case (where h_eq = 0.20 mm), there were both radial and lateral cracking, but, probably in a proportion between the two conditions referred above. The values of AE observed during this test assumed an intermediate position among others.

Results of the G Ratio

The results obtained for G ratio are shown in Fig.(10). It can be seen that the highest value for G ratio was obtained in the third test, which used the highest values for equivalent cutting thickness (h_eq), workpiece speed (v_w) and consequently, the highest h_max. This is because a high value for workpiece speed means a higher level of impacts between the abrasive grain of the grinding wheel and the workpiece. Moreover, the highest value of h_max implies in an increase in the surface damage and in the number of radial and lateral cracks and the material removal process is mostly one of brittle fracture. Consequently, the cutting force and the necessary energy to material removal decrease, due to the superficial damage induced. The abrasive grains are mainly required to remove a layer of material which is already broken. As a result, the wear of the abrasive grains is reduced because the low cutting forces produce little wear in the bond and in the grains of the grinding wheel, resulting in a low volume of grinding wheel used in the test.

For the first test, which used the lowest h_eq value, and, consequently, the lowest h_maxthe G ratio measured was also the lowest. Similarly, for a lower h_eq value (a lower v_w) there are fewer impacts per unit of time between the abrasive grains and the workpiece. The lowest h_max implies in decreasing of subsurface damage. Therefore the workpiece surface and subsurface are not as damaged in terms of the number of cracks as in the case of a highest h_max. Because of this, cutting forces become higher, the abrasive grains are more required than in other tests. As a result, the wear of the abrasive grains is increased because of the higher cutting forces produced in the test.

As expected, in the second test, which used the intermediate h_eq value, the G ratio resulted in an intermediate value between the other two test.

Observations of the Ground Surface

Samples of each test were analyzed using a PHILIPS SEM microscope, model SEM 515. The samples were revested with gold, 10nm thick.

The Figures 11, 12 and 13 present the ground surface of trials using h_eqequal to 0.15, 0.20 e 0.25 mm, respectively.

The Fig.(13) presents the most fragmented surface, with the highest numbers of cracks when comparing to the others Fig.(12 and 11). At this test, were obtained the lowest values of F_tc and AE and the highest value of G ratio (see Figure 10). It confirms the prevailing of the fragile mechanism of material removal, and the increasing of the surface damage as the h_max is increased. The adoption of higher values h_max can cause a increase in the G ratio (through the decrease in the abrasive disc’s wear), but it causes a decrease in the final quality of the ground surface and in its superficial integrity. As the h_max is decreased there is a reduction in the surface damage and very small portions of material are removed by ductile flow, represented by the scratches observed in the surface of the testpiece (Fig.11). Although, the predominant material removal mechanism is brittle fracture.

Conclusions

Based on previous research and on the results obtained in the tests, the following conclusions can be drawn:

The mechanism of material removal in the grinding of ceramic is largely one of brittle fracture. The tangential force required by the process can therefore be reduced. Although, the increase of h_max results in an increase in the surface damage, reducing the mechanical properties of the ground component.

The results obtained for cutting force indicate that as the value of the h_eq parameter increases, due to the increase in workpiece speed (v_w), the cutting force becomes lower. Moreover, the higher the value of h_max there is an increase in the surface damage and in the number of radial and lateral cracks and the material removal process is mostly one of brittle fracture. Consequently, the cutting force and the necessary energy to material removal decrease, and the AE does too, due to the superficial damage induced. The abrasive grains are mainly required to remove a layer of material which is already broken. As a result, the wear of the abrasive grains is reduced because the low cutting forces produce little wear in the bond and in the grains of the grinding wheel, resulting in a increase of the G ratio.

The microscope analysis of the ground surface using the SEM confirmed the results of F_tc, AE and G ratio obtained using the test bed developed. The increase of h_max leads to more damaged surfaces.

To improve large-scale ceramic grinding and achieve cost-effectiveness, with the reduction of the surface damage and, simultaneously increase of the material removal rate, adopting grinding techniques are required, such as grinding under creep mode and high-speed grinding. Decreasing the surface damage, the flexural strength of the advanced ceramics can be improved, increasing its quality and life in service.

Acknowledgments

Thanks to the CNPq and Master Diamond Ferramentas Ltda., DE BEERS do Brasil, KOHLBACK Companies for the material and technical support which they have kindly given to this study.

Manuscript received: September 1999. Technical Editor: Álisson da Rocha Machado.

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Publication Dates

• Publication in this collection
  28 Sept 2001
• Date of issue
  2001

History

• Received
  Sept 1999