Ceramic Industry

Grit Matters

November 29, 2002
Tests have shown that the grit size used in the machining process is an important factor in eliminating strength degradation in advanced ceramic components.

To achieve a cost-effective process and control strength degradation when machining advanced ceramic components, adaptive machining approaches should be used, in which the machining condition is selected according to the actual ceramic material, as well as its intended application.
Dimensional and surface finish requirements of advanced ceramic components are most commonly met by machining (grinding) the component with a diamond wheel. However, diamond-wheel grinding leaves the machined surface with grinding damage, and this damage can have an adverse effect on the strength of the machined components. To evaluate the strength of advanced ceramics, a three- or four-point bending test of type B modulus of rupture (MOR) bars is usually used. The test surfaces of these bars are often prepared using diamond wheel grinding so that the test results will provide an accurate representation of the product’s actual performance. Because grinding condition s such as wheel grit size, depth of cut, wheel speed, table speed, wheel diameter, etc., influence the surface integrity and subsurface condition of the machined specimen, standards from ASTM International (formerly the American Society for Testing and Materials) provide guidelines for some of the grinding conditions used in preparing the test specimens. Recently, ASTM standard C1161 (Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature) was revised.* The main difference between the old standard (C1161-94) and revised standard (C1161-02b) is the grit size of the diamond wheel—the old standard calls for the use of 320-grit wheels, while the new standard recommends the use of 600-grit wheels. As ceramic manufacturers begin adopting the new standard, it will be helpful to know the difference in the flexural strength of MOR bars prepared with the old and the new standards so that they can accurately relate to the flexural strengths obtained with the new standard. Manufacturers also need to understand that the new standard might provide different values when machining different grades of ceramics. While the flexural strengths obtained with both standards will likely be similar for lower grade ceramics with internal flaws, porosity, etc., the strengths for higher-grade ceramics will be quite different.

Table 1
To gain a better understanding of these factors, researchers embarked on a study to investigate the difference in flexural strengths of MOR bars ground according to the guidelines in the old and new ASTM standards for a commercially available high-grade silicon nitride. The results of this study can help ceramic manufacturers better understand their own test results and more easily relate them to their manufacturing processes.

Table 2

Scope of Work

Grinding Conditions

The first set of 20 bars was machined with a 320-grit diamond wheel in accordance with ASTM C1161-94, and the second set of 20 bars was machined with a 600-grit diamond wheel in accordance with ASTM C1161-02b. The grinding conditions are given in Table 1, and the dimensional information of the machined specimens is given in Table 2.

Figure 1. Flexural results for 320- vs. 600-grit diamond wheel machining.
Testing Procedure

The bend tests were performed according to ASTM C1161. All modulus of rupture bars were tested at room temperature in four-point bending (ASTM C1161) on an Instron testing machine using a 1000-lb load cell. In the four-point fixture, the inner and outer spans were 20 mm and 40 mm, respectively. The test was performed at a cross-head speed of 0.508 mm/min. After testing, optical fractography was performed on the tested bars to ascertain the fracture origins. In the majority of cases, the bars seem to have failed from a machining-related flaw near the surface or at the chamfer.

Figure 2. Weibull results for 320- vs. 600-grit diamond wheel machining.

Results and Analysis

The flexural results are shown in Figures 1 and 2 and Table 2, and the Weibull parameters are summarized in Table 3 (p. 26). It can be seen that the average strength of the MOR bars is 936.4 MPa for the bars machined with the 600-grit wheel, versus 794.8 MPa for the bars machined with the 320-grit wheel, which is about a 16% difference. The Weibull modulus is 10.95 versus 7.78, which is about a 29% difference.

Table 3
It is known that grinding conditions can influence the strength of the machined MOR bars, and wheel grit size has been identified as one of the most important parameters. The influence of the grinding parameters on strength can be discussed in terms of one important parameter—the maximum grit depth of cut (hmax), defined as shown in Equation 1. , where vs is the wheel speed, vw is the table speed, a is the depth of cut, d is the wheel diameter, r is the ratio of chip width to average undeformed chip thickness, and C is the number of active cutting points per unit area of the wheel periphery (grit surface density), which depends on the grit size and concentration.1

Equation 1
Finer grit wheels have larger values of C, and coarser grit wheels have smaller values of C. For example, C = 50 lb/mm2 for a 1200-grit wheel, C = 27.6 lb/mm2 for a 320-grit wheel, and C = 11.66 lb/mm2 for a 180 grit wheel.2 The hmax is a measure of the amount of material removed by a single grit when it passes through the grinding zone. A larger hmax means larger machining-related scratches on the ground surface, and therefore larger cracks and a deeper layer of damage created by the grinding process. Equation 1 clearly indicates that the damaged layer will be deeper and that the size of the scratches on the surface will be larger for wheels of coarser grit size. The 600-grit wheel grinding has a smaller grit depth of cut than the 320-grit wheel grinding. Equation 1 also indicates that other grinding parameters—such as wheel speed, work speed and depth of cut—can influence the grit depth of cut and the degree of damage of the ground surface, which eventually determines the strength degradation. For example, the same hmax can be achieved with a coarser grit wheel under a higher wheel speed, but the percentage of fractured surface will be greatly reduced by using the higher grinding speed due to the smaller hmax values at the high wheel speed.3

Figure 3. The effects of internal flaw size on grinding conditions and strength degradation.

Applying the Results to Different Ceramic Materials

Based on the test results, it is understood that the strength of the ground MOR bars is reduced about 16% if the bars are ground with the 320-grit wheel instead of the 600-grit wheel for the chosen silicon nitride. However, these results do not imply that the strength degradation will be 16% for any ceramic ground with a 320-grit wheel, nor do they imply that the strength of the machined MOR bars will be even higher if the ceramic is ground with an 800- or even 1000-grit wheel. Rather, the ceramic’s internal flaw size determines the wheel grit size or the grit depth of cut under which the strength degradation will be totally eliminated. This fact is illustrated in Figure 3. As seen in the figure, Material #2 can be ground without damage and maintain its designed strength of s2 if the grinding conditions are chosen so that the grit depth of cut (hmax) is smaller than the critical value required for that ceramic (hc2). Mayer calculated that the critical grit depth of cut is 0.16 micron for a hot pressed silicon nitride ceramic with designed strength of 800 Mpa.2 This also explains why reaction-bonded silicon nitride ceramics are generally much less sensitive to grinding conditions than sintered reaction-bonded silicon nitride ceramics.4

Applying the Results to Production Operations

It is understood that grinding conditions can influence the strength of machined test specimens and machined components due to the scratches (damages) created on the machined surface and subsurface, and that the size of the scratch is determined by the machining conditions. It is also understood that the same scratch can have a significantly different influence on different ceramics, depending on the internal flaw size of the ceramics. We now need to address two important issues: how to grind test specimens and how to grind components. To test material properties using test specimens, we often follow ASTM C1161 to grind the specimen. It therefore follows that in order to achieve the same surface integrity as the test specimen with a 600-grit wheel in production (based on the new standard), we would need to machine the components under the same conditions used in testing. However, the material removal rate achievable with the 600-grit wheel is very low, and therefore the machining cost will be high. To achieve a cost-effective process and control strength degradation when machining advanced ceramic components, adaptive machining approaches should be used, in which the machining condition is selected according to the actual ceramic material, as well as its intended application.4,5 For example, a given ceramic engine valve might need to be ground with a fine-grit (600-grit) wheel to minimize machining damage and maximize strength, but a coarser-grit (320-grit) wheel could be used to grind the valve on all surfaces except those that are prone to failure, thereby achieving more cost-effective machining. The functionality of the component and the machinability of the material are both key factors in determining the best machining approach.

Maximizing Productivity

The purpose of this study was to compare the old and the new ASTM standards for machining MOR bars. Researchers found that grinding with a 320-grit wheel (ASTM C1161-94) versus a 600-grit wheel (ASTM C1161-02b) lowered the strength value by 16% and the Weibull modulus by 29% on HIPed silicon nitride MOR bars when the other grinding conditions were kept the same. However, the influence of the machining conditions on strength degradation depends on the material being tested. Manufacturers can therefore minimize the effect of the machining process on their components—and maximize their productivity—by using machining approaches in both testing and production that are specifically tailored to their own operations.

References

1. Malkin, S., Grinding Technology – Theory and Applications of Machining with Abrasives, SME, 1989.
2. Mayer, J., “Grinding of Ceramics with Attention to Strength and Depth of Grinding Damage,” Proceedings of the 3rd International Grinding and Machining Conference, 1999, p. 665.
3. Kovach, J. A., Laurich, M. A., Zeigler, K. R., Malkin, S., Sunderland, J. E., Guo, C., Zhu, B., and Ganesan, M., “Development of High Speed Grinding Technology for Ceramic Components,” Proceedings of NAMRI, Vol. 23, 1996, pp. 51-56.
4. Guo, C. and Chand, R.H., “Adaptive Ceramic Machining,” Cutting Tool Magazine, Vol. 50, No. 5, 1998, pp. 90-98.
5. Quinn, G. D, “Overhaul of ASTM C 1161 Flextural Strength Specimens,” American Ceramic Society Bulletin, Vol. 81, No. 5, May 2002, p. 65.

For More Information

For more information about the research presented in this article, contact Chand Kare Technical Ceramics, Div. Chand Associates, 2 Coppage Dr., Worcester, MA 01603-1252; (508) 791-9549; fax (508) 793-9814; e-mail info@chandassociates.com; or visit http://www.chandassociates.com.

*ASTM C1161-02 "Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature," Annual Book of Standards, Volume 15.01, ASTM, West Conshohocken, PA, 2002. (This standard was first adopted on Dec. 27, 1990, and was designated C1161-90, where the last two digits denote the adoption year. A minor revision was made in 1994. Major revisions have been made in 2002, and the current version is designated C1161-02b.)