Scope of WorkGrinding 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.
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.
Results and AnalysisThe 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.
Applying the Results to Different Ceramic MaterialsBased 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 OperationsIt 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 ProductivityThe 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.
References1. 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 InformationFor 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 firstname.lastname@example.org; 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.)