книги / FISMA and the risk management framework the new practice of federal cyber security

where V is the volume of material removed, L the length of travel, P the load, and H the hardness of material. Here, k is the wear coefficient which should remain constant unless the mechanism of wear changes. This would imply a linear relationship between the material removal rate and load. Fig. 4 shows that this is not the case.

A model based on the fracture-dominated process of material removal was proposed by Xu and Jahanmir [24]. They modeled the material removal from scratch tests in ceramics based on fracture mechanics. They considered that microcracks initiated from grain boundary flaws as a result of loading and that the driving force for propagation of these microcracks was the local grain boundary stress. They derived the following equation:

where dV/dn is the volume of material removed per pass, H the hardness, P the load, E the modulus of elasticity, I the grain size, and A and C are constants that depend on the experimental conditions such as relative humidity, lubrication, sliding speed, surface roughness, and indenter shape. This equation implies that material removal rate is directly proportional to the load squared if other variables are held constant. The curves in Fig. 4 were fitted according to the relationship dV/dn The coefficients of determination R2 ranged from 83.33% to 98.5%.

The subsurface damage in alumina in Belt abrasive machining was also greater than in silicon nitride. In addition to the behavior discussed above, temperature rise during abrasive machining is a major factor. Increased contribution to damage would be expected from the thermal effects during machining of the low thermal conductivity material. The role of lubricant in the subsurface damage is significant, as they bring down the temperature at the tool material interface. The lubricants also contribute to higher material removal rates.

CONCLUSIONS

In scratch tests, the material removal rates for alumina and silicon nitride increased with increasing load and after a certain load the increases became much more rapid. The material removal rate for silicon nitride was lower than that for alumina both in scratch test and belt abrasion test. With the increase in belt speed in belt abrasion test, the material removal rate decreased because of heating at the contact surface.

In both scratching and belt abrasion of AlMgB w 70wt.% TiB2 , the mechanism of failure was micro-fragmentation. There was no indication of cracking in {J-BN in both scratching and belt abrasion, instead plastic deformation was exhibited on the surface. The abrasion resistance of AlMgBw70wt.% TiB2 was found to be superior to that ofWC+Co and SiC but lower than that of(3-BN.

The existence and the nature of the cracks in ceramics are controlled by the environment and the mechanical properties of the material.