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

GROOVE WIDTH

Fig. 2. Schematic record of a surface profile with scratch by profilometer.

In order to measure the groove width, scratch profile is recorded by a stylus profilometer to a high resolution. The software routine calculates the surface roughness (Ra) and assigns the zero line of reference which is displayed on the computer screen. The width of the groove is measured with reference to the zero line. The distance between the points where zero line intersects the two sides of the groove is taken to be the width of the scratch, as shown in Fig. 2. The width of each scratch is measured in many locations. The mean scratch width is reported as an indicator of the abrasion resistance of the material.

Belt Abrasion Tests

\;lotor

Lubricant

Level

I

Fig. 3. Schematic arrangement of the experimental setup for belt abrasion test.

This is one of the configurations of the ASTM G132 standard [19). The set-up for multipoint abrasion test consists of a diamond abrasive belt mounted on two rollers which are driven by a variable speed motor (Fig. 3). The specimen is loaded on the rotating belt surface. It is secured

to a spindle which is constrained by a bushing but is free to move in the vertical direction. The friction between the spindle and bushing is minimized by lubrication. In order to avoid clogging of abrasive belt with cutting particles, the whole set-up is positioned in a container filled with a lubricant/coolant.

MATERIALS AND PROPERTIES

Table I. Hardness and Fracture toughness of hard materials studied for abrasion resistance

The hardness and fracture toughness of the materials covered here are given in Table I. In the case of hard materials, these are the most relevant properties for abrasion resistance. Of the materials given in Table 1, Ah03, SiC and Si3N4 are the most common ceramic materials used where high hardness is needed. Cubic boron nitride (1~-BN) is a super hard material with good toughness but is fairly expensive. Included in the discussion is a newer material AIMgB14 which is made harder by the addition of TiB2 while still having reasonable fracture toughness. The interesting point to note here is that with 70 wt% TiB2 in AIMgB 14 along with the increase in hardness, fracture toughness is also increased. The hardness of AIMgBw70 wt.% TiBz is within 10-20% of that of !3-BN and is much higher than that of WC+Co or SiC. Since TiBz proportion in the material with optimum properties is greater than that of AIMgB 14, one could also consider

it as an enhanced TiB2 material.

those from scratch tests where material removal rates were the highest in dry condition [2]. This is attributed to the problem of clogging of abrasive belt with cutting particles in dry abrasion which reduces the abrasion efficiency. The lubricants promote dislodging of abraded particles from the belt. The bonding of abraded fragments to the cutting particles as in the dry condition also increases the rake angle which reduces the cutting action. In single-point scratch tests, these were not the pertinent factors.

Load, N

Fig. 7. Variation of scratch width with load for AIMgB 14 with different TiB2 proportions by weight.

Figure 7 shows the variation of scratch width with load for different TiB2 proportions in AIMgB 14 [22]. There are two observations that deserve to be noticed. Firstly, for any material,

the scratch width increases almost linearly with load. Secondly, for any load, the scratch width decreases with increasing TiB2 proportion up to 70 wt.%. For 80 wt.% TiB2, scratch width is

observed to increase considerably. For this composition, both hardness and fracture toughness are seen to decrease. This behavior supports the generally accepted relationship that a decrease in hardness or fracture toughness leads to the loss of abrasion resistance. At low loads of 20 and 30 N, microchips could be seen on the scratched surface but the critical depth for chip formation was barely reached, as elastic recovery was able to overcome plastic deformation in the grooved track.

s

=. 200

.-::

'C

~150

.c

~

";:

..... 100

~

rJ:J

50

0

Load, N

Fig. 8. Comparison of scratch resistance of AIMgB 14-70 wt.% TiB2 with reference materials listed in Table 1.

Figure 8 provides the comparison of scratch resistance of AIMgB 14-70 wt.% TiB2 with the reference materials (SiC, ~-BN, and WC+Co) given in Table 1 [22]. The boride material is considerably superior in scratch resistance to all the materials except ~-BN.

Figure 9 shows the variation in material removal rate as a function of belt speed in the belt abrasion tests for AIMgBwTiB2 with 0, 30, and 70 wt.% TiB2 [22). Included in this figure are also the plots for WC-Co and ~-BN. It should be noted that wear rate decreases with the increase in belt speed for any given load. With the increase in belt speed from 0.42 to 0.65 mls (A to B; as shown in Fig. 9(a)), there is a large decrease in wear rate for some materials but later from B to C the decrease occurs gradually for all the materials. At A, the belt speed is fairly low and so the heating effect at the interface is negligible. Thus cutting action by the abrasive particles is quite efficient resulting in high wear rate. With the increase in speed to 0.65 m!s corresponding to B, the temperature rise at the interface becomes significant so that the abrasive particles do more plowing and gouging than cutting. As the speed is increased from B to C, there is continuing temperature rise at the cutting interface so that the efficiency of abrasive action decreases. Thus, wear rate continues to decrease from B to C. The AIMgB14 materials with 30 and 70 wt.% TiB2 along with ~-BN exhibit higher abrasion resistance than the other two materials. This is because of their higher hardness (Table I) which offers more resistance to indentation under load. Apart from the difference in hardness, the large difference between the wear rates of boride materials with and without boride is because of the TiB2 phase which is harder than A!MgB 14. The wear rate of WC-Co is the highest because its hardness is the lowest of all the materials.

3:0.006

Belt speed, m/s

(a)

Fig. 9. Variation in material removal rate with belt speed for AlMgB 14 with different TiB2 proportions and the reference materials in Table I when specimens were loaded with (a) 5 N,

(b) 10 N, and (c) 20 N loads on their 9 mm x 3 mm surface. (Continue to the next page)