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

Fig 2. (Continue from the previous page) Force vs. distance scratch curves for each treatment condition.

DISCUSSION

It has been well recognized that the response of material surfaces to scratching involves elastic recovery, plastic deformation, and abrasive wear including shearing and micro-cracking.

The experimental results obtained in the present work showed that, despite the slightly lower toughness of the HTGN samples compared to the solution-annealed ones, nitrogen alloying in the near surface region led to fully ductile fractures during both scratch tests and Charpy impact tests. Besides, the nitrided cases exhibited enhanced resistance to plastic deformation associated to slight decrease in the work hardening rate. Therefore, the increase in specific energy consumed in the scratch tests is mainly due to a decrease in the amount of work done by the contact force when deforming the materials elasto-plastically. This hypothesis is in accordance with the experimentally observed relationship between the specific energy and the total indentation work during depth-sensing indentation tests, which is shown in Fig. 6.

WT, (nJ)

Fig. 6. Relationship between specific energy measured during scratch tests and total indentation work measured during depth-sensing indentation tests.

Regarding the suitability of the single pendulum scratch test to assess the tribological properties of high temperature gas nitrided SS, the experimental results allow pointing out three major qualities: (i) the scratch tests gives a good indication of the elasto-plastic behavior of nitrided cases submitted to contact and cutting stresses, (ii) the results obtained in the analyzed region have a great statistical significance due to its large size (the analyzed area on the sample's surface was circa 0.7 x 17 mm) and (iii) the depth of the scratch is around just ten percent of the hardened case depth.

CONCLUSIONS

1- High temperature gas nitriding strongly improves the scratch resistance of UNS S30403 austenitic SS. Increasing the nitrogen content in solid solution up to 0.5 wt-% leads to an increase in the specific energy from 1770 to 3540 Jig. This can be attributed to the strong hardening effect of nitrogen in solid solution, which does not affect work hardening and toughness significantly.

2 - A linear correlation between the mass loss rate during vibratory cavitation, measured in a previous work, and the machinability (inverse of the specific energy) of high temperature gas nitrided UNS S30403 austenitic SS was observed.

3 - The single pass scratch test is suitable to assess the performance of case hardened HTGN SS, submitted to dynamic contact and cutting stress allowing to measure the abrasion resistance and to estimate the cavitation erosion resistance of these steels.

REFERENCES

1.Gavriljuk V. G. (1996) ISIJ!nt 36 (7), 738-745.

2.Hlinninen H., Romu J., Ilola R., Tervo J. and Laitinen A. (2001) J Mater. Process Tech. 117 (3): 424-430

3.Tervo J., Mater. Sci. Forum 1999; 318-320: 743750.

4.HlinninenH. (1999)Mater. Sci. Forum 318-320:479-488.

5.Berns H., Eul U., Heitz E. and Juse R. (1999) Mater. Sci. Forum 318-320, 517-522.

6.Gavriljuk V. G., Berns H. (1999) High Nitrogen Steels. Berlin: Springer-Verlag.

7.Thomann, U. and Uggowitzer, P. (2000) Wear 239, 48-58

8.Gavriljuk, V. Nitrogen in iron and steel. ISIJ /nt 1996; 36 (7): 738-745.

9.Berns H. (1996) ISJJ Int 36 (7): 909-914.

10.Toro A., Alonso-Falleiros N., Rodrigues D., Ambrosio filho F. and Tschiptschin A. P. (2002) Trans. Indian Inst. Met. 55 (5), 481-487.

II.Berns H., Siebert S. (1993) Proceedings of Int. Conf on High Nitrogen Steels - HNS,

1993, Kiev: Institute of Metal Physics, p. 566-571.

12.Siebert S. Doctoral thesis, Bochum: Ruhr-University, 1994.

13.Berns H. and Siebert S. (1996) ISJJ Int 36 (7), 927-931.

14.Berns H., Bouwman J. W., Eul U., Izaguirre J., Juse R., Niederau H., Tavernier G. and Zieschang R. (2000) Mat.-wiss. U. Werkstofftech 31 (2),152-161.

15.Toro A., Misiolek W., Tschiptschin A. P. (2003) Acta Mater. 51 (12), 3363-3374.

16.Garzon, C. M. and Tschiptschin A. P. (2004) Mat. Sci. and Tech. 51(12), 915-918.

17.Garzon, C. M., Toro A. and Tschiptschin A. P. (2002) Trans. Indian Inst. Met. 55 (4), 255-263.

18.Mesa D. H., Toro A. and Tschiptschin A. P. (2003) Wear 255 (l-6), 139-145.

19.Dos Santos J. F. , Garzon C. M. and Tschiptschin A. P. (2004) Mat. Sci. and Eng.- A 382 (1-2), 378-386.

20.Vingsbo 0. and Hogmark S. (1984) Wear 100 (l-3), 489-502.

21.Jiang J., Yao M., Sheng F. and Gao X. (1995) Wear 181-183 (5), 371-378.

22.Williams J. A. (1996) Tribology lnt 29(8), 675-694.

23.Hu W., LiS., LiS., Sun X. and Guan H. (1999) Tribology lnt 32 (3), 153-160.

24.Paro J., Hanninen H. and Kauppinen V. J Mater. Process Tech. 2001, 119: 14-20.

25.Oliver W. C. and Pharr G. M. (1992) J Mater. Res. 7 (6), 1564-1583.

Fig. A3. The stylus geometry; Square-based 40° truncated pyramid with a O.Sx O.Smm flat top.

294

CHAPTER16

ABRASION OF ENGINEERING CERAMICS, AIMgB14-TiB2 COMPOSITE AND

OTHER HARD MATERIALS

S. BAHADUR and A. AHMED

Department ofMechanical Engineering, Iowa State University

Ames, !A 50011-3020, USA. E-mail: bahadur@iastate.edu

ABSTRACT

The abrasion resistance of hard materials makes these materials particularly suited for harsh situations such as earth moving, mining, rock drilling etc. where contact with abrasive materials is involved. In view of their commercial importance, the abrasion behavior of engineering ceramics such as alumina, silicon carbide and silicon nitride, a-BN, and Al-Mg borides, which are an emerging class of materials, is presented. The latter also include compositions modified by the addition of TiB2 which, when optimized for microstructure and composition, approach the hardness of {1-BN. The abrasion results reported pertain to single point scratching and belt abrasion methods which are described. The material removal in scratch tests was estimated by profilometry and in belt abrasion test by gravimetric measurements. The variation of material removal from abrasion with process variables such as load and belt speed is presented. The mechanisms of abrasion in these materials have been studied with the help of scanning electron microscopy.

INTRODUCTION

Ultra-hard materials are commonly used for abrasion-resistant applications and cutting tools. Such materials are needed in many applications such as earth moving, mining, rock drilling etc. where they come in sliding contact with abrasive materials. In view of the commercial significance of abrasion, many researchers have studied the mechanisms involved in abrasion. Others have studied them with the objective of increasing material removal rates in abrasive machining of hard materials. The problem with enhancing the material removal rates is the surface and subsurface damage that occurs and is detrimental to mechanical properties.

Abrasion is a complex phenomenon affected by hardness, elastic modulus, yield strength, crystal structure, microstructure, and composition. The early work by Khrushchov and Babichev [I] on pure metals showed that abrasion rates were inversely proportional to hardness. With reference to microstructure, it has been shown that austenite and bainite of equal hardness are more abrasion-resistant than ferrite, pearlite, or martensite in steels. In order to be effective for abrasion resistance, the particles precipitated or externally added need to be larger than the abrading particles. The additive particle characteristics that work best for abrasive wear resistance are hard, tough, and blocky. The latter are more effective in terms of reduced crack propagation and breakage as compared to the plate or rod shaped particles. Zum Gahr [2] has

demonstrated that the orientation, size, modulus of elasticity, relative hardness, and brittleness of the second phase in composites are important factors for wear resistance.

Fisher et a!. [3] performed abrasion studies on a series of zirconia samples with constant hardness but varying toughness. They found that the abrasive wear decreased with the fourth power of toughness. This fourth power law does not apply to all materials. Quercia et a!. [4] conducted micro-abrasion test and found that there was a linear variation of wear volume with sliding distance. Mao et a!. [5] studied the abrasion behavior of advanced Ah03-TiC-Co ceramics with varying proportions of the constituents. They found that abrasion resistance depended mainly on fracture toughness, while hardness had merely a secondary effect.

The factors that affect abrasive wear in belt abrasion are the type of abrasive and its characteristics, such as hardness, toughness, angularity, and size [6, 7], speed of contact, unit load of abrasive on the material, humidity, and temperature. The shape of the abrasive particle, together with the load, influences the shape of the groove produced in the material, and the transition from elastic to plastic contact.

The mechanisms of abrasive wear that have been proposed are chipping [8], delamination [9], ploughing, flake formation, and the generation of powdery fragments. The mechanisms depend on contact stress [10, 11] and grain boundary microfracture characteristics of the material [ 12, 13]. In brittle materials, a transition in wear mechanism occurs with increasing load and/or particle size [14]. At low loads or with small particles, fracture may be suppressed and abrasive wear may occur by plastic processes. At higher loads or with larger particles, brittle fracture occurs leading to a much higher wear rate. Gee [15] reported that in the case of hard metals fracture occurred on a fine scale, but in ceramics, fracture occurred on a larger scale often removing large fragments of the material.

Diamond and cubic boron nitride (~-BN) are currently the only established bulk materials with hardness greater than 40 GPa. A new class of competing materials has recently emerged. These are the complex borides of aluminum and magnesium, AIMgB 14, prepared with sub-micron sized second-phase additives. These boride composites have hardness values ranging from 3046 GPa, depending on the size and distribution of the phases. Thus these materials may complement diamond and ~-BN in applications where high hardness is needed. The boride composites may also offer a cost advantage over diamond and ~-BN, if suitable large-scale manufacturing technologies are developed [16-18].

In the present chapter, the abrasion of hard materials is discussed and specific results are presented from our earlier works on the abrasion of ceramics and ultra-hard AIMgB14 composites.

ABRASION TESTS

There are many kinds of tests that have been used for abrasion studies. For specific information on such tests, reference to the ASTM Standards Vol. 03.2 is recommended [19]. This has six standards described for abrasion studies. Since the contact configuration and the contact stresses in these tests vary, the results from different tests are not always in agreement.

The two test methods that are recommended for single-pass and multiple-pass abrasion studies are described below. These were used in the abrasion studies on ceramics and AlMgB 14 materials reported here.

Single-Point Scratch Test

Direction of Motion

Bushing

'===-

Fig. I. Schematic arrangement of the experimental setup for single-point scratch test.

This test method is similar to the ASTM G171 Standard (19] but not identical. The experimental set-up for the test is shown in Fig. 1. It consists of a Rockwell C 120 o spherocone diamond indenter with a 200 Jlm tip radius that is secured to the end of a vertical spindle which slides freely in a long bushing. The friction between the bushing and the spindle is minimized by lubricating the sliding surfaces with lithium grease. The bushing assembly is welded to a nut which traverses linearly as the screw rotates in place; this makes the indenter traverse linearly. The screw is coupled to a motor shaft with a flexible coupling. Limiting switches are installed so as to set the indenter travel. The scratches are spaced apart so as to minimize the effect of damage from adjoining scratches.