ELSEVI E R Wear 205 (1997) 228-230

WEAR

Letter

Comments on the wear mechanisms of impregnated diamond bits

M.Es. Abdel Moneim, S. Abdou Production Engineering Department, Faculty of Engineering and Technology, Suez Canal University. Port $aia~ Egypt

Received ! February 1996; accepted 8 August 1996

Abstract

The evaluation of wear behaviour and tribulogieal properties of impregnated diamond tools were examined under different conditions. The results which were given in Tian and Tian work [ 11 of the mechanics of cutting via impregnated diamond tools deserve some comments which are included.

Keywords: AMasive wear;, Hardness; Diamond bits; Coefficient of friction; Cutting speed

It was interesting to go through the fine paper of Tian and Tian [ ! ] .

There are close similarities between the mechanics of cut- ring via impregnated diamond tools, as described in Ref. [ 1 ] and the mechanics of grinding. Diamond particles are ran- domly distributed inside the segments and are surrounded by metal matrix [ 1 ]. The abrasive wheel can be considered as a special milling cutter in which there are many thousands of small cutting edges embedded in the wheel-shaped matrix [21.

It is unlikely that the metal matrix surrounding the diamond particles will gradually wear away during rock cutting oper- ations in the manner indicated by Tian and Tian [ ! ]. As a matter of fact, such metal matrix is not considered to be involved in actual cutting.

More probably, it is torn away, rather than worn away, when its holding strength is exceeded by the cutting stresses. This holding strength is nominated the tool hardness in grind- ing. The metal matrix holding strength has to be sufficiently high in order to sustain the diamonds particles for reasonable peric~ls. On the other hand, this strength has to be sufficiently low in order to avoid tool face glazing. Such a phenomenon is associated with excessively worn diamond particles.

It is stated [ 3], vaguely without setting any limit, that low thrust forces values lead to the formation of large abrasive wear fiats at the exposed diamond particles. In our opinion, such a glazing phenomenon will proceed until the magnitudes of such forces surpass that of the metal matrix holding strength causing its partial dislodgemenL That will, in turn, expose new sharp diamond particles at higher thrust forces.

One should expect the existence of an, often overlooked, role of tool structure in governing the wear mechanisms of

0043-1648/97/$17.00 © 1997 Elsevier Science S.A. All tights geserved PII S0043-1648 (96) 07325-5

impregnated diamond bits. A closed tool structure means that the diamonds particles are closely embedded in the holding matrix. On the other hand, an open tool structure is achieved when the diamonds particles are fixed far apmt from each other, Needless to say, the Zoo| structure is bound to interfere with tool wear and the results of forces. Tool hardness, i.e. the metal matrix holding strength, will also greatly affect the above results.

We believe that the effects of variations in diamond particle type and cutting tool specifications in cont.'oiling the ensuing cutting forces and tool wear mechanisms of impregnated dia- mond bits merit separate studies.

It is known that diamonds are usually employed to cut materials at very high cutting speeds. We then fail to appre- ciate the wisdom of selecting the unrealistically low single- diamond rock cutting speed of 12revmin - t which was employed in Ref. [ ! ].

Although Tian and Tian [ 1 ] termed their first expe,,'in~ntal pan as single-diamond rock cutting, the cutting tool they employed was described as a single diamond indenter. It is incorrect to simulate the dynamic cutting process with the static indentation process.

However, the performance of the indentation process at very slow speeds in order to imitate diamond cutting wocess may be worth investigating provided that the two processes simulated and their differences are clearly outlined and taken into consideration.

It should be noted that the radi~ cutting forces during single-diamond rock cutting, as outlined in Ref. [ I], are eliminated if the radial stresses vanish. Such stresses may exist even if the diamond indenter is placed in the same horizontal level as the rock sample axis of rotation because

M.$.A. Monci.,n. $. Abdou / Wear 205 f 1997) 228-2.]0 229

of the associated indentation pile up material. It should be noted that the type of hardness number receded in Table I of Ref. [ 1 ] is missing.

The fabrication technique onfline of the single-diamond indenters and the impregnated diamond micro-bits is not clearly defined in Ref. [ ! ].

The heterogeneous nature of rocks is expected to cause considerable fluctuations in the recorded forces during cut- ting. Such fluctuations are not seen in Figs. 5-7 of Ref. [ 1 ]. The magnitudes of the recorded thrust for~.es of these figures have to be multiplied by a factor of 10 in order to obtain the recorded values of the coefficients of friction in Figs. 5-7 of Ref. [ ! ] .

Moreover, the concept of the coefficient of friction in Ref. [ ! ] cutting or indentation tests as a ratio of the tangential force to the normal force may be misleading. Material piling up in the indentation process tests of Ref. [ 1 ] is bound to interfere with the ordinary concept of the coefficient of friction.

It should be noted that the cross-sections of the grooves cut by individual grits (the diamond particles of Ref. [ 1 ] ) will vary in shape owing to the random geometry of the grits [4]. The forces acting on the grits are unlikely to be evenly distributed over the area of contact between the tool and the workpiece [4].

A wear coefficient is taken as the weight loss of impreg- nated diamond bit per distance drilled in Ref. [ I ]. It is not clear whether such weight loss embraces that of diamond particles with the metal matrix or not.

The wear mechanisms of diamond particles and that of holding media are quite different. While there is no scientific significance for combining both types of wear in a single coefficient, it is quite hard to assess them separately. That wear coefficient is not equivalent neither dimensionally nor scientifically to the wear rote per unit sliding distance per unit normal load, contrary to the opinion outlined in Ref. [ 1 ].

We are not in agreement with the statement of Tian and Tian [ 1 ] that the distance drilled is nearly proportional to the product of normal load and cutting velocity. In a similar experimental rock drilling study, Abdel Moneim et al. [5] found empirically that the vertical thrust force (F) and the torque (2") are given by the following empiricalrelationships:

F = BID ml (1)

and

T= b2 Din2

where

bl =/l (N.A)

m , = g l ( . ~ / , A )

(2)

b2=f2(N,A) (3)

m2=g2(N,A ) (4)

From their experimental results, Abdel Moneim et al. [5] concluded that

b~ -- - 3.43 + 111.56.4 + 0.1738.4 T M IN (5)

and

N ml ~ 2.75 - 3.09A 4 2027.66-10.854- 3 (6)

Eqs. (15) and (6) led to a general equation for the vertical thrust force [5].

It could also be shown [5] that

b2 ~ 0.44~ +2.11 X 10-4N (7)

and

N me ~0.71 +0.3,4-14 738.5 - 2.94A -3 (8)

A general equation could be derived for the torque evolved during rock cutting, under the given conditions in Ref. [5], from F.qs. (27) and (8),

Contrary to the work of Turn and Tian [ I ], Abdel Moneim et al. [5] gave the limitations of their experimental work. Such laboratory work [5] was merely a prefimi~ary approach.

In situ tests would be preferable whenever possible [5]. The response of rocks such as those employed in the research work of Abdei Moneim et al. [5] would be expected to be different during petroleum well chilling.

Tian andTian [1] reourded the rate of weight loss versus tool bit feed in Fig. 8(a). Such a drastic increase in the rate of weight loss can be explained in view of the experimental finding in Ref. [5]. According to the findings iHastrated in Fig. 10ofRef. [5], the normal tbn:e ineseases markediywith any increase in the feed rate (penetration per revolution). We are in agreement with the reasoning offered in Ref. [ ! ] for the slight decrease in the weight loss at higherrotational speed which was iiluswated in Fig. 8(b) of Ref. [ ! ].

Abdei Moneim et al. [5] found very pronounced effects for the increase in drill diameter. The fine research work of Tian and Tian [ 1 ] may be extended in order to investigate the effect of bit diameter in governing normal cutting load and torque together with the rate of weight loss.

The exact matrix chemical composition for bit drill in ref. [ i ] is undefined. In Fig. 9 in Ref. [ 1 ], the loss in tungsten carbide should be via a tearing off rather than a wearing off operation, when the cutting forces overcome the holding matrix strength. The effect of feeding speed change axe not clear from Fig. 9(a) and (b) of Ref. [ I ]. The bright spots in Fig.9(b) reveal that WC surfaces are subjeeted to less weight loss than that of Fig. 9(a), yet the penetration feed speed of Fig. 9(b) is greater than that of Fig. 9(a). We believe that these two figures should be interchungecL It is noted that the speed of cutting relevant to Fig. 10 of Ref. [ 1 ] is not given.

Heat is produced at the tool grains-workpiece interfaces as a result of attritional wear of grains which is manifested by the produc0o-_ nf a ~moot.h shiny tool surface [2]. This thermal energy has no fixed value since it depends on various cutting conditions such as tool speed, chip thickness and tool grains and workpiece materials [2].

230 M.s.A. Monein~ S. Abdou I Wear 205 (! 997) 228-230

Tian and Tian [ I ] derived the value of the nominal surface temperature rise at the diamond-rock interface (Tno~). Based on his experimental findings, Abdel Moneim [2] derived a general equation for the maximum temperature rise (0) of the tool-workpiece interface during external cylindrical grinding in the presence of an external grinding load as follows:

0= 468.1 dP 4 exp( - O.O04nc) exp(0.0115D¢)

× exp(0.50 6</) exp( - 0.442P)

where the parameters are d Depth of cut (ram) Dc Workpiece diameter (ram) n,. Workpiece speed (rev rain- J ) P External pressing load (kgf) 0 Maximum tool-workpiece interface

temperature (°C)

References

[ I ] X. T~an and S. Tian, The wear mechanisms of impregnated diamond bits, Wear. 177 (1994) 81-91.

[2] M.Es. Abdel Moneim, The tnbology of the grinding process, an investigation of the temperature increase during gnnding, Wear. 56 (1979) 265-296.

[ 3 ] D. Miller and A. Ball, The wear of diamonds in impregnated diamond bit drilling, Wear. 141 ( 1991 ) 311-320.

[4] M.Es. Abdel Moneim, Heal generation ,n relation to the performance of metal grinding, M.Sc. Diss., Cairo Univ., Egypt, 1968.

[5] M.Es. Abdel Muneim, E.A. Castillo and F.Z. Morales, A fundamental study of rock cutting I, Wear, 61 (1980) 31-41.

Biographies

M.Es. Abdel Moneim: graduated in production engineering from Cairo University 1964. He has published about 60 papers, mostly approved. He became a professor in produc- tion engineering in 1986 at Port-Said, Suez Canal University. His research interests include production engineering, wear properties, coating metals and structure and metal-based composites.

S.M.I. Abdou: graduated from the Production Engineering Department, Faculty of Engineering, Suez Canal University, Port-Said, in 1974. He obtained an M.Sc. in production engi- neering from Suez Canal University. He also receive a Ph.D. on the basis of the Channel System at the Max-Plack-lnstitut ftir Metallforschung and the Institut fiir Metalikunde of the University in Stuttgart, Germany, 1985. He was appointed an associate professor in materials science and metallurgy in 1992 at Suez Canal University, Port-Said. His research inter- ests include precipitation hardening, wear properties, coating metals and structure and metal-based composites. He pub- lished about 27 papers, mostly approved.