Paper V(ii) Formation of wear fragments by fracture processes in abrasive contacts

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Paper V(ii)

Formation of wear fragments by fracture processes in abrasive contacts B. Lamy and T. Mathia

Abrasion and erosion are forms of wear characterized by the scratching of surfaces.In sliding contacts abrasion can arise from existing asperities on one surface, from the generation of a wear fragment, or from the adventitious entry of hard particles from outside the contact. Abrasion of surfaces in relative motion leads to the creation of wear debris conducing sometimes to the formation of a "third body" film in the contact. Experiments were conducted in order to study the creation of'wear fragments by fracture processes. The influence of the geometry of the abrasive tip is analysed. 1 BACKGROUND AND FIRST EXPERIMENTS Abrasive scratching of surfaces produces either brittle scratches with material removal by a fracture process or ductile scratches with plastic cutting and deformation. Hardness, H, is a measure of resistance to deformation, and toughness, Kc, a measure of resistance to fracture. Comparative values of these two parameters could be taken as an indication of brittleness, but H and Kc have different dimensions such that the ratio HIKC has the dimension of (distancer%.However, a definite physical significance may be attached to this (distance) factor. Lawn et a1 (1,2,3) and Hagan (4) measured dimensions of residual impressions and median cracks produced by the Vickers indentation of soda glass as a function of the applied load, and observed a critical indentation dimension below which cracking did not occur. This was also observed in solids as diverse as silicon ( 5 , 6 ) , polycristalline ceramics ( 7 , 8 ) and glass ceramics ( 7 ) . A Fracture mechanics analysis of cracking at indentations gives relationships for the critical load Pc necessary for crack nucleation to take place and for the corresponding minimum crack dimension c :

Here,),oandr,are

In static contact of flat and spherical punches, the mechanisms of crack initiation and propagation were also finely studied : Mouginot and Maugis (9,lO) discuss in details the problem of subcritical crack growth, intrinsic surface energy and fracture toughness. Although defined explicitely in terms of static contact, HIKC appears also to be relevant to sliding contact in case of sharp indenters. Indeed many experimental results suggest that there may be a scale effect during the abrasive wear of semi brittle materials, namely that fracture processes predominate above a critical scratch size, and plastic deformation at smaller sizes. Broese van Groenou and Veldkamp (11) showed that the minimum load for the nucleation of cracks around scratches for brittle materials, is :

geometrical constants.

Therefore, if P is the load on the indenter a n d a a characteristic dimension of the indentationH~(P &CI in hardness measurements), three distinct regions of mechanical behaviour may be identified :

-

the ratio K c ~ His the governing factor in determining the ductile or brittle nature of a static contact problem. The inverse quantity HIKC then becomes the convenient "index of brittleness" (domain of ceramics, optical polymers, glasses, graphites etc).

P(Pc : damage is entirely deformationcontrolled (most metallic and polymeric solids); optimal design calls for maximizing H. - P>Pc : damage is essentially fracturecontrolled and a))c (covalents solids, eg silicon, diamonds and Sic) ; one seeks to maximize Kc. - P z P c : fracture and deformation operate on a comparable scale ; c becomes comparable witha. In this context, the (distance) factor (Kc(H) becomes a parameter of special significance :

where Hs is the scratching hardness and L a geometrical constant. The equation is essentially the same as that given by Lawn (2,3) or Hagan ( 4 ) , and it underlines the basic processes of microplasticity and fracture in both static and scratching experiments. Mathia and Encrenaz (121, when testing soda glass, observed that the "history" of deformation needs to be considered : residual stresses around the scratch lower the fracture stress and the critical load for cracks nucleation depends on the scratching velocity. Lamy (13) used a pendular scratching apparatus to perform single tip abrasion under controlled conditions and to extend to dynamic contact the results obtained by the analytical studies of static indentation. During operation

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a pendulum swung down and an abrasive tip, which was fixed radially to this rigid pendulum, scratched a horizontal specimen at the bottom of the trajectory. The length and depth of the thus-produced-arc-shaped groove could be varied by adjusting the radial position of the tip. The incident velocity of the abrasive tip was given by the release angle of the pendulum. These first experiments were run with half-conical indenters (apex angle g o o ) , the flat surfaces was in front of the indenter, at right angle of the scratching direction during experiments (fig.1). The energy consumed during abrasion was given by the loss of potential energy of the pendulum. As in static indentation, a critical deformation size (ie critical depth D* of the abrasive scratch) was observed in scratching of semi-brittle materials. This depth D* corresponds to the transition from ductile to brittle abrasion (fig.2) and is characterized by the onset of lateral cracking : material removal rate is then very high in comparison with ductile deformation and cutting processes. Different sorts of cracks (12,131 can be distinguished on the glass sample (subsurface, median or lateral cracks). Lateral cracks lead to the phenomenom of chipping. For the polycristalline materials tested, only lateral chipping follows lateral scratching. The fine morphology of lateral brittle cracking depends on grain size and on intergranular cohesion. The critical depth D* depends on the scratching velocity v, (fig. 3), and also on the temperature, particularly in case of polymeric materials. Lamy and Berlie (15) showed that for a given velocity (2m/s), a given temperature (2Ooc), a given indenter (half-conical, the same as described above), the dynamic transition depth D* is related to the ratio Kc/H by the relation :

where4 is a dimensional constant. In these experiments, the relationship between D* and Kc/H is very close to that proposed by the theorical models of static indentation (3,4). The dynamic transition depth D* obtained are plotted against Kc/H (fig.4) for different demibrittle materials. Therefore (14) through sclerometric experiments the relation could be established in the form of the function of the speed v and the temperature T : r

However, as pointed out by Kennedy (16). the effect of the geometry of the abrasive tip is not taken into account. Tests were conducted in order to specify this aspect. 2 PRESENT EXPERIMENTS Pendular scratching tests were run at room temperature (20Oc). Quasi half-conical indenters were used, apex angle 90°, ground slightly more or lessthan the half (precise geometries are shown in fig. 5). Scratches were run with the flat surface in front or at back of the indenters. The incident scratching velocity was

2 m/s. Different materials were tested : - polymethylmethacrylate (PMMA) commercial, - tridirectional carbon-carbon composite material (3D CC). This material consists of carbon fibre yarns periodically and tridirectionally arranged, the carbon matrix fills up the voids between fibre yarns. The principal results are as follows : i) The consumed energy versus the length of the abrasive pendular scratch is plotted in fig. 6 for PMMA. The length of the scratches was varied from 20 mm to 60 mm in present experiments. - The transition from ductile to brittle abrasion is not observed when the flat ground surface is at the back of the indenter during scratching. - Lateral brittle chipping is only observed when the flat surface is in front of the indenter and if the indenter is ground slightly more than the half. Brittle chipping is not observed using indenters ground slightly less than the half. In scratching of semi-brittle materials, the initiation and propagation of lateral fractures appear to be very sensitive to the geometry of the contact indenter-material : this geometry governs the location of the regions of sufficient tensile stress for lateral crack nucleation : cracking occurs if the tension concentrates on directions laterally orientated outside the groove (in case of static contact with a Vickers indenter, tension concentrates on median planes containing the diagonals of the square-shaped surface indentation. ii) In scratching of composite 3DCC the ductile matrix is plastically grooved. The brittle behaviour of the fibre yarns is however only observed when the flat surface is in front of the indenter and if the indenter is ground slightly more than the half. The brittle behaviour leads to lateral brittle chipping of the yarns and also to the breaking up of the interfaces matrix-fibre yarns (fig. 7 ) . iii) F o r the two materials tested, the energy consumed is lower when brittle chipping occurs (fig. 6 , 8 ) . In case of composite 3DCC, the matrix blocks are plastically abraded by cutting (if the flat surface is in front of the indenter) or by ploughing (if the flat ground surface is at the back of the indenter). These observations are in agreement with tests conducted with conical indenters (17). In scratching of ductile materials, the chip-shape, the morphology of material flow, the material removal rate, the energy consumption are very sensitive to the contact geometry (apex angle 8 of the indenter) : cutting occurs if 8 60°, plastic grooving Qccurs if 8) 60°. In case of plastic cutting or grooving, the energy consumed is given by (18) :

<

A : volume of the groove c : geometrical constant p : dynamic hardness s : dynamic shear strength M : material removal rate (fig. 9) The expression 2 p w t a k e s into account the surrounding and underlying deformed material, important in case of ploughing (M-0). When brittle lateral chipping occurs, the energy consumed can be derived from the expres sion of Broese van Groenou and Weldkamp (ll), E depending on both dynamic hardness and toughness.

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3 CONCLUSION

embrittlement. J . Mat. Sci., 1985, 20, 3041-3073

The results of these investigations lead to the following conclusions : - ductile-brittle transition is one of the most important phenomena in abrasion of brittle or semi-brittle surfaces, brittle scratch susceptibility can be related directly to poorer wear resistance. The detachment of particles and wear fragments by fracture processes is very important for the formation of "third body" layers : the rate of material removal by brittle scratching is higher than that obtained by ductile cutting or ploughing ; - knowledge of the brittleness index and of its evolution with speed and temperature are useful in predicting the brittle scratch susceptibility. However, the initiation of cracks is strongly dependant on the geometry of the contact material-abrasive tip (location of tensile stresses for cracks nucleation); - the transition scratching depth could therefore be established in the following form :

11. BROESES VAN GROENOU A. and VELDKAMP J.D.B. Grinding Brittle Materials. Philips Tech. Rev., 1979, 105-118

-

I -

D*=f [IG(c,T ) ,H (\r,T),cOntact

geometry

J

REFERENCES 1.LAWN B.R., JENSEN T. and ARORA A. Brittleness as an Indentation Size Effect. J. Mat. Sci., 1976, 11, 573-575 2.LAWN B.R. and EVANS A.G. A Model for Crack Initiation in Elastic/Plastic Indentation Fields. J. Mat. Sci., 1977, 12, 2195-2199

12. MATHIA T. and ENCRENAZ B. Hysteresis in the abrasive wear of Brittle Solids. Wear, 1981, 73, 205-208

13. LAMY B. Effect of Brittleness Index and Sliding Speed on thenorphology of Surface Scratching in Abrasive or Erosive Processes. Triblology International, 1984, 17 nol, 35-38 14. MATHIA T. and LAMY B. Sclerometric characterisation of nearly brittle material Wear, 1986, 108, 385-399 15. LAMY B. and BERLIE J. Brittleness Analysis of Ceramic and Polymeric Materials by Means of Scratching Experiments. J . Mat. Sci. Letters, 1984, 3, 1069-1070 16. KENNEDY F.E. In Discussion, 12th Leeds-Lyon Symposium on Tribology, Lyon, 1985 17. LAMY B. Influence des PropriCtCs RhCologiques des MatQriaux sur la Morphologie des Ecoulements Superficiels de Matiere et la Formation de Couches de Transfert lors d'un Processus de Frottement. In Microscopic Aspect of Adhesion and Lubrification. Georges J.M. (Ed.), Elsevier, 1982, 599-608 18. LAMY B. Mode d'EnlGvement de Matiere et de DCformation de Surface gCnCr6s lors du Choc Abrasif d'une aspCritC troncoonique CmoussCe sur un plan. MQcanique MatCriaux ElectricitC, 1981, 373, 27-32

3.LAWN B.R.and MARSHALL D.B. Hardness, Toughness and Brittleness : An Indentation Analysis. J . Amer. Ceram. SOC.,1979, 62, 347-350 4.HAGAN J.T. Micromechanisms of Crack Nucleation During Indentation. J. Mat. Sci., 1979, 14, 29752980 5.PUTTICKK.E. Size Effects in Brittle Fracture. In Proceedings of the Third International Conference on Mechanical Behaviour of Materials, Cambridge, 1979, MILLER K.J. and SMITH R.F. (Eds.), ICM 3, 3, 11-17 6.PUTTICK K.E., SHADID M.A. and HOSSEINI M.M. Slze Effects in Abrasion of Brittle Materials. J. Phys. D. : Appl. Phys., 1979, 12, 195-202 7.MARION R.H. Use of Indentation to Determine Fracbhnre TmU$hness. an Fracture Mechanics Applied to Brittle Materials. FREIMAN S.W. (Ed.), ASTM Publication, 1979, 103-111 8.PETROVIC J.J. and MENDIRETTA M.G. Fracture from Controlled Surface flows. In Fracture Mechanics Applied to Brittle Materials. FREIMAN S.W. (Ed.), ASTM Publication, 1979, 83-102 9.MOUGINOT R . and MAUGIS D. Fracture Indentation beneath flat and spherical punches. J. Mat. Sci. 1985, 20, 4354-4376 10.MAUGIS D. Subcritical crack growth, surface energy, fracture toughness, stick-slip and

Fig. 1 The scratching apparatus

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.le

Brittle

c--cI

Ductile cc--c

lA

Transition deDth D *

Fig. 5 Geometry of the quasi-half conical indenteurs Fig. 2 Ductile-brittle transition in pendular scratches

Brittle ( indenter B )

D” (mm)

2

4

6

v(m

s)

40

I

I

45

50

I

55

-

L (mm)

Fig. 6 Energy consumed versus the length of the scratch for PMMA

Fig. 3 Transition depth as a function of the scratching velocity for PMMA

E (J) Polystyrene +-

Ductile 2

’

1

’

PMMA

+-

Polvimide

100

10

% ++/Quartz

1

indenter B)

plycristalline A 1um ina

8

30

Glass

10

100

A h

KC/H ( p m) Fig. 4 The transition depth as a function of the brittleness index

40

Fig. 8 Energy consumed versus the length of the scratch for 3DCC

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Fibre yarn

V

I

)IMatri (ductile)

Interface effect 0.8 mm

Fig. 7 Morphology of the scratch, composite 3DCC

Sb Rate of material removal :

M=

Fig. 9 Cross section of the scratch

Sa- Sb

Sa