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Effect of indentation load on fragmentation of erosion particle tips
115
Wear, 141 (1990) 115-124
Effect of ~den~~on load on ~a~en~t~on particle tips
of erosion
Lamnan Murugesh and Ronald 0. Scattergood Lkpatimsnt of Ma&&&s Science and ~~~~~,
North CarolzizaState Un~v~~~~
Raleigh, NC 27695-7907 (LL5.A.)
(Received October 30, 1989; revised May 31, 1990; accepted June 18, 1990)
Abstract Erosion studies on abmGna of varying hardnesses impacted by AlaOa and Sic erodent particles suggest that particle fragmentation may significantly affect the erosion behavior of ceramics. The effect of load on the fragmentation of impacting particles has been systematically studied here using indentations of individualparticles on sapphire. Replicas of particle tip geometry were made by indenting a diamond-turned copper sample. Scanning electron microscopy of such indentations at varying loads reveals a threshold load at which fragmentation occurs. The threshold load depends on the material properties of the individual particles, and it appears that softer particles fragment at lower loads than harder particles do. The crushing of impacting particles can lead to the blat of particle tips and thus alter the erosion mechanism of hard targets impacted with softer erodents.
The steady state erosion of a brittle target material by solid particle impact has been modeled based on the large amount of literature on indentation fracture mechanics [ 1,2 1. Single-impact studies of damage patterns in alumina ceramics show a central zone of irreversibIe deformation surrounded by median-radial cracks extending into the material and on the surface. Above a certain threshold load, saucer-shaped lateral cracks develop parallel to the material surface on unloading. The radial cracks have been modeled via the stress field resulting from the elastic-plastic contact conditions and the use of equilibrium fracture mechanics, thus bringing in material parameters such as hardness H, elastic modulus E and fracture toughness & [l-3]. Similarly, lateral cracks have been modeled using residual loading-unloading stresses as the driving force for a centre-loaded penny-shaped lateral crack 14, 51. It is these cracks that by their form and growth lead to chipping of the target surface and erosion damage. The role of the impacting particle properties in relation to those of the target material has generally been neglected and it is implicitly assumed that the impacting particle itself is inilnitely rigid and suffers negligible elastic deformation. This assumption is reasonable when brittle materials are eroded with hard sharp, irregularly shaped abrasive particles [6, 71. Tilly and Sage
0043-1648/90/$3.50
0 Elsevier SequoiaPrinted in The Netherlands
116
[81 reported an interaction of the erodent particle and target material in erosion wherein the effect of particle size and velocity on fragmentation of the erodent particles was determined. Recent observations by Wada and coworkers [9, lo] have shown that the properties of the erodent particles affect the erosion and crack morphology of brittle materials. Wada and Watanabe f91 found a sharp increase in the erosion rates when the ratio of the hardness HP of the particle to the hardness Ht of the target was increased. In particular, erosion rates increased dramatically at a ratio of N,/H, very close to unity. Changes in the overall erosion mechanism and crack morphology were noticed when the ratio of HJH, was increased from unity to higher values, the mate&& used being Al,O, and Sic-reinforced AIZO, [IO]. Impact events on these materials using alma particles where &/H, < 1 yielded simple p~c~g-~~ events with the absence of any noticeable lateral cracking. However, when impacted with SIC particles, where H,/Y&> 1, typical lateral crack formation occurred. The velocity exponent n was also lowered when Hp/Ht < 1, in the range 1.3-1.8 compared with values of over 2 obtained for HP/H,> 1. Although not recognized as an HP/W, effect, such transitions in velocity exponents were also reported by Sykes et al. [ 111 on SIC whiskerreinforced alumina samples impacted with AlaO particles. It was seen that the velocity exponents decreased from 2.9 to 0.8 with increasing additions of Sic whiskers up to about 25%. ~vestigations by Morrison et al. [ 121 on the erosion of S&N4reinforced with Sic using AlaO abrasives showed similar effects, although fewer experimental results were available. Erosion testing by Srinivasan and Scattergood [ 131 on a series of sintered aluminas of varying hardnesses found that Hp/Hc emerges as an important parameter that affects the mechanism of material removal. It appeared that some damage accumulation was necessary for fracture initiation when HP/H& 1 and it was suggested that particle crushing or flattening of softer erode&s reduces the efficiency of the erosion process. The overall steady state surface morphology was quite similar for hard VS. soft erodents, but many more impacts were needed to generate the residual stress-driving forces for fracture with softer erodents. Wada and Watanabe [ 141 found that “crashing” of softer abrasive particles leads to some material removal during the erosion of AlaO ceramics. Such results indicate the importance of the HP/H, ratio for erosion of ceramics. A series of normal impact erosion tests on alumina ceramics ranging in hardness from about 10 to 22 GPa using Sic and AlaO particles of sizes 63 and 405 pm at velocities of 40, 60 and 90 m s-’ is currently in progress [ 151. The results confirm the importance of the hardness ratio effect 1161. Scanning electron microscopy (SEM) of single-impact events showed the absence of a lateral cracking mechanism of erosion for the 405 pm Al203 particles at 90 m s- 1 on the hardest of the alumina target materials (sapphire). A plastic punching-type mechanism was seen to occur without any cracking or fragmentation of the sapphire substrate (Pig. 1). Impacts with Sic particles on the same target showed the classical lateral cracking and chipping mechanism of erosion. A depth and width profile of the single impacts using a confocal lsser microscope indicated a shallower average penetration for
117
Fig. 1. Single-impact event of 405 pm E-17 AlSO on sapphire at 90 m s-l, showing a plastic punching-type mechanism without cracking or fragmentation of the sapphire substrate.
the A1203 erodent particle impacts. The steady state erosion surface of sapphire impacted at a velocity of 40 m s-’ with 63 pm AlzO, particles showed the presence of extensive plastic punching similar to that which occurs when ductile targets are eroded with spherical particles [ 171. This, together with energy-dispersive X-ray analysis (EDXA) of single impacts, suggested the existence of some form of fragmentation and embedding of tips of the erodent particles on the surface of the target materials, especially at low velocities and low HP/H, ratios. Further evidence was provided by the presence of hertzian cracks on the steady state sapphire surface impacted with AlzO, particles, which suggests a rounding or blunting of particle tips due to fragmentation. The underlying motivation for the work reported here was to verify the possible effect of increasing load on the fragmentation of erodent particles of different hardnesses, on a standard substrate of sapphire. Using AlsOa and Sic erodents on sapphire gives HP/H, < 1 and HP/H,> 1 respectively. 2. Results and discussion Single particles of AlzO, and Sic with a mean size of 405 pm were mounted using epoxy onto the ends of standard indenter fixtures used in a Micromet microhardness machine. The particles were then indented at loads ranging from 1 to 10 N. Replicas of the particle tip shape were obtained by first indenting a flat, optically smooth diamond-turned copper disc. This was repeated so as to con6rn-1 the shape and size of the initial particle tip. The particle was then used to indent a polished sapphire sample at the same load. A replica of the particle tip was again made on the copper disc so as to obtain information on the fragmentation or crushing caused by indentation on sapphire. This procedure was followed at various loads to define the effect of increasing load on the damage incurred by the particle tip. SEM micrographs were made of the replicas for each series of tests. A slight rotation of the particles in the epoxy used to affix them to the indenter
118
sometimes caused the appearance of spurious shear in the micrographs, as will be noted where appropriate. The indentations were observed using a Hitachi S-530 scanning electron microscope. Further characterization of debris found after indentation was accomplished using EDXA. A Tracer-Northern TN 5500 energy-dispersive spectrometer was used for this purpose. Figure 2 shows a comparison of the abrasive particles of A1203 (Alundum El?) and SiC (Crystolon 37), supplied by Norton. A shape and form analysis of the particles gave the mean properties shown in Table 1. For definitions, refer to ref. 18. The mean section diameter values obtained via intercept lengths tend to be Iess than the average size determined by sieve anaIysis (mesh size) which is reported by the supplier in conjunction with grit size specification. It is seen from Table 1 that the two abrasive particles do not differ in their sizes and shape parameters. Microhardness measurements made using a standard Vickers indenter (400 gf load) indicate that they differ in their hardnesses. The measured hardness of the AlaO, particles was about 16 GPa while the SiC particles had a hardness of about 25 GPa. The single-crystal sapphire sample used had a hardness of about 22 GPa while the hardness of the copper sample used to replicate
(al Fig. 2. 405 pm abrasive particles of (a) SiC and (h) E-17 Al203. TABLE 1 Particle characteristics Property
Supplier, 60 grit &II) Section diameter (pm) Form factor Aspect ratio
Wue Alzos
SiC
405 347f6 0.629 f 0.07 1.591 kO.34
406 345f5 0.646f0.07 1.508 f 0.24
119
A series of control indentations showed that indentations on copper alone do not produce visible particle damage over the range of loads used here. A wide variety of indentation sizes and shapes are possible owing to particle size and shape variation and orientation on the indenter IIxture. Figures 3 and 4 show representative sequences for indentations using A1203 and Sic particles. Figure 6 shows a series of standard Vickers indentations on the copper disc. At a given load, the indentation area of the particle must be comparable with that of the Vickers indentation; otherwise projections of the epoxy mount rest on the surface and partially support the load. Figure 3(a) shows that some fragmentation occurs for AlaO particles at the lowest load of 3 N (some shearing due to particle rotation occurred for the righthand frame in Fig. 3(a)). This was confirmed by many other tests. Continued fragmentation occurs with increasinp load in Fig. 3. EDXA showed that the
(cl Fig. 3. Single-particle indentation of 405 m E-17 AlzOs on a diamond-turned copper sample both before (picture on the left) and replicated after indentation at the same load (picture on the right) on sapphire substrate. The micrographs are taken at loads of (a) 3 N, @) 6.5 N and (c) 10 N. It should be noted that there is fragmentation of the particle tip at a load of 3 N followed by further crushing at the higher load. Also the joining of blunted particle tips at the hlgher load to form the larger indentation should be noted.
Fig. 4. Single-particle indentation of 405 q Sic on a diamond-turned copper sample both before (picture on the left) and replicated after indentation at the same load (picture on the right) on sapphire substrate. The micrographs are taken at loads of (a) 3 N, (b) 6.5 N and (c) 10 N. The fragmentation of the particle tip at the load of 6.5 N followed by shattering at the higher load should be noted.
Fig. 5. Standard Vickers indentations on an optically smooth copper disc at loads of 1, 2, 3, 4, 6.5 and 10 N.
121 sma,ll fragments visible are AlaOs. On the basis of other observations, it is not likely that these fragments come from the sapphire substrate, although this cannot be ~~~~0~s~ established using EDXA. In contrast, SiC particle tips did not show fragmentation until loads reached 6.5 N or higher. A typical sequence is shown in Fig. 4 (the right-hand frame in Fig. 4(a) shows some shear due to rotation). Interestingly, Sic particles sometimes underwent catastrophic shattering at the highest load of 10 N, as is evident in Fig. 4(c). Ca~trop~~ shattering did not usually occur for the A1203 particle, even though they undergo fragmentation at lower loads. EDXA on Sic indentations showed unambiguously that all fragments obtained are SiC and not AI,O, from the sapphire substrate (e.g. fragments in Figs. 4(b) and 4(c)). This reafhrms the conclusion that all fragments observed using either A1203 or Sic indenters are particle fragments. As was already noted, EDXA of the ~den~tions before and after indentation on sapphire indicated the presence of debris from the fragmented particle. A very clear indication of such debris was seen for the A1203particles, and this is shown in Fig. 6. The micrographs were taken for indentations of a particle of A1203 at loads of 4 and 6.5 N. The micrographs show the
F’ig. 6. EDXA of debris located at the ~den~tion of E-17 Al& on copper before and after indentation on a sapphire substrate at the same load. The micrographs show indentations at (a) 4 N and (b) 6.5 N. A clear aluminum peak is found for the 4 N indentation which decreases at the higher load, indicating a threshold load of fragmentation for the particle tip. The rounding of the particle edges is also clearly seen.
122
blunting of the tips of the particles and also reveal the presence of large particle fragments. A well-defined ahuninum peak appears in the X-ray analysis of the replica made after the sapphire indentation (Fig. 6). Similar results also appear at increased loads although the height of the aluminum peak tends to be lower, indicating a lower overall amount of tip ~~en~tion above a threshold load. This suggests that particles are rounding owing to the crushing of the tip above a threshold load. The fact that the debris at increasing loads is found along the periphery of the indentation rather than towards the central region, as was occurring at the lower loads, also seems to lend support to the fact that the particle tips are being rounded off. The indentations and X-ray results shown are found to be reproducible and the X-ray analysis shown is representative of the results found. There is of course a large range of indentation topology because of variations in particle shape. The results obtained lend support to the fact that fra~en~tion and rounding of the individual abrasive particles are likely to occur at certain threshold loads during erosion testing. Details depend on the relative hardness and toughness of the erodent particle compared with the target material. It appears that fragmentation occurs earlier (lower threshold load) when H&Y, is smaller. However, a very large number of tests would be needed to develop quantitative statistics. At higher loads, catastrophic shattering of hard abrasive particles can occur. The particle fragments may in turn affect the overall erosion mechanism. It should be noted that there is no direct comparison of the static indentation loads used here with the dynamic particle impact loads in erosion processes. The quasi-static analysis given in Appendix A suggests, however, that typical impact loads can easily exceed the range l-10 N used in this study. Particle bhmting by fragmentation would cause the individual particles to behave more like rounded spheres impinging on the target surface. Particle blunting can lower the velocity exponents [ I3 1, as was noted earlier. Damage acc~~a~on may be necessary to build up the requisite stresses to produce lateral cracks when softer erodent are used.
Acknowledgments The authors gratefully acknowledge the U.S. Department of Energy for support of this research under Grant DE-FGO5-8~ER~5 115. Useful discussions with S. Srinivasan are also acknowledged. References 1 B. R. Lawn and R. Wilshaw, Indentation fracture: principles and applications, J. Muter. sci, 10 (1975) 1049. 2 A. G. Evans and R. Wiihaw, Quasi-static solid particle damage in brittle solids-I. Observations, analysis and implications, Acta &Z&CX&., 24 (1976) 939. 3 B. R. Lawn, A. G. Evans and D. B. Marshall, Elastic-plastic indentation damage in ceramics: the median-radial crack system, J. Am. &ram. Sot., 63 (9-10) (1980) 574.
123 4 B. R. Lawn, A. G. Evans and D. B. Marshall,Elastic-plastic indentation damage in ceramics: the lateral crack system, J. Am. Cerum. Sot., 6si (11) (1982) 561. 5 A. G. Evans, Abrasive wear in ceramics: an assessment, in A. G. Evans (ed.), The Science of Ceramic Machining and Su@kce Finishing II, Noyes, Park Ridge, NJ, 1979, p. 1. 6 S. M. Wiederhorn and B. R. Lawn, Strength degradation of glass impacted with sharp particles: I, Annealed surfaces, f. Am Ceram. Sot., 62 f1979) 66. 7 A. G. Evans, M. E. Gulden and M. E. Rosenblatt, Impact damage in brittle materials in the elastic-plastic response regime, Proc. R. Sot. London, Ser. A, 361 (1978) 343. 8 G. P. Tilly and W. Sage, The interaction of particle and material behavior ln erosion processes, Wear, 16 (1970) 447. 9 S. Wada and N. Watanabe, Solid particle erosion of brittle materials (Part 3) -The interaction with materialproperties of target andthat of impingementparticle on erosive wear mechanism, Yogyo Kyokai Shi, 95 (6) (1987) 573. 10 S. Wada, N. Watanabe and T. Tani, solid particle erosion of brittle materials (Part Q)-The erosive wear of AlaO,-SiC particle composites, J. C&-am Sot. Jpn., Int. Ed%, 96 (1988) 737. 11 M. T. Sykes, R. 0. Scattergood and 3. L. Routbort, Erosion of Sic-reinforced ahrmina ceramic composites, Composites, 18 (2) (1987) 153. 12 C. T. Morrison, J. L. Routbort and R. 0 Scattergood, Erosion of Sic-whisker reinforced SiaN+ in Advanced Structural Ceramics, Materials Research So&.&g Symp. Proc., Vol. 78, Materials Research Society, Pittsburgh, PA, 1987, p. 207. 13 S. Srinivasan and R. 0. Scattergood, Effect of erodent hardness on erosion of brittle materials, Wear, 128 (1988) 139. 14 S. Wada and N. Watanabe, Solid particle erosion of brittle materials {Part I)-The erosive wear of thirteen kinds of commercial ahrminaceramics, Yo-yo K~okai Shi, 9.5(9) (1987) 835. 15 K. Anand, S. K. Hovis, H. Conrad and R. 0. Scattergood, Flux effects in solid particle erosion, Wear, 118 (1987) 243. 16 L. Murugesh,Effect of erodent and target particle hardnesson erosion of aluminaceramics, M.S. Thesis (in progress), North Carolina State University, 1989. I7 Ronald A. Mayville, Mechanisms of materiel removal in the solid particle erosion of ductile materials, Rep. LBL-7333, 1978 (Lawrence Berkeley Laboratory). 18 J. C. Russ, Computer Aided MicroscopyThe Measum ana! Analysis of Images, North Carolina State University, Raleigh, NC, 1988.
Appendix A Quasi-static analysis can be used to estimate the maximum indentation load transferred by erodent particle impact. While this does not include dynamical effects (strain rate or temperature), it gives an approximate upper bound for the loads. Let the particle shape be represented by the circular cone shown in Fig. Al. D is the diameter of the cone’s base which is assumed equal to the average particle size. $ is the cone apex angle and is a measure of sharpness. The particle is assumed to be perfectly rigid. At a penetration depth z, the radius of the contact area is a. If H is the hardness (flow stress) of the target, then at any position z the load P(z) required to balance the surface tractions is P(z) = 71-wa2= dEz2 tan’ tcr_Inertial forces are ignored in the quasi-static analysis. The plastic work W needed to penetrate to depth z,, and the initial kinetic energy KE of the particle are respectively
124
Fig. Al. Geometry
*mP.x
Iv=
f
P(z)
dz=
0
for cone-shaped
7rHz,,”
impact particle.
tan2 I)
3
and KE=
TDSpU2 48tan$
where p is the particle density and ZI is the velocity. Equating W with the kinetic energy KE gives the maximum penetration depth x,, and maximum load P_ as follows:
213
P max= r~Hz,,,,,~ tan2 3/= dD2v4/3
Typical values for 405 pm AlzO, particles impacting a sapphire target would beH=22GPa,p=3.95x103kgm~3,I)=405~mandv=30ms~’.Substi~ting these values gives P,, = 5 X 104 N. This is very much larger than the l-10 N load range used for the indentations made here. In a real impact situation, only a fraction of the total kinetic energy will actually be transferred to crack driving force, and P,, must be lower. However, the point to note is that impact loads in the range l-10 N are certainly within reason. Particle fragmentation effects of the kind reported here are a likely possibility for erosion conditions.
Wear, 141 (1990) 115-124
Effect of ~den~~on load on ~a~en~t~on particle tips
of erosion
Lamnan Murugesh and Ronald 0. Scattergood Lkpatimsnt of Ma&&&s Science and ~~~~~,
North CarolzizaState Un~v~~~~
Raleigh, NC 27695-7907 (LL5.A.)
(Received October 30, 1989; revised May 31, 1990; accepted June 18, 1990)
Abstract Erosion studies on abmGna of varying hardnesses impacted by AlaOa and Sic erodent particles suggest that particle fragmentation may significantly affect the erosion behavior of ceramics. The effect of load on the fragmentation of impacting particles has been systematically studied here using indentations of individualparticles on sapphire. Replicas of particle tip geometry were made by indenting a diamond-turned copper sample. Scanning electron microscopy of such indentations at varying loads reveals a threshold load at which fragmentation occurs. The threshold load depends on the material properties of the individual particles, and it appears that softer particles fragment at lower loads than harder particles do. The crushing of impacting particles can lead to the blat of particle tips and thus alter the erosion mechanism of hard targets impacted with softer erodents.
The steady state erosion of a brittle target material by solid particle impact has been modeled based on the large amount of literature on indentation fracture mechanics [ 1,2 1. Single-impact studies of damage patterns in alumina ceramics show a central zone of irreversibIe deformation surrounded by median-radial cracks extending into the material and on the surface. Above a certain threshold load, saucer-shaped lateral cracks develop parallel to the material surface on unloading. The radial cracks have been modeled via the stress field resulting from the elastic-plastic contact conditions and the use of equilibrium fracture mechanics, thus bringing in material parameters such as hardness H, elastic modulus E and fracture toughness & [l-3]. Similarly, lateral cracks have been modeled using residual loading-unloading stresses as the driving force for a centre-loaded penny-shaped lateral crack 14, 51. It is these cracks that by their form and growth lead to chipping of the target surface and erosion damage. The role of the impacting particle properties in relation to those of the target material has generally been neglected and it is implicitly assumed that the impacting particle itself is inilnitely rigid and suffers negligible elastic deformation. This assumption is reasonable when brittle materials are eroded with hard sharp, irregularly shaped abrasive particles [6, 71. Tilly and Sage
0043-1648/90/$3.50
0 Elsevier SequoiaPrinted in The Netherlands
116
[81 reported an interaction of the erodent particle and target material in erosion wherein the effect of particle size and velocity on fragmentation of the erodent particles was determined. Recent observations by Wada and coworkers [9, lo] have shown that the properties of the erodent particles affect the erosion and crack morphology of brittle materials. Wada and Watanabe f91 found a sharp increase in the erosion rates when the ratio of the hardness HP of the particle to the hardness Ht of the target was increased. In particular, erosion rates increased dramatically at a ratio of N,/H, very close to unity. Changes in the overall erosion mechanism and crack morphology were noticed when the ratio of HJH, was increased from unity to higher values, the mate&& used being Al,O, and Sic-reinforced AIZO, [IO]. Impact events on these materials using alma particles where &/H, < 1 yielded simple p~c~g-~~ events with the absence of any noticeable lateral cracking. However, when impacted with SIC particles, where H,/Y&> 1, typical lateral crack formation occurred. The velocity exponent n was also lowered when Hp/Ht < 1, in the range 1.3-1.8 compared with values of over 2 obtained for HP/H,> 1. Although not recognized as an HP/W, effect, such transitions in velocity exponents were also reported by Sykes et al. [ 111 on SIC whiskerreinforced alumina samples impacted with AlaO particles. It was seen that the velocity exponents decreased from 2.9 to 0.8 with increasing additions of Sic whiskers up to about 25%. ~vestigations by Morrison et al. [ 121 on the erosion of S&N4reinforced with Sic using AlaO abrasives showed similar effects, although fewer experimental results were available. Erosion testing by Srinivasan and Scattergood [ 131 on a series of sintered aluminas of varying hardnesses found that Hp/Hc emerges as an important parameter that affects the mechanism of material removal. It appeared that some damage accumulation was necessary for fracture initiation when HP/H& 1 and it was suggested that particle crushing or flattening of softer erode&s reduces the efficiency of the erosion process. The overall steady state surface morphology was quite similar for hard VS. soft erodents, but many more impacts were needed to generate the residual stress-driving forces for fracture with softer erodents. Wada and Watanabe [ 141 found that “crashing” of softer abrasive particles leads to some material removal during the erosion of AlaO ceramics. Such results indicate the importance of the HP/H, ratio for erosion of ceramics. A series of normal impact erosion tests on alumina ceramics ranging in hardness from about 10 to 22 GPa using Sic and AlaO particles of sizes 63 and 405 pm at velocities of 40, 60 and 90 m s-’ is currently in progress [ 151. The results confirm the importance of the hardness ratio effect 1161. Scanning electron microscopy (SEM) of single-impact events showed the absence of a lateral cracking mechanism of erosion for the 405 pm Al203 particles at 90 m s- 1 on the hardest of the alumina target materials (sapphire). A plastic punching-type mechanism was seen to occur without any cracking or fragmentation of the sapphire substrate (Pig. 1). Impacts with Sic particles on the same target showed the classical lateral cracking and chipping mechanism of erosion. A depth and width profile of the single impacts using a confocal lsser microscope indicated a shallower average penetration for
117
Fig. 1. Single-impact event of 405 pm E-17 AlSO on sapphire at 90 m s-l, showing a plastic punching-type mechanism without cracking or fragmentation of the sapphire substrate.
the A1203 erodent particle impacts. The steady state erosion surface of sapphire impacted at a velocity of 40 m s-’ with 63 pm AlzO, particles showed the presence of extensive plastic punching similar to that which occurs when ductile targets are eroded with spherical particles [ 171. This, together with energy-dispersive X-ray analysis (EDXA) of single impacts, suggested the existence of some form of fragmentation and embedding of tips of the erodent particles on the surface of the target materials, especially at low velocities and low HP/H, ratios. Further evidence was provided by the presence of hertzian cracks on the steady state sapphire surface impacted with AlzO, particles, which suggests a rounding or blunting of particle tips due to fragmentation. The underlying motivation for the work reported here was to verify the possible effect of increasing load on the fragmentation of erodent particles of different hardnesses, on a standard substrate of sapphire. Using AlsOa and Sic erodents on sapphire gives HP/H, < 1 and HP/H,> 1 respectively. 2. Results and discussion Single particles of AlzO, and Sic with a mean size of 405 pm were mounted using epoxy onto the ends of standard indenter fixtures used in a Micromet microhardness machine. The particles were then indented at loads ranging from 1 to 10 N. Replicas of the particle tip shape were obtained by first indenting a flat, optically smooth diamond-turned copper disc. This was repeated so as to con6rn-1 the shape and size of the initial particle tip. The particle was then used to indent a polished sapphire sample at the same load. A replica of the particle tip was again made on the copper disc so as to obtain information on the fragmentation or crushing caused by indentation on sapphire. This procedure was followed at various loads to define the effect of increasing load on the damage incurred by the particle tip. SEM micrographs were made of the replicas for each series of tests. A slight rotation of the particles in the epoxy used to affix them to the indenter
118
sometimes caused the appearance of spurious shear in the micrographs, as will be noted where appropriate. The indentations were observed using a Hitachi S-530 scanning electron microscope. Further characterization of debris found after indentation was accomplished using EDXA. A Tracer-Northern TN 5500 energy-dispersive spectrometer was used for this purpose. Figure 2 shows a comparison of the abrasive particles of A1203 (Alundum El?) and SiC (Crystolon 37), supplied by Norton. A shape and form analysis of the particles gave the mean properties shown in Table 1. For definitions, refer to ref. 18. The mean section diameter values obtained via intercept lengths tend to be Iess than the average size determined by sieve anaIysis (mesh size) which is reported by the supplier in conjunction with grit size specification. It is seen from Table 1 that the two abrasive particles do not differ in their sizes and shape parameters. Microhardness measurements made using a standard Vickers indenter (400 gf load) indicate that they differ in their hardnesses. The measured hardness of the AlaO, particles was about 16 GPa while the SiC particles had a hardness of about 25 GPa. The single-crystal sapphire sample used had a hardness of about 22 GPa while the hardness of the copper sample used to replicate
(al Fig. 2. 405 pm abrasive particles of (a) SiC and (h) E-17 Al203. TABLE 1 Particle characteristics Property
Supplier, 60 grit &II) Section diameter (pm) Form factor Aspect ratio
Wue Alzos
SiC
405 347f6 0.629 f 0.07 1.591 kO.34
406 345f5 0.646f0.07 1.508 f 0.24
119
A series of control indentations showed that indentations on copper alone do not produce visible particle damage over the range of loads used here. A wide variety of indentation sizes and shapes are possible owing to particle size and shape variation and orientation on the indenter IIxture. Figures 3 and 4 show representative sequences for indentations using A1203 and Sic particles. Figure 6 shows a series of standard Vickers indentations on the copper disc. At a given load, the indentation area of the particle must be comparable with that of the Vickers indentation; otherwise projections of the epoxy mount rest on the surface and partially support the load. Figure 3(a) shows that some fragmentation occurs for AlaO particles at the lowest load of 3 N (some shearing due to particle rotation occurred for the righthand frame in Fig. 3(a)). This was confirmed by many other tests. Continued fragmentation occurs with increasinp load in Fig. 3. EDXA showed that the
(cl Fig. 3. Single-particle indentation of 405 m E-17 AlzOs on a diamond-turned copper sample both before (picture on the left) and replicated after indentation at the same load (picture on the right) on sapphire substrate. The micrographs are taken at loads of (a) 3 N, @) 6.5 N and (c) 10 N. It should be noted that there is fragmentation of the particle tip at a load of 3 N followed by further crushing at the higher load. Also the joining of blunted particle tips at the hlgher load to form the larger indentation should be noted.
Fig. 4. Single-particle indentation of 405 q Sic on a diamond-turned copper sample both before (picture on the left) and replicated after indentation at the same load (picture on the right) on sapphire substrate. The micrographs are taken at loads of (a) 3 N, (b) 6.5 N and (c) 10 N. The fragmentation of the particle tip at the load of 6.5 N followed by shattering at the higher load should be noted.
Fig. 5. Standard Vickers indentations on an optically smooth copper disc at loads of 1, 2, 3, 4, 6.5 and 10 N.
121 sma,ll fragments visible are AlaOs. On the basis of other observations, it is not likely that these fragments come from the sapphire substrate, although this cannot be ~~~~0~s~ established using EDXA. In contrast, SiC particle tips did not show fragmentation until loads reached 6.5 N or higher. A typical sequence is shown in Fig. 4 (the right-hand frame in Fig. 4(a) shows some shear due to rotation). Interestingly, Sic particles sometimes underwent catastrophic shattering at the highest load of 10 N, as is evident in Fig. 4(c). Ca~trop~~ shattering did not usually occur for the A1203 particle, even though they undergo fragmentation at lower loads. EDXA on Sic indentations showed unambiguously that all fragments obtained are SiC and not AI,O, from the sapphire substrate (e.g. fragments in Figs. 4(b) and 4(c)). This reafhrms the conclusion that all fragments observed using either A1203 or Sic indenters are particle fragments. As was already noted, EDXA of the ~den~tions before and after indentation on sapphire indicated the presence of debris from the fragmented particle. A very clear indication of such debris was seen for the A1203particles, and this is shown in Fig. 6. The micrographs were taken for indentations of a particle of A1203 at loads of 4 and 6.5 N. The micrographs show the
F’ig. 6. EDXA of debris located at the ~den~tion of E-17 Al& on copper before and after indentation on a sapphire substrate at the same load. The micrographs show indentations at (a) 4 N and (b) 6.5 N. A clear aluminum peak is found for the 4 N indentation which decreases at the higher load, indicating a threshold load of fragmentation for the particle tip. The rounding of the particle edges is also clearly seen.
122
blunting of the tips of the particles and also reveal the presence of large particle fragments. A well-defined ahuninum peak appears in the X-ray analysis of the replica made after the sapphire indentation (Fig. 6). Similar results also appear at increased loads although the height of the aluminum peak tends to be lower, indicating a lower overall amount of tip ~~en~tion above a threshold load. This suggests that particles are rounding owing to the crushing of the tip above a threshold load. The fact that the debris at increasing loads is found along the periphery of the indentation rather than towards the central region, as was occurring at the lower loads, also seems to lend support to the fact that the particle tips are being rounded off. The indentations and X-ray results shown are found to be reproducible and the X-ray analysis shown is representative of the results found. There is of course a large range of indentation topology because of variations in particle shape. The results obtained lend support to the fact that fra~en~tion and rounding of the individual abrasive particles are likely to occur at certain threshold loads during erosion testing. Details depend on the relative hardness and toughness of the erodent particle compared with the target material. It appears that fragmentation occurs earlier (lower threshold load) when H&Y, is smaller. However, a very large number of tests would be needed to develop quantitative statistics. At higher loads, catastrophic shattering of hard abrasive particles can occur. The particle fragments may in turn affect the overall erosion mechanism. It should be noted that there is no direct comparison of the static indentation loads used here with the dynamic particle impact loads in erosion processes. The quasi-static analysis given in Appendix A suggests, however, that typical impact loads can easily exceed the range l-10 N used in this study. Particle bhmting by fragmentation would cause the individual particles to behave more like rounded spheres impinging on the target surface. Particle blunting can lower the velocity exponents [ I3 1, as was noted earlier. Damage acc~~a~on may be necessary to build up the requisite stresses to produce lateral cracks when softer erodent are used.
Acknowledgments The authors gratefully acknowledge the U.S. Department of Energy for support of this research under Grant DE-FGO5-8~ER~5 115. Useful discussions with S. Srinivasan are also acknowledged. References 1 B. R. Lawn and R. Wilshaw, Indentation fracture: principles and applications, J. Muter. sci, 10 (1975) 1049. 2 A. G. Evans and R. Wiihaw, Quasi-static solid particle damage in brittle solids-I. Observations, analysis and implications, Acta &Z&CX&., 24 (1976) 939. 3 B. R. Lawn, A. G. Evans and D. B. Marshall, Elastic-plastic indentation damage in ceramics: the median-radial crack system, J. Am. &ram. Sot., 63 (9-10) (1980) 574.
123 4 B. R. Lawn, A. G. Evans and D. B. Marshall,Elastic-plastic indentation damage in ceramics: the lateral crack system, J. Am. Cerum. Sot., 6si (11) (1982) 561. 5 A. G. Evans, Abrasive wear in ceramics: an assessment, in A. G. Evans (ed.), The Science of Ceramic Machining and Su@kce Finishing II, Noyes, Park Ridge, NJ, 1979, p. 1. 6 S. M. Wiederhorn and B. R. Lawn, Strength degradation of glass impacted with sharp particles: I, Annealed surfaces, f. Am Ceram. Sot., 62 f1979) 66. 7 A. G. Evans, M. E. Gulden and M. E. Rosenblatt, Impact damage in brittle materials in the elastic-plastic response regime, Proc. R. Sot. London, Ser. A, 361 (1978) 343. 8 G. P. Tilly and W. Sage, The interaction of particle and material behavior ln erosion processes, Wear, 16 (1970) 447. 9 S. Wada and N. Watanabe, Solid particle erosion of brittle materials (Part 3) -The interaction with materialproperties of target andthat of impingementparticle on erosive wear mechanism, Yogyo Kyokai Shi, 95 (6) (1987) 573. 10 S. Wada, N. Watanabe and T. Tani, solid particle erosion of brittle materials (Part Q)-The erosive wear of AlaO,-SiC particle composites, J. C&-am Sot. Jpn., Int. Ed%, 96 (1988) 737. 11 M. T. Sykes, R. 0. Scattergood and 3. L. Routbort, Erosion of Sic-reinforced ahrmina ceramic composites, Composites, 18 (2) (1987) 153. 12 C. T. Morrison, J. L. Routbort and R. 0 Scattergood, Erosion of Sic-whisker reinforced SiaN+ in Advanced Structural Ceramics, Materials Research So&.&g Symp. Proc., Vol. 78, Materials Research Society, Pittsburgh, PA, 1987, p. 207. 13 S. Srinivasan and R. 0. Scattergood, Effect of erodent hardness on erosion of brittle materials, Wear, 128 (1988) 139. 14 S. Wada and N. Watanabe, Solid particle erosion of brittle materials {Part I)-The erosive wear of thirteen kinds of commercial ahrminaceramics, Yo-yo K~okai Shi, 9.5(9) (1987) 835. 15 K. Anand, S. K. Hovis, H. Conrad and R. 0. Scattergood, Flux effects in solid particle erosion, Wear, 118 (1987) 243. 16 L. Murugesh,Effect of erodent and target particle hardnesson erosion of aluminaceramics, M.S. Thesis (in progress), North Carolina State University, 1989. I7 Ronald A. Mayville, Mechanisms of materiel removal in the solid particle erosion of ductile materials, Rep. LBL-7333, 1978 (Lawrence Berkeley Laboratory). 18 J. C. Russ, Computer Aided MicroscopyThe Measum ana! Analysis of Images, North Carolina State University, Raleigh, NC, 1988.
Appendix A Quasi-static analysis can be used to estimate the maximum indentation load transferred by erodent particle impact. While this does not include dynamical effects (strain rate or temperature), it gives an approximate upper bound for the loads. Let the particle shape be represented by the circular cone shown in Fig. Al. D is the diameter of the cone’s base which is assumed equal to the average particle size. $ is the cone apex angle and is a measure of sharpness. The particle is assumed to be perfectly rigid. At a penetration depth z, the radius of the contact area is a. If H is the hardness (flow stress) of the target, then at any position z the load P(z) required to balance the surface tractions is P(z) = 71-wa2= dEz2 tan’ tcr_Inertial forces are ignored in the quasi-static analysis. The plastic work W needed to penetrate to depth z,, and the initial kinetic energy KE of the particle are respectively
124
Fig. Al. Geometry
*mP.x
Iv=
f
P(z)
dz=
0
for cone-shaped
7rHz,,”
impact particle.
tan2 I)
3
and KE=
TDSpU2 48tan$
where p is the particle density and ZI is the velocity. Equating W with the kinetic energy KE gives the maximum penetration depth x,, and maximum load P_ as follows:
213
P max= r~Hz,,,,,~ tan2 3/= dD2v4/3
Typical values for 405 pm AlzO, particles impacting a sapphire target would beH=22GPa,p=3.95x103kgm~3,I)=405~mandv=30ms~’.Substi~ting these values gives P,, = 5 X 104 N. This is very much larger than the l-10 N load range used for the indentations made here. In a real impact situation, only a fraction of the total kinetic energy will actually be transferred to crack driving force, and P,, must be lower. However, the point to note is that impact loads in the range l-10 N are certainly within reason. Particle fragmentation effects of the kind reported here are a likely possibility for erosion conditions.