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Repeated point contact loading on Ce-TZP
Wear, 170 (1993) 247-2.53
Repeated Marie-Odile
247
point contact loading on Ce-TZP* Guillou,
John
Leslie
Henshall
and Robert
Maurice
Hooper
School of Engineering, University of Exeter, Exeter, EX4 4QF (UK) (Received April 5, 1993; accepted August 4, 1993)
Abstract The deformation and fragmentation behaviour in a toughened c&a-stabilized zirconia ceramic have been investigated by using unlubricated repeated metallic point contact loading at room temperature to explore the possibilities of cyclic fatigue effects. All tests were conducted on a purpose-designed and built computer-controlled apparatus. 120” hardened silver steel cones ‘were cyclically loaded on the polished Ce-TZP substrate, and the damage was observed and analysed as a function of the number of cycles for loads of 19.6f9.8 N. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the substrate. A tetragonal --) monoclinic martensitic transformation occurred in the zirconia beneath, and adjacent to, the contact zone. This transformation zone increased in size as the number of cycles increased, even though there was virtually no change in the diameter of the flattened tip. The expansion associated with this phase change in the zirconia caused granular lijiing from the surface, at the edge of the contact zone, that resulted in
intergranular fragmentation and spalling of the substrate. The hardness of the substrate in the contact zone increased by approximately 15% after 2 X 10s cycles. Traces of metal transfer onto the ceramic substrate could be observed only at 2~ ld
cycles and above.
1. Introduction
One of the few ceramics which forms the basis of a major contender for commercial engineeering utilization is zirconia. Practical experience shows that to obtain the benefits of the increased hardness and general wear resistance of ceramics, the fracture toughness, defined as a measure of the materials’ ability to resist the weakening effect of sharp cracks and flaws, requires a value of at least 20 MPa m”.5for reliable performance in highly stressed situations (e.g. the cylinder blocks of engines) [l]. Zirconium-cerium oxides bulk ceramics have been manufactured under carefully controlled conditions to achieve values close to this. A toughening effect, that can be obtained via an energy absorption by a metastable tetragonal to monoclinic martensitic phase transformation, is the fundamental reason for the prominence of the ceria-stabilized polycrystalline tetragonal phase zirconia ceramics (Ce-TZP). This transformation, which occurs under the influence of applied stresses, is accompanied by an increase in volume between 3 and 5%, thus compressing the crack tip. This mechanism enhances toughness since it provides the compressive stress exactly where it is needed. *This research was supported by the UKSciencc and Engineering Council, under Grant No. GRF77739.
0043-1648/93/$6.00
Although these ceramics can now be considered seriously for many potential structural applications because of this effective toughening mechanism; surprisingly, they can be susceptible to degradation under cyclic fatigue loading. The importance of cyclic loading in many potential applications of ceramics, such as in gas turbine and reciprocating engines, has engendered much recent attention to the topic of ceramic fatigue. Initially it was postulated that cyclic fatigue effects in ceramics occurred solely as a result of environmentally assisted crack growth [2]. More recent work [3-51 has shown that cyclic plasticity may occur under variable loading conditions. The mechanisms of fatigue in ceramics are not yet fully understood. Many of the potential application areas of ceramics, e.g. car tappets etc, involve point contact loading. Previous fatigue work in this area has involved using either a conventional Vickers diamond pyramidal indenter [6,7], which is unlikely to be representative of most practical situations, or repeated sliding [3,4]. In this study a new method using 120” metallic conical indenters is proposed. The ways in which repeated metallic point contact loading affects the room temperature microstructure and mechanical properties of a toughened ceria-stabilized tetragonal polycrystalline zirconia (the ceria-stabilized zirconia was supplied by Tenmat, Manchester, UK) are investigated. The reasons
0 1993 - Elsevier Sequoia. All rights resewed
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Guillou
et al. i Reacted
point contact loading an Ce-EZP
for using a softer conical material are (i) that it is related to operational situations where ceramics are subjected to repeated loading by softer materials (e.g. valve seatings in engines), (ii) a harder indenter would of necessity produce immediate and substantial localized plastic deformation to accommodate the displaced volume, (iii) the deformation of the softer indenter ensures that there is good conformity and alignment of the two contact surfaces, which results in a uniform pressure distribution and (iv) the contact area does not vary as the load is cycled in a test, as it would with elastic, i.e. hertzian, contact. The present testing conditions are therefore assumed to reflect the likely commercial application of such ceramic materials.
2. Experimental details 2. I. Materials 2.1.1. Ceramic ~~b~~ate The ce~a-stabi~zed polyc~stalline tetragonal phase zirconia, Ce-TZP, was supplied in the form of 40 mm X 60 mmX5 mm tiles, with batch code reference UCll. The compacts were cold isostatically pressed before firing at 1450 “C. X-ray diffraction analysis showed that the crystal structure was tetragonal, with no detectable monoclinic or cubic zirconia phases. The single edge notched beam & value was 7.7 MPa rn’.’ [S], and the indentation toughness, using the Evans’ analysis [9], was 9.9 MPa rn”.’ [S]. Molten KOH at 400 “C was found to be a very good etchant for Ce-TZP. Polished and etched specimens were examined in the scanning electron microscope operating at 30 kV. The grain sizes were generally observed to be in the range l-2 pm with occasional larger (up to 5 pm) grains. In addition, the link energy dispersive analyser system (EDS), attached to the electron microscope, showed that the occasional enclosed large pores tended to be associated with the impurity elements such as hafnium or barium.
The EDS system with associated ZAF4 software was used to determine the amount of stabilising agent CeO, as 10.45 mol.% for UCll. Referring to the phase diagram proposed by Gupta and Andersson [lo], this result clearly confirms that this Ce-TZP material lies in the face-centered tetragonal region. Prior to fatigue testing, the suitably sized Ce-TZP specimen was cut with an impregnated diamond saw, ground and finally polished down to 4 pm diamond finish. The specimen was then cleaned in an acetone bath i~ediately prior to use. The Knoop hardness (19.6 N applied load) of the undeformed substrate was 8.1 GPa. 2.1.2. Metallic indenter The metallic indenters were machined from either 12 mm diameter silver steel (BS 1407:1970) or 304L stainless steel bars into cones having an apex angle of 120”. The silver steel tips were then hardened by heat treating them for 1 h in a furnace operating at 850 “C, and subsequently quenched in oil. The Knoop hardness values (19.6 N applied load) prior to testing were 7.6 GPa for hardened silver steel and 3.8 GPa for stainless steel. Both types were thereafter carefully ground to a sharp apical angle of 120”. The tips were examined in profile under an optical microscope before each experiment in order to ensure integrity and sharpness, They were thoroughly cleaned ultrasonically in acetone immediately prior to the cyclic fatigue tests. 2.2. The repeated point contact loading method This method, which has been described previously [ll], is based on the room temperature unlub~~ted repeated point contact loading of a ceramic surface with a sharp 120” conical indenter. Figure 1 represents a schematic diagram of the computer-controlled apparatus. The zirconia specimen was clamped to a platform tied on a load cell which in turn was mounted on a rotatable x-y stage. The apparatus was then calibrated immediately prior to use by applying known
Adjustable
Horizontal
beam
Pivot Extension
Adjustobla
- Adjustable
extension-----_-n
Counte~balonce
Fig. 1. A schematic
illustration
height
of the cyclic load fatigue
apparatus.
spring extension
weights
M.-O. Guillou et al. / Repeated point contact loading on Ce-TZP
loads of 0.1-20 N to the load cell and recording the readings with a personal computer (486) using an Amplicon Liveline PC27 A/D card. The platform was split so that the ceramic substrate could be removed after a certain number of cycles for optical examination and replaced in exactly the same position relative to the indenter. The tests were performed in ambient air at 20f 1 “C. Sinusoidal cyclic loading was used with a frequency 2.0 Hz and minimum and maximum loads 9.8 N and 29.4 N respectively. The loading wave form was continuously displayed on the PC and the mean, maximum and minimum applied loads were recorded for each cycle; as well as the total number of cycles, time of test run and corresponding frequency. The metallic tips blunted to a flat conforming surface on first application of the mtium load. During each subsequent cycle, the tips were always in contact with the Ce-TZP substrate. Experiments were carried out to observe not only the damage promoted by an increasing number of cycles but also the flattening of the metallic tips. The test range varied between 1 x lo4 and 4.75 X 105 cycles. The surface damage behaviour was observed after each test by optical means. A new metallic tip was used for each test. Snoop hardness tests were then performed on both the ceramic substrate and the flattened tips. The fracture surfaces in the damaged contact zones were inspected by scanning electron microscopy, together with energy-dispersive X-ray analysis, following gold coating of the ceramic surface. Vickers hardness tests were also carried out in the centre of the deformed impressions in an attempt to evaluate the fracture toughness.
3. Results 3.1. Hardened silver steel cones
The dimensions of the flattened steel tips were measured optically soon after testing in order to obtain the mean contact pressure values P,,,. An average of five measurements was taken per tip. The results, shown in Fig. 2(a), indicate that, at least above 104 cycles, the maximum contact pressures are approximately constant at 3.87 GPa. The corresponding minimum contact pressures are 1.29 GPa. Approximate calculations show that the change in diameter of the contact zone due to elastic deformation is only about 1% of the diameter of the flattened tip. After the compressive cyclic tests had been performed, Knoop hardness tests were undertaken on both the deformed regions on the ceramic surface and on the flattened silver steel tips in order to detect whether work hardening had been produced in either substrate or cone material. The Knoop hardness of the deformed
v
249
Silver Steal lip
D
200
100
Number
300 of cycles
400 (X1000)
(0)
16 ‘;;
14
8 i
12
c E P
E 2
+ Flattened silver steel tip, L-0.25N
10
8
Y
It.,.,.,., 0
100
200
Number
300
of cycles
400
500
(X1000)
(b)
Fig. 2. (a) Maximum contact pressure versus number of cycles. (b) Variations of the Knoop hardness values with increasing number of cycles for both the flattened cones and the deformed Ce-TZP substrate - frequency: 2 Hz - applied cyclic loading: 19.6 f 9.8 N.
substrate was found to increase slightly with increasing number of cycles (see Fig. 2(b)). As shown below, fragmentation and inward displacement of the substrate (after 4.75 x 105 cycles) prevented measurement of its hardness. The Knoop hardness of the flattened metallic tip was observed to reflect this behaviour even more noticeably (see Fig. 2(b)). The amount of damage observed around the edges of the contact zone increased as the number of cycles increased (see sequence of micrographs in Figs. 3(a)-3(g)). Using the scanning electron microscope the fracture path was observed to be intergranular (Fig. 4). The fracture origins associated with spalling were always at the periphery of the contact zones (Figs. 4(a)+d)). A distortion of the Ce-TZP grains was observed prior to fracture. This grain h@ng is clearly visible around the edges of impressions performed at 1 X lo4 cycles and up to 5.7 X lo4 cycles (Fig. 4(a) and
250
M.-O. Guillou et al. I Repeated point contact loading on Ce-TZP
(@I 111 Fig. 3. The effect of number of cycles on the damage caused on Ce-TZP by hardened silver steel indenters - applied cyclic load: 19.61: 9.8 N: (a) 1 x lo4 cycles; (b) 3.77X lo4 cycles; (c) 5.7~ lo4 cycles; (d) 12.75 X 104 cycles; (e) 1.50X l@ cycles; (f) 2.00X l@ cycles; (g) 4.75 X ld qcles.
Fig. 4(b) respectively). This phenomenon was accompanied by intergranular fracture, which became more significant with increasing number of cycles, until considerable spalling occurred (Figs. 4(c)-4(d)). The contact zone was also generally uniformly depressed below the original surface of the ceramic substrate (compare with the scanning electron microscope line-scan in Fig. 5). The depth of the impression increased with increasing number of cycles. In Fig. 5, in the region just outside the contact zone, a surface rumpling which occurred as a result of plastic defo~ation can be seen. Estimated values of the sizes of the plastic zones and impressions with increasing number of cycles are shown in Fig. 6. The values of the latter must be treated with some caution owing to measurement inaccuracies arising from the combination of spalling and the impression sinking into the substrate. The size of the impression made onto the ceramic substrate appeared to decrease with increasing number of cycles (see Fig. 6). In addition, metal adhesion was visible at the edges of the contact zones for tests above l@ cycles (see arrow in Fig. 4(c)). Hence, friction
between the ceramic substrate and the metallic tip would presumably increase as the number of cycles is increased. Although considerable damage of the substrate was observed, in particular around indentations corresponding to 4.75 x 10s cycles (Fig. 3(g)), no trace of zirconia or ceria was found on the flattened hardened steel tip. An attempt to evaluate the fracture toughness in the region of the deformed impressions made by hardened silver Steel tip onto Ce-TZP was made. It was found that the maximum size of an impression that could be fitted into the deformed zone was obtained using a load of 19.6 N. The resulting Vickers indentation fitted in the centre of the region but no cracks emanated from the comers of the impression (Fig. 7(a) and 7(b)}. It was therefore not possible to evaluate an indentation fracture toughness value. 3.2. Stainless steel cones A comparative test was performed using a softer stainless steel cone with the same load range, i.e. 19.6rfi9.8 N and 12.75 X 104 cycles. In this case the
M-O.
251
Guillou et al. / Repeated point contact loading on Ce-TZP
Fig. 5. Line-scan micrograph (from the scanning electron microscope.) showing both the deformed impression sunk into the substrate and the plastic deformation zone surrounding this impression made onto Ce-TZP after 12.75 x 10” cycles using a 120” hardened silver steel tip: frequency, 2 Hz; applied cyclic load, 19.6& 9.8 N, sample tilt, ahout 90”; magnification, x 800 (note the presence of a Vickers indentation in the centre of the deformed region). 240
(C)
(dt
Fig. 4. Details of the edge of contact zone indicating an intergranular fracture mode. Substrate, Ce-TZP, indenting cone, hardened silver steel; frequency, 2 Hz, applied cyclic load, 19.6f9.8 N. (a) 1 X 10’ cycles, (b) 5.7X l@ cycles, (c) 12.75 X lo4 cycles, (d) 4.75 x 16 cycles.
metal tip flattened to a larger diameter, which gave minimum and maximum contact pressures of 0.74 GPa and 2.11 GPa respectively. No cracking or substrate damage was observed. Nevertheless metal was transferred onto the ceramic surface (see Fig. 8). This metal deposit was found to pile-up around the edges of the contact zones. Energy-dispersive X-ray analyses were performed whilst the specimen was observed under the scanning electron microscope. Traces of iron were indeed found, primarily at the edges of the deformed impressions. There was no trace of martensitic transformation in the Ce-TZP in the vicinity of the contact zones in this case.
4. Discussion Since in this series of tests, there is negligible relative displacement of the deformed conical tip, this is a situation where fretting fatigue is unlikely to occur.
I
’
I
8
I
8
200 ? j
160
‘j % E if
120
c
I
/”
-
80
40
0
100
200
300
400
500
Number of cycles (x1000)
Fig. 6. Tbe effect of number of cycles on the dimensions of the impressions and plastic deformation zones produced on Ce.-TZP (UCll) using 120” hardened silver steel cones: applied cyclic load, 19.6 f 9.8 N.
None the less, deformation and fracture due to the cyclic loading can clearly be seen to occur (Fig. 3). It would appear that there is a critical contact pressure required to induce these effects, since they are apparent after only 10“ cycles when the contact pressures are 1.29 to 3.87 GPa (silver steel cones), whereas when they are in the range 0.74 to 2.11 GPa (stainless steel
M.-O. Guillou et al. f Repeated point contact buding on Ce-?ZP
fbf (ai Fig. 7. (a) Scanning electron micrograph showing a 2 kgfdiamond Vickers indentation within the deformed region of an impression made by a 120” hardened silver steel tip onto Ce-TZP after 12.75 X l@ cycles using a frequency of 2 Hz at room temperature - applied cyclic load: 19.61 f 9.8 N. (b) Details of one of the Vickers indentation comers indicating no cracking emanating from it.
Fig. 8. Impression made onto Ce-TZP by a 120” stainless steel cone after 12.75 x 10’ cycles using a frequency of 2 Hz at room temperature. Applied cyclic load: 19.6k9.8 N.
cones), no deformation or fracture is observable after 12.75 x 104 cycles. This process of fatigue damage can be modelled by considering the sequence of events observed whilst cycling with hardened silver steel cones between loads of 9.8 and 29.4 N, i.e. (i) The formation of a tetragonal + monoclinic martensitic transformation zone. (ii) The lateral expansion of the extent of this transformed zone. (iii) Surface steps formed by individual grains lifting-up as a result of the transformation. (iv) Inter-
granular fracture at these surface steps at the periphery of the contact zone. (v) Extension of the fractured p&s around the edges of the contact zone. (vi) Slight metal adhesion at the highest number of cycles.These processes will now be considered in more detail. As outlined in the introduction, the tetragonal +monoclinic martensitic transformation of the zirconia grains involves a volume expansion of 3%-S%. Therefore, it is somewhat surprising to observe such extensive transformation under the compressive loading in the present situation. However, it has been recently shown that loading tetragonal zirconia ceramics in compression also causes permanent ~ansfo~ation f12]. Nevertheless, Heuer et al. f12] stated that the stress required for the tetragonal to the monoclinic transformation to occur in compression is considerably higher than in tension. With increasing number of cycles, the plastic zone forms and spreads outwards from the point contact loading. It is thought that this lateral expansion of the plastic deformation zone in the vicinity of the deformed impression probably occurs as a result of the small plastic strain increments caused by each of the successive cycles. When su~~ient plastic strain has accumulated in grains adjacent to these which have already transformed, this then will initiate the transformation process in these grains. During a standard diamond Vickers test, accoustic emission studies have indicated that the plastic zone was expanding towards the surface during unloading of the indenter [13]. This would suggest that a similar mechanism may occur in the present situation whereby the expansion of the transformation zone occurs during the unloading part of the cycle. At the periphery of the contact zone, the steps formed by grain aping are appreciably higher than elsewhere. The stress components at the surface are generally compressive except for the radial stresses just outside the actual contact zone, which are tensile [14]. The maximum value of this tensile stress occurs at the edge of the contact zone and is given by (1 -2v)p0/2, where p,, is the uniform contact pressure at maximum load, about 3.8 GPa (see Fig. 2(a)), and u is Poisson’s ratio (assumed to be 0.27). This gives a value of about 870 MPa for the maximum radial tensile stress with the present system. There may be surface cracking beneath these uplifted grains, but no definitive observations have yet been possible. It is thought that this tensile stress is responsible also for the cracks which were observed to form and propagate in a circular manner at the edge of the contact zone. It seems likely that the intergranular surface fracture is triggered by the distortion of the grains, with possibly some contribution of simultaneous grain loosening. Once a fracture site has formed, cracks then propagate from this around the edge of the contact zone. The amount of intergranular fragmentation and
M.-O. Guillou et al. / Repeated point contact loading on Ce-TZP
spalling of the transformed grains increases with increasing number of cycles. Cardona et al. [15] have also reported a predominantly intergranular fracture in fatigue tests using bending of notched beams of Ceo,-PSZ . A similar sequence of plastic zone formation and expansion, followed by plasticity-induced microcracking at the edge of the contact zone, has been observed in tests with stainless steel cones on MgO [ll] Adhesive transfer of metal to Ce-TZP has previously been shown to occur [16], and its occurrence in the present tests is quite feasible. However, it is only found after considerable spalling has occurred, and the presence of the induced surface roughness and cracks may assist the adherence and transfer of the metal.
5. Conclusions The surface of the fatigued Ce-TZP specimen was examined after testing by both optical and electron microscopic means. Evidence of surface damage in the form of accumulation of both a plastic deformation zone expanding outwards from the point loading contact area and intergranular fracture followed by spalling was recorded. This article demonstrates that repeated metallic conical indentation is a convenient and useful method for evaluating the susceptibility of a ceramic surface to plastic deformation prior to fragmentation under cyclic point loading. This technique also demonstrates that a softer material (metal) may cause much greater wear of a harder material (ceramic), which is of considerable relevance in tribological applications.
Acknowledgments This research project has been supported by a grant from the UKScience and Engineering Research Council. The authors would like to thank Mr. J. Cooper for his programming skills and Mr. K.N. Smith for his assistance with the construction of the experimental apparatus.
253
References
1 A.H. Cottrell, Will anybody ever use metals and alloys again, in A. Briggs (ed.), The Science of New Materials, Blackwell, Oxford, 1992, pp. 4-31. 2 A.G. Evans and E.R. Fuhler, Crack propagation in ceramic materials under cyclic loading conditions, MetalL Trans., 5 (1974) 27-33. 3 C.A. Brookes, M.P. Shaw and P.E. Tanner, Non-metallic crystals undergoing cumulative work-hardening and wear due to softer lubricated metal sliding surfaces, Proc. R. Sot. London, Sect. A, 409 (1987) 141-159. 4 M.P. Shaw and C.A. Brookes, Cumulative deformation and fracture of sliding surfaces, Wear, 126 (1988) 149-165. 5 R.H. Dauskardt, W. Yu and R.O. Ritchie, Fatigue crack propagation in transformation-toughened zirconia ceramic, J; Am. Ceram. Sot., 70(10) (1987) (=248X252. 6 M. Reece and F. Guiu, Repeated indentation method for studing cyclic fatigue in ceramics, J. Am. Gram. Sot., 73 (6) (1990) 1004-1013. I E. Takakura and S. Horibe, Fatigue damage in ceramic materials caused by repeated indentations, J. Mater. Sci., 27 (1992) 6151-6158. a M.-O. Guillou, Indentation deformation and fracture of hard ceramic materials, Ph.D. thesis, University of Exeter, 1992. 9 A.G. Evans, Fracture mechanics applied to brittle materials, Am. Sot. for Testing and Materials Spec. Tech. Pub. No. 678, ASTM, Philadelphia, PA, 1979, pp. 112-135. 10 T.K. Gupta and C.A. Andersson, Advances in cryogenic engineering materials, Proc. 5th Int. Cryog. Mater. Conf 30, 1984, pp. 367-373. 11 M.-O. Guillou, J.L. Henshall and R.M. Hooper, Indentation cyclic fatigue of single-crystal magnesium oxide, J. Am. Ceram. Sot., 76(7) (1993) 1832-1836. 12 A.H. Heuer, M. Ruhle and D.M. Marshall, On the thermoelastic martensitic transformation in tetragonal zirconia, J Am. Ceram. Sot., 73(4) (1990) 1089-1093. 13 P.E. Reyes-Morel and I. Wei Chen, Stress-biased anisotropic microcrackmg in zirconia polycrystals, J. Am. Ceram. Sot., 73(4) (1990) 1026-1033. 14 K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, 1987, p. 94. 15 D.C. Cardona, P. Bowen and C.J. Beevers, Through thickness fatigue.crack growth in CeOTPSZ (TZP), in R.O. Ritchie, R.H. Dauskardt and B.N. Cox (eds.), Fatigue of Advanced Materials, Proc. Engineering Foundation Int. Conf, Santa Barbara, CA, January 13-18, 1991, pp. 287-305. 16 G.M. Carter, R.M. Hooper, J.L. Henshall and M.-O. Guillou, Friction of metal sliders on toughened zirconia ceramics between 293 and 973 “K, Wear, 148 (1991) 147-160.
Repeated Marie-Odile
247
point contact loading on Ce-TZP* Guillou,
John
Leslie
Henshall
and Robert
Maurice
Hooper
School of Engineering, University of Exeter, Exeter, EX4 4QF (UK) (Received April 5, 1993; accepted August 4, 1993)
Abstract The deformation and fragmentation behaviour in a toughened c&a-stabilized zirconia ceramic have been investigated by using unlubricated repeated metallic point contact loading at room temperature to explore the possibilities of cyclic fatigue effects. All tests were conducted on a purpose-designed and built computer-controlled apparatus. 120” hardened silver steel cones ‘were cyclically loaded on the polished Ce-TZP substrate, and the damage was observed and analysed as a function of the number of cycles for loads of 19.6f9.8 N. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the substrate. A tetragonal --) monoclinic martensitic transformation occurred in the zirconia beneath, and adjacent to, the contact zone. This transformation zone increased in size as the number of cycles increased, even though there was virtually no change in the diameter of the flattened tip. The expansion associated with this phase change in the zirconia caused granular lijiing from the surface, at the edge of the contact zone, that resulted in
intergranular fragmentation and spalling of the substrate. The hardness of the substrate in the contact zone increased by approximately 15% after 2 X 10s cycles. Traces of metal transfer onto the ceramic substrate could be observed only at 2~ ld
cycles and above.
1. Introduction
One of the few ceramics which forms the basis of a major contender for commercial engineeering utilization is zirconia. Practical experience shows that to obtain the benefits of the increased hardness and general wear resistance of ceramics, the fracture toughness, defined as a measure of the materials’ ability to resist the weakening effect of sharp cracks and flaws, requires a value of at least 20 MPa m”.5for reliable performance in highly stressed situations (e.g. the cylinder blocks of engines) [l]. Zirconium-cerium oxides bulk ceramics have been manufactured under carefully controlled conditions to achieve values close to this. A toughening effect, that can be obtained via an energy absorption by a metastable tetragonal to monoclinic martensitic phase transformation, is the fundamental reason for the prominence of the ceria-stabilized polycrystalline tetragonal phase zirconia ceramics (Ce-TZP). This transformation, which occurs under the influence of applied stresses, is accompanied by an increase in volume between 3 and 5%, thus compressing the crack tip. This mechanism enhances toughness since it provides the compressive stress exactly where it is needed. *This research was supported by the UKSciencc and Engineering Council, under Grant No. GRF77739.
0043-1648/93/$6.00
Although these ceramics can now be considered seriously for many potential structural applications because of this effective toughening mechanism; surprisingly, they can be susceptible to degradation under cyclic fatigue loading. The importance of cyclic loading in many potential applications of ceramics, such as in gas turbine and reciprocating engines, has engendered much recent attention to the topic of ceramic fatigue. Initially it was postulated that cyclic fatigue effects in ceramics occurred solely as a result of environmentally assisted crack growth [2]. More recent work [3-51 has shown that cyclic plasticity may occur under variable loading conditions. The mechanisms of fatigue in ceramics are not yet fully understood. Many of the potential application areas of ceramics, e.g. car tappets etc, involve point contact loading. Previous fatigue work in this area has involved using either a conventional Vickers diamond pyramidal indenter [6,7], which is unlikely to be representative of most practical situations, or repeated sliding [3,4]. In this study a new method using 120” metallic conical indenters is proposed. The ways in which repeated metallic point contact loading affects the room temperature microstructure and mechanical properties of a toughened ceria-stabilized tetragonal polycrystalline zirconia (the ceria-stabilized zirconia was supplied by Tenmat, Manchester, UK) are investigated. The reasons
0 1993 - Elsevier Sequoia. All rights resewed
M-0.
248
Guillou
et al. i Reacted
point contact loading an Ce-EZP
for using a softer conical material are (i) that it is related to operational situations where ceramics are subjected to repeated loading by softer materials (e.g. valve seatings in engines), (ii) a harder indenter would of necessity produce immediate and substantial localized plastic deformation to accommodate the displaced volume, (iii) the deformation of the softer indenter ensures that there is good conformity and alignment of the two contact surfaces, which results in a uniform pressure distribution and (iv) the contact area does not vary as the load is cycled in a test, as it would with elastic, i.e. hertzian, contact. The present testing conditions are therefore assumed to reflect the likely commercial application of such ceramic materials.
2. Experimental details 2. I. Materials 2.1.1. Ceramic ~~b~~ate The ce~a-stabi~zed polyc~stalline tetragonal phase zirconia, Ce-TZP, was supplied in the form of 40 mm X 60 mmX5 mm tiles, with batch code reference UCll. The compacts were cold isostatically pressed before firing at 1450 “C. X-ray diffraction analysis showed that the crystal structure was tetragonal, with no detectable monoclinic or cubic zirconia phases. The single edge notched beam & value was 7.7 MPa rn’.’ [S], and the indentation toughness, using the Evans’ analysis [9], was 9.9 MPa rn”.’ [S]. Molten KOH at 400 “C was found to be a very good etchant for Ce-TZP. Polished and etched specimens were examined in the scanning electron microscope operating at 30 kV. The grain sizes were generally observed to be in the range l-2 pm with occasional larger (up to 5 pm) grains. In addition, the link energy dispersive analyser system (EDS), attached to the electron microscope, showed that the occasional enclosed large pores tended to be associated with the impurity elements such as hafnium or barium.
The EDS system with associated ZAF4 software was used to determine the amount of stabilising agent CeO, as 10.45 mol.% for UCll. Referring to the phase diagram proposed by Gupta and Andersson [lo], this result clearly confirms that this Ce-TZP material lies in the face-centered tetragonal region. Prior to fatigue testing, the suitably sized Ce-TZP specimen was cut with an impregnated diamond saw, ground and finally polished down to 4 pm diamond finish. The specimen was then cleaned in an acetone bath i~ediately prior to use. The Knoop hardness (19.6 N applied load) of the undeformed substrate was 8.1 GPa. 2.1.2. Metallic indenter The metallic indenters were machined from either 12 mm diameter silver steel (BS 1407:1970) or 304L stainless steel bars into cones having an apex angle of 120”. The silver steel tips were then hardened by heat treating them for 1 h in a furnace operating at 850 “C, and subsequently quenched in oil. The Knoop hardness values (19.6 N applied load) prior to testing were 7.6 GPa for hardened silver steel and 3.8 GPa for stainless steel. Both types were thereafter carefully ground to a sharp apical angle of 120”. The tips were examined in profile under an optical microscope before each experiment in order to ensure integrity and sharpness, They were thoroughly cleaned ultrasonically in acetone immediately prior to the cyclic fatigue tests. 2.2. The repeated point contact loading method This method, which has been described previously [ll], is based on the room temperature unlub~~ted repeated point contact loading of a ceramic surface with a sharp 120” conical indenter. Figure 1 represents a schematic diagram of the computer-controlled apparatus. The zirconia specimen was clamped to a platform tied on a load cell which in turn was mounted on a rotatable x-y stage. The apparatus was then calibrated immediately prior to use by applying known
Adjustable
Horizontal
beam
Pivot Extension
Adjustobla
- Adjustable
extension-----_-n
Counte~balonce
Fig. 1. A schematic
illustration
height
of the cyclic load fatigue
apparatus.
spring extension
weights
M.-O. Guillou et al. / Repeated point contact loading on Ce-TZP
loads of 0.1-20 N to the load cell and recording the readings with a personal computer (486) using an Amplicon Liveline PC27 A/D card. The platform was split so that the ceramic substrate could be removed after a certain number of cycles for optical examination and replaced in exactly the same position relative to the indenter. The tests were performed in ambient air at 20f 1 “C. Sinusoidal cyclic loading was used with a frequency 2.0 Hz and minimum and maximum loads 9.8 N and 29.4 N respectively. The loading wave form was continuously displayed on the PC and the mean, maximum and minimum applied loads were recorded for each cycle; as well as the total number of cycles, time of test run and corresponding frequency. The metallic tips blunted to a flat conforming surface on first application of the mtium load. During each subsequent cycle, the tips were always in contact with the Ce-TZP substrate. Experiments were carried out to observe not only the damage promoted by an increasing number of cycles but also the flattening of the metallic tips. The test range varied between 1 x lo4 and 4.75 X 105 cycles. The surface damage behaviour was observed after each test by optical means. A new metallic tip was used for each test. Snoop hardness tests were then performed on both the ceramic substrate and the flattened tips. The fracture surfaces in the damaged contact zones were inspected by scanning electron microscopy, together with energy-dispersive X-ray analysis, following gold coating of the ceramic surface. Vickers hardness tests were also carried out in the centre of the deformed impressions in an attempt to evaluate the fracture toughness.
3. Results 3.1. Hardened silver steel cones
The dimensions of the flattened steel tips were measured optically soon after testing in order to obtain the mean contact pressure values P,,,. An average of five measurements was taken per tip. The results, shown in Fig. 2(a), indicate that, at least above 104 cycles, the maximum contact pressures are approximately constant at 3.87 GPa. The corresponding minimum contact pressures are 1.29 GPa. Approximate calculations show that the change in diameter of the contact zone due to elastic deformation is only about 1% of the diameter of the flattened tip. After the compressive cyclic tests had been performed, Knoop hardness tests were undertaken on both the deformed regions on the ceramic surface and on the flattened silver steel tips in order to detect whether work hardening had been produced in either substrate or cone material. The Knoop hardness of the deformed
v
249
Silver Steal lip
D
200
100
Number
300 of cycles
400 (X1000)
(0)
16 ‘;;
14
8 i
12
c E P
E 2
+ Flattened silver steel tip, L-0.25N
10
8
Y
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100
200
Number
300
of cycles
400
500
(X1000)
(b)
Fig. 2. (a) Maximum contact pressure versus number of cycles. (b) Variations of the Knoop hardness values with increasing number of cycles for both the flattened cones and the deformed Ce-TZP substrate - frequency: 2 Hz - applied cyclic loading: 19.6 f 9.8 N.
substrate was found to increase slightly with increasing number of cycles (see Fig. 2(b)). As shown below, fragmentation and inward displacement of the substrate (after 4.75 x 105 cycles) prevented measurement of its hardness. The Knoop hardness of the flattened metallic tip was observed to reflect this behaviour even more noticeably (see Fig. 2(b)). The amount of damage observed around the edges of the contact zone increased as the number of cycles increased (see sequence of micrographs in Figs. 3(a)-3(g)). Using the scanning electron microscope the fracture path was observed to be intergranular (Fig. 4). The fracture origins associated with spalling were always at the periphery of the contact zones (Figs. 4(a)+d)). A distortion of the Ce-TZP grains was observed prior to fracture. This grain h@ng is clearly visible around the edges of impressions performed at 1 X lo4 cycles and up to 5.7 X lo4 cycles (Fig. 4(a) and
250
M.-O. Guillou et al. I Repeated point contact loading on Ce-TZP
(@I 111 Fig. 3. The effect of number of cycles on the damage caused on Ce-TZP by hardened silver steel indenters - applied cyclic load: 19.61: 9.8 N: (a) 1 x lo4 cycles; (b) 3.77X lo4 cycles; (c) 5.7~ lo4 cycles; (d) 12.75 X 104 cycles; (e) 1.50X l@ cycles; (f) 2.00X l@ cycles; (g) 4.75 X ld qcles.
Fig. 4(b) respectively). This phenomenon was accompanied by intergranular fracture, which became more significant with increasing number of cycles, until considerable spalling occurred (Figs. 4(c)-4(d)). The contact zone was also generally uniformly depressed below the original surface of the ceramic substrate (compare with the scanning electron microscope line-scan in Fig. 5). The depth of the impression increased with increasing number of cycles. In Fig. 5, in the region just outside the contact zone, a surface rumpling which occurred as a result of plastic defo~ation can be seen. Estimated values of the sizes of the plastic zones and impressions with increasing number of cycles are shown in Fig. 6. The values of the latter must be treated with some caution owing to measurement inaccuracies arising from the combination of spalling and the impression sinking into the substrate. The size of the impression made onto the ceramic substrate appeared to decrease with increasing number of cycles (see Fig. 6). In addition, metal adhesion was visible at the edges of the contact zones for tests above l@ cycles (see arrow in Fig. 4(c)). Hence, friction
between the ceramic substrate and the metallic tip would presumably increase as the number of cycles is increased. Although considerable damage of the substrate was observed, in particular around indentations corresponding to 4.75 x 10s cycles (Fig. 3(g)), no trace of zirconia or ceria was found on the flattened hardened steel tip. An attempt to evaluate the fracture toughness in the region of the deformed impressions made by hardened silver Steel tip onto Ce-TZP was made. It was found that the maximum size of an impression that could be fitted into the deformed zone was obtained using a load of 19.6 N. The resulting Vickers indentation fitted in the centre of the region but no cracks emanated from the comers of the impression (Fig. 7(a) and 7(b)}. It was therefore not possible to evaluate an indentation fracture toughness value. 3.2. Stainless steel cones A comparative test was performed using a softer stainless steel cone with the same load range, i.e. 19.6rfi9.8 N and 12.75 X 104 cycles. In this case the
M-O.
251
Guillou et al. / Repeated point contact loading on Ce-TZP
Fig. 5. Line-scan micrograph (from the scanning electron microscope.) showing both the deformed impression sunk into the substrate and the plastic deformation zone surrounding this impression made onto Ce-TZP after 12.75 x 10” cycles using a 120” hardened silver steel tip: frequency, 2 Hz; applied cyclic load, 19.6& 9.8 N, sample tilt, ahout 90”; magnification, x 800 (note the presence of a Vickers indentation in the centre of the deformed region). 240
(C)
(dt
Fig. 4. Details of the edge of contact zone indicating an intergranular fracture mode. Substrate, Ce-TZP, indenting cone, hardened silver steel; frequency, 2 Hz, applied cyclic load, 19.6f9.8 N. (a) 1 X 10’ cycles, (b) 5.7X l@ cycles, (c) 12.75 X lo4 cycles, (d) 4.75 x 16 cycles.
metal tip flattened to a larger diameter, which gave minimum and maximum contact pressures of 0.74 GPa and 2.11 GPa respectively. No cracking or substrate damage was observed. Nevertheless metal was transferred onto the ceramic surface (see Fig. 8). This metal deposit was found to pile-up around the edges of the contact zones. Energy-dispersive X-ray analyses were performed whilst the specimen was observed under the scanning electron microscope. Traces of iron were indeed found, primarily at the edges of the deformed impressions. There was no trace of martensitic transformation in the Ce-TZP in the vicinity of the contact zones in this case.
4. Discussion Since in this series of tests, there is negligible relative displacement of the deformed conical tip, this is a situation where fretting fatigue is unlikely to occur.
I
’
I
8
I
8
200 ? j
160
‘j % E if
120
c
I
/”
-
80
40
0
100
200
300
400
500
Number of cycles (x1000)
Fig. 6. Tbe effect of number of cycles on the dimensions of the impressions and plastic deformation zones produced on Ce.-TZP (UCll) using 120” hardened silver steel cones: applied cyclic load, 19.6 f 9.8 N.
None the less, deformation and fracture due to the cyclic loading can clearly be seen to occur (Fig. 3). It would appear that there is a critical contact pressure required to induce these effects, since they are apparent after only 10“ cycles when the contact pressures are 1.29 to 3.87 GPa (silver steel cones), whereas when they are in the range 0.74 to 2.11 GPa (stainless steel
M.-O. Guillou et al. f Repeated point contact buding on Ce-?ZP
fbf (ai Fig. 7. (a) Scanning electron micrograph showing a 2 kgfdiamond Vickers indentation within the deformed region of an impression made by a 120” hardened silver steel tip onto Ce-TZP after 12.75 X l@ cycles using a frequency of 2 Hz at room temperature - applied cyclic load: 19.61 f 9.8 N. (b) Details of one of the Vickers indentation comers indicating no cracking emanating from it.
Fig. 8. Impression made onto Ce-TZP by a 120” stainless steel cone after 12.75 x 10’ cycles using a frequency of 2 Hz at room temperature. Applied cyclic load: 19.6k9.8 N.
cones), no deformation or fracture is observable after 12.75 x 104 cycles. This process of fatigue damage can be modelled by considering the sequence of events observed whilst cycling with hardened silver steel cones between loads of 9.8 and 29.4 N, i.e. (i) The formation of a tetragonal + monoclinic martensitic transformation zone. (ii) The lateral expansion of the extent of this transformed zone. (iii) Surface steps formed by individual grains lifting-up as a result of the transformation. (iv) Inter-
granular fracture at these surface steps at the periphery of the contact zone. (v) Extension of the fractured p&s around the edges of the contact zone. (vi) Slight metal adhesion at the highest number of cycles.These processes will now be considered in more detail. As outlined in the introduction, the tetragonal +monoclinic martensitic transformation of the zirconia grains involves a volume expansion of 3%-S%. Therefore, it is somewhat surprising to observe such extensive transformation under the compressive loading in the present situation. However, it has been recently shown that loading tetragonal zirconia ceramics in compression also causes permanent ~ansfo~ation f12]. Nevertheless, Heuer et al. f12] stated that the stress required for the tetragonal to the monoclinic transformation to occur in compression is considerably higher than in tension. With increasing number of cycles, the plastic zone forms and spreads outwards from the point contact loading. It is thought that this lateral expansion of the plastic deformation zone in the vicinity of the deformed impression probably occurs as a result of the small plastic strain increments caused by each of the successive cycles. When su~~ient plastic strain has accumulated in grains adjacent to these which have already transformed, this then will initiate the transformation process in these grains. During a standard diamond Vickers test, accoustic emission studies have indicated that the plastic zone was expanding towards the surface during unloading of the indenter [13]. This would suggest that a similar mechanism may occur in the present situation whereby the expansion of the transformation zone occurs during the unloading part of the cycle. At the periphery of the contact zone, the steps formed by grain aping are appreciably higher than elsewhere. The stress components at the surface are generally compressive except for the radial stresses just outside the actual contact zone, which are tensile [14]. The maximum value of this tensile stress occurs at the edge of the contact zone and is given by (1 -2v)p0/2, where p,, is the uniform contact pressure at maximum load, about 3.8 GPa (see Fig. 2(a)), and u is Poisson’s ratio (assumed to be 0.27). This gives a value of about 870 MPa for the maximum radial tensile stress with the present system. There may be surface cracking beneath these uplifted grains, but no definitive observations have yet been possible. It is thought that this tensile stress is responsible also for the cracks which were observed to form and propagate in a circular manner at the edge of the contact zone. It seems likely that the intergranular surface fracture is triggered by the distortion of the grains, with possibly some contribution of simultaneous grain loosening. Once a fracture site has formed, cracks then propagate from this around the edge of the contact zone. The amount of intergranular fragmentation and
M.-O. Guillou et al. / Repeated point contact loading on Ce-TZP
spalling of the transformed grains increases with increasing number of cycles. Cardona et al. [15] have also reported a predominantly intergranular fracture in fatigue tests using bending of notched beams of Ceo,-PSZ . A similar sequence of plastic zone formation and expansion, followed by plasticity-induced microcracking at the edge of the contact zone, has been observed in tests with stainless steel cones on MgO [ll] Adhesive transfer of metal to Ce-TZP has previously been shown to occur [16], and its occurrence in the present tests is quite feasible. However, it is only found after considerable spalling has occurred, and the presence of the induced surface roughness and cracks may assist the adherence and transfer of the metal.
5. Conclusions The surface of the fatigued Ce-TZP specimen was examined after testing by both optical and electron microscopic means. Evidence of surface damage in the form of accumulation of both a plastic deformation zone expanding outwards from the point loading contact area and intergranular fracture followed by spalling was recorded. This article demonstrates that repeated metallic conical indentation is a convenient and useful method for evaluating the susceptibility of a ceramic surface to plastic deformation prior to fragmentation under cyclic point loading. This technique also demonstrates that a softer material (metal) may cause much greater wear of a harder material (ceramic), which is of considerable relevance in tribological applications.
Acknowledgments This research project has been supported by a grant from the UKScience and Engineering Research Council. The authors would like to thank Mr. J. Cooper for his programming skills and Mr. K.N. Smith for his assistance with the construction of the experimental apparatus.
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References
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