urna Am, cern,s,%61547-53(303 Influence of Residual Stresses on the Wear Behavior of Alumina/Alumina-Zirconia Laminated Composites Francesco Toschi. Cesare Melandri, Paola Pinasco. Edoardo Roncari, Stefano Guicciardi, and Goffredo de Portu National Research Council (CNR). Institute of Science and Technology for Ceramics (ISTEC). 48018 Faenza(RA). Italy Symmetric structures of laminated ceramic composites were uch cases, the ineffectiveness of increased surface toughness in AlyOyLrOz composite prepared by tape casting. The compos- exaggerated phase transformation in zirconia-based materials and side. This configuration caused residual compressive stresses high contact temperatures reached by the material pairs under very to be induced on the surface due to the different thermal severe operating conditions. The aim of this study is to determine whether laminated pansion coefficients of the various layers, leading to an ceramics with improved apparent surface toughness, conferred by residual stress was determined using the indentation tech nique. The tribological behavior of these laminated structures metrical lamination of Al, O, and AL,O /ZrO, layers, can have a was evaluated using the pin-on-disk method for different loads within a defined range of applied loads and sliding speed terial higher wear resistance than the corresponding stress-free ma and sliding speeds. Comparison with the results obtained from stress-free alumina showed that, within the range of these experimental conditions, the improvement in surface tough ness leads to a reduced friction coefficient and increased wear Il. Experimental Procedure resistance of the composites. Possible wear mechanisms are (1) Preparation of Materials The selected materials were Al, O, and an Al,O /ZrO, compos te(60/40 vol%), which have different shrinkages during sintering 1. Introduction and different thermal expansion coefficients (a 9.0x 10 100×10°℃fom25°to1400c. respectively). These differ- D UE to their high hardness, low specific weight, chemical ences are sufficient to induce residual stresses in the laminated ceramics are widely considered to be excellent candidates for cracking under the experimental conditions described below wear-resistant materials, However, their inherent brittleness causes This study entailed the use of a high-purity (99.7%) alumina ne concern. Because the performances of wear-resistant mmy powder(Alcoa Al6-SG. Alcoa Aluminum Co. New York. NY) rials are mainly related to the properties of thin surface layers. 1.2 with an average particle size of 0.3 um, and a zirconia powde an improvement in surface toughness appears to be an interesting (TZ3Y-S, Tosho Corp, Tokyo, Japan) doped with 3 mol%Y,O way to improve the tribological behavior of ceramics. In fact, since (usually referred to as 3Y-TZP)with an average particle size of 0.3 removal of material in engineering ceramics under sliding condi tions is generally caused by the propagation of surface cracks On the basis of previous experience es. 4.15 the different powders resulting from tensile stresses in the wake of rubbing contact, an were mixed with organic binders, dispersant, plasticizers, and ncrease in apparent surface toughness should lead to an improve solvents to obtain suitable slips for tape casting. Slurry composi ment in wear resistance. It has been observed that when toughness tions were the same for both Al-O, and Al-O / ZrO, composite increases, wear resistance increases by approximately 3 orders of powders. magnitude, measured in air at room temperature at low load and Sheets of pure alumina(hereinafter designated"A")and of the very slow sliding speed. To achieve this goal. efforts have beer mposite alumina/zirconia with a volume ratio of 60/40(herein made to modify the surface properties using zirconia phase after designated"AZ")were uced. A description of the transformation. Another method of improving the surface tough processing procedures can be found elsewhere. Samples with a ness of a material while avoiding the problems related to phase hybrid laminated structure of alumina and alumina/zirconia com transformation" is to produce laminated structures designed to posite (hereinafter referred to as"A/AZ, with seven A and four induce compressive residual stresses at the surface by combining AZ layers)and laminated structures containing I I layers of pure the different thermophysical characteristics (i.e, thermal ex alumina(hereinafter referred to as"AA")were prepared by on and shrinkage on sintering) of the different materials ed,It has been shown,. that although these techniques can stacking eets at 75 C at a pressure of 30 MPa for 30 min increase resistance to static contact damage (i.e. apparent surface The latter batch of samples was prepared to obtain a material toughness), they do not necessarily improve wear resistance. In with the same surface porosity as the A/AZ material, but with zero (or very low) residual stresses In the hybrid samples, the structure was designed to leave the layers of alumina (A)on the two surfaces( Fig. 1)to stimulate compressive surface stresses. Due to butine editor lower thermal expansion coefficient and shrinkage during sinter. ing, the external alumina layers underwent residual compressive To obtain a perfectly symmetrical structure, two A layers were Manuscript No. 187121. Received February 25, 2002: approved April 3. 2003 mber, American Ceramie Society used on each side. This allowed one layer to be removed from each ide
Journal of the American Ceramic Sociery-Toschi et al. Vol, 86, No. 9 A AZ (MA) after sintering after machining Fig. 1. Diagram of sample preparation sintering. Because heating and cooling rates are crucial parameters face material. The pins were cylinders with a diameter of 5 mm determining residual stresses in the structure, the sintering and spherical caps with a surface roughness R=0. 13 um. To cycle was carefully controlled, Details on sintering procedures avoid the effects of the contact temperature (i.e, relaxation of have been reported already. Final dense samples were -2 mm residual stresses and deterioration of the mechanical properties) thick, with layers of -180 um. To obtain a full, dense, stress-free previously observed. 3.25-27 loads and sliding speeds were chosen material for use as a reference, pure monolithic alumina(MA) was in accordance with the values obtained from an analysis of the prepared by cold isostatic pressing (pressure, 150 MPa)and Ashby map, with the aim of keeping the contact temperature sintered at 1600 C for I h. The density of the various samples was measured using the Archimedes method. Theoretical density of the calculated in the worst testing conditions. This was done by ing the theoretical density of the different layers and their number. foreseen tests' in the mathematical model. The additional data The porosity value was then deduced from the relative density required for this calculation were taken from direct measurements values. The grain sizes of the different alumina surfaces were on the samples and from the literature, according to the database standard prEN 623-3 For mechanical and wear tests, the shown in Fig. 2 with the points identifying the experiment surfaces of the specimens were machined to a roughness Ra 0.16±0.03 conditions used in the study, Notwithstanding some approxima (2) Mechanical Properties he experimental parameters selected, the contact temperatures involved in the tests should not exceed 100%C Vickers hardness was measured using a load of 9.81 N, Three different loads(P= 50, 100, and 150 N) and three ccording to European standard prEN 843-4. The theoretical value of 410 GPa o was taken as Youngs modulus of dense selected while the sliding distance was kept constant (15 km). The alumina. For the laminates it was calculated from this value using the polynomial equation proposed by De Portu et al., taking the temperature(22 C)and humidity(70%)of the laboratory atmo- porosity (-3%)present in these structures into account. The sphere were also kept constant. fracture toughness (K,) of the stress-free material (MA)was Before testing, the samples and pins were cleaned in an measured on bars, which were cut and machined from monolithic ultrasonic bath with acetone for 15 min and then dried at 80C for 30 min. After I h of natural cooling, the samples and pins were slice model. The apparent surface toughness of the three differ. weighed using a balance with an accuracy of 10g. After testing of indentation techniques. 23 the samples were cleaned using the same procedure described sing the same loads used to calculate residual stresses. Residual above and weighed again. The specific wear was calculated as stresses at the surface were calculated using the procedure de- weight loss divided by sliding distance and applied load. The dept scribed by Widjaja et al., by measuring the length of the cracks of the wear tracks was measured using a profilometer (Talysurf generated at the corners of the indentation impressions as a Plus, Taylor Hobson, Leicester, U. K ) The torque value Mg, used function of applied loads( 29.4, 49. 1, 98. 1, 147.2, and 196.2 N)on to calculate the friction coefficient(), was continuously moni both the stressed and stress-free material. The maximum load was tored during the tests. hosen, considering that the maximum crack length had to be less Disk surface features before and after the wear tests were han the layer thickness to ensure that all cracks were located in the nalyzed using a scanning electron microscope(SEM). extermal layer. Details on this technique can be found in the literature. (3) Wear Tests Sliding wear tests were conducted on disks using g an inverted pin-on-disk apparatus in unlubricated conditions. Pins of pure dense alumina, obtained from MA samples, were used as counter from Ref. 29
September 2003 Wear Behavior of Al/Al-Zr Laminated Composites 1549 Log(Velocity, m/s) 1.00 Ball on Flat 5.22 lumina on alumina Ho=16.616.6GPa Ra=6.00 To=25°C a=13.613.6mm2/s uo=0.90c1=250.0 L1b= 10.0 L2b= arctan x 四乏u T3 T4 T5 T6 50050505 T8 5.00 H-0.77 Log(Normalized Velocity) 5.00 Fig. 2. Ashby map with the points relative to the test conditions. Ball-on-flat has been selected because pin-on-disk tests using pins with a spherical end can be approximated to that configuration. The symbol O refers to material A/AZ while symbol to AA and symbol V to MA. Il. Results and Discussion Table 1. Mean Values of Grain Size for the Three Different Materials Produced (1) Microstructural Properties For A/AZ hybrid laminates, the mean density was 4.16= 0.0 Mean grain Material 'cm and for AA laminates it was 3. 88+ 0.02 g/cm(96.8% and size (um) 97.3% of the theoretical density, respectively ) For the material MA, the density almost reached theoretical level. Due to the high amount of organic substances used in tape casting, a small amount Alumina in a/AZ of porosity in sintered bodies obtained by this process4..2 is to Alumina laver A be expected. This porosity was almost equally distributed through Composite layer AZ. out the layers. Zirconia (in AZ layer) in hybrid laminate A/AZ 0.35 Figure 3 shows a micrograph of the cross section of the A/AZ aminated structure. Table I shows the mean grain size values measured in the different materials It is evident from Table I that the highest temperature used sInter nolithic alumina results in the formation of slight In the hybrid laminates, the growth of alumina grains in the AZ layers is hindered by the zirconia, as expected. (2) Mechanical Properties and Residual Stresses Calculation Table II shows Vickers hardness values, calculated Y oung s moduli, and fracture toughnesses of the different materials 因射 Table Il. Measured Hardness, Calculated Youngs Modulus, and Surface Toughness for the Different Materials N额的 Material HV (GP: E(GPa K,(MPam)' A/AZ 17.6±0.8 5.37±0.502 164±0.6 3.04±0.2 16.6±0.5 410 3.35±0.43 400 (361 Fig- 3, Optical microscope photograph of a cross section of the hybrid laminate A/AZ. The darker layers are alumina, while the lighter ones are niques for aw an moina with similar microstructure. 31Refemed to alumina layers already re the composite alumina-zirconia
1550 Journal of the American Ceramic Society-Toschi et al Vol 86. No, 9 Indentation tests were performed on the polished surface of the Figure 5 shows the friction coefficient values measured in the laminated structures to measure residual stresses. When suffi stationary state on the various materials for the different experi- ciently high loads are used, the applied model relates the fracture mental conditions toughness to the indentation load and the lengths of the relative The hybrid laminated composite A/AZ was found to have the cracks emanating from the corners of the impressions through the lowest values of H under all experimental conditions, except for following equation: the test conducted at 150 N and 0.15 m/s, for which the value was closer to those of the other two materials. Conversely, the P monolithic alumina generally exhibits the highest values. For KI A/Az and MA materials, the friction coefficient could not be determined when testing at 150N and 0.05 m/s because the pins where K,s=toughness of the stress-free material, x-dimension- less constant(experimentally determined). P= indentation load, Additional relevant information acquired from the data are that and co crack length the friction coefficient of A/AZ is almost constant regardless of the It is possible to evaluate the parameter x by means of the best applied loads. except for P= 150 N and v=0. 15 m/s, where the fit of a regression of the experimental data of P and co, measured wear mechanism differed from that observed under other experi on the MA material and using the value of 3. 61 MPa'mfor K nental conditions. For laminated structures at a load of 100 N. the as measured by the Chevron notch technique. In this case we friction coefficients of the different materials are almost indepen- obtained x=0.089. This is close to the value of 0.080, which can dent of the sliding speed. Under a load of 150 N the friction be obtained from the equation x=S(E/H), as proposed in Ref. coefficients are highest at a sliding speed of 0. 15 m/s. All tested 23. considering $=0.016(suggested by the same authors)to be materials exhibited this behavior. The increase of H results in an he"calibration constant ncrease of the maximum principal tensile stress (oI max)at the When residual stress is present, Eq (1)becomes ontact surface, which is responsible for crack initiation (i.e, to he reduction in critical normal load needed for cracking) and Kr=xcv+ Yom vt (2) two sliding surfaces is determined by several factors: the height of crack length in the stressed material, r= 1.29 the asperities, their hardness, and adhesion of the contacting surfaces. It is easier for the asperities to deform plastically when geometrical factor, and Ores residual stress, whereas the other hardness is lower. Thus, the asperities'height and their effect on symbols have the same meaning as above frictin meanwhile, this Using the data for Kie(measured using the CNB method)and x the contact area between the two counter surfaces of the sliding (obtained previously), regression of the experimental data of P and c through Eq (2)allows us to calculate res, This analysis yielded materials to increase. Consequently, the fact that the material with value of-141 MPa(compres for g. Figure 4 shows the the higher hardness exhibits a lower friction coefficient suggests regression of vickers indentation data for the stress-free material hat the adhesion mechanism is dominant. When the wear is severe MA and hybrid laminate A/AZ. (see the next section). the third body produced during service plays In the laminated composite the load of 147. 15 N was the a role. The data show that when a severe wear regime is present the friction coefficient tends to increase with the severity of the higher loads was longer than the thickness of the external alumina test. This means that the increased amount of debris formed during layer. The model described above assumes half-penny-shape cracks. This is the case of the cracks observed in the section of a large contact surface for the two materials, thus leading to an increase in adhesive friction. As the wear of hybrid laminates uch laminates after indentation and fracture tests results in a lower amount of debris than other materials, the direct consequence Is a lower friction (3) Friction and Wear (B) Wear: Figure 6 shows the specific wear of the various (A) Friction: Observation of friction coefficient patterns ob- rained during te revealed that, in most cases, u started at a data are shown in a semilog plot that reduces differences if high value and reached a stationary condition after a running-in mpared with those expressed as absolute values, it is evident that period of variable duration. When a stationary value was not the wear of hybrid-laminated composite A/AZ is always lower reached (on two occasions for the MA material), the tendency was than the values measured on the other materials, except when the for the friction coefficient to decrease load is 50N and the sliding speed 10 m/s In this case the values for all the studied materials were almost the same. In the more severe wear conditions the depth of the groove produced by the pin was-170 um. that is, the exact length of the longer crack (170 um)induced on the surface by the indentation of a load of 147. 15 250 N used to calculate the residual stress. This means that the surface damage was limited to the external layer, which was -180 um thick and that the calculated compressive residual stress was the effective additional stress operating during the wear test. 141 MPa Wer tom ontical microscope and SEM observations, four main ir mechanisms were identified out of all the samples tested under the various experimental conditions Three of these mechanisms(microcracking, macrocracking, and 口AAz abrasion from the third body) appear at loads of 50 and 100N lor all three sliding speeds (0.05, 0.10, and 15 m/s). A fourth mechanism which induces considerable plastic deformation of the 50100150200250300350 material along the entire wear track, seems to predominate at a Crack length, um load of 150 N and sliding speed of 0. 15 m/s and can be synergetic with the others. and usit mM上个7四可m四m the value of x and the surface residual stress present in the A/AZ layer (s debris. its comminution, and subsequent compaction were caused
Septem Wear Behavior of A//Al-Zr Laminated Composites 1.0 v=005m/s 0.8 0.6 04 008c V=0.10m/s 0.8 0.6 v=0.15ms 0.8 0.6 AAZ 0.4 40 60 80 100 120 140 160 Applied load, N Fig. 5. Mean values of the friction coefficients measured on the various materials for the different experimental conditions. oy cracking of the grains at the early stage of the process. This equal to the local fracture toughness K e The presence of com- ascertainment suggests that microcracking is the main wear mech ressive residual stresses increases the rent surface fracture anism active under the described experimental conditions. When toughness and, since it opposes the tensile stress generated in the the wear mechanism is associated with the formation of surface wake of the sliding contact, it prevents the formation and propa cracks, the laminated composite A/AZ always exhibits(except gation of cracks. under a load of 50 N and sliding speed of 0. 10 m/s)a lower Some cracks were also observed (Fig. 7(a)) in the wear track of pecific wear than the other materials (AA and MA). For a material he A/AZ composite tested at 100 N and 0.05 m/s. However, the with a given flaw size. the critical condition for the onset of severity of the damage caused by this phenomenon was definitely cracking at the surface occurs when the stress intensity factor Kr lower than that observed in the other materials tested under the due to the maximum principal tensile stress (o, max) Is greater or same experimental conditions (Fig. 7(b)) v=005m/s zE0=930 -5 V=0.10m/s ▲…AA 县二 -6 V=0.15ms -5 mild 100 120 Applied load, N Fig. 6. Semilog plot of the disk specific wear of the various materials as a function of the different experimental conditions
1552 Journal of the American Ceramic Sociery-Toschi et al Vol 86. No 9 a ransgrany Intergranular. p “5h Fig 9. Example of the fracture surface observed in the wear tracks of the A/AZ composite. Both intergranular and transgranular fractures are visible This behavior has been observed on all the materials studied (A/AZ). As a result there were fewer fractured grains available for erosive wear of the hybrid-laminated composite, resulting in a lower total specific wear of this material, even under these conditions. Figure 8 shows a typical plastically deformed surface of MA material tested at 150N and 0. 15 m/s. When the fracture occurred, it was both intergranular and transgranular, regardless of the materials(Fig. 9). It is also interesting to note that the wear of the alumina pin on the A/AZ composite was generally lower than that observed for the other pairings(particularly when compared with the MA pairing) In addition to high wear resistance, another important charac teristic useful when selecting materials for tribological use is their Fig. 7. Surface cracking showed by different materials for the same tion. This capability can be associated with the extreme conditions According to Adachi et al., the possibility of this transition occurring is lower when the tested material exhibits high tough 3 Under more severe experimental conditions (150 N and 0. 15 ness. In fact. the superior behavior of the laminated composite s),a pronounced plastic deformation and increase in the A/AZ can also be shown by considering the different wear regimes production of debris were observed. This debris, which acts as ar exhibited by the three materials studied under the various operat abrasive third body, stimulates an increase in wear rate. Under ing conditions. Although a sharp transition with a drastic change in such conditions, the effect of compressive stresses was less wear rate was not observed under the experimental conditions that pronounced and the specific wear increased significantly for all were used, according to the previously proposed limit, the three materials being tested. However, as the cracking of the transition from mild wear to severe wear occurs when the specifi surface(and consequent removal of grains) initially triggered this wear exceeds the values of 10mm(ml hich correspon avalanche-type mechanism. debris production was hindered from to about 4 x 10 g(km.N) in our case. This transition limit is the outset by high toughness in the case of stressed material evidenced as a solid line in Fig. 6. y analyzing the results, we can see (Fig. 6) that the laminate A/AZ still exhibits mild wear even when the applied load reaches 100 N (except with a sliding speed of 0. 15 m/s), while the transition from mild wear to severe wear occurs when the applied load exceeds 50 N in the case of the other two materials from opposites in the system Al,O /ZrO, were prepared by warm pressing and sintering. Using layers of pure alumina and alumina/ zirconia composites, symmetric hybrid laminates were produced Due to the different thermal expansion coefficients and shrinkage ible to stimulate residual stress on the surface by ordering the layers of different materials properly, Using indentation techniques, the value of the 50 surface residual stress was estimated at-141MPa The surface properties(physical, mechanical, and tribological) of the A/AZ laminated composite showed the superiority of the hybrid structure when compared with those of analogous but stress-free material Fig. 8. Plastically deformed debris spread over the surfaces of wear The hardness and toughness of the monophase laminate AA and tracks in the MA material tested at 150 N and 0.15 m/s monolithic MA are almost the same as that of a typical commercial
September 2003 Wear Behavior of Al/Al-Zr Laminated Composites 1553 high-performance alumina. Due to the presence of residual com- 中P.ZCa.D.J.Gren cal Characterization ol pressive tresses, the laminated composite A/AZ exhibits both AOJ∠ rO. Hybrid Lami 534(1998 higher hardness and apparent toughness. From these results it is I. She, S. Scheppokat, R. Janssen, and N Reaction Bonded Three. yer Alumina-Based Composites with Improved Damage Resistance, "J,An evident that the presence of residual compressive stresses leads to Ceram Sxc, 81 (511374-76(1998 an improvement in resistance to damage induced by static contact. H. Tomaszewski, J. Strzeszewski, and w. Gebicki,"The Role of Residual The friction coefficients for the pairings of alumina pins with in Layered Composites of Y- ro, and Al.O, "/. Eur. Ceran. Seve.19 laminated composite A/AZ were always lower than those for materials MA and AA A. Tarlazti, E. Roncari P. Pinasco. S. Guicciardi, C. Melandri. and G, de Portu, Tribological Behaviour of Al O,/ ZrO, Laminated Composites, "Wenr, 244 The data for specific wear show how the presence of compres ive stresses, and therefore an improvement in the apparent surface C Fiori and Gi, de Portu. "Tape Casting: A Technique for Preparing and Studying tance of the alumina when the applied loads and sliding speeds &. ober 19%p. 213-25 in Proceedings of the British Ceramic Society. No38 toughness, plays an important role in improving the wear resis- R. w. Davidge. Shelton, Stoke-on-Trent. U.K. 1986 G. ce Portu. C, Fion, and O. Sbaizero,"Fabrication. Microstructure, an macrocracking (under the experimental conditions used in the study, i. e, low contact temperature). Even under the severest Ceramies, VoL. 24. Science and Technology ef Zinnia Il/ Edited by SSomia Yamam o and H. Yanagida. American Ceramic Society. Columbus OH, 1988. and macrocracking. followed by limited removal of material, and Defects, "/.Am. Ceram, Sox 8018)1929-39(97 Berane Densi ication of rating conditions. the wear behavior of the laminated compos- ite A/AZ was still better than that of the other materials, Micro- Alumina/Zirconia Hybrid in, and G.L.Messing."Const imited abrasion by a third body were identified as wear Alumin al zircon ia Hybrid Laminates: l. viscoelastic stress Computation 1. a. consistent removal of a plastically deformed surface layer and European Standand No, prEN 623-3 (rev. 20X)), Advanced Technical Ceram consequently, a more consistent abrasion mechanism trigger a ics-Monolithic Ceramies--General and Textural Properties, Part 3: Determination transition to severe wear. The production of laminated strue of Grain Size and Size Distribution(Characterized by the Linear Intercept Method) tures with compressive residual stresses at the surface thus European Standard No, prEN 84.34 (rev. 2001). Advanced Technical Ceram cs-Monolithic Cenamics-Mechanical Properties at Room Temperature. 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