Y-TZP sliding reciprocatively against various aluminumoxynitride (AlON) ceramicsCitation for published version (APA):van den Berg, P. H. J., Willems, H. X., & With, de, G. (1993). Y-TZP sliding reciprocatively against variousaluminum oxynitride (AlON) ceramics. Journal of Materials Science, 28(19), 5369-5374.https://doi.org/10.1007/BF00570092

DOI:10.1007/BF00570092

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JOURNAL OF MATERIALS SCIENCE 28 (1993) 5369-5374

Y-TZP sliding reciprocatively against various AION ceramics

P. H. J. VAN DEN BERG, H. X. W l L L E M S Centre for Technical Ceramics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

G. DE W I T H * Philips Research Laboratories, PO Box 80000, 5600 JA Eindhoven, The Netherlands.

The friction and wear characteristics of various yttria tetragonal zirconia polycrystalline (Y- TZP)-aluminium oxynitride (AION) sliding systems were examined. These systems consisted of a Y-TZP sphere reciprocating against three different translucent AION plates. The fully dense AIONs used contained 67.5, 73.0 and 77.5 mol % AI203 and had a grain size of 28 to 56 lam. The tests were performed at a constant track length, two loads, two frequencies and two different time intervals at room temperature. The humidity was controlled by flushing with dry nitrogen. The total vertical displacement and the friction coefficient were measured continuously and sampled. The worn surfaces were observed microscopically. It was concluded that there is no difference in wear behaviour between the three AION types. It was also concluded that the amount of wear increases approximately linearly with load at a frequency of 1 Hz. The tests at 4 Hz show a stochastic transition between relatively mild wear and severe wear. The observations and data were used to derive a wear mechanism.

1. Introduction Ceramics are materials which are still under develop- ment. New types of ceramic are being made and existing ceramics are continuously being improved. The field of applications for ceramics is very wide, ranging from the conventional brick and porcelain cups to the most sophisticated superconducting layers. Research is being conducted to develop materials with very specific characteristics. Once these materials are made, they have to be studied in respect of their properties. Wear-resistant behaviour is one of the fields in which structural ceramics are expected to perform better than other materials. Such ceramics are, for instance, alumina, zirconia, silicon nitride, silicon carbide and sialon. The counterparts of these ceramics in a wear-resistant application can be other ceramics, metals, plastics or composites. Various ma- terial combinations must therefore be investigated to obtain some understanding of the wear behaviour of ceramics.

Zirconias are well-known materials fabricated in several varieties. One of these varieties is yttria tetra- gonal zirconia polycrystalline (Y-TZP). Some of the properties of the material have been described (e.g. [1, 2] ). The material has often been investigated as a wear-resistant material (e.g. [3-5]).

For this study, wear systems of a Y-TZP sphere sliding reciprocatively against three different A1ON plates were investigated. The three A1ONs, which were

processed with different amounts of A120 3, are a new kind of translucent ceramic [6 8]. Wear tests between Y-TZP and A1ON could provide interesting informa- tion about the wear behaviour Of comparable systems like Y-TZP-sialon, or more generally systems of TZP against harder and more brittle ceramics.

2. Experimental procedure The pin material used was in all tests a commercially available Y-TZP sphere (Dynamic Ceramic, Stoke- on-Trent) with a radius Of 2 mm and a polished surface. In Table I the properties of Y-TZP, measured on a plate of the same material from the same supplier, are given. These values are assumed to be comparable to the properties of the spheres. The A1ONs were fabricated at the Centre for Technical Ceramics. De- tails about the processing and characteristics of these A1ONs are given elsewhere [8]. A summary of some of their properties is given in Table I. In all cases the surfaces of the A1ONs were ground.

The wear tests were performed on a pin-on-plate tribometer (Central Technical Workshop, Eindhoven University of Technology). In this set-up, a spherical Y-TZP pin reciprocates with a sinusoidal velocity on the flat AION plate. The track length for the tests was chosen as 10 ram. The pin frequencies, f, of 1 and 4 Hz correspond to maximum velocities of 0.02 and 0.08 m s- 1, respectively. Normal loads, P, of 2 and 8 N

*Also affiliated to the Centre for Technical Ceramics, Eindhoven University of Technology

0022 2461 �9 1993 Chapman & Hall 5369

TABLE I Characteristics of the materials used (most data for AIONs taken from [8]).

Y-TZP AION 1 AION 2 AION 3

A120 3 content (%) - 67.5 77.5 73 9 (g cm-3) 5.86 3.68 3.65 3.67 E (GPa) 208 333 306 314 v 0.32 0.30 0.26 0.25 d (pro) a 1.15 33 56 28 %pb (MPa) b 966 376 408 413 KI~ (MPa ml/2) ~ 7.9 2.3 2.3 2.2 HVzN (GPa) 13.0 HV10 N (GPa) 17.7 16.1 17.7

"Mean linear intercept for A1ONs; for Y-TZP the average max- imum grain size is given. u Measured in a three-point bend test. Measured in a single edge-notch beam test.

were used. The du ra t ion was 24 h for most tests per formed at 4 Hz and 72 h for mos t tests per formed at 1 Hz. Some of the tests were per formed twice. The exper iments were per formed in a flow of dry n i t rogen at r oom temperature . This gives an app rox ima te ly cons tan t env i ronment of less than 1% relat ive humid- ity. The reproducib i l i ty of the testing me thod had been examined in an earl ier s tudy [9].

The to ta l vert ical d i sp lacement of the pin dur ing sl iding was measured by an extensometer (Sangamo DG1). Con t inuous measurement of the vert ical dis- p lacement is not r ecommended in the A S T M s t anda rd [-10] because of the effects of debris and film forma- tion. It does give, however, re levant in format ion abou t the wear system dur ing the test.

The fr ict ion force was measured by a force t rans- ducer. The d isp lacement and friction force were simul- taneous ly sampled under external t r iggering cont ro l with a compute r ized da t a acquis i t ion system. Fu r the r details abou t the da t a acquis i t ion are given else- where [9].

The worn surfaces were visually examined with opt ica l mic roscopy (OM) and scanning electron mic roscopy (SEM). The da ta and observat ions were used to derive a wear mechanism which also explains some phenomena of o ther wear systems.

3 . R e s u l t s The results are presented with examples which are character is t ic of the condi t ions under which the tests were performed. The o ther graphs for tests per formed under compa rab l e condi t ions were similar in shape and reasonably similar in respect of their quant i ta t ive aspects.

Fig. l a and b are examples of a vert ical displace- ment g raph and a friction graph, respectively, for a test per formed at 8 N and 1 Hz for 72 h. All samples showed relat ively mild wear, character is t ic for a pol- ishing mechanism.

The tests per formed at 4 Hz all showed a t rans i t ion of relat ively mild wear to severe wear with a corres- pond ing increase in friction coefficient after a few hours. As an example the graphs of the test at 8 N and 4 Hz are given in Fig. 2a and b, where the t rans i t ion is

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~Tgure 1 Characteristic examples of (a) a vertical displacement graph and (b) a friction coefficient graph of a test performed at 8 N, 1 Hz for 72 h on AION 2. The periodic behaviour is due to a 24 h cycle in temperature.

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Figure 3 Graphical representations of the discontinuities in (a) vertical displacement and (b) friction coefficient occurring during tests performed at 4Hz: (A) A1ON 1, P = 2 N; (O) AION 2, P = 2 N; (D) AION 3, P = 2N; (A) A1ON 1, P = 8 N; ( 0 ) AION 2, P = 8 N; (W) AION 3, P = 8 N. The lower points represents the transition while the higher points represent the end of the test.

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Figure 4 Summary of results from vertical displacement graphs after a sliding distance of(a) 1.73 km and (b) 5.18 km: (A) A1ON 1, P = 2 N; ((3) A1ON 2, P = 2 N; (r~) AION 3, P = 2 N; (A) A1ON 1, P = 8 N; ( 0 ) A1ON 2, P = 8 N; (W) A1ON 3, P = 8 N.

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Figure 5 Summary of results from friction coefficient graphs after a sliding distance of(a) 1.73 km and (b) 5.18 km: (A) AION 1, P = 2 N; (O) AtON 2, P = 2 N; ([2) AION 3, P = 2 N; (A) AION 1, P = 8 N; (0) A1ON 2, P = 8 N; ( I ) A1ON 3, P = 8 N.

indicated by arrows. For other conditions different transition times were observed,

This transition for each experiment can be represen- ted by a line in a displacement time plot as shown in Fig. 3a and b. The starting point of the line is the point at which the regime of severe wear begins, as indicated by the arrows in Fig. 2a and b. The end point repre- sents the end of the test. In two cases it represents the point of total failure of the sample. Although the point at which the transition starts shows some scatter, it can be seen that the average time before the transition occurs is larger for tests at a load of 2 N than for tests at a load of 8 N.

In Fig. 4a and b the results of the vertical displace- ment measurements are summarized in vertical displacement- load graphs after 1.73 and 5.18 kin, re- spectively. The clusters of points at 1 Hz illustrate the approximately linear dependence of vertical displace- ment on load and the independence of the vertical displacement on A1ON type. The highest points for the test performed on A I O N 1 at 1 Hz, and 2 N can be explained by a difference of 5 gm between the first two measurements of the vertical displacement. This could easily be caused, for example, by some foreign par- ticles located accidentally on the wear surface in the initial stage of the test. The values at 4 Hz show a fair amount of scatter, but this is caused by the afore- mentioned scatter in the transition.

The results of the friction measurements are sum- marized in Fig. 5a and b. The points plotted are the

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Fioure 6 Worn surface of A1ON 1 after a test at 8 N, 1 Hz for 72 h as observed by optical microscopy.

values after 1.73 and 5.18 km, respectively. The data for tests at 1 Hz are well within a range of 0.35 to 0.55 and independent of load and A1ON type. The scatter for the data at 4 Hz is again caused by the transition.

An example of a worn surface after a test under intermediate conditions, 8 N and 1 Hz, is shown in Fig. 6. This figure shows the mierostructure of the A1ON due to polishing. Some banding is also visible. In Fig. 7a and b this banding is examined in detail with SEM. These figures clearly show that the banding is formed by an alternation of bands with high and

Figure 7 Worn surface of A1ON 3 after a test at 8 N, 1 Hz for 72 h as observed with scanning electron microscopy: (a) overview, (b) detail.

Figure 8 (a) Worn surface of AION 2 plate after a test at 8 N, 4 Hz for 24 h as observed by optical microscopy. The light spots are due to debris adhering to the plate. (b) Worn surface of the Y-TZP sphere in the same test as observed by scanning electron microscopy. The dark spots are AtON particles adhering to the Y-TZP surface.

low concentrations of grains pulled out, or parts of grains which are pulled out. Fig. 8a and b present characteristic examples of the plate and the sphere, respectively, after a test at 4 Hz. The plate is covered with a layer of what appears to be debris without structure, and the sphere is covered with dark particles which were identified by X-ray energy-dispersive ana- lysis (EDX) as A1ON particles.

4. Discussion In order to illustrate some of the relevant character- istics, a number of observations on this wear system are discussed.

The first part of the wear test is determined by the process of running-in. A conforming contact between the sphere and the plate has to be established, mean- while reducing the local pressure.

The coherence of the grain boundaries and the strength of the material bonds of the A1ONs appear to be very low. This is consistent with the relatively low fracture energy of 8 J m - 2 as calculated from fracture toughness and Young's modulus. A ground surface is full of pits and holes and shows no grinding marks.

This means that the initial wear surface of the A1ONs can lose many A1ON grains or parts of grains of various shapes and sizes.

The hardness of the A1ONs is about 1.3 times the hardness of the Y-TZP. This means that the Y-TZP will not scratch or abrade the A1ON plate. The banding on the AIONs as shown in Fig. 7a and b thus has to be caused by A1ON particles localized at the sphere, either adhered or indented.

The phenomena observed and data obtained were used to develop a model. The experiments performed under intermediate conditions, 8 N and 1 Hz, provide information which explains most of the data. The worn surfaces after tests under these conditions show bands with a high concentration of grain pull-out. These bands are caused by A1ON particles which are positioned at the sphere. The areas in between these bands support the load. These areas are thus supposed to be the wear-determining parts. The main mech- anism of A1ON removal in these areas is polishing. The amount of wear is thus simply linear with load, which is consistent with the results as shown in Fig. 4a and b. Running-in effects appear to have a minor influence in this respect. The friction coefficients are about equal for tests at 1 Hz, 2 N and 1 Hz, 8 N.

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At a frequency of 4 Hz the whole process is greatly enhanced. More A1ON particles are accumulated on the sphere until a threshold is reached and the contact between sphere and plate is practically an A1ON-A1ON contact with regular grain pull-out in the plate and the continuous removal of Y-TZP grains together with A1ON particles from the sphere. A debris layer is formed containing Y-TZP grains and abrasive AION particles.

A comparison with earlier studies on Y-TZP-sialon systems under approximately the same conditions [9] shows that a number of features are found in both systems. The amount of wear at a frequency of 1 Hz is about equal, the banding is found in both systems and a transition to severe wear was also observed in the Y- TZP-sialon system. The wear resistance of the A1ONs at frequencies above 1 Hz is, however, much less than that of the sialons. The correspondence in observed phenomena does provide information which is prob- ably useful in gaining insight into other systems with TZP sliding against harder ceramics.

5. Conclusions 1. There is no difference in wear behaviour between

the three A1ON types. 2. Polishing is the main wear-determining mech-

anism at a frequency of 1 Hz. 3. At 8 N and 1 Hz~ A1ON grains or parts of A1ON

grains are pulled out in bands but this does not influence the vertical displacement which is controlled by polishing.

4. At 4 Hz a transition occurs, at 2 N on average later than at 8 N, to severe wear characterized by

many A1ON particles attached to the Y-TZP sphere, resuiting in mainly A1ON-A1ON contacts.

Acknowledgements This work was partly supported by the Commission for the Innovative Research Program on Technical Ceramics (IOP-TK) of the Ministry of Economic Af- fairs in the Netherlands (IOP-TK research grant 90A211).

References 1. T. MASAKI, J. Amer. Ceram. Soc. 69 (1986) 638. 2. J. L A N K F O R D , R. A. PAGE and L. RABENBERG,

J. Mater. Sci. 23 (1988) 4144. 3. G . W . STACHIOWIAK and G. B. STACHIOWIAK, Wear

132 (1989) 151. 4. I. BIRKBY, P. HARRISON and R. STEVENS, J. Eur. Cer-

am. Soc. 5 (1989) 37. 5. T . E . FISCHER, M. P. ANDERSON, S. JAHANMIR and

R. SALHER, Wear 124 (1988) 133. 6. J .W. McCAULEY and N. D. CORBIN, J. Amer. Ceram. Soc.

62 (1979) 476. 7. H.X. WILLEMS, M. M. R. M. HENDRIX, G. de WITH and

R. METSELAAR, Acc. J. Eur. Ceram. Soc. 10 (1992) 339. 8. ldem, presented at the 2nd ECSC, Augsburg, FRG, September

1991. (Proceedings in press.). 9. P. H. J. van den BERG, G. de WITH, L. DORTMANS,

E. K O K M E I J E R and G.-Z. CAO, J. Mater. Sci. 28 (1993) in press.

10. ASTM G-99, "Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus". (1990) 387.

Received 3 August 1992 and accepted 16 March 1993

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