Tribology International Vol. 28, No. 6, pp. 403-413, Elsevier
Science Ltd. Printed
1995 in Great Britain
temperature friction and r testing of silicon nitride C. Meandri”,
M. G. Gee+, G. de Pot-b’
and S. Guicciardi”
This paper describes the results of unlubricated sliding wear tests carried out on ceramics and composites based on silicon nitride. These were silicon nitride, silicon nitride with titanium carbide, and an electrically conductive silicon nitride with titanium nitride. The tests ‘Nere carried out from room temperature to 1200°C. For all the materials, there was a small increase in friction from room temperature to 3OO”C, followed by a small decrease as the temperature increased, until a sharp increase in friction coefficient at 1200°C. There was a maximum in wear as the temperature was varied for all materials, with almost no wear at room temperature and a!: 1200°C. The wear of the silicon nitride material was signif cantly higher than that of the composite materials at 900°C. Keywords: ceram its
wear testing, silicon nitride,
Introduction Ceram;cs are being used in an increasing number of applicaeions where good wear resistance is essential, and the advantages that accrue from the use of ceramic compo:.rents are not outweighed by the disadvantages that caa come from the use of parts made from strong yet brttle materials such as ceramics in complex engineering systems. In addidon to their high wear resistance, which comes at least in part from their high hardness, ceramics also have many potentially desirable high strength proper1 ies including high temperature wear resistance. A fairhi sizeable body of literature has now developed on the high temperature friction and wear testing of ceramicsiPiO. One of the materials which has perhaps the greatest potential for use in high temperature applications is silicon nitride. This paper describes the results of wear tests that were conducted in a high temperature uniaxial thrust washer test system on three hot-pressed silicon nitride based materials. These included two composite * CNR IRTEC, Italy. ’ NalionGl Physical Laboratory,
materials which contained 30% TIC and TiN, respectively. The main reason for the addition of TiC and TiN to the composite ceramics was to improve the toughness of the material by second phase strengthening, but it was also hoped that the addition of a proportion of titanium containing material to the composites might improve the friction and wear performance of the composites at high temperature by the formation of lubricious films of titaniall. The TiN composite material also had the additional property that it was electrically conductiver*.
Materials Details of the three materials tested and their mechanical properties are given in Table 1. The commercial silicon nitride and sintering aid powders were homogenized in water for 48 h and then freeze dried. The second phase component was then added to the mix and homogenized in isobutyl alcohol. Billets were prepared by hot pressing under vacuum under the conditions given in Table 1r2. The materials were characterized by standard techniques described elsewhere12.
High temperature Table 1 Details Code
and mechanical Procesing
3 A1203, 8 Y203, 30 TIC
3 Ai203, 8 Y,Oar 30 TiN
Hot pressed, 17OO”C, 30 MPa 30 min Hot pressed, 18OO”C, 30 MPa 30 min Hot pressed, 18OO”C, 30 MPa 10 min
C. Melandri properties Density, (Mgm3) (% Th.d.)
Grain size, (pm)
0.6 pm Si3N4 2 pm TiC 0.6 pm S&N4 2pm TiN
While the baseline silicon nitride material was theoretically dense, the composite materials were slightly below theoretical density. The nominal grain size was 0.6 pm for the silicon nitride phase in all materials, and 2 pm for the titanium based second phases (Fig 1). The composites had a lower strength and lower hardness, but significantly improved fracture toughness and increased Young’s modulus in comparison with the baseline silicon nitride. Test system
The test system uses a thrust washer configuration with the test rings oriented horizontally, using rings with the flats of the rings in contact. Details of the test system are given elsewhere13,14. The overall wear was measured continuously throughout the test with a capacitance probe which monitored the movement of the end of the central rod loading the upper stationary specimen. The rotation of the upper stationary specimen was prevented by a small load cell at the base of the central rod, and the friction coefficient was calculated from the force measured by the load cell. The rings had a 19.5 mm outer diameter X 17.5 mm inner diameter. These were prepared with a diamond ground surface with a finish of 0.2 pm Ra. The samples were cleaned before and after testing by immersion in acetone with agitation by an ultrasonic bath for 15 min, drying in an oven at 120°C for 30 min and then left to cool before weighing. Experimental
After cleaning, the specimens were weighed in a Mettler balance with pg resolution before they were mounted in the test specimen. The specimen thermocouple was placed on the upper stationary specimen so that a good measure of the specimen could be obtained. The furnace was then lowered over the specimens and the temperature raised until the specimens were at the required temperature. A soak period of 30 min was allowed to allow the test system to reach thermal equilibrmm before the test was started. 404
Fracture Young’s toughness, modulus (MPa m”.5) (GPa)
The motor was then started with the specimens in contact. The test speed was 0.1 m s-l and the total nominal applied was 20 N. The wear displacement and friction coefficient were recorded throughout all tests by means of a chart recorder and a data logger. The duration of the tests was 6 h, giving a total sliding distance of 2160 m. After the test was completed, the furnace was switched off and allowed to cool to near room temperature before the specimens were removed after carefully collecting all available wear debris. The specimens were than cleaned before weighing to determine the mass change that had occurred during the test. The worn surfaces were examined by scanning electron microscopy, and their by non-contact laser profilometry. The was examined by X-ray diffraction analysis any crystalline phases present. Friction
optical and topography wear debris to determine
Friction In all cases the friction coefficient rose quite quickly at the start of the test before reducing to a level which remained relatively constant for the remainder of the test. In some tests there was a slow increase or decrease’ in the friction values during the remainder of the test, for example the TiC composite test at 900°C but no consistent trend was evident. One unusual feature was the sudden increase in friction coefficient exhibited for the test on the TiC composite at 900°C after 3500 s test duration. The behaviour of the steady state friction was similar for all three materials (Fig 2), with a relatively small increase from room temperature to 300°C followed by a small drop for the composites or level friction coefficient for the baseline material, with a very sharp increase in friction coefficient for all materials at 1200°C. At lower temperature, the friction coefficient for the baseline material was lower than for the composites. 6 September
.!i 1 2 t a, 0.8 8 5 0.6 'i; 0 z 0.4
Fig 2 Steady state friction coejjicient results
Wear The wear displacement traces for the three different materials were all similar, and the results for the baseline silicon nitride are given in Fig 3(a). In most of the tests there was increased wear in the early stages of the test, but the wear rate normally reduced quite quickly to a relatively steady state which remained (a)
1 RT ;
_ _ _
-.. - -.
- - -
.. . ... . ... . / / /
CF 2 0.6 E 8 g 0.4 g
of (a) baseline silicon nitride Fig I .Microstructure material, (b) titanium nitide composite, (c) titanium carbide composite
600 800 Temperature,
Fig 3 Wear displacement results; (a) traces for baseline silicon nitride material, (b) total wear displacement versus temperature
Volume 28 Number 6 September
for the remainder of the test. An exception to this behaviour was the test carried out on the TIC composite at 900°C where a sudden jump in wear occurred after 3500 s of the test. The wear rate (and total wear, Fig 3(b)) increased with temperature to a maximum at 600 or 900°C for all materials. However, although the total wear for all materials was similar at room temperature, 300°C and 12Oo”C, it was very much higher at 900°C (by a factor of three) for the baseline silicon nitride composition.
increase in friction and decrease in wear before another peak occurred (Fig 4(d)). In all three of these tests, this behaviour was not present at the start of the tests, but developed during the tests, and continued until the test was completed. Surface
The worn surfaces were examined by scanning electron microscopy and were found to be very complex in nature. Nevertheless some common themes and trends were evident. Note that the direction of movement in all micrographs was broadly left to right with the exception of Fig 8(c) where the direction of movement was top to bottom.
The mass change results are given in Table 2, and generally confirm the wear displacement results with an increasing magnitude of mass loss as the temperature is increased. However, at 900 and 1200°C most of the mass change measurements in fact registered a mass gain, presumably due to oxidation of the samples. The high wear of the baseline silicon nitride composition at 900°C is borne out by the very large mass loss for this test. It is significant that the bottom specimen always lost more mass than the top specimen. It was also quite dramatic to find that the silicon nitride baseline and titanium carbide composite tests at 1200°C were found to be welded firmly together at the end of the test (i.e. after stopping the rotation and cooling down).
On lightly worn surfaces there were many areas which were smooth and showed features of the underlying microstructure, particularly in the composite materials (Fig 5(a)). Th ere were fine abrasive grooves visible in these areas. Other parts of the lightly worn surfaces were somethat rougher in appearance, with particulate debris visible (Fig 5(b)). Although this debris sometimes clumped together, there were many single particles scattered over the surface in these regions.
Transient effects A feature that was observed in some of the tests was repetitive fluctuations in the friction, sometimes accompanies by similar variations in the wear displacement. Normally, wear and friction traces showed variation of an essentially random nature throughout the test (Fig 4(a)). It is important to note that this variation has a much longer timescale than the rotation of the specimens, which has a period of well under one second.
As the wear increased with the rise in temperature, much more particulate material was evident covering most of the surface (Fig 6(a)). Most of this was contained in rafts of compressed material with some loose material at the trailing edge of the raft. In some cases cracking and fracture of these rafts was observed perpendicular to the direction of motion, suggesting that these structures were not permanent and went through a process of continual breakup and renewal (Fig 6(b)). Th ere were also abrasion grooves present on some rafts.
In many of the tests, however, a much more periodic variation was observed, with all three tests at 1200°C showing an almost sinusoidal variation in friction coefficient with time (Fig 4(b)). One test (the titanium nitride room temperature test) showed an almost steplike behaviour with an increase in friction for about 250 s accompanied by a decrease in wear (specimens moving apart), followed by a similar period where the friction decreased and wear increased (Fig 4(c)). In another test, sharp reductions in friction were accompanied by simultaneous increases (movement of surfaces together) in wear, followed by relatively slow
In some areas where the overlay of particulate material was quite thin, clear evidence was seen of intergranular fracture and pluck-out of material from the surface of the specimen (Fig. (7)).
Table 2 Mass Material
in tests RT
SIN SiN + TIC SiN + TIN
When wear was relatively low, smooth layers were formed over a small fraction of the wear surface. These layers were almost featureless, and occurred in all the room temperature tests, and the two 900°C tests carried out on the composite materials (but not the high wear rate baseline silicon nitride material) (Fig 8). There were great differences in the morphology
-0.2 -1 -0.6
-0.8 -1.6 -0.8
T -19.2 -14.4 -27.5
T is the
-21.4 -15.8 -26.6
-20.6 -19.8 -22.4
B -25.5 -24.1 -30.9
-97.4 0.4 1.3
B is the bottom.
-242 0.6 1.1 welded
B 0.3” 13.4*
4.8 3.9 specimens.
Fig 4 Transient friction and wear displacement recordings for (a) baseline silicon nitride room temperature test, (b) tifanium nitride composite 1200°C test, (c) titanium nitride composite room temperature test, (d) titanium carbide composite 900°C test Tribology
(b) Fig 5 Micrographs from tests carried out at room temperature on titanium carbide composite material
(b) Fig 6 Micrographs from tests carried out at (a) 300°C baseline silicon nitride material, (b) 600°C on titanium nitride composite material
of these layers from one specimen to another, but there was clear evidence for fracture of these layers as a mechanism for the removal of material from the surface (Fig 8(b)). At high temperature, there was considerable evidence for a high rate of oxidation of the surface. Thus the surface of the titanium nitride composite tested at 900°C (Fig 9(a)) shows florets of oxidized material, some of which has been flattened by the action of the opposing surface, which are characteristic of the oxidation of this material at this temperature. Similarly an unworn area of the titanium nitride composite tested at 1200°C shows facetted crystalline growths which are again characteristic of oxidation of the surface at this higher temperature. The silicon nitride specimen that had welded together after the testing was broken apart by striking it with a hammer. It was seen that although most of the surface was covered with oxidized material such as that seen in Fig 9(a), there was a smooth rim which showed some features of the microstructure when examined at higher magnification (Fig 10). It is thought that this region was covered by a very thin layer of liquid during the wear process, which solidified when the specimen was cooled. This is supported by evidence for localized fracture of this thin layer at the interface between the smooth rim and the rest of the surface. 408
Fig 7 Micrograph from tests carried baseline silicon nitride material
out at 300°C on
Cylindrical debris, which has been observed before several times in sliding wear14-17, was observed on worn surfaces from several of the tests (Fig 11). They were always observed on smooth areas, and were about 0.5-l pm in length, and were 0.1-0.2 pm in width. It has been suggested that these cylindrical 6 September
Cb) Fig 8 jWicrographs from tests carried out at (a) 900°C on titanium nitride composite material, (b) room tempei*ature on baseline silicon nitride material
friction and wear testing:
Cb) Fig 9 Micrographs from tests carried out at (a) 900°C on titanium nitride composite material, (b) 1200°C on titanium nitride composite material
wear particles are formed by rolling up self adhered layers of material which has been formed at least in part by tribochemical reaction at the wear interface, and that these cylindrical particles may act to some degree as roller elements and separate the surfaces and reduce friction. One sgnificant observation was evidence for localized melting on the silicon nitride baseline material at the relatively low temperature of 300°C (Fig 12).
The topography of the worn surfaces was examined with a non-contact laser profilometer. The advantage of using an instrument on this type is that since it uses optical measurement methods, the layers of debris present on many of the wear surfaces are not damaged by the passage of a mechanical stylus as would occur in a conventional instrument. The results (Fig 13) show that the surfaces obtained in the test where was generally less than that in However, it is interesting that the 600°C on the baseline silicon nitride significantly smoother surfaces than
roughness of the the wear was low high wear tests. tests at 300 and composition gave the tests on the Tribology
Fig 10 Micrograph from test carried out at 1200°C on baseline silicon nitride material
composite materials; even for the baseline test at 900°C the surface was not significantly rougher than those of the composite samples. However, these samples showed significant generation of form giving a concave shape to the surface of the top sample, and a convex shape to the surface of the bottom sample. Volume
et al. Discussion Transient
A possible cause of the transient effects seen in the friction and wear traces in many of these tests is the breakaway of part of the surface which, until this occurred, was supporting the opposing surface. This concept is supported by the microstructural evidence for intergranular fracture on the surfaces at intermediate temperatures.
Fig 11 Micrographs from test carried out at 300°C on baseline silicon nitride material
However, when the results are examined more closely, two types of behaviour are seen which are somewhat contradictory. In Fig 4(c) a sharp increase in friction was seen followed by a slow reduction for a short while before a relatively quick reduction to a lower level of friction. This was accompanied by a decrease in wear (build up of the surface) as the friction increased. In Fig 4(d) the opposite behaviour was seen with a sharp reduction in friction followed by a slow climb in friction until the next sudden drop in friction. The wear moved in the opposite direction to friction as seen in the earlier case. There are several possible explanations for these transient effects. One is the accumulation of subcritical damage in the surface layers of the wear specimen followed by breakaway of a fragment of material from the surface, allowing the specimens to move suddenly closer together. This explanation is consistent with the effects seen in Fig 4(d), with the slow recovery due to filling of the new cavity with packed debris.
Fig 12 Micrograph from test carried out at room 300°C on baseline silicon nitride material
Another type of mechanism could also be operating, explaining the effects seen in Fig 4(c), with a slow loss of material from the surface, followed by trapping of material between the two surfaces causing the surfaces to move apart and the friction to increase. However, if this mechanism is correct, ‘it is remarkable that the transient response is so regular. Wear
Some surface form was also generated on the surface of the titanium nitride composite tested at room temperature. Wear
The colour of the debris changed dramatically as the composition and test temperature were changed. For the baseline silicon nitride material, the debris was always white, but for the composite materials, black debris was produced at 300°C which gradually lightened as the test temperature was increased (Table 3). The debris was also examined by X-ray diffraction, and it was found in all cases that a lower count rate than expected was obtained, with a relatively large broad band at low angle diffraction angles, showing that there was a large amount of X-ray amorphous material present in all samples of the debris. There were also sharp peaks present indicating the presence of small amounts of silicon nitride and alumina in the debris samples. 410
The two main processes that could contribute to wear are oxidation and fracture. It is thought that the wear and friction that occurred in the tests conducted in this study are largely controlled by the relative balance between the effects of these different processes. A description of the processes of wear and friction that occurred in these tests is given below, but it is clear that at this stage the explanations are largely speculative, and need to be confirmed by additional study. At room temperature, some oxidation of the samples does not occur, and it is likely that the formation of stable oxide products is accelerated by the presence of water vapour in the test environment as the temperature increases. As the ambient temperature increases, the oxidation rate increases, leading to the formation of a larger and larger quantity of oxide material, which at intermediate temperatures is removed from the wear surfaces by the mechanical action of the opposing surface leading to increased rates of wear. 6 September
Fig 13 Three-dimensional height maps of surface of specimens tested at (a) room temperature with titanium nitride material, (b) 300°C with baseline silicon nitride material, (c) 300°C with titanium nitride material, (d) at 600°C with baseline silicon nitride material, (e) at 600°C with titanium carbide material, (f) at 900°C with baseline silicon nitride material
Table 3 Colour of debris produced in wear tests. Note that insufficient debris was generated at room temperature and 1200°C to allow collection of debris Material
SIN SiN + TIC SIN + SiN
White Black Black
White Dark grey Grey
White White Light grey
It should, of course, be noted that much higher flash temperatures will be generated at the localized contacts at the interface between the two surfaces which will also cause increased likelihood of oxidation. At 1200°C the oxide material that was formed was cohesive over most of the contact area, and where contact did occur the glassy phase melted leading to the presence of a viscous liquid phase at the contact Tribology
zone. This possibly protected further wear.
the specimens from
There was little difference in the behaviour of the different materials, exceut for the silicon nitride test which was conducted at 9&l% which wore considerably more than the composite specimens. It is perhaps significant that the silicon nitride material was found to oxidize about four times faster than the composite
Volume 28 Number 6 September
materials in oxidation experiments samples18J9 (Fig 14).
conducted on bulk
The reason for the formation of black debris at intermediate temperatures is likely to be due to the presence of sub-stoichiometric or reduced titanium oxide in the debris. This normally appears black, and the increased lightness as the temperature is increased is due to the more complete oxidation that occurs at higher temperatures. The evidence for localized melting is not surprising in the light of the composition of the materials, which will give a glassy grain boundary phase. It is thought that this glassy phase would melt under the localized heating at the wear interface giving this small amount of molten material. The fact that only isolated areas of localized molten material were found on a small number of the specimens suggests that this mechanism is not a major contributor to the wear that occurred. A more speculative reason for the increase in wear at intermediate temperatures is due to the possibility that the fracture toughness of many ceramics under indentation conditions passes through a minimum as the temperature is increased20J1. Thus the reason for the increased wear at intermediate temperatures would then be due largely to the increased fracture that occurs as the temperature increases. The contribution of this process to wear is supported by the evidence for intergranular fracture and pluck out of grains from the surface, and also by the higher roughness for the composite materials as compared to the baseline silicon nitride samples tested under similar conditions. Whatever the reason, the maximum in wear with temperature that was found in this study confirms earlier results showing similar effects in the wear of other ceramics such as alumina and silicon carbide6v9. The reason for the increase in friction coefficient as the temperature is raised from room temperature to 300°C is likely to be because of the increase in roughness of the surface .as the wear increases with increasing temperature. However, it is interesting to note that although the surface of the baseline silicon nitride samples was normally smoother than the
surfaces of the composite samples tested under similar conditions, there was little difference in the friction coefficients that were measured. At 12Oo”C, the reason for the very high friction coefficients observed for all the materials is the viscous drag imposed by molten glassy phase at the interface between the two samples. The solidification of this glassy liquid as the specimens cooled down after testing is thought to be the reason why some sample welded together after the testing at 1200°C. The similarity of friction results for the different materials is perhaps surprising in the light of other work in the literature which seems to indicate that the presence of titanium in the structure of the material can lead to the reduction of friction coefficient, and thus wear through the lowering of contact stresses2,8,11,22. The reported reduction in friction is reported to be due to the formation of lubritious films of titanium oxide, which have been shown to have a low shear stress under sliding conditions at elevated temperaturesz2. Future
It is clear that further additional work is necessary before a full understanding of the wear and friction behaviour of these materials at high temperature can be developed. Work which is planned includes transmission electron microscopy of the wear debris to determine structure and composition, energy dispersive X-ray analysis and other surface analysis of the wear surfaces to confirm their chemical state, a more careful topographical analysis to confirm the relationships between wear, friction and surface texture, and a study of the hot hardness and indentation fracture of samples of the different materials. Additional high temperature testing under an inert atmosphere would also be useful to ‘give extra information on the exact role of oxidation, but will depend on the availability of resources. Conclusions
In high temperature sliding wear tests carried out on self-mated silicon nitride based composite materials, it was found that there was little difference in the values of friction coefficient that were found for the different materials, with only slightly higher values for the titanium nitride and titanium carbide composites at low temperature.
Nitride 1300 . .. ... .. ..
800 1,000 minutes
Fig 14 Oxidation curves for silicon nitride baseline and silicon nitride composite materials 412
For all materials, there was a small increase in friction from room temperature to 300°C followed by a small decrease as the temperature increased, until there was a sharp increase in friction coefficient at 1200°C. This sharp increase in friction was thought to be due to the formation of viscous interlayers of glassy material at the interface. Indeed, samples were found to be welded together after tests on two of the materials at 1200°C. There was a maximum in wear for all materials with respect to temperature, with almost no wear at room temperature and at 1200°C. The wear of the silicon 6 September
nitride material was sharply higher than the wear of the composite materials at 900°C.
like to thank
for useful discussions concerning
C. Melandri .I. Hard
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