- Email: [email protected]
Wear and microstructure in fine ceramic coatings
221
R. Viiande-Dim,
J Belzunce
and E. Fernandex
ETS de Ingenieros industriale, Area de Ingeniera Mecanicts, Department de Const. e Ingen. de Fab., Centra de Castielio, s/n 3.5’204.G&on, Asturias (Spain) Rincon and M. C. P&ez Insritut de Firrica-Qufmica Rata Sofano+ Cfir
k
Wadrid (Spa&$
(Received June 15, 1990, revised January 7, 1991; accepted February 19, 1991)
Abstract
This paper presents a study of the wear resistance of two ceramic, plasma sprayed coatings of A&O3 and Cr,OB. Tests were carried out using an LWF-1 standard machine, with lineal contact, under dry friction, abrasion and lubrication conditions, The purpose of the tests were to study how load and speed affect material wear. Results show the lower wear rate of the ceramic coating compared with the steel one, as wefl as how remarkably foad affects wear. On the other hand, however, considering the speed ranges used, wear resistance does not depend significantly on speed. The paper proves that the wear process foffows Czichos’ law. At the same time, reformulation of Archard’s equation alfows us to quantify wear using easiJy measurable factors such as pressure, speed, hardness, and those factors typically featuring this type of coatings, e.g&porosity. AIso, a microgra~hj~ study of the coatings carried out by means of a scanning electron microscope has evidenced three stages in the wear mechanism: (a) plastic deformation of particles; (b) crack nucleation and propagation; and (c) loosening of ceramic particles.
1. Introduction
Triboceramics, the science of studying wear resistance in ceramic coatings, has developed greatliy in the last few years. This is mainly because of the interest shown by the industry and also because of the new technologies that have made spraying and fusion of such materials possible. This is also the case with plasma spraying: by means of an electric arc, a mixture of gases is ionized. The energy thus gained by the gases causes the coating material, supplied as powder, to be sprayed at high speed over the substrate, This produces a series of micro weldings which, together with intermofecular cohesion, make adherence resistance of the coatings even higher than 80 N mm-*. The high temperatures reached (16 000 “C) allow smelting of the most heat-resistant ceramic materials. A thin coating spraying over the surface of a mechanical element causes resistance to wear, corrosion, oxidation and heat to improve and does not affect the mechanical properties of the substrate fl, 21. This process is currentIy used in aeronautics, naval sectors, the iron and steel industry, etc. as weII as for aircraft engine components, bearings, shafts, vanes, etc. 13%
0 1991 -
Efsevier Sequoia, Lausanne
222 Various
materials
are used
as coatings
(ceramics,
alloys,
metals,
etc.),
on all kinds
of substrates, and in different spraying environments, e.g. air, inert and vacuum. This remarkable step forward is a result of the improvement in the spraying process and in the control of the sprayed substances, as well as to a better knowledge of their in-service performance derived from ample research in the field [4]. This is so much so that surface technology, linked with the new materials, is likely to have a fundamental role in the next century [S]. Various research groups [G-14] have analyzed the problems derived from ceramic coatings in contact instances from diRerent perspectives such as contact (punctual, superficial, etc.), friction conditions (dry, lubricated, etc.) and using different test parameters (load, speed, etc.). The present paper is refated to a research project carried out on the basis of the interest shown by a company that works on plasma-sprayed ceramic coatings. The aims of the paper are to: (a) determine the wear resistance of ceramic coatings in front of steel under lineal contact, and (b) determine the wear mechanisms of the ceramic coatings at the microscopic level.
An Alpha-LWF-1 tribometer was used which allows lineal contact between a rotating ring (from 0 to 200 rev min-“) and a block that withstands a load of O-2000 N. The machine was fitted with a deposit which enables the rotating ring to spread the 1ubricatioR or abrasion substance over the contact area (Fig. 1). The test specimens used had the recommended geometry and dimensions for linear contact in conformance with ASTM C 77. For the movile specimen, ring-shaped, steel (AISI D2) was used. This steel underwent hardening and tempering treatments to reach a hardness of around 60 HRC. The specimen, or block, consisted of a substrate made of 0.2 carbon steel, a very fine bond layer, and a ceramic coating (Fig. 2). Two different types of coatings were studied, Al,Os and Cr203, the composition and most distinctive features of which are given in Tabfe 1. The coatings were plasma sprayed using METCO 9 MB, 40 kW equipment, following the spraying conditions suggested by. the manufacturer [15]. Porosity was measured by visual counting using an optical microscope.
Fig. 1. Test machine.
223
Fig. 2. Shape and dimensions TABLE
of test specimens
used.
1
Characteristics
of the coatings
Composition
Cr,O, (96%) TiOZ (2%) Others (2%)
AI,03 (98.5%) Si02 (1%) Others (0.5%)
Grain size (pm) Thickness (mm) Hardness (Hv300) Porosity (%) Roughness (Ra, wn) Adherence (MPa) Density (g cm-9
go-105 0.25-0.45 1~~15~
53-68 0.25-0.45 60~8~
7-10 1.5-2
5-7 1.5-Z
X-65 5
59-63 3.3
The adherence resistance of the coatings was determined in conformance with ASTM C-633. A tensile test was carried out on two cylindrical test specimens, one of which was coated, bonded by a monocomponent epoxi-type structural adhesive material (an EC-2214 of 3M was used), the adherence resistance of which was simultaneous~ monitored on similar, but non-coated, test specimens which had the same preparation and testing conditions. Wear resistance tests were performed under dry friction, abrasion and lubrication conditions. For the abrasion test, the material used was alumina powder with an average grain size of 5 mm, mixed with water in a 1:5 weight ratio. In the lubrication tests, 117 cSt and 11.3 cSt oil was used, at temperatures of 38 “C and 100 “C, respectively, and density (25 “C) of 0.882 mg crnm3. To evaluate how load and speed affect the wear process the magnitudes used are within the ranges shown in Table 2.
224 TABLE
2
Ranges of variables ~_I Lubrication
Materials
Chromium oxide I Steel
dv
Abrasive Aluminium 0xide I Steel
Oil
Load (N)
18 45 63 90 136 272 408 544 680 1360 1700
Speed (rev min-‘)
CyClCS
50
100
o-ld 150
200
Wear T I I i
Fig. 3. Evolution
of wear: Czichos’ model.
Once load and speed conditions were fixed and the test specimens, bloc& and ring cleaned with heptane using uftrasonic equ~~rnent~ they were tested far a tied number of cycles, At regufar intervals the test would be stopped and she specimens cleaned using the same procedure again. After that, they were weighed on a 0.1 mg precision balance. Subsequently, the friction test started again and went on for a new fixed number of cycles. During each period the friction load was measured by means of a graphic recorder. The tests ended either when the number of cycles was high enough (lo6 cycles), or once the ceramic coating had disappeared from the contact area.
3. Quantitative models ~ichos’model [ltj] expresses tke re~at~o~sh~pbetween wear and time in a tribosystem by a set of curves whose general shape is shown in Fig. 3,
where N re~~~$e~~ the time in ~~rn~~~ af cycfea. The linear re~~~~~~~~~~ of these equations aflows us ta observe with ease the c#r~cla~~on between the inte~e~i~~ variables and the adaptation of the model to the axperimentaf resuhs in the case of the fine ceramic coatings under study. The evolution of wear sumested by Cxichas does not consider specifically some of the vaiues that may affect wear, such as the material itself, and the conditions of We (load- spkzed,~~~r~~~o~~elc*>X4&&would a~~~~~ be inchSded within the c~~~~~~~~ of the mod&
the load C and hardness N of the material. /3 is a constant which represents the probability of the formation of wear particles that must be determined for each combination ofmatcriais. Bayer [Zs, %if collects fmm a&w authors values characteristic of j3*
This e~re~~~o~ also alfows ~rnpa~~~on beaker the wear (I&, r>,) found in d~~e~e~~ coatings under the same test conditions. In such c~~cs we would obtain WQ2 - P2%/&~1
c9r
During this firststagewear of the ceramic coatings and of the steel ring were compared. Qn the whole, for the three friction cmditians tested (dry, abmsian and l~~~~c~t~~~~ high str$~gth steel was less wear resistant than ceramic coatings, especial& in the lubrication test, as illustrated in Fig. 4 for the case of a Cr207. coating, The perfkrmance of the alumina coatings was similar, although with lower relative values.
The limit fur each of the three wem zones, linked ta a ~~~~~~~~~change in the wear rate, must be determined. With that puqxxe, the wear produced every 500 cycles iv) was measured. This, gra~hi~a~~~represented in Fig. 5, allowed the determination of the number of cycles N from which a new zone starts. The graphs clearly sbow that the tra~s~~~o~from zone Z to zone Xf always occurs in values around 10 #XI cycles
0
woOo
2woo
3@o@oNbyclerrl -WOO
Fig. 4. Wear (lubricated conditions) of steel
compared
with ceramic
coating of C~@J.
under our testing ~~d~t~o~s. Zone IfI was observed o&y when the testing ~~dit~ns were very severe (very high toad in dry and abrasion ~~d~t~ous~. In aft the other instances it did not occur, not even when, owing to the diction of the test, the coating was about to disappear from the friction zone. Such is the case of the test carried out under lubrication conditions shown in Fig. 6, corresponding to an A1203 coating, at 1360 N and 150 rev min-‘. On the other hand, Fig. 5 shows the higher range of AD, in zone 1, from 0 to 10 000 cyctes, and even the presence of some negative values caused by the adherence of steel particles to the ceramic coating, as electron microprobe analyses showed later, The remarkable maierial ~oose~~ug that takes place at this stage, which impfies a high wear rate, is caused by the farge amount of partiefes with tittle adherence present on the outer iayer of the mating. Such particles come ob easily and at random, ihus gj~~ug rise to a si~~~~~t va~abj~~~ in the wear rate. Of outstanding ~m~~rtauce~ however, is the high ~b~sio~ and good adherent of the coatings on the substrate, as welf as their excehent wear pe~o~a~ce, especiafly under high ~ub~cat~on conditions. This is probably because af the high hardness of the ceramic coating and the porosity produced by the plasma spray system.
Test rest&s have agreed with the e~~at~~~ expressed in (4) and (5) of Czkhos’ mudet. The Wing of the data has been ~~er~~~~d~ botk for t&e c~~~~~~ oxide and ahnina ~~~t~~~~~ under the d~~~~e~~friction ~~d~r~o~~,ZLSF&s. 7 and 8 show* On the uther hand, both figures show the ~u~ue~~ of the load, which has turned out to be a relevant factor in the wear process, as expressed in Arebard’s fo~~~at~on* ~everthe~ess~ as regards the speed ranges used in the tests, it is possible to say that the infhzence of speed is not rekvantP ~tho~gh as speed increases wear decreases shghtfy, which agrees with what other authors 161 have stated for compact ceramic materials. At present, research is being carried out on the effect of speed using ranges of 0 to 3000 rev min-“.
200
loo
”
0
400~
2oQo
Fig. 7. Fitting of test results to the running-in coatings under dry conditions.
N(cycles)
bOO0
zone of Czichos’ model for aluminium
oxide
-I
0
soooo
Fig. 8. Fitting of test results to the steady-state coatings under dry conditions.
N(cjcles)
104000
zone of Czichos’ model for aluminium
oxide
4.4. Verification of Archard’s equation Once the densities of the coatings were determined, taking their porosity and microhardness into account, eqn. (8) was used and thus the values that characterize p found: dry, 1.5 X lo-‘; abrasion, 2X 10e4; lubrication, 6 X lo-‘. Considering the values of the factors provided in Table 1 and using eqn. (9) the relationship between Dz/D1 (Dz is the wear in AlzOS and D1 the wear in CrZ03) would range from 0.9 to 1.7. This fact has been verified experimentally, as shown in Fig. 9. Adapting Archard’s equation would provide a simple way to build relative wear scales taking one coating as reference and using the density and microhardness of the layers as measuring parameters.
229
0
Fig. 9. Wear conditions.
100
relationship
Fig. 10. Chromium
5. Mic~st~ctu~
of chromium
oxide coating.
CHwQn-
and aiuminium
200
oxide coatings
Fig. 11. Aluminium
under
the same test
oxide coating.
and f~ctogra~~y
Before submitting the blocks to tests, they were observed with an optical microscope. Figure 10 relates to a CrZ03 coating, and Fig. 11 relates to an A1203 coating. The steel base, the fine bond layer and the ceramic coating can be seen in both photographs. Both ceramic coatings demonstrate a stratified morphology and the presence of porosity, the latter being more accentuated in the CrzO, coating because of its higher melting point. Also obvious is the grain deformation in the union zone, owing to the impact of the spraying. When tested, and previously metallised, the blocks were examined using a scanning electron microscope. Figures 12-16 illustrate the most important aspects observed. Figure 12 represents the limit between two zones - worn and not worn - in a dry test with a chromium oxide coating. In this limit one can observe the stratified structure, which is a consequence of the arrangement of the Cr203 particles. The morphology of this border suggests a brittle-type fracture typical of ceramic materials. Figure 13 shows the worn zone in the same block. One can see the existence of flat zones, polished by rubbing, together with other areas that have not been deformed
Fig. 12. Limit between the worn zone (lower part) and not worn zone (upper part) of a chromium oxide coating, tested under dry conditions with a load of 408 N at 150 rev min _‘. Fig. 13. Flat zone of chromium
Fig. 14. Nehvork
oxide coating,
of cracks in previously
Fig. 1.5. Appearance
of a chromium
plastically
deformed
under compression,
de1 armed zones.
oxide c tating fractured
by wear.
during the test. The generation of these fat zones would suggest a plastic deformation in the Cr,Oa coating, assisted by the hi ;h contact temperatures (taking into account the low thermal conductivity of this ma :erial and the high load used in this test). One of the flat zones, observed in I ‘ig. 13 is shown in magnified form in Fig. 14. Ire generated which in the end provoke the Cracks, apparently of a brittle nature, loosening of particles of material. The surface aspect, once individuai loosening of the particles has begun, can be seen in Fig. 15. Also clearly shown are the rounded f ~rrns of the juxtapositioned particles, indicative of at least a partial fusion during the I xaying process. In the alumina coatings, which are : morphologically similar to the Crz03 coatings, although with a higher melting point, on, can observe a similar behaviour in the wear mechanism. The rounded surfaces of the Ala03 particles, their stratified arrangement, and the progressive brittle failure of the different strata that constitute the hard layer are shown in Fig. 16. From a microscopic study it is possible to conclude that the wear mechanism that affects these thin, plasma-sprayed coatings takes place according to the following stages.
231
Fig. 16. Appearance of an aluminium oxide coating fractured by wear. (I) An initial stage, ~~sist~n~ of the appearance of Aat zones resulting from plastic deformation of the ceramic particles owing to the specific high pressures at the actual contact zones and to the high temperatures in that area. (2) A crack nucleation and propagation stage in the previously deformed area probably caused by material fatigue. (3) A third stage of fracture and individualized loosening of particles. These three stages represent the ~~tinuous cycle of the wear mechanism, where detachment of the deformed particles from the surface allows a second Iayer of ~~darna~ed particles to appear the wear mechanism of which follows the abovementioned process. This process then continues and therefore the wear rate would be constant (zone II is the steady-state zone) until the fayer disappears, This is confirmed by the test results, especially under lubri~~tion conditions (see Fig. 6).
7. Conclusions (1) Wear of the ceramic coatings follows Czichos’ model in the running-in and steady”state zones. The limit between both zones was perfectly defined using the study of the evolution of the wear rate. This parameter turned out to be highly ~nfo~ative for the development of the process. The failure zone only occurred during dry and abrasion tests for high load values, with pressures higher than the adherence resistance and internal cohesion of the substrate. In such instances the process resulted in destruction of the coating. (2) Archard’s equation, involving micro hardness and density, is hefpfuf when setting the differences between the layers and to elaborate the relative wear scales taking one coating as reference, (3) The micrographic study seems to explain the wear mechanism operating in these layers, which takes place as follows. (a) Plastic deformation of ceramic particles under compression, (b) Crack nucleation owing to fatigue and to the irregularity that characterizes the subsurface of the layers. (c) Ind~idu~~d loosening of particles. The same mechanism would be repeated an the new contact layers. This cyclic process would account for the permanence of the steady-state zone, observed particularly during lubrication, until the layer disappears.
232
Acknowledgments This work was undertaken within the framework of the Ministry of Education and Science Research Project, Physical and Chemical Aspects of Wear in Moving and Surface Treated Mechanical Systems, developed jointly between the tribology unit of CSIC (Madrid) and the E.T.S.I.I. at Gijon, of the University of Oviedo, and financed by CICYT. This research was also carried out with the help of the following research project, funded by FICYT, under the auspices of the Principado de Asturias: Study of the Execution and Wear, Corrosion and Heat Resistant Properties of Plasma Sprayed Coatings.
References
5
6 7 8 9 10 11
12 13 14 15 16 17 18 19 20 21
CETIM, Joumh d’lnfotmation du Groupe Franeais de la Ckamique, Senlis, France, 1989. A. R. Nicoll, H. Gruner, G. Wuest and S. Keller, Future developments in plasma spray coating, Mater, Sci. TechnoL, 2 (1986). A. R. Nicoll, Plasma spraying: materials, applications and systems for industrial coatings, Meeting H. School Tech&e G$n, Spain, Nov. 1990. C. Gelinas, B. Champagne and S. Dallaire, Effets des parambtres de projection par plasma sur l’efficacite de deposition et l’adherence des revetements de Cr,O,, Proc. 11th Int. Thermal Spraying Con!, Montreal, Canada, 1986, Pergamon, Oxford, 1986. A. Hanff et al., Plasma surface engineering: technological trends and impacts, in E. Broszeit, W. D. Munz, H. Oechsner, K. T. Rie and G. K. Wolf, Plasma Sugace Engineering, Vol. 1, DGM, 1989, pp. 3-15. J. Denape and J. Lamon, Le comportement en frottement set de ctramiques a hautes performances, in 4gme Congres Europeen de Tribologie Vol. III, Elsevier, Amsterdam, 1985. 3. Hailing, The tribology of surface coatings, particularly ceramics, Proc. Inst. Me&. Eng., 200 (Cl) (1986) 31-40. Zum Gahr, Sliding wear of ceramic-ceramic, ceramic-steel and steel-steel pairs in lubricated and unlubricated contact, Wear, 133 (1989) l-22. T. Nakamura and S. Hirayama, Wear test of grey cast iron against ceramics, Wear, 132 (1989) 337-34.5. Y. Wang, Y. Jin and S. Wen, The inspection of the sliding surface and subsurface of plasmasprayed ceramic coatings using scanning acoustic microscopy, Wear, 134 (1989) 399411. S. M. Shu, V. S. Wang and R. G. Munro, Quantitative wear maps as a visualization of wear mechanism transitions in ceramic materials, Wear, 134 (1989) l-11. M. N. Gardos, On ceramic tribology, Lubr. Erg. (1988) 400-407. K. Kato, Tribology of ceramics, Wear, 136 (1990) 117-133. G. W. Stachowiak, G. B. Stachowiak and A. W. Batchelor, Metallic film transfer during metal-ceramic unlubricated stiding, Wear, 132 (1989) 361-381. Metco Technical Bull., 1988. H. Czichos, Tribologv, Elsevier, Amsterdam, 1978. J. T. Burwell and C. D. Strang, On the empirical law of adhesive wear, J. Appl. Phys., 23 (1952) 18. M. M. Kruschov, Resistance of metals to wear by abrasion; related to hardness, Inst. Mech. Eng. Confi Lubrication and Wear, London, 1957, Inst. Mech. Eng., 1957, pp. 655-659. F. P. Bowden and D. Tabor, The Friction and Lubrication Solids, Vol. II, Clarendon, Oxford, 1964. C. N. Rowe, Some aspects of the heat of absorption in the function of a boundary lubricant, Trans. Am. Sot. Lubr. Eng., 9 (1966) 100-111. I. V. Kragelski, Friction and Wear, Buttetworths, London, 1969.
233 22 23 24 25 26 27
J. Hailing, Principfes of Tti6o@, Macmillan, London, 1975, p. 119. A. D. Sarkar, in J. Hailing (ed.), Weur of MefaLF, Pergamon, Oxford, 1976, pp. 69-73. J. I. Archard, Contact and rubbing of flat surfaces, I. Appl. Phys., 24 (1953) 981-988. R. G. Bayer, Selection and Use of Wear Tests for Coatings, ASTM, Philadelphia, PA, 1981. R. G. Bayer, Prediction of wear in a sliding system, Wear, II (1968) 319-331. R. Vijande, F. J. Belzunce, J. E. Fernandez, M. C. Perez and A. Rincbn, Wear behaviour of plasma sprayed coatings, in D. Dowson, C. M Taylor and M. Godet, 16th Leeds-Lyon Symposium, Lyon, 1989, Elsevier, 1990, pp. 379-386. 28 R. Vijande, Doctoral Thesis, ETSII, Engineering School at Gijon, University of Oviedo, 1989.
R. Viiande-Dim,
J Belzunce
and E. Fernandex
ETS de Ingenieros industriale, Area de Ingeniera Mecanicts, Department de Const. e Ingen. de Fab., Centra de Castielio, s/n 3.5’204.G&on, Asturias (Spain) Rincon and M. C. P&ez Insritut de Firrica-Qufmica Rata Sofano+ Cfir
k
Wadrid (Spa&$
(Received June 15, 1990, revised January 7, 1991; accepted February 19, 1991)
Abstract
This paper presents a study of the wear resistance of two ceramic, plasma sprayed coatings of A&O3 and Cr,OB. Tests were carried out using an LWF-1 standard machine, with lineal contact, under dry friction, abrasion and lubrication conditions, The purpose of the tests were to study how load and speed affect material wear. Results show the lower wear rate of the ceramic coating compared with the steel one, as wefl as how remarkably foad affects wear. On the other hand, however, considering the speed ranges used, wear resistance does not depend significantly on speed. The paper proves that the wear process foffows Czichos’ law. At the same time, reformulation of Archard’s equation alfows us to quantify wear using easiJy measurable factors such as pressure, speed, hardness, and those factors typically featuring this type of coatings, e.g&porosity. AIso, a microgra~hj~ study of the coatings carried out by means of a scanning electron microscope has evidenced three stages in the wear mechanism: (a) plastic deformation of particles; (b) crack nucleation and propagation; and (c) loosening of ceramic particles.
1. Introduction
Triboceramics, the science of studying wear resistance in ceramic coatings, has developed greatliy in the last few years. This is mainly because of the interest shown by the industry and also because of the new technologies that have made spraying and fusion of such materials possible. This is also the case with plasma spraying: by means of an electric arc, a mixture of gases is ionized. The energy thus gained by the gases causes the coating material, supplied as powder, to be sprayed at high speed over the substrate, This produces a series of micro weldings which, together with intermofecular cohesion, make adherence resistance of the coatings even higher than 80 N mm-*. The high temperatures reached (16 000 “C) allow smelting of the most heat-resistant ceramic materials. A thin coating spraying over the surface of a mechanical element causes resistance to wear, corrosion, oxidation and heat to improve and does not affect the mechanical properties of the substrate fl, 21. This process is currentIy used in aeronautics, naval sectors, the iron and steel industry, etc. as weII as for aircraft engine components, bearings, shafts, vanes, etc. 13%
0 1991 -
Efsevier Sequoia, Lausanne
222 Various
materials
are used
as coatings
(ceramics,
alloys,
metals,
etc.),
on all kinds
of substrates, and in different spraying environments, e.g. air, inert and vacuum. This remarkable step forward is a result of the improvement in the spraying process and in the control of the sprayed substances, as well as to a better knowledge of their in-service performance derived from ample research in the field [4]. This is so much so that surface technology, linked with the new materials, is likely to have a fundamental role in the next century [S]. Various research groups [G-14] have analyzed the problems derived from ceramic coatings in contact instances from diRerent perspectives such as contact (punctual, superficial, etc.), friction conditions (dry, lubricated, etc.) and using different test parameters (load, speed, etc.). The present paper is refated to a research project carried out on the basis of the interest shown by a company that works on plasma-sprayed ceramic coatings. The aims of the paper are to: (a) determine the wear resistance of ceramic coatings in front of steel under lineal contact, and (b) determine the wear mechanisms of the ceramic coatings at the microscopic level.
An Alpha-LWF-1 tribometer was used which allows lineal contact between a rotating ring (from 0 to 200 rev min-“) and a block that withstands a load of O-2000 N. The machine was fitted with a deposit which enables the rotating ring to spread the 1ubricatioR or abrasion substance over the contact area (Fig. 1). The test specimens used had the recommended geometry and dimensions for linear contact in conformance with ASTM C 77. For the movile specimen, ring-shaped, steel (AISI D2) was used. This steel underwent hardening and tempering treatments to reach a hardness of around 60 HRC. The specimen, or block, consisted of a substrate made of 0.2 carbon steel, a very fine bond layer, and a ceramic coating (Fig. 2). Two different types of coatings were studied, Al,Os and Cr203, the composition and most distinctive features of which are given in Tabfe 1. The coatings were plasma sprayed using METCO 9 MB, 40 kW equipment, following the spraying conditions suggested by. the manufacturer [15]. Porosity was measured by visual counting using an optical microscope.
Fig. 1. Test machine.
223
Fig. 2. Shape and dimensions TABLE
of test specimens
used.
1
Characteristics
of the coatings
Composition
Cr,O, (96%) TiOZ (2%) Others (2%)
AI,03 (98.5%) Si02 (1%) Others (0.5%)
Grain size (pm) Thickness (mm) Hardness (Hv300) Porosity (%) Roughness (Ra, wn) Adherence (MPa) Density (g cm-9
go-105 0.25-0.45 1~~15~
53-68 0.25-0.45 60~8~
7-10 1.5-2
5-7 1.5-Z
X-65 5
59-63 3.3
The adherence resistance of the coatings was determined in conformance with ASTM C-633. A tensile test was carried out on two cylindrical test specimens, one of which was coated, bonded by a monocomponent epoxi-type structural adhesive material (an EC-2214 of 3M was used), the adherence resistance of which was simultaneous~ monitored on similar, but non-coated, test specimens which had the same preparation and testing conditions. Wear resistance tests were performed under dry friction, abrasion and lubrication conditions. For the abrasion test, the material used was alumina powder with an average grain size of 5 mm, mixed with water in a 1:5 weight ratio. In the lubrication tests, 117 cSt and 11.3 cSt oil was used, at temperatures of 38 “C and 100 “C, respectively, and density (25 “C) of 0.882 mg crnm3. To evaluate how load and speed affect the wear process the magnitudes used are within the ranges shown in Table 2.
224 TABLE
2
Ranges of variables ~_I Lubrication
Materials
Chromium oxide I Steel
dv
Abrasive Aluminium 0xide I Steel
Oil
Load (N)
18 45 63 90 136 272 408 544 680 1360 1700
Speed (rev min-‘)
CyClCS
50
100
o-ld 150
200
Wear T I I i
Fig. 3. Evolution
of wear: Czichos’ model.
Once load and speed conditions were fixed and the test specimens, bloc& and ring cleaned with heptane using uftrasonic equ~~rnent~ they were tested far a tied number of cycles, At regufar intervals the test would be stopped and she specimens cleaned using the same procedure again. After that, they were weighed on a 0.1 mg precision balance. Subsequently, the friction test started again and went on for a new fixed number of cycles. During each period the friction load was measured by means of a graphic recorder. The tests ended either when the number of cycles was high enough (lo6 cycles), or once the ceramic coating had disappeared from the contact area.
3. Quantitative models ~ichos’model [ltj] expresses tke re~at~o~sh~pbetween wear and time in a tribosystem by a set of curves whose general shape is shown in Fig. 3,
where N re~~~$e~~ the time in ~~rn~~~ af cycfea. The linear re~~~~~~~~~~ of these equations aflows us ta observe with ease the c#r~cla~~on between the inte~e~i~~ variables and the adaptation of the model to the axperimentaf resuhs in the case of the fine ceramic coatings under study. The evolution of wear sumested by Cxichas does not consider specifically some of the vaiues that may affect wear, such as the material itself, and the conditions of We (load- spkzed,~~~r~~~o~~elc*>X4&&would a~~~~~ be inchSded within the c~~~~~~~~ of the mod&
the load C and hardness N of the material. /3 is a constant which represents the probability of the formation of wear particles that must be determined for each combination ofmatcriais. Bayer [Zs, %if collects fmm a&w authors values characteristic of j3*
This e~re~~~o~ also alfows ~rnpa~~~on beaker the wear (I&, r>,) found in d~~e~e~~ coatings under the same test conditions. In such c~~cs we would obtain WQ2 - P2%/&~1
c9r
During this firststagewear of the ceramic coatings and of the steel ring were compared. Qn the whole, for the three friction cmditians tested (dry, abmsian and l~~~~c~t~~~~ high str$~gth steel was less wear resistant than ceramic coatings, especial& in the lubrication test, as illustrated in Fig. 4 for the case of a Cr207. coating, The perfkrmance of the alumina coatings was similar, although with lower relative values.
The limit fur each of the three wem zones, linked ta a ~~~~~~~~~change in the wear rate, must be determined. With that puqxxe, the wear produced every 500 cycles iv) was measured. This, gra~hi~a~~~represented in Fig. 5, allowed the determination of the number of cycles N from which a new zone starts. The graphs clearly sbow that the tra~s~~~o~from zone Z to zone Xf always occurs in values around 10 #XI cycles
0
woOo
2woo
3@o@oNbyclerrl -WOO
Fig. 4. Wear (lubricated conditions) of steel
compared
with ceramic
coating of C~@J.
under our testing ~~d~t~o~s. Zone IfI was observed o&y when the testing ~~dit~ns were very severe (very high toad in dry and abrasion ~~d~t~ous~. In aft the other instances it did not occur, not even when, owing to the diction of the test, the coating was about to disappear from the friction zone. Such is the case of the test carried out under lubrication conditions shown in Fig. 6, corresponding to an A1203 coating, at 1360 N and 150 rev min-‘. On the other hand, Fig. 5 shows the higher range of AD, in zone 1, from 0 to 10 000 cyctes, and even the presence of some negative values caused by the adherence of steel particles to the ceramic coating, as electron microprobe analyses showed later, The remarkable maierial ~oose~~ug that takes place at this stage, which impfies a high wear rate, is caused by the farge amount of partiefes with tittle adherence present on the outer iayer of the mating. Such particles come ob easily and at random, ihus gj~~ug rise to a si~~~~~t va~abj~~~ in the wear rate. Of outstanding ~m~~rtauce~ however, is the high ~b~sio~ and good adherent of the coatings on the substrate, as welf as their excehent wear pe~o~a~ce, especiafly under high ~ub~cat~on conditions. This is probably because af the high hardness of the ceramic coating and the porosity produced by the plasma spray system.
Test rest&s have agreed with the e~~at~~~ expressed in (4) and (5) of Czkhos’ mudet. The Wing of the data has been ~~er~~~~d~ botk for t&e c~~~~~~ oxide and ahnina ~~~t~~~~~ under the d~~~~e~~friction ~~d~r~o~~,ZLSF&s. 7 and 8 show* On the uther hand, both figures show the ~u~ue~~ of the load, which has turned out to be a relevant factor in the wear process, as expressed in Arebard’s fo~~~at~on* ~everthe~ess~ as regards the speed ranges used in the tests, it is possible to say that the infhzence of speed is not rekvantP ~tho~gh as speed increases wear decreases shghtfy, which agrees with what other authors 161 have stated for compact ceramic materials. At present, research is being carried out on the effect of speed using ranges of 0 to 3000 rev min-“.
200
loo
”
0
400~
2oQo
Fig. 7. Fitting of test results to the running-in coatings under dry conditions.
N(cycles)
bOO0
zone of Czichos’ model for aluminium
oxide
-I
0
soooo
Fig. 8. Fitting of test results to the steady-state coatings under dry conditions.
N(cjcles)
104000
zone of Czichos’ model for aluminium
oxide
4.4. Verification of Archard’s equation Once the densities of the coatings were determined, taking their porosity and microhardness into account, eqn. (8) was used and thus the values that characterize p found: dry, 1.5 X lo-‘; abrasion, 2X 10e4; lubrication, 6 X lo-‘. Considering the values of the factors provided in Table 1 and using eqn. (9) the relationship between Dz/D1 (Dz is the wear in AlzOS and D1 the wear in CrZ03) would range from 0.9 to 1.7. This fact has been verified experimentally, as shown in Fig. 9. Adapting Archard’s equation would provide a simple way to build relative wear scales taking one coating as reference and using the density and microhardness of the layers as measuring parameters.
229
0
Fig. 9. Wear conditions.
100
relationship
Fig. 10. Chromium
5. Mic~st~ctu~
of chromium
oxide coating.
CHwQn-
and aiuminium
200
oxide coatings
Fig. 11. Aluminium
under
the same test
oxide coating.
and f~ctogra~~y
Before submitting the blocks to tests, they were observed with an optical microscope. Figure 10 relates to a CrZ03 coating, and Fig. 11 relates to an A1203 coating. The steel base, the fine bond layer and the ceramic coating can be seen in both photographs. Both ceramic coatings demonstrate a stratified morphology and the presence of porosity, the latter being more accentuated in the CrzO, coating because of its higher melting point. Also obvious is the grain deformation in the union zone, owing to the impact of the spraying. When tested, and previously metallised, the blocks were examined using a scanning electron microscope. Figures 12-16 illustrate the most important aspects observed. Figure 12 represents the limit between two zones - worn and not worn - in a dry test with a chromium oxide coating. In this limit one can observe the stratified structure, which is a consequence of the arrangement of the Cr203 particles. The morphology of this border suggests a brittle-type fracture typical of ceramic materials. Figure 13 shows the worn zone in the same block. One can see the existence of flat zones, polished by rubbing, together with other areas that have not been deformed
Fig. 12. Limit between the worn zone (lower part) and not worn zone (upper part) of a chromium oxide coating, tested under dry conditions with a load of 408 N at 150 rev min _‘. Fig. 13. Flat zone of chromium
Fig. 14. Nehvork
oxide coating,
of cracks in previously
Fig. 1.5. Appearance
of a chromium
plastically
deformed
under compression,
de1 armed zones.
oxide c tating fractured
by wear.
during the test. The generation of these fat zones would suggest a plastic deformation in the Cr,Oa coating, assisted by the hi ;h contact temperatures (taking into account the low thermal conductivity of this ma :erial and the high load used in this test). One of the flat zones, observed in I ‘ig. 13 is shown in magnified form in Fig. 14. Ire generated which in the end provoke the Cracks, apparently of a brittle nature, loosening of particles of material. The surface aspect, once individuai loosening of the particles has begun, can be seen in Fig. 15. Also clearly shown are the rounded f ~rrns of the juxtapositioned particles, indicative of at least a partial fusion during the I xaying process. In the alumina coatings, which are : morphologically similar to the Crz03 coatings, although with a higher melting point, on, can observe a similar behaviour in the wear mechanism. The rounded surfaces of the Ala03 particles, their stratified arrangement, and the progressive brittle failure of the different strata that constitute the hard layer are shown in Fig. 16. From a microscopic study it is possible to conclude that the wear mechanism that affects these thin, plasma-sprayed coatings takes place according to the following stages.
231
Fig. 16. Appearance of an aluminium oxide coating fractured by wear. (I) An initial stage, ~~sist~n~ of the appearance of Aat zones resulting from plastic deformation of the ceramic particles owing to the specific high pressures at the actual contact zones and to the high temperatures in that area. (2) A crack nucleation and propagation stage in the previously deformed area probably caused by material fatigue. (3) A third stage of fracture and individualized loosening of particles. These three stages represent the ~~tinuous cycle of the wear mechanism, where detachment of the deformed particles from the surface allows a second Iayer of ~~darna~ed particles to appear the wear mechanism of which follows the abovementioned process. This process then continues and therefore the wear rate would be constant (zone II is the steady-state zone) until the fayer disappears, This is confirmed by the test results, especially under lubri~~tion conditions (see Fig. 6).
7. Conclusions (1) Wear of the ceramic coatings follows Czichos’ model in the running-in and steady”state zones. The limit between both zones was perfectly defined using the study of the evolution of the wear rate. This parameter turned out to be highly ~nfo~ative for the development of the process. The failure zone only occurred during dry and abrasion tests for high load values, with pressures higher than the adherence resistance and internal cohesion of the substrate. In such instances the process resulted in destruction of the coating. (2) Archard’s equation, involving micro hardness and density, is hefpfuf when setting the differences between the layers and to elaborate the relative wear scales taking one coating as reference, (3) The micrographic study seems to explain the wear mechanism operating in these layers, which takes place as follows. (a) Plastic deformation of ceramic particles under compression, (b) Crack nucleation owing to fatigue and to the irregularity that characterizes the subsurface of the layers. (c) Ind~idu~~d loosening of particles. The same mechanism would be repeated an the new contact layers. This cyclic process would account for the permanence of the steady-state zone, observed particularly during lubrication, until the layer disappears.
232
Acknowledgments This work was undertaken within the framework of the Ministry of Education and Science Research Project, Physical and Chemical Aspects of Wear in Moving and Surface Treated Mechanical Systems, developed jointly between the tribology unit of CSIC (Madrid) and the E.T.S.I.I. at Gijon, of the University of Oviedo, and financed by CICYT. This research was also carried out with the help of the following research project, funded by FICYT, under the auspices of the Principado de Asturias: Study of the Execution and Wear, Corrosion and Heat Resistant Properties of Plasma Sprayed Coatings.
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