Tribological behaviour of composite materials based on clinker portland reinforced with oxides

Wear 237 Ž2000. 107–115 www.elsevier.comrlocaterwear

Tribological behaviour of composite materials based on clinker portland reinforced with oxides N. Anton J.M. Torralba ´ ) , F. Velasco, M.A. Martınez, ´ Materials Science and Metallurgical Engineering Department, UniÕersidad Carlos III de Madrid, AÕenida de la UniÕersidad 30, E-28911 Leganes, ´ Madrid, Spain Received 19 May 1999; received in revised form 22 September 1999; accepted 22 September 1999

Abstract The wear behaviour of composites based on clinker portland reinforced with 3, 6 and 9% by weight of different oxides Žalumina, magnesia and silica., using a ‘‘pin-on-disk’’ test was studied. All materials were prepared by dry mixing in a ball mill and compacting at 180 MPa using cold isostatic pressing ŽCIP.. The final sintering was carried out at 14008C in air. After sintering, all samples were polished to 0.8 mm and their wear behaviour was evaluated in a tribometer against a spherical pin of alumina Ž1900 HV.. Friction coefficients and wear rates were obtained. The wear behaviour study was completed with a microstructural study by optical and scanning electron microscopy ŽSEM.. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Clinker portland; Pin-on-disk test; Cold isostatic pressing

1. Introduction Industry improves productivity through the fabrication of components of higher endurance, and one way of lengthening the life of structural components is to improve their wear resistance. But this improvement affects the total costs, making the components more expensive, so the tendency is to find new materials with longer working life and more resistance to wear at a lower cost. The design of components and materials leans on two pillars: Ža. verification of the wear in service, and Žb. reducing it to acceptable levels. For this reduction, it is necessary to establish the mechanism of predominant wear w1x. Tribology analyses industrial problems such as the reliability, maintenance and wear of components, and greatly influences costs in many industrial applications w2x. Nonlubricated wear and friction in ceramic materials, depend on the conditions of slip, temperature, and humidity. In a first approach, the abrasion wear of a material could be measured as the inverse of the cutting or milling grades w3,4x. Several studies have reported the influence of

) Corresponding author. Tel.: q34-91-624-99-14; fax: q34-91-62494-30; e-mail: [email protected]

temperature on the wear of ceramics such as Si 3 N4 w5x, SiC w6,7x, Al 2 O 3 w8x and PSZ w1x, as well as the influence of humidity w6,9x. Ceramic components and ceramic matrix composites ŽCMCs. form tribological pairs with other ceramics, metals andror alloys w10x. There are tribological studies of CMCs, normally reinforced with whiskers, such as Al 2 O 3 –ZrO 2 w11x, Si 3 N4 –BN w12x, Si 3 N4 –TiC w13x, Si 3 N4 –TiN w14x, SiC–TiC w14x, B 4 C–TiB 2 –W2 B 5 w15x, Al 2 O 3 –TiN w16x, Al 2 O 3 –graphite w17x, Si 3 N4 –graphite w17x, forming ceramic–ceramic or metal–ceramic pairs. Clinker is normally used for structural applications, as a component of hydrated cement. However, the main idea of this work is the use of nonhydrated clinker as a raw material in low-cost structural ceramics. From the clinkering process products are obtained of adequate chemical and mineralogical composition. In some cases, corrective components may be added. Clinker is composed of at least two-thirds calcium silicates, the remainder being Al 2 O 3 , Fe 2 O 3 and other oxides w18x; the MgO content should be below 5% w19x. Mineralizers such as CaF2 w20x and CaCl 2 , or techniques such as microwave sintering w21x, allow a 100–1508C reduction of the temperature of clinkering, activating the process at temperatures below 13008C w22x. Few comparative works on this material as a structural ceramic consider their sinterability w23,24x, their mechani-

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N. Anton ´ et al.r Wear 237 (2000) 107–115

108 Table 1 Characteristics of clinker portland employed Physical properties

Composition by oxides

Mineralogical composition

ŽFrom Portland Valderrivas. density: 3.13 grcm3 ; specific area: 5586.54 cm2rg; 99% - 45 mm

% SiO 2 , 20.24 % Al 2 O 3 , 6.09 % Fe 2 O 3 , 3.51 % MgO, 1.11 % Sulphates, 1.34 % CaO, 65.72 % CaO free, 1.21 % fire losses, 0.14

% C 3 S, 63.18 % C 2 S, 12.13 % C 3 A, 10.20 % C 4 AF, 10.68

cal properties w25–27x, and their tribological behaviour w28,29x. This study offers an evaluation of the phenomena related to wear of materials based on nonhydrated clinker portland, including a study of friction coefficients and wear rates.

2. Experimental procedure Composite materials based on clinker portland were manufactured by the normal methods of consolidation of conventional and advanced ceramics. Table 1 shows the characteristics of the clinker used as base material and Table 2 shows the characteristics of the added reinforcements. The fabrication process is shown in Fig. 1. The clinker portland was first mixed with the different additives ŽAl 2 O 3 , MgO and SiO 2 ., in amounts of 3%, 6% and 9% by weight, by dry mixing in a ball mill for 30 min, using stainless steel balls of 10 mm diameter. The weight ratio materialrballs was 1r5. The mixtures were checked for adequate homogeneity. The ceramic mixtures, as well as the plain clinker portland, were encapsulated in flexible plastic moulds and the air eliminated to avoid the formation of pores. The samples were consolidated by cold isostatic pressing ŽCIP. in wet bag, at a pressure of 180 MPa. The materials were sintered in air at 14008C, as optimised in Ref. w23x. Finally, the samples were machined to adequate dimensions for the wear tests. The sintering density, following the Archimedes principle w30x, and the hardness Vickers 1 kg ŽHV1 . were

evaluated. A complete microstructural study by optical and scanning electron microscopy ŽSEM. was carried out with a porosity evaluation of the materials using image analysis. To determine the wear behaviour of the materials, the pin-on-disk method was employed. The CMCs were used as disks, and an alumina ball of 1900 HV hardness of 3 mm radius was used as pin. The disks were obtained by cutting and were then polished to a final roughness of 0.8 mm. Table 3 gives the conditions and parameters of the tests which followed recommendations of ASTM G99 w31x. The friction coefficient between the pin and the studied material was measured directly during the test. The wear of each material was evaluated from the mass loss and its conversion to volume loss Žmm3 .. This method is used to compare materials of equivalent density. The equation of conversion is:

Volume loss Ž mm3 . s

Mass loss Ž g . Density Ž grcm3 .

= 1000

Table 2 Characteristics of added reinforcements Additives

Powder characteristics

a-Al 2 O 3

Purity: 99.9%, 98%- 2 mm; density: 3.9 grcm3 ; Alcoa ŽBrazil. Purity: 90.0%; size 95%- 5 mm; density: 3.6 grcm3 ; Panreac ŽSpain. Amorphous; purity: 99%; density: 2.65 grcm3 ; Crosfield Chemicals ŽUK.

MgO SiO 2

Fig. 1. Resume of manufacturing process of CMCs.

Ž 1.

N. Anton ´ et al.r Wear 237 (2000) 107–115 Table 3 Conditions of wear tests Measurements characteristics Friction radius: 6 mm Rotation speed: 159.15 r.p.m. Linear speed: 0.1 mrs

Sliding distance: 377 m Load: 10 N

System characteristics Pin: alumina Ball radius: 3.0 mm Disk: materials ŽCMCs.

Relative humidity: 30% or lower Environment: air, without lubrication Temperature: 20–268C

Then the wear rate was defined by Eq. Ž2.: K Ž mm3 rN m . s

Mass loss Ž g . 3

=1000

Density Ž grcm . =Load Ž N . =distance Ž m. Ž2.

The wear rate of the alumina pin was determined by calculating the volume eliminated, by measuring of the mark on the pin. These results are an average of three tests for each material.

3. Results Figs. 2–4 show the average evolution of the friction coefficient against alumina with the distance. The maximum friction coefficient for all the materials is close to 0.8. Most of the materials present three zones well differentiated as regards wear: the first with a low friction coefficient Žbetween 0.1 and 0.2., the second shows a step up and the last shows maximum values of friction. The most characteristic friction values of each material against alumina are given in Table 4.

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The wear rate as a function of the amount of reinforcement Žalumina, silica and magnesia. against the alumina pin is shown in Fig. 5, together with the wear suffered by the pin is also shown. The composite materials present a slight decrease in wear compared with the plain material, except for those with the addition of silica that present an increase of wear with the amount of reinforcement, although the values are similar. An increase in the pin wear is produced with increased reinforcement with silica and magnesia. In Fig. 6, clinker is seen encrusted on the alumina ball, as evidence of a small transfer of the material from the disk to the pin, and the creation of a third body of an abrasive nature. Fig. 7 shows some of the typical wear tracks of these materials, denoting the influence of the silica on the wear. Scratches Žbrittle behaviour. appear, which are not found on clinker portland. The phenomena of wear can be influenced by the properties of each material. Table 5 presents the main properties of the studied materials that could influence their wear behaviour Ždensity, porosity and hardness.. Fig. 8 illustrates the evolution of the porosity of composite materials compared with that of plain clinker portland. In Fig. 9, the microstructural evolution of the materials with the studied additions.

4. Discussion From the figures showing the evolution of the friction coefficient, it can be affirmed that the addition of particles to clinker modifies the typology of the phenomena of wear produced in the tribological system alumina–clinker. In the case of the material without additions, the response of the

Fig. 2. Friction coefficient of materials reinforced with alumina as compared with plain clinker portland.

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N. Anton ´ et al.r Wear 237 (2000) 107–115

Fig. 3. Friction coefficient of materials reinforced with magnesia as compared with plain clinker portland.

friction coefficient is similar to that of many monolithic materials: the pin Žalumina. is harder than the studied material, so wear is fast, abrasion being the predominant mechanism of the wear. The additions of oxides produce microstructural variations in clinker portland, varying the relationships between existing phases and the appearance of new ones. In some cases, these modifications produce a marked variation in the wear morphology. The alumina reacts with clinker ŽFig. 8b and Fig. 9b., increasing the amount of liquid phase and the quantity of tricalcic aluminate phase ŽC 3 A, 3CaO P Al 2 O 3 ., over those of the base material ŽFig. 8a and Fig. 9a., which directly influences the wear phenomenon. It also generates an

increase in the amount of belitic phase ŽC 2 S, 2CaO P SiO 2 ., lighter in colour, as a secondary reaction with the CaO of clinker, incrementing the content in silica-rich phases. The aluminate phases are presented as forming a net that integrates the structure, a typical phenomenon of liquid phase sintering. With silica addition, a clear reaction is produced between the base material and the reinforcement ŽFig. 9c.: a greater amount of belitic phase ŽC 2 S. appears, against the greater initial amount of alitic phase ŽC 3 S.. A morphologic change occurs in the phases from an equiaxial type Žsimilar to that of plain clinker. to longer grains Žflakes. for high silica additions. There is a decrease in the quantity and size of the porosity as compared with the base material

Fig. 4. Friction coefficient of materials reinforced with silica as compared with plain clinker portland.

N. Anton ´ et al.r Wear 237 (2000) 107–115 Table 4 Resume of friction coefficient results against alumina Composition Clinker portland q3% Al 2 O 3 q6% Al 2 O 3 q9% Al 2 O 3 q3% MgO q6% MgO q9% MgO q3% SiO 2 q6% SiO 2 q9% SiO 2

Average ŽI. a

– 0.162 0.124 0.149 0.153 0.116 0.141 0.128 –a 0.148

Average ŽIII.

Sliding distance Žm.

0.736 0.735 0.757 0.756 0.757 0.735 0.763 0.755 0.728 0.749

0 24.9 166.3 166.3 7.9 107.6 45.7 45.5 0 70.3

a

Without average value in the first zone. Average ŽI. is the average value in the first zone, with minimum friction coefficient; average ŽIII. is average value in the third zone, with maximum friction coefficient; and sliding distance when the change in friction coefficient is produced.

ŽTable 5 and Fig. 8c.. Isolated large-size porosity appears surrounded by belitic formations, starting from an initial particle of added silica that creates a hole on sintering. The reactivity between phases is one of the factors with a clear influence on the wear behaviour, with a more brittle behaviour of the material and therefore a greater wear on increasing the amount of belitic phase. On the other hand, the magnesia reacts apparently with the aluminoferritic phase ŽC 4 AF, 4CaO P Al 2 O 3 P Fe 2 O 3 . or tricalcic aluminate ŽC 3 A. ŽFig. 9d. and not with the other phases. However, it does not present maximal coherence with the base material due to the appearance of porosity associated with the MgO particles and the formation of clusters of reinforcement in certain zones of the microstructure ŽFig. 8d.. In the wear behaviour test with alumina, the addition of reinforcements initially avoids abrasive wear by provoking

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a slip of the pin on the disk with low friction coefficient Žbelow 0.2.. At a certain time, when hard particles are pulled out and the surface uniformity decreases, third body abrasive wear causes a rapid rise of the friction coefficient Ž0.8. and the wear of the material and countermaterial. Although the wear rates of the different materials differ little, it would seem that the number of cycles in which the jump occurs could offer more information about the comparative wear behaviour of these materials. The beginning of the phenomenon of debris dragging and abrasion is clearly represented by the sharp jump in the scheme of the friction coefficient ŽFig. 10.. The materials reinforced with alumina present good coherence between clinker and reinforcement due to the reactions produced, which provoke a quite considerable delay in the friction coefficient ŽFig. 2.. The permanence in this zone of low friction coefficient depends not only on the greater density or the more rounded porosity with the increase of reinforcement, but mainly on the increase of the hardness of the material Žthe formed C 3 A is harder w32x. than the base material and the greater similarity between material and countermaterial. The conjunction of these factors has an important influence on the friction coefficient, but not so much on the wear rate ŽFig. 5.. The influence of the silica with the appearance of a greater number of phases richer in silica means a slight improvement in the friction behaviour. The large porosity associated with silica-rich phases produces an inferior wear behaviour. The material with 9% silica stays longer in the zone with a lower friction coefficient ŽFig. 4., that it is the material with highest density and with low porosity ŽTable 5.. The permanence in the zone of low friction coefficient of materials reinforced with MgO increases in relation to the base material. Porosity clusters are formed in the

Fig. 5. Steady state wear rates of disks and pins of composite materials based on clinker portland and alumina pin.

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Fig. 6. Details of alumina ball used as pin in wear tests. Clinker particles Žoxides and silicates. are seen encrusted on it.

structure and the reinforcement may be pulled out earlier in the case of 9% MgO compared with the other materials. The amount of aluminoferritic phase, acting as a bonding phase between clinker and MgO, remains constant for all MgO additions. So the bond is not possible with 9% MgO, decreasing the cohesion between the particle and base material, which has a direct influence on the friction coefficient that reaches a maximum before lower additions ŽFig. 3.. The wear rate results of the different materials are similar to those of plain clinker. In the wear against

alumina, an increase of the wear rate for the materials reinforced with silica is observed ŽFig. 5., while materials with alumina and magnesia have better wear resistance Ža slightly lower wear rate.. Higher wear of the pin is due to abrasion and pulling out of particles. This phenomena appears when the friction coefficient reaches maximum. The materials with alumina present a lower wear rate with an increase of the amount of this oxide. The wear begins later Žaccording to the friction coefficient. so less debris is produced. Abrasion is not aggressive for the pin nor for the disk with high alumina additions. When the

Fig. 7. Wear tracks for studied materials. Ža. Clinker portland, Žb. detail of Ža., Žc. material with 9% silica and Žd. detail of Žc., scratches produced in wear tracks of materials with silica.

N. Anton ´ et al.r Wear 237 (2000) 107–115

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Table 5 Properties of studied materials in relation to wear behaviour Composition

Sintering density Žgrcm3 .

Hardness HVŽ1.

Reinforcement hardness ŽGPa. 32

Porosity Ž%.

Average size porosity Žmm.

Shape factor

Clinker portland 3% Al 2 O 3 6% Al 2 O 3 9% Al 2 O 3 3% MgO 6% MgO 9% MgO 3% SiO 2 6% SiO 2 9% SiO 2

3.08 3.09 3.11 3.12 3.12 3.17 3.31 3.13 3.16 3.38

309 410 413 338 404 395 381 391 372 380

– 12.00 12.00 12.00 9.20 9.20 9.20 7.12 7.12 7.12

2.52 3.90 3.27 2.57 4.39 2.35 1.47 1.09 0.62 1.03

17.47 25.12 15.30 17.56 14.86 13.09 17.92 14.25 11.90 13.85

0.60 0.67 0.73 0.67 0.67 0.67 0.68 0.61 0.64 0.72

addition is silica, an increase in the belitic phase is produced on increasing the content of this oxide, giving phases with a longer morphology that when eliminated, produce a more aggressive third body abrasion, both of the pin and of the disk. Finally, in the materials reinforced with magnesia, the reinforcement is pulled out at the end of the test, specially in the case of 6% MgO, so the wear is slightly inferior to that of clinker. The phenomenon of pin abrasion is incremented by higher additions where more pulling out is produced and the hardness of the debris is particularly influential ŽFig. 5.. A small transfer of material occurs between disk and pin Žsee Fig. 6., where particles of clinker are seen on the ball, such as calcium silicates and aluminates. Observing

the wear tracks of the materials, better behaviour is found for materials reinforced with alumina, similar to the wear tracks on the plain clinker disk ŽFig. 7a and b., while the materials with silica show scratches and more brittle behaviour that accelerates the wear phenomenon, this being more evident with an increase of the oxide content ŽFig. 7c and d.. In the first case, a smoothing of debris material generates rather than an increase of wear, a ‘‘mattress’’ that avoids this phenomenon, while if scratches are produced, an increment in the wear rate is generated, although materials with silica present lower porosity. Variations occur in the type of wear of composite materials as compared with plain clinker, although in some cases the wear rate increases slightly Žwith silica. and with

Fig. 8. Microstructural evolution of composite materials based on clinker portland. Detail of porosity. Ža. Plain clinker portland, Žb. with 9% alumina, Žc. with 6% silica and Žd. with 6% magnesia. No etching.

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N. Anton ´ et al.r Wear 237 (2000) 107–115

Fig. 9. Microstructural evolution of composite materials based on clinker portland. Phase evolution. Ža. Plain clinker portland, Žb. with 6% alumina, Žc. with 9% silica and Žd. with 3% magnesia. Etched with 13 vol.% acetic acidrethanol.

Fig. 10. Representation of wear abrasive process for these materials.

N. Anton ´ et al.r Wear 237 (2000) 107–115

the other two oxides it is reduced. However, an interesting phenomenon is the permanence in a zone of low friction coefficient that does not present the base material. So these materials could be employed in some industrial applications, particularly if the conditions are not too aggressive Že.g., floor tiles, low-temperature refractory bricks..

5. Conclusions In the evaluation of the wear behaviour of the studied materials, the following wear-related phenomena are observed: Ž1. A small transfer of material occurs between pin and disk, as well as material loss in both materials. The wear mechanism in the most of the cases is three-body abrasion with clinker particles encrustation on the alumina pin. Ž2. The materials present three zones as a function of the test time: the first with a low friction coefficient, corresponding to the slip on the surface of the material; the second with sharp jump; and the third, reaching a high constant friction coefficient, due to the pulling out and dragging of eliminated material Žthird body abrasion.. Ž3. The large number of factors that influence the wear phenomenon do not allow a clear correlation, given the great number of phases these materials present. The wear coefficients are much influenced by the hardness of the material, their porosity and their microstructure with very variable grades of wear. Ž4. In general, reinforcement with alumina and magnesia appears favourable, from the point of view of friction and wear.

Acknowledgements The authors wish to thank Dr. Vicente Amigo´ of Mechanical Engineering Department in Polytechnical University of Valencia ŽSpain. for his help in SEM analysis.

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