WO2015049422A1

WO2015049422A1 - Composite material, process for preparation thereof, use of the composite material, and self-lubricating wear facing part

Info

Publication number: WO2015049422A1
Application number: PCT/FI2014/050752
Authority: WO
Inventors: Simo-Pekka Hannula; Outi SÖDERBERG; Mehmet Erkin CURA
Original assignee: Aalto University Foundation
Priority date: 2013-10-03
Filing date: 2014-10-02
Publication date: 2015-04-09

Abstract

This invention concerns a composite material comprising a hard ceramic matrix having molybdenum (Mo) particles incorporated therein, wherein the molybdenum content in the ceramic matrix is 1–10% by volume with respect of the composite material bulk. The invention also concerns a process for preparing such a composite material. This process comprises the steps of (a) mixing molybdenum powder with powder of ceramic matrix material, and (b) sintering the obtained powder mixture of molybdenum and ceramic matrix material. The invention further concerns a self- lubricating wear facing part suitable for use at temperatures in the range of 20–800°C, the wear facing part comprising at least a layer of, or being made of, a composite material according to the invention.

Description

Composite material, process for preparation thereof, use of the composite material, and self-lubricating wear facing part

Object of the Invention

Composite material comprising a hard ceramic matrix having molybdenum (Mo) particles incorporated therein.

Another object of the invention is a process for preparing the composite material.

A further object of the invention is a specific use of the composite material. A still further object of the invention is self-lubricating wear facing part. Prior art

There is a growing interest in renewal energy and implementing clean and efficient energy solutions into daily applications for sustainability and environmental awareness. Wear and friction related problems have great impact on efficiency of energy consumption [1]. In high temperature applications, protection against wear and reducing friction is a major problem and conventional solutions such as using liquid oils are not effective [2]. When the service temperature is high (> 400 °C) and applied load is above the limit where liquid oils lose their hydrodynamic properties, solid lubricants are used. They are burnished as powders, applied as thin or thick films, or they are used in bulk composites. The main challenges in prior art are covering a large temperature range, resistance to oxidation and humidity, low coefficient of friction (CoF) and especially in composites maintaining the structural strength of the bulk material. It is known that lubrication by metal oxides have a particular advantage over others, they are resistant to oxidation by nature. It is also known that metal oxides can be used directly or used as metals and oxidation takes place during wear process.

In presence of large Mo zones in the microstructure, i.e. when Mo content is 25 vol.% [4], oxidation of Mo increases the wear rate. In certain conditions Mo particles (i.e. > 5 prn) in alumina matrix show brittle like behavior [5], and it is considered the reason for high wear rates [4]. Mild oxidation of the staring powder usually is the reason for this embrittlement. However, smaller Mo particles (1-2 pm) and lower Mo content (1-10 vol.%) have great influence on minimizing it.

Alumina is a cost effective, chemically inert, wear resistant, thermally and electrically insulating, relatively hard material. It has many applications in different engineering systems regarding to its properties. Wear resistant ceramic parts made of alumina already have been utilized in different areas. However, when performing against self-mated surfaces it has relatively high CoF especially at elevated temperatures. Despite some unique features, improving toughness and wear resistance of alumina has been of great interest for a long time. Metals with high melting point are good toughening reinforcements due to their plastic deformability [4, 6-8] and they can improve friction and wear properties [4, 9-10].

Alumina was sintered to 97-98.5% relative density in presence of 25 vol.% Mo by hot pressing at 1550 °C by de Portu et al. [4]. They reported CoF of ~ 0.9 and 0.6 for 10 and 50 N respectively. They observed significant wear when tested against WC above 25 N especially when the particle size of Mo was larger than 5 μιτι. The large particles oxidised more severely and brittle metal oxide was formed on the surface of the particles, thus the wear was more rapid due to brittleness. Another reason was detachment of Mo particles from the alumina matrix when the Mo particle size was higher. Increment of the temperature at the contact region causes, first the formation of M0O3 and then melting and eventually if the generated heat is enough, evaporation of the oxide. Their composites had relatively high CoF but low wear resistance.

Molybdenum can also be used to modify alumina-doped zirconia for tribological improvement. Zhang et al. sintered Zr0₂(3Y) + 5 mol % Al₂0₃ (Y-TZP/A) with Mo content of 10-40 mol% at 1450 °C for 1 h and achieved composites with relative density of 98 % [10]. During hot pressing due to relatively slow heating rate (10 °C/min), the overall process was significantly longer than PECS process. At room temperature CoF of all composites were almost the same, in other words addition of Mo did not have any influence on friction properties. The CoF was 0.86 for Y-TZP/A and between 0.84 and 0.86 in presence of Mo. The same trend was observed at 800 °C for the composites with up to 20 mol% Mo and CoF was about 0.85. It dropped to ~ 0.75 and then to 0.48 only when Mo content was increased to 30 and 40 mol%, respectively. Zhang et al. explained the reason for reduced CoF by homogeneous distribution of Mo particles and lubricating feature of Mo itself.

According to their explanation, homogeneity of distribution of Mo particles was dependent of its amount and was more effective above 20 mol%.

In particulate composites such as MMCs or CMCs, the conventional method is to add/mix reinforcement particles into the matrix, and distribution of these particles in the matrix gives the composite a mixture of properties from both materials.

Functionally graded materials and layered structures are other way of creating a dispersion of attributes. Qi et al. hot pressed laminated AI₂0₃/Mo composites at 1550 °C for 1.5 h and improved mechanical and tribological properties of the material [9]. Hot pressed alumina without any Mo had CoF of 0.9 at RT and about 1.0 at 800 °C under load of 70 N. The laminated AI₂0₃/Mo composite had similar CoF at room temperature but CoF was as low as 0.34 at 800 °C compared to monolithic alumina, and it dropped to 0.25 when the frequency of reciprocating movement of the counter parts was 5 hz. In the laminated composites, the wear mechanism was similar to particulate composites and consisted of oxidation of Mo at an elevated temperature and formation of a lubricious layer on the wear surface. In their analysis of the worn surface by x-ray diffraction (XRD), Qi et al. observed not only Mo0₃ but Mo0_2i8 as well. The CoF was considerably lower, compared to CMCs of Al₂0₃ and Y-TZP/A with Mo, most probably because of much larger and unified regions of (laminated) Mo in the microstructure.

Description of the Invention

In this invention it was surprisingly observed that by using significantly lower amount of Mo amount it is still possible to achieve homogeneous mixtures as well as lower CoF values. The low wear is attributed to the formation of Mo0₃ and Mo₄On in the wear debris and plastic deformation of this layer during sliding against Al₂0₃ counterpart. Formation of Mo0₃ was confirmed by Raman spectroscopy analysis. The composite material according to the invention comprises a hard ceramic matrix having molybdenum (Mo) particles incorporated therein, wherein the molybdenum content in the ceramic matrix is 1-10% by volume with respect of the composite material bulk.

In a preferred embodiment of the composite material composite material of the invention the ceramic matrix comprises Al₂0₃, Zr0₂, MgO, Cr₂0₃, Si₃N₄, SiAION, TiN, SiC, B₄C, or TiC, or a composite thereof. In yet another preferred embodiment of the composite material the ceramic matrix comprises a composite of Zr0₂ and Al₂0₃, at any ratio thereof.

In a particularly preferred embodiment of the composite material the ceramic matrix comprises Al₂0₃.

It is preferable for the composite material of the invention and its' preferred embodiments that the molybdenum content in the ceramic matrix is 10% by volume with respect of the composite material bulk. In a further preferred embodiment of the composite material the molybdenum content in the ceramic matrix is 1-5% by volume with respect of the composite material bulk.

In still a further preferred embodiment of the composite material the molybdenum content in the ceramic matrix is 1-2.5% by volume with respect of the composite material bulk.

It is preferable for the composite material of the invention and its' preferred embodiments that the particle size of molybdenum is 0.1-5 μιτι, preferably 1-2 pm.

It is further preferable for the composite material of the invention and its' preferred embodiments that the ceramic matrix is Al₂0₃ having a particle size of 0.04-5 μιτι, preferably 0.5-1 μιτι. The process of the invention for preparing a composite material according to any one of the claims 1-10, comprises the steps of:

(a) mixing molybdenum powder with powder of ceramic matrix material

(b) sintering the obtained powder mixture of molybdenum and ceramic matrix material.

In a preferred embodiment of the process of the invention the mixing in step (a) is performed by mechanical ball milling. In another preferred embodiment of the process of the invention the sintering in step (b) is performed by electric current sintering using high-pressure moulds, such as graphite, tungsten carbide or silicon carbide moulds.

In a further preferred embodiment of the process of the invention the electric current in step (b) is applied as DC current together with mechanical compression directly to the powder mixture of molybdenum and ceramic matrix material via the mould system.

The sintering in step (b) of the process of the invention is preferably done in vacuum or in a protective gas.

In a particularly preferred embodiment of the process the ceramic matrix material is Al₂0₃. The sintering in step (b) is in that case preferably performed at 1350-1600 °C, preferably 1500-1550 °C, for 5-15 min with a heating rate of 50-200 °C/min and under 50-100 MPa pressure.

The use of a composite material according to the invention comprises the use of the composite material according to any one of the claims 1-10 as self-lubricating material in wear facing applications in a temperature range of 20-800 °C.

The self-lubricating wear facing part suitable for use at temperatures in the range of 20-800 °C, comprises at least a layer of, or is made of, a composite material according to any one of the claims 1-10. In preferred embodiment of the self-lubricating wear facing part of the invention the composite material thereof, when subjected to load, produces a solid lubricant layer comprising at least one molybdenum oxide compound represented by the chemical formula of Mo_n0_3n-i or Mo_n0_3n-2, wherein n = 1, 2, 3..., on the surface subjected to load. If the lubricant layer comprises two or more molybdenum oxide compounds, some of them may belong to molybdenum oxide compounds having the formula Mo_n0_3n-i, and some to molybdenum oxide compounds having the formula Mo_n0_3n-2.

In a further preferred embodiment of the self-lubricating wear facing part of the invention the composite material thereof has a coefficient of friction (CoF) in the range of 0.17-0.40 in a temperature range of 20-800 °C.

When inserting molybdenum (Mo) into a hard ceramic matrix, such as alumina (Al₂0₃), it was surprisingly observed that when the molybdenum content in the ceramic matrix is 1-10% by volume with respect of the composite material bulk it was possible to obtain lower coefficient of friction (CoF) values at a temperature range of from 20-800 °C. The mechanism that is believed to take place is oxidation of Mo into Mo0₃ and Magneli phase oxide (e.g. Mo₄On) during wear at e.g. 400 °C, and formation of a continuous tribo layer of these oxides and reducing CoF as a result.

Mo₄Oii is a member of a homologues series of oxides in the Mo-0 system. Materials as in Mo0_3-x (2<x<3) are characterised with their oxygen deficiency and easy shear planes as a result. They are called Magneli phase oxides. The Mo₄On is believed to be co-responsible for low friction during high temperature wear. However, Mo₄On is believed not to be the only Magneli phase that can be utilized for lowering the friction in the invention. Thus we can utilize Mo0_3-x easy shear oxides in ceramic matrix composites for self lubricating applications.

When sintering e.g. Al₂0₃ and Mo in order to prepare a ceramic composite thereof according to the invention, SPS technique is used. Alternatively, the sintering step in this invention may be performed by hot pressing, hot isostatic pressing or by pressureless sintering.

Further, it is also possible to apply composite material of the invention to a surface (e.g. a wear-surfacing part of some sort), by thermal spraying. In many industrial applications wear resistant or corrosion resistant materials are coated on machine parts by this method for protection. Alumina and related materials are widely applied by thermal spraying for different purposes.

Thermal spraying has different sub-processes but plasma spraying among them is the most suitable for the composite material of the invention.

The self-lubricating ceramic matrix composite materials of the invention can be applied not only by sintering as bulk parts, but also as a protective coating layer on surfaces of wear parts by thermal spraying techniques such as plasma spraying and high velocity oxy-fuel coating spraying (HVOF) in particular[14][15].

Detailed Description of the Invention

In the following the invention is described in more detail by way of example and referring to the following attached drawings.

Figure la is a diagram showing the influence of Mo amount on the CoF of Al₂0₃ composites at RT and 400 °C. Figure lb is a diagram showing the change of CoF for different compositions at increasing temperature.

Figure 2a consists of two photographs as a comparison of wear tracks of pure alumina and Al₂0₃ + Mo at 400 °C.

Figure 2b is a Raman analysis spectrum of the tribo layer of the wear tracks of Al₂0₃ + Mo at 400 °C in figure 2a.

Figure 3 consists of two photographs showing the wear track of Al₂0₃ + 5 vol.% Mo at RT (left) and 400 °C (right) without any measurable wear.

Figure 4a is a diagram showing the evolution of CoF with respect of time for Al₂0₃ with 5 vol.% Mo at 400 °C 5N. Figure 4b is a diagram showing the evolution of CoF with respect of time for Al₂0₃ with 10 vol.% Mo at 600 °C 25N.

Figure 4c is a diagram showing the evolution of CoF with respect of time for Al₂0₃ with 1 vol.% Mo at 800 °C 5N.

Figure 4d is a diagram showing the evolution of CoF with respect of time for Al₂0₃ with 2.5 vol.% Mo at 800 °C 5N. Figure 5 comprises three schematic presentations of the structure of the oxygen- deficient Magneli phases and formation of crystallographic shear planes of (a) Me_n0_3n, (b) Me_n0_3n-i, and (c) Me_n0_3n-₂. (E.Lugscheider et al. 1999, Surf. Coat. Tech.) Figure 6 is a phase diagram of Mo-O. The liquidus temperature of Mo₄On is 818 °C

The ceramic matrix composites of the present invention were produced by a pressure and current assisting sintering technique, which has several names in the literature, i.e., spark plasma sintering (SPS), field assisted sintering (FAST) or pulsed electric current sintering (PECS), or alike [3]. First, molybdenum powder (1-2 pm) was mixed with alumina powder (0.5-1 μιτι) in powder form by mechanical ball milling and then powder mixtures were sintered in PECS using graphite moulds. The pulsed DC current was applied together with mechanical compression directly to powder via mould system to sinter the powder. The sintering was done in vacuum or in protective gas. Rapid heating allowed very short process times (0.5 h) and fully dense (no porosity) Al₂0₃ + 1-10 vol.% Mo composite bulk compacts to be produced. The cylindrical compacts were sintered at 1500 °C for 10 min with a heating rate of 50-200 °C/min and under 50-100 MPa pressure. The surface of the samples was mechanically grinded and polished at RT. The sintered bulk composites had CoF below 0.4 at RT or 400 °C All composites, had CoF lower than pure alumina except for Al₂0₃ + 10 vol.% Mo which had CoF of 0.7 at RT (Fig.l). In pin- on-disk wear tests, against alumina counterpart formation of a very thin but continuous tribo layer was observed at both RT and 400 °C (Fig.2a). Compositional analysis by Raman spectroscopy showed two different oxide phases within this layer, namely Mo0₃ and Mo₄On along with Mo and Al₂0₃ (Fig.2b). During wear due to friction of two counter parts considerable amount of heat is generated, thus metal particles can oxidise even during room temperature wear. If there is enough load and heat oxide phases usually favour the reduction of CoF, by going under plastic deformation. The soft oxide phase (especially the Magneli oxide Mo₄On) is smeared on the worn surface due to easy shear, forming a soft and lubricous tribolayer. If this layer is continuous, it can significantly reduce the CoF and wear rate. In our invention, to the contrary, of the previous reports (Table 1), we used considerably lower amount of Mo and obtained improved friction properties and wear was reduced significantly, too [11]. Homogeneous mixture of two phases was achieved by mechanical ball milling independent of Mo amount. The composites were sintered by PECS which is a much more rapid process compared to hot pressing or other conventional pressure assisted sintering methods, although hot pressing could also be used. We succeeded in consolidation of fully dense (100 % relative density) composites at the same temperature with previous examples where the final relative densities were around 97-98 %. Alumina molybdenum composites did not have any measureable wear when tested against alumina counterpart under load of 5N at room temperature or 400 °C (Fig.3). While the CoF was increased for pure alumina from 0.4 to 0.8 when the temperature was increased from RT to 400 °C, in presence of Mo, CoF was dropped from 0.38 to 0.26 (except for 10 vol.% Mo, which had CoF of 0.72 at RT). The lowest CoF at 400 °C was measured for Al₂0₃ + 5 vol.% Mo, which was 0.26 (Fig.4a).

We tested our materials with different friction test setups and achieved very comparable results independent of equipment features. We had CoF values as low as 0.17 for 10 vol.% Mo at 600 °C (Fig.4b) and under load of 25N, 0.18 and 0.19 for 1 - 2.5 vol.% Mo at 800 °C (Fig.4c,d) under load of 5N, respectively [12].

While applying load of 5N, because of smaller diameter of the counter alumina ball, under a higher pressure at 600 °C the CoF dropped from 0.42 for 1 vol.% Mo addition to 0.36 for 5 vol.% Mo added composite. Under the same loading regime, the friction was reduced enormously at 800 °C and 1 - 2.5 vol.% Mo containing composites had CoF of 0.18 and 0.19 respectively. The CoF was higher for the composites with 5 - 10 vol.% Mo, 0.28 and 0.24 respectively. At 600 °C CoF is higher than at 400 °C, however, the Hertzian pressures are different in each test temperature. At 600 °C when the load is increase to 25N, the CoF of composite with 10 vol.% Mo drops to 0.17, which is 0.46 under 5N. Also CoF drops from 0.36 to 0.28 for 5 vol.% Mo.

We have observed the existence of a sub-stoichiometric oxide phase along with M0O3 by Raman spectroscopy analysis. The observed phase was Mo₄On, which is an intermediate compound that appears during reduction of M0O3 by hydrogen flow at 580 °C It is a member of homologous series of molybdenum oxide that are represented with the chemical formula of Μο_η0_3η-ι. By nature, they have oxygen deficiency that gives a slight distortion from their original lattice (Fig.5). As a result, they have easy shear crystallographic planes in their crystal lattices. Under load these easy shear planes can behave similar to lamellar solids, which have weakly bonded layers stacked on top of each other, and under applied load these bonds easily break and layers slide. In presence of easy shear planes, the bonding is the same with their atomic bonding in the main lattice. However, the shear planes do not dissipate as in lamellar solids and the low friction is maintained by formation of a continuous tribo layer on the materials' surface. Therefore, material removal from the materials' surface is not severe. According to Mo-0 binary phase diagram, the stoichiometric range is very narrow, for this reason their synthesis is very difficult and energy intensive. Use of molybdenum oxide has been of interest as a solid lubricant within different matrix materials [2]. Regarding its low strength M0O3 cannot be used as a reinforcement phase in a composite. Instead having a very high melting point and good ductility Mo was used in different hard matrixes as discussed earlier. Another way of achieving lubricious Mo0₃ and exploiting its low friction properties in alumina molybdenum composites is annealing of the consolidated bulk parts in air, before wear tests in order to oxidize Mo and form Mo0₃ on the surface of Mo particles [13]. This two-step approach is not practical and requires additional energy and time. The mechanism for lowering the friction is also solely dependent on the formation of Mo0₃. We believe formation of Mo₄On is essential for a continuous and lubricious film consisted of alumina and metal oxide. Conventional and low cost ceramic matrix composites of alumina and molybdenum, with high strength and hardness, offer excellent friction properties at room temperature as well as at elevated temperatures when produced by PECS technique. In addition, pressure assisted rapid sintering nature of PECS reduces the production cost of parts made of this composite for wear applications. We believe application of Mo in alumina is a very feasible and effective way of exploiting the low friction property of oxygen deficient oxides, which have clear advantages over lamellar solid lubricants. Therefore, it is very beneficial to use alumina molybdenum composites as self-lubricating material for high temperature applications.

Table 1. Comparison of Published Data

Proces Difference to our

Reference Material Observation

s approach

CoF at RT 0.40, at 400°C

0.26, at 600°C 0.17, At

800°C 0.18. A

continuous tribofilm

consisting of Magneli

Al₂0₃ + oxide and Mo0₃ forms

Our materials 1-10% PECS during wear at 400°C.

Mo No measureable wear at

400°C. Considering the

melting point of Mo₄Ou

and Mo0₃ (Figure 6.),

effective temp, range will

be RT-800°C

G. de Portu, S. Al₂0₃-

Results only at RT; no Guicciardi, C. 25vol% CoF of ~ 0.9 and 0.6 for

high temperature data. Melandri, F. Mo HP 10 and 50 N respectively

The tested counter Monteverde (Ιμιη, 5 at RT

material was WC. (2007) μπι)

The CoF was 0.86 for Y- TZP/A and between 0.84

Zr0₂(3Y) and 0.86 in presence of The matrix was + 5 Mo. The same trend was zirconia-alumina mol% observed at 800 °C for matrix. Uniformity of

Y.-S. Zhang, L- Al₂0₃ (Y- the composites with up the microstructure was T. Hu, J.-M.

TZP/A) HP to 20 mol% Mo and CoF maintained by Chen, W.-M. Liu

with Mo was about 0.85. It increasing Mo content. (2010)

content dropped to ~ 0.75 and Material is tested of 10-40 then to 0.48 only when against Si₃N₄ mol% Mo content was counterpart.

increased to 30 and 40

mol%, respectively Hot pressed alumina

without any Mo had CoF

of 0.9 at RT and about

The alumina

1.0 at 800 °C under load

composites have a of 70 N. The laminated

laminated structure. Mo AI₂0₃/Mo composite had

layers are used instead similar CoF at room

Y.-e. Qi, Y.-S. laminate of dispersed Mo

temperature but CoF was

Zhang, L.-T. Hu d HP particles. The effect of as low as 0.34 at 800 °C

(2012) AI₂0₃/Mo Mo on CoF can be compared to monolithic

benefited only on a alumina, and it dropped

cross sectional surface. to 0.25 when the

The counter material frequency of

for tests was alumina. reciprocating movement

of the counter parts was

5 hz

H. Takene, K. The alumina

Hidenori molybdenum

(2001), (Patent, composites were Isuzu Ceramics annealed in air after

Mo was added 1-50

Res Inst), sintering for forming

AI₂0₃/Mo vol.%. and composites

JP2001058868, Sinterin M0O₃. In our case com posit were sintered at 1800 °C

"Low friction lubricious metal oxides e g in Ar. They are annealed

ceramic and form during high

in air at 600 °C.

production temperature wear, process for where the composites coating layer were aimed to perform, made thereof" in both cases.

Acknowledgements Finnish Funding Agency for Technology and Innovation (Tekes) is acknowledged for the financial support. Ms Ulla Kanerva is thanked for powder preparation and Mr. Simo Varjus is thanked for pin-on-disk tests.

References

[1] K. Holmberg, P. Andersson, A. Erdemir, Global energy consumption due to friction in passenger cars, Tribology International 47 (0) (2012) 221-234.

[2] M.E. Cura, O. Soderberg, S.-P. Hannula, Self-lubricating nano- and

microcomposites for room and elevated temperatures, in: L. Magagnin (Ed.) Engineered metal matrix composites: Forming methods, material properties and industrial applications, Science Nova Publishers Inc., New York, 2013, pp. 193-242.

[3] R. Orru, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Consolidation/synthesis of materials by electric current activated/assisted sintering, Materials Science and Engineering : R: Reports 63 (4-6) (2009) 127-287.

[4] G. de Portu, S. Guicciardi, C. Melandri, F. Monteverde, Wear behaviour of AI203-MO and AI203-Nb composites, Wear 262 (11-12) (2007) 1346-1352.

[5] O. Sbaizero, G. Pezzotti, Influence of residual and bridging stresses on the r- curve behavior of mo- and feal-toughened alumina, Journal of the European Ceramic Society 20 (8) (2000) 1145-1152.

[6] E. Kannisto, M.E. Cura, E. Levanen, S.-P. Hannula, Mechanical properties of alumina based nanocomposites, Key Engineering Materials 527 (2012) 101-106.

[7] LA. Diaz, A. F. Valdes, C. Diaz, A.M. Espino, R. Torrecillas, Alumina/molybdenum nanocomposites obtained in organic media, Journal of the European Ceramic Society 23 (15) (2003) 2829-2834.

[8] T. Rodriguez-Suarez, L.A. Diaz, R. Torrecillas, S. Lopez- Esteban, W. H. Tuan, M. Nygren, J.S. Moya, Alumina/tungsten nanocomposites obtained by spark plasma sintering, Composites Science and Technology 69 (14) (2009) 2467-2473.

[9] Y.-E. Qi, Y.-S. Zhang, L.-T. Hu, High-temperature self-lubricated properties of AI203/MO laminated composites, Wear 280-281 (0) (2012) 1-4.

[10] Y.-S. Zhang, L.-T. Hu, J.-M. Chen, W.-M. Liu, Lubrication behavior of Y- TZP/AI203/MO nanocomposites at high temperature, Wear 268 (9-10) (2010) 1091- 1094.

[11] (submitted - Tribology International) M. E. Cura, X. Liu, U. Kanerva, S. Varjus; A. Kivioja, O. Soderberg, S.-P. Hannula, Friction behavior of alumina/molybdenum composites and formation of Mo03-x phase at 400 °C.

[12] Unpublished data.

[13] H. Takene, K. Hidenori (Isuzu Ceramics Res Inst), 2001, JP2001058868, "Low friction ceramic and production process for coating layer made thereof".

[14] G. Bolelli, L. Lusvarghi, T. Manfredini, F. Pighetti Mantini, E. Turunen, T. Varis, S-P. Hannula. "Comparison between plasma- and HVOF-sprayed ceramic coatings. Part II: tribological behaviour", Int. J. Surface Science and Engineering, VI, p62, 2007. DOI: 10.1504/IJSURFSE.2007.013621.

[15] Turunen, E. ; Varis, T. ; Hannula, S.-P. ; Vaidya, A. ; Kulkarni, A. ; Gutleber, J. ; Sampath, S. ; Herman, H.. "On the role of particle state and deposition procedure on mechanical, tribological and dielectric response of high velocity oxy- fuel sprayed alumina coatings", Materials Science & Engineering: A, v415, pi, 2005. DOI: 10.1016/j.msea.2005.08.226.

Claims

1. Composite material comprising a hard ceramic matrix having molybdenum (Mo) particles incorporated therein, wherein the molybdenum content in the ceramic matrix is 1-10% by volume with respect of the composite material bulk.

2. Composite material according to claim 1, wherein the ceramic matrix

comprises Al₂0₃, Zr0₂, MgO, Cr₂0₃, Si₃N₄, SiAION, TiN, SiC, B₄C, or TiC, or a composite thereof.

Composite material according to claim 2, wherein the ceramic matrix comprises a composite of Zr0₂ and Al₂0₃, at any ratio thereof.

Composite material according to claim 2, wherein the ceramic matrix comprises Al₂0₃.

Composite material according to any one of the claims 1-4, wherein the molybdenum content in the ceramic matrix is 10% by volume with respect of the composite material bulk.

Composite material according any one of the claims 1-4, wherein the molybdenum content in the ceramic matrix is 1-5% by volume with respect of the composite material bulk.

Composite material according to claim 6, wherein the molybdenum content in the ceramic matrix is 1-2.5% by volume with respect of the composite material bulk.

Composite material according to any one of the claims 1-7, wherein the particle size of molybdenum is 0.1-5 μιτι, preferably 1-2 pm.

9. Composite material according to any one of the claims 1-8, wherein the ceramic matrix is Al₂0₃ having a particle size of 0.04-5 μιτι, preferably 0.5-1 pm.

10. Process for preparing a composite material according to any one of the claims 1-9, comprising the steps of:

(a) mixing molybdenum powder with powder of ceramic matrix material

11. Process according to claim 10, wherein the mixing in step (a) is performed by mechanical ball milling.

12. Process according to claim 10 or 11, wherein the sintering in step (b) is

performed by electric current sintering using high-pressure moulds, such as graphite, tungsten carbide or silicon carbide moulds.

13. Process according to claim 12, wherein in step (b) the electric current is

applied as DC current together with mechanical compression directly to the powder mixture of molybdenum and ceramic matrix material via the mould system.

14. Process according to any one of claims 10-13, wherein the sintering in step (b) is done in vacuum or in a protective gas.

15. Process according to any one of the claims 10-14, wherein the ceramic

matrix material is Al₂0₃.

16. Process according to claim 15, wherein the sintering in step (b) is performed at 1350-1600 °C, preferably 1500-1550 °C, for 5-15 min with a heating rate of 50-200 °C/min and under 50-100 MPa pressure.

17. Use of a composite material according to any one of the claims 1-9 as self- lubricating material in wear facing applications in a temperature range of 20-

800 °C.

18. Self-lubricating wear facing part suitable for use at temperatures in the range of 20-800 °C, the wear facing part comprising at least a layer of, or being made of, a composite material according to any one of the claims 1-9.

19. Self-lubricating wear facing part according to claim 18, the composite material thereof, when subjected to load, producing a solid lubricant layer comprising at least one molybdenum oxide compound represented by the chemical formula of Mo_n0_3n-i or Mo_n0_3n-2, wherein n = 1, 2, 3..., on the surface subjected to load.

20. Self-lubricating wear facing part according to claim 18 or 19, the composite material thereof having a coefficient of friction (CoF) in the range of 0.17- 0.40 in a temperature range of 20-800 °C.

21. Self-lubricating wear facing part according to any one of the claims 18-20, the wear facing part comprising at least a layer of a composite material according to any one of the claims 1-9, said layer being coated on the wear facing part by thermal spraying techniques such as plasma spraying and high velocity oxy-fuel coating spraying (HVOF) in particular.