Properties of Sintered Al 2 O 3Cr Composites Depending on the Method of Preparation of the Powder Mixture

Continuous progress in modern science and industry depends on the availability of new effective devices and materials. New generation materials should be characterised by a specified combination of properties, which sometimes exclude one another. Al2O3-Cr composites belong to this group of materials. This study is concerned with the effect of the method of preparation of the starting powders upon the properties of sintered Al2O3-Cr composites. The composites were produced using powder mixtures with various volumetric shares of the starting powders (from 25 to 75vol.%). The mixtures were prepared by conventional mechanical mixing in a ball-mill or by mechanical alloying in a high-energy mill of the attritor type. It has been found that with mechanically alloyed powders the Al2O3Cr composites have better bending strength, hardness and frictional wear resistance.


Introduction
To meet the requirements for machines and devices of increasing efficiency demanded by modern science and technology it is necessary that not only new original constructions should be designed but also new materials of improved mechanical, thermal, electrical and operational properties should be developed.Several of these properties must often occur in combination, which is even more difficult to achieve.
The barrier of unavailability of appropriate materials, which is faced in certain technical fields, has stimulated the continuous and rapid development of composite materials, such as e.g.ceramic-metal composites, which combine good plastic properties of metals with the high mechanical strength of ceramic materials.In this group we can for example mention Al 2 O 3 -Cr composites, which are resistant to thermal shocks and to oxidation at high temperatures (even to 1500 o C), have a high mechanical strength and a high hardness [1,2].These composites are chiefly produced by the sintering technique.
The properties of sintered composite materials chiefly depend on the technological parameters of the sintering process (temperature, pressure, process duration), but also on the geometrical parameters of the starting powders, such as [3]: _____________________________ *) Corresponding author: wydzial4@pan.plgrain size and roughness of the grain surfaces, type and form (particles, fibres) of the reinforcing phase, distribution of the reinforcing phase within the matrix, kind of the matrix, and type of the bonds at the reinforcing phase/matrix interface.The selection of the kind and form of the materials intended to be used for the matrix and reinforcement is determined by the current needs and expectations.The materials selected for a given purpose in turn determine the type of bonds (mechanical, adhesive, diffusion-type, reactive) at the interface between the individual phases [4].We can expect that one of the possibilities of modifying the properties of sintered composite materials may be proper selection of the method of preparation of the powder mixtures.The methods, most often employed, of fabricating ceramic/metal composite powders, include conventional mixing in ball-mills, sputtering of the metal powder accompanied by injection of dispersive ceramic powders (spray forming) and mechanical alloying in high-energy mills [5].The simplest method consists of mixing the powders intended for the matrix and the reinforcement, then pressing them and sintering.This method does not however ensure a uniform distribution of the reinforcing powder throughout the matrix, since the reinforcement particles gather in the outer zone of the mixture.When the grain sizes of the two powders are significantly different, we face another difficulty in that the reinforcing phase forms agglomerates as a result of the intensive action of the van der Waals inter-particle forces [6].
The uniform distribution of the particles of the reinforcing phase within the composite matrix can be achieved by mechanical alloying.In this process (developed by J.S.Benjamin at the INCO Company [7]), which consists of mixing the matrix and reinforcement powders in a high-energy mill, the introduction of the reinforcement powder into the matrix is accompanied by cyclic plastic deformations, cold welding and particle disintegration [8].This method has found wide application in the production of ceramic-reinforced composites based on nickel-alloys [10], copper-alloys [11] and chromium-alloys [12].
The present study is concerned with producing the Al 2 O 3 -Cr composites with various proportions of the starting constituents, and examining their properties.The composites were produced of powders prepared using conventional mixing in a ball mill and powders prepared by mechanical alloying in a high-energy mill of the attritor type.

Experimental procedures
The powders used in the study were a chromium powder with an average grain size of 40µm (NewMet Koch) and an aluminium oxide powder with an average grain size of 1µm.
The proportions of the volumes of the individual powders in the Al 2 O 3 -Cr mixture examined were: - To examine how the powder mixture preparation method affected the properties of Al 2 O 3 -Cr composites, two types of mills were employed: a conventional ball mill with an addition of ceramic pellets and a high-energy mill of the attritor type.
Fig. 1 shows examples of the microstructure of a powder mixture of the composition: 25%Al 2 O 3 + 75%Cr prepared by mechanical mixing and by mechanical alloying.
Microstructural examinations of the mixture of the Al 2 O 3 and Cr powders prepared by milling have shown that the best homogeneity of the mixture is achieved after mixing for 8h.At shorter milling times, the powders are not mixed completely, whereas above 10h, large agglomerates of the ceramic grains form.With mechanical alloying, the optimum mixing time ranges from 16 to 24h depending on the volumetric shares of the powders.Before sintering, the powders were pressed in a steel die under a pressure of 0.2MPa to form samples 10x10mm in size.Then, the samples were densified in an isostatic press (120MPa), placed in a graphite die and sintered in a hot-press (of the ASTRO type) in an argon atmosphere at a temperature of 1400 o C. All the samples, irrespective of their composition, were sintered under a pressure of 30MPa for 60min.

Density of the Al 2 O 3 -Cr composites
The density of the Al 2 O 3 composites was determined by the hydrostatic method.In calculating the theoretical values of the densities for the assumed volumetric shares of the constituents of the composites, the density of alumina oxide was taken to be ρ Al2O3 = 3.97[g/mm 3 ], and the density of chromium to be ρ Cr = 7.19[g/mm 3 ].The values obtained for the composites were: ρ T = 6.385[g/mm 3 ] for the 25Al 2 O 3 -75Cr composites, ρ T = 5.58[g/mm 3 ] for the 50Al 2 O 3 -50Cr composites, and ρ T = 4.775[g/mm 3 ] for the 75Al 2 O 3 -25Cr composites.Based on these data, we determined the relative density of the composites.The densities and the relative densities of the composites are given in Tab.I using the designations: MICRO -the powder mixture prepared by conventional mixing, and MA -the powder mixture prepared by mechanical alloying.

Tab. I
The density examinations have confirmed that, with all the powder mixtures employed, we can produce Al 2 O 3 -Cr composites with a density close to the theoretical value.For the same process parameters, better results were obtained when the powder mixture was prepared by mechanical alloying, but the difference in the relative density of the composites was not significant (within 1%).

Microstructure of the Al 2 O 3 -Cr composites
The microstructural examinations were performed in a stereoscopic microscope.Examples of the results are shown in Fig. 2 The microscopic observations have confirmed that, irrespective of the mixing method employed, the relative density of the Al 2 O 3 -Cr composites is high.We can see small pores in the composite structure, their number being the greatest in the 75%Al 2 O 3 -25%Cr composites.All the samples have a highly homogenous structure.The shape of the chromium powder grains (light areas) and the great differences in their sizes are related to an effect of the mechanical alloying.The grains are elongated and are arranged in the direction perpendicular to the direction of the action of the pressure applied during the sintering process.

Hardness of the Al 2 O 3 -Cr composites
The hardness of the composites was measured using a Vickers hardness-meter, in accordance with the current Standards [13] (load -10kG, loading time -10s).In each sample, the measurement was taken three times.The hardness was determined from the size of the indentation.The average measured values of the hardness are shown in Fig. 3.The results indicate that the hardness increases with increasing proportion of the reinforcing phase (Al 2 O 3 ) in the composite.In the 25%Al 2 O 3 +75%Cr composite, whose average hardness was about 240kG/mm 2 , no effect of the powder mixing technique on the composite hardness was observed.Differences appear in the 50% Al 2 O 3 composites (hardness of about 460kG/mm 2 ) and the 75% Al 2 O 3 composites (830kG/mm 2 ), where the hardness was lower when using conventional mixing technique.With mechanical alloying, the hardness of the composite was higher by about 5%, which can be attributed to the more uniform distribution of the reinforcement within the composite matrix.

Bending strength
The bending strength σ g of the Al 2 O 3 -Cr composites was examined by the three-point method using the ZWICK 1446 strength machine with a 1kN head.The test samples were small bars sized at 2x3x30mm, the support span was 20mm and the head travel speed was 1mm/min.The average results are shown in Fig. 4.
These results indicate that the composites produced of powders prepared by mechanical alloying have a higher bending strength.In the composite composed of 25Al 2 O 3 -75Cr, the bending strength was about 480MPa on average, and exceeded significantly the strength of the composites with the powder blend prepared by the conventional method (slightly below 400MPa).As the alumina oxide share increased, the strength of the composites decreased, which was evidently related with the increased amount of the ceramic phase.However, even with the 75Al 2 O 3 -25Cr powder blend, mechanical alloying of the starting powders appeared to be more beneficial than the conventional technique: when these composites were prepared by mechanical alloying, the bending strength was higher by about 15% (about 350MPa) than it was when the powders were mixed by conventional milling (305MPa).This can be attributed to the more advantageous distribution of the reinforcing phase within the matrix.We can thus conclude that, from the point of view of the bending strength, mechanical alloying is decidedly more advantageous than the conventional method.

Frictional wear resistance
The After the wear test, the composite samples were examined in a Form Talysurf Series 2 scanning profile-meter (Taylor Hobson).The results included: a three-dimensional image of the sample wear (Fig. 5) and the volume of the groove representative of the wear (Tab.II).The indentation length examined was 1mm.As expected, the Al 2 O 3 -Cr composites appear to be highly resistant to frictional wear.Their wear resistance increases with increasing fracture of the alumina oxide in the composite.The wear also depends on the method of preparation of the starting powders: in general, when using the conventional method, the frictional wear resistance was lower than that achieved with mechanical alloying, which can be attributed to the presence of hard ceramic particles in the plastic chromium matrix.This improvement was however more pronounced in the composites with 25% and 50% of the Al 2 O 3 fraction, compared to this effect observed with 75% Al 2 O 3 .

Conclusions
The aim of this study was to find how the method used for mixing the starting powders (conventional milling or mechanical alloying) affected the properties of sintered Al 2 O 3 -Cr composites.The properties examined included: microstructure, density, hardness, bending strength and frictional wear resistance.
Irrespective of the method of powder preparation, the composites appeared to have a high density (96.5-99% of the theoretical value).
Microscopic observations confirmed that the relative density of the composites is high.Their structure is homogeneous.Mechanical alloying of the starting powders diversifies the sizes of the chromium grains and changes their shapes: they become elongated, which distinctly distinguishes them from the Cr grains present in the powder blends prepared by the traditional method.
The composites have a high hardness, dependent on the composition and shape of the starting powders.The highest hardness was shown by the 75%Al 2 O 3 -25%Cr composites: 833kG/mm 2 with the powders prepared conventionally, and 879kG/mm 2 when the powders were mixed by mechanical alloying.These results correspond with the measured values of the relative density, i.e. the higher the density (and thus the smaller number of pores), the higher the hardness.The flexural strength of the composites decreases with increasing fraction of aluminum oxide, which logically results from the increased share of the brittle ceramic phase.The highest strength was achieved when the powders were mixed by mechanical alloying.This can be attributed to the much more advantageous distribution of the reinforcing phase in the matrix in this case.As expected, the Al 2 O 3 -Cr composites show a high resistance to frictional wear, but this property greatly varies depending on the composition of the powder mixture and also on the method of its preparation.
In general, we can conclude that, in the production of sintered Al 2 O 3 -Cr composites, the use of mechanical alloying for the preparation of the powder mixture, instead of mechanical mixing in ball-mills, improves the mechanical and thermal properties of these composites.

Fig. 1
Microstructure of the powder composed of 25%Al 2 O 3 + 75%Cr (vol.%)after: mechanical mixing (a) and mechanical alloying (b).If the share of Al 2 O 3 (reinforcement) in the mixture exceeds significantly the share of the plastic matrix, the mixing process involves all the Cr grains, but only part of the Al 2 O 3 grains.In this case, the powder mixture has a dual structure: it contains Al 2 O 3 -Cr composite grains and a certain number of Al 2 O 3 grains not bound with the matrix.Here, however, the Al 2 O 3 powder agglomerates are not as numerous as those after mechanical mixing.